Anoop Singh · Dheeraj Rathore Editors Biohydrogen Production: Sustainability of Current Technology and Future Perspective Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives 11 R. Miandad, M. Rehan, O.K.M. Ouda, M.Z. Khan, K. Shahzad, I.M.I. Ismail, and A.S. Nizami Abstract Hydrogen (H2) has emerged as a promising alternative fuel that can be produced from renewable resources including organic waste through biological processes. In the Kingdom of Saudi Arabia (KSA), the annual generation rate of municipal solid waste (MSW) is around 15 million tons that average around 1.4 kg per capita per day. Similalry, a significant amount of industrial and agricultural waste is generated every year in KSA. Most of these wastes are disposed in landfills or dumpsites after partial segregation and recycling and without material or energy recovery. This causes environmental pollution and release of greenhouse gas (GHG) emissions along with public health problems. Therefore, the scope of producing renewable H2 energy from domestic and industrial waste sources is promising in KSA, as no waste-to-energy (WTE) facility exists. This chapter reviews the biological and chemical ways of H2 production from waste sources and availability of waste resources in KSA. 11.1 R. Miandad • M. Rehan • K. Shahzad I.M.I. Ismail • A.S. Nizami (*) Center of Excellence in Environmental Studies (CEES), King Abdul Aziz University, Jeddah, Saudi Arabia e-mail: Nizami_pk@yahoo.com; anizami@kau.edu.sa O.K.M. Ouda Department of Civil Engineering, Prince Mohamed Bin Fahd University, Al-Khobar, Saudi Arabia M.Z. Khan Environmental Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, Uttar Pradesh, India Introduction The current world population of 7.2 billion is projected to increase by 1 billion till 2025 with an annual growth rate of 1 % (WHO 2014). The Global South i.e. developing Asia, Middle East, Africa, and Latin American countries is the place where most of this growth will occur due to rapid growth in urbanization and population. As a result, the energy demand is increasing significantly in developing countries, especially in Asia (Ouda et al. 2013), which is expected to increase by 46–58 % with an annual rate of 3.7 % till 2025 (FAO 2010; US-EIA 2007). Fossil fuels are the © Springer India 2017 A. Singh, D. Rathore (eds.), Biohydrogen Production: Sustainability of Current Technology and Future Perspective, DOI 10.1007/978-81-322-3577-4_11 237 238 most relied choice at the moment to fulfill the energy demands (Demirbas et al. 2016). As a consequence, existing reserves of fossil fuels sources are depleting along with global climate change. Therefore, the renewable energy sources are getting more attention to fill the ever increasing energy demand-supply gap. The advances in technologies with lower operational cost and governmental incentives are increasing the growth in renewable energy sector (Nizami et al. 2015a; Nizami et al. 2016; Ouda et al. 2016; Sadaf et al. 2015). Moreover, the national and international protocols such as Kyoto Protocol and Agenda 21 are adding momentum to move from fossil fuels-based economies toward renewable fuels-based economies (Tawabini et al. 2014). The high growth rate of population and urbanization along with raised living standards is also resulting in excessive generation of municipal solid waste (MSW) worldwide. In the next 15 years, the world’s average generation rate of MSW will increase from 1.2 to 1.4 kg per capita per day. Currently, around 2.4 billion tons of MSW is generated every year worldwide that will reach up to 2.6 billion tons by 2025 (UD 2012). The MSW management is not only important for sanitation purposes but also for generation of energy and recyclable materials for revenue and environmental protection (Rathi 2006). Therefore, the sustainable management of MSW is one of the national policy agenda of most developed nations to protect the public health, aesthetic places, and land-use resources (Ouda et al. 2013). 11.1.1 Energy Perspectives of the Kingdom of Saudi Arabia (KSA) The Kingdom of Saudi Arabia (KSA) is located in the Middle East and lies between 16° 22′ and 32° 14′ north latitudes and 34° 29′ and 55° 40′ east longitudes. KSA is one of the world’s largest crude oil producer country. A large socio-economic development has occurred since the last four decades due to oil-export revenue. KSA population is increasing at an annual rate of 3.4 % coupled with high-living standards and urbanization growth (Nizami et al. 2015b). The increase in R. Miandad et al. urbanization was observed from 50 to 80 % from 1975 to 2000, while it has reached up to 85 % in 2010 (Aga et al. 2014). The most urbanized cities of KSA are Riyadh (the capital of KSA), Jeddah, Dammam, Makkah, Medina, Al-Hasa, and Al Taif with the population of 5.2, 3.4, 2, 1.7, 1.2, 1.1, and 1 million respectively (CDSI, 2010). Energy demand of KSA has increased significantly with a rate of 5.6 % from 2006 to 2010 (MEP 2010). The current electricity demand in the country is about 55 GW that is expected to surpass 120 GW by 2032 (Ouda et al. 2015). At present, fossil fuels are the only source to meet all energy requirements of the country. KSA’s government has initiated a program called King Abdullah City of Atomic and Renewable Energy (KACARE) to utilize the indigenous renewable energy resources through science, research, and industry. The ambition of KACARE program is to generate half of the electricity from renewable energy sources, including solar, wind, nuclear, geothermal, and waste-to-energy (WTE) by 2032 (KACARE 2012). 11.1.2 Waste Generation in KSA In KSA, the generation rate of MSW is about 15 million tons per year with an average rate of 1.4 kg per capita per day (Ouda et al. 2015). The Ministry of Municipalities and Local Affair regulates the management of MSW in the country that includes waste collection and disposal to landfill sites (Nizami et al. 2015c). The landfill requirement is extremely high with 2.8 million m2 per year (Ouda et al. 2013). Metals and cardboard are the recycled materials (10– 15 % of total MSW) regulated by informal sector (Khan and Kaneesamkandi 2013). Most of the inuse landfills are approaching to their full capacities and resulting in waste leachate, sludge, odor, and greenhouse gas (GHG) emissions (Ouda and Cekirge 2014). WTE technologies are widely used to recover energy and value-added products (VAP) from different fractions of MSW. The examples of WTE technologies include pyrolysis, gasification, anaerobic digestion (AD), incineration, plasma arc gasification, refuse derived fuel (RDF), and transesterification (Gardy et al. 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives 2014; Ouda et al. 2016; Tahir et al. 2015). In KSA, there is no such WTE or material recovery facility (MRF) exists (Nizami et al. 2015b). 11.1.3 Aim of the Chapter This chapter in its first part reviews the biological and chemical H2 production processes and their advantages and disadvantages, potential waste substrates for H2 production, and technological advances and challenges. In second part, a review of the available waste resources in KSA for H2 production is carried out with an ambition to explore indigenous sources of renewable energy and solve the waste management problems. 239 is required for each process of H2 production in the form of heat or electrolyte (Bhutto et al. 2011). The most common technique for H2 production is the reforming of natural gas (Holladay et al. 2009), while the typical methods of H2 production are fossil fuel non-catalytic partial oxidation and auto-thermal reforming. To further improve these methods, membrane processes, methane selective oxidation, and oxidative dehydrogenation procedures are adopted (Armor 1999). However in recent years, the H2 production from biological methods using renewable resources has gained significant attention (Kapdan and Kargi 2006). 11.2.1 Biological Methods of H2 Production from Waste 11.2 Waste-to-Hydrogen Energy H2 is one of the most abundantly available elements on the earth with highest energy content per unit weight (142 KJ/g) and efficiency of producing electricity (Bhutto et al. 2011). It can be stored in liquid and gas forms and can be converted into different forms of energy. This makes H2 a promising alternative fuel and future energy carrier (Table 11.1). Annually, around 500 billion m3 of H2 is produced globally with 10 % growth rate (Winter 2005); of which 40 % is produced from natural gas, 30 % from heavy oils and naphtha, 18 % from coal, 4 % from electrolysis, and 1 % from biomass (Nath and Das 2003; Kapdan and Kargi 2006). Most of the H2 applications are currently limited to industrial sector, where H2 is used in petrochemical manufacturing, glass purification, hydrogenation of unsaturated fats and vegetable oil and steel processing, and desulfurization and reformulation of gasoline in refineries (Kotay and Das 2008; Kapdan and Kargi 2006). Moreover, H2 is used in metallurgical processes in heat-treating applications for the removal of oxygen (O2) as O2 scavenger. Collectively, 49 % of the produced H2 is used in ammonia production, 37 % is utilized in petroleum refining, 8 % is used in methanol production, and 6 % is utilized in various small applications (Konieczny et al. 2008). H2 can be produced from renewable and nonrenewable sources (Fig. 11.1). A source of energy The biological processes that produce H2 from renewable sources include direct photolysis, indirect photolysis, photo-fermentation, dark fermentation, and microbial electrolysis (bioelectrohydrogenesis). Each process has advantages and disadvantages based on substrate type, process mechanisms, end products, and energy requirements (Table 11.2). The mechanism of biological H2 production was first discovered by Hans Gaffron in the early 1940s, when he found that green algae can either consume H2 as an electron donor in carbon dioxide (CO2) fixation process or produce H2 under anaerTable 11.1 Characteristics of H2 Characteristics Boiling point Liquid density Gas density Heat of vaporization Lower heating value (mass) Lower heating value (liquid, volume) Diffusivity in air Lower flammability limit Upper flammability limit Ignition temperature in air Ignition energy Flame velocity Sequeira and Santos 2010 Unit K kg/m3 kg/m3 kJ/kg MJ/kg MJ/m3 Values 20.3 71 0.08 444 120 8960 cm2/s vol. % (in air) vol. % (in air) °C MJ cm/s 0.63 4 75 585 0.02 270 240 R. Miandad et al. Renewable Sources Water Non-Renewable Sources Solid Fuels Biomass Biological conversion Gasification Liquid Fuels Pyrolysis Natural Gas Reforming Electrolysis Direct Photolysis Steam Reforming Thermochemical water splitting Indirect Photolysis Partial Oxidation Photoelectrolysis Dark Fermentation Autothermal Reforming Sulphur-Iodine Cycle Photo Fermentation Microbial Electrolysis Fig. 11.1 H2 production processes from renewable and non-renewable sources (Armaroli and Balzani 2011) obic conditions in both dark and light (Kumar and Das 2000a, b; Benemann 1996, 1997). H2 can be produced by a number of microorganisms through enzymatic activities (Table 11.3). These microorganisms produce H2 in a variety of different ways due to their diversity in microbial physiology and metabolism. All of these processes offer advantages over the conventional H2 production processes in terms of lower catalyst cost and less energy consumption by using microbial cells and mesophilic operation, respectively (Hallenbeck et al. 2009). The enzymes that control H2 production are called hydrogenase and nitrogenase (Lindberg et al. 2004). 11.2.1.1 Direct Biophotolysis Direct biophotolysis process involves solar energy to split water molecules into hydrogen ions and oxygen by photosynthesis. These hydrogen ions are then converted into H2 by the hydrogenase enzymes (Table 11.4). Different types of algae and cyanobacteria species have been used in producing H2, such as Chlamydomonas reinhardtii (Momirlan and Veziroglu 2005; Holladay et al. 2009), Scenedesmus obliquus (Das and Veziroglu 2008), Chlorococcum littorale (Hallenbeck and Benemann 2002; Hallenbeck et al. 2009), Platymonas subcordiformis (Kumar and Das 2000a, b), Chlorella fusca (Gaffron and Rubin 1942), Anabaena sp., Oscillatoria sp., Calothrix sp., Synechococcus sp., Gloeobacter sp. (Melis and Happe 2001), Anabaena cylindrica (Schulz 1996; Melis and Happe 2001), Anabaena variabilis (Benemann 1997; Miura 1995; Ni et al. 2006; Greenbaum et al. 1983; Ghirardi et al. 2000), etc. Photoautotrophic microorganism such as cyanobacteria or green algae is equipped with chlorophyll A and other pigments to absorb sunlight and use photosynthetic systems (PSI and PSII) to carry out oxygenated photosynthesis. The photons having wavelength shorter than 680 nm are absorbed by the pigments in PSII and generate strong oxidant, which is capable of splitting water into protons H+, electrons e-, and O2. These electrons are transferred to PSI through a series of electron carrier. Similarly, photons having wavelength under 700 nm are absorbed by 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives 241 Table 11.2 Advantages and disadvantages of biological H2 production methods Direct biophotolysis Advantages From water and sunlight, H2 is directly produced An increase of tenfolds in conversion energy in comparison to biomass (trees, crops, grasses, etc.) Indirect biophotolysis H2 is produced by cyanobacteria using water N2 is fixed from atmosphere Photo-fermentation Associated bacteria can use a wide spectral light energy Different organic wastes can be used Substrate conversion efficiencies are high A wide range of substrates can be degraded Dark fermentation Microbial electrolysis It is a simpler and less expensive process A high rate of H2 production is achieved H2 can be produced 24 by 7 without light As substrates, a wide range of carbon sources can be used Electricity or H2 can be made directly from waste sources For H2 economy, it is a promising approach Wastewater, especially effluents with low organic content can be used Sustainable and effective process Disadvantages High intensity of light is required O2 and H2 are produced simultaneously and O2 can be dangerous for the system Even low concentrations of O2 can disturb hydrogenase (e.g. green algae) Photochemical efficiency is lower To stop degradation of H2, hydrogenase enzymes are eliminated In gas mixture, around 30 % of O2 is present H2 is produced at a slow rate On nitrogenase, O2 affects negatively Only 1–5 % conversion efficiency is achieved Due to toxic nature of the substrate (effluent), pretreatment is required High investment costs due to expensive setup installations, and large reactor surface areas For hydrogenase, O2 is inhibitor Lower yield of H2 is achieved The process becomes thermodynamically unfavorable with the increase of H2 pressure CO2 is required to separate from the gas mixture The involved metabolic pathways are not well defined Most of the studies focused on only mixed cultures that are used in already enriched and active microbial fuel cells A low volumetric H2 production occurs when the power densities at the electrode surface becomes low A high voltage negatively affects energy efficiency Bhutto et al. 2011; Kotay and Das 2008; Levin et al. 2004; Das and Veziro 2001 pigments of PSI to further enhance the energy level of electrons. These electrons then reduce oxidized ferredoxin (Fd) and/or nicotinamide adenine dinucleotide phosphate (NADP+) into their reduced forms. It leads to proton gradient across the cellular membrane that drives adenos- ine triphosphate (ATP) production through ATP synthase enzyme. Under special conditions, hydrogenase or nitrogenase enzymes utilize the reducing equivalents to reduce protons for molecular hydrogen evolution (Yu and Takahashi 2007). 242 R. Miandad et al. Table 11.3 Comparison of various biological H2 production methods Process Direct biophotolysis Indirect biophotolysis Photo-fermentation Dark fermentation Two-stage fermentation (darkfermentation+ photolysis) Organism involved Chlamydomonas reinhardtii Anabaena variabilis Rhodobacter sphaeroides Enterobacter cloacae DM 11, Clostridium sp. strain no. 2 Enterobacter cloacae DM 11+ Rhodobacter sphaeroides OU 001 51.20 Mixed microbial flora + Rhodobacter sphaeroides OU 001 Maximum reported rate (mmol H2/L h−1) 0.07 0.36 0.16 64.5–75.6 47.9–51.2 Kotay and Das 2008; Bhutto et al. 2011; Hallenbeck and Benemann 2002 Table 11.4 Chemical reactions involved in the biological H2 production methods Process Direct biophotolysis 2 H2O + light → 2 H2 + O2 Indirect biophotolysis (a) 6H2O + 6CO2 + light → C6H12O6 + 6O2 (b) C6H12O6 + 2H2O → 4H2 + 2CH3COOH + 2CO2 (c) 2CH3COOH + 4H2O + light → 8H2 + 4CO2 Overall reaction 12H2O + light → 12 H2 + 6O2 Photo-fermentation CH3COOH + 2H2O + light →4H2 + 2CO2 Dark fermentation C6H12O6 + 2H2O →2CH3COOH + 4H2 + 2CO2 Microbial electrolysis C6H12O6 + 2H2O → 4H2 + 2CO2 + 2CH3COOH Anode: CH3COOH + 2H2O →2CO2 + 8e- + 8H+ Cathode: 8H+ + 8e − → 4H2 Bhutto et al. 2011; Levin et al. 2004; Das and Veziroglu 2001; Franks et al. 2009 One of the main challenges of this process is to remove the produced O2, as it inhibits hydrogenase enzyme activity and therefore limits H2 production (Hallenbeck and Benemann 2002). Theoretically, direct biophotolysis process is an economical and sustainable method for H2 production by utilizing renewable resources like algae, light, and water. However, the process becomes unavailable at commercial scale due to challenges such as strong inhibition effect of evolved O2 on enzymatic activity (Bhutto et al. 2011), generation and impact of highly explosive H2-O2 mixtures, low conversion efficiencies in different light intensities, low H2 production rates, no waste utilization, and limitations in designing large-scale reactors (Wang and Wan 2009; Das and Veziroglu 2008; Yetis et al. 2000). Therefore, photo-fermentation and darkfermentation processes have advantages for treating waste with H2 production (Tables 11.2, 11.3, and 11.4). 11.2.1.2 Indirect Biophotolysis Indirect biophotolysis process involves two stages coupled with CO2 fixation; O2 is released in the first stage with CO2 fixation and H2 is produced in the second stage (Momirlan and Veziroglu 2005). The reduced substrates such as glycogen and starch accumulate during the photosynthetic O2 evolution and CO2 fixation stage, which are then used in anaerobic conditions to produce H2 and CO2 in the second stage. The conversion of these substrates into H2 in the second step is carried out by the algae or other organisms like photosynthetic or fermentative bacteria. This separation of O2 and H2 production reactions not only helps to overcome the two major problems of direct biophotolysis such as enzyme deactivation and production of explosive 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives gas mixture, but also makes H2 purification relatively easier by using conventional separation methods (Yu and Takahashi 2007). The processes of direct biophotolysis can either be carried out in a single reactor producing O2 and H2 in an alternating cycle or in separate reactors like open ponds and photo bioreactors. 11.2.1.3 Photo-Fermentation Photo-fermentation utilizes photosynthetic bacteria to quantitatively produce H2 and CO2 from various organic substrates, especially organic acids such as acetate and butyrate mediated by nitrogenase, using light energy. This is a simple process and has advantage of high H2 production rate due to bacterial ability in degrading a wide range of substrates. This process has been demonstrated to produce H2 from various organic acids, food, agricultural waste (Hallenbeck and Benemann 2002), and industrial wastewater (Yildiz et al. 1994; Davila-Vazquez et al. 2008). However, pretreatment of substrates originating from waste sources may require the removal of any possible toxicity and dirty color. 11.2.1.4 Dark-Fermentation The dark-fermentation process is a stable process of H2 production due to anaerobic conditions. The bacteria are grown in the dark on carbohydrate-rich substrates to generate electrons. These electrons are then taken up by O2 in aerobic conditions, while they are used by protons to be reduced to H2 molecules in anaerobic conditions. One of the main drawbacks of this process is the production of gas mixture of H2 and CO2 with possible CH4, CO, and H2S that is technically a challenge when used in fuel cells. A variety of substrates can be used in darkfermentation process including simple sugar, starch containing waste, cellulose containing wastes, food industry wastes, wastewater, and waste sludge (Kapdan and Kargi 2006). The process parameters are critical to be controlled for efficient H2 production, such as process temperature, pH, substrate type and composition, type of organic acid produced, reactor configuration, 243 pressure, and residence time (Geelhoed et al. 2010). 11.2.1.5 Microbial Electrolysis The microbial electrolysis is a bioelectrochemical system that generates electrical current to reduce protons to H2 in a process called bioelectrohydrogenesis. The microbial electrolysis cell (MEC) consists of four parts that are anodic chambers, cathodic chambers, electronic separator, and external electrical power source (Liu et al. 2005; Hamelers et al. 2010). The reaction typically utilizes acetate as the electron donor to be oxidized. The electrons and protons are then combined to produce H2 at the cathode (Table 11.4). This process can utilize wastewater and agro-industrial residues containing biopolymers like cellulose and starch to produce H2. The reactions carried out in MEC can be catalyzed both by microorganisms and chemicals like platinum and nickel. The MEC can achieve much higher H2 yields (80–100 %) in comparison to fermentative H2 production (<33 %) (Table 11.5), since it uses electric current to overcome the energy barriers for oxidation of the substrate (Gralnick and Newman 2007). The bio-electrochemical systems recover almost 90 % of the energy using acetate as a substrate that is three times greater than the fermentation process (Catal et al. 2015; Hu et al. 2008). However, the overall performance of the MEC depends upon the physiology of the microorganisms as well as the physicochemical transport processes. Various types of losses such as ohmic resistance, concentration, and conductivity associated with these bioelectrochemical systems adversely affect the overall H2 production rates (Logan et al. 2008). Thus, there still remains a great challenge to keep the electrical potential in balance at both the bioanode and biocathode chambers (Liu et al. 2005). 244 R. Miandad et al. Table 11.5 Comparison of the energy parameters of different biological techniques used for H2 production Systems Direct biophotolysis Indirect biophotolysis Photofermentation Dark fermentation Two-stage fermentation (darkfermentation+ photolysis) Microbial Electrosynthesis System (MES) Energy efficiency (%) 0.07 mmol H2/ (l h) 0.36 mmol H2/ (l h) 0.16 mmol H2/ (l h) 64.5–75.6 mmol H2/(l h) 47.9–51.2 mmol H2/(l h) 90 % at the rate of 0.5 kWh/ m3-H2 References Kotay and Das (2008) Kotay and Das (2008) Bhutto et al. (2011) Bhutto et al. (2011) Hallenbeck and Benemann (2002) Catal et al. (2015) 11.2.2 Chemical Ways of H2 Production from Waste 11.2.2.1 Pyrolysis Pyrolysis is a thermal degradation process that converts the carbonaceous substrates into liquid oil, solid residue (char), and gases in the absence of oxygen at temperatures of 300–650 °C (Manara and Zabaniotou 2012). This is a highly flexible process that can be optimized in accordance to the desired products such as liquid fuel, gases, and char from the specific substrate. This optimization is carried out by regulating the composition of the substrate, temperature and retention time variations in the reaction chamber, catalyst, and substrate particle size. The liquid oil and char are the most investigated products of pyrolysis that are mainly dependent on prevailing temperature conditions (Rehan et al. 2016). The liquid oil as a fuel source is the main product when process is carried out at temperatures of 400–550 °C, while gases are higher when process temperature is greater than 700 °C. Similarly, substrate resident time (from few seconds to 2 h) in the reaction chamber is an important parameter affecting the yields of final products (Chen et al. 2014). H2 can be produced from pyrolysis of different substrates such as sludge, legume straw (Li et al. 2004; Zhang et al. 2011), wheat straw (Hornung et al. 2009), crude beech-wood oil (Davidian et al. 2007), nutshell, olive husk, grape residue, beech wood, straw pellet, and waste plastics (Di Blasi et al. 1999). Kasakura and Hiraoka (1982) reported that 5.5 vol.% H2 is produced from the sludge pyrolysis with 3.65 vol.% CO. According to Chen et al. (2014), the increase of temperature from 500 to 700 °C also increased the gases production from 30–35 to 45–50 vol. %, while Demirbas and Arin (2004) reported the increase of gases from 27–41 to 41–55 vol.% with increase of temperature from 377 to 752°C. In pyrolysis process, the use of catalyst can also increase the gases production. It is reported that by using Ni/Al2O3 and Ni-K/La2O3-Al2-O3 catalysts, H2 production is increased by 45–50 %. Moreover, the substrate particle size increased the gases yield at 28.2, 38.5, 15, 18, 5, and 8 mol% of H2, CO, CO2, CH4, C2H6, and C2H4 respectively, with particle size of 0.45–0.90 mm. Fixed-bed reactor, free-fall reactor, and fluidized bed reactor are commonly used reactor configuration for pyrolysis to produce H2 (Hornung et al. 2009; Li et al. 2004). 11.2.2.2 Gasification Gasification is a thermophilic process used to convert the carbonaceous substrates into H2 at temperature of 800–900 °C in a controlledoxygen environment (Uddin et al. 2013). Different substrate sources such as palm oil waste (Inayat et al. 2012), meat and bone waste (Soni et al. 2009), wood sawdust (Wu et al. 2011), plastic residue (Czernik and French 2006), rich husk (Karmakar and Datta 2011), pellets (Ruoppolo et al. 2012), and pig compost (Wang et al. 2013) are utilized for H2 production. Different catalysts are used to increase the process yield. The use of Ni/MCM-41 catalyst with sawdust in a two-stage fixed bed reactor increased H2 production from 30.1 to 50.6 vol.% (Wu et al. 2011). In another study by Wu and Williams (2009), H2 production from polypropylene (PP) plastic was significantly increased with the use of Ni/Al2O3 catalyst. Asadullah et al. (2001) reported that the use of Rh/CeO2 catalyst produced 1290 H2 per μmol that can be further increased with the increase in temperature. The use of cedar wood 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives with Rh/CeO2/SiO2 catalyst also increased the H2 production at the temperature of 550–700 °C. Lv et al. (2003) reported that with the increase in temperature from 700 to 900 °C, H2 production was increased from 22 to 71 g per kg. 11.3 Waste-to-Hydrogen Energy Potential in KSA Cost effectiveness, availability, high carbon contents, and biodegradability are the key parameters in the selection of H2-producing substrate. Glucose, sucrose, and lactose are the preferable sources of substrate for H2 production (Kapdan and Kargi 2006). Currently there is no WTE or waste-tohydrogen facility exists in KSA, since all of the collected wastes are disposed in landfills or dumpsite untreated. Following is the review of the potential available H2-producing waste sources in KSA. 11.3.1 Organic Fraction of MSW in KSA In 2014, the total MSW generation in KSA was around 15 million tons with an average rate of 1.4 kg per capita per day (Maria 2013). This waste rate is estimated to become double (around 30 million tons) by 2033. The overall generated waste in KSA consists of up to 75 % organic waste, including food waste as the largest waste stream (50.6 % of total MSW) with amount of 7.7 million tons per year and generation rate of 0.71 kg per capita per day (Fig. 11.2). The food waste contains rice, meat, bakery products, and fat with percentages of 38.7, 25, 18.7, and 13 %, respectively. Moreover, bones, fruits, and vegetables are present in food waste with a percentage value of 2.2 %. The chemical composition of food waste shows high percentage of moisture content (38.4 %), carbohydrates (25.6 %), proteins (17.3 %), fats (15.3 %), ash (3.2 %), and fibers (0.3 %) (Abu-Qudais and Abu-Qdais 2000; Khan and Kaneesamkandi 2013; Alruqaie and Alharbi 2012). The other waste fractions of MSW include paper (12 %), plastic (17.4 %), glass (3 %), cardboard (6.6 %), wood waste (2 %), metals (1.9 %), 245 textile (1.9 %), aluminum (0.8 %), leather (0.1 %), and others (3.7 %) (Fig. 11.2). The food waste in three large cities of KSA (i.e., Riyadh, Jeddah, and Dammam) is exceeding 6 million tons per year (Maria 2013). During the month of 2014 Ramadan, 5 thousand tons of food was wasted in first 3 days only in Makkah municipality (Irfan 2014). The alarming news is the wastage of 35–40 % cooked rice annually in KSA with a total loss of 1.6 billion SR (Saudi Gazette 2014). Such waste composition with high fraction of organic contents (up to 75 %) makes it very suitable substrate for biological processes such as dark-fermentation and two-stage anaerobic dark and photo-fermentation to produce H2. For instance, the starch present in food waste can be hydrolyzed to glucose and maltose using enzymes followed by conversion to organic acids and then to H2 (Kapdan and Kargi 2006). Han and Shin (2004) used food waste in dark-fermentation process using a leaching bed reactor for H2 production. The results showed that the process was suitable as initial step for H2 production followed by the methanogenesis process, like a two-stage anaerobic process. 11.3.2 Plastic Waste and Used Oil in KSA Plastic waste is the second-largest waste stream of MSW in KSA with an annual production of 2.7 million tons with an average rate of 0.3 kg per capita per day (Nizami et al. 2015a, b, c; Ouda et al. 2013; Ouda and Cekirge 2014). Currently, only informal sector is involved in plastic waste recycling. However, most of the collected plastic wastes are disposed in landfills or dumpsites untreated. In addition to environmental problems, plastic waste causes operational overburden from its collection to final disposal due to its clogging and non-biodegradable nature (Nizami et al. 2016; Ouda et al. 2016). Recycling through conventional-mechanical techniques such as sorting, grinding, washing, and extraction can recycle only 15–20 % of plastics. Moreover, the plastic waste is polluted with dirt, aluminum foils, food waste, and soil. 246 R. Miandad et al. Fig. 11.2 MSW composition in KSA (Nizami et al. 2015a, b, c; Khan and Kaneesamkandi 2013; Ouda et al. 2016; Nizami et al. 2016) Thermal and catalytic pyrolysis and gasification are one of the WTE technologies used to convert plastic waste into liquid fuel and syngas, respectively (Yuan 2006). H2 is produced from syngas, as it is a combination of H2 and CO. A large fraction of the country’s MSW is also consisted of used oil from household, restaurants, and automobile industry. This waste oil can also be used to produce H2 using pyrolysis and gasification technologies. Kim (2003) used shredded waste tires and waste oil in gasification process as substrates to produce H2. 11.3.3 Slaughterhouse Waste in KSA Millions of animals are slaughtered in KSA every year during the pilgrimage (Hajj) and Ramadan (month of fasting) periods. For example, in 2014 Hajj season, more than 2.5 million animals were slaughtered in KSA to perform Hajj rituals (Amtul 2014). Typically, 12 % waste per body weight is produced from sheep and goat slaughtering, whereas cattle slaughtering produces 38 % waste per body weight (Singh 2013). This waste includes rumen, blood, offal materials, bones, and tallow. Although there is little information available for these waste quantities, but it is evi- dent that animal related-waste quantities are huge in KSA. The slaughterhouse waste has been used in many studies for producing H2 in a two-stage fermentation process (Gomez et al. 2006, 2009). Gomez et al. (2009) produced H2 from sludge and slaughterhouse waste and found that the H2 production was more stable from the process when slaughterhouse waste was used than the sludge waste only. 11.3.4 Agricultural Waste in KSA It is estimated that more than 440 million tons of agriculture residue is produced in KSA every year and most of them is incinerated or disposed of inefficiently (Sadik et al. 2010). Among the agricultural waste, most share comes from date palm trees. There are around 23 million date palm trees in KSA, which produce 780 thousand tons of agriculture residues per year (Al-Abdoulhadi et al. 2011). The agricultural waste can be used as a substrate in the chemical processes such as pyrolysis and gasification and biological processes such as anaerobic dark and photo-fermentation for H2 production. In biological processes, the substrate is pretreated using waste grinding, delignification, and hydrolysis. The hydrolyzed 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives waste is then converted into organic acids and finally into H2 (Kapdan and Kargi 2006). 11.3.5 Industrial Waste in KSA In KSA, there are more than 12 thousand industries working in different sectors that produce large quantities of waste and waste sludge on daily basis (Ouda and Cekirge 2014). In a study on Jeddah industrial sludge, it was estimated that 120 tons of sludge is produced every day on dry solid basis. In 2013, around 200 dry tons of sludge was produced every day in Riyadh. Collectively, the projected amounts of sludge in KSA will be around 1.6 thousand and 1.8 thousand dry tons per day by 2020 and 2025, respectively (SAWEA 2013). The waste sludge contains large quantities of carbohydrates and proteins; thus can be used in biological processes such as photo-fermentation and two-stage anaerobic dark and photo-fermentation for H2 production (Kapdan and Kargi 2006). 11.4 Challenges and Perspectives of Waste-to-Hydrogen Energy Production 11.4.1 Challenges Projections regarding the shortage of fossil fuel reserves in the 21st century enforce researchers to think for alternative renewable energy sources. This has increased the significance of H2 production processes. However to shift from fossil fuelbased economy to H2 energy-based economy, efforts are required to overcome the challenges of H2 production pathways toward optimizing the production processes (Table 11.6). According to Momirlan and Veziroglu (2002), H2 energy production challenges lie in investment cost, storage and delivery, conversion, and end-use applications (Table 11.6). Moreover, according to Hallenbeck and Benemann (2002), utilizing solar energy as a renewable source in biological conversion methods is limited due to its low density and diffusive nature. 247 The advantage of biological processes is the high efficiency of H2 production by utilizing waste materials that are often considered refuse or garbage. However, the challenges in these biological processes are lacking information about microbial activities and their sensitivities to O2 and H2, as they affect the process yield (Table 11.6). Moreover, the microbial inherent properties, limitation of photosynthetic efficiency, and hydrogenase catalytic functions are the microbial challenges of H2 production (Laurinavichene et al. 2006). According to Levin et al. (2004), the rate of H2 production in biological processes is low and requires further process optimization. Kim (2003) examined different carbonaceous wastes for H2 production through steam reforming. These renewable sources are quite cheap but require high-process temperature (1200 °C). On Table 11.6 Challenges in H2 production Barriers in basic science Organism Bacteria do not produce more than 4 mol H2/mol glucose naturally Enzyme Hydrogenase over expression not stable, O2 sensitivity and H2 feedback inhibition Barriers in fermentation process Substrate High cost of suitable substrate (glucose) and low yield using renewable biomass Strain Lack of suitable-industrial strain Process Commercially feasible product yield, incomplete substrate utilization, and sustainable process sterilization Barriers in engineering aspects Reactor design Lack of kinetics/appropriate reactor design for H2 production and light intensity in case of photo-bioreactor Thermodynamics Thermodynamic barrier NAD(P)H→H2 (+4.62 kJ/ mol) H2 storage H2 purification/separation and its storage Bhutto et al. 2011; Kapdan and Kargi 2006; Das and Veziroglu 2001; Kotay and Das 2008; Hong et al. 2013; Rathore et al. 2016 248 the contrary, H2 production through electrolysis can be a cleaner way; however this can be only applied to areas where electricity is cheap (Kapdan and Kargi 2006). Moreover, demineralization of water is required to avoid corrosion and deposition on electrodes (Armor 1999). 11.4.2 Perspectives The eco-friendly and sustainable methods of generating H2 energy will only be possible, if produced from renewable sources (Fig. 11.3). The H2 economy is a sustainable future energy system that will produce electricity and energy carriers utilizing renewable sources. High demand of H2 energy brings the attention of researchers for the development of cost-effective and efficient Fig. 11.3 An overview of various drivers for H2 economy R. Miandad et al. methods for producing H2 as a renewable and sustainable fuel (Kapdan and Kargi 2006). The comparison of H2 production by different processes using conventional fuels and biological systems was carried out by Tanisho (1996), Benemann (1997), Bockris (1981), Kumar and Das (2000a, b), and Benemann (1997). The use of fermentative process can be better than photosynthetic process for H2 production, as it produces various kinds of value-added fatty acids during the process such as lactic acid, acetic acid, and butyric acid. However, if these acids are not separated from photosynthetic process, it may cause water pollution. According to Das and Veziroglu (2001), biological processes are not energy intensive, as they can be carried out at ambient temperature. Gases produced via biological processes are H2 (60–90 v/v) with impurities of CO2 and O2. 11 Waste-to-Hydrogen Energy in Saudi Arabia: Challenges and Perspectives CO2 is water soluble and can be reused as fire extinguisher. A 50 % w/v solution of potassium hydroxide (KOH) is a good CO2 adsorbent, thus can be used for CO2 removal from H2. The presence of O2 may cause fire hazards, thus can be removed by using an alkaline pyrogallol solution. The moisture contents reduce the heating values of H2, thus can be removed by drying and chilling process (Das and Veziro 2001). H2 production is efficiently carried out at small scale using biological systems where substrate is easily accessible to reduce energy consumption and transportation cost. However, its production at industrial scale is still facing process and economic constraints. Therefore, there is a strong need of metabolically engineered microorganism to enhance H2 production (≥4 mol H2/mol of glucose) for large-scale H2 production (Maness et al. 2009). Engineering work is also required to further upgrade the bioreactors for H2 production at commercial scale. 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