Numerical investigation on NOx formation characteristics of ammonia cofiring in a 40 MW coal-fired boiler

LIU Jianing; ZHANG Wenzhen; LIU Xin; XIE Yan; LI Chi; LI Ming; NIU Tao; WANG Heyang

doi:10.13226/j.issn.1006-6772.MDDT25030601

Abstract：Ammonia-coal cofiring is one of the most promising technical routes to realize decarbonation of coal-fired power plants.However,due to the high nitrogen content of NH3,ammonia-coal cofiring may lead to significant increase of boiler NOx emission.This may become one of the key problems restricting the implementation of ammonia cofiring in coal-fired boilers.Therefore,it is imperative to study the NOx formation characteristics of ammonia cofiring in coal-fired boilers,so as to guide the development of effective NOx control methods.The present study numerically investigates the effects of ammonia cofiring ratio(RNH3)on the NOx characteristics of boiler in a 40 MW industrial scale boiler.The simulation results of boiler NOx emission show that,under 20％overfire air flow rate,boiler NOx emission increases first and then decreases with the increase of RNH3,reaching maximum value of 197 mg/m3 when RNH3=5％,and dropping to 95 mg/m3 when RNH3=25％,which is lower than 137 mg/m3 of pure coal combustion.The simulation results are in good agreement with the ammonia cofiring testing results of 40 MW boiler both qualitatively and quantitatively.Such trend of boiler NOx emissions with the increase of RNH3 is attributed to the competing reaction pathways between NO formation and reduction reactions in the process of NH3 combustion.The formation of NO in the boiler is mainly composed of the initial NO formation in the high O2 environment in the early stage of combustion and the NO reduction in the low O2 environment in the later stage of combustion.Boiler NOx emission is jointly determined by the formation and reduction of NO in these two stages.The formation and reduction rates of NO in the furnace both increase with the increase of RNH3.However,in the range of RNH3=0-5％,the formation rate of NO increases faster than that of NO reduction,while in the range of RNH3=5％-25％,the reduction rate of NO reduction increases faster than that of NO formation.As a result,the net formation of NO exhibits an increase-then-decrease trend as RNH3 is increased from 0 to 25％.This is the reason why it was observed in the ammonia-coal cofiring testing of 40 MW boiler that the NOx emission increased first and then then decreased with increase of RNH3.The simulation results reveal the key influence of the competition mechanism between the NO formation and reduction reaction pathways of NH3 combustion on the NOx emission of ammonia-coal cofiring,which is of great significance to realize effective NOx control of ammonia cofiring in large scale coal-fired boilers.