**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 [2–4]. 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 m<sup>3</sup> biogas or about 35 billion m<sup>3</sup> 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 [17–19]. 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, 20–22]. 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

#### *Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications DOI: http://dx.doi.org/10.5772/intechopen.114268*

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 [38–42]. 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 [47–49]. 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 [50–52]. 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 [53–55]. 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/m<sup>3</sup> , 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 m<sup>3</sup> 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 [59–61].

#### **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

#### *Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications DOI: http://dx.doi.org/10.5772/intechopen.114268*

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 smallholder 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 [71–73].

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 [38–42]. 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

*Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications DOI: http://dx.doi.org/10.5772/intechopen.114268*

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:


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**.

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.

*Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications DOI: http://dx.doi.org/10.5772/intechopen.114268*


**Table 1.**

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

#### **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/m<sup>3</sup> . A comparable volume of biogas (1 m<sup>3</sup> ) can be produced by: 0.7 m<sup>3</sup> 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**.

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.

#### **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/m<sup>3</sup> ,1m<sup>3</sup> 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 m<sup>3</sup> of biogas yields around 1.7 kWhe [5, 44, 59]. Biogas output and energy generation from various substrates are compared in **Table 4**.

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.


#### **Table 2.**

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


#### **Table 3.**

*Summary of biogas to electricity conversion systems and technologies.*


*Biogas as a Sustainable Fuel and Feedstock: Properties, Purification, and Applications DOI: http://dx.doi.org/10.5772/intechopen.114268*

#### **Table 4.**

*Electricity produced from biogas yield per ton of fresh matter.*

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.
