复旦大学：《材料失效分析 Materials Failure Analysis》课程教学资源（教学案例）18. Corrosion Evaluation of One Dry Desulfurization Equipment-Circulating Fluidized Bed Boiler

Materials and Design 32(2011)671-681 Contents lists available at ScienceDirect Materials and design ELSEVIER journalhomepagewww.elsevier.com/locate/matdes Corrosion evaluation of one dry desulfurization equipment-Circulating fluidized bed boiler Yi Gong, Zhen-Guo Yang Department of Materials Science, Fudan University, Shanghai 200433, PR China ARTICLE INFO A BSTRACT As a clean fuel combustion technology, circulating fluidized bed(CFB) possesses various advantage Received 12 May 2010 mong them, flexibility in fuels and superiority in desulfurization are the two prominent ones and can Available online 7 August 2010 hereby facilitate sufficient utilization of high-sulfur fuels. But unfortunately, these low-grade fuel always introduce harsh service environment within the Cfb boilers and consequently result in severe degradation extent on relevant equipments, especially the high-temperature sulfur corrosion. In this eywords Failure analysis ent, by nearly ten characterization methods, comprehensive investigation was carried out on a whole CFB boiler during downtime, and special emphasis was particularly laid on the failure components ncluding one perforated nozzle along with its fractured inlet tube for primary air, and one perforated manhole door of refeed valve. Finally, countermeasure and suggestion was put forward, which can pro- vide instructive significance in corrosion prevention for the CFB boilers, even other desulfurization equip- ments, running under similar aggressive conditions in engineering practice. e 2010 Elsevier Ltd. All rights reserved 1 Introduction especially popularized in China in order to sat er natural con- dition of high reserves of low-grade coals [4 Statistically, in the With the increasing demand for energy conservation and envi- year 2008 over three quarters of total installed capacity of China ronmental protection, higher utilization of fossil fuels and lower was fossil power, and among which 66%(379 million kw) was mission of air pollution are presently the two prior concerns to from those plants installed with desulfurization equipments [5- fossil power plants. As for the former one, popularization of new- Hence, normal and safe operation of these FBC and FGD equip- generation ultra-supercritical (USC) boilers is the most effective ments is of critical importance for China [6 and attractive approach, and our previous work carried out an Only in terms of the FBC, three variants have been evolved since integrity evaluation of the dissimilar steels welded joints that are its introduction in 1970s, including bubbling fluidized bed(BFB). often encountered in these USC boilers [1]. with respect to the lat- circulating fluidized bed( CFB)and a hybrid type between BFB and ter one(air pollution). the sulfur pollution, which commonly refers CFB[ 7. Among them, CFB is presently the most universal FBC tech- to the sulfur dioxide, is actually the most hazardous factor result ogy thanks to its relatively higher combustion efficiency than ing in acid rain. So as to relieve the extent of this kind of pollution, BFB. Also for China, she now owns the largest amount and thermal there currently exist two common ways of desulfurization, one is capacity of CFB boilers in the world as well [8. The unique feature of the fluidized bed combustion(FBC) technology and the other is a Cfb boiler compared with the conventional boilers is the added the flue gas desulfurization(FGD) process. FBC is virtually a type equipment called cyclone, which is used to refeed the incompletely f dry desulfurization and desulfurizes simultaneously with ombusted fuel particles and ashes back into the furnace for re-fir bustion under dry condition in furnace, while FGD is a sort of ing, i.e. the circulating function. As a result, fuels can be fully utilized wet desulfurization and needs specific exteriorized FGD equip- and the sulfur in them can be sufficiently eliminated before being ments for desulfurizing amid wet condition. In fact, compared with exhausted. In addition, configurations of CFB boilers usually vary the conventional fossil power plants, the most distinct advantage according to different companies'designs, and the two leading ones of FBC and FGD is their supreme flexibility in fuels, such as coal, are from Foster Wheeler (FW, USA Finland) and gEc-alstom oil, biomass, peat, petrol coke and so on, particularly for those (France)[4. However in fact, high desulfurization efficiency com- w-grade fuels with high sulfur content [2,3]. Consequently, these monly brings about harsh service environment for the CFB boilers two kinds of world-widely used desulfurization technologies are at the same time, especially for those fire high-sulfur fuels like the petrol coke, and will consequently result in degradations on rele- Corresponding author. Tel: +86 21 65642523: fax: +86 21 65103056. ant equipments [9-15. But actually, large amount of the past re- searches mainly focused on the heat transfer efficiency [16-19 0261-3069/s- see front matter o 2010 Elsevier Ltd. All rights reserved. doi:10.1016 mates.2010.08.003

Corrosion evaluation of one dry desulfurization equipment – Circulating fluidized bed boiler Yi Gong, Zhen-Guo Yang * Department of Materials Science, Fudan University, Shanghai 200433, PR China article info Article history: Received 12 May 2010 Accepted 3 August 2010 Available online 7 August 2010 Keywords: Failure analysis Corrosion Fracture abstract As a clean fuel combustion technology, circulating fluidized bed (CFB) possesses various advantages. Among them, flexibility in fuels and superiority in desulfurization are the two prominent ones and can hereby facilitate sufficient utilization of high-sulfur fuels. But unfortunately, these low-grade fuels always introduce harsh service environment within the CFB boilers and consequently result in severe degradation extent on relevant equipments, especially the high-temperature sulfur corrosion. In this event, by nearly ten characterization methods, comprehensive investigation was carried out on a whole CFB boiler during downtime, and special emphasis was particularly laid on the failure components including one perforated nozzle along with its fractured inlet tube for primary air, and one perforated manhole door of refeed valve. Finally, countermeasure and suggestion was put forward, which can pro￾vide instructive significance in corrosion prevention for the CFB boilers, even other desulfurization equip￾ments, running under similar aggressive conditions in engineering practice. 2010 Elsevier Ltd. All rights reserved. 1. Introduction With the increasing demand for energy conservation and envi￾ronmental protection, higher utilization of fossil fuels and lower emission of air pollution are presently the two prior concerns to fossil power plants. As for the former one, popularization of new￾generation ultra-supercritical (USC) boilers is the most effective and attractive approach, and our previous work carried out an integrity evaluation of the dissimilar steels welded joints that are often encountered in these USC boilers [1]. With respect to the lat￾ter one (air pollution), the sulfur pollution, which commonly refers to the sulfur dioxide, is actually the most hazardous factor result￾ing in acid rain. So as to relieve the extent of this kind of pollution, there currently exist two common ways of desulfurization, one is the fluidized bed combustion (FBC) technology and the other is the flue gas desulfurization (FGD) process. FBC is virtually a type of dry desulfurization and desulfurizes simultaneously with com￾bustion under dry condition in furnace, while FGD is a sort of wet desulfurization and needs specific exteriorized FGD equip￾ments for desulfurizing amid wet condition. In fact, compared with the conventional fossil power plants, the most distinct advantage of FBC and FGD is their supreme flexibility in fuels, such as coal, oil, biomass, peat, petrol coke and so on, particularly for those low-grade fuels with high sulfur content [2,3]. Consequently, these two kinds of world-widely used desulfurization technologies are especially popularized in China in order to satisfy her natural con￾dition of high reserves of low-grade coals [4]. Statistically, in the year 2008 over three quarters of total installed capacity of China was fossil power, and among which 66% (379 million kW) was from those plants installed with desulfurization equipments [5]. Hence, normal and safe operation of these FBC and FGD equip￾ments is of critical importance for China [6]. Only in terms of the FBC, three variants have been evolved since its introduction in 1970s, including bubbling fluidized bed (BFB), circulating fluidized bed (CFB) and a hybrid type between BFB and CFB [7]. Among them, CFB is presently the most universal FBC tech￾nology thanks to its relatively higher combustion efficiency than BFB. Also for China, she now owns the largest amount and thermal capacity of CFB boilers in the world as well [8]. The unique feature of a CFB boiler compared with the conventional boilers is the added equipment called cyclone, which is used to refeed the incompletely combusted fuel particles and ashes back into the furnace for re-fir￾ing, i.e. the circulating function. As a result, fuels can be fully utilized and the sulfur in them can be sufficiently eliminated before being exhausted. In addition, configurations of CFB boilers usually vary according to different companies’ designs, and the two leading ones are from Foster Wheeler (FW, USA/Finland) and GEC-Alstom (France) [4]. However in fact, high desulfurization efficiency com￾monly brings about harsh service environment for the CFB boilers at the same time, especially for those fire high-sulfur fuels like the petrol coke, and will consequently result in degradations on rele￾vant equipments [9–15]. But actually, large amount of the past re￾searches mainly focused on the heat transfer efficiency [16–19], 0261-3069/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.08.003 * Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Materials and Design 32 (2011) 671–681 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes

672 le particle hydrodynamics[20-22], and the gas solid separation zles lying on the air distribution plate and the water walls attached mechanisms 23, 24. etc in CFB boilers. on the furnace inner wall, since both of the two components di- In this paper, various types of corrosion degradation such as uni- rectly contacted the corrosive fuels and the flowing particles with- orm corrosion, dew point corrosion, intergranular corrosion, ero- in the furnace and were consequently prone to failure incidents. sive wear, scaling, ablation, etc. were detected in one FW CFB Air distribution plate is commonly installed on the bottom of ler that fired high-sulfur petrol coke and pulverized coal (3: 1. the furnace and the nozzles on it are used to not only support wt%)after five-year service in a petrochemical works in Shangha he bed materials like limestone and fuels but also evenly divide Among them, perforation on a manhole door of the refeed valve and the primary air, seen in Fig. 2a. It can be also learned from fracture on an inlet tube for primary air were the two prior risks. Fig 2a that no obvious corrosion evidences were detected on al- Thus, by means of nearly ten characterization methods, causes of most all the nozzles. However, failure incident was actually discov- the two failure components were detailedly studied. Photoelectric ered on some individual one. As is shown in Fig 2b, for one specifie direct reading spectrometer and metallographic microscope(MM) nozzle, perforation occurred at the juncture between the nozzle were employed to inspect the chemical compositions and the and its inlet tube for primary air, and trace of ablation can also metallographic structures of the matrix metals: X-ray diffraction be observed around the perforation, which may have been caused (XRD), X-ray fluorescence(XRF), ion chromatograph(IC)and ther- by the high temperature effect from the localized accumulating nogravimetric analysis(TGA)were applied to analyze the charac- bed materials. Furthermore, fracture was engendered on this inlet ristics of the corrosion products; scanning electron microscope tube as well, seen in Fig. 2c. Compared with the fracture surface EM)and energy disperse spectroscope(EDS)were used to detect from manual wire cutting(the upper part in Fig. 2c). the fracture the micro morphologies and micro-area compositions of the frac- surface from failure(the lower part in Fig. 2c)was far narrower, tured surfaces Based on the analysis results, causes and mecha- even the width of the narrowest part(0.5 mm)was only one-tenth nisms of the corrosion degradations currently existing were of its original normal value(5 mm), which meant that the fracture discussed. Such a comprehensive corrosion evaluation on a whole was possibly induced by the erosive wear. Besides these, anothe FW CFB boiler whose major fuel was high-sulfur petrol coke was significant phenomenon was that the two locations of the perfora seldom reported in literatures, and it will have a critical significance tion and the fracture were on the same side of the nozzle. In other in both the solution to and the prevention of corrosions for CFB boil- words, the generations of the thinning and fracture on the inlet ers running under similar service conditions in the future. ube, as well as the perforation on the nozzle may have been re- lated to each other. To sum up, although failure took place in just 2 Visual observation one specific nozzle, further investigation was still needed to thor oughly understand its causes and mechanisms for purpose of pre- The total fossil power unit in this event was made up of two sets vention of similar failures in other nozzles in the future of 310 t/h FW CFB boilers and a 100 Mw double extraction con- Installed on the furnace inner wall, the platen water wall is al- densing steam turbine. Further detailed, Fig. 1 presents the sche- ways subjected to severe service conditions including corrosion, matic diagram of the concrete operation procedures of the CFB impact, erosive wear, etc. from fuels and bed materials. Among oilers, which were mainly composed of three systems including them, the erosive wear is the most frequent degradation. As for this the combustion system, the gas-solid separation system and the CFB boiler, that rule was verified as well, seen in Fig 3a. However steam system. The corrosion evaluation was just conducted in according to Fig 3b, the extent of the erosive wear was not serious one of the two CFB boilers, and was successively carried out accord- Meanwhile, no other obvious failure phenomena were discovered ing to the order of left to right'and 'bottom to top, basing on Fig. 1. too. Hence, it can be concluded that water wall of the CFB boiler was basically qualified after service 2. 1. Combustion system Combustion system commonly consists of a primary air cham. 2.2. Gas-solid separation system ber, an air distribution plate, a combustor, a furnace and a coal Gas-solid separation system, which is the largest distinguishing feeding system Evaluation was particularly conducted on the noz- feature of CFB, is made up of a cyclone and a refeed valve Limited petrocoke/coal fu rnace cyclone meston water wall 不不不不不 prum. air air preheater Fig. 1. Schematic diagram of the operation procedures of the Fw CFB boiler

the particle hydrodynamics [20–22], and the gas/solid separation mechanisms [23,24], etc. in CFB boilers. In this paper, various types of corrosion degradation such as uni￾form corrosion, dew point corrosion, intergranular corrosion, ero￾sive wear, scaling, ablation, etc. were detected in one FW CFB boiler that fired high-sulfur petrol coke and pulverized coal (3:1, wt.%) after five-year service in a petrochemical works in Shanghai. Among them, perforation on a manhole door of the refeed valve and fracture on an inlet tube for primary air were the two prior risks. Thus, by means of nearly ten characterization methods, causes of the two failure components were detailedly studied. Photoelectric direct reading spectrometer and metallographic microscope (MM) were employed to inspect the chemical compositions and the metallographic structures of the matrix metals; X-ray diffraction (XRD), X-ray fluorescence (XRF), ion chromatograph (IC) and ther￾mogravimetric analysis (TGA) were applied to analyze the charac￾teristics of the corrosion products; scanning electron microscope (SEM) and energy disperse spectroscope (EDS) were used to detect the micro morphologies and micro-area compositions of the frac￾tured surfaces. Based on the analysis results, causes and mecha￾nisms of the corrosion degradations currently existing were discussed. Such a comprehensive corrosion evaluation on a whole FW CFB boiler whose major fuel was high-sulfur petrol coke was seldom reported in literatures, and it will have a critical significance in both the solution to and the prevention of corrosions for CFB boil￾ers running under similar service conditions in the future. 2. Visual observation The total fossil power unit in this event was made up of two sets of 310 t/h FW CFB boilers and a 100 MW double extraction con￾densing steam turbine. Further detailed, Fig. 1 presents the sche￾matic diagram of the concrete operation procedures of the CFB boilers, which were mainly composed of three systems including the combustion system, the gas–solid separation system and the steam system. The corrosion evaluation was just conducted in one of the two CFB boilers, and was successively carried out accord￾ing to the order of ‘left to right’ and ‘bottom to top’ basing on Fig. 1. 2.1. Combustion system Combustion system commonly consists of a primary air cham￾ber, an air distribution plate, a combustor, a furnace and a coal feeding system. Evaluation was particularly conducted on the noz￾zles lying on the air distribution plate and the water walls attached on the furnace inner wall, since both of the two components di￾rectly contacted the corrosive fuels and the flowing particles with￾in the furnace and were consequently prone to failure incidents. Air distribution plate is commonly installed on the bottom of the furnace and the nozzles on it are used to not only support the bed materials like limestone and fuels but also evenly divide the primary air, seen in Fig. 2a. It can be also learned from Fig. 2a that no obvious corrosion evidences were detected on al￾most all the nozzles. However, failure incident was actually discov￾ered on some individual one. As is shown in Fig. 2b, for one specific nozzle, perforation occurred at the juncture between the nozzle and its inlet tube for primary air, and trace of ablation can also be observed around the perforation, which may have been caused by the high temperature effect from the localized accumulating bed materials. Furthermore, fracture was engendered on this inlet tube as well, seen in Fig. 2c. Compared with the fracture surface from manual wire cutting (the upper part in Fig. 2c), the fracture surface from failure (the lower part in Fig. 2c) was far narrower, even the width of the narrowest part (0.5 mm) was only one-tenth of its original normal value (5 mm), which meant that the fracture was possibly induced by the erosive wear. Besides these, another significant phenomenon was that the two locations of the perfora￾tion and the fracture were on the same side of the nozzle. In other words, the generations of the thinning and fracture on the inlet tube, as well as the perforation on the nozzle may have been re￾lated to each other. To sum up, although failure took place in just one specific nozzle, further investigation was still needed to thor￾oughly understand its causes and mechanisms for purpose of pre￾vention of similar failures in other nozzles in the future. Installed on the furnace inner wall, the platen water wall is al￾ways subjected to severe service conditions including corrosion, impact, erosive wear, etc. from fuels and bed materials. Among them, the erosive wear is the most frequent degradation. As for this CFB boiler, that rule was verified as well, seen in Fig. 3a. However, according to Fig. 3b, the extent of the erosive wear was not serious. Meanwhile, no other obvious failure phenomena were discovered too. Hence, it can be concluded that water wall of the CFB boiler was basically qualified after service. 2.2. Gas–solid separation system Gas–solid separation system, which is the largest distinguishing feature of CFB, is made up of a cyclone and a refeed valve. Limited Fig. 1. Schematic diagram of the operation procedures of the FW CFB boiler. 672 Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681

Gong, Z-G. Yang/ Material and Design 32(2011)671-681 (b) (c) Fig. 2. External appearances of the nozzles (a) layout of nozzles on the furnace bottom(b) perforation on the failure nozzle and (c) fractograph of the failure inlet tube (a) (b) (b) Fig. 4. External appearances of the manhole door A (a) the whole refeed valve, (b) scaling morphology and (c)magnification of scaling substance. to the practical conditions, investigation could only be carried out eral entirely different types of failure phenomena on the two man- on the refeed valve. But actually, perforation of one manhole door hole doors of the refeed valve, further characterization methods of the refeed valve was one of the prominent failures in this event. would be carried out to determine the causes and mechanisms of Fig. 4a displays the external appearance of the refeed valve, them. which owned two manhole doors respectively named A and b on its two sides. It can be learned from Fig. 4b that scaling had occurred on the surface of the refractory on the manhole door 3. Steam syster A, and the scaling substance was green and viscous liquid, seen Like conventional power plants, the Cfb boiler also possesses a in Fig. 4c. With respect to the failures on the manhole door B, be- complete steam to fully utilize the steam heat. the only dif- sides the self-detachment of the refractory on it, two perforations ference of this Fw ceb boiler was that the reheater was substituted could also be found (Fig 5a), and the diameter of the bigger one by the secondary superheater, seen in Fig. 1. Evaluation would be had reached about 10 cm, seen in Fig 5b. Moreover, it is obvious carried out on all the four steam equipments including the primary in Fig. 5c that scaling was also engendered at the edge, but the superheater, the secondary superheater, the economizer and the solid. different from that on the manhole door a. as there were sev Fig 6 shows the external appearances of the primary super heater. whose manhole do covered with brown rust. seen 1 For interpretation of color in Fig 5, the reader is referred to the web version of in Fig cted on even all the steam piping this article as well( Fig. 6b). Hence, it concluded that the only degrada

to the practical conditions, investigation could only be carried out on the refeed valve. But actually, perforation of one manhole door of the refeed valve was one of the prominent failures in this event. Fig. 4a displays the external appearance of the refeed valve, which owned two manhole doors respectively named A and B on its two sides. It can be learned from Fig. 4b that scaling had occurred on the surface of the refractory on the manhole door A, and the scaling substance was green and viscous liquid, seen in Fig. 4c. With respect to the failures on the manhole door B, be￾sides the self-detachment of the refractory on it, two perforations could also be found (Fig. 5a), and the diameter of the bigger one had reached about 10 cm, seen in Fig. 5b. Moreover, it is obvious in Fig. 5c that scaling was also engendered at the edge, but the scaling substance exhibited the morphology as yellow1 and grey solid, different from that on the manhole door A. As there were sev￾eral entirely different types of failure phenomena on the two man￾hole doors of the refeed valve, further characterization methods would be carried out to determine the causes and mechanisms of them. 2.3. Steam system Like conventional power plants, the CFB boiler also possesses a complete steam system to fully utilize the steam heat. The only dif￾ference of this FW CFB boiler was that the reheater was substituted by the secondary superheater, seen in Fig. 1. Evaluation would be carried out on all the four steam equipments including the primary superheater, the secondary superheater, the economizer and the air preheater in this system. Fig. 6 shows the external appearances of the primary super￾heater, whose manhole door was covered with brown rust, seen in Fig. 6a. Likewise, rust was detected on even all the steam piping as well (Fig. 6b). Hence, it can be concluded that the only degrada￾Fig. 2. External appearances of the nozzles (a) layout of nozzles on the furnace bottom (b) perforation on the failure nozzle and (c) fractograph of the failure inlet tube. Fig. 3. External appearances of the platen water wall (a) total morphology and (b) phenomenon of erosive wear. Fig. 4. External appearances of the manhole door A (a) the whole refeed valve, (b) scaling morphology and (c) magnification of scaling substance. 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article. Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 673

74 Y Gong, Z-G Yang/ Materials and Design 32 (2011)671-681 (a Fig. 5. External appearances of the manhole door B(a) two perforations, (b)size of the bigger perforation and (c)scaling on the edge of manhole door. (a) Fig. 6. External appearances of the primary superheater(a)manhole door and(b)steam piping. tion occurred on the primary superheater was uniform corrosion; superheats; on the other hand, decrease of temperature increases the hardness of these ash particles. Thus, the economizer is always Then, observation was conducted on the secondary superheater. subjected to erosive wear in service. As is shown in Fig 8a, trace of Compared with the primary superheater, the manhole door here erosive wear was indeed observed on the piping surfaces in this exhibited no obvious rust phenomenon, seen in Fig. 7a. However, economizer, and they were covered with large amount of ash dust it can be learned from Fig. 7b that uniform corrosion also occurred as well(Fig. 8b). However fortunately, no obvious corrosion was on the steam piping within this superheater detected. This may have been accounted of &s In summary, extent of degradation, ie the uniform corrosion in the fin-type configuration for the piping, which could effectively these two superheaters was not pretty serious. This may have been relieve the corrosion extent on its surfaces. To sum up, as for the relevant to the service conditions of them. As the predominant economizer, no serious corrosion but erosive wear had occurred medium that superheaters contacted was only high temperature on the piping surfaces, and only removing of the ash dust was steam, in other words, no severe corrosive environment would needed be generated under this condition. As a result, merely uniform cor Compared with the superheaters and the economizer, the air rosion that was caused by the effect from high temperature oxida- preheater suffered severer uniform corrosion on its piping wall tion was engendered upon the iron-based piping, and its extent ( Fig. 9a), and lots of brown rust had already scaled off and accumu- was acceptable lated on its bottom(some of it was then collected for further anal- Economizer is usually installed on the lower part of the steam ysis of its chemical compositions), seen in Fig. 9b. In fact, the system. Consequently, on one hand, concentration of the ash parti- service conditions of the air preheater were not as harsh as that cles in it is relatively higher than that in the upper equipments like of the upper steam equipments, why the corrosion extent of it ) Fig. 7. External appearances of the secondary superheater (a)manhole door and(b) steam piping

tion occurred on the primary superheater was uniform corrosion; however its extent was not severe. Then, observation was conducted on the secondary superheater. Compared with the primary superheater, the manhole door here exhibited no obvious rust phenomenon, seen in Fig. 7a. However, it can be learned from Fig. 7b that uniform corrosion also occurred on the steam piping within this superheater. In summary, extent of degradation, i.e. the uniform corrosion in these two superheaters was not pretty serious. This may have been relevant to the service conditions of them. As the predominant medium that superheaters contacted was only high temperature steam, in other words, no severe corrosive environment would be generated under this condition. As a result, merely uniform cor￾rosion that was caused by the effect from high temperature oxida￾tion was engendered upon the iron-based piping, and its extent was acceptable. Economizer is usually installed on the lower part of the steam system. Consequently, on one hand, concentration of the ash parti￾cles in it is relatively higher than that in the upper equipments like superheats; on the other hand, decrease of temperature increases the hardness of these ash particles. Thus, the economizer is always subjected to erosive wear in service. As is shown in Fig. 8a, trace of erosive wear was indeed observed on the piping surfaces in this economizer, and they were covered with large amount of ash dust as well (Fig. 8b). However fortunately, no obvious corrosion was detected. This may have been accounted for the application of the fin-type configuration for the piping, which could effectively relieve the corrosion extent on its surfaces. To sum up, as for the economizer, no serious corrosion but erosive wear had occurred on the piping surfaces, and only removing of the ash dust was needed. Compared with the superheaters and the economizer, the air preheater suffered severer uniform corrosion on its piping wall (Fig. 9a), and lots of brown rust had already scaled off and accumu￾lated on its bottom (some of it was then collected for further anal￾ysis of its chemical compositions), seen in Fig. 9b. In fact, the service conditions of the air preheater were not as harsh as that of the upper steam equipments, why the corrosion extent of it Fig. 7. External appearances of the secondary superheater (a) manhole door and (b) steam piping. Fig. 5. External appearances of the manhole door B (a) two perforations, (b) size of the bigger perforation and (c) scaling on the edge of manhole door. Fig. 6. External appearances of the primary superheater (a) manhole door and (b) steam piping. 674 Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681

Y Gong Z-G Yang/ Materials and Design 32(2011)671-681 (a) (b) Fig 8. External appearances of piping in the economizer(a) header and steam piping and(b)accumulation of ash dust. (a (b) Fig. 9. External appearances of piping in the air preheater (a uniform corrosion and (b)accumulation of rust. was more serious? As the steam temperature in the air preheater steel equaling to Din X15CrNiSi2012 in German Standards. The was relatively lower, the liquid phase concentration of the steam existence of Si element in these two metals can facilitate superie within it was thereby even higher. It is a common sense that resistance to oxidation at high temperature. According to the anal- ron-based materials are most apt to rust under wet and oxygen- ysis results, the two matrix metals were both qualified. rich environment. Consequently, serious uniform corrosion tool Etched in agent of CuSO4 4 g, HCl 20 ml and ethanol 20 ml, the place on the piping wall, but it would not affect the normal service metallographic structures of the inlet tube matrix metal are dis of the whole equipment. played in Fig. 10. As is shown in Fig. 10a, this material exhibited a duplex microstructure of austenite and 8 ferrite. However, as is 3. Failure analysis shown in Fig. 10b, corrosion products had penetrated into the material, i.e. the evidence of intergranular corrosion. As a result, Based on the above investigations, conclusion can be put for- the duplex microstructure would be gradually destroyed with the ard that corrosion extent of the whole fw cfb boiler ncrease of the amount of the corrosion products, and finally result serious enough. Among them, attention should be mainly n intergranular fracture on the boundaries between austenites and two components, i.e. the nozzle with its inlet tube and the ferrites under stresses doors of the refeed valve. Thus, following failure analysis will be fo- cused on them two 3. 1. 2. SEM and eDs After cutting and sampling, cross-section of the fractured inlet tube is shown in Fig. 1la, from which a brown rust layer as well as an obvious width gradient can be observed. The two phenomena 3. 1.1. Matrix metals inspection both verified the assumption mentioned above that the fracture Chemical compositions of the matrix metals of the nozzle and may have been caused by the interaction between corrosion and the inlet tube for primary air are listed in Table 1, which are erosive wear. Further magnified under sEm, two different sorts of respectively in accordance with the requirements of zG3Cr25Ni20 layers that respectively represented the matrix metal ( the com- [25 and 1Cr20Ni14Si2 [26 specifications in Chinese National Stan- pacted part in the middle, light grey color)and the corrosion prod dards. ZG3Cr25N120 represents a kind of heat-resistant cast steel, ucts(the pitted part on two sides, deep grey color)can be seen in while 1Cr20Ni14Si2 is a kind of heat-resistant austenitic stainless Fig. 11b, and the widths of the corrosion products layers had Table 1 Chemical compositions of the nozzle and the inlet tube(wt.). 051 0.20-035 18.0-220 ≤050 1Cr20Ni14Si2 0.20 120-150 50-2 ≤1.50 "l denotes the content lower than 0.5 wt%, the same below

was more serious? As the steam temperature in the air preheater was relatively lower, the liquid phase concentration of the steam within it was thereby even higher. It is a common sense that iron-based materials are most apt to rust under wet and oxygen￾rich environment. Consequently, serious uniform corrosion took place on the piping wall, but it would not affect the normal service of the whole equipment. 3. Failure analysis Based on the above investigations, conclusion can be put for￾ward that corrosion extent of the whole FW CFB boiler was not serious enough. Among them, attention should be mainly paid to two components, i.e. the nozzle with its inlet tube and the manhole doors of the refeed valve. Thus, following failure analysis will be fo￾cused on them two. 3.1. Nozzle 3.1.1. Matrix metals inspection Chemical compositions of the matrix metals of the nozzle and the inlet tube for primary air are listed in Table 1, which are respectively in accordance with the requirements of ZG3Cr25Ni20 [25] and 1Cr20Ni14Si2 [26] specifications in Chinese National Stan￾dards. ZG3Cr25Ni20 represents a kind of heat-resistant cast steel, while 1Cr20Ni14Si2 is a kind of heat-resistant austenitic stainless steel equaling to Din X15CrNiSi20.12 in German Standards. The existence of Si element in these two metals can facilitate superior resistance to oxidation at high temperature. According to the anal￾ysis results, the two matrix metals were both qualified. Etched in agent of CuSO4 4 g, HCl 20 ml and ethanol 20 ml, the metallographic structures of the inlet tube matrix metal are dis￾played in Fig. 10. As is shown in Fig. 10a, this material exhibited a duplex microstructure of austenite and d ferrite. However, as is shown in Fig. 10b, corrosion products had penetrated into the material, i.e. the evidence of intergranular corrosion. As a result, the duplex microstructure would be gradually destroyed with the increase of the amount of the corrosion products, and finally result in intergranular fracture on the boundaries between austenites and ferrites under stresses. 3.1.2. SEM and EDS After cutting and sampling, cross-section of the fractured inlet tube is shown in Fig. 11a, from which a brown rust layer as well as an obvious width gradient can be observed. The two phenomena both verified the assumption mentioned above that the fracture may have been caused by the interaction between corrosion and erosive wear. Further magnified under SEM, two different sorts of layers that respectively represented the matrix metal (the com￾pacted part in the middle, light grey color) and the corrosion prod￾ucts (the pitted part on two sides, deep grey color) can be seen in Fig. 11b, and the widths of the corrosion products layers had al￾Fig. 8. External appearances of piping in the economizer (a) header and steam piping and (b) accumulation of ash dust. Fig. 9. External appearances of piping in the air preheater (a) uniform corrosion and (b) accumulation of rust. Table 1 Chemical compositions of the nozzle and the inlet tube (wt.%). Element C Cr Ni Si Mo Mn Fe Nozzle 0.33 23.64 19.38 1.78 0.51 0.55 53.81 ZG3Cr25Ni20 0.20–0.35 24.0–28 18.0–22.0 62.0 60.50 62.0 Rest Tube 0.09 21.05 11.00 1.54 0.22 0.73 65.37 1Cr20Ni14Si2 60.20 19.0–22.0 12.0–15.0 1.50–2.50 /a 61.50 Rest a ‘‘/” denotes the content lower than 0.5 wt.%, the same below. Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 675