What is Renewable Natural Gas (RNG)?
An overview of the production of renewable natural gas provides a simple insight into how this sustainable fuel source can play an important role in our present and future renewable energy mix. As the only type of renewable fossil fuel, RNG can help power the adoption of solar, wind, and other renewables by providing reliable on-call backup to meet peak demand on the grid.
Renewable natural gas (RNG) is a pipeline-quality gas that is fully interchangeable with conventional natural gas in any and all applications. Produced from methane waste, RNG is a near-net-zero carbon fuel.
Biogas to RNG
As organic matter decomposes, a gaseous byproduct is emitted. This byproduct, known as biogas, offers the same energy composition and profile as conventional natural gas (CNG) – however, just like CNG, biogas must first be purified and processed before it can be injected into the natural gas distribution system.
The conditioning process removes moisture and impurities, along with CO2, sulfur, inert gases, and any propane or butane if present. Once purified, biogas is recharacterized as biomethane or RNG. Note that use in vehicles requires a more refined conditioning process.
The biochemical process that produces biogas can include anaerobic digestion or gasification via thermochemistry, and almost any biomass can be a source of biogas in sufficient quantity. Here are some of the most common biomasses used in large-scale renewable natural gas production today.
According to the U.S. Environmental Protection Agency, landfills are the third-largest source of human-created methane emissions in the United States. As of June 2020, there were over 560 operational projects to capture and purify biogas at landfills in the country, mostly with the aim of using the RNG to power electricity generation.
Landfills are unique in that the construction of an anaerobic digester is not necessary. The digestion process takes place underground.
Animal manure is a major source of biogas that can easily be collected and delivered to an anaerobic digester for optimal methane production. There are hundreds of digesters operating at commercial livestock farms in the United States with future growth predicted as more natural gas-powered farm equipment comes to market.
Wastewater Treatment Plants
The EPA estimates that the solids removed from 100 gallons of wastewater have the potential to produce approximately one cubic foot of biogas. A study by the National Association of Clean Water Agencies and the Water Environment Federation found that U.S. wastewater alone has the potential to meet up to 12% of the country’s total electricity demand.
- Out of 16,000+ wastewater treatment plants in the U.S., only 1,300 currently employ anaerobic digestion to produce biogas for onsite use. This presents a tremendous opportunity for wastewater biogas capture to grow in the coming decade.
As collection methodology improves and more digesters are built, RNG production could extend to a wide range of organic waste sources, including food manufacturers and wholesalers, supermarkets, restaurants, hospitals, industrial or commercial entities, and educational facilities. Crop residue and wood byproducts also serve as good candidates for digestion.
RNG is in its infancy. As our world moves forward, renewable natural gas will play a much larger role in the renewables landscape.