Solids circulation in circulating fluidized beds with low riser aspect ratio and varying total solids inventory

Abstract

This paper presents an experimental study with the aim to understand the relation between the flow conditions – the riser pressure drop and fluidization velocity – in a CFB riser and the net (external) solids flux (Gs [kg/m2s]), applying a riser geometry and overall flow conditions similar to CFB boilers.The experiments are carried out in a CFB unit operated under ambient conditions. The riser has a cross section of 0.7m×0.12m and a height of 8.5m, yielding a riser height-to-width aspect ratio of 10.6 (in the wide dimension), similar to that of CFB boilers. The unit is equipped with densely spaced pressure taps providing a fine resolution of the measured vertical pressure profile along the riser and an automatic system to accurately measure Gs. The experiments cover fluidization velocities of 0.3–7m/s, riser pressure drops of 1.7–10.5kPa and loop-seal fluidization velocities of 0.12–0.54m/s (secondary air flow is not included). These ranges correspond to conditions both with and without a dense bottom region.The results show that Gs is determined by the solids concentration at the riser top, which in turn depends on riser pressure drop and fluidization velocity, and the backflow effect, which depends on the configuration and flow conditions of the loop seal and the exit region. For operating conditions with a dense bottom bed present, Gs is independent of riser pressure drop at any fluidization velocity, whereas when operating without a dense bed an increase in riser pressure drop increases Gs.