Changes in Reaction Rates and Structure of Solid Reactant Concerning the Iron Compounds in the UT-3 Thermochemical Hydrogen Production Cycle

Abstract

Changes in reaction rates of Fe₃O₄ bromination and FeBr₂ hydrolysis under cyclic operation of the UT-3 thermochemical water decomposition cycle were analyzed.
The solid reactant was prepared from Fe₃O₄ powder and ZrSiO₄ powder as binder. The reaction rate equations were determined as follows for the bromination of Fe₃O₄ and hydrolysis of FeBr₂ in the 1st cycle.
bromination : r [mol · m^-3 · s^-1] =-dC_Fe3O4 / dt
= 1.5 × 10^-3C_HBr^1.5C_H2O^-0.5C_Fe3O4
hydrolysis : r [mol· m^-3 · S^-1] =-dC_FeBr2 /dt
= 0.018 C_H2OC_FeBr2^0.5
In cyclic operation, the reaction rate constants of both reactions after 20 cycles were 1.4 times as large as those for the 1st cycle. Analysis of the pore distribution showed that a large amount of reactant Fe-compound in the pellet moved from pores of around 1 μm in diameter to newly appeared pores of around 10 μm during 20 reaction cycles. The BET surface area of Fe₃O₄ after 20 cycles was about twice as large as that for the 1st cycle. It was considered that these structure changes caused the changes in the reaction rate constants.