Combustion Totally Explained

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.
   Direct combustion by atmospheric oxygen is a reaction mediated by radical intermediates. The conditions for radical production are naturally produced by thermal runaway, where the heat generated by combustion is necessary to maintain the high temperature necessary for radical production.
   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:
ť + 2 → + 2

    + 6 → + 2 +
A simpler example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines:
ť 2 + → 2 + heat

The result is simply water vapor.
   In the large majority of the real world uses of combustion, the oxygen (O₂) oxidant is obtained from the ambient air and the resultant flue gas from the combustion will contain nitrogen:
ť + 2 + 7.52 → + 2 + 7.52 + heat

As can be seen, when air is the source of the oxygen, nitrogen is by far the largest part of the resultant flue gas.
   In reality, combustion processes are never perfect or complete. In flue gases from combustion of carbon (as in coal combustion) or carbon compounds (as in combustion of hydrocarbons, wood etc.) both unburned carbon (as soot) and carbon compounds (CO and others) will be present. Also, when air is the oxidant, some nitrogen will be oxidized to various, mostly harmful, nitrogen oxides (NO_x).

Types

Rapid

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released, which often results in a fire. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons.
Combustion is double replacement reaction. On the other hand a chemical reaction is single replacement reaction.

Slow

Slow combustion is a form of combustion which takes place at low temperatures. Respiration is an example of slow combustion.

Complete

In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen. When elements such as carbon, nitrogen, sulfur, and iron are burned, that'll yield the most common oxides. Carbon will yield carbon dioxide. Nitrogen will yield nitrogen dioxide. Sulfur will yield sulfur dioxide. Iron will yield iron(III) oxide. It should be noted that 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. For example, the combustion of methane in air will yield, in addition to the major products of carbon dioxide and water, the minor product carbon monoxide and nitrogen oxides, which are products of a side reaction (oxidation of nitrogen).

Turbulent

Turbulent combustion is a combustion characterized by turbulent flows. It is the most used for industrial application (for example gas turbines, diesel engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

Incomplete

Incomplete combustion occurs when there isn't enough oxygen to allow the fuel (usually a hydrocarbon) to react completely with the oxygen to produce carbon dioxide and water, also when the combustion is quenched by a heat sink such as a solid surface or flame trap. When a hydrocarbon burns in air, the reaction will yield carbon dioxide, water, carbon monoxide, pure carbon (soot or ash) and various other compounds such as nitrogen oxides. Incomplete combustion is much more common and will produce large amounts of byproducts, and in the case of burning fuel in automobiles, these byproducts can be quite unhealthy and damaging to the environment.
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.

Smoldering

Smouldering combustion is a flameless form of combustion, deriving its heat from heterogeneous reactions occurring on the surface of a solid fuel when heated in an oxidizing environment. The fundamental difference between smouldering and flaming combustion is that in smouldering, the oxidation of the reactant species occurs on the surface of the solid rather than in the gas phase. The characteristic temperature and heat released during smoldering are low compared to those in the flaming combustion of a solid. Typical values in smouldering are around 600 °C for the peak temperature and 5 kJ/g-O₂ for the heat released; typical values during flaming are around 1500 °C and 13 kJ/g-O₂ respectively. These characteristics cause smoulder to propagate at low velocities, typically around 0.1 mm/s, which is about two orders of magnitude lower than the velocity of flame spread over a solid. In spite of its weak combustion characteristics, smouldering is a significant fire hazard.

Combustion with other oxidants

Oxygen can be assumed as the oxidant when talking about combustion, but other oxidants exist. Nitrous oxide is used in rockets and in motorsport; it produces oxygen at over 1300 C. Fluorine, another oxidizing element, can produce a combustion reaction, to produce fluorinated products (rather than oxides). For example, mixtures of gaseous fluorine and methane are explosive, just like mixtures of oxygen and methane. Chlorine trifluoride is a strong fluorinating agent that ignites fuels more readily than oxygen.

Chemical equation

Generally, the chemical equation for stoichiometric burning of hydrocarbon in oxygen is as follows: ť

C_xH_y + left(x + fracq'(x,t)p'(x,t)dt

When the heat release oscillations are in phase with the pressure oscillations the Rayleigh Index is positive and the magnitude of the thermoacoustic instability increases. Consecutively 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.

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