(509f) Hydrogen Generation from Hydrogen Sulfide Decomposition

Authors: 
Green, W. H., Massachusetts Institute of Technology

We propose a new method for the processing of hydrogen
sulfide to create elemental sulfur and hydrogen gas. The process is based on a
thermochemical cycle that leverages differences in temperature and reaction
conditions to accomplish the thermodynamically unfavorable hydrogen sulfide
decomposition over two reaction steps. First, hydrogen sulfide is reacted with
iodine in aqueous conditions to produce dissolved hydroiodic acid and elemental
sulfur. The sulfur is filtered out and the concentrated hydrogen iodide and
water mixture is vaporized. The hydrogen iodide is then catalytically
decomposed to hydrogen gas and iodine, which are flash separated. Overall, the
two reaction and separation steps combine to accomplish the decomposition of
hydrogen sulfide to hydrogen and elemental sulfur. This proposed process is
similar to ideas put forth in the 1980s and 1990s by Kalina, Chakma, and Oosawa,
but makes use of more recently discovered thermochemical hydrogen iodide
decomposition methods and catalysts rather than electrochemical or
photoelectrochemical methods.

This process is especially useful when the hydrogen sulfide
was created by the reaction of hydrogen with sulfur-rich hydrocarbons during
hydrodesulfurization. By recovering and recycling the hydrogen gas originally
used to desulfurize the hydrocarbon, the amount of hydrogen gas needed to treat
a sulfur-rich hydrocarbon is dramatically reduced. As larger quantities of
sulfur-rich resources are extracted, methods to efficiently desulfurize
hydrocarbons are increasingly important. Further, this process is inherently
carbon neutral which is an advantage over steam reforming, the current main
source of hydrogen gas.

A process model was created to explore the viability of the
process. The model suggests that hydrogen could be profitably generated in
processes with a maximum temperature of just 300 degrees Celsius. This implies
that the heat required for the decomposition of hydrogen sulfide could be
supplied by solar concentrators or heat integration with other process units. Process
units in the model were defined by literature and experimental results.