JP2998897B2 - Air-fuel ratio control method of heating furnace - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the air-fuel ratio of a heating furnace by a cross limit control method used for controlling the air-fuel ratio of a heating furnace such as a continuous annealing furnace.

[Prior Art] In a heating furnace, control of the ratio of fuel to air, that is, the air-fuel ratio, has been an important issue from the viewpoint of energy saving and prevention of oxidation of materials.

In recent years, with the spread of digital instrumentation, the control of the air-fuel ratio has progressed rapidly, and the ratio control has shifted to the cross limit control, and the controllability of the air-fuel ratio has improved.

This cross limit control is used in industrial combustion furnaces, etc.
In this method, for the purpose of energy saving, pollution prevention, and furnace atmosphere control, both flow command values are limited based on the actual flow values of fuel and combustion air, and the air-fuel ratio is controlled to control the ratio.

For example, Japanese Patent Publication No. 60-14973 discloses a fuel supply system having a fuel flow meter and a fuel flow operation terminal, and a combustion device having an air flow meter and an air supply system having an air flow operation terminal. A detector for detecting a control amount; adjusting means for receiving a fuel flow command signal or an air flow command signal in response to the output signal of the detector; and a predetermined upper limit and lower limit related to the output of the air flow meter. First limiting means for limiting the command signal from the adjusting means; second controlling means having a predetermined upper and lower limit related to the output of the fuel flow meter and limiting the command signal from the adjusting means; Means for receiving the output of the first control means and the output of the fuel flow meter to limit the fuel flow control end; and receiving the output of the second limiting means and the output of the air flow meter to control the air flow rate. Combustion control and means for limiting the end is disclosed.

FIG. 2 is a schematic explanatory diagram of the double cross limit combustion control of the heating furnace.

In the figure, the set value of the fuel flow controller FIC / 1 is regulated by the actual value of the combustion air flow rate, and is a value that, when converted to an air ratio, results in excess air for negative bias α or excess fuel for positive bias β. Limited by Also a combustion air flow controller
The set value of FIC / 2 is also regulated by the actual value of the fuel flow rate, and is limited to a value that becomes an air surplus by a positive bias α or a fuel surplus by a negative bias β when converted into an air ratio.

That is, in this double cross limit method, the air-fuel ratio can be controlled within the range of the set air-fuel ratio + α.

[Problems to be Solved by the Invention] A cross limit control method in a conventional combustion control as described in the above-mentioned Japanese Patent Publication No. 60-14973, and a high / low selector is provided in both control systems of fuel and combustion air. The double cross limit control method, which defines the upper and lower limits of both fuel and combustion air, is very useful for controlling the air-fuel ratio within a certain range. ing.

For example, in a direct-fired furnace with a continuous annealing line, a combustion control system using a cross limit method is often used in order to keep the atmosphere in the furnace in a non-oxidizing state from the viewpoint of preventing oxidation of products. Response speed (combustion load change speed) is poor, that is, the speed of changing the energy input by combustion (flow rate of fuel and combustion air) is slow, so plate temperature deviation occurs and rapid line speed change is possible. However, there was a problem that efficiency could not be improved because of the lack.

An object of the present invention is to improve low responsiveness, which is a drawback of the above-described conventional cross limit control.

[Means for Solving the Problems] The present invention has a disadvantage that the response is not good in the cross limit control system. However, in order to strictly control the air ratio, the ratio between the two control systems is kept constant. It is based on the knowledge that it is not in the complicated crossing mechanism but in the flow characteristics of the control valve.It has a fuel flow control system and a combustion air flow control system, and has a fuel flow control system. The flow rate set value of the flow controller is limited by upper and lower limits determined by the actual value of the combustion air flow rate,
An air-fuel ratio control method for a heating furnace in which a set value of a flow controller of the combustion air flow control system is limited by upper and lower limits determined by an actual value of a fuel flow rate, and an air-fuel ratio is cross-limited within a predetermined range. Detecting the flow rate or opening of each of the flow rate control valves of the fuel flow rate control system and the combustion air flow rate control system, and setting the gain of the corresponding flow rate regulator to a large value in a region where the detected value is small; and This is a control method in which the gain of the corresponding flow rate controller is reduced as the detected value increases, so as to approach a steady-state gain corresponding to a normal maximum flow rate range.

[Operation] In the conventional cross limit control, the reason why the speed of changing the combustion load, that is, the speed of changing the flow rates of the fuel and the combustion air is low is that there is a problem in setting the gain of the combustion and air flow control system.

As shown in FIGS. 1 (a) and 1 (b), the normal flow rate control valve has a small flow rate change ratio with respect to the valve opening on the side close to the valve closing, and has a small ratio with respect to the valve opening on the opening side of the valve. Due to the flow rate characteristic of the valve, in which the rate of flow rate change is large, gain tuning is performed near the normal maximum. Therefore, the gain is insufficient in the low flow rate range, and the response is poor.

Further, in the double cross limit control, since both the fuel and air loops limit the set value of the own loop in accordance with the control amount of the other party, the change of the set value in a low flow rate region where the gain is insufficient. And the combustion load change speed becomes extremely slow.

That is, in the conventional cross limit control, the ratio can be maintained in the target range, but the response is sacrificed.

In the present invention, according to the flow rate characteristics of the flow rate control valve at each operation end of the fuel flow rate control system and the combustion air flow rate control system, a gain schedule of the flow rate regulator is always performed so as to obtain an optimum gain, that is, a flow rate control. By increasing the speed at which the flow rates of fuel and combustion air are changed by changing the gain of the vessel, the flow rate can be controlled with an optimum gain in any flow rate range.

For this reason, the response of the cross limit control becomes faster,
The plate temperature controllability is improved, and there is no limit on the line speed change rate, so that the efficiency is improved.

 Next, examples of the present invention will be described.

[Embodiment] The present invention will be described with reference to an explanatory diagram of an air-fuel ratio control system in a double-cross limit type combustion control of a continuous annealing furnace shown in FIG.

In FIG. 2, FIC / 1 of the fuel flow control system is cascaded from the TIC of the furnace temperature controller, but the upper and lower limit values (α and β are upper and lower limit biases determined from the actual value of the fuel air flow rate). Value) limits the FIC / 1 flow rate setting.

Similarly, the combustion air flow control system FIC / 2 also performs double cross limit control so that the air-fuel ratio falls within a certain range while being limited by the actual value of the fuel flow rate.

In the control systems FIC / 1 and FIC / 2, as shown in the gain scheduling flow rate control in FIG. 3, the gain of the FIC is always optimized depending on the valve characteristics, response, opening degree, fluid pressure, temperature, and the like. By performing gain scheduling as described above, it becomes possible to increase the combustion load change speed.

 FIG. 4 shows the embodiment.

FIGS. 4A and 4B show the case of the conventional control.
That is, the gain of the flow rate regulator is constant regardless of the flow rate range.

FIG. 4 (a) shows that the gain is set to be optimum in the normal maximum flow rate range. In this case, COG P
(Proportional operation) = 0.22, I (Integral operation) = 4, AIR P = 0.18, I
= 10. As described above, setting the control gain in consideration of the controllability in the high flow rate range, that is, a low gain can prevent instability in the high flow rate range, but deteriorates the responsiveness in the low flow rate range. Here, P means a proportional operation, represents a control operation for outputting an output having a magnitude proportional to an input, and
Means an integral operation, and indicates a control operation for outputting an output having a magnitude proportional to the time integral value of the input (JIS-Z-8116-1972).
See Automatic control terminology (general)).

In FIG. 4 (b), the gain is set so as to be optimal in the intermediate flow rate range, and the gain is too high in the high flow rate range. In this case, COG P = 0.46, I = 4, AIR P =
0.50, I = 10. As described above, when the control gain is set in consideration of the controllability in the intermediate flow rate range, that is, when the gain is set to a high gain, the responsiveness in the low flow rate range is improved, but the gain is too high in the high flow rate range and becomes unstable. . As described above, in the conventional control system, the control gain is set to a low gain in consideration of controllability in a high flow rate region at the expense of responsiveness in a low flow rate region to prevent control instability. ing. For this reason, it is difficult to control the flow rate without causing instability with good responsiveness in the entire flow rate range.

On the other hand, in the case of FIG. 4 (c) of the high response type cross limit control of the present invention, the gains of COG and AIR are changed as shown in the figure so that the optimum gain is obtained in each flow rate range. . That is, in accordance with the characteristics of the flow control valve of the fuel flow control system and the combustion air flow control system, the gain of the flow controller is set to a maximum in a small flow rate range of the flow control valve, and the gain is increased as the flow rate increases. Is reduced, and the gain of the flow controller is set to a steady gain corresponding to the reference flow rate near the reference flow rate of the flow control valve. The I gain is fixed at COG, I = 4 and AIR I = 10. That is, when the combustion load is changed from the low flow rate range to the high flow rate range, the gain of the flow rate control becomes the characteristic of the valve,
Temporarily decreases according to response, opening degree, fluid pressure, temperature, etc., and secures controllability in low flow rate range and prevents instability in high flow rate range. Thus, the flow rate is controlled. When the combustion load is changed from the high flow rate range to the low flow rate range, the gain of the flow rate control temporarily increases. As described above, the flow rate regulator controls the gain for controlling the flow rate in accordance with the flow rate characteristics of the valve, the responsiveness, the opening degree, and the pressure, temperature, etc. of the fluid within a range that does not become unstable in any flow rate range. In addition, gain scheduling for changing the combustion load changing speed, that is, changing the energy input amount (the flow rate of fuel and combustion air) so as to be as fast as possible is performed.

In the conventional double cross limit combustion control system shown in FIG. 2, if the gain is increased in order to increase the responsiveness, it becomes unstable in a high flow rate region as shown in FIG. 4 (b).

In the air-fuel ratio control method of the heating furnace in the high response type cross limit control system in which the gain scheduling of the flow control system is performed, the air-fuel ratio is stable without exceeding the upper and lower limit values as shown in FIG. In addition, it has become possible to obtain a response more than twice that of the conventional case.

Although the present invention has been described for the case of an air-fuel ratio control system, the same effect can be obtained in the case of a flow rate control system using another cross limit method.

[Effects of the Invention] According to the heating furnace air-fuel ratio control method using the heating furnace high response type cross limit control method of the present invention, the low response which is a drawback of the conventional cross limit control can be sufficiently improved. In addition, the effect that the temperature controllability is improved and the efficiency can be increased by increasing the response speed is achieved.

[Brief description of the drawings]

1 (a) and 1 (b) are explanatory diagrams showing the flow characteristics of the COG and the air valve, respectively. FIG. 2 is a diagram of a double cross limit combustion control system, FIG. 3 is a diagram of a gain scheduling flow control system, FIG. (A) is an explanatory diagram of a case where the gain is optimal in the case of the conventional control, FIG. 4 (b) is an explanatory diagram of a case where the gain is too high in the case of the conventional control, and FIG. 4 (c) is the present invention. FIG. 4 is an explanatory diagram in the case of the air-fuel ratio control method of the heating furnace of FIG. In the figure, 1: a heating furnace used in the air-fuel ratio control method of the heating furnace of the present invention, 2: burner of the heating furnace.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Katsuhiro Irie, Inventor 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Kazunari Ikegami 1-1-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yoshio Otsuka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-60-82561 (JP, A) JP-A-58-153734 (JP, A) JP-B-60-14973 (JP, B2) JP-B-60-28886 (JP, B2)

Claims (1)

(57) [Claims] 1. A fuel flow control system, comprising: a combustion air flow control system; wherein a flow set value of a flow regulator of the fuel flow control system is limited by upper and lower limits determined by an actual value of the combustion air flow; An air-fuel ratio control method for a heating furnace in which a set value of a flow controller of the combustion air flow control system is limited by upper and lower limits determined by an actual value of a fuel flow rate, and an air-fuel ratio is cross-limited within a predetermined range. Detecting the flow rate or opening of each of the flow rate control valves of the fuel flow rate control system and the combustion air flow rate control system, and setting the gain of the corresponding flow rate regulator to a large value in a region where the detected value is small; and A method for controlling the air-fuel ratio of a heating furnace, wherein the gain of the flow controller is reduced as the detected value increases to approach a steady-state gain corresponding to a normal maximum flow rate range.