LOWERING PEAK TEMPERATURES FOR NUCLEAR THERMOCHEMICAL PRODUCTION OF HYDROGEN
C. W. Forsberg
Oak Ridge National Laboratory* P.O. Box 2008, Oak Ridge, TN 37831-6179, United States of America Tel.: (865) 574-6783; fax: (865) 574-9512; e-mail: forsbergcw@ornl.gov
Dr. Charles Forsberg received his Bachelors of chemical engineering degree from the University of Minnesota and his Doctor of Science degree from the Massachusetts Institute of Technology. He is a professional engineer and a senior scientist at Oak Ridge National Laboratory in the United States of America. Dr. Forsberg has published over 200 articles and reports; holds 9 patents; has served on multiple government, IAEA, and foreign panels; and received multiple awards. In 2002 he received the American Nuclear Society special award for «Advanced Nuclear Power Generation Concepts». This was for development of the Advanced High-Temperature Reactor for hydrogen production.
Forsberg Charles Winfield
The efficient thermochemical production of hydrogen using nuclear heat requires matching the nuclear reactor and the thermochemical processes to convert heat plus water into hydrogen and oxygen. The major challenges are the high temperatures required to produce hydrogen efficiently. Consequently, Oak Ridge National Laboratory, in collaboration with Sandia National Laboratories and the University of California at Berkeley, is investigating nuclear reactor options and thermochemical cycles to minimize those temperatures while efficiently producing hydrogen. We are developing the concept of a molten-salt-cooled Advanced High-Temperature Reactor to produce the heat. The use of a low-pressure liquid coolant minimizes the temperature drops between the hottest fuel elements in the reactor and the thermochemical cycle, thus minimizing peak reactor temperatures. Simultaneously, we are examining the use of inorganic membranes to minimize the temperatures required for the efficient production of hydrogen using the (1) sulfur-iodine, (2) Westinghouse, and (3) Ispra Mark 13 thermo-chemical hydrogen processes.
Introduction
The worldwide demand for hydrogen (H2) is ~50 million tons per year and growing rapidly. Hydrogen is used primarily for production of ammonia for fertilizer and conversion of heavy crude oils into cleaner liquid fuels. An international effort is under way to deliver H2 as a replacement fuel for transport vehicles. Ultimately, the energy required to produce H2 could exceed that for electricity. Consequently, strong incentives exist to develop economic methods to produce H2 using nuclear energy.
Among the leading candidates for low-cost, large-scale H2 production are thermochemical processes. A thermochemical process consists of a set of chemical reactions in which the net result is high-temperature heat plus water yields H2 and O2. Two factors make thermochemical H2 production costs (with nuclear reactors providing the heat) potentially lower than those for electrolysis.
■ Efficiency. Thermochemical processes have potentially greater efficiency because conversion of heat to H2 requires fewer steps than conversion of heat to electricity and electricity to H2.
■ Capital costs. The economics of scale for chemical processes (function of volume) is significantly better than the economics of scale for electrolytic processes (function of area).
If H2 is to be produced economically [1], the nuclear reactor must be matched with the thermo-chemical process. In a recent evaluation [2] of thermochemical cycles, three of the four highest-ranked cycles (Hybrid sulfur, Ispra Mark 13, and sulfur-iodine) were sulfur cycles that have the same high-temperature chemical reactions but different low-temperature chemical reactions. Given these results, we have concentrated our efforts on matching the energy output of the nuclear reactor to the required energy input of these three ther-mochemical processes.
*Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy under contract DE-AC05-00OR22725
The three processes require heat input at peak temperatures of ~850 °C. This condition presents a major challenge. If the chemical process requires 850 °C heat, the nuclear reactor must operate at significantly higher temperatures to allow transfer of the heat from the reactor core to the chemical process. Such temperatures are near the limits of practical current materials. To reduce the material challenges, we have initiated a two-part program to better match the nuclear reactor to the thermochemical cycle: (1) develop a nuclear reactor that delivers heat at 850 °C but that is designed to minimize the peak temperatures within the reactor and (2) modify the high-temperature steps within the thermochemical cycles to lower peak temperatures. This paper discusses the status and results of this research.
Thermochemical production of hydrogen
To understand the challenges of H2 production, some understanding of the thermochemical cycles is required. As noted earlier, three [2] of the four highest-rated processes (fig. 1) have the same high-temperature chemical step that requires ¡1 heat input at >850 °C. The highly endothermic t (heat-absorbing) gas-phase reaction in each of these f processes is as follows: ^
2H2SO4 ^ 2H2O + 2SO3 ^ |
^ 2SO2 + 2H2O + O2 (850 °C). (1) i
c
The three thermochemical processes have dif- $ ferent lower-temperature chemical reactions. The | sulfur-iodine process [2] has two other chemical q reactions that when combined, (1) yield H2 and
High-Temperature Reactions
Oxygen
Heat
h2SO4—H2O+SO3 ~h2o+so2+1/2o2
Base Case
Heat
I 700°C^>
Membrane Separation
h2so4^h2o+so3 -~h2o+so2+%o2
Inorganic Membrane
Low-Temperature Reactions
Hydrogen
Heat
Sulfur Iodine
Hydrogen Hybrid Sulfur
(Westinghouse, GA-22 and Ispra Mark II)
Hydrogen
I Br2+ S02 + 2H20 Electrolysis ♦ 2HBr+H2S04 2HBr -*H2+Br2
Reject Heat
Fig. 1. Sulfur family of thermochemical cycles
щ>
Reject Heat
Ispra Mark 13
O2 from water and heat and (2) recycle all other chemical reagents:
I2 + SO2 + 2H2O ^ 2HI + H2SO4 (120 °C); (2)
2HI ^ I2 + H2 (450 °C). (3)
< The hybrid sulfur process (also known as
i Westinghouse, GA-22, and Ispra Mark 11) has
£ a single electrochemical step that completes the
S cycle [3]:
| SO2 (aq) + 2H2O (1)^ H2SO4 (aq) + H2 (g). (4)
f (Electrolysis: 80 °C)
I The Ispra Mark 13 process has one chemical g reaction and one electrochemical reaction that com™ pletes the cycle:
Br2 (aq) + SO2 (aq) + 2H2O (l)^ ^ 2HBr(g) + H2SO4 (aq) (77 °C); (5)
2HBr (g H Br2 (l) + H2 (g). (6)
(Electrolysis: 77 °C)
In each of these cycles, the high-temperature sulfur trioxide (SO3) dissociation reaction is an equilibrium chemical reaction that requires a catalyst. High temperatures and low pressures drive the reaction towards completion. Fig. 2 shows this equilibrium as a function of temperature.
Detailed studies have concluded that the required process temperatures need to be very high: ~850 °C. After the high-temperature dissociation reaction, all the chemicals must be cooled to near room temperature, the SO2 separated out and sent to the next chemical reaction, and the unreacted sulfuric acid (formed by recombination of SO3 and H2O at lower temperatures) reheated back to high temperatures. Unless the chemical reactions go almost to completion, the energy losses in separations and the heat exchangers to heat and cool all the unreacted reagents (H2SO4) result in a very inefficient and uneconomical process. This phenomenon is illustrated in fig. 3, in which the overall efficiency of one variant of the sulfur-iodine process is shown as a function of temperature [2]. Efficiency is defined as the higher heating value of H2 divided by the thermal energy into the process. In this flowsheet, the process inefficiencies increase so rapidly with decreasing temperature (incomplete reactions) that the process cannot produce H2 at temperatures below 700 °C. The process thus defines the nuclear reactor requirements.
The advanced high-temperature reactor
There are two approaches to developing a nuclear reactor for H2 production. An existing reactor system can be modified to meet the H2 production requirements, or a new reactor system can be developed. At the current time, only one nuclear reactor system, the gas-cooled (helium) reactor,
0-1-.-1-1-.-
600 650 700 750 800 850 900
Temperature, °C
Fig. 2. Equilibrium concentrations of SO3, SO2, and O2 vs temperature starting with 100 moles of SO3
has the potential high-temperature capabilities to provide the heat at sufficient temperatures to drive a H2 production system. This reactor has historically been considered the reactor that would be used to provide high-temperature heat for H2 production [4]. The gas-cooled (helium) reactor was developed for electricity production and uses a coated-particle fuel (see below) and high-pressure helium as a coolant. Several prototype reactors have been built. Last year, Japan began operation of its 30 MW(t) High-Temperature Test Reactor to develop nuclear heat applications, including H2 production. This specific reactor has a peak exit temperature of 950 °C.
Alternatively, a reactor can be designed specifically for H2 production. Given the demanding requirements for H2 production, we are developing a new reactor concept, the Advanced High-Temperature Reactor (AHTR), to match H2 production requirements [5]. This is a joint effort between three organizations in the United States: Oak Ridge National Laboratory (ORNL), Sandia National Laboratories, and the University of California at Berkeley. The AHTR is based on several earlier technological developments:
■ high-temperature, low-pressure molten-salt reactor coolants from the aircraft nuclear propul-
600 700 800 900 1000
Temperature, °C
Fig. 3. Efficiency of the sulfur-iodine process versus temperature
sion program of the 1950s and the molten-salt breeder reactor program of the 1960s;
■ coated-particle gra
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