Heat-Transfer Model of the Rotary Ash Cooler Used in Circulating Fluidized-Bed Boilers

摘要

Rotary ash coolers are widely used in circulating fluidized-bed (CFB) boilers for bottom ash cooling, attributed to their high efficiency and excellent reliability. However, they are mostly still designed empirically, and one of the most important reasons is due to the lack of an appropriate heat-transfer model. In this paper, a comprehensive heat-transfer model was developed and the model parameters were determined by experiments. Against the literature findings, the experimental results showed that the dimensionless thickness of gas film used in the thermal contact resistance calculation is different for different ash particle sizes. The new model was further applied to predict the heat-transfer characteristics for a rotary ash cooler used in a commercial 300 MWe CFB boiler. It was found that the heat-transfer coefficients of ash-air and air-water are close but much lower than that of ash-water, indicating that the heat transfer of ash-water dominates in the total heat transfer. In addition, the ash temperature decreases rapidly in the first half of the roller when the heating surfaces are uniformly arranged along the axis, and the outlet temperatures of ash and cooling water vary approximately linearly with the ash flow rate. In comparison to the industrial data, the model well predicts the axial distributions of the temperature and heat-transfer coefficient and the heat-transfer amount with engineering acceptable accuracy. On the basis of the modeling results, it is suggested that the roller could be more compact by rearranging more heating surfaces into the first half of it.