Thermochemical Decompositions in Hydrogen Production by the "UT-3" Cycle Consisting of Br-Ca-Fe Compounds -Reaction between Magnetite and Hydrogen Bromide-

Abstract

The authors have already proposed a computer-aided search procedure for hydrogen production cycles based on the thermochemical decomposition principle. This procedure led to the formulation of three kinds of thermochemical cycles named UT-1, 2 and 3.
The last cycle consists of four reactions given by
CaBr₂ + H₂O → CaO + 2HBr ( 1 )
Ca0 + Br₂ → CaBr₂ + 1/20O₂ ( 2 )
Fe₃O₄ + 8HBr → 3FeBr₂ + 4H₂O + Br₂ ( 3 )
3FeBr₂ + 4H₂O → Fe₃O₄ + 6HBr + H₂ ( 4 )
At present, a series of experimental studies aiming at the industrialization of this process for H₂ production have been conducted. This paper is to describe the kinetic measurment of the reaction ( 3 ).
Magnetite powders mixed with silica sol were fabricated to a sphere of 10mm in diameter and the resultant sphere was calcined at 900°C in nitrogen stream. Hydrogen bromide gas was diluted with steam to the concentration range from 4.8 to 14.3 mol% for the expected concentration range in the industrialized process. Since the reaction was found to proceed topochemically, two kinetic expressions based on the core model were derived for lower (210-255°C) and higher (225-300°C) temperature ranges, respectively. It was found that the rate of the over-all reaction is controlled by the diffusion of reactants in the product layer and that the calculated effective diffusion coefficient within the layer is 7.23 x 10^-6cm²/s.