In general, energy is defined as the ability to do work. In other words, when an object does something, it means it has energy. Energy plays an important role in the daily life of people. Energy resources (total energy available for use) are divided into two categories: non-renewable energy (such as fossil fuels) and renewable energy (such as biofuels and biogas). Fossil fuels are resources such as oil, gas, coal, etc. These resources have gradually come to existence over hundreds of millions of years (François et al., 2023; Laherrère et al., 2022; Jahanbakhshi et al., 2021). In the process, dead plants and organisms have decayed and been buried under the ground. They have then been compressed by the pressure and internal heat of the earth. Fossil fuels provide about 80–90% of the world's energy, and they are being depleted due to excessive consumption. Fossil fuels took approximately 400 million years to form. But mankind has used something like 80% of all the fossil fuels of the earth in only 60 years from 1960 to 2020 (Yi et al., 2023; Dar and Asif, 2023; Heidari-Maleni et al., 2021).
Biomass is one of the important sources of renewable energy that is obtained from biological materials. Biomass usually includes organic matter that is used to produce electricity or heat (Venugopal, 2022; Jiang et al., 2023). Biological materials include living organisms or their remains. For example, the remains of forest trees, pruned materials from plants and wood chips, animal waste, animal droppings, agricultural product waste, urban waste and food waste can be used as biomass. These materials have the ability to store energy. In fact, during the photosynthesis phenomenon, carbon dioxide is stored in plants through water, soil and air by solar energy and causes their growth and development. This solar energy can be converted into other forms of consumable energy later. Biomass is able to produce electricity, heat, liquid fuels, and gaseous fuels (biogas) and can have various useful chemical applications (Dutta et al., 2023; Periyasamy et al., 2023; Kumar & Agrawal, 2020; Jahanbakhshi & Salehi, 2019).
Biogas is a renewable natural energy source that leaves positive effects on nature and industries. This gas is produced from the decomposition of organic materials, including animal manure, food waste and sewage (Shaibur et al., 2021; Wardle et al., 2021). Manure and waste produce biogas through anaerobic digestion (that is, without the presence of oxygen). About 50–70% of biogas consists of methane and thus it is flammable. Biogas fuel is produced from the combination of methane, hydrogen, and carbon monoxide gases with oxygen, and 21% of air is oxygen. Biogas is the most economical renewable fuel that is used in many countries. This fuel is used for cooking, cooling and heating, electricity generation, waste management, and mechanical power generation (Czekała et al., 2023; Caetano et al., 2022; Fazzino et al., 2021).
The increasing need to obtain biofuels against fossil fuels has created a great interest in producing biogas from biomass sources. Using the anaerobic digestion method to produce biogas is the best way to recover biomass into energy. This technology is used as an effective and reliable method to produce biogas from various organic wastes (such as animal manure, agricultural waste, municipal sewage, etc.). Organic waste left in the environment is an important source of greenhouse gas emissions (such as methane and carbon dioxide). Now, if the methane produced in the process of anaerobic digestion of these wastes is controlled by a method, a large amount of global warming can be avoided. The only possible way to control methane emissions is to use closed vessels (anaerobic digesters) in which biomass is decomposed (Kunatsa and Xia, 2022; Bandgar et al., 2022; Vats et al., 2020). Methane can be used to generate heat, electricity and vehicle fuel. Methane production from organic materials mainly depends on the amount of materials in them that can be decomposed into CH4 and CO2. The fuel value of the produced biogas depends on the percentage of methane, the higher the amount of methane gas, the greater the ability of biogas. There are various types of digesters for anaerobic digestion of organic waste. Anaerobic digestion of organic waste is performed in different digesters in a single-stage, double-stage and multi-stage manner (Yaqoob et al., 2021; Orlando and Borja, 2020; Vats et al., 2020).
It is essential that we use new sources of energy (such as biofuels) instead of fossil sources. New energy systems in the future must rely on structural and fundamental changes in which carbon-free energy sources such as biomass are used. Biogas is one of those renewable energies that are often wasted. In addition to energy production, biogas is able to produce agricultural fertilizers and thus increase the public health level of a society as a disease control measure. Biogas is also a suitable solution for solid waste disposal (Heidari-Maleni et al., 2023; Alao et al., 2022). On the other hand, the accumulation of excreta in animal and livestock keeping centers for long periods of time causes the increase of insects and the transmission of various pollutants to the livestock environment and residential areas. Also, the strong smell of excrement attracts the attention of insects from faraway places to the environment of livestock farms, hence, the possibility of transmission of contamination and animal diseases from other places. Another problem with accumulation of waste in one place is contamination of surface and underground water, which would in turn cause heavy damages to the environment. The use of animal manure in agricultural fields can transmit the disease to other animals in the region if the animal is sick or infected with parasites. However, in addition to producing a significant amount of gas, the use of anaerobic fermentation in animal husbandry creates a healthy environment for animals and human beings. The main flammable element of biogas is methane. Increase in CO2 of the biogas mixture reduces its calorific value and flammability. In this study, we examine the effect of the type of raw material and reactor on improving biogas production. This piece of research pursues the following goals:
1) Management of animal manure and their processing for biogas production.
2) Evaluation and comparison of cow and chicken manure to improving and maximizing biogas production.
3) Evaluation and comparison of different digesters to improving and maximizing biogas production.
4) Investigating the effect of leachate circulation in biogas production.
5) Examining limitations, challenges and future research direction.