Secondary air jet penetration in large-scale CFB

Abstract

This thesis investigates secondary air jet penetration in CFB boilers by using a cold-flow model. Fluid-dynamic scaling using Glicksman’s simplified scaling laws allowed the results to be scaled to hot conditions representative of an industrial boiler. Four different cases with different secondary air velocities were considered and one additional case without secondary was used as a reference. For the four cases, secondary air was injected at two height levels with one additional level representing flue gases. One of the secondary air injection ports was substituted with CO2 as a tracer gas. Different secondary-to-primary air ratios were studied while maintaining a constant total volumetric airflow. The movement of secondary air and solids was evaluated using gas tracing, pressure transducers and solids circulation rate measurements. The results indicate that higher shares of secondary air yield a deeper penetration and dispersion into the riser. However, it was also determined that a higher proportion of secondary air resulted in lower lateral dispersion towards the sides. Regarding the distribution of solids, a dense bed is more easily attained with increased secondary air (and thus reduced primary air). The solids circulation rate was found to decrease for higher proportions of secondary air.