Methods and systems for energy conversion and generation

Abstract

The invention relates to methods and systems of converting electrical energy to chemical energy and optionally reconverting it to produce electricity as required. In preferred embodiments the source of electrical energy is at least partially from renewable source. The present invention allows for convenient energy conversion and generation without the atmospheric release of CO2. One method for producing methane comprises electrolysis of water to form hydrogen and oxygen, and using the hydrogen to hydrogenate carbon dioxide to form methane. It preferred to use the heat produced in the hydrogenation reaction to heat the water prior to electrolysis. The preferred electrical energy source for the electrolysis is a renewable energy source such as solar, wind, tidal, wave, hydro or geothermal energy. The method allows to store the energy gained at times of low demand in the form of methane which can be stored and used to generate more energy during times of high energy demand. A system comprising an electrolysis apparatus and a hydrogenation apparatus, and a pipeline for the transportation of two fluids, is also described.

Claims

1. A method for producing a hydrocarbon or a hydrocarbon derivative from an electrical energy source, said method comprising: a. providing a source of electrical energy; b. using electrical energy from said source to electrolyse water to form hydrogen and oxygen; and c. using hydrogen thereby formed to hydrogenate carbon dioxide to form methane; and wherein the electrolysis is carried out at a temperature of from 100 C. to 1000 C., wherein heat produced by the hydrogenation of carbon dioxide is used to heat the electrolysis reaction, wherein heat in the stream of hydrogen and/or oxygen produced in electrolysis is used to generate steam, and wherein the steam produced is used to generate electricity.

2. A system for producing a hydrocarbon or a hydrocarbon derivative from an electrical energy source, said system comprising: a) a source of electrical energy; b) an electrolysis apparatus electrically coupled to said energy source operable to electrolyse water to form hydrogen and oxygen; c) gas handling means to collect oxygen and hydrogen produced in said electrolysis apparatus; d) a source of carbon dioxide; and e) a hydrogenation apparatus adapted to hydrogenate carbon dioxide to form methane using said hydrogen produced by electrolysis; and wherein the electrolysis apparatus and the hydrogenation apparatus contain heating means and cooling means respectively, and the heating means of the electrolysis apparatus and the cooling means of the hydrogenation apparatus are thermally coupled such that heat energy generated in hydrogenation can be transferred to the electrolysis apparatus, wherein the system is adapted such that the flow of hydrogen and/or oxygen from the electrolysis apparatus is used to generate steam in a boiler, and wherein further the system comprises a steam generator adapted to generate electricity from said steam.

3. A system for producing a hydrocarbon or a hydrocarbon derivative from an electrical energy source, said system comprising: a) a source of electrical energy; b) an electrolysis apparatus electrically coupled to said energy source operable to electrolyse water to form hydrogen and oxygen; c) gas handling means to collect oxygen and hydrogen produced in said electrolysis apparatus; d) a source of carbon dioxide; and e) a hydrogenation apparatus adapted to hydrogenate carbon dioxide to form methane using said hydrogen produced by electrolysis; and wherein the electrolysis apparatus and the hydrogenation apparatus contain heating means and cooling means respectively, and the heating means of the electrolysis apparatus and the cooling means of the hydrogenation apparatus are thermally coupled such that heat energy generated in hydrogenation can be transferred to the electrolysis apparatus, wherein the system is adapted such that the flow of hydrogen and/or oxygen from the electrolysis apparatus is used to generate steam in a boiler, wherein the system is further adapted such that the steam can be fed back into a flow of input water for electrolysis, and the system further comprising at least one steam bridge operable to selectively direct steam through a steam turbine or direct the steam from the high pressure side of the steam turbine to a flow of input water for electrolysis.

4. A combined electrolysis/hydrogenation apparatus for use in the generation of methane and performing electrolysis of water comprising: a high temperature electrolysis apparatus adapted for electrical coupling to a source of electrical energy, said high temperature electrolysis apparatus being operable to electrolyse water at high temperature in an electrolysis chamber using electrical energy from said source to form hydrogen and oxygen, the high temperature electrolysis apparatus comprising an input water feed conduit adapted to carry water to the site of electrolysis; gas handling means to collect oxygen and hydrogen produced in said electrolysis apparatus comprising gas-carrying conduits adapted to carry the oxygen and hydrogen; a hydrogenation apparatus adapted to hydrogenate carbon dioxide to form methane using said hydrogen produced in the high temperature electrolysis apparatus comprising a hydrogenation chamber in which hydrogenation of carbon dioxide occurs; and at least one steam bridge operable to selectively direct steam through a steam turbine or direct the steam from the high pressure side of the steam turbine to the feed conduit of input water for electrolysis; wherein at least a portion of the hydrogenation apparatus is in thermal communication with the feed conduit, such that heat generated in the hydrogenation reaction can heat water to be electrolysed.

Description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(1) Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1a shows a schematic diagram of energy transfer according to the present invention and FIG. 1b shows a particular embodiment of the invention in which time-phased energy transfer is conducted.

(3) FIG. 2 shows a flow chart of one embodiment of the present invention.

(4) FIG. 3a shows a schematic representation of an electrolysis chamber.

(5) FIG. 3b shows a schematic representation of an apparatus for the hydrogenation of carbon dioxide.

(6) FIG. 4a shows a schematic representation of inputs, take-offs and storage capabilities associated with the present invention.

(7) FIG. 4b shows a pipe system according to the present invention for transporting products of the present invention.

(8) FIG. 5 shows a schematic representation of a combined cycle oxy-fuel gas turbine generator with CO.sub.2 recovery.

(9) FIG. 6 shows a combined 2-stage fuel and oxygen production process.

(10) FIG. 7 shows an alternative pipe according to the present invention for use in transporting products.

(11) FIG. 8 shows a scheme for the conversion of methane into other useful products, e.g. petrochemical feedstocks.

(12) FIG. 9 shows an embodiment of time-phased energy transfer from the electricity grid by night (or other time of low demand) into electricity supply and district heating.

GENERAL DISCUSSION OF THE PRESENT INVENTION

(13) Detailed below are various aspects which can be combined to provide a fully-integrated approach to energy storage and generation, preferably without the emission of CO.sub.2 to the atmosphere. This is achieved via the production and subsequent combustion of a hydrocarbon gas as an energy storage medium (which will be referred to at times as a thermogas), by combining the emerging technologies of oxy-fuel combustion, catalytic conversion of CO.sub.2 into methane, and increasingly efficient electrolysis of water into hydrogen and oxygen. It is believed that the approach outlined here represents a method of greatly increasing the usefulness of alternative energy generation and, if implemented on a large scale, has the potential to greatly reduce our dependence on fossil fuels. It also represents a method of storing energy produced at off-peak times for use when demand is high.

(14) Making fuels from various alternative energy sources has been postulated and studied at length for many years in terms such as the hydrogen economy, the methane economy and the methanol economy. The present invention is based on the concept that an energy economy can be based upon a chosen combustible gas or hydrocarbon fuel derived from alternative energies. Furthermore, the present invention extends the concept to integrate it with an energy transfer process. Means will be described to recover CO.sub.2 from electricity production which will then be used to maintain the energy transfer in a recurrent process.

(15) Electricity cannot be readily stored, but combustible fuel and oxygen can, albeit in liquid or solid form. The present invention could deploy vast storage capabilities for fuel and oxygen at strategic locations. Current technology allows for upwards of 200,000 m.sup.3 storage per tank or ship tanker. These storage capabilities can be compared against the commodities required for a 1,000 MW(E) oxy-fuel CCGTEG operating at full load for 12 hours per day for 3 months: approximately 400,000 m.sup.3 of liquid oxygen and 275,000 m.sup.3 of fuel (if liquid methane), whilst approximately 200,000 m.sup.3 of CO.sub.2 stored as a solid would be produced in that time.

(16) Such stored energy being readily available to meet electricity demand, where and when required, has several benefits compared to the current practice of electricity generation. For example where wind energy is used to generate electricity to the grid, it cannot be relied upon to meet the demand at any particular time. In contrast the present invention would use renewables such as wind, wave, tide or solar energy to make fuel and oxygen, e.g. for transfer and storage local to the point of use, subsequently meeting electricity demand when it is required.

(17) The current practice is to extend the electricity grid system to interconnect electricity producing energy sources with places of electricity demand. Sometimes this is not possible or prohibitively expensive. For example the geo-thermal energy of Iceland cannot help to meet electricity demand in America, there being no electricity grid connection between source and demand. The present invention provides ways of securing, storing, transporting and delivering energy from distant alternative sources to electricity demand locations. However, the invention can also be deployed local to the demand using the electricity grid for renewable energy transfer. An example would be overnight production and energy storage followed by electricity supply during days and evenings.

(18) At present District Heating (or Combined Heat and Power) using otherwise wasted heat from electricity generating stations is practiced where possible because it provides means to raise the thermal efficiency of fuel utilisation. However the pollution associated with current fossil power plants all but prohibits their use in urban areas. The present invention can be more acceptable within the urban setting as emissions are virtually zero, and thus District Heating could be more widely practiced achieving further savings in CO.sub.2 emissions.

(19) Water is a precious commodity in certain parts of the world. Desalination methods of water production and supply are widely in use, often by fossil fuel combustion involving CO.sub.2 emissions. The present invention describes how water is produced during the electricity generating process which could then, after conditioning, be made available in various forms as a useful product. The water could be still or carbonated for drinking or demineralised for industrial or other use. Some 3 million liters or so could be produced from a 1,000 MW oxy-fuel CCGTEG running at full load for 24 hours.

(20) The transportation of liquid natural gas and other cryogenic commodities by ship tankers is widely practiced. The present invention could involve such methods as one of the means of transportation of the commodities involved. Another means of transportation could use existing pipeline technology as in the Eurasian pipeline network. Gas transference through the pipelines is by means of pressure boosting plants which typically take in gas at greater than 2 bars and pressurise at up to 10 bars to force the gas to the next booster point or terminal. The present invention could use similar methods except three commodities would typically be involved: fuel, oxygen and CO.sub.2. Moreover one aspect of the invention shows how existing pipelines could be utilised for transference of the three commodities.

(21) FIG. 1a illustrates how energy from renewables 11 can be transferred into electricity and District Heating 12 for supply to consumers.

(22) Means will be shown how water (H.sub.2O) 16 and carbon dioxide (CO.sub.2) 15 can be made into oxygen (O.sub.2) 14 and a combustible fuel, preferably methane (CH.sub.4) 13, by DC electricity 11 derived from renewable energies. The O.sub.2 14 and CH.sub.4 13 are transferred to a heat engine electricity generator 19 to supply electricity to the grid or for local use 12. The heat engine exhaust containing CO.sub.2 and H.sub.2O is cooled to enable separation and compression of the CO.sub.2 for transfer back to the fuel/oxygen production site 18 to continue the process. Hence energy transfer from renewable energy sources into electricity for supply to the grid is made possible with no emissions to atmosphere. The District Heating 12 is optional, but preferable.

(23) In addition to energy, effectively water is transferred from the production site 18 to the supply site 19. Water usage 16 is necessary at the production site 18 but water 17 is made as a product of combustion at the supply site 19.

(24) FIG. 1b shows time-phased energy transference, an embodiment of the present invention, which can optimally be located at a single site. Time-phased is used herein to describe a method or system which allows electrical energy produced at a certain time to be stored (as chemical energy) for release as electrical energy at another time. This is essentially the effect achieved by existing techniques such as pumped storage, but existing technologies are limited by the size of the storage facilities and the cost of infrastructure. The present invention provides a far more flexible, scalable and convenient solution to this problem.

(25) At periods of low demand or over-production (e.g. at night or where renewable production is at high levels due to high winds or the like), electricity is taken from the grid 110 through rectifiers 130 to supply DC electricity to methane and oxygen production plant 128, which will be described in more detail below. CO.sub.2 115 and water 121 taken from the storage tanks 124 and 127 are used in plant 128 to produce methane 117 and oxygen 119 for collection in storage tanks 125 and 126. Make up water 123 would be made available as required to water 121. At periods of high demand (e.g. during the day or when renewable production is low) methane 118 and oxygen 120 would be taken from said storage tanks 125 and 126 and transferred to a heat engine electricity generator 129 (e.g. a CCGT plant or fuel cell) to supply electricity 112 to the grid 110 and heat to district heating 114. The heat engine exhaust is cooled to enable separation and compression of the CO.sub.2 116 into storage tank 124. The water 122, after being degassed of CO.sub.2 would be collected in storage tank 127. Hence energy taken from the grid by night is used to supply emissions free electricity to the grid by day. The benefits of such a system are clear; allowing somewhat sporadic energy generation from renewables to be captured efficiently for use when demand is high, and allowing general electricity production to be smoothed out.

(26) District heating energy can also be provided by night 113 or by day 114. FIG. 9 shows a system for time-phased energy transference in more detail.

(27) The means to achieve the energy transfer referred to above is explained in concept by FIG. 2 where, in the case illustrated, methane is the preferred fuel. Further steps in the flowchart would be necessary for other fuels. Existing technology can enable methane feedstock to be used to produce fuels with higher heat content for the energy transference process but methane is initially preferred because of its compatibility with existing natural gas pipelines and utilities. 1. DC electricity is generated by alternative non CO.sub.2 emitting methods or taken from the grid and used to power electrolysis of H.sub.2O (from local sources), typically on a large scale (21); 2. CO.sub.2 and H.sub.2 are reacted over a metal catalyst to form the preferred combustible fuel CH.sub.4 and H.sub.2O (22); 3. The H.sub.2O produced is preferably recycled into the electrolysis (step 1), while the CH.sub.4 and O.sub.2 are recovered (23); 4. The CH.sub.4 and O.sub.2 are transported to a heat engine electricity generating plant, preferably an oxy-fuel combined cycle gas turbine electricity generator (CCGTEG) of a standardised design (24); 5. Electricity is generated to meet demand by the combustion of the CH.sub.4 in the O.sub.2 (oxy-fuel combustion process) (25); 6. Part of the water produced by the combustion is used as make-up to the Rankine steam cycle; the remaining water can be used as a product as carbonated or still drinking water or as demineralised water (26). 7. The CO.sub.2 produced by the combustion is recovered and transported back for use in the hydrogenation process (step 2) to enable recurrent energy transfer (27).

(28) These steps can be viewed as belonging to one of three sections: the upstream processes (stages 1, 2 and 3); the downstream processes (stages 5 and 6); and transportation of materials (stages 4 and 7). The present invention encompasses the entire system as well as certain individual parts of it.

(29) The core aspects of the various aspects of the present invention are described in detail in the text and in FIGS. 3 to 9 below.

(30) Electrolysis of Water

(31) To obtain the H.sub.2 necessary for the production of methane via the Sabatier process, it is envisaged that large-scale electrolysis of water will be carried out. The basic reaction is given by the equation:

(32) ##STR00002##

(33) Much research has been focused on a means to improve the efficiency of this reaction, as could be expected for such a simple potential route to the valuable commodities hydrogen and oxygen. Recent progress in the development of solid oxide electrolyser systems.sup.[7] has raised strong possibilities for increased efficiency. In particular, the use of yttria-stabilised zirconia (YSZ, Y.sub.2O.sub.3 in ZrO.sub.2), a gastight electrolyte which conducts O.sup.2 ions well at high temperatures, greatly simplifies separation of the hydrogen and oxygen products. Furthermore, it has also been demonstrated that a sizeable improvement in the efficiency of the electrolysis reaction can be obtained if a high operating temperature can be maintained.sup.[8]. Accordingly, it is advantageous that the exothermic Sabatier reaction could be used to supply substantial amounts of heat to the electrolysis cells; for example, by the use of pressurised steam as a medium for heat transfer from the Sabatier reactors, the heated steam then fed (along with that directly produced by the Sabatier reaction) into the electrolysis cells for conversion to H.sub.2 and O.sub.2. In this way, the efficiency of the H.sub.2 generation could be further increased. The hydrogen produced would then be fed back to the Sabatier reactors, giving a continuous two-step process that uses CO.sub.2 and H.sub.2O to produce CH.sub.4 and O.sub.2, summarised by the equation:

(34) ##STR00003##

(35) The CH.sub.4 and O.sub.2 will then be used in the downstream plant (e.g. electricity generation by combustion-fired CCGT) as described below.

(36) FIG. 3a shows a schematic diagram of a basic electrolysis cell. A DC electric potential 34 is applied between the cathode 31 and anode 33, imparting to them net negative and positive charge respectively. Electrons are conducted through the electrolyte solution 32. At the cathode, which is rich in electrons, H.sup.+ ions combine to form H.sub.2, while at the electron-deficient anode, O.sup.2 ions give up electrons and form O.sub.2. These gases are in a different state from the electrolyte 32, and are thus easily drawn off as products.

(37) Generation of Methane

(38) FIG. 3b shows a potential layout of methanation plant for use in conjunction with large-scale electrolysis. H.sub.2 from the electrolysis plant enters through pipe 31 and joins a stream of CO.sub.2 from pipe 32. It is envisaged that the CO.sub.2 entering through pipe 32 will be obtained from the oxy-fuel CCGTEG plant as previously described. The combined stream then passes through pipe 33 into the methanation reactor tube 34, where it reacts across metal catalyst 35 to form CH.sub.4 and H.sub.2O.

(39) The methanation is currently envisaged to take place via the Sabatier process. The reaction of carbon dioxide (CO.sub.2) and hydrogen (H.sub.2) in the presence of a metal catalyst to form methane (CH.sub.4) and water (H.sub.2O) was first demonstrated by Paul Sabatier in the early 20.sup.th Century..sup.[4] Recent efforts to improve the efficiency and commercial viability of the process have been largely driven by its potential applicability in NASA's space program.sup.[5,6], but to the present inventor it is also an ideal means of generating a fundamental thermogas (CH.sub.4) and in the process using CO.sub.2, precisely the gas which we wish to avoid emitting to the atmosphere;

(40) ##STR00004##
with the metal catalyst 35 ideally being the best available using leading-edge technology, currently Ru dispersed on Al.sub.2O.sub.3.

(41) This reaction has been shown to be exothermic, with optimum product yield obtained around 300 C. (As the temperature increases above this point, the back reaction becomes important, reducing yield.) Much research continues to be devoted to optimisation of both the metal catalyst and the reaction conditions. Recently, macroporous Ru-on-Al.sub.2O.sub.3 catalysts have shown to give good selectivity and large surface areas for reaction within a small reactor volume.sup.[5], a promising development which could facilitate the implementation of the Sabatier process within our method. With so many molecules reacting in a small reactor vessel, the issue of heat transfer from the reactor becomes paramount, since if the temperature begins to increase above 300 C., efficiency is lost as described above. Accordingly, long, thin cylindrical reactor tubes have been proposed.sup.[6]. Our proposal intends that the large quantities of heat given out by the Sabatier reactors will be utilised to provide a benefit to another step of the method, as will be described below.

(42) As touched on above, the Sabatier reaction is a highly exothermic process, and suffers from undesirable efficiency loss at temperatures significantly above 300 C. Consequently, the reaction vessel should take the form of the long, narrow reactor tubes 34. Furthermore, these are encased within a pressure vessel 36, in order to maintain them at the optimal reaction temperature of 300 C. or thereabout. The optimum temperature for the Sabatier reaction, in the context of efficiency of the entire system, can be determined through operational trials.

(43) The product stream of CH.sub.4 and H.sub.2O from methanation reactor tube 34 passes through pipe 37 and into condenser 38 in order to separate the CH.sub.4 and H.sub.2O. A stream of external cooling water is passed through the condenser in circuit 39, withdrawing heat from the product mixture and resulting in condensation of the steam. The CH.sub.4 remains in the gas phase and passes out of the condenser through pipe 311. The resulting liquid water is taken off from the bottom of the condenser through pipe 310. In one configuration, this water could be used, possibly along with a top-up supply, as a heat transfer fluid to be passed through vessel 36.

(44) Transportation of Materials

(45) The need to continuously move large quantities of CH.sub.4 and O.sub.2 from upstream to downstream plant, and the parallel return of CO.sub.2 to the upstream, may necessitate the development of large-scale material transportation infrastructure. There are, at present, two principal means for the movement of large quantities of potentially-hazardous fuel: gas pipeline and ship tanker containing liquefied materials, both of which are presently employed on a global scale by the fossil fuel industry. Our investigations indicate that a hybrid approach, combining both methods of transportation, would be the most effective route to address this issue.

(46) The large liquefied natural gas (LNG) tankers currently in operation have a capacity of approximately 250,000 m.sup.3, capable of transporting over 120,000 tonnes of liquefied CH.sub.4 in one shipping.sup.[18]. This would be sufficient to fuel a 1 GW CCGT (of 60% efficiency), running 12 hours a day, for over three months. Alternatively, two full tankers of this capacity, running once a fortnight back and forward from the upstream to downstream plants, would be capable of carrying enough liquid methane to provide electricity for the whole of Scotland by combustion in CCGT plants. It would therefore appear that the ship tanker approach holds excellent potential for the transport of the chosen thermogas (e.g. methane), although this could also be done by gas pipeline, as illustrated below.

(47) The transport of the O.sub.2 and CO.sub.2 also needs to be addressed. Both could theoretically be done by ship tanker: the CO.sub.2 frozen and carried at low temperature in the form of dry ice (CO.sub.2(s)), and the oxygen liquefied and transported in an analogous method to the methane. However, a potentially more elegant solution lies in the use of a pipeline network. Here, the idea is to use four-layer pipeline, as shown in FIG. 4b.

(48) FIG. 4b shows one possible configuration of transference means consisting of a composite pipeline system, transporting oxygen and methane to the CCGTEG plant and returning CO.sub.2 to the fuel production location. Methane is transported through the innermost pipe 41. The CO.sub.2 is divided into two parts, to protect and separate the fuel and the oxygen. One part is passed through pipe 42 to further isolate the methane from the oxygen in pipe 43, the other part being transported through the outermost pipe 44, protecting the oxygen from the outside environment. End connections 45 and 46 facilitate the take-off of the methane and oxygen respectively. The CO.sub.2 is passed into the pipeline through connection 47, with part passing into pipe 42 and part into pipe 44 as described above.

(49) Static and differential pressure sensors between O.sub.2 and CO.sub.2 (48) and between CH.sub.4 and CO.sub.2 (49) monitor the absolute and relative pressures of each gas at all times, providing monitoring and control in order to ensure safe and effective pipeline operation.

(50) FIG. 7 shows a cross-section of an alternative, two-layer pipeline, adapted to carry oxygen and carbon dioxide. The O.sub.2 is injected to the pipeline at the upstream plant, and is pumped in the downstream direction through the inner pipe. Likewise, the CO.sub.2 is pumped back to the upstream from the CCGT plant through the outer pipe, as indicated. Twice as much total cross-sectional area of pipe is required for the O.sub.2 compared with the CO.sub.2, due to the stoichiometry of the combustion reaction. In the case of a longer pipeline, compressor stations would be situated along the route, with entirely separate circuits to ensure no mixing of the gases. This arrangement has the advantage of the pipe containing the high fire-risk gas (O.sub.2) effectively being contained within a blanket of CO.sub.2, further isolating it from the outer environment and thus providing a very useful safety buffer. Each pipeline would be constructed of carefully-selected materials and monitored rigorously to ensure safety.

(51) The rate at which the oxygen is allowed to travel through a pipeline is regulated, with its maximum velocity controlled by the impingement velocity curve (IVC) for the specific pipeline material in use..sup.[19]. This identifies an inverse relationship between pressure and maximum velocity: for example, for a steel pipe, O.sub.2 at 0.6 MPa has a maximum allowed velocity of 7.5 ms.sup.1. The IVC thus implies a maximum transfer rate which depends on the pipeline diameter (and thus the cross-sectional area), multiplied by a constant (P.V, as these are inversely proportional for the curve region in question). In order to fuel a 1 GW CCGT plant of 60% efficiency, a cross-sectional area of 2.09 m.sup.2 is required for oxygen being piped at the limiting conditions of the IVC. This corresponds to an O.sub.2 pipe of internal diameter 1.64 m. This would be surrounded by an outer CO.sub.2 pipeline, which requires a total available cross-sectional area (excluding the O.sub.2 inner pipe) of 1.05 m.sup.2, leading to a total pipeline diameter of 2.04 m.

(52) Generation of Electricity from Thermogas

(53) The large quantities of thermogas and oxygen produced will be transported and/or stored for use to meet electricity demand in a downstream generating plant.

(54) Fossil fuel electricity generating stations are currently multifarious in their design in order to cater for the variety of available fuels. The present invention will provide means to have a single basic design of generating stations to meet electricity demand that need only vary with regard to size.

(55) Power generation is envisaged to be carried out in a combined cycle gas turbine (CCGT), currently in widespread use in natural gas-fired power plants worldwide,.sup.[13,14] but crucially, adapted for oxy-fuel combustion, as described below.

(56) The principle of generation by CCGT is well-understood, and CCGTs are in common use throughout the world. The design combines a primary-loop gas turbine cycle, operating at very high temperature, with a secondary-loop steam turbine cycle which draws its heat from the still-high-T outlet of the gas turbine. In the gas turbine cycle, a working fluid is compressed and passed into a combustion chamber (in our case, burning CH.sub.4 in O.sub.2), where it is heated to very high temperature. This heated, pressurised gas is then expanded and accelerated towards a turbine, which extracts its energy. The output temperature of the flue gas from this turbine is still high, and sufficient to provide heat for the steam turbine cycle. Combining these two cycles can be seen to increase the proportion of the heat generated by combustion of the fuel which is being put to use in electricity generation. Currently, efficiencies of 60% are common for CCGTs. As new materials and designs increase the maximum operating temperature of future-generation CCGTs, it can be expected that efficiencies will increase still further.

(57) Combined cycle gas turbine electricity generators (CCGTEG) with and without District Heating are currently in frequent use burning fossil fuels such as natural gas. Gas is burned commonly with air and the gasses of combustion pass through the gas turbine doing work and producing electricity in the generator. Exhaust gases from the gas turbine then pass into a Rankine steam cycle boiler before being released to the atmosphere via the stack. Steam is formed in the boiler from the heat from the gas turbine exhaust gasses and then used to drive a steam turbine or turbines to produce further electricity. The steam is condensed to water before being returned to the boiler to continue the cycle. This normally occurs in a condenser or by use in District Heating. The present invention can use the CCGTEG in a possible manifestation.

(58) Another, preferred, way of increasing the efficiency of the generating plant would be to run it as a Combined Heat and Power (CHP) system. In this format, the hot water (steam) from the outlet of the turbines would be used to provide district heating for the population of the area around the downstream plant. In this way, much of the waste heat of a standard power plant can be recovered and put to useefficiencies of over 80% are theoretically possible by this method..sup.[15]

(59) Existing Rankine Cycle electrical generating plants have an external water supply to make up losses from the steam cycle. The present invention would be able to supply make up water to the steam cycle of the CCGTEG. In the case where the Rankine cycle uses District Heating to condense its steam and provide water recovery, the system can be self-sustaining in water after plant start-up.

(60) Heretofore, oxy-fuel use has been limited by the availability and cost of oxygen (O.sub.2); for example oxygen extraction from the air would use 15% of the heat content of the fuel. The method specified, by obtaining oxygen directly from the upstream fuel/oxygen production plant, improves the prospects of oxy-fuel electricity generating plant.

(61) The CCGTEG is typically fuelled by natural gas with air supported combustion where the gas turbine exhaust gasses pass into a Rankine steam cycle boiler before passing to atmosphere thereby releasing the CO.sub.2 formed in combustion. The present invention will show how an oxy-fuel CCGTEG and associated plant can be designed to retain the CO.sub.2 of combustion for reuse.

(62) The near universal use of air to support combustion for electricity production has consequences if CO.sub.2 extraction is desired because chemical extraction methods are necessary to separate the CO.sub.2 from the nitrogen, of the combustion air, in the flue gasses. The present invention does not require chemical reaction methods for CO.sub.2 extraction. In stipulating that hydrocarbon fuel combustion is supported with oxygen instead of air, the resultant exhaust gasses, CO.sub.2 and H.sub.2O, are then separable simply by cooling.

(63) Another possible embodiment of the invention could use oxygen-methane electrolytic fuel cells with the option of district heating. The basic fuel cell envisaged would generate electricity by a well known electrochemical reaction when oxygen is passed continually over the cathode and hydrogen is passed over the anode, the hydrogen being derived from steam reforming of hydrogen from methane at temperatures around 760 C.
CH.sub.4+2H.sub.2O=>CO.sub.2+4H.sub.2Steam Reforming:
2H.sub.2=>4H.sup.++4e.sup.Anode Reaction:
O.sub.2+4H.sup.++4e.sup.=>2H.sub.2OCathode Reaction:
2H.sub.2+O.sub.2=>2H.sub.2OOverall Cell Reaction:
CH.sub.4+2O.sub.2=CO.sub.2+2H.sub.2OOverall Reaction:

(64) A hybrid of fuel cell and steam turbine design is also possible with the exhaust from the fuel cell being again CO.sub.2 and steam, at 900 C., being used to drive the turbine generating further electricity. The exhaust from the turbine generator could go to district heating to achieve highly efficient energy transference. The fuels cells would not require the current standard desulphurisation methods essential with fossil fuels because of the complete absence of compounds of sulphur from the upstream plants. Moreover fuel cell life-time enhancement would be expected without the presence of such compounds.

(65) Oxy-Fuel Combustion of CH.sub.4

(66) Several pilot projects have already been completed proving the feasibility of oxy-fuel combustion.sup.[16] and the adaptation of existing gas turbine technology to operate with oxy-fuel.sup.[17]. The biggest drawback has been identified as the cost in power of separating the required oxygen from airestimated at 15% of the total generating capacity of the plant. As can be seen, our method bypasses this problem by obtaining O.sub.2 from the electrolysis at the upstream thermogas-producing plant. In this way, we obtain a large efficiency saving in the downstream plant over existing oxy-fuel combustion projects.

(67) The products of combustion of CH.sub.4 in O.sub.2 are just CO.sub.2 and H.sub.2O, summarised in the equation:
CH.sub.4+2O.sub.2.fwdarw.2H.sub.2O+CO.sub.2

(68) It can be seen that the greenhouse gas carbon dioxide is produced, just as in conventional hydrocarbon-burning power stations. However, the key advantage in using oxy-fuel combustion is that, after condensing out the water vapour, the resulting flue gas will be very pure (>95%) in CO.sub.2, and will be without the nitrous oxides and sulphur dioxide resulting from combustion of impure coal or natural gas in air. This CO.sub.2 will be cooled, before being transported back to the upstream plant (see below), for conversion to methane via the Sabatier process, as illustrated in FIG. 1.

(69) Importantly, it should be realised that although CO.sub.2 arises during the downstream process it will subsequently be consumed in the upstream process. Initially CO.sub.2 from an external source is required to begin the thermogas production process but thereafter no new CO.sub.2 is generated by the process.

(70) The other product of the combustion reaction is H.sub.2O, which can be put to use in several ways. A portion of the H.sub.2O produced by the reaction would be recycled in order to regulate the temperature at the gas turbine inlet, which would otherwise rise to levels requiring the use of very expensive and specially-designed materials in order to operate. The non-recycled remainder of the H.sub.2O holds interesting potential for commercial use. In the absence of pollutant impurities from the standard burning of fossil fuels in air, it could be treated with minerals and distributed as drinking water, or sold in bulk to nearby industries with a demand for pure demineralised water.

(71) FIG. 5 shows a possible layout of the oxy-fuel CCGTEG plant. The fuel inlet stream 51 first passes through a pre-heater 53 where it takes heat from the CO.sub.2 being returned through pipe 526 for transportation to the fuel production site. The heated fuel is then mixed with the O.sub.2 inlet stream 52 and these are burned together in the combustion chamber 54 producing a high-temperature steam/CO.sub.2 mixture 55 which expands through high-pressure gas turbine (HPT) 56 generating electricity in the electricity generator 526. This turbine will be constructed to the current industry-leading standard, and adapted for the high temperatures of oxy-fuel combustion using state-of-the-art materials. The outlet stream 57 from the HPT 56 then passes into a reheat combustor 58, where it is reheated to the inlet temperature of the Intermediate-Pressure Gas Turbine (IPT) 510 through combustion, using a portion of the fuel and oxygen streams diverted to reheat combustor 58 through pipes 530 and 531. This reheated stream 59 then expands through IPT 510 generating further power, and the exhaust stream 511 passes into a heat exchanger 512. Here, it gives up a portion of its heat to the recycled water 523 being used as an atemperator for combustion chamber 54. Such an atemperator is necessary since the burning of CH.sub.4 in pure O.sub.2 will generate extremely high flame temperatures, much higher than the optimal inlet temperatures of the HPT 56.

(72) The steam/CO.sub.2 mix then passes into the heat recovery steam generator 513, where it gives up its heat to the Rankine cycle boiler generating steam which passes through heat recovery steam generator 513 via banks of boiler tubes. This steam is then used to power steam turbine 515, generating further electricity. This turbine like turbine 56 will be constructed to the highest quality possible using well-defined, industry-accepted specifications. It is envisaged that the steam turbine outlet steam 516 can either be taken off for District Heating 517, or passed through a condenser 518 in order to prepare it for return as feed water to the heat recovery steam generator 513. In the case of District Heating, cool water will be returned through pipe 529 from the District Heating system, ensuring that the Rankine Cycle feed water is sustained.

(73) In heat recovery steam generator 513, the steam/CO.sub.2 mixture is separated by cooling. The CO.sub.2 remains in the gas phase and is piped to CO.sub.2 compressor 525, where it is compressed and then passed through pre-heater 53 before being taken off for return to the fuel/oxygen production site. The condensed water from heat recovery steam generator 513 is split into two parts: one part 519 can be taken off to water treatment facility still containing CO.sub.2, while the other part 520 is fed into a degasser 521. From here, the CO.sub.2 taken off 522 is fed back into heat recovery steam generator 513. Some of the degassed H.sub.2O is used as make-up to the feed water for the Rankine cycle steam generator 513. The remainder is split into two parts: H.sub.2O in pipe 523 is fed back through the heat exchanger 512 and then into combustion chamber 54 as an atemperator, while H.sub.2O in pipe 524 is taken off to the water treatment plant for export.

(74) FIG. 6 shows a potential layout of the upstream fuel/oxygen production plant. CO.sub.2 transported from the downstream CCGTEG plant enters the plant through pipe 61 and is heated to 205 C. by start-up heater 648 or by preheater 639 before joining with a stream of electrolysis-produced hydrogen 645 from pipe 643 and non-return valve 644. The combined stream 65, controlled at 205 C., then enters the Sabatier reactor tubes 66 (of which there are several, being fed by a manifold) where the CO.sub.2 and H.sub.2 react over a metal catalyst 67 to form CH.sub.4 and H.sub.2O. The metal catalyst would be the leading industrial standard for this reaction, currently Ru-doped Al.sub.2O.sub.3. The product stream of CH.sub.4 and H.sub.2O passes through pipe 68 to a second heat exchanger 69, where the stream of CH.sub.4 and H.sub.2O gives up part of its heat to the water in pipe 619. The CH.sub.4 and H.sub.2O stream then passes through pipe 610 to condenser 612. A supply of cool water is fed into condenser 612 through pipe 611, cooling the mixture of CH.sub.4 and H.sub.2O to the point where the H.sub.2O condenses to liquid water. The CH.sub.4 leaves the condenser 612 through pipe 613, through which it passes to a CO.sub.2 scrubber 614 used to remove any unreacted CO.sub.2 from the Sabatier reactor tubes 66. The purified CH.sub.4 is then taken off through pipe 615 for transportation to the downstream CCGTEG plant.

(75) The Sabatier reaction needs a temperature of at least 200 C. to proceed and gives its highest product yield at around 300 C., above which increasing temperatures begin to favour the back reaction, reducing product yield and finally, at >500 C., stopping the forward reaction from occurring. The highly exothermic nature of the Sabatier reaction therefore means that, without temperature regulation, the reactor tubes would quickly heat to temperatures that would prevent a sustainable process. To prevent this, the cooled H.sub.2O 619 from condenser 612 is re-circulated to the main heat transfer reactor 625, where it is used as a heat transfer medium to maintain the Sabatier reactor tubes 66 at the optimal reaction temperature of 300 C., although some variance from 300 C. may be acceptable if it has benefits for other aspects of the system. In order to ensure the optimal steam conditions for injection into the heat transfer reactor 625, the H.sub.2O first passes through pipe 617 to heat exchanger 69, where it withdraws heat from the CH.sub.4/H.sub.2O product stream as previously described, and then passes into a steam drum 620. For start-up electric heating 637 will be used. Steam from the steam drum 620 is used to heat the Sabatier reactor tubes 66, to a temperature of 205 C. As the H.sub.2O rises through heat transfer reactor 625 across the Sabatier reactor tubes 66, it is heated to 300 C. The steam continues to rise gaining heat by passing over successively tubes 641 and 642 containing the hot electrolysis products hydrogen and oxygen respectively. Further heating may also be applied by heater 653 for temperature control of the steam entering electrolysis cells 621.

(76) The heated, high-pressure steam then passes into the electrolysis cells 621, illustrated in detail in FIG. 6a. The high-pressure superheated steam enters the cell at the cathode 621a. This is envisaged to be a solid-state electrode providing the best efficiency that current technology is capable of, at present a NiZr cement. A DC electricity supply 621d generated from local renewable energy source(s) will drive the electrolysis of H.sub.2O into H.sub.2 and O.sub.2. The H.sub.2 forms at the cathode 621a and is taken off through pipe connection 621e to pipe 622. The O.sup.2 ions migrate through the solid-state electrolyte 621b to the anode 621c, where they give up electrons and form O.sub.2 molecules. Both the electrolyte and the anode will, like the cathode, reflect the optimal efficiencies available using current technology. At present, the electrolyte is envisaged to be yttria-stabilised zirconia, while the anode would be made from strontium-doped lanthanum manganite. At the anode, the O.sub.2 produced is taken off through pipe connection 621f to pipe 626.

(77) It is inevitable that sizable quantities of unreacted H.sub.2O will also enter the take-off connections 621a and thus pipe 622 and 624. The H.sub.2 will therefore be separated from this steam in a separation chamber using a hydrogen-porous membrane 623. The H.sub.2 will pass through the membrane and then enter pipe 62 and into the steam heating tubes 641 within the vessel 625 exiting via tube 63 into a Rankine cycle boiler 64 before joining the stream of CO.sub.2 as previously described. The steam from the separation chamber 623 is passed back into the vessel 625 through a non-return valve 647 contained in tube 624 thereby to maintain the inlet steam to the electrolysis cells 621 at high temperatures approaching 900 C.

(78) The electrolysis product oxygen exiting from collection manifold 626 is passed into the abovementioned steam heating tubes 642 within the vessel 625 then passing out to the collection manifold 627 and into a Rankine cycle boiler 628 to be cooled to approximately 30 C. as outgoing product at pipe 616.

(79) The Rankine cycle boiler 64 heated by the hydrogen stream 63 has its boiler tubes fed from condensate water pipe 634. Steam is generated in the boiler 64, which passes to steam turbine 629 generating DC electricity in generator 633. In a similar manner, Rankine cycle boiler 628 heated by the oxygen stream 627 is fed by condensate 634 and raises steam for turbine 630 generating further electricity in the DC generator 633. Exhaust steam from the turbines 629 and 630 pass into condensers 631 and 632 respectively to form condensate in 634 to be pumped by feed pumps to boilers 64 and 628 to continue the Rankine cycle.

(80) Yet further gains in efficiency can be achieved by the steam bridges 650 and 649 which connect to pipeline 651 feeding into the main heat transfer reactor 625. This would enable the reduction or elimination of Rankine condenser loss in condensers 631 and 632. The steam bridges 650 and 649 situated within steam turbines 629 and 630 contain valves to divert steam into pipeline 651 and to shut off steam flow to the low pressure side of the turbines 629 and 630. The high pressure side of turbines 629 and 630 would provide steam at 250 psi into the steam bridges, this being the pressure of saturated steam at the temperature required of 205 C. for inlet 625.

(81) In order to pre-heat the CO.sub.2 61 entering the fuel/oxygen production process bled steam 638 from turbine 629 is passed to a heat exchanger 639. Condensate and low enthalpy steam is recovered from the heat exchanger 639 by pipe 640 and recovered into the condenser 631.

(82) The DC generator 633 generates electricity to enable further electrolysis for further product production thereby to increase conversion of energy from the source energy (e.g. non-fossil source) to chemical energy in the methane and oxygen, potentially an efficiency of 70% or even higher, say 80% or even higher, with the additional heat recovery technologies described.

(83) Thus it can be seen that the present invention provides a high efficiency combined electrolysis and Sabatier reaction apparatus and system. Excess heat produced during the Sabatier reaction is used to heat water that is then electrolysed. In order to further elevate the temperature of the water to be electrolysed, the high temperature outputs from the electrolysis process are passed though the water, which makes use of the high heat energy content of the electrolysis products to further raise the temperature of the water input. This increases overall efficiency of the electrolysis process. Furthermore, an additional potential loss of energy in the form of heat is avoided by using the still relatively hot oxygen and hydrogen, even after heating the electrolysis input water, to drive electricity generation via an additional generation stage, e.g. through a Rankine cycle. These features provide a highly efficient system for combining electrolysis and generation of methane. Of course, many of the efficiency increasing features can be used independently from each other, where appropriate, but the optimum efficiency is achieved where all the features are combined.

(84) FIG. 9 shows a system for the transfer of energy from the grid by night into electricity and district heating by day being a possible embodiment of FIG. 1b. The system is characterised by having no emissions to atmosphere at the chosen location. Moreover when the generating assets supplying the grid through the night are independent of fossil fuels (e.g. renewable energy sources) there will be no resultant atmospheric emissions. However, in the general case where some or all of the assets supplying the grid use fossil fuels there will be CO.sub.2 released at their respective source locations.

(85) In FIG. 9 CO.sub.2 from storage tank 960 passes through pipe 91 into start-up heater 948 and pre-heater 939, being heated to 205 C. The heated CO.sub.2 943 is lead through non-return valve 946 to meet a stream of electrolysis-produced hydrogen from pipe 92 and non-return valve 944. The hydrogen temperature is also 205 C. having been cooled in heat exchangers 941 and 94. The combined stream 95 is distributed by a manifold into numerous Sabatier tubes 96 contained within vessel 925 where the CO.sub.2 and H.sub.2 react over a metal catalyst 97 to form CH.sub.4 and H.sub.2O. The product stream of CH.sub.4 and H.sub.2O is collected via an outlet manifold into pipe 98 and then passes into heat exchanger 99 where much of the heat generated by the Sabatier reaction passes into water tubes fed by supply pipe 919 to form steam in steam drum 920. The product stream reduced in temperature at outlet from heat exchanger 99 is led by pipe 910 to be cooled further in condenser 912 from which it exits to pipe 913 into CO.sub.2 scrubber 914 which removes any unreacted CO.sub.2 passing out from the Sabatier tubes 96. Steam formed in the Sabatier reaction is removed from the CH.sub.4 by condenser 912 and the condensate passes into said pipe 919 to be pumped back into the said heat exchanger 99 to supply steam drum 920. The product CH.sub.4 free of any steam or CO.sub.2 is led by the said pipe 913 to be compressed by compressor 914 for storage by night in storage tank 962.

(86) Water from storage tank 963 or demineralised from local sources 901 feeds cooling water to the said condenser 912 via supply pipe 903. After passing through condenser 912 the water joins with the condensate in said pipe 919.

(87) Steam collects above the steam drum's controlled water level to supply steam at 205 C. to said heat exchanger vessel 925. The steam moves up through the vessel 925 and is heated, gaining in superheat, by the exothermic reaction of the Sabatier tubes 96, then further by hot hydrogen from the electrolysis in tubes 941 and then still further by hot oxygen from the electrolysis in tubes 942. A heater 953 is positioned above tubes 942 to enable further control of the steam temperature to be received by electrolysis cells 921.

(88) Two further possible supplies of steam may be provided to vessel 925 in order to conserve heat in the process and minimise losses in converting electrical energy into chemical energy. During electrolysis some steam bypasses the cathode 921e and is redirected back into vessel 925 via 924 and non-return valve 947 and secondly steam can be redirected into vessel 925 from steam bridges 958 and 959 situated within Rankine Cycle steam turbines 929 and 930. Turbines 929 and 930 use steam generated from heat exchangers 94 and 928 fed by the H.sub.2 and O.sub.2 streams subsequent to their exit from vessel 925.

(89) The heated high pressure steam passes into electrolysis cells 921 which can receive rectified DC electricity from the grid and from generator 933 driven by the said turbines 929 and 930. H.sub.2 produced at the cathode 121e together with some steam passes out from collection manifold 922 into molecular filter vessel 928 which allows H.sub.2 to pass into said tube 92 but diverts steam into said tube 924. O.sub.2 is produced at the anode 121f devoid of steam since the solid state electrolyte 121b prevents the passage of gasses. The O.sub.2 is collected at manifold 926 and is distributed by a further manifold into the said tube bank 942 being cooled in said vessel 925 and then is further cooled in heat exchanger 928 and led by pipe 916 to be compressed by compressor 964 into O.sub.2 storage tank 961.

(90) The said heat exchanger 928, heated by the O.sub.2 stream 927, and the said heat exchanger 94, heated by the H.sub.2 stream 93, contain Rankine Cycle boilers which take in feed water 934 and produce steam to steam lines 936 and 935 supplying steam to turbines 930 and 929. The turbines 930 and 929 connect to said DC generator 933 which supplies additional electricity to the electrolysis cells 121 having the effect of augmenting CH.sub.4 and O.sub.2 production.

(91) Steam passes through turbines 930 and 929 into condensers 932 and 931 and the condensate is pumped into condensate lines 934 to continue the Rankine Steam Cycles in the said boilers 928 and 94. Demineralised make up 966 to the condensate lines 934 is provided. Steam turbine 929 provides bled steam 938 to said CO.sub.2 pre-heater 939 which returns the outlet steam by pipe 940 to condenser 931.

(92) The aforementioned steam bridges 959 and 958 situated within turbines 930 and 929 supply 250 psi steam at 205 C. through 951 to vessel 925 but can also provide optional district heating 956 which can supply steam through pipeline 954 and condensate returns through pipeline 955. During start-up, condensers 932 and 931 would be put into service then subsequently valves within the steam bridges would control the diversion of steam to pipe 951 and the partial or complete shut off of steam to the condensers.

(93) Thus O.sub.2 and CH.sub.4 collect in storage tanks 961 and 962 by night, to enable electricity supply to the grid and heat energy to district heating, during day, by the plant described below.

(94) O.sub.2 from said storage tank 961 and CH.sub.4 from storage tank 962 are led by pipes 968 and 969 to combustion chamber 976 and also by pipes 982 and 983 to reheat combustor 977. CH.sub.4 burns in O.sub.2 in the said combustion chamber 976 and being attemperated by cooling water 988 issues out as CO.sub.2 and steam into high pressure gas turbine 972. This working fluid powers turbine 972 and exhausts via 984 and is used as attemperator fluid for said reheat combustor 977. Further CH.sub.4 burns in O.sub.2 in said reheat combustor 977 and is, after being attemperated by said stream 984, directed by pipe 985 into intermediate pressure gas turbine 973 to power the turbine. Turbine 973 exhausts CO.sub.2 and steam via 989 into heat exchanger 957 and then into vessel 978. Vessel 978 functions as a CO.sub.2 and H.sub.2O separator being a heat exchanger containing boiler tubes fed cool water 995. The hot CO.sub.2 and H.sub.2O passing down through vessel 978 generate steam in the boiler tubes 943 being successively cooled in the process until the water condenses. The CO.sub.2 is lead off by pipeline 999 to compressor 971 and recovered by pipe 967 into storage tank 960 to accumulate during the day. The water falls to the well of vessel 978 and into drain 990 when, still containing CO.sub.2 in solution, is led into degasser 981. Mechanical agitators in the degasser 981 separate the CO.sub.2 from the water. The separated CO.sub.2 is passed back by pipe 991 to vessel 978.

(95) Water passes from the degasser 981 to provide make up to the boiler feed water 995 pumped into the Rankine Cycle said boiler tubes 943 contained in said vessel 978. Steam generated in boiler tubes 943 passes into steam turbine 974 via steam pipeline 979. Steam turbine 974 together with said high pressure gas turbine 972 and said intermediate pressure gas turbine 973 drive said compressor 971 and also electricity generator 975. Electricity generator 975 supplies electricity to the grid by day. Steam exhausts from steam turbine 974 into condenser 996 or is diverted in whole or in part to district heating supply 993. Condensate 980 from condenser 996 meets with water returning from district heating 994 to provide boiler feed water in feed pipe 995 to said boiler tubes 943 to continue the Rankine Cycle.

(96) Water from the degasser 981 passes into pipe 986 and a portion taken into pipe 987 to be pumped through tubes in the said heat exchanger 957 as the atemperator coolant 988 to said combustion chamber 976. The remaining portion of water from pipe 986 is fed into pipeline 970 which leads into storage tank 963.

(97) Thus CO.sub.2 and water collect in storage tanks 960 and 963 by day to enable the production of O.sub.2 and CH.sub.4 by night to continue time-phased energy transference.

(98) Production of Methane Derivatives

(99) While the methane produced by the Sabatier reactors represents the basic thermogas, this need not be the only useful product gas obtained from the upstream plant. Indeed, significant research is currently underway aiming to make commercially feasible the conversion of methane into a multitude of alternative chemical feedstock gases. Two significant examples are ethylene, C.sub.2H.sub.4, and longer-chain hydrocarbons.

(100) The manufacture of ethylene (and other C.sub.2+ hydrocarbons) from oxidative coupling of methane has been known for decades, with its reactants the very gases produced in our upscale plant: CH.sub.4 and O.sub.2. Until recently, efforts to make the process economically feasible had stalled: a seemingly unavoidable trade-off between catalyst efficiency and product selectivity limited the yield to levels well below those commercially viable..sup.[9] However, new developments in reactor technology and porous nanomolecule catalysts have re-raised the possibility of this process being developed into a commercial route to ethylene manufacture. Indeed, the OCMOL project.sup.[10] has recently been given substantial funding by the European Union to carry out an extensive five-year project pursuing this aim.

(101) The manufacture of larger-chain hydrocarbons from a methane feedstock has also attracted considerable interest in recent years. For example, WiesnerTech have submitted a patent application for one method of doing this..sup.[11] In many ways their proposed plant set-up resembles the upstream plant we are proposing. In their scheme, the CH.sub.4 produced by the Sabatier reactor would be fed into a partial oxidation reactor, and the output gases then fed into Fischer-Tropsch reactor, in order to create longer-chain hydrocarbons by a well-known process,.sup.[12] which would then be separated and exported to market.

(102) Such developments as those mentioned above can be seen to be entirely compatible with, and in fact complementary to, the scheme we are here proposing, and could evidently be integrated into the upstream plant, providing the capacity for manufacture of a whole range of useful fuels and chemicals some of which are shown in FIG. 8.

CONCLUSIONS

(103) At present less than 2% of the world's electricity demand is met by emissions free renewable energies. This is despite the capacity for generating electricity from such sources being very large indeed. The present invention will enable extensive large scale development of these renewable non-CO.sub.2-emitting energies, e.g. hydro, solar, geothermal, wind, tidal currents, nuclear and others for reliable electricity supply, without the necessity of a direct local connection to the electricity grid. Upon widespread adoption of the invention, the energy sources are sufficiently large as to completely replace fossil fuels for electricity supply. This in turn will enable world economic growth to continue to rise without atmospheric CO.sub.2 levels rising due to electricity supply.

(104) Furthermore the present invention opens new opportunities to improve the ability of the current generation systems to meet changing levels of demand and to provide chemical feedstock for industrial processes.

(105) The invention can be seen to have several potentially significant benefits. These can be categorised as environmental or economic benefits, as described below.

(106) Environmental Benefits

(107) The principal environmental benefit of the approach advocated in this application (and, indeed, one of the main driving factors for its development) is the potential for the complete absence of any CO.sub.2 emissions to the atmosphere, despite the use of relatively cheap and well-established gas-turbine generation methods to ensure a reliable electricity supply. A 2008 European Environmental Agency (EEA) report.sup.[20] estimated that 56.1 kg of CO.sub.2 was emitted to the atmosphere for every gigajoule (GJ) of energy generated by the combustion of natural gas. This corresponds to over 200 tonnes of CO.sub.2 per hour for every 1 GW CCGT plant. Moreover, natural gas is generally considered the cleanest fossil fuel for combustionrates of emission per gigajoule generation for oil, and especially coal, are estimated to be much higher. Therefore, the idea detailed in this application clearly could lead to a dramatic reduction in CO.sub.2 emissions if implemented on a large scale, essentially facilitating the harnessing of the vast quantities of renewable energy available across the globe in a reliable way. In addition, or in the alternative the present invention provides a method by which excess electricity can be captured and stored for later use (in the form of chemical energy), without the emission of any CO.sub.2.

(108) As an example, consider the Pentland Firth, situated to the north of Scotland, between the British mainland and the Orkney Islands. This channel of water is well-known for its quick and powerful tidal runs, and accordingly has been the subject of much interest for years due, to its potential for electricity generation by underwater turbines. Previous estimates using a very simplified model.sup.[21] suggest that the seabed dissipation as the water passes through the channel at its maximum velocity is on the scale of 50 GW. A single full bank of turbines across the 10 km width of the channel could have a potential generating capacity of nearly 15 GW at times of peak tidal power. Clearly, the ability to harness even half of this power would enable electricity generation for the whole of Scotland.

(109) The total power a continuous flow of water travelling with a velocity v, across a width W and depth Z is given by:

(110) $P=\frac{1}{2}.Math..Math.W.Math.Z.Math.v^3$

(111) It can therefore be seen that any potential for increasing the velocity of the water would mean far greater power generation capability. For example, consider halving the 80 m depth of the Pentland Firth at the turbine site. The velocity would double accordingly to ensure the flow rate remained constant. However the power per unit volume of the water would have quadrupled, as this is dependent on v.sup.3 but only linearly on Z.

(112) However, the tidal runs, while predictable, do not always conveniently correspond to the times of peak electricity demandoften, they will occur in the middle of the night. The method of this paper would provide an effective means to make hay while the sun shines: the huge maximum generation capacity at times of peak tidal power could be converted into CH.sub.4 and O.sub.2 stocks, which would then be transported downstream to generating plant for combustion to fire CCGTs to meet demand.

(113) Another powerful example is the case of wind turbine generation. The wind contains great amounts of potential power, but is notoriously unpredictable and therefore unreliable, which has limited its usefulness as a large-scale alternative to fossil fuels. Converting the energy provided by wind turbines into thermogas and oxygen for storage and subsequent downstream generation would enable this difficulty to be effectively overcome. This would greatly increase the viability of large-scale wind turbine projects as a credible alternative to fossil fuel combustion, to meet a significant proportion of overall electricity demand.

(114) To combat the rising environmental and associated economic costs of large-scale CO.sub.2 emissions, the fossil fuel industry is currently investing large sums of money into the development of various carbon capture methods. These essentially involve retrieving the CO.sub.2 from the flue gases at fossil fuel-fired power plants and transporting it for sequestration. This is a difficult and expensive process, however, driving up the operating costs of the plant and reducing profitability; this has prompted the recent research into oxy-fuel combustion, which gives a far cleaner and more readily-separated flue gas mixture as described previously.

(115) The biggest drawback of oxy-fuel combustion methods in their current incarnation is the difficulty of separating the desired oxygen from air, which requires so much energy as to cripple the overall generating capacity of the plant. Furthermore, regardless of the method of carbon-capture used, it is far from certain that CO.sub.2, once sequestered, will remain so in the long term, and not begin to leak to the atmosphere. The methods we propose provide a very elegant solution to this two-fold problem. On the one hand, the O.sub.2 produced during the electrolysis of H.sub.2O at the upstream plant is transported and used at the oxy-fuel combustion stage, removing the need for separation from air. Meanwhile, the CO.sub.2 produced, easily separated from the flue gas mixture due to the use of oxy-fuel combustion, is transported back to the upstream plant for use in the Sabatier reactors to generate more CH.sub.4.

(116) An additional environmental benefit is provided by virtue of the fact that the thermogas would be a manufactured commodity, and therefore would contain almost none of the impurities which are found in geologically-bound fossil fuels. This means that the flue gases from the CCGT combustion would be much cleaner, containing none of the unpleasant and polluting nitrogen oxides (NOx) or sulphur dioxide (SO.sub.2) which generally result from the burning of fossil fuels, particularly coal. The EEA estimates that at present, over a kilogram of SO.sub.2 is emitted for every gigajoule of energy generated by coal and oil-fired power plantsthis corresponds to nearly five tonnes an hour for a 1 GW CCGT plant. Currently, such plants may be releasing anything up to a tonne of NOx an hour, with a substantial percentage of this being NO.sub.2, a gas toxic to humans and a major atmospheric pollutant. SO.sub.2 is also known to be an atmospheric pollutant, and a precursor for acid rain. Eliminating such substantial emissions to atmosphere of these and similar undesirable compounds is therefore a great environmental benefit of our proposed system.

(117) Economic Benefits

(118) In addition to the environmental benefits outlined above, the method we are proposing holds several key economic advantages when compared to similar schemes being piloted or currently in operation. Many of these arise as a natural consequence of the cyclic nature of the process, with comprehensive re-use of materials wherever possible.

(119) The most obvious advantages again lie in the capture and re-use of CO.sub.2 from combustion, and resultant lack of CO.sub.2 emissions to the atmosphere. In addition to the obvious environmental advantages, this can equally be viewed in terms of its potential economic benefits. Across the world, there is a growing trend among governments towards imposing sizeable per-tonne levies on emissions of CO.sub.2, which will push the cost of operating conventional fossil-fuel power plants ever upward..sup.[22] In addition to the increasing levies on CO.sub.2 emissions, the cost of obtaining fossil fuels from geological formations will continue to rise. Increasingly challenging environments need to be accessed and manipulated, at depths and pressures which require greater feats of engineering and hold more potential for disaster. Political factors may also come into play: large amounts of the remaining known oil and gas reserves may lie within states governed by unfriendly or openly hostile regimes, jeopardising a nation's future energy supply. The combination of these factors will make generation from alternative energy sources an increasingly attractive option, provided they can be usefully and reliably harnessed.

(120) Our approach thus should enable the energy industry to invest heavily in large-scale development of renewable resources with an increased measure of confidence. Furthermore, much of the technology required is already well-understood, and energy companies have the expertise in operating it. CCGTs, pipeline construction and operation, and ship tanker transportation are all well-developed methods which have been in extensive use for decades. The existing gas grid could even potentially be modified in order to transport some or all of the process gases required, potentially leading to large savings over the cost of constructing a new network from scratch.

(121) Another major advantage lies in the potential for a universal, standardised design of downstream generating plant, varying only in scale, with a range of turbine sizes (power ratings) possible, but with the plant design always to the same blueprint. This would replace the current multifarious plant designs, required at present due to the range of different fuels burned. Thus, comprehensive research and development efforts could more easily be focused both on optimising the efficiency, and on minimising the construction and operating costs of this single standard design. It could therefore be expected that the cost and build-time of new CCGT generating plant or other suitable plants would decrease substantially over time.

(122) Similarly, there is substantial potential for improving the cost-efficiency of the upstream plant processes. Any improvement in the efficiency of the electrolysis of H.sub.2O, which provides both the O.sub.2 for combustion and the H.sub.2 for the Sabatier reaction, would be most beneficial. Thanks to its potential as a simple route to the materials H.sub.2 and O.sub.2, which have a multitude of uses, much research is indeed focused on improving this reaction,.sup.[25,26] with the development of solid-state electrolysers and research into high-temperature electrolysis two notable recent developments.

(123) Due to its cyclic nature, an increased efficiency in the downstream section of the process will correspond to a large improvement in overall efficiency. For example, an increase in oxy-fuel CCGT efficiency from 60% to 65% would correspond to nearly an 8% reduction in the required amounts of CH.sub.4 and O.sub.2, meaning a corresponding reduction in the quantity of water needing to be electrolysed, and an increase in downstream/upstream generation efficiency of 15% or more. As the technology of oxy-fuel combustion is still in its infancy, the potential clearly exists for substantial improvements to be developed, particularly if major research were to be devoted to this, as would inevitably occur should the idea proposed here gain widespread support.

(124) A further key benefit is provided by the flexibility of the method, and in particular the sites of the upstream and downstream sections of plant. These could be hundreds of miles apart, with a pipeline and ship tanker transportation network connecting them. In this case, thermogas and O.sub.2 would be generated directly from the alternative energy source at the upstream site, and transported to the downstream plant. Here, they would be stored in large tanks until required for combustion at times of high electricity demand. This set-up would enable the siting of downstream plant close to areas of high local electricity demand, enabling generation local to the point of demand as and when required. Construction of the downstream plant close to population centres would furthermore facilitate a CHP setup as described above providing district heating to the local population directly from the steam output and thereby achieving considerably higher efficiencies than those of the CCGT generation-only plant.

(125) As an alternative, particularly where the renewable energy generation site was not particularly far from a population centre or attached to the national grid, the upstream and downstream sections of plant could be integrated at a single site. This would have the considerable economic advantage of bypassing nearly all of the transportation costs, requiring only reduced-scale pipeline construction, along with the capacity to store the levels of thermogas and O.sub.2 produced at peak production times until their use at times of peak electricity demand.

(126) Furthermore, the methods of synthesis described herein provide alternative routes to petrochemicals allowing the replacement of conventional fossil fuel sources. These can be achieved in a manner which allows control of CO.sub.2 emissions. Petrochemical feed stocks to make polymeric materials and other such products can be achieved via synthesis as described above, e.g. from alternative energy sources, and this can actually result in the fixing of CO.sub.2 from which they are made. Upon eventual disposal, e.g. in landfill, the CO.sub.2 would be essentially permanently fixed. Fuels required for transport could be reduced by a partial shift to emissions free electricity, rail transport being the most obvious exponent for ready adoption of such energy. Transportation having large power units, such as shipping, could be adapted for oxy-fuel combustion to recover their exhaust CO.sub.2 in an analogous manner to that described above, i.e. cooling exhaust gases to separate CO.sub.2 from water. Such CO.sub.2 could be returned for further thermogas production, potentially in return for payment. Potentially even small power units, such those in cars, could be converted for oxy-fuel combustion with subsequent CO.sub.2 recovery, though the increased capital cost would be a disincentive. Power units that continue to use air for combustion, including aviation jet engines, could have a supplemental cost added to fuel in order to fund extraction of CO.sub.2 by means of renewable energies.

(127) Thus, in summary, practical methods are made possible of preventing the escalating global CO.sub.2 emissions driven by mankind.

(128) Summary and Future Work

(129) Methods and apparatus by which electricity can be generated reliably and flexibly from alternative energy sources, without emission of CO.sub.2 or sulphurous and nitrogenous pollutants to the atmosphere, via the production and subsequent combustion of gas as an energy storage medium have been described. Aspects of the invention can be combined in an elegant and efficient cyclic process with few waste products, thanks to the use of oxy-fuel combustion in the CCGT generation plant or using fuel cell technology. We believe that the approach outlined here, if implemented in tandem with large-scale development of renewable energy resources, have the potential to greatly reduce our dependence on fossil fuels for electricity generation. Further development work is now required in order to refine and optimise the invention.

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