combustion(redirected from Combustion theory)
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combustion,rapid chemical reaction of two or more substances with a characteristic liberation of heat and light; it is commonly called burning. The burning of a fuel (e.g., wood, coal, oil, or natural gas) in air is a familiar example of combustion. Combustion need not involve oxygen; e.g., hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion. Combustion reactions involve oxidation and reductionoxidation and reduction,
complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation
..... Click the link for more information. . Before a substance will burn, it must be heated to its ignition point, or kindling temperature. Pure substances have characteristic ignition points. Although the ignition point of a substance is essentially constant, the time needed for burning to begin depends on such factors as the form of the substance and the amount of oxygen in the air. A finely divided substance is more readily ignited than a massive one; e.g., sawdust ignites more rapidly than does a log. The vapors of a volatile fuel such as gasoline are more readily ignited than is the fuel itself. The rate of combustion is also affected by these factors, particularly by the amount of oxygen in the air. The nature of combustion was not always clearly understood. The ancient Greeks believed fire to be a basic element of the universe. It was not until 1774 that the French chemist A. L. LavoisierLavoisier, Antoine Laurent
, 1743–94, French chemist and physicist, a founder of modern chemistry. He studied under eminent men of his day, won early recognition, and was admitted to the Academy of Sciences in 1768.
..... Click the link for more information. performed experiments that led to the modern understanding of the nature of combustion. See spontaneous combustionspontaneous combustion,
phenomenon in which a substance unexpectedly bursts into flame without apparent cause. In ordinary combustion, a substance is deliberately heated to its ignition point to make it burn.
..... Click the link for more information. ; heat of combustionheat of combustion,
heat released during combustion. In particular, it is the amount of heat released when a given amount (usually 1 mole) of a combustible pure substance is burned to form incombustible products (e.g.
..... Click the link for more information. .
See C. J. Hilado, Smoke and Products of Combustion (1973); W. C. Gardiner, ed., Combustion Chemistry (1984); F. A. Williams, Combustion Theory (2d ed. 1985).
a complex, rapid chemical transformation that is accompanied by the evolution of a considerable amount of heat and usually by strong luminescence (flame). In most cases it is based on exothermic oxidation reactions of a combustible substance (fuel) with an oxidizing agent. Modern physicochemical theory classifies as combustion all chemical processes associated with rapid transformation and with their acceleration by heat or diffusion, including the decomposition of explosives and ozone, the combination of a number of substances with chlorine and fluorine, and the reaction of many metals with chlorine and of the oxides of sodium and barium with carbon dioxide.
In most cases the chemical reaction of combustion is complex, that is, it consists of a large number of elementary chemical processes. In addition, the chemical transformation during combustion is closely associated with a number of physical processes, such as heat and mass transfer, and is characterized by the corresponding laws of hydrodynamics and gas dynamics. Because of the complex nature of combustion, its overall speed is virtually never the same as the speed of the purely chemical reaction of the system’s reagents; in addition, in the case of heterogeneous processes the speed of combustion often equals the speed of some limiting, purely physical process (vaporization, diffusion, and so on).
The most common property of combustion is the possibility under certain conditions of progressive self-acceleration of the chemical transformation—ignition—which is associated with the accumulation in the reacting system of heat or of the active products of a chain reaction. A characteristic of combustion is its capacity for propagation in space through the transmission of heat or the diffusion of active particles; in the first case the mechanism of flame propagation is thermal, and in the second it is by diffusion. The existence of critical conditions—that is, definite regions of parameter values (mixture composition, pressure, content of impurities, and initial temperature of the mixture) that are characteristic of the particular combustion system outside of which combustion proceeds steadily and inside which it is autoaccelera-ting—is also characteristic of combustion.
The diffusion mechanism of combustion is usually observed at low pressures. Combustion is extensively used in technology to produce heat in furnaces, ovens, and the combustion chambers of motors. So-called diffusion combustion, in which the flame diffusion is brought about by mutual conductive or turbulent diffusion of the fuel and an oxidizing agent, is frequently used.
Two typical stages are characteristic of any form of combustion: ignition and subsequent combustion (afterburning) of the substance to products of complete combustion. The time spent in both stages is the total time of combustion. The main problem in combustion technology is the attainment of the minimum total combustion time with the fullest possible combustion (the most complete heat evolution). In industrial combustion, the physical processes of preparation of the mixture (vaporization, mixing, and so on) are important. The main thermodynamic characteristics of a fuel mixture are its calorific power and theoretical (or adiabatic) combustion temperature, that is, the temperature that could be attained upon complete combustion without loss of heat.
Depending on the state of aggregation of the fuel and oxidizing agent, a distinction is made among homogeneous combustion, which is the combustion of gases and vaporized fuels in a gaseous oxidizing medium (usually atmospheric oxygen); combustion of explosive substances and powders; and heterogeneous combustion, which is the combustion of liquid and solid fuels in a gaseous oxidizing medium or combustion in a system with a liquid fuel mixture and a liquid oxidizing agent (for example, acid).
Homogeneous combustion. The simplest case of homogeneous combustion is the combustion of prepared mixtures. The reactions are mostly chain reactions. Under ordinary conditions of combustion, the decisive factor in the development of chain reactions (chain initiation and propagation) is the preheating of the substance (thermal activation).
An initial energy impulse, most frequently heating of the fuel, is necessary to initiate combustion. Two methods of ignition—self-ignition and forced ignition, or inflammation (by an incandescent body, flame, or electric spark)—are distinguished.
The most important problem in the theory of combustion is the propagation of the flame (a zone of sharp temperature rise and intense reaction). A distinction is made between normal flame propagation, or deflagration, in which the main process is heat transfer, and detonation, in which a shock wave causes ignition. Normal combustion is subdivided into laminar and turbulent types.
A laminar flame has a definite translational velocity with respect to the stationary gas; the velocity depends on the mixture composition, pressure, and temperature and is determined only by chemical kinetics and molecular heat transfer. This normal speed is a physicochemical constant for a mixture.
The rate of propagation of a turbulent flame depends on the flow rate, as well as on the degree and extent of turbulence.
Combustion in a flow (flare process)—the combustion of a flow when it issues from a tube (nozzle) into an open space or chamber—is a form of combustion that is very widespread in technology. A distinction is made between the combustion of a previously prepared mixture and of a fuel and an oxidizing agent that are flowing out separately, where the process is conditioned by mixing (diffusion) of the two flows.
The question of maintenance of the flame at the burner or in the chamber is of great practical importance under conditions of flow combustion. The problem is usually solved either by continuous ignition of the mixture by a special device or by inserting perpendicular to the flow poorly streamlined bodies (stabilizing screens), which provide reverse circulation of the hot products of combustion.
Combustion of explosives. The combustion of explosives is the self-propagation of a zone of an exothermic chemical reaction of decomposition of the explosive or the interaction of its components by the transmission from layer to layer of the energy of reaction in the form of heat. When the gaseous products of combustion can flow away freely from the hot charge, the combustion of the explosives, unlike their detonation, is usually not accompanied by any pressure rise and does not take on the character of an explosion. Condensed explosives, like mixtures of gaseous fuels and oxidizing agents, do not require an external supply of oxygen.
The speed of combustion depends on the nature of the explosive, as well as on pressure, temperature, charge density, and other factors; at atmospheric pressure, it varies from a fraction of a millimeter to several meters per second, depending on the explosive. With detonating powders it is generally tens and hundreds of times greater than for secondary explosives.
Heterogeneous combustion. The vaporization process is very important for the combustion of liquids. The combustion of readily volatile fuels is in practice relevant to homogeneous combustion since, even before combustion, such fuels are completely or almost completely evaporated. Two properties applicable to liquid fuels are distinguished: the flash point and the ordinary self-ignition temperature.
The highly dispersed drop system, for which the laws of ignition and combustion of each separate drop are of decisive importance, is a widespread heterogeneous liquid system. Unlike the case of homogeneous combustion, the ignition stage plays a relatively small part.
In the simplest case the combustion of solids is not accompanied by decomposition of the substance and the evolution of its volatiles (for example, the combustion of metals). The combustion of solid fuels, mainly coal, that consist of carbon with a certain amount of organic substances, which decompose when the fuel is heated and separate as vapor and gas, is of great importance in technology. The thermally unstable part of the fuel is called volatile, and the gases are called volatiles. If the fuel particles are heated rapidly, which is possible with small particles, the volatiles may not be liberated and may burn along with the carbon. Upon slow heating a pronounced change in the initial stage of combustion is observed: at first the volatiles separate and ignite, and then there is ignition and combustion of the solid, so-called coke, residue, which in addition to carbon contains the inorganic part of the fuel (ash).
The catalytic or, more correctly, surface catalytic combustion of gas mixtures belongs to the class of homogeneous-heterogeneous combustion processes: the chemical process can take place both within the volume and on the catalyzing solid surface (for example, platinum). Depending on the actual conditions, the homogeneous or heterogeneous type of combustion can appear. At high temperatures, when volume combustion is rapid, the part played by surface-catalyzed combustion is usually small and can only be appreciable if the mixture flows in small channels, porous materials, or small-particle catalyst charges. The term “flameless combustion” of gas mixtures, which is used in technology, is not always the same as the idea of surface catalytic combustion; rather, it is characteristic of combustion without a luminous flame.
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The burning of any substance, in gaseous, liquid, or solid form. In its broad definition, combustion includes fast exothermic chemical reactions, generally in the gas phase but not excluding the reaction of solid carbon with a gaseous oxidant. Flames represent combustion reactions that can propagate through space at subsonic velocity and are accompanied by the emission of light. The flame is the result of complex interactions of chemical and physical processes whose quantitative description must draw on a wide range of disciplines, such as chemistry, thermodynamics, fluid dynamics, and molecular physics. In the course of the chemical reaction, energy is released in the form of heat, and atoms and free radicals, all highly reactive intermediates of the combustion reactions, are generated.
The physical processes involved in combustion are primarily transport processes: transport of mass and energy and, in systems with flow of the reactants, transport of momentum. The reactants in the chemical reaction are normally a fuel and an oxidant. In practical combustion systems the chemical reactions of the major chemical species, carbon and hydrogen in the fuel and oxygen in the air, are fast at the prevailing high temperatures (greater than 1200 K or 1700°F) because the reaction rates increase exponentially with temperature. In contrast, the rates of the transport processes exhibit much smaller dependence on temperature are, therefore, lower than those of the chemical reactions. Thus in most practical flames the rate of evolution of the main combustion products, carbon dioxide and water, and the accompanying heat release depends on the rates at which the reactants are mixed and heat is being transferred from the flame to the fresh fuel-oxidant mixture injected into the flame. However, this generalization cannot be extended to the production and destruction of minor species in the flame, including those of trace concentrations of air pollutants such as nitrogen oxides, polycyclic aromatic hydrocarbons, soot, carbon monoxide, and submicrometer-size inorganic particulate matter.
Combustion applications are wide ranging with respect to the fields in which they are used and to their thermal input, extending from a few watts for a candle to hundreds of megawatts for a utility boiler. Combustion is the major mode of fuel utilization in domestic and industrial heating, in production of steam for industrial processes and for electric power generation, in waste incineration, and in propulsion in internal combustion engines, gas turbines, or rocket engines.