lecture on fundamental aspects of high temperature ... · lecture on fundamental aspects of high...

Ebara Engineering Review No. 253（2017-4）─　　─1

2. Boilers for waste-to-energy plants

2.1 Characteristics of waste-to-energy plants

Figure 3-11) shows an example of a waste-to-energy plant. The plant shown in the figure uses a grate-type incinerator, a typically used incinerator at present. Other than this type of incinerator, there are fluidized bed furnaces, gasification melting furnaces, etc. The combustion method varies depending on the incinerator type. However, any type of incineration system is almost the same in configuration basically. Waste fed into the pit is burned in the incinerator. The heat is recovered by the boiler, economizer, etc. Then the exhaust gas is purified using environmental measures such as exhaust gas treatment and dust removal, and is discharged from the smokestack.

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 with alkali metals or heavy metals to form chlorides.

AbstractOne 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

[Seminar]

* Ebara Enviromantal Plant Co., Ltd.** Technologies, R&D Division

1. Introduction

Parts 3 and 4 of this lecture discuss high-temperature corrosion associated with incinerators. This part focuses on boilers used in waste-to-energy plants, and the next part, on other equipment.

While thermal power plants practically use steam at 600 °C or higher, in waste-to-energy plants, the steam temperature is around 400 °C, which is significantly lower than the former. Waste-based power is a significant social need as renewable energy, and efficient power generation is also required. However, high-temperature corrosion of the heat transfer tubes is a technical problem, which makes it difficult to use higher-temperature, higher-pressure steam. This part of the lecture explains corrosion of boilers in waste-to-energy plants and corrosion protection.

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**

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


Then, ash containing such chlorides flies from the incinerator to the following equipment, adhering to the heating surface of the boiler. In general, to minimize such ash adhesion, the exhaust gas is turned 180° at the bottom of the boiler to detach adhering ash, with a superheater installed in the third path of the boiler. Figure 3-2 shows an example of ash adhering to the boiler of an actual incineration system for industrial waste. Adhering ash is partially removed from the upper area (a) of the water tube of the boiler. However, no significant difference in appearance is observed among the upper, middle, and lower areas. On the

other hand, in the superheater, ash adhesion greatly varies depending on the area, in general. Ash adhesion is maximized at the superheater entrance (d). In some cases, adhering ash grows in a comb-like shape toward the upstream of the gas flow. Toward the downstream of the gas flow, however, the amount of adhering ash gradually decreases with almost no adhering ash observed at the superheater exit (f) in many cases. Ash adhesion may not only decrease the heat transfer efficiency of boilers but also cause gas blockage in a superheater. Therefore, it is usual that adhering ash is periodically removed during operation using soot blowing which blows steam or the like.

2.2 Characteristics of adhering ash

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 and the water tube side. The photo of ash adhering to the water tube (shown in Figure 3-2 (a) to (c)) indicates that a hard cream-colored substance like a crispy rice bar is observed on the gas side. However, inside the above substance, there is a light green dense substance, inside which there is a reddish brown corrosion product covering the surface of the water tube. The ash adhering to the superheater (shown in Figure 3-2 (d) to (f)) is not as clear as the ash that adheres to the water tube, but the properties of gas side and the tube side are different. Except the ash in Area (f), where the

Water tube

Flue gas

Tube sideGas side

(f)(a) Upper area

(c) Lower area

(b) Middle area (e)

(d)

SH

SH

SH

SH

Boiler diagram：Sampling locations SH：Superheater

Fig. 3-2　Example of ash adhering to the boiler of an actual waste-to-energy plant

Secondary blowerSecondary blower

Waste craneWaste crane

To atmosphereTo atmosphere

Deodorizationunit

Deodorizationunit

Forced draftfan

Forced draftfan

PlatformPlatform

Weighing scaleWeighing scale

Input bunkerInput bunker Storage bunkerStorage bunker

RefusefeederRefusefeeder

Drying stokerDrying stoker

Combustionstoker

Combustionstoker

Post-combustion stokerPost-combustion stoker

Waste hopperWaste hopper

BoilerBoiler

Flow of wasteFlow of wasteFlow of flue gas Flow of flue gas Flow of airFlow of air

Flow of steamFlow of steamFlow of ash and dustFlow of ash and dust

Flow of waterFlow of water

DesuperheaterDesuperheaterSuperheaterSuperheater

EconomizerEconomizerRecycledwater

Bag filterBag filter

IncineratorIncinerator

High-pressure steam headerHigh-pressure steam header

Induced draft fanInduced draft fan

StackStack

Steam turbinegenerator

Steam turbinegenerator

Steam condenserSteam condenser

Ash conveyorAsh conveyor

AshbunkerAshbunker

Solidifiedmaterial pitSolidifiedmaterial pit

From otherincineratorsFrom otherincinerators

Collected dust conveyorCollected dust conveyor

Collected dust andash handling systemCollected dust andash handling system

To the final landfill site

From otherincineratorsFrom otherincinerators

Fallen ashconveyorFallen ashconveyor

Air preheaterAir preheater

CondensatetankCondensatetank

Chemical siloChemical silo

Fig. 3-1　Flowchart of a waste-to-energy plant1)



amount of adhering ash is small, we identified the ash collected at each location as the gas side one or the tube side one, and analyzed them. Table 3-13) shows the analytical results. The ash on the gas side contained almost no component of heat transfer tubes, such as Fe. On the other hand, the ash on the tube side contained a large amount of corrosion products and up to nearly 50 % of the heat transfer tube components. For this reason, these values removed Fe, Cr, and Ni and converted to 100 %. The table also shows the results of analysis on the (collective) ash taken from the equipment following the boiler, which is considered to have passed through the boiler. Comparison between the collective ash and adhering ash indicates that the adhering ash contains higher concentrations of Cl, S, alkali, and heavy metals, which denotes that these elements are likely to be concentrated in the adhering ash. Then, comparison between the tube side ash and gas side ash indicates that the tube side cooled by the heat transfer tube contains higher concentrations of Cl, alkali, and heavy metals, which denotes that these chlorides are likely to be concentrated in lower-temperature areas. In connection with the ash adhering to the water tube, while the surface temperature of the water tube is almost constant regardless of height, the exhaust gas temperature drops as the height decreases. Consequently , Cl and heavy metals are more concentrated in the upper and middle areas, where the exhaust gas temperature is higher, than in the lower

area. The higher the exhaust gas temperature becomes, the higher concentration heavy metal chlorides, especially Pb, tend to have. On the other hand, as well as Ca, S tends to be more concentrated mainly in the ash adhering to the water tube on the gas side. It is considered that S is concentrated as Ca sulfate in the areas where the exhaust gas and surface temperatures are high.

As described above, the temperature range of concentration varies depending on the compound, and chlorides, which have a significant effect on corrosion, are likely to be more concentrated in the areas where the exhaust gas temperature is higher and the temperature of the ash adhering surface is lower. A possible cause of this is that chlorides and other substances existing as vapor in the exhaust gas are precipitated as adhering ash in low-temperature areas. It is likely that chlorides are more concentrated in the tube side, where the temperature is lower than in the gas side, because chlorines volatilize and cannot exist stably on the gas side. The vapor component seems likely to be concentrated by the difference in temperature between the atmospheric gas and the surface to which ash adheres; the higher the gas temperature becomes, the more concentration is promoted. With these facts, adhering ash is affected by both the gas and metal temperatures, changing the ash composition, and as a result, causing the corrosive environment to change. Figure 3-3 shows a photo of

Table 3-1　Analysis results of adhering ash3)

LocationWater tube Superheater Collected

ash N＝11

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

pH 6.2 10.13 5.81 9.82 6.78 8.56



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 known that the corrosion rate greatly increases as ash melts. For example, through lab testing using ash having a melting point of 480 °C, Kawahara et al. verified that there is a large difference in corrosion rate between 450 °C and 500 °C, which are respectively below and above the melting point (Figure 3-4)4). Some descriptions5)-7) have been provided as a corrosion mechanism. One of them is shown in Figure 3-56). The key corrosion reaction is considered to be chlorination based on Cl2 gas. Typical examples of Cl2 generation are shown below:

2NaCl＋SO2＋O2→Cl2＋Na2SO4 ………………… (3-1)4HCl＋O2→2Cl2＋2H2O …………………………… (3-2)4FeCl2＋3O2 →2Fe2O3＋4Cl2 ……………………… (3-3)Formula (3-1) states that Cl2 is generated in the

process where a chloride reacts with SO2 to form sulfate. Formula (3-2) states the Deacon reaction, which is known to proceed using adhering ash as the catalyst8). It is likely that molten ash accelerates these reactions and helps generate Cl2, which is highly

Fig. 3-4　Dependence of corrosion rates on temperature verified through lab testing4)

SiCSiC ChlorideChloride

B: Area where SiC was removed after operationB: Area where SiC was removed after operation

A: Area where SiC peeled offA: Area where SiC peeled off

CementCement

Fig. 3-3　Photo of the furnace wall of the path of grate-type incinerator 1 (area where SiC peeled off)



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 relationship between the molten phase ratio and the corrosion mass loss, determined through burial testing10). As shown in the above figure, it is known that the corrosion rate increases as the molten-phase ratio increases, but the rate decreases once the ratio exceeds a certain level. This decrease is caused by a decrease in gas permeability in association with an increase in molten-phase mass, indicating that supply of gas is essential for corrosion to advance.

The main chlorine components of adhering ash are

alkali sulfate and chlorides, whose eutectic temperature is approximately 510 °C. If various compounds are mixed with them, the melting point is lowered. Typical components of such compounds are heavy metals. Typical heavy metals contained in waste are Cu, Zn, and Pb. Figure 3-711) shows the relationship between the concentration of heavy metals contained in adhering ash and the melting point determined based

Fig. 3-6　Relationship between corrosion mass loss and molten-phase/mass determined through burial testing10)

Cl2 formationCl2 formation

OxidationreactionOxidationreaction

OxidationreactionOxidationreactionChlorinationreactionChlorinationreaction

Diffusion ofCr, Fe, etc.Diffusion ofCr, Fe, etc.

MainreactionsMainreactions

Ni-Cr-Mo-(Nb, Fe) AlloyNi-Cr-Mo-(Nb, Fe) Alloy

MoMo NiNi NbNb FeFe CrCr

Metal temperature fluctuations

Metal temperature fluctuations

Partial molten salt corrosionPartial molten salt corrosion

Corrosion productsCorrosion products

Layer of CrCl2/FeCl3/NiCl2Layer of CrCl2/FeCl3/NiCl2

Layer of Cr2O3/MoO2/Nb2O5Layer of Cr2O3/MoO2/Nb2O5

Layer of NiO / Fe3O4Layer of NiO / Fe3O4

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

Reaction between ash and scaleReaction between ash and scale

Molten ash (repeated adhering and peeling off)Molten ash (repeated adhering and peeling off)

Spraying of high-pressure steamSpraying of high-pressure steam

Destruction and peeling of scaleDestruction and peeling of scale

Entry of molten saltEntry of molten salt

Micro cracksMicro cracks

Entry of molten salt and vapor component

Entry of molten salt and vapor component

Entry ofcombustion gasEntry of

combustion gas

K2SO4N2ClKClCl2O2

K2SO4N2ClKClCl2O2

O2O2 SO3SO3H2OH2OHClHCl

Temperature gradientGas temperature fluctuationsTemperature gradientGas temperature fluctuations

Soot blowingSoot blowing

Fig. 3-5　Scale structure of an alloy of Ni-Cr-Mo-(Nb, Fe) and corrosion mechanism



on TG-DTA (thermogravimetry-differential thermal analysis). As shown in the figure, the melting point drops as the concentration of heavy metals increases. The melting point begins to drop sharply at around a 10 % of concentration and continues to drop until at about 320 °C. Thus, heavy metals drop the melting point of ash, promoting corrosion.

3.2 Example of thickness reduction in actual

equipment (water tube)

In mainstream waste-to-energy plants at present, the steam pressure is about 4 MPa, and the surface

temperature of most water tubes is 300 °C or lower, which means a milder corrosive environment than that of superheaters. Nevertheless, in this environment, due to the high exhaust gas temperature, corrosion may promote thickness reduction. Motoi et al. reported that low-air-ratio combustion promotes thickness reduction of water tubes, which is promoted mainly by destruction of protective scale caused by temperature fluctuations12).

Other than this, as described in section 2-2, a high exhaust gas temperature may concentrate heavy metals, causing thickness to reduce. Figure 3-8 shows the results of analyses based on an EPMA (electron probe microanalyzer) conducted on water tube cross sections that were exposed to high-temperature exhaust gas, resulting in annual thickness reduction of up to 0.4 mm. This analysis says that Cl was found to be concentrated in the interface between the scale and base material in particular, and heavy metals were also detected. The figure shows the results of the analysis of adhering ash collected from a water tube subject to thickness reduction and from a sound tube downstream of exhaust gas. The downstream ash collected from the unaffected tube contained approximately 5 % Cl and

700

650

600

550

500

450

400

350

3000 10 20Content of heavy metals (Pb + Cu + Zn) in adhering ash　%

Industrial waste incinerator A

Incinerator A

Incinerator B

Incinerator C

Melting point of adhering ash　℃

30 40

Fig. 3-7　Relationships between melting point and the heavy metal content in adhering ash11)

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

mass%

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



several percent Pb, and the melting point was higher than 400 °C. On the other hand, the ash collected from the corroded area contained more than 10 % Pb and far more than 10 % Cl. The TG-DTA analysis result indicates that the melting peak is around 330 °C. This probably means that concentrated heavy metals deter iorated the corros ive environment and consequently promoted corrosion.

Corrosion may even advance while the plant is not in operation. Figure 3-9 shows photos of the appearance of a removed water tube. The tube was dry just after reception, but was found to be entirely wet about two weeks after indoor storage. The pH of adhering ash is normally is near neutral (Table 3-1 and Figure 3-8). However, this analysis shows that the sample ash’s pH is approximately 2, which is strongly acidic, and 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.

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 the area starting from a point of 800 mm from the wall toward the center. In the longitudinal direction, thickness

reduction starts to increase at a point of 700 mm from the incinerator wall, while it starts to decrease at a point of 800 mm from the wall. This indicates that the installed protector in the area starting from a point of 800 mm from the wall greatly retards thickness reduction. On the other hand, thickness reduction is the worst around a point of 700 mm probably because of soot blowing. However, that thickness reduction increases in the tertiary and secondary superheaters, which are heavily corroded, but no noticeable thickness reduction is observed in the primary superheater. In other words, soot blowing has a greater influence on more heavily corroded areas. Figure 3-11 shows the comparison of thickness reduction between normal areas and areas

→Area with a protector→Area with a protector

Distance from incinerator wall　mmDistance from incinerator wall　mm

Loss of outer diameter　mm

Fig. 3-10　Tendency of thickness reduction of superheaters (dependence on the distance from the incinerator wall)

Just after receptionJust after reception About two weeks after indoor storageAbout two weeks after indoor storage

pH check resultpH check result

Fig. 3-9　Photos of appearance of a water tube sample



affected by soot blowing in actual equipment, including equipment in other plants14). The broken line in the chart represents points where the thickness reduction in normal areas is equal to the thickness reduction in the areas affected by soot blowing. The larger the thickness reduction, the more the distance from the broken line. While erosion and promotion of corrosion are considered as effects on thickness reduction by soot blowing, it can be determined that the main effect is corrosion promotion due to destruction of protective scale, because more heavily corroded areas are more likely to be affected by soot blowing.

For actual superheaters, thickness reduction is often considered to be the largest at 45° against the gas flow, which has been actually verified in many cases. Figure 3-12 shows an example of a cross section of a superheater, which indicates that thickness reduction is the largest in

the direction of 45° (315°). Figure 3-13 shows the tendency of thickness reduction in the circumferential direction. In both the upper and lower areas of the superheater, thickness reduction is smaller in the direction from 135° to 225°, where the amount of adhering ash is small because of on the downstream side of the gas flow. This indicates that adhering ash promotes corrosion. For the lower area of the tertiary superheater, thickness reduction is larger in the 45° (315°) direction than in 0° direction, where the amount of adhering ash is the largest. This shows the same tendency as in Figure 3-12. This is probably because of gas permeability. In the 0° direction area, to which gas is directly applied, much ash adheres and consequently the gas permeability is low. Therefore, it is likely that thickness reduction proceeds in the 45° direction area, to which ash adheres but does not extremely lower the gas permeability. Figure 3-10 also indicates that adhering ash effectively retards thickness reduction; in the lower area of the tertiary superheater, thickness reduction decreases as the distance to the incinerator wall becomes closer. It is considered that on the wall side of the lower area of the tertiary superheater, the gas flows quickly promoting extreme ash adhesion and causing gas permeability to be extremely lowered. On the other hand, on the upper area of the superheater, ash adheres to the underside of the tube and the upper side is affected by soot blowing. Thus, it is considered that thickness reduction proceeded in the horizontal 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 reduction proceeds in the lower area, which is exposed 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 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 Section 2-2 and pointed out in a reference15), it is likely that a rise in the exhaust gas temperature makes chlorides become more concentrated, resulting in a more severe corrosive environment.

These facts indicate that thickness reduction in

5

4

3

2

1

00 1 2 3

Thickness reduction in unaffected areas　mm

Additional thickness reduction caused by SB

Alloy625HR11NHC-22JHN24

Thickness reduction in the areas affected by SB　mm

Fig. 3-11　Influence of soot blowing (SB) on thickness reduction14)

Gas flow

180°

0°

45°

90°

135°225°

270°

315°

Fig. 3-12　Optical photo of a cross section of a superheater tube taken from a boiler for industrial waste power generation



superheaters is closely related to adhering ash and that exhaust gas alone has a minor influence on corrosion. They also indicate that the exhaust gas temperature 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 depends on each area. Soot blowing promotes thickness reduction, particularly in heavily corroded areas.

Figure 3-14 shows time series changes in thickness reduction in superheaters and a photo of a cross section of adhering ash. Observation of the cross section revealed that the formed scale is porous with a layered structure, which does not provide a dense protective scale; from a macroscopic viewpoint of time, thickness reduction is considered to proceed repeating destruction and regeneration of scales. Therefore, it is probable that the time dependence of thickness reduction did not follow the parabolic rate law described in Part 1, and thickness reduction proceeds linearly in the actual plant as shown in the figure. In addition, as one of the

ResinResin

scalescale

Adhering ashAdhering ash

Ash adhering to a superheater

Superheater tube side

Gas side

Incinerator 1Incinerator 2Tubes taken out for inspection

1.0

0.8

0.6

0.4

0.2

0.00.0 0.5 1.0

Normal operation time (unit of 10000 hours)

Thickness reduction　mm

1.5 2.0

Fig. 3-14　Time series changes in thickness reduction in superheaters and photo of a cross section of adhering ash

Angle against gas flow　°Angle against gas flow　°

Thickness reduction　mm

260 ℃380 ℃

450 ℃

（a）

（b）

（c）

（d）

（e）

（f）

530 ℃

600 ℃

330 ℃

370 ℃

285 ℃

400 ℃

310 ℃

Steam temperature

：Soot blowing

Flue gastemperature

Primary SHSTB410

Secondary SHSUS309J2

Tertiary SHSUS310

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



characteristics of this environment, there is an incubation period when the corrosion rate is low, but after a certain duration has elapsed, the corrosion rate may increase16). Since the surface of a new heat transfer tube is smooth and almost even, adhering ash can be easily removed by soot blowing. If the surface of an aged tube becomes uneven due to corrosion or the like, removal of adhering ash is expected to be difficult. This means that the corrosive environment of the surface is different between new and aged heat transfer tubes. Thus, the incubation period probably includes a time that allows corrosive substances contained in adhering ash to stably stay on the surfaces of heat transfer tubes.

4. Corrosion protection

4.1 Corrosion inhibition method

In recent years, low-air-ratio combustion has been mainly used for incinerators. For their water tubes, corrosive environments become more severe. Accordingly, corrosion protection is often required. As corrosion protection, surface treatment by thermal spraying or cladding of a corrosion-resistant alloy is often used7), 15), 17), 18). Self-fluxing alloys and overlaying are expected to have a long life. However, because these measures cannot be widely used in actual work sites, they are applied to mainly new products in factories. For on-site repair, thermal spraying or the like is applied. However, it allows corrosive components to reach the interface between the coating layer and base material through open pores existing in the coating layer, corroding the base material. As a result, coating layer detachment often occurs. To prevent this, the following are also used: HVOF (high velocity oxygen fuel) thermal spraying with a low porosity or sealing with Al. In a case, for example, where gas flows fast and erosion occurs partially, a protector is provided to the affected areas.

As described in section 3-3, with respect to superheaters, soot blowing is a large factor that accelerates damage and promotes thickness reduction particularly in heavily corroded areas. Accordingly, measures against this are required. The most commonly used countermeasure is installation of a protector. However, if partial pressure of chlorides contained in

exhaust gas is extremely high, protectors may promote thickness reduction3). Rich chloride vapor may be concentrated in gaps between the heat transfer tube and protector, without being removed by soot blowing or the like, staying as highly concentrated chlorine to cause unusual corrosion. In addition, if the gap between the heat transfer tube and protector increases, the surface temperature of the protector increases and promotes corrosion. For this reason, the protector should closely contact the heat transfer tube. If corrosion damage is small, acceleration by soot blowing is retarded. Therefore, it is also possible to clad the surface with a highly corrosion-resistant alloy, such as an Ni-based alloy, to retard corrosion for inhibiting thickness reduction. In addition, instead of soot blowing, there is rapping-based method for removing ash by vibrating heat transfer tubes, which is used in the Tsukui pilot plant of the NEDO project15). However, this method requires more floor space to use horizontal-type superheaters and more construction costs. The steel ball drop method is also put into practical use19).

The corrosive environment for boilers used for waste-to-energy power generation is much more severe than that for thermal power generation because alkali metals and other chlorides are generated in large quantities. Only comparison of the amounts of chlorine contained in the fuel do not make sense, because black liquor, coal, and other fuels may contain an amount of chlorine similar to that of waste. Since the difference between them is the amount of sulfur, increasing that of sulfur is proposed to ease corrosive environments20).

4.2 Prediction of the corrosion rate

As described so far, what hinders improvement of the steam conditions is corrosion of superheaters and increased costs due to corrosion. In other words, to increase the steam temperature, the increase in the revenue from power selling achieved by increased efficiency in power generation must exceed the increase in cost caused by increasing the steam temperature. In short, it is required that the corrosion rate can be predicted based on the design conditions including the steam conditions. The following prediction formula allows you to evaluate the effects of the gas and metal temperatures on corrosion and to determine



the optimal superheater position. As a previous study, the following formula for predicting the corrosion rate and the range of the data used is offered as the valuable results with the NEDO project14).W=10-33.8・［Tg］5.65・［Tm］4.86・［HCl］0.576・　［Cl］0.419・［Cr+Ni+Mo］-0.391・Time …………… (3-4)

W : Corrosion rate (mm)Tg : Gas temperature (°C) 583 to 675 °CTm : Metal temperature (°C) 450 °C and 550 °CHCl : Concentration of HCl in combustion gas (ppm) 568 to 1420 ppmCl : Concentration of Cl in adhering ash (%) 0.28 to 10.5 % Cr + Ni + Mo : Total content of the elements contained

in the material (%) 3.03 to 96.5 % Time : HourThis prediction formula (NEDO formula) was

statistically made according to the thickness reduction data obtained through exposure tests on actual equipment and from demonstration plants, based on the knowledge obtained through lab testing. It is a breakthrough because the corrosion rate in a complicated waste combustion environment can be accurately predicted using relatively simple variables. However, since it is a regression formula, there is a risk to applying it to conditions beyond the data range. The NEDO formula assumes a steam temperature of 500 °C. Compared with the time when the formula was made, separate waste collection has prevailed. Accordingly, the Cl content in fuel tends to decline. Thus, it is not desirable to apply the NEDO formula to typical boilers at present, which use a steam temperature of 400 °C. As a result, the formula must be modified21). Figure 3-15 shows an example of application of the NEDO formula to an existing 400 °C boiler. The solid line in the table represents a relationship of the actually measured values and calculated values being equal, which predicts a risk because the calculated values are significantly lower than the actually measured values. Figure 3-16 shows the results of a change in the NEDO formula variables using the data on an existing incinerator. As shown in the figure, the correction has significantly lessened the discrepancies between the actually measured and calculated values.

In other words, it is found that a sufficiently accurate predict ion is poss ib le with a NEDO formula appropriately modified based on its basic idea.

If the formula is used in the planning phase, the temperature and the alloy composition are design conditions, but the concentrations of HCl and Cl vary depending on the material and other factors. Figure 3-17

and Figure 3-18 show the calculated transfer rates of the exhaust gas and fly ash of Cl in the material22). It is found that the transfer rate varies depending on the incinerator type, and that grate-type incinerators are more likely to allow Cl to transfer to exhaust gas than the other types, and fluidized bed incinerators tend to allow Cl to transfer to fly ash. By understanding these relationships, it is possible to predict the HCl concentration, etc. based on the fuel composition, offering more accurate predictions of corrosion rates. As a result, superheaters with optimized life cycle costs (LCC) can be designed, which contributes to improvement of efficiency using high-temperature steam.

0.0

－0.5

－1.0

－1.5

－2.0

－2.5

－3.0

－3.5－3.5 －3.0 －2.5 －2.0 －1.5 －1.0 －0.5 0.0

log (actually measured value)　mm

Calculated value　= Actually measured value

log (calculated value)　mm

Fig. 3-15　Comparison of actually measured corrosion rates of a superheater and calculated corrosion rates determined by the NEDO formula

0.0

－0.5

－1.0

－1.5－1.5 －1.0 －0.5 0.0

log (actually measured value)　mm

Calculated value = Actually measured value

log (calculated value)　mm

Fig. 3-16　Comparison of actually measured corrosion rates of a superheater and calculated corrosion rates determined by the corrected formula



5. Conclusion

Compared with thermal power generation and other types, waste-to-energy power generation is subject to a significantly severe corrosive environment, which is caused by adhering ash that contains a large amount of chlorides. For a good understanding of corrosion development in the above environment, it is required to understand influences and behaviors of adhering ash. Chlorides transform into fly ash and adhere to heat transfer tubes, resulting in a severe corrosive environment. Lab testing, etc. has demonstrated that molten ash significantly increases the corrosion rate. Corrosion is mainly caused by corrosive Cl2 generated from chlorides contained in adhering ash and/or HCl contained in exhaust gas. Molten salt is considered to promote Cl2 generation. The properties of adhering ash depend on the exhaust gas temperature, the surface temperature of heat transfer tubes, and other factors. For corrosion protection, countermeasures are required to inhibit the corrosive environment. For example, providing the surface treatment to water tubes with a high corrosion-resistance al loy and instal l ing

superheater tubes by taking the gas temperature into account. To optimize the superheater layout, it is necessary to accurately predict the corrosion rate of heat transfer tubes. As a typical formula, the NEDO formula is available. Predictions with sufficient accuracy is possible using a customized NEDO formula while following its basic idea.

In addition, since adhering ash hinders heat transfer and causes blockages, ash is usually removed by soot blowing today. However, because soot blowing has a drawback that it destroys protective scales and promotes corrosion, the areas affected by soot blowing require measures against corrosion, such as a protector.

For further reduction of the LCC and improvement of efficiency of waste-to-energy power generation, development of technologies is required, including corrosion protection technologies for surface treatment, more accurate corrosion prediction technologies, and ash removal technologies as alternatives to soot blowing.

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40%35%30%25%20%15%10%5%0%T

ransfer rate to exhaust gas First analysis

Grate-typeincinerator

Fluidizedbed incinerator

Fluidized-bedgasification meltingincinerator

Second analysis

Fig. 3-17　Transfer rate of Cl in material to exhaust gas22)

100%

80%

60%

40%

20%

0%

Transfer rate to fly ash

First analysis

Grate-typeincinerator

Fluidizedbed incinerator

Fluidized-bedgasification meltingincinerator

Second analysis

Fig. 3-18　Transfer rate of Cl in material to fly ash22)



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