Bottom-bed fluid dynamics - Influence on solids entrainment

Paper in proceeding, 2017

In CFB boilers, the solids concentration along the riser and the external solids circulation are important design parameters, mainly in terms of the heat balance but also influencing the risk of wear on heat transfer surfaces. This work investigates experimentally how the amount of solids entrained from the bottom region of a CFB riser is influenced by the fluidization conditions, including the presence or absence of a dense bottom bed. The paper presents first measurements in a new cold lab-scale unit (3 m tall, 0.45 m2in cross section), which is a scale model of a large utility boiler. The solids inventory consists of glass spheres with a mean size of 112 µm. The operational range covers fluidization velocities between 0.1 and 1.4 m/s and riser pressure drops between 0.2 and 1.5 kPa. The vertical distribution of solids concentration is determined through pressure drop measurements between densely spaced pressure taps (15 in total) along the riser height. The external solids circulation is measured with an automatic valve system in the return leg. The results show that the presence or absence of a dense bed govern how operational parameters influence the characteristics of the solids entrainment from the bottom region. The vertical extension of the splash zone above the dense bed depends strongly on the dense bed height. In the absence of a dense bed, a bottom region with strong solids back-mixing is established which has similarities with the splash region.