Experimental study on combustion characteristics of pulverized coal preheated in a circulating fluidized bed

A new technique to preheat pulverized coal in a circulating fluidized bed was adopted. This technique consists of two stages: the pulverized coal is first self-preheated in a circulating fluidized bed, and then the preheated fuel gas and char particles are burned out in a down-fired combustor under air-staging conditions. Experiments were conducted with two types of coals, two air ratios in the circulating fluidized bed and different air distributions in the combustor. The preheating and combustion processes ran stably under all experimental conditions. For both types of coals, the preheating temperatures in the circulating fluidized bed could be adjusted broadly within 800-950 °C, and the ratios of primary air could be lowered to 0.15. The combustion efficiency and nitrogen conversion were remarkably influenced by the air distribution, and were controlled above 98% and below 10% respectively, although the two targets could not be simultaneously achieved.