The Role of Catalysts in Life Cycle Assessment Applied to Biogas Reforming

Abstract

The real implementation of biogas reforming at an industrial scale to obtain interesting products (like hydrogen or syngas) is a developing research field where multidisciplinary teams are continuously adding improvements and innovative technologies. These works can contribute to the proliferation of green technologies where the circular economy and sustainability are key points. To assess the sustainability of these processes, there are different tools like life cycle assessment (LCA), which involves a complete procedure where even small details count to consider a certain technology sustainable or not. The aim of this work was to review works where LCA is applied to different aspects of biogas reforming, focusing on the role of catalysts, which are essential to improve the efficiency of a certain process but can also contribute to its environmental impact. In conclusion, catalysts have an influence on LCA through the improvement of catalytic performance and the impact of their production, whereas other aspects related to biogas or methane reforming could equally affect their catalytic durability or reusability, with a subsequent effect on LCA. Further research about this subject is required, as this is a continuously changing technology with plenty of possibilities, in order to homogenize this research field.

1. Introduction

1.1. Environmental Issues: Current Overview

There is global concern, from international agencies to society in general, about many different environmental issues that question the sustainability of our current industrial network. Thus, the use of petrol-based products, from their extraction, production, and refining to their energy use, have resulted in many different environmental problems, like a considerable contribution to greenhouse gas emissions, water, and soil pollution, among others [1,2]. These effects are due to the direct use of these compounds, but there are other indirect consequences that are equally important, such as economic dependence and subsequent political instability, as well as the requirement of the shipment or transportation of petrol-based fuels or products [3,4,5]. Therefore, it is not only a matter of sustainability but also a matter of economic survival that has provoked the increasing interest in sustainable practices such as the implementation of green policies, including the circular economy and the sustainable development of certain areas [6], along with efforts by other developed countries (some of them more engaged than others), to safeguard the environment [7,8]. Therefore, there are plenty of actions, from local to global and from institutions (such as the European Union or the United Nations) to individuals, focused on the sustainability of every aspect in our lives [9,10]. The clearest example of this fact are the Sustainable Development Goals (SDGs) [11], which try to foster equality, zero hunger, sustainability, and the protection of soil and water resources.

There are many different technologies that can be implemented to foster sustainability, the circular economy, and green chemistry. Biogas production (normally through anaerobic digestion (AD) from different wastes, such as sewage sludge (SS), agricultural waste, or their combination [12]) is the perfect example for this purpose, with plenty of possibilities to obtain interesting products like hydrogen through reforming processes [13]. In this sense, hydrogen is one of the most valuable energy vectors, with the subsequent valorization of wastes implied in its generation [14].

However, the quality of hydrogen production depends on different factors, such as the kind of source, process, and further treatments to obtain high-purity hydrogen. In this regard, hydrogen can be classified by colors, according to the cleanliness of the whole production [15], as observed in Figure 1.

In this sense, hydrogen can be sorted by color according to the environmental cleanliness, sustainability of the process, and subsequent emissions. Thus, these colors range from black and brown (the most polluting processes) to blue, turquoise, or green (the most sustainable and less polluting ones). As explained in previous works, turquoise hydrogen performs better than gray hydrogen, playing an important role in green transition [16]. It should be noted that there are processes that can be ranked in a certain color depending on specific factors like the use of CO₂ capture techniques. Specifically, the presence or absence of CO₂ capture could involve blue or gray hydrogen production from natural gas. Also, the use of a certain raw material is vital, as biogas is highly related to green hydrogen processes, whereas coal is linked to black hydrogen. Lastly, another determining aspect is the use of green energy for an industry. In this way, when electrolysis is used to produce hydrogen, renewable energies assure the production of green energy, whereas the use of nuclear energy is linked to pink hydrogen (due to the management of nuclear waste). Within the same color, for instance in the case of green or blue hydrogen, there might be different emission levels according to the technologies used [17]. In this sense, CO₂ capture can be highly effective, achieving zero or low CO₂ emissions.

Basically, the color of hydrogen is related to many aspects of hydrogen production. This simple and intuitive labeling points out relevant aspects that are highly esteemed in environmental assessment tools like life cycle assessment (as explained in further sections), like the following:

• Nature of the raw material: Hydrogen obtained from natural gas is not considered as clean as hydrogen obtained from water through hydrolysis, as the source by itself implies certain environmental impacts.
• Origin of the energy used in the process: Depending on the energy used for a specific process to obtain hydrogen, its color might vary. Biogas reforming can be considered green if renewable energy is used in this process, whereas it can be considered blue if other energy sources are selected. The same happens to hydrogen obtained through electrolysis, whose color might vary from pink (for nuclear energy) to green (if renewable energies are chosen).
• Pollution (including evolved gasses but also water and soil pollution) and its management: This is one of the most determining aspects used to rate hydrogen in a certain color. For instance, if CO₂ is released into the atmosphere, it is common to consider the hydrogen produced as black, brown, or gray.

In this work, green hydrogen produced through biogas reforming is considered, which could be of a different color depending on CO₂ management. Indeed, it is assumed that the direct release of carbon dioxide into the atmosphere would downgrade this process to gray hydrogen.

1.2. Assessment of Environmental Impact and Sustainability

As previously mentioned, these concerns by international and local authorities, along with society in general, make the search for sustainable processes essential. Furthermore, it is important not only to choose green industries but also to confirm that their performance is environmentally friendly [18]. This way, any technological solution should be available, with the possibility of choosing different options according to their environmental efficiency, cost, and resulting economic constraints, optimizing any action to reduce impacts as much as possible [19].

For this purpose, there are plenty of methods to determine the suitability of a process, especially at an industrial scale, from an environmental point of view [20,21]. This is a very interesting point, as there are countless research works including emerging green technologies, which should be proven at the industrial scale in terms of efficiency and sustainability. This approval is an essential step for green technologies to compete with traditional industries, especially those based on petrol. In this sense, there are several tools or indicators to assess the environmental impact of a certain industry or process. For instance, the carbon footprint is an interesting indicator that assesses the global greenhouse gas emissions related to an industry or activity. It should be noted that these emissions can be directly or indirectly generated, and it can be considered a part of LCA, as it is usually included in these kinds of assessments. Another interesting indicator is atom economy, whose main philosophy is indicated in Figure 2.

Thus, atom economy would be given by Equation (1).

$$Atom economy={\displaystyle \frac{\sum Molecular weight of desired products}{\sum Molecular weight of reagents}}\times 100$$

(1)

According to this equation (where the molecular weight in the case of basic reactions can be replaced by quantities of desired products or reagents), atom economy ranges from 0 to 100%, with high atom economy values indicating that there is a low percentage of evolved compounds that are undesirable and, therefore, with polluting potential. However, the energy costs (which could be translated into a considerable carbon footprint), as well as the environmental impact of further treatments to increase atom economy, are not covered, which could present a challenge for this tool. Nevertheless, it can be a good indicator to be used as a complement to other methods to determine the environmental impact of different processes.

In a similar way, the carbon and greenhouse gas footprint is an index that indicates the contribution of an industrial activity or production to the levels of these emissions into the atmosphere, normally expressed as kg or tons of CO₂ per a specific unit of production or consumption (that could be equivalent to the functional unit used in LCA, as explained in the following section) [22]. It should be noted that, for a certain process, there might be direct or indirect carbon dioxide emissions, which can be globally considered in this index.

Derived from the abovementioned index, another interesting assessment of the environmental impact of a certain company or entity is the concept of carbon credit (or offset credit), which is commonly defined as the capability of reduction in carbon emissions (or equivalent emissions), normally expressed as metric tons of CO₂. As explained in the following section, LCA assess CO₂ emissions, providing this interesting information that allows companies to offset greenhouse gas emissions due to their typical activities by introducing new, alternative, and green technologies that contribute to global CO₂ emission reduction. Again, LCA can play an important role, as it is possible to draw a comparison of the contribution of innovative technologies to reduce carbon dioxide emissions to check the real carbon credits achieved by implementing them. For instance, biochar production in some stages included during biogas production or reforming could contribute to increase carbon credits with the subsequent mitigation of environmental consequences [23].

Finally, another environmental index is the water footprint, which can be defined in the context of this review as the amount of water consumed or polluted during industrial activities to produce goods, energy, and services, normally expressed in volumetric flow rate units [24]. As explained in further sections, LCA covers this aspect, especially in some processes included in this review such as biogas or methane steam reforming, where the consumption of water can be a challenge due to the energy costs to generate steam and subsequent water consumption. It should be noted that the water footprint concerns different aspects, such as the sphere of influence for the assessment of this indicator, which is an important point in common with LCA, where the delimitation of the process (by establishing boundaries in the system) is vital to understand the environmental impact of a certain process (or technology).

It should be noted that there are other indicators or assessment methods, including atmospheric temperature, ozone levels, or different chemical and biological measurements, which are highly related to the abovementioned concepts. In any case, life cycle assessment can cover or contribute to many of these indicators, as in the case of water or carbon footprints and carbon credits, which are related to the abovementioned SDGs. In the following section, the foundations of LCA, which are essential to understand the contribution of catalysts to a positive environmental assessment, are briefly explained.

1.3. Life Cycle Assessment: Foundations

Thus, compared to the abovementioned approaches, LCA is a complete and thorough methodology to assess the environmental balance of a certain technology or industry, including different biofuels or bioproducts [25]. It allows decisions to be made about the right selection of a certain technological solution among different possibilities [19].

LCA is an extremely useful tool to cover different aspects in industrial production, like the following [19]:

• Corporate strategy: The use of LCA is linked to a thorough analysis of the environmental impact related to industrial products and activities, which is an essential strategy for many companies in strategic sectors such as the energy sector, with subsequent prestige and good branding.
• Research and development: As explained in this review, many research lines are devoted to the improvement of the environmental impact of some steps related to different technologies, and LCA can be a good way to assess the progress of a certain research project or line.
• Design of products or processes: The role of LCA at this point is vital, as the production of items involves multiple previous steps, along with further post-treatments (if necessary), that should be assessed.
• Education: LCA has such great significance that it can be taught at different educational levels, as it is currently considered an important step in the design of products or services.
• Labeling and description of products: There are plenty of rankings of products—for instance, regarding food, according to their nutritional value. In this sense, products can also be labeled according to their impact during production, usage, or disposal. For this purpose, LCA is useful to rank these processes and products.
• Performance of meta-analyses, including the review of LCAs about a certain process or product. These studies could present a global perspective of a specific technology, which usually presents some particularities that can be encompassed by using LCA applied to these variations.

In addition, there are also different applications according to the kind of industry. Indeed, many studies about mature technologies, as well as emerging ones, have included LCA as an important subject, as in the case of microalgal biorefineries [26], biodiesel production from jatropha or waste cooking oil [27,28], bioethanol production [29,30], glycerol reforming to produce H₂ [31], or waste biorefineries [32], among others. In general, LCA covers different aspects of a product, process, or service. On the one hand, it tries to consider not only all the components implied in the production system, including reagents, products, and by-products but also other less-evident factors such as the energy used, efficiency of the process, shipment of goods, or use of fungible materials or catalysts, which will be a determining point in this work. On the other hand, LCA is not only focused on the process but also on the previous steps to carry out every stage and the consequences of industrial activities. As typically said, this method goes from the “cradle to grave”, indicating that every aspect (from extraction, production, manufacture, distribution, use, and posttreatment) of every component involved in the process should be considered, or at least most of it. Thus, the more stages and compounds are considered in LCA, the more complete it is, offering more reliable results [19]. In other words, this tool is suitable for determining which step (or steps) deserves more attention to be optimized from an environmental point of view.

In this sense, LCA is a very adaptable method, with multiple applications in the industry depending on the process. Also, as observed in Figure 3, there are some representative connections between LCA and the so-called Sustainable Development Goals [11], which can be explained as follows:

As observed in this figure, LCA is related to many aspects covered in the SDGs, which are the most representative objectives at an international level. For instance, the following are perfect examples of the adjustment of LCA to the SDGs:

• One of the main aims of LCA is to protect the environment and, subsequently, ensure the good health and well-being of citizens.
• Another interesting aspect is the assessment of pollutants on water, attempting to obtain clean water and protect life in aquatic ecosystems.
• The production and use of affordable and clean energy is another important factor, where LCA has an important role, as energy consumption or production in a certain industry is highly related to CO₂ emissions, for instance.
• Regarding industry innovation and infrastructure, the production of industrial goods and services is going to be more and more controlled from an environmental point of view, whose assessment could be carried out by using LCA to promote clean and green technologies for these purposes.
• Equally, sustainable cities and communities are based on every detail that contributes to environmental conservation, including the transportation or shipment of goods, industrial production, etc. Transportation, among other factors, is normally included in LCA.
• Obviously, responsible consumption and production is the cornerstone of LCA, where terms such as durability, reusability, or recycling are essential to consider a certain process sustainable or not.
• Concerning climate action, life below water, and life on land, the quantity of pollutants (such as some evolved gasses, liquids, or solids) could be a challenge that should be managed, which is an important area of knowledge in LCA. Normally, the lower emissions there are (or the better that management is carried out), the lower the environmental impact on land, water, or climate.

As observed, LCA and the SDGs share many points, guided by their philosophy of going from start to end, pointing out critical aspects such as water, CO₂, or energy management. Equally, the requirements to assess different environmental impacts, including emissions into air, water, or soil, make LCA a suitable tool highly linked to the abovementioned SDGs.

In this sense, there are different standards that are highly related to LCA, like those included in

Table 1. Different standards related to LCA.

Standard	Title/Details	Ref.
ISO 14001	Environmental management systems—Requirements with guidance for use	[33]
ISO 14002-1	Environmental management systems—Guidelines for using ISO 14001 to address environmental aspects and conditions within an environmental topic area. Part 1: General	[34]
ISO 14002-2	Environmental management systems—Guidelines for using ISO 14001 to address environmental aspects and conditions within an environmental topic area. Part 2: water	[35]
ISO 14004	Environmental management systems—General guidelines on implementation	[36]
ISO 14005	Environmental management systems—Guidelines for a flexible approach to phased implementation	[37]
ISO 14020	Environmental statements, programs and products—Principles and general requirements	[38]
ISO 14021	Environmental labels and declarations—Self-declared environmental claims	[39]
ISO 14022	Environmental labeling—Self-declaration environmental claims	[40]
ISO 14024	Environmental labels and declarations—Type I environmental labeling—Principles and procedures	[41]
ISO 14025	Environmental labels and declarations—Type III environmental declarations—Principles and procedures	[42]
ISO 14031	Environmental management—Environmental performance and evaluation—Guidelines	[43]
ISO 14033	Environmental management—Quantitative environmental information—Guidelines and examples	[44]
ISO 14040 *	Environmental management—Life cycle assessment—Principles and framework	[45]
ISO 14044	Environmental management—Life cycle assessment—Requirements and guidelines	[46]
ISO 14045	Environmental management—Eco-efficiency assessment of product systems—Principles, requirements, and guidelines	[47]
ISO 14046	Environmental management—Water footprint—Principles, requirements, and guidelines	[48]
ISO 14048	Environmental management—Life cycle assessment—Data documentation format	[49]
ISO 14049	Environmental management—Life cycle assessment—Illustrative examples on how to apply ISO 14044	[50]
ISO 14050	Environmental management—Vocabulary	[51]

* ISO 14040 series is specifically devoted to LCA, including guidelines, phases of inventory, impact assessment, and interpretation.

. As observed, the ISO 14000 series is a set of standards focused on different environmental aspects, including environmental management, performance evaluation, life cycle assessment, water footprint, etc. These are the foundations for an appropriate LCA, offering a wide range of possibilities of adaptation to each specific case. Consequently, most of these standards can be applicable to assess the environmental impact of technologies such as biogas reforming.

As observed, the different series included in this table cover diverse aspects of environmental management, from foundations of LCA to specific aspects such as labeling, vocabulary, or even examples to clarify various aspects included in these norms. Equally, different guidelines for interesting aspects such as life or eco-efficiency assessment of product systems are covered. In general, these standards are mainly focused on good practices when applying LCA in every aspect related to a process or product, including the main steps that should be accomplished to carry out a complete LCA. Thus, according to

Table 2. Main steps to be considered in LCA [19].

Step	Details
Goal and scope definition	The aim and scope of the study is defined, describing the problem and determining the function of the system, FU 1, and the system boundaries. Also, the base scenario and alternatives are thoroughly described.
Inventory analysis	According to the previous step, where the boundaries are described, the emissions into air, water, and soil are quantified, considering the extraction of renewable and non-renewable raw materials.
Impact assessment	Because of the established emissions, their environmental impact is assessed, depending on the impact category, nature (global warming, ecotoxicity, etc.), damage characterization, etc.
Interpretation of results and uncertainties	In this step, conclusions and recommendations about the previous stages are included, with special attention to the evaluation of the chosen boundaries and assumptions. The environmental impact of a process can be compared with other processes, as well as with social and economic impacts.

¹ Functional unit.

, the main stages in a life cycle assessment are the following.

As in the case of other assessments, the boundaries of the system, that is, the limits within which the environmental impact of a process or production is going to be considered, is an assumption that can simplify the study. However, some valuable information might be missed or ignored if these boundaries are limiting. On the other hand, the concept of functional unit (FU) is very interesting, as it will facilitate the comparison of different LCAs about similar subjects. Thus, it is the quantified performance of a product system for use as a reference unit. In other words, it is a quantifiable and additive reference, like kg of hydrogen in the case of biogas or natural gas reforming [52]. For instance, if hydrogen production is doubled according to this unit, the environmental impact related to the LCA linked to 1 kg of produced hydrogen would be increased by 100%.

As can be inferred from the above, LCA can be easily implemented for a wide range of green technologies, such as biogas reforming. The following sections will deal with the real possibility of LCA applied to this case, including specific cases from which different conclusions can be made.

1.4. Biogas Reforming: Possibilities for LCA

On the other hand, the other aspect of this review—that is, biogas reforming—is equally highly related to these SDGs, which makes its combination with LCA interesting when it comes to sustainability. Thus, some of the SDGs, such as “Affordable and Clean Energy” or “Climate Action”, among others, are especially linked to LCA and biogas reforming. Therefore, LCA applied to biogas reforming offers endless opportunities for the improvement of sustainable development. In this sense, this technology offers a great opportunity for the development of sustainable regions, as multiple wastes can provide biogas through anaerobic digestion that can be used in reforming to produce hydrogen. In this context, the relevance of wastewater treatment plants should be noted, which contributes to the preservation of aquatic ecosystems and the sanitation of water supply. Equally, sewage sludge obtained during anaerobic digestion could present interesting properties once it is treated through hydrothermal carbonization or pyrolysis, obtaining a product with high porosity that presents a wide range of possibilities, like its use as activated carbons for catalyst support or gas adsorption. In other words, reusability, which is in accordance with SDG 12, is another aspect related to biogas reforming, at least at initial stages (that is, biogas production).

However, there are other SDGs that are linked to biogas reforming rather than LCA, like SDG 1 or SDG 8, as the implementation of this technology in rural areas or developing countries could represent a great contribution to the economic growth of these regions, offering an alternative to other energy sources and decreasing energy dependence (which should be combined with LCA to assess the sustainability of these processes, especially in managing wastes to produce energy) [53]. This way, green technologies are relevant in current geopolitical scenarios, as the petrol-based industry is usually used as an economic weapon to penalize commercial agreements when there are political tensions between countries.

Concerning biogas, it is significantly different from natural gas (whose compositions are relatively similar, with methane as the main compound to be upgraded in further processes). Biogas is obtained in AD processes, from natural wastes such as SS or agricultural waste, making biogas a renewable energy. By contrast, natural gas is obtained from geological processes that break down organic material. Consequently, biogas has some interesting advantages, such as the valorization of the abovementioned wastes and, if the suitable technology is used, the production of valuable energy vectors (such as hydrogen) that, along with the control of other by-products such as CO₂ (through its capture or upgrading), could serve as a promising energy source.

As previously explained, biogas reforming could be an interesting area of study for LCA, as it presents interesting challenges like the following (which will be thoroughly studied in further sections):

• The characteristics of biogas play an important role in its reforming. Thorough LCA could consider biogas production through AD depending on the kind of waste used. Consequently, studies from the origin can be carried out.
• Energy consumption and its origin are equally relevant, improving or worsening the environmental performance of biogas reforming.
• Depending on the kind of reforming process, some particularities can be present, such as water consumption in the case of biogas (or methane) steam reforming.
• Apart from these, there are different factors that should be considered, like the use of catalysts, which could improve the catalytic performance of the process; it can also result in additional environmental impact due to its production.
• Thus, depending on the kind of process, the main environmental challenges could be the following: carbon dioxide emissions and their subsequent treatment, methane emissions and their reduction through higher efficiency, catalyst waste and its management, etc.

Accordingly, new studies about the environmental impact of biogas reforming have been conducted, with some of them focused on LCA applied to specific processes where biogas (or methane, its majority compound) is upgraded to hydrogen or syngas. In this sense, the role of different aspects in environmental performance has been covered, with special attention to the most determining pollutants (like CO₂) but also considering the role of catalysts (from their production to their management after use, as well as their positive effect in biogas reforming). LCA offers different approaches or possibilities in biogas production and its reforming, like the following:

• The influence of different wastes and AD conditions on biogas production and their effect on reforming.
• A comparison of different operating conditions during biogas reforming, including the use of various catalysts. Indeed, different technologies based on hydrogen production could be compared under these circumstances to select the most suitable process [54].
• An assessment of the implementation of technologies and their positive effects when it comes to pollutant upgrade or capture.
• Recommendations for the improvement of efficiency and mitigating of environmental impact in biogas reforming, including the implementation of renewable energies for energy supply in the process, etc.

As previously explained, the boundaries of LCA are essential to establish the objective for a certain evaluation of the environmental impact by using this tool. According to the main steps that could be implied in biogas reforming, Figure 4 shows possible boundaries for LCA in this context, including previous steps like anaerobic digestion to produce biogas.

Thus, according to this figure, the following could be inferred:

• It should be noted that boundaries should be perfectly established and sized if different technologies are compared. Thus, a global LCA can reflect a more realistic environmental impact compared to Boundary 5, related to the Fischer–Tropsch process, where the context of syngas production is not covered. In any case, different technologies applied to the Fischer–Tropsch process could be compared by establishing the same boundary in all cases.
• Different boundaries can be overlapped. For instance, Boundaries 2 and 3 (AD and biogas reforming) can be combined if the information of these steps is available.
• In this way, Figure 4 points out different boundaries, like Boundary 1, which includes all the aspects related to the possible use of biogas, apart from biogas reforming. Obviously, this one would be the most complex LCA, but it could also be the most complete one to consider the global environmental impact of the process. Boundary 2 shows aspects related to anaerobic digestion, including the selection and pre-treatment of wastes, the possibility of co-digestion, and the possibility of biogas purification by removing H₂S, among others. Boundaries 3 and 4 cover biogas reforming from different perspectives, with the different possibilities explained in this work and product purification (normally biogas, a combination of hydrogen and carbon monoxide). Finally, Boundary 5 shows a possibility to upgrade biogas through the Fischer–Tropsch process to produce different waxes and biofuels.
• As will be explained in this work, the role of catalysts is present in most of the boundaries included in this Figure. For example, catalysts are essential in biogas steam reforming and in the Fischer–Tropsch process, with an influence in operating conditions (including energy consumption) or selectivity, whereas their durability and efficiency depend on many factors. In essence, these circumstances are normally included in LCA, both in general and specific terms. Therefore, the implementation of a certain technology to improve the environmental impact of a process is normally related to the use of specific catalysts.

1.5. Scientific Interest

Considering these previous ideas, it is no wonder that the interest in environmental issues and sustainable development of industries translates into increasing attention from the scientific community. Thus, Figure 5 shows the evolution of the number of scientific papers over time about the subjects included in this review and their distribution according to their main research field.

According to this Figure, there was a continuous increase in research works related to LCA applications, especially from 2010, which proves the constant interest in this field. On the other hand, it presents a multidisciplinary profile, as these works are framed in several research areas like Environmental Science (22%), Engineering (17%), or Energy (13%), among others. In other words, LCA is a tool that can be easily used by a wide range of scientists and technicians to assess the sustainability of multiple processes, as the study of the environmental impact of these aspects can be easily adapted to each specific case. Therefore, LCA studies can be useful for the industrial implementation of mature technology. In this case, since LCA is a tool devoted to the assessment of environmental impact, it is not surprising that Environmental Science is the field where the majority of these works are published.

Regarding biogas reforming, the interest in this line of research has considerably increased since 2005, equally representing a wide variety of fields, such as Energy (27%), Chemical Engineering (17%), Engineering (13%), Chemistry (13%), or Environmental Science (10%). Also, the role of catalysts is represented in these subjects.

As observed, LCA and biogas reforming share some research areas, especially concerning Energy and Environmental Science, which predict a promising line of research when both concepts are combined. An example of this interconnection could be CO₂ and CH₄ management. As explained in the following sections, this could be a relevant point in LCA applied to biogas reforming, as these compounds result in high environmental impact, especially regarding greenhouse gas effects.

1.6. Aim of This Work

Considering the above, the aim of this review was to carry out a state-of-the-art analysis of works focused on LCA applied to biogas reforming or similar processes, paying special attention to the most recent developments about the following research fields:

• Findings about the application of LCA in biogas reforming in general;
• The role of catalysts in LCA, considering their catalytic performance and influence on biogas reforming and taking into account their environmental impact during extraction, production, use, and management once their useful life is over;
• The influence of operating conditions or other factors related to biogas reforming on the performance of catalysts and, therefore, their impact during LCA;
• General outlook about the main advantages and challenges about catalytic biogas reforming.

1.7. Scope and Bibliometric Analysis

To carry out this review, Clarivate’s Web of Science (WoS) was investigated for all entries in the literature on the topics of life cycle assessment applied to biogas reforming (main entries: “life cycle assessment” and “biogas” and “reforming”; “life cycle assessment” and “biogas”; “biogas” and “reforming”) for the last 10 years (as the research interest in this field has considerably increased for 10 years), with special attention to the last 5-year period (2019–2024) to trace most of the recent research works about this subject. The search, which was conducted from January to June 2024, returned 3683 results, from which up to 209 articles were considered for their inclusion in this work, including information about 124 published works (mainly research works or proceeding papers) in this review.

2. Biogas and Methane Reforming and Its Key Factors in LCA

Apart from the different environmental aspects related to biogas production through AD (which could be equally considered in LCA), and assuming that biogas composition represents high percentages of CH₄ (which would make this process similar to methane reforming), different treatments to upgrade biogas can be found to produce H₂ or syngas (H₂ + CO), such as steam reforming or dry reforming, among others. Hereon, according to the majority of content in CH₄, the chemical routes explained for each biogas upgrade process will be simplified to methane reforming, using both terms (biogas and methane) interchangeably. Also, previous steps such as anaerobic digestion to obtain biogas will be covered in the following subsections.

2.1. Anaerobic Digestion

Before explaining the main steps in different steam reforming processes, and following the philosophy of LCA (that is, “from cradle to grave”), biogas production through anaerobic digestion, as well as the main nature and composition of biogas, should be commented on. Even though this work is devoted to the processes after biogas production, it should be noted that many factors related to biogas can present a considerable influence on its reforming and, especially, on its catalytic performance. According to Table 3, there are many different raw materials that can be used to obtain biogas, such as municipal solid waste [56], wastewater [57,58], food waste [59,60], animal manure [61], or agricultural waste [62], among others. This versatility of raw materials points out the relevance of anaerobic digestion for biogas production, with a positive impact on LCA because most wastes used in this process are valorized during their management.

Thus, AD is essentially a process where microorganisms convert organic matter into biogas (a mixture of CH₄ and CO₂, in general) under anaerobic conditions [63]. Different efficiency rates (from 47 to 70%) can be obtained in this process depending on many factors, such as the method, pH, or microorganisms selected [60,64]. Although there are some specific requirements regarding the kind of waste used in anaerobic digestion, some coincidences or general steps are usually present (see Figure 6), like the following [65]:

• Hydrolysis: This is the first stage in AD, where organic matter and polymers (such as carbohydrates, lipids, or proteins) are decomposed in mono and oligomers (such as glucose, glycerol, or purines, among others) by hydrolytic bacteria (through the action of extracellular enzymes).
• Acidogenesis: In this stage, the abovementioned soluble monomers are converted by acidogenic bacteria to smaller organic compounds such as volatile acids, ketones, or alcohols. In this step, the majority of compounds obtained are acetate, CO₂, and H₂.
• Acetogenesis: In this step, acetogenic bacteria convert the previous acidogenic compounds into hydrogen, CO₂, and acetate, reducing the medium’s pH due to the increase in hydrogen ions, even jeopardizing the performance of acetogenic bacteria when the pH is below 6. This depends on the nature of organic matter, load, and environmental factors when acetic and propionic acids are produced.
• Methanogenesis: This is a crucial step to obtain the highest amount of CH₄ during the whole process. Thus, methane is obtained through the conversion of hydrogen and carbon dioxide by CO₂-reducing and H₂-oxidizing methanogens, whereas acetolactic methanogens use acetate to produce methane.

Figure 6. Anaerobic digestion: main stages and products, including the most determining ones in LCA applied to biogas reforming.

Figure 6. Anaerobic digestion: main stages and products, including the most determining ones in LCA applied to biogas reforming.

At this point, it is interesting to spotlight some aspects of LCA related to the role of catalysts in biogas production and its corresponding reforming, which are also included in Figure 6.

• First, and as explained in the following subsection in more detail, hydrogen sulfide is generated to a greater or lesser extent depending on the waste and AD conditions. The presence of H₂S traces can hinder catalytic biogas reforming, with negative consequences in the global efficiency of the process (and, therefore, a corresponding negative LCA, as different pollutants such as methane or carbon dioxide can be released into the atmosphere in excess).
• Secondly, different digestate phases, apart from biogas, are obtained [66], which should be properly managed or reused to improve LCA at this point. Thus, liquid digestate is an interesting source of nutrients, whereas solid digestate can be transformed into active carbon through different methods like pyrolysis or hydrothermal carbonization (HTC). Biochar could be an interesting way to offset CO₂ emissions, improving the carbon credits of the process [23]. The upgrading of the latter offers different and interesting possibilities due to the versatility of pyrolysis and HTC to obtain products with different porosities (combined with different components such as agricultural waste) [63], being used as the following: adsorbents (which could be a determining solution for H₂S capture), catalyst support for certain processes where relatively low temperatures (below 600 °C) are achieved, or even as an improver of the AD process, due to the fact that these solids can serve as pollutant adsorbers in an AD medium or as a host for microorganisms.
• Finally, biogas can be an important source of hydrogen if the suitable reforming process (along with the corresponding separation technique, like the use of PSA or membrane reactors) is used.

As a result, the correct management of digestates and biogas to improve LCA is important in this stage. Additionally, as happens in many aspects considered in biogas reforming when the philosophy “from cradle to grave” (the gist of every LCA) is considered, there are important details that will have a determining effect on catalytic biogas reforming and its LCA. Thus, as explained in further sections, the quality of biogas is essential to carry out sustainable processes, and the role of solid digestate seems to improve this fact by offering the possibility of increasing methane percentage in biogas (which is highly desirable) and reducing hydrogen sulfide content (which is mandatory for a suitable catalytic performance to avoid catalytic poisoning).

Another interesting aspect concerning the valorization of solid digestate would be the question about which method would be more adequate according to the kind of waste generated. Taking into account the fact that solid digestate usually presents high moisture, the use of pyrolysis would be undesirable, as it requires drying processes, which usually implies high energy consumption (apart from the pyrolytic process). On the contrary, HTC, where water plays an important role, does not require this step (with mild reaction conditions) [67], making this process interesting for a positive LCA.

Therefore, perfect knowledge of the entire process concerning biogas production is essential to understand and improve reforming conditions and their environmental impact. This includes the perfect characterization of biogas, which will be covered in the following subsection. Additionally, in order to obtain a wide framework of the sustainability assessment of biogas systems, different information should be included, such as environmental, geographic, or socio-economic data, among others, to develop a complete and multi-criteria decision-making tool [61].

2.2. Biogas Characteristics and Its Influence on Reforming

As a result of the above, biogas is produced, with around 50–75% CH₄, 25–50% CO₂, and lower amounts of N₂, H₂, and H₂S. This way, one of the great benefits of AD is the possibility to capture and use methane (one of the most problematic greenhouse gasses), which can be the starting point for hydrogen production. Moreover, AD seems to be a very resourceful technology for the valorization of wastes that are difficult to environmentally manage and experience continuous increase, like wastewater [65,68]. As explained in further sections, the efficiency of AD is vital to understand the catalytic performance of different biogas-reforming processes, which will have a great influence on LCA. In short, the following conditions would be desirable from both points of view:

• High CH₄ composition: Considering that this is the most important component in biogas to obtain hydrogen, the ideal situation would be a methane composition near 100%, to avoid the presence of impurities that should be separated or treated in different ways according to their nature and possibilities. However, in further reforming processes, the conversion of methane should be as high as possible to avoid its release into the atmosphere, as it is a greenhouse gas that leads to penalization in the LCA of biogas reforming (specifically) and corresponding waste management through biogas production and hydrogen generation (in general).
• Lower CO₂ composition: Even though CO₂ takes part in some reaction routes to obtain H₂ from CH₄, as explained in further sections, it should be noted that it is one of the typical by-products generated in methane reforming, which should be separated from final hydrogen with additional separation or capture techniques. This is one of the most representative greenhouse gasses, and its release into the atmosphere should be avoided as much as possible.
• Lower N₂ composition: As explained in the case of methane, CH₄ purity is desirable, and nitrogen does not take part in any reforming reaction. Therefore, its minimization in favor of CH₄, when possible, is recommended.
• Zero H₂S composition: The presence of hydrogen sulfide is undesired due to its negative effects at different levels. First, it causes health problems even at low concentrations (ppm); second, it can cause corrosion in facilities; and finally, it provokes poisoning in many typical catalysts (especially Ni-based ones), with negative consequences related to poor methane conversion (that is, methane release into the atmosphere).

It should be noted that some wastes derived from this process, such as sewage sludge, can be valorized and reused in the system. For instance, hydrothermal carbonization could be an interesting process to obtain hydrochar, which could be inserted in AD to host specific microorganisms taking part in the abovementioned steps, as well as be a suitable growing medium through pollutant capture [68]. Consequently, the impact in LCA would be lower, avoiding the release of wastes that are difficult to manage.

Concerning biogas composition found in the literature, as observed in Table 3, due to the heterogeneous composition of raw materials used for biogas production through anaerobic digestion, a wide range of biogas composition can be found, especially concerning key factors such as methane composition (from 18 to 75%) and hydrogen sulfide (from nearly 0 to around 4000 ppm). Even for a certain kind of raw material where biogas is obtained, composition ranges might vary. As a consequence, this initial information is essential to determine the further treatment of biogas (purification and upgrading, mainly), with subsequent changes in LCA according to these changes, which can introduce new processes to be considered in the corresponding assessment. Moreover, bearing in mind the heterogeneity of biogas composition depending on pre-anaerobic digestion conditions (for instance, the presence of certain amino acids like methionine or cysteine in agricultural waste or sewage sludge), reforming systems or at least some components such as H₂S traps should be well dimensioned, considering the possible worst-case scenario.

Considering a biogas suitable for its reforming—that is, with a high CH₄ percentage and low levels of H₂S—there are several processes to produce hydrogen, with the most representative ones being briefly explained in further subsections, steam reforming, dry reforming, and tri-reforming. It should be noted that, apart from biogas composition, each technology applied to its reforming will determine LCA, with some parameters that will be adjusted by using catalysts and, at the same time, will influence the service life and catalytic performance of these heterogeneous catalysts. That is the reason as to why the main inlets and outlets for each process should be, at least briefly, explained (see Figure 7 for a general overview of the case of biogas reforming). In addition, particularities and parts in common will be explained.

Table 3. Main raw materials used in anaerobic digestion to produce biogas and the resulting composition range.

Raw Material	CH4, %	CO2, %	N2, %	H2S, ppm	Ref.
Landfill	25–75	7–60	0–25	0–4000	[13,69,70,71,72]
MSW	55–70	30–60	--	--	[56,69,73]
Biomass/agricultural waste	45–75	25–55	0–5	0–180	[68,72,74]
Organic waste	40–70	30–60	--	--	[68,75]
Sewage sludge	18–75	30–50	2–8.1	0–2000	[68,71,72]

Table 3. Main raw materials used in anaerobic digestion to produce biogas and the resulting composition range.

Raw Material	CH4, %	CO2, %	N2, %	H2S, ppm	Ref.
Landfill	25–75	7–60	0–25	0–4000	[13,69,70,71,72]
MSW	55–70	30–60	--	--	[56,69,73]
Biomass/agricultural waste	45–75	25–55	0–5	0–180	[68,72,74]
Organic waste	40–70	30–60	--	--	[68,75]
Sewage sludge	18–75	30–50	2–8.1	0–2000	[68,71,72]

2.3. Biogas Steam Reforming

Concerning biogas or methane steam reforming (see Equation (2)), it is an endothermic reaction that takes place at high temperatures, normally between 700 and 950 °C, and with pressure values between 5 and 20 bar.

CH₄ +H₂O ↔ CO + 3H₂   ΔH⁰₂₉₈ = +206 kJ/mol

(2)

CO₂ in biogas takes part in a second reaction, as observed in Equation (3).

CH₄ + CO₂ ↔ 2CO + 2H₂   ΔH⁰₂₉₈ = +247 kJ/mol

(3)

The simultaneous methane conversion through these chemical routes can lead to biogas bi-reforming or simply biogas steam reforming [44]. Additionally, the water–gas shift reaction (WGS), as observed in Equation (4), can take place. Both reactions contribute to a higher yield towards hydrogen production and, consequently, higher hydrogen concentrations as a product.

H₂O + CO ↔ CO₂ +H₂   ΔH⁰₂₉₈ = −41 kJ/mol

(4)

The combination of Equations (2) and (4) gives, as a result, Equation (5).

CH₄ + 2H₂O ↔ CO₂ + 4H₂   ΔH⁰₂₉₈ = +165 kJ/mol

(5)

There are different challenges in biogas or methane reforming that need to be solved, which usually represent a problem regarding LCA, as these problems are highly related to efficiency, maintenance, durability, and by-product management, among others. According to the abovementioned equations, the main products obtained in methane steam reforming are H₂, CO₂ (also included in biogas, if biogas steam reforming is carried out), CO, and unreacted CH₄ and H₂O. In these cases, different factors should be considered to optimize LCA applied to biogas steam reforming, like the following:

• High conversions are desired to decrease unreacted CH₄, which would be released into the atmosphere, or should be reintroduced in the system through separation technologies.
• S/C ratio optimization is essential to avoid extra energy costs and water consumption. On the other hand, low S/C ratios could promote coke deposition, with the subsequent deactivation of the catalysts and decrease in efficiency of facilities, as explained in the following sections.
• CO₂ management is required, mainly through capture or conversion.
• Selectivity for hydrogen production is recommended to avoid or reduce the amount of the rest of the undesired products, which could suggest the need for separation stages with their corresponding environmental assessments.

Nevertheless, many of these concerns can be mitigated thanks to the use of catalysts, as explained in further sections with specific examples of the application of LCA to biogas reforming. In this regard, high conversions of methane are achieved by using different catalysts, increasing its conversion above 95% in many cases [76,77]. On the other hand, steam-to-carbon optimization is important to increase the useful life of a catalyst, especially concerning coke deposition, thus reducing their environmental impact and increasing their life cycle. Regarding carbon dioxide capture or conversion, there are some catalytic processes where CO₂ can be valorized thanks to the use of catalysts. Lastly, different catalysts have been proven as a suitable way to improve the selectivity for desired products (in this case, hydrogen), consequently decreasing the amount of undesired products released to the environment [78,79,80].

2.4. Biogas Dry Reforming

The main difference, compared to SR, is the absence of steam during methane decomposition, as shown in Equation (6).

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\leftrightarrow 2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^0 = +248 \mathrm{kJ}/\mathrm{mol}$$

(6)

As observed, methane and carbon dioxide included in biogas composition react to obtain syngas with different CO/H₂ ratios, which could be an interesting way to obtain pure hydrogen through membrane separation or the starting points for further processes such as Fischer–Tropsch synthesis [81]. On the other hand, lower conversions of methane (below 90%) are obtained, with the subsequent release of unreacted CH₄ and CO₂ (whose conversion in this process is higher, up to 95%) if further treatments are not carried out. Therefore, LCA would reveal this negative aspect in these cases.

Obviously, the environmental impact related to water consumption, as well as the energy required to vaporize water to provide a constant steam flow, is omitted in this case, which would simplify LCA. Thus, this reaction normally takes place at lower temperatures compared to steam reforming (from 700 to 800 °C) [72], which could favor a positive LCA. However, this initial advantage compared to the previous reforming process could be neutralized by different factors such as a decrease in efficiency. Also, the absence of steam could favor coke deposition or the Boudouard reaction, as observed in Equation (7).

$$2\mathrm{C}\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+\mathrm{C}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^0 = -171 \mathrm{kJ}/\mathrm{mol}$$

(7)

The details about the decrease in catalytic performance due to coke deposition (along with other factors) will be discussed in further sections.

2.5. Biogas Tri-Reforming

In this process, the reaction of the combination of steam and oxygen with biogas takes place. Thus, some reactions (like the ones described in Equations (2) and (6)) are involved, as well as catalytic partial oxidation and the reverse water–gas shift (see Equations (8) and (9)) [76,82].

$${\mathrm{C}\mathrm{H}}_4+{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{\mathrm{O}}_2\leftrightarrow \mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^0 = -36 \mathrm{kJ}/\mathrm{mol}$$

(8)

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^0 = 41 \mathrm{kJ}/\mathrm{mol}$$

(9)

Equally, as in the case of the previous reforming processes, coke deposition can take place according to the Boudouard reaction (Equation (7)), as well as methane decomposition (Equation 10) and carbon monoxide reduction (Equation (11)).

$${\mathrm{C}\mathrm{H}}_4\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^0 = 75 \mathrm{kJ}/\mathrm{mol}$$

(10)

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\leftrightarrow \mathrm{C}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em} \Delta {\mathrm{H}}_{298}^0 = -131 \mathrm{kJ}/\mathrm{mol}$$

(11)

It should be noted that these reactions contribute to the decrease in the activity of catalysts, with the subsequent decrease in its service life (and the negative impact on LCA). As observed, this method presents, from an LCA perspective, challenges shared with the previous reforming processes, which will be discussed in the following subsection. Basically, knowledge of these particularities will be essential in order to understand and apply suitable technical solutions to improve LCA, with many of them related to catalysts.

In summary, Figure 7 shows, for this specific boundary, the main inputs and wastes generated during biogas reforming in general. As explained in the following subsection, there are some challenges related to LCA applied to this technology—many of them related to the use of catalysts. In this sense, there are different aspects related to the environmental impact of the process that should be controlled or assessed, like the electricity consumed for the production of a certain FU, the amount of water required (especially in SR, where steam is required), the quality of biogas, the use of air, the construction of facilities (including reactor construction, where different materials such as steel, plastics, or compounds to carry out thermal insulation are required), and catalyst production (with the two main components playing an important role, that is, the active phase and the support, along with the corresponding synthesis method). On the other hand, waste generation, including the catalysts, heat generated, and pollutants that are emitted into the atmosphere, should be taken into account in LCA.

As in the case of other hydrogen production pathways, there are different aspects that can be covered in LCA based on biogas reforming, like ecotoxicity, land use, water use, energy resources, ozone depletion, or global warming potential (GWP), among others [83]. The main challenges related to LCA in this context are highly related to the impact in these categories, as explained in the following subsection.

2.6. Main Challenges in LCA Applied to Biogas Reforming

In summary, and according to the previous subsections, the main challenges regarding LCA applied to biogas reforming are the following:

• Characteristics of biogas: As previously mentioned, the ideal composition of a certain biogas should include high concentrations of CH₄ and, especially, the absence of H₂S in order to avoid catalytic poisoning and the subsequent decrease in CH₄ conversion to H₂. As explained in the following sections, the upgrading of biogas could involve further steps that should equally be taken into account in LCA, with a positive or negative response depending on the benefits or problems added to the system.
• Kinds of reforming/operating conditions: As previously explained, and depending on the kind of biogas reforming selected, positive and negative contributions to key aspects of LCA can be obtained. For instance, regarding catalytic performance, steam reforming presents higher coke resistance (with a longer catalyst service life), whereas its vapor consumption (with the subsequent energy cost) is increased. On the contrary, dry reforming seems to be more energy-intensive, but the removal of CO₂ is not required, with a positive impact in the LCA of biogas reforming. On the other hand, different operating conditions (such as the S/C ratio, pressure, or temperature), as explained in further sections, can determine different factors such as coke deposition or hydrogen selectivity, which will determine LCA [15].
• GHG emissions: Depending on the process, variable amounts of CH₄ and CO₂, the two main GHGs involved in this process (and two of the most polluting GHGs [22,84]), are released into the environment due to different factors. On the one hand, high methane conversions are desired, whereas carbon dioxide capture/management is usually required after biogas reforming to improve the atom economy of the process. In any case, reduction in these concentrations is essential to obtain a positive LCA.
• Water consumption: In the case of the SR of biogas, water consumption should be considered in LCA, achieving the optimum S/C ratio for this process in order to avoid excess energy use (when there is excessive steam) or low catalytic efficiency due to coke deposition (when the S/C ratio is not enough to avoid this phenomenon).
• Energy consumption: As in every LCA, the energy footprint is essential to positively assess a specific process. Thus, excess energy consumption would negatively reflect on biogas reforming due to different factors, especially those focused on the high temperature of the process (which can range from 750 to 900 °C) and steam generation. Also, further steps to separate different gasses obtained after biogas reforming (such as PSA) or to upgrade the resulting gas (such as FT) usually involve an increase in energy costs. The use of catalysts could reduce energy costs and the subsequent impact on LCA.
• Efficiency of the process: Because of the previous point, efficiency is fundamental in order to obtain a positive LCA. Thus, high efficiency implies lower consumption of biogas, water, or energy per FU, with a subsequent positive impact on the environment. As explained in the following point, the role of catalysts is important to improve the efficiency in catalytic reforming of methane or biogas.
• Equipment wear: Due to different factors, some components of the facility, such as reactors, resistances, or heat insulation, among others, could be deteriorated over the useful life of the equipment. As a result, certain operating conditions usually become more difficult to keep, like a set temperature in a reforming reactor. This fact results in an increase in energy consumption due to heat loss (among other factors) or even the replacement of certain components, with a negative impact on LCA. In general, it is assumed that the efficiency of a process decreases at a certain annual percentage rate, with further decrease due to the use of extreme operating conditions (high temperatures, pressure, biogas with a certain quantity of H₂S that could promote corrosion, etc.). Additionally, this wear provokes the replacement of these components in shorter periods of time, with a subsequent negative impact on LCA. Mild reaction conditions due to catalysts could partially alleviate this inconvenience.
• Catalyst production and use: The use of catalytic routes in biogas reforming has a strong influence on many different aspects, such as the improvement of its efficiency and the subsequent use of milder reaction conditions, which usually reduces energy and reagent consumption. In turn, methane conversion is improved, with a subsequent decrease in CH₄ emissions into the atmosphere. Regarding CO₂, its concentration is increased, requiring the use of technologies to capture this GHG. Accordingly, the direct use of catalysts represents a positive contribution to LCA, considering other factors such as their production in order to obtain a global assessment of the process.
• Hydrogen leaks or emissions into the atmosphere: Even though it is assumed that hydrogen production technology, compared to fossil fuel technologies, can present a clear advantage with lower environmental impacts, the fact that the permeability of hydrogen is extremely high should be considered, which involves losses in hydrogen production systems. In this sense, the quantification of hydrogen leaks is important in order to completely assess the benefits of LCA, specifically in biogas-reforming systems. Thus, considerable emissions of hydrogen into the atmosphere could provoke its reaction with tropospheric hydroxyl radicals, with the subsequent inefficiency in the oxidation of anthropogenic hydrocarbons like methane. In this regard, depending on the control of the permeability and efficiency process, where hydrogen is used to obtain energy, these technologies can result in a positive or negative LCA compared to traditional ones [85].

In this sense, the role of catalysts in LCA is relevant (due to their production and performance), which should be considered in every LCA applied to biogas reforming from different points of view, as explained in the following section in more detail.

3. Influence of Catalysts on Biogas Reforming in General and LCA in Particular

The role of catalysts in many processes (at laboratory or industrial scales) is essential, as it allows for the completion of a certain chemical reaction at milder reaction conditions, with the subsequent benefits related to energy or time savings, among others. In any case, as previous studies about different processes have pointed out, the role of catalysts in LCA is considerable, and they are used as different inventory items such as in catalyst preparation and reuse, where the main components to produce a catalyst are included, as observed in the case of biodiesel production from Prunus Armeniaca seeds, where an SrO-La₂O₃ catalyst was used [86], or as observed via comparison between a metal and a biochar-based catalyst for biomass gasification, where there were clear and positive differences for the latter, with a 93% decrease in GHG emissions, requiring around 96% less energy [87].

Concerning biogas reforming, there are works where the direct and specific impact of catalysts is clearly explained, showing a positive impact during catalytic performance for hydrogen production from methane through steam reforming, for instance. Thus, Figure 8 shows the influence of catalysts on LCA steam reforming.

In general, there is a pattern that is relatively shared by many processes that can be perfectly applied to biogas reforming, like the following:

• Increase in yield/performance: In general, hydrogen production is the aim of biogas reforming. Thus, an increase in the yield in hydrogen production and a high percentage of methane conversion is a key point to avoid the presence of byproducts in the final gas. Therefore, to obtain high-purity hydrogen, apart from the fact that further technology (like membrane reactors) is required, the absence of by-products or non-reacted methane is desirable to reduce the environmental impact or their further management.
• Use of milder reaction conditions to avoid the continuous wear and tear in equipment due to extreme operating conditions such as high temperature and pressure: It is well known that the use of catalysts in reforming can considerably reduce the operating temperature and pressure. Thus, even a decrease of 100 °C or 1–2 bar can result in a significant extension of the useful life of reforming facilities, with a subsequent maintenance of efficiency to keep specific operating conditions. This fact is not only interesting from an LCA point of view, on account of a lower contribution to environmental impacts due to the fabrication of different components. Also, higher efficiency to keep operating conditions implies lower global energy consumption to produce a certain FU in biogas reforming.
• Higher selectivity for hydrogen production: As in the previous case, production of the desired products is a required situation, avoiding the generation of by-products that should be purified with the subsequent addition of further steps that can present their own environmental impacts (related to production, performance, and management after use, for instance).
• Ability to adapt other processes for waste/bioproduct management: This is a key point on account of the fact that it could allow for the valorization of undesired compounds, with subsequent environmental impact reduction. For instance, if syngas is obtained, catalysts used for the Fischer–Tropsch process (based on transition metals like iron, cobalt, or nickel) could be useful to adapt this technology to CO valorization with subsequent environmental benefits [88].

In a sense, these advantages normally imply a positive LCA answer, as lower amounts of wastes that are difficult to manage are obtained, increasing efficiency and reducing energy costs (and subsequent CO₂ emissions related to energy consumption).

However, the requirement of catalyst production or purification techniques contributes to the complexity of LCA, and this does not necessarily imply that the use of catalysts reduces the environmental impact of the process. There are catalysts based on different active phases (mainly metals such as Ni, Co, Cu, La, etc.), with many of them obtained through calcination from their corresponding salts, like nitrates, which could result in a negative environmental impact from their production due to the release of oxides into the environment. Also, catalyst supports could play an important role (even their shape, porosity, etc.), which could vary the environmental impact per production unit, with surface area being one of the most determining factors to improve catalytic performance [89,90]. Regarding one of the most recurring catalysts—that is, Ni-based ones—their synthesis processes have been thoroughly covered in previous studies, including impregnation, precipitation, or co-precipitation, among others (see

Table 4. Catalyst production and main aspects to be considered in LCA.

Method	Details	Ref.
Impregnation	Low cost and low waste formation. However, a calcination process is required where NOx emissions can be generated.	[92,93,94]
Precipitation	Good particle control, improving the catalytic performance and durability of the catalyst.	[92,95]
Hydrothermal or solvothermal synthesis	In this case, temperature and pressure conditions are controlled, using different solvents such as water or organic compounds.	[92,94]

). In this context, there are different points to consider, like the high environmental impact associated with energy consumption or the possible use of palladium (II) chloride to synthesize Ni-Pd/Al₂O₃ catalysts, which is the most dominant environmental impact in this case [91]. Consequently, the typical beneficial effect of bimetallic catalysts should be considered along with these eventualities during their synthesis.

Equally, there are other novel processes such as microemulsion (where oil–water routes are mainly followed) with promising results, but these are only applied to tri-reforming and dry-reforming, requiring further research. In any case, most of the abovementioned synthesis methods have some relevant and common points regarding LCA.

• Depending on the kind of preparation technique, dispersion of the active phase is important to determine the resistance of the catalyst to deactivation processes (explained in later sections) and its effectiveness during reforming, with a subsequent influence on the release of non-desired products into the environment.
• A possible combination of the original active phase with promoters to improve some characteristics of the final catalyst, especially by reinforcing interactions that make the effect of sintering or coke deposition less effective.
• The use of calcination processes at high temperatures (from 500 to 800 °C, normally), with subsequent energy consumption for a variable range of time (between 2 and 3 to up to 5 and 6, in most cases).
• The products obtained in these processes (normally metal oxides) involve the generation of some problematic compounds that could be emitted into the atmosphere, such as NO_x, with a corresponding negative environmental impact.
• Finally, many of these catalysts require an activation process, usually through heterogeneous redox reactions with gas, like H₂, at high temperatures (500–700 °C).

In this way, these factors should be equally considered in LCA, not only due to the direct environmental impact of the production process but also on account of the characteristics of the methods selected, which will determine the quality of the catalysts, especially regarding particle dispersion throughout the support. As explained in the following sections, these factors have a strong influence on catalyst durability and reusability, which, in turn, is highly related to positive LCA when the catalyst can be reused during longer periods of time.

In this sense, the use of nickel-based catalysts is common for different reforming processes, mainly due to their low price, in searching for alternatives such as the use of noble metals such as Rh, Pt, Pd, or Ru to improve some properties of synthetic catalysts, such as resistance to sintering or coke deposition [94,96].

Another factor to be considered is the possibility of reactivation or reutilization of catalysts, or their management after use. In this sense, it should be noted that these steps would be another aspect to be considered in LCA, assessing their suitability according to the energy required and emissions generated. Along with the durability of the resulting catalyst, these properties could improve LCA by reducing the frequency of catalyst production per FU and the possible negative effects predicted, especially concerning energy consumption and evolved pollutants.

In the case of the synthesis of catalysts, the role of the support is essential (see

Table 5. Different catalyst supports used in biogas reforming [95,97,98,99].

Support	Details
Al2O3	One of the most recurring supports for biogas reforming, due to its price and thermal and mechanical stability.
CeO2	Ceria can also be used as active phase, improving the activity of the catalyst and improving deactivation resistance, especially concerning coke deposition and sintering.
MgO	Highly available, but less stable than alumina and silica. Sensitive to sintering.
SiO2	Often used due to its availability and thermal stability. Presents good pore sizes, although it is prone to carbon deposition.
ZrO2	It has weak interaction with Ni, with strong deactivation due to large active phase particles, among other factors.

for some examples and details), whose interaction with the active phase and the acidic/basic nature will be important to understand the durability of a specific catalyst. Moreover, the scarcity of some supports, apart from the different production system (including the shape of the final catalyst as spherical or ring-like, which will determine the bulk density of the final catalysts) are factors that should be considered in LCA.

As observed, the most popular support is alumina, due to its good properties and availability. Along with SiO₂, it is one of the most used supports for this purpose, as observed in the literature [89,97]. The role of the support in the final heterogeneous catalyst is essential, as it contributes to its suitable catalytic performance. Thus, porosity should be as high as possible to provide good interaction between the solid and gaseous phase in reforming reactions, whereas its strong interaction with the active phase is vital to avoid deactivation processes such as coke deposition, poisoning, or sintering [92,95]. In this sense, it is clear that catalytic support, which usually represents a high percentage (in weight) of the final catalyst, is an important component in the catalytic reforming of biogas.

However, the environmental impact for the extraction/production of raw materials to synthesize these supports should be taken into account in LCA. For instance, in the case of alumina, there are studies about LCA applied to its production through several methods (like the Bayer process and others, such as smelting reduction, reduction roasting, sub-molten salt, or acid leaching, among others), resulting in the finding that obtaining the red mud generated in the alumina industry process is a real challenge, requiring environmental processes and changes in the existing technical framework (including low consumption) [100,101].

Regarding the use of bi or trimetallic catalysts, along with the addition of promoters,

Table 6. Different active phases or promoters for biogas reforming [90,98,102].

Active Phase	Details	CH4 Conversion *
Co	It has been widely used, solely or combined with Ni; low quantities increase catalytic activity and coke resistance, whereas excess Co could present negative effects.	>75%
Cu	Especially in dry reforming, it can promote the coke resistance of Ni-based catalysts.	80–95%
Fe	In the dry reforming of biogas, it is a useful compound combined with Ni, improving the carbon resistance of the resulting catalyst.	>90%
La	Combined with Ni, it offers good results, as higher dispersion during catalyst preparation is achieved.	80–90%
Ni	It is the most used active phase due to its availability and price.	60–78%
Pt	Combined with Ni, inhibits carbon deposition and sintering, promoting the reducibility of Ni.	>85%
Rh	It usually promotes a better catalytic performance when combined with Ni.	>70%
Ru	Its addition to Ni-based catalysts, usually improves methane conversion.	>90%

* These results are conversion ranges, for mono-metallic catalysts in the case of Ni and combinations with Ni in the rest of the cases.

shows the main active phases for biogas reforming. In this sense, the use of these combinations could be interesting for obtaining high-durability catalysts, reducing the effect of different adverse effects such as coke deposition, sintering, or poisoning.

It should be noted that the catalytic performance included in Table 6 is in general terms, as different factors such as temperature, pressure, S/C ratio, or catalyst support were not homogeneous in the studied catalysts covered in this work. Nevertheless, it gives an idea about the positive effect related to the combination of different active phases in order to improve methane conversion. Thus, Ni-based catalysts normally offer better catalytic performance when combined with other promoters when similar operating conditions are selected when it comes to methane conversion. Apart from that, other catalytic performance parameters, like durability, were variable depending on operating conditions and criteria in order to consider an acceptable conversion during several working hours. Nevertheless, in general, Ni-based catalysts offered shorter durability values compared to the combined use with Co, Pt, or Ru [102], with low deactivation percentages at around 10–100 h (from 0 to 5%) [97], due to the abovementioned coke, sintering, and poisoning resistance [90].

In this way, the use of nickel along with different promoters or metallic active phases present a positive effect on the catalytic performance of the corresponding reforming process, whereas their extraction methods (which can be difficult depending on the nature of the element, as in the case of rare-earth production) include processes such as mining, grinding, flotation, solvent extraction, crushing, magnetic separation, precipitation, or electrolysis (among others) [103]. Consequently, these processes have an impact in land use or pollution, as well as a corresponding impact due to the consumption of multiple resources, like large quantities of chemicals [104], depending on the kind of metal or mineral that is extracted [105]. In the case of nickel products, the most relevant stage (demanding considerable energy) is primary extraction and refining, with the highest global warming potential [106].

On the other hand, the use of catalysts has a positive effect in the reforming process. Specifically, many components of reforming facilities can extend their service life thanks to the use of milder reaction conditions. For instance, the use of purification techniques like the use of membrane reactors or pressure swing adsorption could present different impacts, as the former use specific materials for membrane preparation (such as gold), and the second could contribute to higher energy consumption during the process. Thus, the role of catalysts, fostering the useful life of membranes or reducing the cycles to separate product gas, could be positive in this sense [107,108].

In other words, the positive effect of catalysts in a certain process (with the subsequent positive effect on the LCA of this specific process) should be differentiated from its contribution to LCA, where different factors such as catalyst preparation, durability, reusability, and management after use should be taken into account. Therefore, for the implementation of a sustainable process like biogas reforming, concerning the use of catalysts, the industrial design should strike a balance between the positive contribution of the use of catalysts to the process and the environmental impact of catalyst production, use, and management after use, considering the reusability of catalysts as much as possible, along with catalyst regeneration for further processes.

The main challenges of catalytic performance during biogas reforming, according to typical catalysts such as nickel-based ones, have a strong connection to LCA, as these factors could contribute to lowering a positive assessment of the global process. In this way, their relationships with LCA are the following [68,109]:

• Sintering: Due to the use of high temperatures (which equally implies higher energy consumption, with a subsequent negative effect in LCA), the active phase of the catalysts that is homogeneously distributed on a support’s surface can be reunited in bigger clusters or particles, with the subsequent blockage of pores or a decrease in the active surface for biogas reforming. This effect is considerable when the interactions between the active phase and the support are weak. As a consequence, the useful life of the catalyst might be considerably reduced, increasing the effects related to their production per unit of mass on LCA [110].
• Poisoning: The presence of H₂S, apart from obvious health effects and corrosion in industrial facilities (the latter affecting LCA), can provoke the deactivation of the catalyst through poisoning, with a subsequent inefficiency of the catalyst and its continuous replacement, which would multiply the negative impact on LCA per unit of amount of catalyst [111]. Thus, the direct interaction of sulfur compounds with these active sites make them unavailable for the different reaction steps taking place during methane reforming.
• Coke deposition: The continuous conversion of methane results in adsorption and desorption on the catalytic surface, which implies the accumulation of coke (as explained in Equations 7, 10, and 11) that can block active sites and, subsequently, many pores on the catalytic surface [112]. Therefore, the activity of the catalyst gradually decreases over time, with the same effects seen in the case of sintering and poisoning.

These factors can be observed in Figure 9.

As observed in Figure 9a for the case of sintering (at high temperatures), active sites interact with each other to obtain bigger particles that can block pores with available active sites, and these bigger particles present a lower surface area compared to the previous state, with a subsequent decrease in available active sites. Regarding coke deposition (Figure 9b), carbon on active sites block them, as well as many pores with available active sites. Finally, in Figure 9c, the blockage of active sites and pores can be noted through the permanent interaction of sulfur with the active phase. These events usually lead to the same consequences, lower catalytic efficiency and durability, increasing the environmental impact in two main aspects: the release of unreacted methane and further environmental impact from catalyst production and management after its use.

In this sense, a key point for the suitability, according to LCA, of a certain catalyst for this process is its durability and reusability, which could considerably decrease the environmental impact (per unit of production) of its production, exploitation, and management after its use. Another interesting factor could be the addition of the active phase, which should be reduced as much as possible in the final catalyst. Nevertheless, some measures can be taken to increase the useful life of a catalyst, like the following:

• Pre-treatments to remove H₂S from the original biogas (through adsorption, absorption, etc.) [111,113]: As previously explained, the AD process to produce biogas might offer an interesting solution to this challenge, as solid digestate can generate adsorbents (that is, active carbons through pyrolysis or HTC) that can act as traps in H₂S capture units. In this sense, a double solution is offered: first, a waste with difficult environmental management is valorized; second, this valuable product is used in the same system to avoid poor catalytic performance, improving different LCA parameters. In addition, different H₂S valorization technologies could be used in this context, such as thermocatalytic decomposition or H₂S methane reformation, which could solve the abovementioned problem [114].
• Use of modifiers or promoters to improve the performance and durability of traditional catalysts (for instance, La or Mg addition), as previously commented on [89,110,115].
• Selection of optimum conditions: When possible, the use of low temperatures to avoid or slow down sintering (attempting to select temperatures below Tammann and Hüttig temperatures, that is, ½ or 1/3 of the melting point of the metal taking part in the active phase, when the process makes it possible), as well as a good S/C ratio in the case of steam reforming of biogas (normally above 2.5–3.0) to reduce the negative effect of coke deposition on efficiency decrease.
• The implementation of purification techniques that allow for an increase in hydrogen production yield, such as the use of membrane reactors, that would allow for a decrease in temperature (avoiding some problems such as sintering) [109,116]: Also, these purification techniques usually allow for the capture of problematic pollutants such as CO₂, offering a double positive effect in LCA. In this sense, the use of different compounds such as Mg-based or Ca-based sorbents could be an alternative [117].
• The possibility of the regeneration of a spent catalyst: Depending on the deactivation processes previously commented and observed in Figure 9, the possibility of regenerating catalysts, once they are deactivated, will depend on the reversible nature of the deactivation process. Catalysts affected by sintering processes seem to be difficult to regenerate, whereas regeneration due to coke deposition seems feasible, especially using heat treatments and oxidation with air, CO₂, O₂, or steam, among others [81]. Again, it is a matter of assessing if this regeneration process, where energy costs and gaseous compounds are required, offset the removal and post-treatment of spent catalysts.

In summary, factors such as durability, reusability, or efficiency would be improved in the case of catalytic performance. However, the contribution of these upgrades to LCA should also be considered, only accepting those steps that clearly improve the overall assessment.

In this sense, the optimization of catalyst production and performance is essential to contribute to the abovementioned conditions for a positive LCA. In this regard, and considering the previous reasoning, the following points should be considered:

• The right selection of the support, allowing for a high surface area once the final catalyst is generated, along with other properties such as high thermal and mechanical resistance, strong interaction between the support and the active phase, etc.
• The optimum combination, quantity, and proportion of metallic active phases in order to boost the efficiency and durability of the process, avoiding the release of unreacted products. Also, the lowest amount of active phase, keeping its activity as much as possible, is desirable to reduce the impact during its production.
• Also, the optimization of the process according to a specific catalyst is essential, attempting to reduce energy costs but, in turn, enhancing the activity and durability of the synthetic catalyst.

For this reason, multiple optimization studies have been carried out, considering different catalyst supports [89,99,112], bi or trimetallic catalysts [90,98], or variable operating conditions [13,69,71].

In this section, the main aspects related to catalytic biogas reforming have been covered, but it should be noted that there are other steps included in the abovementioned boundaries included in Figure 4 that can also contribute to the improvement of global LCA related to biogas reforming.

In this way, some transition metals such as cobalt, iron, nickel, and ruthenium could be interesting alternatives for a Fischer–Tropsch synthesis if syngas obtained in biogas reforming is not separated to obtain pure hydrogen [118]. Thus, different fractions of biofuels (like Diesel Fraction) could be obtained [119], enhancing the syngas obtained during biogas reforming and, subsequently, improving the LCA of the whole process. That is the reason as to why the interconnection of different technologies is important for obtaining synergistic and positive effects in LCA, as well as a global point of view to assess the real environmental impact of a certain technology. Otherwise, a certain process could present negative LCA by observing a very limiting boundary, preventing the use of a sustainable and feasible alternative by combining pre-existing technologies, some of them with relative technological maturity.

4. Specific Studies

As previously explained, LCA covers a wide range of aspects related to a specific process or product, which can be unmanageable depending on the circumstances. Thus, it is difficult to find a complete LCA work that covers every aspect of an industry, and it is usual to carry out some estimations to simplify these kinds of works.

On the other hand, an interesting approach could be to focus on a certain part of a process, which could be considered the most controversial one from an environmental point of view, in order to establish certain measures to improve the environmental impact of the process as a whole.

Also, there are equivalent studies applied to similar products like natural gas, which can give an idea about some comparisons about different reforming processes under different operating conditions. This way, the research works found in the literature about LCA applied to biogas reforming are scarce and too general or specific, presenting certain particularities due to the following reasons:

• Some works are based on a certain biogas reforming process, with their specific configurations that can be useful for comparison with other similar processes but require further recalculation, as there are many different techniques related to biogas upgrading or reforming.
• They are focused on specific stages within biogas reforming, as in the case of CO₂ management or hydrogen purification processes.
• Several approaches are carried out, simplifying the process by establishing limited boundaries and therefore omitting some aspects that can be considered to have a negligible effect on LCA.
• Some estimations are usually calculated, with subsequent differences between LCA studies applied to biogas.
• LCA can be positive or not depending on several factors, like the kind of energy source (renewable, non-renewable) and its impact on carbon print, the nature of biogas (purity of CH₄, among others), etc.

Specifically,

Table 7. Summary of research works considering LCA related to different aspects applied to biogas or methane reforming.

Process	Details	Ref.
Methane decomposition to produce hydrogen	The use of carbonaceous catalysts presented an interesting environmental performance, leading to a virtually zero-emission process.	[120]
Synthesis of Ni-Pd/Al2O3 catalyst	This work is focused on different production techniques and their corresponding environmental or economic impact.	[91]
Hydrogen production by using a biogas membrane reformer	LCA and economic analysis were carried out, proving that the BIONICO system showed a high biogas conversion, being an alternative for natural gas.	[121]
Comparison of different processes, including methane steam reforming	LCA of different production methods were compared, with methane steam reforming (which could present similarities to biogas steam reforming) offering outstanding LCA performance.	[54]
Comparison of different hydrogen production, from traditional and cleaner resources	LCA of several processes was carried out, including biogas reforming, finding that reforming processes had high hydrogen yields and pointing out the requirements of catalysts, such as coke resistance. Also, if CO2 is not suitably managed, it presents a high GWP.	[122]
Studies about the comparison of natural gas hydrogen production technologies	Methane autothermal reforming offered the lowest life-cycle GHG emissions compared to methane steam reforming.	[52]
LCA of hydrogen production from natural gas comparing emerging technologies	A comparison of methane steam reforming was carried out with and without carbon capture, resulting in considerable amounts of CO2 captured (13.2 per ton H2). On the other hand, more energy was required, not recommending carbon capture, when techno-economic studies were carried out.	[123]

shows the most significative research works about LCA applied to biogas or methane reforming.

It should be noted that these works, apart from the specific role of catalysts on LCA (which is not widely covered currently), are usually focused on other aspects such as carbon dioxide management, energy consumption, or air pollution, among others. Thus, there are works that are specifically focused on the synthesis of different catalysts, omitting the rest of the aspects of industrial processes, whereas there are other works that also include a more comprehensive overview of methane reforming. Again, LCA shows its versatility, with the possibility for comparison of different catalysts according to their environmental performance in the process or depending on their production (both together and separately). Obviously, LCA applied to biogas reforming (including the role of catalysts, which is determining), is a very promising field, where there is a gap that should be accomplished through the contribution of further research works based on global and specific LCA. In this sense, LCA with broad boundaries are required, along with other more specific studies covering biogas reforming in specific boundaries, in order to compare different technologies or catalysts from an environmental point of view.

5. Conclusions

The main conclusions of this review work are the following:

• It has been proven that LCA is a multidisciplinary tool that can be easily adapted to every technology, which plays a vital role in the assessment of sustainable processes like biogas reforming.
• However, there is a wide range of processes involved in biogas or methane reforming that might be different depending on the context, like the use of different sources to obtain biogas or the possibility of hydrogen purification or in combination with other processes. Also, the role of catalysts is important, as observed throughout this review. In this sense, the works included in this review are heterogeneous and focused on global or specific aspects of a certain technology, which makes the research about this subject a promising research field in the near future. Consequently, further research is required to homogenize current results about LCA applied to this field.
• Also, LCA might vary in general factors depending on the budget allocation for a certain technology. For instance, depending on the kind of power supply (especially from renewable energies such as solar or wind energy), CO₂ emissions might considerably be decreased, with a subsequent energy independence that might be especially interesting from an economic standpoint.
• Nonetheless, we face a rapidly changing industry, where the introduction of innovative technologies provokes the continuous assessment of its environmental impact, among other factors. In this sense, the use of catalysts could result in a positive impact on LCA in terms of higher selectivity for desired products or a reduction in undesired products (or their suitable isolation and further management). On the other hand, their reusability, along with their impact during extraction, production, or exploitation, might present challenges that could jeopardize their suitability for sustainable processes like steam reforming.
• An important aspect to be taken into account in LCA in general, and when applied to biogas reforming in particular, is the right selection of certain technologies, especially concerning the reforming process. For instance, the use of steam or dry reforming presents different challenges that can affect LCA, mainly due to the use of steam (or not) during the process, with subsequent changes in global energy demand or the durability of the catalyst used.
• Nevertheless, there are some points in common, regardless of the kind of technology used in biogas reforming, like poisoning due to H₂S and subsequent catalyst deactivation, that should be faced together with the corresponding LCA of the selected capture technology (normally adsorption). Specifically, the use of active carbons produced from sewage during AD is an interesting field for LCA, as different technologies such as pyrolysis or HTC could be assessed according to their environmental impact, with a favorable selection of HTC, as it can be easily adapted to the nature of the abovementioned waste (with high moisture).
• Another interesting point is the implementation of laboratory-scale processes to industrial-scale ones. Specifically, the role of the efficiency of the process, along with the adjustment of these technologies to current environmental regulations, are highly related, only enabling the implementation of innovative technologies at an industrial scale if strict conditions related to the abovementioned factors are fulfilled.
• Thus, one interesting conclusion was that, both in general and when applied to biogas reforming, the implementation of new technologies normally results in more complex LCA, with possibly new challenges derived from these new technologies applied. In this sense, a suitable choice of solutions is essential to avoid shifting the environmental problem. That is the reason as to why some steps related to the valorization of wastes generated during biogas production could be an interesting alternative, as in the case of active carbon production from digestate that can be used to adsorb H₂S, with a subsequent improvement in catalytic performance during biogas reforming. Even for this choice, there are alternatives such as pyrolysis or HTC, with the latter being more appropriate due to the high moisture content of this waste.
• There are various challenges related to this process and a positive life cycle assessment, like hydrogen purification, where the efficiency of hydrogen separation, along with the production of innovative materials, is used to carry out membrane separation. Also, hydrogen leaks in reforming systems, along with efficient fuel cells, should be assessed to avoid a negative LCA. Another crucial factor would be CO₂ management, whose separation or capture is equally necessary. Again, the role of catalysts could mitigate negative impacts on LCA by improving the selectivity for interesting products, and these technologies could reduce the production or proportion of catalysts, even improving their durability or reusability.
• However, some factors such as durability or reusability would determine the suitability of catalytic biogas reforming from an LCA-based point of view. In this way, there are technologies that could contribute to a better performance of catalysts, with subsequent improvement in LCA. In any case, it should be noted that the environmental problem can be moved, as some of these processes cannot be acceptable in LCA. Therefore, the use of technologies that improve LCA applied to biogas reforming in general and its catalytic performance in particular should be considered.
• In this sense, LCA allows for the comparison of different processes involved in biogas reforming, including changes like the use of different catalysts, and it should be an interesting tool to assess the best choice for every technological solution and situation.