Numerical investigation on NOx emission in co-firing with ammonia and pulverized coal in a large-scale boiler

Abstract

Recently, the technology of co-firing ammonia (NH3) with coal in boiler has been paid more and more attention, because NH3 is a carbon-free carrier of hydrogen with lower cost in transportation and storage. In this study, the computational fluid dynamics simulation was performed to investigate the difference in NOx emission behavior between coal firing and NH3 co-firing, and the effects of NH3 injection positions on NOx emission in a large-scale pulverized coal boiler. In this boiler, three stages (lower, middle, and upper) of burners with different heights are installed on both the front and rear walls. According to the simulation results, the condition of NH3 co-firing, in which NH3 is injected into all burners with co-firing ratio (CR) of 20 cal.%, shows a lower NO concentration at outlet than coal firing, because of the DeNOx effect of unreacted NH3 and less char NOx. In addition, in order to investigate the effect of injection position of NH3, the results of three cases, which include NH3 co-firing in all burners, NH3 co-firing in middle burners (CR: single burner 20 cal.%, whole furnace 8 cal.%), and NH3 co-firing in upper burners (CR: single burner 20 cal.%, whole furnace 8 cal.%), were compared. It can be known that NH3 co-firing in upper burners shows a highest NO concentration at outlet, because of quick mixing between NH3 from upper burner and two-stage combustion air. Thus, in order to delay the mixing between NH3 and two-stage combustion air, and ensure enough time for DeNOx reaction of NH3, injecting NH3 into the upper burners should be avoided.