Sulfur oxides (SO x) are primarily sulfur dioxide (SO2). Sulfur dioxide comes from the combustion of fuels with sulfur in them. Sulfur compounds occur naturally in crude oils and most fuels are derived from petroleum. Unless the sulfur is removed, the sulfur in the fuel will also burn, forming sulfur dioxide and other sulfur compounds. Sulfur burning in a fuel contributes to its energy output, but sulfur in a fuel is undesirable because it can cause corrosion of combustion equipment and because of the SO x emissions. Sulfur compounds can also be found in natural gas in the form of hydrogen sulfide (H 2S). Natural gas can be processed to remove sulfur compounds after it is recovered from a well.