JP2024093886A - Boiler System - Google Patents

Abstract

The present invention provides a boiler system capable of stable combustion processing even if the BOG concentration in a gaseous fuel containing BOG changes.
[Solution] A boiler system 1 has a mode setting unit 19 capable of setting a BOG combustion control mode M2 that prioritizes combustion of BOG fuel or an air ratio control mode M3 that burns nitrogen-mixed BOG fuel containing nitrogen at a predetermined air ratio, and a control unit 20 that controls combustion in a boiler 10. When the BOG combustion control mode M2 is set by the mode setting unit 19, the control unit 20 adjusts a gas fuel flow rate adjustment unit 3 to achieve a predetermined combustion amount. When the air ratio control mode M3 is set by the mode setting unit 19, the control unit 20 adjusts the gas fuel flow rate adjustment unit 3 and the liquid fuel flow rate adjustment unit 7 so that BOG fuel and heavy oil are supplied to the boiler 10, and adjusts an air flow rate adjustment unit 12 so that the oxygen concentration Co of the exhaust gas from the boiler 20 becomes a predetermined oxygen concentration Cot.
[Selected Figure] Figure 1

Description

The present invention relates to a boiler system capable of combusting boil-off gas flowing through a free flow line.

When natural gas is transported by ship, it is transported in a liquefied state (liquefied gas) by cooling it to about -160°C under atmospheric pressure. Ships that transport such liquefied gas store the liquefied gas in a liquefied gas storage tank that has an insulating and heat-resistant function that can maintain the inside at an extremely low temperature. However, the liquefied gas in the liquefied gas storage tank generates boil-off gas (hereinafter, simply referred to as "BOG". "Boil-off gas" included in the names of components, terms, names, etc. will also be described as "BOG") due to the heat that penetrates into the liquefied gas storage tank. The liquefied gas storage tank, whose tank pressure has increased due to the generation of BOG, needs to treat the BOG before it reaches the allowable tank pressure. In Patent Document 1, the BOG generated in the liquefied gas storage tank is introduced to a compressor, compressed, and used as fuel for the main gas engine for propulsion and the auxiliary gas engine for onboard power generation. The BOG can also be supplied to a GCU for combustion.

On the other hand, the configuration of Patent Document 1, which requires a compressor, increases the equipment costs and running costs. Therefore, Patent Document 2 discloses a tank pressure control system that includes a free-flow gas supply means for supplying BOG generated in a liquefied gas storage tank to a gas incineration device through a free-flow line, a liquefied gas supercooling device for supercooling the liquefied gas pumped from the tank, and a liquefied gas spraying means for returning the supercooled liquefied gas to the tank. In Patent Document 2, the excess BOG is supplied to the combustion device through a free-flow gas supply line that sends fuel gas to the gas combustion device at the pressure in the liquefied gas storage tank without using a gas compressor. However, Patent Document 2 does not include any description related to the combustion control technology of excess BOG in a gas combustion device.

JP 2018-48607 A JP 2019-163804 A

In recent years, with the growing social demand for SDGs and the like, there is a demand for boiler systems that can efficiently combust and treat BOG, such as methane, which has a high global warming potential, according to the amount of surplus BOG generated, without discharging it into the atmosphere. Therefore, boiler systems must also properly combust and treat BOG mixed with inert gas discharged from a liquefied gas storage tank when the BOG remaining in the liquefied gas storage tank is replaced with inert gas, or when a liquefied gas storage tank filled with inert gas is filled with LNG.

The object of the present invention is to realize a boiler system that can perform stable combustion processing even if the BOG concentration in gaseous fuel containing BOG changes.

A boiler system according to one embodiment of the present invention is a boiler system installed on a ship that includes a liquefied gas storage tank, an inert gas supply unit that supplies inert gas into the liquefied gas storage tank, and a free flow line through which the BOG and inert gas discharged from the liquefied gas storage tank flow. The boiler system includes a boiler capable of burning gaseous fuel containing BOG and liquid fuel, a gaseous fuel pressure detection unit that detects the pressure of the gaseous fuel containing BOG supplied to the boiler, a gaseous fuel flow rate adjustment unit that adjusts the flow rate of the gaseous fuel containing BOG supplied to the boiler, a liquid fuel flow rate adjustment unit that adjusts the flow rate of the liquid fuel supplied to the boiler, an air flow rate adjustment unit that adjusts the flow rate of the combustion air supplied to the boiler, an oxygen concentration detection unit that detects the oxygen concentration in the exhaust gas of the boiler, a mode setting unit that can set one of a BOG combustion control mode that prioritizes the combustion process of the gaseous fuel containing BOG and an air ratio control mode that combusts the gaseous fuel containing BOG containing an inert gas at a predetermined air ratio, and a control unit that controls the combustion of the boiler.

When the BOG combustion control mode is set by the mode setting unit, the control unit adjusts the gas fuel flow rate adjustment unit to achieve a predetermined combustion amount. When the air ratio control mode is set by the mode setting unit, the control unit adjusts the gas fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit so that gas fuel containing BOG and liquid fuel are supplied to the boiler, and adjusts the air flow rate adjustment unit so that the oxygen concentration of the exhaust gas from the boiler becomes a predetermined oxygen concentration.

When it is necessary to reduce the BOG pressure (the priority of the BOG combustion process is high), the above-mentioned boiler system can burn only the BOG fuel in the BOG combustion control mode to increase the amount of excess BOG burned. In addition, when the inert gas is mixed into the gaseous fuel containing BOG due to the inert gas purging in the liquefied gas storage tank, or when BOG is mixed into the inert gas due to the filling of LNG into a liquefied gas storage tank filled with inert gas, the boiler system can continue combustion even if the mixture ratio of BOG and inert gas changes due to the combustion of liquid fuel in the air ratio control mode. Furthermore, the supply amount of combustion air required for the combustion process of the gaseous fuel containing BOG is controlled based on the oxygen concentration of the boiler exhaust gas. This makes it possible to suppress the decrease in boiler efficiency even if the BOG concentration in the gaseous fuel containing BOG changes, and to realize a boiler system capable of stable combustion process.

Note that surplus BOG includes BOG that is not used as fuel in equipment other than boilers, such as the main engine or generator, and BOG that is not recovered in liquefied gas storage tanks through reliquefaction. In addition, BOG that is used as fuel when effectively utilizing the heat recovered by a boiler, such as when operating a boiler to meet steam demand on board the ship, is included in surplus BOG.

From another perspective, the boiler system of the present invention preferably includes the following configuration. The control unit of the boiler system further includes an inert gas mixing determination unit that determines the degree of mixing of an inert gas into the gaseous fuel containing BOG. In the air ratio control mode, when the inert gas mixing determination unit determines that the degree of mixing of the inert gas contained in the gaseous fuel containing BOG is less than a predetermined value, the control unit adjusts the gaseous fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit so that only the gaseous fuel containing BOG is supplied to the boiler. When the inert gas mixing determination unit determines that the degree of mixing of the inert gas contained in the gaseous fuel containing BOG is equal to or greater than a predetermined value, the control unit adjusts the gaseous fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit so that the gaseous fuel containing BOG and liquid fuel are supplied to the boiler.

In the above configuration, even when the air ratio control mode is set, the control unit of the boiler system supplies only gaseous fuel containing BOG to the boiler when the mixture ratio of the inert gas, which is the degree of mixing of the inert gas contained in the gaseous fuel containing BOG, is below a predetermined mixture ratio. This allows BOG combustion to be prioritized when the BOG concentration is high, and the amount of excess BOG burned can be increased.

From another perspective, it is preferable that the boiler system of the present invention includes the following configuration. The control unit adjusts the air flow rate adjustment unit based on the detection result of the oxygen concentration detection unit so that the oxygen concentration (air ratio) of the exhaust gas becomes a predetermined value. The control unit determines the degree of mixing of the inert gas contained in the BOG by the inert gas mixing determination unit based on the adjustment amount of the air flow rate adjustment unit.

In the above-mentioned configuration, when the air flow rate adjustment unit is adjusted so that the oxygen concentration of the exhaust gas becomes a predetermined value based on the detection result of the oxygen concentration detection unit, the control unit of the boiler system calculates the degree of mixing of the inert gas with the BOG in the gaseous fuel containing BOG based on the supply amount of combustion air supplied to the boiler by the inert gas mixing determination unit. This makes it possible to realize a boiler system that can perform stable combustion processing even if the BOG concentration in the gaseous fuel containing BOG changes.

From another perspective, it is preferable that the boiler system of the present invention includes the following configuration. In the BOG combustion control mode, the control unit determines the degree of mixing of the inert gas contained in the BOG by the inert gas mixing determination unit. When the control unit determines that the degree of mixing of the inert gas contained in the BOG is equal to or greater than a predetermined value, the control unit switches to the air ratio control mode and adjusts the gas fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit so that gas fuel containing boil-off gas and liquid fuel are supplied to the boiler.

In the above configuration, when the degree of mixing of the inert gas in the gaseous fuel containing BOG is equal to or greater than a predetermined value, the boiler system supplies liquid fuel according to the mixing ratio of the inert gas in the gaseous fuel containing BOG in order to stably combust the gaseous fuel containing BOG. This makes it possible to realize a boiler system that can stably combust the gaseous fuel containing BOG even if the BOG concentration in the gaseous fuel containing BOG changes.

From another perspective, the boiler system of the present invention preferably includes the following configuration. In the boiler system, when the air ratio control mode is set, the mode setting unit is configured to be able to set one of an inert gas increased combustion control mode and a BOG increased combustion control mode. When the inert gas increased combustion control mode is set by the mode setting unit, the control unit starts main combustion in a state in which the gas fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit are adjusted so that only gas fuel containing BOG is supplied to the boiler. When the BOG increased combustion control mode is set by the mode setting unit, the control unit starts main combustion in a state in which the gas fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit are adjusted so that gas fuel containing boil-off gas and liquid fuel are supplied to the boiler.

In the above-mentioned configuration, in the boiler system, when gaseous fuel containing BOG that can be stably combusted in the boiler is supplied in the air ratio control mode, an inert gas increased combustion control mode is set to increase the amount of BOG combustion processing. Also, in the boiler system, when gaseous fuel containing BOG mixed with inert gas is supplied to the boiler in the air ratio control mode, a BOG increased combustion control mode is set for more stable BOG combustion processing. This makes it possible to realize a boiler system that can perform stable combustion processing even if the BOG concentration in the gaseous fuel containing BOG changes.

From another perspective, it is preferable that the boiler system of the present invention includes the following configuration: When the BOG combustion control mode is set by the mode setting unit, the control unit adjusts the air flow rate adjustment unit so that the oxygen concentration of the boiler exhaust gas detected by the oxygen concentration detection unit becomes a predetermined oxygen concentration.

In the above configuration, when BOG is supplied to the boiler in the BOG combustion control mode, the boiler system adjusts the air flow rate supplied to the boiler according to the composition of the BOG. Thus, the boiler system can supply an appropriate amount of oxygen according to the composition of the BOG to the boiler. This makes it possible to realize a boiler system that can perform stable combustion processing even if the BOG concentration in the gaseous fuel containing BOG changes.

From another perspective, it is preferable that the boiler system of the present invention includes the following configuration. It has a steam pressure detection unit that detects the pressure of steam generated in the boiler. The mode setting unit is configured to be further capable of setting a steam pressure priority control mode that prioritizes the generation of steam in the boiler. When the steam pressure priority control mode is set by the mode setting unit, the control unit adjusts at least one of the gas fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit so that the steam pressure detected by the steam pressure detection unit becomes a predetermined steam pressure.

In the above-described configuration, when steam supply is required, the boiler system can combust gaseous fuel containing BOG while maintaining the steam pressure at a predetermined steam pressure in the steam pressure priority control mode. On the other hand, when the boiler system prioritizes the combustion process of gaseous fuel containing BOG over steam supply, the boiler system can increase the combustion amount of gaseous fuel containing BOG in the BOG combustion control mode or the air ratio control mode. This makes it possible to realize a boiler system that can perform stable combustion process even if the BOG concentration in the gaseous fuel containing BOG changes.

It is possible to realize a boiler system that can perform stable combustion processing even if the BOG concentration in gaseous fuel containing BOG changes.

FIG. 1 is a schematic configuration diagram of a BOG treatment system including a boiler system according to a first embodiment of the present invention. FIG. 2 is a block diagram showing some of the control modes and combustion modes in the boiler system according to the first embodiment of the present invention. FIG. 3 is a block diagram showing a BOG combustion control mode in a modified example of the boiler system according to the first embodiment of the present invention. FIG. 4 is a schematic configuration diagram of a BOG treatment system including a boiler system according to a second embodiment of the present invention. FIG. 5 is a block diagram showing an air ratio control mode in a boiler system according to the second embodiment of the present invention. FIG. 6 is a block diagram showing an air ratio control mode in a boiler system according to the third embodiment of the present invention. FIG. 7 is a block diagram showing control modes and combustion modes in a boiler system according to the fourth embodiment of the present invention.

Each embodiment of the boiler system according to the present invention will be described below with reference to the drawings. In each drawing, the same parts are given the same reference numerals, and the description of the same parts will not be repeated. Note that the dimensions of the components in each drawing do not faithfully represent the actual dimensions of the components and the dimensional ratios of each component.

In the following description, "upstream" means the liquefied gas storage tank T1 side in the piping through which gaseous fuel containing BOG flows from the liquefied gas storage tank T1 to equipment that burns gaseous fuel containing BOG, and means the liquid fuel storage tank T2 side in the piping through which liquid fuel flows from the liquid fuel storage tank T2 to equipment that burns liquid fuel. "Downstream" means the equipment side that burns gaseous fuel or liquid fuel containing BOG in the piping through which gaseous fuel or liquid fuel containing BOG flows.

In the following description, BOG fuel refers to excess BOG that flows through the free flow line, and includes BOG that is not used as fuel in equipment other than boilers, such as the main engine or generator, and BOG that is not recovered in the liquefied gas storage tank by reliquefaction. In addition, when inert gas purging of the liquefied gas tank, etc. is performed, the BOG fuel that flows through the free flow line contains inert gas. In other words, BOG fuel is a gaseous fuel that contains BOG, which is either BOG that contains inert gas or BOG that does not contain inert gas.

In addition, in the following description, the expressions "fixed," "connected," and "attached" (hereinafter referred to as "fixed") include not only cases where members are directly fixed to each other, but also cases where members are fixed via other members. In other words, in the following description, the expression "fixed" includes the meaning of direct and indirect fixation between members.

(Embodiment 1)
(Boiler system)
A boiler system according to a first embodiment of the present invention will be described with reference to Fig. 1 and Fig. 2. Fig. 1 is a schematic diagram of a BOG treatment system 100 including a boiler system 1 according to a first embodiment of the present invention. In the following embodiment, the boiler system 1 burns heavy oil as a liquid fuel, but the present invention is not limited to this. The boiler system 1 may use, for example, kerosene, naphtha, or the like as a liquid fuel.

As shown in FIG. 1, the BOG treatment system 100 has a free flow line 2A through which BOG fuel generated in a liquefied gas storage tank T1 flows, a compressor C connected to the upstream side of the free flow line 2A, a generator Ge that uses the BOG fuel pressurized by the compressor C as fuel, and a boiler system 1 connected to the downstream side of the free flow line 2A. The BOG treatment system 100 may also have a re-liquefied gas recovery system (not shown) on the upstream side of the free flow line 2A that re-liquefies the BOG fuel and recovers it in the liquefied gas storage tank T1. The BOG treatment system 100 may also be configured to supply the BOG fuel pressurized by the compressor C to a BOG supply line 2B of the boiler system 1 described below.

Furthermore, the BOG treatment system 100 has a nitrogen purge function that supplies nitrogen as an inert gas into the liquefied gas storage tank T1 to discharge the BOG fuel in the liquefied gas storage tank T1. The BOG treatment system 100 has a nitrogen storage tank T3, a nitrogen supply line L, a nitrogen supply unit V, and a boiler system 1A.

The free flow line 2A of the BOG treatment system 100 is a pipe that discharges BOG fuel, which is liquefied gas in the liquefied gas storage tank T1 evaporated by heat, to the outside of the liquefied gas storage tank T1. The free flow line 2A is configured to allow the BOG fuel generated in the liquefied gas storage tank T1 to flow in.

The nitrogen storage tank T3 is connected to the liquefied gas storage tank T1 by a nitrogen supply line L. The nitrogen supply line L is provided with a nitrogen supply unit V that switches between an open state in which nitrogen is supplied from the nitrogen storage tank T3 to the liquefied gas storage tank T1 and a closed state in which nitrogen is not supplied to the liquefied gas storage tank T1.

The boiler system 1 is a boiler system capable of combusting BOG fuel flowing through the free flow line 2A. In this embodiment, the BOG treatment system 100 is mounted on a ship that transports liquefied gas. The boiler system 1 is capable of combusting BOG fuel generated in a liquefied gas storage tank T1 that stores liquefied gas on the ship, excluding the amount used by the generator Ge and the amount recovered by a re-liquefied gas recovery system (not shown).

The boiler system 1 has a BOG supply line 2B, a gas fuel flow rate adjustment unit 3, a liquid fuel flow rate adjustment unit 7, a boiler 10, an air flow rate adjustment unit 12, a BOG pressure detection unit 13, a steam pressure detection unit 14, an oxygen concentration detection unit 16, a mode setting unit 19, and a control unit 20.

The BOG supply line 2B of the boiler system 1 is a pipe that supplies the BOG fuel flowing in the free flow line 2A to the boiler 10. The upstream side of the BOG supply line 2B is connected to the free flow line 2A. The downstream side of the BOG supply line 2B is connected to the boiler 10 via the gas fuel flow rate adjustment unit 3. The BOG fuel in the free flow line 2A flows into the BOG supply line 2B due to the pressure of the BOG fuel generated in the liquefied gas storage tank T1. The BOG pressure Pb, which is the pressure of the BOG fuel in the BOG supply line 2B, increases or decreases depending on the relationship between the flow rate of the BOG fuel flowing from the free flow line 2A into the BOG supply line 2B and the flow rate of the BOG fuel flowed downstream by the gas fuel flow rate adjustment unit 3. It is desirable that the BOG supply line 2B is not connected to any equipment other than the boiler 10 that would cause a significant change in the BOG pressure Pb of the BOG supply line 2B.

In addition, in the BOG treatment system 100 of FIG. 1, the pressure of the BOG supply line 2B fluctuates reflecting the relationship between the amount of BOG fuel generated in the liquefied gas storage tank T1 and the flow rate of the BOG fuel used as fuel in the generator Ge, etc. For example, even if the amount of BOG fuel generated in the liquefied gas storage tank T1 does not change, the BOG pressure Pb of the BOG supply line 2B decreases with an increase in the BOG fuel used in the generator Ge, and increases with a decrease in the BOG fuel used in the generator Ge. In this way, the BOG pressure Pb in the BOG supply line 2B fluctuates reflecting the flow rate of the BOG fuel flowing through the free flow line 2A. In other words, the pressure of the BOG supply line 2B fluctuates depending on the relationship between the flow rate of the BOG fuel supplied to the boiler 10 via the BOG supply line 2B and the flow rate of the BOG fuel supplied from the free flow line 2A to the generator Ge.

The gas fuel flow rate adjustment unit 3 is a valve unit that adjusts the flow rate of BOG fuel, which is a gas fuel. The gas fuel flow rate adjustment unit 3 is connected to the BOG supply line 2B. The gas fuel flow rate adjustment unit 3 has a pressure reducing valve 4A, a gas fuel shutoff valve 4B, and a gas fuel flow rate adjustment valve 5.

The pressure reducing valve 4A reduces the BOG pressure Pb. The pressure reducing valve 4A is composed of a direct acting diaphragm type pressure reducing valve, an air driven direct acting pressure reducing valve, a pilot type pressure reducing valve, an electric valve, etc. The pressure reducing valve 4A adjusts the BOG pressure Pb so that the BOG pressure downstream of the pressure reducing valve 4A is equal to or lower than a certain pressure.

The gas fuel shutoff valve 4B shuts off the flow of BOG fuel. The gas fuel shutoff valve 4B is configured as an air-driven valve or an electric valve, etc. The gas fuel shutoff valve 4B is capable of shutting off the flow of BOG fuel.

The gas fuel flow rate control valve 5 adjusts the flow rate of the BOG fuel. The gas fuel flow rate control valve 5 is an air-driven valve or an electric valve whose valve opening can be changed to any opening degree. The gas fuel flow rate control valve 5 is disposed downstream of the gas fuel shutoff valve 4B.

The liquid fuel supply line 6 is a pipe that supplies heavy oil, which is a liquid fuel, to the boiler 10. The liquid fuel supply line 6 is connected to a liquid fuel storage tank T2 that stores heavy oil, and is configured to allow the heavy oil from the liquid fuel storage tank T2 to flow into the liquid fuel supply line 6. The liquid fuel supply line 6 may also be connected to the generator Ge and the main engine Eg, and configured to allow the generator Ge and the main engine Eg to use heavy oil.

The liquid fuel flow rate adjustment unit 7 is a valve unit that adjusts the flow rate of heavy oil. The liquid fuel flow rate adjustment unit 7 is connected upstream of the boiler 10 and downstream of the liquid fuel supply line 6. The liquid fuel flow rate adjustment unit 7 has a liquid fuel shutoff valve 8 and a liquid fuel flow rate adjustment valve 9.

The liquid fuel shutoff valve 8 shuts off the flow of heavy oil. The liquid fuel shutoff valve 8 is configured as an air-driven valve or an electric valve. The liquid fuel shutoff valve 8 is connected to the liquid fuel supply line 6.

The liquid fuel flow rate control valve 9 adjusts the flow rate of heavy oil. The liquid fuel flow rate control valve 9 is an air-driven valve or an electric valve that can change the valve opening to any opening degree. The liquid fuel flow rate control valve 9 is connected downstream of the liquid fuel shutoff valve 8.

The boiler 10 heats water with heat from the combustion of fuel to generate hot water or steam. The boiler 10 has a dual fuel burner capable of burning both BOG fuel and heavy oil. The boiler 10 is configured to be capable of burning BOG fuel, a mixed fuel of BOG fuel and heavy oil, and heavy oil. The boiler 10 recovers heat generated by combustion using water as the heated medium, generates hot water or steam, and supplies the hot water or steam to the inside of the ship through the heat medium line 10a. The boiler 10 exhausts exhaust gas overboard through the exhaust line 15.

The boiler 10 is an auxiliary boiler that is not used to supply power to the main engine of the ship. Therefore, fluctuations in the combustion amount of the boiler 10 do not directly affect the operation of the main engine. The boiler 10 is, for example, a boiler with a steam pressure of about 0.3 MPa to 1.0 MPa. Preferably, the boiler 10 is a boiler with a steam pressure of about 0.5 MPa to 0.7 MPa. The steam generated by the boiler 10 is supplied to steam-using equipment of the ship (heating liquid fuel and cargo, supplying hot water for cleaning, etc., heat source for binary power generation system, steam supply for steam turbine generator) through the heat medium line 10a, and then condensed by a condenser or the like. The boiler 10 can directly combust and process the BOG fuel at a combustion processing amount according to the flow rate of the BOG fuel flowing through the free flow line 2A.

The heat medium line 10a also has an excess steam processing section 10b. The excess steam processing section 10b discharges excess steam in the boiler 10 to a condenser (not shown). The excess steam in the boiler 10 is discharged to the condenser by the excess steam processing section 10b with a valve opening pressure according to the control mode of the boiler 10.

The heat medium line 10a also has an excess steam treatment section 10b. The excess steam treatment section 10b discharges excess steam in the boiler 10 to a condenser (not shown). The excess steam treatment section 10b is composed of an excess steam treatment section (not shown) with a valve opening pressure higher than the combustion stop pressure Pws of the boiler 10 and an excess steam treatment section (not shown) with a valve opening pressure lower than the combustion stop pressure Pws. The excess steam treatment section 10b also includes an excess steam treatment valve that discharges steam when the pressure exceeds a predetermined pressure and a valve that opens and closes a flow path such as a motor valve.

For example, when the combustion stop pressure Pws of the boiler 10 is 0.7 MPa, the valve opening pressure of the surplus steam treatment valve included in the surplus steam treatment unit with the low valve opening pressure is set to 0.65 MPa, and the valve opening pressure of the surplus steam treatment valve included in the surplus steam treatment unit with the high valve opening pressure is set to 0.75 MPa. In the steam pressure priority control mode M1 in which the steam pressure Pw of the steam generated by the boiler 10 is maintained at a predetermined steam pressure Pwt set arbitrarily, the control unit 20 does not discharge the steam to the condenser until the steam pressure Pw reaches 0.75 MPa, which is higher than the combustion stop pressure Pws, by circulating the steam through the surplus steam treatment unit with the high valve opening pressure in the surplus steam treatment unit 10b. Therefore, the boiler 10 stops combustion when the steam pressure Pw of the generated steam reaches the combustion stop pressure Pws, which is lower than 0.75 MPa. In addition, in the BOG combustion control mode M2 in which the boiler 10 combusts the BOG fuel, the control unit 20 causes steam to flow through the surplus steam treatment unit 10b with a low valve opening pressure, and when the steam pressure Pw reaches 0.65 MPa, which is lower than the combustion stop pressure Pws, the control unit 20 discharges the steam to the condenser. Therefore, the boiler 10 continues combustion because the steam pressure Pw of the generated steam does not reach the combustion stop pressure Pws, which is higher than 0.65 MPa.

The air flow rate adjustment unit 12 is an air damper or the like that adjusts the flow rate of the combustion air supplied to the boiler 10. The air flow rate adjustment unit 12 is located in the air supply line 11 that supplies the combustion air to the boiler 10. The air damper, which is the air flow rate adjustment unit 12, can adjust the opening and closing plate to any opening degree by an actuator such as a motor. The flow rate of the combustion air may also be adjusted by adjusting the motor rotation speed of the combustion air blower by an inverter (not shown), or the damper opening adjustment of the air damper may be combined with the adjustment of the motor rotation speed of the combustion air blower.

The BOG pressure detection unit 13 is a sensor that detects the BOG pressure Pb, which is the pressure of the BOG fuel. The BOG pressure detection unit 13 is located upstream of the gas fuel flow rate adjustment unit 3. The BOG pressure detection unit 13 is capable of detecting the BOG pressure Pb in the BOG supply line 2B. The BOG pressure detection unit 13 may be located in the free flow line 2A near the upstream side of the BOG supply line 2B, as long as it can quantitatively detect fluctuations in the BOG pressure Pb in the BOG supply line 2B.

The steam pressure detection unit 14 is a sensor that detects the steam pressure Pw, which is the pressure of the steam generated by the boiler 10. The steam pressure detection unit 14 is connected, for example, to a boiler body (not shown) of the boiler 10. The steam pressure detection unit 14 detects the steam pressure Pw inside the boiler body of the boiler 10.

The oxygen concentration detection unit 16 is a sensor that detects the oxygen concentration Co contained in the exhaust gas from the boiler 10. The oxygen concentration detection unit 16 is located in the exhaust line 15 that discharges the exhaust gas from the boiler 10.

The mode setting unit 19 switches the control mode of the boiler system 1. The mode setting unit 19 allows the operator to set any control mode from the multiple control modes that the boiler system 1 has.

Furthermore, the mode setting unit 19 switches the combustion mode of the boiler system 1. The mode setting unit 19 is configured to allow an operator to set a predetermined combustion mode from at least one combustion mode set in each control mode. The mode setting unit 19 is configured to allow one of the liquid fuel combustion mode Ma, the gaseous fuel combustion mode Mb, and the mixed combustion mode Mc to be set. In addition, the mode setting unit 19 can also be configured as an automatic control in which a predetermined combustion mode is set along with the control mode, and when certain conditions are satisfied, the combustion mode is switched to another combustion mode.

The control unit 20 is a control device that controls the gas fuel flow rate adjustment unit 3, the liquid fuel flow rate adjustment unit 7, the boiler 10, and the air flow rate adjustment unit 12. The control unit 20 has a memory unit 21. The control unit 20 can also acquire data detected by the BOG pressure detection unit 13, the steam pressure detection unit 14, the oxygen concentration detection unit 16, and the like.

The memory unit 21 stores various programs and data for controlling the operation of the gas fuel flow rate adjustment unit 3, the liquid fuel flow rate adjustment unit 7, the boiler 10, and the air flow rate adjustment unit 12, and for acquiring detection data from the BOG pressure detection unit 13, the steam pressure detection unit 14, and the oxygen concentration detection unit 16. The memory unit 21 can store information related to each control mode and combustion mode that can be set in the mode setting unit 19. The memory unit 21 has a BOG combustion control mode memory unit 212 that stores information related to the BOG combustion control mode M2 and an air ratio control mode memory unit 213 that stores information related to the air ratio control mode M3. The memory unit 21 also has a gas fuel combustion mode memory unit 21b that stores information related to the gas fuel combustion mode Mb, and a mixed combustion mode memory unit 21c that stores information related to the mixed combustion mode Mc.

The memory unit 21 stores various set values such as a predetermined steam pressure Pwt and a predetermined BOG pressure Pbt in each control mode memory unit and each combustion mode memory unit. The memory unit 21 stores a predetermined oxygen concentration Cot in the gas fuel combustion mode Mb and the mixed combustion mode Mc in the gas fuel combustion mode memory unit 21b and the mixed combustion mode memory unit 21c. The predetermined oxygen concentration Cot in each combustion mode may be set to a predetermined oxygen concentration Cot according to the combustion stage of the boiler 10 (for example, a predetermined oxygen concentration Cot that is different for high combustion and low combustion).

The control unit 20 is electrically connected to the gas fuel shutoff valve 4B, the gas fuel flow rate adjustment valve 5, the liquid fuel shutoff valve 8, the liquid fuel flow rate adjustment valve 9, the valves constituting the surplus steam treatment unit 10b, and the air flow rate adjustment unit 12. The control unit 20 can send control signals to the gas fuel shutoff valve 4B, the gas fuel flow rate adjustment valve 5, the liquid fuel shutoff valve 8, the liquid fuel flow rate adjustment valve 9, and the surplus steam treatment unit 10b, or to the respective solenoid valves that supply air to the valves if the valves are air-driven valves. The control unit 20 can send a control signal to the air flow rate adjustment unit 12 to change, for example, the damper opening degree of the air damper.

The control unit 20 is electrically connected to the boiler 10. The control unit 20 detects the operating state of the boiler 10 and is capable of transmitting and receiving various measurement data and control signals for controlling the boiler 10. In addition, when the surplus steam processing unit 10b includes an surplus steam processing valve that discharges steam when the pressure exceeds a predetermined value and a valve that opens and closes the flow path in the motor valve, the control unit 20 can transmit a signal to control the motor valve.

The control unit 20 is electrically connected to the BOG pressure detection unit 13, the steam pressure detection unit 14, and the oxygen concentration detection unit 16. The control unit 20 can acquire the BOG pressure Pb detected by the BOG pressure detection unit 13, the steam pressure Pw detected by the steam pressure detection unit 14, and the oxygen concentration Co contained in the exhaust gas of the boiler 10 detected by the oxygen concentration detection unit 16.

The control unit 20 receives the oxygen concentration Co of the exhaust gas from the boiler 10 detected by the oxygen concentration detection unit 16, the adjustment amount of the air flow rate adjustment unit 12, the adjustment amount of the gas fuel flow rate adjustment unit 3, and the adjustment amount of the liquid fuel flow rate adjustment unit 7 per unit time. Based on the oxygen concentration Co contained in the exhaust gas from the boiler 10 detected by the oxygen concentration detection unit 16, the control unit 20 can send a control signal to the air flow rate adjustment unit 12 to change the damper opening of the air damper so that the oxygen concentration Co becomes a predetermined oxygen concentration Cot.

The control unit 20 is electrically connected to the mode setting unit 19. The control unit 20 can acquire a control signal related to the control mode output by the mode setting unit 19. When the control unit 20 acquires a control signal related to the BOG combustion control mode M2 or an air ratio control mode M3, the control unit 20 transmits a predetermined control signal to the gas fuel flow rate adjustment unit 3, the liquid fuel flow rate adjustment unit 7, the boiler 10, and the air flow rate adjustment unit 12 based on information related to the control mode stored in the BOG combustion control mode memory unit 212 or the air ratio control mode memory unit 213.

The control unit 20 is electrically connected to the nitrogen supply unit V. The control unit 22 can receive a signal related to the open/closed state of the on-off valve from the nitrogen supply unit V.

Next, we will explain the BOG combustion control mode M2 and the air ratio control mode M3 in the boiler system 1. Figure 2 is a block diagram showing the control modes in the boiler system 1. Note that the inflow of excess BOG fuel into the BOG supply line 2B continues with fluctuations in the amount of BOG fuel generated.

As shown in FIG. 1 or FIG. 2, in this embodiment, the mode setting unit 19 can set the BOG combustion control mode M2 or the air ratio control mode M3, and output a control signal related to each control mode to the control unit 20. The mode setting unit 19 can also switch between a control signal related to the gas fuel combustion mode Mb or a control signal related to the mixed combustion mode Mc, and output a control signal related to each combustion mode to the control unit 20.

BOG combustion control mode M2 is a control mode that prioritizes the combustion of BOG fuel in the boiler 10. In the BOG combustion control mode M2, when the BOG pressure Pb is equal to or higher than a predetermined BOG pressure Pbt, the boiler system 1 supplies BOG fuel to the boiler 10 and burns the BOG fuel in a gas fuel combustion mode Mb in which only the BOG fuel is combusted in the boiler 10. The boiler 10 supplies the generated steam to the ship. When it is necessary to lower the BOG pressure Pb, the boiler system 1 configured in this manner can burn only the BOG fuel in the BOG combustion control mode M2, increasing the amount of combustion up to the upper limit of the combustion capacity.

When the operator sets the BOG combustion control mode M2 by the mode setting unit 19, the control unit 20 controls the boiler 10 to combust the BOG fuel based on information related to the BOG combustion control mode M2 stored in the BOG combustion control mode memory unit 212. The control unit 20 acquires the BOG pressure Pb, and when the BOG pressure Pb is equal to or higher than a predetermined BOG pressure Pbt, the control unit 20 opens the gas fuel shutoff valve 4B and adjusts the gas fuel flow rate control valve 5 so that the amount of BOG fuel burned in the boiler 10 becomes a predetermined amount based on information related to the gas fuel combustion mode Mb stored in the gas fuel combustion mode memory unit 21b. The control unit 20 also closes the liquid fuel shutoff valve 8 so that heavy oil is not supplied to the boiler 10. When the BOG pressure Pb is less than the predetermined BOG pressure Pbt, the control unit 20 stops the supply of BOG fuel to the boiler 10 by the gas fuel flow rate control unit 3 (see FIG. 1).

The control unit 20 acquires the steam pressure Pw. In the BOG combustion control mode M2, when the excess steam exhaust pressure reaches a value higher than a predetermined steam pressure Pwt and lower than the combustion stop pressure Pws, the control unit 20 opens the excess steam treatment valve of the excess steam treatment unit 10b, which has an opening pressure lower than the combustion stop pressure Pws, maintains the steam pressure Pw below the combustion stop pressure Pws, and continues the combustion process of the BOG fuel (see FIG. 1).

In addition, in the event of a BOG supply abnormality in which the gas fuel flow rate adjustment unit 3 cannot properly adjust the amount of BOG fuel supplied, the control unit 20 closes the gas fuel shutoff valve 4B so that BOG fuel is not supplied to the boiler 10 (see FIG. 1).

The air ratio control mode M3 is a control mode for combusting the nitrogen-mixed BOG fuel when the nitrogen mixing ratio of the nitrogen-mixed BOG fuel discharged from the liquefied gas storage tank T1 varies greatly (for example, the nitrogen mixing ratio varies between 10% and 90% or substantially between 0% and 100%) when the liquefied gas storage tank T1 is purged with nitrogen or when the nitrogen-filled liquefied gas storage tank T1 is filled with LNG (see FIG. 1). In the air ratio control mode M3, the boiler system 1 supports the combustion of the nitrogen-mixed BOG fuel by burning heavy oil, and adjusts the combustion air that has been excessively supplied due to nitrogen being mixed into the BOG fuel to an appropriate range for combustion by controlling the oxygen concentration Co of the exhaust gas to a predetermined concentration.

In the air ratio control mode M3, the boiler system 1 supplies heavy oil and nitrogen-mixed BOG fuel (for example, heavy oil is fixed at 20% of the maximum combustion amount and nitrogen-mixed BOG fuel is up to 80% of the maximum combustion amount) to the boiler 10 while maintaining the supply amount of heavy oil at a predetermined value, and can combust the nitrogen-mixed BOG fuel in the mixed combustion mode Mc in which both fuels are mixed and burned. In the mixed combustion mode Mc, the combustion of the nitrogen-mixed BOG fuel is supported by the combustion of the heavy oil, so combustion can be maintained even when the nitrogen mixture ratio is high. Furthermore, in the mixed combustion mode Mc, the combustion air ratio is maintained within an appropriate range, so combustion can be stabilized and a decrease in boiler efficiency can be suppressed.

When the operator sets the air ratio control mode M3 by the mode setting unit 19, the control unit 20 controls the boiler 10 to combust the nitrogen-mixed BOG fuel and heavy oil based on the information related to the air ratio control mode M3 stored in the air ratio control mode memory unit 213. The control unit 20 opens the gas fuel shutoff valve 4B and adjusts the gas fuel flow control valve 5 so that the nitrogen-mixed BOG fuel is supplied to the boiler 10 based on the information related to the mixed combustion mode Mc stored in the mixed combustion mode memory unit 21c (see FIG. 1). Furthermore, the control unit 20 opens the liquid fuel shutoff valve 8 and adjusts the liquid fuel flow control unit 7 so that heavy oil is supplied to the boiler 10. In other words, the control unit 20 adjusts the gas fuel flow control unit 3 and the liquid fuel flow control unit 7 so that the BOG fuel and heavy oil are supplied to the boiler 10 (see FIG. 1).

The control unit 20 acquires oxygen concentration information of the exhaust gas, including the oxygen concentration Co of the exhaust gas of the boiler 10 detected by the oxygen concentration detection unit 16. The control unit 20 controls the air flow rate adjustment unit 12 so that the oxygen concentration Co of the exhaust gas becomes a predetermined oxygen concentration Cot. When the oxygen concentration Co of the exhaust gas detected by the oxygen concentration detection unit 16 is equal to or greater than the predetermined oxygen concentration Cot, the control unit 20 transmits a control signal to the air flow rate adjustment unit 12 to decrease the flow rate of the combustion air supplied to the boiler 10. On the other hand, when the oxygen concentration Co of the exhaust gas is less than the predetermined oxygen concentration Cot, the control unit 20 transmits a control signal to the air flow rate adjustment unit 12 to increase the flow rate of the combustion air supplied to the boiler 10. In addition, when the oxygen concentration Co of the exhaust gas is equal to the predetermined oxygen concentration Cot, the control unit 20 transmits a control signal to the air flow rate adjustment unit 12 to maintain the flow rate of the combustion air.

In air ratio control mode M3, the boiler system 1 supports the combustion of nitrogen-mixed BOG fuel by burning heavy oil, and by controlling the oxygen concentration Co of the exhaust gas to a predetermined concentration, it is possible to adjust the combustion air that is excessively supplied due to nitrogen being mixed into the BOG fuel to an appropriate range for combustion. In other words, air ratio control mode M3 is a control mode that is particularly effective in the combustion process of BOG fuel when there is a large change in composition (large change in calories) due to nitrogen being mixed into the BOG fuel.

In this way, by switching between the BOG combustion control mode M2 and the air ratio control mode M3, the boiler system 1 can select the appropriate control mode to combust the nitrogen-mixed BOG fuel and supply steam according to the demands on board, even if the steam usage on board or the nitrogen mixing ratio of the nitrogen-mixed BOG fuel due to nitrogen purging fluctuates.

Next, a modified example of the boiler system 1 according to the first embodiment of the present invention will be described with reference to Figures 1 and 3. Figure 3 is a block diagram showing the BOG combustion control mode in the modified example of the boiler system 1.

As shown in FIG. 1, when the BOG combustion control mode M2 is set by the mode setting unit 19, the control unit 20 may control the air flow rate adjustment unit 12 to adjust the oxygen concentration Co of the exhaust gas from the boiler 10 detected by the oxygen concentration detection unit 16 to a predetermined oxygen concentration Cot.

As shown in FIG. 1 or FIG. 3, the control unit 20 stores information related to the adjustment amount of the air flow rate adjustment unit 12 relative to the amount of BOG fuel supplied in the gas fuel combustion mode memory unit 21b so that the oxygen concentration Co of the exhaust gas when BOG fuel having a predetermined composition (e.g., a mixture ratio of methane, ethane, etc.) is supplied to the boiler 10 becomes a predetermined oxygen concentration Cot (see FIG. 1).

When the oxygen concentration Co of the exhaust gas from the boiler 10 is higher than a predetermined oxygen concentration Cot, the control unit 20 determines that the BOG fuel has a composition that requires less combustion air than the BOG fuel having a predetermined composition, or that the BOG concentration in the BOG fuel is low, and adjusts the air flow rate adjustment unit 12 to reduce the flow rate of the combustion air.When the oxygen concentration Co of the exhaust gas is lower than a predetermined oxygen concentration Cot, the control unit 20 determines that the BOG fuel has a composition that requires more combustion air than the BOG fuel having a predetermined composition, or that the BOG concentration in the BOG fuel is high, and adjusts the air flow rate adjustment unit 12 to increase the flow rate of the combustion air.

In the BOG combustion control mode M2, the boiler system 1 configured in this manner can adjust the air flow rate supplied to the boiler 10 according to the composition or BOG concentration of the BOG fuel supplied to the boiler 10. This makes it possible to realize a boiler system capable of stable combustion processing even if the BOG concentration in the gaseous fuel containing BOG and the composition of the BOG fuel change.

(Embodiment 2)
Next, a boiler system 1A according to a second embodiment of the present invention will be described with reference to Fig. 4 and Fig. 5. In the following description, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted, and only the components different from the first embodiment will be described. Fig. 4 is a schematic diagram of a BOG treatment system 100 including a boiler system 1A according to the second embodiment of the present invention. Fig. 5 is a block diagram showing an air ratio control mode M3 in the boiler system 1A.

The boiler system 1A is configured to set the gas fuel combustion mode Mb or the mixed combustion mode Mc in the air ratio control mode M3 based on the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel.

As shown in FIG. 4, the boiler system 1A has a control unit 22. The control unit 22 has a nitrogen mixing determination unit 23, which is an inert gas mixing determination unit. When control is performed to burn at a predetermined air ratio by adjusting the air flow rate adjustment unit 12, the nitrogen mixing determination unit 23 calculates the nitrogen mixing ratio Cn in the nitrogen-mixed BOG fuel supplied to the boiler 10 based on the relationship between the fuel supply amount, the combustion air amount, and the nitrogen mixing ratio Cn. The nitrogen mixing determination unit 23 determines whether the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel supplied to the boiler 10 is equal to or greater than a predetermined nitrogen mixing ratio Cnt as the degree of nitrogen mixing.

The memory unit 21 stores various programs and data for the nitrogen mixing determination unit 23 to calculate the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel being supplied to the boiler 10 based on, for example, the adjustment amount of the air flow rate adjustment unit 12, the adjustment amount of the gas fuel flow rate adjustment unit 3, and the adjustment amount of the liquid fuel flow rate adjustment unit 7.

The control unit 22 can adjust the air flow rate adjustment unit 12 so that the oxygen concentration Co of the exhaust gas becomes a predetermined oxygen concentration Cot (see FIG. 5) based on the detection result of the oxygen concentration detection unit 16. The control unit 22 calculates the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel by the nitrogen mixing determination unit 23 based on the adjustment amount of the air flow rate adjustment unit 12, the adjustment amount of the gas fuel flow rate adjustment unit 3, and the adjustment amount of the liquid fuel flow rate adjustment unit 7 when burning at a predetermined air ratio. The nitrogen mixing determination unit 23 may calculate the nitrogen mixing ratio Cn of the BOG fuel based on the detection result of the nitrogen mixing detection unit 24 (nitrogen sensor or gas calorimeter, gas density meter, etc. (see FIG. 4)) that detects the nitrogen concentration.

As shown in FIG. 4 or 5, in the air ratio control mode M3, the control unit 22 can switch the combustion mode based on the degree of nitrogen mixing in the nitrogen-mixed BOG fuel calculated by the nitrogen mixing determination unit 23. That is, when the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel is smaller than a predetermined nitrogen mixing ratio Cnt, the control unit 22 burns the nitrogen-mixed BOG fuel in the gas fuel combustion mode Mb. Also, when the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel is equal to or greater than the predetermined nitrogen mixing ratio Cnt, the control unit 22 burns the nitrogen-mixed BOG fuel in the mixed combustion mode Mc.

When the operator or the control unit 22 sets the air ratio control mode M3 by the mode setting unit 19, the control unit 22 starts control in the mixed combustion mode Mc based on information related to the air ratio control mode M3 stored in the air ratio control mode memory unit 213. The control unit 22 switches the combustion mode to the gas fuel combustion mode Mb when the nitrogen mixing ratio Cn becomes less than a predetermined nitrogen mixing ratio Cnt, for example, when LNG is filled into the liquefied gas storage tank T1 filled with nitrogen. Also, in the gas fuel combustion mode Mb, the control unit 22 switches the combustion mode to the mixed combustion mode Mc when the nitrogen mixing ratio Cn becomes equal to or greater than the predetermined nitrogen mixing ratio Cnt, for example, when nitrogen purging is performed on the liquefied gas storage tank T1.

The boiler system 1A configured in this manner burns only the nitrogen-mixed BOG fuel when stable combustion is possible with only the nitrogen-mixed BOG fuel based on the nitrogen mixing ratio Cn. In addition, the boiler system 1A burns heavy oil in addition to the nitrogen-mixed BOG fuel when stable combustion processing is difficult with only the nitrogen-mixed BOG fuel based on the nitrogen mixing ratio Cn. Thus, the boiler system 1A can suppress the consumption of heavy oil, increase the combustion processing amount of the nitrogen-mixed BOG fuel, and shorten the combustion processing time of the nitrogen-mixed BOG fuel. This makes it possible to realize a boiler system 1A that can perform efficient combustion processing even if the BOG concentration of the nitrogen-mixed BOG fuel changes.

Here, the control unit 22 switches between the mixed combustion mode Mc and the gaseous fuel combustion mode Mb in the air ratio control mode M3, but the switching between the mixed combustion mode Mc and the gaseous fuel combustion mode Mb may be performed by an operator. For example, the mode setting unit 19 may be provided with a switching unit that switches between the two modes.

2 and 4, a modified example of the boiler system 1A according to the second embodiment of the present invention will be described. The boiler system 1A may be configured to switch from the BOG combustion control mode M2 to the air ratio control mode M3 based on the nitrogen mixture ratio Cn of the BOG fuel.

The control unit 22 determines, via the nitrogen mixing determination unit 23, whether the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel being supplied to the boiler 10 is equal to or greater than a predetermined nitrogen mixing ratio Cnt for each unit time.

When the control unit 22 determines that the nitrogen mixing ratio Cn of the nitrogen in the gaseous fuel containing BOG is equal to or greater than a predetermined nitrogen mixing ratio Cnt in the BOG combustion control mode M2, it switches the control mode to the air ratio control mode M3 and switches the combustion mode to the mixed combustion mode Mc. The control unit 22 adjusts the gaseous fuel flow rate adjustment unit 3 and the liquid fuel flow rate adjustment unit 7 so that nitrogen-mixed BOG fuel and heavy oil are supplied to the boiler 10. The boiler system 1A supports combustion with heavy oil when the nitrogen mixing ratio Cn in the nitrogen-mixed BOG fuel is equal to or greater than a predetermined value. This allows stable combustion processing to be performed even if the BOG concentration in the nitrogen-mixed BOG fuel decreases.

In the air ratio control mode M3, when the nitrogen mixture determination unit 23 determines that the calculated nitrogen mixture ratio Cn of the nitrogen-mixed BOG fuel is less than a predetermined nitrogen mixture ratio Cnt, the control unit 22 may switch the combustion mode to the gas fuel combustion mode Mb in the air ratio control mode M3. The control unit 22 adjusts the gas fuel flow rate adjustment unit 3 and the liquid fuel flow rate adjustment unit 7 so that nitrogen-mixed BOG fuel is supplied to the boiler 10. The boiler system 1A combusts only the nitrogen-mixed BOG fuel when the nitrogen mixture ratio Cn in the nitrogen-mixed BOG fuel is less than a predetermined value. This reduces the consumption of heavy oil and increases the amount of nitrogen-mixed BOG fuel burned, thereby shortening the time required for burning the nitrogen-mixed BOG fuel.

(Embodiment 3)
Next, a boiler system 1B according to a third embodiment of the present invention will be described with reference to Fig. 6. In the following description, the same components as those in the first embodiment will be denoted by the same reference numerals, and a description will be omitted. Only the components different from the first embodiment will be described. Fig. 6 is a block diagram showing an air ratio control mode M3 in the boiler system 1B according to the third embodiment of the present invention.

As shown in FIG. 6, when the air ratio control mode M3 is set by the operator in the mode setting unit 19, the boiler system 1B can set one of the control modes, nitrogen-increased combustion control mode M4 and BOG-increased combustion control mode M5, which are inert gas increased combustion control modes. The nitrogen-increased combustion control mode M4 starts the main combustion of the boiler 10 using only nitrogen-mixed BOG fuel. On the other hand, the BOG-increased combustion control mode M5 starts the main combustion of the boiler 10 using both nitrogen-mixed BOG fuel and heavy oil.

In nitrogen-increased combustion control mode M4, only nitrogen-mixed BOG fuel is supplied to the boiler 10 to start main combustion. In BOG-increased combustion control mode M5, both nitrogen-mixed BOG fuel and heavy oil are supplied to the boiler 10 to start main combustion. After that, the gas fuel combustion mode Mb or mixed combustion mode Mc is set based on the nitrogen mixing ratio Cn of the nitrogen-mixed BOG fuel.

Nitrogen-increased combustion control mode M4 is a control mode that increases the combustion throughput of the nitrogen-mixed BOG fuel in the boiler 10 when the nitrogen mixing ratio Cn in the nitrogen-mixed BOG fuel is less than a predetermined nitrogen mixing ratio Cnt in a state where an increase in the nitrogen mixing ratio Cn is expected due to nitrogen purging to the liquefied gas storage tank T1. On the other hand, BOG-increased combustion control mode M5 is a control mode that makes the start of main combustion in the boiler 10 more stable when the nitrogen mixing ratio Cn in the nitrogen-mixed BOG fuel is equal to or greater than the predetermined nitrogen mixing ratio Cnt in a state where a decrease in the nitrogen mixing ratio Cn is expected due to the supply of LNG to the liquefied gas storage tank T1 that has been replaced with nitrogen by nitrogen purging.

The boiler system 1B configured in this manner can perform fuel processing according to changes in the nitrogen mixing ratio Cn in the nitrogen-mixed BOG fuel that are expected when starting main combustion in the boiler 10, by executing control in the nitrogen-increased combustion control mode M4 or the BOG-increased combustion control mode M5 set by the operator in the air ratio control mode M3. This makes it possible to realize a boiler system 1B that can perform stable combustion processing even if the BOG concentration in the nitrogen-mixed BOG fuel changes.

(Embodiment 4)
Next, a boiler system 1C according to a fourth embodiment of the present invention will be described with reference to Fig. 7. In the following description, the same components as those in the first embodiment will be denoted by the same reference numerals, and a description thereof will be omitted. Only the components different from the first embodiment will be described. Fig. 7 is a block diagram showing a control mode in a modified example of the boiler system 1C according to the fourth embodiment of the present invention.

As shown in FIG. 7, the boiler system 1C is configured to be able to set the steam pressure priority control mode M1 in addition to the BOG combustion control mode M2 and the air ratio control mode M3.

The mode setting unit 25, which switches the control mode of the boiler system 1C, can set one of the steam pressure priority control mode M1, the BOG combustion control mode M2, and the air ratio control mode M3. The mode setting unit 25 can output control signals related to the steam pressure priority control mode M1, the BOG combustion control mode M2, and the air ratio control mode M3 to the control unit 26.

The mode setting unit 25 is configured to be able to set one of the combustion modes: liquid fuel combustion mode Ma, gaseous fuel combustion mode Mb, or mixed fuel combustion mode Mc.

The mode setting unit 25 is configured to be able to switch between the liquid fuel combustion mode Ma and the mixed combustion mode Mc. The mode setting unit 25 is configured to be able to output a control signal related to the liquid fuel combustion mode Ma, a control signal related to the gas fuel combustion mode Mb, or a control signal related to the mixed combustion mode Mc to the control unit 26.

The memory unit 21 further includes a steam pressure priority control mode memory unit 211 that stores information related to the steam pressure priority control mode M1, and a liquid fuel combustion mode memory unit 21a that stores information related to the liquid fuel combustion mode Ma (see FIG. 4).

The control unit 26 transmits a predetermined control signal to the gas fuel flow rate adjustment unit 3, the liquid fuel flow rate adjustment unit 7, the boiler 10, and the air flow rate adjustment unit 12 based on information related to the control mode stored in the steam pressure priority control mode memory unit 211. The control unit 26 also transmits a predetermined control signal to the gas fuel flow rate adjustment unit 3, the liquid fuel flow rate adjustment unit 7, the boiler 10, and the air flow rate adjustment unit 12 based on information related to each combustion mode stored in each memory unit.

Next, we will explain the steam pressure priority control mode M1 in the boiler system 1C.

The steam pressure priority control mode M1 is a control mode that prioritizes the generation of steam in the boiler 10. In the steam pressure priority control mode M1, the boiler system 1C combusts BOG fuel to generate steam so as to maintain the steam pressure Pw of the boiler 10 at a predetermined steam pressure Pwt while supplying the steam generated by the boiler 10 to the inside of the ship in response to requests from on board. In other words, the steam pressure priority control mode M1 is a control mode that prioritizes the stable supply of steam to the inside of the ship in response to requests from on board.

In addition to the BOG combustion control mode M2 and the air ratio control mode M3, the boiler system 1C is configured to include a steam pressure priority control mode M1 that prioritizes steam supply in the liquid fuel combustion mode Ma, the gas fuel combustion mode Mb, or the mixed combustion mode Mc.

When steam supply is required, the boiler system 1C configured in this manner can combust BOG fuel while maintaining the steam pressure Pw at a predetermined steam pressure Pwt in steam pressure priority control mode M1. On the other hand, when the combustion process of BOG fuel is prioritized over steam supply, the boiler system 1C can increase the amount of BOG fuel burned in BOG combustion control mode M2 or air ratio control mode M3. This makes it possible to realize a boiler system that can also be used when steam supply is prioritized.

Other Embodiments
Although the embodiment of the present invention has been described above, the above-mentioned embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and it is possible to carry out the above-mentioned embodiment by appropriately modifying it within the scope of the gist of the present invention.

In each of the above-described embodiments, the BOG supply line 2B may have a pressure regulating valve or the like, and may have a branch or the like that has little effect on the pressure value of the BOG fuel or nitrogen-mixed BOG fuel, as long as the BOG pressure detection unit 13 is configured to be able to quickly detect excess BOG pressure Pb.

In each of the above-described embodiments, the BOG pressure detection unit 13 may detect a physical quantity that can be converted into a pressure value. For example, the BOG flow rate can be converted into BOG pressure by setting a predetermined condition.

In each of the above-described embodiments, the boiler system 1, 1A, 1B, and 1C adjusts the gas fuel flow control valve 5 in the BOG combustion control mode M2 so that the amount of BOG fuel burned in the boiler becomes a predetermined amount. In this case, the predetermined amount of BOG fuel burned in the boiler 10 may be an amount arbitrarily set by the operator.

In each of the above-described embodiments, the gas fuel flow rate adjustment unit 3 may be configured such that the gas fuel flow rate adjustment valve 5 is located upstream of the gas fuel cutoff valve 4B.

In each of the above-described embodiments, the liquid fuel flow rate adjustment unit 7 may be configured such that the liquid fuel flow rate adjustment valve 9 is located upstream of the liquid fuel shutoff valve 8.

In each of the above-described embodiments, the boiler 10 may be a boiler capable of burning fuels other than BOG and heavy oil. For example, in a ship equipped with a vaporizer H (see FIG. 1) for liquefied gas for the main engine Eg, the boiler 10 may burn fuel gasified by the vaporizer H.

In each of the above-described embodiments, the BOG pressure detection unit 13 may be located upstream of the gas fuel flow rate adjustment unit 3.

In each of the above-described embodiments, the gas fuel flow rate control valve 5 and the liquid fuel flow rate control valve 9 may be any valve having an actuator that is driven by any power source, such as a solenoid valve or an air-driven valve.

In each of the above-described embodiments, the boiler systems 1, 1A, 1B, and 1C may adjust the air flow rate adjustment unit 12 based on the value of an inert gas detection unit that detects the concentration of an inert gas such as nitrogen or the flow rate of the inert gas.

In each of the above-described embodiments, the boiler systems 1, 1A, 1B, and 1C may be configured to automatically switch the combustion mode in the steam pressure priority control mode M1 based on the steam pressure Pw, the BOG pressure Pb, or the amount of fuel supplied, etc.

In each of the above-described embodiments, the flow rate of the BOG fuel supplied to the boiler 10 may be configured so that the operator can set the flow rate as desired.

In each of the above-described embodiments, the boiler systems 1, 1A, 1B, and 1C may be configured to adjust the amount of BOG fuel supplied based on the BOG pressure.

In each of the above-described embodiments, the boiler systems 1, 1A, 1B, and 1C may calculate the amount of heavy oil to be supplied to the boiler in the mixed combustion mode Mc based on the BOG concentration in the BOG fuel.

In each of the above-mentioned embodiments, the boiler systems 1, 1A, 1B, and 1C may fix the amount of heavy oil (liquid fuel) burned to a predetermined amount (e.g., 20% of the maximum boiler combustion amount) and vary the amount of nitrogen-mixed BOG fuel burned (e.g., 20% to 80% of the maximum boiler combustion amount) in the air ratio control mode M3. Also, the boiler systems 1, 1A, 1B, and 1C may fix the amount of heavy oil (liquid fuel) burned and the amount of nitrogen-mixed BOG fuel burned to a predetermined ratio (e.g., the amount of heavy oil burned is 20% of the total and the amount of nitrogen-mixed BOG fuel burned is 80% of the total) in the air ratio control mode M3.

In each of the above-described embodiments, the inert gas mixing determination unit may be configured to calculate the degree of inert gas mixing in the BOG fuel that contains the inert gas. The inert gas mixing determination unit may be configured to calculate the degree of inert gas mixing from the inert gas concentration detected by, for example, a gas concentration meter. The inert gas mixing determination unit may also be configured to estimate the degree of inert gas mixing from the gas density of the BOG fuel that contains the inert gas.

Reference Signs List 1, 1A, 1B, 1C Boiler system 2A Free flow line 2B BOG supply line 3 Gaseous fuel flow rate adjustment unit 4A Pressure reducing valve 4B Gaseous fuel shutoff valve 5 Gaseous fuel flow rate adjustment valve 6 Liquid fuel supply line 7 Liquid fuel flow rate adjustment unit 8 Liquid fuel shutoff valve 9 Liquid fuel flow rate adjustment valve 10 Boiler 10a Heat medium line 10b Excess steam treatment unit 11 Air supply line 12 Air flow rate adjustment unit 13 BOG pressure detection unit 14 Steam pressure detection unit 15 Exhaust line 16 Oxygen concentration detection unit 19, 25 Mode setting unit 20, 22, 26 Control unit 21 Memory unit 211 Steam pressure priority control mode memory unit 212 BOG combustion control mode memory unit 213 Air ratio control mode memory unit 21a Liquid fuel combustion mode memory unit 21b Gaseous fuel combustion mode memory unit 21c Multi-combustion mode memory unit 23 Nitrogen mixing determination unit 24 Nitrogen mixture detection unit 100 BOG treatment system T1 Liquefied gas storage tank T2 Liquid fuel storage tank T3 Nitrogen storage tank M1 Steam pressure priority control mode M2 BOG combustion control mode M3 Air ratio control mode M4 Nitrogen-increased combustion control mode M5 BOG-increased combustion control mode Ma Liquid fuel combustion mode Mb Gaseous fuel combustion mode Mc Mixed combustion mode Pb BOG pressure Pbt Specified BOG pressure Pw Steam pressure Pwt Specified steam pressure Pws Combustion stop pressure Co Oxygen concentration Cot Specified oxygen concentration Cn Nitrogen mixture ratio Cnt Specified nitrogen mixture ratio

Claims (7)

A boiler system to be installed on a ship, the boiler system comprising: a liquefied gas storage tank; an inert gas supply unit that supplies an inert gas into the liquefied gas storage tank; and a free flow line through which boil-off gas and the inert gas discharged from the liquefied gas storage tank flow,
The boiler system includes:
A boiler capable of burning gaseous fuel including boil-off gas and liquid fuel;
A gas fuel pressure detection unit that detects a pressure of the gas fuel including the boil-off gas supplied to the boiler;
A gas fuel flow rate adjusting unit that adjusts a flow rate of the gas fuel including the boil-off gas supplied to the boiler;
a liquid fuel flow rate adjusting unit that adjusts a flow rate of the liquid fuel supplied to the boiler;
an air flow rate adjusting unit that adjusts the flow rate of combustion air supplied to the boiler;
an oxygen concentration detection unit that detects an oxygen concentration in exhaust gas from the boiler;
a mode setting unit capable of setting one of a boil-off gas combustion control mode in which a gaseous fuel containing a boil-off gas is given priority in combustion in the boiler and an air ratio control mode in which a gaseous fuel containing a boil-off gas containing an inert gas is burned at a predetermined air ratio;
A control unit that controls the combustion of the boiler,
The control unit is
When the boil-off gas combustion control mode is set by the mode setting unit, the gas fuel flow rate adjusting unit is adjusted to obtain a predetermined combustion amount,
When the air ratio control mode is set by the mode setting unit, the gas fuel flow rate adjusting unit and the liquid fuel flow rate adjusting unit are adjusted so that the gas fuel including the boil-off gas and the liquid fuel are supplied to the boiler, and the air flow rate adjusting unit is adjusted so that the oxygen concentration of the exhaust gas from the boiler becomes a predetermined oxygen concentration.
Boiler system. 2. The boiler system according to claim 1,
The control unit is
Further comprising an inert gas mixing determination unit for calculating a mixing degree of the inert gas contained in the boil-off gas,
In the air ratio control mode,
when the inert gas mixing determination unit determines that the degree of mixing of the inert gas contained in the gaseous fuel containing the boil-off gas is less than a predetermined value, the gaseous fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit are adjusted so that only the gaseous fuel containing the boil-off gas is supplied to the boiler;
when the inert gas mixing determination unit determines that the degree of mixing of the inert gas contained in the gaseous fuel containing the boil-off gas is equal to or greater than a predetermined value, the gaseous fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit are adjusted so that the gaseous fuel containing the boil-off gas and the liquid fuel are supplied to the boiler.
Boiler system. 3. The boiler system according to claim 2,
The control unit is
adjusting the air flow rate adjusting unit based on the detection result of the oxygen concentration detection unit so that the oxygen concentration of the exhaust gas becomes a predetermined value;
The inert gas mixing determination unit determines a mixing degree of the inert gas contained in the boil-off gas based on an adjustment amount of the air flow rate adjustment unit.
Boiler system. 3. The boiler system according to claim 2,
The control unit is
In the boil-off gas combustion control mode, the inert gas mixing determination unit determines a mixing degree of the inert gas in the gas fuel containing the boil-off gas,
when it is determined that the degree of mixing of the gaseous fuel containing the boil-off gas with the inert gas is equal to or greater than a predetermined value, the control mode is switched to the air ratio control mode, and the gaseous fuel flow rate adjustment unit and the liquid fuel flow rate adjustment unit are adjusted so that the gaseous fuel containing the boil-off gas and the liquid fuel are supplied to the boiler.
Boiler system. 3. The boiler system according to claim 2,
The mode setting unit is
When the air ratio control mode is set, any one of an inert gas increase combustion control mode and a boil-off gas increase combustion control mode can be set,
The control unit is
When the inert gas increased combustion control mode is set by the mode setting unit, main combustion is started in a state in which the gas fuel flow rate adjusting unit and the liquid fuel flow rate adjusting unit are adjusted so that only the gas fuel containing the boil-off gas is supplied to the boiler,
When the mode setting unit sets the boil-off gas increased combustion control mode, main combustion is started in a state in which the gas fuel flow rate adjusting unit and the liquid fuel flow rate adjusting unit are adjusted so that the gas fuel containing the boil-off gas and the liquid fuel are supplied to the boiler.
Boiler system. 2. The boiler system according to claim 1,
The control unit is
When the boil-off gas combustion control mode is set by the mode setting unit, the air flow rate adjusting unit is adjusted so that the oxygen concentration of the exhaust gas from the boiler detected by the oxygen concentration detecting unit becomes a predetermined oxygen concentration.
Boiler system. 2. The boiler system according to claim 1,
A steam pressure detection unit that detects the pressure of steam generated in the boiler,
The mode setting unit is
A steam pressure priority control mode in which the generation of steam in the boiler is prioritized may be further set.
The control unit is
when the mode setting unit sets the steam pressure priority control mode, at least one of the gas fuel flow rate adjusting unit and the liquid fuel flow rate adjusting unit is adjusted so that the steam pressure detected by the steam pressure detection unit becomes a predetermined steam pressure.
Boiler system.