A burning, usually in a supply of oxygen to form oxides. The complete combustion of a hydrocarbon yields carbon dioxide and water. The energy associated with the combustion of a mole of a substance is called its heat of combustion.
Combustion or burning is a complex sequence of 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.
In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. For example:
A simpler example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines:
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:
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.
Types
Rapid
Rapid combustion is a form of combustion in which large amounts of heat and light energy are released.
Slow
Slow combustion is a form of combustion which takes place at low temperatures.
Complete
In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen. For example, the combustion of methane in air will yield, in addition to the major products of carbon dioxide and water, the minor products which include carbon monoxide, hydroxyl, nitrogen oxides, monatomic hydrogen, and monatomic oxygen.
Turbulent
Turbulent combustion is a combustion characterized by turbulent flows.
Incomplete
Incomplete combustion happens when there is an inadequate supply of oxygen for combustion to occur completely. 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.
Quality of combustion can be improved by design of combustion devices, such as burners and internal combustion engines.
Smouldering
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.
Chemical equation
Generally, the chemical equation for burning a hydrocarbon (such as octane) in oxygen is as follows:
For example, the burning of propane is:
The simple word equation for the combustion of a hydrocarbon in oxygen is:
If the combustion takes place using air as the oxygen source, the corresponding equations are:
For example, the burning of propane is:
The simple word equation for the combustion of a hydrocarbon in air is:
Fuels
Liquid fuels
Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. 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.Temperature
Assuming perfect combustion conditions, such as an adiabatic (no heat loss and no heat gain) and complete combustion, 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
the heating value the stoichiometric air to fuel ratio λ the heat capacity of fuel and air the air and fuel inlet temperaturesThe 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.
Typically, the adiabatic combustion temperatures for coals are around 1500 °C (for inlet air and fuel at ambient temperatures and for λ = 1.0), around 2000 °C for oil and 2200 °C 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.
Analysis
Combustion analysis is a process used to determine the composition of organic compounds.
Instabilities
Combustion instabilities are typically violent pressure oscillations in a combustion chamber.
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