PRODUCTION OF HYDROGEN USING NUCLEAR ENERGY
C. W. Forsberg
Oak Ridge National Laboratory P.O. Box 2008, Oak Ridge, TN 37831, 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
One of the leading methods for the future production of hydrogen (H2) is nuclear energy. The fundamental characteristics of nuclear energy offer several potential advantages for H2 production: avoidance of the production of greenhouse gases, production of H2 near the final market, economics-of-scale that match the need for H2, and availability of large resources of uranium fuel. Several types of reactors are being considered for H2 production, and several methods exist to produce H2, including thermochemical cycles (heat plus water yields H2 and oxygen) and high-temperature electrolysis (heat plus electricity plus water yields H2 and oxygen). Ultimately H2, not electricity, may be the primary application of nuclear energy. Hydrogen from nuclear energy may in fact become the enabling technology for a large-scale renewable-nuclear economy.
Introduction
The annual world consumption of H2 is ~50 million tons [1], most of which is used for ammonia production (fertilizer) and conversion of heavier crude oils to clean liquid fuels. The rapid growth in demand is a result of decreased availability of light crude oils that do not require extra H2 for conversion to gasoline, with a corresponding increased use of heavy crude oils that require massive amounts of H2 for conversion to gasoline. If the cost goals for automotive fuel cells are reached, the transportation sector may ultimately be fueled by H2. This implies a growth in H2 consumption of one to two orders of magnitude over a period of several decades. Because of these changes, an examination of the use of nuclear energy to produce H2 was undertaken. The use of nuclear energy for H2 production raises three questions:
■ Is nuclear energy compatible with H2 production?
■ How should H2 be produced?
■ Is H2 the future of nuclear energy?
Compability of Nuclear Energy with Hydrogen Production
Each energy technology [2] has a set of characteristics that determine what applications are potentially viable in terms of both technical feasibility and economics. For example, the characteristics of internal combustion engines (small size, high energy output per unit mass, etc.) make them suitable for automobiles. However, the high cost of liquid fuels makes such engines unsuitable for large-scale production of electricity. The viability of nuclear energy for H2 production depends upon the match between the intrinsic characteristics of H2 systems and nuclear energy systems. Four issues are examined: production scale, load factor, H2 transmission, and pipeline infrastructure.
Experience has demonstrated that nuclear energy production in small units is not economi-
cally valuable. If nuclear energy is to be used for economic H2 production, the demand for H2 must match the scale of H2 production from a nuclear reactor. Current «world-class» H2 plants [3] have production capacities of 5.7 million standard cubic meters per day. A new plant has been recently announced with a capacity of 8.5 million standard cubic meters per day (1200 MW of hydrogen energy based on the higher heating value). These plants use steam reforming of natural gas to produce H2. A 2400-MW(t) reactor would be required to produce 8.5 million standard cubic meters of H2 per day. Thus in terms of energy flow, the size of today's H2 production plant is now equivalent to that of a nuclear power plant.
Nuclear power plants are characterized by high capital costs and low operating costs. Good economics are dependent upon maintaining base-load operations with continuous output. The characteristics of the H2 system decouple production from consumption [4]. Hydrogen is currently transported by pipeline and stored in large underground caverns, similar to natural gas. This is a low-cost storage method that, unlike the production of electricity, allows the power plant to produce H2 at full capacity without the need for variable production. Thus, for H2, production characteristics versus time are compatible with nuclear energy.
Nuclear power plant sites are rare and expensive. The need for security, the advantages of using common facilities, and other factors encourage siting multiple reactors at each site. A large electrical transmission line carries about 2 GW. Large H2 pipelines, similar in size to the proposed Alaskan natural gas pipeline, would carry more than 20 GW. Transmission of large quantities of energy in the form of H2 in a few pipelines to urban areas is simpler than construction of large numbers of power lines. Hydrogen production is intrinsically more suitable than electricity for siting large numbers of reactors at a limited number of large sites.
The economic viability of any energy system depends upon the delivered cost of energy, which includes the costs of production, storage, and transportation. If one H2 system has significantly higher costs for transport or storage than another system, such factors can determine the preferred method of H2 production. The average long-term transport costs of H2 produced using nuclear energy will be lower than those for H2 produced from natural gas and many other energy sources. Nuclear power stations are typically located a hundred kilometers from large urban areas, which defines the necessary distance for H2 transport. Natural gas deposits are typically several thousand kilometers from large markets. While most other energy sources require the longdistance transport of H2, the lower transport costs of H2 from nuclear energy provide an economic advantage.
Hydrogen Production
Hydrogen Production Methods. Several methods have been proposed to produce H2 from nuclear power. Electrolysis of water [4] is an established technology that is used to produce H2 in small quantities at dispersed sites. It is not cur- ¡1 rently competitive for the large-scale production of t H2, except where low-cost electricity is available. f Although the conversion of electricity to H2 by ^ electrolysis is an efficient process (80 % efficien- I cy), the efficiency of converting heat (nuclear, fossil, £ geothermal, etc.) to electricity is typically between | 30 and 50 %. Consequently, the total conversion I efficiency of this two-step process from heat to g electricity to H2 is low, between 24 and 40 %. In ™ many industrialized countries, the peak electrical demand is twice the minimum demand. If the off-peak electricity is produced by a source of energy with low fuel costs (such as nuclear), electrolysis may be viable for H2 production. Otherwise, the H2 production costs will be high.
Electrolysis [5] can be performed at high temperatures (700 to 900 °C) to replace some of the electrical input with thermal energy. Because heat is less costly than electricity, the H2 costs via this production method could ultimately be lower than those for traditional electrolysis. Equally important, the high temperature results in better chemical kinetics within the electrolyzer that reduces (1) equipment size and (2) inefficiencies. However, this high-temperature technology is in an early state of development. Hot electrolysis requires collocation of H2 production with the nuclear reactor to provide the heat.
Hydrogen can be produced by direct thermo-chemical processes [6, 7] in which the net reaction is heat plus water yields H2 and oxygen. These are the leading long-term options for production of H2 using nuclear energy. For low production costs, however, high temperatures (>750 °C) are required to ensure rapid chemical kinetics (i. e., small plant size with low capital costs) and high efficiency in converting heat to H2.
Many types of thermochemical processes for H2 production exist. The sulfuric acid processes (hybrid-sulfur, iodine-sulfur, etc.) are currently the leading candidates. In the sulfuric acid processes, the high-temperature endothermic (heat-absorbing) reaction is the thermal decomposition of sulfuric acid to produce oxygen:
H2SO4 ^ H2O + SO2 + 0.502 (850 °C). (1) After oxygen separation, additional chemical reactions are required to produce H2. The leading candidate for thermochemical H2 generation is the iodine-sulfur process, which has two additional chemical reactions:
I2 + S02 + 2H20^2HI + H2S04 (120 °C), (2) and the H2-producing step,
2HI ^ H2 +12 (450 °C). (3)
In addition to the pure thermochemical cycles there are hybrid cycles that include one or more thermochemical steps and a low-power (low-voltage) electrolysis step. The leading candidate is the hybrid-sulfur process [7], which has the same s high temperature step (1) and a different low-¡5 temperature step:
| SO2 (aq) + 2H2O ^ H2SO4 (aq) + H2 (g), (4)
^ (Electrolysis: 80 °C).
Of the advanced methods for H2 generation £ using nuclear power, thermochemical cycles (pure I and hybrid) have received the most attention be-% cause cost estimates [7] indicate that thermochem-g ical H2 production costs are ~70 % those from ™ room-temperature electrolysis. These estimates assume the use of near-term current technology; however, there is the potential for major improvements in thermochemical cycles. In co
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