Fuel Gas Supply System

Abstract

To provide a fuel gas supply system which can constantly maintain the pressure of a gas sending mother pipe for supply to a power plant even if a BOG flow rate supplied to the power plant is varied when adding a compressed BOG to a natural gas which is vaporized by an LNG vaporizer and supplying them to the power plant.SOLUTION: In a fuel gas supply system for controlling an LNG supply flow rate to an LNG vaporizer 3 so as to make a gas pressure in a gas sending mother pipe 6 constant when supplying a natural gas which is vaporized by an LNG vaporizer 3 and a boil-off gas which is compressed by a BOG compressor 4 to a power plant 5 via the gas sending mother pipe 6, a value obtained by subtracting a control margin of the LNG supply flow rate which can compensate for a variation width of a boil-off gas supply flow rate supplied to the power plant 5 by controlling the LNG supply flow rate supplied to the LNG vaporizer 3 from a maximum vaporization capacity of the LNG vaporizer 3 is set as a threshold, and an increase in output of the power plant 5 is stopped when the LNG supply flow rate supplied to the LNG vaporizer 3 is more than the threshold.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel gas supply system that supplies vaporized natural gas and compressed boil-off gas to a power plant via a gas supply header, and relates to a fuel gas supply system that can maintain a constant gas pressure in the gas supply header by compensating for fluctuations in the supply flow rate of boil-off gas to the power plant by controlling the LNG supply flow rate to the LNG vaporizer, even when the maximum delivery rate of the LNG vaporizer corresponds to the maximum load of the power plant's notified output. [Background technology]

[0002] At a storage facility (LNG terminal) that stores LNG to be supplied to a power plant, LNG unloaded from an LNG carrier is stored in an LNG tank 2, as shown in FIG. 4, and then discharged from the LNG tank 2 by an LNG pump (not shown). The LNG is vaporized in an LNG vaporizer 3 and then sent to a power plant 5 via a gas transmission header 6. The LNG tank 2 constantly receives heat from the outside and also from the LNG pump, etc. These heat inputs are consumed as the latent heat of vaporization of the LNG, generating boil-off gas (BOG). To maintain a constant pressure in the LNG tank 2, the generated BOG is suction-compressed (BOG volume adjustment) by a BOG compressor 4 and sent to the gas transmission header 6, where it is combined with natural gas (NG) vaporized in the LNG vaporizer 3 and sent to the power plant 5.

[0003] In order to supply a stable amount of gas to match the consumption amount at the power plant 5, the gas pressure in the gas supply header 6 is kept constant at a predetermined pressure. That is, the required supply flow rate to the LNG vaporizer 3 is determined based on the gas pressure in the gas supply header 6, the BOG supply flow rate supplied from the BOG compressor 4 to the gas supply header, the LNG supply flow rate supplied from the LNG tank 2 to the LNG vaporizer 3, and the fuel consumption amount at the power plant, and the aperture of the flow control valve 7 provided on the inlet side of the LNG vaporizer is controlled so that this required supply flow rate is obtained.

[0004] In such power generation facilities, the notified output (CMW, B) >> C) is set within the range of the maximum load (BMW, A>B) in the environmental impact assessment conducted before construction for the power plant's generating capacity (AMW), and operation is controlled so as not to exceed this notified output. Conventionally, the notified output (CMW) was set to a value well below the maximum load (BMW) in the environmental impact assessment, so even if there were fluctuations in the power plant's consumption or BOG flow rate, it was possible to obtain the required gas supply volume by controlling the LNG supply flow rate to the LNG vaporizer (it was possible to maintain a constant gas pressure in the gas supply header).

[0005] In addition, in contrast to the above-mentioned control on the LNG terminal side, the power plant side is provided with an output increase block circuit that stops the increase in fuel consumption on the power plant side based on conditions unique to the power plant side (when the signal to control the generator output can no longer be confirmed, when the gas turbine burner is turned off, etc.). Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the past, the reported output (CMW) was set low compared to the maximum load (BMW) assessed for environmental impact, which meant that the power plant's output could not be fully utilized as a regulating power source, despite the recent rapid expansion of renewable energy adoption.

[0007] Therefore, there is a demand to raise the notified output of power plants to an output close to the maximum load (BMW) mentioned above (DMW, B ≒ D > C) that allows stable operation of the equipment without causing environmental degradation in the environmental impact assessment, and to establish a system that can respond to output fluctuations that accompany the expansion of renewable energy introduction and contribute to easing the tight supply-demand balance.For this reason, even when the power plant's output is increased to an output close to the maximum load (BMW) in the environmental impact assessment (raising it to DMW), it is necessary to control the pressure of the gas transmission header (main pipe) on the LNG terminal side to always remain constant, so that a stable supply of fuel gas can be supplied to the power plant and stable power generation can be ensured.

[0008] As mentioned above, conventionally, the power plant's notified output was set with a margin relative to the maximum flow rate of the LNG vaporizer (55 t / h x 4 units = 220 t / h), so it was possible to maintain a constant gas pressure in the gas supply header by controlling the LNG supply flow rate to the LNG vaporizer. However, if the notified output is increased and the power plant's equipment is operated near the notified output limit (DMW), the required gas supply volume increases, and there are more cases where the LNG vaporizer's output volume continues to operate at its limit (maximum output of 220 t / h). In other words, the maximum output volume of the LNG vaporizer corresponds to the maximum load of the power plant's notified output. Therefore, even in such a fuel supply system (a fuel supply system in which the maximum discharge volume of the LNG vaporizer corresponds to the maximum load of the power plant's notified output), control is required to maintain a constant gas pressure in the gas supply main pipe.

[0009] The gas flow rate through the gas feed header is calculated as the sum of the gas flow rate from the LNG vaporizer and the BOG flow rate from the BOG compressor, but the BOG flow rate depends only on the pressure of the LNG tank and fluctuates depending on the condition of the LNG tank (LNG receiving work, heat input condition, etc.).For this reason, under normal control, if the BOG flow rate decreases due to a drop in tank pressure, the LNG supply flow rate to the LNG vaporizer is increased to ensure the gas flow rate required by the power plant.

[0010] However, if the LNG vaporizer is already operating at or near its maximum output (220 t / h), there is no room for the LNG vaporizer to further vaporize the LNG (there is no surplus LNG supply flow rate to the LNG vaporizer), and if the BOG flow rate drops, the LNG vaporizer flow rate control will no longer be able to maintain the main line pressure constant (the LNG vaporizer flow rate control will no longer be able to ensure the required main line flow rate), resulting in an inconvenience of a drop in main line pressure.If this drop in main line pressure is left unchecked, it could lead to a trip (shutdown) of the power plant.

[0011] Thus, when the LNG supply facility is operated near its capacity limit, fluctuations in the BOG flow rate become a disturbance that cannot be ignored when controlling the main line pressure at a constant level. Therefore, a new control method is required that will not reduce the pressure in the gas supply header line even when the BOG flow rate changes (a new control method for maintaining the pressure in the gas supply header line at a constant level).

[0012] The present invention has been made in view of the above circumstances, and has as its main object to provide a fuel gas supply system in which natural gas vaporized in an LNG vaporizer and BOG compressed in a compressor are supplied to a power plant via a gas supply header, and which is capable of maintaining a constant gas pressure in the gas supply header regardless of fluctuations in the BOG flow rate supplied to the power plant, even when the maximum discharge rate of the LNG vaporizer corresponds to the maximum load of the power plant's notified output. [Means for solving the problem]

[0013] In order to achieve the above object, the fuel gas supply system of the present invention is designed to maintain a constant gas pressure in the gas transmission header pipe by imposing a predetermined limit on the power plant output, even if the BOG flow rate to be mixed with the natural gas vaporized in the LNG vaporizer fluctuates. In other words, a limit value (threshold value) of the LNG supply flow rate at which the LNG supply flow rate to the LNG vaporizer can be controlled while ensuring a control margin for the LNG supply flow rate that complements the fluctuation range of the BOG flow rate is set in advance, and the increase in fuel consumption at the power plant is stopped while the LNG supply flow rate to the LNG vaporizer exceeds this threshold value.This ensures that the required flow rate in the gas supply header is maintained by controlling the LNG supply flow rate to the LNG vaporizer even if fluctuations occur in the BOG flow rate, and prevents a drop in pressure in the gas supply header.

[0014] More specifically, the fuel gas supply system according to the present invention comprises: The system comprises an LNG vaporizer that vaporizes liquefied natural gas discharged from an LNG tank, and a BOG compressor that compresses boil-off gas generated in the LNG tank, A fuel gas supply system that supplies natural gas vaporized in the LNG vaporizer to a power plant via a gas transmission header and also supplies boil-off gas compressed in the BOG compressor to the power plant via the gas transmission header, an LNG supply flow rate detection means for detecting a flow rate of LNG supplied from the LNG tank to the LNG vaporizer; a BOG supply flow rate detection means for detecting a boil-off gas supply flow rate supplied from the BOG compressor to the power plant; a gas pressure detection means for detecting the gas pressure in the gas supply header; a vaporizer inlet flow rate control means for controlling the LNG supply flow rate supplied to the LNG vaporizer so as to keep the gas pressure in the gas supply header constant based at least on the LNG supply flow rate detected by the LNG supply flow rate detection means, the boil-off gas supply flow rate detected by the BOG supply flow rate detection means, the gas pressure in the gas supply header detected by the gas pressure detection means in the header, and the fuel consumption of the power plant, a threshold setting means for setting a threshold value for the LNG supply flow rate to the LNG vaporizer, the threshold value being a value obtained by subtracting from the maximum vaporization capacity of the LNG vaporizer at least a control amount for the LNG supply flow rate that can compensate for an expected fluctuation range of the boil-off gas supply flow rate supplied from the BOG compressor to the power plant by controlling the LNG supply flow rate by the vaporizer inlet flow rate control means; and an output increase blocking means for stopping an increase in the output of the power plant when the LNG supply flow rate detected by the LNG supply flow rate detecting means becomes greater than the threshold value set by the threshold value setting means with respect to the boil-off gas supply flow rate detected by the BOG supply flow rate detecting means; It is characterized by having:

[0015] Normally, if the flow rate of boil-off gas introduced into the gas supply header decreases for some reason, the decrease in BOG can be compensated for by increasing the supply flow rate to the LNG vaporizer, thereby preventing a drop in the pressure of the fuel gas flowing through the gas supply header.

[0016] However, since the maximum discharge rate of the LNG vaporizer corresponds to the maximum load of the power plant's notified output, if the LNG vaporizer is operated at or near the maximum discharge rate (MAX 220 t / h), the control range in which the LNG vaporizer flow rate control can compensate for a decrease in the BOG supply flow rate will be eliminated or will be narrowed (because there will be no room for the LNG vaporizer to further vaporize LNG). Therefore, if the BOG supply flow rate decreases, this decrease cannot be compensated for by the vaporizer inlet flow rate control means, and it will not be possible to maintain a constant gas pressure in the gas supply header pipe.

[0017] Therefore, a threshold value (upper limit value) for the supply flow rate to the LNG vaporizer is set in advance, ensuring a margin (control margin for LNG supply flow rate) that can be compensated for by flow rate control by the vaporizer inlet flow rate control means even if the flow rate of boil-off gas supplied from the BOG compressor to the power plant fluctuates.The actual LNG supply flow rate to the LNG vaporizer detected by the LNG supply flow rate detection means is compared with the preset threshold value for the boil-off gas flow rate detected by the BOG supply flow rate detection means, and if the actual LNG supply flow rate to the LNG vaporizer becomes greater than the threshold value, the increase in the power plant's output is stopped (the power plant's fuel consumption is prevented from increasing any further, and the power plant's output is simply kept flat or reduced). As a result, the discharge amount from the LNG vaporizer remains flat at or below the threshold value. Even if the BOG flow rate decreases in this state, the LNG vaporizer has a control range for the LNG supply flow rate that corresponds to the range of BOG fluctuations. Therefore, by controlling the flow rate to the LNG vaporizer using the vaporizer inlet flow rate control means, it is possible to reliably compensate for the fluctuations in the BOG flow rate and maintain a constant gas pressure in the gas supply header pipe.

[0018] Here, it is preferable that the threshold value is a value obtained by subtracting a value greater than the difference between the boil-off gas supply flow rate detected by the BOG supply flow rate detection means and the estimated minimum supply flow rate of BOG from the maximum vaporization capacity of the LNG vaporizer. For example, if the boil-off gas flow rate detected by the BOG supply flow rate detection means is 6 t / h and the expected minimum BOG supply flow rate is 0 t / h, the fluctuation range of the boil-off gas flow rate will be 6 t / h, so it may be set to 214 t / h (220 t / h - 6 t / h), which is the maximum vaporization capacity of the LNG vaporizer (e.g., 220 t / h) minus at least this fluctuation range. However, in reality, the fuel consumption on the power plant side will vary depending on the operating conditions and fuel properties on the power plant side, and fluctuations in the gas supply header will vary not only due to fluctuations in the BOG flow rate but also due to other factors. Therefore, it is advisable to set the threshold value to a value (e.g., 212 t / h) that can easily compensate for the fluctuation range of the boil-off gas flow rate, taking these fluctuation ranges into consideration.

[0019] Furthermore, the threshold value of the supply flow rate to the LNG vaporizer should be set smaller as the boil-off gas flow rate detected by the BOG supply flow rate detection means increases, since the BOG fluctuation range (decreasing range) increases as the boil-off gas flow rate detected by the BOG supply flow rate detection means increases. [Effects of the Invention]

[0020] As described above, the fuel gas supply system according to the present invention sets a threshold value for the supply flow rate to the LNG vaporizer by subtracting from the maximum vaporization capacity of the LNG vaporizer at least a control margin for the LNG supply flow rate that can compensate for fluctuations in the boil-off gas supply flow rate detected by the BOG supply flow rate detection means by controlling the LNG supply flow rate to the LNG vaporizer by the vaporizer inlet flow rate control means, and stops increasing the power plant's output when the LNG supply flow rate to the LNG vaporizer exceeds the threshold value. Therefore, even if the BOG flow rate decreases in this state, the LNG vaporizer has a control margin corresponding to fluctuations in the BOG flow, so the decrease in the BOG flow rate can be reliably compensated for by controlling the flow rate to the LNG vaporizer, and the pressure in the gas supply header can be maintained constant. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a block diagram showing an example of the configuration of a fuel gas supply system according to the present invention. [Figure 2] FIG. 2 is a diagram showing threshold values ​​of the LNG supply flow rate to the LNG vaporizer set relative to the BOG flow rate. [Figure 3] FIG. 3 is a flowchart illustrating an example of the control operation of the fuel gas supply system according to the present invention. [Figure 4] FIG. 4 is a block diagram showing the configuration of a conventional fuel gas supply system. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0023] 1 shows the overall configuration of a fuel gas supply system 1 according to the present invention. A storage facility (LNG terminal) for storing LNG to be supplied to a power plant includes an LNG tank 2 for storing LNG unloaded from an LNG carrier, an LNG vaporizer 3 for vaporizing the LNG discharged from the LNG tank 2, a BOG compressor 4 for compressing boil-off gas (BOG) generated in the LNG tank 2, and a gas supply header 6 for delivering the natural gas (NG) vaporized by the LNG vaporizer 3 to a power plant 5 and for mixing the BOG compressed by the BOG compressor 4 with the vaporized natural gas (NG) and delivering the resulting mixture to the power plant 5. A flow control valve 7 for controlling the flow rate of LNG supplied to the LNG vaporizer 3 is provided on the inlet side of the LNG vaporizer 3 (between the LNG tank 2 and the LNG vaporizer 3).

[0024] Therefore, by adjusting the aperture of the flow control valve 7, the supply flow rate of LNG vaporized in the LNG vaporizer 3 is adjusted, which in turn makes it possible to control the gas supply flow rate of the fuel gas obtained by mixing the natural gas vaporized in the LNG vaporizer 3 with the BOG supplied from the BOG compressor 4, i.e., the gas pressure in the gas supply main pipe 6.

[0025] The LNG vaporizer 3 may be a single unit or multiple units arranged in parallel. In this example, five LNG vaporizers with a maximum flow rate of 55 t / h are arranged in parallel, one of which is a spare unit, and the remaining four are in operation, achieving a maximum output of 220 t / h.

[0026] The storage facility is provided with an LNG supply flow rate detector 10 that detects the LNG supply flow rate to the LNG vaporizer 3, a BOG supply flow rate detector 11 that detects the boil-off gas supply flow rate of the boil-off gas pressurized by the BOG compressor 4 that is supplied to the gas supply header 6, and a header gas pressure detector 12 that detects the gas pressure in the gas supply header 6, and the flow control valve 7 is controlled by an LNG main control device 20 described later. The output of the power plant 5 is controlled by a power plant side control device 30.

[0027] The LNG supply flow rate detector 10 and the BOG supply flow rate detector 11 are composed of, for example, differential pressure flow meters, and the flow rate data detected by these flow rate detectors and the pressure data detected by the main pipe gas pressure detector 12 are input to the main pipe pressure control unit 21 of the LNG main control device 20. The header pressure control unit 21 calculates the LNG supply flow rate to be supplied to the LNG vaporizer 3 so as to maintain a constant gas pressure in the gas supply header 6 based at least on the LNG supply flow rate detected by the LNG supply flow rate detector 10, the boil-off gas supply flow rate detected by the BOG supply flow rate detector 11, the gas pressure in the gas supply header detected by the header gas pressure detector 12, and the fuel consumption of the power plant, and controls the opening of the flow control valve 7 using the vaporizer flow rate control unit 25 so as to obtain this calculated LNG supply flow rate.

[0028] Furthermore, the LNG main control device 20 performs processing to set a threshold value for the LNG supply flow rate, which is a limit value for the boil-off gas flow rate detected by the BOG supply flow rate detector 11, to leave a control margin for the LNG supply flow rate that can compensate for expected fluctuations (decreases) by controlling the LNG supply flow rate to the LNG vaporizer 3. If the LNG supply flow rate to the LNG vaporizer exceeds this threshold value, the range up to which the LNG vaporizer reaches its maximum vaporization capacity becomes smaller than the fluctuation range of the BOG supply flow rate, and therefore, it may become impossible to compensate for fluctuations in the BOG supply flow rate by controlling the LNG supply flow rate to the LNG vaporizer. Conversely, if the supply flow rate to the LNG vaporizer is kept below the threshold value, even if the BOG supply flow rate fluctuates, it becomes possible to compensate for fluctuations in the BOG supply flow rate by controlling the supply flow rate to the LNG vaporizer.

[0029] Therefore, the threshold value is set to ensure a control allowance for the LNG supply flow rate that enables the fluctuation range of the boil-off gas supply flow rate supplied from the BOG compressor to the power plant to be compensated for by controlling the LNG supply flow rate, and is stored in advance in the storage unit (threshold information database) 22 as a value as shown in Fig. 2 for the BOG flow rate supplied to the power plant 5. This threshold value is a value obtained by subtracting at least the control allowance for the LNG supply flow rate from the maximum vaporization capacity (220 t / h) of the LNG vaporizer (the difference between the maximum vaporization capacity of the LNG vaporizer and the threshold value is equal to or greater than the fluctuation range of the BOG flow rate), and is set for the BOG supply flow rate as shown in Fig. 2. The system is equipped with a threshold setting unit (FG) 23 that refers to a threshold information database 22 to extract and set a corresponding LNG supply flow rate threshold for the BOG flow rate detected by the BOG supply flow rate detector 11, and a comparison unit 24 that compares the LNG supply flow rate to the LNG vaporizer detected by the LNG supply flow rate detector 10 with the LNG supply flow rate threshold set by the threshold setting unit (FG) 23.

[0030] 2(a), the thresholds stored in the threshold information database 22 are set smaller as the BOG supply flow rate increases, since the expected BOG fluctuation range (the range until the BOG reaches the minimum flow rate, i.e., the range of BOG decrease) increases as the BOG supply flow rate increases, so that even if the BOG fluctuates within this fluctuation range, the required gas delivery flow rate can be supplemented by LNG supply flow rate control. Such thresholds are actually set, for example, as shown in FIG. 2(b), based on the results of field tests.

[0031] In addition, the LNG main control device 20 is equipped with a vaporizer flow control unit 25 that controls the LNG supply flow rate to the LNG vaporizer 3, i.e., the opening degree of the flow control valve 7, to maintain a constant gas pressure in the gas supply header 6 based on the LNG supply flow rate and BOG supply flow rate input to the header pressure control unit 21, the gas pressure in the gas supply header, and the fuel consumption on the power plant side (not shown).

[0032] The power plant side control device 30 is provided with an output increase block circuit 31 that controls the output system of the power plant 5 to suppress an increase in the output of the power plant 5 (controls the power plant output within a range that does not increase the output) when there is a possibility that the gas pressure in the gas supply header 6 cannot be maintained constant by only controlling the LNG supply flow rate to the LNG vaporizer 3 based on the comparison result by the comparison unit 24. Here, the output increase block circuit 31 suppresses the output increase by, for example, blocking a shaft power increase command (governor increase command) using software, thereby blocking a fuel flow rate increase command.

[0033] In the above configuration, the fuel gas supply system 1 (LNG main control device 20, power plant side control device 30) operates as shown in FIG. That is, the header pressure control unit 21 takes in the LNG supply flow rate (PV) to the LNG vaporizer detected by the LNG supply flow rate detector 10, and also takes in the BOG supply flow rate supplied to the gas supply header detected by the BOG supply flow rate detector 11 (steps 52, 54), and sets the corresponding LNG supply flow rate threshold (PH) by referring to the threshold information database 22 based on this detected BOG supply flow rate (step 56).

[0034] Then, the detected LNG supply flow rate (PV) to the LNG vaporizer 3 is compared with the LNG supply flow rate threshold (PH) set by referring to the threshold information database 22, and if the LNG supply flow rate (PV) to the LNG vaporizer is below the LNG supply flow rate threshold (PH), normal LNG supply flow rate control is performed (steps 58, 60) because the gas pressure in the gas supply main pipe can be kept constant by conventional control of the LNG supply flow rate to the LNG vaporizer 3.

[0035] On the other hand, if the LNG supply flow rate (PV) to the LNG vaporizer 3 is greater than the LNG supply flow rate threshold (PH), there is insufficient control margin for the LNG supply flow rate to compensate for fluctuations in BOG, which may result in a drop in pressure in the gas supply header. Therefore, if the LNG supply flow rate (PV) to the LNG vaporizer exceeds the threshold (PH), the power increase block circuit 31, which suppresses an increase in the power output of the power plant 5, is activated to suppress an increase in the power plant's fuel consumption (by maintaining or decreasing the power plant's output), thereby keeping the LNG vaporizer discharge rate at or below the threshold (steps 58 and 62). Therefore, even if the BOG supply flow rate decreases in this state, there is still enough control margin for the LNG supply flow rate to compensate for fluctuations in BOG (because the necessary control margin for the LNG supply flow rate is secured), so the decrease in the BOG flow rate can be reliably compensated for by controlling the LNG supply flow rate to the LNG vaporizer, and the pressure in the gas supply header can be maintained constant. Therefore, even when the maximum discharge rate of the LNG vaporizer corresponds to the maximum load of the power plant's notified output, fluctuations in the boil-off gas supply flow rate to the power plant can be reliably compensated for by controlling the LNG supply flow rate to the LNG vaporizer 3, thereby maintaining a constant gas pressure in the gas supply main pipe.

[0036] In addition, the output increase block circuit 31 is intended to suppress the increase in output of the power plant 5 and ensure that the amount of gas sent (pressure in the gas sending main pipe) can be controlled by controlling the inlet flow rate of the LNG vaporizer.Therefore, when the LNG supply flow rate to the LNG vaporizer falls below a threshold value and can be handled by controlling the LNG supply flow rate to the LNG vaporizer 3, the block on the increase in output of the power plant is released (for example, the block on the increase in output of the power plant 5 is released after a predetermined time (currently 1 second) has elapsed after the LNG supply flow rate to the LNG vaporizer 3 falls below the threshold value), and the normal control of step 60 is restored. [Explanation of symbols]

[0037] 2 LNG tanks 3 LNG vaporizer 4 BOG compressor 5. Power Plant 6 Gas transmission header 7 Inlet flow control valve 10. Vaporizer inlet flow detector 11 BOG supply flow detector 12 Gas pressure detector 22 Threshold Information Database 31 Output increase block circuit

Claims

1. The system comprises an LNG vaporizer that vaporizes liquefied natural gas discharged from an LNG tank, and a BOG compressor that compresses boil-off gas generated in the LNG tank, A fuel gas supply system that supplies natural gas vaporized in the LNG vaporizer to a power plant via a gas feed main pipe, and supplies boil-off gas compressed in the BOG compressor to the power plant via the gas feed main pipe, an LNG supply flow rate detection means for detecting a flow rate of LNG supplied from the LNG tank to the LNG vaporizer; a BOG supply flow rate detection means for detecting a boil-off gas supply flow rate supplied from the BOG compressor to the power plant; a gas pressure detection means for detecting the gas pressure in the gas supply header; a vaporizer inlet flow rate control means for controlling the LNG supply flow rate supplied to the LNG vaporizer so as to keep the gas pressure in the gas supply header constant based at least on the LNG supply flow rate detected by the LNG supply flow rate detection means, the boil-off gas supply flow rate detected by the BOG supply flow rate detection means, the gas pressure in the gas supply header detected by the header gas pressure detection means, and the fuel consumption of the power plant, a threshold setting means for setting a threshold value of the LNG supply flow rate to the LNG vaporizer, the threshold value being a value obtained by subtracting at least a control amount of the LNG supply flow rate, which can compensate for an expected fluctuation range of the boil-off gas supply flow rate supplied from the BOG compressor to the power plant by controlling the LNG supply flow rate by the vaporizer inlet flow rate control means, from the maximum vaporization capacity of the LNG vaporizer; and an output increase blocking means for stopping an increase in the output of the power plant when the LNG supply flow rate detected by the LNG supply flow rate detecting means becomes greater than the threshold value set by the threshold value setting means with respect to the boil-off gas supply flow rate detected by the BOG supply flow rate detecting means; A fuel gas supply system comprising:

2. The fuel gas supply system of claim 1, characterized in that the threshold value is a value obtained by subtracting a value greater than the difference between the boil-off gas supply flow rate detected by the BOG supply flow rate detection means and the estimated minimum BOG supply flow rate from the maximum vaporization capacity of the LNG vaporizer.

3. 3. The fuel gas supply system according to claim 1, wherein the threshold value is set to be smaller as the boil-off gas supply flow rate detected by the BOG supply flow rate detection means increases.

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