JP2007285606A - Heat transfer tube for air preheater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat transfer tube for an air preheater having high durability to low temperature corrosion generated on the heat transfer tube of the air preheater by a corrosive gas and moisture, and high economical rationality. <P>SOLUTION: With respect to the plurality of heat transfer tubes for the air preheater disposed between tube plates of an air preheater main body through which an exhaust gas including the corrosive gas passes, and heating the air while making the air pass therethrough, an area of 2-30% length to the total length from an air inlet side is composed of a corrosion-proof stainless steel tube having a value of corrosion-resisting alloy index GI=-[Cr]+3.6×[Ni]+4.7×[Mo]+11.6×[Cu] of 40 or more, or a Ni-base alloy tube, and an area of the remaining length is composed of a corrosion-proof low alloy steel tube in conformity to ASTM A423 Grade 3 Code Case 2494. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heat transfer tube for a gas-type air preheater used in a power plant, a business boiler, a waste incinerator or the like. More specifically, the present invention relates to a heat transfer tube that is highly durable and excellent in economic rationality against low temperature corrosion caused by a corrosive gas and moisture in a heat transfer tube of an air preheater.

  As a means for utilizing the residual heat of the exhaust gas discharged, a gas-type air preheater (heat exchanger) that exchanges heat between the exhaust gas that has been cooled to a temperature that is not affected by the high-temperature gas in the gas cooling chamber and air is generally used. Many are used.

  FIG. 1 is a block diagram showing an example of the exhaust gas residual heat utilization facility in a waste incinerator, where 1 is an incinerator, 2 is a cooling chamber for cooling the exhaust gas discharged from the incinerator 1, and 3 is a cooling chamber 2. In the air preheater to which the exhaust gas whose temperature has been lowered is sent, the air sent by the blower 4 is heated while passing through the air preheater 3 and is sent to the residual heat utilization facility 5. On the other hand, the exhaust gas that has passed through the air preheater 3 is cleaned by the dust collector 6 and discharged from the chimney 8 to the outside by the blower 7.

  In the residual heat utilization equipment as described above, the heat exchanger 3 is generally of a type called an extra-tube gas type that has less troubles such as blockage due to dust. An example of such an extra-tube gas heat exchanger is shown in FIG. Reference numeral 31 denotes a heat exchanger body having a square cross section through which the exhaust gas passes, and has an exhaust gas inlet 32 and an exhaust gas outlet 33. The side walls (hereinafter sometimes referred to as tube plates) 34a and 34b project outward and open. A plurality of heat transfer tubes 35 are attached in parallel. The duct 36 on the air inlet side has an air inlet 38, and a substantially funnel-shaped air inlet attached to one side wall 34 a of the heat exchanger body 31 so as to cover the side wall 34 a including the opening of the heat transfer tube 35. The duct 37 on the air outlet side is a substantially funnel-shaped air outlet portion having an air outlet 39 and attached to the other side wall 34b so as to cover the side wall 34b including the opening of the heat transfer tube 35. .

  In the heat exchanger 3 as described above, the exhaust gas sent from the gas cooler 2 is guided into the heat exchange body 31 from the exhaust gas inlet 32, and is sent to the dust collector 6 from the exhaust gas outlet 33 through the heat transfer pipe 35. It is done. On the other hand, the air sent by the blower 4 is guided from the air inlet 38 to the air inlet 36, passes through the heat transfer tubes 35, and is discharged through the air outlet 37 and the air outlet 39. During this time, air at about room temperature led from the air introduction part 36 to each heat transfer tube 35 by the exhaust gas of about 200 to 500 ° C. passing through the heat exchanger main body 31 is heated to about 250 ° C. at maximum, and the residual heat utilization equipment Sent to 5.

  By the way, corrosive gas is contained in exhaust gas from business boilers and waste incinerators. Especially when SOx is contained up to 1ppm, sulfuric acid is condensed at 100 ° C or more, and exhaust gas equipment and smoke Corrodes the road violently. This is called low temperature corrosion (sulfuric acid dew point corrosion). In order to reduce low temperature corrosion, in order to keep the casing temperature of equipment and flue as high as possible, the temperature of the exhaust gas is maintained and sufficient heat insulation and heating are performed. However, in the heat exchanger 3 described above, air flows through the heat transfer tube 35 to perform heat exchange. Therefore, the entire surface of the heat transfer tube 35 in contact with the exhaust gas easily becomes lower than the sulfuric acid dew point. Subject to dew point corrosion.

  As a countermeasure against low temperature corrosion of heat transfer tubes, according to Patent Document 1, for example, as shown in FIG. 3, a short insertion tube 40 is closely inserted into the heat transfer tube 35 from the entrance side opening of the heat transfer tube 35, A method of reducing the amount of heat transfer between the exhaust gas and air to suppress a decrease in the surface temperature of the heat transfer tube 35 and preventing low temperature corrosion, that is, a double tube method is also used. Further, in Patent Document 1, as shown in FIG. 4, a gap 54 is provided without bringing the inner tube 52 and the heat transfer tube 35 into close contact with each other, and air is supplied from the air inlet 55 with the tube plate having a double structure. A structure to capture is proposed.

  On the other hand, as a method of adopting a corrosion-resistant pipe excellent in low-temperature corrosion resistance, for example, a method of adopting austenitic stainless steel excellent in sulfuric acid resistance proposed in Patent Document 2 as a steel pipe can be considered.

Japanese Patent Laid-Open No. 07-167585 JP-A-10-168552

  However, when the above-described conventional double tube method is employed (see FIG. 3), it is difficult to insert the inner tube 40 into the heat transfer tubes 35, and the tube plate 34a, the outer plate 41, and the inner tube are inserted. There was a construction problem that welding with 40 was troublesome and required a lot of man-hours. Furthermore, since heat is radiated from the heat transfer tube 35 through the outer plate 41, there is a problem that low temperature corrosion is unavoidable.

  In addition, the conventional technique described in Patent Document 1 has a problem that not only the construction cost is high, but it is difficult to drastically avoid low-temperature corrosion due to heat removal from the tube sheet. In addition, when a corrosion-resistant pipe (austenitic) with excellent sulfuric acid resistance is adopted, not only the material cost is remarkably increased, but other constituent materials such as a tube sheet are carbon steel (ferrite-based), Since the thermal expansion coefficient differs greatly between the heat tube and the constituent material such as the tube plate, it is necessary to join and fix the heat transfer tube to both the tube plates in order to prevent gas leakage, and there is a problem that the design becomes extremely difficult.

  Accordingly, the object of the present invention is to provide a heat transfer tube for an air preheater that is highly durable and excellent in economic rationality against low temperature corrosion that occurs in a heat transfer tube of an air preheater due to corrosive gas and moisture. is there.

  In order to solve the above-mentioned problems, the present inventors diligently studied the corrosion mechanism of the heat transfer tube of the air preheater, and as a result, obtained the following knowledge.

(I) Low temperature corrosion of heat transfer tubes is not uniform. Water corrosion due to a large amount of drain that shows acidity is dominant at a certain length from the pneumatic side end, and sulfuric acid dew point corrosion is dominant at higher temperatures.

(Ii) Austenitic stainless steel or duplex stainless steel showing a GI value ≧ 50, or Ni-based alloy is excellent against water corrosion caused by acidic drains that occur near the air-side end. For sulfuric acid dew point corrosion occurring on the higher temperature side, for example, a sulfuric acid dew point corrosion steel pipe containing about 0.3% Cu and about 0.1% Sb is excellent.

(Iii) A corrosion-resistant stainless steel pipe satisfying the above-mentioned knowledge (ii) is made into a pneumatic side, the remainder is made of a sulfate-resistant low-alloy steel pipe, and one heat transfer pipe is manufactured by a composite steel pipe in which both are friction welded. The test was conducted in place of a carbon steel heat transfer tube. At the tube sheet and joint part, we were concerned about selective corrosion due to dissimilar metal contact corrosion at dissimilar joints. As a result of earnest research on the reason, it was found that the liquid thin film formed by condensation was extremely thin and the battery was not sufficiently formed between different metals.

(Iv) If the length of the highly corrosion-resistant steel pipe is 30% or less of the total length, it was confirmed that there is no harmful effect due to the difference in thermal expansion coefficient.

  The present invention has been made on the basis of the above knowledge, and the gist of the present invention is that a plurality of air preheater main bodies through which exhaust gas containing corrosive gas passes are attached, and the air passes through them. The heat transfer tube for an air preheater that heats the air and has a length region of 2 to 30% with respect to the entire length from the air-filled side is a corrosion-resistant alloy index GI = − [Cr] + 3.6 × [Ni] +4 Corrosion-resistant low alloy consisting of a corrosion-resistant stainless steel pipe or Ni-based alloy pipe having a value of 7 × [Mo] + 11.6 × [Cu] of 40 or more and the remaining length region conforming to ASTM A423 Grade 3 Code Case 2494 It is a heat transfer tube for an air preheater characterized by comprising a steel tube.

  The heat transfer tube for the air preheater of the present invention is excellent in construction cost and material cost, and the structure and construction method of the equipment is almost the same as the conventional one, exhibiting excellent durability against low temperature corrosion, and stabilizing and maintaining the thermal efficiency. Industrial contributions such as reduction of administration costs and reduction of rare alloy usage are extremely significant.

  The present invention overcomes the above-mentioned problems and achieves the object, and specific means thereof will be described with reference to the drawings.

  FIG. 1 is an overall flow diagram of a waste incinerator equipped with an air preheater according to an embodiment of the present invention, FIG. 2 is an overall view of the air preheater, and FIG. 5 is a structural diagram of a heat transfer tube of the present invention. It is.

  Fig. 6 shows the relationship between the steel rust component of the surface layer and the amount of corrosion thinning and the relationship from the pneumatic end to the normal steel pipe (JIS G 3461 STB340) used for one year with the air preheater shown in Fig. 2. Show. In FIG. 6, the amount of corrosion thinning is divided into two regions with a tube length ratio from the air inlet of about 0.2 as a boundary (hereinafter, for convenience, the lower side of the tube length from the air inlet is the region I, and the higher side. It is understood that this is broadly divided into area II). In addition, since iron sulfate is a corrosion product due to sulfuric acid dew point corrosion, and FeOOH is a corrosion product due to water corrosion, it can be seen that water corrosion mainly occurs in region I and sulfuric acid dew point corrosion mainly occurs in region II. . Therefore, it is reasonable to select an alloy that exhibits excellent corrosion resistance in region I and a material that exhibits excellent corrosion resistance in region II.

Area I material:
As the material of the region I, when the value of the corrosion-resistant alloy index, GI = [Cr] + 1.6 × [Ni] + 6.0 × [Mo] + 7.1 × [Cu] is less than 40, a low pH drain Since satisfactory corrosion resistance cannot be obtained due to pitting corrosion or stress corrosion cracking in the entire surface corrosion or crevice due to the above, a corrosion resistant alloy having a GI value of 40 or more is required. High grade austenitic corrosion resistant stainless steel of SUS316 or higher is desirable. Ni-based alloys such as Hastelloy can also be applied.

Area II material:
In region II, since sulfuric acid dew point corrosion is dominant, in order to obtain sufficient corrosion resistance, it is necessary to be a corrosion resistant low alloy steel to which a total of corrosion resistant elements such as Cu and Sb are added to about 1% or less.

Required length of area I material:
The required length of the material in the region I with respect to the entire length of the heat transfer tube is 2% or more in order to obtain sufficient corrosion resistance. Further, if it exceeds 30%, the material cost becomes high, and an increase in the thermal expansion coefficient cannot be ignored in design, so it is limited to 30% or less. When comprehensively considering durability, workability, and material cost, 5 to 20% is preferable.

Size of corrosion resistant high alloy steel pipe and corrosion resistant low alloy steel pipe:
It is desirable that the inner diameter and the outer diameter are the same. In particular, in the case of a structure in which exhaust gas passes through the inner surface of the pipe, it is preferable to prevent the accumulation and blockage of ash near the joint with the same inner diameter.

  The method for joining the dissimilar pipes is not limited in the present invention, but any of welding methods such as butt circumference welding and friction welding, and mechanical joining such as shrink fitting can be applied.

  Hereinafter, the effects of the present invention will be described in more detail with reference to examples. In addition, this invention is not limited to the following Example.

  Using the materials shown in Table 1, the heat transfer tubes shown in Table 2 were manufactured and used for 2 years in an air preheater of a garbage incinerator, then piped out, and the corrosion thinning after rust removal was measured and evaluated. In all cases, the initial plate thickness was 3.7 mm.

  The total length of the heat transfer tube is 5.0 m. The waste incinerator operated continuously, and was suspended once every two months for several days.

  Since Comparative Example 1 is a steel pipe made of ordinary steel (carbon steel) in both regions I and II, corrosion perforations were found in region I, and a thickness reduction of 2 mm or more was observed in region II.

  In Comparative Example 2, since both the regions I and II are made of a sulfate-resistant low alloy steel pipe, the thickness reduction in the region II was good at less than 0.5 mm, but in the region I, corrosion holes were observed.

  In Comparative Example 3, the region I was made of SUS304, and the region II was made of sulfuric acid resistant steel (CRLS) compliant with ASTM A423 Grade 3 Code Case 2494. However, the GI value of the SUS304 of the region I was less than 50. Since it was out of range, corrosion thinning of 1 mm or more, perforation due to pitting corrosion, and stress corrosion cracking were observed in region I.

  In Comparative Example 4, the region I was made of SUS329J1 and the region II was made of sulfuric acid resistant steel (CRLS). However, since the GI value of SUS329J1 is less than 50 and outside the scope of the present invention, the thickness of the corrosion is 1 mm or more in the region I. Was recognized.

  In Comparative Example 5, the region I was a steel pipe made of SUS316 and the region II was made of plain steel, and a thickness reduction of 2 mm or more was observed in the region II.

  Inventive Examples 11 and 12 are 0.3 mm in Region I and less than 0.5 mm in Region II, and it is clear that the estimated corrosion resistance life is more than 8 years, which is longer than that of the Comparative Example.

  Table 3 shows the results of Examples 21 to 24 of the present invention. In invention examples 21 to 24, the region I is defined as UNS. Made of stainless steel (YUS270) compliant with 83254, made of region II made of sulfuric acid resistant steel (CRLS), changing the length of the YUS270 steel pipe, resulting in thinning due to water corrosion occurring on the low temperature side of the CRLS steel pipe Indicates the result of determining whether or not It is clear that the tube length ratio of YUS270 is required to be at least 2%.

It is a block diagram which shows an example of the residual heat utilization equipment of waste gas. It is a figure which shows typically an example of the whole structure of the conventional heat exchanger. It is a figure which shows typically an example of the principal part of the heat exchanger tube of the conventional heat exchanger with sectional drawing. It is a figure which shows typically other examples of the principal part of the heat exchanger tube of the conventional heat exchanger with sectional drawing. It is a structural diagram of the heat transfer tube of the present invention. It is a figure which shows the relationship between the corrosion thinning amount of the normal steel steel pipe used for a long time as a heat exchanger tube of a heat exchanger, the relative strength of a corrosion product, and the distance from an air inlet end.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Incinerator 2 Gas cooling chamber 3 Air preheater 4 Blower 5 Excess heat utilization equipment 6 Dust collector 7 Induction fan 8 Chimney 31 Air preheater main body 32 Exhaust gas inlet 33 Exhaust gas outlet 34a Air introduction side tube plate 34b Air outflow side tube plate 35 Heat transfer tube 36 Duct on the air inlet side (air inlet)
37 Duct on the air outlet side (air outlet)
38 Air Inlet 39 Air Outlet 40 Inner Intubation 41 Outer Plate of Double Tube Sheet 42 Insulating Material 50 Inner Intubation Unit 51 Outer Plate of Double Tube Sheet 52 Inner Intubation 53 Gap 54 Gap 55 Air Inlet

Claims (1)

  A plurality of air preheater heat transfer tubes that are attached between the tube plates of the air preheater main body through which exhaust gas containing corrosive gas passes, and that allow air to pass through them and heat the air, 2-30% length region is a corrosion resistant alloy index GI = − [Cr] + 3.6 × [Ni] + 4.7 × [Mo] + 11.6 × [Cu] with a value of 40 or more. A heat transfer tube for an air preheater comprising: a Ni-based alloy tube, and the remaining length region comprising a corrosion-resistant low alloy steel tube compliant with ASTM A423 Grade 3 Code Case 2494.

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