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.
D.C Electricity
+
-
Thermal Energy
Hydrogen Generation
2H2O + SO2
H2
H2SO4
H2 + H2SO4
Sulphuric Acid
Vaporisation
(Electrolyser)
H2SO4
H2O
SO2
Oxygen Generation
Oxygen recovery
O2
H2O
H2O
SO2
O2
H2SO4
H2O + SO2 + 1/2O2
Thermal Energy
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.
2H2SO4 2H2O + 2SO3 2SO2 + 2H2O + O2 (850C)
(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: 80C)
(ii.)
This is comprised of the individual reactions
Cathode: 2H+ + 2e- → H2
E0
Volts
0.00
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