Experimental study of the effect of friction phenomena on actual and calculated inventory in a small-scale CFB riser

Abstract

Accurate information concerning riser inventory in a fluidized bed is required in some applications such as the calcium looping process, because it is related to the CO2 capture efficiency of the system. In a circulating fluidized bed (CFB), the riser inventory is normally calculated from the riser pressure drop; however, the friction and the acceleration phenomena may have a significant influence on the total riser pressure drop. Therefore, deviation may occur in the calculation from the actual mass. For this reason the magnitude of the friction and the acceleration pressure drop in the entire riser is studied in small-scale risers. Two series of studies were performed: the first one in a scaled cold model riser of the 10kWth facility, and the second one in the 10kWth fluidized bed riser under process conditions. The velocities were chosen to comply with the fluidization regimes suitable for the calcium looping process, namely, the turbulent and the fast. In cold-model experiments in a low-velocity turbulent fluidization regime, the actual weight (static pressure drop) of the particles is observed more than the weight calculated from a recorded pressure drop. This phenomenon is also repeated in pilot plant conditions. In the cold-model setup, the friction and acceleration pressure drop became apparent in the fast fluidization regime, and increased as the gas velocity rose. Within calcium looping conditions in the pilot plant operation, the static pressure drop was observed more than the recorded pressure drop. Therefore, as a conservative approach, the influence of friction pressure drop may be neglected while calculating the solid inventory of the riser. The concept of transit inventory is introduced as a fraction of total inventory, which lies in freefall zones of the CFB system. This fraction increases as gas velocity rises.