JPS62123216A - Optimum combustion control device for recovery boiler - Google Patents

Abstract

PURPOSE:To search the most optimum combustion condition in operation while considering both of a steam generating efficiency and the reducing rate of smelt and carry out combustion control by a method wherein the titled control device is equipped with a heat amount ratio operating means, obtaining the heat amount ratio from the input heat and the output heat of a recovery boiler, an evaluation index operating means, a regulating means and a set value changing means. CONSTITUTION:An input heat and an output heat, obtained by operating units 34, 39, are sent to a subtracting unit 35 to obtain the ratio of the output heat to the input heat, thereafter, are supplied to a weighted sum operating unit 44. The SO2 concentration signal of combustion exhaust gas 7 is being inputted in the weighted sum operating unit 44 and the evaluation index of operating condition in a recovery boiler 2 is obtained by operation. The evaluation index is inputted into an extreme value searching unit 45 and the set value of a primary low air flow amount controller 46 is changed up-and-down so that the evaluation index becomes the greatest. When the evaluation index has the largest value, the set value of the primary low air flow amount is held.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention aims to burn the black liquor produced in the pulp manufacturing process to recover raw materials for cooking chemicals contained in the black liquor, and to utilize the heat capacity of the black liquor to recover raw materials for cooking chemicals contained in the black liquor. The present invention relates to an optimal combustion control device for a recovery boiler that generates steam, and more particularly to an optimal combustion control device for a recovery boiler that controls the combustion state of the recovery boiler by searching for an air flow rate so as to achieve an optimal operating state.

[Prior art] In the cooking process in the paper pulp manufacturing process, it is necessary to separate and extract cellulose (1141 min) and lignin (resin content) from wood, and for this purpose, a chemical agent containing NaOH as a main component is used. is used.

From this cooking process, a waste solution called black liquor is produced, which is a mixture of organic components such as lignin in a dissolved state and inorganic components mainly composed of Na such as Na25O+ and Na2GO3. is discharged.

By the way, the function of the recovery boiler is to burn the black liquor to generate and recover steam that is used for power generation or various factory equipment using the amount of heat held by organic components such as lignin contained in the black liquor. Another function of the recovery boiler is to use the combustion heat of the black liquor to recover the Na2 contained in the black liquor.
The purpose of this method is to reduce SO4 and melt it in the form of Na2S as the main component at the bottom of the furnace to recover raw materials for chemicals used in the cooking process. In other words, the recovery boiler has a dual role of burning the black liquor discharged in the pulp and paper manufacturing process to recover thermal energy and raw materials for chemicals, and has extremely important significance in the operation of pulp and paper mills. have. For this reason, the combustion control of the recovery boiler depends on the steam generation efficiency and the chemical recovery efficiency, that is, the Na2
It is desired to operate in such a way that both the efficiency of reducing SO4 to NaS is high.

FIGS. 7 and 8 are diagrams showing the configuration of the conventional recovery boiler and its combustion control device described above. First, the seventh
The figure shows a recovery boiler, and although not shown, black liquor discharged as waste liquid from the chip cooking process is sprayed into the furnace of the recovery boiler 2 by an injection gun 1. Here, the atomized black liquor is suspended and dried in the furnace by radiant heat and falls to the bottom of the furnace to form a char bed 3. This char bed 3 receives combustion air 4 to 6 and is combusted, and the combustion exhaust gas 7 generated at this time passes through a superheater 8, a drum 9, and an economizer 10, and is subjected to heat exchange. 11 and is discharged from the chimney 12 to the outside of the system as exhaust gas 13. The water supplied to the recovery boiler 2 is heated by the combustion exhaust gas 7 in the economizer 10 to become hot water, and then heated by the bank tube of the drum 9 to produce steam 1.
4 and then sent to a superheater 8 where it is heated and taken out as main steam 15 from the main steam line 16 to the outside of the system.

On the other hand, in the charbet 3 and its vicinity, a high-temperature reducing atmosphere is formed using combustion heat. Na2SO4 mixed into the black liquor becomes Na2 in this high-temperature reducing atmosphere.
It is reduced to S and melted at the bottom of the furnace as a liquid called smelt 17 (chemical for chip cooking), and is produced as smelt spout 18.
more will be recovered. In addition, Na2S that has been mixed into the black liquor
The S○2 gas generated during the thermal decomposition of O4 and the combustion of black liquor is the Na2 gas or Na gas generated at the top of the char bed.
It is captured by 2O fume, immobilized as Na25O+, and similarly reduced to Na2S.

Next, FIG. 8 is a diagram showing a conventional general combustion control device applied to the recovery boiler shown in FIG. 7.

That is, this combustion control device includes a recovery boiler system shown in FIG. The configuration is such that the primary air is supplied to the primary air blowing fan 24 to control the primary air flow rate. This device has an extreme value search section 22
The primary airflow m setting value is searched for using a hill climbing method or the like so that the above-mentioned 502a degrees becomes a minimum, and the search result is supplied to the primary air flow rate controller 23 as the setting value.

The reason for adopting the above-mentioned combustion control means is to directly aim at lowering pollution by lowering SO2 concentration, but indirectly it is thought to aim at increasing the smelt reduction rate. . In other words, the low temperature of 50211 degrees means that the 802 gas produced by the combustion of organic components in the black liquor or the thermal decomposition of Na2SO4 in the black liquor is captured by the Na2 gas or Na2O fume generated from the surface of the charbed, and is not scattered as a gas. (N a2 S 0
It is thought that since the proportion of solidification in the form of smelt 4 is high, it is possible to obtain smelt 17 with a high smelt reduction rate as a result. In addition, regarding the steam generation efficiency, which is another reason for its adoption, we considered this as a secondary consideration, and considered that the higher the hearth temperature, the more the scattering of Na2 gas.
The reduction in concentration represented the realization of a high hearth temperature, which was considered to lead to the realization of a generally high steam generation efficiency.

[Problems that the invention aims to solve]In recent years, as the demand for energy conservation has increased,
The role of recovery boilers as steam generation plants, which have traditionally been considered secondary, is becoming increasingly important. Accordingly, even in the operation of the recovery boiler, it is required to realize and maintain an optimal operating state by considering both the smelt reduction rate and the steam generation efficiency.

In this regard, conventional combustion control devices aim only at maximizing the smelt reduction rate, or at the same time maximize the smelt reduction rate and steam generation efficiency. They were searching for a set value for the primary air flow rate that would minimize the 302 concentration.

However, according to experiments conducted by the inventors of the present application, as shown in FIG. The primary air flows 132 corresponding to the point ports do not necessarily match as conventionally thought, but rather are often different.

Therefore, in a serious sense, it is difficult for conventional devices to simultaneously maximize both conditions, and the recovery boiler is not necessarily operated in an optimal state in terms of steam generation efficiency. On the other hand, if an attempt is made to maximize steam generation efficiency, this will result in a decrease in the smelt reduction rate. In the overall judgment of the inventors regarding the results of the experiment,
The relationship between smelt reduction rate and steam generation efficiency can be theoretically explained as follows. That is, in the recovery boiler, the following chemical reaction takes place during smelt reduction.

Na2304 +40 -+Na2S+C○-136゜5Kca+It is already known that the chemical reaction shown in this formula is an endothermic reaction,
As the reduction reaction becomes more active, the amount of heat absorbed increases, and a phenomenon occurs in which combustion heat that should contribute to steam generation is taken away by the reduction reaction. In addition, this reduction reaction requires unburned carbon (C), but if too much oxygen is supplied in an attempt to promote combustion and increase steam generation efficiency, the unburned carbon necessary for reduction will contribute to reduction. It is lost through oxidative combustion. Therefore, in order to improve the smelt reduction rate, it is necessary to keep the charped 3 in a reducing atmosphere at a temperature that promotes the reduction reaction most without burning it more than necessary. However, such approaches demonstrate that high steam generation efficiency and high smelt reduction rates may be mutually exclusive. The reason why such problems have not been sufficiently clarified or paid attention to is that the steam generation function in recovery boilers has been treated as a secondary function, and for this reason, the steam generation efficiency has decreased to a level that is lower than that of black liquor. This is thought to be due to the fact that the boiler efficiency is not accurately defined in consideration of changes in the amount of combustion heat in the boiler, the heat of reduction reaction m, etc.

The present invention has been made in view of the above-mentioned circumstances, and aims to provide optimal combustion for a recovery boiler that searches for the optimal combustion state for operation and executes combustion control, taking both steam generation efficiency and smelt reduction rate into consideration. The purpose is to provide a control device.

[Means for Solving the Problems] In order to achieve the above objects, the present invention includes a heat ratio calculation means for calculating the heat ratio of heat output and heat input of a recovery boiler;
an evaluation index calculation means for calculating an evaluation index of the overall operating state of the recovery boiler as a function of the heat ratio and at least 502 m degrees of the combustion exhaust gas; and an adjustment means for obtaining a manipulated variable for controlling the combustion state of the recovery boiler; and set value varying means for varying the set value of the adjusting means so that the evaluation index moves toward an extreme value or a target value.

[Function] Therefore, by taking the above-mentioned measures, the steam generation efficiency can be determined as the heat ratio of the heat input brought into the heat output/heat conversion section or recovery boiler and the heat output that can be effectively extracted as steam. By obtaining the efficiency as a measure, detecting the improvement in the smelt reduction rate in the form of a decrease in the SO2 concentration of the combustion exhaust gas, and obtaining the evaluation index of the recovery boiler as a function of the heat ratio and 8021I degree, the steam generation efficiency can be determined. An evaluation index of the operation state of the recovery boiler is obtained by considering both the smelt reduction rate and the smelt reduction rate, and the operation amount for the combustion state υ1111 of the recovery boiler is manipulated so that this evaluation index moves toward the extreme value or the target value. By taking both the steam generation efficiency and the smelt reduction rate into consideration without being biased toward one side, the operation of the recovery boiler can be realized using a favorable weir.

[Example] Hereinafter, an example of the present invention will be described with reference to FIG. In this figure, the same parts as in FIG. 8 are given the same reference numerals and detailed explanations are omitted. That is, in the apparatus of the present invention, a black liquor volumetric flow rate detector 32 and a black liquor density detector 33 are installed in the injection black liquor line 31, and the flow rate signal and density signal detected by these detectors 32 and 33 are input. It is supplied to the thermal calculation section 34. This heat input calculation section 3
Reference numeral 4 calculates the heat temperature, that is, the heat input, brought into the recovery boiler 2 using the flow rate signal and the density signal, and supplies the calculated heat input to the dividing section 5.

Further, a feed water flow type detector 37 and a feed water temperature detector 38 are installed in the water supply line 36, and their respective flow rate signals and temperature signals are supplied to a heat output calculation section 39. Also,
The heat output calculation unit 39 receives a main steam flow rate signal, a main steam pressure signal, and a main steam temperature signal from a main steam flow rate detector 40, a main steam pressure detector 41, and a main steam temperature detector 42 installed in the main steam line 16. signals are provided respectively.

These three heat output calculation units calculate the amount of heat given by combustion of black liquor while the feed water supplied to the recovery boiler 2 is taken out as main steam, that is, the effective amount of heat taken out of the recovery boiler 2, and calculate the above-mentioned division. 35. The output of the heat input calculation part 34 and the output of the heat output calculation part 39 are input to the division part 35, and the heat output/heat input ratio is obtained by dividing the latter by the former.

In addition, 5O2 of the combustion exhaust gas 7 is placed on the downstream side of the electrostatic precipitator 11.
An 802 degree concentration detector 21 for detecting i1 degree is installed, and the 50211 degree signal detected here is supplied to the weighted sum calculation section 44 together with the output of the division section 35. This weighted sum calculating section 46 calculates the evaluation index of the operating state of the recovery boiler 2 by calculating the weighted sum of the output of the dividing section 35 and 30211 degrees. An extreme value search section 45 and a primary low air flow rate controller 46 are provided on the output side of the weighted sum calculation section 44. This extreme value search section 45 is for increasing or decreasing the set value of the primary low air flow rate controller 46 so that the evaluation index of the operating state of the recovery boiler 2 obtained as the output of the weighted sum calculation section 44 moves toward an extreme value. . Further, the primary low air flow m controller 46 controls the primary low air flow rate 4 to a predetermined set value for controlling the combustion state of the recovery boiler 2.

Next, the operation of the apparatus configured as above will be explained.

When receiving the black liquid volumetric flow rate signal and the black liquor density signal from the injection black liquor line 31, the heat input calculation unit 34 multiplies both signals to obtain the weight flow rate of the injection black liquor, and also calculates the weight flow rate of the injection black liquor, and adds a predetermined value to this weight flow rate. The amount of heat input per unit time supplied into the recovery boiler 2 by the injected black liquor is determined by multiplying by the combustion calorific value per unit weight of black liquor.

On the other hand, the heat output calculating section 39 calculates the effective amount of heat given when the water supplied to the recovery boiler 2 is taken out of the system as steam, that is, the amount of heat output per unit time, using the following calculation formula.

Heat output = (main steam flow fliX main steam enthalpy) - (
Feedwater flow rate x Feedwater enthalpy) Main steam enthalpy - f (main steam temperature.

main steam pressure) Water supply enthalpy = g (@water temperature) In the above formula, f and q each represent a certain function.

In other words, main steam enthalpy is a function of main steam temperature and main steam pressure, and feed water enthalpy is a function of feed water temperature, and these relationships are listed in the steam table.
This can be realized by storing this information in advance and calculating it using a microcomputer or the like.

The heat input and heat output calculated by each calculation unit 34 and 39 as described above are sent to the division unit 35,
After determining the heat output/heat input ratio, which is an index representing the steam generation efficiency of the recovery boiler 2, this is supplied to the weighted sum calculation section 44. Furthermore, this weighted sum calculating section 44 is provided with the combustion exhaust gas 7.
The SO2 concentration signal is input, and an evaluation index of the operating state of the recovery boiler 2 is calculated based on the following equation.

Evaluation gfi index - α (heat output/heat input ratio) + β (802 concentration) However, α is a positive weighting coefficient, and β is a negative weighting coefficient. The first term of this evaluation index corresponds to steam generation efficiency,
The first term increases as the steam generation efficiency increases. on the other hand,
The second term of the evaluation index can be considered as corresponding to the smelt reduction rate. In other words, the sulfur content (8 minutes) introduced into the furnace is oxidized in the furnace to generate sulfur oxides, and if the high temperature reduction reaction is sufficiently carried out at the bottom of the furnace, N
A2 gas or Na2O fume is generated at the bottom of the furnace,
These remove the sulfur oxide content generated in the furnace with sodium sulfate (
The degree of SO2'a in the exhaust gas at the boiler outlet tends to decrease, and the smelt reduction rate tends to increase. Therefore, the height of 502i1 degrees becomes one indicator of the smelt reduction rate through the high-temperature reduction reaction state at the bottom of the furnace, and therefore, the second term is negatively weighted because the SO2 concentration tends to decrease as the smelt reduction rate increases. It increases by multiplying by a coefficient. Therefore, this evaluation index tends to increase as both the steam generation efficiency and the smelt reduction rate become higher. In addition, the SO2 concentration is an indicator of the high smelt reduction rate, and it is also directly measured in the boiler 2.
The lower the evaluation index, the more desirable it is from the viewpoint of pollution prevention, and the higher the evaluation index, the more desirable it is for operation.

Furthermore, by changing the weighting coefficient of either the heat output/heat input ratio or the SO211 degree, the weight of the steam generation efficiency and the smelt reduction rate can be changed. In this way, this device takes into consideration the relationship between steam generation efficiency and smelt reduction rate, and also
It is possible to obtain an evaluation index that meets the requirements of the operator by taking into account direct effects such as boiler corrosion prevention effect and environmental conservation effect.

The evaluation index obtained as described above is then input to the extreme value search section 45. This extreme value search unit 45 performs a trial search while increasing and lowering the setting value of the primary low air flow rate controller 46 so that the value of this evaluation index approaches the maximum value, and when the evaluation index is at the maximum value, the primary low air flow rate is Retains the setting value. As the algorithm for this extreme value search, a known algorithm such as the mountain climbing method can be used.

Therefore, according to the above configuration, black liquor is produced in the optimum condition from the recovery boiler operating side, taking into account both steam generation efficiency and smelt reduction rate, and also taking into account the direct effect of 5o211 degree reduction. By searching for a primary low air flow rate for combustion, combustion can be controlled in the recovery boiler in an optimal state.

In addition, the heat input calculating section 34 of the above embodiment uses heavy I! of black liquor. Although the combustion calorific value is calculated based on the amount, if the density of the black liquor does not change much, the combustion calorific value may be calculated based on the volumetric flow rate. In addition, if the weight ratio of organic components such as lignin, which is a combustible substance contained in black liquor, changes due to changes in the properties of black liquor, if this can be estimated using an appropriate method, the flow rate of organic components in black liquor can be estimated. A more accurate amount of heat input can be obtained by calculating the amount of heat generated. Furthermore, if there is a line other than the main steam line 16 as a line for extracting generated steam, the amount of heat extracted from the fluid in that line is added in the heat output calculation. In addition, in the above embodiment, the heat output/heat input ratio is calculated using the heat conversion ratio, but it can be more simply calculated as the ratio between the amount of generated steam and the black liquid product flow rate or the black liquid product flow rate. It is also possible to find the gain and use it as a substitute for the heat output/heat input ratio. Furthermore, if the water ratio in the black liquor varies, the latent heat of evaporation of water can be taken into account, and if the mirabilite (Na2804) fl in the black liquor varies, the amount of endothermic heat generated during the reduction reaction can be determined. The fever and
If it is taken into account in calculating the heat input ratio, the heat output/heat input ratio can be obtained more accurately. Further, if there is a fluctuation in the sensible heat of the combustion air, this may be taken into consideration. Furthermore, in the above embodiment, the primary low air flow rate is used as the operation for controlling the combustion state;
The extreme value search method may be performed using other manipulated variables that cause fluctuations in the combustion state, such as the distribution ratio of primary air to the total air amount, the flow rate of the injected black liquor, and the temperature of the injected black liquor.

Next, FIG. 2 shows another embodiment of the present invention, in which a radiation thermometer 51 is installed on the outer wall of the recovery boiler 2, and the radiation thermometer 51 is used instead of the weighted sum calculation section 44. The furnace temperature measurement signal, the heat output/heat input ratio, and the S○2
may be supplied to the weighted sum calculating section 44A to obtain a weighted sum. The temperature inside the furnace can be measured by appropriately determining the field measurement wavelength of the radiation thermometer 51, such as the surface temperature at the top of the char bed 3 or the flame temperature directly above the char bed. In addition, the radiation temperature system 5
Instead of 1, it is also possible to install a thermal lightning pair or the like in the upper part of the furnace and measure the atmospheric temperature of the gas inside the furnace to know the temperature inside the furnace.

Therefore, according to the above configuration, the evaluation index is heat output and
It is obtained as a weighted sum of three factors: heat input ratio, SO2 degree, and furnace temperature. Since the furnace temperature is an indicator of active combustion, a high furnace temperature contributes to steam generation. In addition, especially if the temperature inside the furnace is at a maximum within the temperature range where the reduction reaction can be carried out properly (e.g. 900°C to 1200°C), both the smelt reduction rate and the steam generation rate will be reduced due to the 802 scavenging effect due to the scattering of Na2 gas. can be improved.

In this way, the furnace temperature can be used as an index for achieving the optimum combustion state.

Next, FIG. 3 is a diagram showing another embodiment of the present invention, in which the CO2m degree 52 of the combustion exhaust gas 7 is set, and instead of the weighted sum calculation section 44 shown in FIG. Said G.O.
The weighted sum calculating section 44B may calculate a weighted sum by calculating the COZ concentration of the two concentration meters 52 by the heat output/heat input ratio and the SO2 concentration signal by m0. In other words, according to the embodiment shown in FIG. 3, the evaluation index of the operating state of the recovery boiler 2 can be obtained as the weighted sum of the heat output/heat input ratio, the 50211 degrees, and the 0021 concentration. Since the COZ concentration is an index of the combustion state in the furnace, the maximum CO211 degree can also be used as an index of the optimal combustion state. Therefore, it is effective to incorporate this into the evaluation index and use it as captured information for comprehensive driving condition evaluation.

Next, in FIG. 4, the weighted sum calculation unit 4 takes in all of the heat output/heat input ratio, SO2 concentration, furnace temperature, and CO2 concentration.
The configuration may be such that the evaluation index is obtained using 4C.

In addition, although the above actual flow example calculates a weighted sum, it is also possible to provide an optimal combustion control device that is more in line with the actual operating situation by using, for example, pressure line calculation. FIG. 5 shows the configuration for this purpose. That is, a function generator 53 corresponding to the SO2 concentration and a function generator 54 corresponding to the heat output/heat input ratio.
These function data may be input to the weighted sum calculation unit 44D, and the extreme value search may be stopped when the target value is reached. That is, in this embodiment, the function generator 44
．． 45 to generate functional data as shown in Figures 6 (A) and (B), and when the heat output/heat input ratio or SO2 concentration reaches the target value, the extreme value search is stopped and the state is maintained. Therefore, in pursuit of extreme values of steam generation efficiency or smelt reduction rate, stable char bed 3
It is possible to prevent the recovery boiler from falling into an unstable combustion state, such as by inhibiting the formation of combustion and repeating periodic combustion failures.

Further, as an applied example of the present invention, it is also possible to take into account constraints such as the maximum limit value of NOX concentration in the calculation of the evaluation index.

[Effects of the Invention] As detailed above, according to the present invention, the steam generation efficiency is improved based on the phenomenon that the combustion state that maximizes the steam generation efficiency of the recovery boiler and the combustion state that maximizes the smelt reduction rate are different. We provide an optimal combustion control device for a recovery boiler that can guide the combustion state of the recovery boiler to a state that meets the requirements of the operator by taking into account both the smelt reduction rate and the smelt reduction rate, and that can stably continue the target operating state. can.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the device of the present invention, FIGS. 2 to 5 are block diagrams explaining other embodiments of the present invention, and FIG. 6 is a function diagram explaining the operation of FIG. 5. The data diagram, FIGS. 7 and 8 are a configuration diagram of a conventional recovery boiler and a configuration diagram of a combustion control device for the recovery boiler, and FIG. 9 is a diagram illustrating problems with the conventional system. 2... Recovery boiler, 3... Char bed, 7...
Combustion exhaust gas, 17... Smelt, 34... Heat input calculation section, 35... Division section, 39... Heat output calculation section, 44.
44A, 44B, 44C, 44D... weighted sum calculation unit,
45...Extreme value search unit, 46...Air flow rate controller. Applicant's Representative Patent Attorney Takehiko Suzue Figure 2 Figure 4 Prisoner Figure 3 Figure 5 (A) CB) Figure 6 Figure 7 Figure 8 Figure 9

Claims (6)

[Claims] (1) By injecting black liquor into the furnace and injecting combustion air into the furnace to combust the black liquor, the raw material for cooking chemicals contained in the black liquor is recovered and by heat exchange with the combustion exhaust gas. In an optimal combustion control device for a recovery boiler that generates steam, a heat ratio calculation means for calculating a heat ratio from heat output and heat input of the recovery boiler; an evaluation index calculation means for calculating an evaluation index of the overall operating state of the recovery boiler based on the SO_2 concentration; an adjustment means for obtaining a combustion state control operation amount of the recovery boiler; 1. An optimal combustion control device for a recovery boiler, comprising: set value varying means for varying the set value of the adjusting means so as to move toward (2) As the evaluation index calculation means, a furnace temperature detection means of the recovery boiler is provided, and the ratio of the calorific value, the SO_2 concentration, and the furnace temperature detected by the furnace temperature detection means are calculated.
2. The optimal combustion control device for a recovery boiler according to claim 1, wherein an evaluation index of the operating state of the recovery boiler is calculated based on a function of the operator. (3) As the evaluation index calculating means, a means for detecting the concentration of CO_2 contained in the combustion exhaust gas of the recovery boiler is provided, and a function of the three factors of the calorific ratio, the concentration of SO_2, and the CO_2 detected by the means for detecting the CO_2 concentration is provided. 2. The optimal combustion control device for a recovery boiler according to claim 1, wherein an evaluation index of the operating state of the recovery boiler is calculated based on the above. (4) The evaluation index calculating means calculates the evaluation index of the operating state of the recovery boiler based on a function of four factors: the heat ratio, the SO_2 concentration, the furnace temperature, and the CO_2 concentration. An optimal combustion control device for a recovery boiler according to scope 1. (5) As the evaluation index calculation means, a target value is set for any one or more of the heat ratio, SO_2 concentration, furnace temperature, and CO_2 concentration, and when any one of these reaches the target value, the The optimal combustion control device for a recovery boiler according to claim 1, which calculates the evaluation index so as not to affect the evaluation index of the operating state of the recovery boiler. (6) The evaluation index calculation means according to any one of claims 1 to 5, wherein the evaluation index of the operating state of the recovery boiler is calculated using the NOx concentration as a constraint condition. Optimal combustion control device for recovery boilers.