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DEVELOPMENT STATUS OF THE HYBRID SULPHUR THERMOCHEMICAL HYDROGEN PRODUCTION PROCESS
Development status of the hybrid sulphur
thermochemical hydrogen production process*
William A. Summers
Savannah River National Laboratory
Aiken, South Carolina
Abstract
The DOE Nuclear Hydrogen Initiative has selected two sulphur cycles, the sulphur iodine (SI) cycle and
the HyS process, as the first priority thermochemical processes for development and potential
demonstration with the next generation nuclear plant. Both cycles share a common high temperature
reaction step – the catalytic thermal decomposition of sulphuric acid. However, they are fundamentally
different in the methods used for the hydrogen production step. Whereas the SI cycle utilises two or
more additional thermochemical reaction steps, the HyS process produces hydrogen (and regenerates
sulphuric acid) in a single electrochemical reaction. As a two-step cycle, HyS is thus the simplest
thermochemical process that has been demonstrated. The process chemistry involves only sulphur
compounds, water, hydrogen and oxygen. It has the potential for high efficiency, competitive cost of
hydrogen, and it has been demonstrated at a laboratory scale to confirm performance characteristics.
This paper will discuss the background, current status and future plans for the development of the
HyS process.
The major challenges for the development of the HyS process are associated with the development of
an efficient, cost-effective electrochemical reactor. The reactor is actually a sulphur dioxide depolarised
water electrolyser (SDE). The Savannah River National Laboratory (SRNL) has adopted proton
exchange membrane (PEM) technology for the electrochemical cell. The advantages of this design
concept include high electrochemical efficiency and small footprint, both of which are crucial for
successful implementation on a commercial scale. Since PEM technology is also the subject of intense
development efforts for use in automotive fuel cells, there is the opportunity for leveraging that work
for improving the SDE.
This paper will discuss the selection, characterisation and performance of the major cell components,
including the PEM electrolyte, the anode and cathode, the gas diffusion layers and the overall cell
design. Over thirty single cell test units have been built and tested, and a three-cell stack, rated at
100 litres per hour of hydrogen, was successfully demonstrated. Ongoing work to address key technical
challenges, including long-term operation without voltage degradation or build-up of elemental
sulphur inside the cell, will be discussed. SRNL has also performed extensive system analysis and has
prepared a conceptual design and cost estimate for a commercial HyS process combined with the
Pebble Bed Modular Reactor nuclear heat source. Results of this study will also be presented.
* The full paper being unavailable at the time of publication, only the abstract is included.
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