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