JP7810092B2 - Fuel Gas Supply System - Google Patents

Description

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 main pipe. Even when the maximum output of the LNG vaporizer corresponds to the maximum load of the power plant's notified output, the fuel gas supply system is capable of maintaining a constant gas pressure in the gas supply main pipe by compensating for fluctuations in the boil-off gas supply flow rate to the power plant by controlling the LNG supply flow rate to the LNG vaporizer.

At a storage facility (LNG terminal) that stores LNG to be supplied to a power plant, LNG unloaded from an LNG ship is stored in LNG tank 2, as shown in Figure 4. It is then discharged from LNG tank 2 by an LNG pump (not shown), vaporized in LNG vaporizer 3, and sent to power plant 5 via gas supply main pipe 6. LNG tank 2 also receives constant heat input from the outside and from the LNG pump, etc., which is consumed as the latent heat of vaporization of the LNG, generating boil-off gas (BOG). To maintain a constant pressure in LNG tank 2, the generated BOG is suction-compressed (BOG volume adjustment) by BOG compressor 4 and sent to gas supply main pipe 6, where it is combined with natural gas (NG) vaporized in LNG vaporizer 3 and sent to power plant 5.

In order to supply a stable amount of gas relative to the consumption amount at the power plant 5, the gas pressure in the gas supply main pipe 6 is maintained constant at a specified pressure. That is, the required supply flow rate to the LNG vaporizer 3 is determined based on the gas pressure in the gas supply main pipe 6, the BOG supply flow rate supplied to the gas supply main pipe from the BOG compressor 4, the LNG supply flow rate supplied to the LNG vaporizer 3 from the LNG tank 2, and the fuel consumption amount on the power plant side, and the aperture of the flow control valve 7 located on the inlet side of the LNG vaporizer is controlled to obtain this required supply flow rate.

In such power generation facilities, the notified output (CMW, B) >> C) is set within the maximum load (BMW, A > B) range determined in a pre-construction environmental impact assessment relative to the power plant's generating capacity (A MW), and operation is controlled so as not to exceed this notified output. Conventionally, the notified output (CMW) was set well below the maximum load (BMW) determined in the environmental impact assessment. Therefore, 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 described above (it was possible to maintain a constant gas pressure in the gas supply header pipe).

In addition to the above-mentioned control on the LNG terminal side, the power plant side is equipped with an output increase block circuit that stops the increase in fuel consumption on the power plant side based on conditions specific to the power plant side (such as when the signal controlling the generator output cannot be confirmed or when the gas turbine burner is turned off).

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

Therefore, there is a demand to raise the power plant's reported output 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, thereby establishing a system that can respond to output fluctuations associated with the expanded introduction of renewable energy and contribute to easing the tight supply-demand balance. For this reason, even when power plant 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, thereby ensuring a stable supply of fuel gas to the power plant and stable power generation.

Conventionally, as mentioned above, the notified output of a power plant was set with a margin for 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 limit of its notified output (DMW), the required amount of gas supply also increases, and there are more cases where the LNG vaporizer's output rate continues to operate at its limit (maximum output rate of 220 t/h). In other words, the maximum output rate of the LNG vaporizer corresponds to the maximum load of the notified output of the power plant.
Therefore, even in such a fuel supply system (a fuel supply system in which the maximum discharge amount of the LNG vaporizer corresponds to the maximum load of the power plant's notified output), control is required to keep the gas pressure in the gas supply main pipe constant.

The gas flow rate through the gas feed main pipe 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 in the LNG tank and fluctuates depending on the condition of the LNG tank (LNG receiving operations, heat input conditions, 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.

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), so if the BOG flow rate drops, the LNG vaporizer flow rate control will no longer be able to maintain constant main line pressure (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 left unchecked, this drop in main line pressure could lead to the power plant tripping (shutdown).

As such, when the LNG supply equipment is operating near its limit, fluctuations in the BOG flow rate become a significant disturbance factor in controlling the main line pressure at a constant level, and therefore new control is required to prevent the pressure in the gas supply header line from decreasing even when the BOG flow rate changes (new control to maintain the pressure in the gas supply header line at a constant predetermined pressure).

The present invention was made in consideration of the above circumstances, and its main objective is to provide a fuel gas supply system that supplies natural gas vaporized in an LNG vaporizer and BOG compressed in a compressor to a power plant via a gas supply main pipe, and that can maintain a constant gas pressure in the gas supply main pipe regardless of fluctuations in the BOG flow rate supplied to the power plant, even when the maximum output volume of the LNG vaporizer corresponds to the maximum load of the power plant's notified output.

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 supply main pipe by imposing a predetermined restriction on the output of the power plant, 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 is set in advance, 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, and the increase in the power plant's fuel consumption is stopped while the LNG supply flow rate to the LNG vaporizer exceeds this threshold value.This ensures that even if fluctuations occur in the BOG flow rate, the required flow rate in the gas supply main pipe is secured by controlling the LNG supply flow rate to the LNG vaporizer, and prevents a drop in pressure in the gas supply main pipe.

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 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;
It is characterized by having:

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.

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

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 the 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 constant or reduced). As a result, the discharge volume from the LNG vaporizer remains constant at or below the threshold value. Even if the BOG flow rate decreases in this state, the LNG vaporizer has a control margin 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 control means, it is possible to reliably compensate for fluctuations in the BOG flow rate and maintain constant gas pressure in the gas supply main pipe.

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 BOG supply flow rate 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, and therefore 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 pipe 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 comfortably compensate for the fluctuation range of the boil-off gas flow rate, taking these fluctuation ranges into consideration.

In addition, the threshold value for the supply flow rate to the LNG vaporizer should be set smaller the higher the boil-off gas flow rate detected by the BOG supply flow rate detection means, since the BOG fluctuation range (decreasing range) becomes larger the higher the boil-off gas flow rate detected by the BOG supply flow rate detection means.

As described above, the fuel gas supply system of the present invention sets a threshold 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, which can be compensated for by controlling the LNG supply flow rate supplied to the LNG vaporizer by the vaporizer inlet flow control means, relative to the boil-off gas supply flow rate detected by the BOG supply flow rate detection means. If the LNG supply flow rate supplied to the LNG vaporizer exceeds this threshold, the increase in power plant output is stopped. Therefore, the LNG vaporizer discharge rate remains flat at the threshold or is below the threshold. Therefore, even if the BOG flow rate decreases in this state, the LNG vaporizer has a control margin corresponding to the BOG fluctuation margin. Therefore, the decrease in the BOG flow rate can be reliably compensated for by controlling the flow rate to the LNG vaporizer, making it possible to maintain a constant pressure in the gas supply header.

FIG. 1 is a block diagram showing an example of the configuration of a fuel gas supply system according to the present invention. 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. FIG. 3 is a flowchart illustrating an example of the control operation of the fuel gas supply system according to the present invention. FIG. 4 is a block diagram showing the configuration of a conventional fuel gas supply system.

Embodiments of the present invention are described below with reference to the drawings.

Figure 1 shows the overall configuration of a fuel gas supply system 1 according to the present invention. The 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 ship, 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 main pipe 6 for delivering the natural gas (NG) vaporized by the LNG vaporizer 3 to the 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 LNG supply flow rate to the LNG vaporizer 3 is provided at the inlet side of the LNG vaporizer 3 (between the LNG tank 2 and the LNG vaporizer 3).

Therefore, by adjusting the opening of the flow control valve 7, the flow rate of LNG supplied to be vaporized in the LNG vaporizer 3 can be adjusted, which in turn controls the flow rate of fuel gas supplied from the BOG compressor 4, which is a mixture of natural gas vaporized in the LNG vaporizer 3 and BOG, i.e., the gas pressure in the gas supply main pipe 6.

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

The storage facility is equipped 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 within the gas supply header 6. The flow control valve 7 is controlled by an LNG main control device 20 (described below). The output of the power plant 5 is controlled by a power plant-side control device 30.

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 main pipe 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 main pipe 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 main pipe detected by the main pipe 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.

The LNG main control device 20 also 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 enough control of the LNG supply flow rate to 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's maximum vaporization capacity is reached will be smaller than the fluctuation range of the BOG supply flow rate, and 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 will be possible to compensate for fluctuations in the BOG supply flow rate by controlling the supply flow rate of the LNG vaporizer.

Therefore, the threshold value is set to ensure a control allowance for the LNG supply flow rate that makes it possible to compensate for the 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, 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.

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

The LNG main control device 20 also has 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 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 rate on the power plant side (not shown).

The power plant control device 30 is equipped with an output increase block circuit 31 that controls the output system of the power plant 5 to suppress an increase in the power plant 5's output (controls the power plant output to a range that does not increase output) when there is a possibility that it will not be possible to maintain constant gas pressure in the gas supply header pipe 6 by controlling the LNG supply flow rate to the LNG vaporizer 3 alone, based on the comparison results of the comparison unit 24. Here, the output increase block circuit 31 suppresses the increase in output by, for example, blocking the shaft power increase command (governor increase command) via software, thereby blocking the fuel flow rate increase command.

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 from this detected BOG supply flow rate (step 56).

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. If the LNG supply flow rate (PV) to the LNG vaporizer is equal to or lower than the LNG supply flow rate threshold (PH), normal LNG supply flow rate control is performed, since the gas pressure in the gas supply header pipe can be kept constant using conventional control of the LNG supply flow rate to the LNG vaporizer 3 (steps 58, 60).

On the other hand, if the LNG supply flow rate (PV) to the LNG vaporizer 3 is greater than the threshold value (PH) of the LNG supply flow rate, there is insufficient control of the LNG supply flow rate to compensate for the fluctuation range of the 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 value (PH), the output increase block circuit 31 that suppresses an increase in the output of the power plant 5 is activated, and the increase in fuel consumption of the power plant is suppressed (by maintaining or decreasing the output of the power plant), thereby keeping the output of the LNG vaporizer at the threshold value or below the threshold value (steps 58, 62). Therefore, even if the BOG supply flow rate decreases in this state, there is still a control margin for the LNG supply flow rate that can compensate for the fluctuation range of the BOG (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, making it possible to maintain a constant pressure in the gas supply header pipe.
Therefore, even when the maximum discharge volume of the LNG vaporizer corresponds to the maximum load of the power plant's reported output, fluctuations in the supply flow rate of boil-off gas 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.

The output increase block circuit 31 is designed to suppress an increase in the power plant 5's output 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 managed by controlling the LNG supply flow rate to the LNG vaporizer 3, the block on the power plant's output increase is released (for example, the block on the power plant's output increase is released after a preset time (currently 1 second) has elapsed after the LNG supply flow rate to the LNG vaporizer 3 falls below the threshold value), and normal control of step 60 is resumed.

2 LNG tank 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 (3)

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: The fuel gas supply system of claim 1, characterized in that the threshold value is a value obtained by subtracting from the maximum vaporization capacity of the LNG vaporizer 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. A fuel gas supply system as described in claim 1 or 2, characterized in that the threshold value is set smaller as the boil-off gas supply flow rate detected by the BOG supply flow rate detection means increases.