JPS63602B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for generating cold energy using liquefied natural gas. (Prior Art) Conventionally, as a method of utilizing liquefied natural gas cold energy, there is liquefied natural gas cold energy power generation. Considering the large amount of liquefied natural gas consumed, the final power generation output differs depending on the various types of cold power generation methods, so the question is how to recover as much of the exergy (maximum effective work) of liquefied natural gas as possible. The key point is how to obtain high output. Known cold power generation methods that use only liquefied natural gas cold energy and do not use other fuels include (1) direct expansion cycle method, (2) secondary medium cycle method, and (3) direct expansion + secondary medium cycle method. The current situation is that a power generation method that can obtain higher efficiency and higher output with a simple equipment configuration is being searched for. (Problems to be solved by the invention) However, the direct expansion cycle method, that is,
A method in which liquefied natural gas is pressurized by a pump, then gasified with seawater, etc., and the natural gas directly drives a turbine has a simple and stable system structure, but it is not possible to lower the pressure at the turbine outlet side. In this case, there is a problem that the effective output is small.
In addition, the secondary medium cycle method uses ethylene, propane, etc. as a secondary medium, exchanges heat with liquefied natural gas, liquefies it with the cold heat, pressurizes the liquefied secondary medium, and then gases it with seawater etc. However, this secondary medium cycle system has the problem that sufficient exergy cannot be recovered. For this reason, a system that combines a direct expansion cycle and a secondary medium cycle has been proposed. As shown in Figure 1, the liquefied natural gas cold power generation system, which combines a direct expansion cycle and a secondary medium cycle, delivers liquefied natural gas at high pressure from a liquefied natural gas storage tank 1 using a pump 2, and condenses it. The secondary medium is condensed, and then the heated liquefied natural gas is transferred to a first heat exchanger 4 such as an open rack vaporizer to heat it with a secondary medium, such as propane, to raise its temperature. The natural gas is vaporized by exchanging heat with the natural gas, and the main turbine 5 is driven by the vaporized natural gas to generate electricity.
The natural gas on the turbine outlet side, whose temperature has decreased due to adiabatic expansion, is heated again by exchanging heat with seawater etc. in the second heat exchanger 6, and then this natural gas is supplied to the outside as fuel from the supply port 10. After the secondary medium condensed through heat exchange with the expansion cycle system and liquefied natural gas is pressurized by the pump 7, it is vaporized by exchanging heat with seawater etc. using a heat exchanger 8 such as an open rack vaporizer. This system is combined with a secondary medium cycle system that drives the auxiliary turbine 9 using a secondary medium to generate electricity. Although this cold power generation method can recover a larger amount of exergy than the direct expansion cycle or secondary medium cycle alone, it still only accounts for about 34% of the effective energy of liquefied natural gas. The present invention has been made in view of the current situation, and aims to provide a method for generating cold energy using liquefied natural gas, which can recover more exergy from liquefied natural gas and obtain larger amounts of electric power. This is the purpose. (Means for solving the problems) The present invention provides, as means for solving the problems, the following:
In a liquefied natural gas cold heat power generation method including a direct expansion cycle system in which liquefied natural gas is pressurized and vaporized and the natural gas drives a main expansion turbine,
A part of the natural gas in the direct expansion cycle system is reliquefied by exchanging heat with liquefied natural gas by a regenerator installed in the direct expansion cycle system, and then returned to the liquefied natural gas flow path on the inlet side of the regenerator. This is what I did. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a description will be given with reference to a system diagram of a liquefied natural gas cold-thermal power generation device used to carry out the method of the present invention. In addition, in the figure, the same reference numerals are given to devices equivalent to those in the apparatus of FIG. 1. The liquefied natural gas cold thermal power generation device according to the present invention has a direct expansion cycle system and a secondary medium cycle system that have the same configuration and function as the device shown in FIG. It has a regeneration cycle system that re-liquefies the natural gas obtained through the process and directly returns it to the expansion cycle system as liquefied natural gas. The regeneration cycle system in the apparatus shown in FIG. 2 includes a regenerator 12 disposed in a liquefied natural gas passage 11 that increases the pressure of liquefied natural gas from a tank 1 with a pump 2 and supplies it to a condenser 3, and a main generator. A part of the gas on the outlet side of the main expansion turbine 5 that drives G ₁ is transferred to the regenerator 1
2, and a regenerator 1 that reliquefies a part of the gas on the outlet side of the main expansion turbine 5.
The pump 14 increases the pressure of the reliquefied natural gas from the regenerator 12 and returns it to the liquefied natural gas passage 11a on the inlet side of the regenerator 12. Reference numeral 15 is a regeneration turbine that is installed as necessary when the discharge pressure of the main expansion turbine, that is, the pressure of the natural gas discharged to the outside of the device cannot be lowered due to the usage. When this regeneration turbine 15 is provided, there is an advantage that the pressure of the regeneration cycle system can be selected regardless of the pressure at which natural gas is delivered to the outside of the apparatus, similar to the embodiment described later. In the above device, the operations of the direct expansion cycle system and the secondary medium cycle system are almost the same as those shown in FIG. Since the temperature has decreased due to adiabatic expansion, the temperature is increased by exchanging heat with seawater etc. in the second heat exchanger 6, and then the natural gas is sent to the outside of the device and the natural gas supplied to the regeneration cycle system. Channel 1 divided into two by channels 13a and 13b
The natural gas passing through 3a can be passed through directly or as shown in the regeneration turbine 15 which drives the generator _G1 .
After working in the regenerator 12 to reduce the pressure and temperature, the regenerator 12
There, the liquefied natural gas exchanges heat with the liquefied natural gas from the tank 1 to raise its temperature, while the liquefied natural gas itself is reliquefied and after being pressurized by the pump 14, the liquefied natural gas at the inlet side of the regenerator 12 is heated. Natural gas flow path 11
a, and is sent to the condenser 3 on the downstream side via the regenerator 12, and again works in the direct expansion cycle system. According to this method, the exergy efficiency can be significantly improved compared to the conventional method shown in FIG. FIG. 3 is a system diagram showing a flow sheet of another embodiment of the apparatus used for carrying out the method of the present invention, in which the natural gas circulating in the regeneration cycle system is used at the outlet side of the first heat exchanger 4, in other words. , a part of the gas on the inlet side of the main expansion turbine is used to generate a generator using this gas.
After driving the regeneration turbine 15 that drives G ₃ ,
The outlet side gas of the regeneration turbine 15 is transferred to the regenerator 12.
The other configuration and operation are the same as those in FIG. 2, except that the natural gas is supplied to the flowchart, but the flow corresponding to the outlet natural gas pressure conditions from high pressure to low pressure is comprehensively shown. Note that, as in the cooling power generation method shown as a comparative example in FIG.
However, in this case, the ratio of the natural gas flowing through the regeneration cycle system to the total natural gas flowing through the direct expansion cycle system, that is, the regeneration ratio, cannot be set high, so This is not preferable because the exergy efficiency is significantly reduced. That is, the temperature of the regenerator and the amount of converted heat (T-
The relationship Q) is shown in Figure 5. In both the method of the present invention and the method of the comparative example, when the regeneration ratio is varied in the range of 0.1 to 0.5, the condensation curve of the recycled natural gas shifts to the right side of the figure as the regeneration ratio increases, as shown by the broken line. Move to. However, the evaporation curve of liquefied natural gas is constant regardless of the regeneration ratio in the comparative example, as shown by the solid line.
In the method of the present invention, the evaporation curve of liquefied natural gas changes as the regeneration ratio changes, as shown by the solid line with the regeneration ratio. Therefore, in the method of the comparative example, when the regeneration ratio is increased, the condensation curve and the evaporation curve intersect at the regeneration ratio of 0.5, and the temperature difference between the regenerated natural gas and the liquefied natural gas disappears. Heat exchange becomes impossible. On the other hand, in the method of the present invention, even if the regeneration ratio is increased, the evaporation curve changes, so even when the regeneration ratio becomes 0.5, the condensation curve and the evaporation curve do not intersect. Even at the point (pitch point) where the temperature difference Δt is minimum, the temperature difference can be maintained between the recycled natural gas and the liquefied natural gas, and heat exchange can be performed. Fifth
As can be seen from the figure, in both the comparative example and the method of the present invention, as the regeneration ratio increases, the Δt (temperature difference) between the two fluids increases.
is smaller, but at the same reproduction ratio, the method of the present invention has a larger Δt at the pinch point (the point where Δt is minimum) than the comparative example. In other words, it can be said that in terms of the design of the regenerator, the method of the present invention allows the regeneration ratio to be larger than that of the comparative example. There is a proportional relationship between the regeneration ratio and the output in the regeneration cycle, and a larger regeneration ratio means a larger output. Therefore, in the liquefied natural gas direct expansion cycle with regeneration excluding the secondary medium cycle, the larger the regeneration ratio, that is, the higher output can be obtained in the method of the present invention than in the comparative example method. On the other hand, if we consider the secondary medium Rankine cycle,
In this case as well, the above explanation applies as is to the output of the reproduction cycle, so here we will consider the output of the secondary medium Rankine cycle. The inlet temperature of liquefied natural gas to the secondary medium condenser in the secondary medium Rankine cycle is determined only by the regeneration ratio, regardless of the comparative example or the method of the present invention. As described in the explanation of the output in the regeneration cycle, the larger the regeneration ratio, the greater the output in the regeneration cycle. However, as the regeneration ratio increases, the temperature of the liquefied natural gas inlet to the secondary medium condenser increases. That is, the exergy available in the secondary medium Rankine cycle decreases, resulting in a decrease in output. Therefore, in the LNG direct expansion with regeneration + secondary medium Rankine cycle system, an increase in output due to the regeneration cycle leads to a decrease in output in the secondary medium Rankine cycle, and vice versa. Therefore, it is necessary to judge which is more effective: aiming to increase the output through the regeneration cycle or increasing the output through the secondary medium Rankine cycle. but,
The maximum exergy that can be recovered in the secondary medium Rankine cycle is limited to the range from the condensing temperature of -40°C to the post-work temperature of 10°C when the secondary medium is propane, so the output increase in the propane cycle is always regenerated. It will not exceed the power reduction in the cycle. This means that the propane Rankine cycle is −40
This is because exergy can be recovered only in a temperature range of ℃ or above, whereas on the regeneration cycle side, exergy can be recovered in a wide temperature range of -124.5℃ or above. That is, it is more effective to perform exergy recovery using the Rankine cycle, which can be used in a wider temperature range. In the end, in the direct expansion with regeneration + secondary medium Rankine cycle method, it is desirable to obtain the output through the regeneration cycle, that is, the LNG Rankine cycle, that is, the method in which the regeneration ratio can be made as high as possible. It can be seen that it is. Example: The amount of liquefied natural gas delivered from the liquefied natural gas storage tank 1 by the pump 2 is 100 tons/hr, the temperature is -160°C, the supply pressure is 50ata, and the natural gas delivery temperature to the outside of the device is +10°C. Pressure 8.0ata
Electric power was generated using the apparatus shown in Figure 2 under these conditions. The results are summarized in the first stage along with the operating conditions of the regeneration cycle system.
Shown in the table. Comparative Examples 1-2 Power was generated using the same equipment as in Example 1 using the apparatus shown in FIG. 1 and the apparatus shown in FIG. The results are also shown in Table 1. The operating conditions of the reproduction cycle system of the apparatus shown in FIG. 4 were set so that the sum of the output in the secondary medium cycle and the output in the reproduction cycle system was maximized.

【table】

[Table] As is clear from the above description, according to the method of the present invention, by adding a regeneration cycle, the same increase in output as in the case of having an independent Rankine cycle can be achieved with a simpler equipment configuration. Also, by returning the regeneration cycle to the inlet side (low temperature side) of the regenerator, there is an approximately 6% increase in power output in the same regeneration system compared to returning the regeneration cycle to the output side (hot side) of the regenerator. Increasing output with a relatively small increase in cost in this way is an extremely effective means in a situation where the thermal efficiency of current thermal power plants is approximately 40%, which has almost reached its limit. It goes without saying that the present invention is not limited to the above-mentioned embodiments, but can be modified in various ways. For example, instead of configuring the main expansion turbine with a single turbine, it may be configured in multiple stages by connecting a high pressure turbine and a medium/low pressure turbine in series. Instead, as shown in FIG. 6, the gas to the regeneration cycle system or the fuel gas to the outside of the device may be taken out from the middle of the main expansion turbine. In addition, in the above explanation, the direct expansion method + secondary medium Rankine method was explained, but the present invention is also effective in the case of only the direct expansion method, and the flow in this case is shown in FIGS. 2 and 3. The secondary medium Rankine cycle system is simply omitted from the flow. Furthermore, the actual power generation amount is, for example, the value obtained by excluding the G ₂ output in the Rankine cycle from the total power generation amount in Table 1.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing a flow sheet of a conventional liquefied natural gas cold power generation device, FIG. 2 is a system diagram showing a flow sheet of an embodiment of a liquefied natural gas cold power generation device according to the present invention, and FIG. Fig. 4 is a system diagram showing a flow sheet of another embodiment of the present invention, Fig. 4 is a system diagram showing a flow sheet of a reference example, Fig. 5 is a graph showing the relationship between temperature and exchanged heat amount in the regenerator, and Fig. The figure is an explanatory diagram showing a modification of the cold-thermal power generation device shown in FIG. 2. 1... Tank, 2, 7, 14... Pump, 3...
... Condenser, 4, 6, 8 ... Heat exchanger, 5, 9, 1
5... Turbine, 12... Regenerator, G ₁ , G ₂ , G ₃
……Generator.

Claims (1)

[Scope of Claims] 1. In a liquefied natural gas cold power generation method including a direct expansion cycle system in which liquefied natural gas is pressurized and vaporized and the natural gas drives a main expansion turbine, the natural gas in the direct expansion cycle system is A part of the liquefied natural gas is reliquefied by heat exchange with the liquefied natural gas in a regenerator installed in the direct expansion cycle system, and then returned to the liquefied natural gas flow path on the inlet side of the regenerator. Cold power generation method. 2. The method according to claim 1, wherein a part of the natural gas in the direct expansion cycle system is pressure-regulated to a predetermined pressure by a regenerative expansion turbine, and then reliquefied by the regenerator.