Combustion is the conversion of primary chemical energy contained in fuels such as coal, oil or wood into heat (secondary energy) through the process of oxidation. Combustion therefore is the technical term for the chemical reaction of oxygen with the combustible components of fuels including the release of energy.
Combustion processes proceed with high temperatures (up to 1000°C and above). The oxygen required for the combustion is supplied as part of the combustion air fed to the process. From that a considerable volume of exhaust gas (flue gas, off gas) is produced together with, depending on the kind of fuel, a certain amount of residues (slag, ash).
Process measurements for combustion optimization analyzing fuel, combustion air and combustion gases in order e.g. to reduce fuel costs, to improve plant efficiency or to extend the lifetime of the plant. These applications are also related to emission control because optimized plant operation normally will cause a reduction of emission levels.
Sigma offers a wide range of combustion analyzers which you can see here.