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Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler
Journal of the Energy Institute xxx (xxxx) xxx
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Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler Xiaohe Xiong, Xing Liu, Houzhang Tan*, Shuanghui Deng MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2019 Received in revised form 25 February 2019 Accepted 26 February 2019 Available online xxx
Because of the updated requirement on ultra-low NOx emission (<50 mg/Nm3), most of Chinese coalfired boilers have to be operated at a low NOx combustion mode. However, for high-sulfur coal, water-cooled wall tubes probably suffer severe corrosion in such a strong reduction atmosphere. This work aims to investigate the high temperature corrosion behavior of water-cooled wall tubes inside a 300 MW boiler unit. A short length of corroded water-cooled wall tube was cut down and was analyzed by various characterization methods to further figure out the detailed corrosion mechanism. The typical corrosion products can be distinguished by blue, black and pale-green. Results showed that blue and black color products were mainly consisted of iron sulfides and iron oxides while the pale-green ones were identified as zinc sulfide. Along the radial direction, a layered structure of corrosion products can be observed. The formation of inner layer resulted from the reaction between iron oxide and hydrogen sulfide. The sulfur element displays a gradual increase trend while the Fe element gives out an opposite trend along the radially outward direction. The intermediate layer comes from the fly ash deposition and the outer layer is formed via condensation and deposition of ferrous sulfide gas on the water-cooled wall. The corrosion in this power plant is typical sulfide type for large amounts of Fe and S element were found in the corrosion products. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: High temperature corrosion Boiler Ferrous sulfide Layered structure Power plant
1. Introduction With the increasingly strict environmental policy coming into effect, more and more Chinese coal-fired plants have completed the ultralow emission retrofit [1,2]. According to the latest pollutant emission standard in China, the NOx emission is required to be reduced below 50 mg/Nm3 [3]. To achieve this goal, most of power plants have to switch the combustion mode into low NOx combustion and operated at lower oxygen atmosphere in primary combustion zone [4]. The long term oxygen-deficient run is apt to incur high temperature corrosion on water-cooled wall tube because of the reducing atmosphere filled with abundant H2S corrosive gases [5]. A direct adverse effect caused by high temperature corrosion is the reduction in tube thickness. With the time going by, the strength of the tube is turning weaker and in extreme case it may even lead to tube burst, which will cause a great economic loss for power plants [6,7]. Hence, the problems related with high temperature corrosion are needed to pay more attention. Due to the complexity in terms of combustion conditions and gas-solid flow, the high temperature corrosion of water-cooled wall tubes is induced and affected by many factors. For coal-fired boilers with a certain operation, coal quality and combustion parameters are viewed as two key factors [8,9]. In terms of coal quality, there is no doubt that the sulfur content plays a dominant role. Large amounts of H2S gas would be formed in oxygen-deficient condition when the high sulfur coal is burned [10]. The H2S gas is active and can penetrate the porous oxidation layer shrouded at the tube wall surface and further corrode the metal iron base. The corrosion rate is almost positively proportional as a function of the concentration of gas phase H2S in the flue gas. If the mole ratio of CO/(CO þ CO2) increases from 8% to 24%, the concentration of gas phase H2S will increase from 0.02% to 0.07%, thus resulting in intensified corrosion on the water-cooled wall [11]. Another coal quality factor is the volatile content. As known that the less volatile content in coal, the more difficult for ignition. When the volatile decrease, the coal particles around the water tube do not have enough time to burn out when jetting into the furnace from the
* Corresponding author. E-mail address: [email protected] (H. Tan). https://doi.org/10.1016/j.joei.2019.02.003 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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burner nozzle [12]. Consequently, a reducing ambience is formed, which creates a liable corrosion condition. Likewise, the coal particle size or fineness is also a vital factor related with burn-out problems, contributing to the formation of the reducing ambience. The other operation parameters include the primary or secondary air velocity, the boiler load, the assembly operation mode of burners, etc. Generally, the wind velocity depends on the coal quality. In China, the coal burned by the plants vary extensively due to the price fluctuation in the coal market [13]. Hence, the operation wind velocity needs corresponding adjustment. The primary wind velocity requires turning down when the coal quality turns worse to ensure the coal particles igniting timely and vice versa [12]. Besides the above two factors, some other conditions such as tube wall temperature and tube materials also can not be ignored. The corrosion rate would be exponentially related with tube wall temperature in 300e500 C. The corrosion rate would be doubled if the temperature increased by 50 C [14]. The big capacity and high parameter unit such as 600/1000 MW unit is more prone to have corrosion trend because of a relatively high wall temperature. In terms of tube materials, these related research are often conducted in the laboratory rather than the field test. Zhao et al. [15] studied the high temperature corrosion rate of T23 and T24 steel materials. Hussain et al. [16] compared four kinds of alloy material commonly used in the plants by simulating the flue gas in the lab and found the following anti-corrosion ability ranking: 625 > HR3C > 347HFG > T92. As a matter of fact, alloy element Cr, Ni content is closely connected with the corrosion resistance ability of the alloy steel. Chatha et al. [17] studied degradation resistance of Ni-20Cr coating on T91 boiler tube steel and found Ni-20Cr-coated steel performed better than the uncoated steel in actual boiler environment. Many suggestive conclusions are obtained through simulated corrosive environment in laboratory. However, laboratory study has its limitations for the simulated condition differed greatly compared to the actual furnace reaction condition. Especially in China nowadays, the coal quality in power plant fluctuated frequently and the operation parameter such as primary or secondary air velocity, oxygen content at furnace outlet, etc. required a synchronous adjustment. The field reaction condition outside the water-cooled wall tube is always kept in change. Hence, it is difficult to construct a same field environment in the lab. Based on this, study on a corroded wall sample is more practical and meaningful. In this paper, a water-cooled wall tube sample suffered from serious corrosion from a coal-fired plant was studied. The microstructures of corrosion products coated on the outer-wall was observed using a scanning electron microscope and the mineral composition, and the chemical composition of corrosion samples were also analyzed by the related analysis methods such as X-ray fluorescence (XRF), X-ray diffraction (XRD) and X-ray spectrometer (FE-SEM/EDS). Combined with the habitual operation mode in this boiler unit, the probable high temperature corrosion causes were listed, aiming to provide some guidance to prevent or slow down the corrosion rate. 2. Materials and experiments 2.1. Corrosion status The high temperature corrosion occurred in a 300 MW boiler, manufactured by Dongfang Boiler (Group) Limited by Share Ltd, subcritical, natural circulation, single furnace, primary reheating, balanced ventilation and solid slag discharge. Six layers of primary air nozzles are arranged at the four corners of the furnace to construct a tangential combustion mode. The water-cooled wall was made from SA-210C carbon manganese steel (GB5310, Chinese Standard, Table 1) with a 63.5 mm external diameter and a 7.5 mm thickness. During a furnace shut-down for maintenance period, the high temperature corrosion phenomenon was observed in the burner area and the most serious corrosive area is the center fireside area of right side wall. Fig. 1 displayed a section of the corroded tube sample cut down from the water-cooled wall. The outer wall surface was wrapped by the corrosion products of non-uniform thickness. Both two sides of the tube showed an obvious thickness reduction phenomenon. The typical fireside tube thickness reduction was nearly 1.5 mm (Fig. 2), occupying 20% of the total water-cooled wall thickness while the backside tube was normal because it was separated by the feedwater and not contacted with the high temperature flue gas. 2.2. Coal properties Two types of coal are used in this plant, bituminous coal and lean coal, with a volatile of 37.23% and 12.74% respectively. The detailed coal quality is shown in Table 2. The coal quality differs hugely that the bituminous coal volatile content is nearly three times of the lean coal volatile. In the daily operation mode, the two coals are both consumed and the lean coal percentage varies from 33% to 50%. The ignition character differed greatly between the two coals. The high volatile is beneficial to ignite. Hence, the operation parameters should make some adjustment when burning these two coals. In addition, both of the two coals sulfur content surpass 0.7%, which increases corrosion risk during long time combustion operation. 2.3. Characterization From the section of the sample tube, three colors can be distinguished by eyes, i.e., the compact dark blue, pale green and black corrosion products. The black corrosion products are powdery and prone to fall off from the tube. Hence, it was hard to be retained at the sample surfaces when the sample tube was cut into small pieces for electron microscope test. Fig. 3 displays other two color corrosion products. The compact dark blue corrosion product shines a metallic lustre character while the pale green one shows lots of fine particles character. The difference in color implied different components in the corresponding corrosion area.
Table 1 Composition of SA-210C (wt %). Metal
C
Mn
Si
P
S
SA-210C
<0.35
0.29e1.06
>0.1
<0.035
<0.035
Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Fig. 1. The corroded water-cooled wall tube.
Fig. 2. The schematic diagram of tube thickness reduction.
Several characterization methods are adopted to further reveal the causes of high temperature corrosion. Micromorphology of were characterized by means of JSM 7000F field emission scanning electron microscope and the element distribution was analyzed combining the dispersive X-ray spectrometer (FE-SEM/EDS). X-ray fluorescence (XRF, S4 PIONEER, Germany) and X-ray diffraction (XRD, X'pert MPD Pro, PANalytical, Netherlands) were used to analyze the element distribution and the chemical components of corrosion products in different color area. 3. Results and discussion 3.1. Elemental and mineral composition analysis Here the composition of different corrosion products was characterized via XRF in order to obtain the elemental content for analyzing corrosion mechanism or process. Fig. 4 presents the XRF and XRD results of the corrosion products. Comparing the dark blue and black corrosion products, the element content distribution is similar, i.e., the first three major elements are sulfur (S), iron (Fe) and oxygen (O), the sum of which take an 84.3%, 70% mass percentage respectively in dark blue and black corrosion products. The next major elements are zinc(Zn), aluminum(Al) and silicon(Si), three of which takes an 8.4%, 20.4% percentage respectively. The XRD results illustrate that the major components of both corrosion products are iron sulfide and iron oxides, such as FeS, Fe7S8, Fe9S10 and Fe3O4. Therefore, these two color corrosion products have similar elements and components. The difference in the color of products may be attributed to different structure for magnetite Fe3O4. The formation of compact magnetite Fe3O4 is probably related with the reaction between ferrous sulfide and oxygen, 3FeS þ 2O2 ¼ Fe3O4 þ 3S
(1)
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Table 2 Proximate analysis of typical coal quality (wt%). Sample
Mt %
Mad %
Aar %
Vdaf %
FCad %
St,ar %
Qnet kcal/kg
Coal-1 Coal-2
5.80 3.60
5.25 3.43
23.36 25.59
37.23 12.74
44.73 61.90
0.75 1.00
5022 5316
Fig. 3. Macro morphology of fragment samples (1.dark blue corrosion, 2. pale green corrosion). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. XRF and XRD results of Corrosion products.
This process is similar to the blueing process for metals processing, occurring at high temperature condition and presents dark blue color character. However, the powder magnetite results from the ferric oxide oxidation, 3Fe2O3 þ O2 ¼ 2Fe3O4
(2)
Generally, the powdery magnetite presents black color character. Apparently different to the two aforementioned corrosion products, the pale green corrosion products are full of abundant zinc, the amount of which is four times relative to that in blue ones and seven times to that in black ones. The sphalerite in the coal [18], the main components of which is zinc sulfide, is a zinc source of flue gas. Inversely, the iron content is obvious less, the amount just occupies 16% Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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relative to that in blue ones and 26% in black ones. In terms of iron source, besides from coal, another source is from the base metal of the water-cooled wall. The pure sphalerite is nearly colorless. When it contains Fe, Ga and other impurity elements, it would present pale green or other color character. Meanwhile, it is noticed that the amount of silicon and oxygen element reaches up higher to 32.5%. In general, the base metal would not contain silicon, oxygen these nonmetal elements to such a large proportion. Therefore, it can be inferred that these nonmetal elements come from the coal, which implied that the operation adjustment is not well and the fly ash in the flue gas washed against the water-cooled wall.
3.2. Micromorphology analysis By means of scanning electron microscope test, more specific structure information of corrosion products can be known. For the reason that black corrosion product is powdery and very easy to fall off from the tube during the process of cutting the sample tube into small pieces for electron microscope test, only dark blue and pale green corrosion products test results are displayed. As shown in Fig. 5, an obvious and overlapped layer structure of the blue corrosion products is observed. This typical morphology may be due to condensation and deposition of hot mineral salt gas. In each layer, enormous spot materials are covered. By further enlarging, some regular hexagonal crystals can be seen. The crystal size is around 20 mm. By EDS analysis, the major elements for regular hexagonal crystals are Fe and S, as the tag 007 shown in Fig. 5. It is presumed to be ferrous sulfide. Meanwhile, it can be observed that these particles with varying sizes scattered on the surface of ferrous sulfide crystal. Because of high carbon content, basically, it is confirmed as unburned coal particles. which overtakes the sulfur content, as the element analysis of tag 008 shown. Besides carbon, the second and third largest element remains sulfur and iron. The extensively distributed ferrous sulfide implied that the major source is from the iron pyrite pyrolysis in coal rather than the oxidation or sulfidation of base iron. The specific reaction is: FeS2 ¼ FeS þ [S]
(3)
The reaction product FeS is released probably as the gaseous form, because of its melting point of 1195 C lowering than the furnace flame temperature. The gaseous FeS would condense or deposit when it encounters the relatively colder surface of water-cooled wall. Generally, the wall temperature is bellow 550 C [19]. After a certain time for condensation and deposition, the overlapped layer structure is formed. Meanwhile, besides gaseous FeS, the very active sulfur atom [S] is released synchronously, which can trigger the starting of the metal tube strong corrosion. Fig. 6 displays the micromorphology and EDS of the pale green corrosion products. Unlike the layer structure of blue corrosion products, the structure of pale green corrosion products is porous and loose. Obviously, pores and spherical fusion ash can be observed. The maximum element content is still sulfur and the components are mainly metal sulfide. From the XRD analysis in Fig. 4, the representative product is zinc sulfide. Except from Zn, other elements such as Na, Al, K, Fe, Ca, Ga, C, O can also be found, verifying that the source is from coal rather than the metal base.
3.3. Line scanning analysis The sample tube was cut into small pieces and further polished and mosaicked into the epoxy resin. The line scanning analysis along radial direction results show that the corrosion products outside the metal iron base can be roughly divided into three layers, i.e., the inner, intermediate and outer layer (the outermost layer is epoxy resin, Fig. 7). Clearly can be seen that the boundary line of different layers. The inner layer is compact while the outer layer is loose. From inside to outside direction, the corrosion layer structure is gradually turning loose. Roughly estimated, the depth of the inner, intermediate and outer layer are 380 mm, 165 mm and 1 mm. Meanwhile, a concave hole can be observed in the inner layer. As the label 032,033 EDS results shown in Fig. 8, no distinguishable difference of components between the inner and outer hole. The major element is also the Fe and S. It can be inferred that the hole is probably from the result of some physical action rather than high temperature corrosion. Fig. 9 displays the different element distribution along radial direction. From the inside to outside direction, the element S is experiencing three steps, which including a gradual rise in the inner layer, a sharp drop in the intermediate layer and then a sudden increase and subsequent stable in the outer layer. The rise step in the inner layer is the result of reaction between iron oxide and hydrogen sulfide. The specific reactions are listed as following: FeO þ H2S ¼ FeS þ H2O
(4)
3Fe2O3 þ 7H2S ¼ 6FeS þ 7H2O þ SO2
(5)
Fe3O4 þ 4H2S ¼ FeS þ Fe2S3 þ 4H2O
(6)
The hydrogen sulfide penetrates the iron oxide layer along with the porous pores. The closer to the metal base, the harder as the reaction going on. Hence, the element S in the inner layer displayed a gradual rise character. A sharp drop in the intermediate layer means that this layer is not result from the chemical reaction but the fly ash deposition. The relative abundant of Si and O element in this layer can verify this point. A small peak of the Si and O element distribution curve in this layer can be observed. The stable step in the outer layer means that element S comes from the ferrous sulfide in flue gas. The high temperature ferrous sulfide gas condensation and deposition on the watercooled wall outer surface for its low temperature. As for element Fe distribution, the content in metal base ranks the most, then undergoes a gradual descend in inner layer then stabilizes in the other two layers. A gradual descend can attribute to the metal Fe high temperature oxidation. The Fe subsequent stabilization in the other two layers illustrates the coal contains some amount of Fe element. Some other metal elements such as Zn, Cu, Ca, Al,.etc. can also be found. Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Fig. 5. Micromorphology and EDS of the blue corrosion products. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Corrosion type and mechanism Generally, high temperature corrosion induced by sulfur-containing species have two types: sulfide and sulfate type [20,21]. The sulfide type is closely related with H2S gas while the sulfate type is connected with sulfate caused by reaction between SO3 and alkali metal oxide such as Na2O, K2O. A vital corrosion intermediate product of sulfate type is Na3Fe(SO4)3 or K3Fe(SO4)3 while the typical corrosion product is FeS for sulfide corrosion. Hence, a criteria to judge the specific corrosion type is the mass ratio of sulfur to oxygen element. In sulfate type, the mass ratio of O to S is 2:1 in an ideal condition from the product chemical formula, or at least, the mass of O is much larger than that of S in a real case. From the above-mentioned analysis, large amount of O is not found. However, on the contrary, the Fe and S element is largely found. Therefore, the corrosion type in this power plant is sulfide type. In practice, the water-cooled wall tube area is not easy to occur sulfate corrosion unless the primary wind washed against water-cooled wall tube severely. But in those convection heat transfer surfaces such as the superheater or reheater area, sulfate corrosion often takes place for the hot sulfate in flue gas would condense and deposit on these heating surfaces. In the oxygen deficient condition, most of the sulfur in coal would be released as gas phase H2S, the amount can exceed 75% [22]. The source of H2S can be generated in the follow reactions. FeS þ H2 ¼ Fe þ H2S
(7)
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Fig. 6. Micromorphology and EDS of the pale green corrosion products. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
S þ H2 ¼ H2S
(8)
FeS2 þ H2 ¼ FeS þ H2S
(9)
The H2S is strongly corrosive and can penetrate the porous iron-bearing oxides to form the iron-bearing sulfides by the reaction (3)e(5). Meanwhile, it can also directly interact with water-cooled wall and cause corrosion by the following action: Fe þ H2S ¼ FeS þ H2
(10)
Besides H2S, the active sulfur atom can also result in detrimental corrosion by reaction with Fe: Fe þ [S] ¼ FeS
(11)
The source of active sulfur atom is listed below: FeS2 ¼ FeS þ [S]
(12)
3FeS2 þ 12C þ 8O2 ¼ Fe3O4 þ 12CO þ 6[S]
(13)
H2S ¼ [S] þ H2
(14)
2H2S þ SO2 ¼ 2H2O þ 2[S]
(15)
2H2S þ O2 ¼ 2H2O þ 2[S]
(16)
Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Fig. 7. The layered corrosion products by line scanning analysis.
Fig. 8. The EDS results in and outer the concave hole.
Fig. 9. Element distribution along radial direction.
Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Fe
Step 1 FeS formation FexOy Base Fe
Step 2
Step 3
Slag formation
Slag fall off
FeS
Step 4 FeS penetration & Slag rebirth FeS
FeS H2S S Unburned carbon Fly Ash
FexOy
9
FeS Fe
FeS
Fe
Fe
Fig. 10. The schematic diagram of high temperature corrosion process.
However, the practical environment is extremely harsh and the high temperature corrosion in water-cooled wall area is rather complicated. A possible corrosion process is deduced in Fig. 10. Here are four steps. First, the fly ash washed against the water-cooled wall because of improper operation adjustment, which is often induced by coal quality fluctuation in large scope. The carbon in coal did not have enough time to burn out and a reducing ambience is formed. The iron oxides wrapped outside iron base are vulcanized by the S-contained species (H2S, S) to produce FeS. Hence, the first step can be summarized as FeS layer formation. Meanwhile, the gaseous FeS in the flue gas would condense and deposit on the water-cooled wall tube surfaces for a relatively cold temperature. With the fly ash washing going on, particles of different sizes undergoing agglomeration and coagulation, a slagging is shaped and suspended on the tube surface. This is step 2, slagging formation. When the slagging grew up to a certain extent, once the complex disturbance such as flue gas washing, thermal stress in the length direction etc. occurred, it would fall off from the tube surface. This is step 3, slagging falls off. Then a new iron base is exposed in the flue gas environment. In a not long period, the new iron base surface would shape a layer of FeS by reaction (10) and (11). Subsequently, the slagging was shaped again and then fell off. This is step 4, corrosion penetration and slagging rebirth. The step 2 to 4 is kept on repeating and the corrosion is conducting toward the radially inward direction. From the above analysis, some points that affect high temperature corrosion can be summarized. The first one is the sulfur content in the coal. Empirically, sulfur content is suggestive to be controlled under 1% for unit safe operation. If condition is permitted, the low sulfur coal is the priority choice. Another factor is that flue gas scours the water-cooled wall. This is directly related with reducing ambience and the slagging growth. The corresponding optimization measures includes keeping coal quality stable, adding essential parameter monitor, using soot blower regularly etc. In addition, water-cooled wall tubes coating technology is also an effective way to relieve the corrosion [23e25]. 5. Conclusions With the increasingly strict environmental policy in China, enormous coal-fired power plants have to operate in low oxygen combustion. The water-cooled wall tube of high temperature corrosion in a plant has been investigated. The corrosion products covered on the outer sample tube is mainly FeS and the corrosion type is typical sulfide type. Dark blue, black three and pale green different color corrosion products can be observed. The major element components of the former two color products are made of sulfur (S), iron (Fe) and oxygen (O). In terms of pale green product, besides S, Fe, O, Zn are the major elements. The line scanning analysis shows that the corrosion products present three layered structures. The inner layer results from the reaction between the iron oxide and hydrogen sulfide, the intermediate layer comes from the fly ash deposition and the outer layer is the results of ferrous sulfide gas condensation and deposition on the watercooled wall. Along the radical direction, the S displays a gradual increase trend while the Fe gives an opposite trend in the inner layer. The H2S in reducing atmosphere adjacent to the water-cooled wall is the major species for inducing high temperature corrosion. To alleviate or prevent high temperature corrosion, some measures including burning low sulfur coal, keeping coal quality stable, adding essential parameter monitor, blowing ash regularly, adopting coating tubes etc. are listed as a reference for the field operation. Conflicts of interest The authors declare no conflict of interest. Acknowledgement The present work was supported by the National Key R&D Program of China (No. 2018YFB0604203). References [1] Hao Zhou, et al., Optimization of ammonia injection grid in hybrid selective non-catalyst reduction and selective catalyst reduction system to achieve ultra-low NOx emissions, J. Energy Inst. 91 (6) (2018) 984e996. [2] Yue Peng, et al., The effect of moisture on particulate matter measurements in an ultra-low emission power plant, Fuel 238 (2019) 430e439. [3] Action plan for energy conservation and emission reduction upgrading and transformation of coal fired plants 2014e2020, National Development and Reform Commission, Ministry of Ecological Environment, National Energy Administration, China, September 12, 2014. EB/OL, http://www.ndrc.gov.cn/gzdt/201409/t20140919_ 626240.html.
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Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler Xiaohe Xiong, Xing Liu, Houzhang Tan*, Shuanghui Deng MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2019 Received in revised form 25 February 2019 Accepted 26 February 2019 Available online xxx
Because of the updated requirement on ultra-low NOx emission (<50 mg/Nm3), most of Chinese coalfired boilers have to be operated at a low NOx combustion mode. However, for high-sulfur coal, water-cooled wall tubes probably suffer severe corrosion in such a strong reduction atmosphere. This work aims to investigate the high temperature corrosion behavior of water-cooled wall tubes inside a 300 MW boiler unit. A short length of corroded water-cooled wall tube was cut down and was analyzed by various characterization methods to further figure out the detailed corrosion mechanism. The typical corrosion products can be distinguished by blue, black and pale-green. Results showed that blue and black color products were mainly consisted of iron sulfides and iron oxides while the pale-green ones were identified as zinc sulfide. Along the radial direction, a layered structure of corrosion products can be observed. The formation of inner layer resulted from the reaction between iron oxide and hydrogen sulfide. The sulfur element displays a gradual increase trend while the Fe element gives out an opposite trend along the radially outward direction. The intermediate layer comes from the fly ash deposition and the outer layer is formed via condensation and deposition of ferrous sulfide gas on the water-cooled wall. The corrosion in this power plant is typical sulfide type for large amounts of Fe and S element were found in the corrosion products. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: High temperature corrosion Boiler Ferrous sulfide Layered structure Power plant
1. Introduction With the increasingly strict environmental policy coming into effect, more and more Chinese coal-fired plants have completed the ultralow emission retrofit [1,2]. According to the latest pollutant emission standard in China, the NOx emission is required to be reduced below 50 mg/Nm3 [3]. To achieve this goal, most of power plants have to switch the combustion mode into low NOx combustion and operated at lower oxygen atmosphere in primary combustion zone [4]. The long term oxygen-deficient run is apt to incur high temperature corrosion on water-cooled wall tube because of the reducing atmosphere filled with abundant H2S corrosive gases [5]. A direct adverse effect caused by high temperature corrosion is the reduction in tube thickness. With the time going by, the strength of the tube is turning weaker and in extreme case it may even lead to tube burst, which will cause a great economic loss for power plants [6,7]. Hence, the problems related with high temperature corrosion are needed to pay more attention. Due to the complexity in terms of combustion conditions and gas-solid flow, the high temperature corrosion of water-cooled wall tubes is induced and affected by many factors. For coal-fired boilers with a certain operation, coal quality and combustion parameters are viewed as two key factors [8,9]. In terms of coal quality, there is no doubt that the sulfur content plays a dominant role. Large amounts of H2S gas would be formed in oxygen-deficient condition when the high sulfur coal is burned [10]. The H2S gas is active and can penetrate the porous oxidation layer shrouded at the tube wall surface and further corrode the metal iron base. The corrosion rate is almost positively proportional as a function of the concentration of gas phase H2S in the flue gas. If the mole ratio of CO/(CO þ CO2) increases from 8% to 24%, the concentration of gas phase H2S will increase from 0.02% to 0.07%, thus resulting in intensified corrosion on the water-cooled wall [11]. Another coal quality factor is the volatile content. As known that the less volatile content in coal, the more difficult for ignition. When the volatile decrease, the coal particles around the water tube do not have enough time to burn out when jetting into the furnace from the
* Corresponding author. E-mail address: [email protected] (H. Tan). https://doi.org/10.1016/j.joei.2019.02.003 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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burner nozzle [12]. Consequently, a reducing ambience is formed, which creates a liable corrosion condition. Likewise, the coal particle size or fineness is also a vital factor related with burn-out problems, contributing to the formation of the reducing ambience. The other operation parameters include the primary or secondary air velocity, the boiler load, the assembly operation mode of burners, etc. Generally, the wind velocity depends on the coal quality. In China, the coal burned by the plants vary extensively due to the price fluctuation in the coal market [13]. Hence, the operation wind velocity needs corresponding adjustment. The primary wind velocity requires turning down when the coal quality turns worse to ensure the coal particles igniting timely and vice versa [12]. Besides the above two factors, some other conditions such as tube wall temperature and tube materials also can not be ignored. The corrosion rate would be exponentially related with tube wall temperature in 300e500 C. The corrosion rate would be doubled if the temperature increased by 50 C [14]. The big capacity and high parameter unit such as 600/1000 MW unit is more prone to have corrosion trend because of a relatively high wall temperature. In terms of tube materials, these related research are often conducted in the laboratory rather than the field test. Zhao et al. [15] studied the high temperature corrosion rate of T23 and T24 steel materials. Hussain et al. [16] compared four kinds of alloy material commonly used in the plants by simulating the flue gas in the lab and found the following anti-corrosion ability ranking: 625 > HR3C > 347HFG > T92. As a matter of fact, alloy element Cr, Ni content is closely connected with the corrosion resistance ability of the alloy steel. Chatha et al. [17] studied degradation resistance of Ni-20Cr coating on T91 boiler tube steel and found Ni-20Cr-coated steel performed better than the uncoated steel in actual boiler environment. Many suggestive conclusions are obtained through simulated corrosive environment in laboratory. However, laboratory study has its limitations for the simulated condition differed greatly compared to the actual furnace reaction condition. Especially in China nowadays, the coal quality in power plant fluctuated frequently and the operation parameter such as primary or secondary air velocity, oxygen content at furnace outlet, etc. required a synchronous adjustment. The field reaction condition outside the water-cooled wall tube is always kept in change. Hence, it is difficult to construct a same field environment in the lab. Based on this, study on a corroded wall sample is more practical and meaningful. In this paper, a water-cooled wall tube sample suffered from serious corrosion from a coal-fired plant was studied. The microstructures of corrosion products coated on the outer-wall was observed using a scanning electron microscope and the mineral composition, and the chemical composition of corrosion samples were also analyzed by the related analysis methods such as X-ray fluorescence (XRF), X-ray diffraction (XRD) and X-ray spectrometer (FE-SEM/EDS). Combined with the habitual operation mode in this boiler unit, the probable high temperature corrosion causes were listed, aiming to provide some guidance to prevent or slow down the corrosion rate. 2. Materials and experiments 2.1. Corrosion status The high temperature corrosion occurred in a 300 MW boiler, manufactured by Dongfang Boiler (Group) Limited by Share Ltd, subcritical, natural circulation, single furnace, primary reheating, balanced ventilation and solid slag discharge. Six layers of primary air nozzles are arranged at the four corners of the furnace to construct a tangential combustion mode. The water-cooled wall was made from SA-210C carbon manganese steel (GB5310, Chinese Standard, Table 1) with a 63.5 mm external diameter and a 7.5 mm thickness. During a furnace shut-down for maintenance period, the high temperature corrosion phenomenon was observed in the burner area and the most serious corrosive area is the center fireside area of right side wall. Fig. 1 displayed a section of the corroded tube sample cut down from the water-cooled wall. The outer wall surface was wrapped by the corrosion products of non-uniform thickness. Both two sides of the tube showed an obvious thickness reduction phenomenon. The typical fireside tube thickness reduction was nearly 1.5 mm (Fig. 2), occupying 20% of the total water-cooled wall thickness while the backside tube was normal because it was separated by the feedwater and not contacted with the high temperature flue gas. 2.2. Coal properties Two types of coal are used in this plant, bituminous coal and lean coal, with a volatile of 37.23% and 12.74% respectively. The detailed coal quality is shown in Table 2. The coal quality differs hugely that the bituminous coal volatile content is nearly three times of the lean coal volatile. In the daily operation mode, the two coals are both consumed and the lean coal percentage varies from 33% to 50%. The ignition character differed greatly between the two coals. The high volatile is beneficial to ignite. Hence, the operation parameters should make some adjustment when burning these two coals. In addition, both of the two coals sulfur content surpass 0.7%, which increases corrosion risk during long time combustion operation. 2.3. Characterization From the section of the sample tube, three colors can be distinguished by eyes, i.e., the compact dark blue, pale green and black corrosion products. The black corrosion products are powdery and prone to fall off from the tube. Hence, it was hard to be retained at the sample surfaces when the sample tube was cut into small pieces for electron microscope test. Fig. 3 displays other two color corrosion products. The compact dark blue corrosion product shines a metallic lustre character while the pale green one shows lots of fine particles character. The difference in color implied different components in the corresponding corrosion area.
Table 1 Composition of SA-210C (wt %). Metal
C
Mn
Si
P
S
SA-210C
<0.35
0.29e1.06
>0.1
<0.035
<0.035
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Fig. 1. The corroded water-cooled wall tube.
Fig. 2. The schematic diagram of tube thickness reduction.
Several characterization methods are adopted to further reveal the causes of high temperature corrosion. Micromorphology of were characterized by means of JSM 7000F field emission scanning electron microscope and the element distribution was analyzed combining the dispersive X-ray spectrometer (FE-SEM/EDS). X-ray fluorescence (XRF, S4 PIONEER, Germany) and X-ray diffraction (XRD, X'pert MPD Pro, PANalytical, Netherlands) were used to analyze the element distribution and the chemical components of corrosion products in different color area. 3. Results and discussion 3.1. Elemental and mineral composition analysis Here the composition of different corrosion products was characterized via XRF in order to obtain the elemental content for analyzing corrosion mechanism or process. Fig. 4 presents the XRF and XRD results of the corrosion products. Comparing the dark blue and black corrosion products, the element content distribution is similar, i.e., the first three major elements are sulfur (S), iron (Fe) and oxygen (O), the sum of which take an 84.3%, 70% mass percentage respectively in dark blue and black corrosion products. The next major elements are zinc(Zn), aluminum(Al) and silicon(Si), three of which takes an 8.4%, 20.4% percentage respectively. The XRD results illustrate that the major components of both corrosion products are iron sulfide and iron oxides, such as FeS, Fe7S8, Fe9S10 and Fe3O4. Therefore, these two color corrosion products have similar elements and components. The difference in the color of products may be attributed to different structure for magnetite Fe3O4. The formation of compact magnetite Fe3O4 is probably related with the reaction between ferrous sulfide and oxygen, 3FeS þ 2O2 ¼ Fe3O4 þ 3S
(1)
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Table 2 Proximate analysis of typical coal quality (wt%). Sample
Mt %
Mad %
Aar %
Vdaf %
FCad %
St,ar %
Qnet kcal/kg
Coal-1 Coal-2
5.80 3.60
5.25 3.43
23.36 25.59
37.23 12.74
44.73 61.90
0.75 1.00
5022 5316
Fig. 3. Macro morphology of fragment samples (1.dark blue corrosion, 2. pale green corrosion). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. XRF and XRD results of Corrosion products.
This process is similar to the blueing process for metals processing, occurring at high temperature condition and presents dark blue color character. However, the powder magnetite results from the ferric oxide oxidation, 3Fe2O3 þ O2 ¼ 2Fe3O4
(2)
Generally, the powdery magnetite presents black color character. Apparently different to the two aforementioned corrosion products, the pale green corrosion products are full of abundant zinc, the amount of which is four times relative to that in blue ones and seven times to that in black ones. The sphalerite in the coal [18], the main components of which is zinc sulfide, is a zinc source of flue gas. Inversely, the iron content is obvious less, the amount just occupies 16% Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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relative to that in blue ones and 26% in black ones. In terms of iron source, besides from coal, another source is from the base metal of the water-cooled wall. The pure sphalerite is nearly colorless. When it contains Fe, Ga and other impurity elements, it would present pale green or other color character. Meanwhile, it is noticed that the amount of silicon and oxygen element reaches up higher to 32.5%. In general, the base metal would not contain silicon, oxygen these nonmetal elements to such a large proportion. Therefore, it can be inferred that these nonmetal elements come from the coal, which implied that the operation adjustment is not well and the fly ash in the flue gas washed against the water-cooled wall.
3.2. Micromorphology analysis By means of scanning electron microscope test, more specific structure information of corrosion products can be known. For the reason that black corrosion product is powdery and very easy to fall off from the tube during the process of cutting the sample tube into small pieces for electron microscope test, only dark blue and pale green corrosion products test results are displayed. As shown in Fig. 5, an obvious and overlapped layer structure of the blue corrosion products is observed. This typical morphology may be due to condensation and deposition of hot mineral salt gas. In each layer, enormous spot materials are covered. By further enlarging, some regular hexagonal crystals can be seen. The crystal size is around 20 mm. By EDS analysis, the major elements for regular hexagonal crystals are Fe and S, as the tag 007 shown in Fig. 5. It is presumed to be ferrous sulfide. Meanwhile, it can be observed that these particles with varying sizes scattered on the surface of ferrous sulfide crystal. Because of high carbon content, basically, it is confirmed as unburned coal particles. which overtakes the sulfur content, as the element analysis of tag 008 shown. Besides carbon, the second and third largest element remains sulfur and iron. The extensively distributed ferrous sulfide implied that the major source is from the iron pyrite pyrolysis in coal rather than the oxidation or sulfidation of base iron. The specific reaction is: FeS2 ¼ FeS þ [S]
(3)
The reaction product FeS is released probably as the gaseous form, because of its melting point of 1195 C lowering than the furnace flame temperature. The gaseous FeS would condense or deposit when it encounters the relatively colder surface of water-cooled wall. Generally, the wall temperature is bellow 550 C [19]. After a certain time for condensation and deposition, the overlapped layer structure is formed. Meanwhile, besides gaseous FeS, the very active sulfur atom [S] is released synchronously, which can trigger the starting of the metal tube strong corrosion. Fig. 6 displays the micromorphology and EDS of the pale green corrosion products. Unlike the layer structure of blue corrosion products, the structure of pale green corrosion products is porous and loose. Obviously, pores and spherical fusion ash can be observed. The maximum element content is still sulfur and the components are mainly metal sulfide. From the XRD analysis in Fig. 4, the representative product is zinc sulfide. Except from Zn, other elements such as Na, Al, K, Fe, Ca, Ga, C, O can also be found, verifying that the source is from coal rather than the metal base.
3.3. Line scanning analysis The sample tube was cut into small pieces and further polished and mosaicked into the epoxy resin. The line scanning analysis along radial direction results show that the corrosion products outside the metal iron base can be roughly divided into three layers, i.e., the inner, intermediate and outer layer (the outermost layer is epoxy resin, Fig. 7). Clearly can be seen that the boundary line of different layers. The inner layer is compact while the outer layer is loose. From inside to outside direction, the corrosion layer structure is gradually turning loose. Roughly estimated, the depth of the inner, intermediate and outer layer are 380 mm, 165 mm and 1 mm. Meanwhile, a concave hole can be observed in the inner layer. As the label 032,033 EDS results shown in Fig. 8, no distinguishable difference of components between the inner and outer hole. The major element is also the Fe and S. It can be inferred that the hole is probably from the result of some physical action rather than high temperature corrosion. Fig. 9 displays the different element distribution along radial direction. From the inside to outside direction, the element S is experiencing three steps, which including a gradual rise in the inner layer, a sharp drop in the intermediate layer and then a sudden increase and subsequent stable in the outer layer. The rise step in the inner layer is the result of reaction between iron oxide and hydrogen sulfide. The specific reactions are listed as following: FeO þ H2S ¼ FeS þ H2O
(4)
3Fe2O3 þ 7H2S ¼ 6FeS þ 7H2O þ SO2
(5)
Fe3O4 þ 4H2S ¼ FeS þ Fe2S3 þ 4H2O
(6)
The hydrogen sulfide penetrates the iron oxide layer along with the porous pores. The closer to the metal base, the harder as the reaction going on. Hence, the element S in the inner layer displayed a gradual rise character. A sharp drop in the intermediate layer means that this layer is not result from the chemical reaction but the fly ash deposition. The relative abundant of Si and O element in this layer can verify this point. A small peak of the Si and O element distribution curve in this layer can be observed. The stable step in the outer layer means that element S comes from the ferrous sulfide in flue gas. The high temperature ferrous sulfide gas condensation and deposition on the watercooled wall outer surface for its low temperature. As for element Fe distribution, the content in metal base ranks the most, then undergoes a gradual descend in inner layer then stabilizes in the other two layers. A gradual descend can attribute to the metal Fe high temperature oxidation. The Fe subsequent stabilization in the other two layers illustrates the coal contains some amount of Fe element. Some other metal elements such as Zn, Cu, Ca, Al,.etc. can also be found. Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Fig. 5. Micromorphology and EDS of the blue corrosion products. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Corrosion type and mechanism Generally, high temperature corrosion induced by sulfur-containing species have two types: sulfide and sulfate type [20,21]. The sulfide type is closely related with H2S gas while the sulfate type is connected with sulfate caused by reaction between SO3 and alkali metal oxide such as Na2O, K2O. A vital corrosion intermediate product of sulfate type is Na3Fe(SO4)3 or K3Fe(SO4)3 while the typical corrosion product is FeS for sulfide corrosion. Hence, a criteria to judge the specific corrosion type is the mass ratio of sulfur to oxygen element. In sulfate type, the mass ratio of O to S is 2:1 in an ideal condition from the product chemical formula, or at least, the mass of O is much larger than that of S in a real case. From the above-mentioned analysis, large amount of O is not found. However, on the contrary, the Fe and S element is largely found. Therefore, the corrosion type in this power plant is sulfide type. In practice, the water-cooled wall tube area is not easy to occur sulfate corrosion unless the primary wind washed against water-cooled wall tube severely. But in those convection heat transfer surfaces such as the superheater or reheater area, sulfate corrosion often takes place for the hot sulfate in flue gas would condense and deposit on these heating surfaces. In the oxygen deficient condition, most of the sulfur in coal would be released as gas phase H2S, the amount can exceed 75% [22]. The source of H2S can be generated in the follow reactions. FeS þ H2 ¼ Fe þ H2S
(7)
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Fig. 6. Micromorphology and EDS of the pale green corrosion products. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
S þ H2 ¼ H2S
(8)
FeS2 þ H2 ¼ FeS þ H2S
(9)
The H2S is strongly corrosive and can penetrate the porous iron-bearing oxides to form the iron-bearing sulfides by the reaction (3)e(5). Meanwhile, it can also directly interact with water-cooled wall and cause corrosion by the following action: Fe þ H2S ¼ FeS þ H2
(10)
Besides H2S, the active sulfur atom can also result in detrimental corrosion by reaction with Fe: Fe þ [S] ¼ FeS
(11)
The source of active sulfur atom is listed below: FeS2 ¼ FeS þ [S]
(12)
3FeS2 þ 12C þ 8O2 ¼ Fe3O4 þ 12CO þ 6[S]
(13)
H2S ¼ [S] þ H2
(14)
2H2S þ SO2 ¼ 2H2O þ 2[S]
(15)
2H2S þ O2 ¼ 2H2O þ 2[S]
(16)
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Fig. 7. The layered corrosion products by line scanning analysis.
Fig. 8. The EDS results in and outer the concave hole.
Fig. 9. Element distribution along radial direction.
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Fe
Step 1 FeS formation FexOy Base Fe
Step 2
Step 3
Slag formation
Slag fall off
FeS
Step 4 FeS penetration & Slag rebirth FeS
FeS H2S S Unburned carbon Fly Ash
FexOy
9
FeS Fe
FeS
Fe
Fe
Fig. 10. The schematic diagram of high temperature corrosion process.
However, the practical environment is extremely harsh and the high temperature corrosion in water-cooled wall area is rather complicated. A possible corrosion process is deduced in Fig. 10. Here are four steps. First, the fly ash washed against the water-cooled wall because of improper operation adjustment, which is often induced by coal quality fluctuation in large scope. The carbon in coal did not have enough time to burn out and a reducing ambience is formed. The iron oxides wrapped outside iron base are vulcanized by the S-contained species (H2S, S) to produce FeS. Hence, the first step can be summarized as FeS layer formation. Meanwhile, the gaseous FeS in the flue gas would condense and deposit on the water-cooled wall tube surfaces for a relatively cold temperature. With the fly ash washing going on, particles of different sizes undergoing agglomeration and coagulation, a slagging is shaped and suspended on the tube surface. This is step 2, slagging formation. When the slagging grew up to a certain extent, once the complex disturbance such as flue gas washing, thermal stress in the length direction etc. occurred, it would fall off from the tube surface. This is step 3, slagging falls off. Then a new iron base is exposed in the flue gas environment. In a not long period, the new iron base surface would shape a layer of FeS by reaction (10) and (11). Subsequently, the slagging was shaped again and then fell off. This is step 4, corrosion penetration and slagging rebirth. The step 2 to 4 is kept on repeating and the corrosion is conducting toward the radially inward direction. From the above analysis, some points that affect high temperature corrosion can be summarized. The first one is the sulfur content in the coal. Empirically, sulfur content is suggestive to be controlled under 1% for unit safe operation. If condition is permitted, the low sulfur coal is the priority choice. Another factor is that flue gas scours the water-cooled wall. This is directly related with reducing ambience and the slagging growth. The corresponding optimization measures includes keeping coal quality stable, adding essential parameter monitor, using soot blower regularly etc. In addition, water-cooled wall tubes coating technology is also an effective way to relieve the corrosion [23e25]. 5. Conclusions With the increasingly strict environmental policy in China, enormous coal-fired power plants have to operate in low oxygen combustion. The water-cooled wall tube of high temperature corrosion in a plant has been investigated. The corrosion products covered on the outer sample tube is mainly FeS and the corrosion type is typical sulfide type. Dark blue, black three and pale green different color corrosion products can be observed. The major element components of the former two color products are made of sulfur (S), iron (Fe) and oxygen (O). In terms of pale green product, besides S, Fe, O, Zn are the major elements. The line scanning analysis shows that the corrosion products present three layered structures. The inner layer results from the reaction between the iron oxide and hydrogen sulfide, the intermediate layer comes from the fly ash deposition and the outer layer is the results of ferrous sulfide gas condensation and deposition on the watercooled wall. Along the radical direction, the S displays a gradual increase trend while the Fe gives an opposite trend in the inner layer. The H2S in reducing atmosphere adjacent to the water-cooled wall is the major species for inducing high temperature corrosion. To alleviate or prevent high temperature corrosion, some measures including burning low sulfur coal, keeping coal quality stable, adding essential parameter monitor, blowing ash regularly, adopting coating tubes etc. are listed as a reference for the field operation. Conflicts of interest The authors declare no conflict of interest. Acknowledgement The present work was supported by the National Key R&D Program of China (No. 2018YFB0604203). References [1] Hao Zhou, et al., Optimization of ammonia injection grid in hybrid selective non-catalyst reduction and selective catalyst reduction system to achieve ultra-low NOx emissions, J. Energy Inst. 91 (6) (2018) 984e996. [2] Yue Peng, et al., The effect of moisture on particulate matter measurements in an ultra-low emission power plant, Fuel 238 (2019) 430e439. [3] Action plan for energy conservation and emission reduction upgrading and transformation of coal fired plants 2014e2020, National Development and Reform Commission, Ministry of Ecological Environment, National Energy Administration, China, September 12, 2014. EB/OL, http://www.ndrc.gov.cn/gzdt/201409/t20140919_ 626240.html.
Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003
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Please cite this article as: X. Xiong et al., Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.02.003