Gas mixing in a multi-stage conversion fluidized bed (MFB) with secondary air injection. Part I: an experimental study

Abstract

This paper investigated the distribution of secondary air after injection into a multi-stage conversion fluidized bed (MFB) cold model. Carbon dioxide (CO₂) was used as the tracer and its concentration was tested. The effects of the velocity of the primary air and secondary air, the particle circulating rate, and the diameter, number and included angle with the central line of the riser of injectors on the distribution of CO₂ were studied. Single- and multi-injector systems were applied, in which different designs of the secondary-air injectors were used. The radial gas dispersion coefficient was calculated by the dispersed plug flow model (DPFM). The concentration profile of the tracer and calculated radial gas dispersion coefficients indicated that lower velocity of primary air, higher velocity of secondary air and particle circulating rate, bigger size of injectors and smaller included angles of injectors helped the gas mixing of the secondary air in the MFB. The tangential injection of secondary air would induce a gathering of gasification agents in the region near the wall, which was undesirable for the operation of the MFB gasifier. The variation of the penetration depth of the secondary air indicated that the penetration depth under multi-injector system was smaller than that under single-injector system when other operational parameters were uniform. Thus, according to numbers of injectors, taking the included angles between injectors and the central line of the riser into consideration, the penetration depth of the secondary air was correlated with operational parameters.