Metal Oxide Chemical Looping Cycles for Coproduction of Hydrogen and Sulfuric Acid

Abstract

Hydrogen production via multi-step thermochemical cycles may be an affordable, low-carbon alternative to steam methane reforming or water electrolysis. Conventional thermochemical water splitting generates hydrogen while coproducing an oxidized product, oxygen, that has little value. Instead, we use elemental sulfur as an input and coproduce sulfur dioxide. This process, which we call Steam Sulfur Reforming (SSR), could function as a value-added substitute for the sulfur burning step of sulfuric acid manufacturing. Steam Sulfur Reforming is demonstrated with experiments using ceria and iron oxide, thermodynamic modeling, and technoeconomic analysis. Sulfur oxidation is favorable at temperatures near 1200 °C, comparable to conventional sulfur burning and lower than many thermochemical water splitting cycles. Experiments with ceria demonstrate stability over dozens of cycles, with sulfur conversions of up to 90% at 1200 °C and steam conversions of up to 50% between 700 and 900 °C. Experiments with iron oxide show the intended cycle but are challenged by sulfidation side reactions and low steam conversion, highlighting opportunities for metal oxide modification. SSR may achieve costs below 2 $/kg H2 with CeO2 or Fe3O4, making the process cost-competitive with fossil hydrogen and a potential drop-in hydrogen replacement for the fertilizer industry.