Experimental study on combustion characteristics and NOx formation of ammonia-coal co-firing in the pilot-scale down-fired combustor

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Ammonia-coal co-firing is a viable low-carbon retrofit for coal-fired power plants, yet the effects of staged combustion and operating parameters on NO x formation remain unclear at the pilot scale. This study experimentally investigates co-firing in the 50 kW pilot-scale down-fired furnace, varying inner and outer secondary air ratio, excess air coefficient, ammonia blending ratio, over-fire air ratio, and thermal load. The results reveal that NO emission is non-monotonic with the ammonia blending. Under the tested range of operating conditions, the 30%-45% range best suppresses NO by balancing fuel-nitrogen input and reduction. An over-fire air ratio of 30% with a 4:6 inner and outer secondary air ratio optimizes combustion and NO x control. Higher excess air improves burnout and reduces CO, but concurrently promotes NO formation. NO rises then falls with load, peaking at 40 kW and dropping at 50 kW due to downstream NH 3 reduction. This study provides quantitative data for air-distribution and load optimization, supporting low-carbon boiler retrofits.