Combustion

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Regenerative thermal oxidizer

Combustion reaction of methane

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Combustion is a high-temperature exothermic chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion in a fire produces a flame, and the heat produced can make combustion self-sustaining. Combustion is often a complex sequence of elementary radical reactions. Solid or liquid fuels undergo rapid pyrolysis or vaporization at the surface and release large quantities of volatile gases.

Combustion is an essential process in many aspects of modern life, from the operation of internal combustion engines in vehicles to the generation of electricity in power plants and the basic process of cooking and heating in homes. Understanding the principles of combustion is crucial for the design and operation of a wide range of equipment and is a central field in the study of chemical engineering, mechanical engineering, and fire safety engineering.

Types of Combustion

Combustion can be classified into several types based on the oxygen supply and the combustion characteristics:

• Complete combustion occurs when there is enough oxygen to allow the fuel to react completely to produce carbon dioxide and water, along with a maximum amount of heat. This type of combustion is desired in gasoline engines and furnaces where a clean and efficient burn is required.
• Incomplete combustion happens when there is insufficient oxygen for the fuel to react completely. It produces carbon monoxide, soot, or other partially oxidized products alongside carbon dioxide and water. Incomplete combustion is less efficient and can be hazardous due to the production of carbon monoxide.
• Spontaneous combustion occurs when a material self-ignites without the application of external heat. This can happen due to chemical reactions, biological processes, or the accumulation of heat due to poor heat dissipation.
• Rapid combustion is a form of combustion in which large amounts of heat and light are produced in a short period, often accompanied by a flame. This is the type of combustion observed in most fires.
• Explosive combustion or detonation occurs when the combustion front moves through the unburned fuel at a speed greater than the speed of sound. Explosives and certain types of engines, like the pulse jet engine, utilize this form of combustion.

Chemistry of Combustion

The basic chemical equation for combustion is typically represented as: \[ \text{Fuel} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{heat} \] However, the actual process is much more complex and involves a series of intermediate reactions and the formation of various intermediates.

The key to the combustion process is the breaking and forming of chemical bonds. Energy is required to break bonds in the fuel and oxygen molecules, and energy is released when new bonds form in the combustion products. The overall process releases energy because the energy required to break the bonds is less than the energy released when new bonds are formed.

Environmental Impact

Combustion processes are a major source of air pollution. They produce carbon dioxide, a greenhouse gas; nitrogen oxides, which contribute to smog and acid rain; and particulate matter, which can affect respiratory health. Reducing the environmental impact of combustion is a significant focus of environmental engineering and sustainable development.

See Also

• Fire
• Flame
• Fuel
• Oxidizing agent
• Carbon dioxide
• Water vapor

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