High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

[Seminar]

Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

Manabu NOGUCHI*, and Hiroshi YAKUWA**

Abstract One of the characteristics of incinerators is that the waste used as fuel contains chlorine in high concentration. The chlorine reacts with alkaline metal, etc., to form a chloride, which is deposited as ash adhesion on boiler heat transfer tubes and the like. When this adhered ash melts, the corrosion environment is significantly deteriorated. In addition, the soot blow used to remove the adhered ash promotes the destruction of the corrosion scale and accelerates the metal loss of the heat transfer tubes. The ash adhesion behavior depends on temperature, and chloride is concentrated where the exhaust gas temperature is high, deteriorating the corrosion environment. Accordingly, a good understanding of the adhesion behavior and an appropriate design for the corrosion environment are required in order to prevent the corrosion of heat transfer tubes. This paper explains the metal loss behavior mainly due to the influence of ash adhesion, and also describes examples of corrosion behavior and corrosion protection in the actual plants. Keywords: High temperature corrosion, Chlorination, Molten salt, Waste to Energy Plant, Superheater, Boiler, Ash adhesion, Predication of corrosion rate

1. Introduction 2. Boilers for waste-to-energy plants

Parts 3 and 4 of this lecture discuss high-temperature 2.1 Characteristics of waste-to-energy plants corrosion associated with incinerators. This part Figure 3-11) shows an example of a waste-to-energy focuses on boilers used in waste-to-energy plants, and plant. The plant shown in the figure uses a grate-type the next part, on other equipment. incinerator, a typically used incinerator at present. While thermal power plants practically use steam at Other than this type of incinerator, there are fluidized 600 °C or higher, in waste-to-energy plants, the steam bed furnaces, gasification melting furnaces, etc. The temperature is around 400 °C, which is significantly combustion method varies depending on the incinerator lower than the former. Waste-based power is a type. However, any type of incineration system is significant social need as renewable energy, and almost the same in configuration basically. Waste fed efficient power generation is also required. However, into the pit is burned in the incinerator. The heat is high-temperature corrosion of the heat transfer tubes recovered by the boiler, economizer, etc. Then the is a technical problem, which makes it difficult to use exhaust gas is purified using environmental measures higher-temperature, higher-pressure steam. This part such as exhaust gas treatment and dust removal, and of the lecture explains corrosion of boilers in waste-to- is discharged from the smokestack. energy plants and corrosion protection. In addition to a large amount of ash, waste contains chlorine and sulfur, which act as oxidizers; the amount of chlorine is much larger than that of sulfur, which is the prominent characteristic of waste2). Chlorine reacts * Ebara Enviromantal Plant Co., Ltd. with alkali metals or heavy metals to form chlorides. ** Technologies, R&D Division

Ebara Engineering Review No. 253（2017-4） ─ ─1 Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

Flow of waste High-pressure steam header Flow of flue gas Flow of air Steam condenser Flow of ash and dust Steam turbine Flow of steam generator Chemical silo Condensate Flow of water tank Boiler Recycled Economizer water To atmosphere

Waste crane Bag filter Superheater Deodorization Desuperheater unit Waste hopper Incinerator

Stack Secondary blower Induced draft fan Drying stoker From other Platform Post-combustion stoker incinerators Combustion Forced draft Refuse stoker fan feeder Collected dust conveyor Weighing scale From other incinerators Collected dust and ash handling system

Fallen ash conveyor Ash conveyor To the final landfill site Air preheater

Input bunker Storage bunker Ash Solidified bunker material pit Fig. 3-1 Flowchart of a waste-to-energy plant1)

Then, ash containing such chlorides flies from the other hand, in the superheater, ash adhesion greatly incinerator to the following equipment, adhering to the varies depending on the area, in general. Ash adhesion heating surface of the boiler. In general, to minimize is maximized at the superheater entrance (d). In some such ash adhesion, the exhaust gas is turned 180° at cases, adhering ash grows in a comb-like shape toward the bottom of the boiler to detach adhering ash, with a the upstream of the gas flow. Toward the downstream superheater installed in the third path of the boiler. of the gas flow, however, the amount of adhering ash Figure 3-2 shows an example of ash adhering to the gradually decreases with almost no adhering ash boiler of an actual incineration system for industrial observed at the superheater exit (f) in many cases. Ash waste. Adhering ash is partially removed from the adhesion may not only decrease the heat transfer upper area (a) of the water tube of the boiler. However, efficiency of boilers but also cause gas blockage in a no significant difference in appearance is observed superheater. Therefore, it is usual that adhering ash is among the upper, middle, and lower areas. On the periodically removed during operation using soot blowing which blows steam or the like.

Tube side 2.2 Characteristics of adhering ash Gas side Water tube Adhering of ash affects corrosion of incinerators in particular. The composition of adhering ash is not constant and is often different between the gas side Flue gas and the water tube side. The photo of ash adhering to (a) Upper area (f) SH the water tube (shown in Figure 3-2 (a) to (c)) indicates that a hard cream-colored substance like a crispy rice SH bar is observed on the gas side. However, inside the

(b) Middle area SH (e) above substance, there is a light green dense substance, inside which there is a reddish brown corrosion SH product covering the surface of the water tube. The ash adhering to the superheater (shown in Figure 3-2 (c) Lower area (d) Boiler diagram (d) to (f)) is not as clear as the ash that adheres to the ：Sampling locations SH：Superheater water tube, but the properties of gas side and the tube Fig. 3-2 Example of ash adhering to the boiler of an actual waste-to-energy plant side are different. Except the ash in Area (f), where the

Ebara Engineering Review No. 253（2017-4） ─ ─2 Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

Table 3-1 Analysis results of adhering ash3)

Water tube Superheater Location Collected (a) (b) (c) (d) (e) (f) ash Side Tube Gas Tube Gas Tube Gas Tube Gas Tube Gas Whole N＝11 Na 5.79 2.75 5.36 2.38 6.00 3.46 15.18 1.42 1.83 1.60 3.51 0.96 Al 1.61 2.56 2.32 2.99 4.01 4.80 2.49 2.59 2.69 2.96 2.60 4.12 Si 2.03 2.40 1.97 2.11 2.02 1.90 5.37 6.22 5.52 6.88 5.85 20.62 S 8.43 12.18 8.74 12.68 11.90 12.60 2.53 3.31 5.55 3.43 5.62 2.17 Cl 12.81 5.16 13.35 4.52 4.40 3.71 23.06 14.32 14.29 11.48 10.25 2.69 K 6.41 0.82 8.66 1.13 6.64 3.37 5.73 2.33 2.67 1.90 3.01 1.16 Ca 6.59 28.46 7.46 26.60 12.37 20.24 21.06 30.44 27.72 29.01 19.96 15.43 Cu 7.69 1.31 8.56 1.60 5.79 3.81 3.98 3.26 7.24 2.37 7.16 0.61 Zn 2.39 1.23 2.38 1.24 2.25 1.81 2.00 1.71 1.78 1.79 2.46 0.70 Pb 22.18 0.09 14.86 0.09 6.85 0.73 0.73 0.31 0.21 0.18 2.86 0.14 Endothermic 344 ℃ 678 ℃ 361 ℃ 658 ℃ 466 ℃ 518 ℃ 527 ℃ 562 ℃ 534 ℃ 536 ℃ N.D. peak pH 6.2 10.13 5.81 9.82 6.78 8.56

amount of adhering ash is small, we identified the ash area. The higher the exhaust gas temperature becomes, collected at each location as the gas side one or the the higher concentration heavy metal chlorides, tube side one, and analyzed them. Table 3-13) shows the especially Pb, tend to have. On the other hand, as well analytical results. The ash on the gas side contained as Ca, S tends to be more concentrated mainly in the almost no component of heat transfer tubes, such as Fe. ash adhering to the water tube on the gas side. It is On the other hand, the ash on the tube side contained considered that S is concentrated as Ca sulfate in the a large amount of corrosion products and up to nearly areas where the exhaust gas and surface temperatures 50 % of the heat transfer tube components. For this are high. reason, these values removed Fe, Cr, and Ni and As described above, the temperature range of converted to 100 %. The table also shows the results of concentration varies depending on the compound, and analysis on the (collective) ash taken from the chlorides, which have a significant effect on corrosion, equipment following the boiler, which is considered to are likely to be more concentrated in the areas where have passed through the boiler. Comparison between the exhaust gas temperature is higher and the the collective ash and adhering ash indicates that the temperature of the ash adhering surface is lower. A adhering ash contains higher concentrations of Cl, S, possible cause of this is that chlorides and other alkali, and heavy metals, which denotes that these substances existing as vapor in the exhaust gas are elements are likely to be concentrated in the adhering precipitated as adhering ash in low-temperature areas. ash. Then, comparison between the tube side ash and It is likely that chlorides are more concentrated in the gas side ash indicates that the tube side cooled by the tube side, where the temperature is lower than in the heat transfer tube contains higher concentrations of Cl, gas side, because chlorines volatilize and cannot exist alkali, and heavy metals, which denotes that these stably on the gas side. The vapor component seems chlorides are likely to be concentrated in lower- likely to be concentrated by the difference in temperature areas. In connection with the ash adhering temperature between the atmospheric gas and the to the water tube, while the surface temperature of the surface to which ash adheres; the higher the gas water tube is almost constant regardless of height, the temperature becomes, the more concentration is exhaust gas temperature drops as the height decreases. promoted. With these facts, adhering ash is affected by Consequently, Cl and heavy metals are more both the gas and metal temperatures, changing the ash concentrated in the upper and middle areas, where the composition, and as a result, causing the corrosive exhaust gas temperature is higher, than in the lower environment to change. Figure 3-3 shows a photo of

Ebara Engineering Review No. 253（2017-4） ─ ─3 Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

SiC Chloride Cement

B: Area where SiC was A: Area where SiC removed after operation peeled off Fig. 3-3 Photo of the furnace wall of the path of grate-type incinerator 1 (area where SiC peeled off) the furnace wall of the first path of municipal waste grate-type incinerator. The gray areas are cement and were coated with SiC, which partially came off during operation (as shown as area A in Figure 3-3). When we removed SiC from an area (area B in Figure 3-3) near area A, white precipitate was found inside the area covered with SiC. The analysis of the precipitate shows that it is a mixture of NaCl and KCl, and suggests that chlorides got into the back side through gaps made by the peeling of SiC, and were precipitated on the cooled cement surface. This result corroborates the assumption that chlorides turn into steam and then become concentrated.

3. Corrosion phenomenon

3.1 Effect of adhering ash on corrosion From a practical viewpoint, the melting point is the most noticeable physical property of adhering ash. It is Fig. 3-4 Dependence of corrosion rates on temperature verified known that the corrosion rate greatly increases as ash through lab testing4) melts. For example, through lab testing using ash having a melting point of 480 °C, Kawahara et al. 2NaCl＋SO2 ＋O2 →Cl2 ＋Na2SO4… ………………… (3-1) verified that there is a large difference in corrosion rate 4HCl＋O2 →2Cl2 ＋2H2O……………………………… (3-2) between 450 °C and 500 °C, which are respectively 4FeCl2 ＋3O2 →2Fe2O3 ＋4Cl2………………………… (3-3)

4) below and above the melting point (Figure 3-4) . Some Formula (3-1) states that Cl2 is generated in the

5)-7) descriptions have been provided as a corrosion process where a chloride reacts with SO2 to form mechanism. One of them is shown in Figure 3-56). The sulfate. Formula (3-2) states the Deacon reaction, which key corrosion reaction is considered to be chlorination is known to proceed using adhering ash as the

8) based on Cl2 gas. Typical examples of Cl2 generation catalyst . It is likely that molten ash accelerates these are shown below: reactions and helps generate Cl2, which is highly

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Soot blowing

Temperature gradient Gas temperature fluctuations Spraying of high-pressure steam Entry of combustion gas Destruction and peeling of scale Entry of HCl O2 H2O SO3 molten salt Molten ash Cl2 formation and vapor (repeated adhering and peeling off) component Entry of molten salt

K2SO4 Reaction between ash and scale N2Cl KCl Micro cracks Cl2 O2

Layer of Cr2O3/MoO2/Nb2O5 and NiO/Fe3O4 Oxidation (CrCl3 and FeCl2 partially contained) reaction

Layer of NiO / Fe3O4 Oxidation reaction Layer of Cr2O3/MoO2/Nb2O5 Chlorination reaction Layer of CrCl2/FeCl3/NiCl2 Diffusion of Cr, Fe, etc. Mo Ni Nb Fe Cr Partial molten salt corrosion Ni-Cr-Mo-(Nb, Fe) Alloy Main Metal temperature Corrosion products reactions fluctuations

Fig. 3-5 Scale structure of an alloy of Ni-Cr-Mo-(Nb, Fe) and corrosion mechanism corrosive, activating the entire corrosion reaction. In addition, as Formula (3-3) states, Cl2 is also generated from corrosion products9). It is probable that the chlorides resulting from chlorination are oxidized to generate Cl2, which causes further chlorination, promoting corrosion. On the other hand, there is concern that molten ash may cause molten salt corrosion. However, in actual plants, the heat transfer tubes are often covered with corrosion scale. For this reason, it is probable that molten salt does not often react directly with a heat transfer tube, and molten salt corrosion occurs partially and/or temporarily in association with scale destruction or exfoliation. The effect of molten ash on corrosion may also involve the corrosion mass loss. Figure 3-6 shows the Fig. 3-6 Relationship between corrosion mass loss and 10) relationship between the molten phase ratio and the molten-phase/mass determined through burial testing corrosion mass loss, determined through burial testing10). As shown in the above figure, it is known alkali sulfate and chlorides, whose eutectic temperature that the corrosion rate increases as the molten-phase is approximately 510 °C. If various compounds are ratio increases, but the rate decreases once the ratio mixed with them, the melting point is lowered. Typical exceeds a certain level. This decrease is caused by a components of such compounds are heavy metals. decrease in gas permeability in association with an Typical heavy metals contained in waste are Cu, Zn, increase in molten-phase mass, indicating that supply and Pb. Figure 3-711) shows the relationship between of gas is essential for corrosion to advance. the concentration of heavy metals contained in The main chlorine components of adhering ash are adhering ash and the melting point determined based

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temperature of most water tubes is 300 °C or lower, 700 Industrial waste incinerator A 650 which means a milder corrosive environment than that Incinerator A 600 Incinerator B of superheaters. Nevertheless, in this environment, due 550 Incinerator C to the high exhaust gas temperature, corrosion may 500 promote thickness reduction. Motoi et al. reported that 450 low-air-ratio combustion promotes thickness reduction 400

350 of water tubes, which is promoted mainly by Melting point of adhering ash ℃ 300 destruction of protective scale caused by temperature 0 10 20 30 40 12) Content of heavy metals (Pb + Cu + Zn) in adhering ash % fluctuations . Other than this, as described in section 2-2, a high Fig. 3-7 Relationships between melting point and exhaust gas temperature may concentrate heavy the heavy metal content in adhering ash11) metals, causing thickness to reduce. Figure 3-8 shows the results of analyses based on an EPMA (electron on TG-DTA (thermogravimetry-differential thermal probe microanalyzer) conducted on water tube cross analysis). As shown in the figure, the melting point sections that were exposed to high-temperature drops as the concentration of heavy metals increases. exhaust gas, resulting in annual thickness reduction of The melting point begins to drop sharply at around a up to 0.4 mm. This analysis says that Cl was found to 10 % of concentration and continues to drop until at be concentrated in the interface between the scale and about 320 °C. Thus, heavy metals drop the melting base material in particular, and heavy metals were also point of ash, promoting corrosion. detected. The figure shows the results of the analysis 3.2 Example of thickness reduction in actual of adhering ash collected from a water tube subject to equipment (water tube) thickness reduction and from a sound tube downstream In mainstream waste-to-energy plants at present, the of exhaust gas. The downstream ash collected from the steam pressure is about 4 MPa, and the surface unaffected tube contained approximately 5 % Cl and

mass% Na S Cl K Ca Cu Zn Pb Melting point pH (℃) Corroded part 4.31 3.52 15.32 4.58 4.89 0.96 1.99 13.83 328 5.79 Uncorroded part 7.97 11.83 5.25 6.95 10.81 0.98 0.78 2.66 426 7.06

Fe O C1 Zn

S Na Ca K

Pb Si Cu SEI

Fig. 3-8 EPMA analysis results of water tube cross sections and adhering ash

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Just after reception About two weeks pH check result after indoor storage Fig. 3-9 Photos of appearance of a water tube sample several percent Pb, and the melting point was higher reduction starts to increase at a point of 700 mm from than 400 °C. On the other hand, the ash collected from the incinerator wall, while it starts to decrease at a point the corroded area contained more than 10 % Pb and of 800 mm from the wall. This indicates that the installed far more than 10 % Cl. The TG-DTA analysis result protector in the area starting from a point of 800 mm indicates that the melting peak is around 330 °C. This from the wall greatly retards thickness reduction. On probably means that concentrated heavy metals the other hand, thickness reduction is the worst around deteriorated the corrosive environment and a point of 700 mm probably because of soot blowing. consequently promoted corrosion. However, that thickness reduction increases in the Corrosion may even advance while the plant is not tertiary and secondary superheaters, which are heavily in operation. Figure 3-9 shows photos of the appearance corroded, but no noticeable thickness reduction is of a removed water tube. The tube was dry just after observed in the primary superheater. In other words, reception, but was found to be entirely wet about two soot blowing has a greater influence on more heavily weeks after indoor storage. The pH of adhering ash is corroded areas. Figure 3-11 shows the comparison of normally is near neutral (Table 3-1 and Figure 3-8). thickness reduction between normal areas and areas However, this analysis shows that the sample ash’s pH is approximately 2, which is strongly acidic, and →Area with a protector corrosion may proceed even when the plant is not in operation. Thus, caution is required when there is condensation on a water tube. 3.3 Example of thickness reduction in actual equipment (superheater) In actual equipment, corrosion develops due to various factors, such as gas flow, variations or fluctuations in the operating conditions, and soot blowing used to remove adhering ash. The following describes the mechanism of thickness reduction based on an investigation13)

conducted on an actual 400 °C superheater. Loss of outer diameter mm Figure 3-10 shows that the loss of the outer diameter of a superheater that has been used for 23 months since the plant started operation depends on the distance from the incinerator wall toward the center of the boiler (1100 mm from the wall). Since soot blowing is performed from the center, a protector is provided in Distance from incinerator wall mm the area starting from a point of 800 mm from the wall Fig. 3-10 Tendency of thickness reduction of superheaters toward the center. In the longitudinal direction, thickness (dependence on the distance from the incinerator wall)

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5 the direction of 45° (315°). Figure 3-13 shows the tendency Alloy625 HR11N of thickness reduction in the circumferential direction. In 4 HC-22 both the upper and lower areas of the superheater, JHN24

3 thickness reduction is smaller in the direction from 135° to 225°, where the amount of adhering ash is small 2 because of on the downstream side of the gas flow. This Additional thickness reduction caused by SB indicates that adhering ash promotes corrosion. For the 1 lower area of the tertiary superheater, thickness

0 reduction is larger in the 45° (315°) direction than in 0° 0 1 2 3 Thickness reduction in the areas affected by SB mm Thickness reduction in unaffected areas mm direction, where the amount of adhering ash is the Fig. 3-11 Influence of soot blowing (SB) on thickness reduction14) largest. This shows the same tendency as in Figure 3-12. This is probably because of gas permeability. In the 0° affected by soot blowing in actual equipment, including direction area, to which gas is directly applied, much ash equipment in other plants14). The broken line in the chart adheres and consequently the gas permeability is low. represents points where the thickness reduction in Therefore, it is likely that thickness reduction proceeds in normal areas is equal to the thickness reduction in the the 45° direction area, to which ash adheres but does not areas affected by soot blowing. The larger the thickness extremely lower the gas permeability. Figure 3-10 also reduction, the more the distance from the broken line. indicates that adhering ash effectively retards thickness While erosion and promotion of corrosion are considered reduction; in the lower area of the tertiary superheater, as effects on thickness reduction by soot blowing, it can thickness reduction decreases as the distance to the be determined that the main effect is corrosion incinerator wall becomes closer. It is considered that on promotion due to destruction of protective scale, because the wall side of the lower area of the tertiary superheater, more heavily corroded areas are more likely to be the gas flows quickly promoting extreme ash adhesion affected by soot blowing. and causing gas permeability to be extremely lowered. For actual superheaters, thickness reduction is often On the other hand, on the upper area of the superheater, considered to be the largest at 45° against the gas flow, ash adheres to the underside of the tube and the upper which has been actually verified in many cases. Figure 3-12 side is affected by soot blowing. Thus, it is considered shows an example of a cross section of a superheater, that thickness reduction proceeded in the horizontal which indicates that thickness reduction is the largest in direction (90° and 270°), which is affected by both sides. If the upper and lower areas of the superheaters were compared, in the primary superheater, thickness 180° reduction proceeds in the lower area, which is exposed 225° 135° to higher exhaust gas and steam temperatures. On the other hand, in both the secondary and tertiary superheaters, the metal temperature is higher in the 270° 90° upper area because the gas flows in parallel with exhaust gas. However, thickness reduction was larger in the lower area. This means that the gas temperature has a large effect on the corrosion rate. As described in 315° 45° Section 2-2 and pointed out in a reference15), it is likely 0° that a rise in the exhaust gas temperature makes

Gas flow chlorides become more concentrated, resulting in a more severe corrosive environment. Fig. 3-12 Optical photo of a cross section of a superheater tube taken from a boiler for industrial waste power generation These facts indicate that thickness reduction in

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380 ℃ （a） 260 ℃ Primary SH STB410 330 ℃ （b） 450 ℃ 370 ℃ （c） Secondary SH SUS309J2 285 ℃ （d） 530 ℃ （e） 400 ℃ Tertiary SH SUS310

310 ℃ Thickness reduction mm （f） 600 ℃ Steam temperature Flue gas temperature ：Soot blowing

Angle against gas flow °

Fig. 3-13 Tendency of thickness reduction of superheaters (dependence on gas flow direction)

superheaters is closely related to adhering ash and that exhaust gas alone has a minor influence on corrosion. Resin They also indicate that the exhaust gas temperature Superheater tube side scale has a large effect on the properties of adhering ash, and the position of superheaters and circumferential direction are significantly related to the adhesion state of ash. For these reasons, the corrosion level largely Adhering ash depends on each area. Soot blowing promotes thickness reduction, particularly in heavily corroded areas. Gas side Ash adhering to a superheater Figure 3-14 shows time series changes in thickness reduction in superheaters and a photo of a cross section 1.0 of adhering ash. Observation of the cross section Incinerator 1 0.8 Incinerator 2 revealed that the formed scale is porous with a layered Tubes taken out for inspection structure, which does not provide a dense protective 0.6 scale; from a macroscopic viewpoint of time, thickness 0.4 reduction is considered to proceed repeating destruction 0.2 and regeneration of scales. Therefore, it is probable that Thickness reduction mm the time dependence of thickness reduction did not 0.0 0.0 0.5 1.0 1.5 2.0 follow the parabolic rate law described in Part 1, and Normal operation time (unit of 10 000 hours) thickness reduction proceeds linearly in the actual plant Fig. 3-14 Time series changes in thickness reduction in superheaters as shown in the figure. In addition, as one of the and photo of a cross section of adhering ash

Ebara Engineering Review No. 253（2017-4） ─ ─9 Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants characteristics of this environment, there is an exhaust gas is extremely high, protectors may promote incubation period when the corrosion rate is low, but thickness reduction3). Rich chloride vapor may be after a certain duration has elapsed, the corrosion rate concentrated in gaps between the heat transfer tube may increase16). Since the surface of a new heat transfer and protector, without being removed by soot blowing tube is smooth and almost even, adhering ash can be or the like, staying as highly concentrated chlorine to easily removed by soot blowing. If the surface of an cause unusual corrosion. In addition, if the gap between aged tube becomes uneven due to corrosion or the like, the heat transfer tube and protector increases, the removal of adhering ash is expected to be difficult. This surface temperature of the protector increases and means that the corrosive environment of the surface is promotes corrosion. For this reason, the protector should different between new and aged heat transfer tubes. closely contact the heat transfer tube. If corrosion Thus, the incubation period probably includes a time damage is small, acceleration by soot blowing is that allows corrosive substances contained in adhering retarded. Therefore, it is also possible to clad the surface ash to stably stay on the surfaces of heat transfer tubes. with a highly corrosion-resistant alloy, such as an Ni- based alloy, to retard corrosion for inhibiting thickness 4. Corrosion protection reduction. In addition, instead of soot blowing, there is 4.1 Corrosion inhibition method rapping-based method for removing ash by vibrating In recent years, low-air-ratio combustion has been heat transfer tubes, which is used in the Tsukui pilot mainly used for incinerators. For their water tubes, plant of the NEDO project15). However, this method corrosive environments become more severe. requires more floor space to use horizontal-type Accordingly, corrosion protection is often required. As superheaters and more construction costs. The steel ball corrosion protection, surface treatment by thermal drop method is also put into practical use19). spraying or cladding of a corrosion-resistant alloy is The corrosive environment for boilers used for often used7), 15), 17), 18). Self-fluxing alloys and overlaying are waste-to-energy power generation is much more severe expected to have a long life. However, because these than that for thermal power generation because alkali measures cannot be widely used in actual work sites, metals and other chlorides are generated in large they are applied to mainly new products in factories. quantities. Only comparison of the amounts of chlorine For on-site repair, thermal spraying or the like is contained in the fuel do not make sense, because black applied. However, it allows corrosive components to liquor, coal, and other fuels may contain an amount of reach the interface between the coating layer and base chlorine similar to that of waste. Since the difference material through open pores existing in the coating between them is the amount of sulfur, increasing that layer, corroding the base material. As a result, coating of sulfur is proposed to ease corrosive environments20). layer detachment often occurs. To prevent this, the 4.2 Prediction of the corrosion rate following are also used: HVOF (high velocity oxygen As described so far, what hinders improvement of the fuel) thermal spraying with a low porosity or sealing steam conditions is corrosion of superheaters and with Al. In a case, for example, where gas flows fast increased costs due to corrosion. In other words, to and erosion occurs partially, a protector is provided to increase the steam temperature, the increase in the the affected areas. revenue from power selling achieved by increased As described in section 3-3, with respect to efficiency in power generation must exceed the superheaters, soot blowing is a large factor that increase in cost caused by increasing the steam accelerates damage and promotes thickness reduction temperature. In short, it is required that the corrosion particularly in heavily corroded areas. Accordingly, rate can be predicted based on the design conditions measures against this are required. The most commonly including the steam conditions. The following prediction used countermeasure is installation of a protector. formula allows you to evaluate the effects of the gas However, if partial pressure of chlorides contained in and metal temperatures on corrosion and to determine

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0.0 the optimal superheater position. As a previous study, the following formula for predicting the corrosion rate －0.5 and the range of the data used is offered as the －1.0 valuable results with the NEDO project14). －1.5 W=10-33.8・［ T g ］ 5.65・［Tm］4.86・［HCl］0.576・ －2.0 0.419 -0.391 －2.5 Calculated value ［Cl］ ・［Cr+Ni+Mo］ ・Time… …………… (3-4) = Actually measured value W : Corrosion rate (mm) log (calculated value) mm －3.0 Tg : Gas temperature (°C) 583 to 675 °C －3.5 －3.5 －3.0 －2.5 －2.0 －1.5 －1.0 －0.5 0.0 Tm : Metal temperature (°C) 450 °C and 550 °C log (actually measured value) mm

HCl : Concentration of HCl in combustion gas (ppm) Fig. 3-15 Comparison of actually measured corrosion rates of 568 to 1420 ppm a superheater and calculated corrosion rates determined by the NEDO formula Cl : Concentration of Cl in adhering ash (%) 0.0 0.28 to 10.5 % Cr + Ni + Mo : Total content of the elements contained

in the material (%) 3.03 to 96.5 % －0.5 Time : Hour This prediction formula (NEDO formula) was －1.0 statistically made according to the thickness reduction log (calculated value) mm data obtained through exposure tests on actual Calculated value = Actually measured value equipment and from demonstration plants, based on －1.5 －1.5 －1.0 －0.5 0.0 the knowledge obtained through lab testing. It is a log (actually measured value) mm breakthrough because the corrosion rate in a Fig. 3-16 Comparison of actually measured corrosion rates of complicated waste combustion environment can be a superheater and calculated corrosion rates determined by the corrected formula accurately predicted using relatively simple variables. However, since it is a regression formula, there is a In other words, it is found that a sufficiently accurate risk to applying it to conditions beyond the data range. prediction is possible with a NEDO formula The NEDO formula assumes a steam temperature of appropriately modified based on its basic idea. 500 °C. Compared with the time when the formula was If the formula is used in the planning phase, the made, separate waste collection has prevailed. temperature and the alloy composition are design Accordingly, the Cl content in fuel tends to decline. conditions, but the concentrations of HCl and Cl vary Thus, it is not desirable to apply the NEDO formula to depending on the material and other factors. Figure 3-17 typical boilers at present, which use a steam and Figure 3-18 show the calculated transfer rates of the temperature of 400 °C. As a result, the formula must be exhaust gas and fly ash of Cl in the material22). It is modified21). Figure 3-15 shows an example of application found that the transfer rate varies depending on the of the NEDO formula to an existing 400 °C boiler. The incinerator type, and that grate-type incinerators are solid line in the table represents a relationship of the more likely to allow Cl to transfer to exhaust gas than actually measured values and calculated values being the other types, and fluidized bed incinerators tend to equal, which predicts a risk because the calculated allow Cl to transfer to fly ash. By understanding these values are significantly lower than the actually relationships, it is possible to predict the HCl measured values. Figure 3-16 shows the results of a concentration, etc. based on the fuel composition, change in the NEDO formula variables using the data offering more accurate predictions of corrosion rates. As on an existing incinerator. As shown in the figure, the a result, superheaters with optimized life cycle costs correction has significantly lessened the discrepancies (LCC) can be designed, which contributes to improvement between the actually measured and calculated values. of efficiency using high-temperature steam.

Ebara Engineering Review No. 253（2017-4） ─ ─11 Lecture on Fundamental Aspects of High Temperature Corrosion and Corrosion Protection Part 3: High Temperature Corrosion and Corrosion Protection of the Boilers in Waste to Energy Plants

40% superheater tubes by taking the gas temperature into 35% First analysis Second analysis account. To optimize the superheater layout, it is 30% necessary to accurately predict the corrosion rate of 25% 20% heat transfer tubes. As a typical formula, the NEDO 15% formula is available. Predictions with sufficient accuracy 10% 5% is possible using a customized NEDO formula while Transfer rate to exhaust gas 0% following its basic idea. Grate-type Fluidized Fluidized-bed incinerator bed incinerator gasification melting incinerator In addition, since adhering ash hinders heat transfer

22) and causes blockages, ash is usually removed by soot Fig. 3-17 Transfer rate of Cl in material to exhaust gas blowing today. However, because soot blowing has a drawback that it destroys protective scales and 100% First analysis Second analysis promotes corrosion, the areas affected by soot blowing 80% require measures against corrosion, such as a protector. 60% For further reduction of the LCC and improvement of

40% efficiency of waste-to-energy power generation,

20% development of technologies is required, including Transfer rate to fly ash corrosion protection technologies for surface treatment, 0% Grate-type Fluidized Fluidized-bed incinerator bed incinerator gasification melting more accurate corrosion prediction technologies, and ash incinerator removal technologies as alternatives to soot blowing. Fig. 3-18 Transfer rate of Cl in material to fly ash22) References

1) N Akiba, T Iguchi,“Stoker-Type Waste Incineration Plant 5. Conclusion for Iwamizawa, Hokkaido Construction and Delivery of Iwamizawa Environment Clean Plaza,” Ebara Engineering Compared with thermal power generation and other Review No. 249 (2015): p.14. 2) M Nakamori (supervisor), High Temperature Corrosion types, waste-to-energy power generation is subject to a and Prevention Methods by Combustion Gas in Boiler significantly severe corrosive environment, which is (Technosystem Co., Ltd., 2012): p.33. 3) M Noguchi, H Cho, C Takatoh, J. T. Bauer, M. C. Galetz, M. caused by adhering ash that contains a large amount of Schütze, Report of the 123rd Committee on Heat Resisting chlorides. For a good understanding of corrosion Metals and Alloys, 54 (2) (2013): p.231. 4) N Orita, Y Kawahara, M Kira, The Technology Development development in the above environment, it is required of Waste to Energy Plants, 34 (2) (High Pressure Institute of to understand influences and behaviors of adhering Japan, 1996): p.36. 5) M Yoshiba, Complicated High-Temperature Corrosive ash. Chlorides transform into fly ash and adhere to Damage and Countermeasure of Heat-Resisting Alloy heat transfer tubes, resulting in a severe corrosive Systems in Aggressive Environment of Advanced Waste-to- Power Plant, 38 (3) (Materia Japan, 1999): p.203. environment. Lab testing, etc. has demonstrated that 6) Y Kawahara,“Recent Trends and Future Subjects on High molten ash significantly increases the corrosion rate. Temperature Corrosion Prevention Technologies in High Efficiency Waste-to-energy Plants,” Zairyo-to-Kankyo 54 (5) Corrosion is mainly caused by corrosive Cl2 generated (2005): p.183. from chlorides contained in adhering ash and/or HCl 7) Y Kawahara,“High Temperature Corrosion in Environmental contained in exhaust gas. Molten salt is considered to and Energy Conversion Equipments,” Zairyo-to-Kankyo 55 (5) (2006): p.172. promote Cl2 generation. The properties of adhering ash 8) Y Takahashi, T Kurumaji, K Yukawa,“Catalytic Action of depend on the exhaust gas temperature, the surface Adhering Ash on High-temperature Corrosion of Incinerators (Project on Development of Technology for High-efficiency temperature of heat transfer tubes, and other factors. Waste-to-energy Power Generation: Part 1 of Development For corrosion protection, countermeasures are required for Materials for Corrosion-resistant Superheaters),” Current advances in materials and processes (CAMP-ISIJ) 7 (3) (1994): to inhibit the corrosive environment. For example, p.670. providing the surface treatment to water tubes with a 9) Y Sato, M Hara, Y Shinada,“Effect of a Small Amount of Hydrogen Chloride on High Temperature Oxidation of Pure high corrosion-resistance alloy and installing Iron,” J. Japan Inst. Met. Mater. 58 (6) (1994): p.654.

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10) Y Kawahara, M Kira,“Effect of Physical Properties of 16) H Notani, S Uno, M Yoshiba,“Study on High Temperature Molten Deposits on High Temperature Corrosion of Alloys Corrosion for Superheater Tube in MSW Incineration in Waste Incineration Environment,” Zairyo-to-Kankyo 46 (1) Boiler,” Takuma Technical Review Vol. 4 (1) (1996): p.35. (1997): p.8. 17) Y Matsubara, A Takeya.“Extending the service life of 11) M Noguchi,“High-temperature Corrosion in Waste-to-Power furnace wall tubes using self-fluxing alloy coatings,” Journal Plant” 71st Technical Seminar by Japan Society of Corrosion of High Temperature Society Vol. 36 (6) (2010): p.281. Engineering (2016): p.48. 18) Y Kawahara,“Present Statuses of the Application of Hot- 12) A. Motoi, T Urabe, M Yoshiba,“Accelerated Thickness corrosion-resistant Coating to Boilers for Waste-to-energy Reduction of Water-Wall Tube in Municipal Solid Waste Power Generation and Assessment of the Durability of the Incinerator due to Low Air-Ratio Combustion,” Zairyo-to- Coating,” Journal of Japan Thermal Spraying Society 38 (2) Kankyo 47 (8) (1998): p.512. (2001): p.73. 13) M. Noguchi, H. Yakuwa, M. Miyasaka, M. Yokono, A. 19) K Kawata, T Izumiya, M Todaka, K Yong Uk, K Kitano, Matsumoto, K. Miyoshi, K. Kosaka and Y. Fukuda: “Removal of Boiler Dust at Power Generations by MSW Experience of superheater tubes in municipal waste Incineration by Shot Cleaning Method,” The 26th Annual incineration plant, Materials and Corrosion, 51, p.774 (2000). Conference of JSMCWM C5-5, (2015): p.367. 14) Project Ledger of“Development of Technology for High 20) Y Harada, Y Ishibashi,“Study of Corrosion Prevention Efficiency in Power Generation, etc. by Conventional Grate- Methods for Municipal-Waste Incineration Plants by type Incinerators,” Development of Technology for High- Corrosion Prevention V Environment Control by Young efficiency Waste-to-energy Power Generation, NEDO report, Engineers,” Material of the Corrosion Prevention Committee http://www.nedo.go.jp/content/100087255.pdf. of Society of Materials Science, Japan 35 (191) (1996): p.27. 15) Y. Kawahara, K. Takahashi, Y. Nakagawa, T. Hosoda and 21) M. Noguchi, H. Cho, H. Sakamoto and Y. Honda, Proc. 61th T. Mizuko: Demonstration Test of New Corrosion-Resistant Jpn, Conf. Materials and Environment, JSCE, D-307 (2014). Superheater Tubings in a High Efficiency Waste-to-Energy 22) M. Noguchi, H. Cho, E. Tanaka, E. Ishikawa and H. Plant, CORROSION 2000, Paper. 265 (2000). Miyayama: Proc. 63th Jpn. Conf. Materials and Environments, in press, JSCE (2016).

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