JP2000292091A - Ceramic tube for heat exchanger - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve heat resistance, corrosion resistance and heat impact resisting properties by forming a ceramic tube for a heat exchanger adapted to recover a heat from an exhaust gas of a melting furnace or the like of a porous ceramic material exhibiting specific values of thermal conductivity and porosity. SOLUTION: A ceramic tube 1 sealing one side surface of a cylinder is projected toward the interior of a gasifying melting furnace from a furnace wall 2, and a pipe 3 is disposed in the tube 1 to constitute the heat exchange. When a melting furnace is operated, the exterior of the tube 1 is exposed with a high temperature combustion gas, and hence when air is sent from the pipe 3, the air can be heated. In this case, the tube 1 is formed of porous ceramics having thermal conductivity of 10 W/m.K or above and porosity of 15% or below. The tube 1 is formed to have a dense SiO2 film on a surface including a crystal phase of SiC, Si3N4 having characteristics of heat resisting impact temperature of 350 deg.C or above by submerging it under water and a flexual strength of 30 MPa or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic tube (heat transfer tube) for a heat exchanger suitable for recovering heat from high-temperature waste gas from an incinerator or a pyrolysis furnace.

[0002]

2. Description of the Related Art Garbage discarded from homes and companies is burned in incinerators of local governments, and the incinerated ash and fly ash contained in the smoke (elements contained: Si, Al, Fe, Ca,
Mg, K, Mn, Cl, Na, and S) contain heavy metal components and toxic contaminants such as dioxin and furan.

Until now, unburned incineration ash after being burned in local government incinerators has been buried as it is in the final disposal site. However, location conditions have become severe, making it difficult to secure a place. In addition, detoxification of harmful pollutants such as dioxin and furan is being regulated strictly by laws and regulations, so incineration ash and fly ash are collected and re-melted to detoxify harmful pollutants. The need for melting furnaces is growing year by year.

[0004] The incinerated ash after being burned in an incinerator, which is turned into slag by high-temperature heat treatment, can be reduced to 1/2 of the incinerated ash.
The high-temperature heat treatment method in this melting furnace is regarded as promising because its volume can be reduced to about 1/4 and harmful pollutants such as dioxin can be decomposed and detoxified by high heat. .

On the other hand, incinerators for municipal waste have been installed for the purpose of incinerating municipal waste to reduce the volume of waste. However, from the viewpoint of effective use of energy, the thermal energy of incinerated waste gas is reduced. In order to recover as much as possible, it is important to recover and utilize the waste heat from the high-temperature waste gas using a heat exchanger.

Conventional heat exchangers have a temperature of 500-60.
Although it was used at 0 ° C., for example, in a pyrolysis gasification and melting furnace that is being commercialized in recent years,
It is operated at a temperature of 0 ° C, and its thermal energy is recovered and used in a region where the temperature of the waste gas is high.
Heat exchange is taking place.

In the pyrolysis gasification and melting furnace, the gasification furnace and the melting furnace are integrated.
Heat-treated at low temperature of ℃ to generate flammable gas and send to melting furnace. In the melting furnace, fly ash, char, and tar sent together with combustible gas are burned at a high temperature of about 1300 ° C to convert fly ash into slag and completely decompose dioxin and the like. It is. After this, the waste gas after high temperature combustion is led to the waste heat boiler, but the heat exchanger is
It is often installed from the melting furnace outlet to the waste heat boiler, and the recovered heat is effectively used for air preheating, a steam generator for power generation, and the like.

The waste gas contains H ₂ O, CO ₂ , O ₂
In addition, a large amount of dust and hydrogen chloride (HCl) gas are contained, and the hydrogen chloride concentration may reach 1500 to 2000 ppm. In addition, the Ca component contained in the dust and the HCl component in the gas are highly corrosive,
Excellent corrosion resistance is required.

In Japanese Patent Publication No. 60-216192, a coating layer of stainless steel or Cr-Ni alloy steel is formed on the surface of a normal steel pipe body, and is used as a heat transfer tube for the heat exchanger.

[0010] Furthermore, a low-temperature gas or the like is passed through the inside of the heat transfer tube and heated to recover heat, but a temperature difference generally occurs between the inner and outer surfaces of the heat transfer tube. Further, since a low-temperature gas is rapidly injected into the heated heat transfer tube, a large thermal shock is applied. Normally, air is most often used as the gas flowing into the heat transfer tube.

[0011]

In the pyrolysis gasification and melting furnace, when ash generated by incineration of refuse is subjected to heat treatment, metal elements such as Cd, Pd and Zn contained in the ash, dioxin, furan and the like are included. In order to decompose harmful pollutants, heat melting treatment is performed at 1200 ° C or higher to render them harmless. Ceramic tubes (heat transfer tubes) used in this melting furnace
Is the molten salt formed by melting the incineration ash, the molten slag steam,
Further, it is exposed to HCl gas or the like. Therefore, Si, Al, Fe, Ca, Na, and Cl in these components gradually penetrate and erode into the material forming the ceramic tube, and the material forming the ceramic tube gradually deteriorates to cause deterioration in strength. Or damage,
Alternatively, the required heat exchange was not performed, and the product could not be used for a long time.

[0012] Generally, ceramic tubes of a heat exchanger are made of copper or copper alloy having high thermal conductivity for low temperature use, and metal materials such as Hastelloy and Inconel are used in a region where the use temperature is 1000 ° C or less. It is 12
There is no suitable material in a portion where the temperature exceeds 00 ° C. and where the corrosion resistance is required and the concentration of hydrogen chloride is high.

In Japanese Patent Publication No. 60-216192, a stainless steel or Cr-Ni alloy steel coating layer is formed on the surface of a normal steel pipe to improve high-temperature corrosion, but it cannot be used in a temperature range of 1200 ° C. .

Therefore, in a temperature environment of about 1200.degree.
Various ceramic materials having excellent corrosion resistance and heat resistance have been devised in a use environment in which about 00 ppm exists. However, most materials are poor in thermal shock resistance due to their dense nature. On the other hand, porous formation is effective for improving the thermal shock resistance, but there is a problem that the thermal conductivity is lowered, the corrosion resistance is extremely deteriorated, and the heated air inside the ceramic tube leaks out of the tube. Since there is no material having both excellent corrosion resistance and heat resistance as well as thermal shock resistance, the present invention
An object of the present invention is to provide a ceramic tube for a heat exchanger which has these necessary characteristics and has a long life.

[0015]

In view of the above, the present invention provides
As a result of various studies, using porous ceramics as the base material,
It improved heat resistance, corrosion resistance, and thermal shock resistance, and found a countermeasure against leakage, which was a disadvantage of porous materials, by forming a SiO ₂ film on the surface and closing the continuous ventilation holes with this. It is a thing.

That is, the present invention provides a heat conductivity of 10 W / m · K.
As described above, the ceramic tube for a heat exchanger is made of a porous ceramic having a porosity of 15% or less.

The above ceramics have a bending strength of 3
It is made of a material containing a crystal phase of SiC or Si ₃ N ₄ having a thermal shock temperature of 350 ° C. or more by a water drop method at 0 MPa or more, and a SiO ₂ film is formed on the surface.

[0018]

[Action] When the ceramic tube sealed on one side, heated by circulating cold air in the tube, but taken out as a heated air, to oxidize the surface of the SiC or Si ₃ N ₄ based porous ceramics in the present invention By forming an SiO ₂ film on the surface of the porous ceramic tube to have a substantially double structure, air in the tube is prevented from leaking.

The SiC or Si ₃ N ₄ based material is 800-
Oxidation starts easily at a temperature of 900 ° C. to form a SiO ₂ film (oxide film). And the SiO ₂ film formed on the surface is
Since the pores near the surface of the porous ceramic are filled, it contributes to preventing leakage. In other words, when air is flowed in the tube and used at an ambient temperature of 1200 ° C. or more, the surface of the SiC or Si ₃ N ₄ material undergoes severe oxidation during use, and a considerable amount of SiO ₂ is contained in the tube. Form. The SiO ₂ undergoes volume expansion due to the reaction, and closes pores existing on the surface, so that leakage hardly occurs. Specifically, in SiC, respectively, 2SiC + 3O ₂ → 2SiO ₂ +
In 2CO Si ₃ N ₄ , Si ₃ N ₄ + 3O ₂ → 3SiO
₂ + 2N ₂ , where SiC, Si ₃ N ₄ and S
Calculating the volume ratio of iO ₂ evidently causes volume expansion after oxidation.

In actual use, since air is introduced into the inside of the ceramic tube and the atmosphere is an oxidizing atmosphere, an SiO ₂ film is usually formed on the inner surface of the tube to provide sufficient leak resistance. If the internal atmosphere gas is an oxidizing gas containing oxygen, an SiO ₂ film is also formed on the outer surface of the ceramic tube, and the leak is less likely to occur in combination with the inner surface SiO ₂ film. This SiO ₂ film is made of SiC or SiC.
₃ becomes N ₄ protective layer suppresses the oxidation proceeds in an oxidizing atmosphere.

As the crystal phase of the SiO ₂ film, quartz,
There are tridymite and cristobalite, both of which are 15
It has heat resistance of 00 ° C. or more and functions well as a protective film.
Above all, tridymite and cristobalite have high heat resistance, and it is more desirable to keep these crystal phases. If the porosity is 15% or more, there is a problem because the air leak in the pipe becomes large, and there is also a problem that the particle size is large, the strength is poor and the handling property is poor, and a problem occurs when the ceramic pipe is set in the main body. Is preferred.

The average pore diameter is about 50 μm or less as a guide. However, the smaller the average pore diameter is, the easier SiO ₂ can be formed, and a more dense film can be formed. As the strength, since it is sometimes fixed by cantilever support, it is important to have a strength of 30 MPa or more.

Since the ceramic tube for a heat exchanger is required to have a high heat conversion efficiency, it is necessary that the material used for the tube has a good thermal conductivity, and is 10 W / m · K or more, preferably 60 W / m · K or more. It is required that the material has a thermal conductivity of K or more. In order to secure 60 W / m · K or more, the ceramic tube may be formed of a material having a SiC crystal phase. On the other hand, to improve thermal shock resistance, Si ₃ N
A material having a _four- crystal phase may be used, but in this case, thermal conductivity is insufficient. Therefore, it is preferable to mix the material with a material having a SiC crystal phase.

When used as a ceramic tube for a heat exchanger, it is exposed to waste gas containing a large amount of corrosive components.
Must have sufficient corrosion resistance, especially Cl, Ca
Although attention is paid to corrosion due to components, if a ceramic tube is formed of a material mainly composed of a SiC crystal phase, Cl,
Since the diffusion of the Ca component can be made as slow as possible, from the viewpoint of corrosion resistance, it is preferable to form the ceramic tube with a single SiC crystal phase. When the ceramic tube is formed by mixing with the Si ₃ N ₄ crystal phase, it is desirable to increase the mixing ratio of the SiC crystal phase as much as possible.

[0025]

Embodiments of the present invention will be described below.

In the heat exchanger shown in FIG. 1, a ceramic tube 1 in which one surface of a cylindrical body is sealed is disposed so as to protrude from a furnace wall 2 of a gasification and melting furnace toward the inside. The pipe 3 is arranged inside. During operation of the melting furnace, the outside of the ceramic tube 1 is exposed to a combustion gas of 1200 ° C. or more, and in this state, if air of about 250 ° C. is sent from the pipe 3, about 550 ° C. inside the ceramic tube 1 It is heated and discharged and acts as a heat exchanger.

Then, the ceramic tube 1 is made to have a thermal conductivity of 10 W / m · K or more and a porosity of 15% as described above.
Hereinafter, the ceramic material is formed of a ceramic material mainly composed of at least one of SiC and Si ₃ N ₄ crystal phases, with a thermal shock temperature of 350 ° C. or higher, a bending strength of 30 MPa or higher by dropping in water. Therefore, even if the ceramics tube 1 is used in a gasification and melting furnace having good thermal shock resistance and large thermal shock, the ceramic tube 1 does not cause a problem such as crack.

Further, an oxide film (SiO ₂ ) exists on the surface of the ceramic tube 1, and a denser SiO ₂ film is used during use.
Since the film is formed, heat exchange can be favorably performed without leakage. Incidentally, the SiO ₂ film may be formed in advance by a method such as coating before use.

Although FIG. 1 shows the ceramic tube 1 having one side sealed, a cylindrical ceramic tube having both ends opened may be used.

When the ceramic tube 1 is manufactured, a ceramic raw material having the above-described crystal phase is used.
It can be obtained by molding into a predetermined shape by a known method such as extrusion molding and baking it under predetermined conditions, or by pressure molding. In any case, by adjusting the composition and the firing conditions, 120
A ceramic tube that can be used under severe conditions at 0 ° C. is obtained.

In order to increase the efficiency of heat exchange, it is preferable that the thickness of the ceramic tube 1 be as thin as possible. However, if the thickness is too small, the strength may be insufficient. The above thickness is optimal, but if the thickness is increased, the efficiency of heat exchange is reduced.
It is better to keep it to 0 mm or less.

It is also possible to use a combination of materials other than the above-mentioned materials. For example, a coating layer made of at least one of SiC and Si ₃ N ₄ is provided on the surface of the ceramic heat pipe 1 with a thickness of 10 to 1000 μm. Before use, a ceramic tube 1 having a multi-layered structure is formed,
Effective heat resistance and corrosion resistance can be obtained. For example, it is of course possible to form the ceramic tube 1 having a multi-layer structure by coating or spraying SiC having good corrosion resistance on the surface of the ceramic tube 1 formed of Si ₃ N ₄ having good thermal shock resistance.

When the above-mentioned coating layer is formed, since the ceramic tube 1 is a porous body, the surface of the ceramic tube 1 has severe irregularities.
The anchor effect also has the effect of increasing the bonding strength of the coating layer.

[0034]

Embodiments of the present invention will be described below.

Example 1 A ceramic tube 1 shown in FIG. 1 was formed of ceramics mainly composed of each crystal phase of Al ₂ O ₃ , mullite, Si ₃ N ₄ and SiC. The dimensions of the ceramic tube 1 is 8
0 mm, an inner diameter of 72 mm, and a total length of 1200 mm. A stainless steel pipe 3 having a diameter of 40 mm is set coaxially with the ceramic pipe 1 inside the ceramic pipe so that air at 300 ° C. can be introduced into the ceramic pipe, and the air is heated by the heat recovery unit simulating apparatus of the waste gas treatment apparatus. It was installed so that heat exchange was possible, and a heat resistance test was further performed. In addition, a heat exchange test was also performed, and a case where air at a temperature of 500 ° C. or more was obtained at the outlet of the ceramic tube was determined to be OK.

In the heat resistance test, as shown in FIG. 1, the ceramic tube 1 was mounted horizontally from the furnace wall 2 to simulate the actual environment, and the atmosphere was a gas temperature of 1200 ° C., an oxygen concentration of 5%, and a carbon dioxide concentration of 95%. Exposure to a gas, air at 250 ° C. was introduced into the ceramic tube to apply a certain thermal stress, and a test was performed. If a crack occurred in the ceramic tube 1, it was determined to be defective (NG). And

At the same time, a 3 × 4 × 45 mm test piece of the same material as each ceramic tube 1 was prepared, and the thermal shock resistance was evaluated using a thermal shock tester. The thermal shock resistance was evaluated by using an underwater drop method in accordance with JIS, and the temperature difference that can be endured was indicated as ΔT (° C.).

The thermal conductivity was measured by a laser flash method using a test piece of φ10 × 2 mm.

The results are shown in Table 1.
₂ O ₃ , mullite and ZrO ₂ have thermal shock resistance of ΔT300
Since the temperature was lower than ℃, cracks were generated, resulting in NG. Also, in the heat exchange test, since the thermal conductivity was 7 W / m · K or less, 500
Air over ℃ was not obtained and it was NG. On the other hand, $ T35
It has been found that there is no problem in thermal shock resistance when SiC or Si ₃ N ₄ material of 0 ° C. or higher is used. Further, in order to obtain heated air of 500 ° C. or more, a thermal conductivity of 10 W / m · K or more is required.

[0040]

[Table 1]

[0041] Example 2 Next, the experimental apparatus of Example 1, Al ₃ O in Example 1
₃ , a heat exchange test was conducted by selecting three materials, SiC and Si ₃ N ₄ .

The test was carried out at an atmosphere temperature of 1200 ° C. and an oxygen concentration of 5%.
It was installed and adjusted so that an air temperature of at least ℃ could be obtained.
At this time, the amount of leakage in the ceramic tube was checked, and if there was a problem, it was indicated by x, and no problem was indicated by ○. After the test, a test piece for analysis was sampled from the ceramic tube, and the SiO ₂ film on the surface of the ceramic tube was visually observed and XR
Checked with D. When the formation of the SiO ₂ film was confirmed, it was indicated by ○, and when it was not confirmed, it was indicated by x.

The results are shown in Table 2, and the porosity is 15
% Or less, the formation of a dense SiO ₂ film was observed in a ceramic tube having a SiC or Si ₃ N ₄ crystal phase as a main component or a ceramic tube having a SiC + Si ₃ N ₄ crystal phase.
It turns out that no leak problem occurs. In addition, Si
The O ₂ film had a crystal phase containing tridymite, cristobalite, and quartz.

[0044]

[Table 2]

[0045]

As described above, according to the present invention, the thermal conductivity is 10 W / m · K or more, the porosity is 15% or less, and the bending strength is 30 or less.
By forming a heat exchanger tube for a heat exchanger with a ceramic porous body having a MPa or higher and a thermal shock resistance of 350 ° C. or higher, the heat transfer tube is excellent in heat resistance, thermal shock resistance, corrosion resistance, and heat transfer, so that it can be favorably used for a long time.

In particular, when used as a ceramic tube of a refuse incinerator / melting furnace or pyrolysis gasification / melting furnace, erosion of corrosive elements in incineration ash components is prevented, strength deterioration and characteristic change are extremely small, and the life is shortened. Can be longer.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a ceramic tube for a heat exchanger of the present invention.

[Explanation of symbols]

 1: Ceramic tube 2: Furnace wall 3: Pipe

Claims (4)

[Claims] 1. A ceramic tube for a heat exchanger, wherein the ceramic tube is made of a porous ceramic having a thermal conductivity of 10 W / m · K or more and a porosity of 15% or less. 2. The ceramic according to claim 1, wherein said ceramic has a bending strength of 30 MPa.
The ceramic tube for a heat exchanger according to claim 1, wherein the thermal shock temperature in the underwater drop method is 350 ° C or more. 3. The ceramic according to claim 1, wherein said ceramic contains at least one phase of SiC or Si ₃ N ₄ crystal phase and has a dense S
heat exchanger ceramics tube according to claim 1, characterized in that it has an iO ₂ film. 4. The method according to claim 1, wherein the dense SiO ₂ film comprises tridymite,
At least one of cristobalite and quartz crystal phases
The ceramic tube for a heat exchanger according to claim 3, wherein the ceramic tube has a crystalline phase or more.