Secondary air jet penetration in large-scale CFB

Sammanfattning

This thesis investigates secondary air jet penetration in CFB boilers by using acold-flow model. Fluid-dynamic scaling using Glicksman’s simplified scaling lawsallowed the results to be scaled to hot conditions representative of an industrialboiler. Four different cases with different secondary air velocities were consideredand one additional case without secondary was used as a reference. For the fourcases, secondary air was injected at two height levels with one additional levelrepresenting flue gases. One of the secondary air injection ports was substituted withCO2 as a tracer gas. Different secondary-to-primary air ratios were studied whilemaintaining a constant total volumetric airflow. The movement of secondary air andsolids was evaluated using gas tracing, pressure transducers and solids circulationrate measurements. The results indicate that higher shares of secondary air yield adeeper penetration and dispersion into the riser. However, it was also determinedthat a higher proportion of secondary air resulted in lower lateral dispersion towardsthe sides. Regarding the distribution of solids, a dense bed is more easily attainedwith increased secondary air (and thus reduced primary air). The solids circulationrate was found to decrease for higher proportions of secondary air.