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Proposed System for Hydrogen Production from Methane Hydrate with Sequestering of Carbon Dioxide Hydrate

2003; ASM International; Volume: 125; Issue: 4 Linguagem: Inglês

10.1115/1.1615795

1528-8994

W. Rice,

CO2 Sequestration and Geologic Interactions

Gas hydrates (clathrates) are molecular compounds with crystalline structure, formed under certain conditions of pressure and temperature for which gas molecules occupy structural lattices formed by water molecules through hydrogen bonding. Many gases, but not all, can form hydrates 1. Methane and carbon dioxide notably can form hydrates in water at high pressure and low temperature and hydrates of both gases are known to exist naturally in sediments and geological structures below the sea 2. Methane (natural gas) hydrate is of very wide occurrence in the geological formations below the sea. It is generally agreed that the amount of energy in the methane hydrate is at least twice the worlds total energy reserves in other fossil fuel forms including gas, oil and coal; estimates of the methane sequestered in methane hydrates range from approximately 1015m3 to 7.6×1018m3, with a consensus value of 21×1015m33456. In general, methane hydrate occurs in porous structures at temperature below about 4°C and pressures corresponding with 300 m or more depth of sea water. The thickness of the methane hydrate deposits ranges from a few meters to as much as 1100 m in different areas. The hydrate in sediments is solid and often forms a cap below which natural gas, mainly methane in a gaseous state, is present 78. Methane hydrate is the stable form of occurrence of methane when water is present at the depths and temperatures at which it exists. The hydrate disassociates into gas and water if the pressure is reduced sufficiently at a fixed temperature or if the temperature is increased sufficiently at a fixed pressure 4. This accounts for the discovery of such hydrates only relatively recently and also suggests ways by which the methane might be produced at the sea surface for use. Projects near the coast of Japan, in the Gulf of Mexico, near the west coast of Canada, and off the coast of South Carolina among others are developing methods for drilling wells and producing methane from the hydrate in the sediments 459. The USDOE has implemented a program for research and development intended to make methane hydrate commercially viable within 20 years 10. The main objective of the program is “to study methane hydrates, ice-like crystals of methane with energy potential equal to more than twice that of all other fossil fuels combined…. There is no existing technology to make extraction commercially viable…” Because the methane hydrate is at sea in deep water, it has been thought that it would be largely ignored until land-based sources of natural gas were nearly exhausted. The use of methane as a fuel is a major contributing factor to the release of carbon dioxide into the atmosphere which is thought to be an important cause of global warming 11. Atmospheric pollution by the burning of fossil fuels and the corresponding release of carbon dioxide have emerged as very pressing problems for civilization. A fuel changeover to hydrogen would eliminate both pollution and carbon dioxide release but no feasible, economical, and plentiful source for the hydrogen without carbon dioxide release has been proposed to this time 12. Herein, a plan is suggested for the production of hydrogen, from methane hydrate, without the release of carbon dioxide to the atmosphere. The processes and techniques to accomplish this are under development or already in use for other purposes. The end result could be an almost limitless source of the transportable, non-polluting fuel, hydrogen, with no release of carbon dioxide or other global warming gas to the atmosphere in either the production or use of it. In addition, alternatively, the methane hydrate can be used to produce vital industrial chemicals, such as ammonia, without atmospheric pollution. The plan described depends in part on the fact that carbon dioxide forms a hydrate more dense than sea water, at sufficiently high pressure and low temperature, such as the conditions in the sea at depths below about 300 m. Thus, carbon dioxide resulting from the process of producing hydrogen from the methane hydrate, might be sequestered in the sea. The feasibility of this as a laboratory process, and to some extent, the practicability, has been shown by investigations in Japan, Norway and the USA, among others 13141516. Schematic diagrams showing the components for the scheme are presented as Fig. 1 and Fig. 2. A large floating platform is used, kept in position by propeller drives, using location information provided by GPS observations. A well is made from the platform and depressurization, probably in combination with heat addition, is used to decompose the hydrate and produce wet methane gas from the well by artesian flow 4; in this way it is predicted that millions of m3/day at a single location can be produced. The methane is passed through a reformer (a well-developed industrial process) to produce hydrogen mixed with carbon dioxide. The hydrogen and carbon dioxide are separated resulting in gaseous hydrogen and carbon dioxide. The carbon dioxide is returned to the sea from the platform through a vertical pipe, terminating in cold water at a depth sufficient to cause carbon dioxide hydrate to be stable. Since it is more dense than sea water, the carbon dioxide is thus sequestered on or in the sea floor as hydrate 13141516. The hydrogen leaving the separator can be liquefied, using conventional gas liquefaction processes. The liquid hydrogen is compact and transportable to land as the valuable end product; there are alternate methods for making the hydrogen transportable 121718. Heat necessary for the reforming process and for producing electrical power needed by equipment, processes and personnel on the platform is obtained by burning part of the methane received on the platform. The combustion products are then separated and the carbon dioxide is added to that produced by the reforming process, to be sequestered in the sea. The technical success of the scheme depends on developing a suitable floating platform, a viable well and production system for the methane hydrate, and a viable process and apparatus for sequestering the carbon dioxide. Development of all of these is in progress to various degrees and in various parts of the world at the present time. The developments needed are engineering tasks, not dependent upon scientific breakthroughs, but they are large scale efforts in very early stages and are very expensive. In addition to engineering development, extensive studies must be made to determine if the production of methane and the sequestering of carbon dioxide can be done with acceptable levels of ecological damage and risks. There are also areas of legal ownership and of costs of implementation that must be explored and judged to be viable and acceptable. Research and development has been reported concerning large floating platforms intended for use as airports, shipping ports, and industrial and housing sites 19202122. These could be adapted to serve as the platform on which to conduct the necessary operations and processes to produce hydrogen from the methane hydrate beneath the sea and sequester the resulting carbon dioxide. The platform must be stable, steerable, resistant to weather extremes and habitable by personnel; these are some of the problems considered in the references cited. Experimental wells to produce methane from the hydrate have been conducted from research ships, platforms on structures supported from the sea-bottom, and land-based rigs with wells diverted into offshore methane hydrate deposits 423242526. The development of floating platforms and of well drilling and production techniques must be coupled in the future to enable the hydrogen production plan described herein to proceed. Various means of producing methane to a platform from the methane hydrate deposits have been considered and described 4. The hydrate decomposes into methane and water, if the temperature is high enough and the pressure low enough. The decomposition (or “melting”) of the hydrate requires that heat be added during the process and the referenced literature suggests ways that this might be accomplished to produce acceptable flow rates of methane to the platform. It has been noted that in the porous structures containing methane hydrate, the hydrate forms an impervious layer below which gaseous methane is trapped (in many instances) 478. The gaseous methane is, of course, equally acceptable for processing on the platform. The necessary techniques for producing methane to the platform from the hydrate and/or the gas below it, are extensions of present deep sea drilling and production methods used in the petroleum industry. The processes that can be used on the platform to produce hydrogen and carbon dioxide from the methane can be readily adapted from those widely used in the chemical industries. Figure 3 is a schematic diagram showing the main elements that are necessary; many “minor” auxiliary devices and processes are needed that are not shown but are available 2728. The methane from the well, separated from water that might be produced with it, first passes to the primary reformer together with steam. In the presence of a nickel catalyst and at a temperature of approximately 800°C (maintained by heat addition to the primary reformer), hydrogen and carbon monoxide result (predominately). The stream then passes to the carbon monoxide shift converter which uses an iron oxide catalyst and operates at approximately 370°C (maintained by extraction of heat from the converter). The resulting stream consists mainly of carbon dioxide and hydrogen but also has a significant component of carbon monoxide. This stream passes to a monoethylene amine (MEA) system in which the carbon dioxide is absorbed and the stream leaving consists of carbon monoxide and hydrogen. The carbon dioxide is subsequently desorbed from the MEA, and the MEA returned for reuse, while the carbon dioxide is directed to the sequestering system. The stream leaving the MEA system enters a cuprous ammonial acetate (CAA) system, where the carbon monoxide is absorbed and the hydrogen is passed out to a hydrogen storage or transport system as the useful product. The carbon monoxide is desorbed from the CAA system and the CAA is returned for reuse, while the carbon monoxide is returned to the input stream of the carbon monoxide shift converter. The primary reforming process is endothermic as noted, and the shift process is exothermic but the net result is that heat energy must be supplied to the overall reforming system, or “plant.” A large amount of additional heat is needed for a power plant to deliver electrical power for uses on the platform. The needed heat can be produced by burning part of the methane delivered to the platform from the methane hydrate. Methane is burned in air to release the needed heat and the carbon dioxide is separated from the combustion gases. It is then added to the carbon dioxide stream leaving the MEA system, to be sequestered. A conventional steam power plant is probably the best choice for delivering electrical power, since steam is needed as well as heat by the reformer, and the remainder is available for power production. An alternate process known as “partial oxidation” is not discussed here but should be considered in subsequent studies. It is an exothermic process for producing hydrogen and carbon dioxide from methane 28. Carbon dioxide hydrate is stable at low temperature and high pressure and is more dense than sea water 13. Several schemes have been proposed for sequestering the carbon dioxide and would use various means of forming the hydrate and delivering it to the sea. The phenomena and complications in accomplishing formation of hydrates are far beyond meaningful discussion here but are reported and presented in the literature 1315. For the purposes of further discussion and example, it is taken herein that the carbon dioxide to be sequestered is caused to go into solution in sea water in a bubble tower on the platform. Sea water is pulled from depth at, say, 10°C and the bubble tower is operated at, say, l5 atmospheres pressure, and the resulting solution is directed downward in a pipe to a suitable depth at which hydrate will form and be stable. The formation of carbon dioxide hydrate from carbon dioxide in solution, at reasonable rates and with little or no gaseous attachments has been reported 29. In this system, the carbon dioxide gas must be compressed to the bubble tower pressure of 15 atmospheres and the sea water solution must be pumped to the same pressure (approximately). One advantage of the scheme is that the compressor power and pump power are less than other proposed schemes since the pumps on the platform must overcome only pipe friction in the vertical pipes since densities of fluids in the pipes are nearly the same as that of the sea water outside of the pipes. Elementary calculations using the chemical equations for the combustion of methane in air, and assuming reasonable efficiencies for the water pump, carbon dioxide compressor, and power plant have been made (by the author). The results indicate that the carbon dioxide sequestering system described above, if successful, will require about 12% of the output of the power plant. The remainder is available for other power requirements on the platform. For each unit mass of methane processed, .25 unit mass of hydrogen will be separated and .75 unit mass of carbon dioxide must be sequestered. It is very important that leakage be minimal, since both carbon dioxide and methane are greenhouse gases and methane is 10-20 times more potent than carbon dioxide as a greenhouse gas. The hydrogen leaving the reforming system, or “plant,” is at near-ambient conditions and is gaseous and voluminous. In order to transport it to points of use on land, it must be made compact. A common way is to liquefy the hydrogen by a widely used industrial process that can be adapted for use on the platform. As a liquid, it can be transported to shore in insulated containers, using cryogenic systems to maintain necessary conditions. Other systems are available, notably transport as a high pressure gas, and transporting it as a solid in metal hydrides 1718. The choice will depend on further development of hydrogen processing and storage systems and on the form adopted for transport and use in an energy economy by vehicles, power plants and industrial processes. Energy will be required on the platform to prepare the hydrogen for transport no matter what transport system is chosen. Energy for that purpose is available from the power plant on the platform. While energy conservation on the platform is desirable to minimize equipment size and costs, additional power required can always be produced by burning methane and sequestering the resulting carbon dioxide. Power use should be minimized also so as not to squander resources even though they seem virtually unlimited. If the scheme described is technically successful, then economic factors are important in determining whether or not it will be widely used. At this stage, estimating capital, operating and other costs would seem to be a futile exercise. Platforms such as the one required have been shown to be economically attractive for other uses. The well should prove to be little or no more costly than presently-used deep offshore drilling and production platforms. The system for sequestering the carbon dioxide should prove to be of low first cost and moderate operating cost. All other required systems and apparatus are presently used with acceptable costs. The scheme described may be used even if it proves to be more expensive than present fuel sources. The benefits of using hydrogen as fuel for transportation vehicles, stationary power plants, and industrial processes, in view of the absence of atmospheric pollution, could outweigh higher fuel costs. For hydrogen to become the universal fuel, as clearly is desirable, much of the mobile and stationary infrastructure must be extensively modified or abandoned. The cost of that could only be borne if extended over a period of years, perhaps 30 years. In the interim, in order to establish the necessary hydrogen production, it is envisioned that the scheme would produce chemicals other than hydrogen, such as ammonia for use in fertilizers, and would gradually be extended to produce hydrogen as the facilities for hydrogen use are developed. Concern has been expressed in the literature concerning the possibility that exploration for methane hydrate and/or the production of methane from hydrate sediments might lead to underwater landslides 30. This could result in a reduction of pressure, and perhaps an increase in temperature, for the hydrate and thus cause an uncontrolled release of methane into the atmosphere. Since methane is a very effective greenhouse gas, this concern will require careful and responsible consideration. Further, it has been suggested that the addition of massive amounts of carbon dioxide hydrate to the seas and/or sea floor might have negative impacts on the plants and animals in the sea 31. This concern also must be addressed and further research is needed to attempt to substantiate counter-arguments and to minimize possible harm during experimentation and subsequent large scale introduction. The term “sequestering” of carbon dioxide as hydrate is inexact. In the sea, under conditions of temperature and pressure corresponding with “stable” hydrate, the hydrate is nevertheless not in equilibrium with regard to carbon dioxide concentration in the surrounding sea water. As a result, carbon dioxide continuously goes into solution in the sea water, from the hydrate. The rate at which this occurs is apparently small 13 and the carbon dioxide is “sequestered” for many years. There is a large literature and ongoing research concerning the ultimate fate of carbon dioxide sequestered in the sea as well as the effects that it may have, especially if the hydrate migrates with ocean currents 1531. There are, of course, other large questions raised by the scheme. Much of the hydrate is far from the countries of use. Present international agreements may not adequately cover the ownership of the hydrate. However, much of the methane hydrate is in coastal waters near huge markets; it is estimated that the methane in hydrate form in deposits in the U.S. Atlantic Margin could satisfy the fuel needs of the USA, at present consumption rates, for more than 100 years. Old alliances of developed countries with the countries having the oil sources would no longer be important; the repercussions of that are not obvious. The enormous cost of changing to a world hydrogen fuel economy must be borne in some way, perhaps offset by the benefits of the cleaner environment. The author expresses his gratitude to Dr. Earl Robbins, Faculty Associate, Department of Computer Science and Engineering, and to Dr. James Beckman, Professor, Department of Chemical and Materials Engineering, both at Arizona State University, who rendered the author valuable assistance using their professional expertise and time. Warren Rice The author received the degree of PhD from the A&M College of Texas in 1958, in Mechanical Engineering. He has since engaged in teaching and research at Arizona State University, and has consulted in diverse fields, mostly with applications of principles of thermodynamics, fluid mechanics and heat transfer. This has included the development of: processes for manufacturing semiconductors, multiple disc type turbomachinery, unconventional multiphase air compressors and refrigeration systems, and air-driven mass transit systems. In retirement from teaching, he lives in the village of Payson, Arizona and continues to examine engineering problems.