Numerical simulations of shell-and-tube heat exchanger and chemical decomposer with straight tube configuration and porous media were performed using FLUENT6.2.16 to examine the percentage decomposition of sulfur trioxide. The decomposition process can be a part of sulfur-iodine (S-I) thermochemical water splitting cycle, which is one of the most studied cycles for hydrogen production. A steady-state, laminar, two-dimensional axisymmetric shell-and-tube model with counter flow and parallel flow arrangements and simple uniform cubical packing was developed using porous medium approach to investigate the fluid flow, heat transfer and chemical reactions in the decomposer. As per the investigation, the decomposition percentage of sulfur trioxide for counter flow arrangement was found to be 93% and that of parallel flow was 92%. Also, a high pressure drop was observed in counter flow arrangement compared to parallel flow. The effects of inlet velocity, temperature and the porous medium properties on the pressure drop across the porous medium were studied. The influence of geometric parameters mainly the diameter of the tube, diameter of the shell and the length of the porous zone on the percentage decomposition of sulfur trioxide in the tube was investigated as well. A preliminary parametric study of the mentioned configuration is conducted to explore effects of varying parameters on the decomposition of sulfur trioxide. From the performed calculations, it was found that the Reynolds number played a significant role in affecting the sulfur trioxide decomposition. The percentage decomposition decreases with an increase in Reynolds number. Surface-to-volume area ratio and activation energy were also the important parameters that influenced the decomposition percentage. A high surface-to-volume area ratio enhances the rate of the chemical reaction and high activation energy decreases the decomposition percentage. The decomposition of sulfur trioxide is calculated and compared for both counter and parallel flow arrangements. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.