Influence of exit geometry in circulating fluidized-bed risers



The influence of exit geometry on the riser axial pressure profile in circulating fluidized beds (CFB) is examined. Experiments were performed in an academic-scale CFB operating at conditions chosen to give dimensional similarity with an industrial CFB combustor. The influence of riser exit geometry was investigated to compare smooth and abrupt exits in terms of three different measures: (1) a dimensionless length of influence, Ω, of the exit along the riser, where Ω = (length of influence)/(riser height); (2) the exit reflection coefficient, Rf, defined by Senior in 1992 where Rf = (downward solids flow in riser)/(upward solids flow in riser) just below the riser exit; and (3) the increase in cross-sectionally averaged solids concentration at the riser exit. Analysis of the new data and results from the literature, lead to the description of solids flow at the exit in terms of a riser exit Froude number (FrR) where FrR = ust/gR. A correlation between the riser exit Froude number and the exit reflection coefficient is presented. A serious correlation between the dimensionless length of influence of the exit bend, measured down the riser, and operating conditions, particle properties, gas properties and riser dimensions is also presented. Both correlations predict the influence of exit geometry (the exit effect) on the riser axial pressure profile. The relationship between operating parameters and the increase in cross-sectional averaged solids concentration at the riser exit is also examined.