JP5277849B2 - Control program, control system, boiler system, and control method - Google Patents

Abstract

Disclosed is a control program for controlling combustion of a boiler group consisting of a plurality of boilers in which the quantity of evaporation can be controlled at stepwise combustion positions. Also provided is a control system and a boiler system. In a control program performing control of a boiler system comprising a boiler group consisting of a plurality of boilers in which the quantity of evaporation can be controlled at stepwise combustion positions and arranged to perform combustion control based on a demanded load, when combustion of any boiler among the boiler group is started, backup control for temporarily raising the combustion position of any one of those boilers which are already under combustion is possible, and when the quantity of evaporation increased by raising the combustion position of a boiler capable of backup control goes above the shortage quantity of evaporation, the timing for starting backup control is controlled depending on the rate of change of at least one physical quantity corresponding to the quantity of evaporation.

Description

  The present invention relates to a control program, a control system, and a boiler system for controlling combustion of a boiler group including a plurality of boilers whose evaporation amount can be controlled at stepwise combustion positions.

  Conventionally, when starting the combustion of any boiler in a group of boilers composed of a plurality of boilers whose evaporation amount can be controlled at stepwise combustion positions and increasing the evaporation amount, There is a case where backup control is performed in order to increase the evaporation amount of the boiler that has already been combusted and to compensate for the steaming time lag of the boiler that starts combustion until the predetermined evaporation amount is reached and steaming becomes possible. is there.

In backup control, it takes time to start combustion and raise the water in the can to the vicinity of the boiling point in a boiler in a combustion stopped state, whereas in a boiler during combustion, the boiler has water near the boiling point. Therefore, this is heated to increase the evaporation amount in a short time.
With regard to such backup control, for example, as shown in Patent Document 1, a technique is disclosed in which followability is improved by performing backup control by increasing the combustion position of a boiler in a low combustion state in a boiler group including three-position control boilers. .
JP-A-1-256704

  However, for example, when performing backup control using a boiler with a large turndown ratio, the amount of evaporation by backup control becomes excessive with respect to the insufficient amount of evaporation, and as a result, the amount of evaporation controlled by backup may cause hunting.

  The present invention has been made in consideration of such circumstances, and when performing backup control in a boiler group composed of a plurality of boilers whose evaporation amount can be controlled at stepwise combustion positions, An object of the present invention is to provide a control program, a control system, and a boiler system capable of stable control while suppressing hunting.

In order to solve the above problems, the present invention proposes the following means.
The invention according to claim 1 includes a boiler group composed of a plurality of boilers whose evaporation amount can be controlled at stepwise combustion positions, and is configured to be controlled in combustion based on a required load. A control program for controlling a boiler system, and when starting combustion of any one of the boiler groups, a backup for temporarily raising the combustion position of any of the boilers that are already in combustion When the amount of evaporation that is increased by increasing the combustion position of the boiler that can be controlled and the backup control is higher than the insufficient evaporation amount, the start timing of the backup control is set to the evaporation amount. It is configured to control in accordance with the speed at which the corresponding at least one physical quantity changes.

  A fourth aspect of the present invention is a control system, comprising the control program according to any one of the first to third aspects.

  The invention described in claim 5 is a boiler system, and includes the control system described in claim 4.

The invention described in claim 6 includes a boiler group composed of a plurality of boilers whose evaporation amount can be controlled at a stepwise combustion position, and is configured to be controlled in combustion based on a required load. A control method for controlling a boiler system, wherein, when starting combustion of any one of the boiler groups, a backup position for temporarily raising the combustion position of any of the boilers that are already in combustion In the case where each evaporation amount that is controlled and can be increased by increasing the combustion position of the boiler capable of the backup control is equal to or greater than the insufficient evaporation amount,
The start timing of the backup control is configured to be controlled according to a speed at which at least one physical quantity corresponding to the evaporation amount changes.

  According to the control program, control method, control system, and boiler system according to the present invention, since the start timing of the backup control is controlled according to the speed at which the physical quantity corresponding to the evaporation amount changes, the evaporation amount may be excessive. As a result, hunting and the like can be suppressed and stable control can be performed.

Invention of Claim 2 is a control program of Claim 1, Comprising: When the said speed is more than predetermined value, the said backup control is performed,
(Insufficient evaporation) ≤ (Evaporation increased by increasing the combustion position of the boiler capable of backup control)
It has the 1st backup control means controlled by the minimum number of boilers which satisfy the above.

  The invention according to claim 7 is the control method according to claim 6, wherein when the speed is equal to or higher than a predetermined value, the backup control is performed by (insufficient evaporation amount) ≦ (the boiler capable of backup control). The first backup control means for controlling by the minimum number of boilers satisfying the evaporation amount increasing by increasing the combustion position) is provided.

According to the control program and the control method of the present invention, when the amount of change in the physical quantity corresponding to the evaporation amount is equal to or greater than a predetermined value and the required evaporation amount is large, the evaporation amount by the backup control is set to the insufficient evaporation amount or more. By doing so, it is possible to minimize the excessive evaporation amount while reducing the required evaporation amount in a short time, and to suppress the occurrence of overshoot and hence hunting.
In this specification, the insufficient evaporation amount includes a difference in evaporation amount obtained by actually subtracting the steam supply amount from the steam supply possible evaporation amount at the combustion position where combustion is instructed.

Invention of Claim 3 is a control program of Claim 1, Comprising: When the said speed is below a predetermined value, the said backup control is performed,
(Insufficient evaporation amount) ≥ (Evaporation amount increased by increasing the combustion position of the boiler capable of backup control)
The second backup control means for controlling by the maximum number of boilers satisfying the above is provided.

  The invention according to an eighth aspect is the control method according to the sixth aspect, wherein when the speed is equal to or lower than a predetermined value, the backup control is performed by (an insufficient evaporation amount) ≧ (a boiler capable of backup control). A second backup control means for controlling by the maximum number of boilers satisfying the increased evaporation amount by increasing the combustion position) is provided.

  According to the control program and the control method according to the present invention, when the amount of change in the physical quantity corresponding to the evaporation amount is equal to or less than a predetermined value and the required evaporation amount is small, the evaporation amount by the backup control is set to be less than the insufficient evaporation amount. However, by minimizing the shortage with respect to the required evaporation amount, the speed at which the insufficient evaporation amount increases can be reduced.

  According to the control program, control system, and boiler system according to the present invention, stable backup control can be performed while suppressing occurrence of hunting in backup control.

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a diagram showing an embodiment of a control system according to the present invention, and reference numeral 1 denotes a boiler system.

The boiler system 1 includes a boiler group 2 composed of a plurality of boilers, a control unit (control system) 4, a steam header 6, and a pressure sensor 7 provided in the steam header 6, and is generated in the boiler group 2. The generated steam is supplied to the steam use facility 18.
The required load in this embodiment is the amount of steam consumed in the steam using facility 18, and the pressure (physical quantity) of the steam in the steam header 6 in which the pressure sensor 7 detects the insufficient amount of steam generated corresponding to this consumed steam amount. It calculates based on P, and the evaporation amount of each boiler is controlled based on the calculated insufficient evaporation amount.

The boiler group 2 includes, for example, five steam boilers, and includes a first boiler 21, a second boiler 22, a third boiler 23, a fourth boiler 24, and a fifth boiler 25.
The first boiler 21,..., The fifth boiler 25 have the same configuration and are in three stages: a combustion stop state, a low combustion state (first combustion position), and a high combustion state (second combustion position). The three-position control boiler can be controlled to a proper combustion position.
Further, in combustion control such as combustion start and backup control, the first boiler 21,..., The fifth boiler 25 are prioritized in this order.
In this embodiment, for example, the control program preferentially performs combustion at the first combustion position over combustion at the second combustion position. That is, until all the boilers start combustion, none of the boilers is shifted to the second combustion position.
Note that the combustion start and the combustion position may be increased according to an order other than the set priority order.

  In addition, the amount of steam that can be generated at each combustion position (evaporation amount) is set for each boiler. For example, the evaporation amount at the first combustion position that increases when combustion is started is 1000 (Kg / h). The increase in the amount of evaporation when shifting to the second combustion position is 2000 kg / h, and the amount of evaporation in the second combustion position, that is, the evaporation capacity is 3000 (kg / h).

  The steam header 6 is connected to the first boiler 21,..., The fifth boiler 25 and the steam pipe 11 and is connected to the steam using equipment 18 and the steam pipe 12, and steam generated in the boiler group 2 is connected to the steam header 6. The steam is supplied to the steam using equipment 18 by adjusting the pressure difference and pressure fluctuation between the boilers.

  The control unit 4 includes an input unit 4A to which an electrical signal is input, a calculation unit 4B, a database 4D, and an output unit 4E. The control unit 4 stores a control program stored in a storage medium (for example, ROM) (not shown). 4B, the evaporation amount is calculated in the calculation unit 4B, the boiler to be controlled for combustion and the combustion position are specified, and a control signal is output to each boiler via the output unit 4E.

The input unit 4A is connected to the pressure sensor 7 by a signal line 13, and a pressure signal detected by the pressure sensor 7 is input thereto.
In addition, the input unit 4A is connected to each boiler by a signal line 16, and information such as a combustion position is input from each boiler.

The calculation unit 4B reads the control program stored in the storage medium, executes the control program, and based on the pressure signal from the pressure sensor 7 sent via the input unit 4A, the steam pressure P in the steam header 6 And the required evaporation amount and insufficient evaporation amount are calculated by comparing the pressure P with the database 4D.
The output unit 4E is connected to each boiler 21,..., 25 by a signal line 14, and a control signal from the calculation unit 4B is output to each boiler via the output unit 4E and the signal line 14. ing.

  The control program according to the first embodiment is configured to control the combustion of the boilers constituting the boiler group 2 based on the required load, and is a first subroutine program for performing backup control (first Backup control means).

  The first subroutine program enables backup control to temporarily raise the combustion position of any of the boilers that are already burning when the combustion of any of the boilers in the boiler group 2 is started. In the case where the combustion position of the boiler capable of backup control is further increased and the evaporation amount is increased, if the evaporation amount of each combustion position capable of backup control is greater than or equal to the insufficient evaporation amount, the backup control start timing Is controlled according to the required increase rate VD of the evaporation amount.

Specifically, when the required increase rate VD of the evaporation amount is equal to or higher than a predetermined value V1 set in advance,
(Insufficient evaporation) ≤ (Evaporation increased by increasing the combustion position of the boiler capable of backup control)
It is configured to perform backup control with the minimum number of boilers that satisfy

Next, the operation of the boiler system 1 will be described with reference to FIGS. 2 and 3.
FIG. 2 is a diagram illustrating an example of fluctuations in the steam pressure P in the steam header 6, where the horizontal axis represents time and the vertical axis represents pressure.
The control pressure zone for controlling the number of boilers is set to a pressure lower than the set pressure P0 as a reference for control, the pressure P1 at which the highest priority boiler starts combustion, and the boiler with the next highest priority starts combustion. Pressure P2 to be performed, and further to the pressure P5, such as the pressure P3 at which the next highest priority boiler starts combustion.

  Further, the pressure difference ΔP1 between the pressure P1 and the pressure P2, the pressure difference ΔP2 between the pressure P2 and the pressure P3, and the pressure difference ΔP3 between the pressure P3 and the pressure P4 are as follows. The pressure corresponds to an evaporation amount of 1000 (Kg / h) at which steam can be supplied (steamed) at the first combustion position.

  Further, the required evaporation rate increase rate VD is a pressure drop rate (a pressure corresponding to the evaporation amount) at the time when combustion of any boiler is newly started in a state where combustion by backup control is not performed. Can be calculated from the pressure VP, and the pressure drop speed VP is acquired as a value obtained by differentiating the pressure P (= dP / dt), for example.

  FIG. 3 is a schematic diagram showing the combustion state of each boiler when the boiler group 2 is backed up using the control program, and each square frame shows the state of the first and second combustion positions of each boiler. The numerical values 1000 and 2000 shown on the left side indicate the amount of increase in the amount of evaporation when shifting to the first combustion position and the second combustion position, respectively.

  FIGS. 3A and 3B show the combustion control states in FIGS. 2A and 2B, and FIGS. 3C1 to 3C show the combustion control states in FIG. It corresponds to the combustion control state in. Note that t1 and t2 in FIG. 2 indicate timings at which the combustion state shifts from (A) to (B) and from (B) to (C), respectively. Further, the arrow indicated by a two-dot chain line in FIG. 2 conceptually indicates that the pressure has increased without reaching the pressure P4 after the pressure P has reached the pressure P3, and the fourth boiler 24 is combusted. Means that did not start.

In addition, the combustion position indicated by shading in FIG. 3 indicates that combustion is in progress, and the boiler that has been subjected to shading and “R” indicates a state until steaming is received in response to a combustion instruction. The boiler with “BK” indicates that backup control is in progress.
Note that the combustion control in FIG. 3 is an example when the required increase rate VD of the evaporation amount is equal to or greater than a predetermined value V1.
Moreover, although the pressure P shown in FIG. 2A is between the pressure P1 and the pressure P2, an example in which one boiler is burning in this state as shown in FIG. 3A. explain.

1) First, the pressure P in the steam header 6 is between the pressure P1 and the pressure P2 as shown in FIG. 2 (A), and the first boiler 21 is burned as shown in FIG. 3 (A). Yes.
2) When the pressure P in the steam header 6 in FIG. 2 (B) becomes lower than the pressure P2, combustion of the second boiler 22 is started as shown in FIG. 3 (B).
When the pressure P reaches the pressure P2 and combustion of the second boiler 22 is started, the required evaporation rate increase rate VD is calculated and compared with a predetermined value V1.
In this example, since the increasing speed VD is equal to or higher than the predetermined value V1,
Backup control is performed by one boiler, which is the minimum number that satisfies (insufficient evaporation amount) ≦ (evaporation amount increased by increasing the combustion position of a boiler capable of backup control).
Specifically, since the second boiler 22 is not steamable, the amount of insufficient evaporation is 1000 (Kg / h), and the amount of evaporation 2000 (Kg / h) increases when the first boiler 21 moves to the second combustion position. h) Since it is the following, the first boiler 21 is shifted to the second combustion position and the backup control is started.
Note that when the increase speed VD is less than the predetermined value V1, for example, backup control is not performed.
3) The pressure P in the steam header 6 in FIG. 2C is lower than the pressure P3. In this case, since the combustion amount in the low combustion state of the three boilers corresponds, the combustion of the third boiler 23 is started as shown in FIG. 3 (C1).
In this case, since the second boiler 22 and the third boiler 23 are not yet steamable, the amount of insufficient evaporation increases to 2000 (Kg / h), but the amount of evaporation 2000 (Kg / h) by the backup control of the first boiler 21. ) Because of the following, the backup control by the second combustion position of the first boiler 21 is continued.
4) FIG. 3 (C2) shows a state in which the second boiler 22 is capable of steaming and the amount of insufficient evaporation is reduced to 1000 (Kg / h).
In this case, the backup control by the second combustion position of the first boiler 21 is maintained in order to perform the backup control by the minimum number of boilers satisfying the insufficient evaporation amount 1000 (Kg / h) or more.
5) FIG. 3 (C3) shows a state where both the second boiler 22 and the third boiler 23 are in the steam supply state. Backup control is canceled because there is no shortage of evaporation.

  According to the control program and the boiler system 1 according to the first embodiment, the shortage evaporation amount can be reduced in a short time by reducing the excess evaporation amount while minimizing the excess evaporation amount while setting the evaporation amount by the backup control to be equal to or more than the short evaporation amount. Occurrence can be suppressed.

Next, a second embodiment of the present invention will be described with reference to FIGS.
The second embodiment differs from the first embodiment in that the control program according to the second embodiment is replaced with the first subroutine program, and a second subroutine program (second back control means) is used. Since the other points are the same as those in the first embodiment, the description of the same parts is omitted.

When the speed VD at which the required evaporation amount increases is less than or equal to the predetermined value V2, the second subroutine program
Backup control is performed by the maximum number of boilers that satisfy (insufficient evaporation amount) ≧ (evaporation amount increased by increasing the combustion position of a boiler capable of backup control).
Further, for example, when the required speed VD at which the amount of evaporation increases is larger than a predetermined value V2, (≦) in the first embodiment is replaced with (<), and the same backup control as in the first subroutine is performed. It is good also as composition which carries out.

FIG. 4 is a diagram showing an outline of the combustion state of each boiler when backup control is performed using the second subroutine program. FIGS. 4A and 4B are views of FIGS. (C1), (C2), and (C3) in FIG. 3 correspond to the combustion control state in FIG. 2 (C).
Note that the combustion control in FIG. 4 is an example when the required increase rate VD of evaporation is equal to or less than a predetermined value V2.

1) (A) is the same as in the first embodiment.
2) In FIG. 2B, the pressure P becomes lower than the pressure P2, and the combustion of the second boiler 22 is started as shown in FIG. 4B.
When the pressure P reaches the pressure P2, a required evaporation rate increase rate VD is calculated and compared with a predetermined value V2.
In this example, since the increasing speed VD is equal to or less than the predetermined value V2, one unit which is the maximum number satisfying (insufficient evaporation amount) ≧ (evaporation amount increased by increasing the combustion position of the boiler capable of backup control). Backup control is performed by the boiler.
Specifically, the amount of evaporation 2000 (Kg / h) that increases when the first boiler 21 shifts to the second combustion position is larger than the amount of insufficient evaporation 1000 (Kg / h), so the backup control is not started.
3) In FIG. 2 (C), the pressure P becomes lower than the pressure P3, and the combustion of the third boiler 23 is started as shown in FIG. 4 (C1).
In this case, the amount of insufficient evaporation increases to 2000 (Kg / h), and the first boiler 21 shifts to the second combustion position and increases to an evaporation amount of 2000 (Kg / h) or more. Is transferred to the second combustion position, and backup control is started.
4) In FIG. 4 (C2), the second boiler 22 can supply steam, and the amount of insufficient evaporation decreases to 1000 (Kg / h).
In this case, since the insufficient evaporation amount is less than the evaporation amount that increases by shifting the first boiler 21 to the second combustion position, the backup control is released.
5) In FIG. 4 (C3), both the second boiler 22 and the third boiler 23 are in the steam supply state.

According to the control program according to the second embodiment, the expansion of the insufficient evaporation amount is delayed and insufficient by minimizing the insufficient evaporation amount relative to the required evaporation amount while keeping the evaporation amount by the backup control below the insufficient evaporation amount. The increase rate VD of the evaporation amount can be reduced.
As a result, stable backup control can be performed when the insufficient evaporation amount is not so large.

  Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

Moreover, in the said embodiment, although the evaporation amount was controlled using the pressure P in the steam header 6 as a physical quantity corresponding to the vapor amount, and the backup control was performed based on the speed at which the pressure P changes, Instead of the pressure P, for example, the boiler group 2 is controlled or backed up based on the parameters of other physical quantities such as the pressure of the steam header 6, the amount of steam used in the steam using equipment 18, the temperature, etc. You may comprise to control.
Further, when calculating a speed that changes in accordance with the evaporation amount, backup control may be performed using changes in a plurality of physical quantities as parameters.

  In the above embodiment, when the required increase rate VD of evaporation is calculated based on the pressure decrease rate VP, the case where the pressure P is obtained by differentiation is described. The average value of the pressure change during a predetermined time before starting the combustion of any boiler may be used as the pressure drop rate VP.

  In the above-described embodiment, the case where the increase rate VD of the insufficient evaporation amount is compared with the predetermined values (constant values) V1 and V2 set in advance. However, instead of the predetermined values V1 and V2 which are constant values, Thus, for example, various parameters such as the pressure P or a function combining them may be compared with the speed VD.

  In the above-described embodiment, the case where the mathematical expressions related to the first subroutine program and the second subroutine program are defined by “≦” and “≧”, respectively, “<” and “>”, respectively. Backup control may be performed by a mathematical formula using.

  Further, the case where the control program according to the first embodiment includes the first subroutine program and the control program according to the second embodiment includes the second subroutine program has been described. The control program may be configured to have both functions related to the first subroutine program and the second subroutine program by changing the ranges to match each other.

  In the above-described embodiment, the case where the insufficient evaporation amount required in the boiler group 2 is calculated in association with the database 4D has been described. However, the evaporation amount may be calculated by calculation.

  In the above-described embodiment, the case where the control according to the present invention is performed by a program has been described. However, it is natural that the control may be performed by analog control using an operational amplifier or the like regardless of the program.

  Moreover, in the said embodiment, although the boiler group 2 which comprises the boiler system 1 demonstrated the case where it comprised with five boilers, the boiler group 2 was comprised with the boilers of two or more arbitrary numbers. Alternatively, some of the five boilers 21,..., 25 constituting the boiler group 2 may be operated when a planned shutdown is caused by, for example, failure or repair. Combustion control may be performed.

  Moreover, in the said embodiment, although the case where the combustion position etc. of the boiler which comprises the boiler group 2 was the same was demonstrated, some or all the maximum evaporation amounts of the boiler which comprises the boiler group 2, The amount of evaporation that increases at the combustion position may be set differently.

  Moreover, although the case where the priority order regarding the combustion start and the backup control is the order of the first boiler 21,..., The fifth boiler 25 has been described, such priority order may be arbitrarily set, for example, in advance. The set priority order may be arbitrarily changed.

  Further, the case where a ROM is used as a storage medium for storing the control program has been described. However, in addition to the ROM, for example, EP-ROM, hard disk, flexible disk, optical disk, magneto-optical disk, CD-ROM, CD-ROM R, a magnetic tape, a non-volatile memory card, etc. can be used. Further, not only the operation of the above-described embodiment is realized by executing the program read out by the arithmetic unit, but an OS (operating system) operating in the arithmetic unit based on an instruction of the program performs actual processing. This includes a case where the operation of the above embodiment is realized by performing part or all of the above. Furthermore, after the program read from the storage medium is written to the memory provided in the function expansion board inserted in the operation unit or the function expansion unit connected to the operation unit, the function expansion is performed based on the instructions of the program. It goes without saying that the case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing and the operation of the above-described embodiment is realized by the processing.

It is a figure showing a schematic structure of a boiler system concerning a 1st embodiment of the present invention. It is a figure explaining the effect | action of the boiler system which concerns on this invention, and is a conceptual diagram which shows an example of the fluctuation | variation of a steam pressure. It is a figure which shows an example of the backup control in the boiler system which concerns on 1st Embodiment. It is a figure which shows an example of the backup control in the boiler system which concerns on 2nd Embodiment.

Explanation of symbols

VD increase rate (speed of increasing required evaporation)
V1, V2 Predetermined value 1 Boiler system 2 Boiler group 4 Control unit (control system)
21, 22, 23, 24, 25 Boiler

Claims (8)

A boiler group composed of a plurality of boilers whose evaporation amount can be controlled at a staged combustion position,
A control program for controlling a boiler system configured to be controlled for combustion based on a required load,
When starting combustion of any one of the boiler groups, backup control that temporarily raises the combustion position of any of the boilers that are already in combustion is enabled,
When each evaporation amount that increases by increasing the combustion position of the boiler capable of the backup control is more than the insufficient evaporation amount,
A control program configured to control the start timing of the backup control in accordance with a speed at which at least one physical quantity corresponding to the evaporation amount changes. A control program according to claim 1,
When the speed is equal to or higher than a predetermined value,
The backup control
(Insufficient evaporation) ≤ (Evaporation increased by increasing the combustion position of the boiler capable of backup control)
A control program comprising first backup control means for controlling by a minimum number of boilers satisfying the above. A control program according to claim 1,
When the speed is below a predetermined value,
The backup control
(Insufficient evaporation amount) ≥ (Evaporation amount increased by increasing the combustion position of the boiler capable of backup control)
A control program comprising second backup control means for controlling by the maximum number of boilers satisfying the above. A control system comprising the control program according to any one of claims 1 to 3 .   A boiler system comprising the control system according to claim 4. A boiler group composed of a plurality of boilers whose evaporation amount can be controlled at a staged combustion position,
A control method for controlling a boiler system configured to be controlled for combustion based on a required load,
When starting combustion of any one of the boiler groups, backup control that temporarily raises the combustion position of any of the boilers that are already in combustion is enabled,
When each evaporation amount that increases by increasing the combustion position of the boiler capable of the backup control is more than the insufficient evaporation amount,
A control method, characterized in that the backup control start timing is controlled in accordance with a speed at which at least one physical quantity corresponding to the evaporation amount changes. The control method according to claim 6, comprising:
When the speed is equal to or higher than a predetermined value,
The backup control
(Insufficient evaporation) ≤ (Evaporation increased by increasing the combustion position of the boiler capable of backup control)
A control method comprising first backup control means for controlling with a minimum number of boilers satisfying the above. The control method according to claim 6, comprising:
When the speed is below a predetermined value,
The backup control
(Insufficient evaporation amount) ≥ (Evaporation amount increased by increasing the combustion position of the boiler capable of backup control)
A control method comprising a second backup control means for controlling by the maximum number of boilers satisfying the above.

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