In the ideal case of complete combustion of a hydrocarbon fuel with stoichiometric air, the exhaust gas would be composed of the chemical species carbon dioxide (CO2), water (H2O) and molecular nitrogen (N2) only. For lean equivalence ratios, molecular oxygen (O2) could be observed among the products as well. However, in real combustion systems there are two reasons that inhibit complete combustion: (i) Elementary chemical reactions never proceed completely into one direction, but they always approach an equilibrium state between products and reactants. Thus, at least a small amount of reactants will remain. (ii) Local boundary conditions such as mixture distribution, temperature and turbulence level are often non-ideal. Therefore, flame extinction, accompanied with unburned or partially burned species, or the formation of entirely new products, e.g. soot or nitrogen oxides, may occur. Consequently, additional components are present in the exhaust gases of combustion engines. These components are carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter which is often approximated as soot. Depending on the quality of the fuel there may also be traces of sulfur oxides (SOx) within the exhaust gas.
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- Pollutant Formation
- verfasst von
Dr.-Ing. Gunnar Stiesch
- Springer Berlin Heidelberg