Combustion

I

Introduction

Combustion, process of rapid oxidation of a substance with simultaneous evolution of heat and, usually, light. In the case of common fuels, the process is one of chemical combination with atmospheric oxygen to produce as the principal products carbon dioxide, carbon monoxide, and water, together with products such as sulphur dioxide that may be generated by the minor constituents of the fuel (see Chemical Reaction). The term combustion, however, also embraces oxidation in the broad chemical sense, and the oxidizing agent may be nitric acid, certain perchlorates, or even chlorine or fluorine. See separate articles on most of the fuels and chemicals mentioned in this article.

II

Energy Release

Most combustion processes release energy, or heat, for the production of power, for use in industrial processes, and for domestic heating and lighting. Combustion is also a means of producing a desired oxidized product, as in the burning of sulphur to produce sulphur dioxide and ultimately sulphuric acid. In addition, it is a method for disposing of wastes.

The energy released by combustion causes a rise of temperature in the products of combustion. The temperature attained depends on the rate of release and dissipation of the energy and the quantity of combustion products. Air is the cheapest source of oxygen, but because air is three-quarters nitrogen by weight, nitrogen becomes the major constituent in the products of combustion, and the rise in temperature is substantially less than if pure oxygen were used. Theoretically, in any combustion, a minimum ratio of air to fuel is required for complete combustion. The combustion, however, can be made more readily complete, and the energy released maximized, by increasing the amount of air. An excess of air, however, reduces the ultimate temperature of the products and the amount of the released energy. Therefore, an optimum air-to-fuel ratio can almost always be determined, depending on the rate and extent of combustion and the final temperature desired. Air with enriched oxygen content or pure oxygen, as in the case of the oxyacetylene torch, may be used to produce high temperatures (see Acetylene). The rate of combustion may be increased by finely dividing the fuel to increase its surface area and hence its rate of reaction, and by mixing it with the air to provide the necessary amount of oxygen to the fuel. If energy must be released extremely fast, as in the case of rockets, the oxidizer may be incorporated directly into the fuel during its manufacture.

III

Solid Fuels

The common solid fuels, in order of heat potential, are coal, coke, wood, sugar cane bagasse, and peat. Combustion of these causes decomposition of the fuel and evolution of the volatile matter as a gas that may burn with a sooty flame. The solid carbonaceous residue burns at a rate determined by the diffusion of oxygen to the surface. This latter combustion requires a higher surface temperature, about 400° to 800° C (752° to 1472° F), which is obtained by radiation of heat from the hot products or from hot surroundings. If the fuel is burned on a grate, air is forced through a bed of solid fuel particles, and the necessary temperature is maintained by interparticle radiation. For more rapid combustion, coal is frequently powdered, mixed with air, and blown into a furnace. Air-to-fuel ratios depend mainly on the type of fuel, 1 kg of a typical bituminous coal requiring a minimum of 11 kg of air for complete combustion; flame temperatures for powdered coals may be of the order of 1540° C (2800° F). If finely powdered coal (coal dust) or any other hydrocarbon is ignited in uncontrolled conditions, the combustion occurs extremely rapidly, approaching that of an explosion. Mine fires caused by coal dust and household fires caused by the combustion of floor dust are examples of rapid combustion processes. See Mining.

IV

Liquid and Gaseous Fuels

Common liquid fuels are fuel oils, petrol, and naphthas derived from petroleum, and, to a lesser extent, coal tar, alcohol, and benzol obtained from coke manufacture. In stationary furnaces, less volatile fuel oils are sprayed through nozzles, with or without air or steam, into the combustion chamber. In an internal-combustion engine, volatile fuels such as petrol or a petrol and alcohol mixture (gasohol) are evaporated and the mixture admitted into the engine cylinder, where combustion is initiated by a spark. In these fuels, from 16 to 23 kg of air are required for complete combustion of 1 kg of fuel. In diesel engines the fuel is injected as an atomized spray into the combustion chamber, where the temperature rise associated with the high compression ratio of diesel engines is sufficient to cause ignition.

Gaseous fuels such as natural gas, refinery gas, and manufactured gases such as producer gas are usually mixed with air before combustion to supply a maximum amount of oxygen to the fuel. The fuel-air mixture then issues from the burner ports at a velocity greater than the velocity of flame propagation to prevent flame flashback into the burner, but not so great a velocity as to blow the flame off the burner. If not premixed with air, these fuels usually burn with smoky, relatively cool flames. Natural gas burned with air can produce flame temperatures in excess of 1930° C (3500° F).

Rockets for space exploration may use liquid fuels such as kerosene and hydrazine, and carry an oxidizer such as liquid oxygen, nitric acid, or hydrogen peroxide. Military rockets such as bazookas use solid fuels, such as cordite, with oxygen incorporated into the fuel; these burn spontaneously on being heated by radiation from the products of combustion.