Boiler and method for controlling boiler

Abstract

An ammonia co-fired boiler (1) serving as a boiler comprises: a burner (20) to which firing air (A1) and at least one among a first fuel (F1) and an ammonia fuel (F2) are supplied; a damper (304); a firing state detection unit (44) that distinguishes and detects firing using the first fuel (F1) and co-firing using the first fuel and the ammonia fuel (F2); and a control unit (40). The control unit (40) is provided with a firing control unit that controls the exhaust gas oxygen concentration to a first oxygen concentration when firing using only the first fuel (F1) is detected by the firing state detection unit (44), and controls the exhaust gas oxygen concentration to a second oxygen concentration lower than the first oxygen concentration when co-firing using the first fuel (F1) and the ammonia fuel (F2) is detected by the firing state detection unit (44).

Description

Boiler, boiler control method

[0001] This application claims priority from Japanese Patent Application No. 2023-209385, filed on December 12, 2023, the contents of which are incorporated herein by reference. The present invention relates to a boiler and a boiler control method.

[0002] By replacing a portion of hydrocarbon fuels, such as natural gas and petroleum fuels, used as boiler fuel with ammonia, which does not generate carbon dioxide, it is expected that global warming can be prevented by reducing carbon dioxide emissions (see, for example, Patent Document 1). When hydrocarbon fuels are combusted, combustion is carried out so that the oxygen concentration in the exhaust gas is equal to or greater than a predetermined value, from the viewpoint of suppressing the generation of carbon monoxide and unburned matter such as soot. On the other hand, when ammonia fuel is combusted, if the oxygen concentration in the exhaust gas exceeds the predetermined value, the generation of nitrous oxide increases, reducing the greenhouse gas (GHG) reduction effect of ammonia combustion.

[0003] Japanese Patent Application Laid-Open No. 2021-185122

[0004] In a boiler using an ammonia-mixed combustion burner, if the air ratio is brought closer to 1 to suppress the heat loss of the exhaust gas, the boiler efficiency improves, but the generation of unburned materials increases. On the other hand, if the air ratio is increased when burning ammonia fuel, nitrous oxide (N2O), which has a high global warming potential, is generated. ２ Therefore, when ammonia fuel is mixed with a hydrocarbon fuel, which has a faster combustion speed than ammonia, a combustion control method is required that can suppress the generation of unburned materials such as carbon monoxide and soot, while also suppressing the generation of nitrous oxide.

[0005] An object of the present invention is to provide a boiler and a boiler control method that can suppress the generation of unburned materials such as carbon monoxide and soot and also suppress the generation of nitrous oxide when ammonia fuel is mixed with a hydrocarbon fuel that has a faster combustion rate than ammonia.

[0006] The present invention relates to a boiler including: a burner to which at least one of a first fuel and ammonia fuel, which has a faster combustion speed than ammonia, and combustion air are supplied; a combustion air adjustment unit that adjusts the amount of the combustion air supplied; and a control unit, wherein the control unit includes a combustion state detection unit that distinguishes between combustion of the first fuel and mixed combustion of the first fuel and the ammonia fuel; and a combustion control unit that, when the combustion state detection unit detects mono-combustion of the first fuel, controls the combustion air adjustment unit so that an oxygen concentration in the exhaust gas after mono-combustion of the first fuel becomes a first oxygen concentration, and, when the combustion state detection unit detects mixed combustion of the first fuel and the ammonia fuel, controls the combustion air adjustment unit so that an oxygen concentration in the exhaust gas after mixed combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration lower than the first oxygen concentration.

[0007] It is also preferable that the first fuel is a liquid fuel, the first oxygen concentration is 4% or more, and the second oxygen concentration is less than 4%.

[0008] It is also preferable that the first fuel is a gaseous fuel, the first oxygen concentration is 3.5% or more, and the second oxygen concentration is less than 3.5%.

[0009] Further, it is preferable that the system further includes an ammonia mixing ratio determination unit that determines a mixing ratio of the ammonia fuel, and the control unit further includes a mixing air ratio storage unit that stores a relationship between the ammonia mixing ratio and the second oxygen concentration such that the second oxygen concentration when the ammonia mixing ratio is high is lower than the second oxygen concentration when the ammonia mixing ratio is low, and when the combustion state detection unit detects mixing of the first fuel and the ammonia fuel, the combustion control unit controls the air ratio during ammonia mixing, based on the ammonia mixing ratio determined by the ammonia mixing ratio determination unit and the relationship between the ammonia mixing ratio and the second oxygen concentration stored in the mixing air ratio storage unit.

[0010] The second oxygen concentration associated with the ammonia co-firing air ratio in the co-firing air ratio storage unit is preferably set based on an oxygen concentration at which an amount of unburned matter generated for each ammonia co-firing ratio clearly increases, and is preferably equal to or higher than the oxygen concentration at which an amount of unburned matter generated clearly increases.

[0011] When the combustion state detection unit detects co-combustion of the first fuel and the ammonia fuel and the ammonia co-combustion ratio determination unit determines that the ammonia co-combustion ratio is 20% or more, the combustion control unit preferably controls the air ratio during ammonia co-combustion, based on the ammonia co-combustion ratio determined by the ammonia co-combustion ratio determination unit and the relationship between the ammonia co-combustion ratio and the second oxygen concentration stored in the co-combustion air ratio storage unit, so that a rate of decrease in the air ratio when the ammonia co-combustion ratio is high relative to the air ratio during first mono-fuel combustion is larger than a rate of decrease in the air ratio when the ammonia co-combustion ratio is low relative to the air ratio during first mono-fuel combustion.

[0012] The present invention relates to a control method for a boiler equipped with a burner capable of co-firing at least one of a first fuel having a combustion rate faster than ammonia and ammonia fuel, and combustion air, the control method comprising: when mono-combustion of the first fuel is detected, controlling the amount of the combustion air so that an oxygen concentration in the exhaust gas after co-combustion of the first fuel becomes a first oxygen concentration; and when co-combustion of the first fuel and the ammonia fuel is detected, controlling the amount of the combustion air so that an oxygen concentration in the exhaust gas after co-combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration, wherein the second oxygen concentration is lower than the first oxygen concentration and is set based on the relationship between the ammonia co-firing ratio and the oxygen concentration at which generation of unburned matter due to combustion clearly increases, and further comprising a step of controlling the second oxygen concentration when the ammonia co-firing ratio is high so that it is lower than the second oxygen concentration when the ammonia co-firing ratio is low.

[0013] According to the present invention, it is possible to provide a boiler and a boiler control method that can suppress the generation of unburned materials such as carbon monoxide and soot (smoke) and also suppress the generation of nitrous oxide when ammonia fuel is mixed with a hydrocarbon fuel that has a faster combustion rate than ammonia.

[0014] 1 is a diagram schematically illustrating the configuration of an ammonia co-firing boiler according to an embodiment of the present invention. FIG. 2 is a diagram illustrating the relationship between the oxygen concentration and the carbon monoxide concentration of the exhaust gas when a gaseous fuel (liquefied natural gas) and ammonia fuel are combusted as the first fuel F1 in the ammonia co-firing boiler according to an embodiment of the present invention. FIG. 3 is a diagram illustrating the relationship between the ammonia co-firing ratio and the oxygen concentration at which the carbon monoxide concentration of the exhaust gas starts to clearly increase, based on the test results shown in FIG. 2. FIG. 4 is a diagram illustrating the relationship between the oxygen concentration of the exhaust gas and the effective ammonia utilization rate (effective ammonia utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reduction amount due to combustion of ammonia fuel) × 100 (%)) when a gaseous fuel (liquefied natural gas) and ammonia fuel are co-firing as the first fuel in the ammonia co-firing boiler according to an embodiment of the present invention. FIG. 5 is a diagram illustrating the relationship between the oxygen concentration of the exhaust gas and the smoke degree (an index of incomplete combustion of oil fuel) when a liquid fuel (heavy oil A) and ammonia fuel are combusted as the first fuel in the ammonia co-firing boiler according to an embodiment of the present invention. 6 is a diagram showing the relationship between the ammonia co-firing ratio and the oxygen concentration at which the smoky degree of the exhaust gas starts to clearly increase, based on the test results shown in FIG. 5. FIG. 7 is a diagram showing the relationship between the oxygen concentration of the exhaust gas and the effective ammonia utilization rate (effective ammonia utilization rate (%)=1-(amount of greenhouse gas emissions as nitrous oxide / amount of greenhouse gas emissions reduced by combustion of ammonia fuel)×100(%)) when a liquid fuel (heavy oil A) as the first fuel and ammonia fuel are co-fired in an ammonia co-firing boiler according to an embodiment of the present invention.

[0015] An ammonia-mixed combustion boiler 1 as a boiler according to an embodiment of the present invention will be described below with reference to the drawings. The ammonia-mixed combustion boiler 1 of this embodiment is a steam boiler that burns fuel to heat water to generate steam and supply the steam to a load device (not shown).

[0016] Fig. 1 is a diagram schematically showing the configuration of an ammonia-mixed combustion boiler 1 according to an embodiment. As shown in Fig. 1, the ammonia-mixed combustion boiler 1 includes a boiler body 10, a burner 20, and a control unit 40. The ammonia-mixed combustion boiler 1 also includes a first fuel supply line 100, an ammonia supply line 200, a combustion air supply line 300, an exhaust stack 400, a feedwater line 500, and a steam supply line 600. In this specification, the term "line" is a general term for a flow path, a passage, a conduit, etc.

[0017] The boiler body 10 includes a lower header 11, a plurality of water tubes 12, an upper header 13, and a combustion chamber B. The boiler body 10 recovers heat from the combustion gas generated by the combustion of fuel in the combustion chamber B, and heats water W1 supplied to the boiler body 10 to generate steam S1.

[0018] The burner 20 combusts a first fuel F1 and an ammonia fuel F2, which have a faster combustion rate than ammonia, in the combustion chamber B of the can body 10. The burner 20 is disposed on top of the can body 10. The burner 20 includes a burner body 21 and a wind box 22. The burner body 21 is connected to a first fuel supply line 100 and an ammonia supply line 200. The wind box 22 is connected to a combustion air supply line 300. In this embodiment, first, the first fuel F1 and combustion air A1 are supplied to the burner to start combustion. Next, ammonia fuel F2 is supplied to the combustion section of the first fuel F1. At this time, a predetermined amount of combustion air A1 corresponding to the total amount of the first fuel F1 and the ammonia fuel F2 is supplied to the burner 20. In this embodiment, the first fuel, which has a faster combustion rate than ammonia, can be a liquid fuel or a gaseous fuel. As the liquid fuel, an oil fuel, an alcohol fuel, or the like can be used. As the gaseous fuel, hydrocarbon gas such as liquefied natural gas (hereinafter referred to as LNG) or liquefied petroleum gas (LPG) can be used.

[0019] The control unit 40 controls combustion in the burner 20 of the ammonia-mixed combustion boiler 1. Details of the control unit 40 will be described later.

[0020] The first fuel supply line 100 supplies a first fuel F1 from a first fuel supply source (not shown) to the burner 20. The upstream side of the first fuel supply line 100 is connected to the first fuel supply source, and the downstream side of the first fuel supply line 100 is connected to the burner 20. The first fuel supply line 100 is equipped with, from the upstream side, a main valve 101, a first fuel flow sensor 102, a first shutoff valve 103, a flow rate adjustment valve 104, and a second shutoff valve 105.

[0021] The main valve 101 is a manual valve that opens and closes the flow path of the first fuel supply line 100. The first fuel flow sensor 102 detects the flow rate of the first fuel F1 flowing through the first fuel supply line 100. The first fuel flow sensor 102 is electrically connected to the control unit 40, and the detection result of the first fuel flow sensor 102 can be acquired by the control unit 40. The first shutoff valve 103 and the second shutoff valve 105 are electromagnetic valves that open and close the flow path of the first fuel supply line 100 to supply or stop the first fuel F1. The first shutoff valve 103 and the second shutoff valve 105 are electrically connected to the control unit 40 and are controlled by signals transmitted from the control unit 40. The flow rate adjustment valve 104 is an adjustment valve that adjusts the flow rate of the first fuel F1 supplied to the burner 20 by adjusting the valve opening. The flow rate adjusting valve 104 is electrically connected to the control unit 40 and is controlled by a signal sent from the control unit 40 based on the detection result of the first fuel flow rate sensor 102 .

[0022] The ammonia supply line 200 supplies ammonia fuel F2 from an ammonia supply source (not shown) to the burner 20. The upstream side of the ammonia supply line 200 is connected to the ammonia supply source, and the downstream side of the ammonia supply line 200 is connected to the burner 20. The ammonia supply line 200 is equipped with, from the upstream side, a main valve 201, an ammonia fuel flow sensor 202, a first shutoff valve 203 serving as an ammonia fuel shutoff valve, a flow rate adjustment valve 204, and a second shutoff valve 205 serving as an ammonia fuel shutoff valve.

[0023] The main valve 201 is configured as a manual valve and opens and closes the flow path of the ammonia supply line 200. The ammonia fuel flow sensor 202 detects the flow rate of the ammonia fuel F2 flowing through the ammonia supply line 200. The ammonia fuel flow sensor 202 is electrically connected to the control unit 40, and the detection result of the ammonia fuel flow sensor 202 can be acquired by the control unit 40. The first shutoff valve 203 and the second shutoff valve 205 are configured as electromagnetic valves and open and close the flow path of the ammonia supply line 200 to supply or stop the ammonia fuel F2. The first shutoff valve 203 and the second shutoff valve 205 are electrically connected to the control unit 40 and controlled by signals transmitted from the control unit 40. The flow rate adjustment valve 204 is an adjustment valve that adjusts the flow rate of the ammonia fuel F2 supplied to the burner 20 by adjusting the valve opening. The flow rate adjustment valve 204 is electrically connected to the control unit 40 and controlled by a signal transmitted from the control unit 40 based on the detection result of the ammonia fuel flow sensor 202.

[0024] The combustion air supply line 300 supplies the combustion air A1 to the burner 20. In this embodiment, the combustion air A1 is supplied to the wind box 22 of the burner 20. The upstream side of the combustion air supply line 300 is connected to a blower 301, and the downstream side of the combustion air supply line 300 is connected to the wind box 22. From the upstream side, the combustion air supply line 300 is equipped with the blower 301, a combustion air flow rate sensor 303, and a damper 304 as a combustion air adjustment unit.

[0025] The blower 301 supplies combustion air A1 to the burner 20. The blower 301 includes a fan and a motor for rotating the fan. The inverter 302 controls the frequency to adjust the motor's rotation speed, thereby adjusting the rotational speed. That is, the inverter 302 is a combustion air adjustment unit that adjusts the amount of combustion air A1 supplied to the burner. In this embodiment, the inverter 302 is electrically connected to the control unit 40 and is controlled by a signal transmitted from the control unit 40. The combustion air flow sensor 303 detects the flow rate of the combustion air A1 flowing through the combustion air supply line 300. The combustion air flow sensor 303 is electrically connected to the control unit 40, and the detection result of the combustion air flow sensor 303 can be acquired by the control unit 40. The damper 304 adjusts the amount of combustion air A1 supplied to the burner 20 by adjusting the opening degree of the damper. Specifically, damper 304 is arranged to be rotatable between a closed state in which the flow path of combustion air supply line 300 is blocked, and an open state in which damper 304 rotates from this closed state by a predetermined angle (e.g., 90 degrees) to open the flow path of combustion air supply line 300. Damper 304 is electrically connected to control unit 40 and is controlled by a signal transmitted from control unit 40.

[0026] The exhaust pipe 400 discharges exhaust gas E1 generated by burning the first fuel F1 and the ammonia fuel F2 in the burner 20 to the outside of the can body 10. The upstream side of the exhaust pipe 400 is connected to the upper part of the circumferential surface of the can body 10, and the downstream side of the exhaust pipe 400 is open to the atmosphere. The exhaust pipe 400 includes, from the upstream side, an exhaust gas temperature detection unit 401 and an exhaust gas detection unit 402.

[0027] The exhaust gas temperature detection unit 401 detects the temperature of the exhaust gas E1 flowing through the exhaust stack 400. The exhaust gas temperature detection unit 401 is electrically connected to the control unit 40, and the detection results of the exhaust gas temperature detection unit 401 can be acquired by the control unit 40. The exhaust gas detection unit 402 detects the components of the exhaust gas E1 flowing through the exhaust stack 400. In this embodiment, the exhaust gas detection unit 402 detects oxygen (O ２ ), carbon dioxide (CO ２), carbon monoxide (CO), nitrogen oxides (NO ｘ ), nitrous oxide (N ２ The exhaust gas detection unit 402 detects oxygen, ammonia (NH), etc. The exhaust gas detection unit 402 is electrically connected to the control unit 40, and the detection results of the exhaust gas detection unit 402 can be acquired by the control unit 40. When detecting oxygen, for example, the exhaust gas detection unit 402 uses a zirconia oxygen sensor, a galvanic cell oxygen sensor, etc.

[0028] The water supply line 500 supplies water W1 from a water supply source (not shown) to the boiler body 10. The upstream side of the water supply line 500 is connected to the water supply source, and the downstream side of the water supply line 500 is connected to the lower header 11. The water supply line 500 is equipped with a main valve 501. The main valve 501 is a manual valve that opens and closes the flow path of the water supply line 500.

[0029] The steam supply line 600 supplies steam S1 from the boiler body 10 to the load equipment. The upstream side of the steam supply line 600 is connected to the upper header 13 of the boiler body 10, and the downstream side of the steam supply line 600 is connected to the load equipment. The steam supply line 600 is equipped with a main valve 601. The main valve 601 is configured as a manual valve and opens and closes the flow path of the steam supply line 600.

[0030] 1, the control unit 40 includes a storage unit 41, a combustion control unit 42, an ammonia co-firing ratio determination unit 43, and a combustion state detection unit 44. The control unit 40 is configured with an arithmetic processor such as a programmable logic controller (PLC), a digital signal processor (DSP), or a field-programmable gate array (FPGA). The various functions of the control unit 40 are realized by executing predetermined software (programs) stored in the storage unit 41, for example. The various functions of the control unit 40 may be realized by a combination of hardware and software, or may be realized solely by hardware (electronic circuits).

[0031] The memory unit 41 stores various setting information. The memory unit 41 also functions as a co-firing air ratio memory unit, storing the relationship between the ammonia co-firing ratio and the second oxygen concentration such that the second oxygen concentration when the ammonia co-firing ratio is high is lower than the second oxygen concentration when the ammonia co-firing ratio is low. The control unit 40 may also include a co-firing air ratio memory unit, separate from the memory unit 41, that stores the relationship between the ammonia co-firing ratio and the second oxygen concentration such that the second oxygen concentration when the ammonia co-firing ratio is high is lower than the second oxygen concentration when the ammonia co-firing ratio is low. Here, the terms "when the ammonia co-firing ratio is high" and "when the ammonia co-firing ratio is low" refer to the relative relationship between the ammonia co-firing ratios. For example, when the ammonia co-firing ratio is set to 10%, 20%, 40%, or 60%, the second oxygen concentration when the ammonia co-firing ratio is 60% is lower than the second oxygen concentration when the ammonia co-firing ratio is 10%. In this case, the second oxygen concentration may be the same at an ammonia mixing ratio of 20% and 40%, for example. In other words, the second oxygen concentration at the maximum ammonia mixing ratio (60% in this case) is lower than the second oxygen concentration at the minimum ammonia mixing ratio (10% in this case), and the second oxygen concentration at ammonia mixing ratios between them decreases as the ammonia mixing ratio increases, or remains the same.

[0032] The combustion control unit 42 controls the combustion of the burner 20. In this embodiment, the combustion control unit 42 starts the blower 301 and adjusts at least one of the inverter 302 and the damper 304, which serve as combustion air adjustment units, to supply combustion air A1 at a predetermined flow rate to the burner 20. The combustion control unit 42 also opens the first shutoff valve 103 and the second shutoff valve 105 and adjusts the aperture of the flow control valve 104 based on the detection result of the first fuel flow sensor 102 so that a predetermined combustion amount of the first fuel F1 is achieved. This starts the combustion of the first fuel F1. The combustion control unit 42 also opens the first shutoff valve 203 and the second shutoff valve 205 and adjusts the aperture of the flow control valve 204 based on the detection result of the ammonia fuel flow sensor 202 so that a predetermined combustion amount of the ammonia fuel F2 is achieved. This starts the combustion of the ammonia fuel F2.

[0033] The combustion state detection unit 44 distinguishes between combustion of the first fuel F1 and mixed combustion of the first fuel F1 and ammonia fuel F2. In the present embodiment, the combustion state detection unit 44 detects that the first fuel F1 is being combusted by detecting, with the first fuel flow rate sensor 102, that the first fuel F1 is flowing through the first fuel supply line 100. The combustion state detection unit 44 detects that the ammonia fuel F2 is being combusted by detecting, with the ammonia fuel flow rate sensor 202, that the ammonia fuel F2 is flowing through the ammonia supply line 200.

[0034] The combustion state detection unit 44 determines that mono-combustion of the first fuel F1 has been detected when only the first fuel flow sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100, and determines that co-combustion of the first fuel F1 and ammonia fuel F2 has been detected when the first fuel flow sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100 and the ammonia fuel flow sensor 202 detects that the ammonia fuel F2 is flowing through the ammonia supply line 200. Note that the combustion state detection unit 44 is not limited to detecting the flow of ammonia fuel F2 by the ammonia fuel flow sensor 202 as described above, and may detect the fuel state by input of a select switch or the like that sets ammonia co-combustion.

[0035] When the combustion state detection unit 44 detects mono-combustion of the first fuel F1, the combustion control unit 42 controls at least one of the inverter 302 and the damper 304, which serve as a combustion air adjustment unit, so that the oxygen concentration in the exhaust gas after the mono-combustion of the first fuel F1 becomes a first oxygen concentration. Furthermore, when the combustion state detection unit 44 detects co-combustion of the first fuel F1 and the ammonia fuel F2, the combustion control unit 42 controls at least one of the inverter 302 and the damper 304 so that the oxygen concentration in the exhaust gas after the co-combustion of the first fuel F1 and the ammonia fuel F2 becomes a second oxygen concentration that is lower than the first oxygen concentration.

[0036] In the present embodiment, when the combustion state detection unit 44 detects mono-combustion of the first fuel F1, the combustion control unit 42 acquires the detection result of the oxygen concentration of the exhaust gas E1 after mono-combustion of the first fuel F1 that is detected by the exhaust gas detection unit 402. Based on the detection result of the oxygen concentration of the detected exhaust gas E1, the combustion control unit 42 controls at least one of the inverter 302 and the damper 304 so that the oxygen concentration of the exhaust gas E1 becomes the first oxygen concentration. In the present embodiment, when the combustion state detection unit 44 detects co-combustion of the first fuel F1 and the ammonia fuel F2, the combustion control unit 42 acquires the detection result of the oxygen concentration of the exhaust gas E1 after co-combustion of the first fuel F1 and the ammonia fuel F2 that is detected by the exhaust gas detection unit 402. The combustion control unit 42 controls at least one of the inverter 302 and the damper 304 based on the detection result of the oxygen concentration of the detected exhaust gas E1 so that the oxygen concentration of the exhaust gas E1 becomes a second oxygen concentration that is lower than the first oxygen concentration.

[0037] As a result, when a hydrocarbon fuel as the first fuel F1, which has a faster combustion speed than ammonia, is mixed with the ammonia fuel F2, it is possible to suppress the generation of unburned materials such as carbon monoxide and soot (smoke) and the generation of nitrous oxide, while also achieving a reduction in the air ratio, which reduces exhaust gas heat loss and improves boiler efficiency.

[0038] As described above, the ammonia fuel flow rate sensor 202 detects the flow rate of the ammonia fuel F2 flowing through the ammonia supply line 200. The ammonia mixing ratio determination unit 43 determines the ammonia mixing ratio based on the detection results of the first fuel flow rate sensor 102 and the ammonia fuel flow rate sensor 202, and transmits the determined ammonia mixing ratio to the combustion control unit 42. In the present embodiment, as an example, determining the ammonia mixing ratio will be described by calculating and determining the ammonia mixing ratio based on the detection results of the first fuel flow rate sensor 102 and the ammonia fuel flow rate sensor 202, but the present invention is not limited to this.

[0039] 2 to 4 are diagrams showing measurement results of exhaust gas characteristics when gaseous fuel (LNG) and ammonia fuel F2 are combusted as the first fuel F1 in the ammonia co-firing boiler 1 according to the embodiment. The boiler combustion tests were conducted for LNG mono-combustion and co-firing of LNG and ammonia fuel F2 (ammonia co-firing ratios of 40% and 60%). The ammonia co-firing ratio is defined as the ratio of the calorific value supplied by ammonia fuel F2 to the total calorific value supplied by LNG and ammonia fuel F2.

[0040] FIG. 2 is a graph showing the relationship between the oxygen concentration and the carbon monoxide concentration of the flue gas E1 when a gaseous fuel (LNG) as the first fuel F1 and an ammonia fuel F2 are combusted in the ammonia co-firing boiler 1 according to the embodiment. In the graph shown in FIG. 2 , the vertical axis represents the relative value, where the carbon monoxide concentration of the flue gas E1 at an ammonia co-firing ratio of 60% and an exhaust gas oxygen concentration of 0.5% is set to 100, and the horizontal axis represents the oxygen concentration [%] of the flue gas E1. As shown in FIG. 2 , in the LNG-only combustion, the carbon monoxide concentration of the flue gas E1 began to increase significantly when the oxygen concentration of the flue gas E1 fell below 2.6%. On the other hand, in the LNG and ammonia co-firing, the carbon monoxide concentration began to increase significantly when the oxygen concentration of the flue gas E1 was less than 1.6%. Furthermore, the point at which the carbon monoxide concentration began to increase significantly shifted toward a lower oxygen concentration as the ammonia co-firing ratio increased.

[0041] Figure 3 is a graph showing the relationship between the ammonia co-firing ratio and the oxygen concentration at which the carbon monoxide concentration in exhaust gas E1 begins to increase significantly, based on the test results shown in Figure 2. In the graph shown in Figure 3, the vertical axis represents the oxygen concentration [%] at which the carbon monoxide concentration in exhaust gas E1 begins to increase significantly, and the horizontal axis represents the ammonia co-firing ratio [%]. As shown in Figure 3, the oxygen concentration at which the carbon monoxide concentration begins to increase significantly decreases as the ammonia co-firing ratio increases. From the approximation line shown in Figure 3, the oxygen concentration in exhaust gas E1 at which the carbon monoxide concentration begins to increase significantly when the ammonia co-firing ratio is 20% is estimated to be 2.1% (air ratio 1.11) (oxygen concentration indicated by a white circle in Figure 3).

[0042] Therefore, based on these phenomena, when co-firing LNG and ammonia fuel F2, operation can be performed with reduced carbon monoxide generation at a lower air ratio than when burning only LNG. Furthermore, the rate of decrease in the air ratio when the ammonia co-firing ratio is high can be made larger than the rate of decrease in the air ratio when the ammonia co-firing ratio is low. For example, the rate of decrease in the air ratio when the ammonia co-firing ratio is 60% can be made larger than the rate of decrease in the air ratio when the ammonia co-firing ratio is 20%. Here, the rate of decrease in the air ratio refers to the rate of decrease in the air ratio during co-firing relative to the air ratio during LNG mono-firing.

[0043] FIG. 4 is a graph showing the relationship between the oxygen concentration of the flue gas E1 and the effective ammonia utilization rate (effective ammonia utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reduction amount due to ammonia fuel combustion) × 100 (%)) when a gaseous fuel (LNG) as the first fuel F1 and ammonia fuel F2 are mixed in the ammonia mixed-fuel boiler 1 according to the embodiment. The "greenhouse gas expenditure reduction amount due to ammonia fuel combustion" is, in other words, the "greenhouse gas emission reduction amount due to reduced hydrocarbon fuel consumption." In the graph shown in FIG. 4, the vertical axis represents the effective ammonia utilization rate [%], and the horizontal axis represents the oxygen concentration [%] in the flue gas E1. As shown in FIG. 4, when the ammonia mixed-fuel ratio is 40% or 60%, the effective ammonia utilization rate drops sharply when the oxygen concentration of the flue gas E1 is 3.5% or higher. Therefore, the ammonia mixed-fuel boiler 1 according to the embodiment is preferably operated at an oxygen concentration of less than 3.5%.

[0044] From the above results, the second oxygen concentration, which is the oxygen concentration of the exhaust gas after co-combustion of the gaseous fuel as the first fuel F1 and the ammonia fuel F2, is preferably less than 3.5%, lower than the first oxygen concentration, and the first oxygen concentration, which is the oxygen concentration of the exhaust gas when the first fuel F1 is exclusively combusted, is preferably 3.5% or higher, from 1.2 to 1.35 (3.5% to 5.4%), which is the target air ratio for small once-through boilers. Furthermore, according to the ammonia co-combustion boiler 1 of this embodiment, the second oxygen concentration, which is the oxygen concentration of the exhaust gas after co-combustion of the gaseous fuel as the first fuel F1 and the ammonia fuel F2, is lower than the first oxygen concentration. This makes it possible to suppress both the generation of unburned matter such as carbon monoxide and the generation of nitrous oxide during co-combustion of the first fuel F1 and the ammonia fuel F2, and further suppress exhaust gas heat loss and improve boiler efficiency.

[0045] 5 to 7 are diagrams showing exhaust gas characteristics when a liquid fuel (heavy oil A) as the first fuel F1 and ammonia fuel F2 are combusted in the ammonia co-firing boiler 1 according to the embodiment. The boiler combustion tests were conducted for the exclusive combustion of heavy oil A and the co-firing of heavy oil A and ammonia fuel F2 (ammonia co-firing ratios of 20% and 40%). The ammonia co-firing ratio is the ratio of the calorific value supplied by ammonia fuel F2 to the total calorific value supplied by heavy oil A and ammonia fuel F2.

[0046] FIG. 5 is a graph showing the relationship between the oxygen concentration of the flue gas E1 and the smoke index (an index of incomplete combustion of oil fuel) when a liquid fuel (heavy oil A) and ammonia fuel F2 are combusted as the first fuel F1 in the ammonia co-firing boiler 1 according to the embodiment. In the graph shown in FIG. 5 , the vertical axis represents the relative value, where the smoke index of the flue gas at an oxygen concentration of 1% and an ammonia co-firing ratio of 20% is set to 100, and the horizontal axis represents the oxygen concentration [%] in the flue gas E1. As shown in FIG. 5 , when using only heavy oil A, the smoke index began to increase significantly when the oxygen concentration of the flue gas E1 fell below 4%. On the other hand, when using the co-firing of heavy oil A and ammonia, the smoke index began to increase significantly when the oxygen concentration of the flue gas E1 was less than 2.5%. Furthermore, the point at which the smoke index began to increase significantly shifted toward a lower oxygen concentration as the ammonia co-firing ratio increased.

[0047] FIG. 6 is a graph showing the relationship between the ammonia co-firing ratio and the oxygen concentration at which the smoke level of flue gas E1 begins to clearly increase, based on the test results shown in FIG. 5 . In the graph shown in FIG. 6 , the vertical axis represents the ammonia co-firing ratio [%], and the horizontal axis represents the oxygen concentration [%] at which the smoke level of flue gas E1 begins to clearly increase. As shown in FIG. 6 , the oxygen concentration at which the smoke level clearly increases decreases as the ammonia co-firing ratio increases. Furthermore, although not shown in FIG. 6 , in boiler combustion tests with ammonia co-firing ratios of 50% and 60%, the oxygen concentration at which the smoke level clearly increases was lower than that at an ammonia co-firing ratio of 40%. From the approximation line shown in FIG. 6 , the oxygen concentration at which the smoke level clearly increases with an ammonia co-firing ratio of 20% is estimated to be 2.9% (air ratio 1.16). Therefore, based on these phenomena, when heavy oil A and ammonia fuel F2 are co-fired, operation can be performed with reduced carbon monoxide generation at a lower air ratio than when only heavy oil A is burned. Furthermore, the rate of decrease in the air ratio when the ammonia co-firing ratio is high can be made larger than the rate of decrease in the air ratio when the ammonia co-firing ratio is low. For example, the rate of decrease in the air ratio when the ammonia co-firing ratio is 40% can be made larger than the rate of decrease in the air ratio when the ammonia co-firing ratio is 20%. Here, the rate of decrease in the air ratio refers to the rate of decrease in the air ratio during co-firing relative to the air ratio when exclusively burning heavy oil A.

[0048] FIG. 7 is a diagram showing the relationship between the oxygen concentration of flue gas E1 and the effective ammonia utilization rate (effective ammonia utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reduction amount due to combustion of ammonia fuel) × 100 (%)) when a liquid fuel (heavy oil A) as the first fuel F1 and ammonia fuel F2 are mixed in the ammonia mixed-fuel boiler 1 according to the embodiment. In the graph shown in FIG. 7, the vertical axis represents the effective ammonia utilization rate, and the horizontal axis represents the oxygen concentration [%] in the flue gas E1. As shown in FIG. 7, the effective ammonia utilization rate drops sharply when the oxygen concentration of the flue gas E1 is 4% or higher. Therefore, the ammonia mixed-fuel boiler 1 according to the embodiment is preferably operated at an oxygen concentration of less than 4%.

[0049] From the above results, the second oxygen concentration, which is lower than the first oxygen concentration, of the exhaust gas oxygen concentration after co-combustion of the liquid fuel as the first fuel F1 and the ammonia fuel F2 is preferably less than 4%, and the first oxygen concentration, which is the exhaust gas oxygen concentration after mono-combustion of the first fuel F1, is preferably 4% or higher, from 1.25 to 1.4 (4.2% to 6%), which is the target air ratio for small once-through boilers. Furthermore, according to the ammonia co-combustion boiler 1 of this embodiment, the second oxygen concentration, which is the exhaust gas oxygen concentration after co-combustion of the liquid fuel as the first fuel F1 and the ammonia fuel F2, is lower than the first oxygen concentration. Therefore, during co-combustion of the first fuel F1 and the ammonia fuel F2, the generation of unburned matter such as soot (smoke) and the generation of nitrous oxide are both suppressed, and further, the exhaust gas heat loss is suppressed, thereby improving boiler efficiency.

[0050] 3 and 6 , which show the relationship between the ammonia co-firing ratio and the oxygen concentration at which unburned matter in the exhaust gas E1 clearly increases, the region where the oxygen concentration of the exhaust gas E1 is higher than the approximate line shown in the figure is a region where there is no clear sudden increase in the concentration of unburned matter generated by the combustion of the exhaust gas E1. Therefore, it is preferable to set the second oxygen concentration for each ammonia co-firing ratio to a region where the oxygen concentration is higher than the approximate line. The combustion control unit 42 controls the reduction rate of the air ratio when the ammonia co-firing ratio is high to be greater than the reduction rate when the ammonia co-firing ratio is low, based on the ammonia co-firing ratio determined by the ammonia co-firing ratio determination unit 43 and the relationship between the ammonia co-firing ratio and the second oxygen concentration stored in the memory unit 41. This suppresses the generation of unburned matter. Specifically, when the combustion state detection unit 44 detects the co-firing of the first fuel F1 and the ammonia fuel F2 and the ammonia co-firing ratio determination unit 43 determines that the ammonia co-firing ratio is 20% or higher, the combustion control unit 42 sets the second oxygen concentration lower than the first oxygen concentration according to the ammonia co-firing ratio. This reduces the air ratio compared to when the first fuel F1 is exclusively burned, thereby improving boiler efficiency. Furthermore, the air ratio during co-firing is reduced only when the ammonia co-firing ratio is 20% or higher, which is sufficient to suppress the generation of unburned materials such as carbon monoxide and soot (smoke). This avoids the risk of increased generation of unburned materials due to a reduced air ratio.

[0051] The ammonia-mixed combustion boiler 1 of the present embodiment described above provides the following effects.

[0052] (1) The ammonia co-firing boiler 1 of this embodiment includes a burner 20 to which at least one of a first fuel F1 and an ammonia fuel F2, which have a faster combustion rate than ammonia, and combustion air A1 are supplied, at least one of an inverter 302 and a damper 304 as a combustion air adjusting unit that adjusts the supply amount of the combustion air A1, combustion of the first fuel F1, and a control unit 40. The control unit 40 includes a combustion state detection unit 44 that distinguishes between the co-firing of the first fuel F1 and the ammonia fuel F2 and detects the exclusive combustion of the first fuel F1 by the combustion state detection unit 44. The combustion control unit 42 controls at least one of the inverter 302 and the damper 304 as a combustion air adjusting unit so that the oxygen concentration in the exhaust gas after the co-combustion of the first fuel F1 and the ammonia fuel F2 becomes a first oxygen concentration when co-combustion of the first fuel F1 and the ammonia fuel F2 is detected by the combustion state detection unit 44, and controls at least one of the inverter 302 and the damper 304 as a combustion air adjusting unit so that the oxygen concentration in the exhaust gas after the co-combustion of the first fuel F1 and the ammonia fuel F2 becomes a second oxygen concentration lower than the first oxygen concentration when the combustion state detection unit 44 detects the co-combustion of the first fuel F1 and the ammonia fuel F2. This enables combustion control that can suppress the generation of unburned materials such as carbon monoxide and soot and also suppress the generation of nitrous oxide during the co-combustion of the first fuel F1 and the ammonia fuel F2, which have a faster combustion rate than ammonia. Furthermore, by suppressing the generation of unburned materials such as carbon monoxide and soot and reducing the air ratio, the heat loss in the exhaust gas is reduced and the boiler efficiency is improved.

[0053] (2) In the ammonia co-firing boiler 1 described in (1), the first fuel F1 is a liquid fuel, the first oxygen concentration is 4% or more, and the second oxygen concentration is less than 4%. As a result, when the first fuel F1, which has a faster combustion speed than ammonia, is a liquid fuel, the exhaust gas oxygen concentration when the first fuel F1 is exclusively burned is controlled to be 4% or more, and the exhaust gas oxygen concentration when the first fuel F1 and the ammonia fuel F2 are co-fired is controlled to be less than 4%, thereby improving boiler efficiency.

[0054] (3) In the ammonia co-firing boiler 1 described in (1), the first fuel F1 is a gaseous fuel, the first oxygen concentration is 3.5% or more, and the second oxygen concentration is less than 3.5%. As a result, when the first fuel F1, which has a faster combustion speed than ammonia, is a gaseous fuel, the exhaust gas oxygen concentration when the first fuel F1 is exclusively burned is controlled to be 3.5% or more, and the exhaust gas oxygen concentration when the first fuel F1 and the ammonia fuel F2 are co-fired is controlled to be less than 3.5%, thereby improving boiler efficiency.

[0055] (4) The ammonia co-firing boiler 1 described in (1) to (3) includes an ammonia co-firing ratio determination unit that determines the co-firing ratio of the ammonia fuel, and a control unit 40 that includes a co-firing air ratio storage unit that stores a relationship between the ammonia co-firing ratio and the second oxygen concentration so that the second oxygen concentration when the ammonia co-firing ratio is high is lower than the second oxygen concentration when the ammonia co-firing ratio is low. When the combustion state detection unit 44 detects co-firing of the first fuel F1 and the ammonia fuel F2, the combustion control unit 42 controls the air ratio during ammonia co-firing based on the ammonia co-firing ratio determined by the ammonia co-firing ratio determination unit and the relationship between the ammonia co-firing ratio and the second oxygen concentration stored in the co-firing air ratio storage unit. As a result, the air ratio when co-firing ammonia fuel F2 and the first fuel F1 is lower than when exclusively burning the first fuel F1, thereby improving boiler efficiency. In addition, the air ratio during co-firing is reduced only when the ammonia co-firing ratio is 20% or higher, which is sufficient to suppress the generation of unburned materials such as carbon monoxide and soot (smoke). This avoids the risk of increased generation of unburned materials due to a reduced air ratio.

[0056] (5) In the ammonia co-firing boiler 1 described in (4), the second oxygen concentration associated with the ammonia co-firing air ratio in the co-firing air ratio storage unit is set based on the oxygen concentration at which the amount of unburned matter generated for each ammonia co-firing ratio clearly increases, and is equal to or higher than the oxygen concentration at which the amount of unburned matter generated clearly increases. This makes it possible to suppress the generation of unburned matter such as carbon monoxide and soot (smoke), and avoid the risk of increased unburned matter generation due to a decrease in the air ratio.

[0057] (6) In the ammonia co-firing boiler 1 described in (4), when the combustion state detection unit 44 detects the co-firing of the first fuel F1 and the ammonia fuel F2 and the ammonia co-firing ratio determination unit determines that the ammonia co-firing ratio is 20% or more, the combustion control unit 42 controls the air ratio during ammonia co-firing based on the ammonia co-firing ratio determined by the ammonia co-firing ratio determination unit and the relationship between the ammonia co-firing ratio and the second oxygen concentration stored in the co-firing air ratio storage unit, so that the rate of decrease in the air ratio when the ammonia co-firing ratio is high relative to the air ratio during first mono-fuel combustion is larger than the rate of decrease in the air ratio when the ammonia co-firing ratio is low relative to the air ratio during first mono-fuel combustion. This improves boiler efficiency.

[0058] (7) The control method for the ammonia co-firing boiler 1 of this embodiment includes the steps of: controlling the amount of combustion air A1 so that the oxygen concentration in the exhaust gas after the co-firing of the first fuel F1 becomes a first oxygen concentration when the mono-combustion of the first fuel F1 is detected; and controlling the amount of combustion air A1 so that the oxygen concentration in the exhaust gas after the co-firing of the first fuel F1 and the ammonia fuel F2 becomes a second oxygen concentration when the co-firing of the first fuel F1 and the ammonia fuel F2 is detected, the second oxygen concentration being lower than the first oxygen concentration and set based on the relationship between the ammonia co-firing ratio and the oxygen concentration at which the generation of unburned matter due to combustion clearly increases; and further including the step of controlling the second oxygen concentration when the ammonia co-firing ratio is high so that the second oxygen concentration is lower than the second oxygen concentration when the ammonia co-firing ratio is low. This makes it possible to suppress the generation of unburned matter such as carbon monoxide and soot during the co-firing of the first fuel F1 and the ammonia fuel F2, which have a faster combustion rate than ammonia. Furthermore, the oxygen concentration in the exhaust gas is lower than when the first fuel F1 is exclusively burned, and the boiler efficiency can be improved.

[0059] Although a preferred embodiment of the ammonia-mixed combustion boiler 1 according to the present invention has been described above, the present invention is not limited to the above embodiment and can be modified as appropriate.

[0060] Although the ammonia co-firing ratio determination unit 43 calculates and determines the ammonia co-firing ratio based on the detection results of the first fuel flow sensor 102 and the ammonia fuel flow sensor 202, the method for determining the ammonia co-firing ratio is not limited thereto. The ammonia co-firing ratio determination unit 43 may calculate the ammonia co-firing ratio based on the aperture of the flow control valve 204, which serves as an ammonia fuel detection unit, and thereby detect the ammonia co-firing ratio. Alternatively, the ammonia co-firing ratio determination unit 43 may determine the ammonia co-firing ratio by calculating it using a calculation formula stored in the storage unit 41 based on the detection result of the combustion state detection unit 44. Alternatively, the ammonia co-firing ratio determination unit 43 may determine the ammonia co-firing ratio based on the detection result of the combustion state detection unit 44, using a data table of ammonia co-firing ratios corresponding to the detection result of the combustion state detection unit 44, which is stored in the storage unit 41. Alternatively, the ammonia co-firing ratio determination unit 43 may determine the ammonia co-firing ratio using a selector switch for setting the ammonia co-firing ratio or by inputting a numerical value of the ammonia co-firing ratio from an input device.

[0061] When the combustion control unit 42 controls combustion using air ratios corresponding to a plurality of ammonia mixing ratios, the mixing air ratio storage unit may store all of the plurality of ammonia mixing ratios and the air ratios corresponding to each of them, or may input an air ratio corresponding to a required ammonia mixing ratio via an input device or the like before performing control and store it.

[0062] Although the ammonia co-firing boiler 1 is preferably operated with the second oxygen concentration, i.e., the exhaust gas oxygen concentration, of less than 4% when the first fuel F1 is a liquid fuel, and with the second oxygen concentration, i.e., the exhaust gas oxygen concentration, of less than 3.5% when the first fuel F1 is a gaseous fuel, in the above example, the present invention is not limited thereto. By further reducing the exhaust gas oxygen concentration by 0.5% during ammonia co-firing, the ammonia co-firing boiler 1 can further improve boiler efficiency, reduce exhaust gas heat loss, and improve the effective ammonia utilization rate, thereby enhancing carbon dioxide reduction effects. For example, as shown in FIG. 4 , when the second oxygen concentration in the gaseous fuel is reduced from 3.5% to 3% during ammonia co-firing in the ammonia co-firing boiler 1, the effective ammonia utilization rate increases from approximately 94.5% to approximately 97%. 7 , when the second oxygen concentration in the liquid fuel is reduced from 4% to 3.5% during ammonia co-firing in the ammonia co-firing boiler 1, the effective ammonia utilization rate improves from approximately 94.5% to approximately 98%. Therefore, the ammonia co-firing boiler 1 can more effectively reduce greenhouse gas emissions by further reducing the flue gas oxygen concentration during ammonia co-firing by 0.5%.

[0063] The combustion state detection unit 44 determines that mono-combustion of the first fuel F1 has been detected when only the first fuel flow sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100, and determines that co-combustion of the first fuel F1 and the ammonia fuel F2 has been detected when the first fuel flow sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100 and the ammonia fuel flow sensor 202 detects that the ammonia fuel F2 is passing through the ammonia supply line 200. However, this is not limited to this. The combustion state detection unit 44 may determine that mono-combustion of the first fuel F1 has been detected based on an open instruction from the combustion control unit 42 to the first shutoff valve 103 and the second shutoff valve 105, and may determine that co-combustion of the first fuel F1 and the ammonia fuel F2 has been detected based on an open instruction from the combustion control unit 42 to the first shutoff valve 103 and the second shutoff valve 105 and an open instruction from the combustion control unit 42 to the first shutoff valve 103 and the second shutoff valve 205. The combustion state detection unit 44 may also determine the combustion state by a select switch that sets the combustion state or by inputting the combustion state from an input device.

[0064] The ammonia fuel may be ammonia or a gas fuel containing ammonia as a main component. Control of the air ratio (oxygen concentration in the exhaust gas E1) by the combustion control unit described herein is particularly effective in stabilizing combustion in an ammonia-mixed combustion burner and improving boiler efficiency in small once-through boilers, small-scale boilers, and fire-tube boilers in which combustion is stopped and started relatively frequently.

[0065] Furthermore, since the present disclosure promotes the use of ammonia as a fuel, which does not emit carbon dioxide, it can contribute, for example, to Goal 7 of the United Nations-led Sustainable Development Goals (SDGs), which is to "Ensure access to affordable, reliable, sustainable and modern energy."

[0066] DESCRIPTION OF SYMBOLS 1 Ammonia co-firing boiler 10 Boiler body 20 Burner 40 Control unit 42 Combustion control unit 43 Ammonia co-firing ratio determination unit 44 Combustion state detection unit 302 Inverter (combustion air adjustment unit) 304 Damper (combustion air adjustment unit) A1 Combustion air F1 First fuel F2 Ammonia fuel

Claims

1. A boiler comprising: a burner to which at least one of a first fuel having a faster combustion speed than ammonia and ammonia fuel, and combustion air is supplied; a combustion air adjustment unit that adjusts the amount of the combustion air supplied; and a control unit, wherein the control unit comprises: a combustion state detection unit that distinguishes between combustion of the first fuel and mixed combustion of the first fuel and the ammonia fuel; and a combustion control unit that controls the combustion air adjustment unit so that an exhaust gas oxygen concentration after the single combustion of the first fuel becomes a first oxygen concentration when the combustion state detection unit detects single combustion of the first fuel, and controls the combustion air adjustment unit so that an exhaust gas oxygen concentration after mixed combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration lower than the first oxygen concentration when the combustion state detection unit detects mixed combustion of the first fuel and the ammonia fuel.

2. A boiler as described in claim 1, wherein the first fuel is a liquid fuel, the first oxygen concentration is 4% or more, and the second oxygen concentration is less than 4%.

3. The boiler according to claim 1, wherein the first fuel is a gaseous fuel, the first oxygen concentration is 3.5% or more, and the second oxygen concentration is less than 3.5%.

4. The boiler according to any one of claims 1 to 3, further comprising an ammonia co-firing ratio determination unit which determines a co-firing ratio of the ammonia fuel, wherein the control unit comprises a co-firing air ratio memory unit which stores a relationship between an ammonia co-firing ratio and the second oxygen concentration such that the second oxygen concentration when the ammonia co-firing ratio is high is lower than the second oxygen concentration when the ammonia co-firing ratio is low, and when the combustion state detection unit detects co-firing of the first fuel and the ammonia fuel, the combustion control unit controls the air ratio during ammonia co-firing based on the ammonia co-firing ratio determined by the ammonia co-firing ratio determination unit and the relationship between the ammonia co-firing ratio and the second oxygen concentration stored in the co-firing air ratio memory unit.

5. The boiler according to claim 4, wherein the second oxygen concentration associated with the ammonia co-firing air ratio in the co-firing air ratio storage unit is set based on an oxygen concentration at which an amount of unburned matter generated for each ammonia co-firing ratio shows a clear increase, and is equal to or higher than the oxygen concentration at which an amount of unburned matter generated shows a clear increase.

6. The boiler according to claim 4, wherein, when the combustion state detection unit detects co-combustion of the first fuel and the ammonia fuel and the ammonia co-combustion ratio determination unit determines that the ammonia co-combustion ratio is 20% or more, the combustion control unit controls the air ratio during ammonia co-combustion based on the ammonia co-combustion ratio determined by the ammonia co-combustion ratio determination unit and the relationship between the ammonia co-combustion ratio and a second oxygen concentration stored in the co-combustion air ratio storage unit, so that a rate of decrease in the air ratio when the ammonia co-combustion ratio is high relative to the air ratio during first exclusive fuel combustion is larger than a rate of decrease in the air ratio when the ammonia co-combustion ratio is low relative to the air ratio during first exclusive fuel combustion.

7. A method for controlling a boiler equipped with a burner capable of co-firing at least one of a first fuel having a combustion speed faster than ammonia and an ammonia fuel, and combustion air, comprising: when mono-combustion of the first fuel is detected, controlling an amount of the combustion air so that an exhaust gas oxygen concentration after mono-combustion of the first fuel becomes a first oxygen concentration; when co-combustion of the first fuel and the ammonia fuel is detected, controlling an amount of the combustion air so that an exhaust gas oxygen concentration after co-combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration, wherein the second oxygen concentration is lower than the first oxygen concentration and is set based on the relationship between the ammonia co-firing ratio and the oxygen concentration at which generation of unburned matter due to combustion clearly increases; and further comprising a step of controlling the second oxygen concentration when the ammonia co-firing ratio is high so that it is lower than the second oxygen concentration when the ammonia co-firing ratio is low.

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