(618a) Reaction Engineering of a Novel Sulfur-Sulfur Thermochemical Water-Splitting Cycle

The Sulfur-Iodine and hybrid (Westinghouse) water-splitting
cycles have suffered from the use of water as reaction media in their low
temperature reactions.  Separation of the various species from an aqueous
medium results in significant energy expenditures and materials challenges in
practical implementation.  For the past few years we have examined an alternative
route where the oxidation of sulfur step takes place in an ionic liquid medium. 
The application of ionic liquid reaction media for the low temperature
reactions presents possible solutions to the problems of excess water removal,
I₂ solidification, and SO₂ solubility.

The observation that carrying out the Bunsen reaction in
ionic liquid media (Eq. 1 below) can be tailored to produce significant amounts
of H₂S gas from the reaction mixture has led to the development of a
novel Sulfur-Sulfur cycle, shown below. 

                                                          4SO₂ + 4I₂ + 8H₂O → 4H₂SO₄
+ 8HI  Eq. 1

                                                              H₂SO₄ + 8HI → H₂S
+ 4I₂ + 4H₂O  Eq.
2

         Net Low Temperature Reaction: 4SO₂ + 4H₂O → H₂S
+ 3H₂SO₄  Eq. 3

                                                                                                    H₂S + 2H₂O → SO₂
+ 3H₂  Eq. 4

                                                               3H₂SO₄
→ 3SO₂ + 3H₂O + 3/2O₂  Eq. 5

The production of H₂S (Eq. 2) following the
Bunsen reaction, a normally undesired side reaction, can be intentionally
enhanced under the appropriate conditions (concentration of I₂ and
water, temperature and reaction time).  The I₂ is regenerated in
this reaction without having to transport HI to a decomposition section, and is
therefore required only in catalytic amounts.  The H₂S spontaneously
desorbs from the reaction medium and can then be steam reformed to produce H₂
and SO₂, (Eq. 4) the latter of which is recycled.  The thermal
decomposition of H₂SO₄ to produce O₂ and SO₂
(Eq. 5) is common to other Sulfur based thermochemical cycles.  The H₂S
generation and steam reformation steps (Eq. 3 and 4) have been demonstrated in
our lab.[1]  An
overall thermal efficiency assuming a steam reformation temperature of 1100 K
is estimated to be 55%.

This presentation will focus on the parametric dependence of
the H₂S production and steam reforming reactions on reactant
concentrations, temperature, and residence time, and details of the estimated
thermal efficiency calculations will be presented.