Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NOx emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO2/O2, and O2 enriched recirculated flue gas. NOx emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O2 concentration on NOx formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NOx emissions has been obtained for air and CO2/O2 in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NOx emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO2/O2. No significant impact of the oxidiser O2 concentration in the CO2/O2 mixture on NOx emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NOx formation and reduction mechanisms showed, that the observed reduction of NOx emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NOx alone, but also to a reduced conversion of fuel-N to NO.