Graduate Thesis Or Dissertation
 

Detailed analysis of the hydrogen sulfide production step in a sulfur-sulfur thermochemical water splitting cycle

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  • The production of hydrogen has been one of the most heavily studied, energy related fields over the past half century, yet few methods are commercially or economically viable and none are currently sustainable. Of those aiming at the sustainable production of hydrogen using renewable resources, perhaps the most widely studied are those attempting to thermochemically split water via various chemical intermediates. These provide an attractive conceptual alternative to other methods due to lower energy input requirements and to the production of the targeted hydrogen and oxygen in separate reaction steps. One of the most widely studied thermochemical cycles is the Sulfur-Iodine cycle, the development of which has recently slowed due to the difficulty in the separation of hydrogen iodine from a hydrogen iodide-iodine-water azeotrope, material compatibility issues, and the perceived need use large amounts of iodine in the process. A modification of the Sulfur-Iodine thermochemical cycle that attempts to avoid those issues along with mitigating the need to process large amounts of water in the cycle was developed, in a cycle we describe as the Sulfur-Sulfur cycle. This new thermochemical cycle can be summarized by the reaction sequence shown below. [see abstract for chemical reactions] Previous work in our group demonstrated the viability of implementing the cycle's low temperature reactions (the first reaction pair, which we call the Bunsen reaction and the Hydrogen Sulfide Production (HSP) reactions) in ionic liquids, which removes the need to process large amounts of water and iodine in the reaction sequence, and minimizes the material compatibility issues, and also demonstrated the feasibility of stream reforming hydrogen sulfide. The present work focuses on an exergetic analysis of the Sulfur-Sulfur cycle, the careful determination of reaction kinetics for the HSP reaction, and developing a model of the kinetics of the low temperature reactions. The exergetic analysis was carried out based on the published thermochemical parameters for the species involved. The analysis showed that the maximum theoretical exergetic efficiency of the Sulfur-Sulfur cycle is nearly 70% with a strong dependence on the reaction temperature of the low temperature reactions. The kinetics of the Bunsen and HSP were investigated through iodine colorimetery and the effect of water was determined. This kinetic data was used for the development of a predictive kinetic model that could accurately monitor the progression of iodine through the reaction system. The work showed that the Bunsen reaction is very fast with an activation energy (E[subscript aB]) of 92.83 kJ/mol and a pre-exponential factor (k₀) of 7.65E+14 min⁻¹, while for the HSP reaction, they were determined to be 117.09 kJ/mole and 7.73E+16 min⁻¹. Integration of these two reactions into a single differential model based on iodine concentration fit the experimental profile extremely well The effect of including a Lewis base other than water in the reaction mixture yielded promising results that warrant future development. Specifically, the rates of both the Bunsen and HSP reactions increased with an increase in the pK[subscript b] of the added Lewis base. A local protocol to recycle the ionic liquid, enabling it to be reused in new experiments, was successfully developed. When the recycled ionic liquid is employed, effects similar to those found through the inclusion of the Lewis base were observed, suggesting that a decomposition product remains in the recycled ionic liquid. This effect could be minimized by acid washing the recycled ionic liquid prior to use.
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