State and Perspectives of Biomethane Production and Use—A Systematic Review

Abstract

In the face of increasingly frequent natural disasters resulting from climate change and disruptions in the supply chains of energy resources, the demand for energy carriers based on locally sourced renewable resources is growing. Biomethane, derived from biomass and having multiple uses in the energy sector, fully meets these conditions. Analyses of the development and spatial distribution of biomethane production plants, the prevalence of methods of its production, and directions of applications, made on the basis of the data gained from official databases and research papers, are the main subjects of the paper. Additionally, the advantages and disadvantages of biomethane production, taking into account the results of the life cycle assessments, and the prospects for development of the biomethane market, facing regulatory and policy challenges, are considered. The results of the review indicate that biomethane production is currently concentrated in Europe and North America, which together generate over 80% of the globally produced biomethane. An exponential growth of the number of biomethane plants and their production capacities has been observed over the last decade. Assuming that the global strategies currently adopted and the resulting regional and national regulations on environmental and socio-economic policies are maintained, the further intensive development of the biomethane market will be expected in the near future.

1. Introduction

Environmental care and the need to ensure the safe functioning of societies, especially in the face of armed conflicts and an uncertain geopolitical situation, make energy transformation and the diversification of energy sources a priority today. The main way to achieve these goals is to replace fossil energy sources with renewable sources, including widely available biomass. The current level of biomass processing technology creates wide possibilities for its energy use, both in stationary installations and in engines powering means of transport [1,2]. A particularly beneficial biomass processing product, from a practical point of view, is gaseous fuel in the form of biomethane. The prefix “bio” indicates that this gas, unlike “petromethane” accompanying crude oil and produced in thermochemical processes, is produced from biomass decomposition.

The term “biomethane” appeared in the scientific literature in the second half of the 1940s [3,4] in reference to the main component of the gas mixture generated as a result of the methane (CH₄) fermentation process of organic matter, although the anaerobic digestion process itself, in which methane is produced, was already well understood at that time. Sir Humphry Davy found that methane is a component of the gases produced during the anaerobic digestion of cattle manure already in 1808, and the first digestion plant worked in a leper colony in Bombay (India) in 1859. The first papers in which the term “biomethane” was used concerned mainly the radioactivity of methane coming from biological and nonbiological sources [3,4,5,6].

According to results from the review of scientometric databases, data of which are shown in Figure 1a, the term “biomethane” began to be visible in scientific literature starting in 2010. Previously, it appeared rarely. In the following years, the number of publications containing this term grew gradually until 2022. The greatest increase in interest in the issue of biomethane took place in the second decade of the 21st century. This issue is still at the center of research interests, although in 2024 the increase in the number of papers was not significant compared to 2022, when the highest number was noted. The average number of papers published per year in the period 2021–2024 is 2–3 times higher (depending on the database) than in the period 2011–2020.

In the 1980s, the term “biomethane” began to take on additional meaning, which was involved with the implementation of solutions enabling new applications of biogas, such as incorporating into the natural gas grid or using in transport. In this sense, biomethane is a gaseous product of biogas upgrading [7] or produced by thermochemical transformation, consisting mainly of methane. According to the definition of the US Department of Energy, biomethane is “a pipeline-quality gas that is fully interchangeable with conventional natural gas and thus can be used in natural gas vehicles”. It is a renewable natural gas (RNG), which has been properly prepared to meet high purity standards [8]. Biomethane typically contains 95–99% methane and 1–6% carbon dioxide (CO₂) [9] and does not differ significantly from natural gas in terms of calorific value CV (the value of this parameter is 33 MJ/m³ and 34–35 MJ/m³ for CH₄ and natural gas, respectively) or other properties, so it can be injected without any transformations to the transmission and distribution infrastructure or end-user equipment [10]. This is the sense in which the term biomethane is used in this paper.

The use of biogas by processing it into a more calorific product has been practiced for about 30 years. In a 1998 report on clean energy sources in transport, Mansson mentioned biomethane produced from biogas as a source of energy in transport, gave the conditions for its use as a transport fuel, and cited the results of tests with biogas-powered buses conducted in Sweden [11]. However, biomethane is still a negligible source of energy on a world scale. Global primary energy consumption in 2023 was estimated at ca. 165,000 TWh, in which natural gas provides over 40,100 TWh [12], i.e., ca. 24%. At that time, the estimated global biomethane production was about 100 TWh (estimated on the basis of CEDIGAZ [13] data on biomethane production in 2023, which was 9.5 bcm). Thus, biomethane represented only 0.25% of global gas demand in 2023.

Scientists’ interest in the production of high-calorie fuel from biogas has been growing since 2010. In the earlier period it was negligible. This is evidenced by the ever-increasing number of publications found in various scientometric databases, containing the terms “biomethane AND upgrading” (Figure 1b). Similarly to the number of papers noted in searching the term “biomethane”, the average number of papers published in the period 2021–2024 is 2–3 times higher (depending on the database) than in the period 2011–2020.

Papers on biomethane published in recent years concern advanced pretreatment technologies aimed at increasing gas production [14,15,16,17,18,19], the development of novel adsorbents and hybrid materials for biogas upgrading [20,21,22], and innovative methods based on injections of hydrogen [23,24,25] or carbon dioxide [26,27] in in situ biomethanation processes, which leads to methane production enhancement [28,29,30]. Details on current research directions in the field of individual biomethane production methods are discussed in Section 3.

Nowadays, many methods of biomethane production are used, and their characteristics are the subject of numerous review papers that have been published over the last dozen or so years [31,32,33,34,35,36,37,38,39,40,41,42,43,44]. Most of the review papers focus on discussing various biogas upgrading methods, but there are also those that take into account selected methods, e.g., membrane [45], water scrubbing [46], pressure swing adsorption [47], chemical absorption [48,49], biological methods [50,51,52], methods using adsorbents [53], or focus on thermochemical methods of biomethane production [54,55]. In some of the papers, the review on the directions of biomethane applications and the quality conditions that it must meet in order to be approved for a specific application was conducted [34,37,56,57,58].

Many authors of papers on biomethane refer in the introduction to the state of dissemination of this biofuel on the energy market. But particular considerations on time-dependent changes in its production and methods of use are few and concern mainly European Union countries [59,60]. A broader perspective was demonstrated by Schmid et al. [61], who presented selected national markets in Europe (Germany, the United Kingdom, and Sweden), Asia (China, Japan, and South Korea), and North and South America (Canada, the United States of America, and Brazil). However, most of the data presented in this paper concern the years before 2018. According to the authors’ best knowledge, there are no papers synthesizing the results of studies aimed at assessing the biomethane production profitability and its impact on the environment.

Due to the numerous review papers published in the scientific literature on biomethane production methods, which were previously mentioned [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61], the authors of this paper did not focus on detailed reviews of the state-of-the-art of biogas cleaning and upgrading technologies and thermochemical methods. They are only presented briefly to provide the paper with a logical order. The aim of the paper is to assess the current state of dissemination and spatial distribution of biomethane production in the world and to present directions of its applications. In addition, the scope of the paper includes an analysis of the advantages and disadvantages of implementing biomethane on the energy market, based on the data available in the literature from real facilities producing biomethane and on the results of the life cycle analysis of biomethane production and use installations.

2. Materials and Methods

In order to find information that was used to prepare this paper (1) scientific peer-reviewed articles indexed in the scientometric databases Scopus, Web of Science, Science Direct, SpringerLink and Google Scholar; (2) official and reliable global and European databases on the production and use of biomethane; and (3) other relevant documents and commonly recognized sources of information made available online by the European Biogas Association (EBA, an international network of national organizations, scientific institutes and companies, committed to the promotion of the deployment of biogas and biomethane production and use [7]), the International Energy Agency (IEA, an international organization affiliated with the Organization for Economic Co-operation and Development) [62], The International Information Center on Natural Gas (CEDIGAZ, the data source referenced by the international organizations, e.g., IEA) [10], The European Network of Transmission System Operators for Gas (ENTSOG, an association responsible for a number of regulatory tasks in Europe) [63], and (4) websites of national government institutions (e.g., US Department of Energy) were searched.

While searching scientometric databases, “biomethane” (or “biogas” in the case of Section 3) was entered as the main keyword. Next, depending on the scope of the sought information, the following detailed secondary terms mentioned below were added (by using the conjunction “and”): “biological methods”, “anaerobic digestion”, “thermochemical methods”, gasification”, “upgrading”, and “H₂S removal (Section 3.1 and Section 3.2); “worldwide production”, “European production”, and “facilities number” (Section 3.3); “biofuel”, “transport fuel”, “gas grid”, “gas pipeline”, “quality standards”, “quality requirements”, “steam reforming”, “Fischer-Tropsch processes”, and “fuel cell” (Section 4); “market development”, production perspectives” (Section 5); carbon footprint”, “life cycle assessment”, “advantages”, and “disadvantaged” (Section 6). Additionally, in order to better understand the concepts and mechanisms of the processes of biomethane production and application, the databases were searched for the following single terms: “chemical methanation”, “biological methanation”, “hydrogenotrophic methanogens”, “acetogenic gas fermentation”, “microbial electrochemical cells”, “microbial electrolysis cells”, and “methanotrophs”.

When preparing the introduction, the search in databases was narrowed down to review articles, adding the term “review” as a detailed keyword.

In general, the focus was on the literature from the last 10 years, but in justified situations, e.g., when it was necessary to obtain an answer to the question about the evolution of research on “biomethane” and the development of biomethane production technology, earlier sources were used, the oldest of which dates back to 1947. Bibliometric data of the papers containing the specified terms mentioned in the Introduction were gathered in January 2025.

3. Biomethane Production Methods and Their Practical Application

The raw material for biomethane production is biomass of various characteristics and origins, including organic waste and by-products, such as animal manure, energy crops, agricultural residues, food waste, sewage sludge, forest residues, and industrial organic residues [64]. However, the way of producing biomethane may vary significantly. It can be based on biological or thermochemical transformations, as a result of which biomass (dry or with various degrees of hydration) is converted into gaseous products, which in turn are further processed to obtain a product consisting mainly of methane. A schematic overview of biomethane production methods is presented in Figure 2.

3.1. Methods of Biomethane Production from Biogas

Biomethane production, which is based on biological transformations, begins with the process of methane fermentation. This process, conducted under strictly anaerobic conditions by the complex consortia of microorganisms, leads to the conversion of biodegradable organic matter contained in biomass into gaseous products, called biogas. The post-fermentation mixture—digestate, consisting of the compounds dissolved in water and solid residues of undecomposed biomass—is the second product of the process. Depending on the type of technology, biomass with various water content can be used in the process. The biomass used in wet anaerobic digestion should have less than 15% of total solids. In contrast to this type of process, operation with biomass with total solids content higher than 15% is classified as dry anaerobic digestion [65]. Anaerobic digestion is usually conducted under mesophilic (20–43 °C, optimal at 35–37 °C) or thermophilic (50–60 °C, optimal at 55 °C) conditions, but it also takes place at lower temperatures (<20 °C), under psychrophilic conditions [66].

Biogas, being a product of the activity of a complex consortium of microorganisms carrying out the processes of hydrolysis, acidogenesis, acetogenesis, and fermentation [64], is a mixture of many components. It typically consists of methane (40–75%), carbon dioxide (25–50%), nitrogen (2–8%), and many components on a trace level, such as hydrogen sulfide (H₂S), ammonia (NH₃), hydrogen (H₂), carbon monoxide, and various volatile organic compounds, e.g., hydrocarbons or siloxanes [67,68]. As a result of the “biogas cleaning” and “biogas upgrading”, this mixture is transformed into biomethane. Biogas cleaning aimed at removing impurities that are undesirable due to the threat they may cause in the natural gas grid or devices in which the gas will be burned. The biogas upgrading involves the removal of CO₂ in order to increase the calorific value and reduce the density to meet a specific Wobbe index [9]. Removing CO₂ can be obtained via its transformation or separation from the other components of biogas [69].

In the biogas purification processes, such as filtration, drying, and desulfurization, solid particles, water vapor, and hydrogen sulfide are removed. These procedures are applied as needed (depending on the concentration of pollutants, the way of using biogas, the planned method of upgrading, and non-technical conditions, e.g., economic). A simple condensation process can be used to eliminate water vapor, usually by decreasing the temperature or increasing the pressure. Methods of adsorption (e.g., with the use of silica gels, activated carbon, or molecular sieves [70]), absorption (e.g., with the use of triethylene glycol), cyclone separators, or moisture traps [32] can also be used.

The selection of the appropriate method of hydrogen sulfide removal also depends on the concentration of the contamination in the biogas and on the method of biogas use, the biogas upgrading method, financial capacity, etc. H₂S can be removed directly inside the digester by in situ H₂S precipitation using iron chloride, which results in the formation of insoluble metal sulfides [35]. However, obtaining gas that meets the strict requirements regarding H₂S content requires the use of more effective methods based on physical, chemical, or biological processes. The gas–liquid contactors (scrubbers) working as packed beds or spray towers, with water or organic solvent (e.g., a mixture of dimethyl ethers of polyethylene glycol, methyldiethanolamine (MDEA) acting as scrubbing liquids, are used in physical and chemical absorption-based methods [33,71]. The addition of substances, such as NaOH and Fe^3+/EDTA (ethylenediaminetetraacetate), to the scrubbing liquid allows the liquid-to-gas ratio due to the acceleration of chemical reactions [32]. Nanofluids are formed by the dispersion of inorganic compounds (e.g., SiO₂ and Al₂O₃) in the form of nanoparticles or graphene oxide and carbon nanotubes into organic solvents (monoethanolamine, diethanolamine, etc.) and also emerged as potential absorbents for H₂S [72]. The advantage of using organic solvents, apart from high H₂S removal efficiency, is the possibility of simultaneous removal of CO₂ [35]. H₂S can also be removed using physicochemical processes (physical adsorption and chemisorption) on solid materials. Oxides of metal (such as Al, Fe, Mn, Co, Cu, and Zn), natural or impregnated activated carbon, and zeolites are frequently used as adsorbents [71,73,74]. New types of absorbents, being the modifications of existing ones or new materials, are still being sought. Some of the recently studied include hematite (Fe₂O₃) activated with copper (II) oxide [75], synthetic zeolites, e.g., produced from bottom ashes [76], catalytic-impregnated activated carbons [35], metal–organic frameworks (MOFs) [77], e.g., MOFs functionalized with polyamines [78], biochar [79,80], steam-activated biochar [81], and carbon-encapsulated zero-valent iron nanoparticles [82].

H₂S can also be removed from biogas due to the activity of chemolithoautotrophs and anoxygenic photoautotrophs [83]. Biological desulfurization can be conducted in biofilters (e.g., with innovative packing materials such as biochar modified with magnetite or waste from cellular concrete) [84,85,86], bioscrubbers [32,35], or biotrickling filters, e.g., working under anoxic conditions and inoculated with activated sludge [87].

It is also possible to use electrochemical methods for H₂S removal from water that leaves the scrubber. Electrochemical sulfide removal in liquid medium can occur through oxidation or electroprecipitation (also called electrocoagulation). Two forms of oxidation processes can be used: direct oxidation at the anodic surface and indirect oxidation with in situ generated oxidants, such as O₂, OH⁻, and Cl₂ [88]. Borgquist et al. [89] have developed a technology based on indirect oxidation form. In their study, the electrochemically generated chlorine oxidized H₂S to solid sulfur. The process was carried out in a scrubber in which biogas contacted a chlorine-containing liquid. In technologies based on electroprecipitation, sacrificial anodes are used to supply metal ions (e.g., iron), which react with sulfide to produce insoluble metal sulfide solids [90].

3.1.1. Biogas Upgrading via CO₂ Separation

Biogas upgrading technologies use different properties of particular biogas components, e.g., solubility in water or specific solvents, differences in particle size or in boiling point. The main purpose of using these methods is CO₂ separation leading to methane concentration. At the same time, other undesirable gas components can be removed, although the effectiveness of their removal varies depending on the method. In the case of some methods, pre-cleaning biogas from trace contaminants is necessary, as their high concentrations negatively affect the effectiveness of the upgrading process and the lifespan of the installation. Depending on the type of process underlying the upgrading method, they are divided into physical, chemical, and biological ones. Technologies such as water scrubbing, organic scrubbing, amine scrubbing, PSA, and membrane separation are well established, while biological processes and cryogenic distillation and are still being developed [91]. The basics of the most commonly used and novel biogas upgrading methods and a brief report of the recent studies of them are discussed below.

Pressure swing adsorption (PSA) is a method based on physical adsorption, in which the difference in particular biogas component particle size is used. CH₄ particles, which are larger than the particles of other gases, do not retain on the sieve micropores and remain in the gas phase, unlike the gas particles with smaller diameters, e.g., CO₂, which are retained on the microsieve pores. The device used in this method works cyclically (adsorption–desorption). Adsorption is carried out at increased pressure (4–10 bars) in a tank (column) where gases contact with an adsorbent that selectively retains CO₂. When the adsorbent is saturated with CO₂, desorption is conducted by reducing the pressure, usually to vacuum [47,92]. The most popular adsorbents are carbon molecular sieves, activated carbons, zeolites, and titanosilicates. Due to the irreversible nature of H₂S adsorption on used adsorbents, it is necessary to pre-treat biogas by removing H₂S before introducing it into the absorption column in order to prevent the disturbances of this upgrading method [92]. It is also advisable to remove water vapor from the biogas before the adsorption.

PSA is considered a mature technology that is commonly used for the separation of gases. Taking into account the biogas upgrading allows us to produce biomethane with high purity at lower energy consumption compared to the other processes [47]. There are many PSA units around the world working at an industrial scale, with a wide range of capacities (from 400–15,000 m³ of biogas upgraded/day). The research conducted in recent years focuses on experimental studies in which innovative designs and adsorbents are tested. Abd et al. [93] studied the non-adiabatic and non-isothermal design of systems and unmodified biomass-based adsorbents. Canevesi et al. [94] tested zeolite 4A adsorbent instead of commercially used carbon molecular sieve in the PSA process. Taking into account the similar operational conditions as used in commercial systems, they obtained a CH₄ concentration of over 98% and biomethane recovery of ca. 90%. The results showed that the examined zeolite can be considered as an alternative to commercial absorbents. Modeling of biogas purification processes, which aims to develop the operating conditions that ensure obtaining high-quality gas [93,95], is the second important direction of PSA technology research. These studies are focused on assessing the possibilities of cost-effectively conducting the process in small installations (<100 m³/day) [96], as well as comparisons of the efficiency and energy consumption of processes carried out in different technological modifications (e.g., conventional PSA, pressure vacuum swing adsorption PVSA, and twin PSA processes) and operational conditions [97]. According to results of comparative studies of Abd et al. [98], amine-absorption technology is more favorable in terms of purity and recovery of products, but the PSA process recorded a lower energy requirement at comparable biomethane purity.

Water scrubbing, a method based on physical absorption, uses the difference in the solubility of biogas components in water. Compressed biogas (7–10 bars) is passed countercurrently to a stream of cold water. This process is usually implemented in a packed column [99]. Due to the contact of gas and water, substances with a high-water solubility coefficient, such as CO₂, H₂S, and NH₃, are dissolved, while dust and microorganisms are retained [47]. Methane, which has a very low solubility in water, is separated from other components. is usually implemented in a packed column Biogas does not require preliminary preparation for this process. Water scrubbing is one of the major technologies implemented for biogas upgrading and one of the most economical methods [100]. Although research is still ongoing to improve its efficiency, especially in the case of small-scale facilities [100]. In recent years, research has focused on the intensification of dissolution of gaseous components and increasing the efficiency of water regeneration. Technological modifications in terms of increasing the solubility of adverse compounds mainly concern the improvement of scrubber operating conditions, e.g., by changeing its dimensions, absorption pressure, and type of packing material. Innovative materials with high surface area and superior mass transfer, such as Raschig super rings made of polypropylene, resistant to aggressive environments, iron wool, calcite marble, and steel wire mesh [99,101,102] were tested as packing materials. Wantz et al. [100] proposed the use of anisotropic packing, the properties of which change with the column height, and the use of vacuum to enhance water regeneration. These operations resulted in obtaining a gas with CH₄ concentration over 97%. The studies were conducted in a full-scale prototype installation developed and implemented on a farm-scale anaerobic digester.

Organic physical scrubbing is a method based on physical absorption, in which the difference in solubility of biogas components in polar or nonpolar solvents is used. The solubility of CO₂ in the organic solvent is significantly higher than in water; thus, the scrubber requires a much lower volume flow than when a water scrubber is used [103]. The gas is compressed to 7–8 bar and cooled to about 20 °C before entering the scrubber to increase the solubility of CO₂. The scrubber is filled with a material that increases the exchange surface. Solvent regeneration is achieved by heating the liquid to about 80 °C [56,91]. Biogas does not need to be pretreated for that upgrading process, although H₂S pre-removal facilitates solvent regeneration [57]. The main commercially available biogas-upgrading processes use the specific solvents Genosorb, Purisol, Selexol, Rectisol, or Sepasolv [37].

Chemical scrubbing, a method based on chemical absorption, uses the difference in the reactivity of biogas components with amines (monoethylamine (MEA), dimethylamine (DEA), methyldiethanolamine (MDEA), diglycolamine (DGA), diisopropanolamine (DIPA), and aminomethyl propanol (AMP) constituting the solvent [104,105]. The study conducted by Morero et al. [106] showed that DGA was the most advantageous compared to the other three amines due to the highest level of biogas purification (CH₄ concentration 97.3%) and the lowest reboiler energy consumption. Additionally, DGA is selective to CO₂ absorption in the presence of H₂S [104]. The CO₂ contained in biogas passing countercurrently to the stream of a solution containing amines reacts with them, forming amine salts. Gas entering the scrubber is compressed to reach the pressure of ca. 2 bar, and temperature ranges from 20 °C to 65 °C, depending on the type of amine selected [56,91]. The saturated absorbent is heated in a regeneration tower, releasing CO₂. Preliminary removal of H₂S from biogas is crucial to prevent amine degradation [32,47,107]. Liquid amine scrubbing has been proven to provide the highest CH₄ purity of >99%, with the highest CH₄ recovery compared to other technologies [5,6,7,97]. According to a comparative study by Jabraeelzadeh et al. [105], the highest total capital costs for implementing the chemical scrubbing system were noted when MEA was used, while the lowest ones were when AMP was used. The difference was estimated to be over 40%. The use of MEA leads to the highest operating profits.

In situ desorption is a method using significantly higher water solubility of CO₂ in comparison to CH₄. Sludge with dissolved carbon dioxide is transported from the bioreactor to the desorption column, where CO₂ is removed from the sludge by use of a countercurrent stream of air or nitrogen. CO₂-free sludge returns to the bioreactor [108].

Membrane separation is a method using differences in the chemical potentials of substances on both sides of the membrane, caused by differences in pressure, concentration, or temperature. The structure of the membrane allows only some molecules to pass to the permeate side, while the other molecules leave the fibers at the retentive side. The ability to pass through the membrane depends on the size and the hydrophilicity of molecules. In the case of biogas components, it increases in the following order: CH₄ < CO₂. Retentate is therefore enriched with CH₄, while the permeate collected on the opposite side of the membrane contains acidic gases [33,47]. Pre-removal of H₂S and H₂O from biogas is recommended in this upgrading method [57].

The membrane technology is widely implemented in full-scale application. The development of these technologies in methane upgrading processes is moving towards the search for new, more efficient, and durable membranes. Koutsiantzi et al. [109] examined the use of commercial hollow-fiber polyimide membranes in a pilot plant for upgrading the biogas in a municipal wastewater treatment plant in Thessaloniki. The results showed an average recovery of CH₄ of 95.7%, and the average CH₄ concentration in the final product was ca. 82.4%. The low purity resulted from the high N₂ concentration, which resulted from the applied method of the H₂S removal from biogas. But N₂ did not affect the CO₂ permeation through the membrane. The expected concentration of CH₄ when assuming N₂ absence in biogas would be 99.8%. According to Rodero et al. [110], 6FDA-based polyimides and polymers of intrinsic microporosity are promising materials for developing membrane technologies in biogas upgrading. Additionally, the formation of mixed-matrix membranes (MMMs) by incorporating fillers such as metal–organic frameworks or zeolites into the polymer matrix significantly increases the overall performance, including CO₂ permeability and CO₂/CH₄ selectivity.

Cryogenic separation is a group of methods in which the difference in conditions (temperature and pressure) of phase changes of CO₂ and CH₄ is used. The condensation-based methods (e.g., cryogenic distillation process and Rayan–Holmes methods), controlled freeze zone (CFZ) method, and anti-sublimation methods can be considered. In the case of the condensation-based method, the difference in boiling points of methane and carbon dioxide is the basis for the separation of these gases. The CFZ method is based on controlled freezing and remelting of CO₂ in a specially designed distillation column [111]. In the anti-sublimation-based method, CO₂ solidification occurs on the heat exchanger surfaces or in a cold box [91,112,113]. One of the solutions used in conventional technologies is gradually lowering the temperature at constant pressure [114]. Gradually, with temperature reduction, water vapor, H₂S, and siloxanes are removed from the biogas, followed by CO₂ with a boiling point of −78 °C (for methane, this parameter is equal to −160 °C) [35,115]. Another technological option of cryogenic separation consists of multistage compression up to 80 bars of preliminary dried biogas [45]. Comparison of the efficiency and energy consumption of three methods of cryogenic separation: Ryan–Holmes method, the dual-pressure low-temperature distillation process, and the anti-sublimation process showed that the dual-pressure low-temperature distillation process has the most favorable features in these respects [112]. The cryogenic process of biogas upgrading attracts attention due to the possibility of producing high-purity biomethane and the potential for integration with the bio-LNG production process. In recent years, technologies based on cryogenic-membrane gas-separation hybrid processes have been implemented for gas separation purposes, which are also effective in the case of biogas upgrading. They are characterized by high cost-effectiveness and energy efficiency [116].

Biological methods include the photosynthetic (photoautotrophic) biogas upgrading method, which involves CO₂ capturing from biogas by eukaryotic microalgae or prokaryotic cyanobacteria microorganisms using light as a source of energy. The carbon contained in CO₂ is then incorporated by these microorganisms into their biomass [51,117,118]. As a result of this process, the methane in biogas is concentrated, and the algae synthesizing various products, regarded as an added value, can be used in the further stages of the process. According to Mendez et al. [119], the maximum CH₄ concentration in the biogas produced from mixed sludge upgraded in the pilot-scale outdoor algal-bacterial system was 91%. Algae technology is considered to be more environmentally friendly and cost-effective than the chemoautotrophic biogas upgrading process due to its capacity to remove CO₂ and H₂S in a single-stage process.

Another biological method of removing CO₂ from biogas is acetogenic gas fermentation. This process involves the conversion of CO₂ to valuable liquids (e.g., acetate, butyrate, ethanol, or higher alcohols) by microorganisms, including homoacetogens (that use hydrogen to reduce carbon dioxide to acetic acid via the acetyl-CoA pathway) and syntrophic acetogens (that use VFAs as substrates for acetic acid production) [50,120].

In recent years, new, cheap, and easily accessible adsorbents have been sought that could be used in small-scale installations or developing countries. Such materials include natural soils, clays, waste-derived materials, activated carbon produced from solid waste, waste iron fillings and biomass ash, compost, and biochars [121,122,123]. Jepleting et al. [121] showed that by appropriate preliminary preparations of these materials, modifications of the surface properties, combining with other materials, and regeneration, their efficiency in the removal of impurities increases, even over 90%. For example, the removal efficiency of iron-rich New Zealand Brown soil was 93.8%. Hybrid sorbents such as bamboo-derived activated carbons, sorbents of metal–organic frameworks, porous slurry formed with zeolite ZIF-8 and isoparaffin C16, and porous liquid mixtures of zeolite Rho with Genosorb solvent were also used for CO₂ removal [124,125,126].

A summary of basic information on the efficiency, costs, advantages, and disadvantages of the conventional biogas upgrading methods is presented in

Table 1. Comparison of biogas upgrading methods [34,35,36,37,39,40,45,56,57,127,128,129,130,131,132,133,134,135,136,137,138,139,140].

Technology	Final CH4 Content [%vol.] CH4 Losses [%]	Advantages	Disadvantages	Investment Cost [€ */Year] Maintenance Cost [€ */Year] **
Pressure swing adsorption (PSA)	95–99 1.5–12.0	Combined removal of CO2, N2, and O2 No need for chemicals Compact technology Possibility of use for a small scale Low energy demand High tolerance for impurities Fast installation Fast regeneration of sorbent	Necessity of prior H2S and H2O removal High investment and operational cost Susceptible to fouling and operating nuisances	1,750,000 56,000
Water scrubbing	95–99 0.5–5.0	Combined removal of CO2, H2S, and NH3 High tolerance for impurities No need for chemicals Flexibility in adjusting capacity by temperature and pressure change Ease of water regeneration	High water volume requirement Slow process High requirement for power Clogging due to bacterial growth Problems with foaming, corrosion and wastewater disposal Necessity to remove water from biomethane	1,000,000 15,000
Organic physical scrubbing	95–99 <1.0–4.0	Combined removal of H2O, H2S, and NH3 Possibility of removing residual CH4 by heating	High investment and operational cost High solvent cost High energy demand for solvent regeneration Expensive for small-scale operation Decrease in process efficiency due to solvent dilution in water	1,000,000 39,000
Chemical scrubbing	97–99 <0.1–4.0	Fast process Low operational cost Ease of solvent regeneration Complete H2S removal	High cost of amine solvents and investment costs High requirement for energy for solvent regeneration Problems with corrosion, foaming and precipitation of salts Problems with toxic for human and environment waste chemicals disposal	2,000,000 59,000
Membrane separation	90–99 <0.5–20.0	Environmentally friendly Fast installation Low energy requirement Compact and simple technology Combined removal of CO2, H2S, and H2O No need for chemicals High reliability Possibility of upgrading even at low gas flows	Strong recommendation for prior H2S and H2O removal High cost of membranes Requirement for multiple modules for high purity Need for membrane replacement every 1–5 years Low membrane selectivity Low efficiency in single step process Possibility of membranes congestion and contamination	2,000,000 25,000
Cryogenic separation	97–99 <0.1–2.0	Environmentally friendly No need for chemicals High purity CO2 production for further use Minimal additional energy requirement for liquid biomethane (LBM) production	High investment and operational costs High energy demand Temperature dependent process efficiency	nd.*** nd.***
Biological upgrading	65–100 nd.***	Environmentally friendly Simple and inexpensive process Low maintenance costs No need for chemicals No waste	Used on a pilot scale High requirement for H2 Not suitable for high H2S concentrations Not recommended for large-scale use Strong recommendation for monitoring and controlling of operational parameters	nd.*** nd.***

* Euro; ** Costs for flow of 1000 Nm³/h biogas; *** no data.

.

3.1.2. Biogas Upgrading via CO₂ Transformation

In contrast to the previously discussed methods for enhancing the calorific value of biogas by removing CO₂ and other components, these methods are based on converting CO₂ into methane through a process known as methanation. Depending on the catalyst used, this process is classified as chemical or biological methanation. Methanation can be performed directly on biogas or indirectly (on the CO₂ stream separated during the processes outlined in Section 3.1.1 [141].

Chemical methanation utilizes catalysts to significantly increase the reaction rate of carbon oxides (CO₂ or CO) with hydrogen (H₂), to produce methane and water (Equations (1) and (2)). This process enables the production of substitute natural gas (SNG) not only from biogas but also, primarily, from synthetic gas (syngas) and captured exhaust CO₂ [142].

4H₂ + CO₂ → CH₄ + 2H₂O    Δ H = −165 kJ      Sabatier reaction

(1)

3H₂ +CO → CH₄ + H₂O     Δ H = −206 kJ

(2)

Catalytic methanation typically occurs within a temperature range of 200 to 550 °C and under pressures ranging from 1 to 100 bars [52,143]. Compared to CO methanation, which is commercially operated at 300 °C and 25 bars, methanation of CO₂ may require more severe operating conditions [144]. The chemical methanation of CO₂ at elevated temperatures (around 400 °C) was first demonstrated by Sabatier and Senderens in 1902 using a nickel (Ni) catalyst [145]. Many other catalysts, such as ruthenium (Ru), rhodium (Rh), cobalt (Co), platinum (Pt), palladium (Pd), manganese (Mn), copper (Cu), and lanthanum (La) supported on various metal oxides such as SiO₂, Al₂O₃, ZrO₂, TiO₂, and CeO₂, have been tested for CO₂ methanation as base catalysts [146,147,148,149,150]. However, Ni-based catalysts remain the most widely used due to their high activity, selectivity, and cost-effectiveness [151]. Generally, the same catalysts are effective for both CO and CO₂ methanation, although CO₂ offers higher selectivity for CH₄ [152].

The main obstacle to the growth and commercialization of methanation processes is the hydrogen acquisition [153], which is largely due to high production costs. The source of hydrogen used in the process significantly impacts both costs and the carbon footprint [92].

The cost of hydrogen production ranges from 1.3 to 2.13 USD/kg H₂, depending on the production method. The methods that allow obtaining renewable fuel, such as gasification or pyrolysis, and those based on biological processes are the most expensive [154]. Due to the wide field of hydrogen applications, competition for hydrogen on the market can be an important limiting factor for the dissemination of methanation processes. Hydrogen is used as an energy source in transport, heat and power production, in metallurgy as a reducing agent (e.g., in iron and steel production), as an agent for hydrocracking and hydrotreating processes in petroleum refining, as a substrate for ammonia production, etc. [155,156,157,158,159]. The transport and storage of hydrogen also pose problems. Hydrogen must be kept under very high compression pressure or very low temperature [160]. Hydrogen liquefaction occurs at a temperature of −253 °C, and at this temperature gas must be stored in an insulated tank. The compressed H₂ requires the use of heavy, pressurized tanks. The cost of transportation is therefore very high [161].

An efficient source of hydrogen is necessary for ensuring stability and continuity of the methanation process. Most often, hydrogen is produced through water electrolysis [92]. However, to maintain the sustainability of the process, the electricity utilized in the process must originate from renewable sources, such as solar and wind [162]. The technology in which the hydrogen used for CO₂ methanation is produced via water electrolysis using energy from a renewable source is referred to as Power-to-Gas (PtG). The PtG technology integrates electrical and fuel systems by storing excess renewable electricity in the form of CH₄ [52]. Combining LBM production with the PtG concept offers a pathway to reduce CO₂ emissions and increase the production of biofuel that replaces natural gas. Studies by Hashemi et al. [163], incorporating exergy and cost analyses, demonstrated that integrated direct methanation of biogas is more advantageous than indirect methanation of CO₂ captured during biogas upgrading processes. The integrated scheme featuring direct biogas methanation increased LBM production by 52.2% compared to a conventional LBM production facility, though with lower exergy efficiency methods. The application of CO₂ methanation in liquefied biomethane (LBM) production plants is a method for reducing their carbon footprint, as the CO₂ captured during biogas upgrading is typically released into the atmosphere. Advantages of direct methanation of biogas using hydrogen from electrolysis over the methanation of carbon dioxide separated from biogas were also noted by Calbry-Muzyka and Schildhauer [141] and Chatzis et al. [52]. The researchers pay attention to lower costs due to the elimination of the step of CO₂ separation and underline the promising role of biomethane in seasonal storage of renewable energy in the natural gas grid.

As part of the research on the Power to Gas (PtG) technology, Mercader et al. [164] examined the suitability of zeolite 5 A for use in Sorption Enhanced Sabatier Reaction (SESaR) technology, which is an alternative to conventional chemical methanation processes. This technology uses the addition of a water absorbent to a catalytic bed. Removing water, which is one of the reaction products, can accelerate the conversion of reagents, even beyond the limits resulting from the reaction equilibrium. The results of the study confirmed that the addition of zeolite LTA 5A to the Ni-Fe/Al₂O₃ catalyst enhanced the performance of the catalyst and minimized CO production.

Biological methanation is also based on the Sabatier reaction (Equation (1)) [52]. However, the sources of H₂ participating in the process vary, ranging from external sources, e.g., water electrolysis, to internal production within biomethanation reactors. Several methods of methanation employing microorganisms are known.

The chemoautotrophic biogas upgrading methods, also called H₂-assisted CO₂ bioconversion, are based on the activity of hydrogenotrophic methanogens that can utilize H₂ to convert CO₂ to CH₄ [51]. Hydrogenotrophic methanogens belong to strictly anaerobic archaea that use H₂ as the electron donor and CO₂ as the carbon source [165]. The most common species of hydrogenotrophic methanogens found in anaerobic digesters belong to the genera Methanobacterium, Methanobrevibacter, Methanothermus, Methanococcus, Methanogenium, and Methanothermobacter [166,167]. Biological methanation occurs under mesophilic or thermophilic conditions (35–65 °C) and at pressures up to 10 bars [168]. It exhibits high tolerance to contaminants like hydrogen sulfide (H₂S) and ammonia (NH₃), reducing the need for extensive biogas pretreatment [52].

According to Strübing et al. [169], highly efficient biomethane production can be obtained in trickle bed reactors (TBRs). Feickert Fenske et al. [170] examined the possibility of improving the efficiency of biological methanation in a pilot-scale trickle bed reactor by changing the gas flow through the reactor towards the plug flow. They stated that applying the feed gas in a top-to-bottom direction and stopping the trickling of process liquid through the packing bed improved the gas flow. The results of biological methanation carried out in a pilot-scale TBR under real conditions of a wastewater treatment plant showed the possibility to obtain long-term production of gas at synthetic natural gas quality [171]. Chatzis et al. [52] made a review of the state of the knowledge and implementation of biological methanation in Europe. They confirmed that the trickle-bed reactor is the most promising reactor type for biomethanation. Thema et al. [172] examined the biological CO₂-methanation in TBR using a pure culture of thermophilic methanogenic archaea Methanothermobacter thermoautotrophicus IM5. Concentration of CH₄ in the final product was over 98%. Adding N₂ as a carrier gas during the start phase, ammonium and sodium sulfide supplied as nitrogen and sulfur sources, and foam control were important factors for improving process efficiency.

The technologies of biological methanation can be divided into (a) in situ methanation, in which H₂ or CO₂ is are injected directly into an anaerobic digester, where it is converted into CH₄ by hydrogenotrophic bacteria, and (b) ex situ methanation, in which hydrogen and biogas are injected into a separated bioreactor in which hydrogenotrophic bacteria convert biogas-derived CO₂ and H₂ into CH₄ [91,173].

Carbon dioxide injection into the anaerobic digester is another type of direct methanation. Research into the mechanisms of this process and its determinants is still ongoing, and the results obtained on a laboratory and pilot scale indicate the significant potential of this method of biomethane production. Many researchers [174,175,176] have shown an increase in the efficiency of biomethane production under the influence of CO₂ injection into anaerobic digesters. Tao et al. [176] observed a stable process of production of biogas with CH₄ content over 90% during digestion of food waste and sewage sludge. However, not all researchers obtained an increase in methane production after CO₂ application. Castel et al. [27], who conducted pilot studies using exogenous CO₂ in biogas plants with standard on-farm feedstocks, showed no demonstrable improvement in biomethane yields. Moreover, they observed a detrimental effect of high-rate CO₂ injection (12 NL/h) maintained for 3 h a week on methane production.

The reduction of CO₂ to acetates by homoacetogens, followed by the conversion into methane via acetoclastic methanogenesis, is a proposed mechanism of the process [177]. Studies indicate that the process requires pH buffering because of changes in CO₂ solubility and the stability of microbial consortiums in the reactor. Further research in the field of methane production methods using CO₂ injection is directed towards modifying the conditions favorable for the development of hydrogenotrophic methanogens. For this purpose, modifiers such as zero-valent iron [29] and bamboo biochar [30], which change the conditions in the reactor, are being studied. Zero-valent iron is an electron donor with better capabilities compared to other iron-based materials. It was observed that CO₂ injection and the introduction of zero-valent iron into the methane fermentation bioreactor increased methane production from food waste by up to 44% compared to the control sample. This was possible due to a significant increase in the efficiency of hydrolysis of proteins, polysaccharides, and lipids, then their acidogenesis. An increase in the number of Methanobacterium was observed [29], which belongs to CO₂-reducing methanogens. The possibility of gaining iron from the waste lowers the cost and is an additional advantage [178]. Biochar improves the conditions of CO₂ solubility and enhances microbial stability, playing the role of pH stabilizer and an electron shuttle. The combined use of biochar and CO₂ injection led to ca. 43% increase in specific methane production [30]. Yang et al. [179] studied the adaptation of bacteria in a fermentation reactor to changes in conditions resulting from changes in the H₂/CO₂ molar ratio. Adapted microorganisms were introduced to an intermittent high-solids anaerobic digestion system. A beneficial effect of bioaugmentation with these highly efficient hydrogenotrophic methanogens on methane production was found. It increased by approx. 34%. The advantage of using CO₂ as a factor intensifying biomethane production compared to H₂ is the greater availability of CO₂, which can also be a by-product of other processes, e.g., biomass combustion. Upcycling of this gas is therefore possible, which contributes to environmental sustainability [174].

Despite the limitations resulting from the distortive effect of H₂ or CO₂ on the activity of methanogenic microorganisms, research indicates that in situ methanation can be successfully used. Although the methanogenesis is not disturbed by hydrogen injection in ex situ methanation, the process is limited by higher installation costs resulting from additional bioreactors [91].

Microbial electrochemical cells (MXC)—the production of methane from CO contained in biogas—is supported by a bioelectrochemical system called electromethanogenesis (EM) [180]. In this process CO₂ is reduced to CH₄ by a biocathode using electrons or H₂ produced by water electrolysis powered by renewable energy [181]. Hydrogenotrophic methanogens were identified as the dominant functioning microorganisms at the cathode in this system [182].

Microbial electrolysis cells (MEC)—in this system, the electrons used for the reduction of CO₂ are released by bacteria during the oxidation of organic compounds at the anode and then combined with protons to produce H₂. The produced H₂ is then utilized at the cathode to reduce CO₂, resulting in the production of biomethane or other feedstocks, such as acetate [50].

Due to the mild temperature and pressure conditions, biological methods are more advantageous, as they significantly reduce energy consumption and equipment requirements. This results in lower costs and a reduced environmental impact. Additionally, biological technologies demonstrate higher tolerance to feed gas contaminants, such as hydrogen sulfide or ammonia, resulting in lower costs of the preliminary gas preparation [52].

3.2. Biomethane Production from Syngas

The production of biomethane, which is fundamentally based on thermochemical transformations, begins with gasification or pyrolysis processes conducted at high temperatures, ranging from several hundred to 1500 °C [183]. For optimal results, the biomass used in these processes should contain no more than 15% water [184]. Thus, the pre-drying step is often needed. In gasification and pyrolysis processes, which occur under different conditions regarding oxygen availability, gasification requires the oxygen supply (in the form of air, pure oxygen, or water steam, but in limited amounts below oxidation stoichiometric values) [184,185]. Pyrolysis occurs in an inert atmosphere [186]. Thermochemical processes yield products of varying phases and chemical compositions. In gasification, gaseous products dominate, primarily consisting of H₂, CO, CO₂, and CH₄ [184]. In contrast, pyrolysis predominantly produces liquid or solid products, with the resulting pyrolysis gas containing H₂, CH₄, hydrocarbon gases (C₂–C₄), CO, CO₂, and H₂S [186]. The gaseous mixture produced from the thermochemical processing of biomass, known as syngas, can be utilized as a source of biomethane through thermochemical or biological conversion [10].

The conversion pathways of syngas to methane proceed via chemical or biological methanation.

The chemical methanation processes of syngas follow the reactions given in Equations (1) and (2). For thermochemical methanation, syngas should be pretreated through tar and water removal, gas conditioning, and cleaning (e.g., H₂S and HCl removal) before the methanation process [183]. After methanation, any remaining CO₂ and water are removed [10]. Chemical methanation processes are widely used to produce substitute natural gas (SNG) from syngas derived from fossil fuels [187,188]. The Swedish experience gained during the long-term operation of a plant producing syngas from woody biomass in Gothenburg—with a production capacity of 20 MW of biomethane delivered to the natural gas grid—demonstrated that the technology in which syngas produced from biomass is converted to CH₄ via chemical methanation can be efficiently implemented on a commercial scale. The quality of the resulting biomethane met the European standard for injection into the natural gas grid [189].

Biological methods for converting the components of synthesis gas (CO, CO₂, and H₂) into methane involve various processes, including acetogenesis, hydrogenotrophic methanation, carboxydotrophic methanation, and acetoclastic methanation [190].

One pathway for biomethane production follows the acetate route, where syngas components are first converted into acetates (acetogenesis), which are then further transformed into methane (acetoclastic methanation). In the initial stage of the process, acetogens such as Acetobacterium woodii and Enbacterium limosum play a key role. The acetates, which are products of their metabolism, are subsequently converted into methane by methanogenic bacteria, such as Methanosarcina barkeri. Another pathway—chemoautotrophic—relies on the ability of hydrogenotrophic methanogens to convert H₂ and CO₂ into CH₄ [191].

The CO present in syngas can be directly converted into CH₄ by carboxydotrophic methanogens or indirectly through its conversion into acetates by carboxydotrophic acetogens, which are then subjected to acetoclastic methanation [191,192].

Aryal et al. [193] investigated the efficiency of biomethane production from syngas using a biological method in a fed-batch trickle-bed reactor system. The study used manure or sludge as the microbiological inoculum. The resulting methanized syngas still contained a high concentration of CO₂ and did not meet the standards for natural gas-quality biomethane. Therefore, exogenous H₂ was introduced into the reactor, resulting in an increase in CH₄ concentration to 95.3 ± 1.0%.

A combined thermochemical and biological approach can also be used in biomethane production, for example, by using the water–gas shift (WGS) reaction to produce CO₂ and H₂ from the reaction of CO with H₂O. This reaction occurs at temperatures ranging from 200 to 500 °C in the presence of a catalyst, often Fe-based or Cu-based [194]. The resulting reaction products are then converted into methane by hydrogenotrophic methanogens [191].

Despite several advantages compared to thermochemical processes—including versatility regarding the proportions of the individual syngas components (CO/CO₂ and H₂), the use of inexpensive biocatalysts, milder operating conditions, greater tolerance to syngas contaminants, and low sensitivity to changes in the C/H ratio [195,196]—biological methanation remains in the laboratory and demonstration stage. One of the main limitations of biological processes is the limited mass transfer rate of H₂ and CO into the reaction environment. It is due to the low solubility of these gases in water and the slow growth rate of anaerobic microorganisms. As a result, volumetric methane production remains low [190].

In summary, the substrates for the methanation process, which result in the biomethane production, can originate from various sources, and processes of different natures can be combined in various configurations. The source of CO₂ may be biogas (CO₂ can be used either in a separated form during upgrading or as a component remaining in the gas mixture), flue gas from biomass combustion, or syngas produced by the thermochemical conversion of biomass. In addition, CO₂ may originate from biological processes, such as alcoholic fermentation or dark fermentation. The source of CO is mainly syngas produced from biomass, while H₂ can be obtained through thermochemical processes (from syngas produced from biomass) or in biological processes such as dark fermentation or light (photo) fermentation. Hydrogen may also be generated via biocatalyzed water splitting (biophotolysis of water), although in this process the substrate is not biomass—the method of hydrogen production is simply of a biological nature. However, water electrolysis is considered the most promising method for hydrogen production. Although such hydrogen is not classified as bio-hydrogen, if the electricity used in its production comes from renewable sources, this hydrogen does not negatively impact the environment (green hydrogen). Additionally, when combined with CO₂ produced from biomass, it enables the production of biofuel.

3.3. Dissemination of Methane Production Methods

The primary method of biomethane production worldwide is biogas upgrading. In 2018, around 90% of biomethane was produced this way [10]. According to IEA data, in 2018, technologies for biomethane production using biomass gasification on a technical scale were operational only in Europe and Central and South America. In Europe, approximately 10% of biomethane was produced this way, while in Central and South America, it accounted for about 12% of the total production in the region [10].

According to IEA Bioenergy Task 37, at the end of 2019, there were 606 biomethane production facilities in the 15 member countries (10 European countries, Canada, South Korea, and Brazil), compared to 428 in 2015 [10,197]. Over four years, the number of biogas upgrading facilities for grid injection or fuel use increased by approximately 40%. The majority of these facilities in Europe are located in Germany, the UK, and Sweden [198]. However, this analysis does not include facilities in non-member countries of Task 37, which numbered 72 at the end of 2019.

According to CEDIGAZ [13], almost 60% of globally produced biomethane was upgraded using water scrubbing and membrane separation technologies. These data aligns with the analysis conducted by IEA Bioenergy Task 37 based on the data from member countries in 2015 and 2019 (Figure 2). The combined share of water scrubbing and membrane separation technologies accounted for approximately 56.1% and 58.4% in 2015 and 2019, respectively. These technologies also dominated among the facilities reported by non-member countries, such as China, Japan, Hungary, Iceland, Norway, Luxembourg, Spain, and the USA.

In 2019, the number of facilities using these technologies was similar, although in 2015, water scrubber installations outnumbered membrane-based ones by 72%. Thus, between 2015 and 2019, there was a 97% increase in membrane-based installations, while water scrubber installations grew by only 20%. It should be noted, however, that these data were incomplete [198]. Additionally, there appears to be little interest in technologies utilizing organic solvents (Figure 3).

As can be seen from the data presented in Figure 4, the water scrubber technology holds the largest global market share (ca. 30%), while the membrane technology has been gaining popularity in recent years (ca. 29%).

Taking into account the data collected by IEA Task 37 [198], which are presented graphically in Figure 5, the uneven dissemination of different types of biogas upgrading technology in a geographical perspective on a global scale can be noticed. The greatest diversity of technologies occurs in Europe, but a large variation in the share of particular technologies in individual countries is also observed in this region (Figure 6).

The unevenness of the installation distribution can be related to economic factors, which relate to the level of economic development of countries [91]. Taking into account the classification of countries by income [199], it can be stated that in China (Asia) and Brazil (South America), which are classified as upper middle-income countries, water scrubber and PSA installations dominate, which are assessed as cheaper (Table 1). However, it is difficult to assess the significance of this factor on the scale of the European continent because all of them are classified as high income. In this case, other factors such as the quality of biogas and the final product and the availability of raw materials necessary to ensure the operation of the installation (e.g., water and energy sources) may be significant [91]. Other factors that may influence choosing the method of biogas upgrading are the availability and competitiveness of fossil fuel prices or legal regulations and other region-specific factors, such as access to raw materials from which biomethane is produced. Access to raw materials depends mainly on the dominant economic sector of the country, climate zone, and land availability [200,201]. Densely populated regions have a greater potential to use municipal solid waste; industrialized regions—to use organic waste streams; agricultural regions—to use agricultural waste (manure, straw) and crops [10,43,200]; and forested regions—to use lignocellulosic biomass in thermochemical processes. Climate and soil type influence plant growth. Regions characterized by a stable, temperate climate may offer more favorable natural conditions for anaerobic digestion, potentially reducing the need for external energy for heating or cooling digesters, leading to lower operating costs [202]. In addition, climate-dependent water accessibility affects the viability of biogas production process [200].

Regional differences are also related to the availability of technical infrastructure, both at the stage of biogas production and the production and distribution of biomethane. The proximity to essential infrastructure such as gas pipelines, the dominant direction of development of automotive technology in the country, and roads for feedstock transport significantly influence the viability of the projects on biomethane production.

As previously mentioned, biomethane is produced mainly from biogas; therefore, a significant cause of the disproportion in the dissemination of biomethane can be seen in the approach to biogas production. This problem was analyzed by Biro et al. [203]. Based on the literature, researchers assessed that the most important factors determining the adoption of biogas production technology on a global scale are social factors, such as public trust, procedural justice, and community involvement. Financial aspects, including incentives, operating costs, and market conditions, were also important, while environmental factors, including greenhouse gas emission reduction, resource compatibility, and impact on land use, were the least important. It was also noted that the importance of individual factors depended on the region. In Europe, the emphasis is on policy alignment, stakeholder management, and advanced infrastructure, while in developing regions, affordability, education, and access to resources are priorities. Additionally, technological and lack of knowledge on how to build installations and operate them, as well as competition with free wood as an energy source, are primarily widespread in developing countries [204].

4. Alternatives of Biomethane Applications

The properties of biomethane vary depending on the substrate that was used for its production and the method of production, while the gas that is to be used for a specific purpose must meet the quality standards that allow for its effective and safe application. Quality standards for biomethane vary between particular countries, depending on the purpose of gas use.

4.1. Transport Biofuel

Like conventional natural gas, biomethane that meets certain quality standards can be used as a transport fuel in the form of compressed biomethane (CBM) or liquefied biomethane (LBM), which are the substitutes of LNG and are also called bio-CNG or bio-LNG. Therefore, it can be applied in natural gas-powered vehicles [8,10] and stored and distributed in the same way [205]. Compressed or liquefied biomethane can be delivered to retail filling stations from a nearby factory or from a further distance in liquefied form (similar to LNG). Biomethane can also be delivered to a gas station via gas pipelines, where the biomethane is mixed with natural gas.

Compressed biomethane has about 275 times higher calorific value than gaseous biomethane, while the liquefaction process allows us to increase the calorific value of the fuel 637 times compared to gaseous biomethane and 2.3 times compared to compressed biomethane in relation to the unit of volume (

Table 2. Properties of different forms of biomethane [198].

Parameter	Gaseous Biomethane	Compressed Biomethane (CBM)	Liquefied Biomethane (LBM)
Methane content [%vol.]	97	97	99.995
Pressure [bar]	1.01325	250	1.03125
and temperature [°C]	15	15	−162
Density [kg/m3]	0.68	186.88	424.14
Energy content, NCV [MJ/m3]	32.96	9063	21,000

). The net calorific value (NCV) of gaseous biomethane is 94–97% of the value of this parameter for natural gas, which is 34–35 MJ/m³ [206].

Compressed biomethane has a lower energy value compared to liquid fuels: about 4 times lower than diesel (36,000 MJ/m³) and kerosene (35,300 MJ/m³), 3.5 times lower than petrol (32,000 MJ/m³), and 2.3 times lower than ethanol (21,100 MJ/m³) (energy values are given according to [207]), which limits its application possibilities in transport. In addition, storage and transport of CBM require special conditions. Due to the high pressure of compressed biomethane, which is 200–300 bars (50–100 bars over the value in a vehicle tank), it is necessary to store it in heavy steel or composite tanks, which limits the possibilities of transport [198]. Despite these problems, such solutions are practiced, e.g., in Reykjavik (Iceland), compressed biomethane produced from municipal waste is supplied to consumers in gas containers, which are made of fiberglass or steel cylinders [208].

Biomethane in the form of LBM provides wider possibilities for use in transport. This form of biomethane has higher energy density, which increases the potential range of LBM-fueled vehicles and makes fuel transport over long distances easier [198]. Liquid biomethane can also be mixed with liquid natural gas (LNG), which is applied in road and marine transport. The net calorific value of LBM and LNG is similar: the NCV of LBM is 21,000 MJ/m³ [198], and LNG is 20,800 MJ/m³ [207]. The requirements specified for biomethane as fuel used in transport in European countries are specified in the standard EN 16723-2:2017 [209]. Considering the need to compress biomethane used for transportation purposes, the dew point required by the 16723-2:2017 standard is lower than in the case of use in the gas network and is minus 2 °C at a pressure of 7 MPa, regardless of the season. The increased requirements in this area are intended to avoid operational problems related to the formation of hydrates and gas hydrates in the fueling station components, occurring after the biomethane compressor [205].

In the US, there is no national quality standard for biomethane used in automotive engines, but The American Biogas Council has developed recommendations for the quality of RNG used in transportation in cases where it is not supplied to fueling stations via a conventional natural gas pipeline. These requirements are less restrictive compared to gas injected into the gas network, including in terms of the lower limit of the heating value, content of oxygen, H₂S, and siloxanes [210].

4.2. Injection into the Gas Grid

Compressed biomethane meeting the appropriate quality standards can be introduced into the gas grid. However, the requirements regarding the quality of gas are not universal. They differ, among others, depending on the type of grid to which the gas is injected. For example, in Europe there are two types of gas grids, characterized by different technical parameters of gas transported in them and having different managers. There are transmission grids, which comprise high-pressure pipelines responsible for transporting gas across extensive distances, managed by transmission system operators (TSOs), and distribution grids composed of low-pressure pipelines designed to supply gas to final consumers, managed by distribution system operators (DSOs) [211]. The differences in the quality of gas injected into these grids concern the values of water dew point, injection temperature, content of total sulfur, O₂, CO₂, and H₂ [212].

In practice, biomethane injection into the grid depends not only on technical factors but also on legislative, logistical, and economic factors. According to EBA assessments [213], the regulations relating to the quality standards of gas injected into the grid, fees for connecting it to the grid, and the possibility of sharing costs between the gas grid operator and the biomethane producer are of particular importance. The costs of connecting to the grid depend largely on the size of the facility and the distance from the existing gas infrastructure.

There are no international standards for the quality of biomethane introduced into the gas grid. The requirements in Europe are specified in standard EN 16723-1:2016 [214]. However, the gas transported in the pipeline, also containing injected methane, should meet the requirements specified in the standard EN 16726:2015+A1:2018 [215]. However, the range and value of parameters specified in EN 16723-1:2016 [214] and EN 6726:2015+A1:2018 [215] differ from each other’s because biomethane, due to its origin, has specific components, such as siloxanes, terpenes, and amines. These components must be monitored because of their potential for creating specific technological and environmental challenges both in the case of incorporating biomethane into the network and its use in transport. For example, silicon-containing organic compounds during combustion are converted into microcrystalline silicon dioxide, which can be deposited on the surfaces of the wires, boilers, or engines [216].

European quality standards developed by the European Committee for Standardisation (CEN) do not, however, constitute mandatory requirements that must be followed by all EU member states. Each European country is allowed to determine its own quality parameters of natural gas or biomethane, and each of them may take into account national regulations, standards, or specific requirements of gas companies in relation to biomethane. Some countries do not take into account the Wobbe index, relative density, or water content in the adopted gas quality standards. The values of the permissible levels of hydrogen, oxygen, sulfur, siloxanes, etc. in particular countries are also different. As it can be seen from a detailed comparison of quality requirements for biomethane in 14 European countries, which were performed by international association MARCOGAZ, the maximum hydrogen content ranges from 0.01% Mol (in transmission grid of the Czech Republic) up to 6% Mol (in France), although in Austria and Denmark it is not taken into account at all; the highest tolerance oxygen levels in biomethane, 1% and 0.6% Mol, are given in Sweden and Italy, slightly lower value—0.5% Mol in Denmark, Switzerland, and the Netherlands (in the latter, in cases with a low-pressure grid <40 bar), in Austria is only 0.02% Mol, but the lowest limit—0.0005% Mol is noted in a case of a high pressure grid in the Netherlands [212]. Differences in quality parameters in individual countries make cross-border trade in biomethane difficult.

In Australia, biomethane should meet the requirements given in Australian Standard AS 4564:2020 [217] for natural gas quality, which hinders the development of the biomethane market, while in the USA, no uniform quality standard for biomethane has been established so far, and specifications for RNG vary depending on gas utilities, and, for example, the limit level of O₂ in this gas ranges from 0.0005 to 1% vol., and the maximum value of CO₂ ranges from 1 to 3% vol. [218].

4.3. Chemical and Biochemical Platform for Various Biorefinery Product

Methane is an important raw material for the chemical industry, including the production of hydrogen, methanol, and other fuels. The industrial use of methane for fuel production involves its conversion into synthesis gas (syngas), a mixture of CO and H₂ in steam or dry methane reforming processes [219] according to Equations (3) and (4), which is then used to produce methanol or synthetic fuels [220].

CH₄ + H₂O ↔ CO+ 3H₂    steam methane reforming

(3)

CH₄ + CO₂ ↔ 2CO + 2H₂    dry methane reforming

(4)

Carbon dioxide generated in steam methane reforming reacts with steam and additional hydrogen is produced as a result [Equation (5)].

CO + H₂O → CO₂ + H₂     water–gas shift reaction

(5)

Steam methane reforming is the main way for industrial hydrogen production [221]. Currently, 70 Mt of hydrogen is produced worldwide, and 76% of it is produced using natural gas, using 6% of the global natural gas consumption [222].

To ensure conditions in which methane can react with steam, it is necessary to maintain a pressure in the range of 3 to 25 bar and a steam temperature of 700–1000 °C [222]. This process causes CO₂ emissions of about 7 kg of this gas per kg of H₂ produced and is the source of almost 3% of global CO₂ emissions in the industrial sector [221]. Replacing, at least partially, petrochemical methane with biomethane, especially in these processes of producing clean energy carriers, will effectively reduce the carbon footprint of the fuel industry [223].

Methane reforming products are a raw material for the production of liquid fuels such as methanol or renewable hydrocarbons—Fischer–Tropsch diesel [224].

Methanol is produced form methane reforming products via the hydrogenation of carbon oxides over a copper oxide, zinc oxide, or chromium oxide-based catalyst [225]:

CO + 2H₂ ↔ CH₃OH

(6)

Renewable hydrocarbons are produced in Fischer–Tropsch processes, which are classified as low-temperature Fischer–Tropsch (LTFT) technology, in which cobalt-based catalysts are used, and high-temperature Fischer–Tropsch (HTFT) technology, in which iron-based catalysts are used. The main products of LTFT processes are paraffins, olefins oxygenates, while in HTFT, alpha-olefins are produced. The products of LTFT processes are upgraded to naphtha or kerosene (diesel oil) via catalytic hydrogenation of olefins and oxygenates or catalytic hydrocracking of wax [225,226].

Because fuels produced in Fischer–Tropsch processes can replace liquid fossil fuels without requirements for engine adaptation, they can be included in “drop-in” type fuels. These fuels are defined as “liquid bio-hydrocarbons that are functionally equivalent to petroleum fuels and fully compatible with existing petroleum refining and distribution infrastructure” [227]. Currently, drop-in fuels are mainly produced by converting oleochemical raw materials, such as vegetable oils or used cooking oils into fully saturated products, but it can be expected that thermochemical technologies, such as gasification, pyrolysis, or hydrothermal liquefaction (HTL), based on widely available biomass raw materials, will play a greater role in the future [227,228].

Methane can serve as a platform in various biochemical processes, such as conversion involving methanotrophs, which leads to the generation different products, e.g., polysaccharides, polyhydroxyalkanoates and proteins, isoprenoids, and vitamins [220,229]. Extracellular polysaccharides (EPSs) can be used in the pharmaceutical, textile, oil, and food processing industries. Polyhydroxyalkanoates (PHAs) accumulated within cells can be used in the biomedical and biodegradable packaging industry [230,231]. Methanotrophs are noted as potential producers of proteins, which can be used in the food and feed industry [232]. Among the other valuable substances produced from methane by methanotrophs is lactate—a substrate in the chemical industry and a precursor of polylactate used for bioplastics production [233]. Methanol can also be produced from methane via a biological oxidation pathway, but because it is quickly converted to formaldehyde as the first intermediate formed during the conversion of methane to carbon dioxide, this process has no practical significance [220].

4.4. Raw Material for Energy Production

Biomethane can be used to produce electricity in a fuel cell. In these devices, the chemical energy of the fuel and oxidant is converted into electrical energy [234]. In this process, there is no combustion of fuel, so the production of pollutants is limited. Fuel cells are galvanic cells in which fuel is supplied to the anode and oxygen/air to the cathode. As a result of the electrochemical reaction of hydrogen and oxygen, electric current, water, and heat are produced [235]. In modern fuel cells, the most commonly used fuel is hydrogen (H₂), but other substances can also be used, e.g., CH₄, ammonia, and methanol.

In practice, various technical solutions for fuel cells are used, such as proton-conducting solid oxide fuel cells (SOFCs), proton ceramic fuel cells (PCFCs), and proton exchange membrane fuel cells (PEMFCs), which have also been tested for methane [236,237,238]. Methane can also be used to produce energy in combined heat and power production systems in gas turbine (GT) devices operating at various scales [239,240].

Heat produced during the combustion of biogas or biomethane can be converted into work and then into power in thermodynamic cycles, e.g., the Kalina cycle (KC) or organic Rankine cycle (ORC). These solutions allow the exploitation of low-temperature heat sources and are also suitable for waste heat recovery applications. They can be used, for example, to increase the efficiency of electricity production in biogas plants, where energy production in CHP engines is characterized by low efficiency [241]. According to Benato and Macor [242], application of ORC can increase the production of net electric power from biogas combustion by 30% compared to the nominal value given on internal combustion engines’ nameplate. The Kalina cycle is a modified Rankine cycle that utilizes a mixture of water and ammonia as the working fluid, while in ORC organic compounds, e.g., chlorofluorocarbons and hydrofluorocarbons, are used as working fluids. Compared to water used in a conventional Rankine cycle, organic compounds have higher molar mass and lower evaporation temperatures. This enables heat recovery from lower temperature sources [243]. Guerron et al. [244] showed on the results of pilot-scale research that regenerative organic Rankine Cycle combined with thermal energy storage based on phase-change materials, which allow us to obtain the thermal inertia in the system, can be used for storage of energy derived from renewable sources.

5. Biomethane Market Evolution and Perspectives

5.1. Current Status and Development

Currently, the main method of biomethane production is biogas upgrading. According to data from the International Energy Agency (IEA) [10], in 2018 over 92% of world biomethane was produced in this way, and ca. 9% of the biogas produced globally was used for this purpose. However, this share varied depending on the region, and so in Central and South America it was about 35%; in North America, about 15%; in Europe, the region that produces the most biogas and biomethane, only about 10%, although in countries such as Denmark and Sweden, these shares were much higher; and in Asia this indicator was the lowest and amounted to 2% [10]. In 2018, the thermochemical path was used only in Africa and Europe (share of 12% and 10%, respectively). Global biomethane production could be significantly increased if technologies based on syngas derived from biomass were to become widespread on the market. Large-scale demonstration plants in Sweden and France have shown the technical feasibility of different gasification pathways to SNG [245].

According to data from the international association CEDIGAZ [13], global biomethane production in 2022 was 7.7 billion cubic meters (bcm) and increased by 20% compared to 2021. It was predicted that production in 2023 would increase to 9.5 bcm (i.e., by 23% compared to 2022) (Figure 7).

Biomethane production is concentrated in Europe and North America. In 2022, about 55% of biomethane was produced in European countries. European production grew almost 20% to 4.2 bcm in 2022 (with 3.4 bcm in the EU), driven by growth mainly in France, Denmark, Italy, and the UK [32]. In 2023, biomethane production in Europe increased to 4.9 bcm [246], which is slightly less than the CEDIGAZ forecast. Data for 2024 are not yet available.

In North America, the leading position in biomethane production is held by the USA, which in 2022 produced 2.5 bcm (in 275 facilities), which constituted about 33% of global production [32]. The share of North America in biomethane production has increased compared to previous years; e.g., in 2018 it amounted to about 29% (Figure 7). Despite the observed increase in biomethane production in the world, it still has a small share in the total demand for gas. According to IEA data, biomethane represented about 0.1% of natural gas demand worldwide [10]. Biomethane has a larger share in meeting the demand for natural gas in European countries. In 2022, it was estimated at an average of 1%, although in some countries it is higher. In Denmark, biogas/biomethane use is already equivalent to almost 40% of gas use, while in Sweden (21%) and Finland (16%) substantial levels are reached [247,248].

The increase in biomethane production is associated with the increase in the number of biomethane production facilities. The change in the number of these plants in Europe in the period 2011–2023 is presented in Figure 8. In the years 2022–2023, the number of biomethane plants increased by almost 30% compared to that given in the European Biogas Association (EBA) report for 2021 [249]. According to the report, over 75% of plants were connected to the transport (19%) or gas distribution networks (58%). In 2018, around 60% of biomethane production facilities worldwide injected biomethane into the gas distribution grid, and another 20% supplied fuel to vehicles. The rest supplied methane for various local end-uses. In 2022, the situation changed in favor of transport. This sector consumed around 45% of the global demand for biomethane [13].

Taking into account the course of changes in biomethane production on a global scale, it can be concluded that the rate of growth of this production in recent years has been much higher than in Europe or North America alone. This indicates a growing contribution of other countries, such as China [13], to the biogas market.

The raw material for biomethane production is mainly agricultural residues, organic municipal waste, and sewage sludge, and since 2017 there have been almost no new plants in Europe based on monoculture crops [249]. According to the latest data, in July 2024 there were 1548 biomethane installations in Europe, which had reached an installed capacity of 6.4 bcm of biomethane per year [246]. The number of installations increased by 226 in half a year (by 17%).

5.2. Prospects for the Development of Biomethane Market

In recent years, there has been a rapid growth in the global market for biogas upgrading equipment—from USD 1.86 billion in 2023 to USD 2.17 billion in 2024, at a compound annual growth rate (CAGR) of 16.6%. Further growth in this area of the economy is expected, to USD 3.93 billion in 2028, at a CAGR of 11.8% [251]. However, despite the lack of technological obstacles, biomethane is at an early stage of commercialization. Nevertheless, a significant increase in biomethane production is expected in the next few years. IEA [248] predicts a doubling of its production value between 2023 and 2027, primarily supported by projects undertaken in Brazil, Europe, India, and North America. According to Emprin [252], potential production of biomethane in 2023 only in Europe is estimated to be 41 bcm. For comparison, the EU natural gas consumption in 2022 was 360 bcm [245]. Taking into account the forecasts of the growth rate of energy demand [253] and biomethane production, a rapid increase in the share of the latter one in meeting the global energy demand can be expected.

These predictions are likely to be confirmed as more and more countries support decarbonization of transport and replacing natural gas with biomethane in their government policies. The following section presents the situation in this area in selected countries on different continents.

Europe: The REpowerEU plan adopted in 2022 [254] targets 35 bcm of biomethane by 2030 [255]. It is 10 times more than the production in 2021. Member States set their targets in the drafts of National Energy and Climate Plans (NECPs). EU policies see biomethane as a key to diversifying the EU’s gas supply and one of the renewable energy priorities, especially when its production is based on waste and organic residues rather than food and feed crops [195]. Several EU member states have adopted biomethane strategies and/or action plans. In 2024, Portugal set a target to replace 18.6% of its natural gas demand with biomethane by 2040, while Romania planned to increase the share of biomethane in the gas network to 5% by 2030 and 10% by 2050 [256].

The European Network of Transmission System Operators for Gas (ENTSOG) indicates that biomethane injection into the gas network provides savings in greenhouse gas emissions without any adaptation cost for end-users. It emphasizes the role of mixing natural gas with biomethane and hydrogen in the process of decarbonizing the gas sector but points out the need to adapt the gas regulations to include hydrogen and strengthen the role of biomethane. It recommends the creation of a trustworthy EU-wide Guarantees of Origin and certificate system, which allows following the environmental impact of a particular source of gases. It recognizes that the emergence of new gases will create challenges for the gas infrastructure, although they assess that the existing gas network is already fit for growing injection of biomethane [257].

America: The United States. The Renewable Natural Gas (RNG) Coalition, which works to promote the production of biogas and biomethane in North American countries, reported that in December 2024 in the USA and Canada, 442 RNG facilities were in operation, 170 were under construction, and 285 were planned. Taking into account that in July 2023, 300 facilities were in operation, the 47% growth within just one and a half years was observed. Further development is planned in this area to obtain the number of facilities 1000 in 2030 and 5000 in 2040 [258].

The basis for the development of the biomethane market in the USA has been the incentives under the Inflation Reduction Act (IRA) of 2022, which became effective on 1 January 2023, including the possibility of obtaining tax credits for investments for entities that want to support the transition to an economy based on clean energy. The expected increase in production is also due to the regulations—the Environmental Protection Agency established biofuel volume requirements and standards for cellulosic biofuel (which primarily apply to biomethane) for 2023–2025 as part of the Renewable Fuel Standard (RFS) program. The new volume targets for cellulosic biofuels increased by 25% in 2023, 29% in 2024, and 33% in 2025, compared to the previous target. Based on the changed targets, biomethane production in the USA could increase to about 4 billion m3 in 2025. However, the problem is the lack of a national standard for biomethane quality. The American Biogas Council (US) [208] has developed a project for a uniform quality standard for renewable natural gas (RNG), based on the experience gathered from dozens of American RNG production plants.

Canada. The quality of biomethane injected into the gas grid in Canada is determined by the National Standard CAN/BNQ 3672-100 Biomethane—Quality Specifications for Injection into Natural Gas Distribution and Transmission Systems issued by The Bureau de normalization du Québec [258]. In order for biomethane to be injected into the natural gas distribution and transmission networks and thus to be approved for the Canadian market, it is necessary that its composition be compatible with that of natural gas. The Canadian Gas Association set a target of replacing 10% of natural gas with RNG by 2030. However, the problem is the low popularity of natural gas-powered vehicles in the country [259].

Brazil. Adopted in 2024, The Fuel of the Future Law aims to promote biogas and biomethane. The Act establishes the National Programme for the Decarbonisation of Natural Gas Producers and Importers and Incentives for Biomethane, the aim of which is to support the development of biomethane production and trade [254]. Additionally, the Brazilian National Biofuel Policy (RenovaBio) emphasizes the strategic role of biofuels in the Brazilian energy matrix and aims to reduce the carbon intensity of the transport sector by 10% by 2028 [10].

Asia: India. In November 2023, India approved mandatory blending of bio-CNG with domestic gas supplies. The requirement is set at 1% of total CNG and domestic pipeline natural gas consumption from 2025 and will gradually increase to 5% from 2028/29. In 2024, the government developed an enabling framework for CBG. This includes providing financial support for biomass aggregation and financial assistance for the development of pipeline connections between bio-CNG plants and the city gas distribution network [256].

China. This country aims to reach 20 bcm/yr of RNG production by 2030 as part of its 14th Five-Year Plan [10]. Biomethane production was 0.3 bcm/yr with the consumption of 331.9 bcm of natural gas in 2022. Only trace amounts of biomethane are injected into the natural gas grid [248]. But the biomethane market in this country is expected to grow at a compound annual growth rate of 12.4% from 2025 to 2030 [260].

Japan. A total of six biogas upgrading plants were operating in 2019. All of these were installed on wastewater treatment plants and utilize the biomethane as transport fuel and use water scrubber technology for biogas upgrading [198,261]. In 2023 in Kagoshima, the first Japanese facility in a waste treatment plant, which produces biomethane used as city gas, was opened [262]. The 11.7% compound annual growth rate of the biomethane market is expected from 2025 to 2030 [263].

Australia: The biomethane market is just starting to develop. The Malabar facility at Wastewater Treatment Plant (New South Wales), operational since 2023, is the first biomethane facility registered under the government-managed GreenPower’s renewable gas certification program [264]. However, the parameters given in the AS4564:2020 standard of natural gas make biomethane injection into the gas grid difficult and expensive. This situation may improve soon, as the AS4564 standard is currently being revised [265].

6. Benefits of Biomethane and Barriers of Its Development

Biomethane production has a number of advantages that can be considered at various levels. From a practical point of view, the most important issue is the possibility of replacing natural gas in all its applications, without the need to modify devices and methods of distribution and storage, using the existing natural gas infrastructure [205,266,267]. The availability of diverse production technologies (biological and thermochemical) allows the use of a wide substrate base, including common lignocellulosic materials. Biomethane can be produced wherever waste and excess biomass are generated. It is therefore a method of diversifying gas supply sources and an alternative to imported natural gas, which is particularly important in countries that do not have their own resources of this fuel. This will reduce dependence on foreign suppliers of natural gas. Additionally, from an economic point of view, the support that biomethane production gives to local entrepreneurs is important [268].

Biomethane can be produced continuously; thus, it is a better version of an alternative energy source, independent of weather and terrain (which is important in the case of wind and solar energy), geological and hydrological conditions (important in geothermal energy and hydroelectric power plants), and time of day (solar energy).

No less important are the environmental benefits associated with biomethane. Biomethane is an important weapon against climate change and a path to decarbonization of the energy and transport sectors [269,270,271]. Based on data from July 2024 for Europe, it was calculated that the total installed capacity of European biomethane can contribute to avoiding nearly 29 million tons of CO₂ emissions [250]. The life cycle assessment conducted by Vinardell et al. [272] for a biomethane production plant based on a biological methanation process showed that biomethane has a lower global warming impact than natural gas when the share of renewable sources in the electricity production exceeds 62%. But they assumed that H₂ used in the process is produced via water electrolysis.

According to Orecchini et al. [273], biomethane as a transport fuel fares better in terms of environmental impact not only compared to fossil fuels but also compared to electric cars and bioethanol. The researchers showed that non-renewable primary energy consumption (NRPEC) for biomethane was almost 3 times lower compared to natural gas, 69% lower than the battery electric vehicle and 55% than bioethanol, but slightly higher compared to biodiesel (+9%). CO₂ emissions in the case of biomethane were reduced 3.55 times compared to natural gas, 176% compared to bioethanol, 124% with respect to biodiesel, and 69% compared to battery electric vehicles. The key element of the environmental impact of biomethane production is the method of biogas upgrading. It was observed that due to large methane losses, membrane technologies have the highest global warming potential (GWP); however, when using waste gas flaring, the GWP is the lowest [9]. According to the Danish Centre for Food and Agriculture (October 2020), biogas production with upgrading to biomethane injected into the gas grid may contribute to a reduction of 55 to 77 kg CO₂ equivalents per GJ of gross energy produced, depending on the type of biogas plant and feedstock used [267].

The added value in the case of biomethane production is the possibility of using waste from biogas production (digestate) in agriculture and soil remediation, which brings benefits in the form of carbon sequestration in the soil and recycling of nutrients. Based on data from July 2024 for Europe, it was calculated that biomethane plants produce 830,000 tons of organic fertilizer per year [250]. Using biomethane and natural gas significantly reduces pollutant emissions (hydrocarbons, carbon monoxide, nitrogen oxides, and particulate matter) compared to gasoline- and diesel-powered engines and is also well below the levels of biodiesel and bioethanol [274].

The following question arises: Since there are no technical barriers in biomethane production and the impact on the environment is beneficial, what barriers prevent the implementation of biomethane? As shown by the economic assessments prepared by the IEA, these are mainly high production costs. As indicated by the data presented in Table 1, the lowest cost was noted in the case of water scrubbing technology, which, however, does not meet the quality standards for the most demanding applications. Chemical methods that ensure the highest methane content can be up to 4 times more expensive. Natural gas prices for transport are still very competitive compared to liquid fossil fuels and are also lower than the costs of biomethane [255]. However, the EBA predicts that biomethane will soon become cheaper than natural gas [275]. The costs of biomethane production consist of many elements. According to Mansson [11], these are the costs of raw materials, distribution (distance from the place of use, length of the gas pipeline), and cars powered by different fuels (gas-driven vehicles are more expensive than diesel-engine vehicles). The cost of biomethane production also depends on the size of the plant. For example, in Sweden, the cost of biomethane production per 1 kWh of upgraded gas was about 3 times higher in a small plant (<100 m³/h raw gas) than in a large one (200–300 m³/h raw gas) [276]. The research by Sun et al. [34] showed that cryogenic technology has high operation and maintenance costs due to its low efficiency. However, in the case of chemical absorption technology, these costs were also high, especially with larger plant size, because of high energy efficiency. The analysis performed by Ferella et al. [277] for a 250 m³/h biomethane plant located in Italy, using the discounted cash flow (DCF) pressure swing adsorption (PSA) technique, showed a positive environmental impact resulting from the use of zeolites synthesized from spent fly ash, but its profitability was confirmed only in some scenarios, in which the construction of a new biomethane plant is assumed, and not the modernization of an existing plant. It was concluded that biomethane can contribute to the development of circular economy models, while the sustainable development goals are achieved only in some scenarios. According to Karne et al. [44], chemical scrubbing is considered to be a promising method of biogas upgrading as it supports high efficiencies of mass transfer and provides high methane purity with less loss. Additionally, it has moderate setup and operating costs.

According to Herno [267], the cost of biomethane production can be reduced between 10% and 16%, depending on plant size and configuration. However, it should be noted that cost reduction may not go hand in hand with environmental benefits. Some pro-environmental measures, i.e., reducing residual CH₄ emissions from the gas stream released from water scrubbers and membrane biogas upgrading plants or pumping manure through pipelines as an alternative to road transport, increase costs. The efforts towards the sustainable biogas upgrading technologies focus in particular on cryogenic separation, new biological techniques, biochar-based upgrading, and hybrid technologies involving two or more different non-integrated methods [56].

Transport and storage difficulties may also be a barrier to the development of biomethane technologies. Biomethane can be transformed into dispatchable energy resources through the application of appropriate storage technologies. Storage of biomethane may be implemented on both small and large scales and in both centralized and decentralized energy systems [278]. However, the most cost-effective and efficient method of biomethane distribution involves injection into the natural gas grid. This approach is particularly justified in countries with substantial biomethane production, such as Germany. Grid injection becomes economically viable when the distance between the biomethane upgrading facility and the gas grid does not exceed 500 m, provided that the injected biomethane complies with the gas grid quality standards [279]. Biomethane can also be stored in existing underground gas storage facilities, such as depleted natural gas or oil reservoirs or salt caverns. These storage options are generally characterized by low capital investment costs and are widely available in numerous countries [280]. In countries with limited access to gas infrastructure, such as Sweden, and restricted availability of underground storage facilities, compressed biomethane (CBM) can be stored in pressurized containers. The primary cost associated with this method is the energy required for gas compression to the necessary pressure levels. This storage system requires the implementation of safety infrastructure, including rupture disks, pressure relief valves, and gas leak detection systems [278]. Liquefied biomethane (LBM) is produced by cooling gaseous biomethane to cryogenic temperatures at near-atmospheric pressure. This process reduces its volume by approximately 1600 times, making LBM particularly suitable as a fuel for vehicles and marine vessels. Additionally, it is non-explosive and non-toxic, enhancing overall operational safety. Adsorbed biomethane (ABM) represents a storage technology in which biomethane is adsorbed onto porous solid materials under relatively low pressures (up to 45 bar). When a storage tank is filled with a suitable adsorbent material, the storage capacity can be increased by up to ten times compared to CBM. Furthermore, ABM technology enhances safety due to the lower operating pressure and reduced risk of explosion. Materials investigated for ABM applications include microporous [281] and nanoporous materials and activated carbon [282,283]. However, all these methods require the use of additional technologies and the allocation of financial resources, which increases overall costs.

Another significant barrier to the development of the biomethane market lies in the area of legal regulations. According to the IEA [252], the main problems in this area concern the absence of long-term regulations, the lack of a national biomethane registry and guarantee of origin system, regulatory difficulties regarding injecting biomethane into gas grid, a lack of consistency in compliance with gas quality standards in particular countries, and long-time procedures for obtaining an operating permit. Thus, despite the incentives to develop the biomethane market resulting from international policies in the field of energy market transformation and climate change mitigation, legal regulations regarding biomethane are not introduced quickly enough to ensure effective implementation of the planned plans. The results of the study conducted by Vinardell et al. [272] showed that exempting the energy production from biomethane from the obligation to acquire carbon emission allowances under the EU Emissions Trading System would provide a competitive advantage to biomethane over natural gas and would become an important factor promoting biomethane production in European countries.

7. Conclusions

Considering the global scale, the biomethane market is developing dynamically, and taking into account the programs supporting the dissemination of alternative energy sources and biofuels in transport adopted in many countries, its further development can be expected (assuming the stability of the adopted policies). However, the production and use of biomethane are not evenly distributed on a global scale. They are concentrated mainly in Europe and North America, although in recent years the share of countries from other continents has been increasing. The influence of various factors, including those related to the diversity of climatic and economic conditions, as well as those resulting from the competitiveness of other energy sources on the local market, ecological awareness, and legal conditions, can be considered as a reason for this diversity.

Actions to promote biomethane must take into account regional and economic differences, which affect the possibilities of producing and using biomethane. The availability of raw materials, production technologies, and possibilities of using biomethane on the local market are the basic issues that must be taken into account. It is necessary to provide long-term financial incentives available to a wide range of interested recipients and to reduce procedural obstacles. In addition, to take into account environmental needs, these incentives should promote the use of waste-based raw materials. Sustaining biomethane production in a geographical context will require countries with highly developed biomethane infrastructure to cooperate and share knowledge and experience with developing countries.

From a technological point of view, further improvement of biomethane production technologies is needed to increase their efficiency and reduce costs so that they can be implemented in developing countries. Biological methods have great potential in this regard. They create opportunities to use raw materials of various origins and can be incorporated into hybrid systems. They are also of particular importance for minimizing the impact on the environment. The popularization of the thermochemical pathway for biomethane production would expand the range of substrates useful for biomethane production to include lignocellulosic raw materials commonly available in many countries.