Numerical calculation with detailed chemistry on ammonia co-firing in a coal-fired boiler: Effect of ammonia co-firing ratio on NO emissions

NH3 co-firing in a commercial 1000 MWe pulverized coal-fired boiler has been considered for reducing CO2 emissions. However, the abundance of fuel-N in NH3 co-firing may cause an increase in NO emissions. The effects of the NH3 co-firing ratio on NO emissions in a commercial boiler are studied through a zero-dimensional numerical calculation with detailed chemistry. A simplified boiler system is composed of a burner zone, an over-fire zone, and two downstream zones. A network of perfectly stirred reactors is used to determine NO emissions in flue gas. The NH3 co-firing ratio is varied from 0 to 80%. NH3 is injected into the flame zone, which is part of the burner zone. As a result of calculation, amount of CO2 emission decreases as an increase in the NH3 co-firing ratio. Since the location where NH3 reacts shifts to the downstream side with an increase in the co-firing ratio, the peak of NO also tends to transfer to the downstream side. Nevertheless, NH3 is almost completely dissociated in the burner zone and unburned NH3 emissions are approximately zero independent of the co-firing ratio. Within the range of 20–60%, NO emissions increase monotonically with the co-firing ratio. However, at 80% co-firing, the NO emission decreases from the value attained at 60% co-firing. With a co-firing ratio higher than 40%, reactions of rich NH3 chemistry, which are characteristic in rich and low-temperature NH3 flame become effective. Due to these reactions, fuel(NH3)–NO production tends to decrease in the location where NH3 reacts.