Researchers at The Ohio State University have used a chemical looping process to produce hydrogen from hydrogen sulfide gas—commonly called “sewer gas”. Hydrogen sulfide is emitted from manure piles and sewer pipes and is a key byproduct of industrial activities including refining oil and gas, producing paper and mining. The process is detailed in a paper in the journal ACS Sustainable Chemical Engineering.

The process uses relatively little energy and a relatively cheap material—iron sulfide with a trace amount of molybdenum as an additive.

Herein, we demonstrate a sulfur looping scheme in a one-reactor system using a low-cost and environmentally safe iron-based sulfur carrier. H₂S decomposition is split into cyclic sulfidation and regeneration of sulfur carriers, which overcomes the inherent thermodynamic constraint, allowing in situ H₂ generation.

We experimentally obtained 24% higher sulfur uptake in 2% Mo-doped iron-based sulfur carriers compared with undoped sulfur carriers. The reaction mechanisms unveiled by the density functional theory indicate that surface hydrogen diffusion is the rate-determining step for sulfidation of the sulfur carrier. Compared with the undoped sulfur carrier, Mo dopant facilitates the surface hydrogen diffusion, thus promoting the overall H₂S conversion.

—Jangam et al.

The study is built on previous work by the same research group using a process called chemical looping, which involves adding metal oxide particles in high-pressure reactors to burn fuels without direct contact between air and fuel. The team first used chemical looping on coal and shale gas to convert fossil fuels into electricity without emitting carbon dioxide into the atmosphere. The initial process used iron oxide to break down the fossil fuels.

The researchers later applied the concept to hydrogen sulfide and invented the SULGEN process, which converts hydrogen sulfide into hydrogen. The researchers found that the pure chemical, iron sulfide, didn’t perform well at the large scales needed for industrial use, said Lang Qin, a co-author on the study and a research associate in chemical and biomolecular engineering at The Ohio State University.

The research team has been trying to identify other inexpensive chemicals that could catalyze that transformation in higher quantities. This study shows that introducing a trace amount of molybdenum into iron sulfide might be an attractive option. That material is relatively inexpensive and easy to acquire, making it an attractive option for larger-scale operations.

It is too soon to tell if our research can replace any of the hydrogen fuel production technologies that are out there. But what we are doing is adjusting this decomposition process and making a valuable product from that.

—Kalyani Jangam, lead author

Resources

• Kalyani Jangam, Yu-Yen Chen, Lang Qin, and Liang-Shih Fan (2021) “Mo-Doped FeS Mediated H₂ Production from H₂S via an In Situ Cyclic Sulfur Looping Scheme” ACS Sustainable Chemistry & Engineering 9 (33), 11204-11211 doi: 10.1021/acssuschemeng.1c03410