Experimental study on weakly caking coal combustion preheated by circulating fluidized bed

In this study, we sought to achieve a stable high-efficiency and low-NOx combustion of weakly caking coal by preheating fuel in a pre-circulating fluidized bed (PCFB). When the operating temperature of the PCFB increased from 954 °C to 1048 °C and the fluidizing gas velocity increased from 2.02 m/s to 3.27 m/s, the stable operating time of the caking coal increased significantly from 290 min to 550 min. Moreover, the caking trend of the preheated char completely disappeared (RI = 0, GR.I = 0). The preheated char particles were obviously bonded together at 954 °C, while the char particles were still clearly separated at 1048 °C. These results indicate that the high preheating temperature and the high fluidizing gas velocity are beneficial to alleviate or even resolve the preheating instability of weakly caking coal. Based on the stable preheating operation, the temperature field and reaction field in the main combustion zone were relatively uniform. Gas-phase nitrogen-containing compounds in the post-combustion chamber existed mainly in the form of NO, and the NO concentration in the reducing zone was zero. With the gradual injection of the burnout air, the NO concentration first increased and then decreased to a stable level. By arranging the burnout air position separating the reducing zone and the oxidizing zone to broaden the reducing zone, NO emissions were further reduced without affecting the combustion effect. And the preheating combustion technology is verified to more flexibly arrange the number of air distribution layers in multilevel air distribution settings.