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.

BIBLIOGRAPHY

Borman, G. L., and Ragland, K. W. (1998). Combustion Engineering. Boston: McGraw-Hill.

Chomiak, J. (1990). Combustion: A Study in Theory, Fact, and Application. New York: Abacus Press.

Combustion and Flame(series). New York: Elsevier.

Combustion Science and Technology (series). New York: Gordon and Breach.

Glassman, I. (1996). Combustion, 3rd ed. San Diego, CA: Academic Press.

International Symposium on Combustion (series). Pittsburgh: The Combustion Institute.

Progress in Energy Combustion Science(series). New York: Pergamon.

Turns, S. R. (2000). An Introduction to Combustion, 2nd ed. Boston: McGraw-Hill.

Williams, F. A. (1985). Combustion Theory, 2nd ed. Menlo Park, CA: Benjamin/Cummings.

Combustion

History

Modern theory

Combustion mechanics

Applications

Environmental issues

Resources

Combustion is the technical term for burning. It is one of the earliest chemical changes noted by humans, partly because of the dramatic effects it has on materials. Today, the mechanism by which combustion takes place is well understood. Combustion is oxidation that occurs so rapidly that noticeable heat and light are produced.

History

Probably the earliest reasonably scientific attempt to explain combustion was that of Johannes (or Jan) Baptista van Helmont, a Flemish physician and alchemist who lived from 1580 to 1644. Van Helmont observed the relationship among a burning material, smoke and flame and said that combustion involved the escape of a “wildspirit” (spiritus silvestre ) from the burning material. This explanation was later incorporated into a theory of combustion—the phlogiston theory—that dominated alchemical thinking for the better part of two centuries.

According to the phlogiston theory, combustible materials contain a substance—phlogiston—that is emitted by the material as it burns. A non-combustible material, such as ashes, will not burn, according to this theory, because all phlogiston contained in the original material (such as wood) had been driven out. The phlogiston theory was developed primarily by the German alchemist Johann Becher (1635–1682) and his student Georg Ernst Stahl (1659–1734) at the end of the seventeenth century.

Although scoffed at today, the phlogiston theory satisfactorily explained most combustion phenomena known at the time of Becher and Stahl. One serious problem was a quantitative issue. Many objects weigh more after being burned than before. How this could happen when phlogiston escaped from the burning material? One possible explanation was that phlogiston had negative weight, an idea that many early chemists thought absurd, while others were willing to consider. In any case, precise measurements had not yet become an important feature of chemical studies, so loss of weight was not an insurmountable barrier to the phlogiston concept.

Modern theory

As with so many other instances in science, the phlogiston theory fell into disrepute only when someone appeared on the scene who could reject traditional thinking almost entirely and propose a radically new view of the phenomenon. That person was the great Frenchchemist Antoine Laurent Lavoisier (1743–1794). Having knowledge of some recent critical discoveries in chemistry, especially the discovery of oxygen by Karl Wilhelm Scheele (1742–1786) in 1771 and Joseph Priestley (1733–1804) in 1774, Lavoisier framed a new definition of combustion. Combustion, he said, is the process by which some material combines with oxygen. By making the best use of precise quantitative experiments, Lavoisier provided such a sound basis for his new theory that it was widely accepted in a relatively short period of time.

Lavoisier initiated another important line of research related to combustion, one involving the amount of heat generated during oxidation. His earliest experiments involved the study of heat lost by a guinea pig during respiration, which Lavoisier called “acombustion.” In this work, he was assisted by a second famous Frenchscientist, Pierre Simon Laplace (1749–1827). As a result of their research, Lavoisier and Laplace laid down one of the fundamental principles of thermochemistry, namely that the amount of heat needed to decompose a compound is the same as the amount of heat liberated during its formation from its elements. This line of research was later developed by the Swiss-Russian chemist Henri Hess(1802–1850) in the 1830s. Hess’s development and extension of the work of Lavoisier and Laplace has earned him the title offather of thermochemistry.

Combustion mechanics

From a chemical standpoint, combustion is a process in which chemical bonds are broken and new chemical bonds formed. The net result of these changes is a release of energy, the heat of combustion. For example, suppose that a gram of coal is burned in pure oxygen with the formation of carbon dioxide as the only product. In this reaction, the first step is the destruction of bonds between carbon atoms and between oxygen atoms. In order for this step to occur, energy must be added to the coal/oxygen mixture. For example, a lighted match must be touched to the coal.

Once the carbon-carbon and oxygen-oxygen bonds have been broken, new bonds between carbon atoms and oxygen atoms can be formed. These bonds contain less energy than did the original carbon-carbon and oxygen-oxygen bonds. That energy is released in the form of heat, the heat of combustion. The heat of combustion of one mole of carbon, for example, is about 94 kcal.

Applications

Humans have been making practical use of combustion for millennia. Cooking food and heating homes have long been two major applications of the combustion reaction. With the development of the steam engine by Denis Papin, Thomas Savery, Thomas Newcomen, and others at the beginning of the eighteenth century, however, a new use for combustion was found: performing work. Those first engines employed the combustion of some material, usually coal, to produce heat that was used to boil water. The steam produced was then able to move pistons and drive machinery. That concept is essentially the same one used today to operate fossil-fueled electrical power plants.

Before long, inventors found ways to use steam engines in transportation, especially in railroad engines and steam ships. However, it was not until the discovery of a new type of fuel (gasoline and its chemical relatives) and a new type of engine (the internal combustion engine) that the modern face of transportation was achieved. Today, most forms of transportation depend on the combustion of a hydro-carbon fuel such as gasoline, kerosene, or diesel oil to produce the energy that drives pistons and moves the vehicles on which modern society depends.

When considering how fuels are burned during the combustion process, “stationary” and “explosive” flames are treated as two distinct types of combustion. In stationary combustion, as generally seen in gas or oil burners, the mixture of fuel and oxidizer flows toward the flame at a proper speed to maintain the position of the flame. The fuel can be either premixed with air or introduced separately into the combustion region. An explosive flame, on the other hand, occurs in a homogeneous mixture of fuel and air in which the flame moves rapidly through the combustible mixture. Burning in the cylinder of a gasoline engine belongs to this category. Overall, both chemical and physical processes are combined in combustion, and the dominant process depends on very diverse burning conditions.

Environmental issues

The use of combustion as a power source has had such a dramatic influence on human society that the period after 1750 has sometimes been called the Fossil Fuel Age. Still, the widespread use of combustion for human applications has always had its disadvantages.

KEY TERMS

Chemical bond— The force or “glue” that holds atoms together in chemical compounds.

Fossil fuel— A fuel that is derived from the decay of plant or animal life; coal, oil, and natural gas are the fossil fuels.

Industrial Revolution— That period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

Internal combustion engine— An engine in which the chemical reaction that supplies energy to the engine takes place within the walls of the engine (usually a cylinder) itself.

Thermochemistry— The science that deals with the quantity and nature of heat changes that take place during chemical reactions and/or changes of state.

Pictorial representations of England during the Industrial Revolution, for example, usually include huge clouds of smoke emitted by the combustion of wood and coal in steam engines.

Today, modern societies continue to face environmental problems created by our prodigious combustion of carbon-based fuels. For example, one product of any combustion reaction in the real world is carbon monoxide, a toxic gas that is often detected at dangerous levels in urban areas around the world. Oxides of sulfur, produced by the combustion of impurities in fuels, and oxides of nitrogen, produced at high temperature, also have deleterious effects, often in the form of acid rain and smog. Even carbon dioxide itself, the primary product of combustion, is causing global climate changes because of the enormous concentrations it has reached in the atmosphere; as of the early 2000s, human industrial combustion (including burning of fuel in cars and to generate electricity) had raised global carbon dioxide levels by almost 40%. The rate of change has been most rapid most recently: between 1960 and 2005, atmospheric carbon dioxide increased from 313 parts per million) to 375 ppm, a 20% increase. By 2006, the consensus view of atmospheric scientists and climatologists was that global climate change was happening rapidly and was indeed caused by human activity. A very small but vocal group of dissident scientists who denied either the reality of climate change or its human origin was still receiving a disproportionate amount of media attention, giving the appearance of a two-sided scientific agreement where there was, in fact, an unusually strong consensus.

See also Air pollution; Chemical bond; Internal combustionengine; Oxidation-reduction reaction.

Resources

Books

Guzzella, Lino and Christopher H. Onder. Introduction to Modeling and Control of Internal Combustion Engine Systems. New York: Springer, 2004.

Kuan-yun Kuo, Kenneth. Principles of Combustion. New York: Wiley Interscience, 2005.

Law, Chun K. Combustion Physics. Cambridge, UK: Cambridge University Press, 2006.

Snyder, Carl H. The Extraordinary Chemistry of Ordinary Things, With Late Nite Labs. New York: John Wiley & Sons, 2004.

David E. Newton

Combustion

Combustion is the chemical term for a process known more commonly as burning. It is one of the earliest chemical changes noted by humans, due at least in part to the dramatic effects it has on materials. Early humans were probably amazed and frightened by the devastation resulting from huge forest fires or by the horror of seeing their homes catch fire and burn. But fire (combustion)—when controlled and used correctly—was equally important to their survival, providing a way to keep warm and to cook their meals.

Today, the mechanism by which combustion takes place is well understood and is more correctly defined as a form of oxidation. This oxidation occurs so rapidly that noticeable heat and light are produced. In general, the term "oxidation" refers to any chemical reaction in which a substance reacts with oxygen. For example, when iron is exposed to air, it combines with oxygen in the air. That form of oxidation is known as rust. Combustion differs from rust in that the oxidation occurs much more rapidly, giving off heat in the process.

History

Probably the earliest scientific attempt to explain combustion was made by Johann Baptista van Helmont, a Flemish physician and alchemist who lived from 1580 to 1644. Van Helmont observed the relationship between a burning material and the resulting smoke and flame it produced. He concluded that combustion involved the escape of a "wild spirit" (spiritus silvestre ) from the burning material. This explanation was later incorporated into the phlogiston theory (pronounced flow-JIS-ten), a way of viewing combustion that dominated the thinking of scholars for the better part of two centuries.

According to the phlogiston theory, combustible materials contain a substance—phlogiston—that is given off by the material as it burns. A noncombustible material, such as ashes, will not burn, according to this theory, because all phlogiston contained in the original material (such as wood) had been driven out. The phlogiston theory was developed primarily by German alchemist Johann Becher (1635–1682) and his student Georg Ernst Stahl (1660–1734) at the end of the seventeenth century.

Words to Know

Chemical bond: Any force of attraction between two atoms.

Fossil fuel: A fuel that originates from the decay of plant or animal life; coal, oil, and natural gas are the fossil fuels.

Industrial Revolution: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

Internal-combustion engine: An engine in which the chemical reaction that supplies energy to the engine takes place within the walls of the engine (usually a cylinder) itself.

Oxide: An inorganic compound (one that does not contain carbon) whose only negative part is the element oxygen.

Thermochemistry: The science that deals with the quantity and nature of heat changes that take place during chemical reactions and/or changes of state (for instance, from solid to liquid or gas).

Although scoffed at today, the phlogiston theory explained what was known about combustion at the time of Becher and Stahl. One serious problem with the theory, however, involved weight changes. Many objects actually weigh more after being burned than before. How this could happen when phlogiston escaped from the burning material? One explanation that was offered was that phlogiston had negative weight. Many early chemists thought that such an idea was absurd, but others were willing to consider the possibility. In any case, precise measurements had not yet become an important feature of chemical studies, so loss of weight was not a huge barrier to the acceptance of the phlogiston concept.

Modern theory

Even with all its problems, the phlogiston theory remained popular among chemists for many years. In fact, it was not until a century later that someone proposed a radically new view of the phenomenon. That person was French chemist Antoine Laurent Lavoisier (1743–1794). One key hint

that helped unravel the mystery of the combustion process was the discovery of oxygen by Swedish chemist Karl Wilhelm Scheele (1742–1786) in 1771 and by English chemist Joseph Priestley (1733–1804) in 1774.

Lavoisier used this discovery to frame a new definition of combustion. Combustion, he theorized, is the process by which some material combines with oxygen. For example, when coal burns, carbon in the coal combines with oxygen to form carbon dioxide. Proposing a new theory of combustion was not easy. But Lavoisier conducted a number of experiments involving very careful weight measurements. His results were so convincing that the new oxidation theory was widely accepted in a relatively short period of time.

Lavoisier began another important line of research related to combustion. This research involved measuring the amount of heat generated during oxidation. His earliest experiments involved the study of heat lost by a guinea pig during respiration (breathing), which Lavoisier called a combustion. He was assisted in his work by another famous French scientist, Pierre Simon Laplace (1749–1827).

As a result of their research, Lavoisier and Laplace laid down one of the fundamental principles of thermochemistry, the study of heat changes that take place during chemical reactions. The duo found that the amount of heat needed to decompose (break down) a compound is the same as the amount of heat liberated (freed, or given up) during the compound's formation from its elements. This line of research was further developed by Swiss-Russian chemist Henri Hess (1802–1850) in the 1830s. Hess's development and extension of the work of Lavoisier and Laplace has earned him the title of father of thermochemistry.

Heat of combustion

From a chemical standpoint, combustion is a process in which some chemical bonds are broken and new chemical bonds are formed. The net result of these changes is a release of energy, known as the heat of combustion. For example, suppose that a gram of coal is burned in pure oxygen with the formation of carbon dioxide as the only product. The first step in this reaction requires the breaking of chemical bonds between carbon atoms and between oxygen atoms. In order for this step to occur, energy must be added to the coal/oxygen mixture. For example, a lighted match must be touched to the coal.

Once the carbon-carbon and oxygen-oxygen bonds have been broken, new bonds can be formed. These bonds join carbon atoms with oxygen atoms in the formation of carbon dioxide. The carbon-oxygen bonds contain less energy than did the original carbon-carbon and oxygen-oxygen bonds. The excess energy is released in the form of heat—the heat of combustion. The heat of combustion of one mole of carbon, for example, is about 94 kilocalories. That number means that each time one mole of carbon is burned in oxygen, 94 kilocalories of heat are given off. (A mole is a unit used to represent a certain number of particles, usually atoms or molecules.)

Applications

Humans have been making practical use of combustion for thousands of years. Cooking food and heating homes have long been two major applications of the combustion reaction. With the development of the steam engine by Denis Papin, Thomas Savery, Thomas Newcomen, and others at the beginning of the eighteenth century, however, a new use for combustion was found: performing work. Those first engines employed the combustion of some material, usually coal, to produce heat that was used to boil water. The steam that was produced was then able to move pistons (sliding valves) and drive machinery. That concept is essentially the same one used today to operate fossil-fueled electrical power plants.

Before long, inventors found ways to use steam engines in transportation, especially in railroad engines and steam ships. However, it was not until the discovery of a new type of fuel—gasoline and its chemical relatives—and a new type of engine—the internal-combustion engine—that modern methods of transportation became common. Today, most forms of transportation depend on the combustion of a hydrocarbon fuel (a compound of hydrogen and carbon) such as gasoline, kerosene, or diesel oil to produce the energy that drives pistons and moves vehicles.

Environmental issues

The use of combustion as a power source has had such a dramatic influence on human society that the period after 1750 has sometimes been called the Fossil Fuel Age. Still, the widespread use of combustion for human applications has always caused significant environmental problems. Pictures of the English countryside during the Industrial Revolution (a major change in the economy that resulted from the introduction of power-driven machinery in the mid-eighteenth century), for example, usually show huge clouds of smoke given off by the burning of wood and coal in steam engines.

At the dawn of the twenty-first century, modern societies continued to face environmental problems created by the enormous combustion of carbon-based fuels. For example, one product of any combustion reaction in the real world is carbon monoxide. Carbon monoxide is a toxic (poisonous; potentially deadly) gas that sometimes reaches dangerous concentrations in urban areas around the world. Oxides of sulfur (produced by the combustion of impurities in fuels) and oxides of nitrogen (produced at high temperatures) can also have harmful effects. The most common problem associated with these oxides is the formation of acid rain and smog. Even carbon dioxide itself, the primary product of combustion, can be a problem: it is thought to be at the root of recent global climate changes because of the enormous concentrations it has reached in the atmosphere.

[See also Chemical bond; Heat; Internal-combustion engine; Oxidation-reduction reaction; Pollution ]

Combustion

Combustion is the chemical term for a process known more commonly as burning. It is certainly one of the earliest chemical changes noted by humans, at least partly because of the dramatic effects it has on materials. Today, the mechanism by which combustion takes place is well understood and is more correctly defined as a form of oxidation that occurs so rapidly that noticeable heat and light are produced.

History

Probably the earliest reasonably scientific attempt to explain combustion was that of Johannes (or Jan) Baptista van Helmont, a Flemish physician and alchemist who lived from 1580 to 1644. Van Helmont observed the relationship among a burning material, smoke and flame and said that combustion involved the escape of a "wild spirit" (spiritus silvestre) from the burning material. This explanation was later incorporated into a theory of combustion—the phlogiston theory—that dominated alchemical thinking for the better part of two centuries.

According to the phlogiston theory, combustible materials contain a substance—phlogiston—that is emitted by the material as it burns. A non-combustible material, such as ashes, will not burn, according to this theory, because all phlogiston contained in the original material (such as wood ) had been driven out. The phlogiston theory was developed primarily by the German alchemist Johann Becher and his student Georg Ernst Stahl at the end of the seventeenth century.

Although scoffed at today, the phlogiston theory satisfactorily explained most combustion phenomena known at the time of Becher and Stahl. One serious problem was a quantitative issue. Many objects weigh more after being burned than before. How this could happen when phlogiston escaped from the burning material? One possible explanation was that phlogiston had negative weight, an idea that many early chemists thought absurd, while others were willing to consider. In any case, precise measurements had not yet become an important feature of chemical studies, so loss of weight was not an insurmountable barrier to the phlogiston concept.

Modern theory

As with so many other instances in science, the phlogiston theory fell into disrepute only when someone appeared on the scene who could reject traditional thinking almost entirely and propose a radically new view of the phenomenon. That person was the great French chemist Antoine Laurent Lavoisier (1743-1794). Having knowledge of some recent critical discoveries in chemistry , especially the discovery of oxygen by Karl Wilhelm Scheele (1742-1786) in 1771 and Joseph Priestley (1733-1804) in 1774, Lavoisier framed a new definition of combustion. Combustion, he said, is the process by which some material combines with oxygen. By making the best use of precise quantitative experiments, Lavoisier provided such a sound basis for his new theory that it was widely accepted in a relatively short period of time.

Lavoisier initiated another important line of research related to combustion, one involving the amount of heat generated during oxidation. His earliest experiments involved the study of heat lost by a guinea pig during respiration , which Lavoisier called "a combustion." In this work, he was assisted by a second famous French scientist, Pierre Simon Laplace (1749-1827). As a result of their research, Lavoisier and Laplace laid down one of the fundamental principles of thermochemistry , namely that the amount of heat needed to decompose a compound is the same as the amount of heat liberated during its formation from its elements. This line of research was later developed by the Swiss-Russian chemist Henri Hess (1802-1850) in the 1830s. Hess' development and extension of the work of Lavoisier and Laplace has earned him the title of father of thermochemistry.

Combustion mechanics

From a chemical standpoint, combustion is a process in which chemical bonds are broken and new chemical bonds formed. The net result of these changes is a release of energy , the heat of combustion. For example, suppose that a gram of coal is burned in pure oxygen with the formation of carbon dioxide as the only product. In this reaction, the first step is the destruction of bonds between carbon atoms and between oxygen atoms. In order for this step to occur, energy must be added to the coal/oxygen mixture. For example, a lighted match must be touched to the coal.

Once the carbon-carbon and oxygen-oxygen bonds have been broken, new bonds between carbon atoms and oxygen atoms can be formed. These bonds contain less energy than did the original carbon-carbon and oxygen-oxygen bonds. That energy is released in the form of heat, the heat of combustion. The heat of combustion of one mole of carbon, for example, is about 94 kcal.

Applications

Humans have been making practical use of combustion for millennia. Cooking food and heating homes have long been two major applications of the combustion reaction. With the development of the steam engine by Denis Papin, Thomas Savery, Thomas Newcomen, and others at the beginning of the eighteenth century, however, a new use for combustion was found: performing work. Those first engines employed the combustion of some material, usually coal, to produce heat that was used to boil water . The steam produced was then able to move pistons and drive machinery. That concept is essentially the same one used today to operate fossil-fueled electrical power plants.

Before long, inventors found ways to use steam engines in transportation, especially in railroad engines and steam ships. However, it was not until the discovery of a new type of fuel—gasoline and its chemical relatives—and a new type of engine—the internal combustion engine—that the modern face of transportation was achieved. Today, most forms of transportation depend on the combustion of a hydrocarbon fuel such as gasoline, kerosene, or diesel oil to produce the energy that drives pistons and moves the vehicles on which modern society depends.

When considering how fuels are burned during the combustion process, "stationary" and "explosive" flames are treated as two distinct types of combustion. In stationary combustion, as generally seen in gas or oil burners, the mixture of fuel and oxidizer flows toward the flame at a proper speed to maintain the position of the flame. The fuel can be either premixed with air or introduced separately into the combustion region. An explosive flame, on the other hand, occurs in a homogeneous mixture of fuel and air in which the flame moves rapidly through the combustible mixture. Burning in the cylinder of a gasoline engine belongs to this category. Overall, both chemical and physical processes are combined in combustion, and the dominant process depends on very diverse burning conditions.

Environmental issues

The use of combustion as a power source has had such a dramatic influence on human society that the period after 1750 has sometimes been called the Fossil Fuel Age. Still, the widespread use of combustion for human applications has always had its disadvantages. Pictorial representations of England during the Industrial Revolution , for example, usually include huge clouds of smoke emitted by the combustion of wood and coal in steam engines.

Today, modern societies continue to face environmental problems created by the prodigious combustion of carbon-based fuels. For example, one product of any combustion reaction in the real world is carbon monoxide , a toxic gas that is often detected at dangerous levels in urban areas around the world. Oxides of sulfur , produced by the combustion of impurities in fuels, and oxides of nitrogen , produced at high temperature , also have deleterious effects, often in the form of acid rain and smog . Even carbon dioxide itself, the primary product of combustion, is suspected of causing global climate changes because of the enormous concentrations it has reached in the atmosphere.

See also Air pollution; Chemical bond; Internal combustion engine; Oxidation-reduction reaction.

Resources

books

Gilpin, Alan. Dictionary of Fuel Technology. New York: Philosophical Library, 1969.

Joesten, Melvin D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Olah, George A., ed. Chemistry of Energetic Materials. San Diego: Academic Press, 1991.

Snyder, C.H. The Extraordinary Chemistry of Ordinary Things. 4th ed. New York: John Wiley and Sons, 2002.

periodicals

Rutland, Christopher. "Probability Density Function Combustion Modeling of Diesel Engine." Combustion Science and Technology 174, no. 10 (2002): 19-54.

other

"Combustion Modelling For Direct Injection Diesel Engines." Proceedings Of The Institution Of Mechanical Engineers 215, no. 5 (2001): 651–663.

David E. Newton

KEY TERMS

Chemical bond

—The force or "glue" that holds atoms together in chemical compounds.

Fossil fuel

—A fuel that is derived from the decay of plant or animal life; coal, oil, and natural gas are the fossil fuels.

Industrial Revolution

—That period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

Internal combustion engine

—An engine in which the chemical reaction that supplies energy to the engine takes place within the walls of the engine (usually a cylinder) itself.

Thermochemistry

—The science that deals with the quantity and nature of heat changes that take place during chemical reactions and/or changes of state.

Combustion

The process of burning fuels. Traditionally biomass was used as fuel, but now fossil fuels are the major source of energy for human activities. Combustion is essentially an oxidation process that yields heat and light. Most fuels are carbon and hydrogen which use oxygen in the air as an oxidant. More exotic fuels are used in some combustion processes, particularly in rockets where metals such as aluminum or beryllium or hydrazine (a nitrogen containing compound) are well known as effective fuels. As rockets operate beyond the atmosphere they carry their own oxidants, which may also be quite exotic.

Combustion involves a mixture of fuel and air, which is thermodynamically unstable. The fuel is then converted to stable products, usually water and carbon dioxide , with the release of a large amount of energy as heat. At normal temperatures fuels such as coal and oil are quite stable and have to be ignited by raising the temperature. Combustion is said to be spontaneous when the ignition appears to take place without obvious reasons. Large piles of organic material, such as hay, can undergo slow oxidation, perhaps biologically mediated, and increase in temperature. If the amount of material is very large and the heat cannot escape, the whole pile can suddenly burst into flame. Will-o'-the-wisps or jack-o'-lanterns (known scientifically as ignis fatuus ) are sometimes observed over swamps where methane is likely to be produced. The reason these small pockets of gas ignite is not certain, but it has been suggested that small traces of gases such as phosphine that react rapidly with air could ignite the methane.

Typical solid fuels like coal and wood begin to burn with a bright turbulent flame. This forms as volatile materials are driven off and ignited. These vapors burn so rapidly that oxygen can be depleted, creating a smoky flame. After a time the volatile substances in the fuel are depleted. At this point a glowing coal is evident and combustion takes place without a significant flame. Combustion on the surface of the glowing coal is controlled by the diffusion of oxygen towards the hot surface. If the piece of fuel is too small, such as a spark from a fire, it is likely to lose temperature rapidly and combustion will stop. By contrast a bed of coals can maintain combustion because of heat storage and the exchange of radiative heat between the pieces. The most intense combustion takes place between the crevices of a bed of coal. In these regions oxygen may be in limited supply which leads to the production of carbon monoxide . This is subsequently oxidized to carbon at the surface of the bed of coals with a faint blue flame. The production of toxic carbon monoxide from indoor fires can occasionally represent a hazard if subsequent oxidation to carbon dioxide is not complete.

Liquid fuels usually need to be evaporated before they burn effectively. This means that it is possible to see liquid combustion and gaseous combustion as similar processes. Combustion can readily be initiated with a flame or spark. Simply heating a fuel-air mixture can cause it to ignite, but temperatures have to be high before reactions occur. A much better way is to initiate combustion with a small number of molecular fragments of radicals. These can initiate chain reactions at much lower temperatures than molecular reactions. In a propane-air flame at about 2000° K, hydrogen and oxygen atoms and hydroxyl radicals account for about 0.3% of a gas mixture. It is these radicals that support combustion. They react with molecules and split them up into more radicals. These radicals can rapidly enter into the exothermic (heat releasing) oxidative processes that lie at the heart of combustion. The reactions also give rise to further radicals that support continued combustion. Under some situations the radicals reaction branch, such that the reaction of each radical produces two new radicals. These can enter further reactions, producing yet further increases in the number of reactions and very soon the system explodes. However the production of radicals can be terminated in a number of ways such as contact with a solid surface. In some systems, such as the internal combustion engine, an explosion is desired, but in others, such as a gas cooker flame, maintaining a stable combustion process is desirable.

In terms of air pollution the reaction of oxygen and nitrogen atoms with molecules in air leads to the formation of the pollutant nitric oxide through a set of reactions known as the Zeldovich cycle. It is this process that makes combustion such an important contributor of nitrogen oxides to the atmosphere.

[Peter Brimblecombe ]

RESOURCES

BOOKS

Campbell, I. M. Energy and the Atmosphere. New York: Wiley, 1986.