Combustion, orburning,[1] is a high-temperatureexothermicredoxchemical reaction between afuel (the reductant) and anoxidant, usually atmosphericoxygen, that produces oxidized, often gaseous products, in a mixture termed assmoke. Combustion does not always result infire, because aflame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. Whileactivation energy must be supplied to initiate combustion (e.g., using a litmatch to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining. The study of combustion is known ascombustion science.
Combustion is often a complicated sequence ofelementaryradical reactions.Solid fuels, such aswood andcoal, first undergoendothermicpyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough thatincandescentlight in the form of eitherglowing or aflame is produced. A simple example can be seen in the combustion ofhydrogen andoxygen intowatervapor, a reaction which is commonly used to fuelrocket engines. This reaction releases 242kJ/mol of heat and reduces theenthalpy accordingly (at constant temperature and pressure):
Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion isstoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, thechemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such ascarbon monoxide,hydrogen and evencarbon (soot or ash). Thus, the producedsmoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures inatmosphericair, which is 78 percentnitrogen, will also create small amounts of severalnitrogen oxides, commonly referred to asNOx, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning orcatalytic converters may be required by law.
Fires occur naturally, ignited bylightning strikes or byvolcanic products. Combustion (fire) was the first controlled chemical reaction discovered by humans, in the form ofcampfires andbonfires, and continues to be the main method to produce energy for humanity. Usually, the fuel iscarbon,hydrocarbons, or more complicated mixtures such aswood that contain partially oxidized hydrocarbons. The thermal energy produced from the combustion of eitherfossil fuels such ascoal oroil, or fromrenewable fuels such asfirewood, is harvested for diverse uses such ascooking, production ofelectricity or industrial or domestic heating. Combustion is also currently the only reaction used to powerrockets. Combustion is also used to destroy (incinerate) waste, both nonhazardous and hazardous.
In complete combustion, the reactant burns in oxygen and produces a limited number of products. When ahydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yieldcarbon dioxide, sulfur will yieldsulfur dioxide, and iron will yieldiron(III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is theoxidant. Still, small amounts of various nitrogen oxides (commonly designatedNO x species) form when the air is the oxidative.
Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example,sulfur trioxide is not produced quantitatively by the combustion of sulfur.NOx species appear in significant amounts above about 2,800 °F (1,540 °C), and more is produced at higher temperatures. The amount ofNOx is also a function of oxygen excess.[2]
In most industrial applications and infires,air is the source of oxygen (O 2). In the air, each mole of oxygen is mixed with approximately3.71 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted toNO x (mostlyNO, with much smaller amounts ofNO 2). On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted tocarbon monoxide, and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel.
The amount of air required for complete combustion is known as the "theoretical air" or "stoichiometric air".[3] The amount of air above this value actually needed for optimal combustion is known as the "excess air", and can vary from 5% for a natural gas boiler, to 40% foranthracite coal, to 300% for agas turbine.[4]
Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however,carbon andcarbon monoxide are produced instead of carbon dioxide.
For most fuels, such as diesel oil, coal, or wood,pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmfulacetaldehyde, and carbon can produce toxic carbon monoxide.
The degree of combustion can be measured and analyzed with test equipment.HVAC contractors,firefighters andengineers use combustion analyzers to test theefficiency of a burner during the combustion process. Also, the efficiency of an internal combustion engine can be measured in this way, and some U.S. states and local municipalities use combustion analysis to define and rate the efficiency of vehicles on the road today.
Carbon monoxide is one of the products fromincomplete combustion.[5] The formation of carbon monoxide produces less heat than formation of carbon dioxide so complete combustion is greatly preferred especially as carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen.[6]
These oxides combine withwater andoxygen in the atmosphere, creatingnitric acid andsulfuric acids, which return to Earth's surface as acid deposition, or "acid rain." Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated withnitrogen oxides is that they, along withhydrocarbon pollutants, contribute to the formation ofground level ozone, a major component of smog.[7]
Breathingcarbon monoxide causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with the risk of heart disease. People who survive severecarbon monoxide poisoning may suffer long-term health problems.[8] Carbon monoxide from the air is absorbed in the lungs which then binds withhemoglobin in human's red blood cells. This reduces the capacity of red blood cells that carry oxygen throughout the body.
Smoldering is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction. Solid materials that can sustain a smoldering reaction include coal,cellulose,wood,cotton,tobacco,peat,duff,humus, synthetic foams, charringpolymers (includingpolyurethane foam) anddust. Common examples of smoldering phenomena are the initiation of residential fires onupholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire) and the persistentcombustion of biomass behind the flaming fronts ofwildfires.
Spontaneous combustion is a type of combustion that occurs by self-heating (increase in temperature due toexothermic internal reactions), followed by thermal runaway (self-heating which rapidly accelerates to high temperatures) and finally, ignition.For example, phosphorus self-ignites at room temperature without the application of heat. Organic materials undergoing bacterialcomposting can generate enough heat to reach the point of combustion.[9]
Combustion resulting in a turbulent flame is the most used for industrial applications (e.g.gas turbines,gasoline engines, etc.) because the turbulence helps the mixing process between the fuel andoxidizer.
Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity
The term 'micro' gravity refers to a gravitational state that is 'low' (i.e., 'micro' in the sense of 'small' and not necessarily a millionth of Earth's normal gravity) such that the influence ofbuoyancy on physical processes may be considered small relative to other flow processes that would be present at normal gravity. In such an environment, the thermal andflow transport dynamics can behave quite differently than in normal gravity conditions (e.g., acandle's flame takes the shape of a sphere.[10]). Microgravity combustion research contributes to the understanding of a wide variety of aspects that are relevant to both the environment of a spacecraft (e.g., fire dynamics relevant to crew safety on theInternational Space Station) and terrestrial (Earth-based) conditions (e.g., droplet combustion dynamics to assist developing new fuel blends for improved combustion,materials fabrication processes,thermal management of electronic systems, multiphase flow boiling dynamics, and many others).
Combustion processes that happen in very small volumes are consideredmicro-combustion. The high surface-to-volume ratio increases specific heat loss.Quenching distance plays a vital role in stabilizing the flame in suchcombustion chambers.
If the stoichiometric combustion takes place using air as the oxygen source, thenitrogen present in the air (Atmosphere of Earth) can be added to the equation (although it does not react) to show the stoichiometric composition of the fuel in air and the composition of the resultant flue gas. Treating all non-oxygen components in air as nitrogen gives a 'nitrogen' to oxygen ratio of 3.77, i.e. (100% −O 2%) /O 2% whereO 2% is 20.95% vol:
where.
For example, the stoichiometric combustion of methane in air is:
The stoichiometric composition of methane in air is 1 / (1 + 2 + 7.54) = 9.49% vol.
The stoichiometric combustion reaction for CαHβOγ in air:
The stoichiometric combustion reaction for CαHβOγSδ:
The stoichiometric combustion reaction for CαHβOγNδSε:
The stoichiometric combustion reaction for CαHβOγFδ:
Various other substances begin to appear in significant amounts in combustion products when theflame temperature is above about1600 K. When excess air is used, nitrogen may oxidize toNO and, to a much lesser extent, toNO 2.CO forms bydisproportionation of CO2, andH 2 andOH form by disproportionation ofH2O.
For example, when1 mol ofpropane is burned with28.6 mol of air (120% of the stoichiometric amount), the combustion products contain 3.3%O 2. At1400 K, theequilibrium combustion products contain 0.03%NO and 0.002%OH. At1800 K, the combustion products contain 0.17%NO, 0.05%OH, 0.01%CO, and 0.004%H 2.[11]
Diesel engines are run with an excess of oxygen to combust smallparticles that tend to form with only a stoichiometric amount of oxygen, necessarily producingnitrogen oxide emissions. Both the United States and European Unionenforce limits to vehicle nitrogen oxide emissions, which necessitate the use of specialcatalytic converters or treatment of the exhaust withurea (seeDiesel exhaust fluid).
The incomplete (partial) combustion of ahydrocarbon with oxygen produces a gas mixture containing mainlyCO 2,CO,H2O, andH 2. Such gas mixtures are commonly prepared for use as protective atmospheres for theheat-treatment of metals and forgas carburizing.[12] The general reaction equation for incomplete combustion of onemole of a hydrocarbon in oxygen is:
Whenz falls below roughly 50% of the stoichiometric value,CH 4 can become an important combustion product; whenz falls below roughly 35% of the stoichiometric value, elementalcarbon may become stable.
The products of incomplete combustion can be calculated with the aid of amaterial balance, together with the assumption that the combustion products reachequilibrium.[13][14] For example, in the combustion of onemole of propane (C 3H 8) with four moles ofO 2, seven moles of combustion gas are formed, andz is 80% of the stoichiometric value. The three elemental balance equations are:
Carbon:
Hydrogen:
Oxygen:
These three equations are insufficient in themselves to calculate the combustion gas composition.However, at the equilibrium position, thewater-gas shift reaction gives another equation:
;
For example, at1200 K the value ofKeq is 0.728.[15] Solving, the combustion gas consists of 42.4%H2O, 29.0% CO2, 14.7%H 2, and 13.9%CO. Carbon becomes a stable phase at1200 K and1 atm pressure when z is less than 30% of the stoichiometric value, at which point the combustion products contain more than 98%H 2 andCO and about 0.5%CH 4.
Substances or materials which undergo combustion are calledfuels. The most common examples are natural gas, propane,kerosene,diesel, petrol, charcoal, coal, wood, etc.
Combustion of aliquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapor that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: itsflash point. The flash point of liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is the minimum temperature at which there is enough evaporated fuel in the air to start combustion.
Combustion of gaseous fuels may occur through one of four distinctive types of burning:diffusion flame,premixed flame,autoignitive reaction front, or as adetonation.[16] The type of burning that actually occurs depends on the degree to which thefuel andoxidizer are mixed prior to heating: for example, a diffusion flame is formed if the fuel and oxidizer are separated initially, whereas a premixed flame is formed otherwise. Similarly, the type of burning also depends on the pressure: a detonation, for example, is an autoignitive reaction front coupled to a strong shock wave giving it its characteristic high-pressure peak and highdetonation velocity.[16]
The act of combustion consists of three relatively distinct but overlapping phases:
Preheating phase, when the unburnedfuel is heated up to its flash point and thenfire point. Flammable gases start being evolved in a process similar todry distillation.
Distillation phase orgaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light.Flames are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
Charcoal phase orsolid phase, when the output of flammable gases from the material is too low for the persistent presence of flame and thecharred fuel does not burn rapidly and just glows and later onlysmoulders.
Efficientprocess heating requires recovery of the largest possible part of a fuel'sheat of combustion into the material being processed.[17][18] There are many avenues of loss in the operation of a heating process. Typically, the dominant loss issensible heat leaving with theoffgas (i.e., theflue gas). The temperature and quantity of offgas indicates its heat content (enthalpy), so keeping its quantity low minimizes heat loss.
In a perfect furnace, the combustion air flow would be matched to the fuel flow to give each fuel molecule the exact amount of oxygen needed to cause complete combustion. However, in the real world, combustion does not proceed in a perfect manner. Unburned fuel (usuallyCO andH 2) discharged from the system represents a heating value loss (as well as a safety hazard). Since combustibles are undesirable in the offgas, while the presence of unreacted oxygen there presents minimal safety and environmental concerns, the first principle of combustion management is to provide more oxygen than is theoretically needed to ensure that all the fuel burns. For methane (CH 4) combustion, for example, slightly more than two molecules of oxygen are required.
The second principle of combustion management, however, is to not use too much oxygen. The correct amount of oxygen requires three types of measurement: first, active control of air and fuel flow; second, offgas oxygen measurement; and third, measurement of offgas combustibles. For each heating process, there exists an optimum condition of minimal offgas heat loss with acceptable levels of combustibles concentration. Minimizing excess oxygen pays an additional benefit: for a given offgas temperature, theNOx level is lowest when excess oxygen is kept lowest.[2]
Adherence to these two principles is furthered by making material and heat balances on the combustion process.[19][20][21][22] Thematerial balance directly relates theair/fuel ratio to the percentage ofO 2 in the combustion gas. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion.[23][24] Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,[25][26] or enriching it in oxygen.[27][28]
Combustion in oxygen is achain reaction in which many distinctradical intermediates participate. The high energy required for initiation is explained by the unusual structure of thedioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in atriplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, withspins aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum. Interaction between the two is quantum mechanically a "forbidden transition", i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, orsinglet oxygen. This intermediate is extremely reactive. The energy is supplied asheat, and the reaction then produces additional heat, which allows it to continue.
Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further to give hydroperoxides, which break up to givehydroxyl radicals. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, andhydroperoxyl. Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced in incomplete combustion. An example isacetaldehyde produced in the combustion ofethanol. An intermediate in the combustion of carbon and hydrocarbons,carbon monoxide, is of special importance because it is apoisonous gas, but also economically useful for the production ofsyngas.
Solid and heavy liquid fuels also undergo a great number ofpyrolysis reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the ongoing combustion reactions. A lack of oxygen or other improperly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.
The rate of combustion is the amount of a material that undergoes combustion over a period of time. It can be expressed in grams per second (g/s) or kilograms per second (kg/s).
Detailed descriptions of combustion processes, from the chemical kinetics perspective, require the formulation of large and intricate webs of elementary reactions.[29] For instance, combustion of hydrocarbon fuels typically involve hundreds of chemical species reacting according to thousands of reactions.
The inclusion of such mechanisms within computational flow solvers still represents a pretty challenging task mainly in two aspects. First, the number of degrees of freedom (proportional to the number of chemical species) can be dramatically large; second, the source term due to reactions introduces a disparate number of time scales which makes the wholedynamical system stiff. As a result, the direct numerical simulation of turbulent reactive flows with heavy fuels soon becomes intractable even for modern supercomputers.[30]
Therefore, a plethora of methodologies have been devised for reducing the complexity of combustion mechanisms without resorting to high detail levels. Examples are provided by:
The kinetic modelling may be explored for insight into the reaction mechanisms of thermal decomposition in the combustion of different materials by using for instanceThermogravimetric analysis.[49]
Antoine Lavoisier conducting an experiment related to combustion generated by amplified sunlight
Assuming perfect combustion conditions, such as complete combustion underadiabatic conditions (i.e., no heat loss or gain), the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on thefirst law of thermodynamics and takes note of the fact that theheat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as theflue gas).
In the case offossil fuels burnt in air, the combustion temperature depends on all of the following:
The adiabatic combustion temperature (also known as theadiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.
Most commonly, the adiabatic combustion temperatures for coals are around 2,200 °C (3,992 °F) (for inlet air and fuel at ambient temperatures and for), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) fornatural gas.[50][51]
In industrialfired heaters,power stationsteam generators, and largegas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air ispercent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used.
Combustion instabilities are typically violent pressure oscillations in a combustion chamber. These pressure oscillations can be as high as 180dB, and long-term exposure to these cyclic pressure and thermal loads reduces the life of engine components. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage to the combustion chamber and surrounding components. This problem was solved by re-designing the fuel injector. In liquid jet engines, the droplet size and distribution can be used to attenuate the instabilities. Combustion instabilities are a major concern in ground-based gas turbine engines because ofNOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce theNOx emissions; however, running the combustion lean makes it very susceptible to combustion instability.
TheRayleigh Criterion is the basis for analysis of thermoacoustic combustion instability and is evaluated using the Rayleigh Index over one cycle of instability[52]
where q' is the heat release rate perturbation and p' is the pressure fluctuation.[53][54]When the heat release oscillations are in phase with the pressure oscillations, the Rayleigh Index is positive and the magnitude of the thermoacoustic instability is maximised. On the other hand, if the Rayleigh Index is negative, then thermoacoustic damping occurs. The Rayleigh Criterion implies that thermoacoustic instability can be optimally controlled by having heat release oscillations 180 degrees out of phase with pressure oscillations at the same frequency.[55][56] This minimizes the Rayleigh Index.
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^Reyes, J.A.; Conesa, J.A.; Marcilla, A. (2001). "Pyrolysis and combustion of polycoated cartons recycling. kinetic model and ms analysis".Journal of Analytical and Applied Pyrolysis.58–59:747–763.doi:10.1016/S0165-2370(00)00123-6.
^John William Strutt, 3rd Baron Rayleigh, Sc.D., F.R.S.., Honorary Fellow of Trinity College, Cambridge; "The Theory of Sound", §322h, 1878:
^A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube",Fourth Symposium (International) on Combustion, The Combustion Institute, pp. 566–574.
^E. C. Fernandes and M. V. Heitor,"Unsteady flames and the Rayleigh criterion" in F. Culick, M. V. Heitor, and J. H. Whitelaw, ed.s,Unsteady Combustion (Dordrecht, the Netherlands: Kluwer Academic Publishers, 1996), p. 4
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Look upcombustion in Wiktionary, the free dictionary.
Poinsot, Thierry; Veynante, Denis (2012).Theoretical and Numerical Combustion (3rd ed.). European Centre for Research and Advanced Training in Scientific Computation. Archived fromthe original on 2017-09-12. Retrieved2011-11-18.
