Boiler and control method for boiler

Abstract

To provide a boiler capable of suppressing generation of unburned substances such as carbon monoxide and soot and suppressing generation of dinitrogen monoxide during mixed combustion of hydrocarbon-based fuel as fuel of which combustion speed is higher than that of ammonia and ammonia fuel, and a control method for the boiler.SOLUTION: An ammonia mixed combustion boiler 1 as a boiler includes: a burner 20 to which at least either of first fuel F1 or ammonia fuel F2 and combustion air A1 are supplied; a damper 304; a combustion state detection section 44 that detects combustion of the first fuel F1 and mixed combustion of the first fuel and the ammonia fuel F2 distinctively; and a control section 40. The control section 40 includes a combustion control section that performs control so that an exhaust gas oxygen concentration becomes a first oxygen concentration when the combustion state detection section 44 detects single combustion of the first fuel F1, and that performs control so that the exhaust gas oxygen concentration becomes a second oxygen concentration lower than the first oxygen concentration when mixed combustion of the first fuel F1 and the ammonia fuel F2 is detected.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a boiler and a method for controlling the boiler.

Background Art

[0002] By replacing a part of hydrocarbon fuels such as natural gas and oil fuel used as boiler fuel with ammonia that does not generate carbon dioxide, prevention of global warming by reducing carbon dioxide emissions is expected (see, for example, Patent Document 1). When burning hydrocarbon fuels, combustion is performed so that the exhaust gas oxygen concentration becomes a predetermined value or more from the viewpoint of suppressing the generation of unburned substances such as carbon monoxide and soot. On the other hand, when burning ammonia fuel, if the exhaust gas oxygen concentration exceeds a predetermined value, the generation of nitrous oxide increases, reducing the greenhouse gas (GHG) reduction effect by ammonia combustion.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a boiler using an ammonia co-firing burner, when the air ratio is made close to 1 to suppress heat loss of the exhaust gas, the boiler efficiency improves, but the generation of unburned substances increases. On the other hand, when the air ratio is increased when burning ammonia fuel, the generation of nitrous oxide (N2O) with a high warming coefficient increases, canceling out the carbon dioxide reduction effect by burning ammonia fuel, which has been a problem. Therefore, combustion control is required that can suppress the generation of unburned substances such as carbon monoxide and soot and can achieve both suppression of the generation of nitrous oxide when co-firing a hydrocarbon fuel as a fuel with a faster combustion rate than ammonia and ammonia fuel.

[0005] The object of the present invention is to provide a boiler and a method for controlling the boiler that can suppress the generation of unburned substances such as carbon monoxide and soot during co-combustion of a hydrocarbon fuel as a fuel with a combustion rate faster than ammonia and an ammonia fuel, and can achieve both suppression of the generation of nitrous oxide.

Means for Solving the Problems

[0006] The present invention includes a burner to which at least one of a first fuel having a combustion rate faster than ammonia and an ammonia fuel, and combustion air are supplied, a combustion air adjustment unit that adjusts the supply amount of the combustion air, and a control unit. The control unit includes a combustion state detection unit that distinguishes and detects the combustion of the first fuel and the co-combustion of the first fuel and the ammonia fuel, and when the exclusive combustion of the first fuel is detected by the combustion state detection unit, controls the combustion air adjustment unit so that the exhaust gas oxygen concentration after the exclusive combustion of the first fuel becomes a first oxygen concentration, and when the co-combustion of the first fuel and the ammonia fuel is detected by the combustion state detection unit, controls the combustion air adjustment unit so that the exhaust gas oxygen concentration after the co-combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration lower than the first oxygen concentration. The present invention relates to a boiler.

[0007] Further, it is 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] Further, it is 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] It also includes an ammonia co - firing rate determination unit for determining the co - firing rate of the ammonia fuel. The control unit includes a co - firing air ratio storage unit that stores the relationship between the ammonia co - firing rate and the second oxygen concentration such that the second oxygen concentration when the ammonia co - firing rate is high is lower than the second oxygen concentration when the ammonia co - firing rate is low. When the combustion state detection unit detects the co - combustion of the first fuel and the ammonia fuel, it is preferable that the combustion control unit controls the air ratio during ammonia co - combustion based on the ammonia co - firing rate determined by the ammonia co - firing rate determination unit, the relationship between the ammonia co - firing rate stored in the co - firing air ratio storage unit, and the second oxygen concentration.

[0010] The second oxygen concentration associated with the ammonia co - firing rate in the co - firing air ratio storage unit is set based on the oxygen concentration at which the amount of unburned substances generated per ammonia co - firing rate shows a clear increase, and it is preferably not less than the oxygen concentration at which the amount of unburned substances shows a clear increase.

[0011] When the combustion state detection unit detects the co - combustion of the first fuel and the ammonia fuel, and the ammonia co - firing rate determination unit determines that the ammonia co - firing rate is 20% or more, it is preferable that the combustion control unit, based on the ammonia co - firing rate determined by the ammonia co - firing rate determination unit, the relationship between the ammonia co - firing rate stored in the co - firing air ratio storage unit and the second oxygen concentration, controls the air ratio during ammonia co - combustion so that the reduction rate of the air ratio when the ammonia co - firing rate is high with respect to the air ratio during exclusive combustion of the first fuel is greater than the reduction rate of the air ratio when the ammonia co - firing rate is low with respect to the air ratio during exclusive combustion of the first fuel.

[0012] The present invention relates to a method for controlling a boiler equipped with a burner capable of co-combusting a first fuel with a combustion rate faster than ammonia, at least one of the ammonia fuels, and combustion air. When exclusive combustion of the first fuel is detected, the amount of the combustion air is controlled so that the exhaust gas oxygen concentration after the exclusive combustion of the first fuel becomes a first oxygen concentration. When co-combustion of the first fuel and the ammonia fuel is detected, the amount of the combustion air is controlled so that the exhaust gas oxygen concentration after the co-combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration. The second oxygen concentration is set based on a relationship between the first oxygen concentration and an oxygen concentration at which an increase in the ammonia co-combustion rate and the generation of unburned substances due to combustion are clearly shown. Further, there is a step of controlling so that the second oxygen concentration when the ammonia co-combustion rate is high becomes lower than the second oxygen concentration when the ammonia co-combustion rate is low. The present invention relates to a method for controlling a boiler.

Effects of the Invention

[0013] According to the present invention, it is possible to provide a boiler and a method for controlling a boiler that can suppress the generation of unburned substances such as carbon monoxide and soot during co-combustion of a hydrocarbon-based fuel as a fuel having a combustion rate faster than ammonia and an ammonia fuel, and can also suppress the generation of nitrous oxide.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0015] Hereinafter, the ammonia co-firing boiler 1 as a boiler according to an embodiment of the present invention will be described with reference to the drawings. The ammonia co-firing boiler 1 of the present embodiment is a steam boiler that burns fuel and heats water to generate steam, and supplies steam to a load device (not shown).

[0016] FIG. 1 is a diagram schematically showing the configuration of the ammonia co-firing boiler 1 according to the embodiment. As shown in FIG. 1, the ammonia co-firing boiler 1 includes a can body 10, a burner 20, and a control unit 40. The ammonia co-firing 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 feed water line 500, and a steam supply line 600. In this specification, the "line" is a general term for a flow path, a route, a pipeline, etc.

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

[0018] The burner 20 burns the first fuel F1 and the ammonia fuel F2 with a faster combustion rate than ammonia in the combustion chamber B of the can body 10. The burner 20 is arranged on the upper part of the can body 10. The burner 20 includes a burner main body 21 and a wind box 22. The first fuel supply line 100 and the ammonia supply line 200 are connected to the burner main body 21. The combustion air supply line 300 is connected to the wind box 22. In this embodiment, first, the first fuel F1 and the combustion air A1 are supplied to the burner to start combustion. Next, the ammonia fuel F2 is supplied to the combustion part 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. Also, in this embodiment, as the first fuel with a faster combustion rate than ammonia, a liquid fuel or a gaseous fuel can be used. As the liquid fuel, oil fuel, alcohol fuels, etc. can be used. As the gaseous fuel, hydrocarbon gases such as liquefied natural gas (hereinafter referred to as LNG) and liquefied petroleum gas (LPG) can be used.

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

[0020] The first fuel supply line 100 supplies the 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, from the upstream side, a main valve 101, a first fuel flow rate sensor 102, a first shut-off valve 103, a flow rate adjustment valve 104, and a second shut-off valve 105.

[0021] The main valve 101 is constituted by a manual valve and opens and closes the flow path of the first fuel supply line 100. The first fuel flow rate sensor 102 detects the flow rate of the first fuel F1 flowing through the first fuel supply line 100. The first fuel flow rate sensor 102 is electrically connected to the control unit 40, and the detection result of the first fuel flow rate sensor 102 can be acquired by the control unit 40. The first shut-off valve 103 and the second shut-off valve 105 are constituted by solenoid valves, open and close the flow path of the first fuel supply line 100, and supply or stop the supply of 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 a signal 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 opening degree of the valve. The flow rate adjustment valve 104 is electrically connected to the control unit 40 and is controlled by a signal transmitted by 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 includes, from the upstream side, a main valve 201, an ammonia fuel flow rate sensor 202, a first shut-off valve 203 as an ammonia fuel shut-off valve, a flow rate adjustment valve 204, and a second shut-off valve 205 as an ammonia fuel shut-off valve.

[0023] The main valve 201 is composed of a manual valve and opens and closes the flow path of the ammonia supply line 200. 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 fuel flow rate sensor 202 is electrically connected to the control unit 40, and the detection result of the ammonia fuel flow rate sensor 202 can be obtained by the control unit 40. The first shut-off valve 203 and the second shut-off valve 205 are composed of solenoid valves, open and close the flow path of the ammonia supply line 200, and supply or stop the 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 a signal 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 opening degree of the valve. The flow rate adjustment valve 204 is electrically connected to the control unit 40 and is controlled by a signal transmitted by the control unit 40 based on the detection result of the ammonia fuel flow rate sensor 202.

[0024] The combustion air supply line 300 supplies the combustion air A1 to the burner 20. In the present embodiment, the combustion air A1 is supplied to the windbox 22 of the burner 20. The upstream side of the combustion air supply line 300 is connected to the blower 301, and the downstream side of the combustion air supply line 300 is connected to the windbox 22. The combustion air supply line 300 includes, from the upstream side, the blower 301, the combustion air flow rate sensor 303, and the damper 304 as the combustion air adjustment unit.

[0025] The blower 301 supplies the combustion air A1 to the burner 20. The blower 301 includes a fan and a motor that rotates the fan, and the rotational speed of the motor can be adjusted by controlling the frequency by the inverter 302, and the rotational speed can be adjusted. That is, the inverter 302 is a combustion air adjustment unit capable of adjusting the supply amount of the combustion air A1 supplied to the burner. In the present 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 supply amount of the combustion air A1 supplied to the burner 20 by adjusting the opening degree of the damper. Specifically, the damper 304 is rotatably arranged between a closed state in which the flow path of the combustion air supply line 300 is blocked and an open state in which it rotates from this closed state by a predetermined angle (for example, 90 degrees) to open the flow path of the combustion air supply line 300. The damper 304 is electrically connected to the control unit 40 and is controlled by a signal transmitted from the control unit 40.

[0026] The exhaust pipe 400 discharges the 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 an exhaust gas temperature detection unit 401 and an exhaust gas detection unit 402 from the upstream side.

[0027] The exhaust gas temperature detection unit 401 detects the temperature of the exhaust gas E1 flowing through the exhaust pipe 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 acquired by the control unit 40. The exhaust gas detection unit 402 detects the components of the exhaust gas E1 flowing through the exhaust pipe 400. In the present embodiment, the exhaust gas detection unit 402 detects oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NO x ), nitrous oxide (N2O), ammonia (NH3), etc. as the components of the exhaust gas E1. The exhaust gas detection unit 402 is electrically connected to the control unit 40, and the detection result of the exhaust gas detection unit 402 can be acquired by the control unit 40. In such an exhaust gas detection unit 402, for example, when detecting oxygen, a zirconia type oxygen sensor, a galvanic cell type oxygen sensor, etc. are used.

[0028] The water supply line 500 supplies water W1 from a water supply source (not shown) to the can 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 provided with a main valve 501. The main valve 501 is constituted by a manual valve and opens and closes the flow path of the water supply line 500.

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

[0030] As shown in FIG. 1, 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 state detection unit 44. The control unit 40 is constituted by an arithmetic processor such as a PLC (Programmable Logic Controller), a DSP (Digital Signal Processor), an FPGA (Field-Programmable Gate Array), etc. Various functions of the control unit 40 are realized by executing a predetermined software (program) stored in the storage unit 41, for example. Various functions of the control unit 40 may be realized by the cooperation of hardware and software, or may be realized by hardware (electronic circuit) only.

[0031] The storage unit 41 stores various setting information. Further, the storage unit 41 stores the relationship between the ammonia co-firing rate and the second oxygen concentration such that the second oxygen concentration when the ammonia co-firing rate is high is lower than the second oxygen concentration when the ammonia co-firing rate is low, as an ammonia co-firing air ratio storage unit. Note that, not limited to this, the control unit 40 may include a combustion air ratio storage unit that stores the relationship between the ammonia co-combustion ratio and the second oxygen concentration separately from the storage unit 41 so that the second oxygen concentration when the ammonia co-combustion ratio is high is lower than the second oxygen concentration when the ammonia co-combustion ratio is low. Here, "when the ammonia co-combustion ratio is high" and "when the ammonia co-combustion ratio is low" indicate the relative relationship of the ammonia co-combustion ratio. For example, when the ammonia co-combustion ratio is set at 10%, 20%, 40%, and 60%, it means that the second oxygen concentration at an ammonia co-combustion ratio of 60% is lower than the second oxygen concentration at an ammonia co-combustion ratio of 10%. At this time, for example, the second oxygen concentrations at ammonia co-combustion ratios of 20% and 40% may be the same value. In other words, the second oxygen concentration at the maximum ammonia co-combustion ratio (here 60%) is lower than the second oxygen concentration at the minimum ammonia co-combustion ratio (here 10%), and the second oxygen concentration at the ammonia co-combustion ratio located therebetween decreases as the ammonia co-combustion ratio increases, or maintains the same second oxygen concentration.

[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, adjusts at least one of the inverter 302 and the damper 304 serving as the combustion air adjustment unit, and supplies combustion air A1 with a predetermined flow rate to the burner 20. Further, the combustion control unit 42 opens the first shut-off valve 103 and the second shut-off valve 105, and adjusts the opening degree of the valve of the flow rate adjustment valve 104 based on the detection result of the first fuel flow rate sensor 102 so that the combustion amount of the first fuel F1 is a predetermined amount. Thereby, the combustion of the first fuel F1 is started. Further, the combustion control unit 42 opens the first shut-off valve 203 and the second shut-off valve 205, and adjusts the opening degree of the valve of the flow rate adjustment valve 204 based on the detection result of the ammonia fuel flow rate sensor 202 so that the combustion amount of the predetermined ammonia fuel F2 is obtained. Thereby, the combustion of the ammonia fuel F2 is started.

[0033] The combustion state detection unit 44 distinguishes and detects the combustion of the first fuel F1 and the co-combustion of the first fuel F1 and the ammonia fuel F2. In this embodiment, the combustion state detection unit 44 detects that the first fuel F1 is being burned by detecting, via 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 burned by detecting, via the ammonia fuel flow rate sensor 202, that the ammonia fuel F2 is flowing through the ammonia supply line 200.

[0034] When only the first fuel flow rate sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100, the combustion state detection unit 44 determines that exclusive combustion of the first fuel F1 has been detected. When the first fuel flow rate sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100 and the ammonia fuel flow rate sensor 202 detects that the ammonia fuel F2 is flowing through the ammonia supply line 200, the combustion state detection unit 44 determines that co - combustion of the first fuel F1 and the ammonia fuel F2 has been detected. Note that the combustion state detection unit 44 may detect the fuel state not only by detecting the flow of the ammonia fuel F2 by the ammonia fuel flow rate sensor 202 as described above, but also by an input such as a select switch that sets ammonia co - combustion.

[0035] When the combustion state detection unit 44 detects exclusive 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 the combustion air adjustment unit, so that the exhaust gas oxygen concentration after exclusive combustion of the first fuel F1 becomes the first oxygen concentration. Further, 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 exhaust gas oxygen concentration after co - combustion of the first fuel F1 and the ammonia fuel F2 becomes a second oxygen concentration lower than the first oxygen concentration.

[0036] In the present embodiment, when the combustion control unit 42 detects exclusive combustion of the first fuel F1 by the combustion state detection unit 44, it acquires the detection result of the oxygen concentration of the exhaust gas E1 after exclusive combustion of the first fuel F1 flowing through the exhaust pipe 400 detected by the exhaust gas detection unit 402. Based on the detection result of the detected oxygen concentration of the 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 control unit 42 detects co-combustion of the first fuel F1 and the ammonia fuel F2 by the combustion state detection unit 44, it 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 flowing through the exhaust pipe 400 detected by the exhaust gas detection unit 402. Based on the detection result of the detected oxygen concentration of the 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 a second oxygen concentration lower than the first oxygen concentration.

[0037] Thereby, when co-combusting the hydrocarbon-based fuel as the first fuel F1 with a combustion rate faster than that of ammonia and the ammonia fuel F2, it is possible to suppress the generation of unburned substances such as carbon monoxide and soot (smoke), and suppress the generation of nitrous oxide, while reducing the air ratio, the exhaust gas heat loss is reduced, and the boiler efficiency can be improved.

[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 co-combustion rate determination unit 43 determines the ammonia co-combustion rate 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 co-combustion rate to the combustion control unit 42. In the present embodiment, as an example, determining the ammonia co-combustion rate will be described by taking an example in which the ammonia co-combustion rate is calculated and determined based on the detection results of the first fuel flow rate sensor 102 and the ammonia fuel flow rate sensor 202, but it is not limited thereto.

[0039] Figures 2 to 4 are diagrams showing measurement results regarding exhaust gas characteristics when gaseous fuel (LNG) and ammonia fuel F2 are burned as the first fuel F1 in the ammonia co-firing boiler 1 according to the embodiment. The boiler combustion test was conducted with exclusive combustion of LNG and co-combustion of LNG and ammonia fuel F2 (ammonia co-combustion rates of 40% and 60%). The ammonia co-combustion rate is defined as the ratio of the supply heat quantity of ammonia fuel F2 to the total supply heat quantity of the supply heat quantity of LNG and the supply heat quantity of ammonia fuel F2.

[0040] Figure 2 is a diagram showing the relationship between the oxygen concentration and carbon monoxide concentration of 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. In the graph shown in Figure 2, the vertical axis represents the relative value when the carbon monoxide concentration of the exhaust gas E1 at an exhaust gas oxygen concentration of 0.5% with an ammonia co-combustion rate of 60% is set to 100, and the horizontal axis represents the oxygen concentration [%] of the exhaust gas E1. As shown in Figure 2, in exclusive combustion of LNG, when the oxygen concentration of the exhaust gas E1 fell below 2.6%, the carbon monoxide concentration of the exhaust gas E1 began to increase significantly. On the other hand, in co-combustion of LNG and ammonia, when the oxygen concentration of the exhaust gas E1 was less than 1.6%, the carbon monoxide concentration began to increase significantly. Also, the starting point of this significant increase in carbon monoxide concentration shifted to the lower oxygen concentration side as the ammonia co-combustion rate increased.

[0041] Figure 3 is a diagram showing the relationship between the ammonia co-combustion rate and the oxygen concentration at which the carbon monoxide concentration of the 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 of the exhaust gas E1 begins to increase significantly, and the horizontal axis represents the ammonia co-combustion rate [%]. As shown in Figure 3, as the ammonia co-combustion rate increased, the oxygen concentration at which the carbon monoxide concentration showed a significant increase decreased. From the approximate straight line shown in Figure 3, the oxygen concentration of the exhaust gas E1 at which carbon monoxide showed a significant increase in the case of an ammonia co-combustion rate of 20% is estimated to be 2.1% (air ratio 1.11) (the oxygen concentration indicated by the open circle in Figure 3).

[0042] Therefore, based on these phenomena, in the co-combustion of LNG and ammonia fuel F2, operation can be carried out to suppress the generation of carbon monoxide at an air ratio smaller than when only LNG is combusted. Also, the rate of decrease in the air ratio when the ammonia co-combustion rate is large can be made larger than the rate of decrease in the air ratio when the ammonia co-combustion rate is small. For example, the rate of decrease in the air ratio when the ammonia co-combustion rate is 60% can be made larger than the rate of decrease in the air ratio when the ammonia co-combustion rate is 20%. Here, the rate of decrease in the air ratio is the ratio of the decrease in the air ratio during co-combustion to the air ratio during exclusive combustion of LNG.

[0043] FIG. 4 is a diagram showing the relationship between the oxygen concentration of exhaust gas E1 and the ammonia effective utilization rate (ammonia effective utilization rate (%) = 1 - (greenhouse gas emission amount as nitrous oxide / greenhouse gas emission reduction amount due to combustion of ammonia fuel) × 100 (%)) when gaseous fuel (LNG) as the first fuel F1 and ammonia fuel F2 are co-combusted in the ammonia co-combustion boiler 1 according to the embodiment. The "greenhouse gas emission reduction amount due to combustion of ammonia fuel" is, that is, the "greenhouse gas emission reduction amount due to reduction in the usage amount of hydrocarbon-based fuel". In the graph shown in FIG. 4, the vertical axis represents the ammonia effective utilization rate [%], and the horizontal axis represents the oxygen concentration [%] in the exhaust gas E1. As shown in FIG. 4, when the ammonia co-combustion rate is 40% or 60%, in both cases, the ammonia effective utilization rate rapidly decreases when the oxygen concentration of the exhaust gas E1 is 3.5% or more. Therefore, it is preferable that the ammonia co-combustion boiler 1 according to the embodiment operates at an oxygen concentration of less than 3.5%.

[0044] From the above results, the second oxygen concentration, which is the exhaust gas oxygen concentration after co-combustion of the gaseous fuel as the first fuel F1 and the ammonia fuel F2, is preferably less than 3.5% which is lower than the first oxygen concentration, and the first oxygen concentration, which is the exhaust gas oxygen concentration during exclusive combustion of the first fuel F1, is preferably 3.5% or more from 1.2 to 1.35 (3.5% to 5.4%) which is the target air ratio of the small-sized once-through boiler. Further, according to the ammonia co-firing boiler 1 of the present embodiment, since the second oxygen concentration, which is the exhaust gas oxygen concentration after co-firing the gaseous fuel as the first fuel F1 and the ammonia fuel F2, is lower than the first oxygen concentration, when co-firing the first fuel F1 and the ammonia fuel F2, it is possible to achieve both suppression of the generation of unburned substances such as carbon monoxide and suppression of the generation of nitrous oxide, and further suppress the exhaust gas heat loss to increase the boiler efficiency.

[0045] Figs. 5 to 7 are diagrams showing exhaust gas characteristics when the liquid fuel (A heavy oil) as the first fuel F1 and the ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment. The combustion test of the boiler was carried out with exclusive firing of A heavy oil and co-firing of A heavy oil and the ammonia fuel F2 (ammonia co-firing rates of 20% and 40%). The ammonia co-firing rate is defined as the ratio of the supply heat quantity of the ammonia fuel F2 to the total supply heat quantity of the supply heat quantity of A heavy oil and the supply heat quantity of the ammonia fuel F2.

[0046] Fig. 5 is a diagram showing the relationship between the oxygen concentration of the exhaust gas E1 and the smoke density (an index of incomplete combustion of oil fuel) when the liquid fuel (A heavy oil) as the first fuel F1 and the ammonia fuel F2 are burned in the ammonia co-firing boiler 1 according to the embodiment. In the graph shown in Fig. 5, the vertical axis is the relative value when the smoke density of the exhaust gas at an exhaust gas oxygen concentration of 1% with an ammonia co-firing rate of 20% is set to 100, and the horizontal axis is the oxygen concentration [%] in the exhaust gas E1. As shown in Fig. 5, in the case of exclusive firing of A heavy oil, when the oxygen concentration of the exhaust gas E1 was less than 4%, the smoke density began to increase significantly. On the other hand, in the co-firing of A heavy oil and ammonia, when the oxygen concentration of the exhaust gas E1 was less than 2.5%, the smoke density began to increase significantly. Also, the starting point of this significant increase in smoke density shifted to the lower oxygen concentration side as the ammonia co-firing rate increased.

[0047] FIG. 6 is a diagram showing the relationship between the ammonia co - firing rate and the oxygen concentration at which the smoke density of exhaust gas E1 starts to increase significantly, based on the test results shown in FIG. 5. In the graph shown in FIG. 6, the vertical axis represents the ammonia co - firing rate [%], and the horizontal axis represents the oxygen concentration [%] at which the smoke density of exhaust gas E1 starts to increase significantly. As shown in FIG. 6, as the ammonia co - firing rate increases, the oxygen concentration at which the smoke density shows a significant increase decreases. Also, although not shown in FIG. 6, in the boiler combustion tests with ammonia co - firing rates of 50% and 60%, the oxygen concentration at which the smoke density shows a significant increase was lower than that at an ammonia co - firing rate of 40%. From the approximate straight line shown in FIG. 6, the oxygen concentration of exhaust gas E1 at which the smoke density shows a significant increase in the case of an ammonia co - firing rate of 20% is estimated to be 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 carried out to suppress the generation of carbon monoxide at an air ratio smaller than when burning only heavy oil A. Also, the rate of decrease in the air ratio when the ammonia co - firing rate is large can be made larger than the rate of decrease in the air ratio when the ammonia co - firing rate is small. For example, the rate of decrease in the air ratio when the ammonia co - firing rate is 40% can be made larger than the rate of decrease in the air ratio when the ammonia co - firing rate is 20%. Here, the rate of decrease in the air ratio is the ratio of the decrease in the air ratio during co - firing to the air ratio during exclusive firing of heavy oil A.

[0048] FIG. 7 is a diagram showing the relationship between the oxygen concentration of exhaust gas E1 and the ammonia effective utilization rate (ammonia effective utilization rate (%) = 1 - (greenhouse gas emission amount as nitrous oxide / greenhouse gas emission reduction amount by combustion of ammonia fuel)×100(%)) when liquid fuel (heavy oil A) as the first fuel F1 and ammonia fuel F2 are co - fired in the ammonia co - firing boiler 1 according to the embodiment. In the graph shown in FIG. 7, the vertical axis represents the effective utilization rate of ammonia, and the horizontal axis represents the oxygen concentration [%] in exhaust gas E1. As shown in Fig. 7, the effective ammonia utilization rate decreases rapidly when the oxygen concentration of the exhaust gas E1 is 4% or more. Therefore, the ammonia co-firing 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 the oxygen concentration of the exhaust gas after co-firing the liquid fuel as the first fuel F1 and the ammonia fuel F2, is preferably less than 4%, which is lower than the first oxygen concentration, and the first oxygen concentration, which is the oxygen concentration of the exhaust gas after exclusive firing of the first fuel F1, is preferably 4% or more from 1.25 to 1.4 (4.2% to 6%), which is the target air ratio of the small once-through boiler. Further, according to the ammonia co-firing boiler 1 of the present embodiment, since the second oxygen concentration, which is the oxygen concentration of the exhaust gas after co-firing the liquid fuel as the first fuel F1 and the ammonia fuel F2, is lower than the first oxygen concentration, when co-firing the first fuel F1 and the ammonia fuel F2, it is possible to suppress the generation of unburned substances such as soot (smoke) and the generation of nitrous oxide, and further suppress the exhaust gas heat loss and improve the boiler efficiency.

[0050] In Figs. 3 and 6 showing the relationship between the ammonia co-firing rate and the oxygen concentration at which the unburned substances in the exhaust gas E1 clearly increase, the region where the oxygen concentration of the exhaust gas E1 is higher than the approximate straight line shown in the figure is a region where there is no obvious rapid increase in the concentration of unburned substances generated by the combustion of the exhaust gas E1. Therefore, the second oxygen concentration for each ammonia co-firing rate is preferably set in a region where the oxygen concentration is higher than this approximate straight line. The combustion control unit 42 controls so that the reduction rate of the air ratio when the ammonia co-firing rate is large is larger than the reduction rate of the air ratio when the ammonia co-firing rate is small, based on the ammonia co-firing rate determined by the ammonia co-firing rate determination unit 43 and the relationship between the ammonia co-firing rate and the second oxygen concentration stored in the storage unit 41. Thereby, the generation of unburned substances can be suppressed. Specifically, when the combustion control unit 42 detects the co-combustion of the first fuel F1 and the ammonia fuel F2 by the combustion state detection unit 44 and the ammonia co-combustion rate determination unit 43 determines that the ammonia co-combustion rate is 20% or more, the combustion control unit 42 sets a second oxygen concentration lower than the first oxygen concentration according to the ammonia co-combustion rate. As a result, the air ratio becomes smaller than when the first fuel F1 is burned alone, and the boiler efficiency can be increased. In addition, the air ratio during co-combustion is reduced only when the ammonia co-combustion rate is 20% or more, which can suppress the generation of unburned substances such as carbon monoxide and soot. Therefore, the risk of an increase in the generation of unburned substances due to a decrease in the air ratio can be avoided.

[0051] According to the ammonia co-combustion boiler 1 of the present embodiment described above, the following effects can be obtained.

[0052] (1) The ammonia co-combustion boiler 1 of the present embodiment includes at least one of the first fuel F1 and the ammonia fuel F2, the combustion speed of which is faster than that of ammonia, the burner 20 to which the combustion air A1 is supplied, and at least one of the inverter 302 or the damper 304 as the combustion air adjustment unit for adjusting the supply amount of the combustion air A1, the combustion of the first fuel F1, and the control unit 40. The control unit 40 includes a combustion state detection unit 44 that distinguishes and detects the co-combustion of the first fuel F1 and the ammonia fuel F2. When the combustion state detection unit 44 detects the exclusive combustion of the first fuel F1, at least one of the inverter 302 or the damper 304 as the combustion air adjustment unit is controlled so that the exhaust gas oxygen concentration after the exclusive combustion of the first fuel F1 becomes 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, the combustion control unit 42 is provided, which controls at least one of the inverter 302 or the damper 304 as the combustion air adjustment unit so that the exhaust gas oxygen concentration 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. Accordingly, during the co - combustion of the first fuel F1 with a combustion rate faster than ammonia and the ammonia fuel F2, it is possible to suppress the generation of unburned substances such as carbon monoxide and soot, and it is possible to achieve combustion control that can also suppress the generation of nitrous oxide. Further, by suppressing the generation of unburned substances such as carbon monoxide and soot and being able to reduce the air - fuel ratio, the exhaust gas heat loss is reduced, and the boiler efficiency can be increased.

[0053] (2) The ammonia co - combustion boiler 1 described in (1) is such that 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%. Accordingly, when the first fuel F1, which has a combustion rate faster than ammonia, is a liquid fuel, by controlling the exhaust gas oxygen concentration when the first fuel F1 is burned alone to be 4% or more, and controlling the exhaust gas oxygen concentration when the first fuel F1 and the ammonia fuel F2 are co - burned to be less than 4%, the boiler efficiency can be increased.

[0054] (3) The ammonia co - combustion boiler 1 described in (1) is such that the first fuel F1 is a gas fuel, the first oxygen concentration is 3.5% or more, and the second oxygen concentration is less than 3.5%. Accordingly, when the first fuel F1, which has a combustion rate faster than ammonia, is a gas fuel, by controlling the exhaust gas oxygen concentration when the first fuel F1 is burned alone to be 3.5% or more, and controlling the exhaust gas oxygen concentration when the first fuel F1 and the ammonia fuel F2 are co - burned to be less than 3.5%, the boiler efficiency can be increased.

[0055] (4) The ammonia co-firing boiler 1 described in (1) to (3) includes an ammonia co-firing rate determination unit that determines the co-firing rate of the ammonia fuel. When the ammonia co-firing rate is high, the second oxygen concentration is lower than that when the ammonia co-firing rate is low. The control unit 40 includes a co-firing air ratio storage unit that stores the relationship between the ammonia co-firing rate and the second oxygen concentration. When the combustion state detection unit 44 detects the 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 rate determined by the ammonia co-firing rate determination unit and the relationship between the ammonia co-firing rate and the second oxygen concentration stored in the co-firing air ratio storage unit. As a result, when co-firing the ammonia fuel F2 and the first fuel F1, the air ratio becomes smaller than when solely burning the first fuel F1, and the boiler efficiency can be increased. Also, only when the ammonia co-firing rate is 20% or more, which can suppress the generation of unburned substances such as carbon monoxide and soot (smoke), the air ratio during co-firing is decreased, so the risk of increased generation of unburned substances due to the decrease in the air ratio can be avoided.

[0056] (5) In the ammonia co-firing boiler 1 described in (4), the second oxygen concentration associated with the ammonia co-firing rate in the co-firing air ratio storage unit is set based on the oxygen concentration at which the amount of unburned substances generated per ammonia co-firing rate shows a clear increase, and is equal to or higher than the oxygen concentration at which the amount of unburned substances generated shows a clear increase. As a result, the generation of unburned substances such as carbon monoxide and soot (smoke) can be suppressed, and the risk of increased generation of unburned substances due to the decrease in the air ratio can be avoided.

[0057] When the co-firing of the first fuel F1 and the ammonia fuel F2 is detected by the combustion state detector 44 and the ammonia co-firing ratio is determined by the ammonia co-firing ratio determination unit to be 20% or more, the combustion control unit 42 determines the ammonia co-firing ratio determined by the ammonia co-firing ratio determination unit, and the relationship between the ammonia co-firing ratio stored in the co-firing air ratio storage unit and the second oxygen concentration. Based on this, when the ammonia co-firing ratio with respect to the air ratio during exclusive firing of the first fuel is large, the reduction ratio of the air ratio is made larger than the reduction ratio of the air ratio when the ammonia co-firing ratio with respect to the air ratio during exclusive firing of the first fuel is small, and the air ratio during ammonia co-firing is controlled. Thereby, the boiler efficiency can be increased.

[0058] (7) The control method of the ammonia co-firing boiler 1 of the present embodiment includes a step of controlling the amount of combustion air A1 so that the oxygen concentration of the exhaust gas after exclusive firing of the first fuel F1 becomes the first oxygen concentration when exclusive firing of the first fuel F1 is detected, and a step of controlling the amount of combustion air A1 so that the oxygen concentration of the exhaust gas after co-firing of the first fuel F1 and the ammonia fuel F2 becomes the second oxygen concentration when co-firing of the first fuel F1 and the ammonia fuel F2 is detected. The second oxygen concentration is set based on the relationship between the ammonia co-firing ratio and the oxygen concentration at which a clear increase in the generation of unburned substances due to combustion is shown, and is lower than the first oxygen concentration. Further, there is a step of controlling so that the second oxygen concentration when the ammonia co-firing ratio is large becomes lower than the second oxygen concentration when the ammonia co-firing ratio is small. Thereby, when co-firing the first fuel F1 with a faster combustion rate than ammonia and the ammonia fuel F2, the generation of unburned substances such as carbon monoxide and soot can be suppressed. Also, the oxygen concentration in the exhaust gas becomes smaller than when the first fuel F1 is burned exclusively, and the boiler efficiency can be increased.

[0059] As described above, the preferred embodiments of the ammonia co-firing boiler 1 according to the present invention have been described. However, the present invention is not limited to the above-described embodiments and can be appropriately changed.

[0060] 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 rate sensor 102 and the ammonia fuel flow rate sensor 202. However, the method for determining the ammonia co-firing rate is not limited to this. The ammonia co-firing rate determination unit 43 may calculate the ammonia co-firing rate based on the opening degree of the flow rate adjustment valve 204 as the ammonia fuel detection unit, and thereby detect the ammonia co-firing rate. Further, the ammonia co-firing rate determination unit 43 may calculate and determine the ammonia co-firing rate using the calculation formula stored in the storage unit 41 based on the detection result of the combustion state detection unit 44. The ammonia co-firing rate determination unit 43 may determine the ammonia co-firing rate from the data table of the ammonia co-firing rate corresponding to the detection result of the combustion state detection unit 44 stored in the storage unit 41 based on the detection result of the combustion state detection unit 44. Also, the ammonia co-firing rate determination unit 43 may determine the ammonia co-firing rate by a select switch for setting the ammonia co-firing rate or by numerical input of the ammonia co-firing rate from an input device.

[0061] When the combustion control unit 42 controls combustion with an air ratio corresponding to each of a plurality of ammonia co-firing rates, the co-firing air ratio storage unit may store all of the plurality of ammonia co-firing rates and the air ratios corresponding to them, or may store the air ratio corresponding to the required ammonia co-firing rate by inputting it with an input device or the like before performing the control.

[0062] When the first fuel F1 is a liquid fuel, the ammonia co-firing boiler 1 is preferably operated with the exhaust gas oxygen concentration, which is the second oxygen concentration, less than 4%. When the first fuel F1 is a gaseous fuel, an example is shown in which the ammonia co-firing boiler 1 is preferably operated with the exhaust gas oxygen concentration, which is the second oxygen concentration, less than 3.5%, but it is not limited thereto. By further reducing the exhaust gas oxygen concentration in the ammonia co-firing by 0.5%, the ammonia co-firing boiler 1 can further improve the boiler efficiency, not only reduce the exhaust gas heat loss, but also improve the ammonia effective utilization rate and enhance the carbon dioxide reduction effect. For example, as shown in FIG. 4, in the ammonia co-firing of the ammonia co-firing boiler 1, when the second oxygen concentration in the gaseous fuel is reduced from 3.5% to 3%, the ammonia effective utilization rate is improved from about 94.5% to about 97%. Further, as shown in FIG. 7, in the ammonia co-firing of the ammonia co-firing boiler 1, when the second oxygen concentration in the liquid fuel is reduced from 4% to 3.5%, the ammonia effective utilization rate is improved from about 94.5% to about 98%. Therefore, the ammonia co-firing boiler 1 can more effectively reduce greenhouse gas emissions by further reducing the exhaust gas oxygen concentration in the ammonia co-firing by 0.5%.

[0063] When only the first fuel flow rate sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100, the combustion state detection unit 44 determines that exclusive firing of the first fuel F1 is detected. When the first fuel flow rate sensor 102 detects that the first fuel F1 is flowing through the first fuel supply line 100 and the ammonia fuel flow rate sensor 202 detects that the ammonia fuel F2 is passing through the ammonia supply line 200, the combustion state detection unit 44 determines that co-firing of the first fuel F1 and the ammonia fuel F2 is detected, but it is not limited thereto. Based on the opening instructions to the first shut-off valve 103 and the second shut-off valve 105 of the combustion control unit 42, the combustion state detection unit 44 may determine that the exclusive combustion of the first fuel F1 is detected, and based on the opening instructions to the first shut-off valve 103 and the second shut-off valve 105 of the combustion control unit 42, and the opening instructions to the first shut-off valve 203 and the second shut-off valve 205 of the combustion control unit 42, it may also determine that the mixed combustion of the first fuel F1 and the ammonia fuel F2 is detected. Further, the combustion state detection unit 44 may determine the combustion state based on a select switch for setting the combustion state or an input of the combustion state from an input device.

[0064] The ammonia fuel may be ammonia or a gas fuel mainly composed of ammonia. In addition, the control of the air-fuel ratio (oxygen concentration of the exhaust gas E1) by the combustion control unit described herein is particularly effective for stabilizing the combustion of the ammonia mixed burner and improving the efficiency of the boiler in a small-scale once-through boiler, a small-scale boiler, and a smoke tube boiler where combustion stop and startup are performed relatively frequently.

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

Description of Reference Numerals

[0066] 1 Ammonia mixed boiler 10 Cylinder body 20 Burner 40 Control unit 42 Combustion control unit 43 Ammonia mixed 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 burner to which a first fuel having a combustion rate faster than ammonia, at least one of ammonia fuels, and combustion air are supplied, A combustion air adjustment unit that adjusts the supply amount of the combustion air, A control unit, and The control unit A combustion state detection unit that distinguishes and detects the combustion of the first fuel and the co-combustion of the first fuel and the ammonia fuel, When the exclusive combustion of the first fuel is detected by the combustion state detection unit, the combustion air adjustment unit is controlled so that the exhaust gas oxygen concentration after the exclusive combustion of the first fuel becomes the first oxygen concentration, and when the co-combustion of the first fuel and the ammonia fuel is detected by the combustion state detection unit, the combustion air adjustment unit is controlled so that the exhaust gas oxygen concentration after the co-combustion of the first fuel and the ammonia fuel becomes a second oxygen concentration lower than the first oxygen concentration. A boiler comprising a combustion control unit.

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

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

4. Comprising an ammonia co-combustion rate determination unit that determines the co-combustion rate of the ammonia fuel, The control unit includes a co-combustion air ratio storage unit that stores the relationship between the ammonia co-combustion rate and the second oxygen concentration so that the second oxygen concentration when the ammonia co-combustion rate is high is lower than the second oxygen concentration when the ammonia co-combustion rate is low. When the co-combustion of the first fuel and the ammonia fuel is detected by the combustion state detection unit, the combustion control unit controls the air ratio during ammonia co-combustion based on the relationship between the ammonia co-combustion rate determined by the ammonia co-combustion rate determination unit, the ammonia co-combustion rate stored in the co-combustion air ratio storage unit, and the second oxygen concentration. The boiler according to any one of claims 1 to 3.

5. The second oxygen concentration associated with the ammonia co-combustion rate in the co-combustion air ratio storage unit is set based on the oxygen concentration at which the amount of unburned substances clearly increases for each ammonia co-combustion rate, and is equal to or higher than the oxygen concentration at which the amount of unburned substances clearly increases. The boiler according to claim 4.

6. When the combustion state detection unit detects 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, Based on the ammonia co-combustion rate determined by the ammonia co-combustion rate determination unit and the relationship between the ammonia co-combustion rate stored in the co-combustion air ratio storage unit and the second oxygen concentration, when the ammonia co-combustion rate is large with respect to the air ratio during exclusive combustion of the first fuel, the combustion control unit controls the air ratio during ammonia co-combustion so that the reduction rate of the air ratio is greater than the reduction rate of the air ratio when the ammonia co-combustion rate is small with respect to the air ratio during exclusive combustion of the first fuel. The boiler according to claim 4.

7. A control method for a boiler comprising a burner capable of co-combusting a first fuel having a faster combustion rate than ammonia and at least one of ammonia fuels and combustion air, When exclusive combustion of the first fuel is detected, a step of controlling the amount of the combustion air so that the exhaust gas oxygen concentration after exclusive combustion of the first fuel becomes the first oxygen concentration; When co-combustion of the first fuel and the ammonia fuel is detected, a step of controlling the amount of the combustion air so that the exhaust gas oxygen concentration after co-combustion of the first fuel and the ammonia fuel becomes the second oxygen concentration, The second oxygen concentration is lower than the first oxygen concentration, set based on the relationship between the ammonia co-combustion rate and the oxygen concentration at which an obvious increase in the generation of unburned substances due to combustion is shown, Furthermore, a control method for a boiler, comprising a step of controlling so that the second oxygen concentration when the ammonia co-combustion rate is large becomes lower than the second oxygen concentration when the ammonia co-combustion rate is small.

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