JPH11293411A - Air heater of trash incineration equipment and steel therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air heater of trash incineration equipment high in dew point resistance and to provide a steel used therefor. SOLUTION: Stainless steel contg., by weight, <=0.1% C, 0.1 to 1.0% Si, 0.1 to 2.0% Mn, 20.0 to 28.0% Cr and 3.0 to 7.0% Mo in steel and satisfying [Cr]+3×[Mo]>=35 is used for the heat transmitting member of an air heater of trash incineration equipment. In the case of duplex stainless steel, it may have a compn. contg. 25.0 to 28.0% Cr, 3.0 to 4.0% Mo and 6.0 to 9.0% Ni, and satisfying [Cr]+3×[Mo]>=35.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air heater used for refuse incineration equipment and a steel material used for the air heater.

[0002]

2. Description of the Related Art In a boiler of a thermal power plant using heavy oil or coal as a fuel, or a heating furnace of a ceramic or steel industry, an air heater for exchanging heat with combustion air to recover sensible heat in exhaust gas is used. Is provided. In particular, the need for sensible heat recovery of low-temperature exhaust gas has increased in recent years from the viewpoint of energy saving measures and environmental measures.

[0003] Sensible heat recovery of low temperature exhaust gas has a problem of dew point corrosion. In particular, in the case of performing heat exchange between normal-temperature air and low-temperature exhaust gas as in an air heater, the problem of dew-point corrosion increases because the heat transfer surface is at a low temperature.

[0004] Dew point corrosion mainly includes sulfuric acid dew point corrosion due to S (sulfur) content and hydrochloric acid dew point corrosion due to Cl (chlorine) content.

[0005] In sulfuric acid dew point corrosion, sulfur contained in fossil fuel is oxidized by combustion to form sulfurous acid gas (SO ₂ ) and further to sulfuric anhydride (SO ₃ ). ₂ SO ₃ ) or sulfuric acid (H ₂ SO
₄ ) Due to the formation of an aqueous solution. When a small amount of SO ₃ is contained in the exhaust gas containing water, the dew point is significantly increased.
At this temperature, the aqueous solution containing sulfuric acid (H ₂ SO ₄ ) condenses.

The concentration of the condensed sulfuric acid aqueous solution is determined by the moisture in the exhaust gas,
It depends on the concentration of sulfuric acid and the temperature of the condensing surface. The sulfuric acid dew point can be calculated from the concentration of SO ₃ and water vapor in the literature (eg
“Thermal Nuclear Power Generation” Vol. 47, no. 8, p. 89
2, FIG. 31). As an example, the dew point of only 10% moisture is 46 ° C., but if this system contains 10 ppm of SO ₃ , the dew point will increase to 135 ° C. and the condensed water will contain about 80% sulfuric acid. In the flue gas whose dew point has risen in this way, moisture condensation occurs at a temperature much higher than estimated from moisture alone, and the high-concentration sulfuric acid solution causes corrosion of the steel. This is sulfuric acid dew point corrosion.

[0007] Regarding waste incineration equipment, from the viewpoint of effective use of energy, an air heater is provided to recover heat from low-temperature exhaust gas.

[0008] The problem of sulfuric acid dew point corrosion is small because the amount of sulfur oxides in the exhaust gas from refuse incineration equipment is very small, but vinyl chloride and the like contained in the refuse are burned to generate hydrogen chloride, and the low-temperature portion of the exhaust gas system To form a hydrochloric acid aqueous solution, and hydrochloric acid dew point corrosion becomes a problem.

The dew point of an aqueous solution containing hydrochloric acid can be estimated from the concentrations of hydrochloric acid and water vapor in a gas, similarly to the sulfuric acid dew point (see, for example, “Thermal Nuclear Power Generation”, Vol. 47, supra).
No. 8, p. 892, FIG. 32, or "Hitachi Shipbuilding Technical Report", Vol. 37, No. 4, p. 202, FIG. 2), the highest hydrochloric acid concentration in a refuse incinerator is about 0.5%, and the dew point of the hydrochloric acid-containing gas at that time is about 80 ° C.

JP-A-7-316745 discloses Cr: 12 to 16%, Ni: 28 to 30%, and C:
u: 1.5 to 3%, Mo: 3% or less, and Cr + 0.
8 ×% Ni + Mo + 0.5 ×% Cu ≧ 39 and -C
r + 0.7 ×% Ni−0.8 × Mo + 1.8 ×% Cu ≧
Stainless steels satisfying No. 9 are disclosed.

In a waste incineration plant, if the exhaust gas temperature is 80 ° C. or higher, there is no problem of hydrochloric acid dew point corrosion, and even if heavy oil or coal containing S is used as the fuel for waste incineration, the exhaust gas temperature is 140 ° C. or higher. It is thought that there is no problem of sulfuric acid dew point corrosion, and ordinary steel, sulfuric acid resistant low alloy steel, stainless steel, etc. have been used as heat transfer materials for air heaters.

[0012]

However, it has been found that, in an actual air heater for refuse incineration equipment, corrosion has progressed more than expected even in the conventional steel type having good corrosion resistance.

As the sulfuric acid-resistant and hydrochloric acid-resistant dew-point corrosion materials, stainless steel disclosed in JP-A-7-316745 may be used. However, the materials disclosed in the publication have a high Ni content. It is an expensive material.

For more severe hydrochloric acid corrosion conditions, C
Ni-based high Mo alloys such as 276 alloy and 625 alloy have high corrosion resistance, but are extremely expensive to apply to large structures such as ducts, chimneys, and air heaters.

Organic materials such as fluororesins and glass materials,
Inorganic coatings are also excellent in terms of corrosion resistance, but are still expensive in terms of total cost including workability and ease of repair.

Based on the above problem recognition, the object of the present invention is to:
An object of the present invention is to provide an inexpensive air heater with good corrosion resistance and its steel material after elucidating the cause of corrosion of the air heater of a refuse incineration plant.

[0017]

DISCLOSURE OF THE INVENTION The present inventors have investigated in detail an air heater which has been damaged by corrosion. As a result, it was found that dew-point corrosion occurred at a much higher temperature than conventionally assumed in air heaters made of ordinary steel, sulfuric acid resistant low alloy steel, and stainless steel in refuse incineration facilities.

For example, steam: 25%, HCl: 700
In an exhaust gas containing 1 ppm and SO ₃ : 1 ppm, the dew point of sulfuric acid can be estimated to be 135 ° C. and the dew point of hydrochloric acid can be estimated to be 70 ° C. But,
In an actual air heater, the temperature is higher than the acid dew point.
The heat transfer tube was punctured due to remarkable corrosion at a part exceeding 0 ° C.

The inventors paid attention to the fact that the surface of the corroded heat transfer tube was covered with combustion ash, and analyzed the chemical composition of this ash. As a result, the combustion ash contained metal chlorides (NaCl, C
aCl ₂ , ZnCl _2, etc.). The inventors have noticed that some of these metal chlorides generate an aqueous solution by absorbing moisture in the exhaust gas and exhibit a so-called deliquescent property.・ Investigation was conducted by humidity.

FIG. 1 shows NaCl, CaCl ₂ and ZnC.
It is a graph showing deliquescence of l _2. As shown in the figure,
The deliquescence depends on the temperature of the exhaust gas and the amount of water vapor, and ZnCl ₂
Is the most deliquescent. When the combustion ash containing ZnCl ₂ adheres to the heat transfer tube, even at a high temperature above the acid dew point, moisture is absorbed and deliquescent from the exhaust gas to form an aqueous solution, and the boiling point of water is reduced to 10%.
Even at a high temperature of 0 ° C. or more, corrosion by the ZnCl ₂ aqueous solution occurs.

Based on this concept, the inventors investigated the corrosion behavior of various steel materials in a high-concentration aqueous chloride solution.

Table 1 shows the chemical compositions of the eight steels used in the corrosion test. Test piece No. 1 in Table 1 7 and 8 correspond to the steel material for the air heater of the present invention.

[0023]

[Table 1]

FIGS. 2 to 5 are graphs showing the corrosion rates of steel materials in various aqueous solutions. The abscissa indicates the number of the type of steel in Table 1, and the ordinate indicates the corrosion rate (g / (m ² · hr)). FIG. 2 is a graph showing the corrosion rate in chloride-free water.

FIG. 3 is a graph showing the corrosion rate in a 55% CaCl ₂ aqueous solution. FIG. 4 is a graph showing the corrosion rate in a 25% NaCl aqueous solution. FIG. 5 shows 80% ZnCl
₂ is a graph showing the corrosion rate in an aqueous solution.

As shown in FIG. 2, in water without chloride and in CaCl ₂ aqueous solution as shown in FIG.
SUS304L (Steel type number 3), SUS316L (N
o. 4), NCF825 (No. 5 of the same), N08320
(No. 6), steel type No. of the present invention. All stainless steels such as 7 to 8 have good corrosion resistance.

As shown in FIG. 4, in the NaCl aqueous solution, the corrosion rate of steel type No. 3 is high, but the corrosion rate is higher than that of steel type No. 3.
-Mo steel (Nos. 4 to 6 and the steel type of the present invention) shows excellent corrosion resistance.

As shown in FIG. 5, in the ZnCl ₂ aqueous solution, the corrosion rate is higher for all steel types as compared with FIGS. Z
Since the nCl ₂ aqueous solution exists even at 150 ° C. exceeding the boiling point of water, the corrosion rate is even higher.

From the above, it was found that in the air heater of the refuse incineration plant, metal chlorides, particularly ZnCl ₂ , in the combustion ash significantly accelerated the corrosion of the heat transfer member.

Next, the inventors used various steel materials for the air heater of the actual refuse incinerator and investigated the corrosion behavior.
As a result, even under the adhesion of combustion ash containing ZnCl ₂ ,
It has been clarified that corrosion does not occur in an air heater using a stainless steel containing a certain range of Cr, Ni, and Mo as a heat transfer member, and the present invention has been completed.

That is, the gist of the present invention is as follows:
In (3). (1) By weight% in steel, C: 0.1% or less, Si: 0.1%
-1.0%, Mn: 0.1-2.0%, Cr: 20.0
２８28.0%, Mo: 3.0-7.0%, [C
r] + 3 × [Mo] ≧ 35, a stainless steel material for an air heater of a refuse incinerator.

(2) Cr: 25.0 to 28.0%, Mo:
A duplex stainless steel containing 3.0 to 4.0% and Ni: 6.0 to 9.0%.
(1) Steel materials for air heaters in refuse incineration facilities as described in paragraph (1).

(3) An air heater of a refuse incineration plant, characterized in that the heat transfer member is made of the steel material described in the above (1) or (2). Here, [Cr] and [Mo] are values in weight%.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the components of stainless steel in the present invention will be described in detail below. In the following description, each component% is% by weight, unless otherwise specified.

In the air heater of the garbage incinerator, Cr in steel is used.
It has been found that the corrosion resistance is determined by the Mo content and the Mo content.

If the content of Cr is less than 20%, even if the content of Mo is large, it does not show good corrosion resistance as a heat transfer member, and if it exceeds 28%, it becomes brittle during use. Preferably it is 22% to 26%.

When Mo is less than 3%, even if Cr is large, good corrosion resistance is not exhibited as a heat transfer member, and when it exceeds 7%, embrittlement occurs during use. Preferably it is 3.5% to 6.5%.

As for Cr and Mo, as described below, in addition to the content of each of them, [Cr] + 3 × [M
o], it was confirmed that the corrosion resistance as a steel material for an air heater is controlled by the index represented by [o].

FIG. 6 is a graph showing the relationship between the Cr and Mo contents of the steel material and the corrosion resistance. In the figure, Cr and Mo are variously changed, and the temperature is 100 ° C., 80% ZnCl 2
_{(2) In an} aqueous solution, with a corrosion rate of 0.05 (g / (m ² · hr)) as a boundary, those with better corrosion resistance are represented as austenite-based, the two-phase system is represented as ●, and those with poor corrosion resistance Represents an austenitic system and × represents a two-phase system. As shown in the figure, Cr and Mo are related to the improvement of corrosion resistance by [Cr] + 3 ×
It can be seen that there is a synergistic effect under the index arranged by [Mo].

FIG. 7 shows [Cr] +3 between low Ni and high Ni.
It is a graph which shows the corrosion rate with respect to x [Mo]. As shown in the figure, if [Cr] + 3 × [Mo] is less than 35, excellent corrosion resistance cannot be obtained. On the other hand, [Cr] + 3 ×
When [Mo] exceeds 49, embrittlement occurs during use, and the material cost also increases. Preferably, [Cr] + 3 ×
[Mo] is preferably 38 to 45.

The types of these stainless steels containing Cr and Mo include ferritic, martensitic, and stainless steels.
An austenitic or two-phase system can be mentioned, but since an air heater is usually manufactured by welding, an austenitic or two-phase system excellent in weldability is desirable.

Further, the stainless steel of the present invention has C: 0.
Contains 1% or less. If it exceeds 0.1%, the corrosion resistance is reduced and the embrittlement is accelerated due to the precipitation of carbides at the welded portion. The lower limit is not particularly defined, but the extremely low carbon content increases the material cost, so that less than 0.01% is not preferable. Preferably 0.
01 to 0.03%.

The stainless steel of the present invention has a Si content of 0.1 to 0.1%.
Contains 1.0%. Si is required to be 0.1% or more in order to obtain clean steel as a deoxidizing agent. However, 1.0%
Excess Si exceeding 1.0% accelerates embrittlement due to precipitation of intermetallic compounds in a weld or the like, so the upper limit is made 1.0%. Preferably it is 0.2-0.5%.

The stainless steel of the present invention has a Mn of 0.1 to
Contains 2.0%. Mn is a deoxidizing agent similar to Si, and is necessary because S that adversely affects hot workability is fixed and stabilized as MnS. However, when Mn exceeds 2%, these effects are saturated and, at the same time, ductility is reduced. Preferably, it is 0.2 to 1.2%.

The stainless steel of the present invention may be added with Nb, Ti, and Zr for suppressing carbide precipitation at a weld. Nb, Ti, and Zr may be any. The content is preferably 0.1 to 1.0% in total of Nb, Ti and Zr. If it is less than 0.1%, carbide precipitation is not sufficiently suppressed, and if it exceeds 1.0%, the effect is saturated and at the same time, corrosion resistance and toughness are reduced by accelerating intermetallic precipitation at a welded portion or the like. N for stabilizing austenite structure
i, N, etc. may be added.

Further, Ca for improving hot workability,
Mg and B may be added. Their total content is 0.
0005-0.01% is preferable. If it is less than 0.0005%, no effect is exhibited, and if it exceeds 0.01%, these elements have a bad effect as nonmetallic inclusions. In addition, Cu, W, etc. for improving acid resistance may be added.

When applied to the austenitic stainless steel of the present invention, Ni must be added in order to obtain a stable austenitic single phase structure. Ni is 22.0-4
5.0%. If it is less than 22.0%, the austenite single phase will not be obtained, and if it exceeds 45%, the material cost increases and it is not economical. Preferably, 24.0-35.0
%.

The inventors further focused on duplex stainless steel, and as a result of study, found that Cr and Mo were changed to Cr: 25.0.
２８28.0%, Mo: 3.0 to 4.0%, Ni: 6.0
It has been confirmed that the use of a duplex stainless steel of up to 9.0% and [Cr] + 3 × [Mo] ≧ 35 is economically advantageous as well as reducing corrosion.

The reason for this is that, in the case of austenitic stainless steel, at least Ni is required to stabilize the austenitic structure under the above-mentioned contents of Cr and Mo.
This is because the content needs to be 2% or more, and the material cost increases. When a two-phase system is used, the Ni content may be 6% or more, which is advantageous in terms of material cost as compared with an austenitic system containing equivalent Cr and Mo.

In the case of duplex stainless steel, if Cr is less than 25%, good corrosion resistance cannot be obtained as a heat transfer member even if Mo is increased. If it exceeds 28%, embrittlement occurs during use.
Preferably it is 26% to 27%.

If Mo is less than 3%, even if the content of Cr is large, good heat resistance is not exhibited as a heat transfer member, and if it exceeds 4%, the cost increases. Preferably it is 3.5% to 4.0%.

Ni is set to 6.0 to 9.0%. If it is less than 6% or more than 9%, an appropriate balance of the two-phase structure cannot be obtained, and the ferrite phase or the austenite phase becomes excessive, respectively, so that excellent corrosion resistance cannot be obtained. Preferably it is 6.5 to 8.5%.

FIG. 8 is a schematic diagram showing an example in which an air heater is installed in an exhaust gas flue. FIG. 8 (a) shows a case where only a heat transfer tube is exposed to exhaust gas, and FIG. Figure (c) shows the case where the heat pipe assembly (header) is exposed to exhaust gas and only the heat transfer tube is exposed to exhaust gas, and the two air heaters are divided into two blocks, a high-temperature side and a low-temperature side. It is.

In FIG. 3A, cool air is supplied to the air heater 1 by an air blower 5, and the heated air is used as combustion air. The air heater 1 includes a heat transfer tube 2, an inlet header 3, and an outlet header 4. The members of the entrance header section 3 and the exit header section 4 are not directly exposed to the exhaust gas from the flue 6. The steel material of the present invention is desirably used for the heat transfer tube 2.

In FIG. 5B, since the members of the inlet header portion 3 and the outlet header portion 4 are directly exposed to the exhaust gas, it is preferable to use the steel material of the present invention also in the header portion.

In FIG. 5C, the temperature of the air heater 1a is higher than that of the air heater 1b. Depending on the temperature condition or the condition of the exhaust gas component, the steel material of the present invention is supplied to the low temperature side air heater 1b.
It is also possible to reduce the cost by using only general heat-resistant steel having no dew point measure for the air heater 1a.

[0057]

EXAMPLE Various steel tubes (outer diameter 45 mm, wall thickness 4 mm, length 2 m) having the chemical composition shown in Table 1 were inserted into the heat transfer tubes of the air heater of the refuse incineration plant shown in FIG. 8 (a). .

The temperature of the flue gas was 350 ° C. upstream of the air heater 1 and had the composition shown in Table 2. Air flow rate is 1.
5Nm ³ / min. Tube, air temperature 30 ° C at inlet, 2 at outlet
The surface temperature of the heat transfer tube of the air heater was 110 ° C. on the air inlet side (upper part of the heat transfer tube 2 in FIG. 8) and 280 ° C. on the air outlet side (lower part of the heat transfer tube 2).

[0059]

[Table 2]

After the test period of two months, the tube was extruded and the corrosion state of the outer surface of the stainless steel tube was examined. The corrosion on the outer surface tended to increase near the air entry side. Table 3 shows the maximum erosion depth obtained by measuring the remaining wall thickness of the steel pipe.

[0061]

[Table 3]

Normal steel (No. 1), low alloy steel (No. 1)
2) and stainless steel with low Cr and Mo contents (N
o. 3 to 6) have large corrosion thinning, whereas Cr: 20.0 to 28.0% and Mo: 3.0 to 0.0% of the present invention.
Stainless steel pipe containing 7.0% and [Cr] + 3 × [Mo] ≧ 35 (No. 7: austenitic, N
o. 8: two-phase system), the erosion depth was extremely small, and it was found to have excellent corrosion resistance as an air heater heat transfer member.

[0063]

The use of the air heater of the refuse incineration plant of the present invention makes it possible to reduce the corrosion of the air heater even in a severely corrosive environment of the refuse incineration plant, and the present invention has great industrial significance. .

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of water vapor in gas and the temperature caused by the deliquescence of various chlorides.

FIG. 2 is a graph showing corrosion rates of various steel materials in water.

FIG. 3 is a graph showing corrosion rates of various materials in a 55% CaCl ₂ aqueous solution.

FIG. 4 is a graph showing the corrosion rates of various steel materials in a 25% NaCl aqueous solution.

FIG. 5 is a graph showing corrosion rates of various steel materials in an 80% ZnCl ₂ aqueous solution.

FIG. 6 is a graph showing the relationship between Cr and Mo contents and corrosion resistance.

FIG. 7 is a graph showing the relationship between [Cr] + 3 × [Mo] and the corrosion rate.

FIG. 8 is a schematic view showing an example in which an air heater is installed in an exhaust gas flue. FIG. 8 (a) shows a case where only heat transfer tubes are exposed to exhaust gas, and FIG. 8 (b) shows heat transfer tubes and heat transfer tubes. In the case where the collecting part (header part) is exposed to the exhaust gas, the same figure (c) shows the case where only the heat transfer tube is exposed to the exhaust gas and the two air heaters are divided into two blocks, a high temperature side and a low temperature side. .

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 1a, 1b Air heater 2 Heat transfer tube 3 Inlet header 4 Outlet header 5 Air blower 6 Flue

Claims (3)

[Claims] C. 0.1% by weight or less in steel
i: 0.1 to 1.0%, Mn: 0.1 to 2.0%, C
Waste incineration equipment characterized by being stainless steel containing [Cr] + 3 × [Mo] ≧ 35, containing r: 20.0 to 28.0% and Mo: 3.0 to 7.0%. Steel for air heater. 2. Cr: 25.0 to 28.0%, Mo:
The steel material for an air heater of a refuse incineration plant according to claim 1, wherein the steel material is a duplex stainless steel containing 3.0 to 4.0% and Ni: 6.0 to 9.0%. . 3. An air heater of a refuse incineration plant, wherein the heat transfer member is made of the steel material according to claim 1.