JPS5817373B2 - Combustion control method using oxygen concentration control in combustion furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a combustion control method for a combustion furnace, and more particularly to a combustion control method using oxygen concentration control.

Normally, when controlling the temperature of a metal heating furnace, etc., different heating temperatures are required, so the set temperature changes significantly and the rate of change in the fuel command is large.

In such a furnace, when an operator suddenly changes the set temperature, a state of excess fuel or excess air temporarily occurs due to the difference in response time between the fuel flow rate control system and the air flow/control system.

Therefore, during normal work, it has been common practice to set a higher excess air rate than necessary in consideration of the safety factor.

For this reason, the fuel cost increases due to a decrease in the furnace temperature due to the excessive amount of combustion air, and the amount of NOx generated increases, which is unfavorable from an environmental and energy saving point of view.

The present invention was made in view of these drawbacks, and its purpose is to suppress the oxygen concentration of the furnace exhaust gas after combustion to a moderately low level, that is, without setting an unnecessarily high excess air ratio. It is an object of the present invention to provide a combustion control method that can provide stable control even when the temperature setting of a furnace is changed, and is thus suitable for reducing NOx and saving energy.

In order to achieve the above object, the present invention measures the oxygen concentration in exhaust gas from a combustion furnace, adjusts the oxygen concentration so that the measured oxygen concentration value is adjusted to a relatively low oxygen concentration set value,
The air-fuel ratio is controlled by the output of the adjustment calculation, and the proportional gain and integral time, which are calculation constants of the proportional operation and integral operation of the adjustment calculation, are automatically changed according to the measured value or set value of the fuel or air flow rate. This is how it was done.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 shows an embodiment of a combustion control device for carrying out the combustion control method of the present invention, in which 1 is a humidity controller for maintaining the humidity of the furnace 14 at a set humidity; is a fuel flow rate controller for adjusting the fuel flow rate according to the command from the humidity controller 1, 3 is a square root calculator, 4 is a transmitter, 5 is an orifice, 6 is a flow rate control valve, 7 is a pipe, and 8 is an air-fuel ratio a setting device; 9, an air flow rate controller for adjusting the combustion air flow rate according to a command from the air-fuel ratio setting device 8; 10, an oxygen concentration controller for controlling the oxygen concentration in the exhaust gas of the furnace 14 to a set concentration; 11; 12 is a selection switch, 12 is a proportional gain and integral time generator, 13 is a combustion zone, 14 is a combustion furnace, 15 is a thermocouple, 16 is an oxygen analyzer, and 17 is an mV/I converter.

The operation of the combustion control device having the above configuration will be explained next.

In addition, in the figure, for convenience of explanation, the combustion furnace is shown as having one combustion zone.

Now, when the temperature of the furnace 14 is set to a certain humidity θsp, the humidity controller 1 receives the measured humidity signal θpv from the thermocouple 15 that measures the humidity of the furnace 14 and calculates the input deviation (θ
The fuel flow rate setting value Gsp of the fuel flow rate controller 2 is given so that sp-θpv) becomes zero.

The fuel flow rate controller 2 has the fuel flow rate measurement value Gpv obtained through a transmitter 4 that converts the differential pressure signal of the orifice 5 into an air signal and a square root calculator 3 that opens the output of the transmitter 4 to the above set value. The opening and closing of the control valve 6 is controlled to match Gsp.

On the other hand, the air flow controller 9 measures the air-fuel ratio μ and the fuel flow setting value G.
The air flow rate setting determined by sp (k sp) is received from the air-fuel ratio setting device 8 and the control valve 6 is controlled so that the measured air flow value Apv matches the set value Asp, as in the case of the fuel flow controller 2. do.

In this way, the fuel flow rate and air flow rate are adjusted according to the desired furnace temperature, but if the air-fuel ratio set value is fixed, when the set temperature θsp of the furnace 14 is changed, the fuel flow rate cannot be controlled. The oxygen concentration in the exhaust gas changes due to the difference in response time between the system and the air flow rate control system.

The relationship between the oxygen concentration of 02%, the amount of NOx generated, and energy loss is as shown in FIG.

Therefore, oxygen concentration control is performed to maintain the oxygen concentration of the in-furnace exhaust gas at an optimal value.

The oxygen concentration controller 10 is for this purpose, and is a PI controller that continuously performs proportional and integral operations.
Oxygen concentration measurement value in exhaust gas detected by 02p
In order to maintain v at the set value 02sp, input deviation (02pv
-02sp), perform proportional and integral calculations,
An air-fuel ratio set value μ is given to the air-fuel ratio setter 8.

Here, when performing proportional and integral calculations, the following well-known formulas are used.

y = Kp (Z+ - f Za t ) TI In the above equation, y corresponds to the air-fuel ratio setting value μ, Z corresponds to the input deviation (02pv-02sp), Kp is the proportional gain, and TI is the integral time It is.

However, if the proportional gain and integral time of the oxygen concentration controller 10 are set to fixed values, if the fuel flow rate increases or decreases, the stability of the control system will be disrupted, and in some cases, an unburned state (black smoke) may occur. or hunting occurs.

In order to solve this problem, the present invention automatically changes the proportional gain and integration time of the oxygen concentration controller according to the flow rate.

That is, the proportional gain and integral time preset by the fuel flow rate measurement value Gpv given via the switch 11 are outputted from the proportional gain/integral time generator 12 and given to the oxygen concentration controller 10.

FIG. 3 shows the relationship between the proportional gain and integration time that should be output with respect to the fuel flow rate measurement value Gpv; as the fuel flow rate increases, a smaller value is output, and as the fuel flow rate decreases, a larger value is output.

The relationship between the proportional gain and integral time with respect to the fuel flow rate is determined in advance based on experiments, as shown in Figure 3, for example, and the proportional gain and integral time are changed according to the measured fuel flow rate value. It has become.

In the embodiment shown in FIG.
is selected, but it is also possible to select Gsp by switching the switch 11.

Since Gpv is controlled to follow Gsp, they both show almost the same value, so either one may be selected.

In this way, the output of the oxygen concentration controller, that is, the air-fuel ratio, is changed according to the oxygen concentration and the fuel flow rate, so that a good combustion state is maintained.

In the above embodiment, the proportional gain and the integral time are changed according to the fuel flow rate signal, but the fuel flow rate control loop and the air flow rate control loop are replaced, and the output of the temperature controller 1 is changed. The set value Asp of the air flow controller is given, the measured air flow value Apv or the set value Asp is used to drive the proportional gain/integral time generator 12, and the output of the air-fuel ratio setting device 8 is used to set the set value of the fuel flow controller. Gsp may also be given.

As is clear from the above description, according to the present invention, the oxygen concentration in the exhaust gas can be maintained at a relatively low desired value, and the calculation constant of the proportional/integral calculation of the oxygen concentration can be adjusted according to the flow rate. As a result, the control system is stable, the occurrence of unburned states can be avoided, and low NOx and energy savings can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of a combustion control device for implementing the combustion control method according to the present invention, and FIG. 2 is a diagram showing the relationship between the oxygen concentration in exhaust gas, the amount of NOx generated, and energy loss. , FIGS. 3a and 3b are diagrams showing the relationship between fuel flow rate, proportional gain, and integration time. 1... Temperature controller, 2... Fuel flow controller, 8...
・Air-fuel ratio setting device, 9...Air flow rate controller, 10...
Oxygen concentration controller, 12... Proportional gain/integral time generator, 13... Combustion area, 14... Combustion furnace, 15...
Thermocouple, 16...Oxygen analyzer, 17...mV/I converter.

Claims (1)

[Claims] 1. When controlling the fuel flow rate and air flow rate supplied to the combustion zone of a combustion furnace according to a desired furnace temperature, one of the fluids is controlled so that the measured flow rate matches the set value from the furnace temperature control means. In the combustion control method, the oxygen concentration in the exhaust gas in the combustion zone, flue, or chimney of the combustion furnace is continuously measured, and a calculation constant is calculated for the deviation between the measured value and a predetermined set value. The air-fuel ratio is calculated by performing proportional and integral calculations using a proportional gain and an integral time, and the proportional gain and integral time are predetermined between the flow rate measurement value or the set value from the furnace temperature adjustment means. Combustion by oxygen concentration control in a combustion furnace, characterized in that the flow rate is automatically changed according to a measured value or a set value according to a relationship, and the other of the flow rates is continuously controlled based on the air-fuel ratio. Control method.