Experimental study on the effects of ammonia cofiring ratio and injection mode on the NOx emission characteristics of ammonia-coal cofiring

Abstract

Ammonia (NH3) cofiring is a promising approach to reduce the CO2 emissions from coal-fired power plants. However, the potential drastic increase of NOx emissions may inhibit the wide implementation of NH3 cofiring. To explore the potential NOx control strategies, the effects of NH3 cofiring ratio (RNH3), NH3 injection mode and overfire air (OFA) rates on the NOx emission characteristics of NH3/coal cofiring are studied in a test furnace that allow for flexible control of the injection positions and combustion environment of NH3. The results show that when NH3 is injected with coal stream (top mixing mode), the NOx emissions increase and then decrease with the increase of RNH3 under all OFA rates. However, when NH3 is fed through the side port of furnace separately (side mixing mode), the NOx emissions exhibit distinct trends under different OFA rates. Under lower OFA rates, the NOx emissions show monotonical decrease with the increase of RNH3, while under higher OFA rates the NOx emissions first decrease and then increase with the increase of RNH3. Consequently, higher OFA rates could produce higher NOx emissions than lower OFA rates. This indicates that air-staging is not always effective in the NOx reduction of NH3 cofiring. Thus, although the side mixing mode of NH3 cofiring exhibits overall better performance in NOx control, caution is needed to accommodate the OFA rate with NH3 cofiring ratio. Although a variety of trends of NOx emissions were observed in this study, the results demonstrate that all these trends are in fact governed by the same mechanism – the competition between the NO-formation and NO-reduction reactions of NH3 in the varying O2 environment in the furnace. By creating appropriate O2 environment in the furnace, the NOx emissions of NH3/coal cofiring could be substantially lower than that of pure coal combustion. The findings of these paper are instrumental in the design and operation of NH3 cofiring system in full scale coal-fired boilers to achieve effective NOx control.