**1. Introduction/Literature Review**

In today's conditions, humanity is losing biodiversity on the planet, living in an era of large-scale environmental pollution, desertification, intensification of negative weather phenomena, loss and pollution of fresh water, and loss of forest resources. Stated above are either results of climate change or causes. It is a well-known fact that humanity has until 2030, when climate change will become irreversible. Against the background of such threatening prospects, as well as due to the depletion of fossil fuels, increasing global energy demand, ever-increasing fuel prices, and military conflicts that disrupt the energy balance of nations, humanity is increasingly focusing on alternative energy sources and efficient renewable energy sources.

Accordingly, in order to find an alternative to fossil fuels, as well as to solve the problem of landfilling of organic waste and reduce the negative impact on the environment

**Citation:** Mirzoieva, T.; Tkach, N.; Nitsenko, V.; Gerasymchuk, N.; Tomashevska, O.; Nechyporenko, O. Development of the Correlation Model between Biogas Yield and Types of Organic Mass and Analysis of Its Key Factors. *C* **2022**, *8*, 73. https://doi.org/10.3390/c8040073

Academic Editors: Indra Pulidindi, Pankaj Sharma and Aharon Gedanken

Received: 25 October 2022 Accepted: 2 December 2022 Published: 5 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in general, the world has been developing innovative approaches to bioenergy, biomaterials, and chemical production from organic waste for a long time. One of the key alternatives to fossil fuels is biofuels. Biomass is a potential renewable source for the production of solid, liquid, and gaseous biofuels. In turn, the modernization of existing technologies for the processing of organic waste into biofuels and its use as a substitute for natural gas or automotive fuel is a trendy direction in research in recent decades around the world. It is widely believed that the most viable option for obtaining energy from organic waste is biogas—a renewable gaseous fuel obtained by the decomposition of organic substances, such as food and animal husbandry waste [1,2].

Biogas consists mainly of methane and carbon dioxide. Combustible methane is the main component of biomass (50–85%), which is the main source of energy. It can be used in many ways, including automotive fuel, as well as for heating and electricity generation. In addition, the production of biogas produces a by-product—digestate, which can be used as biofertilizer. Digestate—or as it is also called, natural fertilizer—contains water, nutrients, and organic carbon suitable for soils [3–5].

The website of the European Biogas Association states that the use of digestate obtained in the process of biogas production as a biofertilizer helps return organic carbon back to the soil and reduces the need for carbon-laden mineral fertilizers [6].

We consider that information about this fact and everything related to biofuel production should continue to be actively disseminated and promoted both in society as a whole and among the farming environment, especially in developing countries.

It should be noted that the production and use of biogas is not an achievement of today; it has deep roots. It is believed that the first human use of biogas dates back to 3000 BC in the Middle East, when the Assyrians used biogas to heat their baths. In turn, the 17th century chemist Jan Baptist van Helmont discovered that combustible gases can be formed from decomposing organic substances. Van Helmont also first added the word «gas» to the scientific dictionary, derived from the Greek word «chaos». The first large anaerobic fermentation plant was built in 1859 in a leper colony in Bombay [1]. In 1884, Louis Pasteur explored the possibility of obtaining biogas from animal waste and proposed it as a fuel for lighting streetlamps [7]. In general, in the past, biogas was widely used as a source of energy in households in Africa and Asia. Despite the rather primitive design, anaerobic boilers have solved the problems of autonomous energy supply of many households in India, Pakistan, Indochina, and others. Later, biogas became a very important part of the energy source for Western Europe and North America [8].

In today's conditions, the development of biogas production can produce a number of positive effects never before seen, some of which are as follows. First, it is important for modern humanity that biogas energy production can reduce greenhouse gas emissions. In particular, the fermentation of manure on a biogas plant significantly reduces greenhouse gas emissions. The benefit of reducing greenhouse gas emissions is enhanced by the processing and use of methane (a strong greenhouse gas) that could otherwise be released into the atmosphere due to the decomposition of organic by-products and wastes.

Paolini et al. [9], for example, noted that the main goal of the biogas industry is to reduce fossil fuel consumption to mitigate the effects of global warming, and, in general, biogas can make a significant contribution to reducing greenhouse gas emissions. An additional advantage of biogas technology is the production of organic fertilizers for agricultural crops through the use of digestate [10]. In general, biogas can be developed on a large-scale die to partnerships between the energy and agricultural sectors. Another argument in favor of biogas is that it is an environmentally friendly renewable energy source. The only time when biogas is depleted is when the production of any type of waste is stopped. It is also a free source of energy [11]. It is produced by anaerobic biodegradation of organic substances. To do this, the waste must enter an environment without oxygen. This can happen naturally or as part of an industrial biogas process [12–14].

The following also apply:


The European Biogas Association's focus on biogas production is a stimulus for rural district development [6]. We consider this aspect to be extremely important for the modern world. In particular, the combination of agricultural activities with renewable energy production using biogas provides three additional benefits: it helps farmers effectively manage their waste and residues, reduces emissions from agriculture, and improves soil quality and biodiversity on agricultural land. In such healthy ecosystems, plants absorb carbon dioxide from the atmosphere, acting as carbon sinks; the digestate used as organic fertilizer returns nutrients to the soil; and methane emissions from livestock enter the controlled environment of the biogas plant, not the atmosphere. Cucui et al. [21] also emphasized that biogas production has a favorable economic effect and can solve waste problems.

The close relationship between agricultural and biogas production is evidenced by the practice of growing cover crops, part of an agronomy system in which an additional second crop is grown before or after the main crop is harvested on the same agricultural land. Cover culture prevents soil erosion and compaction, and promotes the biological, chemical, and physical activity of the soil. As a result, soil quality and fertility are improved, and soils become more resistant to floods and droughts. Cover crops are not ordinary winter crops or pastures, but are sown specifically to protect bare soil in winter and spring after harvesting spring crops. In addition to protecting the soil and its nutrients, cover crops can be of economic importance when used for renewable energy production, such as biogas production.

We fully share the view that bioenergy production from agricultural waste streams and cover crops creates additional business models in the agricultural sector, making agriculture more cost-competitive [6].

In general, in today's information environment there are a lot of data on the benefits of biogas. In our opinion, the most complete set of them is set out, summarized, systematized, and structured in the 2019 report of the World Biogas Association «Global Potential of Biogas». In particular, in this document, the benefits of biogas production are presented in the following main areas: (1) production of renewable energy; (2) climate change mitigation; (3) contribution to the development of the closed-loop economy; (4) improving air quality in cities, the state of water bodies, and soils; (5) contribution to food security; (6) improving

health and sanitation through improved solid waste management; (7) economic development and job creation. Moreover, the report emphasizes that in addition to contributing to the Sustainable Development Goals, the development of organic waste processing in general and biogas production in particular is characterized by the following benefits: (1) the possibility of using diverse and local raw materials; (2) flexibility of scaling—biofuels do not have a minimum scale of realization, and their maximum scale is limited only by the amount of available raw materials; (3) flexible use of biogas—primarily for the production of heat and electricity, as well as fuel for cars; (4) multiple income streams—the source of income can be any main or by-product obtained after waste recycling [22].

Indeed, different types of raw materials are used for biogas production. Scientists and practitioners mostly distinguish four groups of raw materials: crop residues—such as wheat stalks and soybean straw; manure from animals (including cattle, pigs, poultry, and sheep); organic fraction of solid household waste, including industrial waste and sewage sludge [19].

The group of plant nutrient residues includes residues after harvesting wheat, corn, rice, and other cereals; and sugar beets, sugar cane, soybeans, and other industrial and oily crops. The group of solid household waste includes food, paper, cardboard, and wood that are not used in other ways (for example, for composting or processing), and some industrial waste from the food industry. Importantly, some industries, such as food, beverages, and chemicals, produce wet waste with a high content of organic substances, which is a suitable raw material for anaerobic digestion. In such industries, biogas production can have the side benefit of cleaning up waste as well as providing heat and electricity on site. The raw material for biogas production is sewage sludge: a semi-solid organic substance extracted in the form of waste gases from municipal treatment plants. In today's conditions, the largest volumes of biogas are produced from agricultural crops and animal manure.

The development of biogas production in the modern world is quite rapid. Thus, in 2010–2018, due to new directions in biogas use, the volume of biogas production increased by 90% worldwide and further growth is expected, especially based on the use of agricultural crop waste. At the same time, the development of biogas production in the world is uneven, as it depends primarily on the availability of raw materials and on policies that encourage its production and use. A total 90% of world production is accounted for by Europe, China, and the United States. In June 2018, EU institutions agreed on a new Renewable Energy Directive for the next decade, including a legally binding EU-wide target to bring the share of renewable energy to 32% by 2030. Given the above benefits of biogas, it is no exaggeration to say that the biogas sector will contribute to this goal [23].

Considering biogas as the most viable option for obtaining energy from organic waste, it is worth noting that, in fact, there are different technologies to produce biomass, such as gasification (energy conversion performances during the biomass air gasification process under microwave irradiation), pyrolysis (microwave-assisted co-pyrolysis of brown coal and corn stover for oil production), etc. The methane content is significantly varied by many factors [24,25]. These are also promising areas of biomass production, which are increasingly implemented in practice.

It is also worth noting that the biogas sector in Europe, as noted by Vlatka Petravie-Tominac, Nikola Nastav, Mateja Buljubasic, and Bozidar Santek, is quite diverse [26].

In particular, it is well developed in Germany, Denmark, Austria, and Sweden, followed by the Netherlands, France, Spain, Italy, The UK, and Belgium. It is characteristic of the EU countries that they have structured their financial incentives in favor of different types of raw materials depending on national priorities—that is, whether biogas production is considered primarily as a way of processing waste (for example, in the UK, more than 80% of biogas is obtained from landfills and sewage sludge), as a way of obtaining renewable energy (for example, in Germany, 93% of biogas is obtained from agricultural crops and agricultural waste), or their combinations. In other EU countries, different combinations of raw materials are used, depending on the specific circumstances and the availability and prices of certain types of raw materials. For example, Denmark predicts gas networks

will use only biogas in 2034. At the same time, a significant part of biogas is produced from corn, beets, and animal manure [27]. Aware of the potential of biogas, France has already invested in the technologies needed to develop it on its territory. As of 31 December 2021, for example, the country counted 365 installations designed to inject biomethane into natural gas networks, with a capacity of 6.4 terawatt-hours (TWh) per year [28].

The war in Ukraine has given renewable gas a new impetus, with the European Commission proposing to ramp up biomethane production to 35 billion cubic meters (bcm) by 2030, up from 3 bcm in 2020. The advantage of renewable gas is that it can be produced within Europe. In fact, the actions of many modern European institutions are aimed at the following: (1) making European energy and climate policy constant, stable, and inclusive, increasing the competitiveness of renewable energy sources at cost; (2) increasing the production and consumption of constant renewable gas in Europe for all energy sectors, including transport, domestic, and industrial use; (3) ensuring cross-border and intersectoral exchange of renewable gases; (4) achieving recognition of additional domestic socio-economic and environmental benefits of renewable gas production; (5) making anaerobic fermentation an integral part of competitive and constant agriculture and waste management [29–32].

At the same time, the situation with biogas production in different EU countries depends on a number of different factors, such as investment attractiveness (construction of new biogas plants or only modernization of existing ones); guaranteed green tariff price, which is much higher than electricity produced from other sources; national targets; and action plans for renewable energy sources. In addition, with regard to biogas production, each country has its own obstacles that need to be overcome. One such country is Ukraine, a country that is not self-sufficient in terms of energy supply and is characterized by dependence on low-diversified energy imports, which regularly leads to political tensions and is of high socio-economic importance. At the same time, Ukraine is a powerful agrarian state and is a world leader in the production of numerous kinds of agricultural products. Therefore, in today's Ukrainian society, it is widely believed that the production of agricultural biogas is likely to slow down climate change and increase energy selfsufficiency of the state by replacing or supplementing traditional energy sources [33–35].

Evidence for the development of biogas production in Ukraine being given considerable attention is, for example, the functioning of the Bioenergy Association of Ukraine. In his speeches and publications, the Chairman of the Board emphasizes the need to develop biomethane production, which is in line with the idea of a circular economy, as it converts waste streams, such as agricultural by-products or household waste, into energy while providing recycling of nutrients to agricultural land [35]. In turn, analyst Mostova M., analyzing the current state and prospects of biogas in Ukraine, gave impressive figures: in 2012–2019, in biogas capacity in Ukraine was invested about 140 million euros, and the growth rate of biogas capacity in Ukraine in 2019 was almost 3.5 times higher than in 2018 [36]. As a rule, the production of agricultural biogas is analyzed and evaluated from three main points of view: environmental, economic, and social. Meanwhile, in Ukraine, despite the existing understanding of the importance of biogas use and some efforts by government and business, the growth of total renewable energy production, in particular from biomass, is lagging behind the expected growth. This necessitates constant attention and in-depth research by scientists and practitioners.

The purpose of this publication is to try to model the relationship between the output of biogas from different types of organic mass and to specify the factors that affect it.
