Renewable and Sustainable Energy Reviews 156 (2022) 111975 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Exergetic sustainability analysis of municipal solid waste treatment systems: A systematic critical review Salman Soltanian a, Soteris A. Kalogirou b, Meisam Ranjbari c, d, Hamid Amiri e, f, Omid Mahian g, h, i, Benyamin Khoshnevisan j, Tahereh Jafary k, Abdul-Sattar Nizami l, Vijai Kumar Gupta m, n, Siavash Aghaei o, Wanxi Peng a, **, Meisam Tabatabaei p, a, q, r, ***, Mortaza Aghbashlo s, a, * a Henan Province Engineering Research Center for Forest Biomass Value-added Products, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China Department of Mechanical Engineering and Materials Science, Cyprus University of Technology, Kitiou Kyprianou 36, 3041, Limassol, Cyprus c Department of Economics and Statistics “Cognetti de Martiis”, University of Turin, Turin, Italy d ESSCA School of Management, Lyon, France e Department of Biotechnology, Faculty of Biological Science and Technology, University of Isfahan, Isfahan, 81746-73441, Iran f Environmental Research Institute, University of Isfahan, Isfahan, 81746-73441, Iran g School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China h Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK i Renewable Energy and Micro/Nano Sciences Lab, Department of Mechanical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran j SDU Life Cycle Engineering, Institute for Green Technology (IGT), University of Southern Denmark, DK-5230, Odense, Denmark k Engineering Department, International Maritime College Oman, Sohar, Oman l Sustainable Development Study Centre, Government College University, Lahore, 54000, Pakistan m Biorefining and Advanced Materials Research Center, Scotland’s Rural College (SRUC), Kings Buildings, West Mains Road, Edinburgh, EH9 3JG, UK n Centre for Safe and Improved Foods, Scotland’s Rural College (SRUC), Kings Buildings, West Mains Road, Edinburgh, EH9 3JG, UK o Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Italy p Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia q Biofuel Research Team (BRTeam), Terengganu, Malaysia r Microbial Biotechnology Department, Agricultural Biotechnology Research Institute of Iran (ABRII), Agricultural Research, Extension, and Education Organization (AREEO), Karaj, Iran s Department of Mechanical Engineering of Agricultural Machinery, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran b A R T I C L E I N F O A B S T R A C T Keywords: Exergy analysis Sustainability indicators Municipal solid waste Waste-to-energy Incineration Gasification Anaerobic digestion The growing volume of municipal solid waste (MSW) generated worldwide often undergoes open dumping, landfilling, or uncontrolled burning, releasing massive pollutants and pathogens into the soil, water, and air. On the other hand, MSW can be used as a valuable feedstock in biological and thermochemical conversion processes to produce bioenergy carriers, biofuels, and biochemicals in line with the United Nations’ Sustainable Development Goals (SDGs). Valorizing MSW using advanced technologies is highly energy-intensive and chemicalconsuming. Therefore, robust and holistic sustainability assessment tools should be considered in the design, construction, and operation phases of MSW treatment technologies. Exergy-based methods are promising tools for achieving SDGs due to their capability to locate, quantify, and comprehend the thermodynamic inefficiencies, cost losses, and environmental impacts of waste treatment systems. Therefore, the present review paper aims to * Corresponding author. Henan Province Engineering Research Center for Forest Biomass Value-added Products, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China. ** Corresponding author. Henan Province Engineering Research Center for Forest Biomass Value-added Products, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China. *** Corresponding author. Henan Province Engineering Research Center for Forest Biomass Value-added Products, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China. E-mail addresses: pengwanxi@163.com, pengwanxi@henau.edu.cn (W. Peng), meisam_tab@yahoo.com, meisam.tabatabaei@umt.edu.my (M. Tabatabaei), maghbashlo@ut.ac.ir (M. Aghbashlo). https://doi.org/10.1016/j.rser.2021.111975 Received 15 May 2021; Received in revised form 16 November 2021; Accepted 1 December 2021 Available online 10 December 2021 1364-0321/© 2021 Elsevier Ltd. All rights reserved. S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 comprehensively summarize and critically discuss the use of exergetic indicators for the sustainability assessment of MSW treatment systems. Generally, consolidating thermochemical processes (mainly incineration and gasification) with material recycling methods (plastic waste recovery), heat and power plants (steam turbine cycle and organic Rankine cycle), modern power technologies (fuel cells), and carbon capture and sequestration processes could improve the exergetic performance of MSW treatment systems. Typically, the overall exergy efficiency values of integrated MSW treatment systems based on the incineration and gasification processes were found to be in the ranges of 17–40% and 22–56%, respectively. The syngas production through the plasma gasification process could be a highly favorable waste disposal technique due to its low residues and rapid conversion rate; however, it suffers from relatively low exergy efficiency resulting from its high torch power consumption. The overall exergy efficiency values of integrated anaerobic digestion-based MSW processing systems (34–73%) were generally higher than those based on the thermochemical processes. Exergy destruction and exergy efficiency were the most popular exergetic indicators used for decision-making in most published works. However, exergoeconomic and exergoenvironmental indices have rarely been used in the published literature to make decisions on the sustainability of waste treatment pathways. Future studies need to focus on developing and realizing integrated waste biorefinery systems using advanced exergy, exergoeconomic, and exergoenvironmental methods. 1. Introduction glass, waste electrical/electronic devices, batteries/accumulators, and bulky elements (i.e., mattresses and sofas) [1,2]. Generally, organic materials, paper/cardboard, and plastic materials account for more than 70% of the whole MSW (Fig. 1). It should be noted that all the things dumped/landfilled at the local sites, such as construction and demolition waste, wastewater sludge, and harmless industrial wastes, cannot Municipal solid waste (MSW) refers to the commingled or separate waste streams discarded by consumers after using the products, including biowaste (i.e., food residues and garden trimmings), paper/ cardboard, wood products, plastics, textiles, rubber/leather, metals, Fig. 1. Average composition of the MSW generated in some selected countries and the world. United States’ pie chart was redrawn with permission from Ref. [2]. Copyright© 2018 U.S. Environmental Protection Agency (EPA). England’s pie chart was redrawn with permission from Ref. [6]. Copyright© 2017 The Waste and Resources Action Programme (WRAP). China’s pie chart was redrawn with permission from Ref. [7]. Copyright© 2014 Elsevier. The world’s pie chart was redrawn with permission from Ref. [8]. Copyright© 2018 The World Bank (License: Creative Commons Attribution CC BY 3.0 IGO). Note: The provided data cannot be exactly comparable due to the difference in MSW data collection and analysis methods [9]. 2 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 be considered municipal waste [2]. Rapid modernization, urbanization, and industrialization have resulted in the massive MSW generation [3]. As shown in Fig. 2, it is projected that MSW generation will significantly escalate in the near future as countries continue striving for increased industrial development [4,5]. The growing surge in MSW generation has provoked serious economic, environmental, health, and social crises, underlining the necessity of taking urgent action to reduce the waste volume [10]. Traditionally, a large portion of the generated MSW gets recklessly combusted/burned or openly dumped/landfilled, releasing various hazardous substances and consequently triggering irremediable environmental burdens [11]. Uncontrolled solid waste disposal is responsible for about 5% of global greenhouse gas emissions [12]. In addition, improper waste disposal strategies could also result in numerous difficulties for municipalities in small cities with limited landfill space [13]. According to the current legislation and policy, the highest priority direction for waste reduction is the prevention of waste generation, followed by waste re-use, material recycling, energy and fuel/chemical recovery, and controlled disposal (Fig. 3). However, the waste prevention strategy lacks reliable data, rigorous measures, comprehensive information, adequate resources, accepted social/cultural norms, longterm finance, and large organizational capacity [14]. Besides, it heavily depends on the other waste treatment techniques, indicating the significance of waste management as a whole framework. A robust MSW management system not only can minimize waste accumulation but also can produce a broad spectrum of raw materials, energy carriers, transportation fuels, and chemical platforms. Furthermore, it can also conserve energy/natural resources, avoid greenhouse gas/pollutant emissions, and improve public health [15]. Implementing such environmentally-friendly management systems is globally becoming compulsory to curb the adverse environmental consequences of MSW. In line with this trend, the European Waste Framework Directive [1] has mandated to take the required measures to assure that waste management is performed without threatening human well-being, damaging terrestrial and aquatic ecosystems, causing unpleasant noise/odors, and spoiling the landscape. This is why landfilling of untreated MSW is strictly banned in Europe [1]. Moreover, the practical realization of productive waste management systems is consistent with goals 2, 3, 7, and 11, 12, 13, 14, and 15 of the United Nations’ Sustainable Development Goals (SDGs). Accordingly, MSW management systems should turn away from outdated waste disposal techniques to applying advanced treatments in the context of circular bioeconomy [16]. There are various technological options for MSW disposal and treatment, as depicted in Fig. 4. Table 1 also presents the capability of different waste disposal/treatment technologies to handle various components of MSW. The capacity of some common MSW management methods on a global scale is shown in Fig. 5. Landfilling is the most widely practiced MSW treatment method by disposing of about 44% of the total MSW generated worldwide. In addition, 9% of the generated MSW on a global scale is openly dumped without control. Typically, the hydrogen-to-carbon atomic ratio of MSW is higher than biomass feedstocks, making it a suitable feedstock to produce valuable products (Fig. 6). In addition, the oxygen-to-carbon atomic ratio of MSW is close to peat. Different biological (composting, anaerobic digestion, and fermentation), thermochemical (incineration, pyrolysis, gasification, and carbonization), and thermo-mechanical techniques have been introduced and commercialized to valorize MSW into bioenergy carriers, biofuels, biochemicals, and biofertilizers. More specifically, the biodegradable fraction of MSW can be partially converted into biogas in anaerobic reactors and compost in aerobic vessels [27–29]. Landfill gas can also be collected from sanitary landfill sites. The biologically produced methane-rich biogas can then be utilized in power generation facilities (e.g., internal combustion gas engines) to produce heat and electricity [30]. The generated compost can be used for a variety of agricultural and industrial purposes. The combustible portion of MSW can be burned in incineration systems under an oxygen-rich atmosphere to produce thermal energy or can be heated in pyrolysis reactors under an oxygen-free environment to produce bio-oil [31,32]. The evolved bio-oil can be further upgraded into transportation fuels and chemical platforms [33]. The gasification process carried out in an oxygen-limited atmosphere can also be used to convert the combustible part of MSW into syngas [45]. The produced syngas can be subsequently used in power generation systems (i.e., steam boilers, internal combustion gas engines, gas turbines, and fuel cells) to generate heat and electricity, Fischer-Tropsch catalytic synthesis to produce liquid transportation fuels and chemicals, and biological fermentation process to synthesize a wide spectrum of alcohols [46,47]. Furthermore, the combustible fraction of MSW can be converted into refuse-derived fuel by dewatering, crushing, compressing, and pelletizing procedures [48]. The produced refuse-derived fuel can be used to generate electricity or substitute fossil fuels [49]. Overall, thermochemical conversion routes are inseparable elements of advanced integrated MSW management strategies due to their high efficiency and conversion rate [50]. Even though MSW management systems using modern technologies are advantageous from environmental, economic, social viewpoints, the need for large amounts of capital inflows, operation costs, energy sources, and chemical substances is a significant hurdle [51]. Accordingly, the sustainability of MSW management has become a vital issue among researchers, policymakers, and stockholders. There are different techniques, including life cycle assessment (LCA), techno-economic analysis, and thermodynamic methods, to weigh the sustainability level of MSW management systems [52]. The LCA approach can provide decision support information regarding a product/service system by quantifying and assessing its potential lifecycle environmental impacts [53,54]. This environmental sustainability assessment method is carried out in four sequential steps: defining goal and scope, analyzing inventory data, assessing environmental impacts, and interpreting the results [54,55]. The LCA technique has been widely applied to evaluate and compare the environmental features of different MSW management systems [10,54,56]. Choice of system boundaries and freedom of interpretation are the main drawbacks of the LCA method [57]. The techno-economic approach performed by applying the life cycle cost method can offer valuable insights concerning the technical viability and commercial promise of a set of MSW management scenarios. However, it cannot account for the thermodynamic and environmental aspects of MSW management systems, rendering it unsuitable for reliable decision-making [52]. The quantity of the environmental work previously directly and Fig. 2. Projection of MSW generation in different regions worldwide. Redrawn with permission from Ref. [8]. Copyright© 2018 The World Bank (License: Creative Commons Attribution CC BY 3.0 IGO). 3 Renewable and Sustainable Energy Reviews 156 (2022) 111975 S. Soltanian et al. Fig. 3. Waste management priority diagram. Adapted from Article 4 of European Waste Framework Directive [1]. Fig. 4. Various common technological options for disposing and treating MSW [17–23]. Table 1 Capability of different waste disposal/treatment technologies to handle various components of MSW [24,25]. Landfilling Composting Anaerobic digestion Fermentation Thermo-mechanical treatment Incineration Pyrolysis Gasification Plasma gasification Hydrothermal processing Recycling Food waste Garden and other organic wastes Paper and cardboard Wood waste Glass Metals Plastics Textiles Rubber and leather ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ⨯ ⨯ ⨯ ⨯ ✓ ⨯ ⨯ ⨯ ⨯ ✓ ⨯ ⨯ ⨯ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ⨯ ⨯ ⨯ ✓ ✓ ✓ ✓ ✓ ✓ ⨯ ✓ ✓ ✓ ✓ ✓ ⨯ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ⨯ ✓ ⨯ ⨯ ✓ ⨯ ✓ ✓ ⨯ ⨯ ✓ ⨯ ✓ ✓ ✓ ✓ ✓ ⨯ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ⨯ ✓ 4 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 5. Share of common waste management methods in treating the generated MSW globally. Redrawn with permission from Ref. [26]. Copyright© 2012 The World Bank. Fig. 7. Conceptual illustration of exergy at the intersection of energy, environment, and sustainable development. Redrawn with permission from Dincer and Zamfirescu [64]. Copyright© 2016 Elsevier. of waste treatment systems [67]. More specifically, the quantity of exergy destruction of MSW management systems can be translated into resource depletion and economic loss, as shown in Fig. 8 [68,69]. Nevertheless, the exergy concept can be systematically extended to include the economic and environmental constraints, resulting in enhanced exergoeconomic and exergoenvironmental methods [59,70]. The exergetic indicators determined using exergoeconomic and exergoenvironmental techniques can locate, quantify, and explain economic losses and environmental burdens of MSW management systems at the component level [71,72]. Neither exergy analysis nor simple economic accounting/environmental assessment can provide such an invaluable practical guide [73]. Given the capability of the exergy-based methods to deal with the sustainability issue, many reports have emerged on the application of exergetic indicators to evaluate the sustainability aspects of MSW management systems. Accordingly, the present review paper is devoted to comprehensively summarizing and critically discussing the use of Fig. 6. Extended Van Krevelen diagram for various feedstocks, including MSW. The data for different MSW samples are obtained from Refs. [34–44]. indirectly used to generate a product or service is called “emergy” [58]. This thermodynamic method translates all the energy, material, and money flows supplied to the production system into solar emjoules (sej) [59]. The main limitations of the emergy approach are the uncertainty related to the transformity values and the need for some allocation decisions [60]. The energy analysis based on the energy conservation principle is the most widely used sustainability assessment tool in the published literature [61]. The energy approach determines the first-law efficiency of energy systems by considering input, output, and generated energetic streams. The energy accounting method cannot account for the quality of energy and material flows, rendering it insufficient to measure sustainability and make accurate decisions [62,63]. Among the various sustainability assessment tools introduced so far, exergy-based analyses have been proved to be sound and reliable approaches in achieving sustainable development goals. The conceptual illustration of exergy at the intersection of energy, environment, and sustainable development is portrayed in Fig. 7. In simple words, exergy refers to the maximum useful work provided by a thermodynamic system on which it interacts with the environment through reversible processes [65,66]. Exergy analysis is a promising tool for achieving sustainable development goals (SDGs) due to its capability to locate, quantify, and comprehend the thermodynamic inefficiencies Fig. 8. Exergy destruction reflects resource depletion and economic loss. Redrawn and slightly modified with permission from Soltanian et al. [52]. Copyright© 2020 Elsevier. 5 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 exergetic indicators for the sustainability assessment of MSW management systems. In addition, the merits and demerits of exergy-based indices in the sustainability assessment of MSW management systems are highlighted to identify future research directions. It is worth noting that there is no systematic critical review in the published literature focusing on the progress and development of exergy-based methods in MSW management systems to the best of the authors’ knowledge. inclusion and exclusion criteria, only peer-reviewed articles in the English language were included in the research. Accordingly, other document types, such as conference papers, book chapters, editorials, notes, and letters, were excluded from the extracted articles in this stage. As a result, 172 articles remained for further screening. In the third step, the articles were filtered through screening the titles and abstracts to select the papers related to the main focus of this research. In the next step, a more in-depth reading of the full text was carried out on the selected articles to finalize the sample selection, leading to a total of 89 articles. Finally, to ensure sufficient coverage of the selected papers, a snowballing technique [76] was followed to scan the references of the articles collected in the previous step. In the end, 128 articles were picked up as the final sample of the present systematic review. Table 2 provides the details of the search protocol and the data collection process. Authors’ keywords within each research can convey the main idea and scope of the research [77]. On this basis, keyword-based analyses have been widely used by scholars in the waste management domain, such as biodiesel production from waste cooking oil [78], healthcare waste management [79], plastic waste recycling via pyrolysis [80], and waste-to-energy technologies towards a circular economy [81]. Before conducting the keyword analysis in this research, some amendments were made to the keywords of the articles included in the database. In this regard, the full forms and abbreviations and the singular and plural forms of the keywords were merged. Besides, English and American styles of spelling the keywords were unified. As a result, among the 497 keywords within our research database, the top 20 most frequent keywords that appeared in the exergetic sustainability analysis of MSW management research are provided in Table 3. Clearly, exergy, municipal solid waste, and gasification are the top three most frequent keywords with 44, 36, and 21 occurrences, respectively. The next most frequent keywords in this domain are exergy analysis, life cycle analysis, energy, waste heat recovery, thermodynamic analysis, organic Rankine cycle, waste to energy, biomass, biogas, efficiency, and sustainability. In Table 3, the total link refers to the number of keywords that a specific keyword has co-occurred with, and the total link strength shows the total times that a keyword has co-occurred with other keywords. Exergy, municipal solid waste, and gasification have the highest link strength not only in Table 3 but also among all the other keywords which are not reported in this table. Moreover, the co-occurrence network of the keywords was built using the VOSviewer software (version 1.6.16) [82], as illustrated in Fig. 10. A threshold of a minimum of three keyword occurrences was applied to better visualize the keywords, leading to 48 among 497 keywords in the database, as plotted in Fig. 10. In this co-occurrence network, the larger circles, the more occurrences of the keywords. 2. Systematic literature review A systematic literature review was conducted based on the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework [74] to provide the state-of-the-art of using exergetic indicators in the sustainability assessment of MSW treatment systems. To this end, five steps within four phases are considered, as illustrated in Fig. 9. The search protocol employed in the present review to collect relevant articles from the target literature is described in the following paragraphs. Defining a suitable search strategy to capture as many relevant studies as possible has been highlighted by many scholars as one of the most important prerequisites in conducting systematic reviews [75]. In this regard, a structured five-step method was utilized to select the final sample of articles for further consideration in this review. In the first step, the search string was formulated based on different combinations of the keywords “exergy” and “municipal solid waste” as the core keywords of the present research. On this basis, the following search string was designed: (“exergy” OR “exergetic”) AND (“solid waste” OR “food residue” OR “food waste” OR “wood waste” OR “paper waste” OR “textile waste” OR “plastic waste” OR “rubber waste”). In the second step, the Scopus database was selected for record identification and article collection. The initial run returned 222 articles. Given the Table 2 Details of the search strategy. Step Description Search string “exergy” OR “exergetic" AND “solid waste” OR “food residue” OR “food waste” OR “wood waste” OR “paper waste” OR “textile waste” OR “plastic waste” OR “rubber waste" Scopus Article titles, abstracts, author keywords, and keywords plus August 14, 2021 Up to 2021 Peer-reviewed journal articles in the English language Conference papers, book chapters, editorial materials, letters, and non-English documents 172 articles 89 articles 39 articles Database Run on Date of running Years Inclusion criteria Exclusion criteria Initial Result Eligible articles Snowballed articles Final sample Fig. 9. Steps of the search protocol to collect data based on the PRISMA flowchart. 6 128 article S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 respectively. A text mining analysis using the VOSviewer software was conducted on the titles and abstracts of the articles in our pool to unveil the recentness of the terms used within the studied research field. In this regard, first, the terms in the text were identified (a total of 4685 terms), and a selection of the main relevant keywords with a minimum of 3 occurrences was made. Then, considering the publication year of the articles that contain these terms, the average publication year was extracted for each selected term. Fig. 11 shows a timeline from 2013 to 2021, including the selected terms categorized based on the average publication year of the articles in which they appear. Clearly, exergy destruction rate, molten carbonate fuel cell, low carbon power production, environmental life cycle assessment, downdraft gasifier, Table 3 Top 20 most frequent authors’ keywords. Keyword Occurrences Total link Total link strength Exergy Municipal solid waste Gasification Exergy analysis Life cycle assessment Energy Waste heat recovery Thermodynamic analysis Organic Rankine cycle Waste-to-energy Biomass Biogas Efficiency Sustainability Energy recovery Incineration Optimization Plastic waste Solid oxide fuel cell Syngas 44 36 21 18 17 12 12 11 10 10 9 8 7 7 6 6 6 6 6 6 31 27 25 17 19 10 10 13 13 17 15 12 9 4 6 10 10 7 12 12 80 65 55 22 32 33 14 19 15 22 26 16 23 6 9 14 15 8 16 20 Table 4 The most frequent pairs of keywords. Besides, the existence of a link between each pair of keywords indicates the co-occurrence of those keywords, and the thickness of the links refers to the link strength. Table 4 presents the pair of keywords with a link strength of more than 3. Obviously, exergy appears in 7 pairs of most frequent keyword pairs. Exergy and gasification have been found in 10 articles together, followed by exergy and energy appearing 9 times together. Municipal solid waste has been mentioned in the authors’ keywords 7 times with exergy and 7 times with gasification. The pairs of energy and efficiency and exergy and efficiency emerged 6 and 5 times, Keyword 1 Keyword 2 Link strength Exergy Energy Exergy Gasification Efficiency Efficiency Biomass Biomass Energy Energy Exergy Exergy Exergy analysis Life cycle assessment Gasification Exergy Municipal solid waste Municipal solid waste Energy Exergy Exergy Gasification Gasification Municipal solid waste Life cycle assessment Waste to energy Municipal solid waste Municipal solid waste 10 9 7 7 6 5 4 4 4 4 4 4 4 4 Fig. 10. Co-occurrence network of the authors’ keywords. 7 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 11. Timeline of the main subject areas of research in the literature. gasification process, heat recovery unit, municipal solid waste incineration, and global warming potential have been attracting attention recently with average publication years above 2020. On the other hand, subject areas such as waste incineration, waste biomass, cogeneration, metal recovery, biomass fuel, acidification, synthetic natural gas, and waste heat have been identified as the less recent terms in the target literature with an average publication year less than 2018. results reported in the published literature. The exergy rates of various streams involved in the process should first be obtained using the equations provided in Table 5. After that, exergetic indicators can be computed for the main components and the whole plant. The exergetic data obtained can then be used in exergoeconomic and exergoenvironmental analyses. Further to the equations summarized in Table 5, N‒1 auxiliary equations should be defined for exergoeconomic and exergoenvironmental analyses. These equations (F and P rules) can be obtained using the specific exergy costing approach (SPECO) elaborated by Lazzaretto and Tsatsaronis [107]. The environmental impacts of a given component/stream can be obtained using different environmental impact assessment approaches, i.e., Eco-indicator 99, IMPACT 2002+, and ReCiPe methods. 3. Theoretical considerations The procedures of implementing exergy, exergoeconomic, and exergoenvironmental analyses for a bioenergy system have been comprehensively elaborated by Soltanian et al. [52]. The formulas used in the exergetic, exergoeconomic, and exergoenvironmental calculations of a given component of a waste management system under steady-state conditions are tabulated in Table 5 to better understand the 8 Renewable and Sustainable Energy Reviews 156 (2022) 111975 S. Soltanian et al. Table 5 Formulas used in the exergetic, exergoeconomic, and exergoenvironmental calculations of a given component of a waste management system. Formula ∑ ∑ ṁi = ṁe e i e i ∑ ˙ ∑ ∑ Exq,j + ṁi ei = Ẇ + ṁe ee + Ėd e i j Notation(s) Ref. Mass balance of a component ṁ (mass flow rate, kg/s), subscript i (inlet stream), subscript e (exit stream) [52] Energy balance of a component ∑ ∑ ∑ Q̇j + ṁi hi = Ẇ + ṁe he j Application Exergy balance of a component Q̇ (heat transfer rate, kW), Ẇ (work rate, kW), h (specific enthalpy, kJ/kg), subscript j (numerator) e (Total specific exergy, kJ/kg), Ė (exergy rate, kW), subscript d (destruction), subscript q (thermal energy) [83] [52] ˙ q,j = Q̇j 1–T0 Ex Tj e = eph + ech + eke + epe Exergy of thermal energy T (temperature, K), subscript 0 (reference state) [84] Total specific exergy of a stream [52] eph = h–h0 –T0 (s–s0 ) Specific physical exergy of a pure stream Specific physical exergy of a mixed liquid stream Specific physical exergy of a mixed gaseous stream Specific heat capacity of a mixed liquid/gaseous stream Gas constant of a mixed liquid/ gaseous stream Superscripts ph (physical), ch (chemical), ke (kinetics), and pe (potential) ( ) ( ( )) T eph = C T–T0 –T0 ln T0 ( ( )) ( ) T P + RT0 ln eph = C T–T0 –T0 ln T0 P0 ∑ C = x l Cl l R =∑ R l y l Ml ( ) ∑ ∑ ∑ 1 yl e0l + RT0 yl ln(yl ) l l y l Ml ∑l ∑ nm e0m – e0 = –ΔG + nn e0n ech = Product Reactant Specific chemical exergy of a mixed liquid/gaseous stream Standard chemical exergy of an inorganic compound ech = {363.439C + 1075.633H–86.308O + 4.14N + 190.798S–21.1A} × 100 Specific chemical exergy of a solid or liquid organic compound ech = β × LHV Specific chemical exergy of a solid fuel ( )[ ( )] ( ) ( ) H O H N –0.3493 1 + 0.0531 + 0.0493 C C C C ( ) β = O 1–0.4124 C ech = {376.461C + 791.018H–57.819O + 45.473N–1536.242S + 100.981Cl} × 100 1.044 + 0.016 ech = {377.535C + 785.711H–58.446O + 45.682N–1536.242S + 103.486Cl} × 100 ech = {374.642C + 806.343H–57.074O + 48.693N–1533.261S + 101.425Cl} × 100 ech = {376.879C + 787.351H–58.654O + 46.398N–1533.261S + 100.981Cl} × 100 ech = {376.580C + 790.869H–58.475O + 44.639N–1538.180S + 98.566Cl} × 100 1 V2 2 × 1000 1 gz = 1000 eke = epe φ = ψ = Ėe Ėd = 1– Ėi Ėi Ė useful product(s) Ėi Ėd Ėd,tot IP = (1–φ)(Ėi –Ėe ) γ = Ėd = 1–φ Ėi 1 SI = DP ∑ ˙ ∑ ∑ cj Exq,j + ci ṁi ei + Ż = cw Ẇ + ce ṁe ee DP = j Ż = i e Z CRF φ H CRF = I(1 + I)N (1 + I)N –1 cp –cf rc = cf Ż Ż fc = = ∑ Ż + Ċd∑ Ż + cf Ėd ∑ ˙ q,j + bi ṁi ei + Ẏ = bw Ẇ + be ṁe ee bj Ex j i s (specific entropy, kJ/kg K) [85] C (specific heat capacity, kJ/kg K) [86] R (gas constant, kJ/kg K), P (pressure, kPa) [87] x (mass fraction, –), subscript l (numerator) [87] R (universal gas constant, kJ/mol K), y (mole fraction, –), M (molar mass, kg/mol) [88] e0 (standard chemical exergy, kJ/mol) [89] ΔG (Gibbs free energy, kJ/mol), n (mole number, –), subscripts m and n (numerators) [90] C (carbon fraction, –), H (hydrogen fraction, –), O (oxygen fraction, –), N (nitrogen fraction, –), S (sulfur fraction, –), and A (ash fraction, –) [91] LHV (fuel low heating value, kJ/kg) [65, 92] Cl (chlorine fraction, –) [93] – [93] – [93] – [93] – [93] Specific kinetic exergy of a stream V (velocity, m/s) [94] Specific potential exergy of a stream g (gravitational acceleration constant, m2/s), z (height, m) [52] Specific chemical exergy of an MSW stream Specific chemical exergy of a biowaste stream Specific chemical exergy of a woody or paper waste stream Specific chemical exergy of a plastic waste stream Specific chemical exergy of a textile or rubber waste stream Universal exergy efficiency of a componenta Functional exergy efficiency of a componenta Exergy destruction ratio of a component Exergetic improvement potential rateb Depletion factor of a component or a system Sustainability index of a component or a system Cost balance of a component Investment cost rate of a component Capital recovery factor of a component Relative cost difference of a component Exergoeconomic factor of a component e φ (universal exergy efficiency, %) [95] ψ (functional exergy efficiency, %) [96] γ (exergy destruction ratio, %), subscript tot (total) [97] IP (exergetic improvement potential rate, kW) [83] DP (depletion factor, –) [98] SI (sustainability index, –) [98] c (specific cost of exergy, USD/kJ), Ż (investment cost rate, USD/s), subscript w (work) [99] Z (capital investment cost, USD), CRF (capital recovery factor, –), φ (operating and maintenance factor, –), H (annual working time, h) [100] I (real interest rate, –), N (lifespan, year) [101] rc (relative cost difference, –), subscript p (product), subscript f (fuel) [73] fc (exergoeconomic factor, %), Ċ (cost rate, USD/s) [102] [52] (continued on next page) 9 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Table 5 (continued ) Formula Y N×H bp –bf rb = bf Ẏ Ẏ fb = = Ẏ + Ḃd Ẏ + bf Ėd Ẏ = Application Notation(s) Environmental impact balance of a component b (specific environmental impact of exergy, mPts/kJ), Ẏ (environmental impact rate, mPts/s) Environmental impact rate of a component Relative environmental impact difference of a component Exergoenvironmental factor of a component Ref. Y (total environmental impact rate, mPtsc) [103] rb (relative environmental impact difference, –) [104] fb (exergoenvironmental factor, %), Ḃ (environmental impact rate, mPts/s) [105] a The universal exergy efficiency (φ) measures the degree of exergy disposition of a given MSW treatment system due to entropy generation and heat rejection. The value of universal exergy efficiency approaches zero when all the input exergy of an MSW treatment system is wasted because of entropy generation and heat rejection and vice versa. The functional exergy efficiency (ψ ) determines the degree of exergetic profitably and productivity of a given MSW treatment to utilize the supplied exergy according to the predefined purposes. The functional exergy efficiency comes close to 100% when all the input exergy of an MSW treatment system is spent for producing the useful product(s) and vice versa [52]. b The exergy improvement potential rate (IP) measures the possibility of enhancing the exergetic efficiency of a system. c The unit “mPts” refers to milli-points. One Pt is the value of one-thousandth of the annual environmental burden per one European citizen [106]. 4. Municipal solid waste conversion to energy and acid gases (mainly HCl, HF, and SOX) [116]. In such cleaning processes, gypsum might be formed, which should be removed or recovered to mitigate the salt concentration in the wastewater [116]. In addition, incineration residues, such as bottom ash, slag, boiler ash, fly ash, wastewater sludge, and dust, can initiate serious ecological troubles due to their high content of heavy metals. Accordingly, these solid residues should be treated to produce useful substances (i.e., construction materials, ferrous scrap, and non-ferrous scrap) before landfilling, avoiding the formation of harmful leaching [116,117]. Accordingly, advanced thermodynamic methods like exergy-based indices should be applied to improve the sustainability and viability of MSW incineration systems. Notably, exergetic indicators (e.g., exergy efficiency) could be a more reliable sustainability measure for the MSW incineration process than the frequently used indices such as R1 formula efficiency reported in the European Waste Framework Directive, as demonstrated by Grosso et al. [118]. Table 6 summarizes the most important applications of the exergy-based methods in analyzing MSW incineration systems. Zhou et al. [120] showed that integrating incineration with landfilling-composting could yield higher exergy efficiency than the landfilling and landfilling-composting approaches. In addition, the highest exergy efficiency was found for the incinerator, followed by the landfilling and composting processes. The incineration technology can also be integrated with the material recycling systems to compensate for its sustainability shortcomings by simultaneously recovering energy and materials from MSW. In this framework, Laner et al. [68] compared different household solid waste management scenarios, including incineration and bottom ash treatment (first scenario), incineration, bottom ash treatment, and metal recycling (second scenario), incineration, bottom ash treatment, and metal/plastic recycling (third scenario), and incineration, bottom ash treatment, metal/plastic recycling, and anaerobic digestion (fourth scenario) from the exergetic and exergetic life cycle assessment (ELCA) perspectives. The exergetic efficiency values were higher for all the investigated scenarios than the ELCA efficiency values. The exergetic efficiency was increased by including more material recycling and digestion processes. The ELCA performance of the incineration process was improved when it was integrated with the metal recycling process. The observed improvement could be attributed to the fact that the metal recycling process could reduce the need for primary metal production and subsequent cumulative exergy consumption. However, plastic recycling and organic waste anaerobic digestion could not promote ELCA efficiency considerably. In contrast with the ELCA approach, the plastic recycling process was more favorable from the exergetic viewpoint. By calculating the cumulative exergy extraction from the natural environment, Huysman et al. [141] showed that open-loop or closed-loop plastic waste recycling approaches were more resource-efficient compared with landfilling and incineration with energy recovery. In a more detailed analysis carried out by Balcom and Carey [142], it was observed that pre-processing units (particularly Converting MSW into energy is a promising strategy for mitigating the waste volume, dealing with the energy security issue, preserving natural terrestrial/aquatic resources, and minimizing public health risks. The most popular approaches to convert MSW to energy include biological (i.e., landfill gas collection and anaerobic digestion) and thermochemical treatment methods (i.e., incineration, pyrolysis, gasification, and hydrothermal processing) [108]. All the MSW treatment methods practiced should be assessed from the exergetic viewpoints to identify their strengths and weaknesses and devise plans to improve their thermodynamic, economic, and environmental performance. 4.1. Thermochemical treatment of municipal solid waste Thermochemical routes, including incineration, pyrolysis, gasification, and hydrothermal processing, are playing a pivotal role in the energy recovery of MSW [109]. However, these processes intrinsically suffer from high capital/operating costs, greenhouse gas/pollutant emissions, and massive chemical intake [110]. Therefore, advanced sustainability assessment procedures should be considered to alleviate the problems mentioned above. 4.1.1. Municipal solid waste incineration There is a consensus to prefer compact incineration systems over landfilling to treat MSW due to their capability to degrade and immobilize harmful compounds, mitigate waste volume, and exploit the waste chemical potential [7]. In this waste treatment method, the combustible components of MSW are burned to generate heat and electricity. Despite the universality and simplicity of the incineration method, it generates a considerable amount of toxic substances such as dioxins (i.e., polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans), heavy metals (Hg, Cd, As, Pb, and Ni), acid and other gases (HCl, HF, HBr, HI, NOX, SO2, and NH3), and particulate matter [111,112]. Significant technological advances have offered numerous solutions to reduce a considerable portion of pollutant emissions from MSW incineration systems, particularly dioxins [113–115]. However, waste incineration systems consume a huge amount of electricity, heat, support fossil fuels (e.g., natural gas, light oils, and coal), water, exhaust-gas cleaning additives (e.g., limestone, caustic soda, activated carbon, and ammonia), water treatment chemicals (e.g., sodium sulfite), and high-pressure air [116]. The incineration technology can be incorporated into conventional MSW management systems to alleviate the undesirable aspects of ineffective disposal methods, i.e., landfilling. However, the exhaust gas should be cleaned using chemical additives such as limestone, hydrated lime, and hydrogen peroxide to minimize the release of heavy metals (mainly Hg), nitrogen-containing compounds (mainly NOX and NH3), 10 11 Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries Key parameter(s) Reference conditions Exergetic indicator(s) of MSW incineration unit Incineration Biowaste Electricity, heat Commercial Incineration T0 = 298.15 K, P0 = 1.0 atm φ = 9.03% Incineration integrated with bottom ash treatment Incineration integrated with landfilling and composting Separately collected residual waste Electricity, heat, Fe, Al, and Cu scraps NA Collection, transportation, incineration, ash treatment, landfilling Waste flow rate = 7500 tonnes/day, Waste lower heating value = 7.00 MJ/kg NA Mixed MSW (incineration and landfilling), biowaste (composting) Electricity, biogas, compost Commercial Transportation, mechanical separation, landfilling, composting, incineration Incineration combined with district heating NA Heat Commercial Incineration, geothermal heating, natural gas heating, co-firing heating, district heating network Incineration integrated with bottom ash treatment and metal recycling Separately collected metals and residual waste Electricity, heat, Fe, Al, and Cu scraps NA Collection, transportation, incineration, metal sorting, ash treatment, landfilling Incineration integrated with bottom ash treatment and Separately collected metals, plastics, and residual waste Electricity, heat, Fe, Al, and Cu scraps, polyethylene NA Collection, transportation, incineration, metal sorting, plastic sorting, ash treatment, landfilling Waste flow rate = 2900 tonnes/day, Incineration energy recovery efficiency = 80%, Organic waste-tocompost conversion efficiency = 25% Waste lower heating value = 8.80 MJ/kg, Incineration energy recovery efficiency = 95%, Groundwater temperature = 10–55 ◦ C, Supply district heating temperature = 90 ◦ C, Return district heating temperature = 40 ◦ C, System lifetime = 25 years, Salvage-to-capital costs = 10%, Interest rate = 6% NA NA T0 = 298.15 K, P0 = 1.0 bar NA T0 = 276.3 K, P0 = 1.0 atm T0 = 298.15 K, P0 = 1.0 bar T0 = 298.15 K, P0 = 1.0 bar – System exergetic indicator(s) – ψ = 17.1% ψ LCA = 13.9% Remark(s) Ref. The MSW incineration process had higher internal irreversibility compared with the landfill biogas and plasma gasification processes. The incineration process was responsible for the majority of the total exergy destruction. [119] [68] φ = 15.62% φ = 14.0% The incineration process seemed to be a relatively good solution to convert the chemical exergy of waste into electrical exergy. [120] φ = 17.0% rc = 12.92–15.30 (−) fc = 62.0–68.0% φ = 4.2% Compared with natural gas and geothermal heat, the use of MSW had the highest positive impact on the exergoeconomic aspects of the district heating system. [121] ψ = 17.6% ψ LCA = 14.5% Despite the low exergy recovery potential of the metal recycling process, it could slightly improve the exergetic aspect of the MSW disposal system. The plastic recycling process could promisingly enhance the exergetic performance of the MSW treatment systems. [68] – – ψ = 25.2% ψ LCA = 14.3% [68] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 MSW treatment approach S. Soltanian et al. Table 6 Most important applications of the exergy-based methods in analyzing MSW incineration systems. MSW treatment approach Waste fraction(s) metal/plastic recycling 12 Incineration integrated with bottom ash treatment, metal/ plastic recycling, and organic waste anaerobic digestion Separately collected metals, plastics, organic waste, and residual waste Incineration combined with organic Rankine cycle and absorption refrigeration Mixed infectious medical waste and municipal biowaste Incineration combined with organic Rankine cycle, supercritical CO2 power cycle (Brayton cycle), absorption refrigeration, and district heating NA Main product(s) terephthalate, high-density polyethylene Electricity, heat, biogas, digestate Fe, Al, and Cu scraps, polyethylene terephthalate, high-density polyethylene Electricity, cold Electricity, cold key stages inside the investigated boundaries Key parameter(s) Reference conditions NA Collection, transportation, anaerobic digestion, incineration, metal sorting, plastic sorting, ash treatment, landfilling Methane yield = 98 m3/tonne of wet food T0 = 298.15 K, P0 = 1.0 bar Pilot (small) Incineration, organic Rankine power generation, absorption refrigeration NA Commercial Incineration, organic Rankine power generation, supercritical CO2 power generation, absorption refrigeration, heat exchanger of district heating system Waste flow rate = 2.2 tonnes/day, Waste lower heating value = 26.92 MJ/kg, Incinerator temperature = 850 ◦ C, Organic working fluid = R-245fa Waste flow rate = 500 tonnes/day, Waste lower heating value = 7.00 MJ/kg, Organic Rankine cycle isentropic efficiency = 90%, Organic Rankine cycle evaporator pressure = 1500–3000 kPa, Refrigeration solution = LiBr/ H2O, High-pressure generator temperature = 80–85 ◦ C, Minimum temperature of supercritical CO2 cycle = 33–43, Supercritical CO2 cycle compressor isentropic efficiency = 90%, Supercritical CO2 cycle turbine isentropic T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. ψ = 26.9% ψ LCA = 14.1% The anaerobic digestion of organic waste did not seem a good option to be integrated with the MSW incineration process from exergetic viewpoints. [68] – φ = 89.0% Increasing the temperature difference between the hot water of the evaporator and the cold water of the condenser could improve the exergy efficiency of the organic Rankine cycle. [122] – ψ= The heat exchanger of the district heating system coupled with the organic Rankine cycle had the highest exergy efficiency. [123] – 59.29–60.13% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Scale S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries 13 Mixed MSW Electricity Commercial Incineration, steam turbine power generation, flue gas treatment Incineration integrated with steam turbine cycle Mixed MSW Electricity Commercial Incineration, steam turbine power generation Incineration integrated with steam turbine cycle NA Electricity Commercial Incineration, three different steam turbine cycles varying in feedwater preheating source (i.e., external fuelburning, extracted steam from turbines, exhaust gas from MSW incinerator) efficiency = 90%, Pumps isentropic efficiency = 85%, Supply district hot water temperature = 70 ◦ C, Return district hot water temperature = 40 ◦ C Waste lower heating value = 9.72 MJ/kg, Air-to-waste ratio = 1.6, Boiler energy recovery efficiency = 86.2% Waste flow rate = 491 tonnes/day, Waste lower heating value = 9.23 MJ/kg, Steam turbine isentropic efficiency = 85%, Pump isentropic efficiency = 88%, Waste boiler energy efficiency = 85%, Waste boiler pressure = 4.162 MPa, Steam temperature = 420 ◦ C Waste flow rate = 500 tonnes/day, Waste lower heating value = 12.50 MJ/kg, Incinerator temperature = 1100 ◦ C, Waste boiler pressure = 10 MPa, Steam Reference conditions T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa NA Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. – ψ = 20.25% Recovering Fe, Al, Cu, and brass alloys could improve the overall exergy efficiency of the process. [124] φ = 19.33% – A considerable portion of the exergy destruction of the process could be avoided using advanced technologies. [125] Preheating the feedwater using the incinerator exhaust gas showed better exergetic and economic performance than the separate fuel-burning process and the steam extracted from turbines. [126] – φ= 64.0–82.8% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Incineration integrated with steam turbine cycle Key parameter(s) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries Electricity Commercial Incineration, steam turbine power generation Incineration integrated with steam turbine cycle NA Electricity Commercial Incineration, steam turbine power generation Incineration integrated with steam turbine cycle NA Electricity Commercial Incineration, steam turbine power generation with flue gas recirculation temperature = 500 ◦ C, Thermal efficiency of steam recovery unit = 95%, Steam turbine isentropic efficiency = 85%, Pump isentropic efficiency = 85% Waste flow rate = 199 tonnes/day, Waste lower heating value = 9.57 MJ/kg, Incinerator temperature = 950–1100 ◦ C, Turbine adiabatic efficiency = 84%, Pumps and blowers efficiency = 70%, Superheated steam temperature = 360 ◦ C Waste flow rate = 199 tonnes/day, Waste lower heating value = 9.57 MJ/kg, Adiabatic combustion temperature = 850–1200 ◦ C, Turbine adiabatic efficiency = 84%, Pumps and blowers efficiency = 70%, Superheated steam temperature = 360 ◦ C Waste flow rate = 199 tonnes/day, Waste lower heating value = 9.57 MJ/kg, Reference conditions Exergetic indicator(s) of MSW incineration unit T0 = 298.15 K, P0 = 1.0 atm φ = 70.1–72.2% T0 = 298.15 K, P0 = 1.0 atm T0 = 298.15 K, P0 = 1.0 atm φ= 66.75–76.70% φ = 75.7–78.8% System exergetic indicator(s) φ= 31.0–32.0% φ= 28.80–30.89% φ= 34.0–34.4% Remark(s) Ref. Elevating the incineration temperature could slightly increase the overall exergetic performance of the system. [127] Increasing the incineration temperature could improve the exergy efficiency of the incinerator and the whole process. [128] The flue gas recirculation could considerably suppress the exergy destruction of the waste incinerator and [127] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 NA 14 Incineration integrated with steam turbine cycle Key parameter(s) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries Electricity Commercial Incineration, steam turbine power generation with flue gas recirculation Incineration integrated with steam turbine cycle NA Electricity Commercial Incineration, steam generation, steam turbine power generation, flue gas treatment NA Electricity, heat Commercial Incinerator temperature = 950–1100 ◦ C, Turbine adiabatic efficiency = 84%, Pumps and blowers efficiency = 70%, Superheated steam temperature = 360 ◦ C Waste flow rate = 199 tonnes/day, Waste lower heating value = 9.57 MJ/kg, Adiabatic combustion temperature = 850–1200 ◦ C, Oxygen molar percentage in air combustion = 21.0–99.6%, Turbine adiabatic efficiency = 84%, Pumps and blowers efficiency = 70%, Superheated steam temperature = 360 ◦ C Waste flow rate = 600 tonnes/day, Waste lower heating value = 6.80 MJ/kg, Incinerator temperature = 1026 ◦ C, Air-to-waste ratio = 3.6 kg/kg, Steam turbine inlet temperature = 400 ◦ C, Steam turbine isentropic efficiency = 90% Reference conditions Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. acceptably promote the plant exergy efficiency. T0 = 298.15 K, P0 = 1.0 atm T0 = 298.15 K, P0 = 101.3 kPa NA φ= 74.08–89.60% φ= 31.50–37.00% – ψ = 18.9% – ψ = 25.2% Increasing the oxygen molar percentage up to 60% could markedly reduce the exergy destruction of the process. [128] The incineration unit had the highest exergy destruction, followed by the flue gas treatment system. [129] [130] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 NA 15 Incineration integrated with steam turbine cycle Key parameter(s) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale Incineration integrated with steam turbine cycle and district heating 16 Key parameter(s) Incineration, steam turbine power generation, district heating Waste lower heating value = 11.60 MJ/kg, Superheated steam temperature = 420 ◦ C, Superheated steam pressure = 50 bar Waste flow rate = 2000 tonnes/day, Waste lower heating value = 7.50–15.00 MJ/ kg, Steam turbine isentropic efficiency = 80%, Pumps isentropic efficiency = 95% Waste lower heating value = 9.72 MJ/kg, Air-to-waste ratio = 1.6 kg/kg, Boiler energy recovery efficiency = 86.2% Waste lower heating value = 11.60 MJ/kg, Superheated steam temperature = 420–440 ◦ C, Superheated steam pressure = 50–130 bar Waste flow rate = 1344 tonnes/day, Waste lower heating value = 10.00 MJ/kg, Superheated steam pressure = 92 bar, Superheated steam temperature = Incineration integrated with steam turbine cycle and district heating Mixed MSW Electricity, heat Commercial Incineration, steam turbine power generation, district heating Incineration integrated with steam turbine cycle and district heating Mixed MSW Electricity, heat Commercial Incineration, steam turbine power generation, district heating network, flue gas treatment Incineration integrated with steam turbine cycle and district heating NA Electricity, heat Commercial Incineration, district heating, steam turbine power generation with excess air reduction, flue gas condensation, steam reheater Incineration incorporated into natural gas-fueled combined cycle Mixed biowaste, paper, wood waste, plastics textiles, and rubber Electricity Commercial Incineration, gas turbine power generation, steam turbine power generation Reference conditions Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. The highest improvement potential was found for the boiler. T0 = 280–310 K, P0 = 100 kPa Elevating the reference temperature increased the exergy destruction of all the components involved while reducing the overall exergy efficiency of the system. [131] ψ = 28.7% Material recovery and district heating could enhance the exergetic performance of the process. [124] – ψ = 28.8% The excess air reduction, flue gas condensation, and steam reheater could positively affect the overall exergy efficiency of the system. [130] – ψ = 39.0% Using the heat recovery system of a conventional natural gas-fueled combined cycle power plant for preheating the feedwater of the MSW incinerator could lead to improved exergetic, economic, and environmental indices. [132] – ψ= – NA NA T0 = 288.15 K, P0 = 101.3 kPa 34.05–41.80% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 key stages inside the investigated boundaries S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries 17 Mixed MSW Electricity Commercial Incineration, steam turbine power generation Incineration incorporated into coal-fired steam turbine cycle NA Electricity Commercial Incineration, steam turbine power generation Incineration incorporated into coal-fired steam turbine cycle NA Electricity Commercial Incineration, steam turbine power generation 505.8 ◦ C, Natural gas combustor flue gas temperature = 1198 ◦ C Waste flow rate = 500 tonnes/day, Waste lower heating value = 7.00 MJ/kg, Superheated steam pressure = 4.10 MPa, Superheated steam temperature = 400 ◦ C, Waste boiler energy recovery efficiency = 80.74% Waste flow rate = 600 tonnes/day, Waste lower heating value = 7.00 MJ/kg, Superheated steam pressure = 3.90 MPa, Superheated steam temperature = 395 ◦ C, Waste boiler energy recovery efficiency = 78.71% Waste flow rate = 0–1905 tonnes/ day, Waste lower heating value = 2.75 MJ/kg, Waste replacement ratio = 0–20%, Coal flow rate = 826–1032 tonnes/ day, Waste incinerator energy recovery Reference conditions T0 = 288.15 K, P0 = 1.0 atm T0 = 288.15 K, P0 = 1.0 atm T0 = 288.15 K, P0 = 1.0 atm Exergetic indicator(s) of MSW incineration unit – – – System exergetic indicator(s) Remark(s) Ref. ψ = 39.52% ψ we = 28.07% Annexing an MSW incinerator to a coal-fired steam power plant for preheating the feedwater could be an exergetically and economically sound approach. [133] ψ = 38.81% ψ we = 25.46% Preheating the intake air of the MSW-fired boiler and its annexation to a coal-fired steam power plant for preheating the feedwater could be advantageous from the exergetic and economic viewpoints. [134] Increasing the waste-to-coal ratio reduced the exergy efficiency of the integrated system. [135] ψ = ψ we = 37.27–37.72% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Incineration incorporated into coal-fired steam turbine cycle Key parameter(s) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries 18 NA Electricity Commercial Incineration, steam turbine power generation Incineration integrated with geothermal steam turbine cycle, three organic Rankine cycles, and desalination NA Electricity, freshwater Commercial Incineration, geothermal steam turbine power generation, organic Rankine cycle I, organic Rankine cycle II, organic Rankine cycle III, desalination efficiency = 98%, Coal boiler thermal efficiency = 99%, Incinerator temperature = 900 ◦ C, Incinerator pressure = 0.1 MPa, Coal boiler temperature = 1400 ◦ C, Coal boiler pressure = 0.1 MPa Superheated steam pressure = 13.2 MPa, Superheated steam temperature = 535 ◦ C Waste flow rate = 500 tonnes/day, Waste lower heating value = 7.00 MJ/kg, Superheated steam pressure = 3.90 MPa, Superheated steam temperature = 395 ◦ C, Waste boiler energy recovery efficiency = 78.53% Waste flow rate = 4.84 tonnes/day, Waste lower heating value = 27.75 MJ/kg, Geothermal fluid temperature = 205 ◦ C, Geothermal fluid pressure = 2301 kPa, Steam turbine Reference conditions T0 = 288.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW incineration unit – – System exergetic indicator(s) Remark(s) Ref. ψ = 25.89% ψ we = 20.94% The overall exergy destruction of the proposed integration was lower than the sum of the original schemes. [136] The effect of the flash separator pressure on the overall exergy efficiency of the system was more pronounced than the inlet temperature of the steam turbine and the temperature difference of organic Rankine cycle evaporator I. [137] ψ= 17.84–24.85% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Incineration incorporated into biomass-fired steam turbine cycle Key parameter(s) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries 19 Incineration combined with heating, cooling, and power cycles NA Electricity, heat, cold Commercial Incineration, absorption refrigeration, auxiliary heat exchangers Key parameter(s) Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. NA ψ = 30.61% fc = 33.38% ψ = 25.69% The absorption chiller was the most crucial unit in terms of exergetic and exergoeconomic indicators. [138] SI = 1.346 (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 inlet temperature = 350–400 ◦ C, Separator flash pressure = 600–900 kPa, Organic working fluid for cycles I, II, and III = npentane, Organic Rankine cycle evaporator I temperature difference = 5–10 ◦ C, Pinch point temperature differences in heat exchangers = 5 ◦ C, Seawater salinity = 40 g/kg, Annual working hours = 7446 h, Maintenance factor = 1.06 Waste flow rate = 86.4 tonnes/day, Waste lower heating value = 12.50 MJ/kg, Combustion excess air = 80%, Flue gas temperature = 1100 ◦ C, Steam turbine isentropic efficiency = 90%, Pumps isentropic efficiency = 75%, Supply district hot water temperature = 80 ◦ C, Return district hot water temperature = 40 ◦ C, Maximum heat exchanger effectiveness = Reference conditions S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale key stages inside the investigated boundaries Electricity, cold Commercial Steam turbine power generation, organic Rankine power generation, absorption refrigeration Incineration integrated with steam turbine cycle and methanation process Mixed nonrecyclable plastics Electricity, synthetic natural gas, oxygen, CO2 Demonstration Incineration, steam turbine power generation, waste gas treatment, gas conditioning (CO2 capture), proton exchange membrane 85%, Refrigeration solution = LiBr/ H2O, High-pressure generator temperature = 80 ◦ C, Annual working hours = 7446 h, Maintenance factor = 1.06 Waste flow rate = 500 tonnes/day, Waste lower heating value = 7.00 MJ/kg, MSW boiler efficiency = 80.74%, Steam turbine isentropic efficiency = 90%, Organic working fluids = butane, isobutane, isopentane, R245fa, R245ca, and R236ea, Organic Rankine cycle turbine isentropic efficiency = 80%, Organic Rankine cycle evaporation pressure = 700–2800 kPa, Pumps isentropic efficiency = 80%, Absorption temperature = 35 ◦ C, Absorption cycle evaporation temperature = 1–5 ◦ C Waste flow rate = 22.6–29.5 tonnes/ day, Waste lower heating value = Reference conditions T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 1.0 atm Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) – ψ= – ψ = 4.4–26.4% 54.8–64.6% Remark(s) Ref. Increasing the organic Rankine cycle evaporation pressure could positively affect the overall exergy efficiency of the system when working with isopentane, R245ca, R245fa, and butane. [139] Unlike exergy loss, increasing the oxygen content of incineration air increased the exergy destruction of the system. [140] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 NA 20 Incineration integrated with steam turbine cycle, organic Rankine cycle, and absorption refrigeration Key parameter(s) S. Soltanian et al. Table 6 (continued ) S. Soltanian et al. Table 6 (continued ) MSW treatment approach Waste fraction(s) Main product(s) Scale 21 key stages inside the investigated boundaries Key parameter(s) electrolyzer (oxygen and hydrogen production), air separation (oxygen production), methanation unit 46.8 MJ/kg, Adiabatic flame temperature = 1200 ◦ C, Oxygen molar percentage in combustion air = 23.4–92.3%, Methanation temperature = 250 ◦ C, Methanation pressure = 15 bar Reference conditions Exergetic indicator(s) of MSW incineration unit System exergetic indicator(s) Remark(s) Ref. Abbreviations/symbols: ψ : Functional exergy efficiency, ψ LCA : Functional LCA exegetic efficiency or exergetic resource recovery efficiency, ψ we : Waste-to-electricity exergy efficiency, φ: Universal exergy efficiency, fc : Exergoeconomic factor, NA: Not available, rc : Relative cost difference, SI: Sustainability index. Renewable and Sustainable Energy Reviews 156 (2022) 111975 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 The incineration process can be combined with the current district heating and cooling systems to reduce MSW volume effectively while recovering its thermal energy. Baldvinsson and Nakata [121] used exergy and exergoeconomic analyses to compare a residential heating system with a district heating plant. The investigated approaches were powered by natural gas, MSW, and geothermal heat. Unexpectedly, the residential heating system showed better exergetic performance than the district heating system. The district heating plant indicated the highest exergy efficiency when powered by geothermal heat. However, the best exergoeconomic performance was obtained when the system was powered by MSW. The lowest exergetic and exergoeconomic performance indicators were observed for natural gas. Organic Rankine and absorption refrigeration cycles are considered promising candidates for recovering the medium-temperature thermal energy in MSW incineration systems. Using the ELCA approach, Sedpho et al. [149] showed that integrating an organic Rankine cycle with a refuse-derived fuel incinerator could be an efficient MSW treatment strategy from environmental and exergetic viewpoints. Similarly, Chaiyat [150] demonstrated that an incineration-based organic Rankine cycle was a thermodynamically, economically, and environmentally sound option to convert highly infectious medical waste into electricity. Yatsunthea and Chaiyat [122] found that incinerating MSW to produce cold energy through an absorption refrigeration system and generate electricity through an organic Rankine cycle could be a thermodynamically and economically viable strategy. Pan et al. [123] evaluated an integrated MSW-fueled system including an organic Rankine cycle, an absorption refrigeration system, and a supercritical CO2 power cycle (Brayton cycle) from thermodynamic, economic, and environmental standpoints. Elevating the minimum temperature of the supercritical CO2 power cycle had a positive impact on the energy and exergy efficiency values of the system. The MSW boiler had the highest exergy destruction rate, followed by the supercritical CO2 power unit. Running steam power plants using the thermal energy generated by incineration is a popular strategy to exploit the chemical exergy of MSW. In this context, using the Carnot factor calculation, Márcio et al. [151] proved that the exhaust gas exergy of MSW incineration could effectively drive a steam turbine power cycle. Trindade et al. [125] investigated an incineration-based steam power plant powered by MSW using conventional and advanced exergy analyses. It was found that around 8.5% of the total exergy loss of the plant could be avoided using advanced technologies under optimum operating conditions. Efforts to improve the exergetic performance of the process should be concentrated on the MSW boiler having the lowest exergy efficiency and the highest avoidable exergy destruction (Fig. 12). Azami et al. [152] attempted to diagnose the cause of exergy destruction in an MSW boiler consisting of a furnace and a heat recovery steam generator. The furnace was responsible for a significant portion of the exergy destruction of the boiler. The sensitivity analysis indicated that reducing the blow-down water mass flow rate could marginally increase the exergy efficiency of the process. The waste heat exergy of the hot flue gas leaving MSW boilers can be harnessed for heating purposes. Gehrmann et al. [124] showed that the utilization of surplus heat generated in an MSW-fired boiler to provide district heat could increase the overall exergy efficiency of a steam turbine power plant by more than 40%. Ozturk and Dincer [131] revealed that increasing the lower heating value of MSW from 7.5 to 15 MJ/kg could improve the energy and exergy efficiency values of an MSW-fired steam turbine cycle integrated with a district heating system by about 20% and 8%, respectively. An important problem associated with MSW-fueled steam power cycles is the lower temperature of the steam generated in the boiler (typically 400 ◦ C and ~4 MPa). A practical and effective solution to increase the steam temperature and boost the cycle efficiency is to recover the waste heat of the boiler exhaust gas for preheating the cycle feedwater or boiler combustion air. Alrobaian [126] performed energy, exergy, and exergoeconomic analyses to compare three different feedwater preheating approaches annexed to an MSW-fired heat and power Fig. 12. Exergy destruction rate of different units of an MSW-fueled steam turbine power generation plant. Drawn using the data reported by Trindade et al. [125]. extruders) were the main source of exergy consumption in converting plastic waste into new valuable products such as roof tiles. In addition to pre-processing practices, Atta et al. [143] demonstrated that MSW transportation played a critical role in the exergetic sustainability of waste management systems because of its exergy-intensive nature. The ferrous metals (iron) and non-ferrous metals (aluminum, copper, zinc, and brass alloy) of bottom ash of MSW incineration solid residues can be separated by magnets and eddy current separators, respectively [144,145]. In a study by Simon and Holm [146], the chemical exergy of MSW incineration bottom ash was calculated to be about 100 kJ/mol. The highest fraction of bottom ash chemical exergy was found for iron (46.8 kJ/mol), followed by aluminum (37.2 kJ/mol) and calcium oxide (13.3 kJ/mol). The obtained data indicated the importance of iron and aluminum recovery from bottom ash. The chemical exergy of the copper embedded in the bottom ash was as low as 0.64 kJ/kg. This result could be ascribed to the low standard chemical exergy of copper due to its weak affinity for oxidation compared to iron and aluminum. Nevertheless, the bottom ash treatment to recover the copper element could offer considerable benefits since the elemental copper production from natural reserves needs to process huge ores and consume high energy. Laner et al. [68] showed that just above half of the input chemical exergy of bottom ash through Al, Cu, and Fe scraps could be recovered by treating the bottom ash of the incineration process. It has been evidenced that the bottom ash treatment could play a crucial role in improving the exergy efficiency of MSW incineration-based integrated multi-generation plants. The uncertainties associated with external and internal operating factors need to be assessed to ensure the reliability of MSW management systems. In this context, Russo and Verda [147] studied the effects of external (waste composition) and internal (energy consumption) uncertain variables on the exergetic aspects of two different waste management systems. The first system included a mechanical-biological plant to produce refuse-derived fuel and recover metal. The second system was a paper recycling system used for cardboard production. The variations in energy consumption imposed a lower fluctuation on the exergy aspects of both systems compared with the changes in waste composition. This result revealed the ability of waste recycling systems to dampen the uncertainty associated with energy consumption. In continuation, Russo and Verda [148] showed that the selective collection of paper could result in higher variations in the exergy efficiency values of the first system, followed by the selective collection of organic and plastic wastes. Unlike paper and plastic wastes, the higher selective collection of organic wastes could improve the exergy efficiency values of both systems. 22 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 cogeneration cycle. The feedwater was preheated by an external fuel-burning unit (first scenario), by the steam extracted from turbines (second scenario), and by the exhaust gas of the incineration process (third scenario). The third approach showed the highest exergy efficiency and the lowest specific cost of product exergy (Fig. 13). Regardless of the scenarios considered for feedwater preheating, increasing the waste moisture content decreased the overall exergy efficiency of the process. Guo et al. [153] demonstrated that higher moisture content of the degradable organic fraction of MSW resulted in more exergy loss in all the processes involved in its treatment. Tang et al. [154] showed that reducing the moisture content of food waste using the thermal hydrolysis pretreatment and air-drying processes prior to incineration could remarkably promote the exergetic performance of a steam power plant. Integrating MSW-fed incinerators with conventional fossil-fueled power plants can also be another efficient approach to harness energy from the waste while lowering CO2 emission [155]. Chen et al. [134] used the thermal energy of an MSW-fed incinerator to heat the feedwater and the turbine-extracted steam of a coal-fueled steam power plant. The proposed hybrid power plant could improve the waste-to-electricity exergy efficiency compared with the reference plants, i.e., coal- or MSW-fired steam power cycles. In the integrated scheme, the exergy destruction of the coal-fired boiler was immensely higher than that of the MSW-fueled boiler (around 27 times). Carneiro and Gomes [132] combined an MSW-fired steam power cycle with a gas turbine cycle fed by natural gas. The feedwater was preheated using the MSW boiler and the flue gas heat recovery unit. Generally, the proposed system was a plausible option in terms of exergetic, economic, and environmental viewpoints. However, Ye et al. [135] claimed that coupling MSW incineration with a coal-fired power plant negatively affected the thermodynamic and environmental aspects of the baseline coal power cycle. Increasing the waste-to-coal ratio resulted in a linear reduction in the overall exergy efficiency of the integrated scheme, although this modification had no considerable effect on the exergy efficiency of the incinerator and coal boiler. Nevertheless, the economic performance of the plant was significantly improved at higher waste-to-coal ratios. Due to difficulties in the co-firing of MSW and biomass, MSW-fed power plants can be combined with biomass-fired power plants in order to maximize the energy recovery from both feedstocks. In this context, Pan et al. [136] thermodynamically and economically analyzed a consolidated power plant, including an MSW-fueled incineration unit and a biomass-fired power cycle. A fraction of the saturated steam generated in the MSW-fed boiler was used to preheat the incineration fresh intake air. The superheated steam generated in the biomass boiler was used to produce power in high- and low-pressure turbines. The superheated steam of the MSW boiler was also added to the input steam flow of the low-pressure turbine. Compared with the reference schemes (i.e., biomass-fired or MSW-fed power plant), the integrated plant showed lower exergy destruction while promoting the waste-to-electricity exergy efficiency and the overall exergy efficiency of the process (Fig. 14). The exergetic performance of MSW incineration systems can also be boosted by their integration with the thermal power plants driven by other renewable energy resources (e.g., geothermal and solar energy). Heidarnejad et al. [137] exergetically and exergoeconomically assessed a combined geothermal-MSW incineration system consisting of a steam turbine cycle, three organic Rankine cycles, and a desalination unit. The MSW was burned to superheat the steam separated from the two-phase geothermal fluid. The heat of the liquid fraction was also transferred to the first organic Rankine cycle before being injected into the well. The second and third organic Rankine cycles were powered by the heat of the steam expanded in the steam turbine. The thermal energy of exhaust flue gas leaving the MSW incinerator was used to drive the evaporative seawater desalination system. Elevating the flash separator pressure reduced the overall exergy efficiency of the system while increasing the specific cost of product exergy. However, increasing the temperature difference of the first organic Rankine cycle evaporator and the inlet temperature of the steam turbine could improve the exergy efficiency of the process while discounting the specific cost of product exergy. The emission of harmful pollutants is one of the main drawbacks of MSW-fed incineration systems. Flue gas recirculation is a suitable solution to mitigate nitrogen oxides and dioxins of MSW-fed incinerators by reducing the flame temperature [156]. In this regard, Vilardi and Verdone [127] exergetically investigated an MSW-fueled steam turbine power cycle with and without flue gas recirculation at different incineration temperatures. Flue gas recirculation could markedly promote the overall exergy efficiency of the process. Nevertheless, increasing the incineration temperature could marginally improve the overall exergy efficiency of the process. In addition to flue gas recirculation, enrichment of incineration air with oxygen and oxy-combustion of MSW can suppress the formation of NOX and SO2 [157]. These processes can also provide several additional advantages, including (1) reducing the combustor size at a fixed thermal capacity, (2) facilitating the CO2 capture by increasing its concentration in flue gas, (3) enhancing the combustion performance and controllability, (4) improving the burning speed and combustion safety, and (5) decreasing the burnout time and ignition temperature. Vilardi and Verdone [128] explored the effect of combustion air oxygen concentration (ranging from 21 mol% to 99.6 mol%) on the exergetic performance of an MSW-fired steam turbine cycle assisted with flue gas recirculation. Increasing the oxygen concentration could be advantageous from the environmental viewpoint because of a decrease in flue gas flow rate. Nevertheless, increasing the oxygen concentration up to a certain level (i.e., 60 mol%) could only substantially improve the overall exergy efficiency of the process. In fact, increasing the oxygen concentration beyond 60 mol% increased the exergy loss of the air separation process while not significantly improving the power generation rate of the steam turbine. The exergy efficiency of the incineration unit was linearly increased by increasing the oxygen concentration. In a similar context, Rispoli et al. [140] exergetically analyzed a hybrid synthetic natural gas production plant consisting of an MSW oxygen-enriched incineration, steam turbine cycle, proton exchange membrane electrolyzer, gas treatment/conditioning unit, and methanation system. The overall exergy efficiency of the system was increased as the incineration regime was changed from conventional air combustion to oxy-combustion. Fig. 13. Energetic and exergetic efficiencies of three different feedwater preheating methods applied to an MSW-fired combined heat and power cycle. The net power and specific cost of electrical exergy were 18.74 MW & 12.51 USD/ GJ, 23.4 MW & 11.54 USD/GJ, and 24.27 MW & 10.98 USD/GJ when the feedwater was heated using external fuel-burning, turbine-extracted steam, and incineration exhaust gas, respectively. With permission from Alrobaian [126]. Copyright© 2020 Springer. 23 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 14. Exergy flow diagram of (a) sum of the reference plants (MSW-fed and biomass-fired cycles) and (b) MSW-biomass integrated scheme. Redrawn with permission from Pan et al. [136]. Copyright© 2020 MDPI. 24 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 The multi-generation approach is another strategy to maximize the energy recovery from the waste heat in MSW-fired power plants. Nami et al. [138] exergetically and exergoeconomically evaluated an MSW-fed plant generating electricity by a steam turbine cycle, hot water by a district heating system, and cold energy by a lithium bromide-water absorption chiller. The incinerator and boiler showed the highest exergy destruction, followed by the absorption chiller (Fig. 15). The absorber and generator had the lowest exergoeconomic factor values, implying that their investment and irreversibility costs must be balanced. The district heating heat exchanger and steam turbine showed the highest exergoeconomic performance among the plant components. In another study, Lu et al. [139] energetically, exergetically, economically, and environmentally assessed an integrated plant generating electricity via MSW-fired steam turbine and organic Rankine cycles and cold energy via an absorption refrigeration cycle. The effects of six organic working fluids (i.e., butane, isobutane, isopentane, R245fa, R245ca, and R236ea) were investigated on the overall exergy efficiency of the integrated waste-to-energy scheme under the organic Rankine cycle evaporation pressure in the range of 700–2800 kPa. Regardless of the organic evaporation pressure, the isopentane had the highest exergy efficiency, followed by R245ca and R245fa (Fig. 16). In addition, increasing the pressure and temperature of the superheated steam and decreasing the temperature of the generator and absorber of the absorption cycle positively affect the overall exergy efficiency of the system. Overall, the integrated multi-generation scheme showed substantially higher exergy efficiency than the reference MSW-fired steam power plant. The thermal energy derived from MSW incineration can also be used in air turbine power generation systems through heating a pressurized air stream. Using response surface methodology, Mondal et al. [158] Fig. 16. Effects of different organic working fluids and organic evaporator pressures on the overall exergy efficiency of an integrated multi-generation scheme consisting of an MSW-fueled steam turbine cycle, an absorption refrigeration cycle, and an organic Rankine cycle. Redrawn with permission from Lu et al. [139]. Copyright© 2020 Elsevier. exergetically optimized an MSW-fed power generation system consisting of an air turbine cycle and an organic Rankine cycle. The optimum exergy efficiency of 39% was found when the outlet pressure of the air compressor = 4 bar, the inlet temperature of the air turbine = 1100 ◦ C, the hot-side temperature difference of the organic Rankine cycle heat Fig. 15. Exergy flow diagram of a combined cycle heat and power plant consisting of an MSW-based steam turbine cycle, an absorption chiller, and a district heating system. Redrawn with permission from Nami et al. [138]. Copyright© 2020 MDPI. 25 MSW treatment approach Fixed-bed gasifiers Updraft fixed-bed Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries Key parameter(s) Reference conditions Refuse-derived fuel based on biowaste Air Syngas Laboratory Gasification Air-to-waste ratio = 1.91 kg/kg, Equivalence ratio = 0.35 kg/kg, Outlet syngas temperature = 338–382 ◦ C T0 = 298.15 K, P0 = 1.0 atm Air Syngas Pilot Gasification Waste flow rate = 600 kg/day, Waste flow rate = 2.4 tonnes/day, Gasifier temperature = 700–900 ◦ C, Gasifier pressure = vacuum, Equivalence ratio = 0.15–0.35 kg/kg Waste lower heating value = 22.50 MJ/kg, Gasifier temperature = 550–850 ◦ C, Equivalence ratio = 0–0.8 kg/kg Waste flow rate = 600 kg/day, Waste lower heating value = 14.40 MJ/kg, Gasifier temperature = 700–900 ◦ C, Gasifier pressure = vacuum, Steam-to-waste ratio = 0–2.0 kg/kg Waste flow rate = 2.4 tonnes/day, Waste lower heating value = 14.40 MJ/kg, Gasifier temperature = 700–900 ◦ C, Equivalence ratio = 0.15–0.35 kg/kg, T0 = 298.15 K, P0 = 101.3 kPa Fluidized-bed gasifiers Bubbling fluidizedMixed bed biowaste, paper, wood waste, and plastics 26 Bubbling fluidizedbed Mixed food waste, paper, wood waste, plastics, textiles, and rubber Air Syngas Laboratory Gasification Bubbling fluidizedbed Mixed biowaste, paper, wood waste, and plastics Steam Syngas Pilot Gasification Bubbling fluidizedbed Mixed municipal waste CO2 Syngas Pilot Gasification T0 = 298.15 K, P0 = 1.0 atm T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) ψ CGE = 58.0–65.5% – ψ CGE = 47.9–65.8% ψ tar = 1.4–7.2% – φ= 37.69–51.33% ψ= 32.54–47.14% – ψ CGE = 32.2–58.2% ψ tar = 2.9–15.6% – ψ CGE = 50.0–75.0% ψ tar = 1.0–7.0% – Remark(s) Ref. The refuse-derived fuel could outperform torrefied soybean straw and woody fuels in terms of cold gas exergy efficiency. [168] The effect of the reactor temperature on the exergetic performance was more pronounced than the equivalence ratio. [170] The exergy destruction rate was increased at higher reaction temperatures and equivalence ratios due to increased heat transfer and promoted chemical reactions. [171] The exergy efficiency of the MSW gasification process was lower than the woody biomass at different operating conditions. [172] Elevating the CO2-to-MSW ratio could increase the syngas exergy efficiency while reducing the tar exergy efficiency. [173] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Waste fraction (s) S. Soltanian et al. Table 7 Most important applications of the exergy-based approaches in analyzing MSW gasification systems. MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 27 Biowaste Steam Synthetic natural gas, steam Pilot Drying, gasification, flue gas utilization, syngas compression, methanation, water removal, CO2 removal, synthetic natural gas compression Plasma gasifiers Plasma fixed-bed Biowaste Air Electricity, heat Commercial Gasification, combined heat and power generation Plasma fixed-bed Mixed MSW Steam Syngas, tar Demonstration Gasification Plasma fixed-bed Mixed MSW Steam Syngas, tar Demonstration Gasification Plasma fixed-bed Mixed MSW Steam Syngas, tar Demonstration Gasification CO2-to-MSW ratio = 0–1.0 kg/kg Waste flow rate = 240 kg/day, Waste higher heating value = 10.45 MJ/kg, Waste moisture content = 10–20 wt %, Gasifier temperature = 737 ◦ C, Gasifier pressure = 1–15 bar, Methanation temperature = 500–700 ◦ C, Methanation pressure = 15–30 bar Waste flow rate = 7500 tonnes/day, Waste lower heating value = 7.00 MJ/kg, Waste carbon content = 10–60%, Waste moisture content = 10–70% Waste fuel rate = 12–20 tonnes/day, Equivalence ratio = 0.04–0.18 kg/kg, Steam flow rate = 0.02 kg/s, Plasma-to-waste energy ratio = 0.094 Waste fuel rate = 12–20 tonnes/day, Equivalence ratio = 0.06 kg/kg, Steam flow rate = 0.02 kg/s, Plasma-to-waste energy ratio = 0.078–0.109 Waste fuel rate = 12–20 tonnes/day, Reference conditions NA T0 = 298.15 K, P0 = 1.0 atm NA NA NA Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= φ = 0.5–27.2% – 41.7–45.9% ψ= 77.3–88.0% ψ CGE = 34.3–55.2% – ψ= 85.5–87.4% ψ CGE = 40.0–48.5% – ψ= 80.2–87.1% – Remark(s) Ref. The gasification reaction pressure played a significant role in the exergetic performance of the synthetic natural gas process. [174] The higher organic carbon content and lower moisture of MSW could improve the exergy efficiency of the plasma gasification process. [119] Although increasing the equivalence ratio could reduce the tar yield, this parameter negatively affected the exergy efficiency of the plasma gasification process. [175] Even though increasing plasma temperature could promote the char gasification and tar reduction processes, the exergetic performance of the process was negatively affected due to increased electricity consumption. Elevating the steam mass flow rate could improve the [175] [175] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Dual fluidized-bed Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries Key parameter(s) Reference conditions Equivalence ratio = 0.06 kg/kg, Steam-to-air ratio = 0–0.67 kg/kg, Plasma-to-waste energy ratio = 0.094 28 Mixed MSW Steam Syngas Commercial Gasification Plasma-assisted updraft fixed-bed gasifier Mixed MSW collected from residential, institutional, commercial, and industrial sectors Air Syngas Commercial Gasification Air Electricity, heat, cold Commercial Gasification, gas turbine power generation, absorption refrigeration, heat exchanger for district heating Integrated gasification systems Biowaste Downdraft fixed-bed gasification integrated with gas turbine cycle and absorption refrigeration system Waste fuel rate = 86.4 tonnes/day, Waste higher heating value = 25.10 MJ/kg, Steam-to-air ratio = 0–0.56 kg/kg, Torch power = 4.06 MW Waste flow rate = 86.4 tonnes/day, Waste higher heating value = 8.55–16.41 MJ/kg, Waste moisture content = 2.61–57.90, Gasifier temperature = 2500–4000 ◦ C, Gasifier pressure = 1.013 bar, Waste flow rate = 0–2462 tonnes/day, Waste lower heating value = 13.98 MJ/kg, Waste moisture content = 14%, Waste-to-natural gas ratio = 0–1.0 kg/kg, Gasifier temperature = 900 ◦ C, Gasifier pressure = 1 atm, Air compressor isentropic efficiency = 72–89%, System exergetic indicator(s) ψ CGE = 32.5–47.2% NA NA T0 = 273.48–297.82 K, P0 = 101.3 kPa ψ = 33.31% – ψ= 68.64–75.18% – – ψ= 14.98–31.87% Remark(s) Ref. char gasification rate. However, the excessive steam amount could cause higher char oxidation, increasing internal irreversibilities and decreasing the functional exergy efficiency of the process. The plasma gasification of MSW could lead to higher exergy efficiency compared with biomass and woody materials. [176] Increasing the torch power consumption increased the exergy destruction of the gasifier while reducing its exergy efficiency. [177] The syngas combustion chamber showed the highest exergy destruction, followed by the heat recovery steam generator and the gasification reactor. [178] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Plasma Exergetic indicator(s) of MSW gasification unit S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 29 Refuse-derived fuel based on biowaste Steam Electricity, heat, cold, char NA Gasification integrated with gas turbine cycle and absorption and compression refrigeration systems Mixed MSW Steam Electricity, heat, cold, char NA Gasification integrated with gas turbine cycle and absorption and compression refrigeration systems Refuse-derived fuel based on biowaste Steam Electricity, heat, cold, char, syngas NA Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification Gasification integrated with gas turbine cycle and absorption and compression Mixed MSW Steam Electricity, heat, cold, char, syngas NA Gasification, gas turbine power generation, absorption refrigeration, compression Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification Air compressor pressure ratio = 6–18, Gas turbine isentropic efficiency = 78–91%, Gas turbine inlet temperature = 1177–1277 ◦ C, Refrigeration solution = LiBr/ H2O, High-pressure generator temperature = 88 ◦ C Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg Waste lower heating value = 10.00 MJ/kg, Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg, Sold syngas percentage = 0–100% Waste lower heating value = 10.00 MJ/kg, Gasifier temperature = 900 ◦ C, Reference conditions T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) φ= 84.5–87.4% φ= 36.3–59.3% φ = 40.3% φ = 17.5% φ= 85.4–86.5% φ= 49.9–65.0% φ = 39.9% φ = 24.2% Remark(s) Ref. The absorption chiller had the highest irreversibility rate. [179] The mixed MSW resulted in lower overall exergy efficiency compared with the biowaste-based refusederived fuel. [180] Increasing the syngas selling rate could improve the exergy efficiency of the whole plant, while it did not affect the exergetic performance of the gasification process. [179] The biowaste-based refusederived fuel could provide higher overall exergy efficiency compared with the mixed MSW. [180] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Gasification integrated with gas turbine cycle and absorption and compression refrigeration systems Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale refrigeration systems Gasification integrated with gas turbine cycle, supercritical CO2 power cycle, and district heating Mixed MSW Air-oxygen mixture Electricity, heat Commercial 30 Key parameter(s) refrigeration, syngas purification Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg, Sold syngas percentage = 60% Waste flow rate = 108 tonnes/day, Gasifier temperature = 750 ◦ C, Gas turbine inlet temperature = 927–1227 ◦ C, Air compressor pressure ratio = 8–18, Air compressor isentropic efficiency = 87%, Gas turbine isentropic efficiency = 89%, Supercritical CO2 turbine inlet temperature = 550–600 ◦ C, Supercritical CO2 turbine inlet pressure = 75000 kPa, CO2 temperature increase in main heater = 40–80 ◦ C, Supercritical CO2 compressor pressure ratio = 3.0–6.5, Supercritical CO2 compressor isentropic efficiency = 80%, Supercritical CO2 turbine isentropic efficiency = 90%, Supply district heating temperature = 60.5 ◦ C, System lifetime = 20 years, Gasification, gas turbine power generation, supercritical CO2 power generation Reference conditions T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= 31.20–38.48% Remark(s) Ref. The air compressor pressure ratio had the highest impact on the overall exergy efficiency of the system. [181] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 key stages inside the investigated boundaries S. Soltanian et al. Table 7 (continued ) MSW treatment approach Gasification integrated with gas turbine cycle, Stirling engine, supercritical CO2 power cycle, and district heating Waste fraction (s) Mixed MSW Gasifying agent(s) Air-oxygen mixture Main product (s) Electricity, heat Scale Commercial key stages inside the investigated boundaries Gasification, gas turbine power generation, Stirling engine power generation, supercritical CO2 power generation Key parameter(s) 31 T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= 33.21–43.81% Remark(s) Ref. The effects of operating parameters of the Stirling engine (i.e., piston compression ratio and lowest-to-highest temperature ratio) on the overall exergy efficiency were more pronounced than those of the other parameters of the plant, such as the air compressor pressure ratio. [181] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Annual working hours = 8000 h, Interest rate = 12% Waste flow rate = 108 tonnes/day, Gasifier temperature=750 ◦ C, Gas turbine inlet temperature = 927–1227 ◦ C, Air compressor pressure ratio = 8–18, Air compressor isentropic efficiency = 87%, Gas turbine isentropic efficiency = 89%, Stirling engine compression ratio = 1.1–2.0, Stirling engine lowest-to-highest temperature ratio = 0.3–0.7, Supercritical CO2 turbine inlet temperature = 550–600 ◦ C, Supercritical CO2 turbine inlet pressure = 75000 kPa, CO2 temperature increase in main heater = 40–80 ◦ C, Supercritical CO2 compressor pressure ratio = 3.0–6.5, Supercritical CO2 compressor isentropic efficiency = 80%, Supercritical CO2 turbine isentropic efficiency = 90%, Supply district heating Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 32 Refuse-derived fuel based on biowaste Steam Electricity, heat, cold, char, synthetic natural gas, H2 NA Gasification integrated with gas turbine cycle, absorption and compression refrigeration systems, and syngas upgrading Mixed MSW Steam Electricity, heat, cold, char, synthetic natural gas, H2 NA Gasification integrated with gas turbine cycle, absorption and compression refrigeration systems, and syngas upgrading Refuse-derived fuel based on biowaste Steam Electricity, heat, cold, char, syngas, synthetic natural gas, H2 NA Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification, synthetic natural gas production Gasification integrated with gas turbine cycle, absorption and compression refrigeration systems, and syngas upgrading Mixed MSW Steam Electricity, heat, cold, char, syngas, synthetic natural gas, H2 NA Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification, synthetic natural gas production Gasification, gas turbine power generation, absorption refrigeration, compression refrigeration, syngas purification, synthetic natural gas production temperature = 60.5 ◦ C, System lifetime = 20 years, Annual working hours = 8000 h, Interest rate = 12% Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg, Syngas-to-synthetic natural gas percentage = 0–100% Waste lower heating value = 10.00 MJ/kg, Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg, Syngas-to-synthetic natural gas percentage = 34% Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Steam-to-waste ratio = 2.2 kg/kg, Sold syngas percentage = 10%, Syngas-to-synthetic natural gas percentage = 10% Waste lower heating value = 10.00 MJ/kg, Gasifier temperature = 900 ◦ C, Equivalence ratio = 0.03 kg/kg, Reference conditions Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) φ= 83.8–86.5% φ= 41.6–49.7% T0 = 298.15 K, P0 = 101.3 kPa φ = 39.8% T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa Remark(s) Ref. Increasing the syngas-tosynthetic natural gas conversion rate could not significantly impact the exergetic performance of all the units involved in the integrated plant. [179] φ = 18.3% The overall exergy efficiency during operating with the mixed MSW was lower than that of the biowaste-based refusederived fuel. [180] φ = 86.2% φ = 52.2% Increasing the syngas selling rate at a constant ratio of syngas to synthetic natural could enhance the plant exergetic performance. [179] φ = 39.9% φ = 21.6% Increasing the sold fraction of the produced syngas could increase the exergy efficiency of the system. [180] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Gasification integrated with gas turbine cycle, absorption and compression refrigeration systems, and syngas upgrading Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale 33 Key parameter(s) purification, synthetic natural gas production Steam-to-waste ratio = 2.2 kg/kg, Sold syngas percentage = 30%, Syngas-to-synthetic natural gas percentage = 17% Waste flow rate = 4800 tonnes/day, Waste moisture content = 16%, Gasifier temperature = 827 ◦ C, Swing adsorption pressure = 3.7 MPa Waste flow rate = 1900 tonnes/day, Gasifier temperature = 800 ◦ C, Gasifier pressure = 100 kPa, Equivalence ratio = 0.23 kg/kg, Steam-to-waste ratio = 0.1 kg/kg, Gas turbine isentropic efficiency = 88%, Air compressor isentropic efficiency = 88%, Syngas burner temperature = 2119–2371 ◦ C, Dimethyl ether synthesis reactor temperature = 235–245 ◦ C Waste flow rate = 90 tonnes/day, Waste lower heating value = 13.78 MJ/kg, Gasifier temperature = 1000–1300 ◦ C, Steam turbine inlet pressure = 1500–2500 kPa, Fluidized-bed gasification integrated with syngas upgrading Mixed MSW Air H2 Commercial Gasification, catalytic tar reforming, watergas shift reaction, pressure swing adsorption Bubbling fluidizedbed gasification integrated with gas turbine cycle and dimethyl ether and methanol syntheses NA Oxygensteam mixture Electricity, dimethyl ether, methanol Commercial Air separation, gasification or calcium looping gasification, gas turbine power generation, dimethyl ether and methanol syntheses Gasification integrated with steam turbine cycle Mixed combustible MSW Air Electricity Commercial Gasification, steam turbine power generation Reference conditions T0 = 285–310 K, P0 = 100 kPa T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= ψ= 66.23–70.13% ψ= φ= 58.3–67.8% fc = 43.99% ψ = ψ we = 13.60–18.11% fc = 44.63% 55.53–55.61% 46.10–47.40% Remark(s) Ref. The reference temperature did not significantly affect the overall exergy efficiency of the plant. [182] Although the calcium looping gasification process yielded higher dimethyl ether than conventional gasification, it resulted in higher exergy loss and lower exergetic performance. [183] The gasification unit had the highest exergy destruction. [184] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 key stages inside the investigated boundaries S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 34 NA Oxygensteam mixture Electricity Commercial Air separation, gasification, acid gas removal, gas turbine power generation, steam turbine power generation Gasification integrated with gas turbine and steam turbine cycles NA Steam Electricity Commercial Gasification, gas turbine power generation, steam turbine power generation Steam turbine isentropic efficiency = 80%, Combustion chamber temperature = 1000 –1400 ◦ C, Superheater temperature difference = 150–260 ◦ C, Pinch point temperature difference = 180–260 ◦ C, Waste moisture content = 30–40% Waste flow rate = 600 tonnes/day, Waste lower heating value = 6.80 MJ/kg, Gasifier temperature = 985 ◦ C, Gasifier pressure = 2 MPa, Oxygen-to-waste ratio = 0.48 kg/kg, Steam-to-waste ratio = 0.1 kg/kg, Gas turbine isentropic efficiency = 90%, Steam turbine inlet temperature = 451 ◦ C, Steam turbine isentropic efficiency = 90% Waste flow rate = 2016 tonnes/day, Air-to-waste ratio = 0.1 kg/kg, Steam-to-waste ratio = 1.0 kg/kg, Air compressor isentropic efficiency = 85%, Air compressor pressure ratio = Reference conditions T0 = 298.15 K, P0 = 101.3 kPa T0 = 298 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. – ψ = 28.6% The gas-steam combined cycle had the highest exergy destruction, followed by the gasification unit. [129] – ψ= Unlike the pressure ratio of the air compressor, increasing the inlet temperature of the gas turbine could increase the exergy efficiency of the power generation system. [185] 37.17–42.90% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Circulating fluidized-bed gasification integrated with gas turbine and steam turbine cycles Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries Steam Electricity NA Gasification, gas turbine power generation, steam turbine power generation Bubbling fluidizedbed gasification integrated with gas turbine and steam turbine cycles NA Oxygensteam mixture Electricity Commercial Air separation, gasification, gas turbine power generation, steam turbine power generation 6–14, Gas turbine isentropic efficiency = 85%, Gas turbine inlet temperature = 970–1050 ◦ C, Steam turbine isentropic efficiency = 85% Waste lower heating value = 25.02 MJ/kg, Gasifier temperature = 900 ◦ C, Air compressor isentropic efficiency = 85%, Air compressor pressure ratio = 12, Gas turbine isentropic efficiency = 85%, Gas turbine inlet temperature = 1000–1200 ◦ C, Steam turbine isentropic efficiency = 85%, Steam cycle pump isentropic efficiency = 85%, Steam turbine pressure = 30–70 bar Waste flow rate = 1900 tonnes/day, Gasifier temperature = 800 ◦ C, Gasifier pressure = 100 kPa, Equivalence ratio = 0.23 kg/kg, Steam-to-waste ratio = 0.1 kg/kg, Gas turbine isentropic efficiency = 88%, Air compressor Reference conditions T0 = 298 K, P0 = 1.0 bar T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= ψ = 68.83% ψ = 51.30% 34.07–38.60% Remark(s) Ref. The MSW gasification was energetically and exergetically more efficient than using agricultural residues. [186] The gasification unit indicated the highest irreversibility rate. [183] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 NA 35 Fluidized-bed gasification integrated with gas turbine and steam turbine cycles Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach 36 Plasma gasification integrated with gas turbine and steam turbine cycles Waste fraction (s) Mixed MSW collected from residential, institutional, commercial, and industrial sectors Gasifying agent(s) Air Main product (s) Electricity Scale Commercial key stages inside the investigated boundaries Plasma gasification, syngas treatment, gas turbine power generation, steam turbine power generation Key parameter(s) Exergetic indicator(s) of MSW gasification unit NA fc = 83.3–90.3% System exergetic indicator(s) ψ= 22.6–29.8% Remark(s) Ref. The plasma gasification unit contributed to the majority of the total investment cost. The maximum and minimum exergoeconomic factor values for the plasma gasification unit were obtained when it operated with commercial and residential wastes, respectively. [187] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 isentropic efficiency = 88%, Syngas burner temperature = 1434 ◦ C, Steam turbine isentropic efficiency = 88%, High-pressure steam turbine temperature = 500 ◦ C, Intermediatepressure steam turbine temperature = 482 ◦ C, Low-pressure steam turbine temperature = 164 ◦ C Waste flow rate = 75–1000 tonnes/ day, Waste higher heating value = 8.55–16.41 MJ/kg, Gasifier temperature = 2800–4500 ◦ C, Gas turbine inlet temperature = 1418 ◦ C, Air compressor isentropic efficiency = 88.4%, Air compressor pressure ratio = 19.4, Gas turbine isentropic efficiency = 91%, High-pressure steam turbine inlet temperature = 507 ◦ C, High-pressure steam turbine inlet pressure = 120 bar, High-pressure steam turbine Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 37 Mixed biowaste, paper and cardboard, and plastics Air Electricity, heat Demonstration Drying, gasification, gas turbine power generation, steam turbine power generation, district heating isentropic efficiency = 85.8%, Intermediatepressure steam turbine inlet temperature = 510 ◦ C, Intermediatepressure steam turbine inlet pressure = 29 bar, Intermediatepressure steam turbine isentropic efficiency = 90.5%, Low-pressure steam turbine inlet temperature = 255 ◦ C, Low-pressure steam turbine inlet pressure = 4.08 bar, Low-pressure steam turbine isentropic efficiency = 84.4%, System lifetime = 20 years, Annual working hours = 7920 h, Annual inflation rate = 3.51%, Interest rate = 8%, Maintenance factor = 1.10 Waste flow rate = 36 tonnes/day, Gasifier temperature = 750–950 ◦ C, Equivalence ratio = 0.2–0.3 kg/kg, Gas turbine inlet temperature = 1000–1200 ◦ C, Air compressor isentropic efficiency = 85%, Air compressor pressure ratio = 10, Gas turbine isentropic Reference conditions Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 1.013 bar rc = 0.28 (–) fc = 41% System exergetic indicator(s) ψ= 26.6–54.1% rc = 5.22–6.72 (–) fc = 54.14–61.27% Remark(s) Ref. The gasification process indicated better exergetic and exergoeconomic performance at higher gasification temperatures and lower equivalence ratios. [188] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Gasification integrated with gas turbine cycle, steam turbine cycle, and district heating Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach 38 Gasification integrated with gas turbine cycle, steam turbine cycle, and district heating Waste fraction (s) Mixed biowaste, paper and cardboard, and plastics Gasifying agent(s) Air Main product (s) Electricity, heat Scale Demonstration key stages inside the investigated boundaries Drying, gasification, gas turbine power generation, steam turbine power generation, district heating Key parameter(s) Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. T0 = 298.15 K, P0 = 1.013 bar rb = 3.72 (–) fb = 90% ψ = 45.3% The MSW gasification process had the highest impact on the exergoenvironmental performance of the plant. [189] rb = 4.36 (–) fb = 56% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 efficiency = 90%, Steam turbine isentropic efficiency = 75%, Supply district heating temperature = 78 ◦ C, Return district heating temperature = 45 ◦ C, System lifetime = 20 years, Annual working hours = 7200 h, Interest rate = 6%, Maintenance factor = 1.10 Waste flow rate = 36 tonnes/day, Gasifier temperature = 850 ◦ C, Equivalence ratio = 0.25 kg/kg, Air compressor isentropic efficiency = 85%, Air compression ratio = 10, Gas turbine isentropic efficiency = 90%, Steam turbine isentropic efficiency = 75%, Gas turbine inlet temperature = 1000–1200 ◦ C, Supply district heating temperature = 78 ◦ C, Return district heating temperature = 45 ◦ C, System lifetime = 20 years, Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries Steam Electricity Commercial Gasification, gas turbine power generation, steam turbine power generation, organic Rankine power generation Gasification integrated with gas turbine, steam turbine, organic Rankine cycles and absorption refrigeration Mixed MSW Air-oxygen mixture Electricity, cold Commercial Gasification, gas turbine power generation, steam turbine power generation, organic Rankine power generation, absorption refrigeration Annual working hours = 7200 h Waste flow rate = 2016 tonnes/day, Air-to-waste ratio = 0.1 kg/kg, Steam-to-waste ratio = 1.0 kg/kg, Air compressor isentropic efficiency = 85%, Air compressor pressure ratio = 6–14, Gas turbine isentropic efficiency = 85%, Gas turbine inlet temperature = 970–1050 ◦ C, Steam turbine isentropic efficiency = 85%, Organic working fluid = R-113, Organic Rankine cycle turbine isentropic efficiency = 80%, Organic Rankine cycle evaporation pressure = 1500–3500 kPa, Organic Rankine cycle pump isentropic efficiency = 85% Waste flow rate = 3267 tonnes/day, Waste lower heating value = 15.00 MJ/kg, Air-to-waste ratio = 0.3 kg/kg, Gasifier temperature = 650–850 ◦ C, Air compressor pressure ratio = 11–20, Organic working Reference conditions T0 = 298 K, P0 = 101.3 kPa T0 = 300.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= – ψ = 51.15% 38.02–43.91% fc = 33.65–52.91% Remark(s) Ref. Organic Rankine cycle trivially improved the exergy efficiency of the systems. [185] Elevating the air compressor pressure ratio could reduce the overall exergy destruction while increasing the exergoeconomic factor of the plant. [190] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 NA 39 Gasification integrated with gas turbine, steam turbine, and organic Rankine cycles Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 40 NA Oxygensteam mixture Electricity, heat, cold Commercial Gasification, solar collector, gas turbine power generation, organic Rankine power generation, absorption refrigeration, district heating heater Circulating fluidized-bed gasification integrated with gas turbine cycle, steam turbine NA Oxygensteam mixture Electricity, H2 Commercial Air separation, gasification, watergas shift reaction, acid gas removal, pressure swing adsorption, gas turbine power fluid = R134a, Organic Rankine cycle evaporation pressure = 1500 kPa, Compressor isentropic efficiency = 88%, Turbine isentropic efficiency = 90%, Pump isentropic efficiency = 90%, Refrigeration solution = LiBr/ H2O High-pressure generator temperature = 80 ◦ C, System lifetime = 20 years, Annual working hours = 8760 h, Interest rate = 10%, Maintenance factor = 1.06 Waste flow rate = 50.4 tonnes/day, Waste lower heating value = 13.98 MJ/kg, Gasifier temperature = 1500 ◦ C, Air compressor pressure ratio = 10, Gas turbine inlet pressure = 15000 kPa, Organic working fluid = R134a, Refrigeration solution = LiBr/ H2O Waste flow rate = 600 tonnes/day, Waste lower heating value = 6.80 MJ/kg, Gasifier temperature = Reference conditions T0 = 298 K, P0 = 100 kPa T0 = 298.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. – ψ = 52.0% The gasifier showed the highest exergy destruction rate, followed by the air compressor and syngas combustion chamber. [191] – ψ = 46.7% The gasification unit had the highest exergy destruction, followed by the acid gas removal unit. [129] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Gasification integrated with gas turbine cycle, organic Rankine cycle, absorption refrigeration, and district heating Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale cycle, and syngas upgrading 41 Key parameter(s) generation, steam turbine power generation 985 ◦ C, Gasifier pressure = 2 MPa, Oxygen-to-waste ratio = 0.48 kg/kg, Steam-to-waste ratio = 0.1 kg/kg, Water-gas shift temperature = 270–450 ◦ C, Gas turbine isentropic efficiency = 90%, Steam turbine inlet temperature = 455 ◦ C, Steam turbine isentropic efficiency = 90% Waste flow rate = 600 tonnes/day, Waste lower heating value = 6.80 MJ/kg, Gasifier temperature = 985 ◦ C, Gasifier pressure = 2 MPa, Oxygen-to-waste ratio = 0.48 kg/kg, Steam-to-waste ratio = 0.1 kg/kg, Water-gas shift temperature = 270–450 ◦ C, Methanation temperature = 200–700 ◦ C, Gas turbine isentropic efficiency = 90%, Steam turbine inlet temperature = 453 ◦ C, Steam turbine isentropic efficiency = 90% Waste flow rate = 86.4 tonnes/day, Waste lower Circulating fluidized-bed gasification integrated with gas turbine cycle, steam turbine cycle, and syngas upgrading NA Oxygensteam mixture Electricity, synthetic natural gas Commercial Air separation, gasification, watergas shift reaction, acid gas removal, methanation, pressure swing adsorption, gas turbine power generation, steam turbine power generation Gasification integrated with steam turbine NA Oxygen Electricity, synthetic natural gas Commercial Manganeseassisted chemical looping air Reference conditions T0 = 298.15 K, P0 = 101.3 kPa T0 = 298.15 K, P0 = 1.013 bar Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. – ψ = 43.7% The methanation unit contributed to a considerable portion of the total exergy destruction of the system (>6%). [129] φ = 71.33% φ = 43.16% The water-gas shift process with CO2 removal was the most important unit from [192] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 key stages inside the investigated boundaries S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale cycle, chemical looping CO2 capture, and syngas upgrading separation, gasification, steam turbine power generation, watergas shift process with calcium looping CO2 absorption, synthetic natural gas production heating value = 15.05 MJ/kg, Gasifier temperature = 870 ◦ C, Gasifier pressure = 5 bar, Oxygen-to-waste ratio = 1.5–2.0 kg/ kg, Steam-to-waste ratio = 0.05–0.25 kg/kg, Water-gas shift temperature = 650 ◦ C, Methanation pressure = 5 bar, Steam pressure in heat recovery steam generator = 120 bar, Steam turbine isentropic efficiency = 85% Gasifier temperature = 650 ◦ C, Waste moisture content = 30–40%, Organic Rankine cycle turbine isentropic efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 90%, Organic Rankine cycle evaporator pressure = 2500 kPa, Fuel utilization factor = 75–95%, Number of cell = 13, Fuel cell current density = 0.1–1.1 A/cm2, Fuel cell stack temperature Gasification integrated with proton-exchange membrane fuel cell, organic Rankine cycle, thermoelectric generator Biowaste Air, steam Electricity, heat (hot water) Laboratory Gasification, proton-exchange membrane fuel cell, organic Rankine power generation Reference conditions Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. the internal irreversibility viewpoint. T0 = 298.15 K, P0 = 101.3 kPa φ= 71.48–80.90% fc = 57.25–76.38% φ= 4.44–21.83% The steam gasification was more efficient than air gasification in terms of exergy efficiency. The proton-exchange membrane fuel cell had higher exergy destruction and irreversibility-related cost than the gasifier. [193] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Key parameter(s) 42 key stages inside the investigated boundaries S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 43 NA Air, oxygenenriched air, pure oxygen, steam Electricity, heat NA Gasification, solid oxide fuel cell, district heating heat transfer Downdraft fixed-bed gasification integrated with solid oxide fuel cell Biowaste Air Electricity, heat (hot steam) Pilot Gasification, solid oxide fuel cell Downdraft fixed-bed gasification NA Air NA Gasification, solid oxide fuel cell, difference = 2–17 ◦ C Gasifier temperature = 800 ◦ C, Gasifier pressure = 1.013 bar, Steam-to-waste ratio = 1.0 kg/kg, Waste moisture content = 20%, Fuel cell steam-tocarbon ratio = 2.5, Fuel utilization factor = 85%, Number of cell = 5500, Fuel cell current density = 0.05–0.8 A/cm2, Fuel cell operating temperature = 547–847 ◦ C Gasifier temperature = 800 ◦ C, Waste moisture content = 16%, Fuel cell steam-tocarbon ratio = 2.0, Fuel utilization factor = 80%, Number of cell = 11000, Fuel cell current density = 0.05–0.22 A/cm2, Fuel cell operating temperature = 600 ◦ C, Fuel cell stack temperature difference = 70–160 ◦ C, Pump isentropic efficiency = 80%, Steam pressure in heat recovery steam generator = 10 bar Gasifier temperature = Reference conditions Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 101.3 kPa fc = 48.67–61.28% NA fc = 61.04% ψ= NA – ψ= System exergetic indicator(s) ψ= 20.50–47.85% ψ we = 5.68–37.33% 18.60–32.32% 28.09–46.44% Remark(s) Ref. Pure oxygen presented the best exergetic performance among the gasifying agents applied. The solid oxide fuel cell had the highest exergoeconomic factor for all the gasifying agents. [194] The gasification reactor had the highest irreversibility rate. [195] The heat exchanger of the fuel cell had the highest [196] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Downdraft fixed-bed gasification integrated with solid oxide fuel cell Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) integrated with solid oxide fuel cell and absorption refrigeration Main product (s) Scale Electricity, heat (hot water), cold absorption refrigeration 602 ◦ C, Waste moisture content = 16%, Fuel utilization factor = 64–86%, Number of cell = 11000, Fuel cell current density = 0.25–0.61 A/cm2, Fuel cell stack temperature difference = 80–240 ◦ C, Syngas compressor isentropic efficiency = 85%, Air compressor isentropic efficiency = 85%, Pump isentropic efficiency = 85%, Steam pressure in heat recovery steam generator = 20 bar, Refrigeration solution = LiBr/ H2O, High-pressure generator temperature = 127 ◦ C Waste flow rate = 518–864 tonnes/ day, Waste lower heating value = 25.02 MJ/kg, Steam-to-waste ratio = 1.0 kg/kg, Air compressor pressure ratio = 8–13, Number of cell = 1000, Fuel cell current density = 0.26–0.46 A/cm2, Fuel cell operating temperature = 650–750 ◦ C, Gasification integrated with solid oxide fuel cell, gas turbine cycle, and steam turbine cycle NA Steam Electricity Commercial Gasification, solid oxide fuel cell, gas turbine power generation, steam turbine power generation Reference conditions Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. exergy destruction, followed by the gasification reactor. The current density, stack temperature difference, and fuel utilization factor of the fuel cell significantly affected the overall exergy efficiency of the system. T0 = 298.2 K, P0 = 101.3 kPa – ψ= 20.69–25.48% The effect of MSW flow rate on the overall exergy efficiency of systems was more pronounced than those of fuel cell current density, fuel cell operating temperature, and air compressor pressure ratio. [197] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Key parameter(s) 44 key stages inside the investigated boundaries S. Soltanian et al. Table 7 (continued ) MSW treatment approach Bubbling fluidizedbed gasification integrated with plasma converter, solid oxide fuel cell, air gas turbine cycle, steam turbine cycle, chemical looping CO2 capture, and CO2 gas turbine cycle, Waste fraction (s) NA Gasifying agent(s) Oxygensteam mixture Main product (s) Electricity, supercritical CO2 Scale Demonstration key stages inside the investigated boundaries Air separation unit, gasification, plasma converter, solid oxide fuel cell, air gas turbine power generation, steam turbine power generation, chemical looping combustion, CO2 gas turbine power generation Key parameter(s) 45 Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 101.3 kPa ψ = 80.74% System exergetic indicator(s) ψ= 22.56–41.82% Remark(s) Ref. The highest exergy destruction was observed in the plasma-assisted gasifier, followed by the solid oxide fuel cell and chemical looping combustion unit. [198] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fuel cell stack temperature difference = 100 ◦ C, Steam turbine inlet temperature = 2026 kPa Waste flow rate = 10.1 tonnes/day, Waste lower heating value = 19.99 MJ/kg, Waste moisture content = 14.9%, Gasifier temperature = 800 ◦ C, Gasifier pressure = 3.5 bar, Equivalence ratio = 0.37 kg/kg, Steam-to-waste ratio = 0.2–0.6 kg/ kg, Plasma converter operating temperature = 1200 ◦ C, Plasma converter operating pressure = 3.25 bar, Air gas turbine isentropic efficiency = 88%, Air reactor temperature for chemical looping combustion = 850–1100 ◦ C, CO2 gas turbine isentropic efficiency = 88%, Steam turbine isentropic efficiency = 88%, High-pressure steam turbine pressure = 120 bar, Medium-pressure steam turbine pressure = 30 bar, Low-pressure steam Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach 46 Gasification integrated with solid oxide fuel cell, gas turbine cycle, steam turbine cycle, organic Rankine cycle, and absorption refrigeration cycle Waste fraction (s) Mixed MSW except for metals and glass Gasifying agent(s) Air Main product (s) Electricity, cold Scale Commercial key stages inside the investigated boundaries Sorting, gasification, solid oxide fuel cell, gas turbine power generation, steam turbine power generation, organic Rankine power generation, and absorption refrigeration Key parameter(s) T0 = 300.15 K, P0 = 101.3 kPa Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) – ψ= 39.18–74.69% SI = 2.25 fc = 50.63% Remark(s) Ref. The regenerator of the absorption refrigeration cycle had the lowest exergy efficiency, followed by the combustion air preheater. [199] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 turbine pressure = 4 bar, Fuel utilization factor = 65–90%, density = 0.10–0.35 A/cm2, Fuel cell operating temperature = 900 ◦ C, Fuel cell operating temperature = 3.25 bar Waste flow rate = 4752 tonnes/day, Air compressor pressure ratio = 2–20, Air compressor isentropic efficiency = 90%, Gas turbine isentropic efficiency = 85%, Fuel utilization factor = 85%, Fuel cell current density = 0.22–0.82 A/cm2, Fuel cell operating temperature = 850 ◦ C, Fuel cell operating pressure = 1200 kPa, Steam pressure in heat recovery steam generator = 15000 kPa, Steam turbine isentropic efficiency = 80%, Steam cycle pump isentropic efficiency = 90%, Organic Rankine cycle evaporator pressure = 3000 kPa, Organic Rankine cycle turbine isentropic Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach 47 Downdraft fixed-bed gasification integrated with gas turbine cycle, molten carbonate fuel cell, organic Rankine cycle, and carbon capture and sequestration Waste fraction (s) NA Gasifying agent(s) Oxygen Main product (s) Electricity, liquid CO2 Scale Commercial key stages inside the investigated boundaries Gasification, gas turbine power generation, molten carbonate fuel cell, organic Rankine power generation, cryogenic CO2 separation Key parameter(s) Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 101.3 kPa φ = 81.94% rc = 22.37 (–) fc = 23.49% System exergetic indicator(s) ψ= 39.05–47.73% Remark(s) Ref. The fuel cell current density had the most significant impact on the exergy efficiency of the system compared with the gasifier temperature, fuel cell steam-to-carbon ratio, fuel cell CO2 utilization factor, and fuel cell operating temperature. [200] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 90%, Refrigeration solution = LiBr/ H2O, High-pressure generator temperature = 90 ◦ C, Evaporator temperature of absorption refrigeration = 7 ◦ C, Interest rate = 5–14% Waste flow rate = 300 tonnes/day, Waste lower heating value = 12.81 MJ/kg, Waste moisture content = 10%, Gasifier temperature = 700–900 ◦ C, Air compressor pressure ratio = 12, Gas turbine inlet temperature = 1150–1350 ◦ C, Fuel cell steam-tocarbon ratio = 2.5–4.5, Fuel utilization factor = 75%, Fuel cell CO2 utilization factor = 60–90%, Fuel cell current density = 0.025–0.25 A/cm2, Fuel cell operating temperature = 600–700 ◦ C, Organic working fluid = isobutane, Organic Rankine Reference conditions S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 48 NA Oxygen Electricity, liquid CO2 Commercial Gasification, gas turbine power generation, molten carbonate fuel cell, organic Rankine or steam Rankine power generation, cryogenic CO2 separation cycle evaporator pressure = 2880 kPa, Organic Rankine cycle turbine isentropic efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 90%, Minimum cryogenic temperature = –56 ◦ C, Cryogenic pump pressure ratio = 1.5, System lifetime = 20 years, Annual working hours = 7446 h, Interest rate = 12%, Maintenance factor = 1.10 Waste flow rate = 300 tonnes/day, Waste lower heating value = 12.81 MJ/kg, Waste moisture content = 10%, Gasifier temperature = 800 ◦ C, Gas turbine pressure ratio = 12, Gas turbine isentropic efficiency = 85%, Air compressor isentropic efficiency = 85%, Maximum gas turbine temperature = 1500 ◦ C, Fuel cell steam-tocarbon ratio = 3.5, Fuel utilization factor = 75%, Fuel cell current Reference conditions Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 101.3 kPa φ = 81.94% rc = 22.37 (–) fc = 23.49% System exergetic indicator(s) ψ= 39.51–43.95% Remark(s) Ref. The process with the organic Rankine cycle could provide higher exergy efficiency than the process with the steam Rankine cycle. [201] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Downdraft fixed-bed gasification integrated with gas turbine cycle, molten carbonate fuel cell, organic or steam Rankine cycle together with carbon capture and sequestration Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries 49 NA Oxygen Electricity, liquid CO2 Commercial Gasification, gas turbine power generation with exhaust air recirculation, molten carbonate fuel cell, organic Rankine power generation, cryogenic CO2 separation density = 0.11 A/ cm2, Fuel cell operating temperature = 650 ◦ C, Organic working fluid = isobutane, Organic Rankine cycle evaporator pressure = 2880 kPa, Organic Rankine cycle turbine isentropic efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 90%, Minimum cryogenic temperature = –56 ◦ C, Cryogenic pump pressure ratio = 1.5, System lifetime = 20 years, Annual working hours = 7446 h, Interest rate = 12%, Maintenance factor = 1.10 Waste flow rate = 300 tonnes/day, Waste lower heating value = 12.81 MJ/kg, Waste moisture content = 10%, Gasifier temperature = 800 ◦ C, Gas turbine inlet temperature = 1280 ◦ C, Gas turbine pressure ratio = 12, Gas turbine isentropic efficiency = 85%, Air compressor Reference conditions Exergetic indicator(s) of MSW gasification unit T0 = 298.15 K, P0 = 101.3 kPa φ = 81.94% rc = 22.37 (–) fc = 23.49% System exergetic indicator(s) ψ= 37.18–43.25% Remark(s) Ref. The effect of the current density of the fuel cell on the overall exergy efficiency was more marked than that of the fuel utilization factor. [202] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Downdraft fixed-bed gasification integrated with gas turbine cycle, molten carbonate fuel cell, organic Rankine cycle or steam Organic cycle power generation systems including carbon capture and sequestration process Key parameter(s) S. Soltanian et al. Table 7 (continued ) MSW treatment approach Waste fraction (s) Gasifying agent(s) Main product (s) Scale key stages inside the investigated boundaries Key parameter(s) Reference conditions Exergetic indicator(s) of MSW gasification unit System exergetic indicator(s) Remark(s) Ref. 50 Abbreviations/symbols: ψ : Functional exergy efficiency, φ: Universal exergy efficiency, ψ CGE : Cold gas exergy efficiency, ψ tar : Tar exergy efficiency, ψ we : Waste-to-electricity exergy efficiency, Equivalence ratio: Proportion of actual air-fuel ratio to stoichiometric air-fuel ratio; fb : Exergoenvironmental factor, fc : Exergoeconomic factor, NA: Not available, rb : Relative environmental impact difference, rc : Relative cost difference, SI: Sustainability index. Renewable and Sustainable Energy Reviews 156 (2022) 111975 isentropic efficiency = 85%, Maximum gas turbine temperature = 1500 ◦ C, Fuel cell steam-tocarbon ratio = 3.5, Fuel utilization factor = 70–76%, Fuel cell operating temperature = 650 ◦ C, Fuel cell current density = 0.05–0.20 A/cm2, Organic working fluid = isobutane, Organic Rankine cycle evaporator pressure = 2880 kPa, Organic Rankine cycle turbine isentropic efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 90%, Minimum cryogenic temperature = –56 ◦ C, Cryogenic pump pressure ratio = 1.5, System lifetime = 20 years, Annual working hours = 7446 h, Interest rate = 12%, Maintenance factor = 1.10 S. Soltanian et al. Table 7 (continued ) S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 exchanger = 70 ◦ C, the inlet temperature of the organic vapor turbine = 202 ◦ C, and the inlet pressure of the organic vapor turbine = 5 bar. Given the multitude of information mentioned above, the advanced integrated waste-to-energy schemes generating multiple products could exergetically outperform the single-product systems. In addition, using modern technologies could further improve the exergetic performance of integrated MSW treatment systems. In this sense, Houshfar [159] showed that replacing the condensing unit with a thermoelectric generator in an MSW-fueled steam turbine power plant could improve the energy, exergy, economic, and environmental performance of the process. Despite the multitude of advantages of MSW incineration technology, Varbanov et al. [160] claimed that valorizing MSW into chemical platforms (e.g., levulinic acid) could be more exergetically sustainable than MSW incineration practice if the evolved chemical products could be sold at an acceptable price. In another comparative study, Liu et al. [161] showed that MSW gasification could outperform MSW incineration according to the extended exergy accounting concept. In an interesting study, Eboh et al. [130] exergetically investigated a modified MSW-powered steam turbine power plant (base cycle). The base cycle was modified by excess air reduction (Scenario A), flue gas condensation (scenario B), steam reheater (Scenario C), excess air reduction, flue gas condensation, and steam reheater (Scenario D), waste gasification and syngas boiler (Scenario E), flue gas condensation, waste gasification, and syngas boiler (Scenario F), and air preheating (Scenario G). All the developed scenarios outperformed the base cycle in terms of exergy efficiency. The highest exergy efficiency was found for Scenario F, followed by Scenario E. This finding further demonstrated the pivotal role of waste gasification in improving the exergetic performance of MSW management systems. exergetic performance of updraft and downdraft fixed-bed gasifiers during MSW processing (Fig. 17). In general, the updraft fixed-bed gasifier could outperform the downdraft counterpart in terms of exergy efficiency. The air gasification of waste could yield higher exergy efficiency compared with oxygen and steam gasification. Furthermore, the higher volatile content of solid waste could boost the exergy efficiency of the gasification process. Unlike the fixed-bed gasification process, the feedstock and the bed material particles are fluidized in the fluidized-bed gasification process [203]. The main advantage of fluidized-bed gasification technology over other counterparts is its capability to handle a broad spectrum of feedstocks [204]. The fluidized-bed gasification process can also operate at slightly lower temperatures, reducing ash sintering [205]. Nevertheless, exergy-based methods need to be applied to evaluate the efficiency, viability, and sustainability of this MSW treatment method. It is important to note that operating parameters can significantly affect the exergetic performance of the fluidized-bed gasification process. In this context, Couto et al. [170] examined the effects of operation variables, i.e., reaction temperature (700–900 ◦ C) and equivalence ratio (0.15–0.35), on the exergetic performance of a semi-industrial bubbling fluidized-bed gasifier. Generally, the reaction temperature had a greater impact on both syngas and tar exergy contents than the equivalence ratio. Elevating the reaction temperature could linearly increase the syngas exergy content. The recorded improvement could be since the endothermic steam reforming and water-gas shift reactions were promoted at elevated reaction temperatures, resulting in higher CO and H2 yields [171]. The syngas exergy was marginally improved by increasing the equivalence ratio up to 0.25. However, further increasing the equivalence ratio beyond the optimal value (0.25) caused a slight decrease in the syngas exergy content. Notably, elevating the equivalence ratio could cause a uniform decline in CO and H2 yields due to the enhanced oxidation reactions and a constant increase in CO2 yield due to the improved partial combustion [171]. Unlike the syngas exergy content, increasing the reaction temperature and equivalence ratio could reduce the tar exergy value by promoting its cracking. Interestingly, the process exergy efficiency and the syngas exergy content showed similar trends, indicating the dominant impact of the syngas exergy on the exergetic sustainability of the gasification process. In another research, Tang et al. [171] exergetically investigated an air-blown fluidized-bed MSW gasification reactor operating at different temperatures (550–850 ◦ C) and equivalence ratios (0.2–0.8) based on the experimental data. Unlike the findings of Couto et al. [170], the syngas exergy content and the overall exergy efficiency were increased up to 650 ◦ C. However, further elevating the reaction temperature beyond 650 ◦ C negatively decreased the syngas exergy content and the overall exergy efficiency. Increasing the equivalence ratio up to 0.4 could only positively affect the syngas exergetic value and the overall exergy efficiency. Overall, the highest exergy efficiency was obtained at moderate reaction temperatures and equivalence ratios. Couto et al. [173] assessed the effects of reaction temperature (700–900 ◦ C) and steam-to-feedstock mass ratio (0–2.0) on the exergetic parameters of the steam gasification process during operating with MSW and forest residue. In general, the exergy efficiency of the forest residue gasification process was higher than that of MSW under all the investigated conditions. Like the results reported for the air gasification process by Couto et al. [170], elevating the reaction temperature declined the tar exergetic content while increasing the syngas exergetic content and exergy efficiency for both feedstocks. However, increasing the steam-to-feedstock mass ratio up to a certain level could improve the exergy efficiency of the process. The steam-to-feedstock mass ratio for MSW at the maximum exergy efficiency of the process was higher than that for forest residue (1.5 vs. 1.0). In the plastic gasification process, Mojaver et al. [206] showed that increasing the steam-to-plastic waste mass ratio from 1.0 to 3.0 increased H2 and CO2 yields while decreasing CO yield (Fig. 18). Nevertheless, the higher steam-to-plastic waste mass ratio resulted in higher exergy destruction rates (Fig. 18). In general, the 4.1.2. Municipal solid waste gasification The gasification process has been offered as an alternative process to address the environmental concerns of MSW incineration. MSW can be gasified to produce syngas consisting of CO, H2, CH4, and trace amounts of some impurities, i.e., poly-aromatic hydrocarbons, inorganic compounds (NH3, H2S, HCl, HF, HCN, and alkalines), and particulate matter [162]. The gasification process is carried out in a partially oxygenated atmosphere at higher reaction temperatures [163]. Air, oxygen, steam, and CO2 are frequently used as gasifying agents [164]. Internal partial waste combustion and external heat source might be necessary to maintain endothermic gasification reactions. The main challenge associated with the gasification process is the generation of tar or condensable organic compounds, causing blockage and clogging in the downstream equipment [165]. In this regard, the produced syngas should be cleaned, reformed, and purified to yield a pure mixture of H2 and CO. The clean syngas can then be used in internal combustion engines, gas turbines, and fuel cells to generate power and in Fischer-Tropsch synthesis to produce liquid fuels/chemicals [47,166]. MSW is gasified using different technologies, including fixed-bed gasifiers (updraft and downdraft reactors), fluidized-bed gasifiers (bubbling, circulating, and dual-technology reactors), and plasma gasifiers [167]. Although the gasification process is an appealing approach to dealing with MSW and producing renewable energy, its sustainability level should be improved using advanced engineering methods like the exergy concept. Table 7 tabulates the most important applications of the exergy-based approaches in analyzing MSW gasification systems. In a fixed-bed gasification process, the flow rate or velocity of the gasifying agent is not high enough to fluidize the feedstock particles. The gasifying agent and the evolved products percolate inside the bed, undergoing many endothermic reactions to form syngas [162]. Rao et al. [168] exergetically investigated an updraft fixed-bed gasification reactor that processed MSW-based refuse-derived fuel, torrefied soybean straw, and woody biomass. The highest cold gas exergy efficiency was obtained when the system was fed by MSW-based refuse-derived fuel, followed by torrefied soybean straw and woody material. Using the data collected from the published literature, Xiang et al. [169] compared the 51 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 steam gasification of high-density polyethylene indicated the lowest exergy destruction rate compared with other plastic types, i.e., polypropylene, polycarbonate, and polyethylene terephthalate (Fig. 18). In another study, Zheng et al. [207] found that increasing the reaction temperature from 900 to 1200 ◦ C could improve the syngas exergetic value during MSW gasification in CO2. However, increasing the CO2-to-MSW mass ratio beyond 0.5 could augment the syngas exergetic content. Unlike Zheng et al. [207], Couto et al. [173] showed that the syngas exergetic content increased when the CO2-to-MSW mass ratio varied between 0 and 1.0. Nevertheless, according to Couto et al. [173], the highest overall exergy efficiency of the process was obtained when the CO2-to-MSW mass ratio was set at 0.8. Unlike bubbling and circulating fluidized-bed gasifiers, dual fluidized-bed gasification technologies can provide the thermal energy required to sustain endothermic gasification reactions via external char combustion. The thermal energy produced in a separate reactor is then transferred to the gasification reactor using the circulating bed material [208]. The gasification unit applies the bubbling fluidized-bed technology, while combustion is carried out under the fast-fluidizing regime. This process produces high-quality syngas since the produced syngas does not dilute with the flue gas of the combustion reaction. However, the complex heat/mass transfer characteristics and chemical reactions of the dual fluidized-bed gasification technology can make decision-making on its exergetic sustainability aspects more challenging. Vitasari et al. [174] used the exergy concept to analyze a bio-based synthetic natural gas plant employing dual fluidized-bed gasification technology. The effects of feedstock type (i.e., MSW, sludge, and woody biomass) and operating parameters (i.e., gasifier pressure, methanation pressure, and methanation temperature) were investigated on the exergetic performance of dual fluidized-bed gasification, syngas cooling, cleaning and compression, methanation, and syngas conditioning processes. The gasification unit showed the highest exergy destruction rate, followed by methanation and CO2 removing units (Fig. 19). Shehzad et al. [209] observed a similar irreversibility ranking in producing synthetic natural gas using bubbling fluidized-bed gasification technology. According to Vitasari et al. [174], the system indicated better exergetic performance when fed with woody biomass. Increasing the gasification pressure could positively increase the synthetic natural gas yield and the process exergetic performance while lowering the process irreversibility rate. This was because methane-rich and high-quality syngas (with a high hydrogen/carbon monoxide ratio) could be produced at higher gasification pressures. The irreversibility rate of the methanation unit was also reduced at higher gasification pressures. Increasing the pressure of the methanation process deteriorated the exergetic performance of the process when the MSW and sludge were used as feedstocks. However, an opposite trend was observed for woody biomass. Increasing the methanation temperature lowered the exergetic performance of the process when sludge was fed to the system. However, MSW and woody feedstocks did not negatively affect the exergetic performance of the process at higher methanation temperatures. The advanced plasma gasification process is an environmentally friendly technology for disposing of solid wastes and producing valuable products [210]. In the plasma gasification process, wastes are gasified by an external heat source while limiting combustion. The plasma gasification method can effectively convert most of the carbon content of the wastes into syngas [211]. The high temperatures of plasma gasification permit the decomposition of all the heavy molecules of tars, char, and dioxins into gaseous molecules. Therefore, the evolved gaseous stream is very clean, while the bottom of the reactor is nearly free of ash deposits. In addition, most of the heavy metals are confined to the molten slag. The needs for high capital and operation costs and energy demand are significant drawbacks of the plasma gasification process. Accordingly, this cost-intensive and energy-consuming MSW treatment process should be analyzed before real-world implementation using advanced sustainability assessment methods [210]. Zhang et al. [175] energetically and exergetically detailed the key reactions involved in the plasma gasification melting process, i.e., drying, pyrolysis, char gasification, and plasma melting. The pyrolysis reaction showed the highest exergy loss in the plasma gasification process. This issue could be ascribed to the highly irreversible nature of the pyrolysis process, where rapid chemical reactions and fast mass transfer occurred (Fig. 20). The plasma melting process stood in the second rank in terms of exergy loss, mainly due to its high energy loss. In general, the functional exergy efficiency of the plasma gasification process tended to decrease as the equivalence ratio was elevated due to the higher irreversibility rate and the evolved syngas dilution at higher equivalence ratios. In contrast to functional exergy efficiency, the cold gas exergy efficiency was increased by increasing the equivalence ratio since the generated tar was effectively cracked into syngas and lower molecules at elevated equivalence ratios. Like equivalence ratio, increasing the plasma temperature reduced the functional exergy efficiency of the process because of increased electricity consumption. Nevertheless, increasing the plasma temperature could improve the cold gas exergy efficiency by promoting char gasification and tar cracking. Increasing the steam mass flux up to a certain level could improve the functional exergy efficiency of the process. However, further increasing the mass flux beyond the optimal level negatively lowered the functional exergy efficiency of the process. This could be because the elaborated steam mass flux could boost incomplete char gasification, while the excessive steam mass flux could promote char oxidation. Fig. 17. Distribution of exergy efficiency values of updraft and downdraft fixed-bed gasifiers during MSW processing at different a) temperatures and b) equivalence ratios. Drawn using the data reported by Xiang et al. [169]. 52 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 18. Effect of the steam-to-plastic waste mass ratio on the product yield and exergy destruction rate of steam gasification of four different waste plastic types. Redrawn with permission from Mojaver et al. [206]. Copyright© 2021 Elsevier. Jadhao et al. [119] compared three different MSW management strategies, including landfill gas collection, incineration, and plasma gasification, using the exergy concept. Unexpectedly, the landfill gas collection technique had the highest universal exergy efficiency, followed by the plasma gasification and incineration processes. It was argued that the low-moisture and organic carbon-rich waste could promote the exergetic performance of the MSW plasma gasification process. In another study, Janajreh et al. [176] investigated the effects of different feedstocks on the exergy efficiency of the plasma steam gasification process. The exergy efficiency of the process during operating with MSW was higher than those of algae, pine needles, treated and untreated wood, and plywood. However, the exergy efficiency of the process during operating with coal and tire was very higher than that of MSW. Overall, MSW appeared to be an exergetically better option for plasma gasification than biomass and woody materials. Unlike the observations of Jadhao et al. [119], Montiel-Bohórquez et al. [177] claimed that increasing the moisture content of waste could enhance both energy and exergy efficiency values of a plasma-assisted updraft fixed-bed gasifier. This finding could be ascribed to the fact that the thermal degradation rate of the moist MSW under plasma impingement was higher than that of dried one. In addition, increasing the plasma temperature negatively affected the exergy efficiency of the process by increasing torch power consumption. The syngas produced during MSW gasification can be used in combined cooling, heating, and power plants to produce electricity, heat, and cold. Such an integration appears to be an efficient, viable, and sustainable strategy to deal with MSW. Using the ELCA approach, Tang et al. [212] showed that an MSW gasification process integrated with a gas turbine or a gas-steam combined cycle was exergetically and environmentally superior over (i) an MSW incinerator coupled with a steam turbine plant, (ii) an MSW gasification process integrated with a syngas-powered steam turbine cycle, and (iii) an MSW gasification process coupled with a gas engine unit. In another investigation, Asgari et al. [178] exergetically explored a combined cooling, heating, and power system consisting of an MSW-fed air gasifier, a gas turbine cycle fueled by syngas-natural gas mixture, an absorption refrigeration system. Increasing the MSW-to-natural gas mass ratio remarkably decreased the overall exergy efficiency of the system due to the lower calorific value of syngas compared with natural gas. In addition, the higher waste flow rate resulted in higher exergy destruction in the gasifier. On the contrary, the overall exergy efficiency of the system was improved by elevating the pressure ratio of the air compressor and the temperature of the preheated air. Surprisingly, the inlet temperature of the gas turbine had an insignificant effect on the overall exergy efficiency of the system. As expected, an increase in the isentropic efficiency values of both gas turbine and air compressor could notably affect the overall exergy efficiency of the system. Ding et al. [181] studied the effect of incorporating a Stirling engine into a cogeneration plant on its exergetic, exergoeconomic, and environmental performance parameters. The plant included a gasification unit (fueled by MSW, paddy husk, wood, and paper), a gas turbine cycle, and a supercritical CO2 power cycle. For all the feedstocks used, the Stirling engine could boost the overall exergy efficiency of the plant while mitigating its CO2 emission. Unlike the overall exergy efficiency of the system, elevating the piston compression ratio and lowest-to-highest temperature ratio of the Stirling engine could significantly reduce the specific cost of product exergy (Fig. 21). MSW showed the highest overall exergy efficiency of the plant, followed by wood, paddy husk, and paper (Fig. 21). In addition, the order of the specific cost of product exergy was found to be MSW < paddy husk < paper < wood (Fig. 21). Parvez and Khan [186] analyzed the effect of feedstock type (i.e., Fig. 19. The contribution of various stages of the synthetic natural gas process in the total imperfection of the process for three different feedstocks. Redrawn with permission from Vitasari et al. [174]. Copyright© 2011 Elsevier. 53 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 20. Detailed exergy flow diagram of the plasma gasification process. Redrawn with permission from Zhang et al. [175]. Copyright© 2013 Elsevier. MSW, rice husk, sugarcane bagasse) on the exergetic performance of a gas-steam combined cycle integrated with a steam fluidized-bed gasification unit under different operating parameters (i.e., gas turbine inlet temperature and steam turbine pressure). Among the feedstocks used, MSW showed the best exergetic performance, irrespective of the operating conditions investigated (Fig. 21). Unlike steam turbine pressure, increasing the gas turbine temperature could improve the exergy efficiency of the process (Fig. 22). For all the feedstocks studied, the highest exergy destruction was found for the syngas combustion chamber, followed by the gasifier and heat recovery steam generator. Montiel-Bohórquez et al. [187] exergetically and exergoeconomically examined a gas-steam combined cycle integrated with a plasma gasification unit. The gasifier was fueled with various wastes, i.e., residential, institutional, commercial, industrial, and their mixture. The overall exergy efficiency of the process followed the order of commercial > industrial > institutional > mixed > residential. The highest specific cost of electric exergy was found for the commercial waste, followed by the mixed, industrial, institutional, and residential wastes. Unlike the fluidized-bed gasifier reported by Parvez and Khan [186], the plasma gasifier showed the highest exergy destruction among the system units. Casas-Ledón et al. [188] assessed the exergetic and exergoeconomic performance of an integrated MSW management system, including MSW gasification, gas turbine cycle, steam turbine cycle, and district heating 54 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 network, at different gasification temperatures and air equivalence ratios. Unlike the equivalence ratio, elevating the gasification temperature could improve the overall exergy efficiency of the process due to the higher exergetic content of the evolved syngas. Increasing the equivalence ratio increased the quantity of the electrical exergy required for compressing the produced syngas, thereby lowering the overall exergy efficiency of the process. The MSW gasification reactor showed the highest contribution to the total irreversibilities of the process, while the gas turbine was the most exergoeconomically crucial unit due to its high capital and irreversibility cost rates. From the exergoeconomic viewpoint, the capital investment of the plant was dominant over the exergy destruction cost. Unlike the gasification temperature, increasing the equivalence ratio could lower both the exergoeconomic factor and the relative cost difference of the plant. Overall, the most appealing exergetic and exergoeconomic results were obtained at higher gasification temperatures and lower equivalence ratios. Casas-Ledón et al. [189] used exergoenvironmental analysis to investigate the MSW-fueled power plant proposed in their previous study Casas-Ledón et al. [188]. The electricity generated by the steam turbine showed the highest specific environmental impact of exergy. The exhaust gas, gas turbine electricity, and syngas stream had the highest environmental impact rates. The internal irreversibility and pollutant formation in the MSW gasification process played an important role in the exergoenvironmental performance of the whole plant. The MSW gasifier indicated the highest environmental impact because of its massive pollutant flux. The environmental impacts associated with the development of the gasifier and its pollutant flux were significantly higher than the irreversibility-related environmental impact. Ahmad et al. [185] compared the effects of the organic Rankine cycle and feedstock type (MSW and rice husk) on the energetic and exergetic performance of a gas-steam combined cycle integrated with a steam gasifier. The organic Rankine cycle slightly improved the overall exergy Fig. 21. Effects of (a) piston compression ratio and (b) lowest-to-highest temperature ratio of a Stirling engine on the overall exergy efficiency of an integrated system (including a gasification unit, a gas turbine cycle, a Stirling engine, and a supercritical CO2 power cycle) operating with various feedstocks. Redrawn with permission from Ding et al. [181]. Copyright© 2021 Elsevier. 55 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 of the system. Similar to the findings of Parvez and Khan [186] (Fig. 22), MSW could also yield considerably higher exergy efficiency than rice husk. In a similar study, Oko and Nwachukwu [190] exergetically and exergoeconomically analyzed a cogeneration system consisting of various cycles (gas turbine, steam turbine, organic Rankine, and absorption refrigeration) coupled with an air-oxygen gasifier. The exergy efficiency of the system was higher than that recorded by Ahmad et al. [185], probably due to the improvement caused by the absorption refrigeration system. The syngas combustion chamber indicated the largest exergy destruction and the smallest exergoeconomic factor, making its modification and retrofitting necessary. Ghasemi et al. [191] exergetically examined an oxygen-steam gasifier integrated with a steam turbine cycle, an organic Rankine cycle, and an absorption refrigeration system. Parabolic trough solar collectors were used to superheat the steam stream before injecting into the gasifier. Elevating the injected steam temperature increased the exergy destruction inside the gasifier, leading to a reduction in the overall exergy efficiency of the plant. In another study, Habibollahzade et al. [213] integrated a steam turbine power plant running on MSW-based syngas with a solar chimney power plant to resolve the solar energy unavailability issue during nighttime. Even though this integration could considerably increase the power generation rate, there was no substantial improvement in the exergy efficiency of the process. The thermodynamic, economic, and environmental performance of combined cooling, heating, and power plants fueled by MSW-based syngas can be boosted by either selling a fraction of the evolved syngas to other industries through local markets or converting it into valueadded products through integrated synthesizing and upgrading facilities. Kabalina et al. [179] applied exergy and exergoeconomic analyses to assess four different polygeneration district heating and cooling scenarios powered by the refuse-derived fuel gasification process. In addition to three main products (electricity, heat, and cold), char (first scheme), char and syngas (second scheme), char, hydrogen, and synthetic natural gas (third scheme), and char, syngas, synthetic natural gas, and hydrogen (fourth scheme) were produced in the developed scenarios. The second scheme was the most exergetically and exergoeconomically appealing scenario. In addition to refuse-derived fuel, Kabalina et al. [180] used MSW in the next work under different loads of gas turbine and thermal energy and investigated the energetic and exergetic performance of the developed schemes. Regardless of the feedstocks and scenarios considered, the deviation from the nominal power and thermal energy loads decreased the quantities of valuable products and lowered the overall exergy efficiency of the system. Sun et al. [129] exergetically investigated four different MSW treatment scenarios, i.e., incineration coupled with steam turbine power generation (first scenario), syngas-fueled gas-steam turbine power generation (second scenario), syngas-to-hydrogen upgrading combined with gas-steam turbine power generation (third scenario), and syngas-to-synthetic natural gas upgrading integrated with gas-steam turbine power generation (fourth scenario). The third scenario provided the highest exergy efficiency, followed by the fourth, second, and first scenarios. Ozturk and Dincer [182] exergetically examined a hydrogen production plant consisting of an air gasification unit and syngas chemical processing and upgrading (including catalytic tar reforming, high- and low-temperature water-gas shift, and pressure swing adsorption). It could be inferred from the published data that increasing the gasification temperature decreased the hydrogen production rate while augmenting the carbon monoxide and methane production rates. Accordingly, increasing the reaction temperature could increase the overall exergy efficiency of the system while decreasing the hydrogen exergy efficiency. Wu et al. [183] exergetically compared three different schemes, including (i) an MSW gasification integrated with a gas-steam combined cycle, (ii) an MSW gasification integrated with a gas turbine cycle and a dimethyl ether/methanol synthesis unit, and (iii) an MSW calcium looping gasification integrated with a gas turbine cycle and a dimethyl ether/methanol synthesis unit. The first scheme appeared to have the highest exergy efficiency, followed by the second and third schemes. Nevertheless, the lowest specific cost of product exergy was found for the third scheme due to its higher dimethyl ether yield. Integrating MSW gasification systems with advanced technologies such as proton-exchange membrane fuel cells can further enhance their performance. The main advantages of proton-exchange membrane fuel cells are lower emissions, lower operating temperature (~80–200 ◦ C), and shorter start-up time. Using exergy, exergoeconomic, and environmental analyses, Behzadi et al. [193] examined an MSW gasifier integrated with a low-temperature proton-exchange membrane fuel cell, an organic Rankine cycle, a thermoelectric generator. Regardless of the air and steam used as gasifying agents, the exergy efficiency of the process increased by elevating the current density and stack temperature difference of the fuel cell, while this indicator decreased by increasing the fuel utilization factor and waste moisture content. In addition, steam gasification could outperform air gasification in terms of exergy efficiency. Nevertheless, the proton-exchange membrane fuel cell showed the highest exergy destruction and irreversibility-related cost rate during steam gasification. Solid oxide fuel cells are another electrochemical conversion systems that oxidize hydrocarbon fuels by ceramic electrolyte while generating electric current. Fuel flexibility is one of the major merits of solid oxide fuel cells, making them suitable candidates to be combined with MSW Fig. 22. Effects of (a) gas turbine inlet temperature and (b) steam turbine pressure on the overall exergy efficiency of a gas-steam combined cycle integrated with a steam fluidized-bed gasification process operating with various feedstocks. Drawn using the data obtained from Parvez and Khan [186]. 56 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 gasification systems. Hosseinpour et al. [194] energetically, exergetically, and exergoeconomically explored and optimized a solid oxide fuel cell integrated with a downdraft fixed-bed MSW gasification system operating with various gasifying agents used. Pure oxygen could provide the highest exergy efficiency, followed by steam, oxygen-enriched air, and air. The overall exergy efficiency of the process was almost linearly decreased by increasing the fuel cell current density (Fig. 23a). Nevertheless, increasing the fuel cell operating temperature up to a certain level could improve the overall exergy efficiency of the process (Fig. 23b). The gasification reactor showed the highest exergy destruction when operated with air and oxygen-enriched air. The solid oxide fuel cell indicated the highest exergy destruction when the steam agent was used. The solid oxide fuel cell had the highest exergoeconomic factor for all the gasifying agents used. In another investigation, Gholamian et al. [196] demonstrated that an absorption refrigeration system could remarkably augment the energy efficiency (~178%) and exergy efficiency (~50%) of a solid oxide fuel cell system fueled with MSW-based syngas. Elevating the stack temperature difference of the fuel cell system could improve the overall exergy efficiency of the process. However, increasing the fuel utilization factor could only increase the overall exergy efficiency of the process until a certain level (~78.2%). Ghaffarpour et al. [197] exergetically investigated an integrated power generation system consisting of a gasifier, a gas turbine cycle, a solid oxide fuel cell, and a steam turbine cycle. Three different feedstocks (MSW, pine sawdust, and fowl manure) were used in the steam gasification process. The lowest exergetic performance was observed for MSW without considering the operating parameters of the system. Unlike Hosseinpour et al. [194] (Fig. 23a), increasing the fuel cell current density could improve the overall exergy efficiency of the system. However, similar trends were found in both studies (Fig. 23b) when the operating temperature of the fuel cell was modified. In contrast with the feedstock flow rate, increasing the air compressor pressure ratio could promote the exergetic performance of the system. Regardless of the feedstocks and operating conditions applied, the highest irreversibility rate was recorded for the gasification reactor, followed by the syngas combustion chamber. Jiang et al. [198] exergetically explored a plasma-assisted bubbling fluidized-bed gasifier integrated with a high-temperature solid oxide fuel cell, an air gas turbine cycle, a CO2 gas turbine cycle, a steam turbine cycle, and a chemical looping combustion. Increasing the steam-to-waste mass ratio up to around 0.5 could slightly increase the exergy efficiency of the system. Like the findings of Hosseinpour et al. [194], increasing the fuel cell current density linearly decreased the overall exergy efficiency of the system. Quite close to the results of Gholamian et al. [196], increasing the fuel utilization factor could only improve the exergy efficiency of the system up to an optimal value (80%). Elevating the temperature of the chemical looping air reactor up to 1000 ◦ C could increase the overall exergy efficiency of the system. Owebor et al. [199] analyzed an MSW-fed integrated system producing cold and electricity using different low- and high-grade thermal energy systems from exergetic and exergoeconomic viewpoints. The plant included gasification, gas turbine cycle, steam turbine cycle, solid oxide fuel cell, organic Rankine cycle, and absorption refrigeration. The solid oxide fuel cell possessed the highest exergy efficiency, followed by the steam turbine cycle. Incorporating the solid oxide fuel cell, organic Rankine cycle, and absorption refrigeration into the gas-steam combined cycle could increase its exergy efficiency by about 30%. In contrast with Ghaffarpour et al. [197], increasing the air compressor pressure ratio decreased the exergy efficiency of the gas-steam combined cycle and the overall system. The syngas combustion chamber was found to have the lowest exergoeconomic factor due to its highest irreversibility-related cost rate. CO2 emission is one of the main issues of MSW-based heat and power production plants. Accordingly, new carbon capture and sequestration technologies seem necessary for advanced integrated MSW gasification systems to mitigate CO2 emission. Akrami et al. [202] evaluated the exergetic and exergoeconomic performance of two MSW-based power production systems combined with a carbon capture-sequestration system. The first system included a downdraft fixed-bed MSW gasification, a gas turbine operating with syngas, a molten carbonate fuel cell, an organic Rankine cycle, and a cryogenic CO2 separation unit. In the second system, a steam Rankine cycle was considered instead of the organic Rankine cycle. The first system had higher exergy efficiency than the second scheme. The highest exergetic irreversibilities were recorded for the air preheating process in the first system and the waste gasification process in the second system. The molten carbonate fuel cell had the highest exergoeconomic factor in both systems because of its huge capital cost. In continuation, Akrami et al. [200] showed that elevating the gasification and fuel cell operating temperatures could slightly increase the overall exergy efficiency of the first system. However, increasing the current density and steam-to-carbon ratio of the fuel cell deteriorated the exergetic performance of the first system. Increasing the fuel cell CO2 utilization factor up to about 80% could improve the overall exergy efficiency of the first system. Akrami et al. [202] modified the selected system (first case) by thoroughly recirculating the air stream leaving the gas turbine into the syngas combustion chamber to exploit its potential for syngas oxidation. Increasing the current density and fuel utilization factor of the fuel cell decreased the overall exergy efficiency of the system. Unlike the first study [202], the Fig. 23. Effects of (a) fuel cell current density and (b) fuel cell operating temperature on the overall exergy efficiency of a solid oxide fuel cell integrated with a downdraft fixed-bed MSW gasification system operating with various gasifying agents used. Redrawn with permission from Hosseinpour et al. [194]. Copyright© 2020 Elsevier. 57 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 gasifier showed the highest irreversibility (due to the intensification of chemical reactions resulting from air recirculation), followed by the syngas combustion chamber, heat recovery unit, and CO2 cooler condenser. Similar to the first work [202], exergoeconomic analysis showed that the capital cost of the fuel cell system should be reduced even if this modification would lead to an increase in exergy destruction. Lv et al. [192] carried out exergy analysis for an integrated plant converting the syngas derived from an MSW gasification into electricity through a steam turbine cycle and synthetic natural gas through consecutive chemical processes. The plant used the manganese-assisted chemical looping process to separate oxygen from the air and the water-gas shift process with calcium-based chemical looping to absorb CO2 from the syngas. The gasifier had the highest exergy destruction rate, followed by the water-gas shift process with calcium-based chemical looping. Like the findings of Vitasari et al. [174] (Fig. 19), the results of this study demonstrated the necessity of striking an appropriate tradeoff between the environmental benefits and exergetic losses for the chemical CO2 removal processes. hydrothermal processing over other thermochemical methods is the lower emissions of harmful gases to the atmosphere (particularly SOX) [225]. Nevertheless, hydrothermal processing requires a huge amount of energy to heat and pressurize the reaction system, highlighting the need for exergy-based analyses to assess its technical, economic, and environmental viability. Mahmood et al. [226] energetically and exergetically analyzed the hydrothermal oxidation process of food waste at different temperatures with and without enzymatic pretreatment. Although elevating the reaction temperature from 150 to 350 ◦ C could increase biocrude oil yield, this modification reduced the exergy efficiency of the process from 94.7 to 74.4% and 92.2 to 73.3% for the non-enzymatic and enzymatic pretreatment, respectively. It was deduced that the increased biocrude oil could not compensate for the energy consumed to sustain the hydrothermal reaction at higher temperatures. In another study, Stobernack et al. [227] investigated nine different MSW treatment scenarios producing electricity and heat by integrating hydrothermal carbonization with different waste treatment strategies (i.e., anaerobic digestion, composting, incineration, and gasification). Increasing the temperature of the hydrothermal carbonization process from 180 to 220 ◦ C deteriorated the overall exergy efficiency of all the investigated scenarios. Upon the treatment path and reaction temperature, incorporating the hydrothermal carbonization process could improve the overall exergy efficiency up to 77% compared with integrated anaerobic digestion and composting. 4.1.3. Municipal solid waste pyrolysis Pyrolysis can be regarded as an alternative to the incineration and gasification processes to treat MSW. This thermochemical process is carried out in an oxygen-depleted environment at temperatures in the range of 300–700 ◦ C under atmospheric pressure [214,215]. The pyrolysis process converts MSW into bio-oil, charcoal, and gaseous products that can be used to produce various energy carriers, solid fuels/adsorbents, liquid biofuels, and chemical platforms [31,45,216]. The pyrolysis treatment of MSW is more environmentally friendly and technically advantageous than incineration and gasification processes since it produces higher energy value while releasing fewer pollutants [217]. Nevertheless, the high thermal energy demand of the pyrolysis process usually supplied electrically or by burning fossil fuels [218] highlights the importance of exergy-based approaches to assess its sustainability features. Unlike the incineration and gasification processes, the exergetic aspects of MSW pyrolysis have been rarely investigated in the published literature. Zhang et al. [219] exergetically analyzed the pyrolysis of various plastic waste types (i.e., high-density polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and their simulated mixture) in a rotary kiln at the heat carrier filling ratio (i.e., the proportion of heat carrier bed volume to pyrolysis reactor volume) in the range of 0–20%. For the simulated mixture, elevating the heat carrier filling ratio from 0 to 15% increased the exergy efficiency from 59.4 to 66.0%. However, exergy efficiency decreased beyond the optimal heat carrier filling ratio (i.e., 15%). Interestingly, different plastic types showed similar exergy efficiency values under the same fixed operating conditions. 4.2. Biological treatment of municipal solid waste Biological treatment is an indispensable part of traditional and advanced MSW management systems. However, these methods are very time-consuming, energy-demanding, and chemical-intensive. Biological approaches could also cause environmental pollution due to the emission of methane, ammonia, nitrogen oxides, and volatile organic substances [228]. Therefore, advanced sustainability assessment tools like exergy-based methods need to be applied in the design and operation of biological treatment systems to address both energy and environmental issues. The most important applications of exergy-based methods in analyzing the biological treatment of MSW are tabulated in Table 8. 4.2.1. Landfill gas recovery Sanitary landfill sites emit a huge amount of anthropogenic methane and CO2 into the atmosphere. Landfill gas recovery can effectively alleviate the adverse environmental impacts of controlled landfilling [242]. The collected landfill gas can be utilized in internal combustion engines to produce electricity and in cogeneration plants to generate heat, cold, and electricity [243]. Xydis et al. [229] energetically and exergetically analyzed a landfill gas-powered electricity generation system. The chilling unit supplying the cold water for landfill gas dehumidification had the highest exergy loss, followed by the combustion unit. Tozlu et al. [230] exergetically and exergoeconomically studied different components of a turbocharged internal combustion gas engine fueled by landfill gas. The turbocharging turbine showed the highest exergy destruction. The intercooler also had the lowest exergy efficiency. The gas engine displayed the highest exergoeconomic factor and relative cost difference due to its high capital cost. The waste heat of the flue gas exhausting from gas engines is high enough to be used to produce extra heat and power. Tozlu et al. [231] exergetically compared three different schemes integrating a landfill gas-powered engine with (i) a district heating network, (ii) an organic Rankine cycle and a district heating network, and (iii) a supercritical CO2 power cycle and a district heating network. The third scheme was found to be the best option in terms of exergy efficiency. In addition to gas engines, landfill gas can also be burned in boilers to drive steam turbine generators. Sorgulu and Dincer [232] exergetically analyzed a cogeneration plant producing electricity and freshwater. The plant included an MSW sanitary landfill digester, a landfill gas 4.1.4. Municipal solid waste hydrothermal processing The main drawback of incineration, gasification, and pyrolysis technologies is their limited performance in the presence of highmoisture MSW, making necessary the energy-intensive drying process [220]. Hydrothermal processing (hydrothermal gasification, hydrothermal liquefaction, hydrothermal oxidation, and hydrothermal carbonization) is a promising technique to deal with wet MSW without the need for pre-processing [221]. These thermochemical processes are carried out at temperatures and pressures higher than 100 ◦ C and 0.1 MPa in the presence of solvents with and without oxidants/catalysts. The main products of hydrothermal processing are biocrude oil, syngas, and hydrochar, while some aqueous by-products are also generated. The produced biocrude oil can be further used as a feedstock in existing petroleum refining infrastructures to produce valuable chemicals and energy carriers [222]. The evolved syngas can be used to synthesize liquid transportation fuels and fine valuable chemicals using chemical and biological routes. The generated hydrochar can be utilized for soil amendment, carbon sequestration, wastewater pollution remediation, and energy carriers production [223,224]. The main advantage of 58 MSW treatment approach Waste fraction (s) Main product (s) Scale key stages inside the investigated boundaries Key parameter(s) Reference conditions Exergetic indicator (s) of MSW digestion unit Landfill gas Commercial Landfilling Waste flow rate = 1300 tonnes/ day NA φ = 2.92% – Landfill gas, compost Commercial Mechanical separation, landfilling, composting Waste flow rate = 1900 tonnes/ day NA φ = 4.32% φ = 4.65% Electricity, landfill gas Commercial Landfilling, dehumidification, gas engine power generation NA – φ = 33.0% Mixture of municipal, industrial, and medical wastes Mixed MSW Electricity Commercial Landfilling, gas engine power generation, turbocharging Landfill waste disposal rate = 192 tonnes/day, Landfill temperature = 37.7–48.9 ◦ C, Landfill capacity = 1400000 tonnes, Landfill biogas production duration = 20 years Landfill waste disposal rate = 1500 tonnes/day, Landfill temperature = 40–45 ◦ C NA – φ = 47.84% The turbocharging turbine had the highest exergy destruction. [230] Electricity, heat (hot water) Commercial District heating network Exhaust gas temperature = 566 ◦ C NA – ψ = 9.4% [231] Mixed MSW Electricity, heat (hot water) Commercial Organic Rankine power generation, district heating network Exhaust gas temperature = 566 ◦ C NA – ψ = 13.3% The heat exchanger transferring the thermal energy of the flue gas into the district heating water had the highest exergy destruction. The condenser of the organic Rankine cycle had the highest exergy destruction. Mixed MSW Electricity, freshwater Commercial Landfilling, landfill gas conditioning, gasification, syngas cleaning and watergas shift, pressure swing adsorption, steam turbine power generation, multieffect distillation, reverse osmosis MSW flow rate = 216 tonnes/day, MSW lower heating value = 4.81 MJ/kg, Steam turbine isentropic efficiency = 88%, Pump isentropic efficiency = 90%, T0 = 280–310 K, P0 = 100 kPa Increasing ambient temperature decreased the overall exergy efficiency of the plant. [232] Mixed MSW Electricity Commercial Landfilling, gas engine power generation, gasification, steam turbine power generation, organic Rankine power generation Landfill temperature = 23 ◦ C, Waste moisture content = 43.6–53.7%, Landfill gas-to-syngas ratio fed into combustor = 0–20%, Combustion chamber T0 = 296.15 K, P0 = 92.5 kPa The landfill gas appeared to be better fuel for the steam turbine cycle compared with the MSW-derived syngas. [233] Landfill gas recovery Landfilling Mixed MSW Landfilling integrated with composting Landfilling combined with gas engine 59 – ψ= 23.72–34.57% – ψ= 14.30–15.99% Remark(s) Ref. The landfilling process alone could not be an effective strategy in MSW management. Mechanical separation could improve the exergetic performance of MSW disposal systems. The biogas dehumidification process accounted for the majority of the total irreversibility of the system. [120] [120] [229] [231] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Landfilling integrated with turbocharged gas engine Landfilling integrated with gas engine power generation and district heating Landfilling integrated with gas engine power generation, organic Rankine cycle, and district heating Landfilling integrated with biomass gasification, steam turbine cycle, multieffect distillation, and reverse osmosis Landfilling integrated with gas engine, MSW gasification, steam turbine Mixed MSW (landfilling), biowaste (composting) Mixed MSW System exergetic indicator(s) S. Soltanian et al. Table 8 Most important applications of exergy-based methods in analyzing MSW biological systems. MSW treatment approach Waste fraction (s) Main product (s) Scale key stages inside the investigated boundaries cycle, and organic Rankine cycle 60 Landfilling Mixed MSW integrated with gas engine power generation, supercritical CO2 power cycle, and district heating Anaerobic digestion inside the reactor Anaerobic Biowaste digestion coupled with turbocharged gas engine NA temperature = 550 ◦ C, Steam turbine pressure = 650 kPa, Steam turbine temperature = 450 ◦ C, Steam turbine isentropic efficiency = 65%, Steam cycle pump isentropic efficiency = 50% Exhaust gas temperature = 566 ◦ C Reference conditions Electricity, heat (hot water) Commercial Supercritical CO2 power generation, district heating network Electricity, fertilizer Commercial Pretreatment, anaerobic digestion, centrifugal separation, biogas cleaning, biogas compression, refrigeration, dehumidification, pressure regulation, gas engine power generation, engine heat exchanger Waste flow rate = 300 tonnes/ day, Digester temperature = 42 ◦ C, Digester pressure = 1 atm, Primary digester capacity = 1200 m3, Secondary digester capacity = 1500 m3, Engine maximum power = 1042 kW, System lifetime = 30 years, Annual working hours = 8000 h, Interest rate = 10%, Maintenance factor = 1.06 T0 = 293.15 K, P0 = 100 kPa Electricity Commercial Organic Rankine power generation Engine exhaust gas temperature = 567 ◦ C, Exhaust gas outlet temperature from organic Rankine cycle = 80–240 ◦ C, Organic working fluids = toluene, octamethyltrisiloxane, octamethylcyclotetrasiloxane, and n-decane, Organic Rankine cycle pressure ratio = 5, Organic Rankine cycle inlet turbine temperature = 276–330 ◦ C, Organic Rankine cycle turbine NA NA Exergetic indicator (s) of MSW digestion unit System exergetic indicator(s) Remark(s) Ref. ψ = 34.7% The highest irreversibility was obtained for the heat exchanger transferring the thermal energy of the flue gas into the district heating water. [231] φ = 97.4% ψ = 84.9% IP = 20.2 kW rc = 0.49 (−) fc = 94.51% rb = 0.34 (−) fb = 92.06% ψ = 72.8% [88, 104, 234] – ψ= 6.95–16.37% The gas engine power generation unit had the highest exergetic improvement potential, followed by the anaerobic digestion unit. The gas engine power generation unit was the most important unit from the exergoeconomic viewpoint. The gas engine power generation unit possessed the highest operating environmental impact rate, while the digester had the highest manufacturing environmental impact rate. The type of organic working fluid had a significant impact on the exergy efficiency of the process. – [235] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Anaerobic digestion coupled with gas engine and organic Rankine cycle Key parameter(s) S. Soltanian et al. Table 8 (continued ) MSW treatment approach Waste fraction (s) Main product (s) Scale key stages inside the investigated boundaries Electricity, heat, cold NA Anaerobic digestion, biomass pyrolysis, gas engine power generation, engine heat exchanger, heat recovery steam generation, absorption refrigeration Anaerobic digestion coupled with gas engine and Kalina cycle NA Electricity Commercial Kalina power generation Anaerobic digestion coupled with gas engine and supercritical CO2 power cycle NA Electricity Commercial Supercritical CO2 power generation Anaerobic digestion coupled with steam turbine cycle Biowaste Electricity Commercial Anaerobic digestion, steam turbine power generation isentropic efficiency = 85%, Organic Rankine cycle pump isentropic efficiency = 85%, System lifetime = 30 years, Annual working hours = 8040 h, Interest rate = 15%, Maintenance factor = 1.06 Digester temperature = 35 ◦ C, Digester pH = 6.9, Engine compression ratio = 7–12, Engine real-to-design output works ratio = 0.2–1.2, Exhaust gas temperature = 510 ◦ C, Refrigeration solution = LiBr/ H2O Engine exhaust gas temperature = 567 ◦ C, Kalina cycle working fluid = ammonia-water, Ammonia concentration = 0.7–0.9, Kalina cycle pressure ratio = 2.5–3.5, Kalina cycle turbine inlet temperature = 350–450 ◦ C, Kalina cycle turbine inlet pressure = 45–55 bar, System lifetime = 30 years, Annual working hours = 8040 h, Interest rate = 15%, Maintenance factor = 1.06 Engine exhaust gas temperature = 567 ◦ C, Supercritical CO2 cycle pressure ratio = 2.5–4.0, Supercritical CO2 turbine inlet pressure = 185–330 bar, Logarithmic mean temperature difference of CO2 turbine heat exchanger = 9.1–12.9 ◦ C, System lifetime = 30 years, Annual working hours = 8040 h, Interest rate = 15%, Maintenance factor = 1.06 Waste flow rate = 90 tonnes/day, Waste lower heating value = 13.78 MJ/kg, Digester temperature = 55 ◦ C, Combustion chamber temperature = 1000–1400 ◦ C, Reference conditions Exergetic indicator (s) of MSW digestion unit System exergetic indicator(s) NA – ψ= 4.54–45.52% NA NA T0 = 289.15–296.15 K NA T0 = 298.15 K, P0 = 101.3 kPa ψ = 92.8% fc = 81.40% ψ= 19.43–31.92% ψ= 54.00–58.70% ψ = ψ we = 14.5–19.1% fc = 44.99% Remark(s) Ref. Mixing the biogas from the anaerobic digestion with the producer gas from the pyrolysis process could promote the overall exergy efficiency of the system. [236] The ammonia concentration had the highest impact on the exergy efficiency of the system compared with the turbine inlet temperature, turbine inlet pressure, and pressure ratio of the Kalina cycle. [237] The logarithmic mean temperature difference of the CO2 turbine heat exchanger had the highest impact on the exergy efficiency of the process compared with the pressure ratio and turbine inlet pressure of the supercritical CO2 power cycle. [238] The combustion chamber had the highest exergy loss. [184] (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Food waste (kitchen biodegradable waste) 61 Anaerobic digestion coupled with biomass pyrolysis, gas engine, and absorption refrigeration Key parameter(s) S. Soltanian et al. Table 8 (continued ) MSW treatment approach Anaerobic digestion coupled with power generation Anaerobic digestion coupled with Kalina cycle and ejector-assisted refrigeration Waste fraction (s) Food waste Food waste Main product (s) Electricity, heat, digestate, packaging material Electricity, cold Scale key stages inside the investigated boundaries NA Kalina power generation, ejection-based refrigeration Anaerobic digestion coupled with solid oxide fuel cell Biowaste Electricity, heat (hot water) Laboratory Anaerobic digestion, solid oxide fuel cell, heat recovery steam generation Turbine inlet pressure = 1500–2500 kPa, Steam turbine isentropic efficiency = 80%, Superheater temperature difference = 150–260 ◦ C, Pinch point temperature difference = 180–260 ◦ C NA Kalina cycle working fluid = ammonia-water, Kalina cycle turbine inlet temperature = 120 ◦ C, Kalina cycle turbine inlet pressure = 3500 kPa, Refrigerants = R134a, R152a, and R290, Ejector entrainment ratio = 0.15–0.5 kg/kg, Refrigeration cycle evaporator pressure = 293–415 kPa, Refrigeration cycle condenser pressure = 850–1215 kPa, Refrigerant heater superheating degree = 0–15 ◦ C, Diffuser isentropic efficiency = 80%, Pump isentropic efficiency = 60%, Turbine isentropic efficiency = 80% Digester temperature = 55 ◦ C, Waste moisture content = 16%, Fuel cell steam-to-carbon ratio = 2.0, Fuel utilization factor = 80%, Number of cell = 11000, Fuel cell current density = 0.05–0.77 A/cm2, Fuel cell operating temperature = 600 ◦ C, Fuel cell stack temperature difference = 90–200 ◦ C, Pump isentropic efficiency = 80%, Steam pressure in heat recovery steam generator = 10 bar Reference conditions NA T0 = 298.15 K, P0 = 101.3 kPa NA Exergetic indicator (s) of MSW digestion unit – – fc = 24.22% System exergetic indicator(s) Remark(s) Ref. ψ = 63.72% The packing material had the highest exergy value. [239] The entrainment ratio of the refrigeration ejector was the most influential parameter on the overall exergy efficiency of the system. [240] The digestion reactor was the most exergy-destructive unit. [195] ψ= 4.95–11.07% ψ= 34.60–52.51% (continued on next page) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Generation, transportation, sorting, anaerobic digestion, power generation 62 Commercial Key parameter(s) S. Soltanian et al. Table 8 (continued ) NA Generation, transportation, sorting, anaerobic digestion, power generation, animal feed production Commercial Electricity, heat, digestate, packaging material, animal feed Food waste Anaerobic digestion coupled with power generation and animal feed production conditioner, an olive oil waste gasifier, a syngas cleaning/water-gas shift reactor, a pressure swing adsorber, a steam turbine power generator, a multi-effect distillation unit, and a reverse osmosis unit. The mixture (hythane) of biomethane purified in the landfill gas conditioner and the hydrogen upgraded in the pressure swing adsorber was fired in the boiler. The multi-effect distillation unit was derived by the heat extracted from the low-pressure steam in the condenser, while the reverse osmosis unit consumed a portion of the electricity generated in the steam turbine cycle. The syngas cleaning/water-gas shift reactor showed the highest exergy destruction, followed by the gasifier and multi-effect distillation unit. Gallego et al. [233] exergetically evaluated a hybrid power plant including a gas engine power unit (fueled by landfill gas) and steam turbine and organic Rankine cycles (driven by a mixture of landfill gas and MSW-based syngas). Increasing the moisture content of MSW lowered the overall exergy efficiency of the process because of its inefficient conversion through the gasification process (Fig. 24). In addition, raising the fraction of landfill gas to syngas in the combustor could improve the overall exergy efficiency process (Fig. 24), indicating the suitability of landfill gas for driving steam turbine power cycles. Integrating MSW landfilling with composting can also mitigate its devastating emissions (leading to terrestrial/aquatic toxicity and abiotic degradation) while alleviating its acidification and photochemical oxidation problems [244]. Zhou et al. [51] showed that integrating landfilling and composting could outperform landfilling alone in terms of exergy efficiency by more than 80%. The composting unit was also substantially exergetically efficient than the landfill section. Abbreviations/symbols: ψ : Functional exergy efficiency, ψ we : Waste-to-electricity exergy efficiency, φ: Universal exergy efficiency, fb : Exergoenvironmental factor, fc : Exergoeconomic factor, rb : Relative environmental impact difference, rc : Relative cost difference, NA: Not available. [239] Producing animal feed from waste bread could increase the exergy efficiency of the process. – ψ = 74.26% [241] The anaerobic digestion reactor was found to be the most important unit in terms of exergy destruction. φ = 60.4% φ = 61.0% T0 = 272 K, P0 = 1.0 atm Waste collection rate = 110 tonnes/day, Waste lower heating value = 17.40 MJ/kg, Total transportation distance = 1287.5 km/day, Crushing power = 150 kW, Mixer pump power = 21 kW, Digester temperature = 38 ◦ C NA Hydrogen, compost Biowaste Anaerobic digestion integrated with steam methane reforming Commercial Waste collection, transportation, crushing, solid-water mixing, anaerobic digestion, biogas refining, centrifugal separation, composting, biogas steam methane reforming Reference conditions key stages inside the investigated boundaries Scale Main product (s) Waste fraction (s) MSW treatment approach Table 8 (continued ) Renewable and Sustainable Energy Reviews 156 (2022) 111975 Key parameter(s) Exergetic indicator (s) of MSW digestion unit System exergetic indicator(s) Remark(s) Ref. S. Soltanian et al. 4.2.2. Anaerobic digestion of municipal solid waste The anaerobic digestion process is an energetically efficient and economically viable technology compared with landfill gas collection. In addition, more recent evidence shows that the global warming mitigation potential of energy recovery through anaerobic digestion technique is considerably higher than that of landfill gas collection [245]. Barati et al. [88] exergetically analyzed a commercial anaerobic digestion plant equipped with a turbocharged gas engine generator unit using actual operational data (i.e., temperature, pressure, chemical composition, and mass flow rate of various streams and electricity consumption/generation of various components involved in the process). The plant produced electricity and fertilizer from the organic fraction of MSW. The exergy value of the produced fertilizer was almost 5.5 times higher than the exergy value of the generated electric power (Fig. 25). The gas engine generator unit showed the highest contribution to the overall internal irreversibility of the plant, followed by the anaerobic digestion unit. The refrigeration system had the lowest functional exergy efficiency, followed by the gas engine-generator unit and engine heat exchanger. Aghbashlo et al. [104,234] exergoeconomically and exergoenvironmentally examined the MSW anaerobic digestion plant reported by Barati et al. [88]. The specific cost and environmental impacts of exergy for the generated electricity and fertilizer were 26.3 USD/GJ & 11.1 mPts/GJ and 2.3 USD/GJ & 0.4 mPts/GJ, respectively. Increasing the interest rate, plant lifetime, and annual working hours could reduce the specific costs of exergy for both products. Similarly, the specific environmental impacts of exergy for both products could be diminished by extending the plant lifetime and annual working hours. The highest values for the overall cost rate (including investment-related and irreversibility-related cost rates) and the overall environmental impact rate (including component-related and irreversibility-related environmental impact rates) were found for the gas engine generator unit (101.3 USD/h & 37.1 mPts/h), followed by the anaerobic digestion unit (68.4 USD/h & 8.6 mPts/h). Exergoeconomic and exergoenvironmental factors were very low for the gas engine generator unit (<28%), while these indicators were very high for the anaerobic digestion unit (>92%). Accordingly, the exergetic efficiency of the gas engine generator unit should be enhanced to improve the plant exergoeconomically and 63 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 exergoenvironmentally. Besides, the capital costs and development-related environmental impacts of the anaerobic digestion unit should be reduced to make the plant more viable and sustainable. The hot flue gas temperature leaving gas engines is typically between 400 and 600 ◦ C, making it a suitable heat source to drive other power, heating, and cooling systems such as organic Rankine, absorption refrigeration, Kalina, and supercritical CO2 cycles. In this context, Özahi et al. [235] exergetically and exergoeconomically explored an organic Rankine cycle (operating with four different working fluids) driven by the exhaust gas of a gas engine powered by MSW-based biogas. Toluene resulted in the highest exergetic performance compared with other organic working fluids (i.e., octamethyltrisiloxane, octamethylcyclotetrasiloxane, and n-decane). Regardless of the organic working fluid used, elevating the outlet temperature of the exhaust gas from the organic Rankine cycle evaporator could increase the cycle exergy efficiency. Gao et al. [236] exergetically analyzed a combined cooling, heating, and power system (including a gas engine, a heat recovery boiler, and an absorption refrigeration unit) driven by the biogas from food waste anaerobic digestion (first scenario) or the producer gas from biomass pyrolysis (second scenario) or the gas mixture (third scenario). It was found that the main parameters of the gas engine, i.e., compression ratio and real-to-design output work ratio, had direct impacts on the system exergetic performance. More specifically, increasing the engine compression ratio decreased the system exergy efficiency due to a reduction in the engine exhaust gas temperature. On the contrary, increasing the real-to-design output works ratio elevated the engine exhaust gas temperature, improving the system exergy efficiency. At fixed engine parameters, the first scenario tended to generate more electricity than other scenarios due to the higher calorific value of the evolved biogas. However, the first scenario yielded the lowest exergy efficiency, indicating the importance of thermochemical pathways on the exergetic performance of MSW treatment systems. Özahi and Tozlu [237] exergetically and exergoeconomically assessed a Kalina cycle powered by the exhaust gas of five biogas engines. Increasing the concentration of ammonia and the pressure ratio of the Kalina cycle could increase the exergy efficiency and the product exergetic cost rate of the process. Elevating the Kalina cycle turbine inlet temperature had an insignificant impact on the exergy efficiency of the system while remarkably increasing the product exergetic cost rate of the process. In addition, increasing the Kalina cycle turbine inlet pressure reduced the exergy efficiency and the product exergetic cost rate of the process. Tozlu et al. [238] used exergy and exergoeconomic analyses to evaluate a supercritical CO2 cycle generating electricity by utilizing the exhaust gas energy of five biogas engines. Increasing the pressure ratio of the supercritical CO2 cycle until the optimal value of ⁓3.3 could improve the exergy efficiency of the process. Nevertheless, the product exergetic cost rate was constantly increased by elevating the cycle pressure ratio. Increasing the CO2 turbine inlet pressure marginally increased the exergy efficiency and the product exergetic cost rate of the process. Elevating the reference temperature and the logarithmic mean temperature difference of the CO2 turbine heat exchanger negatively affected the exergy efficiency and the product exergetic cost rate of the process. In addition to using in gas engine systems, the MSW-based biogas (or biomethane) produced in anaerobic digesters can be combusted to run power plants. Behzadi et al. [184] exergetically and exergoeconomically compared a steam turbine power plant coupled with either an MSW anaerobic digestion system (first scheme) or an MSW gasification system (second scheme). The first scheme appeared to be a more suitable approach for treating municipal waste from the energetic, exergetic, and exergoeconomic perspectives, as shown in Fig. 26. The highest exergy destruction rate in the first and second scheme was observed for the biogas combustion chamber and the gasification reactor, respectively. Seckin [240] energetically and exergetically examined a cogeneration system including an ammonia-water Kalina cycle and an ejection refrigeration cycle. The Kalina cycle was powered with the biomethane derived from food waste anaerobic digestion, while the refrigeration cycle operated using different refrigerants (i.e., R290, R152a, and R134a). For all the refrigerants considered, increasing the entrainment ratio (i.e., the mass ratio of the low-pressure refrigerant vapor drawn from the evaporator to the high-pressure refrigerant steam entering the primary nozzle) and evaporator pressure of the ejector could improve the overall exergy efficiency of the system. However, elevating the heater superheating degree (i.e., the temperature difference between the superheated refrigerant in the heater and its corresponding saturated vapor) and condenser pressure of the ejector reduced the overall exergy efficiency of the system. The refrigerant R134a provided the best exergetic results, followed by R152a and R290. Instead of combusting the MSW-based biogas, it can be fed into solid oxide fuel cells to generate electricity in an efficient manner. In this vein, Yari et al. [195] compared the energetic, exergetic, and exergoeconomic performance of a solid oxide fuel cell-based heat and power system powered by either the biogas from the thermophilic anaerobic digestion of MSW or the syngas from the downdraft fixed-bed gasification of MSW. Similar to the results reported by Behzadi et al. [184], the digestion-based scheme presented higher exergy efficiency and lower specific cost of product exergy than those of the gasification-based strategy. Importantly, an increase in the fuel cell current density reduced the energy efficiency, exergy efficiency, and specific costs of heat and power exergy (Fig. 27a). However, elevating the stack temperature difference of the fuel cell increased the energy and exergy efficiency values while discounting the specific costs of product exergy (Fig. 27b). The anaerobic digestion and gasification reactors had the highest exergy destructions in the investigated schemes. Unlike digester, the exergoeconomic factor of the gasifier was found to be acceptable due to a good balance between its investment- and exergy-related costs. Using the steam reforming process, the biomethane evolved through anaerobic digestion can be upgraded into a more valuable energy carrier, i.e., hydrogen. Hammond et al. [241] assessed the energetic and exergetic aspects of an anaerobic digestion plant integrated with the methane steam reforming process to produce hydrogen from the MSW-based biogas. Mechanical inefficiencies and wastewater streams accounted for most of the energy losses that occurred in the plant. The largest exergy destruction occurred inside the anaerobic digester, showing the necessity for optimizing its design and operation conditions. The bread present in MSW can be separated before anaerobic Fig. 24. Effects of landfill gas-to-syngas ratio and waste moisture content on the overall exergy efficiency of an integrated plant consisting of a gas engine (fueled by landfill gas) and steam turbine and organic Rankine cycles (powered by a blend of landfill gas and MSW-based syngas). Drawn using the data obtained from Ref. [233]. 64 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 25. Exergy flow diagram of the gas engine-equipped anaerobic digestion plant. Redrawn with permission from Barati et al. [88]. Copyright© 2017 Elsevier. digestion in order to produce animal feed. Vandermeersch et al. [239] conducted exergy analysis, ELCA, and conventional LCA for an anaerobic digestion-based plant (producing electricity, heat, digestate, and package material) without and with bread separation. It was reported that the bread separation could improve the exergetic efficiency of the process while lowering its environmental impacts. In fact, the separated bread could eliminate the exergy losses associated with the agricultural-based animal feed production activities. the life cycle costs and environmental impacts at the component level. Neither life cycle cost analysis nor life cycle environmental impact analysis can provide such decision-support information for researchers, policymakers, and stakeholders. Even though exergy-based analyses are potent tools to design, optimize, and evaluate thermodynamic systems, these approaches cannot resolve all the sustainability issues concerning MSW treatment technologies. Like any other sustainability assessment method, the exergy-based techniques suffer from some inherent limitations. For instance, the results of exergy-based methods are dependent on the choice of the reference environment. The simple exergy analysis identifying the components with the largest irreversibility rate has been considered in most of the published research works. Besides, the widely used conventional exergy-based methods can only identify the units with the highest internal irreversibility, cost loss, and environmental impact while incapable of distinguishing their sources. Accordingly, advanced exergy-based analyses should be considered in developing, analyzing, and optimizing MSW treatment technologies in future research. The SPECO approach used in exergoeconomic and exergoenvironmental analyses considers exergy-based allocation (i.e., P-rule) in analyzing MSW management systems yielding more than one valuable output. Despite the promising features of the SPECO approach, it assigns a similar share of costs/environmental impacts to various (by)products of multifunctional components. This exergy-based allocation methodology controversially considers the same specific cost/environmental impact of exergy for all the (by)products of multifunctional components. Accordingly, alternative allocation procedures should be explored to revise the P-rule of the SPECO approach according to the goal and scope of MSW treatment technologies. In addition, the SPECO method assumes that the specific cost/environmental impact of exergy associated with the exergy removal from a fuel stream is equal to the average specific cost/environmental impact of exergy wherein the removed exergy was delivered to the same stream in upstream components (i.e., F-rule) [107]. However, the F-rule principle of the SPECO approach cannot present a meaningful association between the specific cost/environmental impact of exergy and the energy quality of the flows [246]. This shortcoming can be simply addressed using the methodology proposed by Oyekale et al. [247], where the specific cost/environmental impact of exergy of each stream is linearly correlated with its energy quality level. The mentioned modification in the fuel-product principle can prevent misleading information concerning the cost/environmental impact required to recover waste heat intended to be rejected to the ambient. More specifically, the fuel-product cost principle for the same working substance entering a 5. Challenges and future directions The exergy-based approaches can effectively examine, compare, and assess various MSW management options by providing better insights into their resource use efficiency, economic viability, and environmental sustainability. The exergy concept can reliably normalize and aggregate different energy and material streams in terms of their work potential, thereby facilitating decision-making on the sustainability of MSW management strategies. In addition, the enhanced exergetic methods, i. e., exergoeconomic and exergoenvironmental approaches, can allocate Fig. 26. Energetic and exergetic comparison of a steam turbine power plant coupled with either an MSW gasification system or an MSW anaerobic digestion system. The net output power, heat rate in the steam generator, and specific cost of product exergy were 1.5 MW, 6.9 MW, and 32.4 USD/GJ for the gasifierequipped scheme and 2.1 MW, 9.7 MW, and 30.7 USD/GJ for the digesterequipped scheme, respectively. Redrawn with permission from Behzadi et al. [184]. Copyright© 2018 Elsevier. 65 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 Fig. 27. Effects of (a) current density and (b) stack temperature difference of the fuel cell on the overall exergy efficiency and specific costs of product exergy of a solid oxide fuel cell-based cogeneration system coupled with either a thermophilic digester or a downdraft fixed-bed gasifier. Redrawn with permission from Yari et al. [195]. Copyright© 2016 Elsevier. unit from stream i and leaving through stream e is expressed as Eq. (1): ci ce ⃒ ⃒ ⃒ ⃒ ⃒ T ⃒=⃒ T ⃒ ⃒1– 0 ⃒ ⃒1– 0 ⃒ ⃒ Ti ⃒ ⃒ Te ⃒ parameters in the published literature. Accordingly, due to a lack of harmonized methods in defining dimensionless exergetic parameters, results from different studies can hardly be compared. Functional exergy efficiency, exergoeconomic factor, and exergoenvironmental factor seem to be meaningful dimensionless exergetic indices for fair and accurate comparison of various MSW treatment systems. Different semiempirical and empirical models have been used in the literature to determine the chemical exergy content of MSW. Given the fact that the contribution of chemical exergy to the overall exergy of streams is significant in MSW treatment systems, this issue might negatively impact the accuracy and fidelity of the obtained exergetic results. Accordingly, exergetic calculations and definitions in MSW management processes should be harmonized in future research to make the results more comparable and reproducible. To achieve practical and realistic information from exergy-based (1) Standard exergy analysis does not account for the exergy aspects of the background processes. The mentioned issue could be effectively addressed using the ELCA approach, in which exergy analysis is performed throughout the whole life cycle of a product. Exergy analysis cannot directly account for non-physical flows like labor, capital, and environmental restoration costs. Extended exergy accounting can allocate exergetic values to all physical and non-physical flows [95]. This exergy-based method can express the different forms of physical and non-physical inputs required to make a product based on a common exergy basis (Joule). There is no consensus on the definition of dimensionless exergetic 66 S. Soltanian et al. Renewable and Sustainable Energy Reviews 156 (2022) 111975 6. Concluding remarks analyses, all the downstream, mainstream, and upstream of MSW management systems should be considered in the calculations. However, most published researches have only considered the mainstream while disregarding waste collection/transportation/separation, pollutant remediation, ash treatment, and residue disposal. Notably, applying restricted system boundaries can oversimplify the exergetic calculations while excluding the potential contribution of all relevant processes. Expanded system boundaries can make more inclusive the exergetic results while complicating the required calculations and needing more assumptions. The ELCA and extended exergy accounting approaches seem to be promising methods to deal with the issues mentioned above. The chemical exergy of intermediate products like solid/liquid digestate and unclean syngas has been frequently used in calculating the overall exergetic efficiency of anaerobic digestion- and gasificationbased MSW treatment systems. The obtained high (perhaps unrealistic) exergetic efficiency values might make the comparison unreliable among different MSW treatment scenarios, resulting in controversial decision-making. Accordingly, the final use of intermediate products should be regarded in the exergetic calculations, or the same cut-off criteria should be considered for the boundaries of different MSW treatment systems under investigation. The separation approaches (i.e., at-source and on-site separation) and methods (i.e., biowaste, paper and cardboard, glass, metals, plastics, and other wastes) can significantly affect the thermodynamic, economic, environmental, and social aspects of MSW treatment technologies. Therefore, the effects of separation approaches and methods on the exergetic aspects of MSW management systems should be considered and examined. The single waste management process cannot recover all the energy and material contents of MSW while eliminating its all the potential adverse environmental impacts. These issues can be effectively addressed by developing nearly-zero discharge MSW biorefineries. However, multiple consecutive processes can render MSW management systems more complex and expensive. Therefore, such MSW biorefinery systems should be designed and optimized based on advanced exergetic methods. The MSW characteristics significantly affecting the planning, designing, and operating of its management systems also need to be investigated from the exergetic viewpoints. Extensive literature has dealt with exergy analysis of the integrated MSW treatment systems consolidated with the gasification process. However, far too little attention has been paid to exergetically analyzing advanced gasification-based MSW treatment systems producing transportation fuels and chemical platforms through catalytic FischerTropsch synthesis and mixed alcohols through biocatalytic fermentation [248,249]. Therefore, future research should apply exergy-based analyses to develop advanced gasification-based MSW treatment schemes and assess their sustainability level. In most published studies where the exergy concept has been applied to design, analyze, optimize, and retrofit MSW treatment systems, the simulation data obtained by considering several assumptions, oversimplifications, and approximations have been used. This issue might negatively affect the applicability of the obtained results. Therefore, real-world data should be used in future studies dealing with exergybased methods for developing and investigating MSW management systems. A tremendous amount of low-grade thermal energy is often wasted in MSW management systems. Accordingly, advanced energy recovery systems from low-grade thermal energy should be annexed to MSW management systems to improve their exergetic features. Advanced heat integration and recovery techniques like pinch analysis and self-heat recuperation technology can also enhance the exergetic performance of MSW management systems. The exergetic performance of MSW management systems can also be substantially improved by integrating with other renewable resources such as solar, wind, and geothermal energy. The present paper is devoted to comprehensively reviewing and critically discussing the use of exergetic indicators for the sustainability assessment of MSW management systems. The pros and cons of exergybased methods in the sustainability assessment of MSW management systems are systematically highlighted and critically discussed to identify future research directions. The exergy-based approaches have drawn much attention in MSW management because of the need to dispose of urban wastes in an efficient, viable, and sustainable strategy. The main concluding remarks derived from the published literature could be highlighted as follows: • Compared to biological treatment, thermochemical processing of MSW has been widely investigated using exergy-based methods due to their higher conversion rates. • The overall exergy efficiency of integrated MSW incineration-based power plants typically ranged between 17 and 40%. • The functional exergy efficiency of various integrated MSW gasification-based systems under different operating parameters was generally in the range of 22–56%. • The plasma gasification process seemed to be the least exergetically favorable approach for MSW treatment due to its high electricity consumption. • The reaction temperature, pressure, equivalence ratio, and steam-towaste ratio were the main influential parameters affecting the exergetic performance of the MSW gasification process. More specifically, the gasification temperature had the most significant impact on the exergy efficiency of the process. • The composition and moisture content of MSW could markedly affect the exergetic performance of the incineration and gasification processes. • Integrating the incineration and gasification processes with power generation cycles was found to be an effective solution to enhance the exergetic performance of MSW management systems. • Integrating solid oxide fuel cells with MSW gasification systems and organic Rankine cycles with MSW incineration systems appeared to be exergetically sustainable strategies. • The current density, operating temperature, and stack temperature difference of the fuel cell could markedly affect the exergetic performance parameters of the fuel cell-based MSW treatment systems. • Despite a large-scale investment for treating MSW using the gasification and pyrolysis processes, there is a long way to bring these thermochemical routes to real-world practice because of the heterogeneity of MSW. • The overall exergy efficiency values of MSW anaerobic digestionbased systems typically stood between 34 and 73%. • From the exergetic perspective, landfilling could not be an attractive solution to manage MSW effectively, even for biogas production. • The in-vessel anaerobic digestion process could exergetically outperform landfill gas recovery. • Integrating anaerobic digestion with either gas engine or steam power generation could be an exergetically promising solution to reduce the volume of MSW organic fraction while producing heat, power, and fertilizer. • The biogas cleaning process as a common approach to make possible its injection into the natural gas grid has not been investigated in the published literature from the exergetic viewpoint. • The incinerator (boiler), heat recovery steam generator, gasifier, digester, syngas/biogas combustion chamber, and CO2 removal unit were the main causes of irreversibilities in MSW treatment systems. • A rigorous comparison of the exergetic efficiency values of different MSW treatment scenarios reported in the published literature might not be likely to be achieved due to overlooking the final use of intermediate products in the exergetic calculations. 67 S. 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[39] Bhoi PR, Huhnke RL, Kumar A, Indrawan N, Thapa S. Co-gasification of municipal solid waste and biomass in a commercial scale downdraft gasifier. Energy 2018; 163:513–8. Salman Soltanian: Writing – original draft. Soteris A Kalogirou: Supervision; Investigation. Meisam Ranjbari: Data curation; Software. Hamid Amiri: Methodology; Validation. Omid Mahian: Review & Editing; Methodology. Benyamin Khoshnevisan: Formal analysis, Methodology. Tahereh Jafary: Investigation; Methodology. AbdulSattar Nizami: Supervision; Investigation. Vijai Kumar Gupta: Review & Editing; Methodology. Siavash Aghaei: Visualization; Software. Wanxi Peng: Resources; Funding acquisition; Writing – review & editing. Meisam Tabatabaei: Conceptualization; Resources; Funding acquisition; Supervision; Writing – review & editing. Mortaza Aghbashlo: Conceptualization; Resources; Funding acquisition; Supervision; Writing – review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank Universiti Malaysia Terengganu under International Partnership Research Grant (UMT/CRIM/2-2/2/23 (23), Vot 55302) for supporting this joint project with Henan Agricultural University under a Research Collaboration Agreement (RCA). This work is also supported by the Ministry of Higher Education, Malaysia under the Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP) program (Vot. No. 56052, UMT/CRIM/2-2/5 Jilid 2 (11)). The manuscript is also supported by the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 21IRTSTHN020) and Central Plain Scholar Funding Project of Henan Province (No. 212101510005). 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