In combustion, chemical energy is transformed to thermal and radiant energy.
Combustion (burning) is essentially a chemical reaction between a fuel and oxygen. Atoms in the fuel are held together by chemical bonds. When conditions are right, the atoms start to break apart from one another, setting off a chain reaction that releases more heat, causing more atoms to break away. Do you remember Smokey the Bear’s Fire Triangle?
The interaction of the three sides of the fire triangle (heat, fuel, and oxygen) are required for the creation and maintenance of any fire. When there is not enough heat generated to sustain the process, when the fuel is exhausted or when the oxygen supply is limited, a side of the triangle is broken and the fire is suppressed.
When heat is added to fuel and oxygen, the atoms in the fuel are energized and start to break apart from one another. Once released, these atoms form new bonds with the oxygen. Energy is required to break the chemical bonds in the fuel (an endothermic process). When the new chemical bonds are formed, energy is released (an exothermic process).
In combustion, more energy is released from the new bonds than is required to break the old bonds. This excess energy keeps the reaction going and produces heat and light.
Fossil fuels (coal, petroleum and natural gas) are excellent fuels because they are made up mostly of hydrocarbons (molecules with carbon hydrogen bonds). These molecules react readily with oxygen. The carbon combines with oxygen to form carbon dioxide (CO2). The hydrogen combines with oxygen to form water (H2O).
A primary component of natural gas is methane. Here’s the chemical reaction between methane (CH4) and oxygen which produces carbon dioxide, water, and heat.
A reaction between a hydrocarbon and oxygen yields carbon dioxide and water. But, when there are other elements present, these will become part of the reaction and resulting byproducts. For example, air contains nitrogen. A byproduct of combustion with air will be nitrogen dioxide (NO2). Similarly, the presence of sulfur results in sulfur dioxide (SO2). Depending on the conditions, carbon monoxide (CO) may also be produced. The products of combustion depend on the specifics of all the compounds involved in the reactions.
In all cases, combustion is a chemical reaction that produces products in the form of compounds, heat, and light.
Links
[1] http://chemistry.elmhurst.edu/vchembook/