The hybrid sulfur cycle (often called the Westinghouse cycle) for decomposing water into hydrogen and oxygen has two steps. The sulfuric acid is decomposed into steam and sulfur trioxide, which is further decomposed into sulfur dioxide and oxygen at high temperature (~1100 K). Hydrogen is produced by electrolysis of a sulfur dioxide and water mixture at low temperature, which also results in the formation of oxygen and sulfuric acid.
In this study, separation of decomposed products and internal heat recuperation are examined, and 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 the internal heat recuperation. The analysis is based on currently available experimental data for the electrode potential.
A cycle efficiency of 45.3% [lower heating value (LHV)] appears to be achievable at 1100 K (10 bar, 1100 K, and 60 mol% of H2SO4 for the decomposer, 60 wt% of H2SO4 for the electrolyzer). For a maximum temperature of 1200 K, 50.5% (LHV) appears to be the achievable cycle efficiency (10 bar, 1200 K, and 60 mol% of H2SO4 for the decomposer, 60 wt% of H2SO4 for the electrolyzer). Operation under elevated pressures (70 bar or higher) results in loss of cycle efficiencies due to lower yield of the SO2 in the decomposer but minimizes equipment size and possibly capital cost. However, the loss in efficiency as pressure increases is not large at high temperature (1200 K) compared to that at low temperatures (1000 to 1100 K). Therefore, high-pressure operation for minimizing capital investment would be favored only if the high temperature can be accommodated. The major factors that can affect the cycle efficiency are reducing the electrode overpotential and having structural materials that can accommodate operation at high temperature and high acid concentration.