Boiler, method of controlling a boiler

Abstract

The ammonia co-firing boiler (1) as a boiler comprises: a burner (20) supplied with at least one of a first fuel (F1) and ammonia fuel (F2) and combustion air (A1); a regulating damper (304); a combustion state detection unit (44) that distinguishes between the co-firing of the first fuel and ammonia fuel (F2) and the combustion of the first fuel (F1); and a control unit (40) comprising a combustion control unit that controls the exhaust gas oxygen concentration to a first oxygen concentration when the combustion state detection unit (44) detects that the first fuel (F1) is being burned separately, and controls the exhaust gas oxygen concentration to a second oxygen concentration lower than the first oxygen concentration when the combustion state detection unit (44) detects that the first fuel (F1) and ammonia fuel (F2) are being burned together.

Description

Technical Field

[0001] This application claims priority based on 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 boilers and methods for controlling boilers. Background Technology

[0002] By replacing a portion of hydrocarbon fuels, such as natural gas and petroleum fuels, used as boiler fuel with ammonia, which does not produce carbon dioxide, it is expected that global warming can be prevented by reducing carbon dioxide emissions (see, for example, Patent Document 1). In the case of burning hydrocarbon fuels, combustion is carried out in a manner where the exhaust gas oxygen concentration is above a predetermined value, from the viewpoint of suppressing the generation of carbon monoxide and unburned materials such as coal. On the other hand, in the case of burning ammonia fuel, when the exhaust gas oxygen concentration exceeds the predetermined value, the generation of nitrous oxide increases, which weakens the effect of reducing greenhouse gas (GHG) produced by ammonia combustion.

[0003] Prior art literature

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2021-185122 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] There is a problem that in boilers using ammonia co-fired burners, if the air ratio is kept close to 1 to suppress heat loss from exhaust gases, boiler efficiency increases, but the generation of unburned materials increases. On the other hand, if the air ratio is increased when burning ammonia fuel, the generation of nitrous oxide (N2O), which has a high temperature potential, increases, and the carbon dioxide reduction effect of ammonia fuel combustion is offset.

[0008] Therefore, the following combustion control is sought: when hydrocarbon fuels, which have a faster combustion rate than ammonia, are co-fired with ammonia fuels, the generation of carbon monoxide and unburned materials such as coal can be suppressed, while the generation of nitrous oxide can also be suppressed.

[0009] The purpose of this invention is to provide a boiler and a boiler control method that can suppress the generation of carbon monoxide and unburned materials such as coal when a hydrocarbon fuel, which has a faster combustion rate than ammonia, is co-fired with ammonia fuel, and can also suppress the generation of nitrous oxide.

[0010] Methods for solving problems

[0011] This invention relates to a boiler, wherein the boiler comprises: a burner supplied with at least one of a first fuel with a combustion rate faster than ammonia and an ammonia fuel, and combustion air; a combustion air adjustment unit that adjusts the supply amount of the combustion air; and a control unit comprising: a combustion state detection unit that distinguishes between the co-firing of the first fuel and the ammonia fuel and the combustion of the first fuel; and a combustion control unit that, when the combustion state detection unit detects that the first fuel is being burned exclusively, controls the combustion air adjustment unit to make the oxygen concentration in the exhaust gas after the exclusive combustion of the first fuel a first oxygen concentration, and when the combustion state detection unit detects that the first fuel is being burned exclusively, controls the combustion air adjustment unit to make the oxygen concentration in the exhaust gas after the co-firing of the first fuel and the ammonia fuel a second oxygen concentration lower than the first oxygen concentration.

[0012] In addition, preferably, the first fuel is a liquid fuel, the first oxygen concentration is 4% or more, and the second oxygen concentration is less than 4%.

[0013] In addition, preferably, 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%.

[0014] Furthermore, preferably, the boiler includes an ammonia co-firing rate determining unit that determines the co-firing rate of the ammonia fuel, and the control unit includes a co-firing air ratio storage unit. This co-firing air ratio storage unit stores the relationship between the ammonia co-firing rate and the second oxygen concentration in a manner where the second oxygen concentration is lower when the ammonia co-firing rate is high compared to when the ammonia co-firing rate is low. When the combustion state detection unit detects that the first fuel and the ammonia fuel are co-firing, the combustion control unit controls the air ratio during ammonia co-firing based on the ammonia co-firing rate determined by the ammonia co-firing rate determining unit and the relationship between the ammonia co-firing rate and the second oxygen concentration stored in the co-firing air ratio storage unit.

[0015] Preferably, the second oxygen concentration in the air-fuel ratio storage section that is related to the ammonia co-firing rate is set to an oxygen concentration that shows a significant increase in the amount of unburned material produced for each ammonia co-firing rate, and is above an oxygen concentration that shows a significant increase in the amount of unburned material produced.

[0016] Preferably, when the combustion state detection unit detects that the first fuel and the ammonia fuel are co-fired, and the ammonia co-fire rate determination unit determines that the ammonia co-fire rate is 20% or more, the combustion control unit controls the air ratio during ammonia co-fired based on the ammonia co-fire rate determined by the ammonia co-fire rate determination unit and the relationship between the ammonia co-fire rate stored in the air-co-fire ratio storage unit and the second oxygen concentration, so that the reduction ratio of the air ratio when the ammonia co-fire rate is high relative to the air ratio when the first fuel is burned exclusively is greater than the reduction ratio of the air ratio when the ammonia co-fire rate is low relative to the air ratio when the first fuel is burned exclusively.

[0017] This invention relates to a boiler control method, the boiler comprising a burner capable of co-firing at least one of a first fuel (burning faster than ammonia) and ammonia fuel with combustion air, wherein the boiler control method includes the following steps: when the exclusive combustion of the first fuel is detected, controlling the amount of combustion air to make the oxygen concentration of the exhaust gas after the exclusive combustion of the first fuel a first oxygen concentration; and when the co-firing of the first fuel and the ammonia fuel is detected, controlling the amount of combustion air to make the oxygen concentration of the exhaust gas after the co-firing of the first fuel and the ammonia fuel a second oxygen concentration, the second oxygen concentration being lower than the first oxygen concentration, the second oxygen concentration being set based on a relationship between the ammonia co-firing rate and the oxygen concentration showing a significant increase in the production of unburned matter during combustion, and the boiler control method further includes the following step: controlling the second oxygen concentration when the ammonia co-firing rate is high to be lower than the second oxygen concentration when the ammonia co-firing rate is low.

[0018] Invention Effects

[0019] According to the present invention, a boiler and a boiler control method are provided that can suppress the generation of unburned materials such as carbon monoxide and coal (smoke) when a hydrocarbon fuel with a faster combustion rate than ammonia is co-fired with ammonia fuel, and can also suppress the generation of nitrous oxide. Attached Figure Description

[0020] Figure 1 This is a schematic diagram illustrating the structure of an ammonia co-firing boiler according to an embodiment of the present invention.

[0021] Figure 2 This is a graph showing the relationship between the oxygen concentration and carbon monoxide concentration in the exhaust gas when the gaseous fuel (liquefied natural gas) used as the first fuel F1 and the ammonia fuel are burned in the ammonia co-firing boiler according to the embodiments of the present invention.

[0022] Figure 3 Based on Figure 2The graph shown represents the relationship between the ammonia co-firing rate and the oxygen concentration at which the carbon monoxide concentration in the exhaust gas begins to increase significantly.

[0023] Figure 4 This is a graph showing the relationship between the oxygen concentration of the exhaust gas and the effective utilization rate of ammonia (ammonia effective utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reductions through the combustion of ammonia fuel) × 100 (%) when the gaseous fuel (liquefied natural gas) used as the first fuel is co-fired with ammonia fuel in the ammonia co-fired boiler according to the embodiments of the present invention.

[0024] Figure 5 This is a graph showing the relationship between the oxygen concentration and particulate matter (an index of incomplete combustion of oil fuel) in the exhaust gas when liquid fuel (A heavy oil) as the first fuel and ammonia fuel are burned in the ammonia co-firing boiler according to the embodiments of the present invention.

[0025] Figure 6 Based on Figure 5 The graph shown represents the relationship between the ammonia co-firing rate and the oxygen concentration at which the particulate matter content of the exhaust gas begins to increase significantly.

[0026] Figure 7 This is a graph showing the relationship between the oxygen concentration of the exhaust gas and the effective utilization rate of ammonia (ammonia effective utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reductions through the combustion of ammonia fuel) × 100 (%) when liquid fuel (A heavy oil) as the first fuel is co-fired with ammonia fuel in the ammonia co-fired boiler according to the embodiments of the present invention. Detailed Implementation

[0027] Hereinafter, an ammonia co-firing boiler 1, which is an embodiment of the boiler according to the present invention, will be described with reference to the accompanying drawings. The ammonia co-firing boiler 1 of this embodiment is a steam boiler that generates steam by heating water through fuel combustion, and is used to supply steam to a load device (not shown).

[0028] Figure 1 This is a schematic diagram illustrating the structure of the ammonia co-firing boiler 1 according to the embodiment.

[0029] like Figure 1 As shown, the ammonia co-firing boiler 1 includes a tank 10, a burner 20, and a control unit 40. Furthermore, the ammonia co-firing boiler 1 includes a first fuel supply line 100, an ammonia supply line 200, a combustion air supply line 300, an exhaust stack 400, a water supply line 500, and a steam supply line 600. In this specification, "line" is a general term for flow paths, pathways, pipelines, etc.

[0030] The tank body 10 comprises a lower header 11, multiple water pipes 12, an upper header 13, and a combustion chamber B. The tank body 10 recovers heat from the combustion gases generated by the combustion of fuel in the combustion chamber B and heats the water W1 supplied to the tank body 10 to generate steam S1.

[0031] The burner 20 ignites the first fuel F1 and the ammonia fuel F2, which have a faster combustion rate than ammonia, in the combustion chamber B of the tank 10.

[0032] The burner 20 is located on the upper part of the tank body 10. The burner 20 includes a burner body 21 and an air box 22. A first fuel supply line 100 and an ammonia supply line 200 are connected to the burner body 21. A combustion air supply line 300 is connected to the air box 22.

[0033] In this embodiment, firstly, first fuel F1 and combustion air A1 are supplied to the burner to initiate 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 ammonia fuel F2 is supplied to the burner 20.

[0034] Furthermore, in this embodiment, the first fuel, which burns faster than ammonia, can be either a liquid fuel or a gaseous fuel. As a liquid fuel, oil fuels, alcohol fuels, etc., can be used. As a gaseous fuel, hydrocarbon gases such as liquefied natural gas (hereinafter referred to as LNG) and liquefied petroleum gas (LPG) can be used.

[0035] The control unit 40 controls the combustion of the burner 20 in the ammonia co-firing boiler 1. Details of the control unit 40 will be described later.

[0036] The first fuel supply line 100 supplies 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 includes a main valve 101, a first fuel flow sensor 102, a first shut-off valve 103, a flow regulating valve 104, and a second shut-off valve 105 from the upstream side.

[0037] The main valve 101 is a manual valve used to open and close the flow path of the first fuel supply line 100.

[0038] 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 obtained by the control unit 40.

[0039] The first shut-off valve 103 and the second shut-off valve 105 are composed of solenoid valves and are used to open and close the flow path of the first fuel supply line 100 to supply or stop the first fuel F1. The first shut-off valve 103 and the second shut-off valve 105 are electrically connected to the control unit 40 and are controlled by signals sent from the control unit 40.

[0040] The flow regulating valve 104 is a regulating valve that adjusts the flow rate of the first fuel F1 supplied to the burner 20 by adjusting the valve opening. The flow regulating valve 104 is electrically connected to the control unit 40 and is controlled by a signal sent by the control unit 40 based on the detection result of the first fuel flow sensor 102.

[0041] Ammonia supply line 200 supplies ammonia fuel F2 from an ammonia supply source (not shown) to burner 20. The upstream side of ammonia supply line 200 is connected to the ammonia supply source, and the downstream side of ammonia supply line 200 is connected to burner 20. From the upstream side, ammonia supply line 200 includes a main valve 201, an ammonia fuel flow sensor 202, a first shut-off valve 203 (serving as an ammonia fuel shut-off valve), a flow regulating valve 204, and a second shut-off valve 205 (serving as an ammonia fuel shut-off valve).

[0042] The main valve 201 is a manual valve used to open and close the flow path of the ammonia supply line 200.

[0043] The ammonia fuel flow sensor 202 detects the flow rate of 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 obtained by the control unit 40.

[0044] The first shut-off valve 203 and the second shut-off valve 205 are composed of solenoid valves and are used to open and close the flow path of the ammonia supply line 200 to supply or stop ammonia fuel F2. The first shut-off valve 203 and the second shut-off valve 205 are electrically connected to the control unit 40 and are controlled by signals sent from the control unit 40.

[0045] The flow regulating valve 204 is a regulating valve that adjusts the flow rate of ammonia fuel F2 supplied to the burner 20 by adjusting the valve opening. The flow regulating valve 204 is electrically connected to the control unit 40 and is controlled by a signal sent by the control unit 40 based on the detection result of the ammonia fuel flow sensor 202.

[0046] Combustion air supply line 300 supplies combustion air A1 to burner 20. In this embodiment, combustion air A1 is supplied to the air box 22 of burner 20. The upstream side of combustion air supply line 300 is connected to blower 301, and the downstream side of combustion air supply line 300 is connected to air box 22. Combustion air supply line 300 includes blower 301, combustion air flow sensor 303, and regulating damper 304 as combustion air adjustment unit from the upstream side.

[0047] Blower 301 supplies combustion air A1 to burner 20.

[0048] The blower 301 comprises a fan and a motor that rotates the fan. The motor speed can be adjusted by controlling the frequency via a frequency converter 302, thus adjusting the rotational speed. In other words, the frequency converter 302 is a combustion air regulating unit capable of adjusting the supply amount of combustion air A1 to the burner. In this embodiment, the frequency converter 302 is electrically connected to the control unit 40 and is controlled by signals sent from the control unit 40.

[0049] The combustion air flow sensor 303 detects the flow rate of 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 obtained by the control unit 40.

[0050] The regulating damper 304 adjusts the supply amount of combustion air A1 to the burner 20 by adjusting its opening. Specifically, the regulating damper 304 is configured to rotate between a closed state that blocks the flow path of the combustion air supply line 300 and an open state that opens the flow path of the combustion air supply line 300 by rotating it from the closed state to a predetermined angle (e.g., 90 degrees). The regulating damper 304 is electrically connected to the control unit 40 and is controlled by signals sent from the control unit 40.

[0051] The exhaust stack 400 discharges the exhaust gas E1 generated by the combustion of the first fuel F1 and ammonia fuel F2 in the burner 20 to the outside of the tank 10. The upstream side of the exhaust stack 400 is connected to the upper part of the circumferential surface of the tank 10, and the downstream side of the exhaust stack 400 is open to the atmosphere. The exhaust stack 400 has an exhaust gas temperature detection unit 401 and an exhaust gas detection unit 402 from the upstream side.

[0052] 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 result of the exhaust gas temperature detection unit 401 can be obtained by the control unit 40.

[0053] The exhaust gas detection unit 402 detects the composition of the exhaust gas E1 flowing through the exhaust stack 400. In this embodiment, the exhaust gas detection unit 402 detects oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), and nitrogen oxides (NOx) as components of the exhaust gas E1. x 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 obtained by the control unit 40.

[0054] In such an exhaust gas detection unit 402, for example, when detecting oxygen, a zirconia oxygen sensor, a galvanic cell oxygen sensor, etc., are used.

[0055] Water supply line 500 supplies water W1 from a water source (not shown) to tank 10. The upstream side of water supply line 500 is connected to the water source, and the downstream side of water supply line 500 is connected to the lower manifold 11. Water supply line 500 is equipped with a main valve 501.

[0056] The main valve 501 is a manual valve used to open and close the flow path of the water supply line 500.

[0057] Steam supply line 600 supplies steam S1 from tank 10 to load equipment. The upstream side of steam supply line 600 is connected to the upper header 13 of tank 10, and the downstream side of steam supply line 600 is connected to load equipment. Steam supply line 600 is equipped with a main valve 601.

[0058] The main valve 601 is a manual valve used to open and close the flow path of the steam supply line 600.

[0059] like Figure 1 As shown, the control unit 40 includes a storage unit 41, a combustion control unit 42, an ammonia co-firing rate determination unit 43, and a combustion status detection unit 44.

[0060] The control unit 40 is composed of, for example, a PLC (Programmable Logic Controller), a DSP (Digital Signal Processor), or an FPGA (Field-Programmable Gate Array). The various functions of the control unit 40 are implemented, for example, by executing prescribed software (programs) stored in the storage unit 41. The various functions of the control unit 40 can also be implemented through the coordinated operation of hardware and software, or solely through hardware (electronic circuits).

[0061] Storage unit 41 stores various setting information. In addition, as a mixed air ratio storage unit, storage unit 41 stores the relationship between ammonia mixing ratio and second oxygen concentration in a manner where the second oxygen concentration is lower when the ammonia mixing ratio is higher than when the ammonia mixing ratio is lower.

[0062] It should be noted that, not limited to this, the control unit 40 may also have a combustion air ratio storage unit relative to the storage unit 41, which stores the relationship between the ammonia combustion rate and the second oxygen concentration in a way that the second oxygen concentration is lower when the ammonia combustion rate is high than when the second oxygen concentration is low.

[0063] Here, "high ammonia co-firing rate" and "low ammonia co-firing rate" illustrate the relative relationship of the ammonia co-firing rates. For example, when the ammonia co-firing rates are set to 10%, 20%, 40%, and 60%, it means that the second oxygen concentration at an ammonia co-firing rate of 60% is lower than the second oxygen concentration at an ammonia co-firing rate of 10%. It should be noted that, for example, the second oxygen concentration at ammonia co-firing rates of 20% and 40% can also be the same value. In other words, the second oxygen concentration at the highest ammonia co-firing rate (here, 60%) is lower than the second oxygen concentration at the lowest ammonia co-firing rate (here, 10%), and the second oxygen concentration at ammonia co-firing rates in between decreases or remains the same as the ammonia co-firing rate increases.

[0064] The combustion control unit 42 controls the combustion of the burner 20.

[0065] In this embodiment, the combustion control unit 42 starts the blower 301 and adjusts it to supply combustion air A1 at a predetermined flow rate to the burner 20 via at least one of the inverter 302 and the regulating damper 304 of the combustion air regulating unit. Additionally, the combustion control unit 42 opens the first shut-off valve 103 and the second shut-off valve 105, and adjusts the opening of the flow regulating valve 104 based on the detection result of the first fuel flow sensor 102 to achieve a predetermined combustion quantity of the first fuel F1. Thus, combustion of the first fuel F1 begins.

[0066] Additionally, the combustion control unit 42 opens the first shut-off valve 203 and the second shut-off valve 205, and adjusts the opening of the flow regulating valve 204 based on the detection result of the ammonia fuel flow sensor 202 to achieve the prescribed combustion rate of ammonia fuel F2. Thus, the combustion of ammonia fuel F2 begins.

[0067] Combustion status detection unit 44 detects the difference between the co-combustion of the first fuel F1 and ammonia fuel F2 and the combustion of the first fuel F1.

[0068] In this embodiment, the combustion state detection unit 44 detects that the first fuel F1 is burning by detecting that the first fuel F1 is flowing through the first fuel supply line 100 by the first fuel flow sensor 102.

[0069] The combustion status detection unit 44 detects that the ammonia fuel F2 is burning by detecting that the ammonia fuel F2 is flowing in the ammonia supply line 200 by the ammonia fuel flow sensor 202.

[0070] When the combustion state detection unit 44 detects that only the first fuel F1 is flowing through the first fuel supply line 100, it determines that the combustion of the first fuel F1 is 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 ammonia fuel F2 is flowing through the ammonia supply line 200, it determines that the combustion of the first fuel F1 and ammonia fuel F2 is detected.

[0071] It should be noted that the combustion state detection unit 44 is not limited to detecting the flow of ammonia fuel F2 based on the ammonia fuel flow sensor 202 as described above, but can also detect the fuel state based on inputs such as the setting of the ammonia co-firing selection switch.

[0072] When the combustion state detection unit 44 detects that the first fuel F1 is being burned, the combustion control unit 42 controls at least one of the inverter 302 and the regulating damper 304, which are used as combustion air adjustment units, to make the oxygen concentration of the exhaust gas after the burning of the first fuel F1 reach the first oxygen concentration.

[0073] In addition, when the combustion state detection unit 44 detects that the first fuel F1 and ammonia fuel F2 are mixed and burned, the combustion control unit 42 controls at least one of the frequency converter 302 and the regulating damper 304 to make the oxygen concentration of the exhaust gas after the first fuel F1 and ammonia fuel F2 are mixed and burned to a second oxygen concentration that is lower than the first oxygen concentration.

[0074] In this embodiment, when the combustion state detection unit 44 detects that the first fuel F1 is fully combusted, the combustion control unit 42 obtains the detection result of the oxygen concentration of the exhaust gas E1 flowing in the exhaust stack 400 after the first fuel F1 is fully combusted, as detected by the exhaust gas detection unit 402. Based on the detected oxygen concentration of the exhaust gas E1, the combustion control unit 42 controls at least one of the inverter 302 and the regulating damper 304 to make the oxygen concentration of the exhaust gas E1 reach the first oxygen concentration.

[0075] In this embodiment, when the combustion state detection unit 44 detects that the first fuel F1 and ammonia fuel F2 are mixed and burned, the combustion control unit 42 obtains the detection result of the oxygen concentration of the exhaust gas E1 after the first fuel F1 and ammonia fuel F2 are mixed and burned in the exhaust stack 400, as detected by the exhaust gas detection unit 402. Based on the detected oxygen concentration of the exhaust gas E1, the combustion control unit 42 controls at least one of the frequency converter 302 and the regulating damper 304 to make the oxygen concentration of the exhaust gas E1 a second oxygen concentration that is lower than the first oxygen concentration.

[0076] Therefore, when hydrocarbon fuel F1, which has a faster combustion rate than ammonia, is co-fired with ammonia fuel F2, the air ratio can be reduced while suppressing the generation of unburned materials such as carbon monoxide and coal (smoke dust) and nitrous oxide. This reduces the heat loss of the exhaust gas and improves boiler efficiency.

[0077] The ammonia fuel flow sensor 202 detects the flow rate of ammonia fuel F2 flowing through the ammonia supply line 200 as described above.

[0078] The ammonia co-combustion rate determination unit 43 determines the ammonia co-combustion rate based on the detection results of the first fuel flow sensor 102 and the ammonia fuel flow sensor 202, and sends the determined ammonia co-combustion rate to the combustion control unit 42. In this embodiment, as an example, the ammonia co-combustion rate is calculated and determined based on the detection results of the first fuel flow sensor 102 and the ammonia fuel flow sensor 202, but it is not limited to this.

[0079] Figures 2-4 This is a graph showing the measurement results related to the exhaust gas characteristics when gaseous fuel (LNG) as the first fuel F1 and ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment.

[0080] Boiler combustion tests were conducted using LNG as the sole fuel source and LNG mixed with ammonia fuel F2 (ammonia co-firing rates of 40% and 60%). The ammonia co-firing rate was defined as the ratio of the heat supplied by ammonia fuel F2 to the total heat supplied by LNG and ammonia fuel F2.

[0081] Figure 2 This is a graph showing the relationship between the oxygen concentration and carbon monoxide concentration in the exhaust gas E1 when gaseous fuel (LNG) as the first fuel F1 and ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment. Figure 2 In the chart shown, the vertical axis represents the relative value of the carbon monoxide concentration of exhaust gas E1 when the oxygen concentration of exhaust gas is 0.5% under the condition of 60% ammonia co-firing rate and is set to 100, and the horizontal axis represents the oxygen concentration of exhaust gas E1 [%].

[0082] like Figure 2 As shown, under LNG-only combustion, when the oxygen concentration in exhaust gas E1 is below 2.6%, the carbon monoxide concentration in exhaust gas E1 begins to increase significantly. On the other hand, under LNG-ammonia co-firing, when the oxygen concentration in exhaust gas E1 is below 1.6%, the carbon monoxide concentration begins to increase significantly. Furthermore, the point at which this significant increase in carbon monoxide concentration begins to shift towards the lower oxygen concentration side as the ammonia co-firing ratio increases.

[0083] Figure 3 Based on Figure 2 The graph shown illustrates the relationship between the ammonia co-firing rate and the oxygen concentration at which the carbon monoxide concentration in the exhaust gas (E1) begins to increase significantly. Figure 3 In the chart shown, 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-combustion rate [%].

[0084] like Figure 3 As shown, with increasing ammonia co-firing ratio, carbon monoxide concentration shows a significant increase while oxygen concentration decreases. According to... Figure 3 The approximate straight line shown indicates a significant increase in carbon monoxide at a 20% ammonia co-firing rate, with the oxygen concentration in exhaust gas E1 estimated to be 2.1% (air ratio 1.11). Figure 3 (The hollow circle in the image indicates the oxygen concentration).

[0085] Therefore, based on these phenomena, in the co-firing of LNG and ammonia fuel F2, it is possible to suppress carbon monoxide production with a lower air ratio than when burning LNG alone. Furthermore, the reduction in air ratio is greater when the ammonia co-firing rate is high than when the ammonia co-firing rate is low. For example, the reduction in air ratio is greater when the ammonia co-firing rate is 60% than when it is 20%. Here, the reduction in air ratio refers to the reduction in air ratio during co-firing compared to the reduction in air ratio during LNG-only combustion.

[0086] Figure 4 This is a graph showing the relationship between the oxygen concentration of exhaust gas E1 and the effective utilization rate of ammonia (ammonia effective utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reductions through ammonia fuel combustion) × 100 (%) when gaseous fuel (LNG) as the first fuel F1 and ammonia fuel F2 are co-fired in the ammonia co-fired boiler 1 according to the embodiment. "Greenhouse gas emission reductions through ammonia fuel combustion" refers to "greenhouse gas emission reductions through reductions in the use of hydrocarbon fuels".

[0087] exist Figure 4 In the chart shown, the vertical axis represents the effective utilization rate of ammonia [%), and the horizontal axis represents the oxygen concentration in exhaust gas E1 [%).

[0088] like Figure 4 As shown, when the ammonia co-firing rate is 40% and 60%, the effective utilization rate of ammonia decreases sharply when the oxygen concentration in the exhaust gas E1 is above 3.5%. Therefore, the ammonia co-firing boiler 1 according to the embodiment is preferably operated under conditions where the oxygen concentration is less than 3.5%.

[0089] Based on the above results, the oxygen concentration in the exhaust gas after co-firing the gaseous fuel F1 and the ammonia fuel F2, i.e., the second oxygen concentration, is preferably less than 3.5% lower than the first oxygen concentration. The oxygen concentration in the exhaust gas after the dedicated combustion of the first fuel F1, i.e., the first oxygen concentration, is preferably 3.5% or more, based on the target air ratio of 1.2 to 1.35 (3.5% to 5.4%) set for the small once-through boiler.

[0090] Furthermore, according to the ammonia co-firing boiler 1 of this embodiment, since the oxygen concentration of the exhaust gas after the co-firing of the gaseous fuel of the first fuel F1 and the ammonia fuel F2 is lower than the first oxygen concentration, the boiler efficiency can be improved by suppressing the generation of unburned materials such as carbon monoxide and the generation of nitrous oxide while suppressing the heat loss of exhaust gas during the co-firing of the first fuel F1 and the ammonia fuel F2.

[0091] Figures 5-7 This is a graph showing the exhaust gas characteristics when liquid fuel (heavy oil A) as the first fuel F1 and ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment.

[0092] Combustion tests were conducted on the boiler using heavy oil A as the sole fuel and heavy oil A mixed with ammonia fuel F2 (ammonia co-firing rates of 20% and 40%). It should be noted that the ammonia co-firing rate is defined as the ratio of the heat supplied by ammonia fuel F2 to the total heat supplied by heavy oil A and ammonia fuel F2.

[0093] Figure 5 This is a graph showing the relationship between the oxygen concentration and particulate matter (an index of incomplete combustion of oil fuel) in the exhaust gas E1 when liquid fuel (heavy oil A) as the first fuel F1 and ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment. Figure 5 In the chart shown, the vertical axis represents the relative value of the flue gas dust density when the oxygen concentration in the exhaust gas is 1% and the ammonia co-firing rate is 20%, and the horizontal axis represents the oxygen concentration [%] in exhaust gas E1.

[0094] like Figure 5 As shown, when heavy oil A is burned exclusively, the particulate matter content begins to increase significantly when the oxygen concentration in exhaust gas E1 is below 4%. On the other hand, when heavy oil A is co-burned with ammonia, the particulate matter content begins to increase significantly when the oxygen concentration in exhaust gas E1 is below 2.5%. Furthermore, the point at which this significant increase in particulate matter content begins to shift towards the lower oxygen concentration side as the ammonia co-burning ratio increases.

[0095] Figure 6 Based on Figure 5 The graph shown illustrates the relationship between the ammonia co-firing rate and the oxygen concentration at which the particulate matter content of the exhaust gas E1 begins to increase significantly. Figure 6In the chart shown, the vertical axis represents the ammonia co-firing rate [%), and the horizontal axis represents the oxygen concentration [%) at which the particulate matter content of exhaust gas E1 begins to increase significantly.

[0096] like Figure 6 As shown, with increasing ammonia co-firing ratio, the particulate matter concentration showed a significant increase while the oxygen concentration decreased. Furthermore, although in Figure 6 Not shown in the figure, but in boiler combustion tests with ammonia co-firing rates of 50% and 60%, the smoke dust showed a significant increase in oxygen concentration compared to ammonia co-firing rate of 40%.

[0097] according to Figure 6 The approximate straight line shows a significant increase in particulate matter at a 20% ammonia co-firing rate, with the oxygen concentration of exhaust gas E1 estimated at 2.9% (air ratio 1.16). Therefore, based on these phenomena, in the co-firing of heavy oil A and ammonia fuel F2, operation can be achieved with a lower air ratio than when burning only heavy oil A. Furthermore, the reduction in air ratio is greater when the ammonia co-firing rate is high than when it is low. For example, the reduction in air ratio is greater when the ammonia co-firing rate is 40% than when it is 20%. Here, the reduction in air ratio refers to the reduction in air ratio during co-firing relative to the reduction in air ratio when burning only heavy oil A.

[0098] Figure 7 This is a graph showing the relationship between the oxygen concentration of exhaust gas E1 and the effective ammonia utilization rate (ammonia utilization rate (%) = 1 - (greenhouse gas emissions as nitrous oxide / greenhouse gas emission reductions through ammonia fuel combustion) × 100 (%) when liquid fuel (A heavy oil) as the first fuel F1 and ammonia fuel F2 are co-fired in the ammonia co-firing boiler 1 according to the embodiment. Figure 7 In the chart shown, the vertical axis represents the effective utilization rate of ammonia, and the horizontal axis represents the oxygen concentration [%] in the exhaust gas E1.

[0099] like Figure 7 As shown, the effective utilization rate of ammonia decreases sharply when the oxygen concentration in exhaust gas E1 is 4% or higher. Therefore, the ammonia co-firing boiler 1 according to the embodiment is preferably operated under conditions where the oxygen concentration is less than 4%.

[0100] Based on the above results, the second oxygen concentration, which is lower than the first oxygen concentration, is preferably less than 4% in the exhaust gas after the co-firing of the liquid fuel F1 and the ammonia fuel F2. The first oxygen concentration is preferably 4% or more, based on the target air ratio of 1.25 to 1.4 (4.2% to 6%) set for the small once-through boiler.

[0101] Furthermore, according to the ammonia co-firing boiler 1 of this embodiment, since the oxygen concentration of the exhaust gas after the co-firing of the liquid fuel F1 as the first fuel F1 and the ammonia fuel F2 is lower than the first oxygen concentration, the boiler efficiency can be improved by suppressing the generation of unburned materials such as coal (dust) and nitrous oxide during the co-firing of the first fuel F1 and the ammonia fuel F2, while also suppressing the heat loss of the exhaust gas.

[0102] The relationship between ammonia co-combustion rate and oxygen concentration, which indicates a significant increase in unburned matter in exhaust gas E1, is illustrated. Figure 3 and Figure 6 In the figure, the region with a higher oxygen concentration in exhaust gas E1 compared to the approximate straight line is a region where there is no significant abrupt increase in the concentration of unburned materials generated by the combustion of exhaust gas E1. Therefore, the second oxygen concentration for each ammonia co-combustion ratio is preferably set in the region with a higher oxygen concentration compared to this approximate straight line.

[0103] Based on the ammonia mixing ratio determined by the ammonia mixing ratio determination unit 43 and the relationship between the ammonia mixing ratio stored in the storage unit 41 and the second oxygen concentration, the combustion control unit 42 controls the reduction ratio of the air ratio when the ammonia mixing ratio is high to be greater than the reduction ratio of the air ratio when the ammonia mixing ratio is low. As a result, the generation of unburned materials can be suppressed.

[0104] Specifically, when the combustion state detection unit 44 detects the co-firing of the first fuel F1 and ammonia fuel F2, and the ammonia co-firing rate determination unit 43 determines that the ammonia co-firing rate is 20% or higher, the combustion control unit 42 sets a second oxygen concentration lower than the first oxygen concentration based on the ammonia co-firing rate. This reduces the air ratio compared to burning only the first fuel F1, thereby improving boiler efficiency. Furthermore, by reducing the air ratio during co-firing only when the ammonia co-firing rate is 20% or higher, which suppresses the generation of unburned materials such as carbon monoxide and coal (smoke), the risk of increased unburned material generation due to a reduced air ratio can be avoided.

[0105] The ammonia co-firing boiler 1 of this embodiment, as described above, achieves the following effects.

[0106] (1) The ammonia co-firing boiler 1 of this embodiment includes: a burner 20, which is supplied with at least one of a first fuel F1 and ammonia fuel F2, which has a combustion speed faster than ammonia, and combustion air A1; at least one of a frequency converter 302 and a regulating damper 304 serving as a combustion air adjustment unit, which adjusts the supply amount of combustion air A1; and a control unit 40, which includes: a combustion state detection unit 44, which detects the difference between the co-firing of the first fuel F1 and ammonia fuel F2 and the combustion of the first fuel F1; and a combustion control unit 42, which controls the combustion of the first fuel F1 after the combustion state detection unit 44 detects the first fuel F1 and ammonia fuel F2. When fuel F1 is burned exclusively, the combustion control unit 42 controls at least one of the inverter 302 and the regulating damper 304, which serve as combustion air adjustment units, to make the oxygen concentration of the exhaust gas after the exclusive combustion of the first fuel F1 a first oxygen concentration. When the combustion state detection unit 44 detects that the first fuel F1 and ammonia fuel F2 are mixed and burned, the combustion control unit 42 controls at least one of the inverter 302 and the regulating damper 304, which serve as combustion air adjustment units, to make the oxygen concentration of the exhaust gas after the mixed combustion of the first fuel F1 and ammonia fuel F2 a second oxygen concentration that is lower than the first oxygen concentration.

[0107] Therefore, combustion control can be achieved when the first fuel F1, which burns faster than ammonia, and ammonia fuel F2 are co-fired, suppressing the generation of carbon monoxide and unburned materials such as coal, while also suppressing the generation of nitrous oxide. Furthermore, suppressing the generation of carbon monoxide and unburned materials such as coal and reducing the air-to-gas ratio reduces exhaust heat loss and improves boiler efficiency.

[0108] (2) Based on the ammonia co-firing boiler 1 described in (1), the first fuel F1 is a liquid fuel with a first oxygen concentration of more than 4% and a second oxygen concentration of less than 4%.

[0109] Therefore, by using liquid fuel F1, which has a faster combustion speed than ammonia, the oxygen concentration in the exhaust gas when the first fuel F1 is burned exclusively can be controlled to be above 4%, and the oxygen concentration in the exhaust gas when the first fuel F1 is co-burned with ammonia fuel F2 can be controlled to be less than 4%, thus improving boiler efficiency.

[0110] (3) Based on the ammonia co-firing boiler 1 described in (1), the first fuel F1 is a gaseous fuel with a first oxygen concentration of more than 3.5% and a second oxygen concentration of less than 3.5%.

[0111] Therefore, by using a gaseous fuel, F1, which has a faster combustion speed than ammonia, the oxygen concentration in the exhaust gas when F1 is burned exclusively can be controlled to be above 3.5%, and the oxygen concentration in the exhaust gas when F1 is co-burned with ammonia fuel F2 can be controlled to be below 3.5%, thus improving boiler efficiency.

[0112] (4) Based on the ammonia co-firing boiler 1 described in (1) to (3), the ammonia co-firing boiler 1 is provided with an ammonia co-firing rate determining unit that determines the co-firing rate of the ammonia fuel, and the control unit 40 is provided with a co-firing air ratio storage unit. The co-firing air ratio storage unit stores the relationship between the ammonia co-firing rate and the second oxygen concentration in a way that the second oxygen concentration is lower when the ammonia co-firing rate is high compared with the second oxygen concentration when the ammonia co-firing rate is low. When the combustion state detection unit 44 detects that the first fuel F1 and the ammonia fuel F2 are co-firing, the combustion control unit 42 controls the air ratio during ammonia co-firing based on the ammonia co-firing rate determined by the ammonia co-firing rate determining unit and the relationship between the ammonia co-firing rate and the second oxygen concentration stored in the co-firing air ratio storage unit.

[0113] Therefore, when ammonia fuel F2 is co-fired with the first fuel F1, the air ratio is reduced compared to when the first fuel F1 is burned exclusively, which improves boiler efficiency. In addition, the air ratio is reduced only when the ammonia co-firing rate can suppress the generation of unburned materials such as carbon monoxide and coal (smoke dust) by more than 20%, thus avoiding the risk of increased unburned material generation caused by the reduction of the air ratio.

[0114] (5) Based on the ammonia co-firing boiler 1 described in (4), the second oxygen concentration in the co-firing air ratio storage section that is related to the ammonia co-firing rate is set to an oxygen concentration that shows a significant increase in the amount of unburned material produced under each ammonia co-firing rate, and is above the oxygen concentration that shows a significant increase in the amount of unburned material produced.

[0115] Therefore, it can suppress the generation of unburned materials such as carbon monoxide and coal (smoke), and avoid the risk of increased unburned material generation caused by a decrease in the air-fuel ratio.

[0116] (6) Based on 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 rate determination unit determines that the ammonia co-firing rate is 20% or more, the combustion control unit 42 controls the air ratio during ammonia co-firing based on the ammonia co-firing rate determined by the ammonia co-firing rate determination unit and the relationship between the ammonia co-firing rate stored in the co-firing air ratio storage unit and the second oxygen concentration, so that the reduction ratio of the air ratio when the ammonia co-firing rate is large relative to the air ratio when the first fuel is burned is greater than the reduction ratio of the air ratio when the ammonia co-firing rate is small relative to the air ratio when the first fuel is burned.

[0117] This can improve boiler efficiency.

[0118] (7) The control method of the ammonia co-firing boiler 1 in this embodiment includes the following steps: when the exclusive combustion of the first fuel F1 is detected, the amount of combustion air A1 is controlled so that the oxygen concentration of the exhaust gas after the exclusive combustion of the first fuel F1 becomes the first oxygen concentration; and when the co-firing of the first fuel F1 and ammonia fuel F2 is detected, the amount of combustion air A1 is controlled so that the oxygen concentration of the exhaust gas after the co-firing of the first fuel F1 and ammonia fuel F2 becomes the second oxygen concentration. The second oxygen concentration is lower than the first oxygen concentration. The second oxygen concentration is set based on the relationship between the ammonia co-firing rate and the oxygen concentration that shows a significant increase in the amount of unburned material produced during combustion. Moreover, the control method of the ammonia co-firing boiler 1 includes the following steps: control is performed so that the second oxygen concentration when the ammonia co-firing rate is high becomes lower than the second oxygen concentration when the ammonia co-firing rate is low.

[0119] Therefore, when the first fuel F1, which burns faster than ammonia, is co-fired with ammonia fuel F2, the generation of carbon monoxide and unburned materials such as coal can be suppressed. In addition, the oxygen concentration in the exhaust gas is lower compared to when the first fuel F1 is burned exclusively, which can improve boiler efficiency.

[0120] The above describes a preferred embodiment of the ammonia co-firing boiler 1 involved in the present invention, but the present invention is not limited to the above-described embodiments and can be appropriately modified.

[0121] The ammonia co-firing rate determination unit 43 calculates and determines the ammonia co-firing rate based on the detection results of the first fuel flow sensor 102 and the ammonia fuel flow sensor 202, but the method for determining the ammonia co-firing rate is not limited to this.

[0122] The ammonia co-firing rate determination unit 43 can also calculate the ammonia co-firing rate based on the opening degree of the flow regulating valve 204, which serves as the ammonia fuel detection unit, thereby detecting the ammonia co-firing rate.

[0123] Alternatively, the ammonia co-firing rate determination unit 43 can also calculate and determine the ammonia co-firing rate based on the detection results of the combustion state detection unit 44 using a calculation formula stored in the storage unit 41. The ammonia co-firing rate determination unit 43 can also determine the ammonia co-firing rate based on the detection results of the combustion state detection unit 44, according to a data table of ammonia co-firing rates corresponding to the detection results of the combustion state detection unit 44 stored in the storage unit 41.

[0124] In addition, the ammonia mixing rate determination unit 43 can also determine the ammonia mixing rate based on the selection switch for setting the ammonia mixing rate and the numerical input of the ammonia mixing rate from the input device.

[0125] When the combustion control unit 42 controls combustion based on the air ratio corresponding to each of the multiple ammonia mixing rates, the air-mixing ratio storage unit can store all the multiple ammonia mixing rates and the air ratio corresponding to each of the multiple ammonia mixing rates, or it can store the air ratio corresponding to the necessary ammonia mixing rate by inputting an input device or the like before control is performed.

[0126] An example is shown of an ammonia co-firing boiler 1, in which the oxygen concentration of the exhaust gas (as the second oxygen concentration) is preferably less than 4% when the first fuel F1 is a liquid fuel, and in which the oxygen concentration of the exhaust gas (as the second oxygen concentration) is preferably less than 3.5% when the first fuel F1 is a gaseous fuel, but this is not a limitation. By further reducing the oxygen concentration of the exhaust gas by 0.5% during ammonia co-firing, the ammonia co-firing boiler 1 can further improve boiler efficiency, reduce exhaust gas heat loss, and not only that, improve the effective utilization rate of ammonia, and improve the carbon dioxide reduction effect. For example, as... Figure 4 As shown, in the ammonia co-firing of boiler 1, when the second oxygen concentration in the case of gaseous fuel decreased from 3.5% to 3%, the effective utilization rate of ammonia increased from approximately 94.5% to approximately 97%. Furthermore, as... Figure 7 As shown, in the ammonia co-firing of the ammonia boiler 1, when the second oxygen concentration in the case of liquid fuel is reduced from 4% to 3.5%, the effective utilization rate of ammonia is increased from about 94.5% to about 98%. Therefore, by further reducing the oxygen concentration of the exhaust gas in the ammonia co-firing process by 0.5%, the ammonia co-firing boiler 1 can more effectively reduce greenhouse gas emissions.

[0127] When the combustion state detection unit 44 detects that only the first fuel F1 is flowing through the first fuel supply line 100, it determines that the combustion of the first fuel F1 is 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 ammonia fuel F2 is flowing through the ammonia supply line 200, it determines that the combustion of the first fuel F1 and ammonia fuel F2 is detected, but it is not limited to this.

[0128] The combustion state detection unit 44 can also determine that the combustion of the first fuel F1 is detected based on the opening instructions made by the combustion control unit 42 to the first shut-off valve 103 and the second shut-off valve 105, and determine that the combustion of the first fuel F1 and ammonia fuel F2 is detected based on the opening instructions made by the combustion control unit 42 to the first shut-off valve 103 and the second shut-off valve 105, and the opening instructions made by the combustion control unit 42 to the first shut-off valve 203 and the second shut-off valve 205.

[0129] In addition, the combustion state detection unit 44 can also determine the combustion state based on the selection switch for setting the combustion state and the input of the combustion state from the input device.

[0130] Ammonia fuel can also be ammonia or a gaseous fuel with ammonia as the main component.

[0131] Furthermore, the air ratio (oxygen concentration in exhaust gas E1) control described here by the combustion control unit is particularly effective in stabilizing combustion in ammonia co-firing burners and improving boiler efficiency in small once-through boilers, small-scale boilers, and flue-tube boilers where combustion stops and starts frequently.

[0132] It should be noted that this disclosure promotes the use of ammonia as a fuel that does not emit carbon dioxide, and therefore can contribute, for example, to Goal 7 of the United Nations-led Sustainable Development Goals (SDGs) "Ensuring access to affordable, reliable and sustainable modern energy for all".

[0133] Explanation of reference numerals in the attached figures

[0134] 1. Ammonia co-firing boiler

[0135] 10 tanks

[0136] 20 Burners

[0137] 40 Control Department

[0138] 42 Combustion Control Unit

[0139] 43. Ammonia Co-firing Ratio Determination Department

[0140] 44 Combustion Condition Detection Unit

[0141] 302 Inverter (Combustion Air Conditioning Unit)

[0142] 304 Air Regulating Damper (Combustion Air Adjustment Unit)

[0143] A1 Combustion Air

[0144] F1 First Fuel

[0145] F2 Ammonia fuel.

Claims

1. A boiler, wherein, The boiler has the following features: A burner that is supplied with at least one of a first fuel that burns faster than ammonia and an ammonia fuel, and combustion air; A combustion air regulating unit that adjusts the supply amount of combustion air; and Control Department The control unit includes: A combustion state detection unit that detects the difference between the co-combustion of the first fuel and the ammonia fuel and the combustion of the first fuel; and The combustion control unit, when the combustion state detection unit detects that the first fuel is being burned exclusively, controls the combustion air adjustment unit to make the oxygen concentration of the exhaust gas after the exclusive combustion of the first fuel reach a first oxygen concentration; when the combustion state detection unit detects that the first fuel and the ammonia fuel are being burned together, the combustion control unit controls the combustion air adjustment unit to make the oxygen concentration of the exhaust gas after the mixed combustion of the first fuel and the ammonia fuel reach a second oxygen concentration that is lower than the first oxygen concentration.

2. The boiler according to claim 1, wherein, The first fuel is a liquid fuel. The first oxygen concentration is 4% or higher. 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 higher. The second oxygen concentration is less than 3.5%.

4. The boiler according to any one of claims 1 to 3, wherein, The boiler is equipped with an ammonia co-firing ratio determining unit that determines the co-firing ratio of the ammonia fuel. The control unit includes a combustion air ratio storage unit, which stores the relationship between the ammonia combustion rate and the second oxygen concentration in a manner where the second oxygen concentration is lower when the ammonia combustion rate is high compared to the second oxygen concentration when the ammonia combustion rate is low. When the combustion state detection unit detects that the first fuel and the ammonia fuel are mixed, the combustion control unit controls the air ratio during ammonia mixing based on the ammonia mixing rate determined by the ammonia mixing rate determination unit and the relationship between the ammonia mixing rate stored in the air-mixing ratio storage unit and the second oxygen concentration.

5. The boiler according to claim 4, wherein, The second oxygen concentration in the air-fuel ratio storage section, which is related to the ammonia co-firing rate, is set based on an oxygen concentration that shows a significant increase in the amount of unburned material produced at each ammonia co-firing rate, and is above an oxygen concentration that shows a significant increase in the amount of unburned material produced.

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

7. A method for controlling a boiler, the boiler comprising a burner capable of co-firing at least one of a first fuel with a faster combustion rate than ammonia and ammonia fuel with combustion air, wherein, The boiler control method comprises the following steps: If the burning of the first fuel is detected, the amount of combustion air is controlled so that the oxygen concentration of the exhaust gas after the burning of the first fuel becomes the first oxygen concentration. as well as If co-firing of the first fuel and the ammonia fuel is detected, the amount of combustion air is controlled so that the oxygen concentration in the exhaust gas after co-firing of the first fuel and the ammonia fuel becomes the second oxygen concentration. The second oxygen concentration is lower than the first oxygen concentration. The second oxygen concentration is set based on the relationship between the ammonia co-combustion rate and the generation of unburned material during combustion, which shows a significant increase in oxygen concentration. Furthermore, the boiler control method includes the following steps: The oxygen concentration is controlled to be lower when the ammonia co-firing ratio is high compared to when the ammonia co-firing ratio is low.

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