A hybrid thermochemical electrolytic process for hydrogen production based on the reverse Deacon reaction
Introduction
A hydrogen economy has been proposed as a way of reducing global greenhouse gas emissions while eliminating the need for foreign petroleum imports [1], [2]. However, producing this hydrogen can be problematic, especially when required to do it without generating greenhouse gas emissions, as results from the use of petroleum. Hydrogen is commercially produced via steam methane reforming, a process that both generates carbon dioxide and utilizes natural gas [3]. A more attractive approach consistent with the spirit of the hydrogen economy is to use water as the only chemical feedstock for making hydrogen, with the heat being generated by a renewable energy source. Water is abundant, but renewable energy supplies are not easily harnessed. And the process of converting water to hydrogen is energy intensive. Rather than relying solely upon renewable energy sources such as solar, wind, hydroelectric, and biomass, energy intensive nuclear reactors can supply the power necessary for producing hydrogen from water [4], [5]. Water can be thermochemically decomposed via a series of reactions in a semi-closed cycle. This approach has been investigated by a number of researchers worldwide, but the commonly cited water splitting cycles run at temperatures in excess of , thus requiring a high temperature gas cooled nuclear reactor for supplying heat [6]. As an alternative, Argonne National Laboratory has adopted an approach of seeking water-splitting cycles that have maximum reaction temperatures of less than . This makes it possible to consider a number of lower temperature nuclear reactors, including supercritical water and liquid metal cooled reactors as well as high temperature CANDU reactors.
Section snippets
Reverse Deacon cycle
In an effort to find a relatively low temperature cycle for producing hydrogen, it was decided to consider hybrid cycles that involve an electrolytic reaction in addition to one or more thermochemical reactions. The stipulation would necessarily be that the electrolytic reaction needs to require significantly less energy than water electrolysis. Based on thermodynamic data, the electrolysis of several of the hydrogen halides, including HCl, HBr, and HI satisfy this criterion. Each has a free
Hybrid cycle efficiency analysis
It is envisioned that hydrogen production will eventually occur in cogeneration plants—facilities that produce both hydrogen and electricity. The electricity production will likely be achieved with either a Rankine or a Brayton cycle. For this reason, a meaningful efficiency term should take into consideration not only the absolute value of heat input but also the relative value of each heat stream. Having a Rankine or Brayton cycle nearby provides a means for assigning these relative values.
Experimental methods
Silicalite was selected as the first zeolite material to try as a support, due to its favorable pore size (5.4 Å) and thermal stability. Since it contains virtually no tetrahedra, it is particularly resistant to chemical attack by steam. Powdered silicalite (UOP, HISIV-3000) was dried by heating small samples (1–5 g) to and holding for at least 2 h under flowing dry argon gas. A 9:1 mass ratio of dry silicalite to magnesium chloride powder was then physically mixed and heated rapidly to
Occlusion tests
Seven separate small-scale tests were performed in an attempt to establish a method for absorbing into silicalite. For each test, the silicalite was pre-dried, the mass ratio of silicalite to salt was 9:1, and the salt zeolite mixture was heated to for 16–18 h under dry flowing argon. However, there was a great deal of variability in the extent to which the was adsorbed by the silicalite. The fraction of the salt that was successfully absorbed into the pores of the zeolite and
Summary
From preliminary efficiency estimates and proof of principle experiments, the reverse Deacon cycle appears to be a promising route to low temperature hydrogen production. Silicalite has been investigated as a support for a magnesium-based catalyst in an attempt to solve kinetic and particle size issues. Using zeolites for this purpose requires the ability to load magnesium chloride into the zeolite, sufficient reactivity of occluded magnesium chloride with steam, structural stability of the
Acknowledgements
The authors would like to gratefully acknowledge the contributions of Manuela Serban and Steve Frank in the area of catalyst characterization, Steve Sherman and Mike Goff in the area of program leadership, and Richard Doctor for valuable technical discussions. Support of this research was by the US Department of Energy under contract W-31-109-ENG-38.
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The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory under contract no. W-31-109-ENG-38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.