supplying recovered methane gas having a methane concentration of 30-50% and ventilated methane gas having a methane concentration of 0.3%-0.7% from a coal mine;

generating electric power with an apparatus including an electric power generator coupled to a pilot fuel oil ignition type gas engine having at least one cylinder and a cylinder pressure detector;

diagnosing a combustion condition within the gas engine in response to a cylinder pressure detector signal using a combustion diagnosis apparatus;

adjusting a fuel mixture comprising the recovered methane gas and the ventilated methane gas in the gas engine, using a combustion controller, in response to a combustion condition signal from the combustion diagnosis apparatus;

introducing the fuel mixture into the cylinder using a gas injection device while mixing the recovered methane gas and the ventilated methane gas to define a lean methane/air mixture having a methane concentration of 3-5% and having an air excess ratio not less than 2, such that the gas engine operates to produce electric power; and

registering a carbon dioxide emissions credit for carbon dioxide produced from the generator, defined as a difference in a greenhouse effect index between greenhouse producing gases, on a credit market for trading, wherein the greenhouse producing gases are coal mine methane gas released to the atmosphere and combusted methane gas released to the atmosphere as carbon dioxide.