Sulfur may be present in various fossil fuels at varying levels of concentration and in various forms. Sulfur is found in fossil fuels in elemental form, in molecular form attached to hydrocarbons and as hydrogen sulfide gas (H2S).
Combustion of sulfur contained in fossil fuels generates pollutants such as sulfur dioxide. Environmental regulations at various locations have specified different limits on the sulfur content on marketed fuels. Also, processing of high sulfur containing fossil fuels into end product fuels is a costly affair due to corrosive action of sulfur on processing equipments. Therefore, low sulfur containing fossil fuels are usually in high demand and fetch a high price.
Sulfur content is usually measured in weight percentage (wt %) or in parts per million by weight (ppm wt). Crude oil containing less than 0.5 wt % of sulfur is considered to have low sulfur content and is known as 'sweet crude oil'. Higher sulfur content than 0.5 wt % results in crude oil being known as 'sour crude oil'.
Most of this sulfur content in crude oil occurs in molecular form attache to the hydrocarbons. Small amount is contained as elemental sulfur and in the form of hydrogen sulfide gas.
Usually the sulfur in the form of hydrogen sulfide is separated from hydrocarbons and recovered as elemental sulfur in the sulfur recovery unit (SRU) of a refinery. This sulfur is then sold by the refiners to fertilizer manufacturers.
Coal containing less than 1 wt % of sulfur is considered low sulfur containing fuel, higher than 1 wt % sulfur concentration is considered high for coal.