OPTIMIZATION OF THE HYBRID SULFUR CYCLE FOR HYDROGEN GENERATION

Abstract

The hybrid sulfur cycle (modified from the Westinghouse Cycle) for decomposing water into oxygen and hydrogen is evaluated. Hydrogen is produced by electrolysis of sulfur dioxide and water mixture at low temperature, which also results in the formation of oxygen and sulfuric acid. The sulfuric acid is decomposed into steam and sulfur trioxide, which at high temperature (1100 K) is further decomposed into sulfur dioxide and oxygen. The presence of sulfur dioxide along with water in the electrolyzer reduces the required electrode potential well below that required for electrolysis of pure–water, thus reducing the total energy consumed by the electrolyzer. Further, using only sulfuric acid for the thermochemical processes minimizes the required chemical stock in the hydrogen plant well below that required for the sulfur–iodine pure thermochemical cycle (SI cycle). In this study, ways to optimize the energy efficiency of the hybrid cycle are explored by varying the electrolyzer acid concentration, decomposer acid concentration, pressure and temperature of the decomposer and internal heat recuperation, based on currently available experimental data for the electrode potential. An optimal cycle efficiency of 43.9% (LHV) appears to be achievable (5 bar, 1100 K and 60 mol–% of H[subscript 2]SO[subscript 4] at the decomposer, 70 w–% of H[subscript 2]SO[subscript 4] at the electrolyzer). However, the ideal cycle efficiency is over 70% (LHV), which leaves room to improve the achievable efficiency with further development. For a maximum temperature of 1200 K, 47% (LHV) appears to be the maximum achievable cycle efficiency (10 bar, 1200 K and 60 mol–% of H[subscript 2]SO[subscript 4] for decomposer, 70 w–% of H[subscript 2]SO[subscript 4] for electrolyzer). The ideal cycle efficiency is over 80% (LHV). Operation under elevated pressures (70 bar or higher) results in minimized equipment size and capital cost, but there is loss in thermal efficiency. However, the loss in efficiency as pressure increases is not large at high temperature (1200 K) compared to that at low temperatures (1000–1100 K). Therefore, high pressure operation would be favored only if we can achieve high temperature. The major factors that can affect the cycle efficiency are reducing the electrode over–potential and having structural materials that can accommodate operation at high temperature and high acid concentration.