Coal/NH3 co-firing in a 50-kW self-sustaining combustor test rig: effects of dual-fuel mixing control and air staging

Ammonia–coal co-firing is a promising route for reducing greenhouse-gas emissions while utilizing existing coal infrastructure. However, simultaneously achieving high combustion efficiency, low NOx emissions, and limited unburnt NH 3 remains challenging because pollutant formation is sensitive to fuel-air mixing and air staging. Ammonia–coal co-firing experiments were conducted in a newly developed 50 kW self-sustained combustor. Changing the NH 3 blowing position controlled the mixing state of NH 3 and pulverized coal. The effects of mixing position, separated over-fire air (SOFA) ratio, and NH 3 co-firing ratio on NOx, unburnt NH 3 , CO, and unburnt carbon were evaluated. Results show that an optimized mixing position markedly reduces NOx while maintaining unburnt NH 3 within a controllable range. Increasing the SOFA ratio further reduces NOx. An appropriate NH 3 co-firing ratio improves combustion efficiency and reduces NOx, whereas ratios above 20% significantly increase unburnt NH 3 and unburnt carbon. These results provide guidance for developing efficient, low-emission ammonia–coal co-firing systems.