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 provide instructive significance in corrosion prevention for the CFB boilers, even other desulfurization equipments, running under similar aggressive conditions in engineering practice. 2010 Elsevier Ltd. All rights reserved. 1. Introduction With the increasing demand for energy conservation and environmental 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 newgeneration 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 latter one (air pollution), the sulfur pollution, which commonly refers to the sulfur dioxide, is actually the most hazardous factor resulting 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 combustion under dry condition in furnace, while FGD is a sort of wet desulfurization and needs specific exteriorized FGD equipments 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 condition 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 equipments 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 technology 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-firing, 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 commonly 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 relevant equipments [9–15]. But actually, large amount of the past researches 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 uniform corrosion, dew point corrosion, intergranular corrosion, erosive 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 thermogravimetric analysis (TGA) were applied to analyze the characteristics 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 fractured surfaces. Based on the analysis results, causes and mechanisms 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 boilers 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 condensing steam turbine. Further detailed, Fig. 1 presents the schematic 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 according 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 chamber, an air distribution plate, a combustor, a furnace and a coal feeding system. Evaluation was particularly conducted on the nozzles lying on the air distribution plate and the water walls attached on the furnace inner wall, since both of the two components directly contacted the corrosive fuels and the flowing particles within 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 almost all the nozzles. However, failure incident was actually discovered 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 perforation 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 related to each other. To sum up, although failure took place in just one specific nozzle, further investigation was still needed to thoroughly understand its causes and mechanisms for purpose of prevention of similar failures in other nozzles in the future. Installed on the furnace inner wall, the platen water wall is always 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, besides 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 several entirely different types of failure phenomena on the two manhole 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 difference 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 superheater, 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 degradaFig. 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 corrosion that was caused by the effect from high temperature oxidation 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 particles 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 accumulated on its bottom (some of it was then collected for further analysis 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 oxygenrich 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 forward 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 focused 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 Standards. 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 analysis 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 displayed 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 compacted part in the middle, light grey color) and the corrosion products (the pitted part on two sides, deep grey color) can be seen in Fig. 11b, and the widths of the corrosion products layers had alFig. 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
76 Y Gong, Z-G Yang/ Materials and Design 32 (2011)671-681 a。下 corrosion produets etallographic structures of the fractured inlet tube (a)100x and (b) polished state 200x. 31/TY/7 5E 00010020030040050060070800 0m1002003m40050607m8m Fig. 11. Morphologies and EDS of the cross-section of the fractured inlet tube(a)macroscopic morphology, (b)SEM micrograph, (c) EDS of site A and (d)EDS of site B. reached 50-500 um. Then micro-area chemical compo in site b was the existence of the sulfur element, whose content of the two sites marked as a(matrix metal )and b(corrosion was 5.21% Thus, it can be inferred now that the sulfur related cor- cts) in Fig. 11b were detected by EDS, and the results were rosion may have been mainly blamed for the fracture of the inlet in Fig. 11c and d and Table 2. The most obvious characteristic tube and further discussion is needed to determine its causes Ind mechanisms Chemical compositions of site A and B(wt%). 3. 2. Manhole doors ement C Cr Fe Inspection 08130.1345.7610.6 16.559520.891582263425.675.21 Chenical mma ositions of the matrix metals of the two manhole in table 3. which are in accordance with the
ready reached 50–500 lm. Then micro-area chemical compositions of the two sites marked as A (matrix metal) and B (corrosion products) in Fig. 11b were detected by EDS, and the results were listed in Fig. 11c and d and Table 2. The most obvious characteristic in site B was the existence of the sulfur element, whose content was 5.21%. Thus, it can be inferred now that the sulfur related corrosion may have been mainly blamed for the fracture of the inlet tube, and further discussion is needed to determine its causes and mechanisms. 3.2. Manhole doors 3.2.1. Matrix metal inspection Chemical compositions of the matrix metals of the two manhole doors are listed in Table 3, which are in accordance with the Fig. 10. Metallographic structures of the fractured inlet tube (a) 100 and (b) polished state 200. Fig. 11. Morphologies and EDS of the cross-section of the fractured inlet tube (a) macroscopic morphology, (b) SEM micrograph, (c) EDS of site A and (d) EDS of site B. Table 2 Chemical compositions of site A and B (wt.%). Element C O Si Cr Fe Ni S Site A 12.49 / 0.81 30.13 45.76 10.61 / Site B 16.55 9.52 0.89 15.82 26.34 25.67 5.21 676 Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681
Gong, Z-G. Yang/ Material and Design 32(2011)671-681 loor was probably the ferrous sulfate FeSO4, whose feature color is nical compositions of the two manhole doors(wt.6). In order to confirm the element compositions in the black cor- rosion product, XRF was employed. The results showed that the Manhole door B 87. 229 two primary elements were Fe and S, seen in Table 4. However, GB→Asi6Cu4 the detailed sorts of these substances should be further identified As is shown in Fig. 14a, the black solid corrosion product con- requirements of casting aluminum alloy specifications in GB-ALSi6. sisted of a large amount of compounds. Among them, the ferrous Cua standard of China(27). In it, Si, Cu and Zn elements are used to sulfate hydrate( FeSoa4H2 0)was the predominant one according improve the hardness, castability, corrosion resistance of the mate- to the standard powder diffraction file(PDF)card. It can be easily rere also both qualified. components with matrix metals of iron-based materials in the a n al. Based on the results. matrix metals of the two manhole doors inferred that the ferrous sulfate was the corrosion product of the By using the Keller agent(HF 1.0 mL, HCI 1.5 mL HNO3 2.5 mL lone. and then adhered on the manhole door surfaces meanwhi and H20 95 mL), metallographic structures of the matrix metal of le yellow corrosion product was also analyzed by XRD, seen in the perforated manhole door are presented in Fig. 12. As is shown Fig. 14b. It is clear that the simple substance sulfur was the exclu- in Fig. 12a, the material displayed the typical dendritic microstruc- sive composition, which was originated from sublimation of the um alloy. However, as for the fractured sur- fuels. To sum up the analysis results of XRF and XRD, it testified face, obvious micro cracks had already initiated from its edge, seen that the scaling and the perforation of the manhole doors were also in Fig. 12b. This is a significant evidence of intergranular corrosion partly caused by the sulfur related corrosion. that eventually caused perforation on the manhole door. With respect to the thermal properties of the black corrosion roduct, Fig. 15 displays its TGAresult. It is obvious that there were 3. 2. Corrosion products analysis two turning points in the curve, respectively at temperatures of It can be learned from Figs. 4 and 5 that three different mor- 190C and 280C, and the weight losses of them were 18% and phologies of the corrosion products existed on the surfaces of the 55%. In fact, the three segments of the curve virtually represented manhole doors, i.e. the green liquid(Fig. 4c). the black solid three different steps of the decomposition procedures of the black (Fig. 5b)and the yellow solid(Fig. 5c). Analyses including IC, corrosion product. XRD, XRF and tga would then be successively conducted to inves igate the chemical compositions and the thermal properties of Step one(<190"C): dehydration. Dissolved in deionized water, the green corrosion product re- leased the ions it contained. It is displayed in Fig. 13 that the pre- Table 4 chloride ion, and nitrate radical all did not exceed 0.5 ppm. Thus, -ss black corrosion product(wtx). dominant ion was the sulfate radical, whose concentration was up xRE results of th to 17.74 mg/L i.e. 17.74 ppm. Other ions including fluoride ion, s Cr Mn Fe Ni Cu it can be inferred that the liquid corrosion product on the manhole Wt‰0046011.23.610.14512.03.670.0404Rest a)求器( Fig. 12. Metallographic structures of the perforated manhole door (a)100x and(b) polished state 50 °。20-40608010012010 Fig. 13. lon chromatograph results of the green liquid corrosion product
requirements of casting aluminum alloy specifications in GB-AlSi6- Cu4 standard of China [27]. In it, Si, Cu and Zn elements are used to improve the hardness, castability, corrosion resistance of the material. Based on the results, matrix metals of the two manhole doors were also both qualified. By using the Keller agent (HF 1.0 mL, HCl 1.5 mL HNO3 2.5 mL and H2O 95 mL), metallographic structures of the matrix metal of the perforated manhole door are presented in Fig. 12. As is shown in Fig. 12a, the material displayed the typical dendritic microstructure of casting aluminum alloy. However, as for the fractured surface, obvious micro cracks had already initiated from its edge, seen in Fig. 12b. This is a significant evidence of intergranular corrosion that eventually caused perforation on the manhole door. 3.2.2. Corrosion products analysis It can be learned from Figs. 4 and 5 that three different morphologies of the corrosion products existed on the surfaces of the manhole doors, i.e. the green liquid (Fig. 4c), the black solid (Fig. 5b) and the yellow solid (Fig. 5c). Analyses including IC, XRD, XRF and TGA would then be successively conducted to investigate the chemical compositions and the thermal properties of them. Dissolved in deionized water, the green corrosion product released the ions it contained. It is displayed in Fig. 13 that the predominant ion was the sulfate radical, whose concentration was up to 17.74 mg/L, i.e. 17.74 ppm. Other ions including fluoride ion, chloride ion, and nitrate radical all did not exceed 0.5 ppm. Thus, it can be inferred that the liquid corrosion product on the manhole door was probably the ferrous sulfate FeSO4, whose feature color is exactly green. In order to confirm the element compositions in the black corrosion product, XRF was employed. The results showed that the two primary elements were Fe and S, seen in Table 4. However, the detailed sorts of these substances should be further identified by XRD. As is shown in Fig. 14a, the black solid corrosion product consisted of a large amount of compounds. Among them, the ferrous sulfate hydrate (FeSO44H2O) was the predominant one according to the standard powder diffraction file (PDF) card. It can be easily inferred that the ferrous sulfate was the corrosion product of the components with matrix metals of iron-based materials in the cyclone, and then adhered on the manhole door surfaces. Meanwhile, the yellow corrosion product was also analyzed by XRD, seen in Fig. 14b. It is clear that the simple substance sulfur was the exclusive composition, which was originated from sublimation of the fuels. To sum up the analysis results of XRF and XRD, it testified that the scaling and the perforation of the manhole doors were also partly caused by the sulfur related corrosion. With respect to the thermal properties of the black corrosion product, Fig. 15 displays its TGA result. It is obvious that there were two turning points in the curve, respectively at temperatures of 190 C and 280 C, and the weight losses of them were 18% and 55%. In fact, the three segments of the curve virtually represented three different steps of the decomposition procedures of the black corrosion product. Step one (<190 C): dehydration. Table 3 Chemical compositions of the two manhole doors (wt.%). Element Al Si Cu Zn Fe Mn Manhole door A 87.585 5.541 3.300 2.241 1.224 0.109 Manhole door B 87.229 5.454 3.200 2.256 1.153 0.105 GB-AlSi6Cu4 86.9–91.9 5.0–7.5 3.0–5.0 / / 0.1–0.6 Fig. 12. Metallographic structures of the perforated manhole door (a) 100 and (b) polished state 50. Fig. 13. Ion chromatograph results of the green liquid corrosion product. Table 4 XRF results of the black corrosion product (wt.%). Element Si S Cr Mn Fe Ni Cu O Wt.% 0.0460 11.2 3.61 0.145 12.0 3.67 0.0404 Rest Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 677
Y Gong, Z-G Yang/ Materials and Design 32(2011)671-681 800 600 f叫 w V NW 2/° Fig.14.XRD results of the solid corrosion product (a) the black one and(b)the yellow e one. 3. 23. SEM and EDs Through optical microscope, it can be learned from Fig. 16a that 90 two different morphologies existed on the cross-section of the pe 190℃,829) foration, i.e. the bright silver one and the gray one. In order to fur ther oscopic morphologies ompositions, SEM and edS were employed In Fig. 16b, the bright silver one was compacted while the grey one was pitted. By means of EDS. it was determined that the former one was the matrix me- tal of casting aluminum, and the latter one was the corrosion prod uct containing a high content of sulfur element, seen in Fig. 16c and d and Table 5. This was in accordance with the IC, xrd and XRF re- 280℃45%0) sults, and further identified that causes of the perforation on the manhole door was concerned with sulfur related corrosion Fig. 15. TGA result of the black corrosion 3.3. Air preheater By means of XRF, element compositions of the brown rust accu- mulating on the bottom of the air preheater was determined in Ta- In this step, ferrous sulfate hydrates that performed lik ble 6. In it, Fe and o were the two predominant elements, and green liquid corrosion products dehydrated the crystal consequently it can be confirmed that the rust was iron oxides. seen in Eq.(1)[28, and the weight loss of them was In terms of the concrete compounds, the XRD results displayed in 18% at 190C in Fig. 15 Fig. 17 showed that ferroferric oxide (Fe3 O4) and ferric oxide FeSOg-6H20- FeSO4.4H20+2H20 (1) (Fe2O3)were the two primary ones among the many kinds, which were usually the ultimate products of uniform corrosion[29]. FeSO4. 4H,0 FeSO4. H20+3H20 Step two(190°C~280°): decomposition 4 Discussion After being dehydrated in step one, the ferrous sulfate hydrates 4.1. Fracture of nozzle hen decomposed in this step, particularly for the product Fes O4. H2O. The primary reactions mainly consisted of two succes- As is shown in Fig. 2b that perforation occurred at the juncture sive transformations: firstly the FeSOaH20 decomposed virtually the welded joint between the nozzle and the inlet tube, FeOOH(hydroxyl ferric oxide), seen in Eq. (2): and then the then it can be inferred that unqualified welding may be accounted FeOOH further decomposed to Fe203, seen in Eq.(3). Conse- for this failure incident. As a result, the weakest sites after welding. quently, with generation of the gases as SO2 and SO3, only such as the tiny interspace between the nozzle and the tube, and 65% of the original weight was left at 280C in Fig. 1 or the concaves on the weld seam due to insufficient filler, were most prone to be attacked under aggressive environments. As the 2FeOOH +SO2+ SO3 (2) fuels fired in this CFB boiler were predominantly the high-sulfur 2FeOOH 03+H20 trol coke 5.5%, wt%), also the temperature of the bed mate (3) rials was above 850%C, consequently high-temperature sulfur cor rosion should be primarily blamed for the perforation on the · Step three(>280°: dehydration nozzle. Concretely speaking, initially high-temperature fuels accu- mulated at the weakest sites on the juncture, where the heat could Above 280C, only some remnant FeSO4H20 after step two not be easily released in such semi-closed zones With increase of dehydrated to ferrous sulfate, Eq.(4), and it can be learned in temperature, the surrounding materials were melted, i.e. ablation Fig 15 that nearly no weight loss occurred in this step was engendered near these localized defects. Under this condition, FeSO4·H2O FeSO4+H2O (4) corrosion resistances of the melted materials vanished, and would be subsequently corroded by the enriched sulfur element in the
In this step, ferrous sulfate hydrates that performed like the green liquid corrosion products dehydrated the crystal waters, seen in Eq. (1) [28], and the weight loss of them was about 18% at 190 C in Fig. 15. FeSO4 6H2O !70—100C FeSO4 4H2O þ 2H2O ð1Þ FeSO4 4H2O !95—190C FeSO4 H2O þ 3H2O: Step two (190 C 280 C): decomposition After being dehydrated in step one, the ferrous sulfate hydrates then decomposed in this step, particularly for the product FeSO4H2O. The primary reactions mainly consisted of two successive transformations: firstly the FeSO4H2O decomposed to FeOOH (hydroxyl ferric oxide), seen in Eq. (2); and then the FeOOH further decomposed to Fe2O3, seen in Eq. (3). Consequently, with generation of the gases as SO2 and SO3, only 55% of the original weight was left at 280 C in Fig. 15. 2FeSO4 H2O !>200 C 2FeOOH þ SO2 þ SO3: ð2Þ 2FeOOH !>200 C Fe2O3 þ H2O: ð3Þ Step three (>280 C): dehydration Above 280 C, only some remnant FeSO4H2O after step two dehydrated to ferrous sulfate, Eq. (4), and it can be learned in Fig. 15 that nearly no weight loss occurred in this step. FeSO4 H2O ! 245—310 C FeSO4 þ H2O: ð4Þ 3.2.3. SEM and EDS Through optical microscope, it can be learned from Fig. 16a that two different morphologies existed on the cross-section of the perforation, i.e. the bright silver one and the gray one. In order to further study their microscopic morphologies and micro-area compositions, SEM and EDS were employed. In Fig. 16b, the bright silver one was compacted while the grey one was pitted. By means of EDS, it was determined that the former one was the matrix metal of casting aluminum, and the latter one was the corrosion product containing a high content of sulfur element, seen in Fig. 16c and d and Table 5. This was in accordance with the IC, XRD and XRF results, and further identified that causes of the perforation on the manhole door was concerned with sulfur related corrosion. 3.3. Air preheater By means of XRF, element compositions of the brown rust accumulating on the bottom of the air preheater was determined in Table 6. In it, Fe and O were the two predominant elements, and consequently it can be confirmed that the rust was iron oxides. In terms of the concrete compounds, the XRD results displayed in Fig. 17 showed that ferroferric oxide (Fe3O4) and ferric oxide (Fe2O3) were the two primary ones among the many kinds, which were usually the ultimate products of uniform corrosion [29]. 4. Discussion 4.1. Fracture of nozzle As is shown in Fig. 2b that perforation occurred at the juncture, virtually the welded joint between the nozzle and the inlet tube, then it can be inferred that unqualified welding may be accounted for this failure incident. As a result, the weakest sites after welding, such as the tiny interspace between the nozzle and the tube, and/ or the concaves on the weld seam due to insufficient filler, were most prone to be attacked under aggressive environments. As the fuels fired in this CFB boiler were predominantly the high-sulfur petrol coke (4.5–5.5%, wt.%), also the temperature of the bed materials was above 850 C, consequently high-temperature sulfur corrosion should be primarily blamed for the perforation on the nozzle. Concretely speaking, initially high-temperature fuels accumulated at the weakest sites on the juncture, where the heat could not be easily released in such semi-closed zones. With increase of temperature, the surrounding materials were melted, i.e. ablation was engendered near these localized defects. Under this condition, corrosion resistances of the melted materials vanished, and would be subsequently corroded by the enriched sulfur element in the Fig. 14. XRD results of the solid corrosion product (a) the black one and (b) the yellow one. Fig. 15. TGA result of the black corrosion product. 678 Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681
Y Gong, Z-G. Yang/Materials and Design 32 (2011)671-681 679 c (d)4o 500 000100200 000100200300400 Fig. 16. Morphologies and EDS of the cross-section of the perforated manhole door(a )macroscopic morphology, (b)SEM micrograph, (c)EDS of site A and(d) EDS of site B Table 5 Site a 14.25 9030.13 2.53 1.60 25.31 1297 1840.665342 XRF results of the brown rust(wt%). Element Al Wt% 0.3141.220.744 25302082527.0Rest Fig. 17. XRD results of the brown rust. fuels. Commonly, sulfur is activated at high temperatures and is al- ways in forms of hydrogen sulfide(H2S), mercaptan(RCH2CH2SH) corrosion actually occurred at the same time under which condi- and simple sulfur (S), which are all easy to react with the metal ele- tion, the defects grew increasingly larger and deeper, and eventu- ments in the materials to form metal sulfides, such as FeS, Nis, Mns ally resulted in the perforation and so on. Compared with the most familiar corrosion products After determining the causes of the perforation on the nozzle, the metal oxides, the metal sulfides usually perform four distinct now it was wondered what the actual factors were leading to the features [30. 1. e. the large amount of lattice defects for diffusion, fracture on its inlet tube for primary air. As was detected above the poor thermodynamic stability, the large internal stresses to that the fractured tube had suffered severe thinning of its wall crack, and the ease to form low melting point eutectics like Fe- thickness, seen in Fig. 2c, hence it can be inferred that the fracture FeS, Ni-NiS, FeO-FeS, etc. Consequently, the metal sulfides would was relevant to erosive wear on the pipe. Commonly speaking. easily decompose and therefore enlarge the defects. In addition, three reasons can usually be ascribed to the erosive effect on noz- it was obvious that the ablation and the high-temperature sulfur zles in a CFB boiler, (a)the impact on outside walls of nozzles from
fuels. Commonly, sulfur is activated at high temperatures and is always in forms of hydrogen sulfide (H2S), mercaptan (RCH2CH2SH) and simple sulfur (S), which are all easy to react with the metal elements in the materials to form metal sulfides, such as FeS, NiS, MnS and so on. Compared with the most familiar corrosion products – the metal oxides, the metal sulfides usually perform four distinct features [30], i.e. the large amount of lattice defects for diffusion, the poor thermodynamic stability, the large internal stresses to crack, and the ease to form low melting point eutectics like Fe– FeS, Ni–NiS, FeO–FeS, etc. Consequently, the metal sulfides would easily decompose and therefore enlarge the defects. In addition, it was obvious that the ablation and the high-temperature sulfur corrosion actually occurred at the same time, under which condition, the defects grew increasingly larger and deeper, and eventually resulted in the perforation. After determining the causes of the perforation on the nozzle, now it was wondered what the actual factors were leading to the fracture on its inlet tube for primary air. As was detected above that the fractured tube had suffered severe thinning of its wall thickness, seen in Fig. 2c, hence it can be inferred that the fracture was relevant to erosive wear on the pipe. Commonly speaking, three reasons can usually be ascribed to the erosive effect on nozzles in a CFB boiler, (a) the impact on outside walls of nozzles from Table 5 Chemical compositions on cross-section of the perforated manhole door (wt.%). Element C O Si Al Cu S Site A / 14.25 4.90 30.13 2.53 1.60 Site B 25.31 12.97 5.81 1.84 0.66 53.42 Table 6 XRF results of the brown rust (wt.%). Element Al Si S K Ca Mn Fe O Wt.% 0.314 1.22 0.744 0.225 3.02 0.825 27.0 Rest Fig. 17. XRD results of the brown rust. Fig. 16. Morphologies and EDS of the cross-section of the perforated manhole door (a) macroscopic morphology, (b) SEM micrograph, (c) EDS of site A and (d) EDS of site B. Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681 679
the bed materials. (b) the blow of high-speed air flows from neigh- ment. Actually, it was inevitable that sulfur would sublimate from ring nozzles, and (c)the abrasion on the inside walls of nozzles the fuels under high temperatures and then accumulate on the in- rom the primary air that contains leaked ash slag. The former two ner surfaces of equipments, and consequently bring about sulfur re- are closely related to the conformation design of all the nozzles. lated corrosion. Seen in Fig. 4c, the green liquid substances scaling Since only an individual nozzle failed in this event, thus these on the surface of the refractory were just the corrosion products wo factors could be excluded. Then, it comes to the third one. which were formed due to the high temperature oxidation of sulfur With regards to leakage of the ash slag into the air chamber, it is Concretely speaking, the temperatures within the combustion sys- a common sense that the higher the primary air velocity is, the tem approximately equaled to that in the furnace bed, nearly above lower the probability of leakage is. However in engineering prac- 850C. Under this condition, the sulfur was oxidized to the sulfur tice, high velocity simultaneously brings about high energy con- trioxide sO3, seen in Eq (6). As the surrounding temperatures of umption. Hence, a balanced value of the primary air velocity is the manhole door were near the dew point temperature thus the usually adopted for the optimum efficiency in industry. But actu- SO3 was then converted into the sulfuric acid H2SO4. also in Eq ally, any tiny fluctuations of the bed materials flow would induce (6). Afterwards, this strong aggressive medium corroded the iron- leakage of ash slag into the chamber, and this situation was even based components around the manhole door and produced sul- egarded inevitable 31. What's more, as the sulfur content of phates, particularly the ferrous sulfate FeSO4. This was exactly the the fuels fired in this CFB boiler was particularly high, therefore mechanisms of the dew point corrosion. With the circulating proce even more limestone was needed for desulfurization. From this dure of the fuels, these corrosion products were also transported point of view, more bed materials would increase the probability and some of them then adhered on the refractory of the manhole of leakage as well. Under this condition, the ash slag particles with door. Since the refractory mainly consisted of clay, which per- relatively high content amid the primary air would be sent out formed good corrosion resistance, therefore only scaling rather from the chamber through the inlet tubes, and would bring about than serious corrosion was resulted in on the manhole door a erosive wear effect on the tubes simultaneously Qualitatively. tent of erosive wear in furnaces can usually be expressed in Eq (5) S+02-S02+502-S03+H20-H2SOA Actually, the environment temperature outside the manhole undoC 6) door was close to the room temperature, far lower than that the refractory served. Thus, if the refractory and the manhole door were not conglutinated tightly due to unqualified manufacturing. where E is the erosive wear extent, up, dp and C are respectively the self-detachment of the refractory from the manhole door would velocity, the diameter and the particles concentration in the pri- occur because of the mismatch of their CTEs( coefficient of thermal mary air, K is a constant relevant to materials, t is the running time, expansion). The manhole door B exactly exhibited such situation g is acceleration of gravity. Quantitatively, particles velocity, virtu- As is presented in Fig. 5. scaling of the yellow sulfur as well a ally the primary air velocity was 74.9 m/s in this event according the black corrosion products were both engendered directly on to the design, and the value of the exponent n is commonly adopted the manhole door matrix metal. The former one was easy to under as three based on the past researches. Moreover, referred to the data from the works, compositions of the ash slag were mainly According to the structures and the service conditions of the composed of silica SiOz(48.978, wt% same below), alumina Al203 gas-solid separation system, the unfired fuels were conveyed from 4. 82%)and ferric oxide Fe2 03(7.19%) and their particle sizes ran- the cyclone to the refeed valve through a device called dipleg (33) Within it, there existed three different regions from top to bottom 2.65 mm may not induce erosion on pipes in fluidized bed. Then, the dense region had a high density of fuel solids, and thereby made ash particles with similar sizes? Based on the above [34]. Consequently, this region was usually given another termi inspection, it was also found that the location of the fracture was nology called dead region. As the refeed valve was near the dead re- on the same side with that of the perforation, this evidence may be of critical importance for explanation of thinning on the tubes. gton, thus the refeed valve including the manhole door often When under normal conditions, the primary air from the chamber endured severe erosive wear effect from the high-density solids. Especially for the manhole door, this effect was directly exerted smoothly conveyed through the tubes and then sent out from on the manhole door matrix metal without a refractory that had al- the nozzles. Once a nozzle was perforated, the primary air would ready self-detached. Although in fact, it is a common sense that s outwards through not only the two outlets on both flanks of casting aluminum alloys are usually covered with a passive film the nozzle, but also the perforated hole. As a result, the air flow, of alumina Al203, and consequently exhibit excellent corrosion which was always accompanied with ash slag, was disturbed and resistance. However, under the strong erosive wear effect, the pas- consequently accelerated. This would result in added erosive wear sive film was destroyed and then the fresh material was exposed to effect from the ash particles on the inlet tube, particularly on the the harsh environments. Under this condition, two kinds of reac- side possessing the perforation. Now it can be identified that the localized ablation and the high-temperature sulfur corrosion initi- tions simultaneously took place. As for the aluminum. it was oxi- dized and still formed the alumina: while for the other metal ated by unqualified welding process were the main causes of the perforation on the nozzle, and subsequently the ash slag contained duced the sulfates and other corrosion products, seen in Eqs. (7 in the air flow with accelerated velocity exerted severe erosive wear and ( 8). Virtually, this was an autocatalytic corrosion process since effect on the inlet tube. As a result, the wall thickness of the tube the product H2S04 would corrode the manhole door in return [36- was thinned and eventually fractured 38. Thus, it can be concluded that the black corrosion products were actually the mixtures of FeSO4, FeOOH and so on, which were 4.2. Corrosion and perforation of manhole door in accordance with the XRD results of them. Besides these part of the corrosion products may have also been originated from other Based on the above analysis, the three different kinds of iron-based components around the manhole door and scaled on sion products on the manhole doors all contained the sulfur ele- the manhole door together, just like that happened on the manhole
the bed materials, (b) the blow of high-speed air flows from neighboring nozzles, and (c) the abrasion on the inside walls of nozzles from the primary air that contains leaked ash slag. The former two are closely related to the conformation design of all the nozzles. Since only an individual nozzle failed in this event, thus these two factors could be excluded. Then, it comes to the third one. With regards to leakage of the ash slag into the air chamber, it is a common sense that the higher the primary air velocity is, the lower the probability of leakage is. However in engineering practice, high velocity simultaneously brings about high energy consumption. Hence, a balanced value of the primary air velocity is usually adopted for the optimum efficiency in industry. But actually, any tiny fluctuations of the bed materials flow would induce leakage of ash slag into the chamber, and this situation was even regarded inevitable [31]. What’s more, as the sulfur content of the fuels fired in this CFB boiler was particularly high, therefore even more limestone was needed for desulfurization. From this point of view, more bed materials would increase the probability of leakage as well. Under this condition, the ash slag particles with relatively high content amid the primary air would be sent out from the chamber through the inlet tubes, and would bring about erosive wear effect on the tubes simultaneously. Qualitatively, extent of erosive wear in furnaces can usually be expressed in Eq. (5) [32]: E / un pdpKCs 2g ð5Þ where E is the erosive wear extent, up, dp and C are respectively the velocity, the diameter and the particles concentration in the primary air, K is a constant relevant to materials, s is the running time, g is acceleration of gravity. Quantitatively, particles velocity, virtually the primary air velocity was 74.9 m/s in this event according to the design, and the value of the exponent n is commonly adopted as three based on the past researches. Moreover, referred to the data from the works, compositions of the ash slag were mainly composed of silica SiO2 (48.97%, wt.% same below), alumina Al2O3 (34.82%) and ferric oxide Fe2O3 (7.19%), and their particle sizes ranged from 0–8 mm with an average value of 1.5–2.5 mm. In the literature from Kain et al. [2], alumina and silica with sizes of 0.6– 2.65 mm may not induce erosion on pipes in fluidized bed. Then, why did erosive wear even thinning occur on the inlet tube in this event by ash particles with similar sizes? Based on the above inspection, it was also found that the location of the fracture was on the same side with that of the perforation, this evidence may be of critical importance for explanation of thinning on the tubes. When under normal conditions, the primary air from the chamber is smoothly conveyed through the tubes and then sent out from the nozzles. Once a nozzle was perforated, the primary air would pass outwards through not only the two outlets on both flanks of the nozzle, but also the perforated hole. As a result, the air flow, which was always accompanied with ash slag, was disturbed and consequently accelerated. This would result in added erosive wear effect from the ash particles on the inlet tube, particularly on the side possessing the perforation. Now it can be identified that the localized ablation and the high-temperature sulfur corrosion initiated by unqualified welding process were the main causes of the perforation on the nozzle, and subsequently the ash slag contained in the air flow with accelerated velocity exerted severe erosive wear effect on the inlet tube. As a result, the wall thickness of the tube was thinned and eventually fractured. 4.2. Corrosion and perforation of manhole door Based on the above analysis, the three different kinds of corrosion products on the manhole doors all contained the sulfur element. Actually, it was inevitable that sulfur would sublimate from the fuels under high temperatures and then accumulate on the inner surfaces of equipments, and consequently bring about sulfur related corrosion. Seen in Fig. 4c, the green liquid substances scaling on the surface of the refractory were just the corrosion products, which were formed due to the high temperature oxidation of sulfur. Concretely speaking, the temperatures within the combustion system approximately equaled to that in the furnace bed, nearly above 850 C. Under this condition, the sulfur was oxidized to the sulfur trioxide SO3, seen in Eq. (6). As the surrounding temperatures of the manhole door were near the dew point temperature, thus the SO3 was then converted into the sulfuric acid H2SO4, also in Eq. (6). Afterwards, this strong aggressive medium corroded the ironbased components around the manhole door and produced sulphates, particularly the ferrous sulfate FeSO4. This was exactly the mechanisms of the dew point corrosion. With the circulating procedure of the fuels, these corrosion products were also transported, and some of them then adhered on the refractory of the manhole door. Since the refractory mainly consisted of clay, which performed good corrosion resistance, therefore only scaling rather than serious corrosion was resulted in on the manhole door A. S þ O2 ! SO2 þ 1 2 O2 ! SO3 þ H2O ! H2SO4: ð6Þ Actually, the environment temperature outside the manhole door was close to the room temperature, far lower than that the refractory served. Thus, if the refractory and the manhole door were not conglutinated tightly due to unqualified manufacturing, self-detachment of the refractory from the manhole door would occur because of the mismatch of their CTEs (coefficient of thermal expansion). The manhole door B exactly exhibited such situation. As is presented in Fig. 5, scaling of the yellow sulfur as well as the black corrosion products were both engendered directly on the manhole door matrix metal. The former one was easy to understand, however what was the actual causes of the latter one? According to the structures and the service conditions of the gas–solid separation system, the unfired fuels were conveyed from the cyclone to the refeed valve through a device called dipleg [33]. Within it, there existed three different regions from top to bottom and respectively named inlet, dilute and dense. Just like its name, the dense region had a high density of fuel solids, and thereby made it less efficient for transportation of fuels back into the furnace [34]. Consequently, this region was usually given another terminology called dead region. As the refeed valve was near the dead region, thus the refeed valve including the manhole door often endured severe erosive wear effect from the high-density solids. Especially for the manhole door, this effect was directly exerted on the manhole door matrix metal without a refractory that had already self-detached. Although in fact, it is a common sense that casting aluminum alloys are usually covered with a passive film of alumina Al2O3, and consequently exhibit excellent corrosion resistance. However, under the strong erosive wear effect, the passive film was destroyed and then the fresh material was exposed to the harsh environments. Under this condition, two kinds of reactions simultaneously took place. As for the aluminum, it was oxidized and still formed the alumina; while for the other metal elements like iron, they reacted with the sulfur and finally produced the sulfates and other corrosion products, seen in Eqs. (7) and (8). Virtually, this was an autocatalytic corrosion process since the product H2SO4 would corrode the manhole door in return [36– 38]. Thus, it can be concluded that the black corrosion products were actually the mixtures of FeSO4, FeOOH and so on, which were in accordance with the XRD results of them. Besides these, part of the corrosion products may have also been originated from other iron-based components around the manhole door and scaled on the manhole door together, just like that happened on the manhole 680 Y. Gong, Z.-G. Yang / Materials and Design 32 (2011) 671–681
