Biogas
Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, wastewater, and food waste. Biogas is produced by anaerobic digestion with anaerobic organisms or methanogens inside an anaerobic digester, biodigester or a bioreactor. The gas composition is primarily methane and carbon dioxide and may have small amounts of hydrogen sulfide, moisture and siloxanes. The methane can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used in fuel cells and for heating purposes, such as in cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat.
Biogas can be upgraded to natural gas quality specifications by stripping carbon dioxide and other contaminants. Biogas that has been upgraded to interchangeability with natural gas is called renewable natural gas. RNG can be used as a drop-in fuel in the gas grid or to produce compressed natural gas as a vehicle fuel.
Biogas is considered to be a renewable resource. At a high level, biogas is a carbon-neutral fuel in so far as emissions of carbon dioxide from its combustion are matched by carbon dioxide pulled from the atmosphere to produce biomass. In practice, the carbon intensity of biogas can vary depending on emissions from the production of biomass and the processes used to produce and upgrade biogas. In some applications, the capturing of biogas can avoid emissions of methane reducing overall emissions.
Production
Biogas is produced by microorganisms, such as methanogens and sulfate-reducing bacteria, performing anaerobic respiration. Biogas can refer to gas produced naturally and industrially.Natural
In soil, methane is produced in anaerobic environments by methanogens, but is mostly consumed in aerobic zones by methanotrophs. Methane emissions result when the balance favors methanogens. Wetland soils are the main natural source of methane. Other sources include oceans, forest soils, termites, and wild ruminants.Industrial
Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. This process produces biogas which can be used as a fuel. Industrial biogas production can either be purpose-built such as anaerobic digesters built to process manure and organic waste or can harvest biogas produced as byproduct from landfills or wastewater treatment plants.Biogas plants
A biogas plant is the name often given to an anaerobic digester that treats farm wastes, municipal organic waste and/or energy crops. Industrial biogas plants process organic material in an air-tight tank to create anaerobic conditions. The material is heated to either mesophilic or thermophilic and held for a typical retention time of two to thirty days.These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. During the process, the micro-organisms transform biomass waste into biogas and digestate. Higher quantities of biogas can be produced when the wastewater is co-digested with other residuals from the dairy industry, sugar industry, or brewery industry. For example, while mixing 90% of wastewater from beer factory with 10% cow whey, the production of biogas was increased by 2.5 times compared to the biogas produced by wastewater from the brewery only.
Landfill gas
Landfill gas is produced by wet organic waste decomposing under anaerobic conditions in a similar way to biogas. The waste is covered and mechanically compressed by the weight of the material that is deposited above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. Biogas builds up and is slowly released into the atmosphere if the site has not been engineered to capture the gas. Landfill gas released in an uncontrolled way can be hazardous since it can become explosive when it escapes from the landfill and mixes with oxygen. The lower explosive limit is 5% methane and the upper is 15% methane.The methane in biogas is 28 times more potent a greenhouse gas than carbon dioxide. Therefore, uncontained landfill gas, which escapes into the atmosphere may significantly contribute to the effects of global warming. In addition, volatile organic compounds in landfill gas contribute to the formation of photochemical smog.
Dangers
The air pollution produced by biogas is similar to that of natural gas as when methane is ignited for its usage as an energy source, Carbon dioxide is made as a product which is a greenhouse gas. The content of toxic hydrogen sulfide presents additional risks and has been responsible for serious accidents. Leaks of unburned methane are an additional risk, because methane is a potent greenhouse gas. A facility may leak 2% of the methane.Biogas forms an explosive mixture with air at concentrations between approximately 6% and 22% by volume. To eliminate this hazard during maintenance, empty digesters must be properly ventilated, cleared of gas, and monitored before entry. Crucially, the system must be designed and operated to prevent negative pressure, which can draw air in and create an explosive atmosphere. This is ensured by technical controls, such as automatic shut-off devices, not merely by avoiding a specific minimum pressure.
Frequent smell checks must be performed on a biogas system. If biogas is smelled anywhere windows and doors should be opened immediately. If there is a fire the gas should be shut off at the gate valve of the biogas system.
Composition
The composition of biogas varies depending upon the substrate composition, as well as the conditions within the anaerobic reactor. A 2025 study in Ethiopia concluded that agroecology, biogas plant design, and temperature significantly affect biogas yield quality. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55–75% methane, which for reactors with free liquids can be increased to 80–90% methane using in-situ gas purification techniques. As produced, biogas contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for water vapour content and thermal expansion is easily done via simple mathematics which yields the standardized volume of dry biogas.For 1000 kg of input to a typical biodigester, total solids may be 30% of the wet weight while volatile suspended solids may be 90% of the total solids. Protein would be 20% of the volatile solids, carbohydrates would be 70% of the volatile solids, and finally fats would be 10% of the volatile solids. Biochemical oxygen demand is a measure of the amount of oxygen required by aerobic micro-organisms to decompose the organic matter in a sample of material being used in the biodigester as well as the BOD for the liquid discharge allows for the calculation of the daily energy output from a biodigester.
Contaminants
Sulfur compounds
Toxic, corrosive and foul smelling hydrogen sulfide is the most common contaminant in biogas. If not separated, combustion will produce sulfur dioxide and sulfuric acid, which are corrosive and environmentally hazardous., Other sulfur-containing compounds, such as thiols may be present.Ammonia
is produced from organic compounds containing nitrogen, such as the amino acids in proteins. If not separated from the biogas, combustion results in NOx| emissions.Siloxanes
In some cases, biogas contains siloxanes. They are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or other elements in the combustion gas. Deposits are formed containing mostly silica or silicates and can contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means.Debate
Arguments in favor
High levels of methane are produced when manure is stored under anaerobic conditions. During storage and when manure has been applied to the land, nitrous oxide is also produced as a byproduct of the denitrification process. Nitrous oxide is 273 times more aggressive as a greenhouse gas than carbon dioxide and methane 27 times more than carbon dioxide.By converting cow manure into methane biogas via anaerobic digestion, the millions of cattle in the United States would be able to produce 100 billion kilowatt hours of electricity, enough to power millions of homes across the United States. One cow can produce enough manure in one day to generate 3 kilowatt hours of electricity. Furthermore, by converting cattle manure into methane biogas instead of letting it decompose, global warming gases could be reduced by 99 million metric tons or 4%.
Arguments against
Others environmental groups have argued that manure based biogases are a form of greenwashing. They argue it encourages and subsidies the use of concentrated animal feeding operations and emits other pollutants such as hydrogen sulfide. In 2022, 6 US senators including Bernie Sanders and Elizabeth Warren argued biogas would not be able to succeed without taxpayer dollars and that those would be better used on other methods. They also argued that they may accelerate consolidation in the industry and see farms expand their size specifically to be large enough to receive biogas subsidies. They point to evidence farmers did this following California's rollout of biogas incentive programs.Others have argued the level of funding to biogas is already particularly outsized. For instance, in Wisconsin, just two years of spending on biogas has been higher than 12 years of spending on solar energy. Manufacturing of biogas from intentionally planted maize has been described as being unsustainable and harmful due to very concentrated, intense and soil eroding character of these plantations.