In this study, we have developed a novel model to include the spatial heterogeneity for the combustion of volatiles in the presence of oxygen inside a biomass cookstove (BCS). Three-dimensional computational fluid dynamic simulations of BCS were done to evaluate the dynamics of spatial heterogeneity incurred during biomass combustion. The proposed methodology prescribes the mass release rate of the fuel as a dynamic boundary condition that evolves as per the changing temperature in the void spaces of the packed bed. The temporally evolving mass release rate due to the change in temperatures of the void spaces is calculated iteratively and set as the inlet boundary condition on the particle surfaces using an empirical correlation. The validation studies showed that the time-averaged outlet temperatures predicted from the model are in good agreement with the experimentally obtained values.