Analysis of chemical kinetics of the trade-off between soot and nitrogen oxides in diesel combustion

As suggested by the equivalence ratio-temperature (φ- T) diagram, soot could be burnt off at 1800<T<2 200 K where there is little formation of nitrogen oxides (NOx). Accordingly, nonsooting and low NOx combustion would be possible with moderate rates of exhaust gas recirculation (EGR), and without severe deterioration in combustion efficiency. However, complete burning-off of soot particles needs both sufficiently high temperatures and long residence times that would be favorable for NOx formation. Therefore, a detailed examination of the kinetics of soot particle oxidation and NOx formation processes would be useful. This study employs the detailed mechanisms from gas reactions to soot particle formation and oxidation, and a two-stage model is developed and used to investigate the trade-off between soot and nitrogen oxides emissions. The effects of the equivalence ratio, EGR, ambient pressure and temperature, and initial particle diameter are observed for various residence times. The results show that high rates of NOx formation are unavoidable under conditions where high reduction rates of soot particles are obtained.