Shaping, experimental evaluation and modelling of prototype iron oxide structures for catalytic splitting of sulphuric acid

Abstract

In this work, prototype porous sulfuric acid splitting catalytic structures are prepared entirely from the catalytically active oxides-based phase and evaluated for their performance in a dedicated test reactor.

Sulfuric acid splitting is a crucial step in the sulfuric acid recycling industry, so far implemented via thermal decomposition with fossil fuels. Furthermore, the decomposition of sulfuric acid is the high-temperature step of the Hybrid Sulfur cycle and the Sulfur Iodine cycle that are targeted on H2 production as well as the Solid Sulphur cycle proposed for power generation/heat storage. Typically, in catalytic sulfuric acid splitting, sulfuric acid from a reservoir is vaporized and driven first through a medium-temperature zone where its stoichiometric thermal dissociation into steam and SO3 takes place; the vapours mixture is then passed through the catalytic reactor at higher temperatures where the SO3 splitting is performed. The latter is the highest-temperature 650-1000oC endothermic reaction step where the heat can be supplied by Concentrating Solar Technologies since these temperatures are within the capabilities of state-of-the-art CST tower plants.

State of the art materials investigated for sulfuric acid splitting catalysts include iron oxides. In this work, iron oxide structures of tailored geometries and porosity prepared entirely from the catalytically active phase carrier are presented. The manufacture of such Fe2O3 based structures, lies on the design rationale of circular economy principles; where performant materials must be from elements that are accessible and not in danger of a supply risk. A possible source of such iron-oxide materials are waste fractions (Fe-containing slags) discarded in large-scale from the steel industry.

Iron oxide materials are shaped into monolithic honeycombs and inserted in a tubular lab-scale reactor for a complete evaluation of sulfuric acid splitting potential and performance. The reactor is automated to allow for increased exposure time and cyclability of each structure tested. The performance of the pure-iron oxide extruded monoliths is compared to inert (SiSiC) monoliths coated with a catalyst layer and with structures such as iron oxide foams. The experimental results feed a model describing each structure which is used to drive the design of a pilot-scale reactor for the catalytic splitting of sulfuric acid with renewable heat.

For the first time, unique structures from cheap and abundant materials are prepared and shaped, then evaluated in a dedicated lab-reactor rig, under long-term operation for catalytic sulfuric acid splitting. This work is accompanied by a kinetic and reactor model guiding the design of a scaled-up pilot reactor, relevant to both the sulfuric acid (recycling) industry as well as the solar thermal community for heat storage or hydrogen production.