JP4474429B2 - Waste incinerator and incineration method - Google Patents

Description

  The present invention relates to a method for suppressing the emission of carbon monoxide when gasifying and incinerating waste in a melting furnace having a secondary combustion chamber.

  In incinerators that process waste such as municipal and household waste, the gasification furnace decomposes the waste into pyrolysis gas and solids containing carbonaceous matter and ash, and burns the pyrolysis gas and removes ash. A melting furnace for melting is used. A high temperature of 1200 ° C. or higher is required to thermally melt the ash, but in order to save energy, it has been studied to make the temperature sufficiently high without auxiliary combustion so that self-heat melting can be performed. On the other hand, in a high-temperature melting furnace, the higher the oxygen concentration, the more nitrogen and oxygen in the combustion air react with each other, so that nitrogen oxides called so-called thermal NOx are likely to be generated. There is.

  As one of them, a low air ratio combustion technique is being studied. This suppresses heat loss due to air supply, suppresses the inside of the furnace from being cooled by air supplied from the outside, and increases the temperature inside the furnace. Moreover, in order to reduce the combustion air to be used, the amount of gas that eventually becomes exhaust gas can be reduced. Furthermore, generation of thermal NOx can be suppressed by reducing the amount of air.

  However, low air ratio combustion suppresses the supply of combustion air, and mixing and stirring of the pyrolysis gas and combustion air tend to be insufficient, resulting in hindering complete combustion. Carbon monoxide may be easily generated. For this reason, a method has been studied in which exhaust gas is circulated to promote mixing and stirring of pyrolysis gas and air instead of air (Patent Document 1).

  Specifically, it is desirable that the air ratio in the melting furnace is 1.0 or less. This air ratio is a ratio of an actual air supply amount to a theoretical air amount capable of completely burning the pyrolysis gas and the auxiliary combustor. However, the nature of the waste to be incinerated is not constant, and the amount of pyrolysis gas and char generated in the gasification furnace varies. For this reason, when the melting furnace is actually operated, the air ratio often exceeds 1.0. In this case, since it is high-temperature combustion, the generation of thermal NOx in which nitrogen and oxygen in the air react is remarkable, and it is difficult to suppress the generation of nitrogen oxides.

  On the other hand, for example, as disclosed in Patent Document 2, melting is performed in two or more stages of a main combustion chamber and a secondary combustion chamber, and combustion air is supplied in stages. Is being considered. In this way, the carbon monoxide generated in the melting furnace can be completely burned in the secondary combustion chamber, the amount of carbon monoxide that is finally discharged can be suppressed, and the main combustion chamber can be reduced. By making the air ratio, the production amount of nitrogen oxide itself can be suppressed.

  Another method is to adjust the supply amount of combustion air in accordance with the situation. Incineration of waste is not steady, and the situation changes from moment to moment depending on the content of the waste to be incinerated. The concentration and temperature of gas components in the exhaust gas generated by combustion are measured, and this concentration and temperature are determined in advance. Feedback control is performed to adjust the supply amount of combustion air so as to approach the set value. For example, in Patent Document 3, the oxygen concentration and NOx concentration of the gas discharged from the secondary combustion chamber, the temperature of the secondary combustion chamber are measured, and the order of priority is given to these measurement conditions. The example used as conditions which control the quantity of the combustion air supplied to is described.

  In addition to this, the reaction of the melting furnace and the secondary combustion chamber is examined, the component of the exhausted gas is examined, and the feedback control is performed to control the supply amount of the component supplied upstream from that value. In general, a method for suppressing the amount of carbon monoxide and nitrogen oxides generated is used. For example, Patent Document 4 discloses a waste that is a source of pyrolysis gas depending on the oxygen concentration of a gas to be measured. A method for controlling the input amount of is described.

Japanese Patent Laying-Open No. 2005-201621 JP 2002-031312 A JP 2006-194516 A JP 2006-97916 A

  However, when trying to adjust the supply amount of combustion air at a low air ratio, depending on the increase in the pyrolysis gas supplied from the gasifier, it tends to become extremely oxygen-deficient and carbon monoxide is likely to be generated. Become. Furthermore, if the amount of combustion air used for stirring is temporarily reduced by controlling the supply amount, mixing of pyrolysis gas and combustion air in the combustion chamber becomes insufficient, and carbon monoxide is reduced. May be more likely to occur.

  In addition, since it takes time to measure the concentration with an oximeter and output the measured value, the time lag from when the measured value is found until the amount of air supplied is controlled by adjusting the valve with the feedback mechanism. In the meantime, the carbon monoxide that passed through the incomplete combustion was supposed to be discharged.

  Therefore, the present invention is a waste incinerator having a melting furnace and a secondary combustion chamber and combusting in two or more stages, which is stable even when subjected to instantaneous fluctuations in the nature and amount of waste. The purpose is to incinerate wastes that continue specific combustion and suppress carbon monoxide emissions.

  This invention performs control by a feedback mechanism that measures the oxygen concentration or temperature of exhaust gas discharged from the secondary combustion chamber during normal times and adjusts the amount of combustion air supplied to the secondary combustion chamber according to the measured value. When the oxygen concentration of the gas at the outlet of the melting furnace falls below a predetermined switching set value, the secondary combustion chamber enters the secondary combustion chamber according to the control value by the control device independent of the thermometer or the oxygen concentration of the exhaust gas from the control by the feedback mechanism. When the control for adjusting the supply amount of the combustion air to be supplied is switched and the oxygen concentration of the gas at the outlet of the melting furnace exceeds a predetermined switching set value, the control is returned to the control by the feedback mechanism. It was solved.

  That is, when the concentration of oxygen contained in the gas sent from the melting furnace to the secondary combustion chamber becomes extremely low, the feedback control from the outlet of the secondary combustion chamber ensures that the oxygen in the secondary combustion chamber Carbon monoxide is generated due to incomplete combustion due to the shortage, and the carbon monoxide generated in the melting furnace remains as it is and is contained in the exhaust gas. On the other hand, carbon monoxide has already been discharged from the secondary combustion chamber even if it tries to cope with it by analyzing the components and temperature of the exhaust gas discharged from the secondary combustion chamber and detecting changes. The response is not in time. Therefore, the gas component is examined in the stage before entering the secondary combustion chamber, and if it is determined that incomplete combustion will surely occur, the supply amount control of the combustion air is temporarily controlled at the outlet of the secondary combustion chamber. By switching from the measured value at, the combustion air was quickly replenished so that complete combustion was performed in the secondary combustion chamber, thereby suppressing the emission of carbon monoxide.

  The incinerator for realizing the method is provided with an oxygen concentration meter or a thermometer at the outlet of the secondary combustion chamber, and compares these measuring devices with values set by these measuring devices with preset values. In addition, a control mechanism is provided for adjusting the secondary combustion chamber combustion air regulating valve so that the value approaches the set value, and a melting furnace outlet oxygen concentration meter is also provided at the gas outlet of the melting furnace. A densitometer was connected to the control mechanism. Usually, the secondary combustion chamber combustion air adjustment valve is adjusted based on the control mechanism connected to the outlet of the secondary combustion chamber, and the measured value of the melting furnace outlet oximeter is below the preset switching setting value. The control is switched to the trigger so that the adjustment of the air regulating valve for combustion in the secondary combustion chamber is performed according to the control value by the control device independent of the oxygen concentration meter and the thermometer, and the switching setting in which the measurement value is set in advance. It shall have a switching mechanism which can be switched so that control of an air regulating valve for secondary combustion chamber combustion may be returned when it becomes more than a value. The control value by this independent control device is adjusted so that the secondary combustion chamber combustion air regulating valve is fully opened so that a sufficient amount of combustion air can be supplied to the secondary combustion chamber. But you can.

  In addition, in order to be able to quickly detect a decrease in oxygen concentration at the melting furnace exit, it is not an oxygen concentration meter that uses a zirconia element that is generally used as a conventional oxygen concentration meter and that requires a response speed of about 15 to 20 seconds, When using an infrared laser oximeter with a response speed of about 2 seconds, it is possible to quickly detect a decrease in the oxygen concentration in the gas and increase the supply of combustion air accordingly. Combustion can be more reliably suppressed.

  Furthermore, the exhaust gas discharged from the secondary combustion chamber is circulated to the secondary combustion chamber and used for stirring and mixing of the internal gas. The combustion gas and the combustion air can be sufficiently agitated and completely burned, making it easier to achieve complete combustion.

  By the incineration method according to the present invention, even if there is a sudden change in the quality and quantity of waste to be treated, incomplete combustion in the entire combustion apparatus is suppressed, and carbon monoxide discharged from the waste incinerator Suppress the amount of.

  The present invention will be described below with reference to the embodiment shown in FIG. FIG. 1 is a diagram showing the overall configuration and flow of a waste incinerator according to the present invention.

  The configuration and flow in this embodiment will be described. First, the waste A such as city waste and household waste is pyrolyzed in the gasifier 11. Although not shown, air is actually supplied. Waste A is separated into pyrolysis gas B and solid content C by pyrolysis. The solid content C includes a carbon residue component called char, tar, and ash. These are supplied to the melting furnace 12, and the pyrolysis gas B is burned by the combustion air D supplied in the main combustion chamber 13 in the melting furnace 12 so as to be in a high temperature state, and the solid content C is thermally melted. The heat-melted solid C falls along the wall surface of the main combustion chamber 13 and falls into the lower molten pool 14, and the gas E after combustion is also pushed downward. The solid content C passes through the outlet 15 and is dropped into the water tank 16 to become slag F. After this slag F is cooled in the water tank, it is carried out to the outside. In the main combustion chamber 13, ignition by a burner using a combustion aid is also performed for stable combustion, and combustion air D for the combustion is adjusted by a burner supply air adjustment valve 41.

  On the other hand, the gas E after combustion is sent from the gas outlet 12 a of the melting furnace 12 (hereinafter abbreviated as “melting furnace outlet 12 a”) to the secondary combustion chamber 21. The gas E after combustion includes not only carbon dioxide and nitrogen but also unburned gas that is still a combustible component. It may also contain carbon monoxide due to incomplete combustion. In the secondary combustion chamber 21, the unburned gas and carbon monoxide are completely burned while supplying the secondary combustion air D ′ from the secondary combustion blower 44. The exhaust gas J generated by the complete combustion is cooled through the gas cooling chamber 25, the air preheater 26, and the temperature reducing tower 29 through a series of flues 24 and 24 ′ connected to the induction blower 31. The air preheater 26 preheats the air taken from the outside by the melting furnace combustion blower 27. The preheated air is sent to the melting furnace 12 through the air supply pipe 28 and used as combustion air D. Note that the secondary combustion air D 'does not have to be preheated because it has a cooling effect to suppress overheating of the secondary combustion chamber.

  The exhaust gas cooled so far is further cooled to about 200 ° C. or less when it exits the temperature reducing tower 29. This condenses and solidifies the heavy metal that is contained in the exhaust gas J but has become a gas at a high temperature, and forms a solid dust component. While sucking the cooled exhaust gas by the induction blower 31, the dust component contained in the exhaust gas is collected through the bag filter 30 using a woven fabric or a non-woven fabric as a filter medium. As a result, dust components can be removed, and the purified exhaust gas J 'is discharged out of the apparatus. However, a part of the exhaust gas J ′ is circulated to the main combustion chamber 13 of the melting furnace 12 by the exhaust gas recirculation blower 33 through the exhaust gas melting furnace recirculation pipe 32 branched from the flue 24 ′. Further, the exhaust gas is also circulated to the secondary combustion chamber 21 through the exhaust gas secondary combustion chamber circulation pipe 42.

  The cooling of the exhaust gas J is not limited to the embodiment in which cooling is performed by the gas cooling method shown in FIG. 1, and cooling is performed by recovering heat with a waste heat boiler instead of the gas cooling chamber 25 and the air preheater 26. May be. In this case, water is recovered in the waste heat boiler to obtain water vapor, and the water vapor is used for heating the combustion air D. Further, the exhaust gas cooled thereby is sent to the temperature reducing tower 29, and a part of the purified exhaust gas J ′ obtained through the bag filter 30 is partly converted into the main combustion chamber 13 and the secondary combustion as in the embodiment of FIG. Return to chamber 21.

  In the incinerator for carrying out the incineration method according to the present invention, a melting furnace outlet oxygen concentration meter 17 for measuring the oxygen concentration at the outlet 12a connected to the secondary combustion chamber 21 of the melting furnace 12 is attached. Specifically, it is on the path during which the gas burned in the melting furnace 12 is sent to the gas inlet of the secondary combustion chamber 21. As shown in FIG. 2, the melting furnace outlet oxygen concentration meter 17 includes a transmitter 51 for transmitting an infrared laser, a receiver 52 for receiving infrared light that has passed through the measurement target, and a measurement target from the spectrum of absorbed infrared rays. It is preferable to use an infrared laser type oxygen concentration meter comprising a controller 53 for calculating the oxygen concentration of oxygen. The principle of this infrared laser oximeter is as follows. Every molecule has its own vibration and rotational motion, and each motion state is quantized. For this reason, all molecules can absorb light in a specific wavelength region corresponding to the energy difference between their own quantized motion states and can be excited to a high energy motion state. Therefore, the transmitter emits light in the wavelength range corresponding to the energy that can be absorbed by the oxygen molecules, and the receiver passes the object to be measured and the light in that wavelength range is received by the controller. By measuring whether it is decreasing, the amount of oxygen molecules in the gas to be measured can be calculated.

  By measuring the oxygen concentration using such an infrared laser type oxygen concentration meter, a response can be obtained in a time of about 2 seconds, which is sufficiently faster than a conventional oxygen concentration meter using a zirconia element. . When a conventional zirconia element is used, the response speed is about 15 to 20 seconds, and the infrared laser oximeter has a sufficiently high response speed. In the case of an oximeter using a zirconia element, the gas that has passed through the melting furnace outlet 12a may pass to the secondary combustion chamber 21 during the time until the actual value of the oxygen concentration is measured. However, with an infrared laser oximeter, there is almost no possibility that a gas of that oxygen concentration will pass through the secondary combustion chamber 21 by the time the oxygen concentration is measured, and carbon monoxide emission will be reduced. Can be suppressed.

  In addition, the above infrared laser oximeter can be measured even in a high temperature environment exceeding 1000 ° C. or in an environment with a lot of dust such as fine ash. Further, since the average concentration in the line section through which the laser passes is detected, the measured value is more reliable than the suction type zirconia element oximeter that performs measurement by sucking at one point. For this reason, it is possible to accurately and promptly detect a fluctuating value of the oxygen concentration at the melting furnace outlet 12a. Further, since there is no movable part and no reagent is used, there is an advantage that almost no maintenance is required and stable operation is possible for a long period of time. The controller 53 of the infrared laser oximeter can output the measured oxygen concentration value as a current value or electronic information to the outside.

  Furthermore, in the incinerator for carrying out the incineration method according to the present invention, a thermometer or an oxygen concentration meter is attached to the flue 24 that discharges the exhaust gas J from the secondary combustion chamber 21. Hereinafter, they are referred to as a secondary combustion chamber outlet thermometer 46 and a secondary combustion chamber outlet oxygen concentration meter 46 ′, respectively. In any case, the measurement value can be externally output as a current value or electronic information. As the secondary combustion chamber outlet oxygen concentration meter 46 ', it is preferable to use the above-described infrared laser type oxygen concentration meter because the response speed is high and the operation is easy.

  Combustion air D and secondary combustion air D ′ supplied to the melting furnace 12 and the secondary combustion chamber 21 so that the values measured by these thermometers and oximeters approach the preset values set in advance. Adjust the amount. Among these, the example of the mechanism using the secondary combustion chamber exit thermometer 46 is shown in FIG.

  In the embodiment shown in FIGS. 1 and 3, the supply amount of the combustion air D supplied to the melting furnace 12 is controlled as follows. The measured value of the oxygen concentration is transmitted to the melting furnace outlet oxygen concentration controller 18 from the controller 53 of the melting furnace outlet oxygen concentration meter 17 which is an infrared laser oximeter provided at the melting furnace outlet 12a. The melting furnace outlet oxygen concentration controller 18 compares a predetermined set value with the measured value of the oxygen concentration sent. A control signal corresponding to the difference between the values obtained by this comparison is transmitted to the melting furnace supply air regulating valve 19 through the signal cable 20. When the measured value is larger than the set value, the control signal decreases the amount of air supplied through the melting furnace supply air regulating valve 19, and conversely, the measured value is more than the set value. If it is smaller, the signal is a command to increase the amount of air supplied through the melting furnace supply air regulating valve 19. This signal does not instruct only simple opening and closing, but has numerical information capable of instructing an adjustment amount. This is because as the difference between the set value and the measured value increases, the adjustment amount of the melting furnace supply air adjustment valve 19 must be increased accordingly. This numerical information may be sent as an electronic signal through a cable, or may be indicated by the current value.

  On the other hand, the supply amount of the secondary combustion air D ′ supplied to the secondary combustion chamber 21 is controlled as follows. The secondary combustion chamber combustion air adjustment valve 48 for adjusting the supply amount of the secondary combustion air D ′ is connected to a switch 58 serving as a switching mechanism for switching the control device, and two selectable terminals of the switch 58 are: These are connected to the secondary combustion chamber outlet thermometer 46 and the melting furnace outlet oxygen concentration meter 17, respectively, and are normally set to be connected to the secondary combustion chamber outlet thermometer 46.

  Normally, control is performed by a feedback mechanism that operates according to the measured value of the secondary combustion chamber outlet thermometer 46. The control contents are as follows. First, temperature information is sent from the secondary combustion chamber outlet thermometer 46 to the secondary combustion chamber outlet temperature controller 47, and the preset value of the appropriate temperature is compared with the measured temperature value. If the measured value is lower than the set value, in order to suppress the cooling by the secondary combustion air D ′ and raise the temperature, from the secondary combustion chamber outlet temperature controller 47 to the secondary combustion chamber combustion air adjustment valve 48, Sends a command to close the valve more. On the contrary, if the measured value is higher than the set value, the secondary combustion air D ′ to be cooled is insufficient, and therefore a command to open the secondary combustion chamber combustion air adjustment valve 48 is transmitted. In this way, while adjusting the measured value so as to approach the set value, a sufficient amount of secondary combustion air D ′ is normally supplied to cause complete combustion in the secondary combustion chamber 21. Suppresses emissions of carbon monoxide and nitrogen oxides.

  On the other hand, the oxygen concentration controller 18 at the outlet of the melting furnace is supplied to the secondary combustion chamber 21 when the oxygen concentration is extremely lowered, separately from the set value for adjusting the melting furnace supply air regulating valve 19. A switching set value for switching the control device for controlling the amount of combustion air D ′ from the feedback mechanism is determined. In other words, if the oxygen concentration is extremely reduced, the control is performed after detecting the temperature of the gas discharged from the secondary combustion chamber 21, and therefore incomplete combustion proceeds during that time. To be able to switch to control. The switching setting value serving as a reference for determining an emergency may be the same as the setting value that is a target value for adjusting the melting furnace outlet oximeter 17. Since it is preferable to use only in the case of an emergency in which the oxygen concentration is extremely reduced, it is preferable that the switching setting value for signal switching be lower than the setting value for valve adjustment. When the measured value at the melting furnace outlet oxygen concentration meter 17 falls below the switching set value, the melting furnace outlet oxygen concentration controller 18 instructs the signal switch 56 to send a command to switch the switch 58.

  When the switch 58 is switched, the control of the secondary combustion chamber combustion air regulating valve 48 is temporarily controlled by a control device independent of the secondary combustion chamber outlet thermometer 46. Follow the indicated value. When this switching is performed, the oxygen concentration at the melting furnace outlet 12a, that is, the oxygen concentration in the gas entering the secondary combustion chamber 21, is extremely reduced. A sufficient amount of secondary combustion air D ′ is supplied so as to open and complete combustion in the secondary combustion chamber 21. For this reason, the value indicated by the signal generator 57 is an amount that conforms to the maximum flow rate within a range that allows the maximum flow rate of air to pass through the secondary combustion chamber combustion air adjustment valve 48 or that is possible in the condition of the combustion device. It is preferable to set in advance a fixed value that allows air to pass through.

  Note that the control device that indicates the control value by the independent control device is not limited to the signal generator 57 that indicates a fixed value. For example, the control device may be sufficient depending on the value of the melting furnace outlet oximeter 17. If the supply amount of the secondary combustion air D ′ is supplied in an appropriate amount, the degree of opening of the secondary combustion chamber combustion air adjustment valve 48 may be finely adjusted within a possible range. In addition, the above “independent” means that the control value by the control device may be independent from the value of the secondary combustion chamber outlet thermometer 46, and the secondary combustion chamber connected to the secondary combustion chamber outlet thermometer 46. Even if the control value is indicated by a device that is physically integrated in a device such as the outlet temperature controller 47, the value may be independent and a sufficient amount of oxygen may be supplied.

  After the above switching, if the measured oxygen concentration at the melting furnace outlet 12a exceeds the switching set value for returning the control, it is necessary to urgently increase the supply amount of the secondary combustion air D ′. The melting furnace outlet oxygen concentration controller 18 instructs the signal switch 56 to send a command to switch the switch 58 because it can be returned to normal control. As a result, the control of the secondary combustion chamber combustion air adjustment valve 48 returns to the control by the feedback mechanism according to the value of the secondary combustion chamber outlet thermometer 46. Note that the case where the switching set value is exceeded and the case where the set value is below is not necessarily limited to the case where the threshold value has elapsed, and includes the case where switching is performed when the switching set value is reached.

  The switch setting value when switching the switch 58 to the signal generator 57 side and the switching setting value when returning from the signal generator 57 side to the secondary combustion chamber outlet thermometer 46 side are set to the same value. However, it is preferable to set the switching setting value when returning to a percentage value and increase it by about 0.05 to 0.10%. If the switching setting value at the time of switching is the same as the switching setting value at the time of returning, the measured oxygen concentration exceeds the switching setting value, and as soon as the switch 58 is returned, it is possible to switch again with a slight change in the situation. It may be necessary to satisfy the condition and return the switch 58 to the signal generator 57 side. Thus, when the switch 58 is frequently switched, accurate control becomes almost impossible. On the other hand, if the switching setting value when returning is higher than the switching setting value when switching, even if the oxygen concentration slightly decreases after the switch 58 is returned after satisfying the condition, Therefore, the switch 58 does not need to be switched again immediately.

  The control switching by such a switching mechanism is performed by changing the secondary combustion chamber outlet thermometer 46 to the secondary combustion chamber outlet oxygen concentration meter 46 ′ and the secondary combustion chamber outlet temperature controller 47 to the secondary in the mechanism of FIG. The same operation is performed even when the mechanism as shown in FIG. 4 is used instead of the combustion chamber outlet oxygen concentration controller 47 ′. In other words, the supply amount of the secondary combustion air D ′ is adjusted by a feedback mechanism according to the measured value at the outlet of the secondary combustion chamber at the normal time, and in the event of an emergency when the oxygen concentration at the melting furnace outlet 12a becomes extremely small, The secondary combustion chamber combustion air regulating valve 48 is controlled according to a control value by a control device independent of the combustion chamber outlet oxygen concentration meter 46 '.

  Of the set values for performing the above adjustment, the set value for adjusting the melting furnace supply air adjusting valve 19 is preferably a value for adjusting the air ratio in the main combustion chamber 13 to 0.8 or more and 1.0 or less. Basically, when the air ratio exceeds 1.0, the oxygen concentration becomes too high, and the generation of nitrogen oxides in the main combustion chamber 13 cannot be ignored. On the other hand, when the air ratio is too small and less than 0.8, oxygen is insufficient and this causes incomplete combustion, so that carbon monoxide is likely to be generated. In addition, in order to perform two-stage combustion in the secondary combustion chamber 21 which will be described later, it is preferable that the amount of air in the main combustion chamber 13 is insufficient. The air ratio in the main combustion chamber 13 is a value including not only the air directly supplied to the main combustion chamber 13 but also the air supplied in the gasification furnace 11.

  When complete combustion is performed with the air ratio in the main combustion chamber 13 being 1.0 or less, the oxygen concentration at the melting furnace outlet 12a is theoretically 0%, so oxygen cannot be detected as it is. However, not all of the pyrolysis gas B actually burns, but unburned gas remains, so oxygen remains at the melting furnace outlet 12a, and this remaining amount varies depending on the air ratio in the main combustion chamber 13. Will be. However, since the remaining amount is still small, if the value is set as the control target value, the melting furnace supply air regulating valve 19 is practically fully closed or close to it, and the substantial control of the combustion air D is performed. It becomes difficult. Therefore, either the combustion air D or the exhaust gas J ′ is supplied to a location upstream of the location where the melting furnace outlet oxygen concentration meter 17 is provided, and passes through the melting furnace outlet 12 a. It is preferable to control the amount of combustion air D supplied to the melting furnace by raising the oxygen concentration in the gas, measuring the raised value, and setting a target value. In particular, it is preferable to supply the exhaust gas J ′ so as not to excessively increase the oxygen concentration. For this reason, in the embodiment of FIG. 1, an exhaust gas melting furnace outlet circulation pipe 43 that is branched from the flue 24 ′ or the exhaust gas melting furnace circulation pipe 32 and supplies the exhaust gas to the melting furnace outlet 12 a is provided. Here, raising the bottom means that a gas having a higher oxygen concentration is added to make the value higher than the oxygen concentration of the original gas so that measurement and control can be easily performed.

  The oxygen concentration measured by the melting furnace outlet oxygen concentration meter 17 changes according to the air ratio in the main combustion chamber 13, and the corresponding numerical value is related to the scale and structure of the combustion device, and the exhaust gas. It is determined by the ring flow rate of J ′. Therefore, although the above set value varies depending on the combustion device and other conditions, the oxygen concentration at the melting furnace outlet 12a is 1.0 vol% or more and 2.0 vol% or less, and the air ratio corresponds to 0.8 to 1.0. It is preferable to set the supply amount of the exhaust gas J ′ under such conditions because the supply amount of the combustion air D can be easily controlled.

  On the other hand, it is preferable that the desirable air ratio in the secondary combustion chamber 21 is not determined solely, but is determined by the total amount of air supplied to both the main combustion chamber 13 and the secondary combustion chamber 21. When incomplete combustion occurs in the main combustion chamber 13, a large amount of carbon monoxide or the like is generated. Therefore, a large amount of air needs to be supplied to the secondary combustion chamber 21 to compensate for it. is there. The air ratio of the air supplied to the main combustion chamber 13 and the secondary combustion chamber 21 is preferably 1.3 or less. Within this range, even if carbon monoxide is generated in the main combustion chamber 13, it can be fully burned, while in the secondary combustion chamber 21, which is at a lower temperature than the main combustion chamber 13, this degree This is because an environment in which nitrogen oxides are less likely to be generated can be obtained even with the air ratio. However, if it exceeds 1.3, the amount of exhaust gas will increase more than necessary.

  The preset value set as a target for adjusting the overall air ratio to the above range varies depending on the scale, configuration, and other conditions of the combustion device, but the values that are generally preferred are as follows. It is. First, when a secondary combustion chamber outlet thermometer 46 is provided in the flue 24 that is the outlet of the secondary combustion chamber 21, a preset value set as a target in the secondary combustion chamber outlet temperature controller 47 is a temperature of 900 It is preferable that it is not lower than 1000C and not higher than 1000 ° C.

  On the other hand, when the oxygen concentration meter 46 ′ is provided in the outlet flue 24 of the secondary combustion chamber 21, the set value predetermined as a target for the secondary combustion chamber outlet oxygen concentration controller 47 ′ is 3.5 vol% or more. 7.0 vol% or less is preferable. If it exceeds 7.0 vol%, it is a state where oxygen is excessively supplied even in the set value state, and the thermal efficiency is lowered. On the other hand, if it is less than 3.5 vol%, incomplete combustion tends to occur even in the secondary combustion chamber 21.

  Moreover, it is preferable that the switching set value for switching the control by the switch 58 of the measured value measured by the melting furnace outlet oxygen concentration meter 17 is 1.0 vol% or more and 2.0 vol% or less. This value is raised by the oxygen contained in the exhaust gas J ′ when the exhaust gas J ′ is supplied by connecting the exhaust gas melting furnace outlet reflux pipe 43 to a location upstream of the melting furnace outlet oxygen concentration meter 17. This is the switching setting value for the oxygen concentration. When exhaust gas J ′ is not supplied, the oxygen concentration measured here is extremely low, and it is difficult to make a judgment for switching control. Therefore, the oxygen concentration thus raised is used as a reference. It is preferable to switch.

  In the incinerator according to the present invention, when the adjustment of the supply amount of the secondary combustion air D ′ is controlled by the switching mechanism as described above, the exhaust gas J ′ is circulated in the secondary combustion chamber 21. The As shown in FIG. 1, the exhaust gas J ′ is branched from the flue 24 ′ after dust components are removed by the bag filter 30, and the exhaust gas recirculation blower 33 passes through the exhaust gas secondary combustion chamber recirculation pipe 42. It is returned to the secondary combustion chamber 21. This generates an air flow for sufficiently stirring and mixing the secondary combustion air D ′ whose supply amount varies with the unburned gas contained in the gas sent from the melting furnace 12, and the secondary combustion air D ′. This is because the combustion chamber 21 is excessively heated to suppress the generation of nitrogen oxides generated in a high temperature environment called thermal NOx.

  As a specific configuration for supplying the secondary combustion chamber 21 with the secondary combustion air D ′ and the exhaust gas J ′, a nozzle as shown in FIG. 81 to 83 are assigned to a plurality of stages in the vertical direction, with some of the nozzles assigned to supply the secondary combustion air D ′ and the remaining nozzles assigned to the supply of the exhaust gas J ′. Inject into the chamber 21. It is preferable that these nozzles do not face in the horizontal direction but are inclined downward by about 15 ° ± 5 ° because they are easily mixed in opposition to the gas flow entering the secondary combustion chamber 21. In addition, as shown in the horizontal cross-sectional views of FIGS. 5B to 5D, the introduced gas faces each stage of the nozzles. Further, the respective stages (b) and (c), (c) and (d) are provided so that the nozzle angles are different from each other by 90 degrees so that the supplied gases can be easily mixed with each other. is there. For example, in the example of FIG. 5, the upper nozzle 81 and the middle nozzle 82 are connected to the exhaust gas secondary combustion chamber recirculation pipe 42 to supply the exhaust gas J ′, and the lower nozzle 83 is blown from the outside by the secondary combustion blower 44. The secondary combustion air D ′ is supplied by connecting to the secondary combustion chamber air introduction pipe 45. Although the supply amount of the secondary combustion air D ′ changes, the supply amount of the exhaust gas J ′ is constant, and the exhaust gas J ′ has not yet entered the inlet connected to the melting furnace 12 below the lower nozzle 83. The combustion gas and the air supplied from the lower nozzle 83 are entrained to continuously generate an air flow so that stirring and mixing are sufficiently performed.

  Further, the amount of the secondary combustion air D ′ is changed by the above control. Regardless of this change, the gas supplied to the secondary combustion chamber 21 is always sufficiently stirred. It is preferable that the combustion chamber 21 is always supplied.

  It is desirable that the secondary combustion air D ′ is not locally solidified in the secondary combustion chamber 21 but spreads into the secondary combustion chamber 21 to assist combustion. On the other hand, it is desirable that the exhaust gas J ′ is also supplied into the secondary combustion chamber 21 with sufficient momentum for stirring. For this reason, when supplying the secondary combustion air D ′ and the exhaust gas J ′, the wind speed when being injected from the nozzle is preferably 30 m / s or more for proper mixing. In addition, although this wind speed is so preferable that it is quick, exceeding 50 m / s is not realistic, and 40 m / s or less is suitable.

  The temperature of the exhaust gas J ′ supplied to the secondary combustion chamber 21 is preferably 200 ° C. or higher. If it is lower than 200 ° C., corrosion may occur due to condensation. If it is lower than 200 ° C., the temperature is increased to 200 ° C. or higher by heating. In addition, there is no advantage which heats more than the original discharge temperature.

  The temperature of the gas sent from the main combustion chamber 13 to the secondary combustion chamber 21 is preferably 950 ° C. or higher and 1050 ° C. or lower. If the secondary combustion chamber 21 is too cold, combustion does not proceed, but if it is too high, nitrogen oxides are likely to be generated. Therefore, it is desirable that the secondary combustion chamber 21 itself be cooler than the main combustion chamber 13. However, since the gas emitted from the main combustion chamber 13 is at a high temperature, the exhaust gas J 'is supplied to the melting furnace outlet 12a through the exhaust gas melting furnace outlet recirculation pipe 43 to be cooled to the above temperature.

  When the exhaust gas J ′ is supplied to the melting furnace outlet 12a through the exhaust gas melting furnace outlet circulation pipe 43, the value of the oxygen concentration detected by the melting furnace outlet oxygen concentration meter 17 is oxygen and oxygen contained in the exhaust gas J ′. Therefore, the setting value for adjusting the melting furnace supply air regulating valve 19 in the melting furnace outlet oxygen concentration controller 18 is the same as the switching setting value that is a threshold value for instructing the switching mechanism. In addition, it is preferable to consider the change in value due to the supply of the exhaust gas J ′. When low air ratio combustion is actually performed at an air ratio of 1.0 or less, the oxygen concentration at the melting furnace outlet 12a approaches 0 and it becomes difficult to control from the measured oxygen concentration. By supplying 'and raising the oxygen concentration, it is possible to obtain an effect that control becomes easier.

  However, in this case, it is necessary to provide the melting furnace outlet oxygen concentration meter 17 downstream from the position where the exhaust gas J ′ is supplied from the exhaust gas melting furnace outlet circulation pipe 43 between the melting furnace outlet 12a and the secondary combustion chamber 21. is there. Further, in supplying the exhaust gas J ′ so that the supplied exhaust gas J ′ and the gas sent from the main combustion chamber 13 are sufficiently mixed, the gas sent from the main combustion chamber 13 It is preferable to supply air so as to form an air curtain with respect to the flow. Specifically, a tubular portion connected to the exhaust gas melting furnace outlet reflux pipe 43 is provided on the upper surface portion in the pipe of the melting furnace outlet 12a, and a plurality of gas outlets are provided below the tubular portion. The exhaust gas J ′ ejected obliquely downward from each forms an air curtain so as to block the gas passing through the melting furnace outlet 12a. This prevents the gas after combustion from flowing in as it is, dust is dropped into the water tank 16 by the air curtain airflow, and low-boiling gaseous substances are also trapped by the pipe wall and dust by bypassing the flow path. It can be made easy, and solid content adhesion to downstream equipment can be suppressed. In addition, this makes it possible to efficiently cool the gas.

  Further, according to the present invention, even if some degree of incomplete combustion occurs in the melting furnace 12 and carbon monoxide is generated, it is possible to suppress combustion by completely burning in the secondary combustion chamber 21, but excessive incomplete combustion occurs. In the melting furnace 12, it is preferable that combustion is performed at a temperature of about 1200 ° C. to 1350 ° C. in addition to the air ratio as described above. Therefore, in the embodiment of FIG. 1, the combustion air D supplied to the melting furnace 12 is preheated by the air preheater 26 in the middle of the air supply pipe 28.

  An embodiment in which the incineration method according to the present invention is actually executed will be described below.

(Example)
The configuration and flow of the entire combustion apparatus were performed as shown in FIG. That is, in order to perform two-stage combustion in each of the main combustion chamber and the secondary combustion chamber, the combustion air and the stirring exhaust gas were supplied to each to burn the pyrolysis gas and its unburned gas. Moreover, the gas supplied to a secondary combustion chamber was cooled by supplying exhaust gas to the exit of a melting furnace. As the exhaust gas, the gas discharged from the secondary combustion chamber was cooled by exchanging heat with the combustion air, and then the dust was collected by a bag filter and the dust was removed to circulate. In the main combustion chamber, the fuel was burned using kerosene as a flame retardant.

  Further, the secondary combustion chamber was configured as shown in FIG. 5, and the unburned gas sent from the main combustion chamber 13 was burned together with the secondary combustion air. The residence time in the secondary combustion chamber is 2.0 seconds, and the 50A nozzle as the upper stage nozzle is in a position facing each other, a total of 7 nozzles, 3 and 4, and the 50A nozzle as the middle stage nozzle is in a position facing each other. A total of 6 pipes were provided, 3 each, and these were connected to the exhaust gas secondary combustion chamber recirculation pipe to supply the exhaust gas constantly. Moreover, a total of seven 50A nozzles, three and four, are provided at positions facing each other as the lower nozzles, and these are connected to the secondary combustion chamber air introduction pipe so that the secondary combustion air can be supplied. . The wind speed at each introduction was 30 m / s. In addition, the direction in which the upper, middle, and lower nozzles face each other is shifted by 90 degrees.

  Further, the secondary combustion chamber combustion air regulating valve for adjusting the air supply amount of the secondary combustion chamber air introduction pipe is connected to the switching mechanism having the configuration shown in FIG. 3, and an infrared laser oxygen concentration provided at the outlet of the melting furnace. And a secondary combustion chamber outlet thermometer provided at the outlet of the secondary combustion chamber. In addition, an infrared laser oximeter and a melting furnace supply air adjustment valve were connected.

  The set value, which is the target value to be controlled by the value of the secondary combustion chamber outlet thermometer, is 980 ° C. If the value of the thermometer exceeds this set value, the secondary combustion chamber combustion air adjustment valve should be When the valve is opened and less than the set value, a feedback mechanism is provided to further throttle the secondary combustion chamber combustion air regulating valve. In addition, when the measured value of the infrared laser oximeter provided at the melting furnace outlet is 1.5 vol% or less determined as the switching setting value for switching, the melting furnace outlet oxygen concentration controller switches to the signal switching device. The secondary combustion chamber combustion air regulating valve is fully opened by an instruction from a signal generator indicating a fixed value independent of the secondary combustion chamber outlet thermometer. In addition, when the measured value of the infrared laser oximeter becomes 1.55 vol% or more determined as the switching setting value for returning, the switch switching is instructed again, and the control by the secondary combustion chamber outlet temperature controller is performed. return. Also, set a preset value of 1.5% for the melting furnace outlet oxygen concentration controller, and if the value of the infrared laser oxygen analyzer is less than this setting value, loosen the melting furnace supply air adjustment valve, and conversely infrared laser type If the value of the oximeter exceeds this set value, the melting furnace supply air adjustment valve is throttled.

  In the main combustion chamber, kerosene is supplied as a combustion aid in addition to the pyrolysis gas, and burned by a burner provided in the upper portion of the main combustion chamber to assist the combustion of the pyrolysis gas.

  In addition, the average component of the exhaust gas recirculated to each part of the apparatus was moisture 45.9%, and the oxygen concentration was 13.0 vol%.

The air amount, exhaust gas amount, waste treatment amount, oxygen concentration, and auxiliary agent supply amount supplied to each part of the device when the operation is performed are as shown in FIG. The values of operation are summarized in Table 1. The amount of air for flow is the amount of air used when fluidizing a fluid medium in a gasification furnace and sent to the melting furnace together with the pyrolysis gas. The theoretical amount of air is the theoretical amount of air of 2798 m ³ N / h required for complete combustion of the total amount of pyrolysis gas to be generated, and the air required to burn the auxiliary combustor supplied to the main combustion chamber burner. This is a total amount of 142 m ³ N / h. The melting furnace air amount indicates the total amount of the melting furnace upper air amount supplied to the burner combustion air port and the melting furnace air amount supplied from the upper nozzle and the lower nozzle to the main combustion chamber of the melting furnace. The average value per hour of the quantity supplied by is shown. The melting furnace air ratio is a value obtained by dividing (flowing air amount + melting furnace air amount) by the theoretical air amount. Melting furnace circulating exhaust gas injection amount is 200 m ³ N / h of exhaust gas that is constantly supplied to the main combustion chamber through the middle nozzle (melting furnace injection amount), and exhaust gas that is supplied to the melting furnace outlet and used for cooling. The total amount (melting furnace outlet cooling amount) of 540 m ³ N / h. The secondary combustion chamber circulation exhaust gas injection amount is the amount of exhaust gas introduced from the exhaust gas secondary combustion chamber circulation pipe, and the secondary combustion chamber air amount is the amount of combustion air introduced from the secondary combustion chamber air introduction pipe It is. The amount of auxiliary combustion agent used is the amount of auxiliary fuel supplied to the burner, and the amount of auxiliary combustion air is the amount of combustion air introduced directly into the burner for combustion of the auxiliary combustion agent. The total air ratio is a ratio of (flowing air amount + melting furnace air amount + secondary combustion chamber air amount) to the theoretical air amount. Incidentally, m 3 ^{N /} h denotes the gas volume in terms of standard state, all the volume in the example shown calculated in the standard state. DB (DryBase) indicates a dry gas, and WB (WetBase) indicates a wet gas.

In FIG. 6, the melting furnace upper air amount is the air supplied from the air port provided around the burner provided at the upper part of the main combustion chamber, and the melting furnace air amount is the air amount supplied to the main combustion chamber. is there. Kerosene was used as a combustion aid. The air supplied to the secondary combustion chamber is normally adjusted according to a thermometer, and the minimum supply amount is 1450 m ³ N / h. However, when the oxygen concentration is decreased and the control is switched, the supply amount is higher than the time during which the valve is open.

  The set value for performing the switching is virtually calculated from the amounts shown in FIG. That is, from the components of the gas after combustion at the melting furnace air ratio of 0.82 and the components and amount of the supplied exhaust gas when the amount shown in FIG. Assuming that the oxygen in the exhaust gas is not burning, the hypothetical oxygen concentration of the gas is calculated as 2.4 vol%. The elemental composition of fuel and the like used in this example is as shown in Table 2. Since it is considered that the value at which the oxygen concentration has to decrease and the oxygen supply amount to the secondary combustion chamber must be increased is lower than this, the setting value for switching the control is 1.5 vol%, and the setting value for returning the control is It was set to 1.55 vol%, 0.05% higher than that.

When operating under the above conditions, carbon monoxide (CO) concentration, oxygen (O ₂ ) concentration, nitrogen oxide (NOx) concentration in the exhaust gas discharged from the secondary combustion chamber, and provided at the melting furnace outlet FIG. 7 shows the oxygen concentration (melting furnace outlet O ₂ concentration) measured by the infrared laser oxygen concentration meter. The CO concentration and NOx concentration are both 12% oxygen equivalent values. The amount of carbon monoxide emissions was detected only to the extent that it did not appear on the graph over the entire time period except that a peak appeared only between 6 and 7 o'clock, and the melting furnace, secondary combustion chamber, It was found that carbon monoxide emissions were suppressed by achieving sufficient combustion. Further, the NOx concentration was about 80 ppm or less in terms of the dry gas oxygen concentration of 12%, and it did not jump and become a high value.

(Comparative example)
In the examples, even when the measured value with the infrared laser oximeter was 1.5 vol% or less, the operation was continued by controlling the thermometer as it was without switching the control. FIG. 8 shows the CO concentration, O ₂ concentration, NOx concentration in the exhaust gas discharged from the secondary combustion chamber in this case, and the oxygen concentration measured by the infrared laser oximeter provided at the melting furnace outlet. Although there was almost no difference between the O ₂ concentration and the NOx concentration as compared with the Example, it was found that the CO concentration jumped and increased many times, and sufficient combustion was not possible in the secondary combustion chamber.

The figure which shows the structural example of the incinerator which enforces the incineration method concerning this invention Infrared laser oxygen analyzer configuration diagram Configuration diagram of switching mechanism when secondary combustion chamber outlet thermometer is used Configuration diagram of switching mechanism when secondary combustion chamber outlet oximeter is used (A) Longitudinal sectional view of secondary combustion chamber, (b) AA sectional view of (a), (c) BB sectional view of (a), (d) CC sectional view of (a) The figure which shows supply_amount | feed_rate of air etc. in the case of the test operation in an Example Graph showing _CO, and changes of the concentration of O 2, NOx in the embodiment Graph showing changes in concentration of CO, O ₂ and NOx in Comparative Example

Explanation of symbols

A Waste B Pyrolysis gas C Solid content D Combustion air D 'Secondary combustion air E Gas after combustion F Slag J, J' Exhaust gas 11 Gasification furnace 12 Melting furnace 12a (of melting furnace) Outlet 13 Main combustion Chamber 14 Molten pool 15 Depot 16 Water tank 17 Oxygen concentration meter 18 at the melting furnace outlet Oxygen concentration meter at the melting furnace outlet 19 Melting furnace supply air regulating valve 20, 20 'Signal cable 21 Secondary combustion chambers 24, 24' Chimney 25 Gas cooling chamber 26 Air preheater 27 Blower for melting furnace combustion 28 Air supply pipe 29 Temperature reducing tower 30 Bag filter 31 Induction fan 32 Exhaust gas melting furnace recirculation pipe 33 Exhaust gas recirculation fan 41 Burner supply air regulating valve 42 Exhaust gas secondary combustion Chamber recirculation pipe 43 Exhaust gas melting furnace outlet recirculation pipe 44 Secondary combustion blower 45 Secondary combustion chamber air introduction pipe 46 Secondary combustion chamber outlet thermometer 46 'Secondary combustion chamber outlet oxygen concentration meter 47 Secondary combustion chamber outlet temperature controller 47 'Secondary combustion chamber outlet oxygen concentration controller 48 Secondary combustion chamber combustion air control valve 51 Transmitter 52 Receiver 53 Controller 56 Signal switch 57 Signal generator 58 Switch 81 Upper nozzle 82 Middle nozzle 83 Lower nozzle

Claims (12)

A gasification furnace that thermally decomposes waste into pyrolysis gas and solids;
A melting furnace for burning and melting the pyrolysis gas and solids;
A secondary combustion chamber connected to the gas outlet of the melting furnace and burning unburned gas that has not been burned in the melting furnace among the pyrolysis gas;
A flue for discharging the exhaust gas generated in the secondary combustion chamber to the outside;
A secondary combustion chamber air supply pipe for supplying combustion air from the outside to the secondary combustion chamber;
A secondary combustion chamber combustion air adjustment valve that adjusts the amount of combustion air supplied to the secondary combustion chamber through the secondary combustion chamber air supply pipe;
A thermometer that measures the temperature of the exhaust gas that has exited the secondary combustion chamber, an oxygen concentration meter that measures the oxygen concentration of the exhaust gas, or both,
A feedback mechanism for controlling the secondary combustion chamber combustion air regulating valve so that the value of the thermometer or the oxygen concentration meter approaches a preset set value;
A waste incinerator having
A melting furnace outlet oximeter for measuring the oxygen concentration at the gas outlet leading to the secondary combustion chamber of the melting furnace,
Normally, the secondary combustion chamber combustion air regulating valve is controlled by the feedback mechanism, and when the measured value of the melting furnace outlet oximeter falls below a preset switching set value, the control by the feedback mechanism The control is switched so as to control the secondary combustion chamber combustion air regulating valve in accordance with a control value by a control device independent of a thermometer and the oxygen concentration meter, and the measurement value is set to a preset switching setting value. A waste incinerator having a switching mechanism that, when exceeded, returns to control by the feedback mechanism.   The control device independent of the thermometer and the oxygen concentration meter is a signal generator that instructs to adjust the degree of opening of the secondary combustion chamber combustion air regulating valve to a control value that is a preset fixed value. The waste incinerator according to claim 1.   The waste incinerator according to claim 1 or 2, wherein a switching setting value for returning to the control by the feedback mechanism is set higher than a switching setting value for switching the control.   The waste incinerator according to any one of claims 1 to 3, wherein the melting furnace outlet oximeter is an infrared laser oximeter.   The waste incinerator according to any one of claims 1 to 4, further comprising an exhaust gas secondary combustion chamber circulation pipe that branches off from the flue and supplies the exhaust gas to the secondary combustion chamber.   6. An exhaust gas melting furnace outlet recirculation pipe that branches off from the flue and supplies the exhaust gas to a location upstream of the location where the melting furnace outlet oxygen concentration meter is provided at the outlet of the melting furnace. The waste incinerator according to any one of the above. Waste that pyrolyzes waste into pyrolysis gas and solids, melts and burns them in the melting furnace, burns unburned gas discharged from the melting furnace outlet in the secondary combustion chamber, and discharges exhaust gas Incineration method,
Combustion air supplied to the secondary combustion chamber so that the value of a thermometer that measures the temperature of the exhaust gas or the oxygen concentration meter that measures the oxygen concentration of the exhaust gas approaches a preset value in a normal state. Control the amount of
When the measured value of the melting furnace outlet oximeter for measuring the oxygen concentration at the melting furnace outlet is equal to or less than a preset switching setting value, the control of the amount of combustion air supplied to the secondary combustion chamber, Switching to be performed according to a control value by a control device independent of a thermometer and the oxygen concentration meter, and then, when the measured value exceeds a preset switching setting value, the adjustment is performed using the thermometer or the oxygen meter. A waste incineration method characterized by returning to control by a densitometer value.   The waste incineration method according to claim 7, wherein a control value by a control device independent of the thermometer and the oxygen concentration meter is a preset fixed value.   The waste incineration method according to claim 7 or 8, wherein a switching setting value for returning to the control by the thermometer or the oxygen concentration meter is set higher than a switching setting value for switching the control.   The waste incineration method according to any one of claims 7 to 9, wherein gas in the secondary combustion chamber is mixed and stirred by constantly supplying the exhaust gas to the secondary combustion chamber.   The waste incineration method according to claim 10, wherein a wind speed of the exhaust gas supplied into the secondary combustion chamber is always 30 m / s or more and 40 m / s or less.   The waste incineration method according to any one of claims 7 to 11, wherein the exhaust gas is supplied to a location upstream of the location where the melting furnace outlet oxygen concentration meter is provided at the outlet of the melting furnace.

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