JP2010209591A - Method and system for power generation with low exhaustion of carbon dioxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve both the isolated storage of carbon dioxide and the acquisition of methane gas from methane hydrate with high efficiency. <P>SOLUTION: The carbon dioxide and nitrogen oxides produced by power generation are adsorbed by seawater under high pressure, and the seawater is heated by the waste heat generated by the power generation and injected into a methane hydrate layer H under a sea bottom surface S through a hot water transportation pipe 2. The methane gas dissociated from the methane hydrate by the heat of the seawater is collected, transported to a power generation plant 1 on the sea through a methane gas collection pipe 3, and supplied as a fuel for power generation. The carbon dioxide and nitrogen oxides in the injected seawater are stored in the methane hydrate layer H in an isolated state. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a carbon dioxide low emission power generation method and system, and more particularly, suppresses generation of carbon dioxide itself by power generation using methane gas as fuel, and stores carbon dioxide generated by power generation in a methane hydrate layer on the sea floor. It is related with the technique which suppresses discharge | release to the atmosphere of a carbon dioxide by doing.

  Against the background of fossil energy depletion in recent years, methane hydrate is attracting attention as a next-generation primary energy source to replace conventional fossil fuels such as oil. Methane hydrate is an inclusion compound of methane and water, and is broadly divided into terrestrial methane hydrate that exists under permafrost in high-latitude areas and marine methane hydrate that exists under the sea floor at a depth of 500 m or more. . Of these, the latter ocean methane hydrate is expected to exist in a considerable amount in the waters near Japan, centering on the Nankai Trough basic borehole. Methane hydrate can be decomposed into methane and water by changes in temperature or pressure, and methane gas dissociated from methane hydrate has a carbon dioxide emission per unit calorific value compared to other fossil fuels. It is known that there are few.

  There is a demand not only to enable the use of methane hydrate as an alternative primary energy source, but also to reduce the emission of carbon dioxide emitted from power plants to the atmosphere. The 34th Leaders' Summit Meeting in July 2008 (Hokkaido Toyako G8 Summit) has set a long-term reduction target of “reducing at least 50% of global greenhouse gas emissions by 2050” It has just been done. Under such a demand, a system that does not release carbon dioxide into the atmosphere even if fossil fuels are used is being studied, and carbon dioxide capture and storage technology (CCS) is drawing attention. CCS is a technology that separates carbon dioxide from anthropogenic sources such as power plants from the atmosphere and segregates and stores them in the ground or ocean, and is also being studied by the Intergovernmental Panel on Climate Change (IPCC). According to the IPCC report (2005), it is said that CSS enables 2 trillion tons of underground storage, equivalent to approximately 80 years of global oxygen dioxide emissions. CCS is said to contribute to the reduction of carbon dioxide emissions into the atmosphere in 2100 at a rate of 15 to 55%, and is expected to make a substantial contribution to global warming countermeasures.

  Regarding CCS, the following techniques exist. Carbon dioxide gas discharged from a heat utilization plant such as a power plant is compressed and liquefied, and the liquefied carbon dioxide is stored in a container (storage tank) and transported to a sea or land mining base. Then, carbon dioxide gas is injected from a mining base into a mining point in a methane hydrate layer existing under the sea bottom or underground, and stored in a carbon dioxide hydrate state in this layer. Here, the methane hydrate at the mining site is decomposed into water and methane by the reaction heat generated during the production of carbon dioxide hydrate, and the resulting methane gas is recovered at the mining base and supplied to the heat utilization plant (see below). Patent Document 1).

JP 2000-061293 A (paragraph numbers 0063, 0087)

  Carbon dioxide discharged from the heat utilization plant is injected into the methane hydrate layer in the form of gas, and the methane hydrate near the gas injection part is decomposed by the heat generated when this carbon dioxide gas causes a phase change in the hydrate. There is already something to do as described in the above-mentioned Patent Document 1. However, the technique described in this document does not transport carbon dioxide under the sea bottom in a state of being adsorbed in seawater and inject it into the methane hydrate layer, but this seawater has a function of decomposing methane hydrate. Nor does it use heat.

  The present invention provides a carbon dioxide low emission power generation method and system in consideration of the above.

  The present invention performs power generation using methane gas as fuel, acquires heated seawater in which carbon dioxide generated by the power generation is adsorbed to the heated seawater, and injects the acquired heated seawater into the methane hydrate layer on the seabed. The heat of the injected heated seawater is transferred to the methane hydrate in this layer, and the methane gas dissociated from the methane hydrate by receiving this heat is recovered as fuel, while the carbon dioxide in the injected heated seawater is methane Isolate and fix in the hydrate layer.

  According to the present invention, carbon dioxide generated by power generation is sequestered and stored on the sea floor, so that release of carbon dioxide into the atmosphere can be suppressed. In addition to this, according to the present invention, seawater is used for transporting carbon dioxide toward the seabed, and the seawater is heated and then injected into the methane hydrate layer, so that it can be used as a carbon dioxide transport medium. And the function as a heating medium of methane hydrate can be combined with seawater. Therefore, it is possible to achieve high reduction in carbon dioxide emission due to isolated storage and acquisition of methane gas from methane hydrate.

The whole block diagram of the system which enforces the electric power generation method concerning one embodiment of the present invention. Schematic configuration diagram of the offshore plant constituting the system Phase equilibrium diagram of methane hydrate and carbon dioxide hydrate Overall configuration diagram showing a specific example of a power generation system according to the present embodiment Configuration diagram of carbon dioxide adsorption device (CO2 recovery unit) used in power generation system of specific example as above Illustration of changes in turbine exhaust gas composition when the turbine inlet temperature is changed Explanatory drawing of calculation model of flow in hot water transport pipe Water temperature approximate profile for water depth Explanatory drawing showing the relationship between pipe inner diameter and pipe outlet temperature Explanatory drawing showing the relationship between pipe inner diameter and pipe outlet pressure Explanatory drawing which shows the relationship between design evaluation parameter (= ti / λi) and pipe outlet temperature

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows the overall configuration of a system that implements a power generation method according to an embodiment of the present invention (hereinafter referred to as a “low CO 2 emission power generation system”). In the present embodiment, power is generated using methane gas as fuel, carbon dioxide generated as a result of this power generation is adsorbed and dissolved in seawater, and the seawater on which this carbon dioxide is adsorbed is heated to enter the methane hydrate layer on the sea floor. inject. The heat of the injected seawater decomposes methane hydrate to produce methane gas, which is recovered and used as a fuel for power generation. The carbon dioxide adsorbed on the seawater is changed into liquefied carbon dioxide according to low-temperature and high-pressure environmental conditions in the methane hydrate layer into which the seawater is injected, and is isolated and stored in the methane hydrate layer.

  A power plant floating on the sea (hereinafter referred to as “sea plant”) 1 includes a thermal power generation device using methane gas as a fuel and a carbon dioxide adsorption device (hereinafter referred to as “CO2 recovery unit”) that adsorbs and dissolves carbon dioxide generated by power generation in seawater. And).

  As shown in FIG. 2, the power generator according to the present embodiment includes a compressor (corresponding to a “first compressor”) 101, a combustor 102, and a gas turbine 103. This is a single-stage Brayton cycle type power generator. After the exhaust gas from the gas turbine 103 is cooled by the heat exchanger 104, a part of the exhaust gas is recirculated to the intake side via the gas distributor 105. By recirculating part of the turbine exhaust gas, the turbine inlet temperature is adjusted, and as a result, the air ratio is reduced and the thermal efficiency of the entire system is increased. As described above, by adopting the Brayton cycle as a basic power generation cycle, it is possible to configure a cycle constituent device such as the compressor 101 in a compact manner for installation on the sea. The remaining turbine exhaust gas other than the recirculated gas is pressurized by a compressor (corresponding to a “second compressor”) 106 and then supplied to the CO 2 recovery unit 107. Seawater compressed by the pump 108 is supplied to the CO2 recovery unit 107, and carbon dioxide in the turbine exhaust gas is adsorbed and dissolved in seawater under high pressure. Specifically, turbine exhaust gas is blown into the high-pressure tank of the CO2 recovery unit 107, and compressed seawater is injected and supplied by a nozzle or the like, thereby adsorbing carbon dioxide to the seawater. Such a carbon dioxide adsorption method is known as a physical absorption method. It is possible to increase the solubility of carbon dioxide in seawater by mixing carbon dioxide and turbine exhaust gas under low temperature and high pressure. In this embodiment, the turbine exhaust gas cooled to room temperature is introduced into the CO 2 recovery unit 107 by installing the heat exchanger 109. Here, according to Henry's law, the higher the carbon dioxide partial pressure in the turbine exhaust gas, the higher the solubility. In this way, the carbon dioxide released into the atmosphere is only the carbon dioxide that has not been adsorbed by the seawater in the CO2 recovery unit 107 out of the carbon dioxide in the turbine exhaust gas excluding the recirculation gas. The exhaust gas (residual gas) excluding the adsorbed carbon dioxide is heated by the heat exchanger 109 and then supplied to the gas turbine 110 to be expanded to atmospheric pressure. The generator 111 is driven by the gas turbines 103 and 110, and the electricity generated by the generator 111 is transmitted to a power collection facility on land through the submarine cable 5 (FIG. 1). The gas turbine 103 and the generator 111 constitute a “power generation turbine” according to the present embodiment.

  Returning to FIG. 1, the seawater on which carbon dioxide is adsorbed by the CO2 recovery unit 107 is heated by the heat exchanger 104 (FIG. 2) and then transported to the bottom of the sea bottom S through the hot water transport pipe 2 installed in the sea. Injection into the mining point P in the hydrate layer H. A free gas layer F exists between the methane hydrate layer H and the lower formation U. FIG. 3 is a phase equilibrium diagram of methane hydrate and carbon dioxide hydrate. When methane, carbon dioxide, and water coexist, which state is present among gas, liquid, and hydrate, respectively. Is shown. The marine methane hydrate existence region A is shown as a region on the lower temperature side than the methane hydrate-gas phase equilibrium line indicated by a one-dot chain line. According to Goal et al., Methane hydrate and carbon dioxide hydrate can be obtained by decomposing methane hydrate under the temperature and pressure conditions in region B at a lower temperature side than the carbon dioxide hydrate-gas phase equilibrium line indicated by a solid line. (Naval Goal, “In situ methane hydrate dissociation with carbon dioxide sequestration: Current knowledge and issues”, Journal of Petroleum Science and Engineering, vol. 51, pp. 169-184, 2006). In this embodiment, since methane hydrate is decomposed by a hot water injection method using heated seawater, carbon dioxide in seawater is considered to be stable in the state of liquefied carbon dioxide. A dotted line indicates a carbon dioxide liquid-gas phase equilibrium line.

  Methane gas dissociated from methane hydrate by injection of heated seawater is transported to the offshore plant 1 through the methane gas recovery pipe 3 and supplied to the combustor 102 as fuel for power generation.

  In the following description, a specific example of the low CO2 emission power generation system outlined above will be examined by calculation.

(When considering only carbon dioxide emissions)
FIG. 4 shows the overall configuration of the power generation system, and FIG. 5 shows the configuration of the CO2 recovery unit 107 used in the power generation system.
In this calculation, the product name “HYSYS version 2006.5” which is a steady process simulator is used. HYSYS is a simulator that performs balance calculation on the substance / heat balance in a chemical process. The fluid package (property value database) employs the Peng Robinson-type equation of state, which is considered highly reliable for reactions using hydrocarbons (Ding-Yu Peng and Donald B. Robinson, “A New Two -Constant Equation of State ”, Industrial and Engineering Chemistry Fundamentals, vol. 15, pp.59-60, 1976). In this calculation, it is assumed that a chemical equilibrium state is assumed for a chemical reaction such as combustion, and that methane gas at normal temperature is supplied to the combustor 102 without considering the dissociation process of methane hydrate. In actual power plant design, it is preferable to consider these unsteady factors in order to quantify the change in combustion gas composition over time and the amount of methane gas that can be supplied. The process of this power generation cycle is described below.

1. Air at normal temperature and pressure is compressed to 3 MPa by the compressor 101.
2. A methane gas having a normal temperature of 3 MPa as fuel and compressed air are combusted in the combustor 102.

3. The combustion gas is expanded to normal pressure in the gas turbine 103.
4). The exhaust gas (turbine exhaust gas) from the gas turbine 103 is cooled to room temperature by the heat exchanger 104.

5). Water produced by condensation is separated from the turbine exhaust gas by the gas-liquid separator 121.
6). A part of the turbine exhaust gas is separated by the gas distributor 105 and recirculated to the intake side. The exhaust gas to be recirculated is mixed with air by the gas mixer 122. The remaining turbine exhaust gas other than the exhaust gas to be recirculated is compressed to 5 MPa by the compressor 106.

7). The compressed turbine exhaust gas is cooled to room temperature by the heat exchanger 109.
8). The cooled turbine exhaust gas and the seawater compressed to 5 MPa by the pump 108 are mixed in the CO 2 recovery unit 107 to adsorb carbon dioxide in the turbine exhaust gas to the seawater.

9. The residual gas (high-pressure nitrogen gas) after the carbon dioxide is adsorbed and removed is heated by the heat exchanger 109, while the seawater on which the carbon dioxide is adsorbed is heated by the heat exchanger 104.
10. The heated residual gas is expanded to normal pressure in a gas turbine (expansion turbine) 110, and after energy is recovered, it is released into the atmosphere.

In this calculation, the following assumptions were made for the above process.
In this embodiment, the mass flow rate of methane gas generated from methane hydrate and used for power generation is 1 kg per second, but this flow rate can be appropriately increased or decreased according to the power generation scale and is limited to 1 kg / s. It is not something.

a. The normal pressure is 0.1013 MPa, and the normal temperature is 15 ° C.
b. The composition of air is 79% nitrogen and 21% oxygen.
c. The fuel composition is 100% methane and the flow rate is 1 kg / s.

d. It is assumed that the combustion is complete and the air ratio is 1. As a result, the combustion products are only carbon dioxide and water.
e. The adiabatic efficiency is defined by the following equations (1) and (2) with η _COMP = 88% for the compressor and η _GT = 92% for the gas turbine.

η _COMP = (power in adiabatic compression process) / (power in actual compression process) (1)
η _GT = (output in actual expansion process) / (output in adiabatic expansion process) (2)
The analysis conditions of the process described above are summarized in the following table (1).

  The results of this calculation are shown in Table 2.

  In this calculation, the parameters for determining the performance of the power generation cycle include the number of stages of the carbon dioxide adsorption (recovery) process, the flow rate of seawater, the recirculation ratio of the turbine exhaust gas, and the pressure ratio of the compressor 101. Table 2 The calculation result when changing the value of is shown. The CO2 recovery process is a counter flow type as shown in FIG. 5 in order to increase the CO2 adsorption (recovery) rate. For example, in the CO2 recovery unit 107 shown in FIG. 5, the number of stages of the CO2 recovery process is interpreted as three stages according to the number of the mixing tanks 107a to 107c. Therefore, if the number of mixing tanks is two, the number of stages is two. The recirculation ratio is the ratio of exhaust gas recirculated to the intake side in the entire exhaust gas of the gas turbine 101.

Here, in this calculation, in order to evaluate the basic specifications of the power plant, the power generation output W _{TOTAL of the} entire cycle is a value when the flow rate of methane gas is 1 kg / s. The thermal efficiency η _{TOTAL of the} entire cycle was calculated by the following equation (3).

η _TOTAL = W _TOTAL / LHV _CH4 =
_{= (W GT1 -W COMP1 + W} GT2 -W COMP2 -W PUMP) / LHV CH4 ··· (3)
In the above formula (3), the thermal efficiency η _TOTAL is calculated based on the lower heating value LHV _CH4 (= 50030 kJ / kg) of _methane . This is to facilitate comparison with many existing studies based on low calorific value.

As is apparent from Table 2, it can be seen that the CO2 recovery rate is significantly increased by changing the CO2 recovery process from the second stage to the third stage (Case B). And it turns out that the temperature of the seawater after a heating rises, although CO2 collection | recovery rate reduces a little by reducing the flow volume of seawater (case C). When the recirculation ratio is decreased, the inlet temperature of the gas turbine 101 increases, and as a result, the power generation output W _TOTAL and the thermal efficiency η _{TOTAL of the} entire cycle increase (Case D). It can be seen that by further reducing the recirculation ratio, the power generation output W _TOTAL and the thermal efficiency η _TOTAL can be slightly increased, while the turbine inlet temperature significantly increases (Case E). Finally, it can be seen that when the pressure ratio of the compressor 101 is decreased, the temperature of the seawater after heating increases, while the power generation output W _TOTAL and the thermal efficiency η _TOTAL decrease (Case E).

Thus, there are a plurality of parameters that determine the performance of the power generation cycle. Among the five cases shown in Table 2, it is considered that the setting of case D in which the temperature of the seawater after heating, the power generation output W _TOTAL and the thermal efficiency η _{TOTAL of the} entire cycle are relatively high is appropriate. The overall cycle thermal efficiency η _TOTAL is 41.0%, which is not so high compared to the latest thermal power plant. However, carbon dioxide is recovered at a recovery rate of 91.7%, and the turbine waste heat is used to heat seawater. Since it is used and used for the decomposition of methane hydrate, it is considered that it is not a low value as a whole system. On the other hand, according to Case E, although the highest thermal efficiency η _{TOTAL of} 41.5% is obtained, the turbine inlet temperature is relatively high at 1590 ° C. If the latest technology such as a 1700 ° C. class gas turbine that is currently being developed is put into practical use, it may be realized.

  Dissociation latent heat of at least 56.9 kJ / mol is required to decompose methane hydrate into methane gas and water (Sloan ED Jr., “Clathrate Hydrates of Natural Gas -2nd ed.”, Marcel Dekker, 1998) . Here, since the molecular weight of methane is 16.04 and the flow rate of methane gas set in this calculation is 1 kg / s, the minimum amount of heat necessary for dissociation of methane gas is calculated to be about 3547 kW. On the other hand, considering that the flow rate of seawater in this calculation (Case D) is 200 kg / s, the specific enthalpy of seawater is required to be at least 18 kJ / kg. This corresponds to a specific enthalpy of water of about 4.2 ° C. at normal pressure. Considering that the specific enthalpy of water at 40 ° C at normal pressure is 167.62 kJ / kg, it is possible to supply heat necessary for dissociation thermodynamically by using the seawater assumed for this power generation cycle. I understand that there is. When examining this power generation cycle in more detail, it is necessary to consider the reaction rate between seawater adsorbed with carbon dioxide and methane hydrate, the diffusion rate of seawater in the methane hydrate layer, and the like.

(When considering emissions of carbon monoxide and nitrogen compounds)
In the above calculations, complete combustion was assumed and only carbon dioxide and water were considered as combustion products. In contrast, in this calculation, carbon monoxide (CO) generated by incomplete combustion and nitrogen oxide (thermal NOx) generated by the reaction of nitrogen and oxygen in the combustion gas at a high temperature are considered. Then, I consider the latter nitrogen oxides, especially _{NO, NO 2, N 2 O} , the N 2 _{O 4.} The power cycle process and calculation assumptions are the same as in the above calculation considering only carbon dioxide emissions.
FIG. 6 shows changes in the turbine exhaust gas composition when the turbine inlet temperature is changed. Carbon monoxide (CO) is indicated by a square plot, and NO, N ₂ O ₄ among nitrogen oxides is indicated by a circle or triangle plot, respectively. In each plot, a case where the air ratio λ is set to 1.0 is indicated by a dotted line (a hollow plot), and a case where the air ratio λ is set to 1.1 is indicated by a solid line. The turbine inlet temperature is changed by the recirculation ratio defined in the above simulation. The turbine exhaust gas also contains carbon dioxide and nitrogen, but was omitted to focus on CO and NOx.

According to FIG. 6, it can be seen that when the air ratio λ is changed from 1.0 to 1.1, the generation amount of NO and N ₂ O ₄ increases, but the generation amount of CO decreases. NO ₂ and N ₂ O are not generated in the ppm order indicated by the vertical axis in FIG. And it turns out that there exists a tendency which increases with respect to the raise of turbine inlet temperature about the production amount of both CO and NOx.

  Table 3 shows the exhaust gas composition before and after the treatment by the same physical absorption method (FIG. 5) as in the above-mentioned calculation for the turbine exhaust gas whose turbine inlet temperature is around 1400 ° C. In this calculation, for the sake of simplicity, the adsorption (recovery) process is made one stage.

According to Table 3, it can be seen that the concentration of CO and NO increases after recovery by the physical absorption method regardless of whether the air ratio λ is 1.0 or 1.1. This is because CO and NO are difficult to dissolve in water, the recovery rate is less than 1%, and the gas such as N ₂ O ₄ that is easily soluble in water is recovered and removed, resulting in an increase in their relative concentration in the exhaust gas. it is conceivable that. On the other hand, N ₂ O ₄ is recovered at a recovery rate of 99% or more. As a result obtained from the above, the air ratio λ can be increased to reduce the amount of CO produced, and the air ratio λ can be reduced to reduce the amount of NOx produced.

The CO and NO can be oxidized to CO ₂ and NO ₂ by installing a catalyst at the outlet of the gas turbine 103. These products can be easily dissolved in seawater under high pressure. After heating, these products can be injected into the methane hydrate layer in the state of warm drainage and stored under the sea bottom S. Therefore, it is possible to perform power generation using methane hydrate while suppressing the release of CO and NO into the atmosphere.

  In general, in a single-stage Brayton cycle, increasing the compression ratio of the compressor 101 increases the power generation efficiency. However, in a normal power generation facility, since the amount of NOx emissions increases, there is a limit to the increase in the compression ratio. According to the power generation system according to the present embodiment, generated NOx is also sequestered together with carbon dioxide in the methane hydrate layer, so that a Brayton cycle having a larger compression ratio than that in the present embodiment is constructed, and higher power generation efficiency is achieved. It is possible to obtain

(Analysis of flow in hot water transport pipe)
In the following description, heat loss from the inside of the hot water transport pipe 2 to the outside seawater, which occurs in the process of transporting seawater on which carbon dioxide has been adsorbed to the bottom of the sea bottom S, will be examined. Considering this heat loss, the flow in the hot water transport pipe is analyzed, and the temperature near the outlet of the hot water transport pipe 2 (pipe outlet temperature) is evaluated.
FIG. 7 shows a calculation model used for this analysis. The Nankai Trough foundation drilling, since it has been reported that there is methane hydrate in depth about 1200 m, in this calculation, and set the hot water transport pipe 2 the length L _p of the safe side in 2000 m. Seawater was assumed to flow into the hot water transport pipe 2 at the flow rate and temperature of Case E shown in Table 2. The heat conduction in the wall of the hot water transport pipe 2 is ignored, and the heat transfer coefficient between the pipe outer wall and the surrounding seawater is infinite, that is, the pipe outer wall has a temperature equal to the surrounding seawater.

First, prior to this calculation, the pipe outlet temperature T _bottom is estimated by setting the inner diameter R of the hot water transport pipe 2 to 0.25 m. Since the flow rate of seawater at the pipe inlet is M _in = 200 kg / s, the average flow velocity U _{m of} seawater is 1.02 m / s from the following equation (4).

U _m = M _in / πR ² ρ = 200 / (π × 0.25 ² × 998.2)
= 1.02 (4)
Therefore, the average Reynolds number Re _m in the hot water transport pipe 2 is 5.07 × 10 ⁵ according to the following equation (5).

Re _m = 2U _m R / ν = 2 × 1.02 × 0.25 / (1.005 × 10 ⁻⁶ )
= 5.07 × 10 ⁵ (5)
Therefore, it turns out that the flow in the hot water transport pipe 2 is a turbulent flow. On the other hand, since the Prandtl number Pr = 6.7, the average Nusselt number Nu _m is 1491 according to the following equation (6) using the Ditus-Boelter equation.

Nu _m = 0.023 Re _m ^0.8 Pr ^0.3
= 0.023 × (5.07 × 10 ⁵ ) ^0.8 × 6.7 ^0.3 = 1491 (6)
From the above results, the average heat transfer coefficient _{h m} of the pipe inner wall, a 1789 by the following equation (7).

h _m = Nu _m λ _f /2R=1491×0.6/(2×0.25)
= 1789 (7)
The amount of heat Q _conv lost by convective heat transfer on the inner wall of the pipe is given by the following equation (8), assuming that the temperature of the pipe wall surface, the heat transfer coefficient, and the specific heat of the fluid are constant throughout the pipe 2.

_{Q conv = {(T in -T} out) / (1 / h m)} × 2πRL p
= [{(ΔT ₁ −ΔT ₂ ) / ln (ΔT ₁ / ΔT ₂ )} / (1 / h _m )] × 2πRL _p (8)
Here, T _in is the mixed average fluid temperature in the hot water transport pipe 2, and T _out is the temperature of the surrounding seawater. The numerator on the right side is the logarithmic average temperature difference inside and outside the pipe 2. ΔT ₁ is the difference between the seawater temperature T _top at the pipe inlet and the pipe outer wall temperature T _out, and ΔT ₂ is the difference between the seawater temperature T _{bottom at the} pipe outlet and the pipe outer wall temperature T _out . In this rough estimation, T _top is 51.24 ° C. and T _out is 5 ° C. The above equation (8) holds when the temperature of the pipe wall surface, the heat transfer coefficient, and the specific heat of the fluid can be considered constant.

The change in the enthalpy of seawater from the inlet to the outlet of the hot water transport pipe 2 is given by the following equation (9).
Q _en = ρπR ² U _m {c (T _top −T _bottom ) + (P _top −T _bottom ) / ρ} (9)
In the above equation (9), the first term on the right side represents the enthalpy change due to temperature change, and the second term represents the enthalpy change due to pressure change. Here, according to the energy conservation law, since Q _conv = Q _en , the pipe outlet temperature T _bottom is calculated to be 5.04 ° C. As described above, in the vicinity of the outlet of the hot water transport pipe 2, the temperature of the seawater injected into the methane hydrate layer H and the temperature of the surrounding seawater are substantially equal, and the heat loss in the transport process by the pipe 2 is large. I understand. Therefore, in order to supply heat required for the separation to the methane hydrate, it is considered necessary to attach a heat insulating material around the hot water transport pipe 2.

Based on the above results, the flow in the hot water transport pipe is analyzed using the product name “FLUENT 6.3” which is a thermal fluid analysis tool. The calculation model used in this analysis is the same as that shown in FIG. 7, but it is assumed that a heat insulating material is attached around the pipe 2. The heat transfer coefficient between the outer wall of the heat insulating material and seawater is infinite, and the conditions shown in FIG. 8 are adopted as the boundary conditions of the outer wall of the heat insulating material, and the conditions (U _m = 1.02 _m / s, T _top = 51.24 ° C.). Here, FIG. 8 is an approximation of the water temperature distribution with respect to the water depth around the Nankai Trough basic borehole by an exponential function. Since the Reynolds number in this calculation is 10 ⁵ or more as shown in the above equation (5), the k-ε turbulence model was used as the calculation model.

  9 and 10 respectively show the pipe outlet temperature and the pipe outlet pressure when the pipe inner diameter is changed. The thickness of the heat insulating material is 0.01 m, and the heat transfer coefficient is 0.173 W / m · K.

  According to FIG. 9, it can be seen that the pipe outlet temperature decreases as the pipe inner diameter is increased. This is presumably because, based on the assumption of a constant flow rate, the flow velocity decreases as the pipe inner diameter increases, and the heat loss per unit time increases. On the other hand, according to FIG. 10, it can be seen that the pressure loss increases when the pipe inner diameter is decreased. From these results, it is appropriate that the pipe inner diameter is about 0.5 m.

FIG. 11 shows the pipe outlet temperature when the pipe inner diameter is fixed to 0.5 m and the thickness ti [m] and the heat transfer coefficient λi [W / m · K] of the heat insulating material are changed. With ti / λi [m ² K / W] as a parameter on the horizontal axis, the pipe outlet temperature is shown on the vertical axis. A square plot is a calculation result when there is no heat insulating material. The circle plot A is the calculation result when ti = 0.001, λi = 0.173 (ti / λi≈0.00578), and the equilateral triangle plot B is ti = 0.01, λi = In the case of 0.5 (ti / λi = 0.02), the inverted triangular plot C is obtained when ti = 0.01 and λi = 0.173 (ti / λi≈0.0578). Shows things.

  As is apparent from FIG. 11, the pipe outlet temperature increases as the parameter ti / λi increases. Therefore, the heat insulating material can be selected by setting the target pipe outlet temperature and reading the value of the parameter ti / λi with respect to the target temperature.

  As described above, according to the present embodiment, carbon dioxide discharged from the offshore plant 1 can be isolated and stored under the sea bottom S to suppress the release of carbon dioxide into the atmosphere. In addition, according to the present embodiment, the discharged carbon dioxide is adsorbed and dissolved in seawater, and the seawater S is heated and then injected into the methane hydrate layer H through the hot water transport pipe 2, whereby the sea bottom S It is possible to transport carbon dioxide downward and heat methane hydrate with a single medium (seawater). Therefore, it is possible to achieve high reduction in carbon dioxide emission due to isolated storage and acquisition of methane gas from methane hydrate. Here, by attaching a heat insulating material around the hot water transport pipe 2, it is possible to suppress heat loss from seawater during transport in the sea and to construct a realistic system. And by setting the inner diameter of the hot water transport pipe 2 to about 0.5 m, good results can be obtained with respect to the temperature and pressure of seawater at the pipe outlet, and from the value of the parameter ti / λi for the required pipe outlet temperature. Appropriate insulation can be selected.

  DESCRIPTION OF SYMBOLS 1 ... Offshore plant, 2 ... Hot water transport pipe, 3 ... Methane gas recovery pipe, 5 ... Submarine cable, 101 ... Compressor (1st compressor), 102 ... Combustor, 103 ... Gas turbine, 104 ... Heat exchanger , 105 ... gas distributor, 106 ... compressor (second compressor), 107 ... CO2 recovery unit, 108 ... pump, 109 ... heat exchanger, 110 ... gas turbine, 111 ... generator, S ... sea bottom, H ... methane hydrate layer, P ... mining point.

Claims (29)

Generating electricity using methane gas as fuel;
Obtaining heated seawater in which carbon dioxide generated by the power generation is adsorbed to the heated seawater;
Injecting the obtained heated seawater into a methane hydrate layer on the seabed and isolating and fixing carbon dioxide in the heated seawater in the layer;
Transferring the heat of the heated seawater to the methane hydrate in the layer, and receiving the heat to recover the methane gas dissociated from the methane hydrate;
Supplying the recovered methane gas to the facility for power generation;
A carbon dioxide low emission power generation method.   The power generation method according to claim 1, wherein the heating seawater acquisition step heats seawater using waste heat of the power generation.   The power generation method according to claim 1 or 2, wherein the heating seawater acquisition step adsorbs the carbon dioxide to pressurized seawater.   The power generation method according to any one of claims 1 to 3, wherein the heating seawater acquisition step performs the adsorption of the carbon dioxide in a plurality of stages.   The said heating seawater acquisition step is a power generation method in any one of Claims 1-4 which heat this seawater after making the said carbon dioxide adsorb | suck to seawater, and acquire the said heating seawater.   The said heating seawater acquisition step is a power generation method in any one of Claims 1-5 which acquire the said heating seawater on the sea.   The said heating seawater injection | pouring step is a power generation method in any one of Claims 1-6 which inject | pours the said heating seawater into the said methane hydrate layer through the pipe | tube which gave the heat insulating material.   The power generation method according to claim 7, wherein an inner diameter of the pipe is approximately 0.5 m.   The power generation method according to claim 8, wherein the thickness and the heat transfer coefficient of the heat insulating material are ti and λi, respectively, and the value of ti / λi is in a range of approximately 0.00578 to 0.0578.   The power generation method according to any one of claims 1 to 9, wherein in the heating seawater injection step, carbon dioxide in the heating seawater is isolated and fixed in the layer in a liquid carbon dioxide state.   The power generation method according to claim 1, wherein the power generation step generates power at sea.   The power generation method according to claim 1, wherein the power generation step generates power by a Brayton cycle. The power generation step combusts methane and converts the combustion heat into electrical energy to generate power,
The said heating seawater acquisition step is a power generation method in any one of Claims 1-12 which mix the pressurized seawater and the combustion exhaust gas of the methane, and adsorb | suck the carbon dioxide in the said combustion exhaust gas to seawater.   The power generation method according to claim 13, further comprising a step of recovering energy of the residual exhaust gas from which carbon dioxide has been adsorbed and removed following the heating seawater acquisition step.   The power generation method according to claim 1, further comprising a step of transmitting electricity generated by the power generation through a submarine cable. A power generator using methane gas as fuel,
A carbon dioxide adsorbing device that adsorbs carbon dioxide discharged from the power generation device to seawater;
A heating device for heating the seawater adsorbed with carbon dioxide;
An injection device for transporting the heated seawater toward the seabed and injecting it into the methane hydrate layer of the seabed under a temperature and pressure determined as conditions under which methane hydrate can be decomposed;
A methane gas supply device that is disposed so as to be able to recover the methane gas dissociated from the methane hydrate in the layer by injection of the seawater, and supplies the recovered methane gas to the power generation device,
A carbon dioxide low emission power generation system comprising.   The power generation system according to claim 16, wherein the heating device heats the seawater using waste heat of the power generation device. The power generator is
A first compressor for compressing air;
A combustor for burning a mixed gas of methane gas having a predetermined pressure and air compressed by the first compressor;
A power generation turbine that is connected to the first compressor so as to be drivable, receives and operates the exhaust gas of the combustor, converts the combustion heat of the exhaust gas into electrical energy, and outputs the electric energy; The power generation system of Claim 16 or 17 comprised including.   The power generation system according to claim 18, wherein the heating device includes a heat exchanger that performs heat exchange between the exhaust gas of the combustor and the seawater. It further includes a pump that boosts and delivers seawater,
The power generation system according to claim 18 or 19, wherein the carbon dioxide adsorption device mixes seawater sent from the pump and exhaust gas of the combustor to adsorb carbon dioxide in the exhaust gas to the seawater.   The power generation system according to claim 20, further comprising a gas turbine that operates by receiving the remaining exhaust gas from which the carbon dioxide is adsorbed and removed from the exhaust gas.   The power generation system according to claim 21, further comprising a second compressor disposed so as to be drivable by the gas turbine and compressing the exhaust gas of the combustor before being supplied to the carbon dioxide adsorption device.   The power generation system according to any one of claims 16 to 22, further comprising equipment for installing the power generation device, the carbon dioxide adsorption device, and the heating device on the sea. The injection device comprises:
A hot water transport pipe arranged to be able to transport the heated seawater toward the seabed;
The power generation system according to any one of claims 16 to 23, comprising: a heat insulating material provided so as to be able to suppress heat transfer from the inside of the hot water transport pipe to external seawater.   The power generation system according to claim 24, wherein an inner diameter of the hot water transport pipe is approximately 0.5 m.   26. The power generation system according to claim 25, wherein the thickness and heat transfer coefficient of the heat insulating material are ti and λi, respectively, and the value of ti / λi is in a range of approximately 0.00578 to 0.0578.   27. The power generation system according to claim 16, further comprising a submarine cable that transmits electricity generated by the power generation device. The power generator is a device that generates electricity by burning methane and converting the combustion heat into electrical energy,
In addition to the carbon dioxide, the carbon dioxide adsorbing device adsorbs combustion products other than carbon dioxide generated by the combustion to seawater under high pressure,
The said injection | pouring apparatus is a power generation system in any one of Claims 16-27 which inject | pours the seawater which the said carbon dioxide and combustion product adsorb | sucked into the methane hydrate layer of the said seabed. The power generation system according to any one of claims 16 to 28, wherein the power generation device generates power by a Brayton cycle,
It is provided at the combustion part outlet of the power generator or downstream thereof, and further comprises a catalyst that oxidizes the combustion products generated with the power generation,
The carbon dioxide adsorption device is a power generation system that adsorbs combustion products oxidized by the catalyst to seawater in addition to the carbon dioxide.

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