Combustion

"Burning" redirects here. For the type of injury, seeBurn.For combustion without external ignition, seespontaneous combustion.For the vehicle engine, seeinternal combustion engine.For other uses, seeBurning (disambiguation),Combustion (disambiguation),andFiring (disambiguation).

Combustion,orburning,is a high-temperatureexothermicredoxchemical reactionbetween 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 aflameis only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. Whileactivation energymust be supplied to initiate combustion (e.g., using a litmatchto light a fire), the heat from a flame may provide enough energy to make the reactionself-sustaining.The study of combustion is known ascombustion science.

Theflamescaused as a result of afuelundergoing combustion (burning)

Air pollution abatement equipmentprovides combustion control forindustrial processes.

Combustion is often a complicated sequence ofelementaryradical reactions.Solid fuels,such aswoodandcoal,first undergoendothermicpyrolysisto produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough thatincandescentlightin the form of eitherglowingor aflameis produced. A simple example can be seen in the combustion ofhydrogenandoxygenintowatervapor,a reaction which is commonly used to fuelrocket engines.This reaction releases 242kJ/molof heat and reduces theenthalpyaccordingly (at constant temperature and pressure):

${\ce {2H_{2}(g){+}O_{2}(g)\rightarrow 2H_{2}O\uparrow }}$

Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion isstoichiometricconcerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, thechemical equilibriumof 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,hydrogenand evencarbon(sootor ash). Thus, the producedsmokeis 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 convertersmay be required by law.

Firesoccur naturally, ignited bylightningstrikes or byvolcanicproducts. Combustion (fire) was the first controlled chemical reaction discovered by humans, in the form ofcampfiresandbonfires,and continues to be the main method to produce energy for humanity. Usually, the fuel iscarbon,hydrocarbons,or more complicated mixtures such aswoodthat contain partially oxidized hydrocarbons. The thermal energy produced from the combustion of eitherfossil fuelssuch ascoaloroil,or fromrenewable fuelssuch asfirewood,is harvested for diverse uses such ascooking,production ofelectricityor 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.

Oxidants for combustion have high oxidation potential and include atmospheric or pureoxygen,chlorine,fluorine,chlorine trifluoride,nitrous oxideandnitric acid.For instance,hydrogenburns inchlorineto formhydrogen chloridewith the liberation of heat and light characteristic of combustion. Although usually not catalyzed, combustion can be catalyzed byplatinumorvanadium,as in thecontact process.

Types

Complete and incomplete

See also:pyrolysis

Complete

The combustion ofmethane,ahydrocarbon

In complete combustion, the reactant burns in oxygen and produces a limited number of products. When ahydrocarbonburns 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
_xspecies) 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 trioxideis not produced quantitatively by the combustion of sulfur.NO_xspecies appear in significant amounts above about 2,800 °F (1,540 °C), and more is produced at higher temperatures. The amount ofNO_xis also a function of oxygen excess.

In most industrial applications and infires,airis the source of oxygen (O
₂). In the air, each mole of oxygen is mixed with approximately3.71molof 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".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% foranthracitecoal, to 300% for agas turbine.

Incomplete

See also:Charring

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,carbonandcarbon monoxideare produced instead of carbon dioxide.

For most fuels, such as diesel oil, coal, or wood,pyrolysisoccurs 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 designs of combustion devices can improve the quality of combustion, such asburnersandinternal combustion engines.Further improvements are achievable bycatalyticafter-burning devices (such ascatalytic converters) or by the simple partial return of theexhaust gasesinto the combustion process. Such devices are required byenvironmental legislationfor cars in most countries. They may be necessary to enable large combustion devices, such asthermal power stations,to reach legalemission standards.

The degree of combustion can be measured and analyzed with test equipment.HVACcontractors,firefightersandengineersuse combustion analyzers to test theefficiencyof 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.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.

Environmental problems

These oxides combine withwaterandoxygenin the atmosphere, creatingnitric acidandsulfuric 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 oxidesis that they, along withhydrocarbonpollutants, contribute to the formation ofground level ozone,a major component of smog.

Human health problems

Breathingcarbon monoxidecauses 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 poisoningmay suffer long-term health problems.Carbon monoxide from the air is absorbed in the lungs which then binds withhemoglobinin human's red blood cells. This reduces the capacity of red blood cells that carry oxygen throughout the body.

Smoldering

Smolderingis 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 furnitureby weak heat sources (e.g., a cigarette, a short-circuited wire) and the persistentcombustion of biomassbehind the flaming fronts ofwildfires.

Spontaneous

Spontaneous combustionis a type of combustion that occurs by self-heating (increase in temperature due toexothermicinternal 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 bacterialcompostingcan generate enough heat to reach the point of combustion.

Turbulent

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.

Micro-gravity

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 ofbuoyancyon 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 dynamicscan behave quite differently than in normal gravity conditions (e.g., acandle's flame takes the shape of a sphere.). 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).

Micro-combustion

Combustion processes that happen in very small volumes are consideredmicro-combustion.The high surface-to-volume ratio increases specific heat loss.Quenchingdistance plays a vital role in stabilizing the flame in suchcombustion chambers.

Chemical equations

Stoichiometric combustion of a hydrocarbon in oxygen

Generally, thechemical equationforstoichiometriccombustion of ahydrocarbonin oxygen is:

${\ce {C}}_{x}{\ce {H}}_{y}+\left(x+{y \over 4}\right){\ce {O2->}}x{\ce {CO2}}+{y \over 2}{\ce {H2O}}$

For example, the stoichiometric combustion ofmethanein oxygen is:

${\ce {{\underset {methane}{CH4}}+ 2O2 -> CO2 + 2H2O}}$

Stoichiometric combustion of a hydrocarbon in air

If the stoichiometric combustion takes place using air as the oxygen source, thenitrogenpresent 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:

${\ce {C}}_{x}{\ce {H}}_{y}+z{\ce {O2}}+3.77z{\ce {N2 ->}}x{\ce {CO2}}+{y \over 2}{\ce {H2O}}+3.77z{\ce {N2}}$

where$z=x+{y \over 4}$.

For example, the stoichiometric combustion of methane in air is:

${\ce {{\underset {methane}{CH4}}+ 2O2}}+7.54{\ce {N2-> CO2 + 2H2O}}+7.54{\ce {N2}}$

The stoichiometric composition of methane in air is 1 / (1 + 2 + 7.54) = 9.49% vol.

The stoichiometric combustion reaction for C_αH_βO_γin air:

${\ce {C_{\mathit {\alpha }}H_{\mathit {\beta }}O_{\mathit {\gamma }}}}+\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}\right)\left({\ce {O_{2}}}+3.77{\ce {N_{2}}}\right)\longrightarrow \alpha {\ce {CO_{2}}}+{\frac {\beta }{2}}{\ce {H_{2}O}}+3.77\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}\right){\ce {N_{2}}}$

The stoichiometric combustion reaction for C_αH_βO_γS_δ:

${\ce {C_{\mathit {\alpha }}H_{\mathit {\beta }}O_{\mathit {\gamma }}S_{\mathit {\delta }}}}+\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}+\delta \right)\left({\ce {O_{2}}}+3.77{\ce {N_{2}}}\right)\longrightarrow \alpha {\ce {CO_{2}}}+{\frac {\beta }{2}}{\ce {H_{2}O}}+\delta {\ce {SO_{2}}}+3.77\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}+\delta \right){\ce {N_{2}}}$

The stoichiometric combustion reaction for C_αH_βO_γN_δS_ε:

${\ce {C_{\mathit {\alpha }}H_{\mathit {\beta }}O_{\mathit {\gamma }}N_{\mathit {\delta }}S_{\mathit {\epsilon }}}}+\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}+\epsilon \right)\left({\ce {O_{2}}}+3.77{\ce {N_{2}}}\right)\longrightarrow \alpha {\ce {CO_{2}}}+{\frac {\beta }{2}}{\ce {H_{2}O}}+\epsilon {\ce {SO_{2}}}+\left(3.77\left(\alpha +{\frac {\beta }{4}}-{\frac {\gamma }{2}}+\epsilon \right)+{\frac {\delta }{2}}\right){\ce {N_{2}}}$

The stoichiometric combustion reaction for C_αH_βO_γF_δ:

${\ce {C_{\mathit {\alpha }}H_{\mathit {\beta }}O_{\mathit {\gamma }}F_{\mathit {\delta }}}}+\left(\alpha +{\frac {\beta -\delta }{4}}-{\frac {\gamma }{2}}\right)\left({\ce {O_{2}}}+3.77{\ce {N_{2}}}\right)\longrightarrow \alpha {\ce {CO_{2}}}+{\frac {\beta -\delta }{2}}{\ce {H_{2}O}}+\delta {\ce {HF}}+3.77\left(\alpha +{\frac {\beta -\delta }{4}}-{\frac {\gamma }{2}}\right){\ce {N_{2}}}$

Trace combustion products

Various other substances begin to appear in significant amounts in combustion products when theflame temperatureis above about1600K.When excess air is used, nitrogen may oxidize toNOand, to a much lesser extent, toNO
₂.COforms bydisproportionationof CO₂,andH
₂andOHform by disproportionation ofH₂O.

For example, when1molofpropaneis burned with28.6molof air (120% of the stoichiometric amount), the combustion products contain 3.3%O
₂.At1400K,theequilibriumcombustion products contain 0.03%NOand 0.002%OH.At1800K,the combustion products contain 0.17%NO,0.05%OH,0.01%CO,and 0.004%H
₂.

Diesel enginesare run with an excess of oxygen to combust smallparticlesthat tend to form with only a stoichiometric amount of oxygen, necessarily producingnitrogen oxideemissions. Both the United States and European Unionenforce limitsto vehicle nitrogen oxide emissions, which necessitate the use of specialcatalytic convertersor treatment of the exhaust withurea(seeDiesel exhaust fluid).

Incomplete combustion of a hydrocarbon in oxygen

The incomplete (partial) combustion of ahydrocarbonwith 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-treatmentof metals and forgas carburizing.The general reaction equation for incomplete combustion of onemoleof a hydrocarbon in oxygen is:

${\ce {{\underset {fuel}{C_{\mathit {x}}H_{\mathit {y}}}}+{\underset {oxygen}{{\mathit {z}}O2}}->{\underset {carbon\ dioxide}{{\mathit {a}}CO2}}+{\underset {carbon\ monoxide}{{\mathit {b}}CO}}+{\underset {water}{{\mathit {c}}H2O}}+{\underset {hydrogen}{{\mathit {d}}H2}}}}$

Whenzfalls below roughly 50% of the stoichiometric value,CH
₄can become an important combustion product; whenzfalls below roughly 35% of the stoichiometric value, elementalcarbonmay 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.For example, in the combustion of onemoleof propane (C
₃H
₈) with four moles ofO
₂,seven moles of combustion gas are formed, andzis 80% of the stoichiometric value. The three elemental balance equations are:

• Carbon:$a+b=3$
• Hydrogen:$2c+2d=8$
• Oxygen:$2a+b+c=8$

These three equations are insufficient in themselves to calculate the combustion gas composition. However, at the equilibrium position, thewater-gas shift reactiongives another equation:

${\ce {CO + H2O -> CO2 + H2}}$;$K_{eq}={\frac {a\times d}{b\times c}}$

For example, at1200Kthe value ofK_eqis 0.728.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 at1200Kand1atmpressure when z is less than 30% of the stoichiometric value, at which point the combustion products contain more than 98%H
₂andCOand 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.

Liquid fuels

Combustion of aliquid fuelin 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.

Gaseous fuels

Combustion of gaseous fuels may occur through one of four distinctive types of burning:diffusion flame,premixed flame,autoignitive reaction front,or as adetonation.The type of burning that actually occurs depends on the degree to which thefuelandoxidizerare 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.

Solid fuels

A general scheme ofpolymercombustion

The act of combustion consists of three relatively distinct but overlapping phases:

• Preheating phase,when the unburnedfuelis heated up to its flash point and thenfire point.Flammable gases start being evolved in a process similar todry distillation.
• Distillation phaseorgaseous phase,when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light.Flamesare often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
• Charcoal phaseorsolid phase,when the output of flammable gases from the material is too low for the persistent presence of flame and thecharredfuel does not burn rapidly and just glows and later onlysmoulders.

Combustion management

Efficientprocess heatingrequires recovery of the largest possible part of a fuel'sheat of combustioninto the material being processed.There are many avenues of loss in the operation of a heating process. Typically, the dominant loss issensible heatleaving 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 (usuallyCOandH
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, theNOxlevel is lowest when excess oxygen is kept lowest.

Adherence to these two principles is furthered by making material and heat balances on the combustion process.Thematerial balancedirectly relates theair/fuel ratioto the percentage ofO
₂in the combustion gas. The heat balance relates the heat available for the charge to the overall net heat produced by fuel combustion.Additional material and heat balances can be made to quantify the thermal advantage from preheating the combustion air,or enriching it in oxygen.

Reaction mechanism

Combustion in oxygen is achain reactionin which many distinctradicalintermediates participate. The high energy required for initiation is explained by the unusual structure of thedioxygenmolecule. 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, withspinsaligned, 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 isacetaldehydeproduced 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 ofpyrolysisreactions 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.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 systemstiff. As a result, the direct numerical simulation of turbulent reactive flows with heavy fuels soon becomes intractable even for modern supercomputers.

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 Relaxation Redistribution Method (RRM)
• The Intrinsic Low-Dimensional Manifold (ILDM) approach and further developments
• The invariant-constrained equilibrium edge preimage curve method.
• A few variational approaches
• The Computational Singular perturbation (CSP) method and further developments.
• The Rate Controlled Constrained Equilibrium (RCCE) and Quasi Equilibrium Manifold (QEM) approach.
• The G-Scheme.
• The Method of Invariant Grids (MIG).

Kinetic modelling

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.

Temperature

Antoine Lavoisierconducting an experiment related to combustion generated by amplified sunlight

Assuming perfect combustion conditions, such as complete combustion underadiabaticconditions (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 thermodynamicsand takes note of the fact that theheat of combustionis 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 fuelsburnt in air, the combustion temperature depends on all of the following:

• theheating value;
• thestoichiometric air to fuel ratio${\lambda }$;
• thespecific heat capacityof fuel and air;
• the air and fuel inlet temperatures.

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$\lambda =1.0$), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) fornatural gas.

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.

Instabilities

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 ofNO_xemissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce theNO_xemissions; however, running the combustion lean makes it very susceptible to combustion instability.

TheRayleigh Criterionis the basis for analysis of thermoacoustic combustion instability and is evaluated using the Rayleigh Index over one cycle of instability

$G(x)={\frac {1}{T}}\int _{T}q'(x,t)p'(x,t)dt$

where q' is the heat release rate perturbation and p' is the pressure fluctuation. 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.This minimizes the Rayleigh Index.

See also

Related concepts

• Air–fuel ratio
• Autoignition temperature
• Chemical looping combustion
• Deflagration
• Detonation
• Dust explosion
• Explosion
• Fire
• Flame
• Global warming
• Heterogeneous combustion
• Markstein number
• Phlogiston theory(historical)
• Spontaneous combustion

Machines and equipment

• Boiler
• Bunsen burner
• External combustion engine
• Furnace
• Gas turbine
• Internal combustion engine
• Rocket engine

Scientific and engineering societies

• International Flame Research Foundation
• The Combustion Institute

Other

• Combustible dust
• Biomass burning
• List of light sources
• Open burning of waste
• Stubble burning

Further reading

• Poinsot, Thierry; Veynante, Denis (2012).Theoretical and Numerical Combustion(3rd ed.). European Centre for Research and Advanced Training in Scientific Computation. Archived fromthe originalon 2017-09-12.Retrieved2011-11-18.
• Lackner, Maximilian; Winter, Franz; Agarwal, Avinash K., eds. (2010).Handbook of Combustion, 5 volume set.Wiley-VCH.ISBN978-3-527-32449-1.Archived fromthe originalon 2011-01-17.Retrieved2010-04-29.
• Baukal, Charles E., ed. (1998).Oxygen-Enhanced Combustion.CRC Press.
• Glassman, Irvin; Yetter, Richard.Combustion(Fourth ed.).
• Turns, Stephen (2011).An Introduction to Combustion: Concepts and Applications.
• Ragland, Kenneth W; Bryden, Kenneth M. (2011).Combustion Engineering(Second ed.).
• Baukal, Charles E. Jr, ed. (2013). "Industrial Combustion".The John Zink Hamworthy Combustion Handbook: Three-Volume Set(Second ed.).
• Gardiner, W. C. Jr (2000).Gas-Phase Combustion Chemistry(Revised ed.).