HOMER Grid 1.9

The biomasss substitution ration is the ratio with which the biogas replaces fossil fuel in a cofired generator. If 8 kg/hr of biogas is required to replace 1 kg/hr of fossil fuel to maintain output power, the substitution ratio is 8. This ratio is assumed to be constant, independent of fuel mixture or output power.

If the fossil fuel burned by the generator is gaseous, the substitution ratio is roughly equal to the ratio of the lower heating values of the fossil fuel and biogas. For example, if the cofired generator runs on a mixture of propane and biogas and the LHV of the biogas is one-third that of propane, the substitution ratio is probably approximately 3.

For liquid fossil fuels, the substitution ratio is typically somewhat higher than the LHV ratio because the generator does not operate as efficiently on the mixture of biogas and liquid fuel as it does on liquid fuel alone.

For more information, see the Operation of a Cofired Generator section of the help.