Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications

Abstract

Biogas is a renewable energy resource derived from biomass mainly through anaerobic digestion. Methane (CH4) and carbon dioxide (CO2) and trace elements significantly affect the fuel. Hence, the undesirable components are undesirable and should be removed for various biogas applications. This makes it necessary to purify raw biogas to increase its utility making it suitable for many applications as a sustainable energy resource and process feedstock. Biogas can make substantial contribution to realization of sustainable development goals and the global energy transition. However, the wide-scale use of biogas is seriously constrained by the presence of harmful impurities as well as the low calorific value of biogas. The overall objective of this chapter is to investigate the properties of biogas, its cleaning, upgrading, and its sustainability as a transition energy resource for realization of sustainable development goals and the global energy transition. The chapter provides a comprehensive examination of the potential of biogas as a fuel and potential feedstock for various industrial processes. Furthermore, this study offers a comprehensive examination of the various technological alternatives that can be employed to harness biogas for the purpose of generating heat and electricity.

1. Introduction

Renewable energy sources like biogas and biomethane are important energy carriers when the society seeks to replace fossil fuels with sustainable energy sources [1]. Biogas is a renewable energy resource produced by anaerobic digestion of biomass mainly in the form of municipal waste, farm waste, food waste, and energy crops. Raw biogas typically consists of biomethane (30–80%), carbon dioxide (20–50%), and smaller amounts of nitrogen (1–10%) and varying composition of trace elements namely hydrogen sulfide, ammonia, hydrogen, and various volatile organic compounds depending on the feedstock of biogas. Life cycle assessment of biogas energy resource shows that biogas use effectively reduces greenhouse gas (GHG) emissions [2, 3, 4]. The production and use of biogas as an energy resource helps to diversify the energy systems while at the same time promoting sustainable biomass waste management recycling and disposal [2, 3, 5]. The growing energy demand and concerns over global climate change have increased demand for renewable and green energy sources. Various innovative approaches have been developed for bioenergy production and use as alternatives to fossil fuels as energy sources. Biogas is one of the viable energy options from organic wastes. The sustainability of biogas is enhanced further by production of organic manure as substitute for chemical fertilizers in crop production. Biogas can be used for heating, power generation, fuel, and processing. It could also be used to produce hydrogen, carbon dioxide, and biofuels sustainably. This vast range of applications makes biogas an attractive sustainable energy choice [2, 5, 6].

Over 59 billion m³ biogas or about 35 billion m³ methane equivalent of biogas by volume is produced globally on an annual basis with the European Union accounting for about half of this capacity. The energy generation capacity from biogas globally has expanded rapidly; for example it increased from 65 GW in 2010 to 120 GW in 2019, which is an increase of about 90% [2, 5]. Biogas is an important renewable energy resource whose production and use can aid in mitigation of greenhouse gas (GHG) emissions by substituting fossil fuels and promoting the utilization of biodegradable plant and animal waste feedstocks [7, 8]. As the modern society generates significant wastes, several waste disposal and treatment methods are employed to avoid environmental pollution with anaerobic digestion being one of the popular options as it additionally produces useful energy [9], which is a fundamental requirement for society [10, 11]. International treaties or agreements that commit countries to minimize their carbon footprint like the agenda 21 and Kyoto Protocol are motivating factors for production and use of biogas [12, 13], since they advocate for the transition to renewable and low carbon sources of energy to reduce the global greenhouse gas emissions particularly from the energy sector, which is dominated by fossil fuel sources of energy, with biogas being a very important option [14, 15].

Biogas can be produced from a variety of different types of biodegradable biomass like chicken manure, crop waste, and animal manure, through several processes but mainly by means of controlled anaerobic digestion. Raw biogas has feasible applications like heating and power generation as is, or it can be processed to biomethane and used to produce value-added chemicals. Purified or cleaned biogas has higher utility as a green fuel, and bio natural gas fed into natural gas pipes upon upgrading to remove carbon dioxide [8, 16], while reducing greenhouse gas emissions and contributing a huge potential for use as a renewable resource. In 2014, biogas accounted for almost 8% of Switzerland’s renewable energy generation (excluding hydropower) and 0.29% of the country’s overall energy consumption [17, 18, 19]. Biogas has a potential use as substitute for firewood and thus can protect the forest ecosystems. By 2040, it is projected that biogas will supply clean fuel for almost 200 million people, predominantly in Africa and Asia. Therefore, biogas is a significant player in realization of sustainable development goals [5, 20, 21, 22]. The SDGs and the energy transition to a green and low carbon energy and electricity mix will be much easier to accomplish with biogas as an alternative.

About 2.8 billion people globally rely on primary biomass fuel for their energy needs. Biogas production presents huge potential as a strategy to generate renewable energy products from biomass waste sources with significant economic and environmental benefits. The practical efficiencies indigestion is low, which calls for strategies available to overcome these barriers and create more efficient energy [19, 23]. It is the methane content of biogas that directly influences the energy value of biogas and hence the need to manage the process to ensure high methane composition and additionally control the polluting potential of organic residues having high contents of Biochemical Oxygen Demand (BOD) [24, 25]. With biogas proving to be a sustainable and renewable energy source [26, 27], the growing concerns over greenhouse gas emissions and climate change have generated significant interest in biogas as an energy resource for the global transition [28, 29], hence growing production of biogas from organic waste [30, 31] with common applications being power generation, heating applications like cooking, lighting, and biofuel production with biogas feedstock [6, 32, 33].

Providing modern energy services to their populations is a challenge for many developing nations; for instance, about 450 million people in India did not have access to modern energy for electricity and cooking [34, 35]. Technologies that conserve resources and revenue in agriculture are crucial to achieving sustainable food and energy production [34]. Coal, oil, and gas account for over 60% of global electricity generation, but renewables are steadily increasing their proportion, from 26% in 2015 to 28% in the first quarter of 2020, with variable renewables increasing from 8 to 9% during the same period [36]. Since 1990, the proportion of worldwide electricity output that comes from renewable sources has increased by an average of 2% per year, faster than the annual increase in electricity demand (1.8%). When compared to the annual capacity increases of solar photovoltaics (36.5%) and wind energy (23.0%), biogas’s 11.5% ranked third fastest. The substantial role played by biomass in global energy generation is evidenced by the 9.7% annual growth rate of biofuels since 1990 [37]. Particularly, the relevance of biogas in agriculture and food production cannot be overemphasized. Beyond providing food for all of humanity, agriculture is the primary source of income for more than two-thirds of the world’s population. In addition, nearly 82% of the global population is directly or indirectly involved in smallholder agriculture and other industries, making this economic activity the mainstay of many developing countries [34]. This shows that biogas is crucial to the energy transition and that agriculture and other carbon transformative processes should boost its production and consumption.

Biogas has a significant role to play in the impending energy transition as a renewable and sustainable energy resource because of its high energy conversion potential and widespread availability for power generation, for industrial applications as a renewable process feedstock, and for thermal energy applications [38, 39, 40, 41, 42]. In addition to producing biogas, the anaerobic digestion (AD) process leads to feedstock treatment during the treatment phase, and it can also produce digestate, which is a useful organic fertilizer that can replace chemical fertilizers in sustainable agriculture [43, 44]. An essential part of the global carbon cycle is the creation of biogas through microbial control, as natural anaerobic biodegradation releases 590 to 800 million tons of methane into the atmosphere annually [39, 45, 46]. Numerous different organic feedstocks are anaerobically digested to produce biogas, which can then be enhanced to produce natural gas such as biomethane [47, 48, 49]. The sustainable technique of turning biomass waste into biogas can lead to advantages including less carbon emissions, improved organic waste management, and increased resource use efficiency.

In terms of efficiency and cost, the Stirling engines and internal combustion engines theoretically offer the most viable options for converting biogas to electricity, particularly on a small scale. Comparatively, the electricity generated by internal combustion engines per kilowatt hour is cost competitive and efficient. The engines are available in a wider range of sizes, they are more flexible and efficient, and require less effort to operate and service [50, 51, 52]. For slightly larger plants, gas turbines are commonly utilized for power generation in the capacity range of 3 to 5 MW. Turbines and microturbines have less stringent fuel quality standard requirements compared to internal combustion engines making them ideal prime movers for raw biogas, which has a wide range of harmful impurities [53, 54, 55]. Both the steam turbine and the gas turbine power plants using biogas for power generation encounter higher total capital investment costs Biogas and biomethane provide a sustainable pathway to include rural businesses and communities in the energy transition. This can be accomplished by creating grid-connected electricity and reducing grid load by generating their own electricity and heat.

Biogas as an energy resource has the potential to contribute to sustainable energy transition if carefully produced through the process of anaerobic digestion of organic substrate under carefully regulated conditions and used in a manner that limits leakages of methane, which is a greenhouse gas [56, 57]. The energy content of biogas is contingent upon the methane composition, which is subject to influence from both the production process and the type of substrate employed. The composition of biogas may include elements like sulfur, which can make it inappropriate for use as a fuel in internal combustion engines and certain industrial chemical and thermal processes. The calorific value of biogas demonstrates variability because of its composition, primarily influenced by the relative abundance of methane. The heating value of biogas generally falls within the range of 21–23.5 MJ/m³, suggesting that an approximate equivalence of 0.5–0.6 liters of diesel fuel or roughly 6 kWh of electricity can be attributed to 1 m³ of biogas.

Biogas has significant potential as a sustainable and renewable energy resource, both currently and in the foreseeable future. Biogas energy can make a substantial contribution to the achievement of sustainable development goals and the global transition towards sustainable energy, because of its minimal greenhouse gas emissions, widespread availability, and access to raw materials. The widescale incorporation of biogas into the global energy mix has been constrained due to the presence of harmful impurities and low calorific value of raw biogas [11, 14, 31, 58]. The overall objective of this study is to investigate the properties of raw biogas as a fuel, its cleaning methods without necessarily upgrading to biomethane, and its sustainability as a transition energy resource for realization of sustainable development goals and the global energy transition. This chapter provides a comprehensive examination of the potential of biogas as a fuel and potential feedstock for various industrial processes. Furthermore, this study offers a comprehensive examination of the various technological alternatives that can be employed to harness biogas for the purpose of generating heat and electricity due to its significant energy potential and sustainability [59, 60, 61].

1.1 Problem statement

The heightening need for food and energy resulting from the growth of the global population, particularly in developing countries, has intensified the strain on energy production and consumption. The continued use of fossil fuels threatens humanity with unbearable consequences of climate change due to greenhouse gases, which are causing climate change [18, 34]. Although biogas has a great deal of promise to supply clean energy to both rural and urban populations, the prevalence of inoperable and abandoned biogas facilities has raised concerns about the sustainability of the technology [19, 62]. This poses questions concerning the dependable creation, distribution, and use of biogas fuel [35]. It is technically possible to convert this waste into valuable energy sources, thereby generating additional revenue [57, 63, 64]. The challenges facing biogas applications include lack of technical know-how, limited or lack of incentives and subsidies to make biogas competitive in several countries, and the low calorific value of and presence of harmful impurities in biogas. These challenges can be addressed by developing and accessing appropriate and affordable biogas technologies and development of a comprehensive policy on construction and operation of biodigesters, biogas utilization, and sustainable diversification in biogas energy products and services [2, 5, 6].

The implementation of biogas technology enables the harnessing of renewable energy through the utilization of crop and animal waste. The energy source exhibits significant potential for utilization in electricity generation and as a viable thermal energy solution for residential structures. The ultimate outcome of this process can be efficiently harnessed as a valuable fertilizer in rural agricultural environments and households, leading to a reduction in costs related to waste management and disposal [65]. Biogas energy can improve rural people’s lives and economies. Many small-holder farmers in developing nations burn biomass for disposal, even though it can be used as a fertilizer, and with biogas technology, an impoverished organic fertilizer that is more affordable can be made by smallholder farmers [66]. Designing biogas systems so that they can reliably supply most or 100% of their energy demands requires defining a number of factors [67].

Sustainable farm-level biogas energy requires appropriate infrastructure design and selection [18, 68]. Another thing to consider in such system design is the natural decomposition of agricultural waste that results in the emission of substantial amounts of methane, a potent greenhouse gas, into the atmosphere. For instance, in the year 2015, the exclusive contribution of livestock manure to methane emissions in the United States of America amounted to approximately 10%. However, a mere 3% of the total livestock waste underwent recycling via anaerobic digestion [57]. This presents the main issue in utilizing as it lies in its unsteady production value and variations in quality. These factors can potentially disrupt the generation process or hinder the effectiveness of biogas applications, resulting in reduced reliability [69].

1.2 Rationale of biogas cleaning/purification

Both developed and developing countries face challenges with utilization of significant organic waste and the mounting pressure to replace fossil energy resources like natural gas, oil, and coal with renewable energy sources. Anaerobic digestion of organic biomass is now a mature technology while biogas in its raw form or cleaned and upgraded forms has multiple applications as a bioenergy substitute of fossil fuel sources. The production and utilization of biogas has also been growing with capacity growing by more than double between 2009 and 2022 [70]. Methane fermentation occurs naturally in the process of organic matter decay in oxygen deficient environments like swamps and in landfills leading to natural emission of methane in the atmosphere as a greenhouse gas. Controlled biogas production can be used to reduce these natural emissions [4, 5]. This research presents biogas as a fuel and feedstock for applications like production of biomethane, methanol, syngas, power generation through combustion, or fuel cells, among others [71, 72, 73].

Biogas has a very important role to play in the ongoing energy transition as a renewable and sustainable energy resource because of its high energy conversion potential and the widespread availability for power generation, for industrial applications as a renewable process feedstock, and for thermal energy applications [38, 39, 40, 41, 42]. Additionally, anaerobic digestion (AD) is an effective waste treatment and disposal method while the digestate from biodigesters is a rich organic fertilizer [43, 44]. Anaerobic digestion is a critical process in the global carbon cycle which releases 590 to 800 million tons of methane into the atmosphere annually through uncontrolled biodegradation. These harmful emissions can be mitigated through controlled anaerobic digestion for useful biogas production [39, 45, 46]. The benefits of biogas fuel and its production process include reduced less carbon emissions, better organic waste management, and increased resource use efficiency [39, 74].

Biogas contributes to the sustainable transition as a renewable fuel with multiple applications as a fuel characterized by a high methane content, which is generated through the process of anaerobic digestion of organic substrate under carefully regulated biochemical process conditions [56, 57]. Indigestible carbohydrates, proteins, and lipids are not present in the biomass substrate utilized for biogas production, which can complicate or slow down the digestion [75, 76]. Biogas can also be used as a feedstock for production of biomethane, carbon dioxide, hydrogen, and various biofuels for a wide range of applications including generation of heat and power and feedstock for various industrial processes [77, 78].

The whole world has got about 2.8 billion people who rely on primary biomass fuel like coal, firewood, crop waste, and dried animal waste for their energy needs in cooking and heating, which leads to high-level household pollution [7]. Biogas production presents huge potential as a strategy to generate renewable energy products from biomass waste sources with significant economic and environmental benefits. The practical efficiency in biodigestion is low, which calls for strategies available to overcome these barriers and create more efficient energy systems [23]. It is the methane content of biogas that determines the energetic potential of biogas since it directly influences the calorific value of the fuel hence the need to manage the process to ensure high methane composition. Organic bio digestion additionally reduces the polluting potential of organic residues having high contents of Biochemical Oxygen Demand (BOD), while the substrate can be used as a nutrient rich valuable fertilizer [24, 25].

The human population is steadily growing leading to increased demand for food and energy, which simultaneously aggravates the environmental challenges. Substituting fossil fuels with renewable energy alternatives is currently a major global issue of the twenty first century and is a key sustainable development objective. Implementation of biogas technologies will significantly transform costly, socially sensitive issues and environmentally damaging fossil fuel dependence, environmental pollution, Greenhouse Gas (GHG) emissions, and waste conversion into profitable options like electricity generation, heat production, and production of biofertilizers as well as provision of green vehicular fuel substitute of fossil gas. A national biogas production development program can significantly lead to development of new enterprises, boost income for rural communities, and create new jobs to improve people’s socio-economic well-being [70].

1.2.1 Limitations/challenges facing biogas energy resources

The main challenges facing biogas production and use as an energy resource include high equipment cost and a lack of a governmental incentive programs in many countries, low reliability or lack of guarantee of long-term performance of biogas plants due to technological challenges, unpredictable investment environment, limited biogas distribution and storage capacity for some countries like Sweden, and low cost of commercial fertilizers as well as low cost of fossil fuels like gas and oil in some countries [24]. A study by [7] on household biogas digester use in rural China showed that there was rapid growth in the early part of the century in the use of household biogas digesters but the operations were never smooth because out of 1743 households interviewed, 42% adopted household biogas digesters, but on average they worked for just 6.66 months each year, which is an indicator of underutilization of the digestors.

1.2.2 Benefits of biogas as a renewable energy resource

Biogas has various applications that include production of biofuels as renewable energies; reduction in greenhouse gas emissions; removal of odors and flies, which are generally associated with uncontrolled biomass biodegrading; financial savings and revenue from biogas derived energy products like electricity and biomethane, waste treatment; and recycling of organic waste; reduced air and water pollution; alleviation of rural poverty; and better social conditions of rural or remote settlements, which include gender balance since the women who mainly bear the responsibility of looking for fuel will have an opportunity to engage in other tasks while forests and vegetation will be conserved due to reduced use of firewood and charcoal [2, 6, 79, 80].

The potential benefits associated with use of biogas as an energy resource including generating electrical energy from biogas may include the following:

1. Facilitates decentralized generation, which is done close to consumers with benefits like reduced transmission and distribution costs.

2. Generates extra income and savings from the energy produced from biogas and sold to the grids.

3. A decrease in utility-purchased electricity that must be offset.

4. Creating opportunities for cogeneration, which is a more efficient form of energy exploitation.

5. Reducing greenhouse gas emissions from methane to the atmosphere, which would have been emitted freely to the atmosphere from uncontrolled biodegradation, etc.

The challenges limiting full use of biogas that should be overcome include lack of national generating technologies, the need to clean biogas before use, the challenge of economic feasibilities that requires incentives, and the lack of penalties for possible environmental damages from biogas schemes [24, 25].

2. Composition and properties of biogas

Biogas composition is a function of many factors like process design and the nature of the substrate with the main components of biogas being methane and carbon dioxide, and other minor components in varying proportions [1]. Methane and carbon dioxide are the primary components of the gas mixture known as biogas. Hydrogen sulfide, hydrogen gas, moisture, and siloxanes are a few others. Carbon monoxide, hydrogen, and methane are the combustible components, and they generate heat for a wide range of thermal and electrical uses [81]. The composition of biogas produced from any given feedstock and any given level of control over that process will be unique. However, the main components of biogas are methane (CH₄) and carbon dioxide (CO₂) [53]. With just one carbon and four hydrogen atoms, pure methane has no discernible odor. It is easier to ignite methane than it is to ignite air. A mixture of 5–15% of this substance with air is explosive. While methane alone is non-toxic, inhaling too much of it in a confined space can be fatal because it displaces oxygen. Since it may linger in the air for up to 15 years, methane is a potent greenhouse gas; on a 20-year time frame, it is nearly 20 times more effective at trapping heat than carbon dioxide [82, 83]. Because of the severe environmental concerns associated with the development and processing of biogas in the absence of effective regulation, this highlights the necessity for regulated bio-deration of organic wastes.

Biogas has multiple applications beyond just energy generation, including use as a feedstock in the petrochemical, hydrogen, and synthesis gas industries. Methane and carbon dioxide are the primary components of biogas, but other contaminants such as ammonia (NH₃), water vapor, hydrogen sulfide (H₂S), nitrogen (N₂), methyl siloxanes, oxygen, halogenated volatile organic compounds (VOCs), hydrocarbons, and carbon monoxide (CO) are often present as trace elements. For safety and efficiency reasons, it is necessary to filter out certain trace elements before using biogas as a fuel. Hydrogen sulfide and carbon dioxide are two examples. It is important to find ways to reduce the high operational expenses and energy consumption associated with biogas upgrading so that it can be used as a fuel [12, 13, 84].

2.1 Composition of biogas

Biogas is a mixture of gases consisting of methane and carbon dioxide as the main components, in addition to several trace elements like carbon monoxide, hydrogen sulfide, hydrogen gas, moisture, and siloxanes. Carbon monoxide, hydrogen, and methane are the combustible constituents with methane content being the main determinant of heating value of biogas. Combustion of biogas generates heat for various thermal and electricity applications with related emissions, mainly carbon dioxide, a greenhouse gas. Biogas composition of biogas varies based on the substrate used and the process control applied [2, 3].

Methane is the main constituent of biogas that generally accounts c 50–80% composition, while other constituents are carbon dioxide (20-45%), water vapor (2%), and trace gases like O₂ N₂, NH₃ H2 H₂S [56, 77]. Biogas may also contain siloxanes formed in anaerobic decomposition of materials commonly found in detergents and soaps. The composition varies with feedstock and process, for example, landfill has average methane composition of about 50%, while advanced waste treatment technologies yield 55%–75% methane content. Reactors with free liquids can generate biogas with 80%-90% methane content using in-situ gas purification techniques [2, 85, 86]. The average biogas composition is shown in Table 1.

	Element	Composition (%)
1	Methane	30–80%
2	Carbon dioxide	20–50%
3	Nitrogen	0.5–10%
4	Ammonia	0–0.05%
5	Moisture	2–8%
6	Hydrogen	0–1%
7	Hydrogen sulfide	0.1–3%
8	Oxygen	0.1–3%
9	Ammonia	0–0.5%
10	Carbon monoxide	0–0.1
11	R2SiO	0–0.5 mg/m^3
12	CxHy	0–1%

Table 1.

Average composition of biogas [2, 16, 25, 49, 53, 56, 66, 87, 88, 89].

Based on the data presented in Table 1, it can be observed that the predominant constituents of biogas consist of methane and carbon dioxide. The relative proportions of these components generally vary between 30 and 80%, with the specific range being contingent upon factors such as the quality of the feedstock utilized and the efficacy of process management.

2.2 Thermodynamic properties of biogas

The H₂S concentration can be reduced through chemical, biological, and physical means, whereas the water content can be reduced through condensation in gas storage or along the gas stream [42, 77]. With rising methane content, biogas has a calorific value that varies between 5000 and 7000 kcal/m³. A comparable volume of biogas (1 m³) can be produced by: 0.7 m³ of natural gas; 0.6 kilograms each of kerosene, gasoline, and butane; 3.5 kg each of wood and dung briquettes; 4 kWh each of electricity and carbon; and 0.43 kg each of butane [39, 90]. At 0.1013 Mpa and 273 K, biogas consists of 60% methane and 40% carbon dioxide. Molar mass 16.04, specific heat capacity 2.165 kJ/kg. K, and ignition temperature between 650 to 750°C are shown for biogas in Table 2.

	Property	Symbol	Value
1	Specific heat capacity	Cp	2.165 kJ/kgK
2	Molar mass	M	16.04
3	Gas constant	R	0.518 kJ/kg
4	Normal density	g	1.2 g/l
5	Critical density	gc	320 g/l
6	Relative (to air) density	gr	0.83
7	Calorific value of biogas	LCV	22.6 Mj/m^3
8	Critical temperature	T	−2.5 °C
9	Critical pressure	p	7.3–8.9 Mpa
10	Flammability limit content in air	v	6–12%
11	Ignition temperature	T	650-750°C

Table 2.

Thermodynamic properties of biogas [77, 90, 91].

From Table 2, it is noted that the thermodynamic properties of biogas are competitive with those of other fuels. Due to its advantages for the environment, biogas is a desirable fuel despite having somewhat worse thermal properties than fossil fuels [71, 92, 93]. Table 3 lists the biogas equivalents of various fuels with remarks.

	Device/; technology	Application	Remarks
1	Fuel cell	Electricity generation	Though expensive, fuel cells are more efficient
2	Hydrogen production	Biomethane can be converted to hydrogen for use as combustion fuel or electricity and process chemicals.	Renewable hydrogen can be produced from biogas or biomethane by dry reforming, steam reforming, or hydrolysis
3	Biomethanation	Biomethane can be fed to natural gas supply as substitute for natural gas	Renewable replacement of fossil natural gas is feasible with use of biomethane
4	Diesel engine	Biomethane can be used as a diesel engine fuel in either dual fuel mode or pure gas engines	Diesel engines have more fuel flexibility and efficiency and can easily use biofuels as fossil fuel substitutes
5	Gas/petrol engine	Biomethane can be used as a fuel for petrol or gasoline engines with little or no modification	The gas/petrol engines are less efficient engines compared to diesel engines but are simpler and easier to convert to biogas engines
6	Stirling engine	Stirling engines are also called hot air engines.	The Stirling engines can operate on a wide variety of fuels due to high adaptability
7	Gas turbine	Based on size, gas turbines can be micro, small, or large gas turbines in open, closed, or combined cycle configuration	Turbines are simple in construction, are versatile, and can use raw biogas as well as biomethane and are easy to operate.
8	Cogeneration	In cogeneration, biomethane is burned to simultaneously produce useful heat and electricity.	Cogeneration with biomethane as a fuel can be applied on various conversion systems like Stirling engines, diesel engines, gas turbines, hydrogen, and fuel cells to increase system efficiency
9	Trigeneration	Trigeneration refers to generation of electricity and both heating and cooling from same fuel/energy resource simultaneously	Trigeneration is costly and complex but the most effective method of energy conversion

Table 3.

Summary of biogas to electricity conversion systems and technologies.

2.3 Energy and electricity potential of biomass

Biogas can be used for different purposes, including electricity and heat production (cogeneration), heat only, power only, biomethane, fuel for vehicles, high tech process energy, and in the chemical industry as feedstock material. With average caloric value of calorific value of biogas being 21–23.5 MJ/m³, 1 m³ of biogas is an equivalent of 0.5–0.6 liters of diesel fuel or 6 kWh in energy content. But due to inefficiencies and losses, 1 m³ of biogas yields around 1.7 kWhe [5, 44, 59]. Biogas output and energy generation from various substrates are compared in Table 4.

	Type	Electricity produced per ton fresh matter (kW·h)	Biogas yield per ton fresh matter (m^3)
1	Fat	1687.4	826–1200
2	Maize silage	409.6	200/220
3	Chicken litter/dung	257.3	126
4	Food waste (disinfected)	224.6	110
5	Municipal solid waste	207.2	101.5
6	Fruit wastes	151.6	74
7	Cattle dung	122.5	55–68
8	Horse manure	114.3	56
9	Sewage sludge	96.0	47
10	Pig slurry	23.5	11–25

Table 4.

Electricity produced from biogas yield per ton of fresh matter.

From Table 4, it is noted that different feedstocks have different biogas potential with fats having the highest potential followed by maize silage and chicken droppings. Pig slurry and sewage sludge have some lowest values of biogas potential. The conversion factors are 35% electrical efficiency, and 55% methane content [27]. When compared to maize silage, chicken manure, and sterilized food waste, fat has the highest biogas yield and largest biogas and electricity potential.

3. Energy content and potential of biogas

The energy content of biogas varies but a typical normal cubic meter of methane has a calorific value of around 10 kWh, but CO₂ being noncombustible has zero, making methane the ultimate determinant of the energy value of biogas. At 60% methane composition, then, the energy content of biogas would be less than 10 kWh per normal cubic meter, as CO₂ and other noncombustible impurities absorb instead of generating heat [1, 2, 94]. The primary factor influencing the thermal energy content of biogas is the methane composition within the gas mixture. The volumetric composition of methane in biogas widely varies based on factors like the feedstock used and process control [49, 95, 96]. This disparity suggests that biogas's LHV ranges between 16 and 28 MJ/m³, rather than remaining stable at a single value. Combustion of biogas releases thermal energy that can be used for cooking or other uses like internal combustion engines [3]. Raw biogas has undesirable components like hydrogen sulfide, ammonia, and various volatile organic compounds whose presence and composition are determined by the feedstock used and process control applied [33, 97].

3.1 Biogas for engine applications

Biogas can be used as a fuel internal combustion engine although it requires some modifications on the engine design and ignition to handle gaseous biogas fuel instead of the traditional liquid fuels. Biogas has higher octane rating (MON = 130) enabling it to sustain higher compression ratios without engine knocking, hence better engine performance [51, 98, 99, 100]. Historically, the use of biogas as an engine fuel started during the Second World War (2WW), as Germany and its Nazi territories had challenges sourcing fuel vehicles and farm machinery [44, 62]. This forced them to resort to sewage gas and biogas from manure and digesters, which were bottled and used as fuel during the crises.

Biogas is an efficient, sustainable, and environmentally friendly fuel that can perform optimally in spark ignition engines modified to suit its specific properties. The performance and utility of biogas as engine fuel is increased by methane enrichment or the addition of hydrogen to improve its flame quality [1]. The main challenge with biogas as engine fuel is the gaseous nature, which means low volumetric density. Biogas as an engine fuel is improved by biogas upgrading, methane enrichment, prechamber combustion, alteration of ignition parameters, higher compression ratio, and addition of hydrogen for better performance and emissions management [98, 101].

The following are some guidelines to consider when selecting biogas engine fuel.

1. The amount of methane present must be high.

2. The amount of water and carbon dioxide (CO₂₎ should be minimized to maximize energy production based on high calorific value.

3. Acids produced by the condensation or combustion of sulfur corrode metals by eating away at their surface and should therefore be removed.

3.2 Energy content and equivalents of biogas

Biogas is an odorless and colorless gas that burns with a clear blue flame like natural gas but has a lower calorific value of 20–26 MJ/m³ (537–700 Btu/ft³) compared to natural gas whose caloric value is 39 MJ/m³ (1028 Btu/ft³) [6, 33, 94]. Energy equivalents of biogas are presented in Table 5.

	Alternative fuel	Biogas Equivalent	Remarks
1	1 kg of petrol	2.5 m^3 of biogas	Has less energy content than petrol
2	1 kg of carbon	2.33 m3 of biogas	Carbon has higher energy value than biogas
3	1 liter of kerosene	2.0 m^3	Biogas can limit fossil fuel dependence and use
4	1 kg of fuel oil	1.42 m^3 of biogas	Has lower energy content than fuel oil
5	1 m^3 of natural gas	1.43 m^3 of biogas	Has lower energy content, may need purification
6	1 kg charcoal	0.5 m^3 biogas	Biogas use limits forest destruction
7	1 kg of firewood	0.29 m^3	Biogas can be used to avoid deforestation
8	1 kWhr of electricity	0.25 m^3 of biogas	Biogas can substitute grid electricity use
9	1 kWhr of electricity	0.25 m^3 of biogas	Biogas can substitute grid electricity use
10	1 kg of dried cow dung	0.1 m^3	Biogas has more energy value than dried cow dung
11	1 kg of manure briquette	0.083 m^3 of biogas	Biogas has more energy value

Table 5.

Energy equivalents of biogas [77, 90, 91].

From Table 5, it is noted that biogas has a higher energy value than manure briquettes, but it has a lower energy content than fossil fuels. But it is a potential substitute for fossil fuels and cuts down on forest clearance at the same time.

The calorific value of the biogas generally varies between 22.5 and 25 MJ/m³, or 6.25 and 10 kWh/m³ assuming methane has heating value of 35.8 MJ/m³. The energy content can be further enhanced if the raw biogas is upgraded to biomethane [24, 25]. Biogas with 55% CH₄ has an average calorific value of about 21.5 MJ/m³, compared to pure methane (CH₄), which has a calorific value of about 35.56 MJ/m³, which is one of the main reasons for upgrading by removal of carbon dioxide (CO₂) from raw biogas [4, 5, 70]. Table 6 shows the energy value of biogas compared to other fuels like butane, natural gas, and propane.

	Fuel/element	Calorific value (MJ/m^3)
1	Butane	117.23
2	Propane	92.11
3	Methane	35.56
4	Natural gas	31.82
5	Biogas	22.5 to 25
6	Biomethane	23.02

Table 6.

Calorific value of common gaseous fuels.

From Table 6, it is noted that biogas has the lowest calorific value compared to methane, propane, butane, natural gas, and biomethane. Therefore, upgrading to biomethane can improve its calorific value making it more competitive thermodynamically.

4. Biogas purification/cleaning

The quality of biogas fuel for use in power generation and other applications can be improved using a variety of techniques. By guaranteeing constant fermentation through continuous supply of feedstock of the right quality and digestion at the ideal parameters of C/N ratio, PH, dilution, and digestor temperature, biogas output can be optimized. A little amount of oxygen or air can be injected into the headspace of a storage fermenter to help hydrogen sulfide (H₂S) be oxidized by microorganisms, leading to the removal of a significant portion of the sulfate from the gaseous phase. This technique is the most affordable and popular way to desulfurize biogas, removing up to 95% of the sulfur [27, 102].

The second method involves filtering the water with iron oxide and activated carbon as part of an external chemical treatment. Iron hydroxide is used in a reversible procedure that regenerates the filter by adding oxygen [99]. As an alternative, adsorption materials like iron-rich soils or leftovers from the steel or aluminum industries can be used. The governing equation is given by the relation.

$$\mathit{Fe}{\left(\mathit{OH}\right)}_2+H_{2}S\to \mathit{FeS}+2C{H}_{2}O$$E1

Activated carbon filters are used as standard components are provided for the system [77, 103].

Raw biogas is generally corrosive making it necessary to carefully select and maintain equipment material and to treat raw biogas. The corrosive nature of biogas is because of the presence of traces of hydrogen sulfide. The internal combustion engine and micro-turbine applications recommend allowable H₂S levels in the biogas, of 10 ppm or 0.0001% by volume for internal combustion engines and 5000 ppm. For microturbine. Different countries have also imposed different quality standards e.g., in Brazil the standard emission for sulfates according to CONAMA (the national environmental agency) is limited to 1.0 mg of S L⁻¹. Air pollutants containing sulfur include SO₂, SO₃, H₂S, and sulfates. Ammonia is also a corrosive element in raw biogas and can be released in the form of nitrogen oxides as a product of combustion. Nitrogen oxides are also harmful to human health and to the environment [24, 25].

5. Biogas applications

Biogas can be utilized in its raw, cleaned, or upgraded form for several applications as an energy resource and as a feedstock for several chemical processes. These applications include production of biomethane, hydrogen production, fuel cell energy source, production of biofuels, power generation, and thermal applications as a heat source [2, 5]. Biogas is used in combined heat and power (cogeneration), heating, feedstock for biomethane production used as a direct substitute of natural gas in applications like automotive fuel, industrial process heating, and feedstock in chemical industries [59, 96]. The various applications of biogas are discussed below.

5.1 Biomethane production

Biomethane is a product of biogas or syngas upgrading having superior properties to biogas, and a desirable substitute of natural gas. Compressed biomethane can be utilized as cooking gas in industrial settings, distributed by injecting it into natural gas mains, and packaged in containers or cylinders for home use. The main barrier is the processing cost, which is inversely proportionate to the type of technology used. The term “bio-CNG” refers to compressed fuel with a high methane content. To make bio-CNG, pure biogas that contains more than 97% methane is compressed to 20–25 MPa. Compressed bio-CNG has the same fuel characteristics, economics, engine performance, and emissions as regular CNG. Due to its high-octane number, bio CNG, like conventional CNG, offers outstanding thermal efficiency. As a result, it can be utilized as a direct substitute for regular compressed natural gas in gas pipes and other applications, such as a natural gas fuel source [104].

5.2 Hydrogen production

Hydrogen is an ideal raw material for a sustainable energy transformation, but with the challenge being where and how to get hydrogen from renewable sources. Renewable hydrogen can be produced using renewable energy sources and usually produced via water electrolysis [105]. Biogas has applications beyond electricity and biomethane production, as, through steam reforming, it can be used to manufacture green hydrogen, in a process where a catalyst refines and separates the hydrogen from the gas stream [41, 105]. The most common method used to manufacture hydrogen is by steam-reforming of natural gas, followed by pressure-swing adsorption to remove impurities. Small steam reformers in biogas plants are in commercial operation [105]. The biogas has low heating coefficient due to high composition of non-combustibles like carbon dioxide and water vapor which reduce the energy content [105]. Upon removal of carbon dioxide and water molecules, the methane (CH₄) can be used for hydrogen synthesis and bio-fuel production. Methane can be split to hydrogen molecules (H₂) in a process that can be done in a steam/methane reformer. In the process, high pressure and temperature steam is combined with the methane (CH₄) to produce flow of hydrogen molecules and CO molecules [48, 106].

It is through thermochemical processes of hydrocarbons that large-scale hydrogen production is manufactured through the reforming process. Biomethane has significant potential application in hydrogen manufacture as a substitute of fossil natural gas as a raw material for reforming processes. The demand for renewable hydrogen production is set to grow significantly due to concerns over fossil fuels depletion and greenhouse gas emissions, and associated concerns over global climate change. The selection of the reforming process is influenced by the availability of capital, hydrogen demand, purity hydrogen and the composition of the biogas feedstock used [8, 107].

Biogas can be used in fuel cells for power generation. This technology promises to play a key role in the hydrogen economy for sustainable energy transition. Hydrogen fuel cells can be used as prime movers in electric vehicles just like batteries do, in addition to application in power generation [21, 108]. Biomethane derived from biogas can be used as a source for renewable hydrogen, for stationary fuel cells in power generation and fuel cells as prime movers for electric vehicles (FCEVs). The hydrogen-powered FCEVs are environmentally attractive since they have no tailpipe emissions making them clean transport option and substitute for fossil fuel-powered vehicles [2, 107].

Use of biomethane for hydrogen production can increase energy sustainability for energy applications like fossil fuels. Hydrogen can be manufactured by autothermal reforming (ATR), electrolysis, or methane reforming (SMR) [109]. Biomethane can be used as a substitute for natural gas, which will provide a hedge against growing demand for natural gas [107].

Hydrogen fuel can be used to reduce emissions from engines that are widely used in transportation. Hydrogen fuel cells promise to provide an alternative to internal combustion (IC) engines particularly due to the clean exhaust emissions, renewal nature of the fuel, and higher efficiencies. Hydrogen fuel cell vehicles can achieve widespread acceptance except for existing challenges like waste heat removal in mobile applications [110].

5.3 Production of biofuels

The transport sector is important since it accounts for about 14% of the global greenhouse gas emissions [111]. Liquefied biomethane is not only an effective fuel for generators and other large machinery, but it may also be utilized as a building block in the production of other fuels and chemicals. As an alternative to fossil fuels and several other processed transport fuels, biomethane is already being used by several countries [50, 112].

Biofuels include the Bio-CNG, which is compressed biomethane like CNG in properties with industrial, automotive, and domestic applications. The process needs removal of impurities likes water, N₂, O₂, H₂S, NH_3, and CO₂ to achieve composition of >97% CH₄, <2% O₂ at 20–25 MPa. Bio-CNG occupies less than 1% of the volume at standard conditions [112, 113].

Biomethane can also be used in the industry as transport fuel by liquefying it at a high pressure ranging from 0.5 to 15 MP [4]. Through biological or chemical processes, biomethane made from biogas can be converted into methanol, diesel, LPG, and gasoline. As can be seen in the picture below, methane is partially oxidized to produce methanol [2].

$${\mathit{CH}}_4+0.5O_2\to C{H}_3\mathit{OH}∆H^0=-128\mathit{kJ}/\mathit{mol}$$E2

In another method, methane is biologically converted from biomethane to methanol by using methanotrophic bacteria used in methanol production through the action of methane monooxygenase (MMO) enzyme [5].

Methanol can also be produced by reforming methane to syngas then followed by catalytic conversion of syngas to methanol as shown below [6].

$$2H_2+\mathit{CO}\to C{H}_3\mathit{OH}+2H_2∆H^0=-91\mathit{kJ}/\mathit{mol}$$E3

$$3H_2+{\mathit{CO}}_2\to C{H}_3\mathit{OH}+H_{2}O∆H^0=-49\mathit{kJ}/\mathit{mol}$$E4

Then, the methanol-to-gasoline process can be utilized to convert the methanol into gasoline. Biogas or biomethane can be turned into methanol using the dry reforming, steam reforming, partial oxidation reforming, autothermal reforming (ATR), or Fischer-Tropsch (FT) process. Syngas, the main byproduct of the biomethane reforming procedure, can be used to produce a wide variety of long-chain hydrocarbons [114].

1. Dry Reforming

   In dry reforming, CO and H₂ are produced by the reaction of methane (CH₄) and carbon dioxide (CO₂). The process uses CH₄ and CO₂ which are both greenhouse gases making it very attractive. However, the endothermic reaction reduces heat emitted in CO₂ production. Dry reforming is an effective method of creating synthesis gas with an H₂/CO ratio close to 1 is dry reforming [115]. The syngas ratio (H₂/CO = 1) produced by dry reforming is lower than that of steam reforming. In this reaction, the water gas shift reaction (WGS) influences H₂/CO ratio by decreasing it because of the reverse reaction that oxidises hydrogen to water. Through partial oxidation of methane with feeding water, the H₂/CO ratio is maintained between 1 and 2. This improves the responsiveness for shifting water and gas ahead. Due to the exothermic nature of partial oxidation, the energy requirement of the process is significantly reduced [114]. Dry reformation occurs within a temperature range of 700–1000°C [114, 116].

   $${\mathit{CH}}_4+{\mathit{CO}}_2\to 2\mathit{CO}+2H_2∆H^0=247\mathit{kJ}/\mathit{mol}$$E5

2. Steam Reforming and Water Shift Reaction

   This process combining methane in biomethane with water vapor generates CO and H₂ in the presence of a catalyst. The process is endothermic and takes place between 650 and 850°C, to produce hydrogen yield of 60–70% [115]. Steam reforming takes place between 700 and 900°C. The two-step chemical reaction is shown below.

   $${\mathit{CH}}_4+H_{2}O\to \mathit{CO}+3H_2∆H^0=206\mathit{kJ}/\mathit{mol}$$E6
   $$\mathit{CO}+H_{2}O\to {\mathit{CO}}_2+H_2∆H^0=-41\mathit{kJ}/\mathit{mol}$$E7

   The process of steam reforming is often followed by a water shift reaction to improve hydrogen generation.

3. Partial Oxidation Reforming (POR)

   Compared to steam reformation, which is very endothermic, this process produces hydrogen at a lower energy cost due to its mild exothermicity. H₂ and CO are produced by the partial oxidation at atmospheric pressure and between 700 and 900°C partial oxidation reforming. The H₂/CO ratio of 2 yield is achieved in full conversion with reduced soot formation. Methane reacts with oxygen to form carbon dioxide (CO₂) due to a decrease in CO selectivity. The high exothermicity of the combustion causes hotspots to emerge in the reactor bed and coke to deposit on the catalyst [115]. In this process, methane is oxidized to syngas as demonstrated below.

   $${\mathit{CH}}_4+0.5O_2\to \mathit{CO}+2H_2∆H^0=-25.2\mathit{kJ}/\mathit{mol}$$E8

4. Autothermal Reforming (ATR)

   Combining POR and SR in the presence of carbon dioxide results in autothermal reforming. Autothermal reforming (ATR) is a process wherein steam reforming occurs in a catalytic zone heated by heat generated by partial oxidation in the reactor. The process does not need external heating and the reactor is easy to stop and restart. Compared to partial oxidation reaction, the hydrogen yield is higher and consumes less oxygen [115].

5.3.1 Upgrading syngas

Dry reforming results in Syngas, which must be devoid of carbon dioxide before being fed into the Fischer-Tropsch reactor. Amines are highly selective in their ability to absorb carbon dioxide. Other applications of this technology are separation of CO₂ from flue gases, natural gas cleaning, and large-scale upgrading of biogas. Common solvents used in the process are alkanolamines like monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) [32, 117].

5.3.2 Fischer-Tropsch (FT) process

The Fischer-Tropsch (FT) synthesis, named after the German inventors Franz Fischer and Hans Tropsch, is a process used to manufacture liquid hydrocarbon fuels like coal-to-liquids (CTL) and/or gas-to-liquids (GTL) based on source of syngas [32, 117]. Fischer-Tropsch synthesis (FT synthesis) can be used to convert biomethane and natural gas into fuels at an industrial scale [114]. The Fischer-Tropsch (FT) process converts syngas to products like LPG, diesel, and jet fuels [117].

The Fischer-Tropsch synthesis (FT-synthesis) polymerizes the carbon and hydrogen atoms in syngas or biomethane to create long chain molecules. The process is run over iron or cobalt catalyst at 20–30 bars [14] in an overall exothermic process leading to polymerization of CH₂ to hydrocarbons with long chains called syncrude. The various reactions in Fischer-Tropsch process are summarized below.

$$\left(2n+1\right)\ast H_2+n\ast \mathit{CO}\to C_n{H}_{2n}+n\ast H_{2}O$$E9

$$2n\ast H_2+H_2+n\ast \mathit{CO}\to C_n{H}_{2n}+n\ast H_{2}O$$E10

Reactors used include multi-tubular fixed bed, circulating fluidized bed, fixed fluidized bed, and slurry reactor. The reactions for the slurry reactor conditions are 20–30 bar, and 200–300°C while the syngas H₂/CO ratio of 1–1.8 [114, 117]. For high temperature synthesis, fluidized-bed FT reactors are used to generate light hydrocarbons in the form of gaseous hydrocarbons and gasoline and generally have higher output. The catalysts used are Fe and Co, which are sensitive to sulfur compounds in syngas [117].

5.3.3 Biofuels from biomethane

Various biofuels can be made from biomethane for the transport sector e.g. methanol, compressed biogas (CBG), hydrogen, liquid biogas (LBG), dimethyl ether, and Fischer-Tropsch (FT) fuels [118]. Compressed biogas (CBG), liquefied biogas (LBG), syngas (used to make hydrogen, methanol, dimethyl ether (DME), and Fischer-Tropsch (FT) diesel), and biomethane (upgraded biogas) are all possible fuels that can be made through various processes [6, 33, 118].

5.4 Biomethane for gas and power grids

In many countries, governments have come up with national support schemes to promote the biomethane market. Support mechanisms include feed-in support schemes, green gas products, and quota obligations as market drivers in Europe. Biomethane production for many countries is based on organic waste as feedstock, but for Germany, which dominates Europe’s feed-in market, it is based on energy crops. In Germany, the main driver is the feed-in tariff for renewable electricity through the Renewable Energy Sources Act (EEG). Biomethane support schemes mainly rely on mass balancing systems or ‘book and claim’ certificates. Existing mass balancing systems can contribute to international market development through the creation of common standards. As biomethane becomes popular and relevant in the energy systems, integration into power and gas grids and shift away from subsidies to markets and competition with natural gas will become major issues [119, 120].

Biomethane should meet some standard specifications with respect to storage and transport before it can be practically injected into existing natural gas networks. The presence of various components in different concentrations makes it difficult to inject biogas to the grid hence the need for upgrading. Pipeline designers should know what the exact thermodynamic properties of a gas mixture are, particularly in terms of density, and heating value, which may tend to vary greatly in biogas [16].

Biomethane has a very important role to play in the transition to renewable sources of energy. Demand-based production of biomethane for power generation directly links the gas grid and the electricity grids, which can help in balancing the power grid. The gas grid will shift from fossil fuel distribution to provide energy balancing service provider with short-term as well as seasonal storage options. There is increasing integration of decentralized biomethane feed-in into the gas grid for the gas grid infrastructure thus introducing new challenges. There are examples in Germany of facilities that feed in more biomethane to the local distribution network than the total discharge, which leads to the need to compress the excess gas and transfer it to a higher level [119].

Production of biomethane from energy crops has a negative impact on agriculture. On the other hand, use of digestion residues as fertilizers close to biogas production site improves local nutrient cycles. Production of energy-intensive nitrogen fertilizers and use of declining global phosphorous reserves can be avoided by use of bio-fertilizer from the digesters [51, 62].

Biomethane is the most efficient biofuel in terms of fuel production equivalent per area of crop land needed and is therefore expected to perform a larger role in the fuel/energy market because of government support, growing use in NGVs and reduction in GHG emissions. There is growing awareness of biomethane and a shift in perception from regarding biomethane as a sub-branch of biomass production to an independent renewable energy resource. And legislation and strategies are recognizing biomethane as an independent energy resource [119, 121].

The main sustainability challenge facing biomethane market is cost of subsidies and need for free market competition with fossil natural gas, which can be accelerated if the market price of natural gas rises. The European cap-and-trade for greenhouse gas emissions GHGs is another driving factor for the future. Since use of biomethane omits GHG emissions, there will not be compensation or penalties in the form of GHG certificates [119, 122].

The evolution of biomethane markets is expected to create its own demand and supply and enable and exchange between different countries since the green gas product market opened to international trade. Countries tend to create their own set of biomethane support schemes to address individual situations and therefore designed to address the priorities and challenges of specific countries. For the biomethane market to grow, countries should open their support schemes to biomethane imported from neighboring countries to encourage international trade in biomethane [6, 119].

5.5 Electricity from biogas and biomethane

Biogas can be used as fuel for power generation in various prime movers. They include internal combustion engines, gas turbines of varying sizes, and fuel cells, among others. The efficiency can be improved through combustion and conversion in set ups like cogeneration and tri-generation schemes [2, 50]. Diesel engines can run biogas as a direct substitute of natural gas. Biogas can also be upgraded to biomethane for applications like substitution of natural gas [123, 124]. Diesel engines can be run effectively either on pure diesel or in dual fuel mode with biogas and fossil fuel like diesel and petrol [123, 125].

Onsite electricity from biogas can be used directly to avoid or limit electricity power imports from the grid while excess generation can be used for a wide in fuel cells [5, 8, 126]. Various pathways for use of biomethane for power generation are summarized in Table 3 below.

From Table 3, biomethane can be used through various conversion technologies with varying characteristics in thermal and electricity generation. The conversion can be done in cogeneration, trigeneration, and open conversion systems. Prime movers that can use biogas include internal combustion engines, gas turbines, fuel cells, and Stirling engines as well as production of fuels for application in transport, heat, and electricity generation [89].

In the transport sector, biomethane has a double role to play in emissions reduction i.e. as a direct fuel substitute of fossil fuels and as feedstock for production of biofuels/chemicals through the Fischer-Tropsch (FT) Process e.g. diesel, jet fuel, and gasoline, and through reforming processes to produce hydrogen and methanol [2, 100].

Biogas production, its upgrading, and use are associated with some limited greenhouse gas emissions like CO₂, CH₄, and N₂O whose quantities vary with the technology applied and the source of biogas or feedstock used. The use of biomethane reduces the negative environmental impact and pollution potential as a substitute for fossil fuel energy sources. Additionally, the anaerobic digestion and gasification used to produce biogas and syngas help keep the environment clean and healthy as a means of waste disposal [87, 127, 128]. Figure 1 shows pathways for biogas use as a renewable source of energy.

Figure 1.

Biogas applications.

From Figure 1, it is observed that upon cleaning and purification, biogas has got many applications like bio-CNG applications, CNG, heat and production, and reforming to produce syngas and fuels by the Fischer-Tropsch (FT) Process. Other energy and process products are methanol, ethanol, higher alcohols, and gasoline.

Diesel engines, petrol and oil-fired engines, turbines, microturbines, and Stirling engines are all viable options for converting biogas into mechanical power for use in power generation. Biogas can be used as a fuel in both spark ignition (petrol) and compression ignition (diesel) engines with varying degrees of modification. Internal combustion engines with a dual fuel mode can be used with minimal or no adaptation, in contrast to those that undergo a full engine conversion to petrol. Biogas is an important component in the development of sustainable energy since it may be used in fuel cells for direct conversion to electricity, and as a feedstock for the manufacture of hydrogen and transportation fuels. Biogas can also be used as a feedstock for manufacture of the Fischer-Tropsch (FT) fuels, in addition to being utilized for direct energy generation, cooking, and lighting. The biogas is cleaned and purified, then it is reformed into syngas and partially oxidized to create methanol, which can be used to make petrol. Alcohols, jet fuel, diesel, and petrol may all be made from syngas using the Fischer-Tropsch process [2, 5].

6. Sustainability of biogas production and applications

Sustainable biogas systems incorporate systems for treatment of waste, environmental protection, conversion of low value material to higher value material, and power generation, heat, and advanced gaseous biofuels. Biogas and anaerobic digestion systems are dispatchable and hence can support intermittent renewable electricity [86]. Biogas adds value to the three pillars of sustainable development dimensions (SDDs) i.e., the first pillar of SDD is “Economic.” SDG number 1 is related to this pillar, “No poverty,” where biogas contributes by supplying affordable biofertilizer and eliminating the complex supply chain of fertilizer. Biogas contributes to SDG number 2, which is about “Zero hunger,” by generating new jobs through new businesses creation and increasing the yield of crops due to affordable fertilizer, which enhances the soil fertility by replenishing lost nutrients and carbon. Biogas fuel contributes to SDG number 3, which is about “Decent work and economic growth,” by increasing the gross domestic product through economic utilization of waste. Biogas also contributes to the SDG number 4, which is about “Industry, Innovation, and infrastructure,” by building sustainable infrastructure and providing electricity to small-scale industries at local level [5, 8, 129].

6.1 Environmental and health impact of biogas energy resource

Biogas environmental benefits are valid and sustainable alternatives to fossil fuels that can reduce greenhouse gas (GHG) emissions and can enhance energy security. Biogas enables exploitation of agricultural and zootechnical byproducts and municipal wastes, due to their lower impact on the environment compared to other combustion-based energy options [130]. Biogas can reduce greenhouse gas emission by more than 100% by considering decreased use of fertilizer on account of substrate use and control of leakages of methane and nitrous oxides from biogas energy systems [1, 130, 131]. Biogas has environmental and health impacts to be considered before its selection and application as a fuel. Biogas has negative impacts on human health and on the air quality. Biogas has components that are potentially toxic to human health and the environment, by forming toxic substances in the process of combustion, or formation of toxic substances during photochemical aging in the atmosphere [39, 79]. Methanethane, which is odorless, consists of one atom of carbon and 4 atoms of hydrogen. It is lighter than air and is highly flammable and hence should be handled with care since it can form explosive mixtures with air at concentrations of 5 to 15%. Methane is not toxic but causes death due to asphyxiation through oxygen displacement in an enclosed environment [44, 126]. Methane is a very powerful greenhouse gas, which is about 20 times more potent than carbon dioxide and can remain in the atmosphere for up to 15 years. Therefore, uncontrolled biogas generation and handling is harmful to the environment and hence effort should be made to ensure controlled biodegradation of organic wastes to produce biogas energy resources [3, 122].

6.2 Life cycle assessment of biogas

The life cycle assessment of biogas shows that by deploying biogas technologies, energy related greenhouse gas emissions can effectively be reduced and hence reduce the climate impact of energy consumption. The use as well as production of biogas also help diversify energy systems while promoting sustainable waste management practice because anaerobic digestion also results in waste treatment [3, 132, 133]. Therefore there are significant environmental, economic, and social benefits of biogas production, using the circular agricultural waste utilization model [7, 39, 79]. Biogas is a renewable energy carrier used in electricity generation, in heat production, and as a transportation fuel. Most biogas is produced by anaerobic digestion of animal and plant materials [79, 99].

Health and environmental concerns over biogas use often hamper the social acceptance of biogas. Environmental factors considered in the use of biogas are direct emissions of gaseous pollutants e.g. nitrogen oxides (NO_x), impact of using waste biomass and digestate, nitrogen release and soil fertility, and methane leakages [130, 131, 134].

The biogas circular economy has multiple functions like waste treatment, reduction in greenhouse gas emissions, environmental production, and energy applications. The restorative and generative structure of circular economy seeks to maintain the products and materials always used and their maximum value [49, 64]. Biogas generation occurs at the last stage of anaerobic digestion with digestate, as the byproduct having valuable use as biofertilizer. Therefore biogas production is a value addition process for biowaste [30]. Figure 2 shows the main concept of circular economy.

Figure 2.

Life cycle stages in the linear, recycling, and circular economy models.

From Figure 2, it is noted that substitution of fossil fuels by biogas leads to significant environmental improvements. Generation of 1 MJ of electricity from biogas equivalently substitutes 0.4 and 0.9 MJ of fossil fuel and thus heat and power production reduces greenhouse gas emissions (GHG), by 75–90%. The emissions reduction in emissions is reduced further by substituting chemical fertilizers with digestate. Utilizing manure or food industry waste as a feedstock for biogas production is one of the feasible ways to reduce greenhouse gas emissions by values as high as 95%. Additional positive impacts include reduction in impacts like eutrophication and acidification dangers [19, 48, 62, 135]. Hence, biogas use is a strategy to control environmental problems that include eutrophication, acidification, air pollution, and greenhouse gas emissions with maximum benefits being realized through proper design and location of biogas systems [135].

Studies show that using biomethane as vehicle fuel can reduce greenhouse gas emission reductions of 27–62% as compared to emissions from conventional natural gas passenger vehicles or 41–70% compared to conventional petrol vehicles. The main challenge is limited suitable biomass to support widespread deployment of power-to-methane systems using biogenic carbon. Available biomethane production options offer attractive means to reduce greenhouse gas emissions in a future energy system with large amounts of intermittent renewable power generation [126, 136, 137].

Numerous studies have been undertaken to evaluate the environmental impacts of producing power from biogas from the anaerobic digestion of agricultural products and waste over their entire life cycles. Studies on the anaerobic digestion using tomato waste, maize silage, slurry, and tomato waste as feedstocks and for cogeneration applications suggested that systems using animal slurry offer best options, except for ecotoxicity to marine and terrestrial environment. The studies also indicate that it is more feasible to have smaller plants that use slurry and waste rather than big plants using maize silage to operate efficiently [12, 48, 138]. Electric power from biogas is environmentally more sustainable than grid electricity for many biogas schemes. However, compared to natural gas, biogas electricity is not always the best option in terms of impacts. Biogas also has higher impacts than other renewables, like solar photovoltaics, wind and solar. Overall, though, biogas electricity can help minimize the GHG emissions relative to a fossil-intensive electricity mix, although some other impacts may increase. Further analysis shows that, if the main objective is to mitigate climate change, then other renewables like wind, solar, and hydro offer greater potential to reduce GHG emissions than biogas. However, the environmental sustainability of biogas is much improved by cogeneration when heat is used, especially in terms of global warming potential, summer pollution, and the loss of ozone layer and abiotic resources.

6.3 Biogas in the circular economy

Circular economy refers to an economic concept that is the reverse of the linear system whose main objective is to minimize waste through reuse or recycling. Realizing a circular economy is a challenge for many countries because many biogas plants are generally small energy production facilities, having a smaller impact to waste management [86, 139]. Rural areas have an opportunity to diversify agriculture activities by adding value to the socio-economic systems in additional. Biogas plants should be implemented in a way that properly communicates with local communities to participate in the transformation. The operation of biogas plants is generally compatible with the goals of a circular economy that makes use of local energy resources. Locations for new biogas plants should minimize the need for substrate transport to increase their local character. Successful implementation of largescale biogas systems can help in achieving the Sustainable Development Goal 7 on achieving affordable and clean energy, and Goal 12 on responsible consumption and production [139].

Avoiding open digestate storage reduces methane emissions, and controlling how much of it is dispersed on land reduces ammonia emissions and other negative environmental effects [140]. Figure 3 depicts the layout of a biogas plant in a circular design.

Figure 3.

Functions of biogas in the circular economy.

From Figure 3, the biogas role in the circular economy is presented. The first stage involves the substrate collection and preparation to feed the biodigesters where biogas is produced with digestate as a byproduct.

The traditional scenario model is such that biowastes from agricultural and livestock farming are disposed thus creating a cost for the household or the firm disposing of the waste [30]. In the circular economy scenario, manure and agricultural wastes are used to produce biogas and digestate as fertilizer, which help preserve the environment since biogas energy is an alternative to fossil energy and has socio-economic value to farmers and the biogas producer [30, 86].

Biogas production offers treatment and management of bio digestible animal and plant waste, which sterilizes waste and produces useful manure and sewage leading to significant volume and mass reduction, which effectively reduces the cost of waste treatment and disposal [44, 141, 142]. Use of biogas as a fuel significantly reduces greenhouse gas emissions while the use of animal manure as biodigester feedstock reduces methane emissions from the manure storage and use. Carbon dioxide (CO₂) emissions from biogas have a lower global warming potential than methane produced by fossil fuel. Therefore, biogas production and use reduce greenhouse gas emissions in accordance with the Kyoto protocol and Paris agreements. The negative effect is that reduced use of manure increases use of chemical fertilizers for fertility or nutrient replenishment to the soil [2, 30]. A few of the potential sustainability issues with employing agricultural wastes for biogas production are competition with feeding in animal husbandry and possible loss of nutrients in agricultural land [70].

To consider biogas applications like use as a vehicle fuel as sustainable requires a sustainability criterion to be met along the entire production chain, beginning with primary production to final use and disposal. Sustainability demands that they should not destroy areas of high biological value or generate excessive GHG emissions. For biofuels to decrease greenhouse gas emissions, the sustainability criteria require that the associated greenhouse gas emissions (GHG) emissions over the life cycle should be at least 35% lower compared to fossil fuels [1, 134].

Bio-based economy-enabled technologies lead to green electricity and heat generation and fossil fuel substitution in transport and power generation, and manufacture more value-added products and byproducts. Bio digestion industrial operations, agriculture, and other anthropogenic activities such as food waste (FW) can produce valuable energy sources, nutrient-rich manure, and specialty chemicals. Several anaerobic and microbial interventions sustain biomass valorization and related processes, leading to more efficient bio methanation [85].

7. Conclusion

Resource overuse, climate change, and energy autonomy are challenges facing the volatile world. Biogas production and use represents a model example of the circular economy, which is an economy that offers a path for sustainable growth amid growing energy demand and production requirements. Anaerobic digestion is a biological process where biodegradable matter is degraded by the action of microorganisms in absence of oxygen to produce biogas, which is a clean fuel from renewable feedstocks with potential use in heat and electricity generation. Factors affecting bio digestion include type of raw materials, temperature, pH, mixing, level preparation and pretreatment, residence time, redox effect, and hydraulic pressure etc. The efficiency of anaerobic digestion can be evaluated in terms of bio-methane potential, gross energy yields, process energy demand, net energy yield, and energy efficiency. Energy crops are the most used substrate for biogas production.

Concerns about greenhouse gas emissions and depletion of fossil fuels have increased demand for sustainable renewable sources. Biogas production is one of the innovative and viable methods for production of energy from biowastes and biomaterials, with an extra benefit of producing biofertilizer to support sustainable agriculture. Huge quantities of biodegradable waste are generated by humanity globally, which can pose a serious threat to human and animal health and their environment. There are many wastes disposal and treatment methods employed to avoid environmental pollution. Among the many options, anaerobic digestion is a popular non-conventional energy source used to produce renewable energy from biodegradable biomass.

Biogas is a product of the anaerobic digestion process with many applications as in generation of renewable energy. The main component of biogas with energy value is methane, but it has impurities like moisture, carbon dioxide, siloxanes, hydrogen sulfide, siloxanes, hydrocarbons, oxygen, ammonia, oxygen, carbon monoxide, and nitrogen whose presence is undesirable as they reduce the calorific value of biogas and create operational problems in the energy systems. This necessitates biogas cleaning and application of multi-stage technologies to produce upgraded biogas called biomethane. Biomethane gas is a flexible and easy to store fuel with similar properties and applications to natural gas with no need to modify any equipment settings for natural gas devices and equipment.

The factors influencing the process and quality of biogas produces include the C/N (carbon nitrogen ratio, the pH, dilution of feedstock, composition and nutritive value of feedstock, residence period, mixing and stirring, temperature, presence of toxicants, loading time, and redox conditions. The main challenges facing biogas production and use include lack of technical training on biogas production and use and lack of subsidies and incentives. Therefore, besides easy access to appropriate and affordable biogas technologies, there is a need for developing a comprehensive policy on construction and operation of biodigesters as well as utilization of biogas and sale of biogas energy products and services. Biogas education and exposure, availability of simple and affordable biodigesters, and facilitating legal and policy environment are key tools needed to increase the adoption of biogas technology and its sustainable use.

Raw or cleaned biogas that is free from harmful contaminants has significant energy value and applications. The multifunctional uses of biogas produce include the production of biofuels and renewable energies, reduction in greenhouse gas emissions mainly methane and carbon dioxide, reduction in nuisances resulting from odors and flies associated with uncontrolled biodegrading of biomass, economic or financial savings for farmers, sustainable waste treatment and recycling of organic waste, minimization of water and air pollution, rural poverty alleviation, and better social conditions of rural or remote settlements like gender balance, as women who bear the responsibility of looking for firewood are relieved of the struggle, and conservation of forests and vegetation cover due to reduced use of firewood and charcoal.

Conflict of interest

The authors declare that they have no conflict of interest.

Funding

There was no external funding provided for this research and publication charges were paid by the Durban University of Technology.

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Availability of data

The research has provided all data and information used and did not use any undeclared data and information. However, any datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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