The Westinghouse Process
The Westinghouse Process is one of the ‘Sulphur family’ of thermochemical cycles being considered for the generation of hydrogen. It is a sulphur cycle using hybrid electrochemical thermochemical process for decomposing water into hydrogen and oxygen.
The Westinghouse Sulphur Cycle is a two-step thermochemical cycle for decomposing water into hydrogen and oxygen. Sulphurous acid and water are reacted electrolytically to produce hydrogen and sulphuric acid. The resultant sulphuric acid is vaporised to produce steam and sulphur trioxide, with the latter compound being subsequently reduced at higher temperatures into sulphur dioxide and oxygen. Following the separation of the water and sulphur dioxide for recycle to the electrolyser, oxygen is available as a process by product.
Figure 1 A Simplified Flowsheet of the Westinghouse Sulphur Cycle (Goossen et. al 2003)
The sulphur cycle was originally selected for its potential to achieve high thermal efficiencies while using common inexpensive chemicals. The amount of basic information needed in a process development effort is reduced because the properties of sulphur and its compounds are well documented. Importantly, sulphur assumes a variety of valence states, thereby facilitating it use in oxidation-reduction reactions important to the thermochemical cycle. Sulphur compounds are abundant, inexpensive and of known toxicity. Processes, equipment, catalysts, literature and a distribution infrastructure already exist. Finally the product hydrogen and oxygen streams can be made available under high pressure and at high purity, which is desirable for downstream use.
Process Description
Figure 2 Simplified Diagram of Westinghouse Sulphur Cycle (Forsberg et. al 2003)
The process in its most general form consists of two chemical reactions; one for producing oxygen and the other for producing hydrogen. The production of oxygen occurs via the thermal reduction of sulphur trioxide obtained from sulphuric acid.
2H2SO42H2O + 2SO32SO2 + 2H2O + O2 (850C)(i.)
The equilibrium for Reaction (i.) lies to the right at temperatures above 1000K. Catalysts are available for accelerating the rate of sulphur trioxide reduction to sulphur dioxide and oxygen. The process is completed by using the sulphur dioxide from the thermal reduction step to depolarize the anode of a water electrolyser. The overall reaction occurring electrochemically is
SO2(aq) + 2H2O(l) H2SO4(aq) + H2(g) (Electrolysis: 80C) (ii.)
This is comprised of the individual reactions
Cathode: 2H+ + 2e- → H2
Anode: H2SO3 + H2O → 2H+ + H2SO4 + 2e--0.17 Volts
The net result of Reactions (i.) and (ii.) is the decomposition of water into hydrogen and oxygen. Sulphur oxides are involved as recycling intermediates. Although electrical power is required in the electrolyser, much smaller quantities than those necessary in conventional electrolysis are needed. The theoretical voltage to decompose water is 1.23V, with many commercial electrolysers requiring over 2.0V. The power requirements for Reaction (ii.) (0.17 Volts at unit activity for reactants and products) are thus seen to be less than 15% of those required in a conventional electrolysis. This dramatically changes the theoretical heat and work required to decompose water and leads to high thermal efficiencies.
Brecher, L., Spewock, S., & Warde, C., (1977) The Westinghouse Sulphur Cycle for the thermochemical decomposition of water, International Journal of Hydrogen Energy, 2: 7-15, Pergamon Press ltd. 1977
Carty, R., et al, (1977) Process sensitivity studies of the Westinghouse sulphur cycle for hydrogen generation, International Journal of Hydrogen Energy, 2: 17-22, Pergamon Press ltd. 1977
Farbman, G., (1979) Hydrogen Production by the Westinghouse Sulphur Cycle Process: Program Status, International Journal of Hydrogen Energy, 4: 111-122, Pergamon Press ltd. 1979
Forsberg, C., Bischoff, B., Mansur, L., Trowbridge, L., Tortorelli, P., (2003) A Lower Temperature Iodine-Westinghouse-Ispra Sulphur Process for Thermochemical Production of Hydrogen, Submitted Manuscript to U.S Government (Global 2003 Manuscript number 87682) by Oak Ridge National Laboratory.
Goossen, J., Lahoda, E., Matzie, R., and Mazzoccoli, J., (2003) Improvements in the Westinghouse process for hydrogen production, Paper printed by the Westinghouse Electric Company Science and Technology Department 1344 Beulah Road, Pittsburgh, PA 15235-5083
Lin, S., & Flaherty, R., (1983) Design studies of the sulphur trioxide decomposition reactor for the sulphur cycle hydrogen production process, International Journal of Hydrogen Energy, 8: 589-596, Pergamon Press ltd. 1983