Industrial scale testing on the combustion and NOx emission characteristics of ammonia cofiring in a 40 MWth coal-fired boiler

Abstract

This paper reports the first industrial scale testing on ammonia (NH3) cofiring in a 40 MWth coal-fired boiler under high ammonia cofiring ratio (ENH3, percent of thermal input) ranging from 0 % to 25 %. A full-scale cofiring swirl burner is designed to inject NH3 gas with pulverized coal through the primary air pipe of the burner. The effects of ammonia cofiring ratio and key boiler operating parameters on boiler NOx emission and fuel burnout are examined. The results show that the boiler can establish stable ignition and combustion under all ammonia cofiring conditions in this study. The burnout of coal under ammonia cofiring conditions are better than those of coal combustion cases. Overfire air (SOFA) has significant influences on boiler NOx emission and burnout of coal and NH3. It is found that there exists optimal SOFA ratio in the range of 15–20 % within which both boiler NOx emission and NH3 slip can be well controlled. Under 20 % SOFA ratio, boiler NOx emission tends to increase and then decrease with the increase of ENH3 reaching the peak value under ENH3 = 5 %. As a result, the NOx emission can be reduced to the level lower than that of coal combustion case under ENH3 > 20 %. Boiler excess O2 also shows significant influences on boiler NOx emission and NH3 slip. With the decrease of boiler O2, boiler NOx emission decreases significantly while NH3 slip increases rapidly. It is found that there exists optimal boiler O2 in the range of 3.0–3.7 % within which both boiler NOx emission and NH3 slip can be kept at relatively low levels. Injection of NH3 from the side wall of furnace is also tested in this study. The NOx emission shows similar trends with the change of ENH3 in the range of 0–20 %. However, above this range the NOx emission tends to increase with further increase of ENH3. The results of this paper demonstrate the technical feasibility of NH3 cofiring in large-scale coal-fired boilers under high cofiring ratios and provide important guidance to the operation of boilers.