WO2024166542A1 - Gas circulation system and transportation machine - Google Patents

Abstract

Provided is a gas circulation system in which carbon dioxide gas discharged in a power station or an industrial plant, etc., is utilized for collection of natural gas without being released into the atmosphere, and in which there is no particular need for, inter alia, a liquefaction equipment for natural gas and/or carbon dioxide gas.　The gas circulation system 1 is provided with: a transportation machine 2a for transporting, from a collection facility 70 for collecting natural gas at a collection site 90 to a power station 80 at which the natural gas is utilized, the natural gas collected by the collection facility 70; and a transportation machine 2b for transporting, from the power station 80 to the collection facility 70 or a carbon-dioxide-utilizing facility, carbon dioxide gas discharged in the course of power generation at the power station 80. The transportation machine 2a transports the natural gas in a compressed state from the collection facility 70 to the power station 80. The transportation machine 2b transports the carbon dioxide gas in a compressed state from the power station 80 to the collection facility 70 or the carbon-dioxide-utilizing facility. The carbon dioxide gas transported by the transportation machine 2b is supplied as supply gas to the carbon-dioxide-utilizing facility or the collection site 90 at which the collection facility 70 carries out collection.

Description

Gas circulation systems and transport machinery

The present invention relates to a gas circulation system and transport machinery that can suppress the release of carbon dioxide gas into the atmosphere from power plants, industrial plants, etc., and can transport the gas without the need for special facilities for liquefying natural gas or carbon dioxide gas.

In thermal power plants, natural gas buried underground is extracted and liquefied to produce liquefied natural gas (LNG) and other fuels, which are burned in a combustor to generate gas, which is then used to drive a turbine to generate electricity. In the process, thermal power plants emit exhaust gases containing large amounts of carbon dioxide gas. The carbon dioxide gas contained in the exhaust gases is separated from the exhaust gases and then released into the atmosphere.
Industrial plants also use natural gas in the manufacturing process, resulting in the release of carbon dioxide gas.

However, due to concerns that carbon dioxide gas is a cause of global warming, efforts have been made in various fields in recent years to reduce the amount of carbon dioxide gas released into the atmosphere, and efforts to curb carbon dioxide gas emissions are becoming more active worldwide.

In light of this situation, a system has been proposed that uses methane gas obtained from methane hydrates present under the seabed or underground as fuel, in which carbon dioxide gas produced by the combustion of methane gas is injected into the methane hydrate layer and immobilized as carbon dioxide gas hydrate, thereby making it possible to reduce carbon dioxide emissions into the atmosphere (see Patent Document 1).

JP 2000-61293 A

In the system using methane hydrate as fuel disclosed in Patent Document 1, methane gas recovered from a mining site is liquefied at a mining base and then transported to a heat utilization system by a methane gas transport ship, and carbon dioxide gas generated in the heat utilization system is liquefied and then transported to the mining base by a carbon dioxide gas transport ship. The carbon dioxide gas transported to the mining base is heated to high temperature and pressure in a boiler and sent to a methane hydrate layer, where methane hydrate is decomposed into water and methane gas, of which only the methane gas is recovered. The recovered methane gas is transported again to the heat utilization system by a methane gas transport ship, and this series of steps is repeated.
In this way, this system sends carbon dioxide gas into the methane hydrate layer as a heat source for decomposing the methane hydrate, and causes the water produced by the decomposition of the methane hydrate to react with the carbon dioxide gas, thereby fixing it in the methane hydrate layer as carbon dioxide gas hydrate, thereby reducing the release of carbon dioxide into the atmosphere and curbing the progress of global warming.

Incidentally, natural gas has generally been liquefied and transported as liquefied natural gas when it is transported to thermal power plants, etc. When natural gas is liquefied, its density increases significantly, making it possible to transport large amounts of natural gas over long distances using a relatively small number of ships.
On the other hand, there is a problem in that it would require the installation of natural gas liquefaction facilities at mining bases and liquefied natural gas regasification facilities at thermal power plants, which would incur enormous costs.

In addition, the system disclosed in Patent Document 1 requires the installation of equipment for liquefying natural gas, etc., as well as equipment for liquefying carbon dioxide gas and equipment for gasifying carbon dioxide gas at high temperature and pressure, and considering the maintenance costs of the equipment, this is not realistic.

The present invention has been made to solve the above problems, and aims to provide a gas circulation system that can reduce the release of carbon dioxide gas emitted at power plants into the atmosphere and transport natural gas or carbon dioxide gas without the need for liquefaction equipment, and transport machinery for use in this system.

The gas circulation system according to the present invention comprises a first transport means for transporting the natural gas collected by a collection facility from the collection facility where the natural gas is collected at a collection point to a natural gas utilization facility that utilizes the natural gas, and a second transport means for transporting carbon dioxide gas emitted during the utilization process of the natural gas utilization facility from the natural gas utilization facility to the collection facility or the carbon dioxide utilization facility, the first transport means transporting the natural gas in a compressed state from the collection facility to the natural gas utilization facility, the second transport means transporting the carbon dioxide gas in a compressed state from the natural gas utilization facility to the collection facility or the carbon dioxide utilization facility, and supplying the carbon dioxide gas transported by the second transport means as a supply gas by the collection facility to the collection point or the carbon dioxide utilization facility.

In this way, in the present invention, the first transportation means transports natural gas from the extraction facility to the natural gas utilization facility, and the second transportation means transports carbon dioxide gas from the natural gas utilization facility to the extraction facility, and the carbon dioxide gas transported by the second transportation means is supplied to the extraction point by the extraction facility or as supply gas to the carbon dioxide utilization facility. This means that the carbon dioxide gas discharged from the natural gas utilization facility can be reused at the extraction facility or carbon dioxide utilization facility, and carbon dioxide gas discharged from the natural gas utilization facility is not released into the atmosphere, thereby contributing to the prevention of global warming.

In addition, because carbon dioxide gas transported from the natural gas utilization facility is supplied to the extraction point, if the extraction point is a small or medium-sized gas field, an abandoned gas field, or an oil field, the internal pressure at the extraction point that decreases as natural gas or crude oil is extracted can be compensated for by carbon dioxide gas, making it possible to improve or regenerate the production capacity of natural gas or crude oil, and to fully recover natural gas, etc.

Furthermore, compared to LNG carriers, which require measures to prevent liquid swaying during transport, natural gas and carbon dioxide gas are transported in a compressed supercritical or gaseous state, so measures to prevent swaying of the transport means are not necessary, making shipbuilding design easier and reducing transport constraints.

In the gas circulation system according to the present invention, the first transport means transports the natural gas in a supercritical state from the extraction facility to the natural gas utilization facility, and the second transport means transports the carbon dioxide gas in a supercritical state from the natural gas utilization facility to the extraction facility or the carbon dioxide utilization facility, as necessary.

In this way, in the present invention, the natural gas and carbon dioxide gas are transported in a compressed supercritical state, so that the density can be made the same as in a liquefied state, and the natural gas and carbon dioxide gas can be transported with a transport efficiency comparable to that of the liquefied state.

The gas circulation system according to the present invention is, as necessary, provided with a heat exchange means for exchanging heat between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the natural gas utilization facility by the second transport means.

In this way, in the present invention, the heat exchange means exchanges heat between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the natural gas utilization facility, so that the natural gas is cooled by the lower temperature carbon dioxide gas. This makes it possible to cool the natural gas without installing cooling equipment that requires a certain amount of energy supply, or by reducing the load on the cooling equipment, thereby significantly reducing production costs and improving the efficiency of natural gas transportation.

The gas circulation system according to the present invention is, as necessary, equipped with an evaporation means for performing heat exchange between the working fluid and the natural gas and evaporating the working fluid to obtain a gaseous working fluid, a power conversion means which is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condensation means for condensing the working fluid into a liquid phase by performing heat exchange between the gaseous working fluid discharged from the power conversion means and the carbon dioxide gas.

In this way, in the present invention, heat is exchanged between the working fluid and natural gas or carbon dioxide gas, and a power conversion means driven by the gas phase working fluid is provided between the evaporation means and the condensation means. Therefore, by using the flow of the gas phase working fluid to generate power using the power obtained by the power conversion means, it is possible to supply electricity to the collection facility and the equipment of the first and second transportation means, etc., and production costs can be reduced.

The gas circulation system according to the present invention is provided, as necessary, with a heat exchange means for exchanging heat between the carbon dioxide gas generated in the natural gas utilization facility and the natural gas transported from the extraction facility by the first transport means.

In this way, in the present invention, the heat exchange means exchanges heat between the carbon dioxide gas generated at the natural gas utilization facility and the natural gas transported from the extraction facility, so that the carbon dioxide gas is cooled by the lower temperature natural gas. This makes it possible to cool the carbon dioxide gas without installing cooling equipment that requires a certain amount of energy supply, or by reducing the load on the cooling equipment, thereby making it possible to transport the carbon dioxide gas more efficient and at lower cost.

The gas circulation system according to the present invention is, as necessary, equipped with an evaporation means for performing heat exchange between the working fluid and the carbon dioxide gas and evaporating the working fluid to obtain a gaseous working fluid, a power conversion means which is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condensation means for performing heat exchange between the gaseous working fluid discharged from the power conversion means and the natural gas to condense it into a liquid phase.

In this way, in the present invention, heat is exchanged between the working fluid and natural gas or carbon dioxide gas, and a power conversion means driven by the gas-phase working fluid is provided between the evaporation means and the condensation means. Therefore, by generating electricity using the power obtained by the power conversion means by utilizing the flow of the gas-phase working fluid, it becomes possible to supply electricity to natural gas utilization facilities and the equipment of the first and second transportation means, and production costs can be reduced.

In the gas circulation system according to the present invention, the first transport means or the second transport means transports the natural gas and/or the carbon dioxide gas in a pressure vessel formed of at least fiber-reinforced plastic, as necessary.

In this way, in the present invention, natural gas and/or carbon dioxide gas is stored in a pressure vessel formed at least of fiber-reinforced plastic and transported by the first transport means or the second transport means, so that the gas can be stored at a higher pressure than conventionally, and natural gas and carbon dioxide gas can be transported with a high efficiency comparable to that of conventional liquefied natural gas transportation.

In addition, these pressure vessels are a fraction of the weight of conventional steel vessels, which allows for significant weight reduction in transport equipment, contributing to reduced fuel consumption, shorter transport times, and longer transport distances.

The transport machine of the present invention transports natural gas extracted at a extraction facility in a compressed state from the extraction facility to a natural gas utilization facility, and/or transports carbon dioxide gas emitted during the utilization process of the natural gas utilization facility in a compressed state from the natural gas utilization facility to the extraction facility or carbon dioxide utilization facility.

The transport machine according to the present invention transports the natural gas extracted at the extraction facility in a supercritical state from the extraction facility to the natural gas utilization facility, and/or transports the carbon dioxide gas emitted during the utilization process of the natural gas utilization facility in a supercritical state from the natural gas utilization facility to the extraction facility or the carbon dioxide utilization facility, as necessary.

The transport machine according to the present invention is provided with a heat exchange section for exchanging heat between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the natural gas utilization facility, as necessary.

The transport machine according to the present invention is equipped, as necessary, with an evaporator in which the heat exchange section exchanges heat between the working fluid and the natural gas and evaporates the working fluid to obtain a gaseous working fluid, a turbine that is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condenser that condenses the working fluid into a liquid phase by exchanging heat between the gaseous working fluid discharged from the turbine and the carbon dioxide gas.

The transport machine according to the present invention is provided with a heat exchange section for exchanging heat between the carbon dioxide gas generated at the natural gas utilization facility and the natural gas transported from the extraction facility, as necessary.

The transport machine according to the present invention is equipped, as necessary, with an evaporator in which the heat exchange section exchanges heat between the working fluid and the carbon dioxide gas to evaporate the working fluid and obtain a gaseous working fluid, a turbine that is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condenser that exchanges heat between the gaseous working fluid discharged from the turbine and the natural gas to condense it into a liquid phase.

The transport machine according to the present invention transports the natural gas and/or carbon dioxide gas, as necessary, by storing them in a pressure vessel made of at least fiber-reinforced plastic.

1 is a schematic diagram of a gas circulation system according to a first embodiment of the present invention. FIG. 2 is a schematic diagram of a collection facility in the gas circulation system according to the first embodiment of the present invention. 1 is a schematic diagram of a power plant in a gas circulation system according to a first embodiment of the present invention. FIG. 4 is a schematic diagram of a gas circulation system according to a second embodiment of the present invention. FIG. 5 is a schematic diagram of a collection facility in a gas circulation system according to a second embodiment of the present invention. FIG. 5 is a schematic diagram of a power plant in a gas circulation system according to a second embodiment of the present invention. FIG. 5 is a schematic diagram of a transport machine in a gas circulation system according to a second embodiment of the present invention. FIG. 5 is a schematic diagram of a gas circulation system according to a third embodiment of the present invention. FIG. 11 is a schematic diagram of a transport machine in a gas circulation system according to a third embodiment of the present invention. FIG. 13 is a schematic diagram of a collection facility in a gas circulation system according to a fourth embodiment of the present invention. FIG. 11 is a schematic diagram of a power plant in a gas circulation system according to a fourth embodiment of the present invention. FIG. 13 is a schematic diagram of a collection facility in a gas circulation system according to a fifth embodiment of the present invention. FIG. 13 is a schematic diagram of a power plant in a gas circulation system according to a fifth embodiment of the present invention.

(First embodiment of the present invention)
A gas circulation system according to a first embodiment of the present invention will be described below with reference to Figures 1 to 3. Throughout this embodiment, the same elements are designated by the same reference numerals.

The gas circulation system 1 according to this embodiment includes a transport machine 2a as the first transport means for transporting natural gas collected by a collection facility 70, which collects natural gas at a collection point 90 such as an unused small-to-medium-sized gas field or an abandoned gas field, to a natural gas utilization facility that utilizes the natural gas, and a transport machine 2b as the second transport means for transporting carbon dioxide gas emitted during the utilization process of the natural gas utilization facility from the natural gas utilization facility to the collection facility 70 or the carbon dioxide utilization facility.

The collection facility 70 may also be a facility for collecting natural gas from associated gas or flare gas generated in an oil field serving as a collection site.
Furthermore, the collection facility 70 may be installed either at sea or on land.

The natural gas utilization facility is a power plant 80 that generates electricity using natural gas, or may be an industrial plant that uses natural gas and emits carbon dioxide gas. In the following, the power plant 80 will be described as an example of the natural gas utilization facility.
If the power plant 80 and the _CO2 gas recovery unit 82 described below cannot be constructed on land, they can be installed on a ship or barge, and the ship or barge can be moored at sea.

The carbon dioxide gas generated at the power plant 80 may be transported by the transport machine 2b to the collection facility 70 or to other collection facilities serving as carbon dioxide utilization facilities.
The other extraction facilities, like the extraction facility 70, may be facilities that currently extract natural gas, or may be facilities that have been used as natural gas extraction facilities in the past.
The carbon dioxide gas transported by the transport machine 2b to the extraction facility 70 or another extraction facility is supplied as a supply gas. Specifically, the supply gas is gas for injection into the extraction site 90 by the extraction facility 70 or into an extraction site by another extraction facility, or gas for underground storage at an extraction site by another extraction facility. The supply gas may also be an industrial raw material used in another carbon dioxide utilization facility.

The transport machine 2a transports the natural gas in a compressed state from the extraction facility 70 to the power plant 80, and the transport machine 2b transports the carbon dioxide gas in a compressed state from the power plant 80 to the extraction facility 70.
When natural gas and carbon dioxide gas are transported by sea, the transport machines 2a and 2b are ships such as tankers. The transport machines 2a and 2b may be transport vehicles, airplanes, etc., in addition to ships.

The extraction facility 70 mainly comprises a natural gas extraction unit 72 that extracts natural gas from the extraction site 90 through a production well 71, a natural gas refining unit 73 that removes acid gas and moisture contained in the extracted natural gas, and a natural gas compression unit 74 that compresses the extracted natural gas.
The collection facility 70 also includes a _CO2 gas injection section 75 that injects carbon dioxide gas transported from the power plant 80 by the transport machine 2b into the collection site 90 via an injection well 76.

The natural gas collection unit 72 pumps natural gas from the collection point 90 to the collection facility 70 using pressure or a pumping pump via the production well 71 that has reached the collection point 90.

The natural gas pumped up by the natural gas collection section 72 is sent to the natural gas purification section 73. The natural gas sent to the natural gas purification section 73 has acidic gases such as carbon dioxide gas and hydrogen sulfide and moisture removed. The devices for removing acidic gas and moisture are well known in the art, so details thereof will be omitted.

The refined natural gas is compressed to a predetermined pressure in the natural gas compression section 74, and the compressed natural gas is stored in an empty pressure vessel 3a loaded onto the transportation machine 2a through a distribution pipe.
The compressed natural gas may be stored in a pressure vessel 3a at the extraction facility 70 and then loaded onto the transport machine 2a.

The natural gas compressed in the natural gas compression section 74 is cooled appropriately or kept at an appropriate temperature depending on the transport temperature.

On the other hand, carbon dioxide gas transported from the power plant 80 by the transport machine 2b and stored in the pressure vessel 3b is sent from the transport machine 2b to the _CO2 gas injection section 75 through a distribution pipe, and is injected into the collection point 90 through the injection well 76 by means of self-pressure or an injection pump.

The materials constituting the pressure vessels 3a, 3b that contain the compressed natural gas and compressed carbon dioxide gas are not particularly limited as long as they have sufficient pressure resistance and corrosion resistance against the corrosive impurities contained in natural gas and carbon dioxide gas, but examples include fiber-reinforced plastics such as carbon fiber reinforced plastic (hereinafter also referred to as CFRP) and glass fiber reinforced plastic.

For example, CFRP containers can be made compact through a high-pressure resistant design that takes advantage of the fiber strength of CFRP. For example, a 300 L container with a design pressure of 100 MPa can have an outer diameter of 560 mm and a length of 2,865 mm. This makes it possible for the CFRP container to transport compressed natural gas and compressed carbon dioxide gas using transport machines 2a and 2b.

The weight of a fiber-reinforced plastic container is about one-tenth of that of an iron container having the same pressure resistance, and the fiber-reinforced plastic container is also excellent in fuel efficiency in the transport machines 2a, 2b.
Furthermore, fiber-reinforced plastic containers are resistant to corrosion caused by water, hydrogen sulfide, carbon dioxide gas, etc., and are airtight enough to prevent hydrogen molecules from passing through.

As described above, natural gas and/or carbon dioxide gas is stored in pressure vessels 3a, 3b made of at least fiber-reinforced plastic and transported by transport machine 2a or transport machine 2b, so the gas can be stored at a higher pressure than in the past, and natural gas and carbon dioxide gas can be transported with a high efficiency comparable to the conventional transport of liquefied natural gas.

In addition, these pressure vessels 3a, 3b are a fraction of the weight of conventional steel vessels, which allows for a significant reduction in the weight of the transport machinery 2a, 2b, contributing to reduced fuel consumption, shorter transport times, and longer transport distances.

Furthermore, natural gas and carbon dioxide gas can be kept airtight at room temperature in a high-pressure supercritical state during transportation, eliminating the need for measures to prevent boil-off (loss of gas due to evaporation), which is an issue when transporting liquefied natural gas.

Table 1 below shows the transport efficiency of methane gas, the main component of natural gas, depending on the pressure and temperature. Table 1 also shows the transport efficiency of methane gas compressed at 20 MPa, which is a pressure that can be withstood by conventional iron containers.
Here, the transport efficiency is the ratio of the density of compressed methane gas to the density of liquefied methane (temperature -162°C, pressure 0.1 MPa (atmospheric pressure)) under conditions of a temperature of 25°C or -30°C (= (density of compressed methane gas/density of liquefied methane) x 100 (%)).

Table 2 below shows the transport efficiency according to the pressure and temperature of carbon dioxide gas. Table 2 also shows the transport efficiency of carbon dioxide gas compressed at 20 MPa, which is a pressure that a conventional iron container can withstand.
Here, the transport efficiency is the ratio of the density of compressed carbon dioxide gas to the density of liquefied carbon dioxide (temperature -20°C, pressure 2 MPa) under conditions of a temperature of 31.1°C (critical temperature) or 0°C (= (density of compressed carbon dioxide gas/density of liquefied carbon dioxide) x 100 (%)).

As can be seen from Table 1, the transport efficiency of methane gas compressed at 20 MPa, which is the pressure that conventional iron containers can withstand, is 37%, which is far below the transport capacity of liquefied methane. In contrast, the transport efficiency of methane gas compressed at 50 MPa exceeds 60%, and it is found to have nearly double the transport capacity compared to when compressed at 20 MPa.
In addition, the transport efficiency of methane gas compressed at 100 MPa exceeds 80%, and even at low temperatures (-30°C), the transport efficiency exceeds 90%, demonstrating a transport capacity comparable to that of liquefied methane.

As can also be seen from Table 2, at 0°C and 31.1°C (critical temperature), the transport efficiency of carbon dioxide gas compressed to 20 MPa and 50 MPa, which are pressures that conventional iron containers can withstand, exceeds 85%, and in particular, the transport efficiency at 50 MPa and 0°C is 107%, demonstrating that it has a transport capacity comparable to that of liquefied carbon dioxide.

Here, we consider the volume of natural gas and carbon dioxide gas transported.

The chemical reaction equation for methane combustion is given below:
_CH4 + _2O2 → _2H2O + _CO2
As shown in the above formula, 1 mol (44 g) of carbon dioxide gas is produced from 1 mol (16 g) of methane gas, resulting in 2.75 times the amount of carbon dioxide gas produced by mass.

For example, when methane gas (natural gas) is transported at 70 MPa and -3°C (density 323 kg/ ^m3 ) by transport machine 2a to a natural gas utilization facility (power plant 80) for utilization, and the discharged carbon dioxide gas is transported to a collection facility 70 or carbon dioxide utilization facility by transport machine 2b having the same transportation capacity, the carbon dioxide gas needs to be compressed to a density of 888.3 kg/ ^m3 (= 323 kg/ ^m3 × 44 g/16 g).

Carbon dioxide gas has a density of 880 kg/ ^m3 at 20 MPa and 31.1°C (supercritical state), and when the pressure is increased to 30 MPa, the density increases to 900 kg/ ^m3 at 40°C, 990 kg/ ^m3 at 20°C, and 1110 kg/ ^m3 at -20°C.
For this reason, when transporting methane gas (natural gas) and carbon dioxide gas using transportation machinery with the same transportation capacity, by transporting methane gas (natural gas) in a supercritical state of 70 MPa and -3°C on the outbound journey and transporting carbon dioxide gas in a supercritical state of 30 MPa and 40°C or less on the return journey, it becomes possible to transport natural gas from the extraction facility 70 to the natural gas utilization facility (power plant 80) with a transportation efficiency of approximately 80% compared to liquefied methane, and to transport all of the carbon dioxide gas produced by the utilization of the transported natural gas to the extraction facility 70 or the carbon dioxide utilization facility.

As described above, natural gas and carbon dioxide gas can be transported in a compressed supercritical state, making it possible to achieve a density similar to that of a liquefied state, and natural gas and carbon dioxide gas can be transported with a transport efficiency comparable to that of a liquefied state.

The power plant 80 mainly comprises a gas turbine 81 fueled by natural gas, a _CO2 gas recovery unit 82 that recovers carbon dioxide gas from the combustion gas discharged from the gas turbine 81, and a _CO2 gas compression unit 83 that compresses the carbon dioxide gas recovered in the _CO2 gas recovery unit 82.

The gas turbine 81 includes a combustor 81a that burns natural gas as fuel, an air compressor 81b that compresses and supplies air to the combustor 81a, and a turbine 81c that is driven by the combustion gas generated in the combustor 81a.

The combustor 81a combusts natural gas supplied via a delivery pipe from a pressure vessel 3a transported from the extraction facility 70 by a transport machine 2a with air compressed by an air compressor 81b through a reaction between the gas and the air. The combustion gas generated by the combustor 81a is sent to a turbine 81c.

The turbine 81c is driven by the combustion gas supplied from the combustor 81a. The turbine 81c rotates with the combustion gas to generate rotational kinetic energy, which is then converted by the generator 84 into electric power, generating electric power.
The combustion gas that has passed through the turbine 81c is sent to a _CO2 gas recovery section 82.

In the above, the power plant 80 is described as using a gas turbine 81 to generate electricity, but the power plant 80 may use other power generation methods.

The combustion gas sent to the _CO2 gas recovery section 82 has carbon dioxide gas recovered by a conventionally known method, for example, a chemical absorption method, and the carbon dioxide gas vaporized by heating is sent to a _CO2 gas compression section 83.
The combustion gas from which the carbon dioxide gas has been recovered is released into the atmosphere.

The carbon dioxide gas supplied to the _CO2 gas compression section 83 is compressed to a predetermined pressure, and this compressed carbon dioxide gas is stored in an empty pressure vessel 3b loaded onto the transport machine 2b through a delivery pipe.
The compressed carbon dioxide gas may be stored in a pressure vessel 3b at the power plant 80 and then loaded onto the transport machine 2b.

The carbon dioxide gas compressed in the _CO2 gas compression section 83 is appropriately cooled or kept at an appropriate temperature depending on the transportation temperature.

The electricity generated by the generator 84 is transmitted to various locations via a power transmission cable (not shown).

In this way, in the gas circulation system 1 according to this embodiment, the transport machine 2a transports natural gas from the extraction facility 70 to the power plant 80, and the transport machine 2b transports carbon dioxide gas from the power plant 80 to the extraction facility 70, and the carbon dioxide gas transported by the transport machine 2b is supplied to the extraction point 90 by the extraction facility 70 or as supply gas to the carbon dioxide utilization facility. This means that the carbon dioxide gas discharged from the power plant 80 can be reused at the extraction facility 70 or the carbon dioxide utilization facility, and carbon dioxide gas discharged from the power plant 80 is not released into the atmosphere, thereby contributing to the prevention of global warming.

In addition, because the carbon dioxide gas transported from the power plant 80 is supplied to the collection site 90, if the collection site 90 is a small or medium-sized gas field, an abandoned gas field, or an oil field, the internal pressure at the collection site 90 that decreases as natural gas or crude oil is collected can be compensated for by the carbon dioxide gas, making it possible to improve or regenerate the production capacity of natural gas or crude oil, and to fully recover natural gas, etc.

In addition, compared to LNG transport ships and the like, which require measures to prevent liquid swaying during transport, natural gas and carbon dioxide gas are transported in a compressed supercritical or gaseous state, so measures to prevent swaying of the transport machinery 2a and 2b are not necessary, making shipbuilding design easier and reducing transport constraints.

Furthermore, in the case of liquefied natural gas, a large amount of energy is consumed during the production and transportation process because liquids at an extremely low temperature of -162 degrees are handled, which means that carbon dioxide emissions are unavoidable. However, because natural gas is transported in a compressed supercritical or gaseous state, the generation of carbon dioxide can be reduced.

In the above, the transportation of natural gas is described as being performed by the transport machine 2a, and the transportation of carbon dioxide gas is described as being performed by the transport machine 2b. However, a single transport machine (e.g., transport machine 2a) may travel between the extraction facility 70 or another extraction facility and the power plant 80 to transport natural gas and carbon dioxide gas.
In this case, in the collection facility 70, after the transport machine 2a supplies carbon dioxide gas to the _CO2 gas injection section 75, natural gas is sequentially stored in the emptied pressure vessel 3a from the natural gas compression section 74. In the power plant 80, after the transport machine 2a supplies natural gas to the gas turbine 81, carbon dioxide gas is sequentially stored in the emptied pressure vessel 3a from the _CO2 gas compression section 83.

Second Embodiment of the Invention
In the gas circulation system according to the first embodiment described above, the natural gas generated at the collection facility and the carbon dioxide gas generated at the power plant are simply transported by transport machinery to the power plant and the collection facility, respectively. However, without being limited to this, as shown in Figures 4 to 6, the natural gas collected at the collection facility can be heat exchanged with the carbon dioxide gas transported from the power plant, and the carbon dioxide gas generated at the power plant can be heat exchanged with the natural gas transported from the collection facility.
In the following description, the description of the configuration that overlaps with the first embodiment will be omitted.

The gas circulation system 1 according to this embodiment includes a heat exchange unit 4 as heat exchange means that exchanges heat between the natural gas collected by the collection facility 70 and the carbon dioxide gas transported from the power plant 80 by the transport machine 2b, and a heat exchange unit 5 as heat exchange means that exchanges heat between the carbon dioxide gas generated at the power plant 80 and the natural gas transported from the collection facility 70 by the transport machine 2a.

The heat exchange unit 4 includes a first heat exchanger 40 that exchanges heat between the heat medium and natural gas, a second heat exchanger 41 that exchanges heat between the heat medium and carbon dioxide gas, and a pump 42 that sends the heat medium discharged from the second heat exchanger 41 to the first heat exchanger 40.

The natural gas collected from the collection point 90 is compressed through the natural gas compression section 74 and then sent to the first heat exchanger 40, where it is heat exchanged with the heat medium introduced into the first heat exchanger 40. The natural gas that has been heat exchanged with the heat medium and has a lower temperature is stored in the pressure vessel 3a of the transportation machine 2a through a delivery pipe.
On the other hand, the heat medium that has been heat exchanged with the natural gas is sent to the second heat exchanger 41 .

The second heat exchanger 41 is supplied with carbon dioxide gas transported from the power plant 80 by the transport machine 2b.
The heat medium that has been heat exchanged with the natural gas is introduced into the second heat exchanger 41 and is subjected to heat exchange with lower temperature carbon dioxide gas.
The heat-exchanged carbon dioxide gas is discharged from the second heat exchanger 41 and sent to the _CO2 gas injection section 75, and is injected into the collection point 90 via the injection well 76.

The heat exchange unit 5 includes a first heat exchanger 50 that exchanges heat between the heat medium and carbon dioxide gas, a second heat exchanger 51 that exchanges heat between the heat medium and natural gas, and a pump 52 that sends the heat medium discharged from the second heat exchanger 51 to the first heat exchanger 50.

The carbon dioxide gas compressed in the _CO2 gas compression section 83 is sent to the first heat exchanger 50, and is heat exchanged with the heat medium introduced into the first heat exchanger 50. The carbon dioxide gas, which has been heat exchanged with the heat medium and has a lower temperature, is accommodated in the pressure vessel 3b of the transport machine 2b via a delivery pipe.
On the other hand, the heat medium that has been exchanged with the carbon dioxide gas is sent to the second heat exchanger 51 .

The second heat exchanger 51 is supplied with natural gas transported from the extraction facility 70 by the transport machine 2a.
The heat medium that has exchanged heat with the carbon dioxide gas is introduced into the second heat exchanger 51 and is exchanged heat with lower temperature natural gas.
The heat-exchanged natural gas is discharged from the second heat exchanger 51 and sent to the gas turbine 81 .

If the cooling of the natural gas in heat exchange section 4 and the cooling of the carbon dioxide gas in heat exchange section 5 are not sufficient, the natural gas and carbon dioxide gas may be further cooled according to the transport temperature after the heat exchange between them.

In addition, heat exchange between the natural gas and the carbon dioxide gas may be performed directly, without the use of a heat transfer medium, via a heat transfer medium such as metal.

In this way, in the gas circulation system 1 according to this embodiment, the heat exchange section 4 exchanges heat between the natural gas extracted at the extraction facility 70 and the carbon dioxide gas transported from the power plant 80, so that the natural gas is cooled by the lower temperature carbon dioxide gas. This makes it possible to cool the natural gas without installing cooling equipment that requires a certain amount of energy supply, thereby significantly reducing production costs and improving the transport efficiency of the natural gas.

In addition, because the heat exchange unit 5 exchanges heat between the carbon dioxide gas generated at the power plant 80 and the natural gas transported from the collection facility 70, the carbon dioxide gas is cooled by the lower temperature natural gas, and the carbon dioxide gas can be cooled without installing cooling equipment that requires a certain amount of energy supply, making the transportation of carbon dioxide gas more efficient and less costly.

The heat exchange units 4 and 5 may be installed in the extraction facility 70 and the power plant 80, respectively, or may be installed in the transport machines 2a and 2b.
For example, when natural gas and carbon dioxide gas are transported back and forth between the extraction facility 70 and the power plant 80 using only the transport machine 2a, the heat exchange unit 4 installed in the transport machine 2a can also be used as a heat exchange unit in the extraction facility 70 and the power plant 80.

Specifically, in the collection facility 70, as shown in Fig. 7(a), the carbon dioxide gas stored in the pressure vessel 3a is sent to the heat exchanger 4 in the transport machine 2a and heat-exchanged with the natural gas collected in the collection facility 70, and then sent to the _CO2 gas injection section 75 via the connection section 61. The natural gas compressed through the natural gas compression section 74 is sent to the heat exchanger 4 in the transport machine 2a via the connection section 60 and heat-exchanged with the carbon dioxide gas transported from the power plant 80 by the transport machine 2a, and then stored in the pressure vessel 3a, which has been emptied by discharging the carbon dioxide gas.
In the power plant 80, as shown in Fig. 7(b), the natural gas stored in the pressure vessel 3a is sent to the heat exchanger 4 in the transport machine 2a, where it is heat exchanged with the carbon dioxide gas generated in the power plant 80, and then sent to the gas turbine 81 via the connection part 62. The carbon dioxide gas compressed through the _CO2 gas compression part 83 is sent to the heat exchanger 4 in the transport machine 2a via the connection part 63, where it is heat exchanged with the natural gas transported from the extraction facility 70 by the transport machine 2a, and then stored in the pressure vessel 3a, which has been emptied by discharging the natural gas.

In addition, if the cooling of the natural gas in heat exchange section 4 and the cooling of the carbon dioxide gas in heat exchange section 5 are not sufficient, the natural gas and carbon dioxide gas may be further cooled according to the transport temperature after the heat exchange between them.

Third embodiment of the present invention
In the gas circulation system according to the second embodiment described above, heat is exchanged between the natural gas collected at the collection facility and the carbon dioxide gas transported from the power plant, and also between the carbon dioxide gas generated at the power plant and the natural gas transported from the collection facility. However, the present invention is not limited to this, and as shown in FIG. 8, the compressed natural gas and compressed carbon dioxide gas can be introduced into an expander to lower the temperature before the heat exchange.
In the following description, the description of configurations that overlap with the above-described embodiments will be omitted.

The gas circulation system 1 according to this embodiment includes an expander 6 between the transport machine 2b, which transports carbon dioxide gas generated at the power plant 80 to the collection facility 70, and the heat exchange unit 4, and an expander 7 between the transport machine 2a, which transports natural gas collected at the collection facility 70 to the power plant 80, and the heat exchange unit 5.

At the collection facility 70, the natural gas collected at the collection facility 70 is compressed in the natural gas compression section 74, becoming a higher temperature gas and being introduced into the heat exchange section 4, and at the same time, the carbon dioxide gas transported from the power plant 80 is depressurized by the expander 6, becoming a lower temperature gas (Joule-Thomson effect), and being introduced into the heat exchange section 4.
In the expander 6, the pressure energy of the compressed carbon dioxide gas is converted into rotational energy when the carbon dioxide gas is decompressed. This rotational energy is utilized to generate electricity in a generator (not shown).

On the other hand, in the power plant 80, the carbon dioxide gas generated in the power plant 80 is compressed in the _CO2 gas compression section 83, becoming a higher temperature gas and being introduced into the heat exchange section 5, and the natural gas transported from the extraction facility 70 is decompressed by the expander 7, becoming a lower temperature gas (Joule-Thomson effect), and being introduced into the heat exchange section 5.
In the expander 7, similarly to the expander 6, the pressure energy of the compressed natural gas is converted into rotational energy when the natural gas is decompressed. This rotational energy is utilized to generate electricity in a generator (not shown).

In addition, the expansion devices 6 and 7 may be expansion valves that simply reduce the temperature by reducing the pressure.

Here, an example of the pressure and temperature of the natural gas and carbon dioxide gas in the main part of the gas circulation system 1 according to this embodiment is shown.
As a premise, it is assumed that the natural gas is transported by the transport machine 2a under conditions of 70 MPa and -3°C, and the carbon dioxide gas is transported by the transport machine 2b under conditions of 30 MPa and 33°C.

At the extraction facility 70, the natural gas passes through a natural gas compression section 74 and is supplied to the heat exchange section 4 at 70 MPa and 60°C. The natural gas introduced into the heat exchange section 4 is heat exchanged with carbon dioxide gas transported from the power plant 80 by the transport machine 2b, and is cooled to 70 MPa and 40°C. It is further cooled in a cooler (not shown) to 70 MPa and -3°C, stored in the pressure vessel 3a, and transported to the power plant 80 by the transport machine 2a.
On the other hand, carbon dioxide gas transported from the power plant 80 by the transport machine 2b at 30 MPa and 33°C passes through an expansion valve (expander 6) to be heated to 10 MPa and 25°C, and is supplied to the heat exchange section 4. The carbon dioxide gas introduced into the heat exchange section 4 is heat exchanged with natural gas to be heated to 10 MPa and 38°C, and is supplied to the _CO2 gas injection section 75.

On the power plant 80 side, the carbon dioxide gas is supplied to the heat exchanger 5 at 30 MPa and 72°C through the _CO2 gas compressor 83. The carbon dioxide gas introduced into the heat exchanger 5 is heat exchanged with the natural gas transported from the extraction facility 70 by the transport machine 2a to be heated to 30 MPa and 33°C, stored in the pressure vessel 3b, and then transported to the extraction facility 70 by the transport machine 2b.
On the other hand, the natural gas transported from the extraction facility 70 by the transport machine 2a at 70 MPa and -3°C is passed through an expansion valve (expander 7) to be heated to 10 MPa and -30°C, and is supplied to the heat exchanger 5. The natural gas introduced into the heat exchanger 5 is heat exchanged with carbon dioxide gas to be heated to 10 MPa and 27°C, and is supplied to the gas turbine 81.

In this way, by providing expanders 6 and 7 that reduce the pressure of the natural gas and carbon dioxide gas transported by transport machines 2a and 2b, the efficiency of heat exchange can be improved by the temperature drop that accompanies the reduction in pressure. In addition, the pressure energy of the compressed natural gas and carbon dioxide gas during the reduction in pressure can be used to obtain power for power generation. This makes it possible to generate power and supply it to various facilities, thereby reducing production costs.

The expanders 6 and 7 may be installed together with the heat exchangers 4 and 5 in the extraction facility 70 and the power plant 80, respectively, or may be installed in the transport machines 2a and 2b.

Furthermore, the natural gas compression unit 74 and the _CO2 gas compression unit 83 may not be installed in the extraction facility 70 and the power plant 80, but may be installed in the transport machines 2a and 2b together with the heat exchange units 4 and 5 and the expanders 6 and 7.
For example, when natural gas and carbon dioxide gas are transported back and forth between the extraction facility 70 and the power plant 80 using only the transport machine 2a, the gas compression unit, heat exchange unit 4, and expander 6 installed in the transport machine 2a can also be used in combination as the gas compression unit, heat exchange unit, and expander in the extraction facility 70 and the power plant 80.

Specifically, in the collection facility 70, as shown in Fig. 9(a), the carbon dioxide gas stored in the pressure vessel 3a is sent to the expander 6 in the transport machine 2a to be decompressed, then introduced into the heat exchanger 4, where it is heat exchanged with the natural gas collected in the collection facility 70, and sent to the _CO2 gas injection section 75 via the connection section 61. The natural gas refined through the natural gas refining section 73 is sent to the gas compression section 8 in the transport machine 2a via the connection section 60 to be pressurized, then introduced into the heat exchanger 4, where it is heat exchanged with the carbon dioxide gas transported from the power plant 80, and stored in the pressure vessel 3a that has been emptied after discharging the carbon dioxide gas.
In the power plant 80, as shown in Fig. 9(b), the natural gas stored in the pressure vessel 3a is sent to the expander 6 in the transport machine 2a to be decompressed, then introduced into the heat exchanger 4, where it is heat exchanged with the carbon dioxide gas generated in the power plant 80, and sent to the gas turbine 81 via the connection part 62. The carbon dioxide gas recovered via the _CO2 gas recovery part 82 is sent to the gas compression part 8 in the transport machine 2a via the connection part 63 to be pressurized, then introduced into the heat exchanger 4, where it is heat exchanged with the natural gas transported from the extraction facility 70, and stored in the pressure vessel 3a, which has been emptied after the natural gas has been discharged.

(Fourth embodiment of the present invention)
In the gas circulation system according to the second embodiment, heat is exchanged between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the power plant, and the carbon dioxide gas generated at the power plant and the natural gas transported from the extraction facility. However, without being limited to this, as shown in Figures 10 and 11, in the process of heat exchange between the natural gas and the carbon dioxide gas, the working fluid alternates between the gas phase and the liquid phase, and this can be utilized to drive a turbine and generate electricity.
In the following description, the description of configurations that overlap with the above-described embodiments will be omitted.

The gas circulation system 1 according to this embodiment includes a heat exchange unit 4 that performs heat exchange between the natural gas collected by the collection facility 70 and the carbon dioxide gas transported from the power plant 80 by the transport machine 2b, and a heat exchange unit 5 that performs heat exchange between the carbon dioxide gas generated at the power plant 80 and the natural gas transported from the collection facility 70 by the transport machine 2a.

The heat exchange unit 4 includes an evaporator 43 as the evaporation means for performing heat exchange between a working fluid consisting of a low boiling point medium and natural gas, and evaporating the working fluid to obtain a gas-phase working fluid, a turbine 44 as the power conversion means that is operated by receiving the gas-phase working fluid and converts the thermal energy contained in the working fluid into power, a condenser 45 as the condensation means that condenses the working fluid into a liquid phase by performing heat exchange between the gas-phase working fluid discharged from the turbine 44 and carbon dioxide gas, and a pump 46 that sends the liquid-phase working fluid discharged from the condenser 45 to the evaporator 43.
Of these, the evaporator 43, the turbine 44, the condenser 45 and the pump 46 are well-known devices similar to those used in a general power cycle, and therefore a description thereof will be omitted.

The natural gas collected from the collection point 90 is compressed through the natural gas compression section 74 and then sent to the evaporator 43, where it is heat exchanged with the liquid-phase working fluid introduced into the evaporator 43. The natural gas, which has been heat exchanged with the liquid-phase working fluid and has a lower temperature, is stored in the pressure vessel 3a of the transportation machine 2a through a delivery pipe.
On the other hand, at least a portion of the liquid-phase working fluid that has been heat exchanged with the natural gas is converted to a gas phase and sent to the turbine 44 .

The turbine 44 is rotated by the gas-phase working fluid supplied from the evaporator 43, and generates rotational kinetic energy. The generator 47 converts this rotational kinetic energy into electric power, thereby generating electricity.
The gas-phase working fluid that has passed through the turbine 44 is supplied to a condenser 45 .

The condenser 45 is supplied with carbon dioxide gas transported from the power plant 80 by the transport machine 2b.
The gas-phase working fluid discharged from the turbine 44 is introduced into the condenser 45 and undergoes heat exchange with the lower temperature carbon dioxide gas, thereby becoming a liquid-phase working fluid.
The heat-exchanged carbon dioxide gas is discharged from the condenser 45 and sent to a _CO2 gas injection section 75, and is injected into a collection point 90 via an injection well 76.

The heat exchange unit 5 includes an evaporator 53 as the evaporation means for performing heat exchange between a working fluid consisting of a low boiling point medium and carbon dioxide gas, evaporating the working fluid to obtain a gas-phase working fluid, a turbine 54 as the power conversion means which is operated by receiving the gas-phase working fluid and converts the thermal energy contained in the working fluid into power, a condenser 55 as the condensation means which condenses the gas-phase working fluid discharged from the turbine 54 into liquid phase by heat exchange between the gas-phase working fluid and natural gas, and a pump 56 which sends the liquid-phase working fluid discharged from the condenser 55 to the evaporator 53.

The carbon dioxide gas compressed in the _CO2 gas compression unit 83 is sent to the evaporator 53 and is heat exchanged with the liquid-phase working fluid introduced into the evaporator 53. The carbon dioxide gas, which has been heat exchanged with the liquid-phase working fluid and has a lower temperature, is accommodated in the pressure vessel 3b of the transport machine 2b via a delivery pipe.
On the other hand, at least a part of the working fluid that has exchanged heat with the carbon dioxide gas is turned into a gas phase and sent to the turbine 54 .

The turbine 54 is rotated by the gas-phase working fluid supplied from the evaporator 53, and generates rotational kinetic energy. The generator 57 converts this rotational kinetic energy into electric power, thereby generating electricity.
The gas-phase working fluid that has passed through the turbine 54 is supplied to a condenser 55 .

The condenser 55 is supplied with natural gas transported from the extraction facility 70 by the transport machine 2a.
The gaseous working fluid leaving the turbine 54 is introduced into the condenser 55 where it is subjected to heat exchange with lower temperature natural gas, thereby becoming a liquid-phase working fluid.
The heat-exchanged natural gas is discharged from the condenser 55 and sent to a gas turbine 81 .

The heat exchange units 4 and 5 according to this embodiment can be used in combination with the expanders 6 and 7 according to the third embodiment described above. By combining the expanders 6 and 7, more power can be extracted and electricity can be generated, which is expected to further reduce production costs.

In this way, the gas circulation system 1 according to this embodiment exchanges heat between the working fluid and natural gas or carbon dioxide gas, and has a turbine 44 driven by gas-phase working fluid between the evaporator 43 and the condenser 45. Therefore, by using the flow of the gas-phase working fluid to generate power using the power obtained by the turbine 44, it is possible to supply electricity to the collection facility 70 and the transport machines 2a and 2b, etc., thereby reducing production costs.

In addition, heat is exchanged between the working fluid and natural gas or carbon dioxide gas, and a turbine 54 driven by gas-phase working fluid is provided between the evaporator 53 and the condenser 55. By using the flow of gas-phase working fluid to generate power using the power obtained by the turbine 54, it becomes possible to supply electricity to the power plant 80 and the transport machinery 2a, 2b, etc., thereby reducing production costs.

Fifth embodiment of the present invention
In the gas circulation system according to the first embodiment described above, the natural gas generated at the collection facility and the carbon dioxide gas generated at the power plant are simply transported by transport machinery to the power plant and the collection facility, respectively. However, without being limited to this, as shown in Figures 12 and 13, the system can also be configured to produce hydrogen using the natural gas generated at the collection facility and the carbon dioxide gas generated at the power plant.
In the following description, the description of configurations that overlap with the above-described embodiments will be omitted.

The gas circulation system 1 according to this embodiment includes a heat exchange unit 100 that performs heat exchange between the natural gas collected by the collection facility 70 and seawater taken from the sea in the collection facility 70, an evaporation unit 110 that introduces the seawater discharged from the heat exchange unit 100 into a predetermined evaporation space that has been decompressed to a pressure lower than the saturated vapor pressure of the seawater to perform flash evaporation, a heat exchange unit 120 that performs heat exchange between the water vapor discharged from the evaporation unit 110 and carbon dioxide gas transported from the power plant 80 by the transport machine 2b, and a hydrogen production unit 130 that produces hydrogen gas using the desalinated water discharged from the heat exchange unit 120.

The heat exchange section 100 exchanges heat between the natural gas extracted by the extraction facility 70 and seawater taken from the sea through an intake pipe 101, cooling the natural gas with the seawater while heating the seawater with the natural gas.
The cooled natural gas is stored in a pressure vessel 3a of a transportation machine 2a via a distribution pipe.
The natural gas may be compressed after being cooled in the heat exchange unit 100. That is, the natural gas compression unit 74 may be disposed between the heat exchange unit 100 and the transportation machine 2a.
The heated seawater is then sent to the evaporator section 110 .

The seawater introduced into the heat exchange section 100 is, for example, warm seawater from the ocean surface.
The seawater may be supplied to the heat exchange section 100 after air and the like contained in the seawater is removed by a deaeration device (not shown).

The evaporation section 110 has an evaporation space therein that communicates with the heat exchange section 120, and is equipped with a hollow reduced pressure vessel 111 in which this evaporation space is kept at a reduced pressure lower than the saturated vapor pressure, and an injection section 112 that is disposed within the reduced pressure vessel 111 and injects seawater introduced from the outside into the evaporation space of the reduced pressure vessel 111 in the form of a mist, water droplets, a water film, a water column, etc.
The evaporation section 110 obtains water vapor by flash evaporating the seawater injected from the injection section 112 in an evaporation space within the reduced pressure vessel 111 .

In addition, a pressure reduction exhaust device (not shown) is connected to the reduced pressure container 111 of the evaporation section 110, and the evaporation space in the reduced pressure container 111 is adjusted to a pressure lower than the saturated vapor pressure of water at the same temperature as the seawater to be evaporated in the reduced pressure container 111, and the temperature at which the water in the seawater changes from the liquid phase to the gas phase (evaporates) in the reduced pressure container 111 is maintained lower than the temperature at atmospheric pressure.
As a result, a portion of the seawater introduced into the reduced pressure vessel 111 changes from liquid phase to gas phase and is desalinated.

The heat exchange section 120 directly or indirectly condenses the water vapor discharged from the evaporator section 110 with carbon dioxide gas transported from the power plant 80 by the transport machine 2b, while directly or indirectly heating the carbon dioxide gas transported from the power plant 80 with the water vapor discharged from the evaporator section 110.
The heated carbon dioxide gas is discharged from the heat exchange section 120 and sent to the _CO2 gas injection section 75, and is injected into the collection site 90 via the injection well 76.
The condensed water (fresh water) is discharged from the heat exchange section 120 and sent to the hydrogen production section 130 .

The hydrogen production unit 130 includes a confluence unit 131 where the water discharged from the heat exchange unit 120 is temporarily stored, an electrolysis unit 132 that electrolyzes water to obtain hydrogen, a first gas-liquid separator 133 that separates water from the hydrogen gas generated in the electrolysis unit 132, a hydrogen gas recovery unit 134 that recovers the hydrogen gas discharged from the first gas-liquid separator 133, and a second gas-liquid separator 135 that separates water from the oxygen gas generated in the electrolysis unit 132.

The electrolysis unit 132 can be a known PEM (Polymer Electrolyte Membrane) type water electrolysis hydrogen production device, so details will be omitted, but it can be a series-arranged multiple cells in which a proton-conductive polymer membrane such as a perfluorosulfonic acid-based polymer electrolyte membrane or a fluorine-based polymer electrolyte membrane is bonded to both sides with iridium oxide or platinum-based metal as an electrode catalyst. Hydrogen gas is generated from the electrode catalyst that serves as the cathode, and oxygen gas is generated from the electrode catalyst that serves as the anode.

The hydrogen gas discharged from the electrolysis section 132 is sent to the first gas-liquid separator 133 .
The first gas-liquid separator 133 separates the hydrogen gas discharged from the electrolysis section 132 into hydrogen gas and water, and the separated hydrogen gas is sent to the hydrogen gas recovery section 134, while the separated water is returned to the circulation line and sent to the junction section 131, where it is mixed with the water supplied from the heat exchange section 120.

The oxygen gas discharged from the electrolysis section 132 is sent to the second gas-liquid separator 135 .
The second gas-liquid separator 135 separates the oxygen gas discharged from the electrolysis unit 132 into oxygen gas and water, and the separated oxygen gas is recovered in an oxygen gas recovery unit (not shown) or released into the atmosphere. On the other hand, the separated water is returned to the circulation line again and sent to the junction unit 131, where it is mixed with the water supplied from the heat exchange unit 120.

The hydrogen production section 130 may also include a heat exchanger, a non-regenerative polisher, a final filter, etc., not shown, between the second gas-liquid separator 135 and the junction section 131.

The gas circulation system 1 according to this embodiment also includes a heat exchanger 140 in the power plant 80 that exchanges heat between carbon dioxide gas discharged from the power plant 80 and seawater taken from the sea, an evaporation unit 110 that introduces the seawater discharged from the heat exchanger 140 into a predetermined evaporation space that has been decompressed to a pressure lower than the saturated vapor pressure of the seawater to perform flash evaporation, a heat exchanger 150 that exchanges heat between the water vapor discharged from the evaporation unit 110 and natural gas transported from the extraction facility 70 by the transport machine 2a, and a hydrogen production unit 130 that produces hydrogen gas using the desalinated water discharged from the heat exchanger 150.

The heat exchange section 140 directly or indirectly cools the carbon dioxide gas discharged from the power plant 80 with seawater taken from the sea through a water intake pipe 141, while directly or indirectly heating the seawater taken from the sea with the carbon dioxide gas discharged from the power plant 80.
The cooled carbon dioxide gas is stored in the pressure vessel 3b of the transport machine 2b via a delivery pipe.
The carbon dioxide gas may be compressed after being cooled in the heat exchanger 140. That is, the _CO2 gas compressor 83 may be disposed between the heat exchanger 140 and the transport machine 2b.
The heated seawater is then sent to the evaporator section 110 .

The heat exchange section 150 directly or indirectly condenses the water vapor discharged from the evaporation section 110 with the natural gas transported from the collection facility 70 by the transport machine 2a, while directly or indirectly heating the natural gas transported from the collection facility 70 with the water vapor discharged from the evaporation section 110.
The heated natural gas exits the heat exchange section 150 and is sent to the gas turbine 81 .
The condensed water (fresh water) is discharged from the heat exchange section 150 and sent to the hydrogen production section 130 .

As described above, the system is equipped with an evaporation section 110 that desalinates seawater using thermal energy from natural gas and carbon dioxide gas, and a hydrogen production section 130 that produces hydrogen gas from fresh water, so the produced hydrogen gas can be used as fuel and can be used as an energy source for cooling pressure vessels 3a, 3b that contain natural gas or carbon dioxide gas during transportation by transport machines 2a, 2b.

The hydrogen gas may be converted into electrical energy at the extraction facility 70 or power plant 80 and loaded onto the transport machines 2a and 2b, or may be loaded into the transport machines 2a and 2b in a gaseous state together with a fuel cell. The energy extracted from the hydrogen gas can be used for cooling the pressure vessels 3a and 3b, etc.

In the above embodiments, natural gas and carbon dioxide gas are directly supplied and received between the transport machines 2a, 2b and the collection facility 70 or power plant 80, but they may be supplied and received between the transport machines 2a, 2b and the collection facility 70 or power plant 80 via storage tanks for various gases. For example, when natural gas or carbon dioxide gas is transported by sea, in the above second embodiment, a ship or barge equipped with a storage tank for natural gas or carbon dioxide gas may be moored at sea between the heat exchange units 4, 5 and the transport machines 2a, 2b, and natural gas and carbon dioxide gas may be supplied and received between the heat exchange units 4, 5 and the transport machines 2a, 2b via the storage tank. This allows natural gas and carbon dioxide gas to be continuously and stably supplied and received.

Furthermore, the above embodiments can be used in appropriate combinations.

1 Gas circulation system 2a, 2b Transport machinery 3a, 3b Pressure vessel 4, 5 Heat exchange section 6, 7 Expander 8 Gas compression section 40, 50 First heat exchanger 41, 51 Second heat exchanger 42, 52 Pump 43, 53 Evaporator 44, 54 Turbine 45, 55 Condenser 46, 56 Pump 47, 57 Generator 60-63 Connection section 70 Collection facility 71 Production well 72 Natural gas collection section 73 Natural gas refining section 74 Natural gas compression section 75 CO ₂ gas injection section 76 Injection well 80 Power plant 81 Gas turbine 81a Combustor 81b Air compressor 81c Turbine 82 CO ₂ gas recovery section 83 CO ₂ gas compression section 84 Generator 90 Collection point 100, 120, 140, 150 Heat exchange section 101, 141 Water intake pipe 110, evaporation section 111, reduced pressure vessel 112, injection section 130, hydrogen production section 131, junction section 132, electrolysis section 133, first gas-liquid separator 134, hydrogen gas recovery section 135, second gas-liquid separator

Claims (14)

1. a first transport means for transporting the natural gas collected by a collection facility from the collection facility that collects the natural gas at a collection point to a natural gas utilization facility that utilizes the natural gas;
   and a second transport means for transporting carbon dioxide gas discharged during the utilization process of the natural gas utilization facility from the natural gas utilization facility to the collection facility or the carbon dioxide utilization facility;
   the first transport means transports the natural gas in a compressed state from the extraction facility to the natural gas utilization facility;
   The second transportation means transports the carbon dioxide gas in a compressed state from the natural gas utilization facility to the collection facility or the carbon dioxide utilization facility;
   A gas circulation system, comprising: a carbon dioxide gas transported by the second transport means as a supply gas to the collection site by the collection facility or to the carbon dioxide utilization facility.
2. 2. The gas circulation system according to claim 1,
   the first transport means transports the natural gas in a supercritical state from the extraction facility to the natural gas utilization facility;
   The gas circulation system, wherein the second transport means transports the carbon dioxide gas in a supercritical state from the natural gas utilization facility to the extraction facility or the carbon dioxide utilization facility.
3. 2. The gas circulation system according to claim 1,
   A gas circulation system comprising a heat exchange means for exchanging heat between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the natural gas utilization facility by the second transport means.
4. 4. The gas circulation system according to claim 3,
   the heat exchange means comprising: evaporation means for effecting heat exchange between the working fluid and the natural gas and evaporating the working fluid to obtain a gaseous working fluid; power conversion means which is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power; and condensation means for effecting heat exchange between the gaseous working fluid discharged from the power conversion means and the carbon dioxide gas to condense the working fluid into a liquid phase.
5. 2. The gas circulation system according to claim 1,
   A gas circulation system comprising a heat exchange means for exchanging heat between the carbon dioxide gas generated in the natural gas utilization facility and the natural gas transported from the collection facility by the first transport means.
6. 6. The gas circulation system according to claim 5,
   the heat exchange means comprising: evaporation means for effecting heat exchange between the working fluid and the carbon dioxide gas and evaporating the working fluid to obtain a gaseous working fluid; power conversion means which is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power; and condensation means for effecting heat exchange between the gaseous working fluid discharged from the power conversion means and the natural gas, thereby condensing it into a liquid phase.
7. 2. The gas circulation system according to claim 1,
   A gas circulation system, characterized in that the first transportation means or the second transportation means transports the natural gas and/or the carbon dioxide gas in a pressure vessel formed of at least fiber reinforced plastic.
8. A transport machine characterized by transporting natural gas extracted at an extraction facility from the extraction facility to a natural gas utilization facility in a compressed state, and/or transporting carbon dioxide gas emitted in the process of utilizing the natural gas utilization facility from the natural gas utilization facility to the extraction facility or the carbon dioxide utilization facility in a compressed state.
9. 9. The transport machine according to claim 8,
   A transportation machine that transports the natural gas collected at the collection facility from the collection facility to the natural gas utilization facility in a supercritical state, and/or transports the carbon dioxide gas discharged during the utilization process of the natural gas utilization facility from the natural gas utilization facility to the collection facility or the carbon dioxide utilization facility in a supercritical state.
10. 9. The transport machine according to claim 8,
    a heat exchange unit that exchanges heat between the natural gas extracted at the extraction facility and the carbon dioxide gas transported from the natural gas utilization facility.
11. The transport machine according to claim 10,
    the heat exchange unit comprises an evaporator that performs heat exchange between the working fluid and the natural gas and evaporates the working fluid to obtain a gaseous working fluid, a turbine that is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condenser that performs heat exchange between the gaseous working fluid discharged from the turbine and the carbon dioxide gas to condense the working fluid into a liquid phase.
12. 9. The transport machine according to claim 8,
    A transportation machine comprising: a heat exchange unit for exchanging heat between the carbon dioxide gas generated in the natural gas utilization facility and the natural gas transported from the natural gas extraction facility.
13. 13. The transport machine according to claim 12,
    the heat exchange unit comprises an evaporator that performs heat exchange between the working fluid and the carbon dioxide gas and evaporates the working fluid to obtain a gaseous working fluid, a turbine that is operated by receiving the gaseous working fluid and converts the thermal energy contained in the working fluid into power, and a condenser that performs heat exchange between the gaseous working fluid discharged from the turbine and the natural gas, thereby condensing it into a liquid phase.
14. 9. The transport machine according to claim 8,
    A transportation machine, characterized in that the natural gas and/or the carbon dioxide gas is accommodated in a pressure vessel formed at least of fiber reinforced plastic and transported.
    
    

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