Investigation of solids circulation in a cold model of a circulating fluidized bed

Abstract

Design of a fluidized bed gasification plant requires a good understanding of how operating parameters influence solids circulation, because this affects the heat transfer that sustains the reactions in the system. In this paper, the effect of various operating parameters, such as primary and secondary fluidizing airflows, on solids circulation within a scaled-down cold model of a circulating fluidized bed (CFB) was investigated. An operational map was developed to show regions of stable CFB operation and boundaries of unsatisfactory CFB operation, when fluidization becomes highly unsteady or inefficient due to gas bypass. Under stable operation, it was observed that solids circulation increased with an increase in primary and secondary fluidizing airflows and solids inventory of the plant. Provided the loop seal was fluidized properly, any changes in operating parameters resulted in the solids flow adjusting accordingly to maintain a proper gas seal and ensure stable operation. The operating parameters also affected the pressure drop across various points in the CFB, essentially due to redistribution of solids within the system. A new semi-empirical model was developed to estimate the ratio of solids that exit the CFB riser to solids that recirculate back into it. This model was based on considerations of the outlet geometry and solids inertia, in particular the ability of the solids to resist the change in direction of the airflow as it curves towards the exit of the CFB riser.