JP5353033B2 - Corrosion prevention method for exhaust gas cooler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing the corrosion of an exhaust gas cooler by which the corrosion of an exhaust gas cooler part is prevented by reducing NOx concentration incorporated in the exhaust gas at the preheating time with a simple structure, thereby reducing the maintenance cost of the exhaust gas cooler and extending the service life. <P>SOLUTION: An exhaust gas duct 12 is constituted of; a first horizontal part 12a whose one end is connected to the upper part of a vacuum-degassing vessel 10; an inclining part 12b whose one end is connected to another end of the first horizontal part 12a and which inclines in slanting lower part; and a second horizontal part 12c whose one end is connected to another end of the inclining part 12b and whose another end is connected to the exhaust gas cooler 11. A connection part between the first horizontal part 12a and the inclining part 12b is made to be a first bending part P1 having the bending angle &alpha;1 to the horizontal surface, and the connecting part of the inclining part 12b and the second horizontal part 12c, is made into a second bending part P2 having the bending angle &alpha;2 to the horizontal surface. Level difference D of the bending, is made to be &ge;1.0 and &lt;4.0 times of the inner diameter of the first horizontal part 12a, and the bending angles &alpha;1, &alpha;2, are defined as &lt;45&deg;. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

The present invention relates to a method for preventing corrosion of an exhaust gas cooler caused by NOx contained in exhaust gas generated when preheating a vacuum degassing tank in vacuum degassing treatment.

The molten steel that has undergone primary refining in the converter is degassed using a vacuum degassing apparatus for decarburization or for the purpose of removing dissolved gases such as hydrogen and nitrogen. In the degassing treatment, after the lower part (immersion tube) of the vacuum degassing tank is immersed in the molten steel in the pan, the air in the vacuum degassing tank is sucked so that the vacuum degassing tank is in a vacuum state. Thus, the molten steel is sucked into the vacuum degassing tank, and the gas components in the molten steel are released into the vacuum degassing tank.

The vacuum degassing tank has a high temperature due to heat received from the molten steel at the time of degassing the molten steel, but the temperature rapidly decreases during standby. When the next molten steel is degassed in a state where the temperature of the vacuum degassing tank is lowered, the adhesion of the splash and the metal is remarkable, which causes the operation to be hindered. Therefore, in preparation for the next degassing treatment of molten steel, the vacuum degassing tank is preheated using a heating means such as a burner.

As a technique related to preheating of the vacuum degassing tank, Patent Document 1 is characterized in that a shut-off valve is provided in the position immediately adjacent to the vacuum degassing tank of the exhaust gas duct for guiding the exhaust gas from the vacuum degassing tank to the exhaust gas cooler. An invention of a heat retaining device for a vacuum degassing tank is disclosed.

Further, in Patent Document 2, in a vacuum degassing tank in which an exhaust pipe is connected to the upper part of the tank body and a dip pipe is attached to the lower part of the tank body, the lower inner wall of the exhaust pipe becomes lower and larger as it moves away from the tank body. Thus, a technique for inclining is disclosed.

Japanese Patent Publication No. 6-89395 JP 2004-68060 A

However, in the heat retaining device described in Patent Document 1, since it is necessary to attach a shut-off valve to the exhaust gas duct, there is a problem that the shut-off valve is easily damaged by heat during preheating and the maintenance cost of the shut-off valve increases. Further, there is a problem that the exhaust gas during preheating leaks from the shut-off valve damaged under high heat and enters the exhaust gas duct, causing corrosion of the exhaust gas cooler.
On the other hand, in the case of the vacuum degassing tank described in Patent Document 2, since the area of the exhaust pipe heated by radiant heat increases, the refractory constituting the exhaust pipe is damaged and the life of the exhaust pipe is reduced. There is. Further, since the structure of the exhaust pipe becomes complicated, the manufacturing cost increases. In addition, since the splash flowing into the exhaust pipe flows down the inclined surface and accumulates in the lower part, there is a problem that it takes time to remove the accumulated metal.

In order to solve the above problem, a vacuum degassing apparatus in which a vacuum degassing tank and an exhaust gas cooler are connected by a horizontal exhaust duct has been proposed. FIG. 7 shows a schematic diagram of a vacuum degassing apparatus having a horizontal exhaust duct. When the vacuum degassing tank 10 is preheated by the burner 16 inserted through the upper lid of the vacuum degassing tank 10, the exhaust gas generated by the combustion is sucked into the exhaust duct 72 to form a partial flow C, This partial flow C flows along the upper surface of the exhaust duct 72 to the exhaust gas cooler 11, is cooled by the exhaust gas cooler 11, and forms a large circulating flow that flows back through the lower surface of the exhaust duct 72.
In addition, a refrigerant pipe 15 through which cooling water circulates is disposed in the exhaust gas cooler 11, and dew condensation generated on the surface of the refrigerant pipe 15 passes through a drain 13 connected to the lower end of the exhaust gas cooler 11. It is drained into the seal pot 14.

The formation process of the circulating flow will be described based on the thermal flow rate analysis result shown in FIG. The exhaust gas C 1 heated by the burner 16 rises in the exhaust duct 72 and collides with the upper surface of the exhaust duct 72. A part C2 of the exhaust gas C1 colliding with the upper surface of the exhaust duct 72 is branched and flows to the exhaust gas cooler 11 along the upper surface of the exhaust duct 72. The exhaust gas C3 that has flowed into the exhaust gas cooler 11 is cooled and descends, and becomes a flow C4 that returns to the vacuum degassing tank 10.

In the combustion preheating, oxygen and nitrogen in the air sucked into the vacuum degassing tank 10 cause a chemical reaction, and NOx is generated. When the exhaust gas becomes a circulating flow, the amount of air sucked with combustion preheating increases, so the generated NOx also increases, and the NOx concentration in the exhaust gas becomes extremely high. The NOx in the exhaust gas dissolves in the dew condensation generated in the exhaust gas cooler 11 to become a nitric acid aqueous solution having a low pH, and abruptly corrodes the exhaust gas cooler portion including the exhaust gas cooler 11, the drain 13, and the seal pot 14.

The present invention has been made in view of such circumstances, and with a simple structure, the NOx concentration contained in the exhaust gas during preheating is reduced to prevent corrosion of the exhaust gas cooler, thereby reducing the maintenance cost of the exhaust gas cooler. And it aims at providing the corrosion prevention method of the exhaust gas cooler which aims at lifetime extension.

In order to achieve the above object, the present invention is based on NOx contained in exhaust gas generated during preheating of the vacuum degassing tank in a vacuum degassing treatment apparatus in which a vacuum degassing tank and an exhaust gas cooler are connected by an exhaust duct. A method for preventing corrosion of the exhaust gas cooler, wherein the exhaust duct is provided with two or more bent portions at four or less portions, and at least a part of the bent portions has a height from the bent portion by the vacuum degassing. It is characterized by being below the bent part located on the tank side and / or above the bent part located on the exhaust gas cooler side from the bent part.

In the present invention, the exhaust duct connecting the vacuum degassing tank and the exhaust gas cooler is bent in the downward direction at 2 or more and 4 or less (a part of the bent portions may be horizontal) to be described later. As described above, the flow of the exhaust gas toward the exhaust gas cooler along the upper surface of the exhaust duct can be cut off at the bent portion. In addition, since it becomes difficult to form a circulating flow, the amount of air sucked with combustion preheating decreases, and the amount of NOx generated decreases. As a result, the NOx concentration in the exhaust gas is greatly reduced, and corrosion of the exhaust gas cooler can be prevented.

In the exhaust gas cooler corrosion prevention method according to the present invention, the vertical distance between the bent portion located closest to the vacuum degassing tank and the bent portion located closest to the exhaust gas cooler, The inner width of the exhaust duct is preferably 1.0 times or more and less than 4.0 times.
The vertical distance between the bent portion closest to the vacuum degassing tank and the bent portion closest to the exhaust gas cooler (hereinafter referred to as “bent step”) is 1. If it is less than 0 times, the exhaust gas partially flowing in the direction of the exhaust gas cooler is liable to form a circulation flow, which is not preferable. On the other hand, when the bending step is 4.0 times or more the internal width of the exhaust duct, the exhaust resistance may increase and hinder vacuum refining.

In the present invention, the NOx concentration in the exhaust gas can be greatly reduced by bending the exhaust duct connecting the vacuum degassing tank and the exhaust gas cooler in the downward direction at 2 or more and 4 or less . As a result, corrosion of the exhaust gas cooler part is prevented, so that the maintenance cost of the exhaust gas cooler part can be reduced and the life can be extended.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.

FIG. 1 shows a schematic diagram of an exhaust duct constituting a vacuum degassing apparatus to which a method for preventing corrosion of an exhaust gas cooler according to an embodiment of the present invention is applied.
The exhaust duct 12 includes a first horizontal portion 12a having one end connected to the upper portion of the vacuum degassing tank 10, and an inclined portion 12b having one end connected to the other end of the first horizontal portion 12a and inclined obliquely downward. The second end of the inclined portion 12b is connected to the other end of the inclined portion 12b, and the other end of the inclined portion 12b is connected to the exhaust gas cooler 11.

The connecting portion between the first horizontal portion 12a and the inclined portion 12b is a first bent portion P1 having a bending angle α1 with respect to the horizontal plane, and the connecting portion between the inclined portion 12b and the second horizontal portion 12c is relative to the horizontal plane. The second bent portion P2 has a bent angle α2. That is, the second bent portion P2 is positioned below the first bent portion P1 located on the vacuum degassing tank 10 side from the second bent portion P2, and the first bent portion P1 is the first bent portion P1. It is located above the second bent part P2 located on the exhaust gas cooler 11 side from the part P1.

Here, the bending angles α1 and α2 are less than 45 degrees. When the bending angles α1 and α2 are 45 degrees or more, the exhaust resistance increases and it takes time for the vacuum degassing process.
Further, the bending step D, which is the vertical distance between the bent portion P1 located closest to the vacuum degassing tank 10 and the bent portion P2 located closest to the exhaust gas cooler 11, is an inner diameter of the first horizontal portion 12a. 1.0 times or more, preferably 1.5 times or more and less than 4.0 times.

In addition, as for the horizontal distance H from the end of the 1st horizontal part 12a to the exhaust gas cooler 11 center part, 6-20 m is preferable. If the distance H is less than 6 m, the exhaust gas partially flowing in the direction of the exhaust gas cooler 11 reaches the exhaust gas cooler 11 and causes corrosion of the exhaust gas cooler 11. On the other hand, if the distance H exceeds 20 m, the exhaust resistance increases and it takes time for the vacuum degassing process.

Here, the corrosion prevention mechanism of the exhaust gas cooler according to the present invention will be described.
FIG. 2 is a schematic diagram showing the flow of exhaust gas in the exhaust duct 12 having the shape shown in FIG. The exhaust gas G1 heated by the burner rises in the first horizontal portion 12a and collides with the upper surface of the inclined portion 12b. Most of the exhaust gas G1 that has collided with the upper surface of the inclined portion 12b becomes a partial flow that returns to the vacuum degassing tank 10 along the upper surface of the first horizontal portion 12a, and a part G2 of the exhaust gas G1 branches to form the inclined portion 12b. It flows downward along the upper surface of. The branched flow G2 is cooled and descends and collides with the lower surface of the inclined portion 12b. Most of the exhaust gas G3 colliding with the lower surface of the inclined portion 12b becomes a partial flow returning to the vacuum degassing tank 10 along the lower surface of the inclined portion 12b, and a part G4 of the exhaust gas G3 is branched to form the second horizontal portion 12c. It flows in the direction of.

Thus, in the method of the present embodiment, the exhaust duct 12 becomes the partial flows G1 and G3 that return to the vacuum degassing tank 10 before most of the exhaust gas reaches the exhaust gas cooler 11, and a circulation flow is not easily formed. Become. For this reason, the amount of air suction accompanying combustion preheating decreases, and the amount of NOx generated decreases. As a result, NOx in the exhaust gas cooler 11 is significantly reduced, and corrosion of the exhaust gas cooler portion can be prevented.

FIG. 3 is a graph showing changes in the concentrations of NOx and O ₂ in the exhaust gas cooler obtained by heat flow rate analysis. From the figure, it can be seen that in the case of a horizontal exhaust duct, the NOx concentration in the exhaust gas cooler increases rapidly after 10 minutes. Along with this, the O ₂ concentration also increases. On the other hand, in the exhaust duct according to the present invention, it can be seen that the NOx concentration in the exhaust gas cooler hardly increases and is a very low value. Moreover, since the combustion efficiency is high, the O ₂ concentration is also a very low value.

Next, an exhaust duct constituting a vacuum degassing apparatus to which the corrosion prevention method for an exhaust gas cooler according to another embodiment of the present invention is applied will be described.
4A and 4B show the case where there are three bent portions, and FIGS. 4C to 4E show the case where there are four bent portions, respectively.
In the exhaust ducts 22 and 52 shown in FIGS. 4A and 4D, the inclined portions 22b and 52b between the first horizontal portions 22a and 52a and the second horizontal portions 22c and 52c are projected downward. In the exhaust ducts 32 and 62 in FIGS. 4B and 4E, the inclined portions 32b and 62b between the first horizontal portions 32a and 62a and the second horizontal portions 32c and 62c are convex upward. Yes. Further, in the exhaust duct 42 of FIG. 4C, the intermediate portion of the inclined portion 42b between the first horizontal portion 42a and the second horizontal portion 42c is horizontal.

The bending angles α1, α2, α3, and α4 at the bent portions P1, P2, P3, and P4 are less than 45 degrees, and are positioned closest to the bent portion P1 and the exhaust gas cooler 11 that are closest to the vacuum degassing tank 10. A bending step D, which is a vertical distance between certain bending portions P3 and P4, is 1.0 times or more and less than 4.0 times the inner diameter of the first horizontal portions 22a, 32a, 42a, 52a, and 62a.
In addition, since the exhaust resistance in a vacuum degassing process will increase and a refinement will be hindered when a bending part exceeds four places, within four places are desirable.

Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations described in the above-described embodiments, and is considered within the scope of the matters described in the claims. Other embodiments and modifications are also included. For example, in the above embodiment, the cross section of the exhaust duct is assumed to be circular, but may be rectangular.

Using the number of bent portions of the exhaust duct and the bent step D as parameters, a test was conducted on the NOx concentration in the exhaust gas cooler. Table 1 shows the test conditions and test results. The bending step D in Table 1 is the ratio to the inner diameter of the exhaust duct, and the corrosion index is the ratio to the corrosion area of the conventional example.
In addition, FIG. 5 shows the dimensions of the exhaust duct (when there are two bent portions) at that time, and FIG. 6 shows a graph of test results showing the relationship between the bent step D and the corrosion index.

Table 1 shows the following.
In the case of the horizontal duct of the conventional example, since a large circulation flow reaching the exhaust gas cooler is formed, NOx is continuously generated and corrosion proceeds. On the other hand, Examples 1 to 5 of the present invention in which two bent portions are provided have a lower corrosion index than the conventional example. On the other hand, in Comparative Example 1 in which one bent portion is provided, a large circulation flow reaching the exhaust gas cooler is not formed by providing the bent portion, but the exhaust gas cooler has a structure located below the exhaust duct. There is a problem that dust accumulates. Further, in Comparative Example 2 in which five bent portions are provided, the corrosion index becomes very low by providing a large number of bent portions, but the pressure loss (exhaust resistance) in the exhaust duct increases and the vacuum degassing treatment time increases. There is a problem.

When the examples are compared with each other, as is apparent from FIG. 6, it can be seen that the corrosion index decreases as the bending step D increases. Moreover, about an Example, there exists correlation (FIG. 6) with the bending level | step difference D and a corrosion index.
In Example 4 in which the bending step D is 0.5 times the inner diameter of the exhaust duct, the exhaust gas that partially flows in the direction of the exhaust gas cooler easily forms a circulation flow, and therefore has a corrosion index as compared with other examples. It is high. In Examples 1, 2, 3, and 5, since a sufficient bending step D is secured in the exhaust duct, a large circulation flow reaching the exhaust gas cooler is not formed, the amount of NOx in the exhaust duct is small, and the exhaust gas cooler Little corrosion occurs. However, in Example 5, since the bending step D is 4.0 times the inner diameter of the exhaust duct, the pressure loss of the exhaust duct is large, and the vacuum degassing processing time becomes long.

It is a schematic diagram of the exhaust duct which comprises the vacuum degassing processing apparatus to which the corrosion prevention method of the exhaust gas cooler which concerns on one embodiment of this invention is applied. It is a schematic diagram for demonstrating the flow of the waste gas in the same exhaust duct. Is a graph showing the change in concentration of NOx and O ₂ in the exhaust gas in the cooler. (A)-(E) are the schematic diagrams of the exhaust duct which comprises the vacuum degassing processing apparatus to which the corrosion prevention method of the exhaust gas cooler which concerns on other embodiment of this invention is applied, respectively. It is a schematic diagram of the exhaust duct used for the test. It is a graph of the test result which shows the relationship between the bending level | step difference D and a corrosion index. It is a schematic diagram for demonstrating the corrosion mechanism of an exhaust gas cooler. It is a schematic diagram for demonstrating the flow of the waste gas in a horizontal exhaust duct.

Explanation of symbols

10: vacuum degassing tank, 11: exhaust gas cooler, 12: exhaust duct, 12a: first horizontal part, 12b: inclined part, 12c: second horizontal part, 13: drain, 14: seal pot, 15: refrigerant Piping, 16: burner, 22: exhaust duct, 22a: first horizontal portion, 22b: inclined portion, 22c: second horizontal portion, 32: exhaust duct, 32a: first horizontal portion, 32b: inclined portion, 32c: second horizontal portion, 42: exhaust duct, 42a: first horizontal portion, 42b: inclined portion, 42c: second horizontal portion, 52: exhaust duct, 52a: first horizontal portion, 52b: inclined Part, 52c: second horizontal part, 62: exhaust duct, 62a: first horizontal part, 62b: inclined part, 62c: second horizontal part, 72: exhaust duct, D: bending step, P1: first Bending part (bending part), P2: second bending part (bending part) P3, P4: bent portion, α1, α2, α3, α4: bending angle

Claims (2)

In a vacuum degassing apparatus in which a vacuum degassing tank and an exhaust gas cooler are connected by an exhaust duct, the exhaust gas cooler is prevented from corroding due to NOx contained in the exhaust gas generated during preheating of the vacuum degassing tank. There,
The exhaust duct is provided with 2 to 4 bent portions, and the height of at least a portion of the bent portion is lower than the bent portion located on the vacuum degassing tank side from the bent portion, and / or A method for preventing corrosion of an exhaust gas cooler, characterized by being located above the bent portion located on the exhaust gas cooler side from the bent portion. The corrosion prevention method for an exhaust gas cooler according to claim 1, wherein a vertical distance between the bent portion closest to the vacuum degassing tank and the bent portion closest to the exhaust gas cooler is defined as the exhaust gas. A method for preventing corrosion of an exhaust gas cooler, wherein the inner width of the duct is 1.0 times or more and less than 4.0 times.

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