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

Flameless combustion is a well known measure to reduce NO x 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 CO 2 or H 2 O is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with O 2 . In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily CO 2 and H 2 O , and the thereby reduced concentration of O 2 in the reaction zone the local partial pressures of CO 2 and H 2 O become significantly higher than that for O 2 . Therefore, the char reaction with CO 2 and H 2 O 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.