Combustion
The flames caused as a result of a fuel undergoing combustion (burning)

Combustion (English pronunciation: /kəmˈbʌs.tʃən /) or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid phase.

In a complete combustion reaction, a compound reacts with an oxidizing element, such as oxygen or fluorine, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH4 + 2 O2CO2 + 2 H2O + energy
CH2S + 6 F2CF4 + 2 HF + SF6[discuss]

A simple example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines:

2 H2 + O2 → 2 H2O(g) + heat

The result is water vapor.

Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present such as carbon monoxide and pure carbon (soot or ash). Additionally, any combustion in atmospheric air, which is 78% nitrogen, will also create several forms of nitrogen oxides.

Contents

Types

Complete vs. incomplete

Incomplete combustion, the reactant burns in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, nitrogen will yield nitrogen dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide.

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 in combustion of sulfur. Nitrogen oxides start to form above 2,800 °F (1,540 °C) and more nitrogen oxides are produced at higher temperatures. Below this temperature, molecular nitrogen (N2) is favored. It is also a function of oxygen excess.[1]

In most industrial applications and in fires, air is the source of oxygen (O2). In air, each mole of oxygen is mixed with approximately 3.76 mole of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to NOx, usually between 1% and 0.002% (2 ppm).[2] Furthermore, when there is any incomplete combustion, some of carbon is converted to carbon monoxide. A more complete set of equations for combustion of methane in air is therefore:

CH4 + 2 O2CO2 + 2 H2O
2 CH4 + 3 O2 → 2 CO + 4 H2O
N2 + O2 → 2 NO
N2 + 2 O2 → 2 NO2

Incomplete

Incomplete combustion will only 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.

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 harmful acetaldehyde, and carbon can produce toxic carbon monoxide.

The quality of combustion can be improved by design of combustion devices, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process. Such devices are required by environmental legislation for cars in most countries, and may be necessary in large combustion devices, such as thermal power plants, to reach legal emission standards.

The degree of combustion can be measured and analyzed, with test equipment. HVAC contractors, firemen and engineers use combustion analyzers to test the efficiency of a burner during the combustion process. In addition, the efficiency of an internal combustion engine can be measured in this way, and some states and local municipalities are using combustion analysis to define and rate the efficiency of vehicles on the road today.

Smoldering

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, charring polymers including polyurethane foam, and dust. Common examples of smoldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires

Rapid

Container of ethanol vapour mixed with air, undergoing rapid combustion
Container of ethanol vapour mixed with air, undergoing rapid combustion

Rapid combustion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons. Sometimes, a large volume of gas is liberated in combustion besides the production of heat and light. The sudden evolution of large quantities of gas creates excessive pressure that produces a loud noise. Such a combustion is known as an explosion. Combustion need not involve oxygen; e.g., hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion.

Turbulent

Combustion resulting in a turbulent flame is the most used for industrial application (e.g. gas turbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

Microgravity

Colourized gray-scale composite image of the individual frames from a video of a backlit fuel droplet burning in microgravity.

Combustion processes behave differently in a microgravity environment than in Earth-gravity conditions due to the lack of buoyancy. For example, a candle's flame takes the shape of a sphere.[3] Microgravity combustion research contributes to understanding of spacecraft fire safety and diverse aspects of combustion physics.

Micro Combustion

Combustion processes which happen in very small volume are considered as micro combustion. Quenching distance plays a vital role in stabilizing the flame in such combustion chambers.

Chemical Equation

Generally, the chemical equation for stoichiometric burning of hydrocarbon in oxygen is

C_xH_y + \left( x + \frac{y}{4} \right) O_2 \rightarrow \; xCO_2 + \left( \frac{y}{2} \right) H_2O

For example, the burning of propane is

C_3H_8 + 5O_2 \rightarrow \; 3CO_2 + 4H_2O

Generally, the chemical equation for stoichiometric incomplete combustion of hydrocarbon in oxygen is as follows:

\left( z \right) C_xH_y + \left( z \left(\frac{x}{2} + \frac{y}{4} \right) \right) O_2 \rightarrow \; z \cdot xCO + \left( \frac{z \cdot y}{2} \right) H_2O

For example, the incomplete combustion of propane is:

2C_3H_8 + 7O_2 \rightarrow \; 2C + 2CO + 8H_2O + 2CO_2

The simple word equation for the combustion of a hydrocarbon in oxygen is:

\textrm{Fuel} + \textrm{Oxygen} \rightarrow \; \textrm{Heat} + \textrm{Water} + \textrm{Carbon\ dioxide}

If the combustion takes place using air as the oxygen source, the nitrogen can be added to the equation,as and although it does not react, to show the composition of the flue gas:

C_xH_y + \left( x+ \frac{y}{4} \right) O_2 + 3.76 \left( x+ \frac{y}{4} \right) N_2 \rightarrow \; xCO_2 + \left( \frac{y}{2} \right) H_2O + 3.76 \left( x + \frac{y}{4} \right) N_2

For example, the burning of propane is:

C_3H_8 + 5O_2 + 18.8N_2 \rightarrow \; 3CO_2 + 4H_2O + 18.8N_2

The simple word equation for this type of combustion is hydrocarbon in air:

\textrm{Fuel} + \textrm{Air} \rightarrow \; \textrm{Heat} + \textrm{Water} + \textrm{Carbon\ dioxide} + \textrm{Nitrogen}

Nitrogen may also oxidize when there is an excess of oxygen. The reaction is thermodynamically favored only at high temperatures. Diesel engines are run with an excess of oxygen to combust small particles that tend to form with only a stoichiometric amount of oxygen, necessarily producing nitrogen oxide emissions. Both the United States and European Union are planning to impose limits to nitrogen oxide emissions, which necessitate the use of a special catalytic converter or treatment of the exhaust with urea.

Fuels

Liquid fuels

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

Solid fuels

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

  • Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
  • Distillation phase or gaseous 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 or solid phase, when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly and just glows and later only smoulders.
A general scheme of polymer combustion

Reaction mechanism

Combustion in oxygen is a radical chain reaction where many distinct radical intermediates participate.

The high energy required for initiation is explained by the unusual structure of the dioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a triplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, whose spins are 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, or singlet oxygen. This intermediate is extremely reactive. The energy is supplied as heat. The reaction produces heat, which keeps it going.

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 give hydroxyl radicals. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced in incomplete combustion. An example is acetaldehyde produced in the combustion of ethanol. An intermediate in the combustion of carbon and hydrocarbons, carbon monoxide, is of special importance because it is a poisonous gas, but also economically useful for the production of syngas.

Solid fuels also undergo a great number of pyrolysis reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the combustion reactions. A lack of oxygen or other poorly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

Temperature

Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions (i.e., no heat loss or gain), the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the flue gas).

In the case of fossil fuels burnt in air, the combustion temperature depends on all of the following:

The adiabatic combustion temperature (also known as the adiabatic 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 λ = 1.0), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) for natural gas.

In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent 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 180 dB, 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 of 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 of NOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the NOx emissions; however, running the combustion lean makes it very susceptible to combustion instabilities.

The Rayleigh Criterion is the basis for analysis of thermoacoustic combustion instabilities and is evaluated using the Rayleigh Index over one cycle of instability:[citation needed]

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.[4][5] When the heat release oscillations are in phase with the pressure oscillations, the Rayleigh Index is positive and the magnitude of the thermo acoustic instability increases. On the other hand, if the Rayleigh Index is negative, then thermoacoustic damping occurs. The Rayleigh Criterion implies that a 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.[citation needed]

Rate of Combustion

Rate of combustion is the amount of mass of a material that goes through combustion over an amount of time. It can be expressed in g/s or kg/s.

See also

Related concepts
Machines and equipment
Measurement techniques
Social applications and issues

References

  1. ^ The formation of NOx. Alentecinc.com. Retrieved on 2010-09-28.
  2. ^ CHP Emissions. Northeastchp.org. Retrieved on 2010-09-28.
  3. ^ Shuttle-Mir History/Science/Microgravity/Candle Flame in Microgravity (CFM) – MGBX. Spaceflight.nasa.gov (1999-07-16). Retrieved on 2010-09-28.
  4. ^ 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.
  5. ^ 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

External links


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Synonyms:

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  • combustion — Combustion …   Thresor de la langue françoyse

  • COMBUSTION — La combustion recouvre des phénomènes très variés. La définition même du terme répond à deux tendances différentes. La première lui donne un sens très général. Elle englobe «combustion vive» et «combustion lente». La combustion de l’air dans les… …   Encyclopédie Universelle

  • combustion — COMBUSTION. s. f. Action de brûler entièrement. C est la combustion de l air qui fait la flamme. La combustion des cadavres étoit très dispendieuse chez les Romains. [b]f♛/b] Il se dit d Un incendie qui détruit totalement un édifice. J ai vu la… …   Dictionnaire de l'Académie Française 1798

  • Combustion — Com*bus tion (?; 106), n. [L. combustio: cf. F. combustion.] 1. The state of burning. [1913 Webster] 2. (Chem.) The combination of a combustible with a supporter of combustion, producing heat, and sometimes both light and heat. [1913 Webster]… …   The Collaborative International Dictionary of English

  • combustión — sustantivo femenino 1. Uso/registro: elevado. Acción y resultado de quemarse o arder un cuerpo: La combustión de la madera del almacén no permitió a los bomberos apagar el fuego. La combustión del carbón de la bodega llenó en seguida la casa de… …   Diccionario Salamanca de la Lengua Española

  • combustion — Combustion. s. f. Division, brouillerie & tumulte. Grande combustion. horrible combustion. tout le Royaume estoit en combustion. il a mis tout l Estat, toute la ville, toute la famille en combustion …   Dictionnaire de l'Académie française

  • combustión — Proceso de quemar u oxidación que se puede acompañar de luz y calor. El oxígeno por sí mismo no arde, pero facilita la combustión. La velocidad de combustión está determinada tanto por la concentración de oxígeno como por su presión parcial …   Diccionario médico

  • combustión — (Del lat. combustĭo, ōnis). 1. f. Acción y efecto de arder o quemar. 2. Biol. Oxidación de los alimentos en los seres vivos. 3. Quím. Reacción química entre el oxígeno y un material oxidable, acompañada de desprendimiento de energía y que… …   Diccionario de la lengua española

  • combustion — early 15c., from O.Fr. combustion (13c.), from L. combustionem (nom. combustio) a burning, noun of action from pp. stem of comburere to burn, from com , intensive prefix (see COM (Cf. com )), + *burere, faulty separation of amburere to burn… …   Etymology dictionary

  • combustion — [n] explosion; on fire agitation, candescence, disturbance, flaming, ignition, kindling, oxidization, thermogenesis, tumult, turmoil; concepts 521,676,724 …   New thesaurus

  • combustion — ► NOUN 1) the process of burning. 2) Chemistry rapid chemical combination with oxygen, involving the production of heat and light …   English terms dictionary

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