Boiler fuels Fuels are used to heat water or produce steam from boilers . The common fuels fall into one of four categories are:- Solid Liquid Gaseous Electricity The first three are used most commonly to fire boilers and are composed mainly of carbon and hydrogen with small amounts of sulphur and incombustible material. All the three fuels are easily available for industrial , commercial and domestic use. Electricity may also be used as a second generation fuel and is used to heat small steam and hot water boilers where emissions are prohibited.