phd dissertation on biogas

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Technologies, challenges and perspectives of biogas production within an agricultural context. The case of China and Africa

• Open access
• Published: 28 February 2021
• Volume 23 , pages 14799–14826, ( 2021 )

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• Rufis Fregue Tiegam Tagne 1 ,
• Xiaobin Dong 2 ,
• Solomon G. Anagho 1 ,
• Serena Kaiser 3 &
• Sergio Ulgiati   ORCID: orcid.org/0000-0001-6159-4947 3 , 4  

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The use of fossil fuels in modern economies has been a success because of the low cost of fossil resources. However, the depletion of fossil reserves, the increase in waste production and global warming concerns have led to increased research on the production of biofuels from renewable resources. Waste production is steadily increasing in quantity and constantly changing in quality, creating enormous risks for the environment and, consequently, for the health of the population. This situation is much more worrying in developing countries, in particular because of the considerable delay in the field of the conversion and recovery of biomaterials, due to their difficulty in approaching the problem in a way that fits their context. The composition of such wastes and residues, rich in organic matter, allows their conversion via biochemical mechanisms, thus constituting an effective solution to address the environmental problems of their disposal. Anaerobic digestion remains a valuable and effective technology for transforming these biomaterials into biogas. The present review focuses on technologies, challenges and areas of application of biogas, especially in China and some African countries, in order to promote the large-scale use of biogas for electricity generation and biofuels. Results point out that China is more used to this technology, while African countries still rely on traditional and less advanced technologies, thus hampering the potential derived from the large availability of biomaterials. Both realities, however, share similar backgrounds about the dimension of the biogas plants and their non-commercial purposes, even if China is recently shifting toward the adoption of a different model. These considerations are used in the article to open an interesting new scenario of political alternatives which may provide a way out from poverty and economic dependence, within the framework of a wider circularity.

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1 Introduction

Sustainable development is currently one of the most up-to-date concepts in the debate between developed and developing countries, as most of these countries have focused on their economic growth while neglecting the imperatives of environmental protection. With the expression “sustainable development” it is meant the use of the available resources without jeopardizing the well-being of future generations (Bertrand and Rocher 2007 ).

Thus, at COP 21, member countries of the United Nations set as their objective to reduce their carbon emissions by 30% by the year 2030 compared to those of 1990, and to reach the carbon neutrality in 2100 (United Nations 2015 ). In the same direction, the Chinese authorities are prioritizing the reduction of greenhouse gas emissions, with the declared goal of reducing global warming. One of the solutions to the carbon emission problem is to develop renewable energies as alternatives to fossil fuels. Among the alternative energy sources, lignocellulosic and waste biomasses are considered carbon–neutral fuels because the CO 2 released from their combustion is integrated into the virtuous cycle of photosynthesis of plants (US Environmental Protection Agency 2015 ). Agricultural waste-to-energy bioconversion technology can make a significant contribution to electricity demand relative to fossil fuel supply (Nixon et al. 2017 ). These agricultural wastes can be assimilated to woody material, animal excrement, manure and associated wastewater (Almomani et al. 2019 ).

These biomaterials can be transformed by technological processes to obtain biochemicals and bio-energies, among which biogas can be used as a biofuel or for electricity production (Pan et al. 2018 ). Biogas is produced by anaerobic digestion, a process that decomposes organic matter, using a set of microorganisms in anaerobic conditions, to produce biogas and digestate (Yang et al. 2015 ). Anaerobic digestion technology is a biochemical process used for the biological treatment of wastes producing sustainable energy and reducing greenhouse gas emissions (Almomani and Bhosale 2020 ).

Biogas is considered as renewable energy because it comes from organic waste that would not be valued elsewhere. Biogas production is therefore part of a circular economy. It can be upgraded locally to produce electricity, heat or both simultaneously in combined cycles. The use of biogas in the place of fossil fuels as energy source in industries gives rise to a much more eco-friendly, carbon-free and sustainable environment (Wall et al. 2018 ). Industries and households are continuously using biogas for heating and for producing hot water. Food industries requiring very clean fuel, whose combustion does not generate odors and particles, are increasingly using biogas. In many low- and middle-income countries of Africa and Asia, biogas produced in small digesters is used in rural areas for heating, cooking or lighting (Lohri et al. 2017 ). Biomethane can be used in other applications, such as catering, industrial kitchens and bakeries, where the heating has to be instantaneous and continuous and needs to be rapidly controlled.

Biogas in a non-enhanced form is obtained from cogeneration and trigeneration processes and can be used in power plants to generate heat and electricity, to power absorption chillers for cooling purposes (Kaparaju and Rintala 2013 ; Persson et al. 2014 ). Electricity from the power plants can be used locally, or it may be provided to the electricity network or grid, as well as for district heating. Thus, the use of heat from the combustion of biogas brings in very important economic and environmental benefits as it is renewable (Hengeveld et al. 2016 ) and suitable for several purposes.

Biogas is also used as vehicle fuel, after treatments to remove hydrogen sulfide, dust particles and CO 2 , generating biomethane with increased energy content. The improved clean biogas is used as fuel for cars, buses and trucks of different sizes. Several countries present well-developed infrastructure for gas vehicles, and it is possible to easily refuel natural gas powered vehicles (Florio et al. 2019 ) . The upgraded biogas is completely interchangeable with its fossil equivalent. The real challenge for the transport sector lies in the abandonment of fossil fuels and the decarbonization of energy.

Fuel cells are another potential use of biogas to produce heat and energy. Biogas steam reforming generates "green" hydrogen (Minh et al. 2018 ). Fuel cells combine hydrogen with oxygen from air to produce electrical energy and co-product water vapor and heat (Erin 2019 ; Zucaro et al. 2013 ). Fuel cells are recognized as very reliable tools and are often used as an emergency power source. Cogeneration systems with fuel cells (Bargigli et al. 2010 ) are presently used in hospitals, campuses, and remote telecommunication stations to generate heat and electricity, but also for transportation and as an electric generator in some residential homes.

China's energy generation has increased rapidly since 1980. However, demand is more and more outstripping supply because of the vast size of the country, where it is difficult to bring energy to remote and isolated rural areas. According to data from the National Development and Reform Commission (NDRC), total electricity production increased by 6.5 percent in 2017 to 6.310 billion kWh (John and Mathewsand 2016 ). This energy is obtained mainly via coal combustion, which provides the bulk of the energy, delivering about 66%, of the total energy consumption of the country. The second source is oil and other liquid fuels accounting for nearly 20% of consumption, hydroelectric sources (8%), natural gas (5%), nuclear energy (nearly 1%) and additional renewable energies (more than 1%) (María 2018 ). Concerning renewable resources, China has huge amounts of residues generated from agricultural activities and forest exploitation. These agricultural wastes contain materials like cellulose, hemicellulose and lignin which biodegrade and can be easily digested in the process of anaerobic digestion (Almomani 2020 ). It also generates large amounts of animal manure and municipal solid waste. All of these can be used as raw materials for producing biogas. It can be noticed that these “natural resources” do not compete with food items for which the cost keeps rising, especially that of staple foods. China is considered the first country in terms of agricultural production: It produces wheat, rice, potatoes, peanuts, millet, cotton and more. Also, livestock is the second most important component of agricultural production in China (Xuchuan et al. 2018 ) as it is the largest producer of hogs, chickens and eggs in the world. As an example, Fushan Farm located in Hangzhou in Zhejiang Province is composed of 32.47 hectares of rice, 4 hectares of tea bushes, 13.7 hectares of aquatic protection and 7.3 hectares of ponds for fish farming. The farm also produces 30,000 egg laying hens, 150,000 broilers and 8000 pigs each year; this results in a daily spill of 15 tons of solid waste and 70 tons of wastewater, thereby representing a huge amount of pollution (Zhang et al. 2015 ). These main products that can be consumed, marketed or processed by agro-food industries create enormous quantities of waste that are most often removed by open burning or by natural decomposition for a long time (Rufis et al. 2019 ).

Similar problems also arise in Africa, where electrification levels are still significantly low, compared to the population growth and barely reaching 42% (Vintila et al. 2019 ). By 2050, the continent will have a population of at least 2 billion people, 40% of whom will live in rural areas. In 2010, almost 57% of the population of Africans did not have access to electricity (État des Villes Africaines 2010 ). In sub-Saharan Africa, more than 600 million people do not have access to electricity supply, and the situation will substantially remain constant until 2030, if nothing changes (Joseph et al. 2019 ). Cameroon, like many other sub-Saharan African countries, is still facing a significant problem of access to electricity. For example, in Cameroon, only 3 million of its 20 million inhabitants have access to electricity at a rate of 15%. Total electricity production in Cameroon is around 305 kWh per inhabitant for a gross electricity consumption of 6.2 TWh (Vintila et al. 2019 ). Yet the country has abundant resources which may become renewable energy sources and may also play a major role in supplementing the growing energy demands, while also contributing to socioeconomic development. Cameroon is an important subregional agricultural player, where the rural sector is a driving force of the national economy, ensuring the food security of population in a concern for sustainable development. It mainly produces maize, millet, sorghum, rice, cassava, macabo, taro, yam, sweet and Irish potatoes, peanut, bean and soy, hence producing huge waste. Their management may add value to the national economy (Vintila et al. 2019 ). In the interest of sustainable development, the adoption of renewable energy is an important issue for many developing African countries. In addition, biogas could be a fast remedy to fix the pressure arising from low electrification of rural communities, by burning it to generate power at a low cost. The depletion of fossil fuels, the increase in agricultural residues, the threats from the accumulation of municipal waste, and the need for a sustainable environment and sanitation all justify the urgency to set up improved biogas technologies.

This work will mainly focus on a comparison of biogas energy technologies and challenges in China and Africa. The study stems from the interest of Africa and China in the technology of biogas as a multipurpose provider of energy for the production of sustainable fuel, electricity, heat and fertilizer. Biogas production and use show benefits in the environmental (less greenhouse gas emission), economic (low cost of produced commodities), sociocultural (employment) and legal–institutional (standards and regulations) sectors.

Of course, creating a comparison between China and selected African countries may raise some problems in terms of dimensions, industrialization and economic strategy and orientation, but this paper does not intend to make comparisons in absolute terms: What is interesting is the similarity in the role of agricultural production, as well as the similar growing demand of electrification and the wide part of population still excluded from it because living in rural areas. These similarities suggest that the path that China is following toward the adoption of more renewable energies may be a solution for African countries, too. Following the development of policies for biogas production in China and how they were integrated within the existing economic systems (agriculture, industry, etc.), similar or diverging and revised pathways could be traced for the development of the biogas technologic and regulatory systems in Africa. In this perspective, Cameroon is chosen as a case study of existing technologies and variables. This work is not comparing absolute numbers of totally different contingencies, but aims at establishing an example of development of suitable strategies.

2 Methodology and historical background

2.1 the biogas technological background in china and africa: an historical perspective.

In China, as well as in Africa, the implementation of biogas production has been very much small-scale oriented so far, being the total amount of biogas production mainly represented by small plants located in remote areas where there is no connection to the national electric grid. This means that the biogas production had no commercial purpose until a recent shift, while the American and European contexts have developed almost exclusively commercial large-scale production (Kemausuor et al. 2018 ). However, as anticipated above, China has recently changed its direction, and is now adopting a more commercial-oriented strategy (Kemausuor et al. 2018 ).

So, on the one hand, we have a similar development of biogas production in China and in the African continent. On the other hand, it is important to consider the recent Chinese shift and the level of top-down organization of the Chinese context, which surely represents an easier management of any modification in the political choices about energy.

Leaving aside the considerations about how desirable this centralization can be from the democratic point of view, it is all the same possible to accept that African countries may be inspired by the Chinese experience: Governments may use renewable sources as an opportunity to regain the control of strategic sectors.

In this sense (production of biogas through anaerobic digestion), widening the proportions of biogas plants may become interesting for African countries which, on the contrary, now mainly rely on foreign private investors for renewable energies: As a matter of fact, currently there is a wide profit which very seldom provides advantages to the countries and the continent (IEA 2018 ).

It may seem a paradox, but the “Chinese lesson” could represent a way for Africa to gain more economic independence from many ex-colonial countries and from China itself, which is currently one of the main economic investors in Africa. However, it is important to specify that the current link between China and Africa in the energy field is more related to the purchase of Chinese coal by the African countries: Chinese investments in Africa are more concentrated within the field of services than in renewable energies (Chen et al 2018 ). In this regard, we can be more precise concluding that the possibility of investing in bigger biogas plants represent the opportunity for African countries to become both more independent from the European ex-colonial countries’ investments in renewable energies and from the necessity to buy coal from China.

One of the examples we may consider to analyze the management of a larger biogas implementation in China is represented by the “Hebei Rural Renewable Energy Development Project,” started in 2015 and lasting until 2021 for sustainable biogas production and utilization. It includes a large-scale biogas facilities management with six installations in Hebei region for the conversion of agricultural wastes into stable clean energy for rural residents ( https://projects.worldbank.org ). From the indicators published by the World Bank, the project has allowed access to biogas supply to 2,300 rural households and is expected to create access to further 58,780 households by the end of December 2020. The biogas production has also generated a wide reduction in CO 2 emissions ( https://www.worldbank.org/en/results ) and this indicator is expected to grow, as well. This project may be a good example of introduction of new criteria in clean energy production, to involve external investments in a way which contributes to the improvements of local communities, within a productive interplay of central Government planning and the due importance to the territorial dimension.

This study was conducted with reference to the technologies of installed biogas digesters, the problems and challenges envisaged between China and Africa. China has developed and used biogas technology for almost three centuries so far (Lei et al. 2016 ). Since the 1980s, its government has integrated the development of biogas technology into the national program (Jiang et al. 2011 ). The capacity of biogas production in China by domestic digesters ranged from 4.5 Giga cubic meters to 16 Giga cubic meters between 2003 and 2013, reaching 8 000 Ktoe oil equivalent. In recent years, the Chinese have introduced various models of biogas digesters of different categories, depending on the circumstances of local environment. The main contributor to biogas production in China is rural biogas plants. The round pressure biogas plant was the oldest form of biogas digester used, and it was the most popular amongst the Chinese farmers. In recent years, other forms of digesters have been developed, an example of which is the Strong back-flow biogas digester (He et al. 2013 ). About 40 million digesters have been installed in China, while each year, 6 million are planned to be constructed, each using a totally or partially prefabricated fixed dome design. On the international scene, the use of prefabricated hybrid technology seems to be on the rise because it can be rapidly be installed, and it has the advantage of being able to be readily recovered by investors in the event of non-repayment of loans. However, there are very few reports on studies related to the technological innovations of large-scale biogas engineering projects in China. Actually, most of the high-level biogas engineering technology in China comes from developed countries (Lei et al. 2016 ).

Africa is currently undergoing rapid economic growth, sustained transformation, rapid population growth and diversified economic activities. For the growth to be sustainable, massive investments in the energy sector are required. The current rate of power production in Africa is very low (33% less than China), especially in the countries of the sub-Saharan Africa region (IAE 2019 ). Figure  1 shows the production of electricity in China and Africa broken down into different primary sources. It can clearly be seen that, while coal dominates the primary energy supply in China, biomass is still the main power source in Africa (although fossil contribution is also relevant).

Production of electricity in China and Africa from different sources in 2017 (IEA 2019 )

Natural resources such as biomass in Africa are still exploited in traditional ways, because of little or no policies on biomass energy and also due to very low levels of investment. The biogas technology emerged in Africa in recent decades has been supported by projects and the training of technicians (Cyimana et al. 2013 ). But this technology is characterized in Africa by low level of access, low capacity in utilization and lack of maintenance. With the help of NGOs, several countries like Rwanda, Ethiopia and Tanzania have developed more than 17,000 digesters between 2007 and 2012 (Cyimana et al. 2013 ). Some countries such as Morocco, Tunisia and Botswana, which are near to deserts and hence have unfavorable geographical positions, have been engaged on installing biogas digesters. Currently, South Africa runs about 200 working biogas digesters (Patrick et al. 2016 ). This shows that biogas technology is undergoing considerable development in Africa. Many African countries such as Burundi, Botswana, Burkina Faso, Ivory Coast, Ethiopia, Ghana, Guinea, Lesotho, Namibia, Nigeria, Rwanda, Zimbabwe, Tunisia, Cameroon, Morocco, Tanzania, and Uganda have developed and installed large-sized and medium-sized domestic biogas digesters to address the energy crisis in their territories (Amigun et al. 2012 ). Slaughterhouse waste, industrial waste, agricultural residues, household waste and animal excrement are used as raw materials in these various digesters (Mshandete and Parawira 2009 ). Figures  2 and 3 show some digesters used, respectively, in Africa and China (Patrick et al. 2016 ). Since 2008, several partnership programs have taken the initiative to support biogas projects in Africa. For example, China (BIOMA), provides technical support to national biogas programs (Renwick et al. 2007 ) even if, as said above, this does not represent the main investment field for China in Africa. This firm has trained several experts and technicians in the domain of biogas from a number of African countries. Countries such as Senegal, Burkina Faso, Ethiopia, Tanzania, Uganda and Kenya, which belong to the Biogas Partnership Program in Africa (BPPA), have built 70,000 biogas digesters in 2013, funded by Netherlands (Clemens et al. 2018 ). Because of national biogas programs, there is a rapid increase in the number of biogas plants in Africa. These national programs were born in countries such as Rwanda, Tanzania, Kenya, Uganda, Ethiopia, Cameroon, Benin and Burkina Faso. These states aim at building more than 10,000 biogas digesters in the coming years (Clemens et al. 2018 ). However, to maintain the rapid growth in the number of digesters and deal with technical, environmental, financial and social issues, there is the crucial and urgent need to provide scientifically rigorous answers to questions such as available substrates, efficiency, technology development, and personnel training.

Examples of digesters used in Africa

Examples of digesters used in China

Moreover, as already said, showing the positive consequences of the investments in biogas, this article aims at supporting the idea that governments should rely more on their own resources and ventures, thus creating the conditions for a wider independence from investors and raw materials coming from abroad.

2.2 Biogas technologies in China and in Africa

The biogas currently produced in China is mainly used for the production of heat, electricity, and compost for fertilization. Biomethane from biogas upgrading is used today in China in vehicles as biofuel (Deng et al. 2017 ).

Household biodigesters are the most widely used in Africa, as said above. The biogas currently produced in Africa is used for cooking and lighting. Digestate is used as a fertilizer for soil fertilization and as a pesticide for the protection of crops against pests and other harmful insects (EDE 2013 ).

The People's Republic of China (PRC), located in Eastern Asia, covers an area of about 9.6 million km 2 . It is the third largest country in the world, with only Russia and Canada being bigger than it. It is the most populous state on the planet, with nearly 1.4 billion inhabitants. It shares borders with many countries: Russia and Mongolia in the North, North Korea in the East, Vietnam, Laos, Burma (Myanmar), India, Bhutan and Nepal in the South, Pakistan, Afghanistan, Tajikistan, Kyrgyzstan and Kazakhstan in the West. The Chinese population is still very rural. In 2018, the density of the Chinese population was 148.35 inhabitants per km 2 (World Bank 2020 ). China ranks second in economic power in the world. Its economy is mainly based on agriculture, manufacturing and energy. It is the largest producer and consumer of energy. Energy is very vital for the economic development of all countries. The gross electricity consumption in China averaged 6990 TWh in 2018.

The fossil fuel reserves and domestic production capacity remain insufficient in front of the colossal demand of the population (John and Mathewsand 2016 ). China relies a lot on oil and gas it imports from the Middle East and Africa. They are mostly transported by oil tankers along maritime shipping lines to meet the demand of the population. This strong dependence of the country on foreign oil, the problems associated with the environment due to the massive use of coal, and the need to fight against climate change have oriented the Chinese energy strategy toward a greater diversification of the energy mix and increased the use of renewable energy (Zhang and Zhang 2014 ). Today, China faces a challenge: continuing its economic growth to improve the life of its 1,380 million inhabitants and meet their expectations, but at the same time reducing pollution from coal and limiting its emissions of greenhouse gases. For this to occur, the Chinese government has set as a goal to increase the amount of non-fossil energy to at least 15% of the national energy consumption by 2020 and 20% by 2030 (John and Mathewsand 2016 ). Among alternative sources to fossil fuels, biomass, which is organic matter, generally agricultural or forestry by-products that can be used directly to produce heat by combustion or indirectly, after having been transformed into various biofuels, is considered to be predominant.

Africa, the second largest continent in the world, accounts for 10% of the world's population (Amigun et al. 2008 ). In this respect, biofuels, in particular biogas, are considered as the most powerful alternatives to fossil fuels (Adeniyi et al. 2007 ; Ayhan 2008 ). Biogas production is highly developed in America, Asia, and Europe. However, the African continent and more specifically the sub-Saharan Africa region has experienced a very slow development of this technology in recent decades, despite significant efforts at the individual, institutional, national and international levels (Lynd et al. 2015 ). This delay is due principally to the lack of appropriate infrastructure and the limit on raw material caused by poor farming practices (USDA/FAS 2008 ).

2.3 Methods used

Many scientific works have already been published about biogas production technology. The various works cited within the framework of this study were listed in the sites available online (Academia. edu, Mendeley library) making an inventory on the potential production of biogas in Africa and China. Indeed, the present study took into account the scientific results and data published in journals and books and indexed in Scopus, Web of Science, Google Scholar and other databases available in internet between the years 2000 and 2020. In addition, articles from UN, IEA, Chinese government and African governments are included. Selected articles make reference to anaerobic digestion technology in China and Africa. In total, 95 scientific works were referenced, most of which referred to in the following, distributed as follows:

Articles: 50 published works

Thesis: 3 published works

World Bank: 2 published works

UN: 2 published works

IEA: 2 published works

Book chapters: 5 published works

Conference papers: 7 published works

Conference reviews: 9 published work

Book chapters: 8 published works

Reviews: 15 published works.

About the political and economic framework, reports and articles from the World Bank databases have been used to support the idea that renewable sources of energy are taken into consideration even by international and supranational organizations as path for a development which is not only environmentally feasible, but also economically advantageous and fair.

Of course, this does not mean that the concept of what is fair about development is expressed by the values and purposes of the World Bank or other similar organizations, but the critical use of their databases can be a precious resource for researchers.

3.1 General results about China and Africa

China holds important reserves of different types of biomass and biogas, uses less expensive and less devastating technologies: This pushes China to further develop biomass and biogas projects. China is currently the world leader in renewable energy. Biogas production in China is already widely used and can still be well developed to provide green energy. It is also a highly sustainable energy for the country. This form of energy accounts for about 1.2% of China's total energy consumption. In 2010 the Chinese Government set two main objectives: The first one was that 40 million households (about 16% of all rural households) would use biogas and their number was expected to double to 80 million by 2020. In 2008, about 40 million digesters were in service for biogas production. The second was to combine biogas technology with agricultural production and environmental protection. The production of biogas is in perpetual evolution. According to the International Energy Agency (IEA 2019 ), the country produced 630 megawatts of biogas in 2018. Figure  4 shows the evolution of biogas production in People’s Republic of China between 2010 and 2018 according to IEA ( 2019 ).

Evolution of biogas production in China between 2010 and 2018 in Megawatt (IEA 2019 )

China has an abundance of biomass resources. Few of them are effectively used today. These resources are mainly obtained from forest residues, animal wastes, industrial and municipal biodegradable wastes. China produces about 700 million tons of straw a year, distributed as follows: About 37% of this total is maize, 28% rice and 20% wheat. The remaining 15% is derived from various other crops. In addition, China produces 300 million tons of forestry waste, which is immediately useable for fuel production (Chi et al. 2017 ). The country is also expected to produce 210 million tons per year of municipal solid waste by 2020. It could produce between 2 to 10 billion cubic meters of methane if 60% of this municipal waste disposed of as for landfill were used for methane production. Table 1 gives the potential quantity of residues for biogas production (Liu et al. 2008 ).

China has been known for many years for its ability to use organic waste for biogas production. However, it is surprising that, despite the success of anaerobic digestion for the production of biogas, the application of biogas technology at relatively large scale for small communities is still essentially limited today. This limit can also be due to the lack of raw materials, because the widely available biomass in China is also used in the pulp and paper making industries. Yet the country has other resources that do not yet have any commercial value, like waste from hospitals such as food waste and sewage, unless already managed as municipal waste. Some authors indicate as potential substrates also body parts, blood, chemical drugs, medical devices and syringes, which are very rich in organic matter and can be used as feedstock for biogas production, although possibly raising ethical problems and lack of respect of local cultures (Mohammed et al. 2017 ). The number of hospital centers in China has increased dramatically from 305,000 to 961,830 in space of 10 years and consequently, medical waste has also increased rapidly (Li et al. 2014 ). To enhance the biomass for large-scale application, careful management of hospital waste will increase the potential for biogas production.

According to Novozymes (Susan et al. 2016 ), China could become a world leader in the production of second generation (2G) biofuels. According to a report by the International Energy Agency, 2G biofuels account for about 90% of all biofuels used today. Thus, wastes could represent 2.4 to 2.8% of China's total energy consumption. China is now building new socialist villages under the current Eleventh Five-Year Plan (Lifan 2016 ). The guidelines for building such villages are the development of production system, maintaining a clean environment and innovation to save resources. The development of biogas production corresponds to that program. China has developed many types of household biogas digesters since the 1980s to overcome the weaknesses of traditional brick and concrete digesters (Lei et al. 2020 ) . Among all the traditional digesters, the Chinese dome digester is the most popular one due to its reliability. It has become a standard for the design of domestic prefabricated digesters like the Puxin digester (Jegede et al. 2019 ). The most common prefabricated biogas digesters in China are flexible plastic digesters and composite material digesters (Lei et al. 2020 ) . Flexible plastic digesters are very efficient due to their low cost and ease of implementation and use. Composite material digesters have many advantages, such as ease of movement, long-term durability and high productivity (Lei et al. 2020 ) . The major constraint to the program is the lack of technical expertise in managing and maintaining the biogas digesters.

Biogas still remains an unknown energy source in some African countries, despite several attempts for its use that have been undertaken since the forties, also because it is not part of their sociocultural and economic context. A very limited number of African countries had adopted biogas technology in 2005 with a number of biogas plants, although insignificant compared to what was achieved on the other continents (Mshandete 2009 ). In an effort to improve biogas production in Africa, an African initiative was launched in 2007 to install biogas digesters in at least 2 million households by 2020 (Van Nes et al. 2007 ; Ukpabi 2008 ). Under this initiative, there was an increase in the number of biogas plants in 2010, with Tanzania alone having about 4000 units (Ocwieja 2010 ). However, only about 60% of these plants were functional, while the others displayed a lower level of satisfaction for reasons such as planning and construction errors, low awareness of the community and lack of an adequate culture of maintenance, (Ocwieja 2010 ). Composite material digesters are relatively new in African countries (Lei et al. 2020 ) . Almost all African countries still use traditional brick and concrete digesters. Table 2 shows some African countries that had adopted biogas technology in 2005 (Mshandete 2009 ).

About 95% of the population in sub-Saharan Africa, uses traditional biomass for cooking and heating, with access to modern energy systems for cooking being very low. In North Africa a very low percentage, 10 million out of more than 200 million people, of population relies on traditional biomass. Biogas in Africa is produced by waste from agricultural activities (crops and livestock), bio waste from municipal activities and bio waste from agro-industrial activities. According to the World Bank report 2015 , 62% of the population of sub-Saharan Africa and 36% of the population of North Africa live in rural areas, whose energy needs are very different from those of the urban population. A report by SNV and the International Institute of Tropical Agriculture, published in 2007, estimated the technical potential of national biogas plants in Africa at 18.5 million, while current ones are estimated at less than one hundred thousand. This scale and implementation model has already been adopted in Asian countries, as China has about 40 million digesters. Biogas production could therefore constitute a viable means for overcoming the energy shortage in African countries, thus constituting a brake on social and economic development. The African continent with its hot climates and tons of waste generated could develop high performances in organic matter anaerobic digestion (Amigun et al. 2012 ). Table 3 gives some comparisons of the biogas situation in China and Africa.

3.2 The case of Cameroon

3.2.1 the geography and energy situation of cameroon.

As a Central African country, Cameroon is located between the 2nd and 13th degrees of north latitude and the 9th and 16th degree of east longitude. With a total area of 475,650 km 2 , of which 466,050 km 2 are continental and 9600 km 2 are maritime, its surface area is about 5% of the total surface area of China. It is open to the Atlantic Ocean over a distance of 280 km. It is bounded to the north by Lake Chad, northeast by the Republic of Chad, to the east by the Central African Republic, to the south by the Republic of Congo, the Republic of Gabon and the Republic of Equatorial Guinea and to the West by the Federal Republic of Nigeria. This geographical location of Cameroon explains the diversity of natural environments, from pre-Sahel to the high western lands, through the coastal lowlands and the highlands of medium altitudes. This mosaic of natural environments justifies the variety of Cameroonian agricultural products (Tchatat 2014 ). Cameroon's population in 2015 was approximately 24 million, making it the most populous country in Central Africa. The Cameroonian economy is essentially based on agricultural activity (agriculture, livestock, fisheries and forestry) and oil (Yotchou 2014 ). Cameroon has substantial renewable energy resources such as solar, wind, biomass, geothermal and hydropower. Hydropower is the main source of energy used in industries with 75% of electricity generation and little attention is given to other renewable energies. Major sources of energy that the country has are coal, oil, hydro, and biofuels (EUEI-PDF 2013 ). Electricity production in 2017 in Cameroon was 11,295 ktoe, with 58% produced mainly from biofuels and waste (IEA 2019 ). Figures  5 and 6 show, respectively, the main sources of primary energy and consumption in Cameroon in 2017 (IEA 2019 ).

Primary sources of energy in Cameroon in 2017 (%) (IEA 2019 )

Final uses of energy in Cameroon in 2017 (%) (IEA 2019 )

The majority of the energy used in Cameroon comes mainly from the biomass resources originating from agricultural activities, livestock and food industries. Cameroon is recognized as the third country with the largest biomass in sub-Saharan Africa (Mus’ud et al. 2015 ).

Overall, Cameroon's energy consumption is more than 65% met by traditional energies (wood, coal, etc.), 21% by petroleum products and 14% by electricity according to National Energy Action Plan for Poverty Reduction (PANERP) (Ministry of Energy and Water Resources Cameroon, 2010 ). Nearly all poor households in Cameroon use firewood to meet their energy needs for cooking. Barely 6% of energy resources are exploited, while an important part of the households and partners of the industrial sector in the country do not have access to electricity and the available electric grid, which requires heavy investments, and is characterized by intermittent performance. The challenge of Cameroon's electrification program is to ensure an adequate supply of energy throughout the country at the lowest cost. The government aims by 2020 to achieve a national electrification rate of 48%, an access rate of 75% electricity and a rural electrification rate of 20%. This work can contribute to the achievement of these objectives. Although the consumption of wood poses enormous environmental and health problems, this energy resource remains the most consumed in the country. Table 4 tells us about the most polluting sectors in Cameroon. It shows that the combustion of fossil fuels is the main source of GHG emissions, followed by the transport sector.

Commercial production of biofuels is still not significant in Cameroon, although the objective of the Government of Yaoundé is to produce at least 77 million liters bioethanol and 74 million liters of biodiesel in 2020 (UNIDO 2016 ). The biogas production technology is not yet well developed in Cameroon. However, there is domestic and artisanal production of biogas in some parts of the country, like the North where in 2011 the Cameroonian Society of Hygiene and Sanitation (HYSACAM) inaugurated its first biogas production plan, and it was based on household waste (Erasmus et al. 2018 ).

There is no information on research on biogas technology in Cameroon, although a number of digesters have been installed in the country. In addition, published works on laboratory biogas digesters is not known. There is no data available on the amount of methane produced from installed biogas digesters or on ways to improve the quality of methane produced. In addition, research on biogas technology by biogas specialists is hampered by lack of funds, which has a negative impact on biogas programs in the country. Table 5 gives the potential of agricultural biomass residues available in Cameroon (Ngnikam et al. 2016 ).

3.2.2 Crop and livestock farming in Cameroon as sources of clean energy, social justice and women’s empowerment

Cameroon's natural environment makes it possible to divide the country into three major areas of agricultural concentration: the northern zone and Adamaoua, the western highlands and the southern forest. By possessing all the geographical, and climatic zones, and the vegetation that is found on the African continent, Cameroon is often called Africa in miniature (Ngnikam et al. 2016 ). The government, through the Ministry of Agriculture and Rural Development, intends to continue implementing an emergency plan to increase agricultural production. Cameroon produces mainly maize, millet, sorghum, cassava, macabo, taro, yams, sweet potatoes and potatoes. It is the largest producer of coffee and cocoa in Central Africa (Vintila et al. 2019 ). With regard to livestock, the country produces 7 million cattle annually, 8 million small ruminants, 2 million pigs and 50 million poultry. The agricultural sector remains a priority for the government in terms of coverage of domestic needs and fight against poverty (Tchatat 2014 ). Cameroon is often presented as the breadbasket of Central Africa. It enjoys great climatic diversity (equatorial climate, humid tropical and dry tropical) and an ecology that allows it to produce a varied range of agricultural commodities. The dynamism of Cameroonian agriculture makes it possible to satisfy the food needs of the local populations and some of those from the countries of the sub region (Chad, Republic of Central Africa, Gabon, Congo and Equatorial Guinea).

Despite the government's efforts in the agricultural sector, Cameroon faces a high cost of factors of production, in particular because of a supply of energy much lower than the domestic need would require. Yet the country produces annually billions of tons of waste from agricultural and agro-industrial activities and from hospitals that may solve huge problems associated with environmental pollution and low energy supply, by embarking upon biogas production. While contributing to the reduction of the energy deficit and the creation of numerous employment opportunities, the exploitation of these renewable energies or biofuels would constitute new outlets to ensure an important source of income in the whole country.

Livestock farming deserves a particular attention, since it is not only one among the sources of materials to be used for the production of biogas, but also an activity which has created the conditions for the empowerment and emancipation of a relevant number of women in Cameroon. Since the 1980s, indeed, a severe economic crisis negatively affected many families, whose men decided to go to the big cities to look for a job in the urban context. After this, women had to take care of livestock farming, activity in which they had been previously involved only as assistant of their husbands. This job represented the possibility for women to earn the necessary resources for themselves and for their families, and of course it turned into an opportunity to build their own emancipation. Nowadays, livestock farming is mainly managed by women (Kam Kah 2013 ). This result opens a series of very interesting scenarios about the creation of new projects involving livestock farming as a source for the production of biogas: Projects may involve a wide protagonism of local women, whose empowerment would be not only related to their economic independence, but also to the creation of a virtuous alternative to fossil energy.

4 Discussion

4.1 biogas production: challenges and economic benefits for the actors within a circular economy framework.

This section describes the benefits of biogas in a circular economy framework. Biogas is the center of an ecologically based economy that is emerging in China and may spread to several countries of the world. While the agricultural economy intensifies, there are still many farms, of large or medium sizes, in the suburbs of large cities. The multifunctionality in the use of biogas includes the waste treatment, the protection of the environment by reducing emissions of greenhouse gases, the generation of electricity, heat and gaseous biofuels. Anaerobic digestion can break down a multitude of organic wastes to produce biogas, and a wide range of other products, thereby, contributing significantly to the circular economy. Restorative and generative structure of circular economy aims at maintaining the products and materials they use and their maximum value at any time of the cycle (Sariatli 2017 ). Figure  7 shows the main concept of circular economy. The circular pattern around a biogas plant is shown in Fig.  8 . Biogas production is the last stage of anaerobic digestion of the raw material, in which a renewable energy vector (biogas) is produced together with a digestate, to be used as a biofertilizer. Therefore, biogas production is a way to recycle waste for added value.

Life cycle stages in the linear, recycling and circular economy models (Van Buren et al. 2016 )

Functions of biogas in the circular economy (ADEME 2013 )

Without a biogas production scenario, the traditional model is set on the use of wastes coming from agricultural and livestock farming activities to fertilize the land; the rest of manure and other materials is considered to be disposed of, thus creating a cost for the household or the firm from which the material is originated (Yazan et al. 2018 ). Within a circular economy scenario in which manure and agricultural wastes are used to produce biogas and digestate as fertilizer, there are benefits not only for the preservation of environmental resources, due to the fact that the energy which is produced is alternative to fossil energy, but also for the economic incomes of both the farmer and the biogas producer. Of course some variables like the distance between the farm and the biogas plant, the dimension of both the structures and other elements may affect the positivity of this income (Yazan et al. 2018 ). A study has shown that cooperation between the two actors represent the best solution in terms both of environment preservation and economic benefit (Yazan et al. 2018 ).

4.2 Biogas as a basis for energy independence of a country

Fossil energy is still widely used around the world. It derives from coal, petroleum and natural gas reserves which are located in relatively small geographical areas. Hence, many countries depend on a small number of fossil fuel exporting countries for their energy supply. Substituting these fossil fuels with some biological/renewable energy types would better balance the energy supply situation around the world, thereby making countries and regions more energy self-sufficient. Biogas production is becoming more reassuring for any municipality to become more energy independent and for the different local industries to benefit from greater energy production themselves. Centralized energy production systems are more sensitive to the risk of interruption of energy distribution in the event of storms or other natural events (Fagerström et al. 2018 ). Faced with these instabilities, the energy supply of the second world economy, China, is very far from being assured because China depends heavily on oil and gas imports from the Middle East and Africa. To ensure the country's economic growth, Beijing has to cover growing energy needs. Becoming increasingly dependent on imports, a sense of deep anxiety has raised among Chinese leaders, together with the fear that the country's economic growth could be slowed down by potential supply disruptions and unpredictable price increases. Biogas generation of electricity could balance energy production when the networks are vulnerable, for example during major weather events (Persson et al. 2014 ).

4.3 Advantages of biogas for the environment: perspectives for next future’s countries’ policies

Cameroon is a country which is projected as an emerging country by 2035 (EB 2019 ). Therefore, a trajectory will be built concerning its energy policy, in particular bioenergy, to ensure sustainable development. To reduce global warming, the Chinese authorities are putting priority in reducing greenhouse gas emissions. China has been the world's largest producer and consumer of coal since the early 1980s, accounting for nearly half of the global market. China's dependence on this energy source, with the exploitation of nearly 12,000 mines, largely explains the country's high greenhouse gas emissions and pollution problems, of course also coupled with its large population. The country accounts for 28% of CO 2 emissions worldwide, according to the International Energy Agency (IEA 2019 ). To solve this problem, China must invest heavily in renewable sources of energy among which, of course, biogas may play an important role. Increased energy efficiency and China's goal of increasing environmental sustainability is expected to result in a smaller share of coal importation (Wang and Kaare 2019 ). In the last years, the Chinese government increased its attention to the questions of environment and green energy and promulgated a law called "law on the protection of the environment." Further, the development of bioenergy is requested to be an important goal in China's 13th Five-Year Plan (Liu et al. 2015 ). All of this should translate into good prospects for the Chinese biogas industry.

4.3.1 Reduction of greenhouse gas (GHG) emissions from fossil sources

Disposing of agricultural waste by means of open fires after the harvest season releases various greenhouse gases and fine substances into the atmosphere that contribute to the problem of global warming and air pollution (Almomani and Bhosale 2020 ). Biogas is considered a "cleaner" household fuel because it does not contribute to air pollution compared to fossil fuel (Poushali and Milind 2020 ). Biogas production may have a significant impact on reducing GHG emissions (Liebetrau et al. 2017 ). This reduction is the main challenge of switching from fossil fuels to renewable energies. The use of biogas has a beneficial consequence on the greenhouse effect: It makes it possible to burn the methane produced during the fermentation of the waste and thus prevent additional greenhouse gas with very high climate change potential from being produced from fossil fuels and released to the atmosphere. The rationale of this process is the observation that it is impossible to prevent the production of methane because of agricultural and especially livestock production all day long. Since methane global warming potential is 25 times higher than CO 2 , it is scandalous to release it into the atmosphere (Fagerström et al. 2018 ).

Biogas from anaerobic digestion is a multifunctional technology that offers solutions, particularly for the treatment and management of digestible waste from crops and animal origin, manure and sewage. A study by the French Environment and Energy Management Agency (ADEME) predicts that by 2020, biogas will contribute to a significant reduction in fossil greenhouse gas emissions (ADEME 2006 ). The estimate of this reduction in greenhouse gases is 751,000 tons of CO 2 equivalent. The use of manure as raw material for anaerobic digestion reduces CH 4 emissions generated from the storage and use of manure itself. Practically, this can lead to carbon-negative energy and fuels, because the CO 2 emitted during the combustion of biogas has a lower global warming potential than methane emitted into the atmosphere from fossil fuel (Murphy et al. 2004 ). The use of biogas also makes it possible to reduce greenhouse gas emissions in accordance with the Kyoto protocol (Singh et al. 2017 ). However, subtracting manure to agricultural use will lead to an increased use and production of chemical fertilizers, causing environmental damage in their generation and application.

4.3.2 Carbon dioxide sequestration potential by the anaerobic fermentation process

A major advantage of the use of biogas is the possibility to uptake and sequester CO 2 , in order to keep the global temperature below 1.5 °C (EASAC 2018 ). For the sake of clarity, the methane obtained during the digestion process is accompanied by about 30% of carbon dioxide. When biogas is recovered as a biofuel in transport, CO 2 must be separated from the CH 4 . Future energy systems, within a circular economy perspective, may use the CO 2 released as raw material for food and biomaterials production. It could, for example, be used to increase the concentration of carbon dioxide in greenhouses (when its purity is high), thereby increasing crop and food production. It could also be used as a support in cooling systems or as a raw material for chemical production. Pure carbon dioxide could also be used in the food industry for the production of sparkling water and in breweries to add carbonated bubbles to beer.

To produce microalgae in closed-loop systems, significant amounts of carbon dioxide are captured from biogas and then converted into biomethane (Xia et al. 2015 ). The production of biogas on a domestic scale in Africa has contributed to the reduction of respiratory and eye diseases linked to the use of firewood or charcoal, an improvement of the living environment of households through better management of cow dung and domestic wastewater, in particular by connecting toilets to biodigesters.

4.4 Water purification through anaerobic digestion implementation

Water is a common national heritage which the State will have to protect and carefully manage, and make easily accessible to the population. In Cameroon, 53% of households are exposed to diseases carried by water through the use of wells for drinking. Similarly, China has considerable water resources, but given the size of its population (about 1.3 billion), availability per capita is very limited. Water pollution is becoming a widespread phenomenon in the cities and countryside of some riverine areas due to major works of heavy industrialization in the country, the accelerated and uncontrolled suburbanization (Lu and Zhongguo 2000 ).

Anaerobic digestion is an effective method of reducing high biological oxygen demand (BOD) in the effluents. Biological oxygen demand is a measure of the amount of oxygen used by microorganisms during the biochemical oxidation of the organic matter. For example, wastewater from dairy products have high BODs; with values being as high as 25 to 40 times more than that in domestic wastewater. About 70% to 90% of it can be removed by anaerobic processes at a lower cost, when compared to aerobic systems.

The production of biogas on a domestic scale in Africa has contributed to the reduction of respiratory and eye diseases linked to the use of firewood or charcoal, an improvement of the living environment of households through better management of cow dung and domestic wastewater, in particular by connecting toilets to biodigesters.

4.5 Agricultural waste management and energy generation in an anaerobic digestion framework

Thanks to anaerobic digestion, farmers will be able to reduce their greenhouse gas emissions, store and process their waste, diversify their activities and thus secure a portion of their income by reselling the energy they have produced. The use of biogas digestate for soil fertilization significantly reduces the use of chemical fertilizers. The minerals present in the substrates are conserved, and the predominantly organic nitrogen in the raw manure effluents becomes mainly mineralized in the digestates. It is thus more easily assimilated by plants, and water pollution is reduced. The use of digestates in the rural areas significantly reduces odors and flies that result from the use of raw slurry, thereby greatly improving on the quality of air and life in the areas. An article (Gobar Gas 2018 ) in the Nepal Times points out that the successful biogas program in Nepal not only has provided farmers with clean fuel, but has also helped conserve forests and provide high quality fertilizer for their crops; the result is a worldwide repayment, to avoid the burning of firewood, which releases carbon dioxide CO 2 [a greenhouse gas] into the atmosphere (Kandal 2002 ). Multifunctional uses of biogas bring cross-benefits in the areas of energy, transport, agriculture and waste management, as well as in all sectors of the environment and climate (AL Seadi et al. 2018 ). In Africa and China, we can observe farmers in the agricultural field who have adopted the biodigester based on an integration of livestock and agriculture by valuing the by-products from these two activities (valuation of millet stalks and other agricultural wastes for the production of animal feed; production of organic manure for the soil).

To summarize, the multifunctional uses of biogas produce savings in: (1) the production of biofuels and renewable energies; (2) the reduction of greenhouse gas emissions (CH 4 , CO 2 ); (3) the reduction of nuisances due to odors and flies; (4) financial savings for farmers; (5) sustainable treatment and recycling of organic waste; (6) the reduction of air and water pollution; (7) the improvement of local and rural economies; the improvement of social conditions, among which gender balance has a particular importance. Figure  9 shows the process of producing biogas from agricultural waste.

Detailed mechanism of the biogas production from agricultural waste (Liu et al. 2011 )

5 Conclusions

The use of biogas as renewable energy has excellent prospects. An important factor in encouraging the spread of biogas technology is the establishment of an appropriate legal framework. For example, in March 1999, Germany carried out a major tax reform which established taxes on energy sources and carbon emissions, while having a tax exemption on the use of renewable energy sources.

To maximize the benefits of biogas as a source of energy, China surely needs to incorporate advanced technologies for purifying and compressing methane, and it needs to build new engines that work effectively and efficiently using methane. Such policies may contribute to making green energy profitable and are crucial to the ultimate success of green energy programs.

Anaerobic digestion allows the production of renewable and efficient energy, while improving the environmental performance of farms. It helps to reduce chemical fertilizers and improve soil quality. China, just as Cameroon and many other African countries, has an abundance of resources from agricultural activities which are largely untapped. The use of these biological resources will grow in absolute and relative terms in the coming years. In an effort to attain a sustainable development, this waste can be transformed by biotechnological methods for producing renewable energy. They can contribute significantly to reducing greenhouse gas emissions, for the improvement of soil fertility, and for the reduction of deforestation in Africa by offering an alternative fuel to wood for various applications such as cooking and heating, thereby ensuring a sustainable environment and bringing in potential financial and social gains for the local population.

In addition to all this, as already said several times above, the introduction of a wider, more solid and independent program of biogas plants installation may represent the opportunity for many African countries to avoid both the dependence from coal—mainly coming from China—and from European capitals investing and gaining profits from the local renewable sources of energy.

Abbreviations

Conference Of the Parties

National Development and Reform Commission

Kilowatt hour

Non-governmental organizations

Biogas Institute of Ministry of Agriculture

Biogas Partnership Program in Africa

People Republic of China

International Energy Agency

Netherlands Development Organization

International Institute of Tropical Agriculture

National Energy Action Plan for Poverty Reduction

Information Processing for Energy Policies for Ecodevelopment

Greenhouse gas

French Environment and Energy Management Agency

• Anaerobic digestion

Biological oxygen demand

Cameroonian Society of Hygiene and Sanitation

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Acknowledgements

This research was supported by the Geography Faculty of the Beijing Normal University through the 2019 Short Program Course on Resources and Environmental Management. The authors would like to thank all the people who supported this study. Sergio Ulgiati and Serena Kaiser also acknowledge the funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie Innovative Training Networks (H2020-MSCA-ITN-2018) scheme, Grant Agreement Number 814247 (ReTraCE project).

Open access funding provided by Università Parthenope di Napoli within the CRUI-CARE Agreement. This research was supported by the Geography Faculty of the Beijing Normal University through the 2019 Short Program Course on Resources and Environmental Management. The authors would like to thank all the people who supported this study. Sergio Ulgiati also acknowledges the funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie Innovative Training Networks (H2020-MSCA-ITN-2018) scheme, Grant Agreement Number 814247 (ReTraCE project).

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Tagne, R.F.T., Dong, X., Anagho, S.G. et al. Technologies, challenges and perspectives of biogas production within an agricultural context. The case of China and Africa. Environ Dev Sustain 23 , 14799–14826 (2021). https://doi.org/10.1007/s10668-021-01272-9

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A critical review of biogas production and usage with legislations framework across the globe

S. abanades.

1 Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France

H. Abbaspour

2 Department of Biology, Faculty of Biological Science, North Tehran Branch, Islamic Azad University, Tehran, Iran

3 School of New Technologies, Iran University of Science & Technology, Tehran, Islamic Republic of Iran

4 Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar, Asaam 788010 India

M. A. Ehyaei

5 Department of Mechanical Engineering, Pardis Branch, Islamic Azad University, Pardis New City, Iran

F. Esmaeilion

6 Department of Energy Systems Engineering, School of Advance Technologies, Iran University of Science & Technology (IUST), Tehran, Iran

M. El Haj Assad

7 Sustainable & Renewable Energy Engineering Department, University of Sharjah, Sharjah, United Arab Emirates

T. Hajilounezhad

8 Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO USA

D. H. Jamali

9 School of Environment, College of Engineering, University of Tehran, Tehran, Iran

10 R, L. Applied Thermodynamic, National Engineering School of Gabes, University of Gabes, Gabes, Tunisia

H. A. Ozgoli

11 Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Sh. Ehsani Rad St., Enqelab StParsa SqAhmadabad Mostoufi RdAzadegan Highway, 3313193685 Tehran, Iran

12 Department of Energy Engineering, Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University (IAU), Daneshgah Blvd, Simon Bolivar Blvd, 1477893855 Tehran, Iran

13 Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah, UAE

E. H. Bani-Hani

14 Department of Mechanical Engineering, School of Engineering, Australian College of Kuwait, Kuwait City, Kuwait

This review showcases a comprehensive analysis of studies that highlight the different conversion procedures attempted across the globe. The resources of biogas production along with treatment methods are presented. The effect of different governing parameters like feedstock types, pretreatment approaches, process development, and yield to enhance the biogas productivity is highlighted. Biogas applications, for example, in heating, electricity production, and transportation with their global share based on national and international statistics are emphasized. Reviewing the world research progress in the past 10 years shows an increase of ~ 90% in biogas industry (120 GW in 2019 compared to 65 GW in 2010). Europe (e.g., in 2017) contributed to over 70% of the world biogas generation representing 64 TWh. Finally, different regulations that manage the biogas market are presented. Management of biogas market includes the processes of exploration, production, treatment, and environmental impact assessment, till the marketing and safe disposal of wastes associated with biogas handling. A brief overview of some safety rules and proposed policy based on the world regulations is provided. The effect of these regulations and policies on marketing and promoting biogas is highlighted for different countries. The results from such studies show that Europe has the highest promotion rate, while nowadays in China and India the consumption rate is maximum as a result of applying up-to-date policies and procedures.

Introduction

From the 1980s onward, the striking jump in global energy consumption has been largely driven through fossil energy resources. Generally, oil, coal, natural gas, electricity, nuclear energy, and renewable energies have shared 33, 27, 24, 7, 4, and 4% of total primary energy proportion in the whole world in 2018, respectively. Approximately, 85% of the world's primary energy consumption has been supplied by fossil fuels in 2018 (BP. 2019 ; Ghasemian et al. 2020 ).

The conversion of biomass to energy has been promoting from 65 GW in 2010 to 120 GW in 2019 due to climate change, reasonable energy prices, distributed generation increase, and environmental aspects, in recent years. Wastes with high moisture content are more compatible with conversion by anaerobic digestion, landfill, and digestion technologies. The global amount of biogas plant capacity was about 19.5 GW at the end of 2019. Organic wastes are the most common feedstocks to produce biogas from wastes, including domestic wastes (food, fruits, and vegetables) or public moist wastes (cafes and restaurants, daily markets, and companies’ biological wastes), due to significant moisture content and high degradability. These input materials are classified as OFMSW, which represents the organic fraction of municipal solid waste (Antoine Beylot et al. 2018 ; A. Luca C.R. 2015 ).

Biogas is inherently renewable, on the contrary to fossil fuels, because it is generated from biomass, and this source is practically a reserve of the solar energy via photosynthesis process. Anaerobic digestion (AD) biogas will not only enhance a country's energy basket status but also contribute significantly in conserving natural resources and protecting the environment (Teodorita Al Seadi DR 2008 ).

Biogas is naturally composed of biogenic material. This biogas, which occurs naturally, spreads into the ambient, and its major component, methane, plays a serious detrimental role in global warming (Bochmann and Montgomery 2013 ). Methane has been used as important fossil fuel and converted to generate power, transportation, and heating, over the past decades. Nowadays, the major portion of methane consumption and utilization comes from natural gas resources, but the production of bio-methane from waste recovery approaches has been meaningfully increased. Its production potential has been improved by 4% over 9 years (from 2010 to 2018). At present, about 3.5 Mtoe of biomethane is produced around the world and the potential for biomethane production today is over 700 Mtoe (Edenhofer et al. 2011 ). Of course, this does not mean that methane conversion is feasible from all kinds of natural resources. In other words, infrastructures for biogas development extremely rely on specific equipment and the availability of control and management systems. Therefore, a sustainable industry can be installed and implemented to generate bio-energy from renewable and green natural resources (Bochmann and Montgomery 2013 ).

Developed countries use advanced large-scale plants for utilizing biogas. Biogas is regularly applied to generate heat, power, and electricity. Also, several industrial applications for its utilization in biogas plants as a substitute to natural gas are being progressed. Based on the analyzed data, a continuous increase in biogas production has been observed due to the global policies and programs. Since 0.5% proportion of renewable energies contribution that is about 12.8 GW (IRENA RES. 2015 ) is supposed to be achieved in 2020 for transportation sectors, bio-fuel production has been considered as the main source of this plan in different regions. It is noteworthy that biogas production should not be developed as a food production threat. For this reason, biofuels are mainly generated from cellulosic and lignin wastes (Nicolae Scarlat and Fahl 2018 ; Angelidaki et al. 2018 ).

A wide global market of biogas has been conspicuously promoted for the previous decades in various countries. Moreover, the advanced biogas production technologies have been supported by domestic or international supportive rules, such as research, design, and development (RD&D) financial funds, subsidization, and guaranteed electricity purchase contracts to make a competitive market against conventional energy suppliers (Teodorita Al Seadi DR 2008 ).

According to Fig.  1 , the different utilizations of the biogas technology offer a multi-purpose solution to generate the required energy of the industrial or social sectors. Biogas is mainly consumed for combined heat and power (CHP) plants, hydrogen production units, and advanced energy systems such as fuel cells.

Overview of biogas utilization

Generally, in the European Union (EU) and North America (NA), biogas plants came to be developed more than in other continents for the last 40 years. The main advantages of the units located in the mentioned regions are industrial scale, energy efficiency, and high complexity level. Biogas production was considered by academic centers and governments owing to its potential in response to different global challenges. It should also be pointed out that using biogas technologies allows industries to eliminate greenhouse gases (GHGs) emissions and waste disposal pollutions, while it provides a broad spectrum of energy utilization such as heat, electricity, and transportation purposes, based on its renewable nature.

There are various strategies around the world for producing biogas from agricultural products. In Germany, for example, the production of cheap agricultural products that require low processing (with no outcomes for consumers) provides feedstock for biogas plants. New policies recommend the use of crops and plant residents, life stocks remaining, and landfill use (IRENA RES. 2015 ).

This review focuses on proposing a comprehensive analysis of the recent biogas technologies progress, aiming advances toward wastes conversion to produce electricity, heat, and other forms of energy carriers. It reports the current and future AD conversion technologies, as well as examines accessible details in the literature about feedstock categories, pretreatment approaches, process development, and its yield to increase production efficiency. Furthermore, suggested future biogas application trends and directions for efficient ways of energy generation from wastes are other main outputs of this study. Also, the present review highlights the emerging biogas technologies which are promoted to distribute biomethane and biofuel production, especially the production of hydrogen from biogas is the innovative insight in the mentioned field.

The structure of the present research is as follows: “Biogas Applications” reports extensive data on the up-to-date status of biogas consumption in energy generation, energy storage, and transportation. Biogas development levels around the world, regulations, and historical progress are expressed in “Biogas utilization in various parts of the world” section. Also, the characterization of the feedstocks and additives, pretreatment, process types, and related techniques are described in “Recent progress in biogas production” section. The novel technologies are indicated with their advantages and constraints for each section. Eventually, the conclusion and predictive tendencies for future research are explained in the last section.

Thus, this work represents a comprehensive review of the biogas in terms of a renewable energy source for both production and applications. The procedures for production and applications are up to date. Researchers' work in 2020 is presented where they used the most updated technologies which help other research agencies to continue from this end. The review of the development of the biogas industry and utilization covers 20 years of information. Moreover, a review of the international recent policies and regulations relevant to biogas management is provided. Based on that, a suggested policy based on international guidelines and international conventions is proposed.

Methodology

Published research papers and data on biogas sources, production, and applications are collected from the literature. These sources cover the years from 1997 till 2020 to summarize the current situation and development relevant to biogas. A review of policies and regulations on national and international levels is presented. Regulatory entities in the world that issue guidelines instruction to organize the biogas market are presented. This review showed the increase of world awareness regarding this source of energy by introducing the most updated policies in many countries. Based on all of the above, a proposed framework and policy is presented.

An introduction shows the necessity of biogas as a source of renewable energy is presented. The increasing demand for biogas in the energy section showed to be increased in the coming years. Biogas production process and the sources to get the biogas are presented. The sources vary from agricultural to animal wastes which are the richest biogas sources however, other sources such as wastewater treatment plants, and landfill disposal sites.

Applications of biogas and its contribution to the total national energy sector are presented. These applications range from energy conversion, producing alternative fuels, electricity generation, etc. Traditional methods of biogas production are presented with developments of such methods. New technologies and methods for production and purification of biogas are described.

Biogas applications

Biogas is globally considered as traditional off-grid energy. Biogas can also be utilized to generate electricity. The various applications of biogas are described below.

Electricity generation

Power generation from biomass is currently the most popular and growing market worldwide, due to technological improvements, decreasing reliance on fossil-based energy, and reduction of greenhouse gases (GHG) emissions. Biogas has the potential for electricity generation in power plants by internal combustion engines (ICEs) or gas turbines (GTs) as the two most commonly used power generation methods. Micro gas turbines are also an attractive method due to lower NOx emissions and flexibility to meet various load requirements. Multiple microturbines sizing from 70 kW to over 250 kW can be employed to meet low/medium power load demands. The electricity can provide the required power to the adjacent industries and companies. With the development of electric cars, another state-of-the-art application, especially in developed countries like Germany, is the utility of electricity for e-vehicles of a connected car-sharing association (Scarlat et al. 2018 ).

The major benefit of on-site electricity generation is to prevent transport losses and to increase reliability due to the independence from a centralized grid mostly run by traditional fossil fuels. It also brings extra economical profit by providing the required in-house power demand and selling the extra electricity (Scarlat et al. 2018 ).

Heat generation

Biogas can be directly combusted in boilers for heat generation only. It is feasible to slightly modify natural gas boilers to operate with biogas. As farm biomass is a major biogas production source, the generated heat can be used for heating the digesters, farm buildings like housing units for pigs/sties, greenhouses, as well as aquafarming, cooling/refrigeration of farm products, and drying purposes. The drying process in agricultural businesses, such as drying of digestate, woodchip, grain, herbs, and spices, is a remarkable added value to the farm economy (Herbes et al. 2018 ).

Available heat for external use, representing nearly 30–50% of generated heat, can be sold to a nearby district to be used for district heating/cooling like heating swimming pools. Also, an absorption chiller can be a potential candidate to better use heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling power with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

Combined heat and power (CHP) generation

Concurrent generation of heat and electricity by CHP systems is an operational approach to upgrade the energy conversion efficiency of biogas. When only converting biogas to electricity or heat, just a minor fraction of energy contained in biogas is used. Characteristically, in these types of systems, associated power conversion productivity is somewhere in the region of 30 to 40%, while it is diminished by employing biogas as an alternative for refined and purely natural gas (Saadabadi et al. 2019 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed before. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid and the extra heat can be sold to the local district utilization.

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively. In this case, some thermal energy is used to heat the process and about 2/3 is used for external uses. In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

There are three common ways to produce heat and power from biogas including Gas-Otto engines, Pilot-injection gas motor, and Sterling motors (Teodorita Al Seadi DR 2008 ). In EU, four-stroke engines and ignition oil diesel engines contributed roughly the same in CHPs at somewhere in the vicinity of 50%, each (Dieter Deublein 2008 ). Biogas is also employed in gas turbines, microturbines, and fuel cells (discussed in detail in `` Fuel cells '' section ) for CHP applications (Kaparaju and Rintala 2013 ; Nikpey Somehsaraei et al. 2014 ).

CHP plants offer the advantage of high-temperature exhaust gas from the electricity generation subsystem (ICEs or GTs) as a source of valuable heat for many heating purposes already discussed. Although the electricity generation efficiency of simple plants is only 20–45% (Muche et al. 2016 ), a larger portion of energy (around 60% of the utilized energy (Damyanova and Beschkov 2020 )) is converted to heat that is reused by heat recovery systems; making it more attractive when there is a high heat demand. This considerably enhances the system efficiency and improves the payback period of plants, making the distributed generation the most common biogas application. The extra electricity could be supplied for the national grid, and the extra heat can be sold to the local district utilization. Also, an absorption chiller can be a potential candidate to better use the extra heat through CHP, in addition to cooling power (tri-generation). It can convert heat into cooling with high efficiencies of up to 70% (Rümmeli et al. 2010 ).

A CHP cycle has sufficient productivity that has an efficiency up to 90%, while it can produce 35% and 65% of the generated electricity and heat, respectively (Shipley et al. 2009 ). In some proposed models for biogas-based power plants, the use of generated heat is ignored and the focus is only on generating electricity. Without any doubt, this approach has no economic justification and must use all its thermal potential.

Upgrading to biomethane

If biogas is upgraded and purified to biomethane, it can be fed into natural gas grid to be used for heating purposes, power generation, or to provide fuel for compressed natural gas (CNG) and even natural gas vehicles (NGV). A significant benefit of biomethane is that it can be stored to meet peak demands (Herbes et al. 2018 ). The two major steps to produce biomethane are upgrading methane content up to 95–97% followed by a cleaning process to eliminate water vapor, hydrogen sulfide, oxygen, ammonia, siloxanes, carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen (Ryckebosch et al. 2011 ). Biogas upgrading is performed by physical and chemical technologies such as adsorption, absorption, cryogenic and membrane separations, and gas separation membranes as well as biological technologies (in situ and ex situ (Kapoor et al. 2019 )). Although biological methods are emerging, suggesting an enormous technological potential, they are not widely used in industry since they are generally much slower, have low rates of reaction/synthesis, and require long startup period that made them less economically feasible, while physicochemical methods are common due to technological advancements and implementations (Scarlat et al. 2018 ).

Upgrading biogas to biomethane or renewable natural gas (RNG) is on a hot trend in developed countries especially in North America among oil and gas companies for decreasing GHG emissions and using the carbon credit. There are also other environmental and economical benefits in smaller scale to farmers, municipalities, and counties for waste management and profitable contracts with gas utility companies. Biomethane market for transportation purposes equaled to 160 cubic meter per year in 2015 Eurostat.European Statistics ( 2019 ).

Transportation fuel

Biogas converted to biomethane (through upgrading and cleaning) can be readily used in natural gas-powered vehicles as another option for fossil natural gas. Using biomethane as transportation fuel results in remarkably low GHG emissions that make it a suitable source of renewable fuel. Biomethane turns out to be a great fit to replace fossil-based fuels in terms of environmental and economic considerations (Scarlat et al. 2018 ). However, the overall efficiency is extremely improved when biomethane is utilized in advanced hybrid or fuel cell vehicles (FCVs) in comparison to current biodiesel or ethanol-powered ICE vehicles (Faaij 2006 ).

Generally, biogas can be improved to transportation fuels (bio-CNG) that can be stored for future use, in the form of liquefied biogas (LBG), syngas/hydrogen, methanol for gasoline production, ethanol, and higher alcohols (Yang et al. 2014 ). Compression and liquefaction are common physical methods to convert biogas into bio-CNG and LBG, while the dominant chemical approach to obtain syngas is catalytic reforming. If Fischer–Tropsch synthesis (FTS) or fermentation is employed, syngas may be converted into a variety of alcohols like methanol, ethanol, and butanol (Yang et al. 2014 ). This fuel alternative has already been applied within the European Union and the USA. As an example, many vehicles run on biogas in the urban public transport (in Sweden and Germany) either as 100% methane (CBG100) or mixed with natural gas (e.g., CBG10 and CBG50) (Damyanova and Beschkov 2020 ; Yang et al. 2014 ).

Hydrogen production

Hydrogen displays many promising potentials for renewable energy and the chemical industry due to its high potential for energy production. Hydrogen offers the biggest share of energy per unit mass (121.000 kJ/kg). The hydrogen council suggests about 18% contribution of total final energy utilization by 2050. Hydrogen is best employed in fuel cells as an emerging energy application to produce electricity, heat, and possibly water. Furthermore, there are many applications in chemical industries for hydrogen, including food treatment, hydrogenation methods, production of ammonia and methanol, Fischer–Tropsch synthesis, pharmaceutical manufacturing, among others (Armor 1999 ).

Technically, hydrogen (H 2 ) can be released from the BSR (biogas steam reforming) process. This process has temperature flexibility in the range of 600 to 1000° C, which also includes catalytic techniques. (Holladay and J., King, D.L., Wang, Y. 2009 ; Alves and C.B., Niklevicz, R.R., Frigo, E.P., Frigo, M.S., Coimbra-Araújo, C.H. 2013 ). The main difference between BSR and SMR (steam methane reforming) is the presence of carbon dioxide in the feedstock. This factor increases the sensitivity to carbon production in the process. The produced carbon can deposit in the active phase of the catalyst to create deactivation.(Gioele Di Marcoberardino et al. 2018 ). Furthermore, fed gas can affect the hydrogen separation unit. In this case, PSA (pressure swing absorption) and VPSA (vacuum PSA) are the most common methods of purifying the system for hydrogen-rich reformate or syngas (Ugarte and P., Lasobras, J., Soler, J., Menéndez, M., Herguido, J. 2017 ; Ahn and Y.W., Lee, D.G., Kim, K.H., Oh, M., Lee, C.H. 2012 ). The potential of hydrogen production from all landfill sources in the USA is probably between the total potential of 16 million tons of methane from raw biogas and 4.2 million tons of hydrogen (Milbrandt GSaA. 2010 ). Biogas production systems have a capability for production from 100 Nm 3 /h for small-scaled agricultural to a few 1000 Nm 3 /h for large-scaled municipal waste landfills; furthermore, occasionally, not all biogas may be converted to the desired hydrogen and further biogas valorization can coexist in the system. Therefore, the capacity considered for BSR should be in the range of 50 and 1000 Nm 3 H 2 /h (Doan Pham Minh et al. 2018 ).

Hydrogen is clean transportation fuel, while as discussed earlier syngas may be used as a feedstock for alcohol production. With new advancements in reforming procedures, biogas can now be directly improved to syngas by dry or steam reforming without the necessity to remove carbon dioxide (Yang et al. 2014 ).

Fuel cells are probably the cutting-edge application of biogas. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes. Also, fuel cells can be utilized in large-scale power plants, power distribution generators, buildings, small-scaled and portable power supply apparatus for microelectronic equipment, and secondary power components in vehicles (Alves et al. 2013 ).

Fuel cells can use the chemical energy of hydrogen and oxygen without any intermediaries to deliver electricity and heat (A. Trendewicz R.B. 2013 ). In this case, there are only a small number of fuel cell-based power plants (most of which are pilots) that generate electrical power from biogas. (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ). Fuel cells exhibit high electrical efficiency of 60% (in power generation only mode) and thermal efficiency of up to 40% (in CHP applications) (Pöschl et al. 2010 ), but can easily be integrated with other power generation systems like gas turbines or microgas turbines to further improve their performance. Also, biogas fueled integrated solid oxide fuel cell (SOFC)-CHP offers a modern energy system that can address both heat and power generation demands for decentralized grids with drastically higher electrical efficiencies (Wongchanapai et al. 2013 ; Safari et al. 2020 ; Safari et al. 2020 ). Such high efficiency compared to other common combustion technologies is a result of not being limited by thermodynamic Carnot efficiency. SOFCs are more tolerant to fuel impurity and flexibility; hence offering better integration with biogas systems (Wasajja et al. 2020 ). This highlights their key role in enhancing the highly efficient generation of electricity from biogas, which demonstrates significant environmental and economic merits. However, for the use of biogas as fuel in fuel cells, a cleaning procedure seems essential to eliminate biogas impurities such as H 2 S, siloxanes, and other volatile organic compounds (VOCs) that have harmful impacts on fuel cell operation.

Furthermore, hydrogen produced from biogas can directly feed fuel cells. The reforming practice can be succeeded either internally employing fuel cells or externally by a catalytic pre-reformer. The three chief techniques for methane conversion are steam reforming, partial oxidation (POX), and dry reforming. Besides, mixed approaches like autothermal reforming (ATR) (mixed steam reforming and methane POX) are applicable. In a pilot plant constructed in Barcelona, Spain named “Biocell project”, biogas from a WWTP was employed in two categories of a fuel cell. The first was proton-exchange membrane fuel cell (PEMFC) that entailed exterior gas cleaning and reforming unit. Biogas has also been added into a SOFC after the cleaning process. This pilot plant is intended for 2.8 kWe. Electrical and thermal effectiveness for the SOFC pilot plant was 24.2 and 39.4%, respectively, which are considerably more than those for the PEMFC pilot plant (S. Ali Saadabadi ATT, Liyuan Fan, Ralph E.F. Lindeboom, Henri Spanjers, P.V. Aravind. 2019 ; Arespacochaga and CV, C. Peregrina, C. Mesa, L. Bouchy, J. Cortina 2015 ).

Biogas development in various parts of the world

The worldwide biogas industry has increased more than 90% between 2010 and 2018, while further growth is still expected. The International Renewable Energy Agency (IRENA) reported that the overall potential for the biogas industry in 2018 could provide 88 Tera Watt per hour (TWh) of biogas each year. Installed electricity generated from biogas reached 18.1 GW in 2018, against 8.2 GW in 2009 (Agency 2019 ). Over 20% of electricity produced in the entire biopowered production is generated from biogas, with a share of 4% of heat generation worldwide.

Among different countries throughout the world, Europe plays a pivotal role in biogas electricity generation. In 2017, Europe contributed to over 70% of the world biogas generation representing 64 TWh, followed by North America accounting for 15 TWh (in which the US participation was over 85% in entire North America). Asia produced 4 TWh followed by Eurasia with 1.7 TWh, South America with 953 GWh, and Africa biogas production accounted for 89 GWh (Scarlat et al. 2018 ; Agency 2019 ).

In terms of thermal energy production, biogas is turning to be a more significant source of heat, in which around 4% of the worldwide bioheat in 2015 was generated by biogas. In the EU, biogas produced 127 TJ of heat, which corresponds to almost 50% of entire biogas use in the EU (Scarlat et al. 2018 ). In Demark, the electrical power cost produced by biogas is 0.056 EUR/kWh in a CHP unit or injected into the grid (Seadi and J. 2019 ).

Biogas utilization differs significantly in various countries around the world. This varies from several small-scaled biogas plants providing heat in China and India to large-scale plants generating electricity as well as upgrading into biomethane as fuel, mostly in Sweden (McCabe et al. 2018 ).

Nanyang in China is one of the top biogas cities in the globe due to its location in the center of a rank soil zone. Since corn is abundant, other types of cereals can be employed for producing biogas (Dieter Deublein 2008 ; Lei Zheng 2020 ).

In China, biogas plants are classified as medium scale with the volume of digester equaled to 300 cubic meters and large scale with a capacity of 500 cubic meters, with daily biogas production in the range of 150 to 500 cubic meters per day (Song and C., Yang, G., Feng, Y., Ren, G., Han, X. 2014 ). The governmental support for domestic digester has been stopped since 2015. More backing would make large-scale biogas plants and bionatural gas schemes (Ndrc 2015 ). Chinese biogas industry reported that 41.93 million biogas digesters were built (containing centralized biogas source for houses), for almost 200 million recipients, in which 14.5 billion m 3 biogas is produced per year (China Statistics Press 2018 ).

In India, around 2.5 Mio biogas plants are operating, with a medium digester volume of 3–10 m 3 . Based on the circumstances, the plants produce 3–10 m 3 biogas daily, adequate to deliver a regular farmer family with energy for food preparation, heating, and lighting. Also, more than 1.2 million households employ small-scaled AD and 100,000 family-sized AD units have been installed between 2016 and 2017. Over 35,000 biogas plants have been constructed with governmental investments (MNER 2016 ).

Japan is a pioneer in the use of biogas, with increasingly using AD to produce biogas and manage municipal waste in the last decade. The development is such that only Japan uses thermophilic AD (Abbasi et al. 2012 ).

Up to 2008, over 70 plants have been constructed in Russia, over 30 in Kazakhstan, and a single plant in Ukraine. In Ukraine, bioreactors with 162,000 m 3 volume have been previously installed in sewage treatment units (M. R. Atelge DK, Gopalakrishnan Kumar, Cigdem Eskicioglu, Dinh Duc Nguyen, Soon Woong Chang, A. E. Atabani, Alaa H. Al-Muhtaseb, S. Unalan. 2018 ).

It should be noted that some nations employed biogas as a practical tool for waste management, mostly to decrease the detrimental effects of municipal waste or wastewater. Likewise, a broad range of various technologies are employed from simple digesters to expanded granular sludge blanket (EGSB) digesters (McCabe et al. 2018 ).

Biogas technology and industry

The biogas industry varies significantly in the various parts of the world. Different countries have been advanced in several types of biogas systems mainly premised on different environment as well as energy demand and supply chain. The UK, Australia, and South Korea employed landfill sites to achieve a considerable portion of their produced biogas, while in Switzerland and Sweden, using decomposition of sewage to generate biogas is prevailing. Denmark utilizes mainly manure due to its abundance and availability. In Germany, UK and Sweden most of the biogas generation arises from food waste (McCabe et al. 2018 ; Union 2015 ; Association WB.Global Potential of Biogas 2019 ).

In farm-based biogas production, China and Germany are recognized as world leaders since about 24,000 small-scale plants exist in China and nearly 8000 agriculture plants in Germany. Similarly, France, Holland, Austria, and Italy employed considerable farm-based biogas plants (Union 2015 ). Moreover, the scale of plants ranges from small household units to larger plants using feedstocks such as household waste, industrial waste, and manure to generate both heat and electricity (Union 2015 ). Studies revealed that in Asia and Africa, most of the installed biogas plants were family-sized (Kemausuor et al. 2018 ). China and India have dominated the microscale biogas industry in the world. At this time, Thailand takes benefits from more than 1700 biogas plants and more than 150 plants of industrial waste. The Thai government has attempted to expand industrial wastewater technology that has the potential of 7800 TJ/y biogas production (Tonrangklang et al. 2017 ). The ministry of energy of Nepal (Government of Nepal Ministry of Energy WRaI.Biogas. 2020 , 2020 ) has reported that most of the villages about 2800, out of the total 3915 in all 75 districts of Nepal, have small-scale or household biogas production systems. Primarily two categories of plants have been constructed in Nepal. These are the floating-drum plant based on the Indian style and fixed-dome plants with a flat floor, cylindrical digester, and a dome prepared by concrete. Among 50 million microscale digesters operating in various parts of the world, 42 million are installed in China and another 4.9 million in India. The statistics from the World Biogas Association (WBA) have shown that there are only 700,000 biogas plants installed in Asia, Africa, and South America (Association WB.Global Potential of Biogas.2019. 2019 ).

In terms of large-scale plants, about 7000 large-scale biogas systems are operating in China. Europe, in 2017, had a share of 17,783 plants, while Germany was dominating the European biogas industry with 10,971 plants followed by Italy with 1665 plants, France with 742, Switzerland, and the UK with 632 and 613 plants, respectively (Association 2018 ). The World Biogas Association data mentioned about 2200 anaerobic digesters large-scale plants in the USA, able to generate 977 MW (Association WB. International Market Report 2018 ).

Another application of biogas relies on upgrading to biomethane. Although being comparatively a novel technique, it achieves widespread utilization worldwide. Some biogas upgrade plants are employed to produce vehicle fuel, while others deliver it into the local or national grids Association WB.Global Potential of Biogas ( 2019 ).

Africa is a region with abundant and diverse resources for biogas production, though it has accomplished small progress in the sector. Although the continent has made considerable achievements in small-scale biogas plants, profitable biodigesters still require further development (Kemausuor et al. 2018 ). In Africa, harvest and livestock farmers, small to medium and large food treating businesses, wastewater, sanitation, and municipalities running institutes, as well as municipal waste management organizations, are considered as potential candidate employers of large-scale biogas technology. Moreover, schools, institutions of higher education, hospitals, and commercial buildings have the potential to benefit from biogas technologies and facilities (Parawira 2009 ). Excluding South Africa, insufficient scientific literature has reported technology development of the commercial biogas system in Africa. In the Southern parts of Africa, developed technologies are the lagoon, plug low, and up-flow sludge blanket (UASB) (Mutungwazi et al. 2018 ).

Biogas production and utilization

In this section, biogas production from wastewater treatment plants (WWTP), biowaste digestion, agricultural products (largely manure and energy crops), waste stream from different industries, and landfill gas are considered. In Europe, Germany has dominated the industry by far in which its annual production is accounted for 120 TWh followed by the UK with 25 TWh and 9 TWh in France. Denmark and the Netherland's production capacity is around 4 TWh and the remaining countries share is less than 3 TWh (Bioenergy 2019a ).

In Germany, the total gross electricity and heat production from biogas is about 33 TWh/year and 18.8 TWh/year, respectively. Based on statistics revealed by the Federal Ministry for Economic Affairs and Energy of Germany, a considerable amount of the biogas was utilized for electricity production (58%) and heat production (33%), and approximately only 1% was used as a vehicle fuel (Bioenergy 2019a ).

In 2018, about 32% of entire renewable heat used in the UK was produced by anaerobic digestion technology, of which 9 TWh/year was produced by biomethane, 2 TWh/year by biogas and CHP accounted for 918 GWh/year, while 2681 GWh of electricity was generated by the sector (Association ADaB. ADBA annual report 2019. 2018 ).

In France, total electricity production from biogas was about 1.8 TWh/year at the end of 2017, simultaneously total heat generated accounted for 1.7 TWh/year, which demonstrates nearly equal portion for both heat and electricity. Regarding heat production, the agriculture sector accounts for an indispensable portion, while in electricity production, the landfill has a pivotal role with 953 GWh/year followed by agriculture with 765 GWh/year (Bioenergy 2019a ).

In Denmark, the biogas sector provides 5% of the entire energy consumption of which biogas plants contribution is 60% and the rest relies on wastewater treatment plants and landfill sites. The Danish Energy Agency states that due to several support schemes such as upgrading biogas to Natural gas, biogas employment for process purposes in the industrial sectors, etc. results in promoting biogas utilization through the country (Agency and Biogas in Denmark 2019 ). Total Danish biogas production at the end of 2018 was reported to be about 1763 GWh/year in which the agriculture sector (both centralized and farm plant types) showed the largest contribution with 1367 GWh/year. 66% of produced biogas energy (which corresponds to 1150 GWh) is used to provide electricity, followed by upgrading plants with 17% portion and heat generation with 16% Bioenergy IEAI.Denmark Country Report -2019 ( 2019 ). In the Netherlands, in 2017, two co-digestion and municipal waste plants had the largest share in production, and the final use of biogas (3034 TJ heat was produced solely with municipal waste, while co-digestion had a pivotal role in electricity production representing 1825 TJ) Bioenergy IEAI.The Netherlands Country Report -2019 ( 2019 ).

In Sweden, 48% of biogas production corresponds to co-digestion plants followed by WWTPs (37%), the remaining being produced by the other plant types such as landfills, industrial facilities, and farm-based. In terms of utilization, the upgrading or transport sector represented a considerable portion (65%) followed by heat (19%), while electricity production share was almost 3% (Bioenergy 2019b ).

In Asia, China plays a significant role with 98.4% of biogas production between non-OECD countries. Primary infrastructures such as advanced industry and socioeconomic conditions have a profound impact on biogas generation and utilization growth. Small-scale and household biogas systems have been widely developed by countries like India and Bangladesh. Various researches prove that there are plenty of resources for producing biogas in developing countries when barriers such as socioeconomic, climate conditions, and appropriate technology have been addressed accurately. Several biogas plants in the range of medium to large scale have been launched in China and India (Mittal et al. 2019 ; Jiang et al. 2011 ; Gu et al. 2016 ).

In the USA, over 2200 biogas plants are operated, among which 250 AD on farms, 1269 wastewater recovery plants employing an AD, and 66 independent plants that use food waste as feed and 652 landfill gas projects. The America Biogas Council has revealed that there is still an enormous potential for developing the biogas industry in the USA where it is possible to achieve 103 trillion kWh/year (Council 2019a ). California ranks first in biogas production potential among all the 50 states in the USA (Council 2019b ), followed by Texas (Council 2019c ).

The power generation from biogas is estimated to be 9731 million kWh and 6574 million kWh electricity for California and Texas states, respectively. In California, the manure system has the highest potential with about 900 biogas plants, while currently 38 manure plants are operated with 156 wastewater facilities in Texas. (Council 2019b , c ).

In Canada, bioenergy currently provides approximately 26.7% of Canadian entire renewable energy market, the highest share is from burning solid biomass (23.1%), followed by the liquid biofuels (2.4%), and biogas (1.2%) (Canada 2019 ). In Canada, total installed plants for biogas production are estimated to be around 150. Most production takes place in landfills with 45 plants (share of 30%), followed by the agriculture sector with 37 plants (share of 24.7%) and WWTPs with 31 plants (20.7% production portion) (Association WB.Canada Market Report. 2019 . 2019 ).

Based on the Canadian Biogas Association data, at the end of 2018, about 195 MW of electricity and 400,000 GJ of Renewable Natural Gas (RNG) were generated (Biogas and Potential. 2019 . 2019 ). Biogas is utilized for providing heat and electricity, delivering to a nearby user using a pipeline, converting into electricity and connecting to the grid, or refining to RNG based on circumstances such as the landfill site location, and the energy demand of plants. In this regard, approximately 50% of the produced biogas is converted into power, with the rest going to combined heat and power (CHP) application (about 25%), heat (only 10%) and RNG (about 4%), and electricity and RNG (about 1%) (Association WB.Canada Market Report.2019. 2019 ).

In Australia, at the end of 2017, generated electricity from biogas industry was approximately 1200 GWh, which is equivalent to almost 0.5% of the entire electricity generation of the country, while biogas potential electricity generation was estimated as 103 TWh, equal to almost 9% of Australia’s entire energy consumption (Australia and Biogas opportunities for Australia. 2019 ).

The main use of biogas in Australia is for electricity with the greatest share for landfills (53.7%), followed by biowaste and WWTPs (40% and 33.3%, respectively). Heat is used in the industrial sector with a share of 30% and afterward the WWTPs with a share of 26.2%. In CHP applications, agriculture plants have the largest portion (50%), followed by the biowaste and the WWTPs (equal share of about 20% each). Between 40–50% of the excess biogas is flared at agriculture, industries, and landfills. Twenty percent of WWTPs and biowaste are no biogas upgrading plants in Australian’s biogas industry (Bioenergy IEAI.Australia Country Report. 2019 . 2019 ).

In Africa, South Africa has the largest share of installed biogas plants with about 700 plants, while only 300 plants might have been in operation as of 2007, while it can generate 148 GWh electricity from estimated biogas potential by appropriate investment and implementation schemes (Kemausuor et al. 2018 ).

Various industrial trends in the biogas production have been introduced to improve quantitative and qualitative properties of the biogas. Yet, the accomplishments of AD intended for advanced investments will increase from the low charge of feedstock accessibility and the broad range of practical set ups of the biogas (i.e., heating, electricity power, and fuel form). The remained parts of slurry from biogas production procedure have the potentials to be improved to be used as fertilizer to enhance the sustainability. Produced biogas could be employed to generate power for integrated or isolated systems in the rural and urban regions and are deemed to be economical favorable. The employed processes of AD, modern trends accompanied by included advantages and disadvantages are also demonstrated more details and progress on the way to producing biogas in a sustainable approach. Obtained results from previous researches indicated that the present amount of biogas production confirms that regarded approaches would have main influence on the energy utilization in upcoming times. The impression contains diminished release of pollutants to the atmosphere guarantees that the global warming prevention. Nevertheless, the current trend of the biogas production varies in diverse countries, either in production or the sources (landfill, AD, sewage sludge, or thermochemical methods). The involvement of biogas to the domestic natural gas utilization varies differently, around 4% on standard values; however, it raised 12% in Germany. The major nations in the biogas production in the European Union are France, Italy, Germany, Czech, and UK. Germany stands as the European frontrunner with a biogas production of 329 PJ and a contribution of 50% of total in the EU. It’s reasonable to surmise that, based on the provided data from various researches, it has been declared that given the growing need and available technology, European Union countries, and especially Germany and Sweden, will be pioneers in the development, operation, and production of biogas in the world. Table ​ Table1 1 indicates the biogas plants, upgrading units, and their upgrading capacities in certain EU countries (Lampinen 2015 ; Backman and Rogulska 2016 ; Esmaeilion et al. 2021 ).

Biogas plant in EU selected countries and their specifications

Recent progress in biogas production

Producing biogas is a key option in the energy sector of various countries. There is a wide variety of raw materials for utilization in biogas plants. In this case, obtaining a stable state in plants is a crucial concern that influences the prices and additives. Another important issue in the biogas plants is that their products should be attractive in terms of value and efficiency (Chen et al. 2012 ). Recent progress in the field of biogas production can be divided into three categories: feedstock and additives, pretreatments, and processes.

Feedstock and additives

The organic matters are the main feedstocks in the biogas plant, which can fall into different categories. Evaluating the potential of biogas production based on organic matters from rural regions has been investigated. The highly fermentative wastes can decrease the quantity of feedstock in biogas plants (Pawlita-Posmyk and Wzorek 2018 ).

Microalgae with satisfactory features is a potential option for feedstock in biogas systems. In comparison with other biomass resources, microalgae has better efficiency, more convenient production, and higher content of lipid and polysaccharide that make it a flexible choice in biogas plants (Wu et al. 2019 ). Kaparaju et al. (Kaparaju et al. 2009 ) explored the production of biogas from sugars released from wheat straw with the aid of hydrothermal pretreatment based on the biorefinery procedure. In this case, the pretreatment process increased the gas yield by 10%.

For achieving sustainable progress, the global trend of energy production is moving to the waste-to-energy (WTE) method which has multilateral benefits. Currently, biomass resources are being employed to generate energy. All around the world, biomass satisfies around 50 exajoule of the entire energy demand annually (Steubing et al. 2010 ; Ferreira et al. 2017 ; Ahmadi et al. 2020 ).

A broad spectrum of waste types can be consumed as a feedstock in biogas units by anaerobic digestion (AD) technology. Huge amounts of lignocellulosic waste could be collected from agricultural and municipal resources. The most common types of waste and residuals that can be used in the biogas sector are animal manures and dungs, muck and slurry, domestic/municipal wastewater (sewage), mud (sludge), urban garbage or municipal solid waste (MSW), and food substances loss. Table ​ Table2 2 indicates the power generation and associated yields of biogas production by accessible resources (Waste-to-energy 2015 ; Stucki et al. 2011 ).

Comparison between different resources in terms of biogas yield and electricity generation

Considered efficiency for electricity production is 35% in CHP

To enhance the yield of biogas production, utilization of additives is an acceptable method. Specifications of these components can be varied based on their biological or chemical properties under various conditions. With the aid of these materials, desirable conditions for bacteria could be provided. However, biocenosis features are vital for achieving the ideal concentration (Demirel and Scherer 2011 ).

Using salts with Mg and Ca improves methane production efficiency with low slurry foaming (Sreekrishnan et al. 2004 ). For stabilizing pH fluctuations and reducing the contents of NH 3 and H 2 S, several types of additives have been studied (Kuttner et al. 2015 ). Furthermore, using zeolite compounds has the potential to intensify the quantity of biogas production by 15%, also the addition of CaCO 3 can improve this yield by 8%. Adding biological additives increased the production rate of biomethane and biogas by optimizing AD (Vervaeren et al. 2010 ). Using biological additives is a common way of increasing biogas production yield. Yi Zheng et al. (Zheng et al. 2014 ) stated that by adding enzymes to lignocellulosic biomass, biogas production was enhanced by 34%. Vervaeren et al. (Vervaeren et al. 2010 ) reported that by adding homo and hetero-fermentative bacteria to maize components, production yield increased by 22.5%. With the addition of fungi compounds (e.g., ceriporiopsis subvermispora ATCC 96,608) to the yard trimmings, methane production increased by 154% (Zhao 2013 ). The alternative options for biological additives are chemical compounds. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc., can improve the associated biogas production yield. Chandra et al. reported the effects of using NaOH as an additive to the wheat straw. Obtained results presented that yield of methane could be improved by up to 112% (Chandra et al. 2012 ). Badshah et al. investigated the diluted H 2 SO 4 properties, added to the sugarcane bagasse, which could increase the production rate by up to 166% in comparison with pre-additive treatments (Badshah et al. 2012 ).

The impact of activator addition on the biogas quality slurry is investigated in Indonesia (Ginting 2020 ), the study started by adding new bioactivator prepared from agricultural wastes such as bananas, papayas, and pineapples waste with an additional of chicken intestines where the bacteria in the chicken intestine are effective at work. The addition of the activator resulted optimally in the work where stable gas production was achieved. The slurry at the end of the production process was a liquid fertilizer ready to use. The study showed the best concentration of the activator in the production process of both the slurry and the biogas.

Pretreatment

Predominantly, there are two wide-ranging classifications for biogas production upgradation, ex situ, and in situ techniques, while most of the methods focus on ex situ approaches. Some of the conventional ex situ treatments are adsorption, catalytic processes (e.g., biological or chemical), membrane gas permeation, desulfurization, scrubbing, and absorption. Sarker et al. ( 2018 ) overviewed the in situ biogas production upgrades.

With the help of the in situ method, the associated cost concerning cleaning techniques could be reduced and the quality of produced biogas improved in the same vein. Nevertheless, the in situ method is limited to the empirical state and prototype models. Figure  2 summarizes various types of biogas upgrading methods (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 ).

Biogas improvement by ex situ and in situ techniques (Sarker et al. 2018 ; Bassani et al. 2016 ; Rachbauer et al. 2016 ; Lemmer et al. 2015 )

The pretreatment productivity influences the associated bioprocess efficiency of lignocellulose. Pretreatment techniques are intended to make AD faster, enhancing the yield of the biogas, and producing a broad range of usable substrates.

Figure  3 indicates the mentioned effects of pretreatment processes. By considering efficiency, economy, and application as objective functions, optimization of pretreatment processes is a necessitated aim. Pretreatment should be operative in eradicating the structural obstacles of associated polymers with lignocellulose (it should be noted that the cellulose and hemicellulose constituents are in this classification), through exposing these substances to microbial decay efforts, which increases the biomass degradation and consequently enhances the biogas yield (Spyridon et al. 2016 ).

Pretreatment effects on the value of anaerobic digestion ( b ) and yield of CH 4 ( c ) (Achinas et al. 2017 )

There are crucial requirements in common designs of biogas plants for increasing the rate of gas production. Recently, innovative designs of biogas plants have been introduced (e.g., Konark, Deenbandhu, and Utkal Models) (Sreekrishnan et al. 2004 ; Kalia and Singh 2004 ; Abouelenien et al. 2010 ; Prasad et al. 2017 ) in which the design parameters changed to increase productivity and effectiveness in cost factors. In these concepts, by implementing optimum measurements in regarded shapes (similar to the spiral shape), the index of gas storage volume was enhanced by 33–50%, while the related costs were reduced by 10–15%.

The hydrolysis of a high proportion of non-biodegradable compositions from MSW (which is intractable by AD) can be performed by microwaving or autoclaving (Pecorini et al. 2016 ). In another study, by applying pressure to biowaste in the pretreatment procedure, biogas yields were improved significantly (Micolucci et al. 2016 ). The most desirable condition in the pretreatment of biomass is to provide an ideal environment for breaking down the feedstock substances to the sugars that are fermentable, by increasing the accessibility for microorganisms. This process leads to eradicating the lignin endurance and declining the cellulose’s crystalline formation (Micolucci et al. 2016 ). Table ​ Table3 3 presents the merits and demerits of various pretreatment technologies.

Merits and demerits of pretreatment techniques

By implementing fast pyrolysis pretreatment, biogas production has been increased (Wang et al. 2016a ). This innovative approach in thermochemical pretreatment with the aid of a lower temperature fast pyrolysis (LTFP) to enhance the performance of the AD process has been introduced, in which corn stover was used as a primary substance.

During the pretreatment procedure, a fluidized bed pyrolysis reactor applied high-temperature gas flow at 200 °C. To improve the efficiency, different strategies in the pretreatment section were performed (e.g., characteristics analysis, assessing crystal concentration of the corn stover components). Comparing the results obtained between pre- and post-treatment, the production efficiency of methane increased by about 18%. In thermochemical pretreatment, chemical bonds in substances would be broken by implementing the thermo-physical process. Biogas production and hydrolysis of celluloses are affected by the degradation of hemicellulose and lignin (Cara et al. 2006 ). Thus steam explosion falls into this category (Bauer et al. 2014 ). In this method, biomass is subjected to high-temperature steam at 240 °C, so that after a long time, morphological and chemical transformations in biomass can occur (Biswas et al. 2011 ). Another pretreatment method to upgrade the biomass is the Torrefaction process which is applied to produce a higher amount of hydrophobic fuel with a fixed range of carbon content. The operational temperature for this process is from 200 to 300 °C in a stable environment (Mafu et al. 2016 ; Sarkar et al. 2014 ). Fast pyrolysis is an additional pretreatment that was highly used in the field of biofuel production. In this case, by reducing the temperature (around 200 °C) lignin and hemicellulose could be wrecked. Nonetheless, there is no study demonstrating an increase in biogas production (Bridgwater 2012 ; Y-m et al. 2009 ). Rodriguez et al. (Rodriguez et al. 2017 ) investigated different pretreatments for grass in biogas production sectors. The obtained results revealed that all pretreatments could increase biogas production by around 50% even though all of them suffer from high energy consumption.

The ultrasonic pretreatment process is an innovative and practical technique in the pretreatment section. This process increases the efficiency of sludge dewatering, stability of the digestion, solids solubility, and rate of biogas production. The outcome of this method is a digestate containing a low share of residual organic materials. The ultrasonication modifies the biological, chemical, and physical specifications of the sludge. Some of these variations are pathogen reduction, settling velocity improvement, and protein concentrations increase (Cella et al. 2016 ; Liu et al. 2015 ; Feng et al. 2009 ).

By applying this pretreatment, the rate of CH 4 production increased by 34% (up to 80% of energy consumption in the pretreatment unit is reachable by produced methane) (Mirmasoumi et al. 2018 ). The Lysis centrifuge consists of a method focused on centrifuge which initiates partial destruction in sludge cells. This strategy can improve biogas production by 15–26% with thickened sludge resources. This practice is suitable in pretreatment processes (for dewatering) and does not impose any extra load on the system for extra operations (Dohányos et al. 1997 ).

Biological pretreatment is an alternative for thermal and chemical pretreatment that is composed of different stages like enzymatic hydrolysis, using fungi additives and thermal phased AD (TPAD). Among named processes, TPAD has attracted attention. The benefits of this biological pretreatment are lower energy consumption and higher biogas production in comparison with other methods (Zhen et al. 2017 ; Bolzonella et al. 2012 ).

By comparing the results between thermal and autohydrolysis pretreatments, the production of biogas in the biological procedure is considerably lower than in the thermal pretreatment (26% and 45%, correspondingly). The dominant conditions of autohydrolysis pretreatment were reported to be at 55 °C for 12–24 h compared with 170 °C for half an hour for thermal pretreatment (Carvajal et al. 2013 ). In this field, the highest yield achieved in biogas production was investigated by Bolzonella et al. (Bolzonella et al. 2012 ) by applying the pretreatment at 70 °C for 2 days, with associated yield increasing by up to 145%. It is noteworthy to mention that many studies have investigated the combined pretreatment for increasing the biogas production yield (Liu et al. 2018 ; Bao et al. 2015 ; Chan et al. 2016 ; Abelleira-Pereira et al. 2015 ; Wang et al. 2014 ; Bentayeb et al. 2013 ) however, this is out of the scope of this study.

The biogas production can be categorized into two main fermentation process which are dry and wet processes. For the digestion by wet process, the overall solids concentration in the fermenter is lower than 10%. To treat solid substrates, using liquid manure for achieving pumpable slurry is necessary. On the other hand, in the dry digestion, the overall concentration of solids in the fermenter is ranging from 15 to 35%. The stability in the wet digestion processes is higher than in dry methods. In the agricultural section, wet digestion practices are more widespread (Weiland 2010 ).

The biogas production procedure includes four important phases which are hydrolysis, acidogenesis, acetogenesis, and methanogenesis as can be seen in Fig.  4 .

Diagram of the biogas production procedures by AD (Mao et al. 2015 ; Visvanathan and Abeynayaka 2012 )

For developing methane fermentation, diverse associations of bacteria are needed, which are aceticlastic and hydrogenotrophic methanogens, syntrophic acetogens, fermentative bacteria, and homoacetogens. The balanced contribution between them increases the efficiency of biogas production and the AD process (Chen et al. 2016 ). There is a specific type of AD that involves anaerobic membrane bioreactors (AnMBRs), which increases the quantity of biogas production by membrane specifications. By considering the techno-economical parameters of AnMBRs, the efficiency of biogas production has the potential to be increased dramatically (Chen et al. 2016 ). Figure  5 shows the different types of AnMBR technologies.

The biogas production processes by different types of AnMBR technologies

The methanogenic organisms have a negative instinct for sluggish growing, and also the complexities of microbial in the systems have caused difficulty in the functioning of biogas fermenters. An innovative concept of integrating the anaerobic bioprocess with membrane breakdown practice through a membrane bioreactor (MBR) allowed augmenting the biomass concentration through a bioreactor. With an anaerobic membrane bioreactor (AnMBR), high hydraulic load, and adequate mixing brought sustainability for high cell concentrations (Wang et al. 2011 ).

The AnMBRs have a special feature for providing satisfactory retention of active microorganisms. This specification leads to optimal productivity and favorable resistance against toxic substances. Furthermore, high concentrations in the final product and easy separation of biomass and products (by micro-/ultra-filtration) have been added to its benefits (Ylitervo et al. 2013 ). Obtained results revealed that methane yield in biogas production was up to 0.36 l CH 4 /g chemical oxygen demand (COD) and methane content reached 90% (Liao et al. 2010 ).

Wang et al. (Wang et al. 2011 ) discussed the developing approaches for the biogas sector in China and presented every aspect of this technology including the AnMBRs. Ylitervo et al. reviewed the MBR strategy for producing ethanol and biogas and explained the progress in MBRs (Ylitervo et al. 2013 ). Minardi et al. (Minardi et al. 2015 ) reported various applications of the membrane in biogas technologies and purification methods. Mao et al. (He et al. 2012 ) investigated the latest trends in biogas production by AD and AnMBRs. To improve the efficiency of AD, numerous investigations have been focused on various configurations (like single- or multiple-stage reactors).

The latest studies considered the breakdown of the AD method into two groups. For example, acetogenesis–methanation and hydrolysis–acidogenesis are accomplished in unconnected reactors, which can enhance the rate of the conversion process of organic matters to CH 4 , although the high prices associated with these types of systems are a critical issue (Yu et al. 2017 ).

More stability and improved efficiency are the outcomes of utilizing multiple-stage bioreactor systems. These types of systems allow for different conditions to be implemented. Obtained results from (Colussi et al. 2013 ) revealed that the two-step AD of corn requires a greater oxygen demand. Marín Pérez et al. (Pérez and Weber 2013 ) stated that the AD physical parting into two phases established the acceptance of various procedure settings for a particular bacteria type, which increased the degradation rate of organic materials. For preventing ammonia inhibition, the two-stage AD of MSW has been implemented (Yabu et al. 2011 ).

A study conducted in 2008 evaluated the one- and two-stage AD in terms of performance. Results showed that the two-stage process had an advanced yield of CH 4 production (Park et al. 2008 ). Figure  6 shows the schematic of multi-stage AD technology.

Standard diagram of a multiple-stage scheme of AD technology

A two-stage AD system has a suitable potential to process a variety of residuals with high microbiological contents. Blonskaja et al. (Blonskaja et al. 2003 ) stated that by using a two-phase AD for distillery waste, a higher rate of methane would be produced. Kim et al. (Kim et al. 2011 ) implemented a four-phase scheme for activated slurry, which allowed extraordinary digestion productivity. The latest improvements in the utilization of molecular biology implements have developed the utility of included microorganisms and the knowledge of the AD practice. Bioindicators and innovative eco-physiological considerations are the ultimate enhancements of the chemical indexes for monitoring and controlling the stability of the AD process (Lebuhn et al. 2014 ). AD process with renewable feedstock has been introduced as a forthcoming method for biogas production. The biogas chiefly consisted of CH 4 (60%) and CO 2 (35–40%). (Abdeshahian et al. 2016 ).

With the aid of the pyrolysis process, pyrolysis gas from biomass resources can be produced. Pyrolysis gas consists of carbon monoxide, hydrogen, carbon dioxide, and extra gases in minor quantities, e.g., methane and some specific components. The biomass resources are lignocellulosic biomass, MSW, lignite, and digestate. The most important advantage of pyrolysis is that the organic components (specifically the relatively dry and gradually biodegradable biomass that is not appropriate for the AD process) can be converted to pyrolysis gas (Luo et al. 2016 ). In the pyrolysis gas production, a methanation process is essential. Traditional catalytic methanation needs high pressure and temperature (230–700 °C) and a metal catalyst, which imposes high cost with low energy efficiency (Guiot et al. 2011 ). Li et al. (Li et al. 2017a ) have investigated the new approach for employing pyrolysis products as a reservoir of carbon for biogas production. In this study, the effects of different parameters on biomethanation of pyrolysis gas have been assessed.

Different strategies, i.e., hydrothermal pretreatment (HTPT), ultrasonic method, alkaline method, and a combination of them, have been used for the dewatering of biomass materials. By considering every aspect of their functions, HTPT has provided the intended benefits (e.g., hot compressed water utilization and decomposing extracellular polymeric substances) (Park et al. 2017 ; Ruiz-Hernando et al. 2015 ).

A prototype for combining hydrothermal pretreatment with pyrolysis and AD process for cogeneration of biogas and biochar has been presented (Li et al. 2018 ). In the hydrothermal pretreatment (HTPT) stage, by heating sludge at 180 °C for half an hour, the water content fell significantly (from 85 to 33%) and dewaterability improved. After that, filtration outputs were subjected to mesophilic AD without any interruption at an approximate temperature of 37 °C. An up-flow anaerobic sludge-bed reactor has been used for biogas production to be consumed in the hydrothermal pretreatment section. Concurrently, for producing heavy biochar, a rotary kiln has been utilized for filter cake pyrolysis at about 600 °C. The considered configuration included a boiler, a pressure filter, a cooling chamber, and a hydrothermal reactor. The sludge was fed into the first reactor (A) and for diluting the sludge, some water was added (20%). In the next reactor, the superheated steam raised the temperature of the sludge (190 °C). By discharging the steam to the first reactor, the pressure in the second reactor decreased (less than 0.11 MPa) and drained steam used for preheating the input sludge (Li et al. 2017b ).

Hübner et.al (Hübner and Mumme 2015 ) proposed a design for biogas production by using aqueous liquor from digestate pyrolysis. In the applied conditions, three main liquors were produced by the pyrolysis process (at 330, 430, and 530 °C) under four chemical oxygen demand (COD) concentrations (3, 6, 12, and 30 g.L −1 ). At 3 g.L −1 , 6 g.L −1 , and 12 g.L −1 a considerable increase in biogas has been observed. Besides, an important feature was that the biogas production in this process did not need any additives.

The studies based on the microbiology field are developing the concept of hydrolytic microbes and biogas production correlation. These types of investigations focused on the hydrolytic microorganisms’ involvement in biogas units, metabolism types, and their functionality in regarded processes. Azman et al. (Azman et al. 2015 ) studied the participation of anaerobic hydrolytic microbes in biogas production from lignocellulosic (by considering microbiological features). Nina Kolesáarová et al. (Kolesárová et al. 2011 ) examined the possibilities for producing biogas with biodiesel by-product as a feedstock in various phases. Yang et al. (Yang et al. 2014 ) presented a membrane gas-permeation for biogas upgrading. In this study, the authors implemented polymer membranes to upgrade biogas production. Furthermore, Miltner et al. (Miltner et al. 2017 ) reviewed innovative technologies in purification and production of biogas. Kiros Hagos et al. (Hagos et al. 2017 ) presented an anaerobic co-digestion (AcoD) process for producing biogas from various diverse biodegradable organic sources. The digestate (fermentation residue) had a high content of moisture that should be dried for increasing the nutrient concentration and decreasing the transported mass. In this case, using a solar greenhouse dryer in tandem with heat recovery from combined heat and power and a microturbine provided a logical opportunity to eradicate the undesirable moisture content. The hybrid case had the potential to reduce moisture content by up to 80% (Maurer and Müller 2019 ). Owing to the faster reaction rates and higher productivity, the thermophilic digestion method is more satisfactory than mesophilic digestion. The mesophilic digestion method leads to a low methane yield and the related biodegradability is relatively poor. On the other hand, these systems represent enhanced stability and higher concentration in bacteria distribution. Unexpected thermal fluctuations affect methanogens performance; as a result, any extreme variation in temperature is undesirable. In this case, it is better to coat the facilities of biogas plants with insulators to control the digester temperature. By building sun-facing biogas units, the effect of cold winds would be eradicated. The integrated system consisted of a solar system and a biogas plant, which provided satisfactory results in gas yield values during cold seasons (Horváth et al. 2016 ).

Therefore, it is reasonable to surmise that biogas production has been influenced by different parameters and factors, including pretreatment processes, feedstock, and additives features, and process technologies. Provided data appear to confirm the following summary of key points.

• Production of biogas is an approach for biomass treatment and can help energy generation sustainably. Proper potentials for fossil fuel replacements increased the attention to biogas upgrading and advanced pretreatment methods. The biogas pretreatment procedure has two main steps: 1. biogas cleaning methods and 2. biogas upgrading method. With the help of these strategies, the lignin layer would be broken and the biomass turns to a suitable feedstock for the digestion process, while the porosity increases simultaneously. Hereon, the biogas yield would be improved (based on the feedstock types and associated technologies, obtained rates would be different).
• There are different techniques for biogas upgrading that each one has a specific contribution based on the applied commercial technologies. Waster scrubber, chemical scrubber, membrane pressure swing adsorption, and organic physical scrubber contributed the most accounting for 35%, 21%, 20%, 17%, and 5%, respectively.
• An extensive variety of compositions has been evaluated and observed for biogas production. Crop biomasses (wheat, barley, etc.), organic wastes (MSW, agro-industry wastewaters, animal manners, etc.), crop residues (wheat straw, barley, or rice straw, etc.), and non-conventional biomass (microalgae or glycerol) fall into this category. Using a wide variety of chemical additives like NaOH, Ca(OH) 2 , NH 4 OH, H 3 PO 4, etc. can improve the biogas production yield. Using additives can improve the AD process stability and lead to up to 40% higher methane yield.

Recent research has been conducted using carbon membranes for biogas upgrading (Lie 2005 ) where the gas separation process was faster. The selected membranes were thin carbon layers with a thickness of less than 1  μ m supported on ceramic tubes with a length of 0.50 m. Permeation tests using these membranes showed that the CO 2 molecules permeate 50 times faster than CH 4 molecules. By using such membranes, a typical gas mixture consisted of 0.6 of CH 4 is enriched with CH 4 by only one step separation process to more than 0.9 at 1.20 MPa. The membranes showed excellent mechanical properties after a one-month test. The same membranes are used to separate other gases in the biogas mixture such as H 2 S gas. Such new technologies helped a lot in the biogas industrial process in terms of cost reduction and energy consumption compared to classical technologies such as scrubbing.

As mentioned in this part, process, pretreatment, and feedstock are the main influential parameters for biogas production. For obtaining the highest yield in this term, forming a proper balance between these factors has a significant impact on efficiency. Biomass comprises carbohydrate matters, proteins substances, fats, cellulose, and hemicellulose, which could be employed as raw materials for biogas production. In the existing method, co-substrates improved gas yield by increasing the organic content. Distinctive co-substrates contain organic wastes from agriculture-linked productions, food leftover, and gathered municipal wastes from houses. The composition and yield of biogas production be determined by the feedstock and co-substrate category. Although carbohydrates or proteins demonstrate quicker transformation degrees than fats, it is stated that the second one provides more biogas yield (Achinas et al. 2017 ; Braun 2007 ). To keep away from process non-fulfillments, pretreatment is essential. Employing pretreatment approaches improves the degradation of substrates and then the process productivity. Chemical, thermal, mechanical, or enzymatic procedures can be used to accelerate the decomposition method, while this doesn’t unavoidably affect an advanced yield (Putatunda et al. 2020 ; Mshandete et al. 2006 ).

Policy and framework conditions

The biogas industry expands and develops as it represents an alternative source of energy and has a direct influence on the economy. Worldwide many countries organize the market of biogas through policies and regulations. Well-prepared policies prosper the market of biogas as a renewable energy source.

For instance, by EU policies, instructions, and strategic planning, the portion of renewable energies from 2005 to 2015 increased from 9% up to 16.7%, which is predicted to rise to 20% until 2020 (Irena 2018 ).

Keeping these long-term policies with continuous revision and evaluation is a leader in the world of biogas utilization and marketing (Al Seadi et al. 2000 ; Torrijos 2016 ).

Several organizations and governments such as the European Biogas Association (EBA) and the European Parliament and Council legislated regulations in this regard (Xue et al. 2020 ). The role of such organizations is to prepare new relevant policies for upcoming issues and update existing policies to satisfy the market needs and fluctuations and to harmonize the environment and investment. For example, in the UK there are 118 renewable energy policies compared to 7, 28, and 32 in Denmark, Italy, and German, respectively. This is a common framework, and it is used to write their national policies for organizing renewable energy in Europe. Despite the presence of common European directives such as (2009/28/EC) that considered the biogas production from of agronomic deposits and organic trashes and its application in producing power and heat, several EU nations established their energy markets and biomass sources. These countries issued policies to satisfy their own needs and priorities which known as the National Renewable Energy Action Plan.

China has more than 25 energy policies to manage renewable energy. These policies support the renewable market. Biogas was among these renewable energy sources that benefit from such policies and regulations to develop. The Chinese government governmental parties such as the State Council (SC), the Party Central Committee (PCC), and the Ministry of Agriculture and Rural Affairs (MARA) of China participated extensively in developing policies, regulations, and instruction relevant to the progress of biogas (Gu et al. 2016 ; Hua et al. 2016 ; Wang et al. 2016b ). There is a policy about the development necessities of biogas in rural zones that must be updated every year since 2004 it is called Central Document No. 1 (Ndrc 2017 ).

To point out some Chinese policies, the policy of Measures for the Administration of Rural Biogas Construction National Debt Projects (Trial) was developed in 2003 by its Ministry of Ecology and Environment (MEE) and Rural Biogas Project Construction Fund Management Measures was developed by both MARA and Ministry of Finance (MF) of China in 2007.

Recently in 2019, there has been two policies that resulted in numerous ideas to follow the significant growth of farming and rustic zones, the experimental work program establishing waste-free cities by the state council (SC), and the friendly waste of rustic facilities of biogas production by (MARA). All of these policies regarding the handling, management, utilization, and safe disposal technologies are developed by many authorities in china to avoid the work duplication that might retard the international investment and privatization programs. Clear organization between different authorities helps in generating national priorities and smooth management this includes agricultural activities, finance, trade, and scientific research. For example, introducing a fixed premium subsidy enabled the development of biogas and green gas projects, where a finite budget for a subsidy was determined first. Another country introduced what so-called bioeconomy, especially for such projects. Finally, the harmonized sales tax (HST) is paid on purchases/expenses related to commercial construction and operation of biogas facilities which is also called (input tax credits).

Policy framework

It is important in any policy development to have certain targets to be achieved. These targets are dependent on national priorities, so it is changed from one country to another despite having common targets. Examples of targets can be achieving sustainable development for environment elements, communicate clear standards and regulations for wastes management, environmental laws to regulate the relevant processes and etc. To achieve these targets, action plans revised on a yearly basis to evaluate and update the current regulations for future use.

In general, there are five phases to develop a certain biogas policy, which are:

Phase I creating one regulatory body to coordinate the efforts of all stakeholders who can affect/ be affected by the activities of biogas management, by selecting one focal point to help in decision making and future development. This focal point can be from government or from non-governmental organizations (NGOs). This will unify the efforts to get national priorities and plans.

Phase II developing comprehensive and clear instructions and requirements for biogas production that includes management of raw materials, biogas, safe disposal of biogas wastes, preparing environmental impact assessment (EIA) for current and future facilities, applying the waste hierarchy which is referred to as 5Rs (responsibility, reduce, reuse, recycle, recover), issuing the license and permit to work, and applying the periodic environmental audit.

It is also important to apply the proximity principle for the newly constructed biogas plants and make sure to have a centralized biogas plant where all raw materials from different sources can reach it. The idea behind the one huge centralized biogas plant is to make the audit, monitoring, waste collection, transportation, packaging, labeling, storing and/or and safe disposal as easy as possible and make sure that the best and correct disposal method is applied, for example, in Denmark the Danish Government introduced a total ban on landfilling organic or combustible wastes in 1997 (Al seadi T. 2017 ).

Phase III providing incentives and subsidizing to encourage the facilities to produce biogas and increase its contribution to the economy which is referred to as the green economy and to encourage the partnership with the private sector. Such a program will help the facilities that deal with biogas in rehabilitation activities and waste management. Such incentives and subsidy include tax-free period, free consultation, reduced tariff for raw materials used in the manufacturing procedure, and decrease in the payback period increasing the return on investment of the coming projects which in turn will help in mitigation the biogas sustainability challenges. Contingency plans for unexpected challenges must be considered. The sectors of energy and renewable energy are exposed to many parameters that can affect the energy market such as wars, natural disasters, and nowadays the pandemic of COVID-19 where the prices of oil are dropped drastically (Hübner and Mumme 2015 ) and negative effects are imposed on the industry.

Phase IV providing scientific support to the projects of biogas and waste-to-energy plants. This is necessary to use the best environmental practices (BEP) and the best available techniques (BAT). This can be achieved by technology transfer and scientific research, where each plant must have a research and development (RD) department. Ministry of higher education or any relevant authority in the countries with cooperation with industry can provide funds to universities and plants for more research to utilize the waste in producing energy.

Phase VI participating in international conventions and agreements. Each country must participate in international activities and international conventions relevant to waste management and W-t- E initiatives such as Basel conventions (Basel, 2020) that regulate the transboundary movements of hazardous wastes. This participation is important to make use from the experience of each other and to get the consultation from international experts and to get fund for environmental projects from international agencies such as the German Technical Cooperation Agency (GTZ), Japan International Cooperation Agency (JICA), and United States Agency for International Development (USAID),

Phase VII training and awareness programs, where the concerned parties of W-to-E activities prepare training programs for its staff in the fields of waste management, national and international laws, environmental auditing, risk assessment/management, inspection and licensing. Also, the awareness program for the public is important to educate the people in cleaner production and relevant environmental issues. The role of universities is also important to introduce courses for undergraduate and postgraduate students to raise awareness and support scientific research.

Conclusion and recommendation

With the new applications of biogas, the worldwide biogas industry has increased by more than 90% between the years 2010 and 2018, while further growth is still expected. However, the biogas industry varies significantly in different locations over all the world. Different countries have developed several types of biogas systems which are mainly dependent on different environments as well as on energy demand and supply chain. In this study, the production processes and specific applications of biogas in recent years were reviewed and discussed. In the lack of oxygen, the disintegration of organic material produces biogas that mostly consists of carbon dioxide and methane. In recent years, the exploitation of biogas and the expansion of its potential applications have gained popularity due to factors like climate change, reasonable energy prices, and an increase in distributed generation. Biogas also traditionally known as an off-grid energy resource and can be used in various applications consisting of electricity production and CHP systems. The following key points are summarized from the study:

• It is envisioned that the extraction of intrinsic chemical energy of biomass with an efficient AD process can be achieved with proper microbial resource management. Further, advanced monitoring and control of the AD process are needed for the hour for decision making to improve the conversion productivity of the procedure by decreasing the loss of potential methane production due to imbalances of biomass charging rate.
• A sustainable circular economy can be created through biomass utilization by recycling organic residues including nutrients in order to bring it back to the society as energy and fuel.
• Upgradation of the existing technology for efficient conversion of biomass-based organic residues to biomethane and its utilization as a substitute natural gas or vehicle fuel is the trending research scope.
• Hydrogen production using a biogas reforming system with high efficiency is one of the recent applications of biogas. The progress in the application of hydrogen as a clean fuel especially for vehicles is very promising.
• Another cutting-edge application of biogas is fuel cells. Recent advances in fuel cells resulting in low emissions (CO 2 , NO x ) and high efficiency make them suitable for power generation and transportation purposes.
• Even though the conversion of biomass to biogas through AD has already become a touchable reality in many countries, high financial risks linked to its establishment seek higher financial incentives from the policymakers for sustainable shifting of existing technologies.

Failure of the extraction/utilization of renewable energy sources does not sanction the researchers to explore further, but to transfer any sustainable technology from laboratory to the market seeks ground-breaking effort of the researchers and incentives from the policymakers to handle wisely the transition period of partial/full replacement(s)/modification(s) of the existing technologies/ infrastructures, and social acceptance of the simplified—and perhaps definitive—application of the renewables.

Acknowledgements

No financial support exists in this paper.

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