The paper suggests the calculation method of combustion when solid and gaseous fuels are cofired. The calculation is suggested to carry out per unit of heat output released during combustion but not per unit of mass or volume of fuel. The method has been tested for evaluation of greenhouse gas emissions during combustion of different fuel types and during waste and landfill gas cofiring. During coal and waste combustion, the CO2 content in products of combustion is around 0,05 m3/MW, for natural gas combustion - 0,028 m3/MW which is 1,8 times lower than for coal firing. During biogas combustion, the carbon dioxide content in the combustion products depends on the methane concentration in biogas and is around 0,04...0,06 m3/MW. The increased CO2 emission during biogas combustion in comparison to natural gas is explained by a high carbon dioxide content in biogas itself. Having in mind that biogas creates a significant greenhouse effect due to methane presence in it the landfill gas utilization, which would have been released into the atmosphere anyway, leads to greenhouse gas emissions decrease by 0,6...1,9 m3/MW of CO2 equivalent.