JP5310817B2 - Heating furnace atmosphere control method - Google Patents

Description

  The present invention relates to atmosphere control of a heating furnace in which low air ratio combustion is performed.

A heating furnace in which a pair of regenerative burners is arranged heats the regenerator by discharging exhaust gas from the other regenerative burner through the regenerator when burning one of the regenerative burners. . Then, by frequently changing the state of both the regenerative burners at intervals of several tens of seconds to several minutes, combustion and exhaust gas discharge are alternately performed, and the heat storage body heated by the exhaust gas is used for combustion. Preheated when air passes. Thereby, high exhaust heat recovery efficiency is achieved and energy saving is achieved.
However, in such a heating furnace, when heating an object to be heated, fuel gas and combustion air are supplied in a mixed state, and combustion is performed near the surface of the object to be heated. The surface of the substrate is easily oxidized, and an oxide film remains on the surface of the object to be heated. For this reason, in particular, when heat-treating expensive steel materials such as stainless steel and titanium, a technique for avoiding the formation of an oxide film by burning a regenerative burner at a low air ratio (low oxygen) has been proposed.

Japanese Patent Laid-Open No. 2001-082736 (page 3, FIG. 1)

  By the way, when the combustion of the regenerative burner in the furnace is shifted from low air ratio combustion (combustion with an air ratio of less than 1) to high air ratio combustion (combustion with an air ratio of 1 or more), or from high air ratio combustion to low air If the air ratio is changed in a short time when shifting to specific combustion, combustion of the regenerative burner may become unstable, or oxygen and carbon monoxide may be mixed and burnt rapidly. However, in the conventional technology, no means for solving such a problem has been proposed.

  The present invention has been made in view of the above-described circumstances. When the combustion of the regenerative burner in the furnace is shifted from low air ratio combustion to high air ratio combustion, or from high air ratio combustion to low air ratio combustion. The atmosphere control method of the heating furnace that can stabilize the combustion of the regenerative burner and avoid the occurrence of abrupt combustion by increasing or decreasing the air ratio at a predetermined ratio when shifting The purpose is to propose.

In the heating furnace atmosphere control method according to the present invention, the following means are employed in order to solve the above problems.
The present invention relates to an atmosphere control method for a heating furnace in which a pair of regenerative burners are alternately burned to heat-treat an object to be heated, and the pair of regenerative burners are alternately switched and burned, and the pair of regenerative burners is The allowable concentration range of carbon monoxide in the heating furnace is regulated according to the air ratio of the combustion air to be burned.
According to the present invention, since the appropriate concentration of carbon monoxide in the furnace is determined in conjunction with the air ratio, it is possible to avoid the occurrence of rapid combustion due to an increase in carbon monoxide in advance.
Also, in the case where the allowable concentration range of oxygen in the heating furnace is specified according to the air ratio of the combustion air for burning the pair of regenerative burners, the appropriate oxygen concentration is obtained in conjunction with the change in the air ratio. It is possible to avoid sudden combustion due to the increase in advance.
Also, if the carbon monoxide concentration or oxygen concentration in the furnace deviates from the allowable concentration range, a warning is issued or the operation of the heating furnace is emergency stopped, so that sudden combustion can be avoided immediately. it can.

  According to the present invention, since the appropriate concentration of carbon monoxide in the furnace is determined in conjunction with the air ratio, it is possible to avoid the occurrence of rapid combustion due to an increase in carbon monoxide in advance.

It is a conceptual diagram which shows the system configuration | structure of a soaking furnace. It is a figure which shows the ratio of the change of air ratio. It is a figure which shows a proper carbon monoxide concentration and a proper oxygen concentration. Diagram showing proper oxygen concentration

Hereinafter, embodiments of the heating furnace of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram showing a system configuration of a soaking furnace.
The soaking furnace 10 includes a furnace body 12 having four sides and a floor surface surrounded by a furnace wall formed of a refractory such as heat-resistant concrete, and a furnace lid 20 that opens and closes an opening formed above the furnace body 12. With. The side wall of the furnace body 12 is provided with a regenerative burner (regenerative burner) 30 composed of a pair of burners 30A and 30B including heat storage bodies 32A and 32B therein.
The regenerative burner 30 is a burner that heats and pressurizes using waste heat of air and gas, and can reduce the amount of gas used. The heat storage bodies 32A and 32B are composed of ceramics formed in a honeycomb shape.
A fuel gas line 50 that supplies fuel gas and a combustion air line 60 that supplies combustion air are connected to the burners 30A and 30B. The combustion air is piped so as to pass through the heat storage bodies 32A and 32B from the air supply fan 64 and to be supplied to the burners 30A and 30B.
Thereby, each burner 30A, 30B mixes the supplied combustion gas and combustion air and burns them, and also supplies the fuel gas flow control valves 52A, 52B and the combustion air line 60 provided in the fuel gas line 50 to each other. The combustion state and the furnace temperature are controlled by adjusting the flow rate of the provided combustion air flow control valves 62A and 62B.
Further, an exhaust gas line 70 for exhausting exhaust gas generated in the furnace is connected to each burner 30A, 30B, and the exhaust gas in the furnace passes through the heat storage bodies 32A, 32B and is sucked into the exhaust fan 74 to the outside. Piped to be released. The exhaust gas line 70 is provided with exhaust gas flow control valves 72A and 72B.
The furnace body 12 is provided with an exhaust bypass 80 for exhausting the exhaust gas in the furnace to the outside without passing through the burners 30A and 30B, and through an exhaust gas cooler 82 and a bypass flow control valve 84 for cooling the exhaust gas. The exhaust gas is connected to the exhaust gas line 70 immediately before the exhaust fan 74.
Further, a carbon monoxide concentration sensor 92, an oxygen concentration sensor 94, a furnace pressure sensor 96, and a temperature sensor 98 (all not shown) are provided in the furnace, and the measurement information is sent to the control unit 90 (not shown). Sent. And the control part 90 commands each fan 64,74, each flow control valve 52,62,72,84, and each burner 30A, 30B, and controls the combustion operation, atmosphere, etc. of the soaking furnace 10. FIG.

Next, the operation of the soaking furnace 10 and the regenerative burner 30 will be described.
First, when a steel material such as stainless steel is carried into the furnace, the fuel gas flow regulating valve 52A and the combustion air flow regulating valve 62A are opened based on a command from the control unit 90, and the burner 30A is burned. . At this time, the fuel gas flow regulating valve 52B and the combustion air flow regulating valve 62B are closed.
Then, by burning the burner 30A, the inside of the furnace is heated, and the exhaust gas is sucked into the burner 30B to heat the heat storage body 32B. The exhaust gas sucked into the burner 30B is deprived of heat by the heat storage body 32B, cooled to about 200 ° C., and discharged to the outside. At this time, the exhaust gas flow control valve 72B is opened, while the exhaust gas flow control valve 72A and the bypass flow control valve 84 are blocked.
And when setting time (for example, 30 seconds) passes, combustion can be switched from burner 30A to burner 30B. That is, the fuel gas flow regulating valve 52A and the combustion air flow regulating valve 62A are blocked, while the fuel gas flow regulating valve 52B and the combustion air flow regulating valve 62B are opened.
Further, the exhaust gas flow control valve 72A is opened, and the exhaust gas flow control valve 72B is blocked.
Then, the combustion air supplied to the burner 30B is preheated when passing through the heat storage body 32B, and is raised to a temperature close to the furnace temperature. On the other hand, the burner 30A sucks the exhaust gas and superheats the heat storage body 32A.
By repeating such a combustion cycle repeatedly, the inside of the furnace is heated to about 1300 ° C., and the combustion operation is continued so as to maintain the state.

FIG. 2 is a diagram showing changes in the air ratio in the furnace.
As the combustion operation (combustion cycle) proceeds, the air ratio of the combustion air supplied from the combustion air line 60 to each burner 30A, 30B is changed from 1.0 or more to 1.0 as shown in FIG. Reduce to less than.
Specifically, the air ratio is changed from about 1.2 to 1.3 to about 0.7 to 0.8. Thereby, combustion of each burner 30A, 30B shifts from high air ratio combustion (or normal combustion) with an air ratio of 1.0 or more to low air ratio combustion with an air ratio of less than 1.0.
The air ratio (also referred to as excess air ratio) is a ratio (ratio) of air actually supplied during combustion to air theoretically required for combustion.
For this reason, low air ratio combustion with an air ratio of about 0.7 to 0.8 results in so-called incomplete combustion (low oxygen combustion), but minimizes an oxide layer formed on the surface of stainless steel or the like. Therefore, there is an advantage that the scale loss of the product is reduced.
When the soaking process is terminated, as shown in FIG. 2B, the combustion of each burner 30A, 30B is stopped after returning from the low air ratio combustion to the high air ratio combustion again.

By the way, when transitioning from high air ratio combustion to low air ratio combustion, and transition from low air ratio combustion to high air ratio combustion, attention must be paid to the transition time (time required for air ratio change). Don't be. That is, when shifting from high air ratio combustion to low air ratio combustion, since complete combustion shifts to incomplete combustion, combustion tends to become unstable, and in the worst case, combustion may stop. .
In addition, at the time of transition from low air ratio combustion to high air ratio combustion, carbon monoxide filled in the furnace and the inflowing air (oxygen) are mixed and abrupt combustion occurs. In particular, if the combustion of each burner 30A, 30B is stopped as it is without shifting from the low air ratio combustion to the high air ratio combustion, there is a possibility of burning as soon as the furnace lid 20 is opened.
Therefore, the atmosphere in the furnace is controlled by changing the air ratio of the combustion air supplied from the combustion air line 60 to the burners 30A and 30B as follows.

First, the transition from high air ratio combustion to low air ratio combustion will be described.
In the transition from high air ratio combustion to low air ratio combustion, combustion stability becomes a problem. Therefore, the air ratio is lowered while monitoring the stability of combustion. Specifically, the reduction rate of the air ratio is determined by experiments or the like so that the flow rates of the combustion air and fuel gas, the temperature and pressure in the furnace do not change rapidly. That is, if the air ratio changes suddenly and combustion becomes unstable, the temperature and pressure changes in the furnace also change abruptly, and the flow rates of the combustion air and fuel gas also change.
Thereby, air ratio can be reduced, aiming at stabilization of combustion of each burner 30A, 30B.

Next, the transition from low air ratio combustion to high air ratio combustion will be described.
In the transition from low air ratio combustion to high air ratio combustion, the occurrence of rapid combustion due to the reaction between carbon monoxide and oxygen becomes a problem. Therefore, the time required for the change in the air ratio (that is, the time required for changing the air ratio from about 0.7 to about 1.3) is determined when the burners 30A and 30B are burned with the minimum combustion amount. Set it longer than the time when the gas is completely replaced.
The minimum combustion amount is the minimum combustion amount (combustion gas usage amount) that each burner 30A, 30B can stably burn, and is, for example, a combustion amount of about 1/10 of the maximum combustion amount.
The time for completely replacing the gas in the furnace is the time required to replace the gas in the furnace with the exhaust gas accompanying combustion, and differs depending on the size of the furnace and the capability of the burner.
As a result, even if each burner 30A, 30B burns at the minimum combustion amount, the increased air (oxygen) is surely consumed (combusted), so oxygen and carbon monoxide in the furnace are The occurrence of rapid combustion due to mixing can be avoided.
The time taken for the change in the air ratio can be determined by the size of the furnace and the capability of the burner, and is, for example, about 5 minutes. Therefore, the open angle of the combustion air flow regulating valves 62A and 62B is gradually widened over 5 minutes to increase the air ratio.
As described above, the opening angle of the combustion air flow regulating valves 62A and 62B is set not only to the so-called open control in which the air ratio is controlled by setting the opening time, but gradually opening the combustion air. The carbon monoxide concentration and the oxygen concentration may be measured by the carbon monoxide concentration sensor 92 and the oxygen concentration sensor 94 to perform feedback control.

Furthermore, sudden combustion can be avoided by using the carbon monoxide concentration sensor 92 and the oxygen concentration sensor 94. That is, the concentration of carbon monoxide in the furnace can be determined from the composition of the combustion gas, the air ratio during combustion, and the temperature in the furnace, so if there is a large difference between the calculated value and the measured value, rapid combustion will occur. It can be judged that there is.
Therefore, the carbon monoxide concentration is monitored during the combustion operation (all operations such as high air ratio combustion, low air ratio combustion, low air ratio combustion, transition to high air ratio combustion, and fire extinguishing).

FIG. 3 is a diagram showing an appropriate carbon monoxide concentration.
As a representative example, a case where the combustion operation is performed at an air ratio of 0.7 will be described. When the combustion operation is performed at an air ratio of 0.7 and the exhaust gas temperature is 1200 ° C., the dissociation equilibrium carbon monoxide concentration is calculated to be about 10%. Therefore, when the carbon monoxide concentration greatly exceeds or falls below 10%, it can be determined that the combustion is abrupt.
For example, the upper limit is a dissociation equilibrium carbon monoxide concentration when the combustion operation is performed at an air ratio of 0.6, and the lower limit is a combustion operation at an air ratio of 0.8. The dissociation equilibrium carbon monoxide concentration can be obtained.
Furthermore, as a threshold value for emergency stop of the combustion operation (rapid combustion), the dissociation equilibrium carbon monoxide concentration when the combustion operation is performed at an air ratio of 5 can be used.
In this way, the difference (that is, the threshold value) between the calculated value of the carbon monoxide concentration that is determined to be rapid combustion and the actually measured value is defined based on the air ratio at the time of combustion. The threshold value for emergency stop is defined similarly.
Note that the method of defining the threshold is not limited to the above method. For example, the upper and lower 2% of the dissociation equilibrium carbon monoxide concentration obtained by calculation may be set as a threshold value.

Further, the oxygen concentration in the furnace can also be obtained from the exhaust gas temperature and the air ratio.
Therefore, the oxygen concentration is monitored during the combustion operation. As in the case of carbon monoxide, the difference (that is, the threshold value) between the calculated value of the oxygen concentration determined to be rapid combustion and the actual measurement value is defined based on the air ratio at the time of combustion.
FIG. 4A is a diagram showing an appropriate oxygen concentration when the air ratio is 0.5. FIG. 4B is a diagram showing an appropriate oxygen concentration.
Within the allowable range shown in FIG. 4A (within hatching), oxygen does not burn. That is, there is no sudden combustion. Therefore, the combustion operation is performed so that the oxygen concentration in the furnace is within the hatching in the figure. Since the appropriate oxygen concentration varies depending on the air ratio, the allowable oxygen concentration is defined based on the air ratio and the exhaust gas temperature. Since the heat resistance temperature of the exhaust gas line 70 and the like is 400 ° C., the exhaust gas temperature can actually define the allowable oxygen concentration based only on the air ratio with the exhaust gas temperature being 400 ° C. as a reference.
And when an allowable oxygen concentration is prescribed | regulated based only on an air ratio on the basis of the time when exhaust gas temperature is 400 degreeC, it will become like FIG.4 (b).

The exhaust gas temperature is measured at the rear end of the burner, that is, at the portion where the fuel gas line 50 and the combustion air line 60 are connected. This is because rapid combustion occurs at this position.
Thus, by defining and monitoring appropriate values of the carbon monoxide concentration and the oxygen concentration during the combustion operation in conjunction with the air ratio, it is possible to avoid sudden combustion. In addition, when there is an abnormality in the carbon monoxide concentration and the oxygen concentration, it is conceivable that a warning alarm is issued or the combustion operation is stopped as described above.

  Note that the operation procedures shown in the above-described embodiment, or the shapes and combinations of the components are examples, and can be variously changed based on process conditions, design requirements, and the like without departing from the gist of the present invention. is there.

10 ... soaking furnace (heating furnace)
30A, 30B ... Burner (Regenerative burner)
DESCRIPTION OF SYMBOLS 50 ... Fuel gas line 60 ... Combustion air line 70 ... Exhaust gas line 90 ... Control part 92 ... Carbon monoxide concentration sensor 94 ... Oxygen concentration sensor

Claims (3)

In an atmosphere control method for a heating furnace in which a heated object is heated by alternately burning a pair of regenerative burners,
While alternately switching and burning the pair of heat storage burners,
An atmosphere control method for a heating furnace, wherein an allowable concentration range of carbon monoxide in the heating furnace is defined according to an air ratio of combustion air for burning the pair of regenerative burners. Defining an allowable concentration range of carbon monoxide in the heating furnace and an allowable concentration range of oxygen in the heating furnace according to an air ratio of combustion air for burning the pair of regenerative burners. The atmosphere control method for a heating furnace according to claim 1.
  The heating according to claim 1 or 2, wherein when the carbon monoxide concentration or the oxygen concentration in the furnace deviates from the allowable concentration range, a warning is issued or the operation of the heating furnace is stopped. The furnace atmosphere control method.

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