Behavior of Fouling Deposits Formed on a Probe with Different

Article pubs.acs.org/EF

Behavior of Fouling Deposits Formed on a Probe with Different Surface Temperatures Hao Zhou,* Bin Zhou, Hailong Zhang, and Letian Li State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: Zhun Dong (ZD) coal, which is characterized by high sodium levels, was burned in a pilot-scale furnace to investigate the behavior of fouling deposits on an oil-cooled probe with different surface temperatures. In this study, the fouling deposits were collected at 1298 K furnace temperature. In addition, the inlet temperatures of the conduction oil were set as 543, 493, and 443 K, which were comparable to the feedwater temperature in the coal-fired boiler economizer. The surface temperatures of the deposition probe varied in the ranges of 733−594, 714−571, and 671−507 K. A digital imaging technique was used to monitor variations in deposit thickness and deposit morphology over time for the three cases. The results revealed that the probe surface temperature had a significant effect on the growth, mineralogy, and microstructure of the fouling deposit. The stable thicknesses of the three fouling deposits fluctuated in the ranges of 9.63−9.82, 12.48−12.54, and 15.31−15.53 mm in the 543, 493, and 443 K cases, respectively. In addition, a low probe surface temperature resulted in the absence of significant sintering in the fouling deposit for the 443 K case.

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deposition.13 A new ash deposit growth model was developed by Mendes et al. to evaluate the characteristics of actual deposits in a utility boiler.14 Lawrence et al. proposed a new coal characterization index based on thermomechanical analysis.15 The results revealed that this index could predict boiler slagging more accurately than conventional indices. Waclawiak et al. also investigated two-dimensional (2D) modeling of powdery, medium-temperature deposit formation on superheater tubes, with results indicating that this numerical approach could predict deposit shape as a function of the boiler operating time.16 Rushdi et al. used a mechanistic approach to predict ash deposition behavior and trends.17 Richards et al. investigated variations in deposit surface temperature, heat flux through the reactor wall, and deposit thickness with time.5 Nevertheless, the modeling of ash deposition growth is not very accurate because of many physical and chemical processes involved. A digital image technique can make it up, which can precisely online monitor ash deposition growth. For the experimental studies, Su et al. proposed that the growth rate (mm/h) was better correlated than the buildup rate (g/h) with the fouling propensity of blended coals.18 Shimogori et al. recognized that fine particles and alkalis had significant effects on heat-transfer deterioration in the initial stage of ash deposit formation.2 Li et al. used laser Doppler velocimetry to measure the particle deposition patterns in a bend.19 Vuthaluru et al. applied an air-cooled probe to collect the ash deposits at different furnace temperatures and quantified the deposit samples by measuring the weight of them at the end of the experiments.20 Mischa et al. evaluated the effect of the probe surface temperature on the formation rate of ash deposition by measuring the weight of deposit samples on a probe.21 In

INTRODUCTION Coal is expected to be the core fossil fuel in the coming decades in China because of its abundant reserves. However, some issues related to the use of coal in power plant boilers need to be resolved. It is generally acknowledged that coal contains several kinds of mineral matter in various forms. Coal combustion in power plant boilers will result in operating and environmental problems, which are principally due to the existence of inorganic elements in coal.1 Ash deposition on heat exchanger surfaces during coal combustion in utility boilers is an issue of common concern.2 The deposition not only reduces the overall heat exchanger capacity because of the low thermal conductivity but also causes erosion of heat-transfer tubes and blockage of flue.3,4 All of the factors will lead to decreased efficiency, diminished electrical production, low system availability, and increased cost of maintenance.5−9 Ash deposition is impacted by a lot of physical and chemical factors, such as the tube surface temperature, tube material, coal type, furnace temperature, reaction atmosphere, ash transport mechanisms, and furnace flow field.10 Inorganic components of coal can be transported into fly ash during combustion. It is known that three transport mechanisms control the ash species deposition on heat-transfer surfaces from flue gas, i.e., inertial impaction, diffusion, and thermophoresis.11 Laursen et al. proposed that inertial impaction was the most important mechanism for large particle (>10 μm) deposition on tubes, turbulent diffusion was the main transport mechanism enabling fine particles (15 25 175 >15

4.30 3.70 1.81 0 6.80 5.69 0 4.02 11.49 0

0.086 0.062 0.036 0 0.105 0.047 0 0.161 0.066 0

39.22 3.58 3.39 0 29.67 1.98 0 43.79 3.01 0 7705

stable thickness (mm)

stable heat flux (kW/m2)

9.63−9.82

90.70

12.48−12.54

45.56

15.31−15.53

35.37

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formation did not occur in the outer deposit layer in the 443 K case. Therefore, deposit porosity and thermal conductivity did not change significantly in the stable stage in the 443 K case. The stable deposit thicknesses for the three cases were 9.63− 9.82, 12.48−12.54, and 15.31−15.53 mm (see Table 3), and the stable heat fluxes of the three cases were 90.70, 45.56, and 35.3 kW/m2, respectively. It can be concluded that stable thickness increases with decreasing the probe surface temperature. However, the stable heat flux appears to have the opposite tendency. These results are confirmed by the morphology of the deposits in the three cases, as shown in Figure 9. The deposit in the 543 K case has undergone melting,

deposit rate in stage 3 was slightly less than that in stage 2 of the 543 K case. All of the heat fluxes through the deposition probe declined dramatically in stage 1 for the three cases (see Figure 8). The ratios of heat flux reduction to deposit thickness increment were as high as 39.22, 29.67, and 43.79 kW mm−2 min−1 for 543, 493, and 443 K cases, respectively, as given in Table 3. It revealed the thermal conductivity of the initial layer deposit was relatively low. The initial layer was commonly characterized by a particulate, porous, and fragile structure, which consists of voids and discrete solid particles, as mentioned in section 3.1. This structure and porosity resulted in relatively low thermal conductivity of the initial layer. Moreover, the initial layer to a large extent determined the overall thermal conductivity of the deposit. Consequently, heat flux through deposition probe decreases evidently in the initial stage for all three cases. However, in the 543 K case, heat flux presented a moderate decrease with deposit growth in stages 2 and 3, as shown in Figure 8, and the corresponding ratios of heat flux decrement to thickness augmenter of stages 2 and 3 were 3.58 and 3.39 kW m−2 mm−1, respectively. The values are distinctly less than that of stage 1. It is well-known that the deposit surface temperature increases with deposit growth. The higher temperature causes the outmost layer of the deposit to be sintered and even to be entirely fused. This sintering process can result in neck formation, and subsequent densification and shrinkage of deposit particles by sintering can give rise to a significantly increased density of the deposit material. Therefore, thermal conductivities of the sintered layer and the slag layer were significantly greater than that of the initial layer, and the corresponding ratios of heat flux decrement to thickness augmenter of stages 2 and 3 were markedly less than that for stage 1. Meanwhile, melt formation in the outermost layer (corresponding to stage 3) resulted in pore filling. The thermal conductivity of the slag layer can be greater than that of the sintered layer. Consequently, the ratio of heat flux decrement to thickness augmenter for stage 3 was slightly less than that of stage 2. In addition, heat flux and thickness both varied slightly at a stable stage, giving rise to aggregation of a number of data symbols into an ellipse, as shown in Figure 8. This may have been caused by continuous formation and fracture of the porosity structure into the deposit during a stable stage. Furthermore, in the 493 and 443 K cases, the heat flux also decreased moderately with deposit growth in stage 2, as illustrated in Figure 8. The corresponding ratios of heat flux variation to thickness increment for these two cases were 1.98 and 3.01 kW mm−2 min−1, respectively. These results were significantly less than those in the initial stage for these two cases. This phenomenon resulted from the occurrence of sintering in stage 2 for these two cases, which caused densification of deposit particles in the outer deposit layer. The thermal conductivity of deposit material of stage 2 was also dramatically greater than that of the initial stage. In consequence, the ratios of heat flux decrement to thickness increase in stage 2 in these two cases were markedly less than in stage 1. In the 493 K case, a number of data symbols were also aggregated into an ellipse in the stable stage, as shown in Figure 8. However, this phenomenon did not occur in the 443 K case. This may have resulted from the low probe surface temperature in the 443 K case, which caused the deposit surface temperature to remain lower than the melting temperature of the deposit material. Consequently, liquid-state sintering and pore structure

Figure 9. Morphology of the three deposits at the last moment obtained by the image monitoring system.

with a large molten fraction at the surface in the stable stage. The deposit in the 493 K case has been intensely sintered. However, the deposit in the 443 K case presents a pine-needle structure with no significant sintering. Consequently, the effective heat conductivity of these deposits follows the order: 543 > 493 > 443 K. In consequence, the stable heat flux in the 543 K case was the greatest among these cases. 3.3. Chemical Composition of the Fouling Deposits. The chemical composition of the three deposits was quantitatively analyzed by EDX analysis, with the results shown in Table 4. These results indicate that the three deposits were rich in Fe, Ca, Si, and Al and less rich in Na, K, Mg, and Ti. In the 543 K case, the concentration of K decreased along the deposit growth direction. However, the Fe content increased in the thickness direction. Using X-ray diffraction (XRD) analysis as described in section 3.4, no iron-bearing crystalline phase was detected. This result may have been caused by the reaction of iron oxides with clay minerals to form 7706

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Table 4. Chemical Compositions of Ash Deposits Identified by SEM−EDX case 543 K

493 K 443 K

layer layer layer layer layer layer layer

1 2 3 1 2 1 2

Na2O

K2O

TiO2

Al2O3

CaO

SiO2

Fe2O3

MgO

2.174 2.539 2.190 1.408 1.819 2.978 2.068

0.117 0.260 0.059 0.123 0.075 0.333 0.089

1.528 1.814 1.453 1.016 1.586 1.889 1.740

15.745 15.078 16.062 13.948 14.895 19.386 17.045

26.130 24.508 24.748 29.622 28.394 20.905 23.708

45.590 47.020 45.500 44.440 43.170 48.040 48.104

6.551 6.938 7.496 7.659 7.921 5.250 5.662

2.162 1.830 2.488 1.779 2.128 1.207 1.585

formation. The sticky condensed K-based components act as an adhesive, which encourages the semi-molten Ca-containing aluminosilicate fly ashes to stick to the deposit surface. Consequently, Ca, Al, and Si were the most abundant elements in the three deposits. A comparison of the components of coal ash and deposit is shown in Figure 10c. It is clear that Al2O3 and SiO2 concentrations in the deposit were greater than those in the coal ash. This reveals that Al2O3 and SiO2 play an important role in deposit formation with the production of aluminosilicates. On the other hand, Na, Mg, and Ca appeared to be depleted in the deposit compared to the coal ash. 3.4. Crystallographic (XRD) Analysis of Fouling Deposits. To obtain further insight into the mineralogy of the collected fouling deposits, XRD analysis was used to identify mineral phases of the deposits. For the 543 K case, the major detected mineral phases in the initial layer (layer 1) were gehlenite, quartz, kyanite, and orthoclase, as shown in Figure 11a. The internal sintered layer (layer 2) was rich in gehlenite, quartz, and albite, and the principal constituents of the external sintered layer (layer 3) were gehlenite, quartz, and anorthite. Quartz intensity decreased with the deposit thickness direction from the initial layer to the external sintered layer. It may result from the transition of this crystalline phase and partial melting into the glassy phase. Furthermore, the gehlenite peak in the internal sintered layer was the highest among the three layers. This suggests that this mineral phase continued to form in the internal sintered layer, with part of it melting into the glass phase in the external sintered layer because of high temperatures. Orthoclase and albite mineral phases may result from a reaction between aluminosilicates and alkali-based condensates. Additionally, the occurrence of gehlenite implies a chemical reaction between quicklime and aluminosilicates. The formation of gehlenite can be explained by eqs 3 and 4 below.

a low-melting-point eutectic. In consequence, the deposit was rich in Ca-bearing aluminum silicates. This result was proven by the XRD patterns depicted in Figure 11, which indicated that the deposit contained an abundance of gehlenite. A comparison of the bulk compositions of the deposit and coal ash is presented in Figure 10a. It is obvious that each deposit layer in the 543 K case contained more SiO2, Al2O3, and Fe2O3 than the ash. This observation indicates that these three elements are very important to deposit formation. However, the Na2O content of the collected deposit was depleted. It is generally acknowledged that sodium-containing species in coal are volatile and end up with NaCl in flue gas during coal combustion.26 Then, these NaCl vapors can form submicrometer particles by homogeneous or heterogeneous condensation on silicon−calcium-based fly ash particles before they adhere to the deposition probe.27 After this, sulfur-bearing compounds in the fly ash particles react with condensed NaCl to form K2SO4, or the condensate reacts with silicon to form sodium silicates. According to ash composition analysis, ZD coal ash is rich in sodium compounds, which account for 8.31 wt % of the total mass. However, chlorine is much less abundant in coal ash. Consequently, large quantities of sodium species end up with fly ash particles or in bottom deposits. Mg appears to exhibit a similar phenomenon to Na because the deposit contains less magnesium oxide than coal ash. Other elements present no clear difference between coal ash and deposit. In the 493 K case, the concentration of K decreased with the deposit growth direction. Nevertheless, the iron content increased in the thickness direction. This phenomenon was similar to deposits in the 543 K case. Other elements did not change evidently along the deposit thickness direction. Figure 10b shows a comparison of bulk compositions of coal ash and fouling deposits. Layers 1 and 2 of the deposits contained more SiO2, CaO, and Fe2O3, which suggests that these three elements are extremely important to the formation of the initial and sintered layers. Nevertheless, no Fe-bearing mineral phase was detected in the XRD patterns illustrated in Figure 12. This result reveals that iron oxide may have reacted with aluminosilicate to form eutectics with low fusion temperature, which cannot be detected by XRD analysis. Na and Mg showed similar results to those in the 543 K case, and the concentration of aluminum oxide in the coal ash was equal to that in the deposits, as shown in Figure 10b. In the 443 K case, the concentrations of Al2O3, Na2O, K2O, and TiO2 decreased with the deposit growth direction. However, the contents of CaO and Fe2O3 increased with the thickness direction. Moreover, the Si content increased slightly in the deposit growth direction. In conclusion, the variations in potassium content in the layer structure for the three cases indicate that condensation of K-containing vapors is the major facilitator for initial layer

CaCO3 → CaO + CO2

(3)

Al 2O3·SiO2 + 2CaO → Ca 2Al 2SiO7

(4)

Anorthite was present only in layer 3, which indicates that this mineral phase was formed at relatively high temperatures. However, Unsworth et al. demonstrated that a solid-state reaction is the major formation mechanism for the anorthite phase;28 this reaction can be expressed by eq 5. Al 2O3·2SiO2 + CaO → CaAl 2Si 2O8

(5)

Figure 12 shows the results of XRD analysis of the deposit collected in the 493 K case. The dominant components of the initial layer (layer 1) were gehlenite, quartz, albite, and ussingite. The appearance of albite in the initial layer may have resulted from a chemical reaction between Na-bearing condensate and aluminosilicate-containing fly ash particles. The sticky albite can promote adhesion of fly ash particles to the deposit surface. Layer 2 contained similar crystalline phases to 7707

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Figure 10. Comparison of bulk composition between ash and deposits for the three cases.

Figure 11. XRD patterns of the deposit sample collected in the 543 K case.

this crystalline phase has a relatively low melting temperature. Because of the higher temperature of the sintered layer in comparison to the initial layer, the quartz melted into a silica amorphous phase. The XRD analysis of the fouling deposit formed in the 443 K case is presented in Figure 13. Layer 1 was rich in gehlenite, quartz, nepheline, sanidine, and aragonite. However, the major

the initial layer, except for ussingite. This implies that ussingite can melt into a glassy phase at high temperatures. In addition, the gehlenite peak increased with the deposit growth direction, as shown in Figure 12. This indicates that gehlenite continued to form with the deposit growth and that its melting temperature is relatively high. Meanwhile, quartz intensity decreased in the deposit growth direction, which reveals that 7708

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Figure 12. XRD patterns of the deposit sample collected in the 493 K case.

Figure 13. XRD patterns of the deposit sample collected in the 443 K case.

mineral phases detected in layer 2 were gehlenite, nepheline, anorthite, and ussingite. In addition, the quartz peak was highest in the initial deposit layer in the 443 K case, as illustrated in Figure 13a. This suggested that a low probe surface temperature could facilitate quartz formation. Raask reported that the silica-bearing amorphous phase can be produced by reduction of SiO2 to SiO during coal combustion according to eq 6 below.29

deposit growth. Higher temperatures can promote the formation of gehlenite according to the formation mechanism expressed in eqs 3 and 4. 3.5. Microstructure of Fouling Deposits through SEM Analysis. Panels a−c of Figure 14 present the microstructure of a deposit cross-section in the 543 K case. It is clear that each deposit layer had a significantly different microstructure. Layer 1 (the initial layer) presented no evident agglomerates or bridge formations, as depicted in Figure 14a. At the same time, the void space between the ash deposit particles was filled by epoxy resin. The initial layer was formed by condensation of alkali-containing vapors and impact of Si−Ca-based fly ash particles. Because this layer was adjacent to the deposit probe surface, it did not sinter significantly. In consequence, the deposit particles in this layer were isolated from their neighbors or contacted each other weakly. This result implied that the heat conductivity of the initial layer was relatively low, which was verified by the results shown in Figure 8 that the heat flux decreased significantly in stage O−A. In Figure 14b, the epoxy resin area has decreased markedly and has been replaced by a more compact morphology. Moreover, a more continuous phase was presented by severe sintering of ash particles. The internal sintered layer (layer 2) contained many regular pores

SiO2 + C → SiO + CO

(6)

Volatile SiO can be oxidized as SiO2 in the flue gas, and subsequently, the vapor can recondense as silica fumes.29 Consequently, a low probe surface temperature can facilitate the formation of quartz in the initial layer. This result can explain the observation that the quartz peak was highest in the initial deposit layer in the 443 K case. Quartz intensity decreased with the deposit growth direction from the initial layer to the sintered layer. It may result from the higher temperature of the sintered layer compared to the initial layer, which impeded silica fumes from condensation on the deposit surface. This could also explain the similar quartz concentration change phenomena in the other two cases. The increasing gehlenite peak indicates that gehlenite continues to form with 7709

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Figure 15. Electron micrographs of the cross-section of the deposit for the 493 K case.

matrix. These may have been caused by shrinkage of deposit particles at high sintering temperatures. Panels a and b of Figure 16 depict SEM images of crosssectional deposit structures in the 443 K case. The two deposit

Figure 14. Electron micrographs of the cross-section of the deposit for the 543 K case.

with diameters less than 200 μm. Liquid-state sintering occurred in this layer. This sintering process is characterized by shrinkage and densification of deposit particles. Because of the relatively low thermal conductivity of the initial layer, the surface temperature of the internal sintered layer (layer 2) increased with deposit growth. Layer 2 could have been largely fused, which embedded the voids in a continuous deposit matrix. Therefore, the sintered layer contained an abundance of small pores. Figure 14c illustrates clear fusion and agglomeration in layer 3. The two zones can be easily distinguished by their different colors and brightness. The light gray areas represent the molten fraction of the deposit, and the darker gray areas denote epoxy resin. The ash particles have melted completely, forming a continuous solidified phase. The external sintered layer (layer 3) contains several huge voids with diameters larger than 500 μm. This layer corresponds to the later stages of sintering. The surface temperature of the external sintered layer (layer 3) was found to increase further with deposit growth. Because of the higher surface temperature, the deposit material was completely melted, as shown in Figure 9a. In consequence, the deposit material in this layer was compact, and some macropores appeared in the slag layer. In conclusion, the dense microstructures of the last two layers (layers 2 and 3) suggest that the thermal conductivities of these two layers were higher than that of the initial layer (layer 1). Electron micrographs of cross-sectional structures in the fouling deposit in the 493 K case are shown in panels a and b of Figure 15. The two layers can be observed to have significantly different microstructures. As illustrated in Figure 15a, epoxy resin areas filling in the voids between the ash deposit areas, similar to those shown in Figure 14a, can be seen in the SEM image of the initial layer (layer 1) in the 493 K case. Nevertheless, the ash deposit areas were starting to connect with each other to form bridges. In Figure 15b, the ash deposit areas have substantially melted, forming a more compact morphology than that of the inner layer. This microstructure resulted in relatively high thermal conductivity in layer 2 compared to layer 1. In addition, macropores and small voids with spherical shapes were found to coexist in the deposit

Figure 16. Electron micrographs of the cross-section of the deposit for the 443 K case.

layers had similar microstructures, with no evident agglomeration formation. In addition, the voids between the deposit areas were filled by epoxy resin in these two layers. This implies that the effective heat conductivity of this deposit was relatively low. However, the deposit particles in the sintered layer were starting to connect with each other to form bridges, as shown in Figure 16b. In comparison to the initial layer, the sintered layer presented a more continuous phase, which may have been caused by occurrence of solid-state sintering in this layer. Solidstate sintering is caused by chemical potential or difference in free energy between the neck and particle surface. Because of the low probe surface temperature in the 443 K case, this sintering was a slow process, with no melting involved. However, sintering caused an increased contact area between the particles in layer 2 compared to layer 1.

4. CONCLUSION ZD coal, which is characterized by high sodium levels, was tested in a pulverized coal-fired furnace to investigate the behavior of fouling deposits on a probe with different surface temperatures by means of digital imaging techniques. During the experiments, fouling deposits were collected at 1298 K furnace temperature. The surface temperatures of the deposition probe varied in the ranges of 733−594, 714−571, and 671−507 K with deposit growth. A relationship between fouling deposit behavior and probe surface temperature has been obtained as follows: (i) The probe surface temperature has a significant influence on fouling deposit growth. In the 543 K case, the growth process of the fouling deposit is composed of four stages; the corresponding growth rates were 0.086, 0.062, 0.036, and 0 mm min−1. However, the growth process of the fouling deposit in the 493 K case contained only three 7710

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stages, with growth rates of 0.105, 0.047, and 0 mm min−1 corresponding to stages 1, 2, and 3, respectively. In the 443 K case, the fouling deposit growth process was also composed of three stages. The corresponding deposit rates of the three stages were 0.161, 0.066, and 0 mm min−1. The stable thicknesses of the three fouling deposits fluctuated in the ranges of 9.63−9.82, 12.48−12.54, and 15.31−15.53 mm in the 543, 493, and 443 K cases, respectively. It can therefore be concluded that the stable thickness increases with decreasing the probe surface temperature. (ii) The crystalline phase of the fouling deposit has a clear relationship with the probe surface temperature. The fouling deposits in the 543 K case were rich in gehlenite, quartz, kyanite, orthoclase, albite, and anorthite. In the 493 K case, the dominant identified components in the fouling deposit were gehlenite, quartz, albite, and ussingite. In the 443 K case, the main components of the fouling deposit were gehlenite, quartz, nepheline, sanidine, aragonite, anorthite, and ussingite. Concentrations of all compounds in the fouling deposits varied significantly in the deposit thickness direction. (iii) Changes in the probe surface temperature had an evident impact on the microstructure of fouling deposits as determined by SEM analysis. A low probe surface temperature resulted in an absence of significant sintering in the fouling deposit in the 443 K case.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87952598. Fax: +86-571-87951616. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51476137), the National Basic Research Program of China (973 Program) (2015CB251501), and the Zhejiang Provincial Natural Science Foundation of China (LZ12E06002).

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