What Is Biogas?

Biogas is a combustible gas mixture produced when microorganisms break down organic matter in the absence of oxygen. It consists mainly of methane and carbon dioxide, with trace quantities of hydrogen sulfide, ammonia, water vapor, nitrogen, and siloxanes. Because it is generated from crop residues, manure, sewage sludge, food waste, and municipal solid waste rather than from fossil deposits, biogas is classified as a biofuel and is counted as a renewable energy carrier in most national accounting frameworks. Its methane fraction usually falls between 45 and 75 percent by volume, which gives a lower heating value of roughly 16 to 28 megajoules per cubic meter, well below the 36 MJ/m3 of pipeline natural gas.

The technology sits at the intersection of environmental engineering, agricultural waste management, and power generation. Capturing methane that organic waste would otherwise release to the atmosphere delivers a climate benefit independent of the energy produced, since methane has a far higher near-term warming potential than the carbon dioxide released when it is burned.

Anaerobic Digestion

Biogas is produced in a sealed vessel called a digester, where a mixed microbial consortium works through four stages: hydrolysis of polymers into soluble sugars, amino acids, and fatty acids; acidogenesis into volatile fatty acids and alcohols; acetogenesis into acetate, hydrogen, and carbon dioxide; and methanogenesis, in which archaea convert those products into methane. The US Environmental Protection Agency's AgSTAR program describes the reactor configurations built around this biology, from covered lagoons and plug flow digesters to complete mix and fixed film systems. Operating temperature separates mesophilic digestion near 35 degrees Celsius from thermophilic digestion near 55 degrees Celsius, the latter running faster and destroying more pathogens at the cost of tighter process control. Retention time, organic loading rate, carbon-to-nitrogen ratio, and pH stability determine whether a digester produces steadily or sours through volatile fatty acid accumulation.

Composition, Cleanup, and Upgrading

Raw biogas is corrosive and variable, so it is conditioned before use. Hydrogen sulfide is removed by iron sponge, biological scrubbing, or activated carbon to protect engines and pipeline steel, moisture is condensed out, and siloxanes are stripped from landfill gas because they form abrasive silica deposits on engine valves. Removing carbon dioxide raises the methane fraction above 90 percent and yields biomethane, a gas interchangeable with natural gas. As the International Energy Agency notes in its introduction to biogas and biomethane, the common upgrading routes are water scrubbing, pressure swing adsorption, chemical and physical solvent scrubbing, and membrane separation, each with a different energy penalty and methane slip.

Feedstocks and Resource Potential

Digester output depends heavily on what is fed to it. Fats, oils, and greases yield far more methane per tonne of volatile solids than fibrous crop residue, which is why co-digestion of food processing waste with manure is common practice. Landfill gas, collected through wells drilled into closed cells, is a separate and large source with a lower and more variable methane content. A US Department of Energy resource assessment of biofuels and bioproducts from wet and gaseous waste streams inventories these sources across landfills, wastewater treatment plants, animal operations, and food waste, and estimates the quantity available from each.

Applications

Biogas has applications in a range of sectors, including:

• Combined heat and power generation using reciprocating engines, microturbines, or fuel cells
• Injection of upgraded biomethane into natural gas distribution networks
• Compressed and liquefied renewable natural gas for heavy vehicle fleets
• On-farm heating, drying, and electricity supply paired with digestate used as fertilizer
• Wastewater treatment plants that offset their own process energy demand
• Household and community digesters supplying cooking fuel in rural areas