Modeling of Wall-to-Bed Heat Transfer in a Supercritical Water Fluidized Bed by the Packet Approach

Supercritical water fluidized bed (SCWFB) is a promising reactor to gasify coal or biomass with low pollution. In the present paper, attempts are made to model wall-to-bed heat transfer in a SCWFB by the packet renewal theory developed in traditional fluidized beds. The key parameters, such as the packet residence time near the wall, the fraction of total time with packet contact, and packet voidage, which are required for the packet approach, are obtained by the homemade capacitance probe for high temperature and pressure. To complete the wall-to-bed heat transfer model in a SCWFB, the physical properties of the packet are chosen properly and seven kinds of thermal conductivity models for the mixture are tested. A simplified spherical particle model is adopted to describe the thermal resistance between the wall and the packet. Then, a comparison is made between heat transfer coefficients by the model based on the packet renewal theory and heat transfer coefficients by the empirical correlation [ Zhang Int. J. Multiphase Flow, 2018, 109, 26−34 ]. It is found that they could match well with each other. The best fit can be observed when the Zehner and Schltinder thermal conductivity model is adopted, the relative error of which is 5.6% and the maximum relative error of which is less than 15%. Moreover, wall-to-packet heat transfer is considered to be dominant in a SCWFB.