Effects of particle size on bed-to-surface heat transfer in bubbling fluidized bed heat exchangers of 550 MWe ultra-supercritical CFB boiler

Fluidized bed heat exchangers (FBHEs) are widely used as result of the scale-up in the capacity of circulating fluidized bed (CFB) boilers. In this study, the effect of particle size on the bed-to-surface heat transfer in the bubbling FBHE of a 550 MWe ultra-supercritical CFB boiler is investigated. The bed-to-surface heat transfer coefficient (hw) is experimentally obtained via field test; the results show that hw increases with decreasing particle sizes. A packet renewal model is proposed using the correlation of hydrodynamic parameters based on the experimental data of particles whose sizes approximate those of particles in the FBHE. The predicted hw value exhibits good agreement with that obtained via field test to within ±18.5%; however, it fails to predict the effect of particle size on hw. A single particle model is also adopted to evaluate hw. The derived results agree well with field test results, the predicted hw increases with decreasing particle sizes. In this work, it is observed that a finer particle size, which can be achieved by improving the cyclone efficiency, can enhance heat transfer in the FBHE. The results of this study are anticipated to offer engineering guidance for the design and operation of large-scale CFB boilers.