Effect of bed particle size on heat transfer between fluidized bed of group b particles and vertical rifled tubes

Abstract

The effect of bed particle size on the local heat transfer coefficient between a fluidized bed and vertical rifled tubes (38mm-O.D.) has been determined in a large-scale circulating fluidized bed (CFB) reactor. Bed particles with different Sauter mean particle diameter within the range of 0.219–0.411mm and particle density in the range of 2650–2750kg/m3 were used as bed material in this heat transfer study. A gas fluidized bed furnace with 27.6×10.6m cross-section above refractory line and 48m in height was used. Air coal firing conditions at the membrane wall in the form of water tubes welded with lateral fins corresponded to a suspension density covering the range of 1.36–6.22kg/m3, furnace temperatures in the range of 1080–1164K, a superficial gas velocity varied from 2.99 to 5.11m/s and solids circulation flux covered a range of 23.3–26.2kg/(m2s). For these operating conditions, the heat transfer analysis of CFB reactor with detailed analysis of bed-to-wall heat transfer coefficient along furnace height was investigated. In this work, the overall heat transfer coefficient was estimated using a mechanistic heat transfer model based on cluster renewal approach. The experimental results show that: (i) higher heat transfer coefficients along furnace height were found under finer bed particles size dp<0.241mm, (ii) heat transfer data confirms strong dependency of the overall heat transfer coefficient on suspension density and also hydrodynamic conditions within CFB furnace, (iii) for small bed particles, dp<0.233mm, the particle convection component plays dominant role in heat transfer mechanism, (iv) for large bed particles, dp>0.366mm, the effect of particle size on relative contribution of radiation from dispersed phase become essential with particle diameter increasing, and (v) for all bed particles with diameters in the range of 0.240–0.411mm, the gas convection heat transfer coefficient between the fluidized bed (Geldart B particles) and the rifled tubes increased as the bed particles size increased.