A three dimensional transient model was developed to simulate the pyrolysis process of coal in fixed bed reactor. The model considered a number of aspects associated with coal pyrolysis such as the evaporation and condensation of physical water, the release of volatiles, secondary cracking of pyrolysis tar and the variation of porosity. Conductive, radiative and convective types of heat transfer were all taken into account in the model. The phase transformation of physical water was modeled using the Lee model (Lee, 1980) available in the commercial CFD software Fluent. The multiple independent parallel reaction (MIPR) model was applied to predict the generations of volatile products. The secondary cracking of tar was described using the model proposed by Wurzenberger et al. (2002). The model was implemented in Fluent and validated through modeling the pyrolysis process of low-rank sub-bituminous coal in four laboratory-scale and externally heated fixed-bed reactors with or without internals. The predicted temperature evolutions of coal charge were in all quantitative agreements with experimental results. And the influences of internals on the yield of tar were also successfully captured by the simulations. (C) 2019 Elsevier Ltd. All rights reserved.