In the second step, the uranium reaction product and the excess Na2CO3 are ... thermochemical uranium cycle based on the following chemical reactions was ...
Carbonate Thermochemical Cycle for the Production of Hydrogen Juan Ferrada, Jack Collins, Les Dole, Charles Forsberg,‡ M. Jonathan Haire, Rodney Hunt, Ben Lewis, Ray Wymer, and Jennifer L. Ladd-Lively Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6166 ‡
Massachusetts Institute of Technology Cambridge, Massachusetts 02139-4307 Paper to be presented at the National Hydrogen Association (NHA) Annual Conference and Hydrogen Expo Columbia, South Carolina March 30–April 3, 2009 ABSTRACT A modeling and experimental effort has indicated the viability of a novel carbonate thermochemical cycle (CTC) for the production of hydrogen using uranium. Changes in the uranium oxidation state can be used to decompose water to produce hydrogen. The CTC has a small number of process steps that occur spontaneously at near-ambient pressures and form stable nonvolatile compounds. Well-established equipment and processes are used throughout. The peak temperature within the cycle is below 700°C—a temperature achievable with existing hightemperature nuclear reactors and some solar systems using commercially available materials. In this thermochemical cycle, triuranium octoxide (U3O8) is initially reacted with sodium carbonate (Na2CO3) and steam to generate hydrogen and sodium diuranate (Na2U2O7) at 600°C and above. The remaining steps in the cycle use well-known chemical processing steps from the uranium industry. In the second step, Na2U2O7 is dissolved using an alkaline carbonate solution, and the resulting uranyl solution is passed through an anion-exchange resin (Dowex 1-X) to separate uranium from sodium. The Na2CO3 can be recovered from the uranium-free raffinate. The uranium is then stripped from the resin using ammonium carbonate [(NH4)2CO3]. The resulting uranium solution is dried to produce ammonium uranyl carbonate [UO2CO3·2(NH4)2CO3] and then heated to 400°C to generate U3O8, oxygen, ammonia (NH3), carbon dioxide (CO2), and water (H2O). The U3O8, NH3, CO2, and H2O can be recycled and used in the next H2 production cycle. Oxygen is a by-product of the cycle. This cycle has several advantages over competing processes. The key advantages are as follows: •
the proposed chemical mechanisms do not require volatile hazardous chemicals (with the exception of NH3, which is a product of the process), and
•
the process operates at temperatures that are compatible with common materials of construction.
This paper describes the process, the thermodynamic analysis that was initially used to develop the process, and some experimental results.
1.
INTRODUCTION
Thermochemical processes represent promising large-scale means of producing hydrogen because the energy input is in the form of lower-cost heat rather than more-expensive electricity. In a thermochemical process, heat plus water (H2O) yields hydrogen (H2) and oxygen (O2). Ideally, all other chemicals within the process should be fully recycled. A recent assessment1 evaluated over 100 thermochemical cycles, but only a few were considered potentially viable. To be practical, a thermochemical cycle should (1) operate at high thermodynamic efficiency, (2) use a practical heat source, (3) have a small number of process steps to simplify equipment requirements and reduce costs, (4) not require a significant inventory of volatile or hazardous chemicals that can cause on-site or off-site risks, and (5) operate at temperatures that allow relatively inexpensive construction materials. The leading candidate thermochemical cycles are variations of the sulfur–iodide (SI) and transition metal–chlorine cycles. These thermochemical cycles have three serious limiting characteristics: (1) they have nominal operating temperatures of ~850°C and/or high pressure to greater than 20 atm; (2) they require exotic construction materials to withstand corrosive reagents and high temperatures; and (3) they use large inventories of hot, hazardous, pressurized, volatile reagents that represent potential off-site accident risks. Several of these processes (such as the hybrid sulfur processes) also require an electrochemical step that requires the input of moreexpensive electricity to complete the cycle. Therefore, a need exists for better thermochemical cycles. A review of the literature indicates many studies of thermochemical cycles but almost no consideration of using uranium in these cycles. In most studies, uranium was excluded either because it was considered expensive or because of its radioactivity. However, from a chemical perspective uranium exists in many valence states and has overlapping 6d and 5f electrons and a complex chemistry—exactly the chemistry one would investigate for development of a thermochemical cycle. The nuclear fuel cycle involves mining natural uranium (0.7% 235U) followed by enrichment of the uranium (3 to 5% 235U) in the 235U isotope for use in nuclear fuels. The by-product is depleted uranium. Worldwide, there is over a million tons of excess depleted uranium (~0.3% 235U) available at little or no cost. The depleted uranium is primarily 238U with a half-life of several billion years (i.e., its radioactivity is extremely low, thus making it credible to develop a thermochemical cycle). Based on these considerations, an investigation was undertaken to find and develop a thermochemical cycle based on uranium. This paper describes the thermochemical cycle that was developed, the thermodynamic analysis, and the experimental work.
2.
PROCESS DESCRIPTION
Oak Ridge National Laboratory (ORNL) has developed a carbonate thermochemical cycle (CTC)2 using uranium, a multivalent element from the actinide series. Figure 1 shows the current flowsheet. The CTC uses valence-state transitions and the formation of oxidized and reduced uranium species to split H2O for the production of H2. Uranium is capable of more than one valence state under relatively mild temperatures and pressures. Other candidate multivalent elements such as transition metals may also be appropriate for this process.
Fig. 1. Flowsheet for the CTC for the production of hydrogen. In the CTC process, uranium valence changes drive the decomposition of water to hydrogen and oxygen. In this process, water is added to triuranium octoxide (U3O8) and sodium carbonate (Na2CO3). Under mild pressures (~1 atm), an excess of carbon dioxide (CO2) in an inert carrier gas, such as argon (Ar), is added to the system. When these reactants are held at 600 to 700°C, hydrogen production occurs. Steam (H2O), excess CO2, Ar, and H2 can be drawn from the chemical reaction vessel. Although not demonstrated in this work, the H2 can be separated from these gases by selective membranes or pressure-swing adsorption and/or cryogenic separation methods. The steam is condensed to recover heat, and the excess CO2 and carrier gases are recycled. Only this first step in the CTC process is unique; the remaining two steps are conventional processes in the uranium processing industry.3 In the second step, the uranium reaction product and the excess Na2CO3 are dissolved with alkaline carbonate. This uranium solution is passed through a column of Dowex 1-X. The uranium is loaded onto the ion-exchange resin while the sodium passes through the column. The uranium is then stripped from the resin using a solution of ammonium carbonate [(NH4)2CO3]. The uranium is in the form of ammonium uranyl carbonate [UO2CO3·2(NH4)2CO3]. In the third step, the thermal decomposition of the ammonium uranyl carbonate generates U3O8, O2, NH3, CO2, and H2O. Additional process operations, which have not been studied, are used to clean and prepare the H2 stream for subsequent delivery. Other material regeneration processes require optimization to enable use of recovered heat. The CTC process has no electrolysis step, and the various uranium compounds are nonvolatile. The CO2 and Ar gases are noncorrosive and nontoxic, while the steam and NH3 are corrosive gases. The gases will all be recycled. The NH3 will be trapped and converted into nonvolatile (NH4)2CO3 within the process. With these CTC process materials and operating conditions, common materials are adequate for use in construction of the process equipment.
3.
THEORETICAL DEVELOPMENT OF THE CTC
The CTC was conceived by members of ORNL’s Nuclear Science and Technology Division, who had examined an earlier proposal for a uranium-based thermochemical cycle.4 A thermochemical uranium cycle based on the following chemical reactions was proposed: U3O8 + H2O + 3CO2(g) → 3UO2CO3 + H2(g)
(1)
3UO2CO3 → 3UO3 + 3CO2(g)
(2)
3UO3 → U3O8 + 0.5O2
(3)
The thermodynamics of this cycle were analyzed using Outokumpu’s HSC 5.0 Chemistry software computer code version 5.11.5 The thermodynamic analysis indicated that reaction (1) would not occur spontaneously because the free energy (ΔG) is never negative. This lack of a reaction was confirmed experimentally. This led to the investigation of a series of alternative cycles using uranium. A modification of the above cycle generated a more thermodynamically viable set of chemical equations, shown below: U3O8 + 9CO2(g) + 12NaOH(a) → 3UO2(CO3)34-(a) + 12Na+(a) + H2(g) + 5H2O(l)
(4)
3UO2(CO3)34-(a) + 12Na+(a) → 3UO3 + 6Na2O + 9CO2(g)
(5)
3UO3 + 6Na2O + 6H2O(g) → U3O8 + 12NaOH(a) + 0.5O2(g)
(6)
Free energy of the first equation is negative for temperatures below 200°C. The UO2(CO3)34-(a) is formed in a narrow range of temperatures around 80 to 120°C, but with extremely low yield. Likewise, the production of H2 occurs around 350°C, but with low yield. In addition, a significant amount of competition exists among the species produced. The UO2(CO3)34-(a) was kept in the reaction equation because CO2 is needed for the reaction to proceed to any significant extent. This CO2 requirement suggests that the optimal reaction pathway proceeds through the tricarbonate species. Analysis indicated that perhaps the addition of a reducing agent into the mixture of reactants would improve the hydrogen yield. Simulation of the process with hydrogen at 450°C gave a production efficiency of ~25%. Finally, a modification of the basic cycle using Na2CO3, instead of NaOH, with 3 mol of excess CO2 was examined. Reaction 4 thus becomes the following: U3O8 + 3CO2(g) + 6Na2CO3 + H2O → 3UO2(CO3)34-(a) + 12Na+(a) + H2(g)
(7)
The input to the HSC code for this reaction is given in the table below, and the graphic results are shown in Fig. 2. Hydrogen production, as shown by the red line, is favorable above 300°C, and it should reach a maximum at approximately 625°C.
Input to HSC code CO2(g) H2O Na2CO3 U3O8
Moles 3.000 55.500 6.000 1.000
Fig. 2. Thermodynamic equilibrium for modified base case (1 atm).
4.
EXPERIMENTS
A conceptual design of the equipment used in the hydrogen generation step is shown in Fig. 3. In the actual experiments, a fourth furnace was used to remove trace amounts of O2 from the Ar supply. Oxygen must be eliminated from the system because U3O8, Na2CO3, and O2 can react spontaneously to form Na2U2O7 and reduce the H2 yield. The Ar was passed through zirconium metal pieces, which were heated to 490°C. The flow rates of Ar and CO2 were maintained using MKS mass flow controllers. Typical flow rates for Ar and CO2 were 300 and 75 mL/min, respectively. The Ar and CO2 gases were combined and then passed through a prefilter, which was designed to remove any reductants, such carbon monoxide (CO). The prefilter used copper oxide (CuO), which was heated to 500°C. The feed gases were then passed through a stainless steel vessel, which contained deionized H2O. A heating tape was wrapped around the water container, and the water was heated to 70–90°C. The Ar, CO2, and H2O vapor passed through a heated transfer line to an alumina tube in a tube furnace. A stainless steel reactor cannot be used, because high-temperature H2O corrosion of the steel can also generate H2 and impede the desired reaction. The alumina tube contained an alumina boat with a mixture of
either U3O8 or uranium dioxide (UO2) and Na2CO3. The molar ratio of Na2CO3 to U3O8 was 6 to 1, while the molar ratio of Na2CO3 to UO2 was 2 to 1. In most experiments, UO2 was used because its H2 generation per gram of sample is 3 times higher than that of its U3O8 counterpart. The typical sample size of the Na2CO3 and UO2 was 4 to 8 g. The reactor was heated between 600 and 675°C for 11 to 21 h. The gases from the alumina reactor were passed through a condenser, which was cooled to −10°C with the aid of a refrigerated recirculator. The condenser removed most of the water vapor. The remaining gases were directed through the H2 and CO detector, which used CuO heated to 500°C. Reduction of CuO to copper metal (H2 + CuO → Cu + H2O or CO + CuO → Cu + CO2), determined by weight loss, was the primary method used to measure the amount of reductant produced. Under these test conditions, thermodynamics favor the production of H2 over CO. For the smaller samples of UO2 and Na2CO3, a typical weight loss for the CuO in the H2 detector was 9 to 18 mg for 14 h at 675°C. As expected, the rate of H2 generation increased as the reactor temperature was raised. The appearance of copper metal, as shown in Fig. 4, confirmed the generation of a reductant. The solids were removed from the reactor and weighed. The solids became orange as the reaction continued. X-ray diffraction analysis of the orange solids indicated that the reaction product was primarily Na2U2O7. If any black starting solids remained, the solids were reacted longer to generate more hydrogen.
Water Reactor U3O8 + Na2CO3
CuO 500 oC
H2O Ar CO2
500 oC
CuO (CO removal)
Ar CO2
H2 H2O CO2 Ar
CO2 Ar
H2 CO2 Hydrogen Ar Carbon Monoxide Detector Cooled Condensers
Fig. 3. Conceptual design of the equipment used in the hydrogen generation step.
Fig. 4. Copper metal from the H2 and CO detector. When the hydrogen production reaction was completed, the orange reaction products were transferred to a Teflon container. Dilute sodium hydroxide solution was added to the container, and the solids and liquid were mixed with the aid of the stir bar. Carbon dioxide gas was bubbled
through the solution. Uranium(V and VI) compounds are soluble in alkaline carbonate. In contrast, U(IV) species will not go into solution. Essentially all of the orange solids went into solution when the pH and liquid volume were sufficient. The lack of solids indicated that the H2 production reaction had reached completion. A resulting uranyl carbonate solution is displayed in Fig. 5. This solution was then passed through a column containing Dowex 1-X ion-exchange resin. Uranium was loaded on the resin while the sodium passed through the column. The loading of the ion exchange continued until a faint yellow color was observed in the effluent. The uranium-loaded column was then washed with deionized H2O to remove any soluble sodium in the column. The uranium was stripped from the ion-exchange column using a concentrated solution of (NH4)2CO3. This stripping process continued until the effluent became colorless. The Dowex 1-X was then rinsed with deionized H2O and the resin reloaded with uranium from the uranyl carbonate solution. The same sample of Dowex 1-X resin was used for several loading and stripping cycles. The uranium from the ion-exchange process was in the form of UO2CO3·2(NH4)2CO3. The solution was evaporated to dryness using a hot plate. The dried solids were placed in a tube furnace and heated to 400°C in air. The thermal decomposition of the UO2CO3·2(NH4)2CO3 generates U3O8, O2, NH3, CO2, and H2O. The presence of U3O8 was confirmed by X-ray diffraction analysis.
Fig. 5. Uranyl carbonate solution from the dissolution of Na2U2O7. The U3O8 from this decomposition was accumulated until a sufficient amount of U3O8 was available for another H2 production reaction. The recycled uranium was mixed with fresh Na2CO3 so that the molar ratio of Na2CO3 to U3O8 was 6 to 1. When this mixture was heated to 700°C with CO2 and H2O for 17 h, the weight loss for the H2 was 83.7 mg, which is very close to the theoretical maximum. Therefore, this test result indicated the presence of a carbonate cycle and improved kinetics with the recycled uranium.
5.
CONCLUSIONS AND FURTHER WORK
Thermodynamic modeling indicates that the CTC process is viable and that H2 should be generated in the range of 350 to 650°C. Several tests have confirmed that H2 is produced at temperatures of above 600°C using this CTC process. All of the process steps to recycle the
uranium have been performed previously on a large scale and have been successfully repeated in this effort. Recycled uranium with fresh Na2CO3, CO2, and H2O has been used to produce additional H2 to demonstrate a closed cycle with improved kinetics for the second H2 generation step. While the CTC experiments demonstrated a complete cycle for thermochemical H2 production, the process must be tested for multiple numbers of complete cycles, with identification and measurements of chemical compounds at each step. Further, analysis of the results of this work is limited because of the imposed mass-transfer constraints arising from the small-scale batch apparatus used in the work. Additional work under experimental conditions in which mass transfer is not limiting is highly desirable.
6.
REFERENCES
1. L. C. Brown, J. F. Funk, and S. K. Showalter, Initial Screening of Thermochemical Water-Splitting Cycles for High Efficiency Generation of Hydrogen Fuels Using Nuclear Power, GA-A23373, General Atomics, San Diego, April 2000 2. J. Collins, L. Dole, J. Ferrada, C. Forsberg, M. J. Haire, R. Hunt, B. Lewis, and R. Wymer, U.S. Patent Application 11/874,958, Carbonate Thermochemical Cycle for the Production of Hydrogen, 2008. 3. C. R. Edwards and A. J. Oliver, Uranium Processing: A Review of Current Methods and Technology, JOM, 52(9): 12 (2000). 4. L. O. Williams, Hydrogen Power: An Introduction to Hydrogen Energy and Its Applications, Pergamon Press, New York, 1980. 5. Outokumpu HSC Chemistry for Windows, User’s Guide Version 5.1, Outokumpu Research Oy, Information Service, Fin-28101 Pori, Finland, 2002.
