Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman California State University Fresno, CA 93740
Gas Hydrates: From Laboratory Curiosity to Potential Global Powerhouse Robert E. Pellenbarg* Chemistry Division, Naval Research Laboratory, Washington DC 20375;
[email protected] Michael D. Max MDS Research, Suite 302, 1211 Connecticut Ave. NW, Washington DC 20036
What Is Hydrate? In the early 1820s, John Faraday, working in England, was investigating a newly discovered gas, chlorine. He easily repeated the earlier experiments of Humphry Davy (1) in which gaseous chlorine and water formed solid chlorine hydrate upon exposure to the “late cold weather”. These pioneering efforts are the first reported synthetic reference to a class of associative compounds now known as gas hydrates (2). Chlorine hydrate has persisted as a laboratory curiosity (3) because its ease of formation lends it to laboratory demonstration. Hydrates (water molecules are host) are a subgroup of clathrates. Generally, a clathrate is a compound formed by the inclusion of (guest) molecules in voids of the crystal lattice of another (4) (Fig. 1); clathrates can form when both host and guest molecules are present. Clathrates, forming under proper conditions of temperature and pressure, display only weak hydrogen or van der Waals chemical bonding between the host and guest molecules. There are many examples of clathrates (Table 1), which are also known as “container compounds” (5). The generic name, clathrate, is taken from the Latin word clathratus, which means “enclosed by bars or grating” (6, 7). Water, of course, is ubiquitous on planet Earth, a fact of major geological significance with regard to natural hydrates. Conventional chemical wisdom holds that a chemical compound consists of atoms linked, or bonded, to one another in a definite ratio, yielding a fixed atomic structure for the molecule. Thus, hexane is C6H14, or a chain of 6 carbon atoms A
Table 1. Examples of Clathrates Host
Guest
Urea
Straight-chain hydrocarbons
Thiourea
Branched-chain and cyclic hydrocarbons
Dinitrodiphenyl
Derivatives of diphenyl
Phenol
Hydrogen chloride, sulfur dioxide, acetylene
Water (ice)
Halogens, noble gases, sulfur hexaflouride, low molecular weight hydrocarbons, CO2, SO3
Nickel dicyanobenzene,
Benzene, chloroform
Clay minerals (molecular sieves) Hydrophilic substances Zeolites
Wide range of adsorbed substances
Graphite
Oxygen, hydrocarbons, alkali metals (in sheetlike cavities and buckyballs)
Cellulose
Water, hydrocarbons, dyes, iodine
with 14 hydrogen atoms attached. Either ionic (as in sodium chloride, NaCl) or covalent (as in hexane) bonds serve to hold these and countless other chemical entities together as gases, liquids, or solids. Further, most pure chemical compounds exhibit a regular molecular structure. This ordering on the molecular level is manifested by a crystal form, a material characteristic in all compounds save certain supercooled fluids such as glass. Some crystals of simple compounds, such as salt, are dense solids and can be represented by a series of spheres laid adjacent to one another in a 3-dimensional array. However, a variety of more complex molecules can B
Figure 1. Models of crystal and hydrate lattices showing (A) type I hydrate and adjacent methane molecule (atomic radii not to scale but size of lattice voids and guest methane molecule are to proportional scale) and (B) guest methane molecule inserted in the hydrate water host crystal lattice.
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crystallize under proper conditions of temperature and pressure to give a rather open, void-filled structure (4 ). Often the open space in the crystal lattice remains just that: open space. However, under certain conditions, these crystalline voids can be occupied by guest molecules of such size and configuration that the guest molecule fits into the crystalline voids formed by the host molecule. Since a host lattice has a well-defined structure with a clearly defined void volume in the lattice, a clathrate can exhibit a definite formula, if the voids are completely occupied by guest molecules, quite analogous to that of a true chemical (bonded) compound. This state of affairs, that is, a combination of atoms with a fixed ratio in the mix implying a strong covalent or ionic bonding, does not exist between the weakly bonded host– guest molecules in clathrates/hydrates. Clathrates can form spontaneously under proper conditions of pressure and temperature. Both host material, which can crystallize into an open lattice structure, and a guest molecule of suitable size and molecular conformation to fit into the resulting lattice voids are required to complete the clathrate crystalline structure. Methane hydrate is dependent on pressures usually in excess of one atmosphere (STP), which forces a re-ordering of the lattice water molecules into a 3-dimensional array with voids large enough to accommodate the guest methane molecules.
Table 2. Some Physical Properties of Water Ice and Methane Hydrate (9, 11, 13, 14) Property
Ice
Dielectric constant at 273 K
94
Hydrate ~58
NMR rigid lattice 2nd moment of H2O protons (G2)
32
33 ± 2
Water molecule reorientation time at 273 K / µs
21
~10
Diffusional jump time of water molecules at 273 K / µs 2.7
>200
Isothermal Young's modulus at 268 K, 109 Pa
9.5
~8.4
Speed of longitudinal sound at 273 K / km s᎑1
3.8
80
Transit time / µs ft ᎑1
3.3
92
Velocity ratio Vp /Vs at 272 K
1.88
1.95
Poisson's ratio
0.33
~0.33
Bulk modulus at 272 K
8.8
5.6
Shear modulus at 272 K
3.9
2.4
Bulk density / g cm᎑3
0.916 0.912
Adiabatic bulk compressibility at 273 K, 10–11 Pa
12
14
Thermal conductivity at 263 K /W m᎑1 K ᎑1
2.23
0.49 ± 0.02
Methane Hydrate: A Scientific Hot Topic Methane hydrate is nonstoichiometric in that the crystal structure of the hydrate can be established without all the methane lattice sites being occupied. One cubic meter of saturated methane hydrate contains 164 m3 of methane (at STP) and 0.87 m3 of water (8). In most oceanic hydrate, some 150 m3 of gas may be released from 1 m3 of hydrate because of incomplete filling of available lattice sites. On the other hand, some of the host voids available for methane may be filled with other gases, such as ethane or propane. This substitution will increase the stability of the material while lowering the number of methane molecules per unit cell required to stabilize the hydrate. Crystallization forces methane molecules into tightly packed lattice sites, compressing the methane. Methane hydrate has the highest energy density of any naturally occurring form of methane (184,000 Btu/ft3 for the hydrate and 1,150 Btu/ft3 for methane gas [STP]; the energy density of LNG is about 430,000 Btu/ft3). Thus, methane hydrate is an attractive economic target as a source of methane. The density of methane hydrate is about 0.91 g/cm3; this may vary minutely according to the degree of methane saturation of the hydrate lattice and the local incorporation of other molecules (e.g., H2S), taking the place of methane in the lattice. The heat of hydrate formation (exothermic) and the heat of hydrate dissociation (endothermic) are equal in absolute magnitude but are of opposite sign. A nominal value for methane hydrate formation enthalpy at 273 K is 54 kJ/mol (measured, 9). Methane hydrates have a constantpressure heat capacity of 257 kJ mol᎑1 K᎑1 (10, 11). The heat of solution (absorption) of methane gas is 13.26 kJ/mol (12, 11). The thermal conductivity of a hydrate–sediment mixture is 2.2–2.8 W/m-K; the conductivity of a water-ice sediment mixture is 4.7-5.8 W/m-K (Table 2). The presence of
Figure 2. Gas hydrate phase diagram, showing the stability fields of the water–ice–methane–hydrate system with respect to temperature and total pressure and no effect from geothermal heat (diagonal hatching). The presence of CO2, H2 S, ethane, or propane with methane in the hydrate will shift the hydrate phase boundary line to the right, increasing the P–T field in which methane hydrate is stable (8). Permafrost and oceanic hydrate deposit show the main potential economic regions in P–T space.
methane in the water molecule lattice modifies these figures from those of pure water-ice. Oceanic methane hydrates are stable in a region nearly parallel to the sea floor, the hydrate stability zone (HSZ), which is a region of hydrate thermodynamic equilibrium that extends downward from the sea floor in water depths greater than ~400 m to a depth determined by heat rising from within the earth, the local geothermal gradient. The sea floor temperature and the geothermal gradient, which determine the heat available in the ocean–sea floor system, control the
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
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local thickness of the HSZ. Hydrostatic pressure at the sea floor at any particular place where hydrate is liable to form is the main variable in the pressure–temperature system. As a guide, note that hydrostatic pressure increases 1 atm for every 10-m increment in depth. Methane hydrate has a broad stability range (Fig. 2). Within the HSZ, the conductive transfer of heat between the warmer marine sediment and rocks below and the cold oceanic waters above is in pseudo-equilibrium within the pressure–temperature field at any particular depth at a given location. When the sea floor is encountered, temperature increases with depth in the sediment. The base of the HSZ is the buried hydrate phase boundary; hydrate is stable above this boundary and gas plus water is stable below (i.e., higher temperature regime). Both shallow, biogenic gas derived mainly from bacteriological decay of organic matter buried along with the sediment and deeper-sourced, thermogenic gas produced by thermal cracking of higher molecular weight hydrocarbons have been found within hydrate deposits. Methane formed in deeper sediments tends to migrate upward because of its buoyancy, becoming trapped within and beneath the hydrates. When hydrate becomes more deeply buried owing to continued sedimentation, it will become unstable as the base of the HSZ passes upward via geothermal heat flow from below. The gas will then pond beneath the hydrate where the hydrate above forms an impervious seal; but where it can, it will pass upward into a new HSZ, where it is may be recaptured through formation of a new cycle of hydrate. Interest in Hydrates: Industry and Energy In the 1930s and 1940s, the natural gas industry recognized a serious problem associated with crystalline, waxlike deposits that formed spontaneously and clogged natural gas and petroleum pipelines, especially in colder regions of the country. The pipeline material was stable and was an economic nuisance (15). Research identified this “stuff ” as hydrates of mixed hydrocarbon gases (e.g., mostly methane, with ethane, propane and/or butane) that have a stability field much broader than hydrate of pure methane. The simple solution, which is still a first line of defense against unwanted hydrate formation, was to dry the natural gas: methane hydrates cannot form without water. In 1964, naturally occurring methane hydrate was discovered in permafrost terrane in Siberia, and successful experiments to develop methane extraction techniques followed shortly. Oceanic gas hydrates were discovered soon afterward (16 ). The recognition of gas hydrate in deep ocean sediments was a major contribution of the Deep Sea Drilling Project. This effort confirmed earlier, tentative hydrate identification from seismic reflection records (17–20). The examination of actual deep-core samples related the seismic response to occurrence of the hydrate. Earth appears to posses immense quantities of methane. In conventional hydrocarbon deposits, this methane is commonly mixed with other hydrocarbon gases and liquids, and other gases such as carbon dioxide. Methane in permafrost hydrate deposits (Fig. 2) is commonly associated with conventional hydrocarbon deposits. Most methane in oceanic hydrate deposits, however, is not associated with petroleum 898
liquids, and most methane in oceanic hydrate does not appear to have been produced by higher-temperature thermogenic processes. Biogenic processes are the major source of methane in hydrate, particularly in oceanic sediment regimes. Methane hydrate on the planet contains (at least) some 104 gigatons (Gt) of carbon, approximately twice the carbon contained in the fossil fuel deposits of petroleum, coal, and free methane gas combined. Of critical importance is the fact that methane hydrate has a stability field that is matched by widespread ambient conditions on the earth. Thus, in the deep sea (cold and high pressure), in the Arctic permafrost (very cold, and moderate pressure), or even in areas with a high methane partial pressure (pipelines in the winter), methane hydrate will form spontaneously and persist as a stable solid. Response to Recognition of Extensive Natural Deposits of Methane Hydrate Scientific meetings focused on methane hydrate began with the first national gas hydrate meeting in Washington, DC, in the spring of 1991. This meeting was sponsored by the Naval Research Laboratory, Washington, DC, the U.S. Department of Energy Federal Energy Technology Center (F.E.T.C.) in Morgantown, WV, and the U.S. Geological Survey, Woods Hole, MA (21). The Third International Conference on Gas Hydrates, convened in Salt Lake City, UT, in July 1999, showcased current understanding of hydrate. Some 250 technologists from 19 nations, approximately one-third each from academe, industry, and government, gathered to examine the status of and trends in gas hydrate research and development. Academic participants reported on the latest successes associated with bringing hydrates into the laboratory, a traditionally difficult task considering the stability conditions needed to contain hydrate. Representatives from industrial R&D affiliates discussed advances in preventing hydrate plugs from forming in gas-bearing pipelines. The Federal government, especially the Department of Energy, described in detail the efforts in place to implement a U.S. national gas hydrate R&D effort. Further, the U.S. Navy reported on new industrial applications of hydrate technology such as the possibility of using specially engineered methane hydrates as fuel for a radically redesigned, more robust fleet. Overall, the conference helped define what will be needed to develop the promise locked in the vast deposits of methane hydrate on the planet and how to access these deposits in as environmentally benign a manner as possible. In 1984 the U.S. Department of Energy F.E.T.C. established the first national study of the possibility of recovering methane from hydrate. This program was terminated in 1993, but was restarted in 1997. Japan (1995) and India (1996) have also established well-funded national programs to develop methods for assessing and exploiting gas hydrate in their respective EEZ as a fuel source. Korea, China, and European Union countries (principally Germany) have followed suit and established hydrate research programs. Japan is currently in the lead in methane hydrate research and development and has completed a detailed drilling effort in Canada’s MacKenzie Delta to refine techniques needed to fully characterize the extent and nature of hydrate deposits. The Canadian and U.S. Geological Surveys and the U.S. Department of Energy participated. The experience and results gained are
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
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being applied to the Nankai Trough hydrate deposits adjacent to the Japanese mainland. Both Japan and India have earmarked tens of millions of dollars annually to proceed with their respective national gas hydrate efforts. The geopolitical implications of energy independence for Japan or India and for their relations with the rest of the world are staggering. Both energy security concerns and the prospect of abundant new energy resources are driving the current interest in methane hydrate. Petroleum fuel currently dominates and underpins global economic activity. Petroleum, however, is a finite resource. Further, there are clear political problems associated with the distribution of petroleum resources on the planet. Because of the world abundance of methane and the natural limitations to petroleum, a methane-based economy will inevitably supplant the current petroleum-based economy. The only question is when and where will it develop first. Methane as a fuel offers clear advantages over oil or coal: immense resource potential, ease of transport via in-place distribution infrastructure, less carbon dioxide release per unit volume burned, no release of sulfur or nitrogen oxides, and so forth. Further, methane hydrate serves as an analog for other gas hydrate species. Carbon dioxide hydrate is increasingly examined as a potential storage medium for carbon dioxide produced by combustion of fossil fuels in general. Studies are examining the feasibility of using liquid carbon dioxide or the corresponding hydrate to sequester carbon dioxide captured from fossil fuel combustion. U.S. Navy scientists have defined the concept of using methane hydrate as the basis of a new technology to desalinate seawater (U.S. Patent 5,873,262 issued 27 Feb 1999). Clearly, the future of methane hydrate research and development is full of promise. Only within the past 20 years has the scientific community come to realize that there is in fact enough methane on the planet to underpin a gas-based economy. Immense reservoirs of methane occur as gas hydrate, newly recognized deposits of which are much more uniformly scattered around the globe. The Middle East has no monopoly on gas hydrate deposits as it does for petroleum supplies. Hydrate deposits potentially large enough to allow for energy independence occur in the EEZs of at least two major industrial nations, the USA and Japan, and are likely to occur adjacent to most coastal oceanic states. There is clear consensus that there is a lot of methane as hydrate in the sediments of the world ocean. Less clear is the exact nature of such deposits, and details of the technology that would be required to tap such immense reservoirs of clean-burning fuel. However, the industry view is that if and when hydrates are to be tapped for their energy potential, technology challenges will be overcome, just as industry mastered petroleum exploitation in increasingly deep land locations and offshore in deep water, for example. To place the energy promise of methane hydrate in perspective, consider the situation of the Blake Ridge region off the coast of Georgia, USA. The Blake Ridge has been well characterized and holds at least 1,000 trillion cubic feet of methane gas in a single deposit. Yet the USA consumes only about 22 trillion cubic feet (TCF) of gas per year; the Blake Ridge deposit alone could supply U.S. needs for some 50 years if a significant proportion of the methane in hydrate there is recovered! It must be acknowledged that the apparent dispersed nature of the hydrate in very clay rich sediments (i.e., low
porosity) of the Blake Ridge area may pose significant problems to methane recovery there. Other hydrate deposits have also been identified in waters of the USA’s EEZ (22). Clearly, hydrate-derived methane offers an unparalleled promise as an energy resource. The methane could be used directly as a fuel, or be converted to methanol or higher molecular weight organic fluids. Summary The promise of methane from hydrate has encouraged various governmental entities to initiate preliminary R&D efforts focusing on hydrate. Japan and India are proceeding with well-funded efforts, and the USA has established, via Congressional mandate, a smaller national program. The geopolitical implications of a new energy paradigm based upon energy independence supported by a gas-based industry are of potentially immense importance for the USA and other nations now having an oil-based economy. Carefully designed and implemented national efforts with effective cooperation among government, academe, and industry at the national level can lead to such independence based on the promise of methane hydrate. Acknowledgment We gratefully acknowledge support from the U.S. Department of Energy (grant document DEA12697FT34238), which supported preparation of this article. Literature Cited 1. Davy, H. Philos. Trans. R. Soc. 1811, 101, 155. 2. Faraday, M. Philos. Trans. R. Soc. 1823, 22A, 160–189; http:// dbhs.wvusd.k12.ca.us/Chem-History/Faraday-Chlorine-1823. html (accessed Feb 2001). 3. Pauling, L.; Marsh, R. E. Proc. Natl. Acad. Sci. USA 1952, 38, 112–118. 4. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1966. 5. Cram, D. J. Nature 1992, 356, 29–36. 6. Barrer, R. M.; Stuart, W. I. Proc. R. Soc. London A 1957, 243, 172–189. 7. Brown, J. F. Sci. Am. 1962, 207 (1), 82–92. 8. Kvenvolden, K. A. In The Future of Energy Gases; Howell, D. G., Ed.; U.S. Geological Survey Professional Paper 1570; U.S. Government Printing Office: Washington, DC, 1993; pp 279–291. 9. Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Dekker: New York, 1997. 10. Handa, Y. P. J. Chem. Thermodyn. 1986, 18, 891. 11. Sloan, E. D. Clathrate Hydrates of Natural Gases; Dekker: New York, 1990. 12. Water: A Comprehensive Treatise, Vol. 2; Franks, F.; Reid, D. S., Eds.; Plenum: New York, 1973. 13. Davidson, D. In Natural Gas Hydrates—Properties, Occurrence and Recovery; Cox, J., Ed.; Butterworth: Woburn, MA, 1983; pp 1–16. 14. Prensky, S. E. A Review of Gas Hydrates and Formation Evaluation of Hydrate-Bearing Reservoirs; Paper GGG, presented at meeting of the Society of Professional Well Log Analysts, Paris, France, June 26–29, 1995.
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Chemistry for Everyone 15. Hammerschmidt, E. G. Ind. Eng. Chem. 1934, 26, 851. 16. Tucholke, B. F.; Bryan, G. M.; Ewing, J. J. Am. Assoc. Petrol. Geolog. Bull. 1977, 61, 698–707. 17. Bryan, K. M.; Markl, R. G. Microtopography of the BlakeBahama Region; Lamont Geological Observatory Technical Report No. 8 (CU-8-66-NOpBSR); Columbia University: New York, 1966. 18. Paull, C. K.; Dillon, W. P. The Appearance and Distribution of Gas Hydrate Reflector Off the S.E. United States; U.S.G.S. Open File Report 80-88; U.S. Geological Survey: Denver, CO, 1981.
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19. Kvenvolden, K. A.; Barnard, L. A. Initial Reports of the Deep Sea Drilling Project 1982, 76, 353–368. 20. Kvenvolden K. A.; McDonald, T. J. EOS 1982, 63, 101. 21. Max, M. D.; Dillon, W. P.; Malone, R. D. Proceedings: Report on National Workshop on Gas Hydrates, U.S.G.S. National Center, Reston, VA, April 23–24, 1991; U.S. DOE/METC 91/6/24, DE 91016654; U.S. Department of Energy: Washington, DC, 1991. 22. Collett, T. Am. Assoc. Petrol. Geol. Bull. 1993, 77, 793– 812.
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
