Combustion is the process of rapid oxidation of a substance accompanied by the simultaneous evolution of heat and, usually, light. In the combustion of common fuels, the process is a chemical reaction with atmospheric oxygen. The principal products of the reaction are carbon monoxide, carbon dioxide and water. Small amounts of other compounds such as sulfur dioxide may also be generated by minor constituents of the fuel.
Most combustion processes release energy, or heat, usually for the purpose of generating power. Examples include stationary power plants for the production of electricity or an engine for driving automotive machines. The partial combustion of a solid fuel, such as coal, produces a gaseous fuel, which is usually more convenient to use. Combustion is also a method of producing an oxidation product, for example, the burning of sulfur to produce sulfur dioxide.
The energy released by combustion causes an increase in the temperatures of its products. The degree of temperature increase is dependent upon the rate at which energy is released and dissipated, as well as the amount of combustion products. Air is the least expensive source of oxygen, but since it is composed of three-fourths nitrogen by weight, nitrogen is a major constituent of its reaction products. This results in a substantially lower temperature increase than if pure oxygen were used. Theoretically, for complete combustion to occur there is a minimum ratio of air to fuel required. Increasing the amount of air will result in more complete combustion and maximize the energy released. However, an excess of air will reduce the temperature of the products and the amount of energy liberated. Therefore, depending on the desired extent of the combustion and final temperature, there is almost always optimum air-to-fuel ratio.
In order of potential heat, the most common solid fuels are coal, coke, wood, sugarcane bagasse and peat. The combustion of these results in the decomposition of the fuel and the evolution of a volatile gas. The rate at which oxygen diffuses to the surface determines how quickly the solid carbonaceous residue burns. A surface temperature in the range of 4,000° C to 8,000° C is required for this combustion to occur. These temperatures are usually produced by radiation of heat from the hot products or from hot surroundings. In cases where the fuel is burned on a grate, air is forced through a bed of solid fuel particles and inter-particle radiation maintains the required temperature. To achieve more rapid combustion, powdered coal is often mixed with air and blown into the furnace. Air-to-fuel ratios are dependent on the type of fuel. One pound of bituminous coal requires 11 pounds of air for complete combustion. Powdered coals produce flame temperatures of the magnitude of 15,400° C.
The most common liquid fuels include fuel oils, gasoline and naphthas derived from petroleum. To a lesser extent, coal tar, alcohols and benzol derived from coke are also used as fuel. Less volatile fuel oils are sprayed into the combustion chamber of stationary furnaces. The more volatile fuels such as gasoline are used in internal combustion engines. The gasoline is evaporated and introduced into the engine cylinder where combustion is initiated by a spark. If an explosively rapid combustion occurs, pressure fluctuations within the cylinder may produce a “knocking” sound. This is often controlled by mixing a catalyst such as tetraethyl lead with the fuel. These fuels require 16 to 23 pounds of air per pound of fuel for complete combustion. In a diesel engine, fuel is injected into the combustion chamber as an atomized spray. The high compression ratio of the diesel engine causes the temperature to rise sufficiently to cause ignition.
Gaseous fuels, including natural gas, refining gas and manufactured gas are typically premixed with air prior to combustion. This provides the maximum amount of oxygen to the fuel. To prevent flashback into the burner, the fuel-air mixture is issued from the burner ports at a velocity greater than that of the flame's propagation, but not so rapidly that the flame is blown off the burner. Unless they are premixed with air, these fuels burn with relatively cool flames and produce a substantial amount of smoke. When burned with air, natural gas is capable of producing flame temperatures exceeding 19,300° C.