boiler

Abstract

To provide a boiler which can make an air ratio converged within a proper range at any of combustion amounts.SOLUTION: A boiler comprises: a blowing path for sending air to a boiler main body; a fuel supply path for supplying fuel to the boiler main body; a first regulation valve arranged at the fuel supply path, and regulating an opening according to a pressure difference between a first position of the blowing path and a second position at a downstream side rather than the first position; a second regulation valve arranged at a downstream side rather than the first regulation valve in the fuel supply path, and regulating the opening; and a control part for controlling a combustion amount in amounts including the first combustion amount to a second combustion amount which is larger than the first combustion amount, and regulating the opening of the second regulation valve. A reference opening being a reference of the opening of the second regulation valve is defined in advance according to the combustion amount which is controlled by the control part, and the control part controls the opening of the second regulation valve on the basis of the reference opening which is defined according to the controlled combustion amount.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a boiler. [Background technology]

[0002] Conventionally, there are boilers equipped with a mechanical proportional valve that adjusts the opening based on the flow rate (pressure) of combustion air on a fuel supply line that supplies fuel to a nozzle (see, for example, Patent Document 1). Mechanical proportional valves have fewer parts than electronic proportional valves, making them less prone to failure and safer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-2787 Summary of the Invention [Problem to be solved by the invention]

[0004] Mechanical proportional valves installed in conventional boilers typically have a maximum opening angle set to provide an appropriate air ratio for the combustion air flow rate at the maximum combustion rate, and a minimum opening angle set to provide an appropriate air ratio for the combustion air flow rate at the minimum combustion rate. On the other hand, for intermediate combustion rates greater than the minimum combustion rate but less than the maximum combustion rate, the opening angle is set between the preset minimum and maximum opening angles according to the actual combustion air flow rate, leaving the air ratio to be determined by the actual combustion air flow rate. As a result, for intermediate combustion rates, the air ratio cannot be kept within the appropriate range. This can result in a low air ratio that exceeds a certain value, leading to nitrogen oxide (NOx) concentrations in the exhaust gas (hereinafter sometimes referred to as "NOx values"), or a high air ratio that exceeds the flammability limit (threshold), resulting in a fire. Particularly given the growing environmental concern in recent years, there is a need to ensure that the air ratio remains within the appropriate range at all possible combustion rates in order to reduce air pollution by regulating the emission concentrations and emissions of air pollutants such as NOx.

[0005] The present invention has been devised in view of the above circumstances, and its object is to provide a boiler that can converge the air ratio within an appropriate range regardless of the combustion amount. [Means for solving the problem]

[0006] In order to achieve the above object, a boiler according to one aspect of the present invention includes an air passage for sending air into a boiler body, a fuel supply passage for supplying fuel to the boiler body, a first adjusting valve provided in the fuel supply passage and adjusting its opening degree in accordance with a pressure difference between a first position of the air passage and a second position downstream of the first position, a second adjusting valve provided in the fuel supply passage downstream of the first adjusting valve and adjusting its opening degree, and a valve for controlling a combustion amount between a first combustion amount and a second combustion amount greater than the first combustion amount. and controls the supply of air to the boiler body in an amount corresponding to the controlled combustion amount. and a control unit that controls the opening degree of the second adjusting valve, wherein a reference opening degree that serves as a basis for the opening degree of the second adjusting valve is determined in advance according to the combustion amount controlled by the control unit, and the control unit controls the opening degree of the second adjusting valve based on the reference opening degree that is determined according to the combustion amount to be controlled.

[0007] According to the above configuration, while using a first regulating valve that adjusts its opening according to the pressure difference, the standard opening of the second regulating valve is set to an opening that puts the air ratio within the appropriate range according to the combustion amount, and by adjusting the flow rate of fuel supplied to the boiler body using the second regulating valve, the air ratio can be converged into the appropriate range regardless of the combustion amount.

[0008] Preferably, the first regulating valve is predetermined to have a first opening degree at a first differential pressure between the first position and the second position that can be obtained when the first regulating valve is operated under control at the first combustion amount, and a second opening degree at a second differential pressure between the first position and the second position that can be obtained when the first regulating valve is operated under control at the second combustion amount, and when the second regulating valve is operated under control at a third combustion amount between the first combustion amount and the second combustion amount, the second regulating valve has an opening degree between the first opening degree and the second opening degree that corresponds to the differential pressure between the first position and the second position, and the reference opening degree of the second regulating valve is a third opening degree that is determined according to the first combustion amount, a fourth opening degree that is determined according to the second combustion amount, and a fourth opening degree that is determined according to the third combustion amount. A fifth opening degree is determined according to the combustion amount, and the first opening degree of the first regulating valve and the third opening degree of the second regulating valve are determined to be opening degrees that will result in a specific state where the air ratio is below a threshold value and the nitrogen oxide concentration in the exhaust gas is below a predetermined value when the valves are controlled to operate at the first combustion amount, the second opening degree of the first regulating valve and the fourth opening degree of the second regulating valve are determined to be opening degrees that will result in the specific state when the valves are controlled to operate at the second combustion amount, and the fifth opening degree of the second regulating valve is determined to be opening degrees that will result in the specific state when the valves are controlled to operate at the third combustion amount, even if the opening degree of the first regulating valve is an opening degree that corresponds to the pressure difference between the first position and the second position.

[0009] According to the above configuration, not only when the combustion amount is controlled to the first combustion amount and the second combustion amount but also when the combustion amount is controlled to the third combustion amount, the opening degree of the first adjusting valve can be controlled to correspond to the differential pressure, while the second adjusting valve can be controlled to a specific state.

[0010] Preferably, a reference opening degree different from the reference opening degrees determined according to each of the first combustion amount and the second combustion amount is determined for a third combustion amount between the first combustion amount and the second combustion amount.

[0011] According to the above configuration, by determining the standard opening corresponding to the third combustion amount regardless of the standard opening corresponding to each of the first combustion amount and the second combustion amount, it becomes possible to precisely adjust the air ratio to a more appropriate value according to the combustion amount.

[0012] Preferably, the reference opening determined according to the first combustion amount and the reference opening determined according to the second combustion amount are both determined to be an opening that is neither fully closed nor fully open.

[0013] According to the above configuration, the reference opening corresponding to the third combustion amount can be made larger or smaller than the reference openings determined for the first combustion amount and the second combustion amount, respectively, allowing for more flexible adjustment. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram illustrating a schematic configuration of a boiler. [Figure 2] (a) and (b) show an example in which the air ratio is not within the appropriate range during the intermediate combustion stage when the fuel flow rate is adjusted using only the first regulating valve, and (c) and (d) show an example in which the air ratio is within the appropriate range during all combustion stages, including the intermediate combustion stage, when the fuel flow rate is adjusted using the first regulating valve and the second regulating valve. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. First, a schematic configuration of a boiler according to this embodiment will be described with reference to FIG.

[0016] 1, the boiler 1 includes a boiler body 2, a blower 3 that sends air into the combustion chamber of the boiler body 2 through an air passage 30, an exhaust passage 4 that discharges exhaust gas from the boiler body 2, a fuel supply passage 5 that supplies fuel to the boiler body 2, a water supply line (not shown) that supplies water to the boiler body 2, and a control unit 6 that controls the operation and behavior of the boiler 1. Note that, although an example in which the fuel is gas will be described, the fuel is not limited to a gas such as gas, and may be a liquid such as oil.

[0017] The fuel supply passage 5 is connected to the air passage 30. The fuel supplied from the fuel supply passage 5 is mixed with air blown from the blower 3 in the air passage 30 and supplied to the burner 20 in the boiler body 2.

[0018] Air supplied from blower 3 is supplied as combustion air to burner 20 in boiler body 2 via air duct 30. The flow rate of combustion air is adjusted by providing damper 7 in air duct 30 and adjusting the position (opening degree) of damper 7, or alternatively or additionally by changing the rotation speed of the fan of blower 3 using an inverter.

[0019] The control unit 6 controls the combustion stage to one of a plurality of combustion stages with different combustion amounts, for example, a low combustion stage (first combustion amount), a medium combustion stage (third combustion amount), or a high combustion stage (second combustion amount). The control unit 6 controls the combustion stage according to the set target steam pressure and the steam pressure in the steam header. The control unit 6 controls the blower 3 and damper 7 so that the conditions (for example, rotation speed, frequency, and opening) correspond to the controlled combustion stage, and supplies the amount of combustion air according to the combustion stage.

[0020] The fuel supply passage 5 is provided with an on-off valve 11 (solenoid valve) for opening and closing the passage, a first regulating valve 12 for adjusting the flow rate of the fuel to be supplied, and a bypass line downstream of the first regulating valve 12, in which an orifice 16 and a second regulating valve 17 are provided in parallel. The first regulating valve 12 functions as a regulating valve that can adjust the flow rate of the fuel to be supplied to the boiler body 2 according to the flow rate of the combustion air to be supplied, and also has a shutoff function. In this embodiment, the first regulating valve 12 is exemplified by a governor that is mechanically and automatically adjusted to an opening degree according to the flow rate of the combustion air to be supplied, but is not limited to this as long as the regulating valve is adjusted to an opening degree according to the flow rate of the combustion air to be supplied.

[0021] In this embodiment, a combustion air decompression member 8 such as a punched metal is provided in the air passage 30 downstream of the damper 7. At a first position P1 upstream of the combustion air decompression member 8, the air passage 30 communicates with a first adjustment valve 12 provided on the fuel supply passage 5 via a first communication passage 14. At a second position P2 downstream of the combustion air decompression member 8, the air passage 30 communicates with the first adjustment valve 12 provided on the fuel supply passage 5 via a second communication passage 15.

[0022] The first adjusting valve 12 is configured so that its opening degree changes according to the pressure difference between the first communication passage 14 and the second communication passage 15 (which is basically the same as the pressure difference before and after the combustion air decompression member 8 in the air passage 30). The first adjusting valve 12 is a pressure equalizing valve whose opening degree is mechanically adjusted so that the introduced pressure difference and the pressure of the supplied fuel (secondary pressure) are equalized. A branch passage 13 is provided from the flow path downstream of the governor. The first adjusting valve 12 can adjust its opening degree so that the secondary pressure obtained from the branch passage 13 corresponds to the pressure difference before and after the combustion air decompression member 8 in the air passage 30 to which the fuel is introduced. As a result, if the pressure difference before and after the combustion air decompression member 8 increases, the flow rate of fuel that can pass through the first adjusting valve 12 increases, and if the pressure difference before and after the combustion air decompression member 8 decreases, the flow rate of fuel that can pass through the first adjusting valve 12 decreases.

[0023] Regarding a regulating valve that adjusts its opening according to a pressure difference, such as the first regulating valve 12 in this embodiment, the minimum and maximum openings of the regulating valve are generally set to an initial setting (also called zero span) so that when the boiler is operated at a minimum combustion amount (e.g., a low combustion stage), the minimum and maximum openings are set to a value that supplies fuel such that the air ratio (also called the air-fuel ratio) is within an appropriate range for the flow rate of combustion air at the pressure difference between the first position P1 and the second position P2. When the boiler is operated at a maximum combustion amount (e.g., a high combustion stage), the minimum and maximum openings are set to a value that supplies fuel such that the air ratio is within an appropriate range at the pressure difference between the first position P1 and the second position P2. This allows combustion at an appropriate air ratio at the minimum and maximum combustion amounts. The appropriate air ratio refers to an air ratio that is neither too low, which would cause the NOx concentration in the exhaust gas to exceed a certain value, nor too high, which would cause a misfire due to the combustion limit being exceeded. The state in which the air ratio is appropriate is also referred to as a specific state.

[0024] In contrast, when the boiler is operated by controlling it to an intermediate combustion volume between the minimum and maximum combustion volumes (for example, at the intermediate combustion stage), the control valve opening is left to chance based on the initially set minimum and maximum openings, the combustion air flow rate at the minimum and maximum combustion volumes, and the actual pressure difference at that intermediate combustion volume, and as a result, the air ratio is also left to chance.The control valve opening when operated by controlling it to an intermediate combustion volume is, for example, a value calculated from (maximum opening - minimum opening) ÷ (combustion air flow rate at maximum combustion volume - combustion air flow rate at minimum combustion volume) x (combustion air flow rate at intermediate combustion volume - combustion air flow rate at minimum combustion volume) + minimum opening. Therefore, when the combustion amount is intermediate, the air ratio cannot be brought within the appropriate range, and there is a risk that the air ratio will be too low, causing the NOx concentration in the exhaust gas to exceed a certain value, or that the air ratio will be too high, exceeding the combustion limit and causing a misfire.

[0025] A specific example will now be described with reference to Figures 2(a) and 2(b). Figures 2(a) and 2(b) show an example in which the air ratio during the medium combustion stage is not within the appropriate range when the fuel flow rate is adjusted only by a governor such as the first adjusting valve 12. In Figures 2(a) and 2(b), the horizontal axis shows the high combustion stage, the medium combustion stage, and the low combustion stage, and the vertical axis shows the combustion amount for each combustion stage, the pressure difference when operating at that combustion amount, the governor opening corresponding to the pressure difference, and the air ratio. In Figure 2, the combustion amount during the high combustion stage is 100%, the combustion amount during the medium combustion stage is 60%, and the combustion amount during the low combustion stage is 25%. 2(a) and 2(b), when the boiler is controlled to operate in the high combustion stage, the possible differential pressure between the first position P1 and the second position P2 is "a," and the maximum opening of the first regulating valve that supplies fuel with an air ratio in the appropriate range for "a" is initially set to "80%." When the boiler is controlled to operate in the low combustion stage, the possible differential pressure between the first position P1 and the second position P2 is "c," and the minimum opening of the first regulating valve that supplies fuel with an air ratio in the appropriate range for "c" is initially set to "30%." Note that the percentage of opening indicates the ratio of the valve to full opening.

[0026] Under such premises, in Fig. 2(a), when the boiler is controlled to operate in the medium combustion stage, the differential pressure becomes "b1", and the opening degree of the first regulating valve becomes "58%", which is an opening degree that changes according to the "b1" without any further adjustment. As a result, the fuel supplied is too much compared to the flow rate of the combustion air being supplied, and there is a risk that the air ratio is too low and the concentration of NOx in the exhaust gas exceeds a certain value. This is an example of such a case. Also, in Fig. 2(b), when the boiler is controlled to operate in the medium combustion stage, the differential pressure becomes "b2" (for example, b2 < b1), and the opening degree of the first regulating valve becomes "50%", which is an opening degree that changes according to the "b2" without any further adjustment. As a result, the fuel supplied is too little compared to the flow rate of the combustion air being supplied, and there is a risk that the air ratio is too high and combustion exceeds the limit and misfires. This is an example of such a case. Thus, when adjusting the fuel flow rate only with a governor such as the first regulating valve 12, although the opening degree of the first regulating valve can be initially set so that the air ratio is within the appropriate range in the high combustion stage and the low combustion stage, in the intermediate combustion stage, it becomes an opening degree that changes according to the initially set minimum and maximum opening degrees, the flow rate of the combustion air at the minimum and maximum combustion amounts, and the actual differential pressure at the intermediate combustion amount, etc., and the air ratio also changes without any further adjustment. As a result, it cannot always be said that the air ratio can be converged within the appropriate range.

[0027] Therefore, in the present embodiment, the maximum flow rate of the fuel that can be supplied (passed) by the first regulating valve 12 is adjusted according to the differential pressure, a second regulating valve 17 is provided on the downstream side of the first regulating valve 12, and the opening degree of the second regulating valve 17 is adjusted to a reference opening degree according to the combustion amount. Specifically, the minimum and maximum opening degrees of the first regulating valve 12 are initially set to larger opening degrees (opening degrees that allow more fuel to pass compared to the flow rate of the combustion air being supplied) when adjusting only with the first regulating valve 12, and by adjusting the opening degree of the second regulating valve 17 to an opening degree according to the combustion amount, the flow rate of the fuel supplied to the boiler body 2 among the fuel passing through the first regulating valve 12 is adjusted. As the reference opening degree of the second regulating valve 17 according to the combustion amount, an opening degree at which the air ratio is within the appropriate range is predetermined when the opening degree of the first regulating valve 12 becomes an opening degree according to the actual differential pressure.

[0028] Regarding the initial setting of the first regulating valve 12, when the boiler 1 is operated by controlling it to the minimum combustion amount (e.g., low combustion stage), the minimum opening is initially set to an opening that allows a fuel flow rate greater than the fuel flow rate at which the air ratio is within the appropriate range at the differential pressure between the first position P1 and the second position P2 that can be achieved at that time.When the boiler 1 is operated by controlling it to the maximum combustion amount (e.g., high combustion stage), the maximum opening is initially set to an opening that allows a fuel flow rate greater than the fuel flow rate at which the air ratio is within the appropriate range at the differential pressure between the first position P1 and the second position P2 that can be achieved at that time.When the boiler 1 is operated by controlling it to the intermediate combustion stage, the opening is set to a value that depends on the initially set minimum and maximum openings, the combustion air flow rate at the minimum and maximum combustion amounts, the actual differential pressure at the intermediate combustion stage, etc. Based on these assumptions, a standard opening degree for the second regulating valve 17 is determined in advance for each combustion amount (high combustion stage, medium combustion stage, low combustion stage) so that the air ratio during combustion in the boiler 1 is within an appropriate range, and the control unit 6 controls the opening degree of the second regulating valve 17 based on the standard opening degree determined in advance according to the combustion amount to be actually controlled. Note that the standard opening degree of the second regulating valve 17 for each combustion amount is calculated and specified in advance at the design stage or installation stage of the boiler, and is stored in advance in the memory unit of the control unit 6, etc.

[0029] Here, a specific example will be described with reference to Figures 2(c) and 2(d). Figures 2(c) and 2(d) show an example in which the air ratio is within the appropriate range in all combustion stages, including the intermediate combustion stage, when the fuel flow rate is adjusted by the first regulating valve 12 and the second regulating valve 17. Figure 2(c) shows an example in which the air ratio is converged within the appropriate range by adjusting the aperture of the second regulating valve 17, as compared to the example shown in Figure 2(a). Figure 2(d) shows an example in which the air ratio is converged within the appropriate range by adjusting the aperture of the second regulating valve 17, as compared to the example shown in Figure 2(b). Note that Figures 2(c) and 2(d) show the aperture of the second regulating valve 17 in addition to the items on the vertical axis shown in Figure 2(a) and other figures.

[0030] In Figures 2(c) and 2(d), when the boiler is controlled to operate in the low combustion stage, the minimum opening of the first regulating valve 12 is initially set to "40%" (first opening) for the differential pressure "c," and when the boiler is controlled to operate in the high combustion stage, the maximum opening of the first regulating valve 12 is initially set to "100%" (second opening) for the differential pressure "a." In this way, larger openings are initially set compared to Figure 2(a) and other figures. In addition, when the boiler is controlled to operate in the low combustion stage, the opening of the second regulating valve 17 is set to "75%" (third opening), and when the boiler is controlled to operate in the high combustion stage, the opening of the second regulating valve 17 is set to "80%" (fourth opening). The opening degree of the second regulating valve 17, "75%," is the opening degree at which the air ratio during combustion in the boiler 1 falls within the appropriate range when the boiler is controlled to operate in the low combustion stage and the opening degree of the first regulating valve 12 is set to the minimum opening degree of "40%." The opening degree of the second regulating valve 17, "80%," is the opening degree at which the air ratio during combustion in the boiler 1 falls within the appropriate range when the boiler is controlled to operate in the high combustion stage and the opening degree of the first regulating valve 12 is set to the maximum opening degree of "100%." For this reason, Figures 2(c) and 2(d) show that the air ratios in the low combustion stage and the high combustion stage are both within the appropriate range.

[0031] On the other hand, in Fig. 2(c), when the boiler 1 is operated under control in the medium combustion stage, as also illustrated in Fig. 2(a), the differential pressure becomes "b1", and an example is shown where the opening degree of the first regulating valve 12 becomes "77%", which is an opening degree left to follow according to the "b1". Further, in Fig. 2(d), when the boiler 1 is operated under control in the medium combustion stage, as also illustrated in Fig. 2(c), the differential pressure becomes "b2" (for example, b2 < b1), and an example is shown where the opening degree of the first regulating valve 12 becomes "70%", which is an opening degree left to follow according to the "b2". Even when the opening degree of the first regulating valve 12 becomes an opening degree left to follow in this way, as the opening degree of the second regulating valve 17 corresponding to the medium combustion stage, an opening degree (the fifth opening degree) at which the air-fuel ratio during combustion in the boiler 1 is within the appropriate range is determined. Specifically, in the case of Fig. 2(c), "71%" is determined as the reference opening degree of the second regulating valve 17 corresponding to the medium combustion stage, indicating that the air-fuel ratio in the medium combustion stage is within the appropriate range. Also, in the case of Fig. 2(d), "78%" is determined as the reference opening degree of the second regulating valve 17 corresponding to the medium combustion stage, indicating that the air-fuel ratio in the medium combustion stage is within the appropriate range. Thereby, as shown in Fig. 2(c) and Fig. 2(d), it is shown that not only in the low combustion stage and the high combustion stage, but also the air-fuel ratio in the medium combustion stage is within the appropriate range.

[0032] As described above, in the present embodiment, as shown in Fig. 1, while using the first regulating valve 12 that adjusts the opening degree according to the differential pressure, the second regulating valve 17 is provided on the downstream side of the first regulating valve 12. As illustrated in Fig. 2(c) and Fig. 2(d), the reference opening degree of the second regulating valve 17 is determined to be an opening degree at which the air-fuel ratio becomes within the appropriate range according to the combustion amount (combustion stage), and the flow rate of the fuel supplied to the boiler 1 is adjusted by the second regulating valve 17. Thereby, the air-fuel ratio can be converged within the appropriate range at any combustion amount.

[0033] 2(c) and 2(d), the minimum opening of the first regulating valve 12 and the reference opening of the second regulating valve 17 corresponding to the low combustion stage are set to openings that keep the air ratio within the appropriate range when controlled and operated in the low combustion stage. The maximum opening of the first regulating valve 12 and the reference opening of the second regulating valve 17 corresponding to the high combustion stage are set to openings that keep the air ratio within the appropriate range when controlled and operated in the high combustion stage. Furthermore, the reference opening of the second regulating valve 17 corresponding to the medium combustion stage is set to openings that keep the air ratio within the appropriate range when controlled and operated in the medium combustion stage, even if the opening of the first regulating valve 12 is left to chance depending on the pressure difference. As a result, not only when the combustion chamber is controlled to operate at the low combustion stage or the high combustion stage, but also when the combustion chamber is controlled to operate at the medium combustion stage, the opening of the first regulating valve 12 is left to chance according to the pressure difference, but the second regulating valve 17 can be used to converge the air ratio to within the appropriate range.

[0034] In this embodiment, the reference opening for the second adjusting valve 17 corresponding to the intermediate combustion stage is set to "71%" in FIG. 2(c) and "78%" in FIG. 2(d). These values are different from the reference opening for the low combustion stage, "75%," and the reference opening for the high combustion stage, "80%," respectively, in FIG. 2(c) and FIG. 2(d). In this way, a reference opening that is independent of the reference openings for the low combustion stage and the high combustion stage is set as the reference opening corresponding to the intermediate combustion stage, so that the air ratio can be precisely adjusted to a more appropriate value according to the combustion amount.

[0035] In this embodiment, as shown in FIGS. 2(c) and 2(d), the reference openings of the second adjusting valve 17 corresponding to the low combustion stage and the high combustion stage are set to 75% and 80%, respectively, neither of which is fully closed nor fully open. Therefore, the reference opening corresponding to the intermediate combustion stage can be set larger or smaller than the reference openings for the low combustion stage and the high combustion stage. For example, the reference openings are set to 71% in FIG. 2(c) and 78% in FIG. 2(d), allowing for more flexible adjustment. For example, the reference openings corresponding to the low combustion stage and the high combustion stage could be set to 100%, but in that case, the reference opening corresponding to the intermediate combustion stage could only be adjusted to a smaller opening than the reference openings corresponding to the low combustion stage and the high combustion stage. This avoids this restriction.

[0036] The present invention is not limited to the above-described embodiment, and various modifications and applications are possible. Modifications of the above-described embodiment that can be applied to the present invention will be described below.

[0037] In the above embodiment, an example has been described in which second regulating valve 17 is controlled to an opening based on a reference opening corresponding to the combustion amount. However, this opening may also be corrected by control device 6 as needed depending on the operating environment, circumstances, and the like during operation. Control device 6 may, for example, continually detect the air ratio and the NOx concentration in the exhaust gas, and adjust the opening of second regulating valve 17 controlled according to the combustion amount based on the detected air ratio and NOx concentration in the exhaust gas. For example, control device 6 may increase the fuel supply amount when the air ratio is high and the combustion limit is approaching, and decrease the fuel supply amount when the NOx concentration in the exhaust gas is high because the air ratio is low. Alternatively, or in addition, control device 6 may adjust the opening of second regulating valve 17 controlled according to the combustion amount depending on, for example, the fuel temperature, the combustion air temperature, and the like.

[0038] In the above embodiment, the boiler 1 has been described as an example in which the boiler is controlled to one of a number of combustion stages with different combustion amounts (so-called multi-position control) depending on the set target steam pressure and the steam pressure in the steam header, but this is not limited to this, and the boiler may be proportionally controlled to achieve an arbitrary combustion amount depending on the set target steam pressure and the steam pressure in the steam header.

[0039] In the above embodiment, an orifice 16 is provided in parallel with the second regulating valve 17. It is desirable that this orifice 16 has a pressure loss that falls within a range where safe combustion is possible (or the air ratio is within an appropriate range) even if the second regulating valve 17 fails when the valve is fully open or fully closed. This allows the boiler 1 to continue operating even if the second regulating valve 17 fails.

[0040] In the above embodiment, an example has been described in which second adjusting valve 17 is controlled to an opening degree based on a reference opening degree corresponding to the combustion amount, but the opening degree of second adjusting valve 17 at the time of ignition is desirably set larger than both the reference opening degree determined according to the low combustion stage and the reference opening degree determined according to the high combustion stage. Since ignition is difficult when the air ratio is high, misfires can be prevented by increasing the opening degree of second adjusting valve 17 at the time of ignition to temporarily increase the amount of fuel supplied.

[0041] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0042] 1 boiler 2 Boiler body 3. Blower 4 Exhaust passage 5 Fuel supply path 6 Control Unit 7 Damper 8 Combustion air pressure reducing element 11 On-off valve 12 First adjusting valve 13 Fork in the Road 14 1st communication passage 15 2nd communication passage 16 Orifice 17 Second adjusting valve 20 Burner 30 Air duct

Claims

1. an air duct for sending air into the boiler body; a fuel supply passage for supplying fuel to the boiler body; a first adjusting valve provided in the fuel supply passage and configured to adjust an opening degree in accordance with a pressure difference between a first position of the air blow passage and a second position downstream of the first position; a second adjusting valve provided downstream of the first adjusting valve in the fuel supply passage and configured to adjust an opening degree of the second adjusting valve; a control unit that controls a combustion amount within a range including a first combustion amount and a second combustion amount greater than the first combustion amount, controls so that an amount of air corresponding to the controlled combustion amount is supplied to the boiler body, and controls an opening degree of the second adjustment valve, a reference opening degree serving as a reference for the opening degree of the second adjusting valve is determined in advance according to a combustion amount controlled by the control unit, The control unit controls the opening degree of the second adjusting valve based on a reference opening degree determined according to a combustion amount to be controlled.

2. the first adjusting valve is predetermined to have a first opening degree at a first differential pressure between the first position and the second position when operated under control at the first combustion amount, and a second opening degree at a second differential pressure between the first position and the second position when operated under control at the second combustion amount, and when operated under control at a third combustion amount between the first combustion amount and the second combustion amount, the first adjusting valve has an opening degree between the first opening degree and the second opening degree and corresponding to the differential pressure between the first position and the second position, a third reference opening degree of the second adjusting valve is determined in accordance with the first combustion amount, a fourth reference opening degree is determined in accordance with the second combustion amount, and a fifth reference opening degree is determined in accordance with the third combustion amount, a first opening degree of the first regulating valve and a third opening degree of the second regulating valve are set to opening degrees that, when operated under control at the first combustion amount, result in a specific state in which the air ratio is equal to or less than a threshold value and the nitrogen oxide concentration in the exhaust gas is equal to or less than a predetermined value; a second opening degree of the first adjusting valve and a fourth opening degree of the second adjusting valve are determined to be opening degrees that result in the specific state when the valves are operated under control at the second combustion amount, 2. The boiler of claim 1, wherein the fifth opening degree of the second adjusting valve is set to an opening degree that results in the specific state when the boiler is operated by controlling the third combustion amount, even if the opening degree of the first adjusting valve is an opening degree that corresponds to the pressure difference between the first position and the second position.

3. 3. The boiler according to claim 1, wherein a third combustion amount between the first combustion amount and the second combustion amount is set to a standard opening different from the standard openings set according to each of the first combustion amount and the second combustion amount.

4. A boiler as described in any one of claims 1 to 3, wherein the reference opening determined according to the first combustion amount and the reference opening determined according to the second combustion amount are both determined to be an opening that is neither fully closed nor fully open.

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