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CLA THR ATE-BAS ED FUEL STORAGE AND TRANSPORT MEDIA: POT ENT IAL IM PA CTM. D. Max, Code7420, R. E. Pellenb arg, Code6101, Naval Research Labo ratory, Washington,D.C. 20375

Synthetic hydrate fuel, methane, gasINTRODUCTIONClatrhrates, particu larly methane and other hydrocarbon gas hydrates, have been known aslaboratory curiosities since chlorine hydrate (Cl2.6HzO),was reported (Faraday, 1823) . I nthe 1930s and 1940 s the na tura l gas industry had problems with the forma tion of acrystalline, wax-like substance in natu ral gas trans port pipes. This ma terial clogged the lin esand research was focusedon understanding h e origin and physical che mistry of the m aterial 90that i ts appearance in pipelines could be minimized. Methane hydrates are now recognized asbeing very widespread in marine sediments and in permafrost regions, and may constitute thelargest store of fixed carbon on earth (Kvenvolden, 1993 ) . Our pre sen t knowledge aboutmethane hydrate physical ch em istry, and the potentia l large volumes of recoverable methanefrom natu rally occurring sources argues strongly tha t methane is likely t o be the fuel o f thefuture, especially if the aspect of compressing methane with in a clathrate crystal lattice canutilized on an industrial scale.There is cu rre ntl y an increasing intere st in methane as a fuel because the technology fo rhandling it as a fuel, and the direct (e.g., on-site combustion for heating) and ind irect(el ec tric ity generation) energy co nversion technologies are w ell understood and cost effective.Additionally, because methane contains more hydrogen atoms fo r each carbon ato m in it smolecule than any other hydrocarbon fuel, less carbon d ioxide is produced upon combustion.Also, gas field methane s usua lly relativ ely pure and relative ly easy t o purify. Its use as a fue lis thus m ore environmentally benign than other more complex hydrocarbon fuels or coal.Methane also produces much less carbon dioxide pe r mole than alcohols, where OH substitutesfor one molecule of H,and much less than in liqu id petroleu m and oil basedfuels.Methane (n atu ra l gas) produced fro m conventional gas deposits s plen tiful, easily delivered(as a gas) to the user by an in-place domestic dis trib uti on system, and as a fuel, methane i sclean burning and has a respectab le heat content. The prospe ct of methane recovery from vastoceanic gas hydrate depo sits, however, argues for an almost inde fini te supply of methane, therecovely o f which will probably speed the development of the gas-energy economy to replacethe cur ren t oil-based economy. In add ition to this development being ecolog ically sound, oil maybe viewed be tte r as an ind us tria l feed stock than as a direct fuel, so long as a convenient,alternate source of energy such as methane s madeava ilable.Methane is particula rly amenable to transp ort and handling as a gas in pipelines and transpo rtto point use in pipes wit hin contiguous land areas. In fa ct, most of the early work int o thechemistry of methane hydrates was undertaken by the gas transp ort indu stry because hydrateswere fo rming and clogging gas pipelines even a t rela tively high tempera tures and moderatepressures. Current technology frequently requires tha t methane fue l be moved as e ithe rcompressed gas or as liquefied gas, as when natura l gas is imported to he U.S. distribution gridfro m foreig n gas fields. Of course, many fixed-site utilizatio ns fo r na tur al gas (e.g. spaceheating, ele ctric al power gene ration, or cooking) rely exclusively on gaseousmethane as a fuelstock. Where technical or geographic diffic ultie s proh ibit the use of piped distri but ion ,however, o ther means of distrib utin g gas must be developed or use. Storage of methane (e.g.,compressed gas) at th e p oint of use may also a problem so long as a continuous piped supply isnot available.Both com pressed gas and liqu ified gas, as tran sport media, possess serious safety concernsassociated wi th the f lamma bil i ty of the mate rial (compressed natura l gas) or the coldtemperatures and ultima te flammability/potentially explosive nature of the liquefied medium.This pape r suggests and examines a new application of clathrate chem istry , which could have asignificant impact on methane fuel use and distribu tion if implemented. We call atten tion to apotential thir d alternative for bulk gas transport and point-of-use storage, which would beenergy dense, fairly stable, non-flammable in bulk, easy to rans port, and pote ntially useableas-is for mo tor fuels.SYNTHETIC METHANE CLATHRATE FUEL (SMCF)Natu rally occu rring methane hydrates are no t stable at sea lev el am bient temperatures andpressures. However, it is not intended o use pure methane hydrate as the basis fo r the new fue ltransport andstorage media. Current experimental results show tha t hydrates can be abricatedboth fro m natu ral gas more dense than methane (de Boer e t al, 1985 ) with variable physicalproperty ranges tha t are stable we ll above the normal methane hydrate P-T stab ility field(Sloan, 1990) . Also, in the course of producing synth etic methane hydrate, me tastab ilitiesabout the liquidus line exist (Ste rn e t al., 19961, which may point toward controllingmetastab ility ranges of methane hydrate rather than expanding the methane hydrate sta bili tyfield. This broader stab ility of natu rally occurring mu ltiple gas clathrates, poorfy undemoodme tasta bility, and relative ease wi th which synth etic methane (based) h ydra te can be formed,leads us tosug ges tthat research fabricating sp ecial prop erty methaneclathrates is feasible andthat research should be undertaken tofab ricate a new methane fuel storage andtransport media.

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The gas to be transpo rted would be carried as a stabilized water-gas hydrate, or as a clathrateuti lizi ng selected pro ba bly gaseous) additives which could expand the stab ility field for pu remethan e-pure water clathrates well beyond that of natural methane hydrate or even som eof theothe r natu ral hydrates th a t are stable nearer standard T-P (Fig. 1). It is clear thatdevelopment and adoption o f a clathrate-based fuel transportation/distribution system, toaugment the in-place domestic gaseous-state fuel distr ibutio n complex, would offer manyadvantagesaboveand beyond hose associatedwith safety.0- -

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Figure 1. Natural h ydrate phase boundaries fo r dif fere nt commongases. Fro mMakogon (1 98 8). Replotted wit h temperature in normal scale and pre ss ure -depth in meters seawater. 0 is atmo spheric pressure a t sea level. CH4,methane; C2H& ethane; C3H8, propane; QH10, butane, the highest mo lecu larweight of the pa raffin gases, which mosteasily forms clathrates. 032, carbonDioxide; H2S, hydrogen su lph ide .

Although the energy density of methane clathrate is low compared with common liquid fuels(Table 1), its potential energy density is actually greater than a similar volume of liquidmethane, and up t o 1 64 times (Kvenvolden, 19 93 ) the same volume of methane gas (at STP).The compression factor is obtained becausemethane molecules are forced closer together in thecrystalline solid m ethane hydrate than is obtained by any other f orm of methane compression.For our energy conversion fac tor we use 1 60 X compression factor, although it is unlikely thatthe indus trial synthetic fu el will actually have a compression fac tor that high, because it i sconserva tively less than the max imum anticipated and results in even numbers ap propriate fo rpreliminary estimation.

IMethane-water solid I CH4( H,O),

)I Fuezn of I Formula 1 Density g/cc 1 Energy Content 1 Energy ContentI I Btu / lb I Btu/ft3I------ 1.O 277 * 4

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______iMethane Gas I CH4 1 7 x 1 0 - 4ItMethane-water solid(natural hydratepotential)Octane(gaso1ine) GH,, 0 7 0JP-5'1 C,4HN 0 7 7

CH4( H20), 1 - 1 01 -

277 * 4 / 1 8 4 , 0 0 0 * 4 * 6

19,000 * 3 840,000 * 318,500 * 3 930,000 * 3

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Table 1. Energy con tent of various Hydroc arbon Fuels. 1. Less volatile j e tfuel used by Navy, mandatedfor use on carriers toreduce danger of explosion.* ? boiling point -1 61 C. * 3 , STP Conditions, gas phase. *4, Energy may beconsumed producing gaseous metha ne from these forms or in containing them.*5 . Combustion products are HzO and Q. Energy content takes into accountenergy requ ired t o decomposehydrate t o Hz) and CH4; this figure representsenergy content after conversion at 150 volumes of methane in hydrate pe r

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volume o f m ethaneat STP , *6. Total potential energy co nten twith no accounthydrate per 1 volume of methane a t STP (engineering may reduce the energyrequirements for dissociation from specially fabricated clathrate or natu ralhea t sinks may be used as an energy source ).

It must be pointed ou t tha t the precise nature of SMCF is no t known because it has yet to bedesigned and fabricated. Thus, th e energy den sity, energy losses upo n fabrica tion andsubsequent gasification, and the equivalen t energy of methane aft er conve rsion, in addition to th eCost of the conversion and other engineering necessary fo r an SMCF system need t o be knownbefore a commercia l value can be placed on the SMCF media. The poten tial energy con tent ofnaturally occurring methane hydrate is high enough t o allow for some system or usagediminution and stil l remain an att rac tive new fuel storage and transp ort media. Thus, if anenergy effic ien t means fo r gasifying synthe tic methane cla thra te fuel (SMCF), can be found, i tmay prove to be a more efficie nt means of compressing methane than liqu ific ati on .Because it is un likely that the energy density of a clathrate-based fuel m edia will eversignificantly approach tha t of l iquid petroleum fuels, the clath rate fuel is clearly notappropriate for all vehicles. For instance, vehicles wi th small volumes capacity for fuelstorage, such as private mo tor vehicles and aircraft, where weigh tlvolum e is a major factor,are not likely end-point users. Larger platforms, however, such as ships and possibly hig h -speed tra ins which could be made environmentally benign (w ith respect t o noise of energygeneration and exhaust), might be possible end-users, especially when th e othe r attri bu tes ofclathrate based fue l media, such as inh ibitin g uncontrolled fires and explosion s in commerc ialapplications and explosion dam ping and deflec ting in m ilit ar y applications, are taken intoconsideration.The proposed safer tran spo rt system utilizes gas hydrates (clathra tes) which are phys icalassociations of water ice and low molec ular weigh t gas molecules (e.g. methane, ethane, propaneor butane). These clathrates fo rm spo ntaneously when wate r and a suitable low molecularweight gas (e.9. methane, carbon dioxide, hy drogen sulfide, chlorin e) are mixe d a t suitabletemperatures (generally low) and pressures (generally moderate). Indeed, the older lite ra tu recontains many references t o gas hydrates forming spontaneously in natu ral gas transm issionpipelines, and often blocking them; this poten tial situatio n requires the drying o f gas prior t~pipeline transportation (DOE, 19 87 ).Research into the low pressure species has main ly concerned todeve loping techniques hat w i Iallow for indu stria l capability to efficie ntly dissolve, or gasify hydrates. Where bondinginterac tion between guest and host molecules might be enhanced somewhat, gas that no rm all ydoes no t hydrate, suc h as hydrogen, may be bound in to specially formulated hydrates . If hostcavities were t o be lined with groups having a high hydrogen bonding character, such ashydroxyl or amino groups, other factors, such as the solub ility parameter of the host, would beof less impo rtance. Increased hydrog en bonding powe r might also be induced by charging guestmolecules prior t o exposure to hydrate lattice, or th roug h the use of magnetic field cha rging(moving the fuel in a field, pu lsing a field, or mov ing a field w ith respect to the orientation ofthe hydrate). Release of gas could be induced throug h heating, lowe ring o f pressure, o relectronic stimu lation tha t would produce effects simila r t o hat of microwaving food(where thefrequency of the microwave is specific t o water molecules).FUEL SYSTEM REQUIREMENTSThe proposed SMCF storage and transpo rt system would consist of three separa te components:(1 ) Formation Module, (2 ) Transport Vessels, and ( 3 ) Gasseparation Unit.(1 ). Hydrate Form ation Module (HFM). Methane hyd rates are stable unde r moderatepressures, and low temperatures (Fig. 1). The HFM will consist of a pressure vessel intowhich are pumped stabilize r, wate r spray, and methane; the P-T conditions of formation arepresently uncertain but may be differen t from those neededfor stab ility of the special hydrateduring storage. Recent research shows tha t the methane hydrate forms immediately uponmixing water w ith th e gas, when the system is within the stab ility field of the clathrate (P eterBrew er, MBARI, pers. comm., November, 19 96 ). Once th e hydrate is formed , the ma teria lwould be removed from the HFM, and transferred t o the transp ort vessel for movem ent o pointof use or distr ibution.(2 ) Hydrate Transp ort Vessel (HTV). The Kn/ would consist of a insulated container whichcould contain th e stable special hydrate at ambient to moderate pressures. The insulation wouldmore than likely consist of plastic foam such as is used by the refrigera tion indus try; vacuumjackets would be avoidedbecauseof Cost andsafetyconcerns. The HTV could be fabricated in anydesired shape, and might evolve t o be conformal t o the hull or some inte rior stru ctu ral embers of the platform using the stored gasas a fuel, for example, in the double hu ll space of aship.(3 ) Gas S eparation Un it (GSU). The GSU could be inte gra l to th e HTV, r separate, as mandatedby the ultimate use of the released fuel gas. The clathrates are unstable in t he presence ofelevated heat; the h ydrate could be decomposedby d irec t heating (e.g. a clathra te slush would betransfe rred t o a heatedvessel: gas evolves fro m the slush and escapesfor use, and the wate r

I taken of dissociation energy requirements basedon 16 0 volumes of methane in

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from th e hydrate is discarded or retained for use in making more hydrate later). Altern atively,the hydrate slush could be sprayed w ith water, the heat in which would be sufficient mdecomposethe hydrate. In eith er case, the evolved gasw ould be routed to a device (e.g. engine)which could use the com bustible g asas a fuel.It must be notedtha t there is no inherent reason why the units listed above would necessarily beseparate components. For example, the storage vessel could contain ntegral su bse ctio ns whichwould allow both formation of anddecompos itionof thegas hydrate right in the KTV. Further, itis technically possible to design and build an interna l combustion engine which would usehydrate as the only, o r ma jority , fuel; such a system would be similar to wate r- injectiontechnology as applied t o inte rn al com bustion engines, as in some experim ental figh ter planeengines andrac e cars.CONCLUSIONS AND DISCUSSI NWe have considered the feas ibility of forming gas hydrates on demand, the ut ili ty of doing S(safe transportation of m ethane as a fue l), and poten tial end uses of gas moved as a hydrate(decom position nto g asf or com bustion, or design of engines tooperate on hydrate itself). It i sappropriate t o examine the effec ts of adding a clathrate-based fuel on the current energyeconomyof a developedsociety.Where wou ld the SMCF sys tem be applied and how w ould it develop? These questions cannot beful ly answered because: 1. the eng ineering possibilities have yet to be explored, 2. the effectcmma rket forces cannot be assessed beyond observing that the technology and po tential fuelhandling system s largely exist or can be developeda t low cos t, and, 3. government regulationstha t would apply (bu t do not ye t ex ist) could either inhibit or promote development of both theSMCF its elf and a wo rld gas economy.In the broadest sense, a SMCF-based fuel econo myw ould be akin t o the system b asedo n liq uidhydrocarbons. Specially formulate d clathrates wou ld be transported in th e form of slush i nmuc h the same way as present liquid hydrocarbons. Transp ortation is solid form co uld uti liz emuc h of the presen t containe r-handling equipment and facilities including muc h of the sea, ra iI,and road equipm ent already in existence. Produ ction as slush (Najafi and Schaetzle, 1 9 8 9 )wou ld also allow pumped dis trib ut ion . Moreover, where safety concerns are paramount, theSMCF might be used because of could greatly enhance safety; even in the presence of openflames, methane s evolved slowly from hydrates through breakdown of the crystal structure.This means tha t all of the gas or liquid methane available as an explosive com ponen tin prese ntconventional methane storage media can only be evolved a t a rate a t which it could feed a f i re ,bu t no t an explosion with ou t f ir s t collecting e volved gas. In addition, upon gas evolution,sub stan tial qua ntities of water are also produced, whose presence could be engineered toinhibitaccidental ignition attribute s of the system.A dedicated clathrate-bas ed metha ne fue l economy, in existence and developing, w ould driv eexploration and development t o uti lize the vast quan tities of methane tha t are only now beingrecognized as presen t on the p lanet (Max and Lowrie, 199 6). Current conservative estimatesindicate that naturally occurring methane hydrates contain at least twice the amount of fixedcarbon as do conventional methane, liquid hydrocarbons, and coal, combined, on Earth(Kvenvolden, 19 93) , and it is un likely that this fuel source will remain untapped, especially i fan SMCF sys tem can be de veloped at a reasonable cost.Re fe rencesDe Boer, R.B., Houbolt, J.J.H.C. & Lagrand, J. 198 5. Form ation of gas hydrates in a permeablemedium. GeologieenMijnbouw, M, 4 5 - 2 4 9 .DOE, 19 87. Gas hydrates Technology Status Report. Departme nt of Energy DOE /MET C-870 246(DE87001 27) 54pp.Faraday, M. 18 23. On fluid chlorine. Philosop hical Transa ctions Royal Society of London 22A ,Kvenvo lden, K.A. 19 93 . A p rim er on gas hyd rate. In: Howell, D.G. et al. (eds). The fut ure ofenergy gases.USGS Profess ional Paper 1 57 0, 2 7 9 -2 9 1 .Makogon, Yu.F. 19 88 . Gas -hydrate accu mu lation s and perm afros t development. In: Senneset, K.(ed). P erma frost. Fif th International Conference Proceedings, 95 -1 01.Max, M.D. & Lowrie, A. 19 96 . Methane hydrate: A fron tier fo r exploration of new gasresources. Journa l o f Petroleu m Geology,19, 1 5 6.

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Najafi, M. & Schaetzle, W.J. 19 89 . A nalysis of clath rate solidification. W C H 2 7 8 1 -Sloan, E.D., Jr. 19 90 . Cla thrate H ydra tes of Natu ral Gases. Marcel Dekker, Inc., New York andBasel. 64 1 pp.3 /89 /0000 , 1839 -1 844 .

Stern, L.A., Kirby, S.H. & Durham, W.B. 19 96. Peculiarities of methane clathra te hydrateformation and solid-state deform ation, including pos sible superheating of water ice. Science,22& 1 8 4 3 - 1 8 4 8 .466

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