The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions has lead to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) Nuclear Hydrogen initiative was investigating thermochemical cycles for hydrogen production using high-temperature heat exchangers. in this study a three-dimensional computational model of high- temperature heat exchanger and decomposer for decomposition of sulfur trioxide by the sulfur-iodine thermochemical water-splitting cycle with different packed bed designs has been done. The decomposer region of the bayonet heat exchanger also called as silicon carbide integrated decomposer (SID) is designed as the packed bed region. Cylindrical, spherical, cubical and hollow cylindrical pellets have been arranged inside the packed bed. The engineering design of the packed bed was very much influenced by the structure of the packing matrix, which was governed by the shape, dimension and the loading of the constituent particles. Staggered and regular packing methods are used for packing the pellets in the packed bed region. The numerical model is created using GAMBIT and fluid, thermal and chemical analyses were performed using FLUENT. The decomposition percentage of sulfur trioxide is found for the packed bed region with different pellets and the numerical results obtained is compared with the experimental results. A comparison is made for the decomposition percentage of SO(3) for the packed bed approach and the porous media approach. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.