For the design and operation of fixed bed gasifiers the understanding of the occurring physicochemical phenomena is essential. In this work, to study the fluid flow, heat and mass transfer as well as chemical reactions a particle-resolved CFD-model is presented. The two-dimensional approach was validated against experiments and simulations for different conditions were conducted and the resulting local temperatures, species mass fractions and carbon mass flux are discussed. Furthermore, the basic two-dimensional approach is compared to three-dimensional representation of packings considering two different configurations, namely random packed bed and simple cubic packed bed. The random packed bed was generated by a sedimentation utilizing Discrete-Element-Method. Furthermore, conditions which can be found in a British Gas Lurgi (BGL) gasifier were applied to simulate carbon conversion in a packing at a pressure of 40 bar. It was found that the results from the basic model are in good agreement with those of the random packed bed. Furthermore, the 3-d distribution of particles in a bed shows significant influence on the fluid flow, heat and mass transfer which is discussed through the flame length. (C) 2017 Elsevier Ltd. All rights reserved.