Biogas is a by-product of anaerobic digestion a process in which microorganisms degrade organic material, such as dairy manure, in the absence of oxygen to produce biogas and digestate. Biogas is composed mostly of methane, which is the main constituent of natural gas. Biogas can be directly burned to produce heat, fed to a generator (with a previous cleaning of hydrogen sulfide) to produce electricity, cleaned, and injected into the natural gas grid, or cleaned and compressed to be used as a transportation fuel. Digestate is the degraded organic material that exits the digester after the digestion process. Digestate contains nutrients that can be used as an organic fertilizer. Although many of the nutrients change form, the total amount of nutrients remains nearly constant.
Anaerobic digestion happens in an anaerobic digester, a sealed reactor that maintains the optimal conditions to accelerate and maximize the production of methane. In the U.S., digesters are mostly installed in agricultural farms where they have manure readily available as the primary feedstock. On average, methane production from manure can range from 0.08 to 0.39 cubic meters per kilogram of dry manure from beef and pig animals, respectively and the energetic content of methane is 35 MJ or 9.2 KWh per cubic meter (LHV).
There are currently over 300 anaerobic digesters in operation or under construction on farms in the U.S. Some of the most popular digester designs are plug-flow, complete mixed, and covered lagoons. Most of the existing agricultural anaerobic digestion systems in the U.S. generate electricity for the grid, but new projects are targeting renewable natural gas (RNG) production, driven by incentives from the Renewable Fuel Standard (RFS) and the California Low Carbon Fuel Standard. Anaerobic digestion produces renewable energy while also reducing odors, pathogens, antibiotics, and greenhouse gas emissions.
Visit our Energy Generation page to know more about MREC related anaerobic digester projects and resources to other great informational pages.
- American Biogas Council
- Anaerobic Digestion – U.S. Environmental Protection Agency (U.S. EPA)
- Landfill gas and Biogas – U.S. Energy Information Administration (EIA)
- Biomass Resource Data, Tools, and Maps – National Renewable Energy Laboratory (NREL)
- Biogas Upgrading and Waste-to-Energy -National Renewable Energy Laboratory (NREL)
- An introduction to biogas and biomethane – International Energy Agency (IEA)
- Bioenergy – Renew Wisconsin