Enhancement of Particle Segregation By Air-Staging in a Circulating Fluidized Bed

Coal gasification is endothermic and slow reaction between gas and solid. It requires high solid holdup and uniform temperature profile in a circulating fluidized gasifier. Air-staging means the total air to the riser is split into the primary and secondary introduced at different location. It affects the flow structure and thermal balance in a gasifier, and it is a promising method to meet the requirements of coal gasification in a CFB. The regulation effect of the secondary air on solid circulating flow rates, axial profiles of solid holdup and particle size distribution was studied in a small circulating fluidized bed test rig.

The CFB test rig consists of a riser with an internal diameter of 0.15 m and a height of 3 m, a cyclone, a loop seal. Three layers of the secondary air nozzles locate at 0.3 m, 0.46 m and 0.7 m, respectively, above the air distributor. The secondary air is introduced from layers in different combination modes. The ratio of secondary air to total air, the distribution mode and the distribution ratio between layers are considered the key factors in the experiments. To adjust solid circulating flow rates in the standpipe, aeration rate in the loop seal was also studied. For fixed secondary air flow rate, the secondary air introduced from several layers together results in an especially strengthened particle segregation phenomenon. Compared with the experiment without air-staging, solid holdup decreases in the dense phase zone and increases in the dilute phase. Axial profile of solid holdup is more uniform. The distribution ratio also has effect on flow structure. The results can be used for design of CFB gasifier and optimizing operating parameter to promote gasification reaction.