JPH0551806B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a combustion method and a combustion device in a radiant tube burner, and particularly preferably, the flow rate of combustion air in primary combustion and secondary combustion is made different. The aim is to effectively reduce the amount of _NOx produced by performing two-stage combustion within the radiant tube.

(Prior art) When oxidation of the material to be treated is not desired during heat treatment, such as in a continuous annealing furnace for steel strips, the inside of the furnace is made into a non-oxidizing atmosphere such as (N ₂ + H ₂ ) gas, and As a heating method, a radiant tube burner, which is an indirect heating method, is generally used.

This burner has a radiant tube (generally
Since combustion is carried out in a very narrow space (with a diameter of 100 to 200 mm), the disadvantage is that a large amount of _NOx is produced compared to a direct-fired burner that burns in a large space.

In this regard, the inventors previously provided a combustion method and apparatus in a radiant tube burner that can reduce the amount of _NOx produced in Japanese Patent Application Laid-Open No. 57-14106. An overview of this technology will be explained based on FIG. 10. Reference numeral 1 indicates a part of a radiant tube, and a burner 2 is attached to the end of this tube. Combustion gas is supplied from the supply port 3 to the gas chamber 4, and is connected to gas pipes (3 to 6).
Gas is ejected into the tube 1 from a gas nozzle 7 which opens into the burner head 6 through the tube 5. On the other hand, the combustion air passes through the tube 1 from the supply port 8 and is ejected from the primary air nozzle 10 formed by the inner surface of the tube and the flange portion 9 of the burner head 6 to become external air (primary air). Further, the remaining combustion air is ejected from a secondary air nozzle 12 of a secondary air pipe 11 provided in the burner head 6 and becomes secondary air (inner flow air). The combustion gas ejected from the gas nozzle 7 and the primary air ejected from the primary air nozzle 10 cause primary combustion, and then the unburned gas and the secondary air ejected from the secondary air nozzle 12 cause secondary combustion. It is burned. In this case, the ratio of the primary air amount to the total air amount should be 35 to 80%, the momentum ratio of the combustion gas to the primary air should be 4.0 or more, and the length of the secondary air pipe should be 200 mm or more. This suppresses the production of _NOx .

(Problems to be Solved by the Invention) Recently, high-temperature annealing has become popular, for example, in continuous annealing furnaces for steel strips.

In other words, in the past, steel strips were mainly made of low carbon steel, but recently there has been a tendency for ultra-low carbon steel (C≦100ppm), which has a high annealing temperature, to be subjected to continuous annealing. However, with advances in various control technologies within the furnace (temperature, tension, speed, etc.), high-speed threading has become possible. That's what I'm starting to aim for. Furthermore, attempts have been made to annealing stainless steel at a higher annealing temperature by continuous annealing for ordinary steel as described above.

In order to respond to such high-temperature annealing, it is sufficient to increase the amount of combustion in the radiant tube, but if the amount of combustion is simply increased, as mentioned above, the inside of the tube is very narrow, so the combustion As the temperature of the gas increases, thermal
There was a limit to such high-load combustion because the amount of NO _X generated increased and the environmental standard value for NO _X (150 ppm or less) could not be satisfied.

Although it may be possible to deal with this by adding more burners, this not only increases equipment costs but also has the disadvantage of increasing the number of piping and valves, making maintenance difficult. Furthermore, if the furnace is already installed, there are other problems such as the need for complicated remodeling work.

This invention was developed in view of the above circumstances, and uses a combustion method in a radiant tube burner that can effectively suppress the amount of _NOx generated even during high-load combustion. The purpose of the present invention is to propose a suitable combustion device.

(Means for Solving the Problem) Now, as disclosed in Japanese Patent Application Laid-Open No. 57-14106, the inventors have determined that the momentum ratio of combustion gas to primary air, η η=M _G /M _AO , is 0.4 or more. However, with this _technology , the effect saturates when the momentum ratio η exceeds 1.0.
No further reduction in _NOx could be expected. Moreover, the actual momentum ratio could only be increased to a maximum of about 1.7.

Therefore, the inventors developed a device that can achieve such a high momentum ratio based on the idea that a further reduction in NO _x could be achieved by increasing the momentum ratio η. As a result of intensive research including the above, it was found that the amount of _NO

In other words, when the momentum ratio η is further increased,
They obtained the knowledge that the amount of NO _X produced can be further reduced.

This invention is based on the above knowledge.

That is, this invention introduces combustion air into a cylindrical body having a burner head at its tip disposed within a radiant tube, and a secondary air nozzle provided at the center of the burner head and an arrangement surrounding the secondary air nozzle. From the primary air nozzle,
First, the combustion gas is ejected as secondary air and primary air, and from 3 to 6 combustion gas nozzles provided in the burner head so as to surround the secondary air nozzle.
In a combustion method in which primary combustion is performed with primary air and then secondary combustion is performed with secondary air, the number of primary air nozzles is the same as the number of combustion gas nozzles, and the ratio of the amount of primary air to the amount of combustion air is This is a combustion method in a radiant tube burner, in which the momentum ratio of the combustion gas to the primary air is set at 35 to 80% and at least 1.7.

The present invention also provides a cylindrical combustion device having a burner head at its tip, which includes a secondary air nozzle protruding from the center of the burner head, and a secondary air nozzle provided in the burner head to surround the secondary air nozzle. 3 to 6 combustion gas nozzles, and the same number of combustion gas nozzles provided on a concentric circle
By providing a secondary air nozzle and arranging a throttle plate for ejected air near the tip of the primary air nozzle, a pipe structure is created in which the cross-sectional area of the flow passage at the tip of the nozzle is larger than that of the constriction part. This is a combustion device in a radiant tube burner.

(Function) Now, as a method for increasing the momentum ratio η, the following ~ can be considered.

Increase the flow rate of combustion gas.

Decrease the flow velocity of primary air.

However, since the combustion gas used in steelworks is generally C gas, B gas, etc., there is a limit to the gas supply pressure, and therefore there is a limit to the gas flow rate.

Regarding one, it is possible to increase the opening area of the primary air nozzle, but if the area is simply increased, the ratio of the primary air amount to the total air amount will change and fall outside the optimal ratio range. Therefore, if the area of the primary air nozzle is increased, the area of the secondary air nozzle must be increased accordingly, but as mentioned above, the diameter of the radiant tube is fixed, so this method also applies. There is a limit, and the optimal momentum ratio that can be achieved with this method is only about 1.7, as mentioned above.

In contrast, the present invention increases the momentum ratio η by maintaining the ratio of the amount of primary air to the total amount of air within a predetermined range and also by making the flow velocity of the primary air smaller than that of the secondary air. death,
This has achieved low _NOx emissions.

(Example) Hereinafter, an example of the present invention will be specifically described based on the drawings. Figures 2 to 4 show a burner according to the invention incorporated in the end of a radiant tube.
Shown in local cross section, partially enlarged view, and longitudinal cross section, number 13 in the figure is a radiant tube, and number 14 is a radiant tube burner attached to the end thereof. Reference numeral 15 is a cylindrical body with a high degree of roundness, and a gas chamber 16 is provided at one end of the cylindrical body, and a burner head 17 is provided at the other end. A combustion gas supply port 18 is connected to the gas chamber 16, and the combustion gas supplied from this port flows through the gas chamber 16 into a plurality of (3 to 6) gas pipes 19 and opens into the burner head 17. The gas is ejected into the tube 13 from the gas nozzle 20.
Reference numeral 21 denotes a combustion air supply port connected to the cylindrical body 15, and the air supplied from this port passes through the cylindrical body and is transferred from a primary air nozzle and a secondary air nozzle, which will be described later, to primary air,
It is designed to be blown out into the tube 13 as secondary air. In the burner head 17, 22 is a burner plate attached to the end of the cylindrical body 15, 23 is a primary air pipe, 24 is a throttle plate, 2
5 is a primary air nozzle, 26 is a secondary air pipe provided in the center, and 27 is a secondary air nozzle.

Now, 35 to 80% of the amount of air supplied from the air supply port 21 mentioned above is ejected into the tube from this primary air nozzle 25, but at this time, the cross-sectional area of the flow path at the nozzle tip is larger than that at the constriction part. is also larger, so the flow velocity of the air jetted from the nozzle tip is slower than the flow velocity inside the cylindrical body 15. Meanwhile, the remaining air (20-65% of the total air amount)
will be ejected from the secondary air nozzle 27 at the same flow rate. The combustion gas ejected from the gas nozzle 20 and the primary air ejected from the primary air nozzle 25 perform primary combustion, and then the unburned combustion gas and the secondary air nozzle 27
Secondary combustion is performed by the secondary air ejected from the combustion chamber.

By the way, if uneven combustion occurs in the circumferential direction in the burner head, a locally high temperature area will be generated, leading to an increase in thermal _NOx . Therefore, it is necessary to avoid uneven combustion in the circumferential direction as much as possible.
The primary air nozzles 25 and the gas nozzles 20 are arranged concentrically and at equal pitches, as shown in FIG.
Furthermore, the primary air nozzle 25 and the gas nozzle 20
It is preferable that the mutual spacing between them is also equal.

In addition, the flow rate distribution of primary air and secondary air is 1
The ratio of the total opening cross-sectional area of the aperture plate 24 of the secondary air nozzle 25 to the opening cross-sectional area of the secondary air nozzle may be adjusted to fall within a predetermined range.

Furthermore, since the flow velocity of the primary air is determined by the ratio of the opening cross-sectional area at the throttle part of the throttle plate 24 and the opening cross-sectional area at the tip of the primary nozzle 25,
This may also be determined so that the momentum η becomes a desired value.

With the above configuration, the momentum ratio of 1.7 or more, which was difficult to achieve with conventional burners, can be increased to η: 1.7 or more, about 4.0, as shown in FIG. 1 above in this invention. In particular, if η is set to 2.5 or more, an extremely excellent NO _x reduction effect can be obtained.

The conventional burner shown in FIG. 10 and the burner according to the present invention shown in FIG. 2 are installed in the W-shaped radiant tube 1 shown in FIG. 5, respectively.
Figure 6 shows the results of a survey on the amount of _NOx produced.

In the figure, the horizontal axis indicates the maximum temperature in the longitudinal direction of the radiant tube, and the vertical axis indicates the amount of NO _x produced in terms of 11% O ₂ .

As is clear from the figure, if the radiant tube temperature is the same, the amount of _NOx produced in this invention is about 50 ppm lower.

Furthermore, in order to satisfy the environmental standard value (NO _X ≦150ppm), the conventional radiant tube temperature
Whereas it was only possible to raise the temperature to around 950℃,
In this invention, the radiant tube temperature can be raised to 1100°C or higher.

Another embodiment is shown in FIGS. 7-9.

Figure 7 shows a tapered air pipe 28 without using a throttle plate to reduce the flow velocity of the primary air.
This is what was used. Further, in this embodiment, the primary air ejected from the air nozzle is not parallel to the normal line of the burner plate, so there is an advantage that the primary combustion with the combustion gas can be smoothly diffused.

FIG. 8 is a development of FIG. 7, in which the primary air nozzle 25 is also provided with a constriction part 29 to further change the direction of the ejected air.

Figure 9 shows the air pipe 30 tilted outward.
This is designed to promote mixing of air and gas, and similar to the ones shown in Figs. 7 and 8, the amount of _NOx produced can be further reduced by about 10 ppm compared to the one shown in Fig. 2.

(Effects of the Invention) Thus, according to the present invention, the amount of _NOx generated can be reduced to 2/3 to 3/4 of the conventional amount, and environmental standards can be fully satisfied even when high-load combustion is performed.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the momentum ratio of combustion gas to primary air and the amount of _NOx produced.
2, 3 and 4 are respectively a local sectional view, a partially enlarged view and a longitudinal sectional view of a preferred combustion device according to the present invention; FIG. 5 is a front view of a W-type radiant tube; and FIG. Graphs 7 to 9 showing the relationship between the maximum temperature in the longitudinal direction of the radiant tube and the amount of _NO A cross-sectional view showing a preferred example, FIG. 10, is a partial cross-sectional view of a conventional combustion burner. 13...Radiant tube, 14...Radiant tube burner, 15...Cylindrical body, 16...
Gas chamber, 17... Burner head, 18...
...Combustion gas supply port, 19...Gas pipe, 20
... Gas nozzle, 21 ... Combustion air supply port,
22... Burner plate, 23... Primary air pipe, 24... Throttle plate, 25... Primary air nozzle, 26... Secondary air pipe, 27... Secondary air nozzle, 28... Tapered air pipe,
29... Aperture section.

Claims (1)

[Claims] 1. Introducing combustion air to a cylindrical body having a burner head at the tip disposed within a radiant tube,
The secondary air nozzle provided at the center of the burner head and the primary air nozzle surrounding the secondary air nozzle eject secondary air and primary air, respectively.
Combustion gas is ejected from 3 to 6 combustion gas nozzles provided in the burner head so as to surround the secondary air nozzle, and primary combustion is performed first with primary air, and then secondary combustion is performed with secondary air. In the method, the number of primary air nozzles is the same as the number of combustion gas nozzles, the ratio of the primary air amount to the combustion air amount is 35 to 80%, and the momentum ratio of the combustion gas to the primary air is A combustion method in a radiant tube burner, characterized in that the ratio is 1.7 or more. 2. The combustion method according to claim 1, wherein the flow velocity of the primary air is lowered than the flow velocity of the secondary air. 3. The combustion method according to claim 1 or 2, wherein the central flow of primary air is injected outward with respect to the axis of the cylindrical body. 4. A cylindrical combustion air device with a burner head at the tip, including a secondary air nozzle protruding from the center of the burner head, and 3~3mm air nozzles provided on the burner head to surround the secondary air nozzle. By providing six combustion gas nozzles and the same number of primary air nozzles provided concentrically with the combustion gas nozzles, and arranging an ejection air throttle plate near the tip of the primary air nozzle, A combustion device for a radiant tube burner, characterized in that the cross-sectional area of the flow passage at the tip of the nozzle is larger than that of the throttle part. 5. The combustion device according to claim 4, wherein the ejection direction of the primary air central flow is in a direction expanding outward with respect to the axis of the cylindrical body.