JP7214940B2 - Heating furnace air volume control method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for properly controlling the burner air flow rate supplied to a burner installed in a heating furnace. In particular, the present invention relates to a method for controlling the amount of air in a heating furnace, which calculates the amount of air entering the heating furnace from a path other than the regular supply route, and causes the burner to burn properly based on the calculated amount of entering air. be.

A cast slab, such as a slab, manufactured in an ironworks may be heated to a predetermined temperature using a direct-fired heating furnace. In such a heating furnace, a burner for heating the inside of the heating furnace is generally controlled to keep the air ratio constant. Here, the air ratio is the ratio of the air actually supplied to the theoretical air amount, assuming that the minimum amount of air theoretically required for complete combustion is "theoretical air amount". . The reason why such a concept of air ratio is used is that in a heating furnace, intruding air may enter the furnace from other than the normal supply route.

For example, even if there is constantly air entering from the outside of the above-described heating furnace, the effect of the entering air will be small if the combustion load of the burner is large. However, when the combustion load of the burner is small, the influence of the intruding air on the combustion of the burner becomes large, and the air ratio of the burner changes, resulting in an increase in the oxygen concentration in the furnace. When the oxygen concentration in the furnace increases in this way, the amount of generated _NOx increases, and conversely, the problem arises that the unburned gas concentration increases due to the shortage of the air amount in the furnace.

In order to solve such problems, the techniques of Patent Document 1 and Patent Document 2 have been proposed.
First, in Patent Document 1, continuous hot rolling is provided with a regenerative switching combustion burner in at least two combustion control zones, and a detector for detecting the oxygen concentration in the exhaust gas in the exhaust gas induction system of the combustion control zone. A continuous steel material heating furnace using a continuous multi-zone walking beam for heating steel materials to a predetermined temperature, wherein the oxygen concentration is detected in the induced exhaust gas header pipe for each combustion control zone having the regenerative switching combustion burner. A continuous steel heating furnace is disclosed in which the oxygen concentration detector and control means for controlling the air ratio in the furnace based on the detected value of the detector are installed. In this technology, the air ratio control amount is calculated from the oxygen concentration in the furnace, the fuel flow rate and the fuel air amount, and the air ratio is controlled by controlling the fuel air amount so as to eliminate the deviation from the set air ratio. suppresses the generation of _x .

On the other hand, Patent Document 2 discloses a combustion control method for combustion equipment for obtaining optimum combustion in a combustion equipment where outside air can enter. Based on the deviation between the measured oxygen concentration value and the target set value, a procedure for obtaining the first air-fuel ratio for eliminating the deviation, and the measured value of the carbon monoxide concentration similarly measured on the outlet side of the combustion zone and the target set value, a procedure for obtaining the second air-fuel ratio for eliminating the deviation, a procedure for obtaining a weighting factor according to the fuel flow rate, the reactor pressure and the combustion state, and using the weighting factor a procedure for obtaining a set air-fuel ratio by weighted averaging the first and second air-fuel ratios with the above; a procedure for obtaining a required combustion air amount by multiplying the theoretical air amount by the set air-fuel ratio; and the required combustion air amount; A method of controlling combustion in a combustion facility is disclosed, comprising:

This technology eliminates the deviation between the theoretical air ratio calculated from the fuel flow rate, the oxygen concentration on the combustion side, and the unburned gas concentration measured as a weighted average, and the target value. An optimum combustion state can be obtained by calculating the air ratio and performing control according to it.

JP 2007-285564 A JP-A-61-49928

The technique of Patent Document 1 described above is applicable only within a range where the influence of intruding air is small,
It does not directly deal with intruding air, which is a factor in increasing the oxygen concentration in the furnace. Therefore, in actual control (control when there is an influence of intruding air), the time constant is large and the hunting of the air ratio becomes large. In other words, there is a problem that the correction width has to be reduced, and the effect of reducing the oxygen concentration is also reduced.

In addition, the technique of Patent Document 2, like the technique of Patent Document 1, controls the air ratio without calculating the intruding air amount, so the correction width must be small, and the oxygen concentration reduction effect is also small. It has the same problem as in Patent Document 1 that it becomes. Furthermore, the technique of Patent Document 2 uses a weighting factor calculated based on the in-furnace combustion state, and this weighting factor is determined by the user's value judgment.

In other words, in the technique of Patent Document 2, the influence of the in-furnace oxygen concentration and the in-furnace unburned gas concentration on the air ratio is expressed using weighting coefficients. However, with this method of setting weighting factors, the generality of the weighting factors is lost depending on the user's value judgment, and an error occurs in the target value of the air ratio, which makes it impossible to obtain the optimum combustion state in the heating furnace. There is
In addition, the techniques of both patent documents correct the air ratio with the in-furnace oxygen concentration, but do not obtain the amount of infiltrating air, which is the original factor, so the time constant (time delay) is large, and the air ratio is changed. Hunting increases and it is not possible to control the air ratio appropriately. Therefore, it is necessary to limit the correction width to a small value, and there is a problem that the effect of reducing the oxygen concentration is also reduced.

Therefore, the applicant directly calculates the amount of intruding air and uses a function obtained by subtracting the calculated intruding air from the set air ratio as the objective function, thereby controlling the burner combustion air flow rate by a feedforward method without time delay. I came up with the idea. As a result, the burner air ratio can be controlled at a constant value with high accuracy, and as a result, the optimal combustion state in the furnace can be constantly maintained.

In addition, the applicant has proposed that the burner combustion air flow rate is fed with a small time constant by using the function for calculating the burner air ratio with the infiltration air amount, which is the original factor of the oxygen concentration change in the furnace, as the independent variable, as the objective function. We have derived a technology that can be controlled by the forward method. As a result, it became possible to control the burner air ratio to a constant value with high accuracy, and to maintain the optimal combustion state in the furnace steadily.

Furthermore, in Patent Documents 1 and 2, the oxygen concentration is measured to deal with the influence of the intruding air. The problem is the delay in dealing with In response to this point, the applicant has come up with a technique that can quantify the amount of air infiltration in real time without stopping the operation of the heating furnace by using the infiltration air calculation method, and as a result, early detection of defects is possible. made it possible.

In summary, in order to solve the problems in controlling the burner air ratio in the heating furnace, which cannot be solved by Patent Documents 1 and 2, in the present invention, in a heating furnace where outside air can enter, air entering the heating furnace The purpose of this study is to propose a burner air ratio control method for an industrial furnace that can reduce the concentration of unburned gas and suppress the amount of generated NOx by determining the amount of infiltrated air and controlling the amount of burner air according to the determined amount of infiltrated air. do.

In order to solve the above problems, the method for controlling the amount of air in a heating furnace of the present invention employs the following technical means.
That is, the method for controlling the amount of air in a heating furnace according to the present invention provides a heating furnace having a plurality of zones in which a gas burner for heating a slab to a predetermined temperature is provided in a continuous manner, wherein a gas burner is provided for each of the plurality of zones. Calculates the amount of air that enters the zone from other than the air line connected to the gas burner, and controls the burner air flow rate that is supplied to the gas burner from the air line connected to the gas burner according to the calculated amount of intruded air. Based on the oxygen concentration in the furnace, the fuel flow rate of the gas burner, the burner air flow rate, and the temperature in the furnace, using the conservation of mass equation that considers unsteady changes in the concentration of each gas in the furnace and downstream advection, It is characterized by calculating the amount of intruding air.

Preferably, the burner air flow rate is controlled using a function for calculating a target burner air ratio with the intruding air amount as an independent variable.

According to the technology of the present invention, in a heating furnace where outside air can enter, the amount of air entering the heating furnace is determined, and the burner air amount is controlled according to the determined amount of entering air, thereby realizing an optimal combustion state. , it is possible to reduce the concentration of unburned gas and suppress the amount of NOx generated.

It is the figure which showed the heating furnace of this embodiment typically. It is the figure which showed the calculation flow which calculates the air ratio of a burner. It shows the control flow for controlling the heating furnace.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described based on the drawings. FIG. 1 is a diagram schematically showing the heating furnace of this embodiment.
The heating furnace 1 has an inlet for carrying a slab (material to be rolled) such as a slab into the furnace, and an outlet for carrying the slab out of the furnace. The slab continuously carried in through the carry-in port is carried in the furnace in the direction of the arrow in FIG. The slab carried into the furnace through the carry-in port is heated in the furnace and extracted through the carry-out port of the heating furnace 1 .

The cast slab carried out from the heating furnace 1 is then transported to a hot rolling mill and hot rolled into a hot rolled material. In the heating furnace 1, a plurality of gas burners 5 are provided along the transport direction (line direction) of the slab as a heating unit for heating the transported slab.
The heating furnace 1 has a plurality of zones arranged continuously from the inlet side to the outlet side. Specifically, the heating furnace 1 has a preheating zone 2 (preheating zone) for preheating the slab carried in at normal temperature, and further heating the preheated slab to about 1000 ° C. in order from the inlet side. A heating zone 3 (heating zone) and a soaking zone 4 (soaking zone) for further heating the slab heated in the heating zone to eliminate temperature unevenness are provided along the conveying direction of the slab. In other words, the heating furnace 1 has three zones, the preheating zone 2, the heating zone 3, and the soaking zone 4, so that they are adjacent to each other along the conveying direction, and the slab and atmospheric gas can move between the zones. It is a combination of

Each zone is provided with a line (piping) for supplying air and CO gas, which is fuel, and is connected to a gas burner 5 respectively. Specifically, an air line for supplying air is connected to the gas burner 5 arranged in the preheating zone 2 . This air line is provided with a valve for controlling communication of air and an air flow meter for measuring the flow rate of air, and the flow rate _Qa1 of air supplied through the air line is measured. The end of this air line is connected to the gas burner 5 .

Similarly, a fuel line for supplying fuel is connected to the gas burner 5 arranged in the preheating zone 2 . Like the air line, this fuel line is also provided with a valve for controlling fuel communication and a fuel flow meter for measuring the flow rate of fuel, and the flow rate _Qf1 of fuel supplied through the air line is measured. The end of this fuel line is also connected to the gas burner 5 .
Zones other than the preheating zone 2, that is, the heating zone 3 and the soaking zone 4 are similarly provided with oxygen lines and fuel lines, and valves and flow meters are provided on the respective lines. . Both the valve and the flowmeter described above measure the flow rate of air and fuel before being supplied into the heating furnace 1 .

On the other hand, in the zone, a gas burner 5 is provided at the tip of the air line and the fuel line. This gas burner 5 is provided in each of the preheating zone 2, the heating zone 3, and the soaking zone 4, and a furnace thermometer and a furnace oxygen concentration meter are attached near the gas burner 5 in each zone. there is
For example, taking the preheating zone 2 as an example, the temperature in the preheating zone 2 is measured by an in-furnace thermometer provided in the preheating zone 2, and the measured temperature data is taken into a process computer, PLC, or the like. Further, the oxygen concentration in the preheating zone 2 is measured by an in-furnace oxygen concentration meter provided in the preheating zone 2, and the measured oxygen concentration data is taken into the process computer, PLC, or the like. The process computer, PLC, etc. also take in the values of the air flow meter of the air line and the value of the fuel flow meter of the fuel line. Based on the values obtained in this way, the valves of each line are adjusted according to the control method described later. The heating zone 3 and the soaking zone 4 other than the preheating zone 2 are also provided with similar measuring means and data taking-in means, so that each zone can be controlled.

Between the preheating zone 2 and the heating zone 3, a flow path is formed to flow the gas preheated in the preheating zone 2 to the heating zone 3, and between the heating zone 3 and the soaking zone 4. is formed with a flow path for flowing the gas heated in the heating zone 3 to the soaking zone 4 . Through this flow path, not only the slab but also the gas can move between zones in the order of "preheating zone 2→heating zone 3→soaking zone 4". That is, if the gas composition of the preheating zone 2 can be calculated, the gas composition of the heating zone 3 into which the gas of the preheating zone 2 flows can also be calculated. Further, if the gas composition of the heating zone 3 is known, the gas composition of the soaking zone 4 into which the gas flows through the flow path can also be calculated. In this manner, in the three zones described above, if the state within a certain zone can be grasped, the states within the remaining zones can be calculated sequentially, and finally the state of the entire heating furnace 1 can be determined. It becomes possible.

Next, the method of controlling the burner air ratio of the heating furnace 1 considering the amount of intruding air according to the present invention will be described. This control method is implemented by a program that operates inside the aforementioned process computer, PLC, or the like.
As described in "Background Art", conventionally used methods for controlling the burner air ratio of the heating furnace 1 limit the correction width in order to avoid hunting of the air ratio due to O ₂ concentration correction. It can only be changed within a maximum range of ±0.5% based on the set air ratio (especially Patent Document 2, etc.).

On the other hand, in the control method of the present invention, the intrusion air flow rate in each zone is calculated from the O ₂ concentration, the fuel and the burner air flow rate, and the difference between the target air ratio and the intrusion air ratio is used as the objective function. The amount of air to be supplied is strictly determined. Therefore, it is possible to keep the inside of the heating furnace 1 at the target air ratio. Therefore, it is possible to suppress the generation of NO _X and reduce the concentration of unburned gas accompanying the reduction of the excessive oxygen concentration region in the furnace due to air intrusion.

A specific method for controlling the amount of air in the heating furnace 1 of the present invention will be described below.
The method for controlling the amount of air in the heating furnace 1 of this embodiment calculates the amount of air in the gas burner 5 according to the formula shown in FIG.
Specifically, the calculation formula in FIG. 2 described above is for calculating the state in the furnace for any one of the three zones.

For example, in the case of heating zone 3, the suffix x is "2" indicating heating zone 3. In the heating zone 3, the in-furnace oxygen concentration C _O2,2 measured by the in-furnace oxygen concentration meter in the heating zone 3, the in-furnace temperature T ₂ measured by the in-furnace thermometer in the heating zone 3, and the air line The air flow rate Q _a,2 supplied to the gas burner 5, the fuel flow rate Q _f,2 supplied from the fuel line to the gas burner 5, and the like are measured and input to the calculation formula.

In addition, the calculation formula formulates the relationship between the gas supplied to the zone and the gas discharged from the zone for each zone according to the concept of conservation of mass. The gases supplied to or discharged from this zone are O ₂ , N ₂ , CO, CO ₂ , H ₂ O as shown in composition i, and the unsteady changes and concentrations of these gases downstream Taking into account advection, the mass flow rate m _O2 ^L _,2 of the incoming air in the case of heating zone 3 and the concentration C _i,2 of each gas exiting heating zone 3 (i=O ₂ , N ₂ , CO, CO ₂ , H ₂ O), and further the concentration C _i,2 of each gas is converted into the volumetric flow rate Q ₂ .

Strictly speaking, the mass flow rate m _O2 ^L _,2 of the intruding air described above indicates the amount of oxygen contained in the amount of intruding air. Since the actual intrusion air amount in zone i includes gases other than oxygen, it is defined by the following formula.

becomes. For example, in the case of air, air can be considered to consist only of 21% oxygen and 79% nitrogen, so the nitrogen mass flow rate m _N2 ^L _,i in zone i is the oxygen mass flow rate m _O2 ^L _{, i} can be shown as follows.

When the amount of intruding air in zone i considering gases other than oxygen is calculated in this way, it can be shown as follows.

That is, if the above-described calculation formula is used, the mass flow rate m _O2 ^L _,x of the air entering each of the preheating zone 2, the heating zone 3, and the soaking zone 4 (x=1: preheating zone 2 , x=2: heating zone 3, x=3: soaking zone 4). Therefore, if the amount of air supplied from the air line to each zone is adjusted in accordance with the calculated mass flow rate of intruding air (if the amount of air supplied is reduced in accordance with the mass flow rate of intruding air), each Combustion can be performed in the optimum combustion state in the zone, and it is possible to reduce the concentration of unburned gas and suppress the amount of NOx generated.

In actual operation, the amount of air in the heating furnace 1 is controlled according to the flow chart of FIG.
As shown in FIG. 3, first, "gas property setting" is performed. This "gas physical property setting" is a setting such as a physical property related to each gas. This setting can be stored in advance in the control unit or the like, or can be input. After "setting gas properties", next, perform "operation data input".

"Operation data input" is operation data consisting of various conditions related to operation, such as the volume of the heating furnace 1, the furnace temperature, the upstream zone, or the composition and flow rate of the gas supplied into the zone from the inlet. It is to be entered. After "operation data input" is performed, next, "gas density calculation" is performed.
"Calculation of gas density" is to calculate and obtain the density of gas in each zone based on gas properties and operation data. After the "gas density" is calculated in this way, in the "burner combustion gas", the in-furnace oxygen concentration C _O2,x measured by the in-furnace oxygen concentration meter and the in-furnace thermometer in the heating zone 3 Measure the furnace temperature _Tx , etc., and determine the "infiltration air amount" using a formula in the next "intrusion air amount calculation".

After the "intrusion air amount" is calculated for each zone in this way, the "air amount" supplied to the gas burner 5 is adjusted based on the calculated "intrusion air amount" so as to enable ideal combustion. Set a target value.
When the target value of "air amount" is set, combustion is performed with the target "air amount".
Combustion with the target "air amount" is nothing other than controlling the heating furnace 1 with the air ratio λ _B obtained by the following equation. _QL is the intrusion air flow rate obtained by the method described above.

λ _B =1−Q _L /(A _o q) (1)
Here, Q _L : amount of intruding air, A _o : ratio of air flow rate required for complete combustion to fuel flow rate, q: fuel flow rate.
An actual machine test was conducted using the calculated air ratio λ _B , and the average CO concentration and average NO _x concentration measured in the heating furnace 1 were actually measured. As a result of the actual measurement, the applicant confirmed that it is possible to reduce the concentration of unburned gas and suppress the amount of generated NOx.

From the above, it is considered that the air amount control method of the present embodiment can suppress the increase in the NO _x concentration and reduce the unburned CO concentration. It is judged that an economic effect can be expected.
It should be noted that the embodiments disclosed this time should be considered as examples and not restrictive in all respects. In particular, matters not explicitly disclosed in the embodiments disclosed this time, such as operating conditions, operating conditions, various parameters, dimensions of components, weights, volumes, etc. Instead, a value that can be easily assumed by a person skilled in the art is adopted.

REFERENCE SIGNS LIST 1 heating furnace 2 preheating zone 3 heating zone 4 soaking zone 5 gas burner

Claims (2)

In a heating furnace provided with a plurality of connected zones equipped with gas burners for heating a slab to a predetermined temperature,
For each of the plurality of zones, the amount of air that intrudes into the zone from a line other than the air line connected to the gas burner is calculated, and the air line connected to the gas burner is directed to the gas burner according to the calculated amount of intrusion It controls the burner air flow rate to be supplied,
Based on the oxygen concentration in the furnace, the fuel flow rate of the gas burner, the burner air flow rate and the furnace temperature , the intruding air 2. The method for controlling the amount of air in a heating furnace according to claim 1, wherein the amount of air is calculated . 2. The method for controlling the amount of air in a heating furnace according to claim 1 , wherein the burner air flow rate is controlled using a function for calculating a target burner air ratio in which the intruding air amount is an independent variable.

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