Heat transfer characteristics in a large-scale BFB with immersed horizontal tube bundles

What is it about?

This work investigates heat transfer characteristics in a bubbling fluidized bed with a submerged superheater tube bundles under conditions of an integrated fluidized bed heat exchanger (Integrated Recycle Heat Exchanger, Intrextm) of a large-scale circulating fluidized bed boiler. The effect of mean bed particle size, normalized suspension density, and fluidizing number on the average heat transfer coefficient between the immersed horizontal tubes and the bed was evaluated. The physical parameters of bed particles and fluidizing air in the external heat exchanger were measured at different CFB unit loads. Bubble fraction and contacting time of emulsion phase on the heat transfer surface were calculated on the basis of operating data of fluidized bed heat exchanger. A mechanistic heat transfer model was used to predict the heat transfer coefficient. Depending on emulsion density and the local dynamics of gas and bed particles, the average heat transfer coefficient (havg) varied in a range of 255-381 W/(m2K), showing an increasing trend with the decrease in bed particle size. Moreover, havg decreased with increasing emulsion contact time on the tube surface with the reduction of the solids mixing. Obtained results were compared with literature data.

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Why is it important?

The type of heat transfer data presented in the work provides useful information for the designer of heat exchangers for CFB reactors. Our heat transfer study fully verifies the packet-renewal model which gives accurate predictions of averaged bed-to-tube heat transfer coefficient with the presence of bubbles.