Pilot-scale study on NO emissions from coarse coal combustion preheated by circulating fluidized bed

•Stable low-NOx combustion of coarse pulverized coal in pulverized coal boiler is achieved.•High-calorific coal gas and higher physical sensible heat greatly improved ignition performance of coarse coal.•Coarse coal combustion preheated by circulating fluidized bed is in MILD combustion mode.•Effects of burnout air positions in strong reductive zone on NO emissions can be ignored. Achieving stable low-NOx combustion of coarse pulverized coal could improve the energy efficiency of the combustion system. In this study, a pilot-scale experiment on coarse coal (0–1 mm) combustion preheated by circulating fluidized bed is conducted. The coarse coal could achieve a long-term stable preheating, and the assisted combustion by the high-calorific preheated coal gas (3.06 MJ/Nm3) and the higher physical sensible heat (754 °C) greatly improved the ignition performance and the combustion stability of the preheated fuel. Meanwhile, the release ratio of fuel-nitrogen during the preheating was 36.56%, while the concentrations of NOx in the preheated coal gas were all 0 ppm. During the stable combustion, the volume-based average temperature and the volume-based temperature fluctuation coefficient in the main combustion zone was approximately 1112 °C and 8.89%, and the combustion efficiency was approximately 95%. The NOx concentrations in the strong reductive zone were almost 0 ppm. As the reaction atmosphere along the combustion path gradually varied from reducing to oxidizing, fuel-nitrogen was mainly oxidized to NO, and the NO emissions was approximately 68 ppm (114 mg/m3 @6%O2). In addition, the effects of burnout air positions in strong reductive zone on NO emissions could be ignored.