Analysis and modelling of solid flow in a closed loop circulating fluidized bed with secondary air injection

The goal of the work presented here is to analyse the influence of secondary air on the solid circulating phenomenon in a closed loop circulating fluidized bed (CFB; by “closed loop”, we mean a free solid circulating system as shown in one of the figures). Variables of such a system are “gas velocity”, gas distribution ratio between “primary” and “secondary” levels and solid inventory. Solid circulating flow rate, solid mean concentration in the column and local gas–solid flow properties are measured for a large set of operating conditions. The analysis of results obtained lets us develop a simple model for estimating the solid circulating flow rate in industrial CFB. In this model, the suspension flow pattern is divided into three components: “dilute”, “cluster” and “wall flow” phases. The velocity and the concentration of each phase, as well as the solid flow rate, are estimated by means of a simple force balance including some basic hypothesis. The results obtained show that the model fits experimental values quite well. However, it is imprecise for low values of air velocity (here <1.5 m/s).