Introduction
Biogas generated from the anaerobic digestion (AD) of livestock manure is a reliable source of renewable energy and is most commonly used to generate electricity (US EPA AgSTAR Database 2021). In more recent years, biogas has increasingly been refined and used as a “drop in” fuel to replace natural gas in transportation applications. Financial incentives from multiple credit programs operated by the US federal government (e.g., Renewable Fuel Standard [RFS]) and by states like California, Oregon, and Washington that have clean fuels standards (also called low carbon fuels standards) are driving this trend.
When biogas is upgraded to biomethane and then used directly to replace fossil fuel natural gas (FFNG), it is commonly referred to as renewable natural gas (RNG) (US EPA 2021). Digester gas that is used in a cogeneration engine must be cleaned or “scrubbed” of some of its impurities to avoid damage to downstream mechanical equipment. However, upgrading biogas to RNG for transportation fuel via pipeline injection or direct vehicle fueling requires a more significant purification process. This publication will explore the differences between biogas and RNG, the impurities that must be removed, and the technological approaches currently available to refine and upgrade raw digester gas to fuel quality.