AbstractA comprehensive three-dimensional mathematical model is developed for studying the microwave-assisted pyrolysis of biomass. Kraft Lignin is considered as biomass feedstock in the model, and a mixture of lignin and char, is used as the sample for pyrolysis. A lumped kinetic model which considers three lumped pyrolysis products (gas, liquid and remaining solid fractions) is coupled with the governing equations for the microwave field, heat transfer, mass transfer, Darcy fluid flow and a transient numerical analysis is performed. The distribution of electric field in the microwave cavity, and the distribution of electric field, temperature, and pyrolysis products within the lignin sample are presented. The lignin sample is predicted to undergo volumetric heating when subjected to microwave heating. Accordingly the reaction zone extends from the center of the biomass sample bed towards the outer surface. Preliminary evaluation of the applicability of the model for assessing the effect of different parameters on the microwave pyrolysis of lignin is also carried out.