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Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers
Journal of the Energy Institute xxx (2017) 1e8
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Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers Jinqing Wang a, *, Yichao Yuan a, Zuohe Chi b, Guangxue Zhang b a
Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China b College of Metrological Technology and Engineering, China Jiliang University, Hangzhou 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2017 Received in revised form 4 August 2017 Accepted 30 August 2017 Available online xxx
To address the fouling problem in boilers fired with high-sodium coal (HSC), a composite ceramic coating was developed using the slurry method and applied on 15CrMo steel. After sintering, the composite ceramic coating had a dense structure and was well bonded to the substrate forming a metallurgical damascene structure. The fouling and thermal shock resistance of the ceramic coating to sodium sulfate was found be excellent. After five fouling cycles, the uncoated steel had a surface fouling rate of 21.9%, compared to 0% for the coated steel. The composite ceramic coating also demonstrated an appreciable fouling resistance to sodium sulfate. Similar to the flaky oxide film formed on the uncoated steel surface, only a few fine sodium sulfate particles penetrated the microcracks generated during the sintering of the ceramic coating. However, remarkable anti-fouling results were achieved when the composite ceramic coating was applied in an industrial boiler fired with a Zhundong HSC blend. © 2017 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: High-sodium coal Anti-fouling Boiler efficiency Ceramic coating
1. Introduction High-sodium coal (HSC) refers to a type of coal with a sodium content (i.e. ash content) greater than 2%. HSC reserves are found primarily in China, Australia, the United States and Germany [1]. In China, vast amounts of HSC, up to 390 billion tonnes, are mainly found in the Zhundong region of Xinjiang. This HSC is, thus, referred to as Zhundong coal. Based on the current annual coal output, coal reserves in the Zhundong region can meet the demands of the entire country over the next 100 years [2]. In addition, Zhundong coal can be mined cheaply and is a source of readily available energy [3]. Notably, Zhundong coal is highly volatile and reactive and has a low ash and sulfur content. However, the sodium content in Zhundong coal is generally greater than 2% and, in some mining areas, it can exceed 10%, which is far greater than that of typical types of coal [4]. Sodium in coal is an important contributor to the fouling and slagging of the surface of the heat exchanger in boiler systems during the combustion process. For this reason, the use of Zhundong coal as a fuel is extremely limited. In recent years, researchers have studied the effect of the type of sodium-rich compounds present in coal on the fouling of heat exchangers [5e7], the migration pattern of sodium during the combustion process [4,8e10] and the fouling mechanism of HSC [11e15]. Previous research has revealed that sodium oxides contained in the coal first undergo sublimation in the high-temperature combustion environment and, then, condense onto the wall surface of the heat exchanger. Finally, the sodium oxides react with sulfur dioxide, aluminum oxide and iron oxide contained in the flue gas, forming a dense sticky deposition layer consisting of various types of sulfates, such as sodium sulfate, compound sodium sulfate and sodium pyrosulfate. This layer can capture a large amount of fly ash, forming hightemperature sticky ash deposits [1]. Quite a large number of studies have been conducted on different methods to control the formation of the sticky HSC deposition layer. Some researchers have suggested the use of additives, such as kaolin [16,17], diatomaceous earth [18], bauxite [19] and coal-fired fly ash [19,20], which can physically absorb or chemically react with the sodium in the gas phase, thereby reducing the gas-phase sodium concentration and its subsequent deposition on the surface of the heat exchanger. Others have investigated sodium removal treatments in order
* Corresponding author. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.joei.2017.08.003 1743-9671/© 2017 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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to minimize the amount of sodium released during the coal burning process [1,21]. Currently, fouling problems associated with the use of HSC are primarily addressed by blending HSC with coals of low fouling tendency [13,22]. However, the proportion of HSC in the coal blend is inevitably kept at relatively low levels. Most of the previous studies have focused on reducing the amount of sodium in the flue gas to prevent the formation of the sticky deposition layer. However, the fouling and slagging in coal-fired boilers can also be mitigated by minimising the adhesion between this layer and the surface of the boiler, hence, limiting the formation of the deposition layer. To this end, a fouling-resistant coating technique can be employed to protect the steel surface. The coating applied to the heating surface of a boiler needs to have a relatively low surface energy, appropriate density and thickness and no chemical affinity with the deposition layer. Coating techniques have been widely used to improve the wear and corrosion resistance of the heating surface of boilers [23e25]. However, the employment of coating techniques to mitigate the fouling and slagging problems in boilers is less common. In view of the aforementioned problem, in the present study a composite ceramic coating, based on ultrafine hexagonal boron nitride (h-BN) and graphite powders, was developed and applied on the surface of 15CrMo steel sheets, which are typically used in boiler superheaters. The fouling and thermal shock resistance of the composite ceramic coating were thoroughly investigated. 2. Materials and methods 2.1. Preparation of ceramic slurry The raw aggregates used to prepare the composite ceramic coating consisted of ultrafine hexagonal boron nitride (h-BN) powder (purity: >99%; mean particle size: 500 nm) and graphite powder (purity: >99%; mean particle size: 1 mm). Potash water glass (P), with a silica modulus of 2.7, was used as a binder. To ensure the stability of the coating on storage and during spraying and solidification, traces of an emulsifier, dispersant and defoamer were also added in the slurry. h-BN has a lamellar structure, similar to graphite, with strong BeN covalent bonds. It has high temperature and oxidation resistance, high thermal conductivity, and excellent resistance to corrosion from acids, alkalis, and vitreous slag [26,27]. The coating prepared in the present study also contained a small amount of flaky graphite. The graphite consists of layers of carbon atoms, with every two layers being connected by van der Waals forces, which are relatively weak. As a result, graphite has relatively good lubricity and low macroscopic expansion coefficient. Therefore, small additions of graphite into the coating could improve its thermal shock resistance [28]. Ultrafine powders of both h-BN and graphite were used in order to promote sintering reactions at lower temperature as a result of their high surface area and low surface energy. Potash water glass, used as the binding agent in the ceramic slurry formulation, forms SieOeSi bonds upon heating above 200 C resulting in films with a three-dimensional network structure and excellent water resistance. Additionally, it can form strong bonds with the ceramic aggregates and moderately strong bonds with the substrate. The slurry preparation process is as follows: The initial slurry was prepared and mixed based on the chemical composition listed in Table 1. The slurry was then thoroughly mixed with the rest of the additives to form a homogenous mixture, followed by wet milling in a planetary ball mill for 2 h. Finally, the slurry was filtered and stored, completing the ceramic slurry preparation. 2.2. Preparation of ceramic coating The substrate used throughout the experiments was the 15CrMo steel, which is the most common material used in boiler superheaters. A 15CrMo steel sheet was cut into thin square sheets (20 20 3 mm). The substrate material was then subjected to a sandblasting treatment to increase its surface roughness with the aim of strengthening the bond between the substrate and the coating. The ceramic slurry was applied on the substrate surface by spraying. Subsequently, the specimens were sintered at 550 C for 6 h in a muffle furnace. The solidification temperature was selected based on the pipe wall temperature typically measured on the flue gas side of the superheater in the boiler. Given that the temperature of the pipe wall roughly rises at the rate of 3 C/min as the boiler starts up, the same heating rate was applied during sintering of the ceramic coating. Finally, the sintered specimens were cooled to room temperature in air. 2.3. Thermal shock resistance testing The thermal shock resistance of a coating reflects the strength of the bond between the coating and the substrate, and it is also an indication of the compatibility of thermal expansion coefficients of the two materials. In addition, the thermal shock resistance of a coating directly depicts its spalling resistance to temperature changes and it is a determining factor for its overall quality. In the present study, thermal shock resistance testing was performed using the water-cooling method. Each sintered specimen, prepared using the aforementioned process, was immediately placed at 550 C in a muffle furnace. Specimens remained in this environment for 10 min, after being quickly removed and placed in a beaker filled with water. Then, specimens were removed from water and were allowed to cool to room temperature in air. Upon drying of specimens, the process was repeated until complete damage of the coating layer was achieved. This was evidenced by the formation of cracks on the surface or peeling and spalling of the coating. The number of thermal shock cycles after which the coating began to spall off the substrate and when 1/3 of the coating had spalled off the substrate were recorded.
Table 1 Chemical composition of the ceramic slurry. Raw materials
BN
Graphite
Potash water glass
Water
Additives
wt%
12
1
18
65
5
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2.4. Fouling resistance testing Sodium sulfate is the main component of the sticky deposition layer [29]. Therefore, the present study investigated the fouling resistance of the composite ceramic coating using sodium sulfate as the fouling medium. When a fouling medium adheres to the surface of a specimen, the reflection coefficient of the specimen surface changes, with larger changes associated with higher fouling rates. Based on this principle, the fouling resistance of the coating was evaluated by measuring the changes in the reflection coefficient of the coating surface after fouling. In practice, the adhesion between the heating surface and the deposition layer is provided by the oxide film layer formed due to the oxidation of the steel sheet under high temperature conditions. For this reason, each uncoated steel sheet specimen was first oxidised under high temperature conditions (550 C) in a muffle furnace to allow the formation of a dense oxide film on its surface prior to fouling resistance testing. Each uncoated specimen was removed from the muffle furnace after 24 h of oxidation and the reflection coefficient of its surface was determined using a reflectoscope. This process was repeated until there was no appreciable difference between two consecutive measurements of the reflection coefficient. Given the high-temperature oxidation resistance of the composite ceramic coating, the coating surface changed slightly as the oxidation process progressed. Therefore, the coated specimens were not subjected to the high-temperature oxidation treatment. Regarding fouling testing, the initial reflection coefficients of the uncoated and coated steel specimens were determined. Fine particles of sodium sulfate were then applied on the entire sample surface by using a hairbrush. As evidenced by previous experiments, the required quantity of fine particles of sodium sulfate is 0.2 g. The sodium sulfate powder-treated samples were placed at 550 C in a muffle furnace, where graphite powder had already been placed. The furnace was closed to create a reducing atmosphere, approximating the combustion atmosphere in real furnaces. After 24 h, the specimens were removed from the muffle furnace and allowed to cool to room temperature. Subsequently, an air gun (nozzle diameter: 5 mm) was used to blow compressed air (pressure: 0.4 MPa) into the sodium sulfate powdertreated surface of each specimen for 1 min. The air gun nozzle outlet was 20 cm away from the specimen. After the air-blowing treatment, the reflection coefficient of each specimen was measured. Then, samples were again treated with sodium sulfate powder and the process was repeated five times. The fouling resistance of each specimen was calculated using Eq. (1):
X¼
jA0 Ai j 100% A0
(1)
where X represents the rate of variation of the reflection coefficient of the specimen, used to describe the surface fouling rate of the specimen; A0 represents the reflection coefficient of the specimen prior to fouling; and Ai represents the reflection coefficient of the specimen after the ith attempt of fouling. Three uncoated steel sheet specimens and three coated steel sheet specimens were subjected to fouling resistance testing. The mean values of the fouling rates of these specimens were used for subsequent analysis. The morphology and chemical composition of the specimens subjected to fouling were evaluated using a scanning electron microscope (SEM) (Zeiss Supra 55S), and an X-ray energy dispersive spectrometer (EDS) (OXFORD X-Max 20), respectively. 3. Results and discussion 3.1. Microstructure of ceramic coatings Fig. 1 shows the surface morphology of the steel sheet before and after sandblasting. As seen in Fig. 1b, the roughness of the steel sheet surface increased after the treatment. This can significantly improve the bonding strength of the coating. When sandblasting the pipe walls of a boiler onsite, the slag and fouled layers adhered to the pipe walls can also be removed. Therefore, sandblasting can also help achieve cleaning of the surfaces. Fig. 2a shows the surface morphology of the sintered composite ceramic coating. A small number of microcracks was also observed. This could be due to water evaporation and decomposition of the additives contained in the slurry during sintering or/and due to different thermal expansion coefficients between the coating and the steel substrate [30]. As seen in Fig. 2b, the sintered slurry was highly vitreous due to the presence of potash water glass which helped form strong bonds between the ceramic particles. Therefore, the coating was relatively densified and was well attached to the substrate, almost forming a metallurgical damascene structure. Therefore, spalling of the coating would not be likely to occur easily.
Fig. 1. Surface morphologies of the steel sheet before (a) and after (b) sandblasting.
Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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Fig. 2. Surface (a) and cross-sectional (b) morphologies of the composite ceramic coating.
3.2. Thermal shock resistance The coating began to spall off the substrate after 22 thermal shock cycles with 1/3 of the coating spalling off the substrate after 43 thermal shock cycles. Clearly, the as-prepared composite ceramic coating possessed excellent thermal shock resistance. This superior performance can be attributed to the flaky structure of the h-BN and graphite used in the ceramic formulation [26e28]. As seen in Fig. 3, defective spots appeared on the surface of the coating after 22 thermal shock cycles (Fig. 3b). As the number of thermal shock cycles increased, the number of defective spots on the surface of the coating gradually increased (Fig. 3c). Delamination of the coating occurred after spalling, indicating that only the surface layer of the coating spalled off; the remaining coating stayed attached to the surface of the steel substrate. Thus, the steel substrate was not exposed, and the coating continued to elicit a protective effect. 3.3. Fouling resistance Fig. 4 shows the surface fouling rate curves of the uncoated and coated steel sheet specimens exposed to sodium sulfate. The surface fouling rate of the uncoated steel sheet specimens increased rapidly to 15.9% after a single fouling cycle and then increased gradually as the number of fouling cycles increased. The surface fouling rate of the uncoated steel sheet specimens was essentially fixed at 20.6% during the first four fouling cycles. In comparison, the surface fouling rate of the coated specimens remained essentially 0% even after multiple fouling cycles, indicating good stability of the ceramic coating during the fouling process. Overall, the coating demonstrated excellent fouling resistance to sodium sulfate at 550 C. Fig. 5 depicts the microscopic morphology of the uncoated and coated steel sheet subjected to sodium sulfate fouling. As shown in Fig. 5a, a relatively large number of sodium sulfate particles adhered to the uncoated steel sheet, resulting in increased fouling rate. This was due to the precipitation of sodium sulfate particles in the oxide film formed on the surface of the specimen under high temperature (Fig. 5b). Notably, an external force, such as compressed air, could not remove the sodium sulfate particles in the film because of its flaky structure. In the case of coated specimens, no sodium sulfate particles were observed (Fig. 5c). Fine sodium sulfate particles were only detected in some of the microcracks on the coating surface as shown in Fig. 5d. This is attributed to the formation of microcracks during sintering of the ceramic coating, which helped encapsulate the sodium sulfate particles. Therefore, appropriate measures should be taken to minimize the formation of microcracks during the sintering process. Based on the aforementioned results, it can be concluded that the composite ceramic coating improved significantly the fouling resistance of steel sheets to sodium sulfate. This was mainly attributed to the relatively low surface energy and dense structure of the ceramic coating as well as to its chemical incompatibility with sodium sulfate. In general, ceramic materials have significantly lower surface energies compared to metallic materials and can, therefore, be used as nonstick agents. Given that the ceramic coating used in the present study had a lower surface energy compared to the 15CrMo steel substrate, sodium sulfate could not easily adhere to the surface of coated specimens. As a result, the coated steel sheets exhibited excellent fouling resistance to sodium sulfate. In addition, because ultrafine powders were used as aggregates in the ceramic slurry, the sintered coating had a relatively dense structure, thereby, effectively minimising the formation of pores on the surface of the coating. Consequently, fine sodium sulfate particles did not adhere to its surface. Another main factor determining the fouling resistance of the coating to sodium sulfate was the chemical incompatibility of those two components. Iron, the main element in 15CrMo steel, reacts with sodium sulfate at high temperatures according to the following reaction [31]:
Fe þ Na2 SO4 /FeS þ Fe2 O3 =Fe3 O4 =FeO þ Na2 O
(2)
Therefore, there is chemical affinity between the sodium sulfate particles and the steel surface, resulting in a relatively strong bonding between the two. For this reason, applying compressed air was not sufficient to remove the sodium sulfate particles fouling the steel surface. However, when ceramic coating was applied on the steel surface, the adhesion of sodium sulfate particles was relatively weak. This can be attributed to the presence of h-BN, the main component of the composite ceramic coating, which is chemically inert and does not easily react with other materials. Thus, in this case, most of the sodium sulfate particles found on the surface of coated steel specimens could be easily removed by compressed air.
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Fig. 3. Macroscopic surface morphology of the ceramic coating at different stages of the thermal shock resistance testing: (a) before initiation of the testing, (b) upon formation of first cracks and (c) after 1/3 of the coating spalled off the substrate.
Fig. 4. Surface fouling rate curves of the uncoated and coated steel sheet specimens exposed to sodium sulfate at 550 C as a function of the fouling time. Each fouling cycle comprised of 24 h.
3.4. Engineering applications of the coating In the present study, the composite ceramic coating was sprayed onto the heating surface of a boiler using a Zhundong HSC blend as fuel. The maximum blending ratio of the Zhundong HSC was 70%. The properties of coal samples used are given in Table 2. Coal A and Coal B represent the coal typically used in the plant and the Zhundong HSC, respectively. Zhundong HSC, i.e. coal B, had a high water and volatile content, low ash content, and medium calorific value. The alkali content of the Zhundong HSC ash was also significantly higher, with the Na2O content being a high as 7.4%. Therefore, when the boiler was fired with the Zhundong HSC, serious fouling and slagging problems occurred on the cooling water and platen superheater walls. A photograph and an infrared (IR) thermography image of the coated heating surface near the lowest secondary air nozzle in the boiler system are shown in Fig. 6. A significant difference in the fouling resistance between the uncoated and coated areas of the heating surface was observed. The coated surfaces were relatively clean, whereas ash deposits visibly covered the uncoated surfaces. As seen from the IR thermography image (Fig. 6b), the temperature of the pipe wall in the uncoated area was higher by 90 C compared to the coated area. This is associated with the lower heat transfer capacity of the ash deposits on the uncoated surfaces. Overall, the efficiency of the heat exchanger was enhanced thanks to the superior fouling resistance of the composite ceramic coating. The monthly service time of the soot blower and the amount of water used in the desuperheater were assessed after one year of operation of the boiler. As seen in Table 3, the monthly service time of the soot blower was reduced by 58.2%e116 h after applying the ceramic coating. This sharp decrease suggests that the ceramic coating made the furnace more resistant to slagging. Before the spraying of the coating, severe slag build-up in the furnace caused the rise of flue-gas temperature at the furnace outlet. Consequently, the temperature difference within the superheater increased, leading to higher-temperature superheated steam and increased amounts of desuperheating water. Namely, prior to ceramic coating application, the monthly usage of water in the desuperheater was 25,099 t. However, this was decreased by 75.2% since the furnace slagging was greatly improved after the application of the ceramic coating. Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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J. Wang et al. / Journal of the Energy Institute xxx (2017) 1e8
Fig. 5. SEM micrographs of the surface of the sodium sulfate-fouled uncoated steel sheet ((a) 100; (b) 1000) and coated steel sheet ((c) 1000; (d) 5000). Table 2 Properties of the two different coal samples used to fire an industrial boiler.
Proximate analysis (wt%) Mar Aar Vdaf FCar Elemental analysis (wt%) Car Har Nar Oar Sar Ash fusion temperature (K) Deformation temperature Softening temperature Flow temperature Ash composition (wt%) SiO2 Al2O3 Fe2O3 TiO2 CaO MgO SO3 K2O Na2O
Coal A
Coal B
12.30 21.80 37.86 40.95
25.00 8.54 36.04 42.51
51.96 2.83 0.68 9.81 0.61
51.77 2.71 0.39 10.88 0.71
1403 1553 1623
1473 1488 1499
50.10 8.27 21.27 0.89 6.35 3.56 4.58 1.58 0.96
35.65 11.40 14.50 4.45 10.70 10.69 0.02 1.84 7.40
Fig. 6. Photography (a) and IR thermography image (b) of the ceramic coating used in real-life boiler system.
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Table 3 Monthly service time of the soot blower and the amount of desuperheating water before and after spraying of the ceramic coating after one year of boiler operation. Parameter
Before spraying
After spraying
Monthly average of superheated steam flow (t$h1) Monthly average of service time of the soot blower (h) Monthly average usage of desuperheating water (t)
421.5 278 25,099
546.0 116 6233
4. Conclusions A composite ceramic coating using the slurry method was developed and applied on the surface of 15CrMo steel to investigate the fouling and thermal shock resistance of the coating. In addition, the coating was sprayed on a boiler fired with a Zhundong high-sodium coal (HSC) blend. The following conclusions can be derived: (1) Upon sintering of the ceramic slurry, a vitreous ceramic coating with a dense structure was formed. This can be attributed to the coalescence of the ceramic powders used, i.e. hexagonal boron nitride (h-BN) and graphite, in the presence of potash water glass. Water evaporation and decomposition of the additives present in the slurry during sintering, coupled with differences in the coefficients of thermal expansion between the coating and the steel substrate, resulted in the formation of a small number of microcracks. However, overall, the coating and the steel substrate exhibited a metallurgical damascene structure and were well bonded. (2) The coating began to spall off the substrate after being subjected to 22 thermal shock cycles, and 1/3 of the coating had spalled off the surface after 43 cycles. Hence, the coating had good thermal shock resistance. (3) The fouling rate of the uncoated steel sheet increased rapidly after it was subjected to sodium sulfate fouling, reaching 21.9% after five fouling cycles. In comparison, the coated steel sheet surface was not fouled after five fouling cycles. The excellent fouling resistance of the composite ceramic coating to sodium sulfate was mainly due to its relatively low surface energy, dense structure and the chemical incompatibility between the coating and sodium sulfate. The flaky oxide film on the uncoated steel sheet surface could easily encapsulate some of the fine sodium sulfate particles. In the case of coated steel surfaces, it was observed that a few fine sodium sulfate particles had also penetrated the microcracks formed during the sintering process. In both cases, these particles could not be removed by compressed air. (4) The composite ceramic coating had an appreciable anti-fouling effect when applied to a boiler fired with a Zhundong HSC blend. Boiler operational data collected within one year show that the monthly average service time of the soot blowers was decreased by 58.2%, and the monthly average usage of water in the desuperheater was reduced by 75.2%. Hence, the ceramic coating developed throughout this study has great potential to be applied in real-life boiler systems to increase their overall efficiency. Acknowledgements This work was supported by the National Natural Science Foundation of China (51408574), the Department of Education of Zhejiang Province in China (Y201534242), and the Public Projects of Zhejiang Province (2017C31043). References [1] S.Y. Zhang, C. Chen, D.Z. Shi, J.F. Lv, J. Wang, X. Guo, A.X. Dong, S.W. Xiong, Situation of combustion utilization of high sodium coal, Proc. CSEE 33 (2013) 1e12. [2] J. Li, X.G. Zhuang, X. Querol, O. Font, N. Moreno, J.B. Zhou, Environmental geochemistry of the feed coals and their combustion by-products from two coal-fired power plants in Xinjiang Province, Northwest China, Fuel 95 (2012) 446e456. [3] H.J. Ge, L.H. Shen, H.M. Gu, T. Song, S.X. Jiang, Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier, Fuel 159 (2015) 107e117. [4] X.Y. Zhang, H.X. Zhang, Y.J. Na, Transformation of sodium during the ashing of Zhundong coal, Procedia Eng. 102 (2015) 305e314. [5] S.A. Benson, P.L. Holm, Comparison of inorganic constituents in three low-rank coals, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 145e149. [6] C.L. Han, Modes of occurrence of sodium in coals, J. Fuel. Chem. Technol. 27 (1999) 575e578. [7] A.R. Manzoori, P.K. Agarwal, The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur low rank coals, Fuel 71 (1992) 513e522. [8] S. Srinivasachar, J.J. Helble, D.O. Ham, G. Domazetis, A kinetic description of vapor phase alkali transformations in combustion systems, Prog. Energy Combust. 16 (1990) 303e309. [9] S.H.D. Lee, F.G. Teats, W.M. Swift, D.D. Banerjee, Alkali-vapor emission from PFBC of Illinois coals, Combust. Sci. Technol. 86 (1992) 327e336. [10] G.Y. Li, C.A. Wang, Y. Yan, X. Jin, Y.H. Liu, D.F. Che, Release and transformation of sodium during combustion of Zhundong coals, J. Energy Inst. 29 (2015) 48e56. [11] P. Radhakrishnan, V. Zakkay, A. Agnone, Alkali and gas emissions from PFB combustion of lignite, Combust. Sci. Technol. 50 (1986) 271e281. [12] Z.H. Xiao, T.K. Shang, J.K. Zhuo, Q. Yao, Study on the mechanisms of ultrafine particle formation during high-sodium coal combustion in a flat-flame burner, Fuel 181 (2016) 1257e1264. [13] X.B. Wang, Z.X. Xu, B. Wei, L. Zhang, H.Z. Tan, T. Yang, H. Mikul ci c, N. Dui c, The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing, Appl. Therm. Eng. 80 (2015) 150e159. [14] X.B. Qi, G.L. Song, W.J. Song, S.B. Yang, Q.G. Lu, Effects of wall temperature on slagging and ash deposition of Zhundong coal during circulating fluidized bed gasification, Appl. Therm. Eng. 106 (2016) 1127e1135. [15] X.J. Wu, X. Zhang, K. Yan, N. Chen, J.W. Zhang, X.Y. Xu, B.Q. Dai, J. Zhang, L. Zhang, Ash deposition and slagging behavior of Chinese Xinjiang high-alkali coal in 3 MW th pilot-scale combustion test, Fuel 181 (2016) 1191e1202. [16] X.B. Wang, H.Y. Wu, S.H. Deng, L. Zhang, B. Wei, H.Z. Tan, A mechanism study on the decomposition of sulfate in Zhundong coal with high sulfur content in coal ash, in: G. Yue, S. Li (Eds.), Clean Coal Technology and Sustainable Development: Proceedings of the 8th International Symposium on Coal Combustion, Springer Singapore, Singapore, 2016, pp. 101e106. [17] K.Q. Tran, K. Iisa, B.M. Steenari, O. Lindqvist, A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector, Fuel 84 (2005) 169e175. [18] S.H.D. Lee, I. Johnson, Removal of gaseous alkali metal compounds from hot flue gas by particulate sorbents, J. Eng. Gas. Turb. Power 102 (1979) 397e402. [19] M. Bl€ asing, M. Müller, Investigation of the effect of alkali metal sorbents on the release and capture of trace elements during combustion of straw, Combust. Flame 160 (2013) 3015e3020.
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[20] H. Wu, M.S. Bashir, P.A. Jensen, B. Sander, P. Glarborg, Impact of coal fly ash addition on ash transformation and deposition in a full-scale wood suspension-firing boiler, Fuel 113 (2013) 632e643. [21] B.M. Jenkins, R.R. Bakker, J.B. Wei, On the properties of washed straw, Biomass. Bioenergy 10 (1996) 177e200. [22] J.B. Li, M.M. Zhu, Z.Z. Zhang, K. Zhang, G.Q. Shen, D.K. Zhang, The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace, Fuel Process. Technol. 149 (2016) 176e186. €m, High temperature corrosion of thermally sprayed NiCr and FeCr coatings [23] T. Varis, D. Bankiewicz, P. Yrjas, M. Oksa, T. Suhonen, S. Tuurna, K. Ruusuvuori, S. Holmstro covered with a KCleK2SO4 salt mixture, Surf. Coat. Tech. 265 (2015) 235e243. [24] J. Chen, Y. Ninomiya, H. Naganuma, Y. Sasaki, M. Noguchi, H. Cho, Y. Ueki, R. Yoshiie, I. Naruse, Development of thermal spraying materials through several corrosion tests for heat exchanger tube of incinerators, Fuel Process. Technol. 141 (2015) 216e224. €rfel, R.S. Neumann, A. Kranzmann, Corrosion of uncoated and alumina coated steel X20CrMoV12-1 in H2OeCO2eO2 and air at 600 C, [25] W. Schulz, M. Nofz, M. Feigl, I. Do Corros. Sci. 68 (2013) 44e50. [26] J. Eichler, C. Lesniak, Boron nitride (BN) and BN composites for high-temperature applications, J. Eur. Ceram. Soc. 28 (2008) 1105e1109. [27] S. Mahathanabodee, T. Palathai, S. Raadnui, R. Tongsri, N. Sombatsompop, Effects of hexagonal boron nitride and sintering temperature on mechanical and tribological properties of SS316L/h-BN composites, Mater Des. 46 (2013) 588e597. Visloguzova, L.V. Serova, Study of carbon-containing refractories for steel smelting production units, Refract. Ind. Ceram. 48 (2007) [28] I.D. Kashcheev, K.G. Zemlyanoi, E.A. 313e316. [29] T.H. Yang, X.P. Kai, Y. Sun, Y.G. He, R.D. Li, The effect of coal sulfur on the behavior of alkali metals during co-firing biomass and coal, Fuel 90 (2011) 2454e2460. [30] Z.Q. Yan, X. Xiong, P. Xiao, F. Chen, H.B. Zhang, B.Y. Huang, SieMoeSiO2 oxidation protective coatings prepared by slurry painting for C/CeSiC composites, Surf. Coat. Tech. 202 (2008) 4734e4740. [31] L.Q. Shi, On the possibility of a Na2SO4-Na2O eutectic melt developing on metals coated with Na2SO4 deposit in oxygen/air at intermediate temperatures, Corros. Sci. 37 (1995) 1281e1287.
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Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers Jinqing Wang a, *, Yichao Yuan a, Zuohe Chi b, Guangxue Zhang b a
Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China b College of Metrological Technology and Engineering, China Jiliang University, Hangzhou 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2017 Received in revised form 4 August 2017 Accepted 30 August 2017 Available online xxx
To address the fouling problem in boilers fired with high-sodium coal (HSC), a composite ceramic coating was developed using the slurry method and applied on 15CrMo steel. After sintering, the composite ceramic coating had a dense structure and was well bonded to the substrate forming a metallurgical damascene structure. The fouling and thermal shock resistance of the ceramic coating to sodium sulfate was found be excellent. After five fouling cycles, the uncoated steel had a surface fouling rate of 21.9%, compared to 0% for the coated steel. The composite ceramic coating also demonstrated an appreciable fouling resistance to sodium sulfate. Similar to the flaky oxide film formed on the uncoated steel surface, only a few fine sodium sulfate particles penetrated the microcracks generated during the sintering of the ceramic coating. However, remarkable anti-fouling results were achieved when the composite ceramic coating was applied in an industrial boiler fired with a Zhundong HSC blend. © 2017 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: High-sodium coal Anti-fouling Boiler efficiency Ceramic coating
1. Introduction High-sodium coal (HSC) refers to a type of coal with a sodium content (i.e. ash content) greater than 2%. HSC reserves are found primarily in China, Australia, the United States and Germany [1]. In China, vast amounts of HSC, up to 390 billion tonnes, are mainly found in the Zhundong region of Xinjiang. This HSC is, thus, referred to as Zhundong coal. Based on the current annual coal output, coal reserves in the Zhundong region can meet the demands of the entire country over the next 100 years [2]. In addition, Zhundong coal can be mined cheaply and is a source of readily available energy [3]. Notably, Zhundong coal is highly volatile and reactive and has a low ash and sulfur content. However, the sodium content in Zhundong coal is generally greater than 2% and, in some mining areas, it can exceed 10%, which is far greater than that of typical types of coal [4]. Sodium in coal is an important contributor to the fouling and slagging of the surface of the heat exchanger in boiler systems during the combustion process. For this reason, the use of Zhundong coal as a fuel is extremely limited. In recent years, researchers have studied the effect of the type of sodium-rich compounds present in coal on the fouling of heat exchangers [5e7], the migration pattern of sodium during the combustion process [4,8e10] and the fouling mechanism of HSC [11e15]. Previous research has revealed that sodium oxides contained in the coal first undergo sublimation in the high-temperature combustion environment and, then, condense onto the wall surface of the heat exchanger. Finally, the sodium oxides react with sulfur dioxide, aluminum oxide and iron oxide contained in the flue gas, forming a dense sticky deposition layer consisting of various types of sulfates, such as sodium sulfate, compound sodium sulfate and sodium pyrosulfate. This layer can capture a large amount of fly ash, forming hightemperature sticky ash deposits [1]. Quite a large number of studies have been conducted on different methods to control the formation of the sticky HSC deposition layer. Some researchers have suggested the use of additives, such as kaolin [16,17], diatomaceous earth [18], bauxite [19] and coal-fired fly ash [19,20], which can physically absorb or chemically react with the sodium in the gas phase, thereby reducing the gas-phase sodium concentration and its subsequent deposition on the surface of the heat exchanger. Others have investigated sodium removal treatments in order
* Corresponding author. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.joei.2017.08.003 1743-9671/© 2017 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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to minimize the amount of sodium released during the coal burning process [1,21]. Currently, fouling problems associated with the use of HSC are primarily addressed by blending HSC with coals of low fouling tendency [13,22]. However, the proportion of HSC in the coal blend is inevitably kept at relatively low levels. Most of the previous studies have focused on reducing the amount of sodium in the flue gas to prevent the formation of the sticky deposition layer. However, the fouling and slagging in coal-fired boilers can also be mitigated by minimising the adhesion between this layer and the surface of the boiler, hence, limiting the formation of the deposition layer. To this end, a fouling-resistant coating technique can be employed to protect the steel surface. The coating applied to the heating surface of a boiler needs to have a relatively low surface energy, appropriate density and thickness and no chemical affinity with the deposition layer. Coating techniques have been widely used to improve the wear and corrosion resistance of the heating surface of boilers [23e25]. However, the employment of coating techniques to mitigate the fouling and slagging problems in boilers is less common. In view of the aforementioned problem, in the present study a composite ceramic coating, based on ultrafine hexagonal boron nitride (h-BN) and graphite powders, was developed and applied on the surface of 15CrMo steel sheets, which are typically used in boiler superheaters. The fouling and thermal shock resistance of the composite ceramic coating were thoroughly investigated. 2. Materials and methods 2.1. Preparation of ceramic slurry The raw aggregates used to prepare the composite ceramic coating consisted of ultrafine hexagonal boron nitride (h-BN) powder (purity: >99%; mean particle size: 500 nm) and graphite powder (purity: >99%; mean particle size: 1 mm). Potash water glass (P), with a silica modulus of 2.7, was used as a binder. To ensure the stability of the coating on storage and during spraying and solidification, traces of an emulsifier, dispersant and defoamer were also added in the slurry. h-BN has a lamellar structure, similar to graphite, with strong BeN covalent bonds. It has high temperature and oxidation resistance, high thermal conductivity, and excellent resistance to corrosion from acids, alkalis, and vitreous slag [26,27]. The coating prepared in the present study also contained a small amount of flaky graphite. The graphite consists of layers of carbon atoms, with every two layers being connected by van der Waals forces, which are relatively weak. As a result, graphite has relatively good lubricity and low macroscopic expansion coefficient. Therefore, small additions of graphite into the coating could improve its thermal shock resistance [28]. Ultrafine powders of both h-BN and graphite were used in order to promote sintering reactions at lower temperature as a result of their high surface area and low surface energy. Potash water glass, used as the binding agent in the ceramic slurry formulation, forms SieOeSi bonds upon heating above 200 C resulting in films with a three-dimensional network structure and excellent water resistance. Additionally, it can form strong bonds with the ceramic aggregates and moderately strong bonds with the substrate. The slurry preparation process is as follows: The initial slurry was prepared and mixed based on the chemical composition listed in Table 1. The slurry was then thoroughly mixed with the rest of the additives to form a homogenous mixture, followed by wet milling in a planetary ball mill for 2 h. Finally, the slurry was filtered and stored, completing the ceramic slurry preparation. 2.2. Preparation of ceramic coating The substrate used throughout the experiments was the 15CrMo steel, which is the most common material used in boiler superheaters. A 15CrMo steel sheet was cut into thin square sheets (20 20 3 mm). The substrate material was then subjected to a sandblasting treatment to increase its surface roughness with the aim of strengthening the bond between the substrate and the coating. The ceramic slurry was applied on the substrate surface by spraying. Subsequently, the specimens were sintered at 550 C for 6 h in a muffle furnace. The solidification temperature was selected based on the pipe wall temperature typically measured on the flue gas side of the superheater in the boiler. Given that the temperature of the pipe wall roughly rises at the rate of 3 C/min as the boiler starts up, the same heating rate was applied during sintering of the ceramic coating. Finally, the sintered specimens were cooled to room temperature in air. 2.3. Thermal shock resistance testing The thermal shock resistance of a coating reflects the strength of the bond between the coating and the substrate, and it is also an indication of the compatibility of thermal expansion coefficients of the two materials. In addition, the thermal shock resistance of a coating directly depicts its spalling resistance to temperature changes and it is a determining factor for its overall quality. In the present study, thermal shock resistance testing was performed using the water-cooling method. Each sintered specimen, prepared using the aforementioned process, was immediately placed at 550 C in a muffle furnace. Specimens remained in this environment for 10 min, after being quickly removed and placed in a beaker filled with water. Then, specimens were removed from water and were allowed to cool to room temperature in air. Upon drying of specimens, the process was repeated until complete damage of the coating layer was achieved. This was evidenced by the formation of cracks on the surface or peeling and spalling of the coating. The number of thermal shock cycles after which the coating began to spall off the substrate and when 1/3 of the coating had spalled off the substrate were recorded.
Table 1 Chemical composition of the ceramic slurry. Raw materials
BN
Graphite
Potash water glass
Water
Additives
wt%
12
1
18
65
5
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2.4. Fouling resistance testing Sodium sulfate is the main component of the sticky deposition layer [29]. Therefore, the present study investigated the fouling resistance of the composite ceramic coating using sodium sulfate as the fouling medium. When a fouling medium adheres to the surface of a specimen, the reflection coefficient of the specimen surface changes, with larger changes associated with higher fouling rates. Based on this principle, the fouling resistance of the coating was evaluated by measuring the changes in the reflection coefficient of the coating surface after fouling. In practice, the adhesion between the heating surface and the deposition layer is provided by the oxide film layer formed due to the oxidation of the steel sheet under high temperature conditions. For this reason, each uncoated steel sheet specimen was first oxidised under high temperature conditions (550 C) in a muffle furnace to allow the formation of a dense oxide film on its surface prior to fouling resistance testing. Each uncoated specimen was removed from the muffle furnace after 24 h of oxidation and the reflection coefficient of its surface was determined using a reflectoscope. This process was repeated until there was no appreciable difference between two consecutive measurements of the reflection coefficient. Given the high-temperature oxidation resistance of the composite ceramic coating, the coating surface changed slightly as the oxidation process progressed. Therefore, the coated specimens were not subjected to the high-temperature oxidation treatment. Regarding fouling testing, the initial reflection coefficients of the uncoated and coated steel specimens were determined. Fine particles of sodium sulfate were then applied on the entire sample surface by using a hairbrush. As evidenced by previous experiments, the required quantity of fine particles of sodium sulfate is 0.2 g. The sodium sulfate powder-treated samples were placed at 550 C in a muffle furnace, where graphite powder had already been placed. The furnace was closed to create a reducing atmosphere, approximating the combustion atmosphere in real furnaces. After 24 h, the specimens were removed from the muffle furnace and allowed to cool to room temperature. Subsequently, an air gun (nozzle diameter: 5 mm) was used to blow compressed air (pressure: 0.4 MPa) into the sodium sulfate powdertreated surface of each specimen for 1 min. The air gun nozzle outlet was 20 cm away from the specimen. After the air-blowing treatment, the reflection coefficient of each specimen was measured. Then, samples were again treated with sodium sulfate powder and the process was repeated five times. The fouling resistance of each specimen was calculated using Eq. (1):
X¼
jA0 Ai j 100% A0
(1)
where X represents the rate of variation of the reflection coefficient of the specimen, used to describe the surface fouling rate of the specimen; A0 represents the reflection coefficient of the specimen prior to fouling; and Ai represents the reflection coefficient of the specimen after the ith attempt of fouling. Three uncoated steel sheet specimens and three coated steel sheet specimens were subjected to fouling resistance testing. The mean values of the fouling rates of these specimens were used for subsequent analysis. The morphology and chemical composition of the specimens subjected to fouling were evaluated using a scanning electron microscope (SEM) (Zeiss Supra 55S), and an X-ray energy dispersive spectrometer (EDS) (OXFORD X-Max 20), respectively. 3. Results and discussion 3.1. Microstructure of ceramic coatings Fig. 1 shows the surface morphology of the steel sheet before and after sandblasting. As seen in Fig. 1b, the roughness of the steel sheet surface increased after the treatment. This can significantly improve the bonding strength of the coating. When sandblasting the pipe walls of a boiler onsite, the slag and fouled layers adhered to the pipe walls can also be removed. Therefore, sandblasting can also help achieve cleaning of the surfaces. Fig. 2a shows the surface morphology of the sintered composite ceramic coating. A small number of microcracks was also observed. This could be due to water evaporation and decomposition of the additives contained in the slurry during sintering or/and due to different thermal expansion coefficients between the coating and the steel substrate [30]. As seen in Fig. 2b, the sintered slurry was highly vitreous due to the presence of potash water glass which helped form strong bonds between the ceramic particles. Therefore, the coating was relatively densified and was well attached to the substrate, almost forming a metallurgical damascene structure. Therefore, spalling of the coating would not be likely to occur easily.
Fig. 1. Surface morphologies of the steel sheet before (a) and after (b) sandblasting.
Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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Fig. 2. Surface (a) and cross-sectional (b) morphologies of the composite ceramic coating.
3.2. Thermal shock resistance The coating began to spall off the substrate after 22 thermal shock cycles with 1/3 of the coating spalling off the substrate after 43 thermal shock cycles. Clearly, the as-prepared composite ceramic coating possessed excellent thermal shock resistance. This superior performance can be attributed to the flaky structure of the h-BN and graphite used in the ceramic formulation [26e28]. As seen in Fig. 3, defective spots appeared on the surface of the coating after 22 thermal shock cycles (Fig. 3b). As the number of thermal shock cycles increased, the number of defective spots on the surface of the coating gradually increased (Fig. 3c). Delamination of the coating occurred after spalling, indicating that only the surface layer of the coating spalled off; the remaining coating stayed attached to the surface of the steel substrate. Thus, the steel substrate was not exposed, and the coating continued to elicit a protective effect. 3.3. Fouling resistance Fig. 4 shows the surface fouling rate curves of the uncoated and coated steel sheet specimens exposed to sodium sulfate. The surface fouling rate of the uncoated steel sheet specimens increased rapidly to 15.9% after a single fouling cycle and then increased gradually as the number of fouling cycles increased. The surface fouling rate of the uncoated steel sheet specimens was essentially fixed at 20.6% during the first four fouling cycles. In comparison, the surface fouling rate of the coated specimens remained essentially 0% even after multiple fouling cycles, indicating good stability of the ceramic coating during the fouling process. Overall, the coating demonstrated excellent fouling resistance to sodium sulfate at 550 C. Fig. 5 depicts the microscopic morphology of the uncoated and coated steel sheet subjected to sodium sulfate fouling. As shown in Fig. 5a, a relatively large number of sodium sulfate particles adhered to the uncoated steel sheet, resulting in increased fouling rate. This was due to the precipitation of sodium sulfate particles in the oxide film formed on the surface of the specimen under high temperature (Fig. 5b). Notably, an external force, such as compressed air, could not remove the sodium sulfate particles in the film because of its flaky structure. In the case of coated specimens, no sodium sulfate particles were observed (Fig. 5c). Fine sodium sulfate particles were only detected in some of the microcracks on the coating surface as shown in Fig. 5d. This is attributed to the formation of microcracks during sintering of the ceramic coating, which helped encapsulate the sodium sulfate particles. Therefore, appropriate measures should be taken to minimize the formation of microcracks during the sintering process. Based on the aforementioned results, it can be concluded that the composite ceramic coating improved significantly the fouling resistance of steel sheets to sodium sulfate. This was mainly attributed to the relatively low surface energy and dense structure of the ceramic coating as well as to its chemical incompatibility with sodium sulfate. In general, ceramic materials have significantly lower surface energies compared to metallic materials and can, therefore, be used as nonstick agents. Given that the ceramic coating used in the present study had a lower surface energy compared to the 15CrMo steel substrate, sodium sulfate could not easily adhere to the surface of coated specimens. As a result, the coated steel sheets exhibited excellent fouling resistance to sodium sulfate. In addition, because ultrafine powders were used as aggregates in the ceramic slurry, the sintered coating had a relatively dense structure, thereby, effectively minimising the formation of pores on the surface of the coating. Consequently, fine sodium sulfate particles did not adhere to its surface. Another main factor determining the fouling resistance of the coating to sodium sulfate was the chemical incompatibility of those two components. Iron, the main element in 15CrMo steel, reacts with sodium sulfate at high temperatures according to the following reaction [31]:
Fe þ Na2 SO4 /FeS þ Fe2 O3 =Fe3 O4 =FeO þ Na2 O
(2)
Therefore, there is chemical affinity between the sodium sulfate particles and the steel surface, resulting in a relatively strong bonding between the two. For this reason, applying compressed air was not sufficient to remove the sodium sulfate particles fouling the steel surface. However, when ceramic coating was applied on the steel surface, the adhesion of sodium sulfate particles was relatively weak. This can be attributed to the presence of h-BN, the main component of the composite ceramic coating, which is chemically inert and does not easily react with other materials. Thus, in this case, most of the sodium sulfate particles found on the surface of coated steel specimens could be easily removed by compressed air.
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Fig. 3. Macroscopic surface morphology of the ceramic coating at different stages of the thermal shock resistance testing: (a) before initiation of the testing, (b) upon formation of first cracks and (c) after 1/3 of the coating spalled off the substrate.
Fig. 4. Surface fouling rate curves of the uncoated and coated steel sheet specimens exposed to sodium sulfate at 550 C as a function of the fouling time. Each fouling cycle comprised of 24 h.
3.4. Engineering applications of the coating In the present study, the composite ceramic coating was sprayed onto the heating surface of a boiler using a Zhundong HSC blend as fuel. The maximum blending ratio of the Zhundong HSC was 70%. The properties of coal samples used are given in Table 2. Coal A and Coal B represent the coal typically used in the plant and the Zhundong HSC, respectively. Zhundong HSC, i.e. coal B, had a high water and volatile content, low ash content, and medium calorific value. The alkali content of the Zhundong HSC ash was also significantly higher, with the Na2O content being a high as 7.4%. Therefore, when the boiler was fired with the Zhundong HSC, serious fouling and slagging problems occurred on the cooling water and platen superheater walls. A photograph and an infrared (IR) thermography image of the coated heating surface near the lowest secondary air nozzle in the boiler system are shown in Fig. 6. A significant difference in the fouling resistance between the uncoated and coated areas of the heating surface was observed. The coated surfaces were relatively clean, whereas ash deposits visibly covered the uncoated surfaces. As seen from the IR thermography image (Fig. 6b), the temperature of the pipe wall in the uncoated area was higher by 90 C compared to the coated area. This is associated with the lower heat transfer capacity of the ash deposits on the uncoated surfaces. Overall, the efficiency of the heat exchanger was enhanced thanks to the superior fouling resistance of the composite ceramic coating. The monthly service time of the soot blower and the amount of water used in the desuperheater were assessed after one year of operation of the boiler. As seen in Table 3, the monthly service time of the soot blower was reduced by 58.2%e116 h after applying the ceramic coating. This sharp decrease suggests that the ceramic coating made the furnace more resistant to slagging. Before the spraying of the coating, severe slag build-up in the furnace caused the rise of flue-gas temperature at the furnace outlet. Consequently, the temperature difference within the superheater increased, leading to higher-temperature superheated steam and increased amounts of desuperheating water. Namely, prior to ceramic coating application, the monthly usage of water in the desuperheater was 25,099 t. However, this was decreased by 75.2% since the furnace slagging was greatly improved after the application of the ceramic coating. Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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Fig. 5. SEM micrographs of the surface of the sodium sulfate-fouled uncoated steel sheet ((a) 100; (b) 1000) and coated steel sheet ((c) 1000; (d) 5000). Table 2 Properties of the two different coal samples used to fire an industrial boiler.
Proximate analysis (wt%) Mar Aar Vdaf FCar Elemental analysis (wt%) Car Har Nar Oar Sar Ash fusion temperature (K) Deformation temperature Softening temperature Flow temperature Ash composition (wt%) SiO2 Al2O3 Fe2O3 TiO2 CaO MgO SO3 K2O Na2O
Coal A
Coal B
12.30 21.80 37.86 40.95
25.00 8.54 36.04 42.51
51.96 2.83 0.68 9.81 0.61
51.77 2.71 0.39 10.88 0.71
1403 1553 1623
1473 1488 1499
50.10 8.27 21.27 0.89 6.35 3.56 4.58 1.58 0.96
35.65 11.40 14.50 4.45 10.70 10.69 0.02 1.84 7.40
Fig. 6. Photography (a) and IR thermography image (b) of the ceramic coating used in real-life boiler system.
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Table 3 Monthly service time of the soot blower and the amount of desuperheating water before and after spraying of the ceramic coating after one year of boiler operation. Parameter
Before spraying
After spraying
Monthly average of superheated steam flow (t$h1) Monthly average of service time of the soot blower (h) Monthly average usage of desuperheating water (t)
421.5 278 25,099
546.0 116 6233
4. Conclusions A composite ceramic coating using the slurry method was developed and applied on the surface of 15CrMo steel to investigate the fouling and thermal shock resistance of the coating. In addition, the coating was sprayed on a boiler fired with a Zhundong high-sodium coal (HSC) blend. The following conclusions can be derived: (1) Upon sintering of the ceramic slurry, a vitreous ceramic coating with a dense structure was formed. This can be attributed to the coalescence of the ceramic powders used, i.e. hexagonal boron nitride (h-BN) and graphite, in the presence of potash water glass. Water evaporation and decomposition of the additives present in the slurry during sintering, coupled with differences in the coefficients of thermal expansion between the coating and the steel substrate, resulted in the formation of a small number of microcracks. However, overall, the coating and the steel substrate exhibited a metallurgical damascene structure and were well bonded. (2) The coating began to spall off the substrate after being subjected to 22 thermal shock cycles, and 1/3 of the coating had spalled off the surface after 43 cycles. Hence, the coating had good thermal shock resistance. (3) The fouling rate of the uncoated steel sheet increased rapidly after it was subjected to sodium sulfate fouling, reaching 21.9% after five fouling cycles. In comparison, the coated steel sheet surface was not fouled after five fouling cycles. The excellent fouling resistance of the composite ceramic coating to sodium sulfate was mainly due to its relatively low surface energy, dense structure and the chemical incompatibility between the coating and sodium sulfate. The flaky oxide film on the uncoated steel sheet surface could easily encapsulate some of the fine sodium sulfate particles. In the case of coated steel surfaces, it was observed that a few fine sodium sulfate particles had also penetrated the microcracks formed during the sintering process. In both cases, these particles could not be removed by compressed air. (4) The composite ceramic coating had an appreciable anti-fouling effect when applied to a boiler fired with a Zhundong HSC blend. Boiler operational data collected within one year show that the monthly average service time of the soot blowers was decreased by 58.2%, and the monthly average usage of water in the desuperheater was reduced by 75.2%. Hence, the ceramic coating developed throughout this study has great potential to be applied in real-life boiler systems to increase their overall efficiency. Acknowledgements This work was supported by the National Natural Science Foundation of China (51408574), the Department of Education of Zhejiang Province in China (Y201534242), and the Public Projects of Zhejiang Province (2017C31043). References [1] S.Y. Zhang, C. Chen, D.Z. Shi, J.F. Lv, J. Wang, X. Guo, A.X. Dong, S.W. Xiong, Situation of combustion utilization of high sodium coal, Proc. CSEE 33 (2013) 1e12. [2] J. Li, X.G. Zhuang, X. Querol, O. Font, N. Moreno, J.B. Zhou, Environmental geochemistry of the feed coals and their combustion by-products from two coal-fired power plants in Xinjiang Province, Northwest China, Fuel 95 (2012) 446e456. [3] H.J. Ge, L.H. Shen, H.M. Gu, T. Song, S.X. Jiang, Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier, Fuel 159 (2015) 107e117. [4] X.Y. Zhang, H.X. Zhang, Y.J. Na, Transformation of sodium during the ashing of Zhundong coal, Procedia Eng. 102 (2015) 305e314. [5] S.A. Benson, P.L. Holm, Comparison of inorganic constituents in three low-rank coals, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 145e149. [6] C.L. Han, Modes of occurrence of sodium in coals, J. Fuel. Chem. Technol. 27 (1999) 575e578. [7] A.R. Manzoori, P.K. Agarwal, The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur low rank coals, Fuel 71 (1992) 513e522. [8] S. Srinivasachar, J.J. Helble, D.O. Ham, G. Domazetis, A kinetic description of vapor phase alkali transformations in combustion systems, Prog. Energy Combust. 16 (1990) 303e309. [9] S.H.D. Lee, F.G. Teats, W.M. Swift, D.D. Banerjee, Alkali-vapor emission from PFBC of Illinois coals, Combust. Sci. Technol. 86 (1992) 327e336. [10] G.Y. Li, C.A. Wang, Y. Yan, X. Jin, Y.H. Liu, D.F. Che, Release and transformation of sodium during combustion of Zhundong coals, J. Energy Inst. 29 (2015) 48e56. [11] P. Radhakrishnan, V. Zakkay, A. Agnone, Alkali and gas emissions from PFB combustion of lignite, Combust. Sci. Technol. 50 (1986) 271e281. [12] Z.H. Xiao, T.K. Shang, J.K. Zhuo, Q. Yao, Study on the mechanisms of ultrafine particle formation during high-sodium coal combustion in a flat-flame burner, Fuel 181 (2016) 1257e1264. [13] X.B. Wang, Z.X. Xu, B. Wei, L. Zhang, H.Z. Tan, T. Yang, H. Mikul ci c, N. Dui c, The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing, Appl. Therm. Eng. 80 (2015) 150e159. [14] X.B. Qi, G.L. Song, W.J. Song, S.B. Yang, Q.G. Lu, Effects of wall temperature on slagging and ash deposition of Zhundong coal during circulating fluidized bed gasification, Appl. Therm. Eng. 106 (2016) 1127e1135. [15] X.J. Wu, X. Zhang, K. Yan, N. Chen, J.W. Zhang, X.Y. Xu, B.Q. Dai, J. Zhang, L. Zhang, Ash deposition and slagging behavior of Chinese Xinjiang high-alkali coal in 3 MW th pilot-scale combustion test, Fuel 181 (2016) 1191e1202. [16] X.B. Wang, H.Y. Wu, S.H. Deng, L. Zhang, B. Wei, H.Z. Tan, A mechanism study on the decomposition of sulfate in Zhundong coal with high sulfur content in coal ash, in: G. Yue, S. Li (Eds.), Clean Coal Technology and Sustainable Development: Proceedings of the 8th International Symposium on Coal Combustion, Springer Singapore, Singapore, 2016, pp. 101e106. [17] K.Q. Tran, K. Iisa, B.M. Steenari, O. Lindqvist, A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector, Fuel 84 (2005) 169e175. [18] S.H.D. Lee, I. Johnson, Removal of gaseous alkali metal compounds from hot flue gas by particulate sorbents, J. Eng. Gas. Turb. Power 102 (1979) 397e402. [19] M. Bl€ asing, M. Müller, Investigation of the effect of alkali metal sorbents on the release and capture of trace elements during combustion of straw, Combust. Flame 160 (2013) 3015e3020.
Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003
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J. Wang et al. / Journal of the Energy Institute xxx (2017) 1e8
[20] H. Wu, M.S. Bashir, P.A. Jensen, B. Sander, P. Glarborg, Impact of coal fly ash addition on ash transformation and deposition in a full-scale wood suspension-firing boiler, Fuel 113 (2013) 632e643. [21] B.M. Jenkins, R.R. Bakker, J.B. Wei, On the properties of washed straw, Biomass. Bioenergy 10 (1996) 177e200. [22] J.B. Li, M.M. Zhu, Z.Z. Zhang, K. Zhang, G.Q. Shen, D.K. Zhang, The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace, Fuel Process. Technol. 149 (2016) 176e186. €m, High temperature corrosion of thermally sprayed NiCr and FeCr coatings [23] T. Varis, D. Bankiewicz, P. Yrjas, M. Oksa, T. Suhonen, S. Tuurna, K. Ruusuvuori, S. Holmstro covered with a KCleK2SO4 salt mixture, Surf. Coat. Tech. 265 (2015) 235e243. [24] J. Chen, Y. Ninomiya, H. Naganuma, Y. Sasaki, M. Noguchi, H. Cho, Y. Ueki, R. Yoshiie, I. Naruse, Development of thermal spraying materials through several corrosion tests for heat exchanger tube of incinerators, Fuel Process. Technol. 141 (2015) 216e224. €rfel, R.S. Neumann, A. Kranzmann, Corrosion of uncoated and alumina coated steel X20CrMoV12-1 in H2OeCO2eO2 and air at 600 C, [25] W. Schulz, M. Nofz, M. Feigl, I. Do Corros. Sci. 68 (2013) 44e50. [26] J. Eichler, C. Lesniak, Boron nitride (BN) and BN composites for high-temperature applications, J. Eur. Ceram. Soc. 28 (2008) 1105e1109. [27] S. Mahathanabodee, T. Palathai, S. Raadnui, R. Tongsri, N. Sombatsompop, Effects of hexagonal boron nitride and sintering temperature on mechanical and tribological properties of SS316L/h-BN composites, Mater Des. 46 (2013) 588e597. Visloguzova, L.V. Serova, Study of carbon-containing refractories for steel smelting production units, Refract. Ind. Ceram. 48 (2007) [28] I.D. Kashcheev, K.G. Zemlyanoi, E.A. 313e316. [29] T.H. Yang, X.P. Kai, Y. Sun, Y.G. He, R.D. Li, The effect of coal sulfur on the behavior of alkali metals during co-firing biomass and coal, Fuel 90 (2011) 2454e2460. [30] Z.Q. Yan, X. Xiong, P. Xiao, F. Chen, H.B. Zhang, B.Y. Huang, SieMoeSiO2 oxidation protective coatings prepared by slurry painting for C/CeSiC composites, Surf. Coat. Tech. 202 (2008) 4734e4740. [31] L.Q. Shi, On the possibility of a Na2SO4-Na2O eutectic melt developing on metals coated with Na2SO4 deposit in oxygen/air at intermediate temperatures, Corros. Sci. 37 (1995) 1281e1287.
Please cite this article in press as: J. Wang, et al., Development and application of anti-fouling ceramic coating for high-sodium coal-fired boilers, Journal of the Energy Institute (2017), http://dx.doi.org/10.1016/j.joei.2017.08.003