Iron oxide-based catalytic structures as enablers of a thermochemical cycle based on solid sulphur for long-term storage of solar thermal energy

Abstract

This work is part of a wider project aiming to demonstrate direct storage of solar energy into solid elemental sulphur. The concept as originally proposed by General Atomics (Norman J.H. US Patent 4 421 734 1978) is a combination of three process steps that interconvert sulphuric acid to sulphur. In the first step sulphuric acid (H2SO4) is first evaporated and then sequentially decomposed into sulphur dioxide and oxygen. This sulphuric acid splitting is the highest-temperature (650-1000C) endothermic reaction step of the cycle. The heat needed can be supplied by Concentrating Solar Technologies (CST) since these temperatures are within the capabilities of state-of-the-art CST tower plants. In the second step sulphur dioxide disproportionation sulphur dioxide reacts with water to produce sulphuric acid and elemental sulphur. Hence a significant proportion of the solar energy used to decompose the sulphuric acid is stored in sulphur for virtually unlimited time. In the third step sulphur can be combusted on demand to release this stored solar energy as heat at temperatures in excess of 1200C suitable for gas turbines and high-efficiency combined cycle power generation. The present work comprises an integrated approach on catalytic sulphuric acid splitting. Typically sulphuric acid is vaporized and driven first through a medium-temperature zone wherein it is thermally dissociated into steam and SO3; the vapours mixture is then passed through a catalytic reactor at higher temperatures where the SO3 splitting to SO2 and oxygen is performed. Atomistic-scale simulations like Density Functional Theory (DFT) were employed to investigate the binding strength of SO3 on various catalysts surface culminating to a plausible reaction mechanism. In parallel a large number of iron oxide-based compositions - which are the state-of-the-art inexpensive splitting catalytic materials - single or mixed with other cations were investigated. They were shaped into spherical granules catalytically tested and physico-chemically characterized before and after exposure to the reaction environment. Selected compositions were then shaped into larger-size honeycombs that underwent long-term (> 300 hours) catalytic testing in a suitably designed catalytic reactor test rig. Several of them demonstrated not only close-to-equilibrium conversion but also remarkable corrosion resistance under the reaction s hostile environment. The next goals are on the one hand to incorporate a large amount of iron-oxide-based inexpensive recycled materials into these catalytic structures and on the other hand to use the results for the design of a pilot-scale reactor for sulphuric acid catalytic splitting with renewable heat.