Biogas is a mixture of different gases produced by anaerobic decomposition (with methanogen or anaerobic organisms), or fermentation, of biomass – organic material (including animal dung, human sewage, food waste, crop residues, and industrial and municipal wastes). Biogas is a renewable energy source. It is composed primarily of methane (up to 60%), which is the combustible component, carbon dioxide, and hydrogen sulfide.
Biogas has a high calorific value and can be converted into electricity and heat. The residue of fermentation is digestate, a liquid material, completely odorless and with a very high agronomic value, with improved characteristics compared to the starting material.
The production of biogas can take place in two ways: through the process of anaerobic respiration, without the aid of oxygen, or, naturally, through aerobic respiration, from large quantities of different types of biomasses such as: biological waste, livestock waste, etc.
Biogas is produced in an air-tight container, called an anaerobic digester, biodigester or a bioreactor. Is used as a fuel to heat stoves, lamps, run small machines, and to generate electricity. The residues of biogas production are used as a low-grade organic fertilizer. Biogas fuels do not usually cause any pollution to the atmosphere, and because they come from renewable energy resources they have great potential for future use.
Biomass used for biogas production can be of animal, plant or waste origin. They are mostly agricultural waste biomass or organic waste, originating from several sectors:
- zootechnical (livestock wastes);
- agro-industrial (agricultural and animal by-products);
- agricultural production (crop wastes and residues);
- the organic fraction of solid organic waste or material collected from the separate collection of organic waste;
- dedicated crops (silage, corn, triticale, sorghum, rye).
Depending on the biomass used in the energy production process, the types of pre-treatment also vary, as well as those following the biogas production process.
The use of waste biomass is certainly one of the main advantages of biogas that allows to produce energy in a completely natural form from waste materials that otherwise would not be used by any other industry.
An ecological advantage in the use of biogas is to prevent the diffusion into the troposphere of methane emitted naturally during the decomposition of carcasses and plants: methane is in fact one of the most powerful greenhouse gases and therefore its degradation into CO2 and water by combustion is desirable. The emission of 1 kg of CH4, in a time horizon of 100 years, is equivalent to emit 25 kg of CO2 (IPCC 2007).
The advantages of biogas are, therefore, multiple and significant:
- It enhances and exploits waste biomass and by-products to produce energy, reducing the environmental impact from waste treatment and that caused by the distribution of traditional fossil fuels;
- It contributes to the realization of the green economy foreseen for the future, to the achievement of a circular economy model and to a more sustainable energy use, attentive to the safeguard of the environment;
- It allows to decrease carbon dioxide emissions. The combustion of biogas does not originate additional CO2 compared to that already used previously by plant or animal biomasses of departure, unlike fossil fuels that produce it from scratch;
- It reduces the emission of methane gas, which has a negative impact on the environment. In fact, methane is one of the main greenhouse gases and has a great impact on increasing the greenhouse effect;
- It allows for energy diversification. The use of biogas reduces dependence on traditional energy sources and is one of the clean and sustainable energy choices;
- In addition to generating electricity, it also produces heat. In fact, biogas can be used both to generate electricity and to meet heating demand;
- It can be supplied in a continuous form as biomass energy can be regulated at will and can be stopped when desired, just like energy from fossil sources;
- Biogas plants are easier to build and the technologies implemented are unsophisticated and more readily available. There is therefore a reduction in costs and a lower investment in their construction compared to other renewable energy plants;
- Once the biogas has been purified of impurities and the CO2 has been removed, it can be transformed into biomethane. It is then transported and used through traditional infrastructure, allowing countries to reduce emissions in some difficult sectors, such as heavy industry and freight transport.
Therefore, even biogas, despite the many advantages listed above, presents problems related to both production and use:
- To power a 1 MW power plant, at least 300 hectares are required, the minimum adoptable area. It is therefore necessary to have large amounts of land available;
- The sewage used gives off unpleasant odors: it is therefore necessary that the plants are located far enough from population centers to ensure a state of comfort to citizens;
- Transport: if the plant is located far away, adequate means of transport will be needed to transport the raw materials, as well as the final products. High traffic involves high carbon dioxide emissions;
- The use of waste biomass and by-products for gas production is not straightforward. Their high fiber concentration can limit digestion by bacteria, which then implies problems related to blending and surface crusting leading to increased self-consumption and reduced plant output.
Like any other technology, biogas has problematic aspects. If we exclude the plants that exploit the biogas produced by the decomposition of organic products from landfills, a large number of biogas plants use animal slurry combined with plant products in a variable ratio, since the yield of biogas is optimized by mixing several types of organic products.
First of all, therefore, for this type of power plants (the most widespread), there is the problem of the raw material. In fact, to feed a 1 MW power plant using only specially cultivated products requires about 300 Ha of available land. Therefore, if this were done on a large scale for many thousands of hectares on valuable agricultural land already used for human or animal consumption, land would be taken away from food production. It is therefore essential that the authorities limit the percentage and type of area that can be cultivated with biomass, in order to maintain a balance between crops dedicated to food or animal feed and crops dedicated to energy production. At the same time, it should also be considered that recent years have been characterized by a progressive abandonment of land due to the low profitability of agriculture and competition from foreign countries.
The substitution of low-income crops with biomass corn or similar plants has allowed many companies to survive this moment of crisis. However, this poses the problem of converting agricultural land for food purposes into agricultural land for energy purposes. In these cases, since the plants needed for fermentation are not intended for human consumption and since what matters is the yield, producers are more encouraged to increase fertilizer and pesticide treatments, thus increasing the environmental impact of the crops involved.
Another problem is related to the bad smells emitted by the fermentation of vegetables and/or the associated slurry. The problem can be solved by a correct management of the plant, in fact the tanks to work must be completely sealed. Many of these plants, usually to exploit the excess heat in a district heating network, are being built far from the areas of production of sewage and close to homes, resulting in heavy discomfort for the population. This involves, among other things, a movement of thousands of trucks only locally because the plants are fed by short supply chain with a consequent decrease in pollution resulting from transport over long distances.