Energy & Fuels, Vol.34, No.8, 9620-9631, 2020
Heat-Transfer-Corrected Isothermal Model for Devolatilization of Thermally Thick Biomass Particles
An isothermal model is commonly used in computational fluid dynamics (CFD) modeling of biomass devolatilization in fluidized beds. However, the particle internal heat transfer, which is neglected by the isothermal model, influences significantly the devolatilization process for large biomass particles. To consider the effect of internal heat transfer, a heat-transfer-corrected isothermal model is introduced by comparing the differences between an isothermal model and a nonisothermal model. Two correction coefficients for external heat transfer, H-T, and reaction rate, H-R,H-i were defined to correct the conventional isothermal model. The predictions of the heat-transfer-corrected isothermal model and the nonisothermal model were in good agreement with the experimental data for both thermally thick (Biot number, Bi >= 1.0) and thermally thin (Bi < 1.0) biomass particles, while the conventional isothermal model gave reasonable results only for thermally thin biomass particles. The heat-transfer-corrected isothermal model was further implemented in a CFD model to simulate biomass devolatilization in a batch bubbling fluidized bed. Compared to the conventional isothermal model, the heat-transfer-corrected isothermal model had similar computational efficiency, but it predicted a lower heating rate and a lower devolatilization rate, which were in good agreement with the observations from single-particle modeling.