(159c) Convert Hydrogen Sulfide to Hydrogen By Iron Sulfide Via a Novel Thermo-Catalytic Sulfur Capture Process

Hydrogen sulfide (H₂S) is a by-product of natural gas sweetening and crude oil desulphurization, and it is a potent pollutant in environment, human health and many industrial activities. The use of liquid solvent absorption along with Claus process that convert H₂S to H₂O is the current commercial process. However, such process requires special solvents for selective H₂S absorption and a large input of energy and several process units to convert H₂S to H₂O and sulfur. As hydrogen (H₂) is one of the most extensively used chemical intermediates in industry and an option of clean energy, its global demand keeps on rising. An advanced H₂S processing technology that can convert H₂S to H₂ and elemental sulfur is therefore highly desired.

In this work, we adopt the concept of chemical looping and develop a thermo-catalytic sulfur capture (TCSC) process in a simple two-step manner: sulfidation and regeneration. In the sulfidation step, iron sulfide (FeS_x) reacts with H₂S to produce H₂ while it is converted to a higher sulfidation state (FeS_y, x<y) by capturing sulfur. The captured sulfur is then released by passing an inert gas like nitrogen (N₂) at a higher temperature than the sulfidation reactor in the regeneration step, where FeS_x is regenerated. Therefore, the net result is production of H₂ and sulfur from H₂S. The application of this technology reduces the number of processing units drastically as compared to the existing methods, and also produces H₂ which improves the heating value of the gas stream. However, the reaction kinetics of pure FeS_x with H₂S were found to be slow, and methodologies to improve its reactivity becomes key to the success of this process. First-principles density functional theory (DFT) simulations were performed to unravel the reaction mechanism of H₂S on FeS_x. DFT simulations further pointed out that using molybdenum (Mo) as a dopant can modify the surface electronic structure significantly, and hence lower the energy barrier of the rate-limiting step substantially. The simulation results were validated in fixed bed experiments by comparing the H₂S conversion between FeS_x and Mo-doped FeS_x under the same operating conditions, where Mo-doped FeS_x indeed shows superior reactivity. Findings from this study would pave a way for further improvement of H₂S separation technology.