Combustion

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CH
₄ + 2 O
₂ → CO
₂ + 2 H₂O + energy

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

2 H
₂ + O
₂ → 2 H₂O(g) + heat

The result is water vapor.

Complete combustion is almost impossible to achieve. 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 percent nitrogen, will also create small amounts of several forms of nitrogen oxide, commonly referred to as NOx, at high temperatures.

Types

Complete vs. incomplete

See also: pyrolysis

In complete 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 (N
₂) is favored. It is also a function of oxygen excess.

In most industrial applications and in fires, air is the source of oxygen (O
₂). In air, each mole of oxygen is mixed with approximately 3.76 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to NO
_x, usually between 1% and 0.002% (2 ppm). 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:

CH
₄ + 2 O
₂ → CO
₂ + 2 H₂O

2 CH
₄ + 3 O
₂ → 2 CO + 4 H₂O

N
₂ + O
₂ → 2 NO

N
₂ + 2 O
₂ → 2 NO
₂

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 stations, 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

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. 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. 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 volumes are considered micro-combustion. The high surface-to-volume ratio increases specific heat loss. Quenching distance plays a vital role in stabilizing the flame in such combustion chambers.

Chemical equations

Stoichiometric combustion of a hydrocarbon in oxygen

Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:

$\mathrm {C} _{x}\mathrm {H} _{y}+\left(x+{\frac {y}{4}}\right)\mathrm {O_{2}} \rightarrow \;x\mathrm {CO_{2}} +\left({\frac {y}{2}}\right)\mathrm {H_{2}O}$

For example, the stoichiometric burning of propane in oxygen is:

$\mathrm {C_{3}H_{8}} +\mathrm {5O_{2}} \rightarrow \;\mathrm {3CO_{2}} +\mathrm {4H_{2}O}$

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

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

Stoichiometric combustion of a hydrocarbon in air

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

$\mathrm {C} _{x}\mathrm {H} _{y}\ +\ \left(x+{\frac {y}{4}}\right)\mathrm {O_{2}} \ +\ 3.76\left(x+{\frac {y}{4}}\right)\mathrm {N_{2}} \rightarrow \;x\mathrm {CO_{2}} \ +\ \left({\frac {y}{2}}\right)\mathrm {H_{2}O} \ +\ 3.76\left(x+{\frac {y}{4}}\right)\mathrm {N_{2}}$

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

$\mathrm {C_{3}H_{8}} +\mathrm {5O_{2}} +\mathrm {18.8N_{2}} \rightarrow \;\mathrm {3CO_{2}} +\mathrm {4H_{2}O} +\mathrm {18.8N_{2}}$

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

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

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 have limits to nitrogen oxide emissions, which necessitate the use of a special catalytic converter or treatment of the exhaust with urea (see Diesel exhaust fluid).

Incomplete combustion of a hydrocarbon in oxygen

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

$z\mathrm {C} _{x}\mathrm {H} _{y}+z\cdot \left({\frac {x}{2}}+{\frac {y}{4}}\right)\mathrm {O_{2}} \rightarrow \;z\cdot x\mathrm {CO} +\left({\frac {z\cdot y}{2}}\right)\mathrm {H_{2}O}$

For example, the incomplete combustion of propane is:

$\mathrm {2C_{3}H_{8}} +\mathrm {7O_{2}} \rightarrow \;\mathrm {2C+2CO+8H_{2}O+2CO_{2}}$

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

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

Fuels

Substances or materials which undergo combustion are called fuels. The most common examples are kerosene, diesel, petrol, charcoal, coal, wood, etc.

A good fuel is one which is readily available, is cheap, burns easily in air and at a moderate rate, has a high calorific value and is environment friendly. There is probably no fuel which can be considered an ideal fuel.

Liquid fuels

Combustion of a liquid 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: 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 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 and heavy liquid 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.

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).

Temperature

Antoine Lavoisier conducting an experiment related combustion generated by amplified sun light.

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 heating value;
• the stoichiometric air to fuel ratio ${\lambda }$;
• the specific heat capacity of fuel and air;
• the air and fuel inlet temperatures.

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

In industrial fired heaters, power station 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 instability.

The Rayleigh Criterion is 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 thermo acoustic instability is maximised. 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.

See also

Related concepts

• Air-fuel ratio
• Autoignition temperature
• Deflagration
• Detonation
• Dust explosion
• Explosion
• Fire
• Flame
• Flue gas emissions from fossil fuel combustion
• Flue gas stacks
• Gas stoichiometry
• Heat of combustion
• Markstein number
• Phlogiston theory (historical)
• Pyrolysis
• Pyrophoric
• Smouldering
• Soot
• Spontaneous combustion
• Stoichiometry
• Chemical looping combustion

Machines and equipment

• Boiler
• Water heater
• Cyclone furnace
• Bunsen burner
• External combustion engine
• Industrial furnaces
• Gas turbine
• Internal combustion engine
• Jet engine
• Rocket engine
• Rotary combustion engine
• Staged combustion cycle (rocket)

Measurement techniques

• Calorimeter
• Coherent anti-Stokes Raman spectroscopy (CARS)
• Laser Doppler velocimetry
• Laser-induced fluorescence
• Particle image velocimetry
• Thin filament pyrometry

Social applications and issues

• Cooking
• Cooking oil
• Global warming

External links

• Thierry Poinsot and Denis Veynante (2012). Theoretical and Numerical Combustion (Third ed.). European Centre for Research and Advanced Training in Scientific Computation (CERFACS). {{cite book}}: External link in |title= (help).
• Maximilian Lackner, Franz Winter and Avinash K. Agarwal (Editors) (2010). Handbook of Combustion, 5 volume set. Wiley-VCH. ISBN 978-3-527-32449-1. {{cite book}}: |author= has generic name (help); External link in |title= (help)