Stresses in a Cylindrical Wood Particle Undergoing Devolatilization in a Hot Bubbling Fluidized Bed

A mathematical model capable of estimating the temperature and stress distribution in a devolatilizing cylindrical wood particle is proposed. The model assumes wood as orthotropic and comprises a thermal submodel and a stress submodel. It takes into account the contributions to stress by the temperature rise, shrinkage during drying and devolatilization, and mechanosorption. The thermal submodel estimates the temperature distribution and shrinkage as a function of space and time to be used as inputs to the stress submodel. Calculations have been carried out for cylindrical wood particles of aspect ratio l/d = 1 and with dimensions of 10, 20, and 30 mm. The model predictions compared with the results of an available drying model are found to be satisfactory. During devolatilization of the wood particle, the model predicts high compressive stresses at the center while those at the surface are tensile in nature and moderate in magnitude. A modified von Mises criterion predicts failure at the axial center of the cylinder, with the size of the crack formed by the failure increasing from 0.4 to 1.4 mm in diameter as the size of the wood particle increases from 10 to 30 mm. The predicted crack radius and the time of crack initiation are compared with the measurements carried out on wood particles undergoing devolatilization in a fluidized bed combustor. The predicted crack radius and time of crack initiation are found to be sensitive to the magnitude of the mechanical properties.