The United States is the world’s leading natural gas producer thanks to its abundant natural gas resources across the country. In addition, new discoveries and extraction methods have led to a dramatic rise in shale gas development.
As part of a comprehensive energy strategy, the U.S. Department of Energy is committed to safe development of the nation’s natural gas resources. Among its efforts, the U.S. is investing in innovative research projects, exploring ways to develop natural gas from methane hydrates, and supporting deployment of natural gas-powered alternative fuel vehicles, which are ideal for high-mileage, centrally fueled fleets that operate within a limited area.
Renewable natural gas (RNG) is a pipeline-quality gas that is fully interchangeable with conventional natural gas and therefore can be used in natural gas vehicles. RNG, also known as biomethane, is natural gas produced by the decomposition of organic matter under anaerobic (oxygen-free) conditions. RNG is essentially biogas 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).
Biomethane, which is another term for this purified pipeline-quality fuel, refers to biogas that has also been cleaned and conditioned to remove or reduce non-methane components such as water, carbon dioxide and hydrogen sulfide. This processed biogas is used as a replacement for traditional natural gas to generate combined electricity and heating for power plants—not in vehicle applications.
Major sources of RNG are landfills, animal manure, and solid waste extracted during wastewater treatment. The term “renewable” is used to describe this natural gas because it is derived from waste that is continuously produced by present-day activities. These waste sources naturally produce methane as they decompose, so RNG production captures methane that would otherwise escape into the atmosphere.
The National Renewable Energy Laboratory estimates that the United States could generate up to 420 billion cubic feet of renewable natural gas per year from waste sources, or around 1.5% of current U.S. natural gas consumption.
Renewable natural gas can also be generated from a range of plant materials and a range of other processes that are currently being developed.
Basically, shale gas is natural gas – primarily methane – found in shale formations, some of which were formed 300-million-to-400-million years ago during the Devonian period of Earth’s history. The shales were deposited as fine silt and clay particles at the bottom of relatively enclosed bodies of water. At roughly the same time, primitive plants were forming forests on land and the first amphibians were appearing. Some of the methane that formed from the organic matter buried with the sediments escaped into sandy rock layers adjacent to the shales, forming conventional accumulations of natural gas which are relatively easy to extract. But some of it remained locked in the tight, low permeability shale layers, becoming shale gas.