A cofired generator can operate on a mixture of fossil fuel and biogas. The fossil fraction (xfossil) is the ratio of fossil fuel used by the generator in dual-fuel mode to that required to produce the same output power in pure fossil mode. With compression-ignition (diesel) engines, it is necessary to maintain a minimum fossil fraction to ensure proper ignition.
For a more complete explanation of the fossil fraction and the operation of a cofired generator, see the article Operation of a Cofired Generator.
Generator Minimum Fossil Fraction