Combustion Characteristics and Pollutant Emissions During NH ₃ -Coal Co-Firing in a 4 MW Pilot-Scale Combustion Facility

Ammonia-coal co-firing represents a pivotal low-carbon transition pathway for existing coal-fired boilers. Because previous work has focused largely on fundamental studies and has provided limited guidance for industrial deployment, an NH3-coal co-firing experimental investigation was carried out on a 4 MW pilot-scale test bench that faithfully reproduces the structure and operating processes of an industrial Π-type boiler. The effects of NH3-fuel ratio (0–100%), fuel injection strategy, nozzle arrangement, and oxygen-supply conditions on combustion behavior and pollutant emissions were comprehensively assessed. Results show that increasing the NH3-fuel ratio fundamentally alters combustion. The transition from pure coal combustion to pure ammonia combustion led to the reductions in flame visibility and main-zone temperature accompanied by a monotonic rise in NOx emissions from 146 mg·m−3 to 760 mg·m−3, driven primarily by higher inputs of fuel-bound nitrogen. A moderate NH3-fuel ratio of 40% produced the most favorable pulverized-coal burnout, whereas both higher and lower proportions degraded combustion efficiency. Diversion of half of the NH3 stream to a separate overfire-air burner lowered NOx but caused a pronounced increase in CO emissions to 232 mg·m−3, which indicates deterioration of combustion stability. By comparison, the NH3 and coal nozzles separated by an axial distance of 50 mm led to decreases in both NOx and CO. Reduction of overall air supply significantly suppressed elevated NOx levels at high NH3-fuel ratios but increased CO and N2O release. When flue gas O2 concentrations rose from 2.5% to 4.0%, NOx emissions lowered from 760 mg·m−3 to 235 mg·m−3 under pure NH3 combustion. These results support the advancement of NH3-coal co-firing toward industrial application and offer a practical reference for subsequent large-scale deployment.