Int. J. Hydrogen Energy, Vol. 17, No, 10, pp. 783-788, 1992. Printed in Great Britain. 0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. © 1992 International Association for Hydrogen Energy. DESIGN OF SOLID REACTANTS AND REACTION KINETICS CONCERNING THE IRON COMPOUNDS IN THE UT-3 THERMOCHEMICAL CYCLE R. AMIR, T. SATO, K. YOKO YAMAMOTO, T. KABE and H. KAMEYAMA Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei-shi, Tokyo 184, Japan (Receivedfor publication 19 May 1992) Abstract--A preparation method has been studied for Fe compound reactant pellets, which are required to be highly reac- tive and durable to operate successfully in the UT-3 thermochemical water-decomposition process. Reactant pellets were prepared by calcination of a pelletized mixture of magnetite powder (reactant), inert ceramic particles of silica and zirconia powder (support substance) and such additives as cellulose and graphite powders (to give porosity). The reactant pellets were prepared by adding both graphite and zirconia powders into the raw materials until the pellets showed the highest reactivity and durability. It was shown that the pore volume of pellets increased linearly from 0.07 to 0.35 ml g-~ with the graphite content in the raw material mixtures, and that pellets prepared with 20 wt% graphite were five times as reactive as those prepared without graphite. Kinetics measurements were made for bromination of hematite and magnetite, and hydrolysis of iron bromide. NOMENCLATURE C~ concentration of the solid reactant si [mol m 3] C O initial concentration of the solid reactant sl [mol si m -3 ] Cg, concentration of gaseous reactant g~ [mol m -3] k~ rate constant based on unit volume of pellet for the reaction (i) [m 3 s -t mol -~] m,n numbers of reaction orders with respect to solid and gaseous reactant, respectively r~ reaction rate for the reaction (i) [mol s -~ m -3] T reaction temperature [K] t reaction time [h] X conversion of the solid reactant = C,/C~ [--] X' conversion of the pellet = (amount of Fe in produced FeBr~)/(total of Fe in the pellet) 1. INTRODUCTION The UT-3 cycle consists of four main reactions among Fe, Ca and Br compounds [see equations (1-4) in Table 1 ]. Since the Fe compound is converted to hematite (Fe203) during the calcination stage of the preparation process, the pretreatment reactions (6) and (4) are necessary in order to convert the hematite into magnetite (Fe304). Takiyama et al. [ 1 ] reported that some of the Br2 produced in reaction (3) reacts with unreacted magnetite [see equation (5)]. Since all the reactions are gas-solid interactions, the reaction gases are simply passed individually over four separate fixed beds of the solid reactants, with the gases being alternated. During this cycle, the iron changes repeatedly between its bromide and magnetite forms. However, these two compounds are greatly different in molar volume. It is therefore necessary for the reactant pellets to be strong enough to endure the volume changes of the Fe compound as well as to be reactive, but this pro- perry has not been exhibited. Possible reasons for this are crystal growth of the Fe compound and the fragility of the support substance. The reactant Fe compound pellets were prepared from a mixture of the magnetite powder, inert ceramic particles of silica and zirconia (used as the support substance) and additives such as cellulose (to develop macropores). Aihara et al. [2] reported that addition of graphite improved the reactivity of the reactant Ca compound pellets in reactions (1) and (2). It is expected that the addition of graphite may also be effective in improving reactivity in the case of Fe compounds by developing macropores in the pellets. Thermochemical data may also suggest some possi- ble strong and stable support materials. This paper reports improvements in the reactivity and durability of Fe compound pellets brought about by adding graphite and zirconia powders to the raw materials, and also reports current data on reaction rates of Fe compounds with 20 wt% additive graphite. 2. EXPERIMENTAL 2.1. Materials The raw materials of the Fe reactants were obtained either from commercially available powder or laboratory 783