Hydrodynamics of a circulating fluidized bed operated with different secondary air injection devices

Abstract

A cold model of a circulating fluidized bed combustor having a 0.120 m i.d. and 5.75 m high riser, was operated by splitting the total gas flow rate in a primary air stream fed through the bottom distributor and a secondary air stream laterally injected at a fixed level along the riser. Two different feeding devices were used: in one case the injection of secondary air stream occurred over the whole circumference of the riser through a slot; in the other the injection occurred through four opposite ports. Solids used were 89 μm Ballotini. Overall superficial gas velocity was fixed at 6 m/s. Solids mass flux was alternatively equal to 35 and 55 kg/(s m 2). The ratio between secondary and primary air flow rates was kept at a value of 1. An isokinetic probe was used to measure radial profiles of upward and downward solids mass fluxes and those of tracer gas concentration. A stream of CO 2 was mixed with the secondary air stream before the injection and used as tracer gas. For each set of operating conditions, axial profile of voidage and radial profiles of solids mass flux and CO 2 concentration were measured along the riser. Results confirm that lateral air stream remarkably affects the hydrodynamics in the regions below and above the gas inlet. They also show that the mechanism of interaction between the secondary air stream and the gas-solids suspension rising from the primary zone completely changed when the injection device was changed. Solids and tracer gas distribution along the riser were noticeably different.