Abstract A mathematical model is proposed to predict the primary fragmentation of wood in a bubbling fluidized bed combustor. The model is based on the stress contribution due to temperature gradient, physical shrinkage during drying and devolatilization, and volatiles pressure. An order of magnitude estimate of bed material impact is also made. The model comprises of a 2D thermal sub-model and a 1D stress sub-model. The thermal model is anisotropic in nature and is numerically solved while an analytical solution is obtained for the orthotropic stress model. A new criterion to predict primary fragmentation is proposed. The model predicts the possibility of fragmentation, timing of fragmentation and its location within the wood particle. The model predictions are compared with experimental data obtained in a bubbling fluidized bed combustor at 1123 K, using cylinders made of Casuarina equisetifolia wood. Cracks in the wood are initiated due to shrinkage stresses while the impact with the inert bed material assists in further weakening at the cracks and thereby causing primary fragmentation. Influence of volatiles pressure is negligible.