**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

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

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, NH3, 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].

$$\text{CH}\_4 + \text{0.5}\,\text{O}\_2 \to \text{CH}\_3\text{OH} \\ \Delta H^0 = -\text{128 kJ/mol} \tag{2}$$

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].

$$\text{CH}\_2 + \text{CO} \rightarrow \text{CH}\_3\text{OH} + 2\text{H}\_2 \qquad \qquad \Delta H^0 = -\Re \, k \text{J/mol} \tag{3}$$

$$\text{CH}\_2 + \text{CO}\_2 \rightarrow \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \qquad \qquad \Delta H^0 = -49 \text{ kJ/mol} \tag{4}$$

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].

#### i. *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].

$$\text{CH}\_4 + \text{CO}\_2 \rightarrow 2\text{CO} + 2\text{H}\_2 \qquad \qquad \Delta H^0 = 247 \text{ kJ/mol} \tag{5}$$

#### ii. *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.

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

$$\text{CH}\_4 + \text{H}\_2\text{O} \rightarrow \text{CO} + \text{3H}\_2 \qquad \qquad \Delta H^0 = 206 \text{ kJ/mol} \tag{6}$$

$$\text{CO} + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{H}\_2 \qquad \qquad \Delta H^0 = -41 \,\text{kJ/mol} \tag{7}$$

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

#### iii. *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.

$$2\text{ }CH\_4 + 0.5\text{ }O\_2 \rightarrow CO + 2H\_2\Delta H^0 = -25.2\text{ kJ/mol}\tag{8}$$

#### iv. *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.

$$(2n+1)\*H\_2 + n\*CO \to C\_nH\_{2n} + n\*H\_2O\tag{9}$$

$$2n\*H\_2 + H\_2 + n\*CO \rightarrow C\_nH\_{2n} + n\*H\_2O\tag{10}$$

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

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

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 biofertilizer 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.

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].

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

## **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 (NOx), 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.

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

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

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.

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 socioeconomic 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].

**Figure 3.** *Functions of biogas in the circular economy.*

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