Combustion

views updated

COMBUSTION

"Combustion" is a term often used synonymously with "burning." However, a distinction can be made that explains why combustion is more than just burning. To burn something is to set it on fire. To combust something is to subject the material (or fuel) to the process of rapid oxidation that leads to the consumption of both the material (or fuel) and the oxidizer (usually the oxygen in air) with the release of heat and light. (Usually the oxidizer is oxygen but there can be nonoxygen species, that under certain circumstances fit the definition of an oxidizer being a substance that can accept electrons in a chemical reaction.) Fires and burning involve combustion, but not all combustion involves fire in the form of visible, hot flames. There are flames that are invisible but release heat, and there are flames that emit light but have so little evolution of heat that they are called "cool flames." By making this distinction between burning and combustion, many features of combustion such as ignition, extinction, and flames can each be discussed separately from a scientific perspective.

Combustion is the entire process by which something is oxidized. It is part of the use of gasoline or diesel fuel in automobiles and trucks, as well as part of propulsion in aircraft either in jet engines or propeller engines. This latter association is so often made that the propulsive devices in aircraft are called combustors. Similarly, furnaces and boilers, that often involve flames for the production of heat, are combustion devices involving many of the elements of the complete process. Incinerators, too, are commonly associated with combustion of fuel in the form of waste materials. Other common manifestations of combustion are house, forest, and chemical fires; explosions of flammables; and air pollution from cars and incinerators.

Because of the beneficial and adverse aspects of combustion, it is necessary to better understand it (i.e., all the components in the process of oxidizing a fuel with attendant heat and light).

COMBUSTION SCIENCE

Flames

A flame is a thin region of rapid, self sustaining oxidation of fuel that is often accompanied by the release of large amounts of heat and light. Flames are what we most commonly associate with combustion. One part of combustion science focuses on the different ways flames can be formed and the scientific and practical consequences of each.

Premixed Flame. For this type of flame, the fuel and oxidizer—both gases—are mixed together before flowing to the flame zone (the thin region of the flame). A typical example is the inner core of a Bunsen burner (Figure 1), or combustion in an automobile engine cylinder. (In general, a burner is the part of the combustion device that supplies fuel and sometimes air and where the flame is produced and stabilized.)

Diffusion Flame. When the fuel and oxidizer are initially unmixed and then mix in a thin region where the flame is located, the flame is called a diffusion flame (Figure 2). The word diffusion is used to describe the flame because the fuel and oxidizer are mixed on the molecular level by the random thermal motion of the molecules. An example of a diffusion flame is a candle flame or flares at an oil refinery.

Laminar Versus Turbulent Flames. Premixed and diffusion flames can be either laminar or turbulent gaseous flames. Laminar flames are those in which the gas flow is well behaved in the sense that the flow is unchanging in time at a given point (steady) and smooth without sudden disturbances. Laminar flow is often associated with slow flow from small diameter tubular burners. Turbulent flames are associated with highly time dependent flow patterns, often random, and are often associated with high velocity flows from large diameter tubular burners. Either type of flow—laminar or turbulent—can occur with both premixed and diffusion flames.

Droplets/Sprays. Flames can also be established with fuels that are initially liquids. A typical example is the flame around a droplet of hydrocarbon fuel such as diesel fuel. Droplets can burn individually, with a gaseous diffusion flame surrounding the evaporating liquid fuel center (Figure 3). When many droplets are combined into an array, a spray is formed. Burning of the droplets in the spray may consist of a continuous flame if the droplets completely evaporate or of isolated flames around each droplet if fuel evaporation is slow. An intermediate situation occurs when incomplete evaporation takes place. The types of flames formed depend on the fuel droplet sizes and spacing.

Liquid Pool Flames. Liquid fuel or flammable spills often lead to fires involving a flame at the surface of the liquid. This type of diffusion flame moves across the surface of the liquid driven by evaporation of the fuel through heat transfer ahead of the flame. If the liquid pool or spill is formed at ambient conditions sufficient to vaporize enough fuel to form a flammable air/fuel mixture, then a flame can propagate through the mixture above the spill as a premixed flame.

Solid Fuel Flames. The flames from the combustion of solids such as coal and wood are the result of a combination of processes including the burning of gases that have been released from the heated solid (devolatization) that burn in the gas phase as diffusion flames. The remaining nonvolatile material, the char, then is oxidized on its surface and in its pores as oxygen diffuses into the interior. If the particles are large, devolatization and char burning occur simultaneously.

Very small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning—devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume.

Other seemingly solid fuel flames such as those from the burning of plastics are actually more like liquid pool flames because the plastic melts and volatizes ahead of the advancing flame front.

Ignition

Even if a fuel and oxidizer are present in proportions that could sustain combustion, nothing may occur unless the combustible mixture is brought to the right conditions by an ignition source. Typical ignition sources are spark plugs in car engines, pilot flames in gas stoves, and matches for lighting barbecues. In order for an ignition source to be effective, it has to raise the temperature of the combustible mixture enough so that combustion can continue after the ignition source is removed. This means that the amount of heat added during the ignition process must be adequate to overcome any heat loss, to the engine walls for example, and still raise the temperature of the gas region to a value high enough to cause the flame to propagate.

Extinction

Once propagating, flames will continue to propagate unless they are extinguished or quenched. An obvious cause of extinction is the depletion or cessation of fuel flow. Flames can be extinguished by heat loss (e.g., by passage through very small passageways that accentuate heat loss), through smothering by water or chemical fire extinguishers that slow the combustion process, or blowing the flame away with high velocity flows. Flames can also be extinguished by removing one of the reactants, such as air.

Detonations

These are types of combustion waves (actually shock waves—extremely thin regions in which flow properties such as pressure and temperature change enormously—sustained by combustion) that consume fuel at supersonic speeds and create very large pressure and temperature increases. Detonations are formed only under special conditions that convert an ordinary flame propagating through a combustible mixture to a detonation. When these special conditions exist, then the detonation can have devastating consequences (e.g., explosions in buildings containing natural gas from leaks or in mines filled with natural gas as a result of the mining operation).

FUELS FOR COMBUSTION

Fuels for combustion are initially gases, liquids, or solids. A fuel initially in one phase may be transformed into another during the burning process (i.e., liquids vaporized to gases). The factors involved in the selection of the fuel phase or its physical and chemical characteristics for an application such as burning in an automobile or jet aircraft involve many different considerations such as price, availability, and source.

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel.

The combustion characteristics of liquid fuels are similarly determined by measures of their ability to sustain a flame. Two measures of the combustion characteristics of liquid fuels especially related to safety are flash point and autoignition temperature. The flash point is the maximum temperature at which a liquid fuel can be maintained in an open vessel exposed to air before which it will sustain a flame in the presence of a pilot flame. The autoignition temperature is a similar concept except no pilot flame is present. The autoignition temperature is the maximum temperature at which a liquid fuel can be maintained in an open vessel exposed to air before which the fuel bursts into flame without the presence of an external ignition source.

Solid fuels, unlike gases and liquids, are entirely characterized by their composition. For example, coal can be characterized by its carbon, hydrogen, oxygen, sulfur, and nitrogen content. The water and mineral content of coal are also important means of differentiating coals from various sources.

THE CHEMISTRY OF COMBUSTION AND ITS EFFECT ON THE ENVIRONMENT

The most commonly used fuels for combustion are hydrocarbons, materials that are compounds of only hydrogen and carbon. Occasionally, fuels such as alcohols, that contain oxygen, are burned. When hydrocarbon fuels with or without oxygen are burned in air (combusted) to completion, the products are water, from the hydrogen part of the fuel, and carbon dioxide, from the complete conversion of the carbon part. If oxygen is present in the fuel, it shows up in the final product as part of either the water or carbon dioxide.

Carbon dioxide has been implicated as a contributing factor in global warming. Increased global warming has been associated with increased release of carbon dioxide into the atmosphere attributed in part to an increase in the combustion of hydrocarbon fuels. Carbon dioxide is an inevitable consequence of the complete combustion of hydrocarbons in air. If combustion devices are made more efficient, less fuel is required and less carbon dioxide is released into the atmosphere.

Unlike carbon dioxide and water that are the inevitable by products of complete combustion of hydrocarbons, species such as carbon monoxide, ethene, toluene, and formaldehyde can be emitted because combustion has been interrupted before completion. Many factors lead to emissions from incomplete combustion. Emitted unburned hydrocarbons and carbon monoxide are regulated pollutants that must be eliminated. In automobiles with spark ignited engines, these emissions are almost entirely removed by the catalytic converter.

Soot particles, that are comprised primarily of carbon and hydrogen in an 8 to 1 ratio and are about 20-50 nm in diameter when first formed (coagulation and surface growth ultimately leads to a chain of soot spheres much larger than 50 nm), are the result of incomplete combustion of hydrocarbons. Soot, too, is a regulated combustion pollutant and is a particular problem with diesel engines. The black clouds emitted from the vertical exhaust stacks of trucks are laden with soot particles. The nature of diffusion flames precludes a practical way to reduce soot emissions in the diffusion flame processes that occur in diesel engines.

In contrast to carbon monoxide, small hydrocarbon molecules and soot that result from incomplete conversion of the hydrocarbon fuels, nitric oxide and nitrogen dioxide, are noxious emissions that result from the oxidizer—air. However, fuel components that contain nitrogen may also contribute, in a lesser way, to the formation of the oxides of nitrogen.

The nitrogen component of air, normally inert and unreactive, reacts at the very high temperatures of combustion. It reacts in a series of simple steps with atomic and molecular oxygen to yield NO, nitric oxide, that is subsequently converted to NO₂, nitrogen dioxide, in the atmosphere. Nitrogen dioxide, is a regulated, undesirable emission because it forms a brownish haze, leads to acid rain, and is a component of photochemical smog. Both nitric oxide and nitrogen dioxide can be considered inevitable byproducts of high-temperature combustion in air. The concentration of NO emitted into the atmosphere can be reduced either by lowering the temperature of combustion by various engineering techniques (with negative effects on performance) or through catalytic conversion to molecular nitrogen in postcombustion cleanup as is done in automobiles.

RESEARCH ACTIVITIES

The following synopsis of current research activities in the field of combustion is organized around the list of papers presented at the 27th International Symposium on Combustion (1998) under the auspices of The Combustion Institute.

Elementary Reaction Kinetics, Kinetic Mechanisms, Models, and Experiments

Examining the details involved in the oxidation and pyrolysis (thermal decomposition) of fuel molecules is very important. The results of these research activities will permit predictions about the chemicals emitted during incomplete combustion because reaction rate constants and chemical pathways will be evaluated and determined.

Laminar Premixed Flames

Research in this area focuses on understanding the chemical, thermal, and fluid-mechanical (behavior of fluids) structure of these types of flames. Recent advances in computer based modeled flames requires the knowledge developed in this type of research for calibration, validation, and prediction.

Laminar Diffusion Flames

Here, too, computer based predictions about the nature of these flames require information about the chemicals and science of diffusion flames for the predictions to be accurate. The predictions are made accurate by comparison with measured chemical species concentrations, measured temperatures, and flow characteristics.

Premixed Turbulent Combustion and Nonpremixed Turbulent Combustion

Because many practical flames are turbulent (spark ignited engine flames, oil field flares), an understanding of the interaction between the complex fluid dynamics of turbulence and the combustion processes is necessary to develop predictive computer models. Once these predictive models are developed, they are repeatedly compared with measurements of species, temperatures, and flow in actual flames for iterative refinement. If the model is deficient, it is changed and again compared with experiment. The process is repeated until a satisfactory predictive model is obtained.

Incineration, NO_x (NO and NO₂) Formation and Control, Soot Formation and Destruction

Environmental consequences of combustion are still a high priority requiring investigation of the chemistry and process effects on the emissions. Effective means of eliminating the pollutants is also a subject of further research.

Gas Turbines, Diesel Combustion, and Spark Ignition Engines

Application of combustion science to practical power source devices is one of the ultimate aims of developing a fundamental understanding of combustion. Using combustion science to improve performance through design changes and engineering techniques is an ongoing research subject.

Droplet and Spray Combustion and Pool Fires

The combustion of liquids is a fertile area for further study. Kowledge of the combustion science of individual droplets as well as groups of droplets helps improve performance of devices that rely on spray burning, particularly diesel engines. Understanding of the science of liquid pool fires potentially effects safety during spills.

High-Speed Combustion, Metals Combustion, and Propellants

Research in these specialized areas is aimed at developing improved and new methods for advanced propulsion. For example, an understanding of high-speed combustion is used in the development of supersonic ram jet engines, which are simple alternatives to conventional turbojet engines. Knowledge of metals combustion is relevant to improving the use of metal additives in solid propellants to increase impulse and stability. In general, the study of propellant combustion aids in the development of more stable and longer range rocket engine performance.

Catalytic and Materials Synthesis

These two research areas share the common characteristic of involving inorganic solids in the combustion process. Catalytic combustion research focuses on using the solid to facilitate the oxidation of well-known fuels such as hydrogen and methane. Materials synthesis research focuses on using combustion as a means to react the solids either with each other or a gas, such as nitrogen (which in this case acts as an oxidizer), to make new solid materials.

Microgravity Combustion

Microgravity refers to the environment of extremely low gravity commonly known as a weightless environment. Under microgravity conditions, combustion phenomena that are affected by gravity, such as flames, behave differently than at Earth gravity conditions. Research in this area focuses on using the special microgravity conditions to understand, by contrast, basic combustion processes on Earth.

Fire Safety Research

Research is conducted to increase understanding of the science of combustion specifically as it relates to fires involving homes and plastics, wood, and large-scale spills. This is helpful in the development of fire prevention and extinction techniques.

Detonations

Examining the conditions for the formation and propagation of detonation waves is relevant to special applications of detonations to propulsion as well as safety.

Coal and Char Combustion

The importance of coal as an energy source motivates further research into the combustion characteristics and chemical kinetics of both coal and the material that remains after devolatization, char. Further research will aid in making coal a cleaner and more efficient energy source.

Fluidized Beds, Porous Media Fixed Bed Combustion, and Furnaces.

Specialized practical configurations for combustion have a number of practical applications such as coal burning for energy production. The study of these specialized combustion setups is necessary for better application.

Kenneth Brezinsky

See also: Catalysts; Coal, Production of; Conservation of Energy; Explosives and Propellants; Heat Transfer.