Thermochemical H2

Thermochemical CyclesWater will not decompose into hydrogen (H2) and oxygen (O2) unless temperatures exceed 3000 K. This is impractical for two main reasons: 1) lack of suitable or inexpensive materials of construction for the actual reactors to handle such temperatures exists; and 2) hydrogen and oxygen can easily recombine to form water [1]. Thermochemical cycles split water into separate streams of hydrogen and oxygen by means of a series of reactions at temperatures that can be achieved via nuclear or concentrated solar heat (on the order of ~1000˚C). A thermochemical water splitting cycle can be conceptualized as a thermal engine that works to create chemical potential energy in the form of H2, which can recombine with oxygen to form water, which releases usable energy. This conceptual engine can take heat energy from a hot reservoir, utilize this energy to split water, and reject the remaining energy to a cold reservoir [2]. The required heat can be generated by either solar concentrators or nuclear reactors. Since a pure thermal decomposition of water is impractical, thermochemical cycles serve to decrease the required temperatures by means of a series of reactions (typically 3 reactions). In identifying potential cycles, a major metric of consideration is cycle efficiency, which is defined as the energy released upon water formation divided by the energy required by the thermochemical cycle:

Cycle Efficiency formula
Where formula 2= heat of formation of water at standard conditions (the energy that is released when hydrogen bonds with oxygen), [kJ]= heat inputs, [kJ]W = work inputs to system (pumping, compression, etc), [kJ]= efficiency of conversion of various pumps, compressor, etc. [3]In 1969, an “International Round Table on Direct Production of Hydrogen with Nuclear Heat” was held at Ispra, Italy. Twenty-four cycles resulted from this initiation of study [4]. The Ispra project resulted in bringing the concept of thermochemical cycles from a purely theoretical idea to a potentially viable method for sustainable hydrogen production. At present, there have been nearly 300 proposed cycles [5]. Though thermochemical cycles have yet to be scaled-up to any sort of commercial capacity, they are regarded as potentially being highly efficient, sustainable, inexpensive, large scale hydrogen production methods. In general, the selling point behind such methods is that they theoretically require only water and heat, while all intermediate reactants and products are recycled in a closed process. Similarly, the only outputs of a thermochemical process are hydrogen and oxygen. Though promising, thermochemical cycles are not as easy to implement as they look on paper. Undesirable side reactions, complicated separation schemes, and corrosion are just some of the problems still plaguing thermochemical hydrogen production and hindering a demonstrable laboratory-scale process from one that is commercially viable on a large-scale. The most heavily studied purely thermochemical cycle is the Sulfur-Iodine Cycle, shown in, which was studied extensively by General Atomics throughout the 1970s and 1980s. Presently at Oregon State, the Yokochi Group in the Department of Chemical, Biological, and Environmental Engineering is investigating alternative solvents for the highlight reaction in Figure 1, the Bunsen Reaction, in order to streamline downstream separation processes and thereby significantly improve overall process efficiency. This cycle is promising; theoretical calculations show that this cycle can be up to 51% efficient [3].

Thermochemical cycle
Figure 1: The Sulfur-Iodine Thermochemical Cycle

References

[1] Licht, S. Thermochemical solar hydrogen generation. Chem. Commun., 2005, pp. 4635-4646.
[2] Abraham, B, and Schreiner, F., “General Principles Underlying Chemical Cycles Which Thermally Decompose Water into the Elements”, Ind. Eng. Chem. Fundamen., 2004, 13, no. 4, 305-310.
[3] Goldstein, S., Borgard, J.M., and Vitart, X. Upper bound and best estimate of the efficiency of the iodine sulphur cycle. Intl. Jour. Hydrogen Energy, 2005, 30, pp. 619 – 626.
[4] Funk, J. E., and Reinstrom, R. M. “Energy Requirements in Production of Hydrogen from Water”, Ind. Eng. Chem. Process Des. Dev., 1966, 5, no. 3, 336-342
[5] Abanades, S., Charvin, P., Flamant, G., and Neveu, P. Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy. Energy, 2006, 31 2805-2822.

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