JP4126765B2 - Coal fired boiler fuel oil mixed ratio setting method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for setting a fuel oil mixed combustion ratio in a coal fired boiler.
[0002]
[Prior art]
In general, a coal fired boiler has a configuration in which a large number of pulverized coal burners 2 and a fuel oil burner 3 such as light oil are disposed on the side of a boiler body 1 as shown in FIG.
[0003]
The pulverized coal burner 2 is connected via a pulverized coal pipe 6 to a mill 5 that pulverizes coal supplied from a coal feeder 4 to form pulverized coal, and the fuel oil burner 3 includes A fuel oil pipe 8 provided with a fuel oil flow rate adjusting valve 7 is connected midway.
[0004]
In the example of FIG. 5, six mills 5 are provided, and each of the mills 5 has eight pulverized coal burners 2 (only one of them is shown in FIG. 5). The fuel oil burners 3 are concentrically provided at the axial center of each pulverized coal burner 2.
[0005]
The coal fired boiler is provided with a control device 11 that outputs a coal supply command 9 to the coal feeder 4 of each mill 5 and outputs a fuel oil flow command 10 to the fuel oil flow adjustment valve 7. Yes.
[0006]
As shown in FIG. 6, the control device 11 multiplies the coal-based combustion amount command 12 corresponding to the boiler load command by the fuel oil mixed combustion ratio setting value 13 to obtain the fuel oil flow rate command 10 to obtain the fuel oil flow rate adjustment valve. 7 and a subtractor 15 that subtracts the fuel oil flow rate command 10 from the combustion amount command 12 and obtains a coal supply amount command 9 and outputs it to the coal feeder 4. Yes.
[0007]
During the operation of the coal fired boiler as described above, the multiplier 14 of the control device 11 multiplies the coal-based combustion amount command 12 corresponding to the boiler load command by the fuel oil mixed combustion ratio set value 13 to obtain the fuel oil flow rate command 10. Is output to the fuel oil flow rate adjusting valve 7, the opening degree of the fuel oil flow rate adjusting valve 7 is adjusted, and a required amount of fuel oil such as light oil is injected from the desired fuel oil burner 3 into the boiler body 1. The fuel oil flow rate command 10 is subtracted from the combustion amount command 12 in the subtractor 15 to obtain the coal supply amount command 9 and output to the coal feeder 4, so that the required amount of coal is supplied to the desired coal supply. It is injected into the boiler body 1 from the desired pulverized coal burner 2 as pulverized coal via the mill 5 from the machine 4 and burned.
[0008]
When performing the co-firing operation of pulverized coal and fuel oil in the boiler body 1, the number of coal feeders 4 to be operated is appropriately selected according to the boiler load and the operation status at that time, and the desired desired value is selected. The pulverized coal is injected from the pulverized coal burner 2 corresponding to the coal feeder 4, and the amount of pulverized coal corresponding to the coal supply amount command 9 is supplied into the boiler body 1 as a whole, while the pulverized coal is injected. The fuel oil burner 3 that is concentric with the pulverized coal burner 2 is not used, and the number of fuel oil burners 3 to be used among the other fuel oil burners 3 is appropriately selected. Fuel oil is injected from the oil burner 3, and an amount of fuel oil corresponding to the fuel oil flow command 10 as a whole is supplied into the boiler body 1.
[0009]
In addition, the coal supply command 9 needs to be within the range of the minimum coal supply amount and the maximum coal supply amount determined by the number of operating coal feeders 4 according to the boiler load, The fuel oil flow rate command 10 is determined based on the minimum fuel oil flow rate determined from the minimum usable number of fuel oil burners 3 (usually about four) and the number of usable fuel oil burners 3 according to the number of operating coal feeders 4. The fuel oil co-firing ratio setting value 13 must be within the range of the maximum fuel oil flow rate determined from the maximum number, and the fuel oil co-firing ratio set value 13 is determined by the operator according to the coal supply amount command 9 and the fuel oil flow rate command 10. Are set within a permissible range, and are set manually so as not to hinder the operation of the mill 5 or the control of the fuel oil burner 3.
[0010]
[Problems to be solved by the invention]
However, as described above, the fuel oil co-firing ratio set value 13 is simply set manually by the operator, and if the co-firing operation of pulverized coal and fuel oil is performed in the boiler body 1, the boiler load changes. The coal supply amount command 9 may deviate from the range of the minimum coal supply amount to the maximum coal supply amount, or the fuel oil flow rate command 10 may deviate from the range of the fuel oil minimum flow rate to the fuel oil maximum flow rate. In some cases, the operation could not be continued.
[0011]
In view of such circumstances, the present invention sets the fuel oil mixed combustion ratio set value set by the operator to the fuel oil mixed combustion ratio upper limit even when the boiler load or the like changes during the mixed combustion operation of pulverized coal and fuel oil in the boiler body. And the fuel oil co-firing ratio lower limit, the coal feed command is kept within the range from the minimum coal feed amount to the maximum coal feed amount, and the fuel oil flow rate command is changed from the fuel oil minimum flow rate to the fuel oil maximum flow rate. It is intended to provide a method and apparatus for setting a fuel oil co-firing ratio of a coal fired boiler that can be kept within the above range and that can continue operation smoothly.
[0012]
[Means for Solving the Problems]
The present invention obtains the minimum amount of coal supply based on the number of operating coal feeders, subtracts the minimum amount of coal supply from the coal-based combustion amount command to obtain the maximum fuel oil flow rate, and based on the number of operating coal feeders The fuel oil maximum flow rate is obtained, and the lower one of the fuel oil maximum flow rate and the fuel oil maximum flow rate obtained from the minimum coal supply amount is selected as a fuel oil flow rate upper limit, and the fuel oil flow rate upper limit is set as the combustion amount. While dividing by the directive to obtain the upper limit of fuel oil mixed combustion ratio
Obtain the maximum amount of coal supply based on the number of operating coal feeders, subtract the maximum amount of coal supply from the combustion amount command to obtain the minimum fuel oil flow rate, and determine the minimum fuel oil flow rate determined from the minimum number of fuel oil burners that can be used And the fuel oil minimum flow rate obtained from the maximum coal supply amount is selected as the lower limit of the fuel oil flow rate, the fuel oil flow rate lower limit is divided by the combustion amount command to obtain the fuel oil mixed combustion ratio lower limit,
A fuel oil co-firing ratio setting value set by an operator falls within a range between the fuel oil co-firing ratio upper limit and the fuel oil co-firing ratio lower limit. .
[0013]
Further, the present invention is a first function generator for obtaining and outputting a minimum amount of coal supply based on the number of operating coal feeders,
A subtractor that subtracts the minimum coal supply amount from a coal-based combustion amount command and obtains and outputs a maximum fuel oil flow rate;
A second function generator for obtaining and outputting a maximum fuel oil flow based on the number of operating coal feeders, a maximum fuel oil flow output from the second function generator, and a maximum fuel oil flow output from the subtractor; A low selector that selects the lower one and outputs it as the upper limit of the fuel oil flow rate,
A divider that divides the fuel oil flow rate upper limit output from the low selector by the combustion amount command and obtains and outputs a fuel oil mixed combustion ratio upper limit;
A third function generator for obtaining and outputting the maximum amount of coal supply based on the number of operating coal feeders;
A subtractor that subtracts the maximum coal supply amount from the combustion amount command and obtains and outputs a minimum fuel oil flow rate;
A high selector that selects a higher one of the minimum fuel oil flow rate determined from the minimum usable number of fuel oil burners and the minimum fuel oil flow rate output from the subtractor and outputs the lower limit of the fuel oil flow rate;
A divider that divides the fuel oil flow rate lower limit output from the high selector by the combustion amount command and obtains and outputs a fuel oil mixed combustion ratio lower limit;
A fuel oil co-firing ratio of a coal fired boiler, comprising a high / low limiter that keeps a fuel oil co-firing ratio set value set by an operator within the range between the upper limit of the fuel oil co-firing ratio and the lower limit of the fuel oil co-firing ratio It relates to the setting device.
[0014]
According to the above means, the following operation can be obtained.
[0015]
In the method for setting a fuel oil co-firing ratio of a coal fired boiler according to the present invention, when the coal fired boiler is operated, a minimum coal supply amount is obtained based on the number of operating coal feeders, and the minimum coal supply is determined from a coal-based combustion amount command. The fuel oil maximum flow rate is obtained by subtracting the amount, and the fuel oil maximum flow rate is obtained based on the number of operating coal feeders, and the fuel oil maximum flow rate obtained from the fuel oil maximum flow rate and the minimum coal supply amount The lower one is selected as the upper limit of the fuel oil flow rate, and the upper limit of the fuel oil flow rate is divided by the combustion amount command to obtain the upper limit of the fuel oil mixed combustion ratio, while the maximum coal supply based on the number of operating coal feeders The fuel oil minimum flow rate is obtained by subtracting the maximum coal supply amount from the combustion amount command, and the fuel oil minimum flow rate determined from the minimum usable number of fuel oil burners and the maximum coal supply amount are obtained. Fuel The higher one of the minimum flow rates is selected as the lower limit of the fuel oil flow rate, the lower limit of the fuel oil flow rate is divided by the combustion amount command to obtain the lower limit of the mixed fuel oil ratio, and the mixed fuel oil ratio set by the operator The set value is within a range between the upper limit of the fuel oil mixed combustion ratio and the lower limit of the fuel oil mixed combustion ratio.
[0016]
In the fuel oil co-firing ratio setting device of the coal fired boiler of the present invention, when operating the coal fired boiler, the first function generator determines the minimum coal supply amount based on the number of operating coal feeders, and outputs it to the subtractor. In the subtracter, the minimum fuel supply amount is subtracted from the coal-based combustion amount command and the maximum fuel oil flow rate is obtained and output to the low selector, and the second function generator determines the maximum fuel oil based on the number of operating coal feeders. The flow rate is obtained and output to the low selector, and the lower one of the maximum fuel oil flow rate output from the second function generator and the maximum fuel oil flow rate output from the subtractor in the low selector. The fuel oil flow rate upper limit is selected and output to the divider. In the divider, the fuel oil flow rate upper limit output from the low selector is divided by the combustion amount command to obtain the fuel oil co-firing ratio upper limit, and to the high / low limiter output It is.
[0017]
At the same time, the third function generator obtains the maximum amount of coal supply based on the number of operating coal feeders and outputs it to the subtractor. In the subtractor, the maximum amount of coal supply is subtracted from the combustion amount command. The minimum flow rate is obtained and output to the high selector, and the higher one of the minimum fuel oil flow rate determined from the minimum usable number of fuel oil burners in the high selector and the minimum fuel oil flow rate output from the subtractor is higher. The fuel oil flow rate lower limit is selected and output to the divider, and the fuel oil flow rate lower limit output from the high selector is divided by the combustion amount command in the divider to obtain the fuel oil co-firing ratio lower limit to the level limiter. Is output.
[0018]
The fuel oil co-firing ratio setting value set by the operator is input to the high / low limiter. Here, the fuel oil co-firing ratio setting value is assumed to be changed if the boiler load is changed. When the ratio upper limit is exceeded, the fuel oil co-firing ratio set value is lowered to the fuel oil co-firing ratio upper limit and output from the high / low limiter, while the fuel oil co-firing ratio set value becomes smaller than the fuel oil co-firing ratio lower limit. In this case, the fuel oil mixed combustion ratio set value is raised to the lower limit of the fuel oil mixed combustion ratio and output from the high / low limiter.
[0019]
As a result, even if the boiler load changes, the coal supply command deviates from the range of the minimum coal supply amount to the maximum coal supply amount, or the fuel oil flow command ranges from the fuel oil minimum flow rate to the fuel oil maximum flow rate. There is no fear that you will not be able to continue driving.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0021]
FIG. 1 shows an example of an embodiment of the present invention. In the figure, the same reference numerals as those in FIGS. 5 and 6 denote the same components, and the basic configuration is shown in FIGS. Although it is the same as the conventional one shown in the figure, the feature of this illustrated example is that, as shown in FIG. 1, a first function generator 18 for obtaining and outputting a minimum coal supply amount 17 based on the number of operating coal feeders 16. When,
A subtractor 19 that subtracts the minimum coal supply amount 17 from the coal-based combustion amount command 12 and obtains and outputs a maximum fuel oil flow rate 18 ′;
A second function generator 21 for obtaining and outputting the maximum fuel oil flow rate 20 based on the number of operating coal feeders 16;
A low selector that selects the lower one of the maximum fuel oil flow rate 20 output from the second function generator 21 and the maximum fuel oil flow rate 18 ′ output from the subtractor 19 and outputs the selected fuel oil flow rate upper limit 22. 23,
A divider 25 for dividing the fuel oil flow rate upper limit 22 output from the low selector 23 by the combustion amount command 12 to obtain and output a fuel oil mixed combustion ratio upper limit 24;
A third function generator 27 for obtaining and outputting a maximum coal supply amount 26 based on the number of operating coal feeders 16;
A subtractor 29 for subtracting the maximum coal supply amount 26 from the combustion amount command 12 to obtain and output a minimum fuel oil flow rate 28;
A high selector 32 that selects the higher one of the minimum fuel oil flow rate 30 determined from the minimum usable number of the fuel oil burners 3 and the minimum fuel oil flow rate 28 output from the subtractor 29 and outputs it as the lower limit 31 of the fuel oil flow rate. When,
A divider 34 for dividing the fuel oil flow rate lower limit 31 output from the high selector 32 by the combustion amount command 12 to obtain and output a fuel oil mixed combustion ratio lower limit 33;
The control device 11 is additionally equipped with a high / low limiter 35 for keeping the fuel oil mixed combustion ratio set value 13 set by the operator within the range between the fuel oil mixed combustion ratio upper limit 24 and the fuel oil mixed combustion ratio lower limit 33. .
[0022]
A function as shown in FIG. 2 is input to the first function generator 18, and this function determines the minimum amount of coal supply required to operate one mill 5. The minimum coal supply amount 17 is increased or decreased in proportion to the increase or decrease of the number of operating coal feeders 16. However, the minimum coal supply amount 17 when the number of operating coal feeders 16 is one and two is the same value because the mill 5 in the illustrated example operates at least two or more. Because.
[0023]
Further, a function as shown in FIG. 3 is input to the second function generator 21, and this function reduces the number of fuel oil burners 3 that can be used when the number of operating coal feeders 4 increases. Therefore, the fuel oil maximum flow rate 20 is increased or decreased in an inversely proportional manner to the increase or decrease of the number of operating coal feeders 16. However, the maximum fuel oil flow rate 20 when the number of operating coal feeders 16 is one and two is the same value because the mill 5 in the illustrated example operates at least two or more. Because.
[0024]
Furthermore, a function as shown in FIG. 4 is input to the third function generator 27, and this function determines the maximum amount of coal supply when one mill 5 is operated. Therefore, the maximum coal supply amount 26 is increased or decreased in proportion to the increase or decrease of the number of operating coal feeders 16. However, the maximum coal supply amount 26 when the number of operating coal feeders 16 is one and two is the same value because the mill 5 in the illustrated example operates at least two or more. Because.
[0025]
As shown in FIG. 1, between the low selector 23 and the divider 25, there is a fuel oil flow rate upper limit 22 output from the low selector 23 and a fuel oil flow rate lower limit 31 output from the high selector 32. A high selector 36 that selects the higher one of the two and outputs the same to the divider 25 is provided. This is because the upper limit of the fuel oil flow rate 22 becomes lower than the lower limit of the fuel oil flow rate 31 due to some failure. In order to avoid reversing the magnitude relationship between the fuel oil mixed combustion ratio upper limit 24 and the fuel oil mixed combustion ratio lower limit 33 in the case of a failure, the fuel oil flow lower limit 31 is replaced with the fuel oil flow upper limit 22. Of course, the fuel oil flow rate upper limit 22 output from the low selector 23 is output to the divider 25 as it is in a normal state.
[0026]
Next, the operation of the illustrated example will be described.
[0027]
During operation of the coal fired boiler, the first function generator 18 obtains the minimum coal supply amount 17 based on the number of operating coal feeders 16 and outputs it to the subtractor 19, and the subtractor 19 outputs the coal-based combustion amount command 12. The minimum fuel supply amount 17 is subtracted from the fuel oil maximum flow rate 18 'and output to the low selector 23, and the second function generator 21 determines the maximum fuel oil flow rate 20 based on the number of operating coal feeders 16. Between the maximum fuel oil flow rate 20 output from the second function generator 21 and the maximum fuel oil flow rate 18 ′ output from the subtractor 19 in the low selector 23. The lower one is selected and output to the divider 25 via the high selector 36 as the fuel oil flow rate upper limit 22, and the fuel oil flow rate upper limit 22 output from the low selector 23 in the divider 25 is the combustion amount. Fuel oil mixed combustion ratio upper limit 24 divided by decree 12 is obtained and output to the high and low limiter 35.
[0028]
At the same time, the third function generator 27 obtains the maximum coal supply amount 26 based on the number of operating coal feeders 16 and outputs it to the subtractor 29, which then outputs the maximum coal supply from the combustion amount command 12. The amount 26 is subtracted and the minimum fuel oil flow 28 is obtained and output to the high selector 32. The high selector 32 determines the minimum fuel oil flow 30 determined from the minimum number of usable fuel oil burners 3 and the subtractor 29. The higher one of the fuel oil minimum flow rates 28 to be output is selected and output to the divider 34 as the fuel oil flow rate lower limit 31, and the fuel oil flow rate lower limit 31 output from the high selector 32 in the divider 34 is Divided by the combustion amount command 12, the fuel oil mixed combustion ratio lower limit 33 is obtained and output to the height limiter 35.
[0029]
Although the fuel oil mixed combustion ratio setting value 13 set by the operator is input to the high / low limiter 35, the fuel oil mixed combustion ratio setting value 13 is temporarily changed because the boiler load is changed. When the fuel oil mixed combustion ratio upper limit 24 is exceeded, the fuel oil mixed combustion ratio set value 13 is lowered to the fuel oil mixed combustion ratio upper limit 24 and is output from the high / low limiter 35 to the multiplier 14. The fuel oil mixed combustion ratio upper limit 24 output from the high / low limiter 35 is multiplied to the coal-based combustion amount command 12 according to the command to obtain the fuel oil flow rate command 10 and output to the fuel oil flow rate adjustment valve 7, The degree of opening of the fuel oil flow rate adjusting valve 7 is adjusted, and a required amount of fuel oil such as light oil is injected from the desired fuel oil burner 3 into the boiler body 1 and burned. Quantity command The fuel oil flow rate command 10 is subtracted from 2 and a coal feed amount command 9 is obtained and output to the coal feeder 4, and a desired amount of coal is passed through the mill 5 from the desired coal feeder 4 as desired pulverized coal. It is injected from the charcoal burner 2 into the boiler body 1 and burned.
[0030]
On the other hand, when the fuel oil mixed combustion ratio set value 13 becomes smaller than the fuel oil mixed combustion ratio lower limit 33, the fuel oil mixed combustion ratio set value 13 is raised to the fuel oil mixed combustion ratio lower limit 33 and the high / low limiter 35 multiplies. 14 is multiplied by the fuel oil mixed combustion ratio lower limit 33 output from the high / low limiter 35 to the coal-based combustion amount command 12 corresponding to the boiler load command in the multiplier 14 to obtain the fuel oil flow rate command 10. Is output to the fuel oil flow rate adjusting valve 7, the opening degree of the fuel oil flow rate adjusting valve 7 is adjusted, and a required amount of fuel oil such as light oil is injected from the desired fuel oil burner 3 into the boiler body 1. The fuel oil flow rate command 10 is subtracted from the combustion amount command 12 in the subtractor 15 to obtain the coal supply amount command 9 and output to the coal feeder 4, so that the required amount of coal is supplied to the desired coal supply. From machine 4 It is burned through and being ejected from the desired pulverized coal burner 2 as pulverized coal into the boiler body 1 Le 5.
[0031]
As a result, even if the boiler load changes, the coal supply command 9 deviates from the range of the minimum coal supply 17 to the maximum coal supply 26, or the fuel oil flow command 10 changes to the fuel oil minimum flow 28 (or 30) does not deviate from the range of the maximum fuel oil flow rate 18 ′ (or 20), and there is no fear that the operation cannot be continued.
[0032]
Thus, even when the boiler load or the like is changed during the co-firing operation of pulverized coal and fuel oil in the boiler body 1, the fuel oil co-firing ratio upper limit 24 and the fuel oil co-firing ratio are set to the fuel oil co-firing ratio set value 13 set by the operator. By being within the range of the lower limit 33, the coal supply amount command 9 is within the range of the minimum coal supply amount 17 to the maximum coal supply amount 26, and the fuel oil flow rate command 10 is set to the fuel oil minimum flow rate 28 (or 30). Can be kept within the range of the maximum fuel oil flow rate 18 ′ (or 20), and the operation can be continued smoothly.
[0033]
It should be noted that the method and apparatus for setting the fuel oil co-firing ratio of the coal fired boiler of the present invention is not limited to the above illustrated example, and various modifications can be made without departing from the scope of the present invention. It is.
[0034]
【The invention's effect】
As described above, according to the fuel oil mixed combustion ratio setting method and apparatus of the coal fired boiler according to the present invention, even if the boiler load or the like changes during the mixed combustion operation of pulverized coal and fuel oil in the boiler body, the operator By setting the fuel oil co-firing ratio set value to be within the range between the fuel oil co-firing ratio upper limit and the fuel oil co-firing ratio lower limit, the coal supply command is kept within the range from the minimum coal supply amount to the maximum coal supply amount, In addition, the fuel oil flow rate command can be kept within the range of the fuel oil minimum flow rate to the fuel oil maximum flow rate, and an excellent effect that the operation can be continued smoothly can be achieved.
[Brief description of the drawings]
FIG. 1 is a control block diagram illustrating an example of an embodiment of the present invention.
FIG. 2 is a diagram representing a function input to the first function generator shown in FIG. 1;
FIG. 3 is a diagram representing a function input to the second function generator shown in FIG. 1;
FIG. 4 is a diagram representing a function input to the third function generator shown in FIG. 1;
FIG. 5 is a schematic configuration diagram of an example of a coal fired boiler.
FIG. 6 is a control block diagram of a conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Boiler body 2 Pulverized coal burner 3 Fuel oil burner 4 Coal feeder 5 Mill 7 Fuel oil flow rate adjustment valve 9 Coal feed amount command 10 Fuel oil flow rate command 12 Combustion amount command 13 Fuel oil co-firing ratio set value 16 Number of coal feeders operated 17 Minimum coal supply amount 18 First function generator 18 ′ Maximum fuel oil flow 19 Subtractor 20 Maximum fuel oil flow 21 Second function generator 22 Fuel oil flow upper limit 23 Low selector 24 Fuel oil co-firing ratio upper limit 25 Divider 26 Maximum coal supply 27 Third function generator 28 Fuel oil minimum flow 29 Subtractor 30 Fuel oil minimum flow 31 Fuel oil flow lower limit 32 High selector 33 Fuel oil co-firing ratio lower limit 34 Divider 35 High / low limiter

Claims (2)

Obtain the minimum coal supply based on the number of operating coal feeders, subtract the minimum coal supply from the coal-based combustion amount command to obtain the maximum fuel oil flow, and determine the maximum fuel oil flow based on the number of operating coal feeders. The lower one of the maximum fuel oil flow rate and the maximum fuel oil flow rate determined from the minimum coal supply amount is selected as the fuel oil flow rate upper limit, and the fuel oil flow rate upper limit is divided by the combustion amount command. While seeking the upper limit of the fuel oil co-firing ratio,
Obtain the maximum amount of coal supply based on the number of operating coal feeders, subtract the maximum amount of coal supply from the combustion amount command to obtain the minimum fuel oil flow rate, and determine the minimum fuel oil flow rate determined from the minimum number of fuel oil burners that can be used And the fuel oil minimum flow rate obtained from the maximum coal supply amount is selected as the lower limit of the fuel oil flow rate, the fuel oil flow rate lower limit is divided by the combustion amount command to obtain the fuel oil mixed combustion ratio lower limit,
A fuel oil co-firing ratio setting method for a coal fired boiler, wherein a fuel oil co-firing ratio setting value set by an operator is within a range between the fuel oil co-firing ratio upper limit and the fuel oil co-firing ratio lower limit. A first function generator for obtaining and outputting a minimum amount of coal supply based on the number of operating coal feeders;
A subtractor that subtracts the minimum coal supply amount from a coal-based combustion amount command and obtains and outputs a maximum fuel oil flow rate;
A second function generator for determining and outputting the maximum flow rate of fuel oil based on the number of operating coal feeders;
A low selector that selects a lower one of the maximum fuel oil flow rate output from the second function generator and the maximum fuel oil flow rate output from the subtractor and outputs the lower limit as a fuel oil flow rate upper limit;
A divider that divides the fuel oil flow rate upper limit output from the low selector by the combustion amount command and obtains and outputs a fuel oil mixed combustion ratio upper limit;
A third function generator for obtaining and outputting the maximum amount of coal supply based on the number of operating coal feeders;
A subtractor that subtracts the maximum coal supply amount from the combustion amount command and obtains and outputs a minimum fuel oil flow rate;
A high selector that selects a higher one of the minimum fuel oil flow rate determined from the minimum usable number of fuel oil burners and the minimum fuel oil flow rate output from the subtractor and outputs the lower limit of the fuel oil flow rate;
A divider that divides the fuel oil flow rate lower limit output from the high selector by the combustion amount command and obtains and outputs a fuel oil mixed combustion ratio lower limit;
A fuel oil co-firing ratio of a coal fired boiler, comprising a high / low limiter that keeps a fuel oil co-firing ratio set value set by an operator within the range between the upper limit of the fuel oil co-firing ratio and the lower limit of the fuel oil co-firing ratio Setting device.

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