Introduction
A significant "Hydrogen Economy" is predicted that will reduce our dependence on petroleum imports and reduce pollution and greenhouse gas emissions (Ref 1). Hydrogen is an environmentally attractive fuel that has the potential to displace fossil fuels, but contemporary hydrogen production is primarily based on fossil fuels. This industry produces hydrogen for use in production of fertilizers, in oil refineries to lighten heavy crude oils and produce clearer-burning fuels, and for other industrial uses, primarily by steam reformation of methane. In the USA, this hydrogen industry produces 11 million tonnes of hydrogen a year with a thermal energy equivalent of 48 GWt. In so doing, it consumes 5% of the US natural gas usage and releases 74 million tonnes of CO2. Transition to a Hydrogen Economy will require significant expansion in the production and use of hydrogen. Use of hydrogen for all our transportation energy needs would require a factor of 18 more hydrogen than currently used. Use of hydrogen for all our non-electric energy needs would require a factor of 40 increase. Clearly, new sources of hydrogen will be needed. Hydrogen produced from water using nuclear energy can be one of the sources, and would avoid use of fossil fuels and greenhouse gas emissions.
Hydrogen could be produced from nuclear energy by several means (Ref 2). Electricity from nuclear power can separate water into hydrogen and oxygen by electrolysis. The net efficiency is the product of the efficiency of the reactor in producing electricity, times the efficiency of the electrolysis cell, which, at the high pressure needed for distribution and utilization, is about 75-80%. For LWRs with 32% electrical efficiency the net efficiency is about 24-26%. If an advanced high temperature reactor with 48% electrical efficiency is used, the net efficiency could be about 36-38%. Thermochemical water-splitting processes offer the promise of heat-to-hydrogen efficiencies of ~50%.
We have recently completed a three-year project for the US DOE whose objective was to "define an economically feasible concept for production of hydrogen, using an advanced high-temperature nuclear reactor as the energy source" (Ref 3). Thermochemical water-splitting, a chemical process that accomplishes the decomposition of water into hydrogen and oxygen, met this objective. The goal of the first phase of this study was to evaluate thermochemical processes which offer the potential for efficient, cost-effective, large-scale production of hydrogen, and to select one for further detailed consideration. We selected the Sulphur-Iodine cycle. In the second phase, we reviewed all the basic reactor types for suitability to provide the high temperature heat needed by the selected thermochemical water splitting cycle and chose the helium gas-cooled reactor. In the third phase we designed the chemical flowsheet for the thermochemical process and estimated the efficiency and cost of the process and the projected cost of producing hydrogen. These results are summarized in this paper.
Thermochemical water-splitting
Thermochemical water-splitting is the conversion of water into hydrogen and oxygen by a series of thermally driven chemical reactions. The Sulphur-Iodine cycle is a prime example of a thermochemical cycle. It consists of three chemical reactions, which sum to the dissociation of water.
I2 + SO2 + 2H2O =>2HI + H2SO4 (120°C) (Exothermic) (1)
H2SO4 =>SO2 + H2O + 1/2 O2 (~850°C) (Endothermic) (2)
2HI =>I2 + H2 (~350°C) (Endothermic) (3)
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H2O =>H2 + 1/2 O2 (4)
Energy, as heat, is input to a thermochemical cycle via one or more endothermic high-temperature chemical reactions. Heat is rejected via one or more exothermic low temperature reactions. All the reactants, other than water, are regenerated and recycled. In the S-I cycle most of the input heat goes into the dissociation of sulphuric acid, (2). Sulphuric acid and hydrogen iodide are formed in the exothermic reaction of H2O, SO2 and I2 (1) and the hydrogen is generated in the decomposition of hydrogen iodide (3).
In phase one of the DOE-supported study described in Ref 3, General Atomics, Sandia National Laboratories and University of Kentucky carried out a search of the world literature on thermochemical water-splitting cycles. We located and catalogued 822 references and identified 115 separate thermochemical water-splitting cycles. We evaluated these against quantifiable screening criteria and selected the 25 most promising for detailed technical evaluation. We studied the chemical thermodynamics of these cycles and prepared preliminary engineering block flow diagrams to evaluate practicality. We focused our attention on pure thermochemical cycles and chose the Sulphur-Iodine (S-I) cycle, shown on Figure 1, as the one best suited for high efficiency, practical application to a nuclear heat source.
The Sulphur-Iodine cycle was invented at General Atomics in the mid 1970s and first described in Ref 4. In this cycle, iodine and sulphur dioxide are added to water, forming hydrogen iodide and sulphuric acid in an exothermic reaction (1). Under proper conditions, these compounds are immiscible and can be readily separated. The sulphuric acid can be decomposed at about 850°C releasing the oxygen and recycling the sulphur-dioxide (2). The hydrogen iodide can be decomposed at about 350°C, releasing the hydrogen and recycling the iodine (3). The net reaction is the decomposition of water into hydrogen and oxygen (4). The whole process takes in only water and high temperature heat and releases only hydrogen, oxygen and low temperature heat. All reagents are recycled; there are literally no effluents. Each of the major chemical reactions of this process was demonstrated in the laboratory at GA. Work was done for application of this cycle to heat supplied by nuclear, solar and fusion energy sources. Decomposition of sulphuric acid and hydrogen iodide involves aggressive chemical environments. Materials candidates were chosen and corrosion tests performed to select preferred materials. The high temperature sulphuric acid decomposition reaction was demonstrated in the laboratory and in the Solar Power Tower at the Georgia Institute of Technology. A complete laboratory scale S-I cycle test loop was recently operated successfully in Japan (Ref 6). A schematic for the process is shown on Figure 2.
The S-I cycle does require high temperatures, but offers the prospects for high efficiency conversion of heat energy to hydrogen energy as shown on Figure 3.
Selection of nuclear reactor
Sandia National Laboratories evaluated various nuclear reactors for their ability to provide the high temperature heat needed by the S-I process, and to be interfaced safely and economically to the hydrogen production process (Ref 5). The recommended reactor technology should require minimal technology development to meet the high temperature requirement and should not present any significant design, safety, operational, or economic issues.
We will use an intermediate helium loop between the reactor coolant loop and the hydrogen production system. This assures that any leakage from the reactor coolant loop will not contaminate the hydrogen production system or expose hydrogen plant personnel to radiation from the primary loop coolant. It also assures that the corrosive process chemicals cannot enter the core of the nuclear reactor. The heat exchanger interface sets the boundary conditions for selection of the reactor system. The principal requirement is the temperature requirement for the Sulphur-Iodine cycle, which must account for the temperature drop between the core outlet and the point of application in the hydrogen production system. We assumed a reactor outlet temperature of 950°C. This should give a peak process temperature of ~900°C and a process efficiency of 52%.
The reactor coolant becomes a primary consideration for determining which concepts are most appropriate. The reactor/coolant types considered include pressurised water-cooled reactors, boiling water-cooled reactors, alkali liquid metal-cooled reactors, heavy liquid metal-cooled reactors, gas-cooled reactors, organic-cooled reactors, molten salt-cooled reactors, liquid-core reactors, and gas-core reactors. The reactor types were assessed against the five requirements and five important criteria given in Table 1.
Based on this assessment, and upon evaluation of the relative development requirements for candidate reactors, the following conclusions and recommendations were made:
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PWR, BWR, and organic-cooled reactors – not recommended: cannot achieve the high temperatures needed.
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Liquid-core and alkali metal-cooled reactors – significant development risk due to materials concerns at the high temperatures needed.
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Heavy metal and molten salt-cooled reactors – promising, but significant development needed.
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Gas-cooled reactors – baseline choice, only modest development needed for helium gas-cooled reactor.
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Gas-core reactors – not recommended, too speculative.
Helium gas-cooled reactors are recommended as the baseline choice for a reactor heat source for a Sulphur-Iodine thermochemical cycle for hydrogen production.
The Modular Helium Reactor
Selection of the helium gas-cooled reactor for coupling to the S-I hydrogen production process allows us to propose a design concept and do preliminary cost estimates for a system for nuclear production of hydrogen. The latest design for the helium-gas cooled reactor is the Gas Turbine-Modular Helium Reactor (Ref 7). This reactor consists of 600 MWt modules that are located in underground silos. The direct-cycle gas turbine power conversion system is located in an adjacent silo, as shown in Figure 4.
This new generation of reactor has the potential to avoid the difficulties of earlier generation reactors that now have stalled nuclear power in the United States. The GT-MHR has high temperature ceramic fuel and a core design that provide passive safety. A catastrophic accident is not possible. Under all conceivable accident conditions, the reactor fuel stays well below failure conditions with no actions required by the plant operators or equipment. By avoiding the need for massive active safety back-up systems, the capital cost of the GT-MHR is reduced. The high temperature fuel also allows high efficiency power conversion. The gas turbine cycle is projected to give 48% efficiency.
The high helium outlet temperature also makes possible the use of the MHR for production of hydrogen using the S-I cycle. By replacing the gas turbine system with a primary helium circulator, an intermediate heat exchanger, an intermediate helium loop circulator and the intermediate loop piping to connect to the hydrogen production plant, the GT-MHR can be changed into the "H2-MHR", as shown in Figure 5.
Economics estimates
We have estimated the economics of hydrogen production from nuclear energy using the S-I thermochemical cycle. The Gas Turbine-Modular Helium Reactor has a predicted "overnight" capital cost of US$975/kWe or US$468/kWt (Ref 7). We excluded the cost of the turbo-generator and included an intermediate heat exchanger, circulators and piping. We assumed a US$45/kWt premium to provide 950°C for the S-I process instead of the 850°C outlet temperature of the GT-MHR. We determined the capital cost of the hydrogen plant equipment and chemical inventory. We estimated the total capital and operating costs of the integrated hydrogen plant to calculate the cost of the hydrogen produced. The capital costs are for a 2400 MWt "nth of a kind" plant producing approximately 800 tonnes of hydrogen per day, enough to support a large oil refinery, or almost 1.5 million fuel cell cars. The costs include all direct and indirect costs, plus interest during construction. The reactor operating costs include all fuel cycle costs (fuel, conversion, enrichment, fabrication, waste disposal and decommissioning) plus normal operation and maintenance costs. The hydrogen plant operating costs include normal operation and maintenance costs plus the cost of high purity water. All costs are in 2002 funds. Since both the reactor and the hydrogen plant are capital intensive, the hydrogen cost estimates using several different capital recovery factors (CRF) are shown in Table 2.
The cost of producing hydrogen from natural gas by steam reformation of methane depends strongly on the cost of the natural gas, which is used for both the feedstock and the energy source. At the current natural gas cost of about US$6/MBtu, steam reformation can produce hydrogen for about US$1.40/kg. If carbon capture and sequestration is required, an estimated cost of US$30/tonne of CO2 could add 20¢/kg of H2 to the cost of hydrogen from methane. Nuclear production of hydrogen using the Modular Helium Reactor could thus be competitive at today’s prices for natural gas. As the price of natural gas rises with increasing demand and decreasing reserves, nuclear production of hydrogen would become more and more cost effective.
Conclusions
Nuclear energy is an attractive potential source of hydrogen for the Hydrogen Economy. A large hydrogen market already exists and it is growing rapidly to provide increasing amounts of hydrogen to oil refineries for upgrading heavy crude oils and producing clean-burning products. This market is expected to continue growing at ~10%/yr, doubling by 2010 and doubling again by 2020. To transition to a "Hydrogen Economy" would take still more hydrogen. New sources of hydrogen will be needed. Our recent DOE-supported study of nuclear production of hydrogen identified the Sulphur-Iodine thermochemical water-splitting cycle coupled to the Modular Helium Reactor (the H2-MHR) as an attractive candidate system for hydrogen production.
The estimated costs presented in this paper show that hydrogen production by the H2-MHR could be competitive with current techniques of hydrogen production from fossil fuels if CO2 capture and sequestration is required. This favorable situation is expected to further improve as the cost of natural gas rises.
Nuclear production of hydrogen would allow large-scale production of hydrogen at economic prices while avoiding the release of CO2. Nuclear production of hydrogen could thus become the enabling technology for the Hydrogen Economy.
Acknowledgements
Work supported by General Atomics and US Department of Energy under Grant No. DE-FG03-99SF21888.