Combustion is the rapid chemical combination of oxygen with the combustible elements of a fuel, resulting in the production of heat. Combustion is accomplished by mixing fuel and air at elevated temperatures. The air supplies oxygen, which unites chemically with the carbon, hydrogen, and a few minor elements in the fuel to produce heat.

Steam has been generated from the burning of a variety of fuels. In addition to the common fossil fuels of coal, oil, and natural gas, today an increased amount and varied supply of waste and by-product fuels are used, such as municipal solid waste (MSW), coal mine tailings, and biomass wastes such as vine clippings and bagasse, a sugar cane by-product. MSW also can have a large percentage of biomass because it contains yard waste. These fuels must be burned and their combustion products properly handled. They create a unique challenge in their use because the fuel quality is significantly reduced due to the fuel s lower heating value and poor combustion characteristics. In addition, such fuels often present more restrictive emission limitations. The designs of the boilers are therefore unique to the combustion of each of these fuels.