Methane (CH₄) is produced in small quantities from fuel combustion due to incomplete combustion of hydrocarbons in fuel. CH₄ emissions are usually an indication of inefficiency in the combustion process. The production of CH₄ is dependent on the temperature in the boiler/kiln/stove. In large efficient combustion facilities and industrial applications, the CH₄ emission rate is very low. In smaller combustion sources, emission rates are often higher, particularly when smoldering occurs. The highest rates of CH₄ emissions from fuel combustion occur in residential applications (small stoves and open burning).

CH₄ emissions from mobile sources are a function of the CH₄ content of the motor fuel, the amount of hydrocarbons passing unburnt through the engine, the engine type, and any post-combustion controls. In vehicles without emission controls the amount of CH₄ emitted is highest at low speeds and when the engine is idle. Poorly tuned engines may have a particularly high output of CH₄.

Table 1-7. IPCC CH₄ Default Emission Factors