Bed-to-surface heat transfer in a BFB oxy-fuel combustor

Abstract

This work presents an experimental determination of heat transfer coefficient between the dense zone of a bubbling fluidized bed combustor and an immersed heat exchanging surface in oxy-fuel combustion mode, air mode being used for a comparison. The experimental work was carried out using a 30 kW scale autothermal bubbling fluidized bed combustor with real wet flue gas recirculation. A lightweight ceramic aggregate was used as the bed material and wood pellet as the fuel, the testing range of the bed temperature and fluidization velocity was within a typical range of fluidized bed combustors. For heat transfer measurement purposes, a single-loop water-cooled heat exchanger was developed. Convection and radiation were evaluated for both combustion modes. The convection in air mode reaches about 120 to 180 W/m2/K, while in the oxy-fuel mode it reaches about 180 to 230 W/m2/K. Radiation is not particularly affected by the combustion mode, reaching 80 to 120 W/m2/K for both modes.The correlation equations to determine convection were examined and compared with the experimental data. The general shortcoming of most of the available equations is a disagreement in trend between the predicted and experimentally measured convection correlated with the fluidization velocity. The Packet model, which was identified to predict this trend correctly, was fitted with the experimental data to determine new constants. The modified model is capable of predicting convection with a 2 and a 9 % deviation for air and oxyfuel combustion modes, respectively.