Renewable energy options from biogas

Anaerobic digestion (AD) of dairy manure and other organic material (e.g., food waste) produces biogas that can be used for renewable energy options. The options in use today include generating electricity and heat using a combined heat and power (CHP) system and producing renewable natural gas (RNG). Other systems have been proposed to work with AD or on their own; these include hydrothermal liquefaction (HTL) to produce biocrude oil, pyrolysis to produce biochar, and pelletizing manure for combustion. CHP systems operating on biogas can use different prime movers, including the common reciprocating internal combustion engine, a gas turbine (e.g., microturbine), and a fuel cell. Each of these requires a level of raw biogas conditioning but can operate on the natural biogas composition of approximately 50 to 60 percent methane and 40 to 50 percent carbon dioxide that an AD produces. RNG requires more extensive biogas conditioning and compression to produce a product gas containing typically greater than 97 percent methane for use in compressed natural gas (CNG) vehicles or in place of pipeline natural gas