On the influence of the char gasification reactions on NO formation in flameless coal combustion

Flameless combustion is a well known measure to reduce NOxNOx emissions in gas combustion but has not yet been fully adapted to pulverised coal combustion. Numerical predictions can provide detailed information on the combustion process thus playing a significant role in understanding the basic mechanisms for pollutant formation. In simulations of conventional pulverised coal combustion the gasification by CO2CO2 or H2OH2O is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with O2O2. In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily CO2CO2 and H2OH2O, and the thereby reduced concentration of O2O2 in the reaction zone the local partial pressures of CO2CO2 and H2OH2O become significantly higher than that for O2O2. Therefore, the char reaction with CO2CO2 and H2OH2O is being reconsidered. This paper presents a numerical study on the importance of these reactions on pollutant formation in flameless combustion. The numerical models used have been validated against experimental data. By varying the wall temperature and the burner excess air ratio, different cases have been investigated and the impact of considering gasification on the prediction of NO formation has been assessed. It was found that within the investigated ranges of these parameters the fraction of char being gasified increases up to 35%. This leads to changes in the local gas composition, primarily CO distribution, which in turn influences NO formation predictions. Considering gasification the prediction of NO emission is up to 40% lower than the predicted emissions without gasification reactions being taken into account.