LNG Cold and Heat Power Generation Using Hybrid Working Fluids

Abstract

When LNG cold energy power generation is performed using a mixed working fluid consisting of carbon dioxide and ethane according to the present invention, waste heat in the process can be utilized, and the amount of natural gas consumed can be significantly reduced compared to conventional power generation processes.

Description

Technical Field

[0001] The present invention relates to LNG cold-heat power generation using a mixed working fluid.

Background Art

[0002] The liquefied natural gas (hereinafter, LNG) imported by Korea is about 40 million tons or more per year, making it the third largest importer in the world after Japan and China. As a single receiving base, the scales of the Pyeongtaek and Incheon receiving bases of Korea Gas Corporation are the first and second largest in the world. Korea, Japan, and China import natural gas liquefied at -163°C or lower under conditions close to normal pressure.

[0003] The reason for liquefying natural gas is that the volume of LNG decreases to about 1 / 600 of that under normal temperature and pressure conditions, making storage and transportation easier. However, in order to liquefy natural gas at -163°C or lower, a refrigerator and electrical energy for driving the refrigerator are required. Although it varies depending on the liquefaction process, Table 1 below compares the required power for liquefying 1 kg / h of LNG by liquefaction process.

[0004]

Table 1

[0005] When LNG in a low-pressure state is vaporized by seawater, no power can be obtained. LNG cold energy power generation means that after using a pump to increase the pressure in the LNG state and then exchanging heat with seawater for vaporization, high-pressure natural gas can be obtained, so a considerable amount of power can be obtained by a turbine. At this time, the power consumed by liquid pumping is relatively small compared to the power obtained by the operation of the turbine using the high-pressure natural gas.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention aims to provide a mixed working fluid that can improve the effect of conventional LNG cold energy power generation.

Means for Solving the Problems

[0007] The present invention provides an LNG cold energy power generation including a working fluid composed of carbon dioxide and ethane; a pump; an evaporator; a turbine; and a condenser.

[0008] The molar ratio of the carbon dioxide and ethane is preferably 85-95:15-0.5.

[0009] It is preferable to use the working fluid in a closed Rankine cycle.

[0010] The supply pressure of the supplied LNG is preferably adjusted to the pressure at which it is in a saturated liquid state.

[0011] The temperature at the rear end of the condenser is preferably adjusted to the saturated vapor temperature of the LNG.

[0012] The pressure at the rear end of the pump is preferably adjusted to the critical pressure of the mixed working fluid.

[0013] Preferably, the cycle includes two turbines and a heater therebetween.

[0014] The heater preferably heats the working fluid so that it is not condensed.

[0015] It is preferable that the evaporator or the heater or both utilize waste heat in the process.

Advantages of the Invention

[0016] When performing LNG cold energy power generation using the mixed working fluid according to the present invention, waste heat in the process can be utilized, and the consumption of natural gas can be significantly reduced compared to conventional power generation processes.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0018] 〔Best Mode for Carrying Out the Invention〕 The present invention provides LNG cold energy power generation including a working fluid composed of carbon dioxide and ethane; a pump; an evaporator; a turbine; and a condenser.

[0019] 〔Modes for Carrying Out the Invention〕 Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0020] In order to describe the present embodiment, when adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are added to the same components as much as possible even if they are shown on other drawings. Further, in the description of the present invention, when it is determined that a specific description of a related known configuration or function may obscure the gist of the present invention, the detailed description thereof is omitted. The accumulation ratio is not applied to the drawings referred to below.

[0021] In the description of the components of the present invention, terms such as first, second, A, B, (a), (b), etc. can be used. Such terms are for distinguishing the components from other components, and the essence, order, or sequence of the components is not limited by such terms.

[0022] When a component is described as being "connected", "coupled", or "joined" to another component, it should be understood that the component may be directly connected or directly joined to the other component, but there may also be another component "connected", "coupled", or "joined" between the components.

[0023] Also, when a component such as a layer, film, region, plate, etc. is said to be "on" or "above" another component, this should be understood to include not only the case where it is directly above the other component, but also the case where there is another component in between. Conversely, when a component is said to be "directly above" another part, it should be understood to mean that there is no other part in between.

[0024] FIG. 1 shows a conceptual diagram of the simplest open Rankine cycle for LNG cold thermal power generation using a working fluid.

[0025] According to FIG. 1, LNG under a pressure condition slightly higher than the normal pressure near -162°C becomes high-pressure LNG after being pressurized by a pump. Then, when heat-exchanged with seawater, the LNG evaporates and undergoes a phase change into high-pressure natural gas. When a turbine is operated using the high-pressure natural gas, power can be produced thereby.

[0026] Tokyo Gas Co. utilized 10 tons / h of LNG to produce approximately 290 kW of power.

[0027] Since the composition of LNG was not accurately known, the Typical gas composition was used from the LNG composition imported by Korea Gas Corporation as shown in Table 2.

[0028]

Table 2

[0029]

Table 3

[0030] Tokyo Gas Co. obtained a power generation effect of 442 kW using 10 tons / h of LNG cold heat by LNG cold heat power generation using propane as the working fluid in a closed Rankine cycle as shown in Figure 3.

[0031] Figure 4 shows a flow sheet that realizes a closed Rankine cycle using PRO / II with PROVISION.

[0032] The computer simulation results using PRO / II with PROVISION in Fig. 4 are summarized and arranged in Table 4 below.

[0033] [Table 4] According to Table 4, the net power obtained from the cold energy of 1 ton / h of LNG is 35.768 kW. Furthermore, since the temperature at the rear end of the working fluid evaporator is 120°C, it is necessary to use steam to evaporate the working fluid. To obtain steam, combustion of natural gas is required. In Table 2 above, the molecular weight of LNG is 17.924 kg / k-mole and the GHV is 10,450 kcal / Sm3. Therefore, the mass flow rate of LNG required to supply the heat amount of 1.2046×106 kcal / h, which is the heat duty of the working fluid evaporator, is 92.24 kg / h. This means that 92.24 kg / h of natural gas per hour is consumed for LNG cold energy power generation using propane as the working fluid.

[0034] The selection conditions for the working fluid for application to the closed Rankine cycle utilizing LNG cold energy according to the present invention are as follows.

[0035] First, a working fluid that can operate with a high pressure at the rear end of the pump is good. This is related to the critical pressure of the working fluid. The pressure at the rear end of the pump is generally pressurized to near the critical pressure.

[0036] Second, the lower the temperature at the rear end of the working fluid condenser due to heat exchange with LNG, the more advantageous it is. This is because the pressure at the rear end of the expansion valve can be immediately reduced, and the expansion ratio in the turbine can be increased, so that more power can be obtained. The temperature at the rear end of the working fluid condenser is related to the freezing point of the working fluid. Since the rear end of the working fluid condenser is immediately connected to the pump, there is a constraint that the temperature above the freezing point of the working fluid must be maintained.

[0037] Thirdly, it is more advantageous if the temperature is lower at the rear end of the working fluid evaporator. In the case of Fig. 4, when propane is used as the working fluid, the temperature at the rear end of the evaporator was 120°C. In this case, since low-pressure (LP) steam must be used to evaporate the working fluid, consumption due to the combustion of natural gas occurs.

[0038] In order to select a working fluid that satisfies such conditions, Table 5 below summarizes some basic physical property values for several working fluid candidates.

[0039] [Table 5] Among the working fluid candidates shown in Table 5 above, in terms of critical pressure, carbon dioxide is most advantageous at 73.83 bar. In terms of freezing point, the ethane component is most advantageous at -182.8°C. In this case, since it is lower than the supply temperature of LNG, there is a disadvantage that the cold heat of LNG due to the complete evaporation of LNG cannot be fully utilized. And finally, the critical temperature of the working fluid is more advantageous if the temperature is lower when it is completely evaporated in the working fluid evaporator after being pressurized to near the critical pressure in the pump. In this case, since the critical temperature of ethylene is 9.19°C, which is low, it can be said to be the most advantageous.

[0040] Table 6 below shows the composition, temperature, and pressure conditions of LNG used in the present invention.

[0041] [Table 6] In order to fully utilize the latent heat of LNG, the supply pressure of LNG was reduced to 0.605 MPaG at which it becomes a saturated liquid state. It was found that the temperature at which LNG completely evaporates under this condition is -54.125°C. At this time, in order to fully utilize the cold heat of LNG so that the temperature at the rear end of the condenser of the mixed working fluid differs from the temperature after the evaporation of LNG by only 3°C, the composition of carbon dioxide and ethylene to reach -51.125°C was preferably 10 mol% of carbon dioxide and 90 mol% of ethylene.

[0042] In Table 5 above, a power generation process using LNG cold heat was devised by using a mixed working fluid with a composition of 90 mol% of carbon dioxide and 10 mol% of ethane respectively in a process as shown in Fig. 5. Here, E3 is a condenser for the mixed working fluid, and since it exchanges heat with LNG, it is in fact the same heat exchanger as E4 here.

[0043] The following Table 7 intensively shows some physical properties of carbon dioxide and ethane.

[0044]

Table 7

[0045]

Table 8

[0046] According to Table 8 above, the net power obtained from the cooling heat of 1 ton / h of LNG is 36.573 kW. This is slightly higher than the net power of 35.768 kW obtained when propane is used as the working fluid. It can be seen that the temperature at the rear end of the working fluid evaporator is 69.215°C. It can be seen that this is a very low temperature compared to 120°C, which is the temperature at the rear end of the evaporator of the propane working fluid. It can be seen that this is a temperature at which the waste heat available in the process can be utilized to a low extent. Also, this process has the advantage that it does not need to consume 92.24 kg / h of natural gas compared to the power generation process that utilizes the LNG cooling heat with propane as the working fluid.

[0047] The above description is merely illustrative of the present invention, and those with ordinary knowledge in the technical field to which the present invention pertains can make various modifications without departing from the essential characteristics of the present invention.

[0048] Therefore, the embodiments disclosed in this specification are for illustrative purposes rather than for limiting the present invention, and the scope of the idea of the present invention is not limited by such embodiments. The protection scope of the present invention shall be construed according to the following claims, and all technologies within the equivalent scope should be construed as being included in the scope of the rights of the present invention.

Claims

1. An LNG cold heat power generation including a working fluid composed of carbon dioxide and ethane, a pump, an evaporator, a first and a second turbine, and a condenser, using the working fluid in a closed Rankine cycle in which the working fluid flows through the pump, the evaporator, the first turbine, the second turbine, and the condenser in this order, and further including a heater located between the first turbine and the second turbine for heating so that the working fluid is not condensed.

2. The LNG cold heat power generation according to Claim 1, characterized in that the molar ratio of carbon dioxide and ethane is 85 to 95:15 to 0.

5.

3. The LNG cold heat power generation according to Claim 1, characterized in that the supply pressure of the supplied LNG is adjusted to a pressure at which it becomes a saturated liquid state.

4. The LNG cold heat power generation according to Claim 1, characterized in that the temperature at the rear end of the condenser is adjusted to the saturated vapor temperature of the LNG.

5. The LNG cold heat power generation according to Claim 1, characterized in that the pressure at the rear end of the pump is adjusted to the critical pressure of the mixed working fluid.

6. The LNG cold heat power generation according to Claim 1, characterized in that waste heat in the process is utilized by the evaporator or the heater or both.

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