JPS5925851B2 - Power recovery method using liquefied natural gas vaporization and cold heat using the cascade Rankine cycle - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention efficiently utilizes effective energy between LNG and a high heat source fluid by forming two types of Rankine cycles in a cascade in the regasification of liquefied natural gas (hereinafter abbreviated as LNG). It is often related to methods of recovering power.

LNG has attracted attention for its non-polluting properties, and the amount of LNG introduced in Japan has exceeded 10 million tons per year.

As is well known, this LNG is stored in a liquid state at -160°C, but in order to use it as a fuel for power generation or as a raw material for city gas, it must be pressurized to a predetermined pressure and then regenerated into a gaseous state at room temperature by some method. It is necessary to gasify it.

This regasification requires a large amount of heat, about 20 million kilocalories, for 100 tons of LNG, and conventionally, a so-called open rack vaporizer, which uses seawater as a heating source, is used. In most cases, the effective cold energy of LNG is unnecessarily released into seawater without being recovered.

However, from the viewpoint of energy conservation, efforts have recently been made to actively develop ways to recover the effective cold energy of LNG as power.

The methods include: (1) A method in which LNG is vaporized using a single component such as propane as a medium, and power is recovered by the Rankine cycle of a single component medium using seawater as a high-humidity heat source (Japanese Patent Application Laid-Open No. 53-119) 1
(No. 26003) (2) LN using a mixture of nitrogen and light hydrocarbons as a medium
A method of vaporizing G and recovering power using the Rankine cycle of this mixture medium (Japanese Patent Laid-Open No. 17401/1983) (3) After increasing the pressure of LNG to a high pressure, vaporizing it with seawater etc., and reducing the pressure to a predetermined pressure with a turbine. How to recover power.

(4) A method of recovering power using the Playton cycle, which uses nitrogen as a heat medium, uses LNG as a low heat source, and burns a portion of regasified LNG as a high heat source.

(5) There is a method of recovering power using an open gas turbine method in which air is used as a heat medium, LNG is used as a low heat source, and a portion of regasified LNG is burned as a high heat source.

For methods (4) and (5), regasified LN
Since the combustion gas, which is partially fueled by G, is used as a high heat source, the output is high from the viewpoint of a generator, but from the standpoint of energy conservation, the heating source requires fuel,
It consumes its own fuel, so it cannot necessarily be said to be advantageous.

On the other hand, methods (LL (2) and (3)) use seawater or the like as a high heat source, and there is no self-fuel consumption, so it can be said that they are excellent from the viewpoint of overall energy saving.

Regarding methods (1), (2), and (3),
In order to efficiently recover cold energy, LN
It is important to minimize the loss of effective energy associated with heat exchange between G and the heat medium and between seawater and the heat medium, and this is done by minimizing the humidity difference in each heat exchange as much as possible. Achieved by doing.

However, regarding method (1), in heat exchange between the heat medium and LNG, since the heat medium is a single component, in order to exchange heat while minimizing the temperature difference, as shown in Figure 1, , it is necessary to use multiple stages, which is disadvantageous in terms of cost.

Also, if we look at the so-called direct expansion method in (3), L
Since iNG directly exchanges heat with seawater, which is a high heat source fluid, the temperature difference is large, and the effective energy loss in this heat exchange is quite large, making it inefficient.

On the other hand, in method (2), by adjusting the composition of the mixed heat medium, it is possible to keep the humidity difference during heat exchange between 10NG and the heat medium to a small level, and to reduce the effective energy loss during this heat exchange. I can do it.

The T-Q curve obtained by method (2) is shown in Figure 2, but L
Since the main focus is on heat exchange between NG and the mixed heat medium, there is also effective energy that cannot be recovered, as shown by the shaded area, in the heat exchange between seawater, which is a high heat source, and the mixed heat medium. are doing.

The present invention aims to solve the above-mentioned drawbacks, and particularly to improve the method (2), by providing a second heating medium between the high heat source and the mixed heating medium, which is shown in the shaded area in FIG. By constructing a Rankine cycle and exchanging heat between the high-pressure mixed medium and the low-pressure second medium, that is, by cascading each Rankine cycle using two types of media, effective energy loss is reduced and power recovery is further improved. It is something that causes an increase.

In the cascade, the LNG heated by exchanging heat with the low-pressure mixed medium in a multi-fluid heat exchanger is further heat-exchanged with the high-pressure mixed medium and the second low-pressure medium.

As a result, the amount of heat exchanged by the second medium increases, and the effective cold energy of the LNG can be further recovered as motive power.

The composition of the heat carrier in the first Rankine cycle is L
The composition of the NG varies depending on its vaporization, pressure, and humidity conditions of an external high heat source such as seawater, so that the temperature difference between the LNG and the first heat medium in the heat exchanger is about 3 to 10 degrees Celsius throughout.
They are also selected taking into consideration the overall economic efficiency.

The heat medium composition of the second Rankine cycle is determined based on the composition of the heat medium of the first Rankine cycle determined according to the above criteria, its evaporation pressure, the temperature conditions of the external high heat source, etc., while considering the overall economic efficiency. The power generation from the second Rankine cycle turbine is determined to be a dog.

In addition, regarding the components constituting each heat medium, whether or not they are easy to manufacture or obtain on-site is also a factor in selection.

Hereinafter, the present invention will be explained in detail with reference to FIG. 3 showing one embodiment of the present invention.

However, the present invention is not limited to this.

The conditions for sending regasified natural gas are 100 tons/hour 8.5 Yu/cfItG and 10℃, and the temperature of seawater, which is a high heat source, is 1.
Figure 3 shows the case where the temperature was 5°C.

-160°C LNG is pressurized from storage facility 1 by pump 2 and sent out to line a of multifluid heat exchanger 3.
is heated by exchanging heat with the first mixed heat medium in line C,
It vaporizes and becomes -20°C.

Furthermore, the line a' of the multi-fluid heat exchanger 4 exchanges heat with the line of the second mixed heat medium d to be heated to -6°C, and after being heated with seawater in the heat exchanger 5, the temperature is raised at a predetermined temperature. After warming, it is sent to the demand side.

The mixed heating medium which is the first medium on this side is methane 39
．． 49 mol%, ethane 37.59 mol%, furopane 16
．． 23 mol%, butane 6.69 mol%.

Pressure from the first turbine 6: 4.8 kg/ail
G first medium enters line C of multifluid heat exchanger 3;
It is cooled by exchanging heat with the LNG in line a and the high-pressure first medium in line b, and is completely condensed.

The low-pressure first medium thus liquefied is pumped to the pump 7 for 14k
The pressure is increased to g/crttG, and it becomes a high-pressure first medium,
Similarly to the heating of LNG, line b of the multifluid heat exchanger 3,
It is heated to 10° C. in line b' of the multifluid heat exchanger 4 and in the seawater heat exchanger 8, and is completely vaporized.

The vaporized high pressure first medium has a pressure of 13.2 ky/ff.
Enters the first turbine b at l G, 4.8 kg/cr
It reduced the pressure to G and gave 6,310kW of power.
It becomes a low-pressure first medium and forms a first Rankine cycle.

On the other hand, the second heat medium consists of components with a higher boiling point than the first medium, in this case, 5.28 mol% of ethylene and 94.76 mol% of propane. The low-pressure second medium is cooled by heating line a' and line b' in line d of the multifluid heat exchanger 4.
It is condensed into a completely liquid phase and pumped at 6.5 k by pump 10.
After increasing the pressure to g/crrLG, it becomes a high-pressure second medium, and is heated and vaporized by seawater in the heat exchanger 11.

The completely vaporized high pressure second medium is transferred to the second turbine 9
The pressure is reduced from 5.7 Y/cat G to 3.4 kg/cviG, and a power of 480 kW is provided.
It becomes a medium and forms a second Rankine cycle.

An example of the above cascade Rankine cycle is shown in FIG. 4 as a temperature-enthalpy diagram.

In the above example, the high heat source fluid is 15°C.
However, from the spirit of the present invention, the high heat source fluid is not limited to seawater (for example, it is effective for even higher temperature heating sources such as waste heat steam, and in some cases In some cases, this system may be possible without installing the heat exchangers 5 and 8.

As described above, in the method of vaporizing liquefied natural gas using the cascade Rankine cycle of the present invention and recovering power by cold heat, the low-pressure cooling curve almost matches the evaporation curve of the liquefied natural gas to be gasified. Select a mixture of hydrocarbons of numbers 1 to 6 or a mixture of halogenated hydrocarbons whose boiling point is similar to that of the hydrocarbons, and minimize the loss of effective energy accompanying heat exchange between the first medium and the liquefied natural gas. (a) as a second medium having a low-pressure cooling curve that substantially matches the evaporation curve of the first medium;
Selecting a single substance or mixture of hydrocarbons having 1 to 6 carbon atoms, (0) a single substance or mixture of halogenated hydrocarbons having a boiling point similar to the hydrocarbon described in (a), or (c) ammonia. This minimizes the energy loss between the light heat source such as seawater and the first medium, and also reduces the outlet temperature of the regasified natural gas and the first medium by the first and second multi-fluid heat exchangers. By raising the seawater, the freezing conditions when using seawater for heat exchange can be relaxed, making it possible to use a normal shell-and-tube type heat exchanger and reducing costs.

The recovery power when the second Rankine cycle is not constructed is 1
Compared to 6310 kW for 00 tons/hour of LNG, a total of 6790 kW can be recovered by incorporating the second Rankine cycle.

[Brief explanation of the drawing]

Figure 1 shows a temperature-heat load diagram for vaporizing liquefied natural gas using a Rankine cycle using a conventional single-component heat medium.
Figure 2 is a temperature-heat load diagram for vaporization of liquefied natural gas using a Rankine cycle using a conventional mixed component heat transfer medium.
FIG. 3 is a schematic system diagram of an apparatus utilizing the present invention, and FIG. 4 is a temperature-enthalpy diagram of the medium and natural gas cycle of FIG. 3. 1...Storage equipment, 2...Pump, 3,
4... Multifluid heat exchanger, 5... Heat exchanger, 6... Turbine, 1... Pump,
8... Heat exchanger, 9... Turbine, 1
0...Pump, 11...Heat exchanger.

Claims (1)

[Claims] 1. A mixture of hydrocarbons having 1 to 6 carbon atoms or a halogenated hydrocarbon whose boiling point is similar to that of the hydrocarbons, using liquefied natural gas as a low heat source, and whose low pressure cooling curve is similar to that of gas. A first medium having a composition substantially matching the evaporation curve of the liquefied natural gas to be converted is subjected to a first Rankine cycle, and in the first Rankine cycle, in a first multifluid heat exchanger,
exchanging heat between the liquefied natural gas and the high pressure first medium with the low pressure first medium, and disposing a first turbine to recover power;
(a) A single substance or a mixture of hydrocarbons having 1 to 6 carbon atoms, or (b) a single substance or a mixture of halogenated hydrocarbons having a boiling point similar to the hydrocarbon described in (a), or (c)
Ammonia, whose low pressure cooling curve is the same as the high pressure first one.
A second medium having a boiling point higher than that of the first medium, which almost matches the evaporation curve of the medium, is subjected to a second Rankine cycle, and the liquefied natural gas heated in the first Rankine cycle is heated at high pressure in a second multi-fluid heat exchanger. Simultaneously with the first medium, heat is exchanged with a low-pressure second medium to cascade the first Rankine cycle and the second Rankine cycle, and a second turbine is disposed in the second Rankine cycle to recover power and liquefy natural A power recovery method using liquefied natural gas vaporization and cold heat using a cascade Rankine cycle, which is characterized by vaporizing gas. 2 The first medium introduces nitrogen or hydrogen into the hydrocarbon mixture,
Alternatively, a method for recovering power by vaporizing and cooling liquefied natural gas using a cascade Rankine cycle according to claim 1, wherein the halogenated hydrocarbon contains nitrogen. 3. A power recovery method using liquefied natural gas vaporization and cold heat using a cascade Rankine cycle according to claim 1 or 2, wherein the second medium contains nitrogen in the hydrocarbon mixture.