This study investigates the effect of shrinkage on the heat-transfer-controlled processes of tabular (slab), cylindrical, and spherical geometries. A model is used to simulate the product yield, pyrolysis time, particle size, specific heat capacity, and Biot number for analyzing the effect of shrinkage on different geometries. Spherical, tabular, and cylindrical biomass briquettes with an equivalent diameter of 0.0002 -0.1 m are used for simulation. The proposed model is validated using the experimental product yield data obtained from the two-stage gasifier in this study and the experimental data reported in the literature. Thermal waves are observed with an increase in particle size. The tabular geometry exhibits the maximum number of thermal waves, whereas the spherical geometry exhibits the least number of thermal waves. The shrinking effect favors the formation of volatiles and gases, whereas the non shrinking effect favors the formation of char. The product yield remains unchanged with an increase in the specific heat capacity and Biot number. However, the production of volatiles and gases increases in the order sphere > cylinder > tabular, whereas the production of char increases in the order tabular > cylinder > sphere for the entire specific heat capacity range. (C) 2018 Elsevier Ltd. All rights reserved.