Biogas and Biomethane Circular Economy

1 INTRODUCTION

1.1 Biogas & Biomethane

Biomass is a renewable energy resource. It is the fourth largest primary energy resource and a potentially clean energy resource [1] with numerous benefits, locally and globally. As is well known, lignocellulosic materials are available everywhere at low and stable costs. It is mainly waste materials containing an abundance of carbohydrates and non–competitive with food chains. Biomass as an energy resource requires a conversion process to be used as a fuel. Anaerobic digestion (AD) is one of the most effective biological conversion processes of biomass into biofuel. It breaks down organic matter by microorganisms and enzymes in an oxygen-free environment to produce biogas.

Biogas is a mixture of methane, carbon dioxide, and small quantities of other gases arouse by the anaerobic digestion of organic matter in an anoxic environment. Biogas’ precise composition depends on the type of feedstock and the production pathway [2]; these include the following leading technologies: biodigesters, landfill gas recovery systems, and wastewater treatment plants. Biogas’ methane content typically ranges from 45% to 75% by volume, with most of the remainder being CO2. This variation means that biogas’ energy content can vary; the lower heating value (LHV) is between 16 and 28 megajoules per cubic metre (MJ/m3). Biogas can be used directly to produce heat and electricity or as an energy source for cooking.

Biomethane (also known as “renewable natural gas”) is a near-pure source of methane produced either by “upgrading” biogas — a process that removes any CO2 and other contaminants present in the biogas) — or through the gasification of solid biomass followed by methanation: upgrading biogas, or thermal gasification of solid biomass followed by methanation. Biomethane has an LHV of around 35 MJ/m3. It is equivalent to natural gas and so can be used unchanged in distribution infrastructure or end-user equipment and is appropriate for use in natural gas vehicles.

Figure 1. The multiple production pathways for biogas and biomethane. Source: https://www.iea.org/reports/outlook-for-biogas-and-biomethane-prospects-for-organic-growth/an-introduction-to-biogas-and-biomethane

The generation and use of biomethane can provide new opportunities for society at multiple levels. However, some challenges and non-technical barriers can be found, such as the lack of public acceptance for biomethane plants and the current flaws of legislative and normative management guidance and support.

1.2 What is Circular Economy and why do we need it?

The linear economy model became dominant with the rise of consumer society and mass production; conversely, the circular economy model endeavours to close the flows of materials and energy circulating in the economy. Several strategies could be used to achieve this: reducing the quantities of materials and energy used to produce goods, extending their lifespan through sharing, repair and reuse, or recycling the materials they contain at the end of their life, according to an endless cycle.

According to most projections, the world population will continue to grow in the coming decades. This increase would be associated with an increase in the world’s energy demands. Moreover, the generation of waste in a region tends to rise with population and economic growth: increasing industrial production, population growth and urbanisation have increased the consumption of goods and services. This has resulted in an increased solid waste generation [3], specifically in urban areas, in which waste includes public and household solid waste and is called municipal solid waste (MSW).

The circular economy has been defined as to take, make, use, and reuse and reuse again and again. This refers to products that can be reused or reprocessed into another product. However, the last case alternative is to convert the product to some form of energy. The ultimate purpose is to move away from waste disposal to the more efficient management of waste. It is essential to convert all waste to resources—moving from that linear economy to a circular one. That is where waste-to-energy initiatives make a difference as biogas is a sustainable, clean energy option for the future.

Equitable, sustainable societies must increasingly be based on the efficient use of materials occurring in waste flows and on the production of energy and other by-products from these natural resources. The recovery of energy from waste in circular economy models is integral to close the materials and energy loops. Among the potential energy forms to be stemmed from bio-wastes, biogas is of great interest due to its capability to transform organic feedstocks into biomethane and produce a fermentate that can be used as a valuable agricultural/horticultural fertiliser.

Figure 2. The role of biogas in a circular economy. Source: https://www.compostnetwork.info/policy/circular-economy/

2 APPLICATIONS

2.1 Anaerobic digestion in the current global scenario

AD is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen [4]. One of the end products is biogas, which can be combusted to generate electricity and heat or be processed into renewable natural gas and transportation fuels.

A range of anaerobic digestion technologies is converting food waste, livestock manure, municipal wastewater solids, fats, oils and grease, industrial wastewater and residuals, and various other organic waste streams into biogas on a continuous basis. Separated digested solids can be directly applied to cropland, composted, utilised for dairy bedding, or converted into other products. Nutrients in the liquid stream, the digestate, can be used in agriculture as fertiliser.

The process begins with bacterial hydrolysis of the materials to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.

There are many different anaerobic digester systems commercially available. Some of them are the following: 

• Covered anaerobic lagoon digester — Sealed with a flexible cover, with methane recovered and piped to the combustion device;
• Plug flow digester — Long, narrow concrete tank with a cover (rigid or flexible). The tank is built partially below grade to limit the demand for supplemental heat;
• Complete mix digester — Enclosed, heated tank with a mechanical, hydraulic, or gas mixing system;
• Dry digester — Upright, silo-style digesters made of concrete and steel with rigid cover.

The progress of biogas has been uneven across the world, as it counts not only on feedstocks being procurable but also on policies that boost its production and use. Europe, China and the United States account for 90% of the global output [5]. Likewise, Germany, the U.K., Sweden, Switzerland, and the U.S. were the largest biogas producers as vehicle fuel in recent years.

Figure 3. Biogas production by region and by feedstock type, 2018. Source: https://www.iea.org/data-and-statistics/charts/biogas-production-by-region-and-by-feedstock-type-2018

Europe is currently the largest producer of biogas. Here, most biogas is used for electricity (62%) followed by heat (27%). Biomethane used directly in the transport sector or injected into the grid to be used in the built environment (for heating or cooking mainly) contributes to about 11% of generated energy from biogas. Upgrading biogas to biomethane is significant in Sweden, the Netherlands and Germany; although in absolute terms, the latter produces by far the largest amount — 33 PJ or 788 ktoe of biomethane [6].

In China, household-scale digesters in rural areas, which are intended to increase access to clean cooking fuels, account for around 70 per cent of installed biogas capacity today. Moreover, the National Development and Reform Commission issued a guidance document in late 2019 specifically on biogas industrialisation and upgrading to biomethane, supporting biomethane in the transport sector.

In the United States, biogas’ primary pathway has been through landfill gas collection, which accounts for almost 90% of its biogas. Agricultural waste represents a growing subject of matter since domestic livestock markets are responsible for nearly one-third of methane emissions in the U.S.  This country is also leading the way globally in using biomethane in the transport sector due to both state and federal support.

Around half of the remaining provision comes from developing countries in Asia, notably Thailand and India. The former produces biogas from the waste streams of its pig farms, biofuel industry and cassava starch sector. India aims to establish around 5,000 new compressed biogas plants over the following years. Brazil and Argentina have also supported biogas through auctions; Brazil has seen most production coming from landfills, but there is also potential from vinasse, a by-product from the ethanol industry. A clear picture of today’s biogas consumption in Africa is made more difficult by a lack of data.

2.2 Transport

Today, the transport sector is responsible for a third of global energy demand and one-sixth of global Greenhouse Gas (GHG) emissions [7]. The use of fossil fuels currently dominates this sector. Turning to biogas as vehicle fuel can reduce emissions in the transport sector between 60 and 80 per cent compared to fossil-fuelled baseline.

Different institutions highlight the technologies available – both established and newly emerging – for biogas production and biogas-driven vehicle fleets. The International Renewable Energy Agency (IRENA) has evaluated costs, performance and sustainability aspects and outlined the best practices worldwide to accelerate the adoption of this key clean transport fuel. When sufficiently purified, it can be used in place of fossil-based gas to drive natural gas vehicles (NGVs) or dual-fuel vehicles.

Around 500 plants worldwide generate approximately 50 petajoules (PJ) of such upgraded biogas, also known as biomethane. Countries are promoting biogas-powered cars, vans, and buses through a combination of tax cuts, procurement subsidies, and biogas injection into the natural gas system. In particular, China, France, the U.K., and Scandinavian countries have strongly supported the transition to biogas in the transport sector.

Cost reduction remains the critical challenge for achieving sustainable transport. Seeking synergies with other power and transport technologies could considerably reduce biogas production costs.

2.3 Electricity from sanitary landfills

One of the options less costly and more used throughout the world for the disposal of urban solid waste are landfills. These landfills are widely used in developing countries, and consist of large anaerobic reactors where the input is solid waste and the output is biogas and slurry. The biogas is produced by anaerobic digestion of organic matter and has high energy potential (due to its high percentage of methane). Therefore, it can be used in various applications such as heat generation, electricity, or fuel production for vehicles. The energy use of biogas from landfills reduces the environmental impacts of this structure because it employs the best land used to construct the landfill and reduces greenhouse gas emissions into the atmosphere.

The production of biogas in landfills occurs through anaerobic digestion. According to technical literature [8], biogas composition is approximate 55%–70% methane (CH4) by volume; 30%–45% carbon dioxide (CO2); 80–100 ppmV ammonia (NH3), 1000–3000 ppmV hydrogen sulfide (H2S) and other hydrocarbons (<100 ppmV). Traces of siloxanes can also be found.

Upon introducing MSW into the landfill, the organic components begin to undergo a series of chemical reactions. In the presence of atmospheric air near the landfill surface, natural organic components are oxidised aerobically, producing CO2 and H2O. However, the principal reaction in landfills is anaerobic digestion. This occurs in three main stages.

(1) Fermentative bacteria perform hydrolysis of complex organic matter insoluble molecules;

(2) These molecules are converted into simple organic acids, CO2 and H2 by acid-producing bacteria. The primary acids produced are: Acetic acid (CH3COOH), propionic, butyl and ethanol;

(3) Fermentative bacteria carry out hydrolysis of complex organic matter, transforming it into soluble molecules;

(4) These molecules are converted into simple organic acids, CO2 and H2 through acid-producing bacteria. The primary acids produced are: acetic acid (CH3COOH), propionic, butyl and ethanol;

(5) Finally, CH4 is formed by methanogenic bacteria, both by breaking down these acids in CH4 and CO2 (Eq. 1) and by the reduction of CO2 by H2 (Eq. 2):

Landfill gas production passes through four phases, as described below.

• Phase I – First aerobic phase: Aerobic decomposition occurs with oxygen consumption present in the waste at the time of disposal. At this stage, the production of CO₂ is verified.
• Phase II – Non-methanogenic anaerobic: Anaerobic activity becomes predominant. During this period, a peak occurs in the concentration of carbon dioxide and hydrogen production is relevant. At this stage, there is still the substitution of N₂, which can, however, be reproduced in smaller quantities through denitrification.
• Phase III – Unstable methanogenic anaerobic: An increase is seen in CH₄ concentration and stabilisation at a very high value. At this stage, the concentrations of CO₂ and N₂ are reduced to final concentrations.

This gas has several applications, such as the generation of heat, electric energy and the production of vehicle fuels. Among the difficulties in the application of this gas are the correct prediction of the gas collected and the presence of impurities in its composition. Great potential lies in electric energy generation from biogas; when harnessed, this significantly improves landfills’ environmental performance and reduces the energy liabilities of the goods and products that generated the waste sent to the landfill.

Figure 4. Diagram of an electrical energy power plant in a sanitary landfill. Source: https://doi.org/10.1016/j.scs.2018.04.029

Biogas produced in landfills can be considered a renewable fuel and stands out as an excellent energy generator by contributing to distributed generation. In this form of generation, energy is produced close to consumer centres, reducing the risks and losses, which can be characteristic of the centralised generation of extensive transmission lines. Landfills perform well on global warming emission reductions, ozone layer depletion, human toxicity, acidification and abiotic depletion when using biogas for electrical generation.

The power levels, Pel, and energy, E, which can be generated each year through energy conversion of the biogas, is calculated using Eqs. 3 and 4.

Where: P_el = Electric power in kW; Q = Biogas flow to be used in m³ s^-1; η = Energy conversion efficiency; E_C = Landfill gas collection efficiency; LHV = Lower calorific value of biogas = 22 * 10⁶ J m³; 8,760 = Number of hours per year; C_F = Capacity factor; E = Energy available annually in kWh y^-1.

The EC value varies with the characteristics of the landfill. The value suggested by USEPA [9] is 75%, while a frequent value is also around 55.5%. A representative schematic of a biogas plant installed in a landfill is presented in Fig. 3. As shown in figure 4, the components required for the plant are:

(1) Gas collection drains;

(2) Piping to transport the collected gas;

(3) Combustion burner for unused biogas;

(4) Gasometer for storage and gas flow control;

(5) Gas treatment unit;

(6) Compressor for pumping and raising gas pressure;

(7) Energy conversion unit for the conversion of biogas chemical energy into mechanical energy; and finally

(8) Electric generator for conversion into electric energy.

2.4 Organic fertilisation

Generally, the final output products of the AD process are biogas and digestates. As we have already mentioned, biogas can be directly used in several applications or upgraded and either injected into a gas network or used as a transportation fuel in compressed natural gas-fuelled vehicles. Digestate, the digested effluent of the biogas production process, consists of the feedstock materials after extraction of biogas.

The digestate generated usually comes in liquid and solids streams and can be further separated. It contains the remaining nutrients which did not digest in the digestion process, such as ammonium and phosphates [10]. The liquid stream has the potential to be used in agriculture as a bio-fertiliser and others. In contrast, the solid stream can be composted, utilised for dairy bedding or applied directly to cropland.

Because of the content of easily accessible macro- and micronutrients, digestate is a valuable crop fertiliser, suitable to be used in the same way as raw animal slurries. Recycling as fertiliser is considered the most sustainable utilisation of digestate, as it can provide benefits for society in general and the environment in particular and help preserve limited natural resources such as fossil resources of mineral phosphorus. For use as fertiliser, the digestate needs to be of the highest quality and free of pathogens, chemical and physical impurities and pollutants; this can be achieved by using AD feedstock of controlled quality.

Regulatory frameworks, aimed to guarantee the production of high-quality digestate, thus are implemented in countries with developed biogas sectors such as Germany, Denmark, Austria, Sweden, Switzerland and the U.K [11]. The regulatory frameworks are regularly updated and increasingly restrictive, in line with new knowledge and experience.

Wastewater also contains various nutrients that are harmful to water resources, which can be used to produce a potential fertiliser. Many inorganic constituents like ammonium, phosphate, and magnesium, present in a large quantity of waste, can be used to make a suitable fertiliser. Recycling waste materials for energy production could protect and save the environment and reduce energy consumption and cost.

2.5 Pricing

The cost of biogas production varies significantly and depends on the substrate’s parameters and the possibilities to allocate the resulting digestate in the surrounding agricultural area. For most biogas generated, however, the price is higher than that of the energy sources they compete with (natural gas, diesel, etc.). Compared to other renewable energy sources, i.e. solar PV and wind, biogas has the advantage that it can be used to provide flexible power production, including in times of low wind and solar intensity [6].

Financing solid waste management systems is a big barrier, even more so for continued O&M costs than for capital investments, and they need to be taken into account upfront. Operating costs for advanced waste management, including storage, transportation, treatment, and recycling, typically reach USD 100 per tonne in high-income countries. Lower-income nations spend not as much on waste operations in absolute terms, with prices spanning USD 35 per tonne and often higher [12], but recovering costs is much more difficult for them.

Waste management is labour intensive, and transportation payments alone are in the range of USD 20–50 per tonne. The cost-to-return ratio for waste facilities varies dramatically across income brackets. Usage payments range from an average of USD 35 a year in low-income countries to USD 170 per year in high-income countries, with high-income countries earning full or almost full expense recovery.

In other words, cost return for waste services differs drastically across income levels. User fee criteria may be fixed or variable based on the type of consumption being billed. Typically, local governments cover about 50 per cent of the waste system investments, and the remaining proceeds mainly from national government subsidies and the private sector.

3 STUDY CASES

3.1 What are the opportunities for municipalities?

The world generates more than 2 billion tonnes of municipal solid waste annually, with at least 33 per cent of that not being managed in an environmentally safe manner [12]. Globally, most of it is dumped or disposed of in some form of a landfill. Municipalities are recommended to assess their potential biogas sources as well as options to develop sustainable opportunities. These can reinforce their energy independence and surrounding rural development, increase the share of locally produced clean energy, and, at the same time, reduce the environmental impact of waste streams.

Figure 5. Global share of different treatment and disposal methods for municipal waste. Source: https://datatopics.worldbank.org/what-a-waste/trends_in_solid_waste_management.html

Urban-level initiatives are fundamental for successfully managing and utilising waste streams, thereby transitioning to more effective and efficient circular economy models at the local, regional, national and global levels.

For example, virtually every EU Member State has gas storage facilities in place, a natural gas infrastructure for transport and gas quality codes and regulations, all crucial prerequisites for biomethane deployment and growth. Nevertheless, only a limited number of Member States are upgrading biogas into biomethane and injecting it into the grid.

As part of the Circular Economy Package, published by the European Commission in 2015, several legislative proposals on waste were published. The overarching objective was formulated as ‘Turning waste into a resource is a necessary element of increasing resource productivity and closing the loop in a circular economy. These included several issues relevant for biogas, such as a binding target to reduce landfill to a maximum of 10% of municipal waste by 2030 and a ban on landfilling of separately collected waste.

The biomethane potential for several European cities has been estimated [13]; a large share of the identified feedstock could be used as vehicle fuel, helping the EU achieve its Paris Agreement commitments within the transport sector. Theoretically, the organic fraction of municipal solid waste (OFMSW) can provide sufficient biomethane to reduce fossil-fuel-based GHG emissions substantially. The same is true for utilising by-products (e.g. animal manure, harvest residues, and other wastes from the agro-industry) in biomethane production and the related fermentate-based fertilisers.

The feedstock availability in the Municipality of Rome can assure production of biomethane equal to 37.6 million m3: 26,306 thousand m3 from the organic fraction of municipal solid waste (OFMSW) and 11,281 thousand m3 from by-products). That is sufficient to fuel about 28,200 natural gas vehicles (NGVs) with an overall reduction of GHG emissions equal to 47 thousand tons of CO2eq/year [14]. As a result, the whole fuel demand for transportation in this territory can be entirely satisfied.

Since innovation in the biogas value chain can have a range of benefits (e.g. increased GHG savings, cost reduction), continued efforts into R&D of biogas production, conversion into biomethane and the application of biogas are recommended. Worldwide diffusion of biogas-related knowledge and expertise can be enhanced by setting up a common platform for best practices related to biogas technologies, applications and policies, targeted at municipalities and policy decision-makers.

3.2 Germany

The leading biomethane producing country in Europe is Germany, with about 196 biomethane plants yielding about 122,000 m³ h−1 biomethane, which is used to directly substitute natural gas. The utilisation of biomethane in the German heating and transport sectors is framed by laws and regulations, encompassing, i.e. quotas, tax reliefs and sustainability requirements, but not through direct financial incitement [15]. Instead, it is dependent on customers willing to buy a more environmentally friendly product.

The combination of revenues from biomethane marketing (use for power, heat or fuel applications) and revenues from biogenic carbon dioxide marketing are seen as options to secure an ongoing biomethane production and decrease dependencies on governmental support. However, it is evident that other measures are needed to ensure continuous biomethane production in Germany.

3.3 Sweden

Sweden already uses biomethane as a vehicle fuel, in which the municipalities use environmentally-friendly buses, cars and trucks. They also use it to fuel public transport and separate collection vehicles. Additionally, private actors can benefit from tax exemptions and transport fuel certificates.

In Sweden, the biomethane sector is well developed despite restrained gas infrastructure: biomethane is typically distributed by trucks rather than by the grid [7]. The Swedish forest still holds a significant resource potential for transport fuels. It is also the feedstock for large industry branches, e.g. pulp and paper, sawn wood products, and solid biofuels. Forest-derived methane is compatible with the existing distribution infrastructure in large parts of Sweden’s more densely populated regions since it is interchangeable with vehicle gas.

Sweden aims at attaining a vehicle fleet independent of fossil fuels by 2030 [16]. A government inquiry has proposed a definition of this target: i) the vehicle fleet can technically function without fossil energy carriers, and ii) fossil-free energy carriers are available in sufficient quantities. The target is thus vague; the vehicle fleet must not necessarily be fossil-free. Nonetheless, it is clear that the 85% share of fossil fuels in transport during 2014 is far from the target.

Local targets play an important role in Sweden since a significant share of the financial resources is managed within municipalities and counties. For example, Stockholm County aims at fossil-free public transport by 2030, including road and rail traffic.

3.4 Rio & Sao Paulo, Brazil

The city of Rio de Janeiro generates approximately 8,600 tonnes a day of MSW. Around 40% of this total amount is directed to the Caju’s Waste Transhipment Unit, from where it is transported to Seropedica’s Landfill (80 km far) by transhipment trucks. The OFMSW corresponds to about 53% of the RJ- MSW composition [17]. The inadequate disposal of it generates systemic environmental impacts, such as GHG emissions, surface and underground water contamination by leachate, attraction and proliferation of disease vectors, and impairment to public health.

Due to all these impacts, diverting organic wastes from landfills is currently a municipal goal. Furthermore, the management and appropriate treatment of organic waste is essential to meet the strategic objectives of the Brazilian Solid Waste Policy and the Brazilian Policy on Climate Change. To meet these directives, methanization of the OFMSW has been highlighted as a chief alternative because the following main benefits can be accomplished: i) mitigation of GHG emissions and leachate pollutants; ii) reduction of final disposal costs; iii) production of biogas, a renewable energy source; iv) and production of biosolids, a soil conditioner and agricultural fertiliser.

The city launched a collaborative action called “Biomethanisation Unit – Treatment of the organic fraction of urban solid residues by anaerobic digestion for energy generation and organic compost production”. It aims at developing and implementing the first Brazilian and Latin American technology to treat the organic fraction of urban solid waste, regardless of whether it was segregated at the source or not.

The unit can treat 35 tons per day, which is the equivalent of serving a population of 70,000 people per day. It can produce 10-20 tons of organic compounds per day and generate 3,150 Nm³ of biogas/day or up to 2,408 MWh / year of electricity. Through this pilot initiative, the city intends to identify and validate optimal operational parameters and establish a new business model for the sector, contributing to the fight against climate change.

Figure 6. Termoverde Caieiras power plant in Sao Paulo metropolitan region. Source: https://agenciabrasil.ebc.com.br/es/economia/noticia/2016-09/brasil-inaugura-planta-termoelectrica-que-utiliza-combustible-renovable

But the largest thermoelectric plant in Brazil powered by renewable fuel was inaugurated in 2016 in the city of Caieiras, metropolitan region of São Paulo. Termoverde Caieiras has an installed capacity of 29.5 megawatts (MW) and generates electricity from methane gas from the decomposition of waste deposited in landfills. Considering the possible losses, the average power generation of the new plant should reach 26 MW per hour, enough to supply a city of 300 thousand inhabitants.

4 PROMISING TECHNOLOGIES FOR AGRI-WASTE VALORISATION

Some treatment technologies can be applied for the conversion of agricultural wastes to energy or value-added products (Table 1). These are commercially available and are classified into mechanical, thermochemical and biochemical technologies, as discussed below.

Table 1. Comparison of technologies for agri-waste valorisation.

4.1 Pelletisation

Pelletisation is a process of producing high density, solid energy carrier from biomass. It consists of multiple steps, including raw material pretreatment, pelletisation and post-treatment. Sawdust, wood shavings, wood wastes, agricultural residues like straw, switchgrass etc., are commonly used raw materials. Pellets are widely used in heating, cooking, power plant and boiler applications.

4.2 Thermochemical conversion

Combustion, pyrolysis and biomass gasification are the three major thermochemical biomass conversion processes available [18].

Combustion

Combustion is a relatively widespread and straightforward commercial technique of burning biomass in the presence of air. In this process, the chemical energy present inherently in biomass can be transformed into mechanical power, heat or electricity. This can be achieved by using different conversion devices such as boilers, furnaces, steam turbines etc.

A simple example of a combustion technique is a boiler. The biomass is burnt to produce high-pressure steam to run the turbine connected to a generator to produce electricity. Generally, the efficiency of conversion in combustion technology ranges from 20 to 40%.

Biomass gasification

Biomass gasification is the process of converting biomass into a mixture of combustible gases primarily consisting of carbon monoxide, nitrogen, methane, hydrogen etc., using biomass gasifiers. Gasification takes place at high temperatures ranging from 800 to 1100 °C, under controlled oxidation of biomass.

The intermediate process includes drying, pyrolysis, combustion and reduction. It can be used directly for thermal applications or as a fuel in internal combustion engines for producing mechanical or electrical power.

Gasification produces syngas for producer gas (containing approximately 40–70% H2, 15–25% CO, and 1–2% CO2), having a lower calorific value in the range of 4–5 MJ/m3. Syngas is an intermediate in generating synthetic natural gas, ammonia or methanol and other value-added chemicals. Gasifiers in the range of 20 – 500 kW are commonly used for producing biomass power in India.

Pyrolysis

Pyrolysis is the process of conversion of biomass to solid (biochar), liquid (bio-oil), and non-condensable gases (H2, CH4, CO, CO2, N etc.). It takes place at higher temperatures in the range of 300–800 °C in the absence of oxygen. The composition of the pyrolysis products depends on the process conditions mainly, temperature and residence time. By using slow pyrolysis, the production of biochar can be maximised, which is preferred for agronomic applications, and by employing the flash pyrolysis process, more bio-oil can be produced. Bio-oil can be used for running engines. There are some problems relating to bio-oils corrosion and stability as it starts undergoing degradation after the pyrolysis is over. Bio-oil up-gradation may be required in some instances, which can be done by reducing the oxygen content and eliminating the alkalis employing catalytic cracking of the oil and hydrogenation.

4.3 Biochemical conversion

Apart from anaerobic digestion, bioethanol fermentation and transesterification for biodiesel production are the two alternative biochemical conversion processes used for biomass treatment.

Fermentation

The fermentation process is used for the production of bioethanol, mainly using glucose from sugars (molasses, sugarcane), starch (wheat, corn, grains) or cellulose (wood, grass). The biomass is pulverised, and the starch is converted to sugars by the enzymes, then the yeast converts the sugars to ethanol. The use of sugarcane is quite widespread in India for making bioethanol. Distillation based purification of bioethanol is an intensive energy-consuming step, in which around 450 L of bioethanol is produced by 1000 kg of dry corn. The solid residue obtained from this process can be used as a feed for cattle, and bagasse obtained from sugarcane can be used as a fuel for boiler or biomass gasification.

Transesterification

Biodiesel is a form of diesel produced from biomass using a catalyst (generally NaOH or KOH) and an alcohol (typically methanol or ethanol) through the transesterification process.

Biodiesel is a long-chain alkyl (methyl, propyl or ethyl) ester and can be used as a vehicular fuel or running diesel engines. It can be obtained from several feedstocks such as vegetable oils, animal fats, waste cooking oils, non-edible oils etc., through the esterification and transesterification process.

Non-edible oils are more suitable feedstocks for biodiesel as they cannot be used for nourishment purposes. Pongamia and Jatropha are two common oilseeds used for biodiesel production. Glycerol is a by-product of transesterification reaction that finds several cosmetics, pharmaceutical and industries, and biodiesel applications.

5 CONCLUSION

Since innovation in the biogas value chain can have a range of benefits (e.g. cost reduction, cut in GHG emissions), continued efforts into R&D of biogas production, conversion into biomethane and the application of biogas are recommended.

The biomethane sector is mainly developed in Europe, with a dominant position being played by Germany. This country has opted to use biomethane in combined heat and power plants; however, recent changes in subsidies will influence future biomethane usage patterns.

Global dissemination of biogas-related knowledge and expertise can be improved by setting up a platform for best practices related to biogas production technologies, applications and policies, targeted at farmers, economic actors, municipalities and policy decision-makers.

6 REFERENCES

[1] Global Potential of Sustainable Biomass for Energy

[2] Biogas Production – an overview

[3] Bagheri, M., Esfilar, R., Golchi, M. S., & Kennedy, C. A. (2020). Towards a circular economy: A comprehensive study of higher heat values and emission potential of various municipal solid wastes. Waste Management, 101, 210-221.

[4] What is Anaerobic Digestion?.

[5] An introduction to biogas and biomethane – Outlook for biogas and biomethane: Prospects for organic growth – Analysis

[6] Kampman, B., Leguijt, C., Scholten, T., Tallat-Kelpsaite, J., Brückmann, R., Maroulis, G., … & Elbersen, B. (2017). Optimal use of biogas from waste streams: an assessment of the potential of biogas from digestion in the EU beyond 2020.

[7] https://www.irena.org/publications/2017/Mar/Biogas-for-road-vehicles-Technology-brief

[8] Biogas Production From Solid Waste Landfill. Ivan FS Santos, Regina M Barros, and Geraldo L Tiago Filho, Federal University of Itajubá (UNIFEI-MG), Itajubá, Brazil. 2018 Elsevier Inc.

[9] USEPA, 2005. United States Environmental Protection Agency .In: Landfill Gas Emissions Model. (LandGEM). Version 3.02 User’s Guide. USEPA. EPA-600/R-05/047.

[10] The production of biogas, biodiesel as high-value bio-based product and multiple bio-products through an integration approach of the anaerobic digestion and fermentation processes. Raid Alrefai, Alla M. Alrefai, Khaled Y. Benyounis, and Joseph Stokes, Dublin City University, Dublin 9, Ireland. 2019 Elsevier Inc.

[11] Al Seadi, T., Drosg, B., Fuchs, W., Rutz, D., & Janssen, R. (2013). Biogas digestate quality and utilisation. Biogas Handbook.

[12] Trends in Solid Waste Management

[13] M. Prussi, M. Padella, M. Conton, E.D. Postma, L. Lonza, Review of technologies for biomethane production and assessment of EU transport share in 2030, J. Clean. Prod. 222 (2019) 565e572, https://doi.org/10.1016/j.jclepro.2019.02.271.

[14] D’Adamo, I., Falcone, P. M., Huisingh, D., & Morone, P. (2021). A circular economy model based on biomethane: What are the opportunities for the municipality of Rome and beyond?. Renewable Energy, 163, 1660-1672.

[15] Horschig, T., Welfle, A., Billig, E., & Thrän, D. (2019). From Paris agreement to business cases for upgraded biogas: Analysis of potential market uptake for biomethane plants in Germany using biogenic carbon capture and utilisation technologies. Biomass and Bioenergy, 120, 313-323.

[16] Government of Sweden. (2013) A vehicle fleet independent of fossil fuels 84. SOU, Stockholm.

[17] Ornelas-Ferreira, B., Lobato, L. C. S., Colturato, L. F. D., Torres, E. O., Pombo, L. M., Pujatti, F. J. P., … & Chernicharo, C. A. L. (2020). Strategies for energy recovery and gains associated with the implementation of a solid state batch methanization system for treating organic waste from the city of Rio de Janeiro-Brazil. Renewable Energy, 146, 1976-1983.

[18] Kapoor, R., Ghosh, P., Kumar, M., Sengupta, S., Gupta, A., Kumar, S. S., … & Pant, D. (2020). Valorisation of agricultural waste for biogas based circular economy in India: A research outlook. Bioresource technology, 304, 123036.