A fuel-air or fuel-oxygen mixture is partially combusted resulting in a hydrogen rich syngas. Most fossil fuels are combusted with ambient air (as differentiated from combustion with pure oxygen). In the main combustion chamber, the propellants are combusted completely to produce thrust. The gases methane, hydrogen, and carbon monoxide () can be combusted or oxidized with oxygen. The particles and any combustible gases may be combusted in an "afterburner". Near the mines were gob piles that spontaneously combusted. Its energy density is however low reflecting the fact that it represents partially combusted methane. Cogeneration can be applied to any fuel which is combusted for energy. Natural gas is frequently combusted in gas turbines as well as boilers. More fuel is injected into this air where it is again combusted. Gas output may be combusted for electricity production. During industrial processes, the biomass combusted or processed re-releases the CO into the atmosphere. When the coal is combusted, it creates an alkaline dust. In some cases, the PCBs are combusted as a solution in kerosene. About of CO are produced when a gallon of pure ethanol is combusted.