Combustion

• Type of physical science: Chemistry
• Field of study: Chemical reactions

Combustion of a compound involves the incorporation of oxygen, either from the atmosphere or from another compound containing it. It is an exothermal process that leads to flames when a gaseous medium is produced, and to explosion when, from a small amount of reactants, a large amount of gaseous products are formed.

Overview

The burning of any substance, whether gaseous, liquid, or solid, is referred to as combustion. The reaction is always an oxidation-reduction reaction, since changes in the oxidation state are involved. The reactants are described as the oxidizer and the fuel. Often, the oxidizer is oxygen, which can be in its free state (for example, in atmospheric air) or part of another compound (for example, nitric acid, HNO3, or ammonium perchlorate, NH₄ClO₄) that makes it available for the combustion to occur. Fluorine (a non-oxygen-carrying substance) can also serve as the oxidizer when reacted with hydrogen the latter acts as the fuel to produce hydrogen fluoride. Although oxygen is not combustible, it actively supports combustion, but its presence is not required if another oxidizing agent, such as fluorine in the above reaction, is present. The decomposition of a compound with emission of light and heat can also be classified as combustion. Such examples include ozone (to form oxygen gas), hydrogen peroxide (to produce water and oxygen gas), and acetylene (which yields carbon and hydrogen). The minimum temperature required to initiate or cause self-sustained combustion in any substance in the absence of a spark or flame is called the autoignition point, or autoignition temperature. The temperature at which a liquid or volatile solid gives off vapor sufficient to form an ignitable mixture with the air near the surface of the liquid is known as the flash point.

Subjecting a solid to combustion involves several stages. First, the volatile material that is trapped in the solid is driven out and is burned in the presence of air. The heat produced will provide the burning of the solid residue, unless the amount of oxygen available is not enough, or the heat is dissipated by irradiation, at which stages combustion stops.

The combustion of liquids involves the same procedure, which is greatly aided by the higher vapor pressure of the liquid. The heat produced from the initial stage is used to evaporate more liquid, which in turn will undergo burning with the air.

Gases are generally more flammable. Nevertheless, the collision between molecules is not enough for spontaneous combustion to occur at ordinary temperatures. Raising the temperature leads to an increase in molecular collisions. This however, still is not adequate to initiate combustion. The occurrence of collisions leads to the formation of intermediates that are very unstable and prone to exothermic reactions. These intermediates can be atoms or free radicals, and are capable of generating a series of branched-chain reactions that lead to combustion. The chain reactions often are suppressed by radical scavengers that force radicals to react with one another, thus reducing their abundance. When the rate of radical or atom formation far exceeds the rate of their dissipation, however, molecular collisions increase drastically in frequency, and heat is produced uncontrollably. Since the dissipation of heat is not efficient enough, an explosion known as thermal explosion will occur. Slow combustion occurs when the formation of atoms or radicals is carefully controlled.

When a heated uniform body of gas is subjected to a spark, an exothermic reaction front, or wave, is produced, spreading from the point of initiation and often showing luminosity (light emission). This phenomenon, which is usually seen in combustion, is the flame, and it can be categorized in various ways. When fuel and oxidizer are mixed before they burn, they are called premixed; if they are produced when fuel and oxidizer mix and burn simultaneously, they are called diffusion flames. Flames can also be classified according to shape, time behavior (stationary or moving), flow regime (laminar or turbulent), buoyancy (forced or natural convection), and flow complications (swirling flow and crosswind). Spray combustion differs from a premixed, combustible gaseous system in that it is not uniform in composition. The fuel is present in the form of discrete liquid droplets, which may have a range of sizes and different velocities from that of the main stream of gas.

Heat is not always necessary for combustion to occur. Lower temperatures are adequate when catalysts are present. These are species that reduce the minimum amount of energy needed for the reaction to occur, yet do not get consumed or incorporated in the structure of final products. As a result, the reactants dissociate on their surface and produce the atoms and radicals necessary for the reaction to occur.

When elements are subjected to combustion with oxygen, the products are oxides.

Thus, magnesium yields magnesium oxide (MgO), while aluminum produces aluminum oxide (Al₂O₃). On the other hand, combustion of organic compounds leads to carbon monoxide (CO) or carbon dioxide (CO₂), (depending on the amount of oxygen present) and water. Thus, glucose (C₆H₁₂O₆) produces carbon dioxide and water, in a reaction that is the reverse of photosynthesis.

Spontaneous combustion may occur at or even below room temperature for one or more reasons. Substances that are sensitive to oxidation, such as phosphorus, will ignite by simple exposure to air. Often, heat can be built up from bacterial activity, for example in compost and sewage sludge. Finally, auto-oxidation, a spontaneous, self-catalyzed oxidation occuring in the presence of air, can create heat accumulation that will produce the combustion of ignitable materials, such as fish oils.

Explosions are often associated with combustion reactions. This occurs if the products formed are of much larger volume and the amount of energy released is high. Chemical explosives are metastable compounds that can react in a combustion or detonation manner. In combustion, the rate at which the reaction front advances is governed by the heat conduction and convection of heat from the hot products to the unreacted explosive. In detonation, the rate of reaction front depends on the so-called detonation (shock) wave, which is driven by the high-pressure products of the reaction, and therefore the damage is more significant. Increase of the reaction rate can convert combustion to detonation and vice versa.

Applications

Combustion in all its various forms is the reaction that sustains life. The chemical energy present in carbohydrates produced by plants can be used by higher organisms that eat them. The energy is released in an exergonic and flameless process, with the carbohydrates serving as fuels. The oxidizing agent is oxygen, to which electrons are transferred from the oxidized species in the so-called electron transport process.

Burning of fossil fuel has been of great importance to humankind. It is by way of this process that humans can produce industrial products, travel, and provide heat and air-conditioning in their buildings. Burning fuel (coal, natural gas, or oil) leads to the production of carbon dioxide and water and a large amount of energy output. At the same time, problems of pollution have reached alarming levels in big cities, which can be divided into brown-air (Los Angeles, Athens) and gray-air cities (Chicago, New York, London). The former cities suffer from nitric oxide (NO) and its oxidized products (the yellow gas nitrogen dioxide, NO₂, and photochemical smog), produced by the extensive use of automobile engines. The latter are affected by the particulates (dust, soot) and sulfur dioxide (SO₂) produced by the burning of coal and oil in electric power generation, manufacturing, and heating.

An application of the combustion of solid material is the burning of coal. The first stage involves the formation of carbon monoxide (CO) by the burning of the fine particles of coal by means of the air stream. Carbon monoxide further reacts with air to produce carbon dioxide (CO₂) via the heat produced by the first stage. Carbon dioxide will form only if adequate oxygen is available, and only if the heat produced after the initial step is not lost by irradiation or any other means. This is precisely the reason why heating from burners should take place in adequately aerated places, to avoid the formation of high concentrations of carbon monoxide.

When large amounts of solid fuel are stocked, the possibility of spontaneous combustion is likely. The process starts (for example, in a mine) by the slow oxidation of the flammable material (for example, methane gas) often created by microorganisms. Since the heat cannot be dissipated, a chain reaction ensues that produces higher and higher amounts of heat, which eventually leads to spontaneous burning and explosions. As a precaution, therefore, coal is normally stored in shallow piles to prevent the heat of oxidation from accumulating.

When a new compound is prepared, one of the first items of interest is the determination of its formula. This is often done by taking an accurately weighed sample and reacting it with oxygen to produce carbon dioxide, water, and nitrogen dioxide. The process, known as combustion analysis, was of extreme significance in the past and is still extensively used, despite the development of modern techniques such as spectroscopy.

The flame of the Bunsen burner is an example of a contrived premixed flame. When the flow of the unburned gas and the propagation of the flame in the burner are exactly balanced, the flame stands still. The flame cone lengthens when the gas flow is increased, and it shortens when it is decreased. When the gas flow is made too high, stabilization fails and the flame is swept away. On the other hand, the flame can be initiated in a finite body of gas rather than a continuing stream. This process finds great application in spark-ignited internal-combustion engines, which require high-effective flame speeds to accomplish efficient, high-speed operation.

Diffusion flames find a much wider use in technical applications. Thus, in furnaces, boilers, gas heating devices, and turbine engines, the burner flames are designed for continuous combustion.

Combustion of a metal with oxygen is a phenomenon that occurs routinely in everyday life. For example, oxygen can combine with iron in the presence of water to form rust, while magnesium burns with oxygen, emitting a great deal of heat and intense light, which makes it applicable in the operation of flashbulbs. In flame photometry, the presence of several metals can be detected in minimal quantities (microgram of a metal per gram of sample) by dissociation of the evaporated salt into gaseous atoms and subsequent excitation, which releases light of characteristic colors. Lithium salts produce a red flame, sodium salts give a brilliant yellow color, and potassium salts impart a violet flame. The method is the precursor of the modern techniques of plasma emission spectrometry and atomic absorption spectrophotometry.

The use of spectroscopy in combustion processes has been proven to be exceptionally helpful in determining flame temperatures and combustion intermediates and mechanisms. In this experimental technique, data about the flames are collected without interfering with the combustion process. The band spectra are studied with a spectroscope (prism, grating) or interferometer, and the dispersed radiation is detected photographically or via a photomultiplier tube.

The importance of gas formation in explosions is clearly displayed in the decomposition of nitroglycerin. Upon sudden impact, 4 moles of the unstable liquid nitroglycerin produce 29 moles of stable gaseous products, which, as gases, have the tendency to occupy a larger volume. Moreover, a large quantity of energy is produced, which increases the gaseous volume, thus producing a pressure surge and a damaging shock wave. Similarly, trinitrotoluene, TNT, a solid at room temperature, also decomposes upon impact to the same devastating effect.

Two moles of TNT produce 20 moles of gaseous products.

Context

Since the Industrial Revolution, there has been a dramatic increase in the burning of fossil fuels such as coal, natural gas, oil, and gasoline. Since that time, the carbon dioxide content of our atmosphere has risen because of the burning of fossil fuels. It is feared that a continued rise in the carbon dioxide content could cause a dangerous increase in what is known as the greenhouse effect.

Carbon dioxide allows light energy from the sun to pass through the atmosphere, but it prevents lower-energy heat radiation from escaping back into space. This would result in a gradual warming of the earth's atmosphere, which would cause the Antarctic ice sheet to melt or slip into the ocean, thus raising the sea level to levels dangerous to many major cities.

Another concern attributed to combustion is the formation of large quantities of nitrogen and sulfur oxides produced in large cities by the extensive consumption of fuel. These by-products, together with the solid residues such as fly ash, are responsible for the photochemical smog that has destroyed vegetation and shortened the lives of thousands of people with pulmonary conditions.

The use of polymers in modern life has also increased the risk of fire, especially in construction and fabrics. Modern technology has, however, modified the polymer to the extent that minimized fragmentation occurs, which in turn reduces the free radicals that sustain combustion. Thus, the synthesis of flame-retardant materials has saved many lives as well as billions of dollars in property damage.

Spray combustion as a source of energy has been established since the 1880s as a powerful method of burning relatively involatile liquid fuels. It remains the major way of burning heavy fuel oils, even though they can now be burned in fluidized bed combustors.

The reaction of metals with oxygen is advantageous in many instances. The use of magnesium in flashbulbs, aluminum as a structural component, and iron's oxidation with an acetylene flame in the cutting of steel are typical examples. The combustion of aluminum with ammonium perchlorate has helped the launching of rockets and the space shuttle by providing the necessary thrust.

Finally, the role of combustion in explosives is paramount. It was Alfred Nobel who first introduced liquid nitroglycerin for rock blasting in 1863, replacing black powder (a mixture of charcoal, sulfur, and potassium nitrate) as the only industrial explosive. It was also Nobel who, in 1865, devised the detonator and, a decade later, the first dynamites and blasting gelatin.

The impact of these developments on industry and warfare has led not only to destructive uses but also to advances in mining, road construction, and the location of oil deposits.

Principal terms

AUTOIGNITION POINT (OR TEMPERATURE): the minimum temperature required to initiate or cause self-sustained combustion in any substance in the absence of spark or flame

COMBUSTION: an exothermic oxidation reaction that often occurs with organic compounds (to produce carbon dioxide or monoxide and water) and metals (to yield oxides)

FLAME-RETARDING AGENT: a substance applied to a combustible material to reduce or eliminate its tendency to ignite when exposed to a low-energy flame

FLAMMABILITY: the ability of a material to ignite, either in a high-temperature environment or with a spark or flame

FLASH POINT: the temperature at which a liquid or volatile solid gives off sufficient vapor to form an ignitable mixture with the air that is in contact with the surface

PYROLYSIS: transformation of a compound into another one by heat application alone, without oxygen participation

Bibliography

Glassman, Irvin, et al. Combustion. 5th ed., Academic P, 2014.

Grier, Joseph M. Combustion: Types of Reactions, Fundamental Processes and Advanced Technologies. Nova Science Publishers, 2014.

Lindsay, Jack. Blast-Power and Ballistics: Concepts of Force and Energy in the Ancient World. Barnes & Noble, 2009.

Strahle, Warren C., and William A. Sirignano. Introduction to Combustion. Routledge, 2020.

Radel, Stanley R., and Marjorie H. Navidi. Chemistry. Subsequent ed., West Group, 1994.

Williams, Alan. Combustion of Liquid Fuel. Butterworths, 1990.