JPS63243627A - Injected black liquid particle size control system in heavy oil mixed firing recovery boiler - Google Patents

Abstract

PURPOSE:To maintain proper injected black liquid particle size to always ensure an optimum firing condition by detecting the flow rate of supplied heavy oil or combustion air so as to modulate the temperature of the injected black liquid. CONSTITUTION:The black liquid temperature regulator 37 is manually set in accordance with black liquid temperature, properties, etc. As the flow rate of heavy oil 5 injected from a heavy oil burner 4 into the furnace 1 changes, the compensation value by a black liquid temperature compensator 35 is caused to be changed with the heavy oil flow rate. With an increase in heavy oil flow rate, the compensation value is reduced, the compensated setting value from an adder 36 becomes smaller, a black liquid temperature regulator 32 outputs an operation signal to close a steam flow rate regulator valve 31, the heating intensity by a heater 27 is reduced, and so, the black liquid temperature is reduced. Due to the temperature reduction, the dryness of black liquid particles sprayed from a black liquid burner 2 is lowered, and the dry condition in the furnace maintains an optimum condition for firing. As a result, proper injected black liquid particle size can be maintained to always ensure the optimum firing condition.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a droplet size control device for injected black liquor in a heavy oil co-firing recovery boiler used in a paper valve production process.

(Prior art) Generally, a recovery boiler used in the paper production valve process is used to generate steam and recover chemical raw materials for chip cooking by burning black liquor produced as waste liquid in the chip cooking process. It is. In particular, when the amount of steam generated is changed depending on the load connected to a recovery boiler such as a turbine, a so-called heavy oil co-firing recovery boiler is used, which burns heavy oil at the same time as black liquor.

In this type of heavy oil co-firing recovery boiler, when changing the flow rate of the main steam generated depending on the load, only the heavy oil flow rate is changed, and the black liquor flow rate is not changed. This is because when the black liquor flow rate is changed, the response of steam generation is slower than when the heavy oil flow rate is changed, and changes in the black liquor flow rate impair the stability of combustion in the charbed, and the heavy oil co-firing recovery boiler is This is because they become a hindrance to the stable operation of the plant.

Now, in order to stably operate such a heavy oil co-firing recovery boiler in a good condition, it is necessary to maintain a good combustion condition in the char bed as described above. For this purpose, it is important to maintain the droplet size of the jetted black liquor appropriately. In other words, if the droplet size of the injected black liquor is too large, the black liquor droplets will not dry well and will fall onto the char bed without sufficient water evaporation. Not only is there a risk of poor combustion due to the loss of the required heat, but there is also a risk that the formation of a high-temperature reducing atmosphere necessary for the reduction reaction may be inhibited.

Furthermore, the amount of charbed accumulated may become too large, causing a portion of the charbed to collapse, making the combustion state unstable. On the other hand, if the droplet size of the injected black liquor is too small, drying will occur too early and the black liquor droplets will burn before landing on the char bed, resulting in insufficient heat generation on the char bed. However, there is a risk that the formation of a high-temperature reducing atmosphere will be inhibited, and the amount of scattered dust will increase, resulting in an increase in the amount of dust adhering to the boiler tube, which may impede operations.

Therefore, it is necessary to control the droplet size of the jetted black liquor so that it lands on the char bed in an appropriate dry state, but the droplet size of the jetted black liquor must be directly measured. It is difficult to control. However, the black liquid injection mechanism is the same,
and black liquor concentration and temperature.

It is known that, if the pressure and properties are the same, the droplet size changes depending on the temperature of the black liquor. In other words, when the black liquor temperature is increased, the viscosity and surface tension of the droplet after ejection become smaller, resulting in a smaller droplet size, while when the black liquor temperature is lowered, the viscosity and surface tension of the droplet become larger, and the droplet size becomes smaller. Particle size increases.

For this reason, in conventional heavy oil co-firing recovery boilers, a black liquor heater and a black liquor temperature detector are installed in the injection black liquor line to the recovery boiler to heat the black liquor, and the injection black liquor temperature is controlled to a set value. A droplet size control device is configured by providing a jet black liquor temperature control device, and an operator can manually change the set value of the jet black liquor temperature control device according to the concentration and properties of the jet black liquor. The particle size was controlled.

However, in a heavy oil co-firing recovery boiler, the amount of heavy oil input is usually changed at any time in order to change the amount of steam generated depending on the load, but at this time the radiant heat from the heavy oil combustion section changes at any time. When the radiant heat changes, the dryness of the black liquor changes even if the droplet size is the same. Therefore, the appropriate value of the droplet particle size changes depending on the amount of heavy oil input into the heavy oil co-firing recovery boiler. However, conventional control devices do not take into account the radiant heat from the heavy oil combustion section that accompanies the input amount of heavy oil.

(Problems to be Solved by the Invention) As described above, in this type of conventional droplet size control device, radiant heat from the heavy oil combustion section does not affect the dryness of suspended black liquor. However, since control was not performed in consideration of this radiant heat, it was not possible to maintain an appropriate droplet size, and when the load was changed, an appropriate combustion state could not be obtained. was there.

Therefore, the present invention has developed a heavy oil co-fired recovery boiler that can maintain an appropriate droplet size of injected black liquor against changes in radiant heat from the heavy oil combustion section in a heavy oil co-fired recovery boiler, and that can always ensure optimal combustion conditions. An object of the present invention is to provide a droplet size control device for injected black liquor in a recovery boiler.

[Structure of the Invention] (Means for Solving the Problems) The invention of Part 1 of the present application generates steam by burning the black liquor produced as waste liquor in the chip cooking process and at the same time burning heavy oil. In a heavy oil co-firing recovery boiler for recovering chemical raw materials for chip cooking contained in the heavy oil co-firing recovery boiler, a heavy oil combustion-related amount detection means for detecting the flow rate of heavy oil input to the heavy oil co-firing recovery boiler or the air flow rate for heavy oil combustion; black liquor temperature changing means for changing the temperature of the black liquor injected into the heavy oil co-firing recovery boiler based on the heavy oil flow rate or the heavy oil combustion air flow rate detected by the combustion-related quantity detection means. Droplet size control.

Further, the invention of the present application tube 2 is an injection black liquid that changes the injection pressure of the black liquor injected into the heavy oil co-firing recovery boiler based on the heavy oil flow rate or the heavy oil combustion air flow rate detected by the heavy oil combustion related amount detection means. This is a droplet size control device for jetting black liquor, which is equipped with a liquid pressure changing means.

(Operation) By implementing the first invention, even if the amount of heavy oil input into the heavy oil co-firing recovery boiler or the air flow rate for heavy oil combustion changes and the radiant heat due to the combustion of heavy oil changes, the optimum droplet size can be maintained. The temperature of the injected black liquor is automatically adjusted to maintain the temperature.

Moreover, by implementing the second invention, the pressure of the jetted black liquor is automatically adjusted so that the optimum droplet size can be maintained against changes in radiant heat.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of a first embodiment of the present invention. First, the heavy oil co-firing recovery boiler will be explained, but since the boiler itself is exactly the same as the conventional boiler, this explanation will be incomplete. In the figure, reference numeral 1 denotes a furnace of a heavy oil co-firing recovery boiler.
At the same time, heavy oil 5 is sprayed into the upper part of the furnace 1 by the heavy oil burner 4. The black liquor 3 spread in the furnace 1 is suspended and dried while falling through the furnace, and is deposited on the bottom of the furnace to form a char bed 6. Also, a part of the black liquor 3 floating in the char bed 6 and inside the furnace is used as black liquor combustion air through the air input lower, and the char bed bottom air 10. The combustion is carried out by the charbed top air 11 and the furnace top air 12.

On the other hand, the heavy oil sprayed into the furnace is combusted by the heavy oil combustion air 14 introduced from the air inlet 13. The combustion exhaust gas 15 generated by the combustion of the black liquor 3 and the heavy oil 5 passes through a superheater 16, a drum 17, and an economizer 18, and is subjected to heat exchange, and further passes through one electrostatic precipitator to the chimney 20. is discharged from the system. The water 2) supplied to the heavy oil co-firing recovery boiler is heated by the combustion exhaust gas 15 in the economizer 18 to become hot water, and then heated by the bank tube of the drum 17 to become steam.
It is roughly superheated in a superheater 16 and taken out as main steam 22 from a main steam line 23 to the outside of the system.

In addition, a high-temperature reducing atmosphere using combustion heat is formed in the char bed 6 and its vicinity, and the black liquor 3
The chemical components therein are reduced in this reducing atmosphere and recovered from the smelt buffet 24 as smelt 25. The above is the configuration of the heavy oil co-firing recovery boiler.

Next, a droplet size control system for jetted black liquor will be described. The black liquor injection line 26 for supplying the black liquor 3 to the black liquor burner 2 includes a black liquor heater 27 for heating the black liquor;
A black liquor temperature detector 28 is installed to detect the temperature of the black liquor flowing through this line 26. black liquor heater 2
7 is the black liquor 3 by the steam 3o flowing through the steam line 29.
The amount of heating is determined by the opening degree of the steam flow 1ul1 regulating valve 31 provided in the steam line 29. Further, the black liquor temperature signal from the black liquor temperature detector 28 is input to a black liquor temperature regulator 32, and the opening degree of the steam flow rate regulating valve 31 is controlled by the output of the black liquor temperature regulator 32.

On the other hand, a heavy oil flow rate detector 34 is installed in a heavy oil line 33 for supplying heavy oil 5 to the heavy oil burner 4, and a heavy oil flow rate signal is input to a black liquor temperature compensator 35. The black liquor temperature compensator 35 is a functional unit having input/output characteristics as shown in FIG. The signal is input to adder 36. The adder 36 adds a black liquor temperature compensation value corresponding to the heavy oil flow rate output from the black liquor temperature compensator 35 to the black liquor temperature setting 1 preset by the black liquor temperature setting device 37. The added output is input to the black liquor temperature regulator 32. Thus, the black liquor temperature regulator 32 performs control calculations so that the black liquor temperature detected by the black liquor temperature detector 28 becomes equal to the set value compensated by the adder 36.
The steam flow rate control valve 31 is operated based on the calculation result.

Next, the operation of the device of this embodiment will be explained.

The set value of the black liquor temperature in the black liquor temperature setting device 37 is manually set by an operator based on the black liquor concentration, black liquor properties, and the like.

In this state, when the flow rate of the heavy oil 5 sprayed into the furnace 1 from the heavy oil burner 4 changes, the compensation value of the black liquor temperature compensator 35 changes in accordance with the heavy oil flow rate. Now, if the heavy oil flow rate increases, the compensation value will decrease, and the compensated set value from the adder 36 will become smaller. As a result, the black liquor temperature regulator 32 outputs an operation signal in the direction of closing the steam flow rate control valve 31, the amount of heating in the heater 27 is reduced, and the black liquor temperature is lowered. In other words, as weight*vLm increases, the radiant heat from the heavy oil combustion section increases, but as the black liquor temperature decreases, the dryness of the black liquor droplets sprayed from the black liquor burner 2 decreases, so the dryness in the furnace decreases. The dry state is maintained at an R-suitable state.

In this way, according to the device of this embodiment, even if the heavy oil flow rate changes and the radiant heat from the heavy oil combustion section changes accordingly, the black liquor temperature setting value can be adjusted appropriately according to the change in the amount of radiant heat. Since the value can be automatically compensated for, the drying state of the black liquor droplets can always be maintained in an optimal state regardless of changes in the flow rate of heavy oil. Therefore, the combustion state of the char bed 6 can be more stabilized, and the heavy oil co-firing recovery boiler can always be operated in a good state.

Next, a second embodiment of the present invention will be described.

FIG. 3 is a system diagram showing the main components of the second embodiment, and the same parts as in FIG. 1 are given the same reference numerals and detailed explanations are omitted. In this embodiment, a jet black liquor pressure detector 41 and a jet black liquor pressure regulating valve 42 are added to the jet black liquor line 26, and the black liquor pressure signal from the jet black liquor pressure detector 41 is transmitted to the jet black liquor line 11i5. This injection black liquor pressure v
The opening degree of the injection black liquor pressure control valve 42 is controlled by the output of the four-node 43.

In addition, a heavy oil flow rate detector 34 provided in the heavy oil line 33
The heavy oil flow rate signal is input to the incomplete differential calculator 44 in addition to the black liquor temperature compensator 35. This incomplete differential calculator 4
4 has the input/output characteristics shown in FIG. is input. This injection black liquor pressure compensator 45 has the input/output characteristics shown in FIG. 46. The adder 46 adjusts the injection black liquor pressure compensator 45 to the injection black liquor pressure set value preset by the injection black liquor pressure setting device 47.
The set value is compensated by adding a compensation value of the injection black liquor pressure corresponding to the heavy oil flow rate outputted from the injection black liquor pressure regulator 43, and the added output is inputted to the injection black liquor pressure regulator 43.

Therefore, the injection black liquor pressure regulator 43 performs control calculation so that the injection black liquor pressure detected by the injection black liquor pressure detector 41 becomes equal to the set value compensated by the adder 46,
The injection black liquor pressure control valve 41 is operated based on the calculation result.

In the second embodiment configured as described above, when the flow rate of heavy oil flowing through the heavy oil line 33 changes, immediately after the change in the flow rate of heavy oil, the system operates to compensate not only the black liquor m degree but also the injection black liquor pressure. do. That is, when the heavy oil flow rate signal from the heavy oil flow rate detector 34 changes, an output is obtained from the incomplete differential calculator 44 that is proportional to the amount of change only immediately after the change in the heavy oil flow rate, and then gradually decreases, and this output is equal to the injection black liquor pressure. Compensator 45
given to. Therefore, when the heavy oil flow rate increases, the black liquor pressure compensation value decreases in proportion to the heavy oil flow rate increase immediately after the heavy oil flow rate increases, and thereafter operates so that the compensation value gradually approaches zero. Therefore, in the adder 46, the injection black liquor pressure setting device 4 is manually set by the operator.
The compensation value of the injection black liquor pressure compensator 45 is added to the setting value of 7 to compensate the setting value, and the injection pressure regulator 43 compensates the injection black liquor pressure detection value of the injection pressure detector 41. The valve opening degree of the injection black liquor control valve 42 is operated so as to be equal to the value.

In this way, according to this embodiment, even if the radiant heat from the heavy oil combustion section changes with a change in the heavy oil flow rate, the black liquid pressure is changed at the same time as the black liquid temperature in accordance with the change in the amount of radiant heat. This allows the droplet size of the jetted black liquor to be maintained at an appropriate value. Generally, when comparing the responsiveness to set values, the responsiveness to temperature is slower than the responsiveness to flow rate or pressure. For this reason, in the first embodiment in which only the black liquor m degree is changed, if the heavy oil flow rate changes suddenly, the black liquor temperature may not respond in time, and the black liquor droplets may not be able to be maintained at an appropriate particle size. be. However, according to this second embodiment, for a while immediately after the heavy oil flow rate changes, the black liquor pressure is changed to maintain the appropriate droplet size, so the black liquor temperature control is controlled. During the delay in response, compensation can be made using the black liquor pressure, and when the compensation based on the black liquor temperature reaches a predetermined value and is completed, the black liquor pressure compensation can be set to zero and the system can be switched to a mode where black liquor temperature compensation is the main component. Therefore, the droplet size can be maintained at an appropriate value even in the face of sudden changes in the flow of heavy oil, thereby making it possible to further stabilize the combustion state of the char bed 6.

Next, a third embodiment of the present invention will be described.

FIG. 6 is a main structure diagram of the third embodiment, and the difference between this embodiment and the second embodiment is that a heavy oil flow rate control valve 51 is added to the heavy oil line 33 to detect the heavy oil flow rate. The opening degree of the heavy oil flow rate control valve 51 is controlled by the output of the heavy oil flow rate regulator 52 which inputs the heavy oil flow rate signal from the fuel oil flow rate signal from the fuel oil flow rate controller 34. A compensation value from a heavy oil flow rate compensator 53 that inputs the output of the black liquor pressure compensator 45 to calculate a heavy oil flow rate compensation value;
A compensated set value is provided by adding the set value set by the heavy oil flow rate setting device 54 by an adder 55. here,
As shown in FIG. 7, the heavy oil flow rate compensator 53 includes a black liquor flow rate calculator 6 that calculates a change in the black liquor flow rate that should be brought about by a change in black liquor pressure according to the output from the injection black liquor pressure compensator 45.
1, a calorific value calculation unit 62 that calculates a change in heat generation m due to a change in the black liquor flow rate, and a heavy oil flow rate detector 63 that converts a heavy oil flow rate value that brings about an equal calorific value change in a direction that cancels out this calorific value change. It consists of

In this embodiment configured as described above, when a compensation value is output from the injection black liquor pressure compensator 45 due to a change in the heavy oil flow layer, the black liquor pressure changes due to the compensation value by the action of the heavy oil flow rate compensator 53. The amount of change in the black liquor flow rate that would have been caused by this is calculated, and the amount of change in calorific value due to this change in black liquor flow rate is calculated, and roughly speaking, the amount of change in calorific value that is equal in magnitude is calculated in the direction of canceling out the change in calorific value. The heavy oil flow concavity that brings about this is converted into
This heavy oil flow fltliil is applied to an adder 55.

Then, in the adder 55, the heavy oil flow rate value from the heavy oil flow rate compensator 53 is added to the load master signal related to the heavy oil co-firing recovery boiler from the heavy oil flow rate setting device 54 or to the heavy oil flow rate setting device set by the operator. The heavy oil flow rate control valve 51 is operated by the heavy oil flow rate regulator 52 so that the heavy oil flow rate value detected by the heavy oil flow rate detector 34 matches the added value.

As described above, according to this third embodiment, when the jetting black liquor pressure is changed in order to maintain the droplet size appropriately,
Along with this change, the black liquor flow rate changes, causing a change in calorific value, but since the heavy oil flow l is controlled in a direction that cancels out the change in calorific value, it is possible to reliably compensate for the change in calorific value. can. Therefore, by controlling the black liquor pressure at the same time as the black liquor temperature, not only can the appropriate black liquor droplet size be maintained at all times, but also the calorific value can always be maintained at the desired value, improving the stability of the heavy oil co-firing recovery boiler. More improved vines.

Note that the present invention is not limited to the above embodiments.

For example, in each of the above embodiments, the case where the heavy oil flow rate is used as the load related to the heavy oil co-firing recovery boiler is shown, but for example, the air amount of the heavy oil combustion air 14 or the main steam line 22
The main steam flow rate taken from . Further, the flow rate of water supplied to the heavy oil co-firing recovery boiler may be used, and furthermore, a master signal indicating the load of the heavy oil co-firing recovery boiler may be directly used. In addition, the second. In the third embodiment, the black liquor pressure regulator 43 was used for the purpose of changing the pressure of the injected black liquor, but by installing a black liquor flow rate regulator and changing the flow rate of the injected black liquor, the pressure can be indirectly adjusted. A similar effect can be obtained by changing the . Further, in each of the above embodiments, the operator manually sets the set value using the black liquor temperature setting device 37, but the set value of the black liquor temperature may be automatically determined by measuring the black liquor concentration, etc. Needless to say, a configuration including a means may also be used.

It goes without saying that various other modifications can be made without departing from the spirit of the invention.

[Effects of the Invention] As detailed above, according to the present invention, it is possible to maintain an appropriate droplet size of injected black liquor against changes in radiant heat from the heavy oil combustion section in a heavy oil co-firing recovery boiler. It is possible to provide a device for controlling the droplet size of injected black liquor in a heavy oil co-firing recovery boiler that can always ensure an optimal combustion state.

[Brief explanation of the drawing]

Figure 1 is an overall configuration diagram of the first embodiment of the present invention;
The figure is an input/output characteristic diagram of the black liquor temperature compensator, Figure 3 is a diagram of the main part configuration of the second embodiment of the present invention, Figure 4 is an input/output characteristic diagram of the incomplete differential calculator, and Figure 5 is FIG. 6 is an input/output characteristic diagram of the injection black liquor pressure compensator, FIG. 6 is a characteristic diagram of main parts in the third embodiment of the present invention, and FIG. 7 is a specific configuration diagram of the heavy oil flow rate compensator. 1... Furnace, 3... Black liquor, 5... Heavy oil, 6...
Char bed, 14... Air for heavy oil combustion, 26...
Black liquor line, 27... Black liquor heater, 28... Black liquor temperature detector, 32... Black liquor temperature regulator, 33... Heavy oil line, 34... Heavy oil flow rate detector, 35. ...Black liquor temperature compensator, 36°46.55...Adder, 41...
Black liquor pressure detector, 45... incomplete differential calculator, 46.
... Injection black liquor pressure compensator, 52... Heavy oil flow rate regulator,
53...Heavy oil flow compensator. Applicant's agent Patent attorney Takehiko Suzue Figure 2 Time Figure 4 Input Figure 5

Claims (6)

[Claims] (1) In a heavy oil co-firing recovery boiler that generates steam by burning black liquor produced as waste liquid in the chip cooking process and simultaneously burns heavy oil and recovers the chemical raw material for chip cooking contained in the black liquor, the above-mentioned A heavy oil combustion related amount detection means for detecting the flow rate of heavy oil input to the heavy oil co-firing recovery boiler or the air flow rate for heavy oil combustion, and a heavy oil combustion related amount detection means that detects the heavy oil flow rate or the air flow rate for heavy oil combustion detected by the heavy oil combustion related amount detection means. 1. A droplet size control device for injected black liquor in a heavy oil co-firing recovery boiler, comprising: black liquor temperature changing means for changing the temperature of the black liquor injected into the heavy oil co-firing recovery boiler. (2) The black liquor temperature changing means includes a black liquor temperature setting means for setting the temperature of the black liquor injected into the heavy oil co-firing recovery boiler, and a black liquor temperature setting means for setting the black liquor temperature setting means. black liquor set temperature compensating means for compensating based on the heavy oil flow rate or heavy oil combustion air flow rate detected by the heavy oil combustion related amount detecting means; A droplet size control device for injected black liquor in a heavy oil co-firing recovery boiler as set forth in claim (1), comprising a black liquor temperature adjusting means for regulating the temperature of the liquid. (3) In a heavy oil co-firing recovery boiler that generates steam by burning black liquor produced as waste liquid in the chip cooking process and simultaneously burns heavy oil and recovers the chemical raw material for chip cooking contained in the black liquor, the above-mentioned A heavy oil combustion related amount detection means for detecting the flow rate of heavy oil input to the heavy oil co-firing recovery boiler or the air flow rate for heavy oil combustion, and a heavy oil combustion related amount detection means that detects the heavy oil flow rate or the air flow rate for heavy oil combustion detected by the heavy oil combustion related amount detection means. black liquor temperature changing means for changing the temperature of black liquor injected into the heavy oil co-firing recovery boiler; 1. A droplet size control device for black liquor injected in a heavy oil co-firing recovery boiler, characterized by comprising an injected black liquor pressure changing means for changing the injection pressure of black liquor injected into the recovery boiler. (4) The injection black liquor pressure changing means includes an injection black liquor pressure setting means for setting the injection pressure of black liquor injected into the heavy oil co-firing recovery boiler, and a black liquor injection pressure setting means for setting the injection pressure of black liquor injected into the heavy oil co-firing recovery boiler, and black liquor setting pressure compensating means for compensating the liquid setting pressure based on the heavy oil flow rate or the heavy oil combustion air flow rate detected by the heavy oil combustion related quantity detection means; and the black liquor setting compensated by the black liquor setting pressure compensating means. A droplet size control device for an injected black liquor in a heavy oil co-firing recovery boiler according to claim (3), characterized in that the device comprises a black liquor pressure adjusting means for adjusting the pressure of the injected black liquor. . (5) The injection black liquor pressure changing means includes an injection black liquor flow rate setting means for setting the flow rate of black liquor injected into the heavy oil co-firing recovery boiler, and a black liquor injection flow rate setting means for setting the black liquor injection flow rate setting means. black liquor set flow rate compensating means for compensating the set flow rate based on the heavy oil flow rate or the heavy oil combustion air flow rate detected by the heavy oil combustion related quantity detection means; and the black liquor set flow rate compensated by the black liquor set flow rate compensating means. A droplet size control device for injected black liquor in a heavy oil co-firing recovery boiler according to claim (3), characterized in that the device comprises a black liquor flow rate adjusting means for adjusting the flow rate of the injected black liquor. (6) The injection black liquor pressure setting means is configured to use a compensation value outputted by the black liquor set pressure compensating means, a compensation value outputted by the black liquor set flow rate compensating means, or a compensation value outputted by the black liquor set flow rate compensating means, or a compensation value output by the black liquor set flow rate compensating means, or the black liquor injected into the heavy oil co-firing recovery boiler. Claim (3) characterized in that a heavy oil flow rate compensating means is provided for compensating the heavy oil flow rate injected into the heavy oil co-firing recovery boiler based on either the black liquor flow rate or a flow rate equivalent index. Alternatively, the droplet size control device for injected black liquor in a heavy oil co-firing recovery boiler according to item (4) or item (5).