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

• Studied the effects of NH 3 cofiring ratio, injection mode and OFA on NO x emission. • NO x exhibited a variety of trends under different injection modes and OFA rates. • Revealed all the different NO x trends are governed by the same NO mechanism. • Found air-staging is not always effective in the NO x reduction of NH 3 cofiring. Ammonia (NH 3) cofiring is a promising approach to reduce the CO 2 emissions from coal-fired power plants. However, the potential drastic increase of NO x emissions may inhibit the wide implementation of NH 3 cofiring. To explore the potential NO x control strategies, the effects of NH 3 cofiring ratio (R N H 3 ), NH 3 injection mode and overfire air (OFA) rates on the NO x emission characteristics of NH 3 /coal cofiring are studied in a test furnace that allow for flexible control of the injection positions and combustion environment of NH 3. The results show that when NH 3 is injected with coal stream (top mixing mode), the NO x emissions increase and then decrease with the increase of R N H 3 under all OFA rates. However, when NH 3 is fed through the side port of furnace separately (side mixing mode), the NO x emissions exhibit distinct trends under different OFA rates. Under lower OFA rates, the NO x emissions show monotonical decrease with the increase of R N H 3 , while under higher OFA rates the NO x emissions first decrease and then increase with the increase of R N H 3 . Consequently, higher OFA rates could produce higher NO x emissions than lower OFA rates. This indicates that air-staging is not always effective in the NO x reduction of NH 3 cofiring. Thus, although the side mixing mode of NH 3 cofiring exhibits overall better performance in NO x control, caution is needed to accommodate the OFA rate with NH 3 cofiring ratio. Although a variety of trends of NO x 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 NH 3 in the varying O 2 environment in the furnace. By creating appropriate O 2 environment in the furnace, the NO x emissions of NH 3 /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 NH 3 cofiring system in full scale coal-fired boilers to achieve effective NO x control. [ABSTRACT FROM AUTHOR]