Gaseous fuels have been available for many years and are stored in a variety of different ways.

Liquified petroleum gas (LPG) was the first gas generally used for internal combustion engines in vehicles as well as for small-scale thermal applications such as cooking. Whilst highly convenient and still used in many vehicles, LPG comprises a mixture of butane and propane which is not optional for many applications where a methane-based fuel is preferred for its combustion and safety characteristics.

Natural gas overcomes the disadvantages of LPG, and compressed natural gas (CNG) has been used as a transportable fuel in vehicles for many years. However, the weight and cost of substantial CNG storage have prevented its success in the market place other than for relatively short range vehicles such as buses. CNG powered vehicles have generally only been justified on environmental grounds, and the success of CNG as an alternative fuel for vehicles is unlikely to be realised using the available storage systems unless these environmental issues are taken into account.

Liquified natural gas (LNG) provides the highest energy density of the currently used transportable gas storage systems. However, the production of LNG is extremely expensive because gas quality has to be of an acceptable standard and the process is very capital intensive.

To date, LNG systems are primarily suited to bulk transportation. While investigations have been made into LNG-powered vehicles, such systems have not so far proved commercially viable owing to the capital restraints associated with LNG storage.

In the light of the aforementioned disadvantages of the prior art, it has been necessary to investigate other forms of storage of natural gas in order to more satisfactorily provide the benefits of this methane-based fuel.

Natural gas forms hydrates in the presence of water at appropriate combinations of pressure and temperature. Natural gas hydrates have been proposed as a high energy density natural gas fuel source, but they have not been used as a fuel outside of a laboratory.

International patent application WO 99/19283 provides a useful discussion on natural gas hydrates and explains that, while there is extensive documentation relating to gas hydrate production processes, less attention is paid in the literature to devices and methods for storing and re-gassifying the hydrates. WO 99/19283 proposes one means for storing and re-gassifying the natural gas hydrates, but it is intended to be used in fixed equipment such as existing refinery, chemical or power generation plant systems or with a dedicated boiler for producing steam.

US patent specification 5,806,316 also proposes fixed apparatus for forming a gas hydrate as well as for the controlled delivery of pressurised gas resulting from the decomposition of the hydrate to a turbine engine coupled to a generator for the production of electricity or to some other installation.

US patent specification 5,964,093 proposes another fixed storage tank for gas hydrates in which the hydrates are decomposed by exposure to sunlight.

For transportation purposes, natural gas hydrates have been perceived as best suited to a fuel source for bulk transportation, such as in ships, due to the difficulty in meeting transient surges in demand for fuel in, for example, vehicle internal combustion engines.

US Patent 5,540,190 was the first proposal for on-board vehicle storage of natural gas hydrates, and describes a storage tank for storing the hydrates at a pressure preferably between 450 and 850 psig containing a series of hollow metal plates capable of having cooling fluid and heating fluid circulate through them to, respectively, form and decompose the hydrates. Any free natural gas within the tank is used to start and warm the engine of the vehicle, but there is no suggestion of how the system will meet transient surges in demand for fuel, such as when rapid acceleration of a vehicle is required.

According to the present invention there is provided a fuel system for an energy conversion device, comprising : a first tank for storing solid hydrate of natural gas; means for controlling the decomposition of the solid hydrate into natural gas and aqueous liquid; a first fuel line for supplying natural gas resulting from the decomposition of the solid hydrate from the first tank; a second tank for storing compressed natural gas; a second fuel line from the second tank for supplying natural gas therefrom; and a controller for releasing natural gas from said second tank through said second fuel line in response to transient surges in fuel demand above the supply of natural gas in the first fuel line from the first tank.

Advantageously, the second tank is supplied with natural gas from the first tank by way of a fuel line connecting the two tanks, but this is not essential. However, it will enable relatively smaller containers to be adopted for the second tank since, if the supply of compressed natural gas from the second tank runs low, it is readily supplemented by natural gas from the first tank. For fuel systems in accordance with this embodiment in which the natural gas hydrates are stored in the first tank at a lower pressure than the compressed natural gas in the second tank, means will be provided in the fuel line between the two tanks for compressing natural gas to the desired storage pressure. The compressing means may comprise, for example, two compressors in series, optionally with means for cooling the compressed fuel between the two compressors.

The aforementioned fuel line connecting the first and second tanks advantageously extends from the first fuel line.

The controller is conveniently a pressure controller which is preferably adapted to sense the pressure of natural gas in the first fuel line and to open a valve in the second fuel line in response to a reduction in pressure below a predetermined level. The second fuel line may be adapted to supply natural gas from the second tank to the first fuel line, in which case the pressure controller advantageously senses pressure in the first fuel line upstream of this junction with the second fuel line.

The second tank advantageously stores the compressed natural gas at a pressure buffer typically in excess of about 500 psig, but the upper pressure is limited merely by storage vessel cost and the cost of compressing the gas. The storage pressure may vary according to the use of the fuel.

For example, for a gas turbine, the pressure in the second tank would typically be 2 or 3 times the fuel pressure required for the turbine of, typically, 300 to 700 psig, ie. about 600 psig to about 2000 psig in the second tank. Advantageously, for an internal combustion engine the natural gas is stored in the second tank at a lower pressure than the usual storage pressure of 1000 to 3000 psig for vehicle-based CNG fuel systems. Preferably, for a fuel system for an internal combustion engine, the compressed natural gas is stored in the second tank at a pressure in the range 60 to 750 psig.

The first tank may be capable of storing the natural gas hydrate at pressures up to 2,000 psig, as described in US 5,540,190, or more but lower pressures may advantageously be used at reduced temperatures to facilitate filling the tank with pre-formed hydrate. Natural gas hydrate may be stored at atmospheric pressure at temperatures just below 0°C, for example-8 to -10°C, and storage pressures at particular temperatures may be reduced, by the use of suitable additives to the hydrate mixture. Natural gas hydrate and a process and apparatus for producing same are described in International patent applications PCT/AU00/00719 and PCT/AU00/00973.

Pure natural gas hydrate will decompose into natural gas and water, but any additives to the hydrate mixture are likely to separate with the water as an aqueous liquid.

The usual minimum pressure in the first tank is ambient or less. The pressure of the nautral gas may then require boosting by a compressor or other means to the pressure at which the natural gas will be required by the energy conversion device, for example about 30 psig or less at the inlet to an internal combustion engine. Alternatively, therefore the pressure in the first tank will be 0 to 120 psig, most preferably 5 to 80 psig, above the required inlet pressure for the energy conversion device. Thus, for an internal combustion engine, the first tank is preferably adapted to store the solid hydrate at atmospheric pressure or at a pressure up to about 150 psig.

The natural gas hydrates may be stored in the first tank in a variety of different forms and may be formed in situ, for example by spraying, or be formed prior to introduction into the first tank Since forming the hydrates in situ in the first tank may take excessive time, the natural gas hydrates are advantageously preformed for introduction into the first tank and may be in the form of a slush or slurry, particles or large pieces such as bricks, blocks or slabs. The term"solid hydrate"shall be understood to encompass natural gas hydrate in the form of a slush or slurry, that is solid hydrate in a non-pathetic carrier liquid which maintains sufficient flowabilitv at low temperature. Suitable carrier liquids include iso-propyl alcohol, other alcohols, brine (CaCl2, NaCl), crude oil, petroleum condensates and their derivatives such as hexane, and silicon liquids. Most advantageously, the natural gas hydrates are supplied as preformed particles since this may permit lower pressures to be adopted during filling and simplify means for introducing them to the first tank, allowing, for example, a shut-off valve and/or a rotary valve to be adopted. Using larger pieces may require a substantial part of the tank to be opened during filling.

In-situ formation of the hydrates in the first tank may require additional cost and facilities associated with the tank which in a vehicle fuel system will add additional weight which is redundant when the vehicle is operating. Such facilities may include, for example, supplementary chilling facilities, water circulation pumps and spray units. Advantageously, therefore, these additional facilities are associated with the filling system rather than the first tank. In one embodiment, the first tank is interchangeable. Thus, an empty first tank may be removed from a vehicle and be replaced by one which has been filled at a static hydrate formation plant. In another embodiment, the hydrate is provided in a replaceable thin-walled container, such as of a flexible membrane, and an empty container is removed from the first tank and replaced with a full one.

The means for controlling the decomposition of solid hydrate may comprise a heat exchanger in the first tank connected to a chilling system. The heat exchanger may take any suitable form but preferably at least partially lines the walls of the tank which may themselves be insulated.

In one embodiment, the in-tank heat exchanger has a dimple plate structure. The heat exchanger is connected to a chilling system which should be capable of ensuring there is minimum boil-off of the hydrate in the first tank during periods of low fuel demand. Cooling of the chilling system may be provided by, for example, a refrigerant or an absorption chiller. The chilling of the in-

tank heat exchanger may be controlled by a pressure sensor monitoring the pressure in the first tank or, preferably, in the first fuel line.

Advantageously, means is provided for heating the natural gas hydrate in the first tank. Such means for heating may take any of a variety of forms and is preferably controlled by a pressure sensor monitoring pressure in the first tank or in the first fuel line.

For example, the means for heating may comprise a storage vessel and a nozzle in the first tank for spraying a medium in the storage vessel into the first tank. Such a medium may be heated by any suitable means, for example by waste heat from the energy conversion device during operation of the fuel system and electrically during start-up. The storage vessel may be connected to the first tank for receiving, as the medium, residual liquid from the decomposition of the natural gas hydrate in the first tank Alternatively, or in addition, the means for heating may comprise a heat exchanger in the first tank connected to a source of heat. The source of heat is advantageously a cooling system for the energy conversion system or, for example, electrical heating. Again, electrical heating may be used merely during start-up. In a preferred embodiment, the heat exchanger is also connected to a chilling system, for example as previously described.

In the case of a solid hydrate stored in the form of a slush or slurry, it may be advantageous to allow the hydrate to decompose externally of the first tank, between the first tank and the first fuel line or in the first fuel line. In this case heating means, preferably in the form of a heat exchanger controlled as described above, may be provided in the first fuel line adjacent the first tank or between the first tank and the first fuel line to facilitate the decomposition of the hydrate.

Preferably, means is provided for separating the carrier liquid from the natural gas and water.

Means may be provided in the first fuel line for at least partially separating out aqueous liquid, water and/or water vapour from the re-gassified natural gas. Moisture in the natural gas is useful in both internal combustion engine and fuel cell applications. In particular, in internal combustion engines, moisture in the fuel can reduce nitrogen oxides (NOx) by reducing the peak temperatures in the primary combustion zones, can reduce the propensity for pre-ignition by increasing the ignition energy required, and can increase specific power by increasing the

expansion of the combustion product.

The present invention is believed to make the use of natural gas hydrates as a fuel source much more practical than has hitherto been the case where transient surges in demand are experienced.

The fuel system may be used with, for example, internal combustion engines, gas turbine engines and/or fuel cells as well as other energy conversion devices requiring natural gas as a fuel. While a particularly advantageous use of the fuel system is in vehicles, it may also be used in water vessels, in static applications such as power generation, and, for example, aircraft where the efficient storage of the fuel is especially convenient.

One embodiment of a fuel system for an energy conversion device in accordance with the present invention will now be described by way of example only with reference to the accompanying drawing which is a schematic layout of the fuel system for use with a vehicle internal combustion engine.

Referring to the drawing, the fuel system 10 is connected to a vehicle engine (not shown) at 11.

An insulated tank 12 is provided for storing natural gas hydrate at a pressure typically of about 30 psi and a minimum temperature of about-100°C. Higher temperatures and higher or lower pressures may be adopted, for example a temperature up to about 0°C and ambient pressure with the use of suitable additives in the hydrate mixture, but the lower pressure gas storage reduces the likelihood of instantaneous gas release in an accident as well as the cost and weight of the tank.

The hydrate may be formed in situ by spraying natural gas and water into the chilled tank 12, but preferably the hydrate is preformed at a static plant as granules with a particle size between 0. 5mm and 100mm which are introduced to the tank through a valve 14 following removal of a fuel filler cap 16. The fuel may be supplied from a source 18 through a rotary valve 20 once a shut-off valve 22 has been opened.

The storage tank 12 may be formed of stainless steel or other materials capable of coping with the temperature and pressure and can take any of a variety of forms. Preferably, the tank is a twin-walled structure and may have external insulation 13. The internal wall may act as a heat

exchanger, optionally with heat exchange fluid being supplied to the space between the walls.

Alternatively, the space between the walls may act as insulation, possibly by use of a vacuum.

As illustrated, the inner wall 24 of the twin wall tank 12 is a dimple plate heat exchanger and the space between the walls is connected to a heat exchange fluid system 26 containing a first heat exchanger 28 for warming the fluid and a second heat exchanger 30 for chilling the fluid.

A suitable fluid for the heat exchange circuit 26 is a light hydrocarbon such as a high altitude jet fuel or, less advantageously due to its lower heat transfer properties, a gas heat exchange medium.

The heat exchange fluid circuit 26 comprises a return line 32 from the heat exchanger 24 in the tank 12 having a pump 34 therein. The return line 32 leads to a valve 36 for directing the heat exchange fluid through either the hot circuit heat exchanger 28 or the cold circuit heat exchanger 30 according to the pressure of natural gas in a natural gas conduit 38 leading from the tank 12.

The pressure is sensed by a pressure controller 40. Heat exchange fluid directed by the valve 36 through the hot circuit heat exchanger 28 passes via a conduit 42 to an inlet 44 to the heat exchanger 24 in the tank 12. Heat exchange fluid directed by the valve 36 through the cool circuit heat exchanger 30 in the heat exchange fluid circuit 26 is directed via a conduit 46 to the inlet 44.

The cold circuit heat exchanger 30 is connected to a chiller loop 48 which may be either a refrigerant chiller or an absorption chiller capable of providing the required cooling to the tank 12. The refrigerant load may be relatively low with suitable tank insulation and out-of-tank formation of the hydrates.

The hot circuit heat exchanger 28 is in an evaporator loop 50 in which hot fluid, usually water with additives, circulates from the engine cooling system 52. The flow of fluid around the evaporator loop 50 is controlled by a valve 54 which is itself controlled by the pressure sensor 40. Thus, when there is at least a predetermined pressure of natural gas in the natural gas conduit 38, the pressure controller 40 closes the valve 54 at the same time as the valve 36 directs the heat exchange fluid in the circuit 26 through the cold circuit heat exchanger 30 rather than through the hot circuit heat exchanger 28.

The evaporator loop 50 is also connected to a heat exchanger 56 in the natural gas conduit 38 which is adapted to heat the natural gas being supplied to the engine to a temperature that meets the requirements of the engine. Fluid flow through the loop 58 to the heat exchanger 56 is controlled by a valve 60 which is actuated by a temperature sensor 62 in the natural gas conduit 38.

The natural gas conduit 38 is connected to an inlet screen 64 in the storage tank 12 to ensure that no solid particles of hydrate leave the tank. Natural gas conduit 38 then leads to a separator 66 and filter 68 via a pressure indicator 69, to recover water from the re-gasified natural gas in the conduit 38. For applications where moist gas is not acceptable, a dehydration module or other dryer (not shown) may be fitted in series with the separator 66 and filter 68. Water recovered from the separator 66, filter 68 and optional dryer passes into a drain 70 where it may be collected for subsequent use.

The natural gas conduit 38 then leads via the heat exchanger 56 to a pressure regulator 72 to ensure that fuel supplied to the engine is at the desired pressure. Upstream of the pressure regulator 72 is a branch 74 leading to a pressure relief valve 76 which relieves pressure in the natural gas conduit 38 in the case of, for example, a failure in the chiller loop 48. Natural gas discharged through the relief valve 76 is consumed in a thermal reactor 78 to ensure that no natural gas is discharged to atmosphere.

The fuel system described above in accordance with the drawing provides natural gas from the tank 12 on demand. As the pressure in the natural gas conduit 38 drops below a predetermined level due to demand by the engine or excessive cooling of the tank 12, heat is supplied to the in-tank heat exchanger 24 via the evaporator loop 50 and hot circuit heat exchanger 28 to decompose more solid hydrate in the tank. The decomposition may be accelerated by means of, for example, an optional electrical heater or hot fluid spray (not shown) in the tank 12, also controlled by the downstream pressure, but at times of great transient demand for natural gas by the engine, even this may not be enough. For those applications where a direct contact fluid spray is used to accelerate the hydrate decomposition, it may be beneficial to drain fluid from the hydrate storage tank 12 into a fluid storage tank (not shown) and then recirculate it as a heated fluid spray into the storage tank 12.

In order to satisfactorily meet transient surges in demand for natural gas, a branch line 80 from the conduit 38 supplies pressurised natural gas to a second storage tank 82 via a one way valve 84 and compressor means 86. As illustrated, the compressor means comprises two small stage compressors 88 and 90 with an inter-cooler 92 between them. The natural gas may be stored in the second tank 82 at a pressure of about 600 psig, lower than normal CNG fuel storage systems, and because the tank 82 is only intended to supply natural gas during surges in demand, for example when the engine is accelerating quickly, the second tank may be considerably smaller than in known CNG fuel storage systems, having a volume of, for example 3 cu ft per 100HP (about 0.13m3 per 100 KW). The tank 82 may be of known construction, for example with a cylindrical shape and formed of stainless steel or epoxy lined carbon steel.

Supply of natural gas from the second tank 82 to the engine is by way of a return line 94 to the conduit 38 and under the control of a valve 96 which is opened automatically by a pressure controller 98 sensing the pressure of natural gas in the conduit 38 upstream of the return line 94.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope as defined by the appended claims.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word"comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge.