Coal combustion characteristics in a circulating fast fluidized bed

The effect of coal size (0.73–1.03 mm), excess air ratio (1.0–1.4), operating bed temperature (750–900‡C), coal feeding rate (1–3 kg/h), and coal recycle rate (20–40 kg/h) on combustion efficiency, temperature profiles along the bed height and flue gas composition have been determined in a bubbling and circulating fluidized bed combustor (7.8 cm-ID x 2.6 m-high). Combustion efficiency increases with increasing excess air ratio and operating bed temperature and it decreases with increasing particle size in the bubbling and circulating fluidzing beds. In general, temperature profiles and combustion efficiency are more uniform and higher in a circulating bed than those in bubbling bed. Combustion efficiency also increases with increasing recycle rate of unburned coal in the circulating bed. The ratio of CO/CO₂ of flue gas decreases with increasing bed temperature and excess air ratio, whereas the ratio of O₂(CO + CO₂) decreases with bed temperature in both bubbling and circulating fluidized beds.