A 50 NL-H-2/h hydrogen production scale sulfur-iodine (SI) thermochemical process test facility to be operated under a pressurized environment has been constructed in Korea. This study focused on the catalyst-packed type HI thermal decomposer and bayonet type H2SO4 thermal decomposer, which are the key components for the 50 NL-H-2/h SI test facility. To sustain a closed loop operation of the SI process, the catalyst particles packed in the top of the H2SO4 thermal decomposer are protected from coming into contact with the inlet sulfuric acid aqueous solution, and the production molar ratio of oxygen discharged from the H2SO4 thermal decomposer to hydrogen discharged from the HI thermal decomposer always satisfies 0.5/1. Based on the design specifications and mass balance of the SI integrated process of 50 NL-H-2/h, numerical calculations for the two decomposers were done to evaluate their start-up and static behaviors. Based on the results of these calculations, it is predicted that the boiling interface of the sulfuric acid solution is located around 280 mm from the bottom of the electric furnace to heat the bayonet type H2SO4 thermal decomposer, which is far from the catalyst-packed region. It is also proposed that the proper operating temperature of the HI thermal decomposer to satisfy the hydrogen production rate of 50 NL-H-2/h and sustain the closed loop operation of the SI process is 587 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.