Renewable Natural Gas (Biomethane) Production
Renewable natural gas (RNG), or biomethane, is a pipeline-quality gas that is fully interchangeable with conventional natural gas and thus can be used in natural gas vehicles. RNG is essentially biogas (the gaseous product of the decomposition of organic matter) that has been processed to purity standards. Like conventional natural gas, RNG can be used as a transportation fuel in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). RNG qualifies as an advanced biofuel under the Renewable Fuel Standard.
Biogas is produced from various biomass sources through a biochemical process, such as anaerobic digestion, or through thermochemical means, such as gasification. With minor cleanup, biogas can be used to generate electricity and heat. To fuel vehicles, biogas must be processed to a higher purity standard. This process is called conditioning or upgrading, and involves the removal of water, carbon dioxide, hydrogen sulfide, and other trace elements. The resulting RNG, or biomethane, has a higher content of methane than raw biogas, which makes it comparable to conventional natural gas and thus a suitable energy source in applications that require pipeline-quality gas.
Biogas from Landfills
Landfills are designated locations for disposal of waste collected from residential, industrial, and commercial entities. Landfills are the third-largest source of human-related methane emissions in the United States, according to the U.S. Environmental Protection Agency (EPA). Biogas from landfills is also called landfill gas (LFG), as the digestion process takes place in the ground rather than in an anaerobic digester. As of March 2015, there were about 645 operational LFG projects in the United States, according to EPA. However, most of these projects use biogas to produce electricity rather than power natural gas vehicles.
Learn about these LFG alternative fuel transportation projects:
- Waste Management's Altamont Landfill near Livermore, California (see the case study)
- St. Landry Parish Landfill in Washington, Louisiana
- Joint Water Pollution Control Plants in Los Angeles County, California
Biogas from Livestock Operations
Biogas recovery systems at livestock operations can be used to produce renewable natural gas. Animal manure is collected and delivered to an anaerobic digester to stabilize and optimize methane production. The resulting biogas can be processed into RNG and used to fuel natural gas vehicles.
As of January 2015, there were about 247 anaerobic digester systems operating at commercial livestock farms in the United States. Most of these facilities use biogas for electricity generation. A few farms are using biogas to produce transportation fuel, including Hilarides Dairy in California and Fair Oaks Dairy in Indiana. EPA's AgSTAR database provides more information about the use of such systems in the United States.
Biogas from Wastewater Treatment
Biogas can be produced during the digestion of solids removed in the wastewater treatment process. According to EPA estimates, this biogas potential is about 1 cubic foot of digester gas per 100 gallons of wastewater. Energy generated at U.S. wastewater treatment plants (WWTPs) could potentially meet 12% of the national electricity demand, according to the National Association of Clean Water Agencies. This could spur some production of RNG for vehicle use as well.
There are more than 16,000 WWTPs in the United States, and about 1,500 employ anaerobic digestion to produce biogas that is used on site. The Janesville Wastewater Treatment Plant in Wisconsin is an example of a plant that uses biogas to produce RNG for use in vehicles.
Other Sources of Biogas
Other sources of biogas include organic waste from industrial, institutional, and commercial entities, such as food manufacturing and wholesalers, supermarkets, restaurants, hospitals, and educational facilities to name a few. Learn about the Sacramento BioDigester, the largest anaerobic digestion system of its kind in North America.
Biogas can also be produced from lignocellulosic material (such as crop residues, woody biomass, and dedicated energy crops) via thermochemical conversions, co-digestion, and dry fermentation. These technologies are underway in Europe, with limited applications in the United States.