Investigation of the Influence of the Furnace Temperature on Slagging

Aug 26, 2014 - The calculated results for ash deposit viscosity revealed that the ..... The deposit growth rates for the six stages in the 1573 K case...
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Investigation of the Influence of the Furnace Temperature on Slagging Deposit Characteristics Using a Digital Image Technique Hao Zhou,* Bin Zhou, Letian Li, and Hailong Zhang State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: This research investigated the effect of the furnace temperature on ash deposition behavior using digital image techniques and combined chemical equilibrium calculations with the improved model to calculate ash deposit viscosities at different temperatures. Three different furnace temperatures were selected: 1473, 1523, and 1573 K. In addition, the chemical components of the deposit samples were analyzed by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDX). The results revealed that the growth process of the ash deposit at 1573 K was significantly different from those at 1523 and 1473 K. Meanwhile, it revealed that a low temperature can facilitate an increase in deposit thickness. The chemical equilibrium calculations indicated that the main crystalline phases in the three deposits were anorthite and hematite. The calculated results for ash deposit viscosity revealed that the viscosity of ash deposits decreased with increasing the temperature.

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INTRODUCTION As is well-known, China has large coal reserves. In consequence, the primary source of electric power will be coal-fired power plants in next few decades. However, coal contains many inorganic species in a variety of forms. The variety will result in severe operating and environmental issues when coal is burned in coal-fired plants.1 In particular, ash-related fouling and slagging are particularly serious problems for power station boilers. The relatively low heat conductivity coefficient of deposits gives rise to a reduction in the overall heat-transfer coefficient.2 Meanwhile, ash deposits also lead to reduction in power generation capacity, unscheduled outages, erosion of boiler tubes, decreased system reliability, and increased generating costs.3−6 As known to all, the formation of ash deposits contains plenty of physical and chemical processes. Consequently, these deposits can be influenced by a lot of factors, for example, the ash chemical composition, type of mineral existing in ash, ash melting temperature, furnace temperature, material of the tube, tube surface temperature, flow fields of the furnace, ash particle size, and corresponding temperature.7 Inorganic elements in coal will transform into the fly ash during pulverized coal combustion in boilers. According to early research,8 ash deposition on tubes from flue gas was controlled by three transport mechanisms: inertial impaction, thermophoresis, and diffusion. Rushdi et al. investigated that Brownian motion and thermophoretic force were the major transport mechanisms that induce fine particles to deposit on heat-exchanger tubes.9 The deposition of a large particle is controlled by turbulent diffusion and inertial impaction. Many investigations of ash deposition behavior have been conducted in recent years, involving both theoretical and experimental work. Among theoretical studies, for instance, a two-dimensional (2D) model of ash deposit formation was developed, an improved deposition model was proposed in combination with a comprehensive combustion code, and chemical equilibrium calculations were used to predict ash deposition behavior.2,10−15 Among experimental studies, air- or © 2014 American Chemical Society

water-cooled probes have been used to investigate the ash deposition behavior.16−21 Moreover, Kupka et al. applied online weighing technology to evaluate ash deposit growth.22 However, a limited number of studies have monitored the growth process of ash deposition online. In this study, a charge coupled device (CCD) has been applied to monitored the growth morphology of ash deposition online. Meanwhile, the images of the slagging deposit have been processed by a digital image technique to quantitate the ash deposition. The purpose of the present work was to evaluate the ash deposit characteristics of Shanmei (SM) coal at different furnace temperatures in a 300 kW coal-fired furnace through digital image techniques. As is well-known, the furnace temperature strongly influences deposit viscosity. Additionally, deposit strength and growth rate are significantly affected by deposit viscosity. Consequently, chemical equilibrium calculations combined with an improved Urbain model were used to obtain ash deposit viscosities under different furnace temperatures. This work will strongly promote insights into the mechanisms of deposit formation.

2. EXPERIMENTAL SECTION 2.1. Furnace and Experimental Conditions. The slagging tests were carried out in a 300 kW furnace burning coal, and the corresponding schematic diagram was depicted in Figure 1. The furnace was mainly composed of a combustion chamber (length of 3950 mm and inner diameter of 350 mm) with a 350 kWth swirl burner for pulverized coal located on the top. In addition, ash deposition samples were collected by an ash deposit sampling system during the experiments. When the experimental condition was stable, the deposition tube was placed into the center of the combustion chamber, and it is 1910 mm distant from the burner nozzle (see Figure 1). Additionally, the growth process of the ash deposit was monitored online by a CCD monitoring system. The fuel feeding rate of the coal feeder was fixed at 45 kg/h once the experiments arrived at their expected conditions. Received: July 22, 2014 Published: August 26, 2014 5756

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Figure 1. Sketch map of the furnace system.23 The furnace was described in more detail in our previous study.23 In this study, the furnace temperatures were set to 1473, 1523, and 1573 K and the range of the oxygen content in flue gas was between 4.0 and 5.0% by volume during the experiments. 2.2. Deposit Sampling Technology and Image Sampling System. The ash deposition tube made of stainless steel mainly consisted of two sections: the annular tube cooled by oil and the deposit sampling probe. The length and outer diameter of the tube are 76 and 40 mm (see Figure 2a), respectively. Additionally, the sampling probe and tube were connected by a screw thread during the experiments. During the experiments, a heat conduction oil heater was used to keep the temperature of cooling oil at 503 K. The inner and outer surface temperatures of the deposit sampling probe were measured by two K-type thermocouples, which were installed in small holes located on the end face of the probe (see Figure 2b). A CCD monitoring system was used in this work to capture the growth morphology of the ash deposit. The corresponding sketch map of the system is depicted in Figure 2c. The system was principally composed of four sections: (1) CCD camera, (2) camera shield, (3) camera lens, and (4) protective tube. Our early study23 illustrated the monitoring system in greater detail. 2.3. Digital Image Technique. During the tests, the growth morphology of the ash deposit was captured by the image sampling system in video form. Then, the image processing software was used to extracted 24 bit images from the recorded video. Subsequently, the slag images then underwent edge-detection processing. The parameters of circles in the edge images can be calculated by Hough transform, as shown in Figure 4. To simplify the calculations, the maximum height of the ash deposit was taken to represent deposit thickness and was expressed as h. In this study, the outer diameter of the deposition probe, D1, was assumed as a constant during the experiments. Furthermore, PD represents the number of pixels of D1, and the number of pixels of the deposit thickness is expressed as Ph. The image processing software can easily obtain the values of PD and Ph. Therefore, the following expression can be applied to obtain the deposit thickness h:

h=

PhD1 PD

experiments. The corresponding properties of coal ash are given in Table 1. Apparently, the ash contains a great quantity of SiO2, Al2O3, and CaO, which account for 89.93 wt % of the total. In addition, the feed coal was ground by a fan mill with a particle size less than 70 μm before the slagging tests. 2.5. Heat Transfer through the Deposition Probe. In this study, fly ashes deposited mainly on the upside of the probe. In addition, the appearance of the deposition sample resembled a half ellipse in shape, as depicted in Figure 2b. In consequence, the heat flux through the deposition probe could be calculated using the heat conduction model for the cylindrical wall. To simplify the calculation, heat conduction is supposed toward the radius direction of heat flux, i.e., heat transfer through point A to point B in Figure 2b. In consequence, heat flux through the deposition probe can be calculated by eq 2 q=

λ(t 2 − t1) r ln

() r2 r1

(2)

where λ denotes the heat conductivity of the probe material, t2 and t1 represent the outside and internal surface temperatures of the probe, r1 denotes the length between the center of the probe and point B in Figure 2b, r2 denotes the length between the center of the probe and point A in Figure 2b, and r denotes the semi-diameter of the deposition probe. Figure 3 illustrates the corresponding variations of t2 versus time for the three cases. 2.6. Chemical Equilibrium Calculations. It is generally acknowledged that furnace temperatures significantly affect the melting behavior of ash deposits. The melting characteristic of the ash deposit will greatly influence the deposit growth rate and deposit viscosity. Therefore, to investigate the influence of the furnace temperature on ash deposition melting characteristics, a chemical equilibrium calculation was carried out using the FactSage software, version 5.2. In the calculations, the volume fractions of O2, CO2, CO, and N2 gas are fixed, and the corresponding values are 5, 16.0, 0.2, and 78.1%, respectively. Table 1 shows the ash constituents applied in the calculation. In addition, the conditions of the chemical equilibrium calculations are shown in Table 2. Furthermore, a slag−liquid system has been selected as the slag system for the calculation. The corresponding species of a slag−liquid system have been listed in Table 3. The mass

(1)

2.4. Coal Samples. SM coal produced from Shanxi in China, which has large reserves, was selected as the feed coal for slagging 5757

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Figure 2. (a) Sketch map of the deposition probe, (b) feature of the deposition sampling probe, and (c) schematic diagram of the image sampling system.23 percent concentration of molten slag in the overall ash is determined by the calculation with 10 K increments. The corresponding temperature range is from 1073 to 1873 K.

For the other two cases, the deposits also presented a distinctly layered structure, as shown in panels b and c of Figure 5. Additionally, the widths of the three deposits were found to increase with the furnace temperature. 3.2. Deposit Growth for Different Furnace Temperatures. Because of the long duration of the experiments, the CCD camera produced numerous deposit images. Therefore, digital image techniques were used to determine deposit thickness by extracting slagging images from the video. In this research, the image extraction frequency was one image per 5 min. The variations of deposit thickness with time for the three furnace temperatures are illustrated in Figure 6. The corresponding changes of heat flux through the probe versus thickness are shown in Figure 8. Table 4 gives the corresponding growth parameters for the three cases. As seen

3. RESULTS AND DISCUSSION 3.1. Physical Characteristic of Ash Deposits. The duration of each experiment was 4 h in this study. After the experiments, all of the ash deposits on the probe were collected and taken for analysis. Corresponding cross-sections of the three deposits are shown in Figure 5. The deposit of the 1473 K case appears as a distinctly stratified structure with different hardness and colors. According to this result, the deposit mainly consists of three layers: the first layer (initial layer), the second layer (sintered layer), and the third layer (slag layer). Moreover, the sintering degree of the three layers increases with the deposit thickness direction from layer 1 to layer 2. 5758

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Table 1. Properties of the Feed Coal coal sample

SM coal

heating value (MJ/kg) moisture (wt %, ad) proximate analysis (wt %, db)

ultimate analysis (wt %, db)

ash fusion temperature (K)

Figure 3. Variations of t2 versus time for the three cases. ash composition (wt %)

from Figure 6, the growth curves of the three cases contain several segments according to their different slopes. The growth processes of the deposit for the 1473 and 1523 K cases were composed of four stages: the first stage (initial stage), the second stage (sintering stage), the third stage (slagging stage), and the fourth stage (stable stage). The results are consistent with the conclusion stated in section 3.1 that both the slag deposits for the 1523 and 1473 K cases had a three-layer structure. In the 1573 K case, because of the appearance of shedding at 88 min, the deposit growth process consisted of six stages: the first stage (stage OA), the second stage (stage AB), the third stage (stage BC), the fourth stage (stage DE), the fifth stage (stage EF), and the sixth stage (stage FG). The occurrence of shedding resulted from the high furnace temperature in the 1573 K case. A high furnace temperature resulted in a higher molten fraction in the ash deposit and a lower viscosity of the ash deposit (lower adhesive strength). The corresponding morphologies of the ash deposit before and after shedding as recorded by the CCD camera are shown in Figure 7. Apparently, it can be found that the ash deposit is lightly sintered. According to the study by Zbogar et al.,8 the spontaneous shedding may result from thermal stresses, erosion, and gravity force. In addition, it can be seen that

HHV ash volatile matter fixed carbon C H O N S Cl IT ST HT FT Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO2 Fe2O3

29.74 3.14 9.81 33.15 57.04 72.42 4.38 12.11 0.77 0.31 0.20 1466 1472 1476 1478 0.74 1.08 20.22 48.58 0.25 1.26 21.23 0.73 0.25 5.65

Table 2. Conditions of Chemical Equilibrium Calculations temperature (K) gas composition (%)

ash composition (wt %)

1090−1873 78.1 N2 O2 5.0 CO2 16.0 CO 0.2 SiO2, MnO2, CaO, Fe2O3, K2O, MgO, Na2O, TiO2, P2O5, and Al2O3

the removal of deposit material occurs within the deposit mass. Consequently, it may be mainly caused by the thermally induced stresses (i.e., by combustion fluctuations) and

Figure 4. Description of the digital image technique: (left) slag image and (right) edge image. 5759

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Table 3. Species of a Slag−Liquid System code

species

data

436 437 438 439 440 441 442 443 444 445

MgO (SLAG A) FeO (SLAG A) MnO (SLAG A) Na2O (SLAG A) SiO2 (SLAG A) TiO2 (SLAG A) Ti2O3 (SLAG A) CaO (SLAG A) Al2O3 (SLAG A) K2O (SLAG A)

FACT FACT FACT FACT FACT FACT FACT FACT FACT FACT

phase FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG FACT-SLAG

A A A A A A A A A A

Figure 5. Cross-sections of deposit samples formed at different cases.

mechanically induced tensions (i.e., by mechanical fluctuations).24 Meanwhile, the experimental condition was comparably stable in the later 160 min. Combustion fluctuations and mechanical fluctuations did not take place. There was no fracture inside the deposit. In consequence, this shedding did not repeat in the later 160 min. The deposit growth rates for the six stages in the 1573 K case were 0.075, 0.138, 0.062, 0.076, 0.065, and 0 mm/min, corresponding to stages 1, 2, 3, 4, 5, and 6, respectively. The deposit growth rates of the four stages for the 1523 K case were 0.061, 0.095, 0.047, and 0 mm/min, respectively. The deposit growth rates of the four stages of the 1473 K case were 0.037, 0.106, 0.058, and 0 mm/min, respectively. It can be found that there is no shedding that happens in the 1523 and 1473 K cases. Zbogar et al. showed that the deposit shedding is significantly affected by the deposit strength.8 According to the study by Piroozmand et al.,25 the mechanical strengths could be divided into three categories: the tube−deposit adhesion strength, the bend strength, and the tensile strength. The process of deposit removal is mainly dependent upon the tensile strength. This strength increases with the increased temperature.26 Therefore, the tensile strength of the three cases is sorted in the following sequence: 1473 < 1523 < 1573 K. However, the experimental conditions of the 1473 and 1523 K cases are more stable than the 1573 K case during the experiments. Therefore, there will be no fracture inside the two deposits formed at 1523 and 1473 K cases. Consequently, the deposit growth plateaued at a lower temperature but did not shed. It is interesting to observe that the deposit growth rate in

Figure 6. Variation of the deposit thickness versus time for the different furnace temperatures.

stage 1 increased with the furnace temperature, as illustrated in Figure 6. Furthermore, heat fluxes decreased dramatically in stage 1 for all three cases (see Figure 8). The ratios of reduction in heat flux to increase in thickness in stage 1 were as high as 5760

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Table 4. Parameters of Deposit Growth for the Different Furnace Temperature Cases case 1573 K

1523 K

1473 K

stage of growth stage stage stage stage stage stage stage stage stage stage stage stage stage stage

1 2 3 4 5 6 1 2 3 4 1 2 3 4

time Δt (min)

thickness increment (mm)

slope (mm/min)

ratio of heat flux variation versus thickness increment (|Δq|/Δh, kW m−2 mm−1)

20 25 40 30 95 >10 20 30 135 >25 35 40 155 >15

1.49 3.45 2.46 2.29 6.17 0 1.21 2.85 6.39 0 1.29 4.24 9.09 0

0.075 0.138 0.062 0.076 0.065 0 0.061 0.095 0.047 0 0.037 0.106 0.058 0

154.01 10.90 8.92 34.47 7.73 0 116.96 19.34 11.40 0 130.42 5.14 6.09 0

stable thickness (mm)

stable heat flux (kW/m2)

10.05−10.17

156.72

10.47−10.56

152.88

14.62−14.76

114.88

Figure 7. Slagging images before and after shedding.

154.01, 116.96, and 130.42 kW m−2 mm−1 for the 1573, 1523, and 1473 K cases, respectively. Afterward, heat flux decreased moderately with increasing deposit thickness for all three cases. This may be because the deposit growth was in the sintering stage, which results in shrinkage and compacting of deposit particles. Meanwhile, the coefficient of heat conductivity of the deposits will increase significantly. In addition, for the 1523 and 1473 K cases, the data symbols in the stable stage were agglomerated into an ellipse, as illustrated in panels b and c of Figure 8, and their corresponding thicknesses fluctuated within a narrow range, as shown in Figure 6. This phenomenon may have resulted from the balance between viscous forces, gravity, and surface force, which act on the deposit surface. When the experimental condition achieved the steady state, the deposit thickness would increase to the maximum. The value fluctuated in a small range. In this stage of deposit growth, the viscous deposit surface continued to collect both sticky fly ash particles and condensing materials. Meanwhile, the deposit may be removed by erosion, thermal shock, and melting. Furthermore, the loss of deposit mass is balanced by the continued deposition material. As seen from Table 4, the stable thicknesses were 10.05− 10.17, 10.47−10.56, and 14.62−14.76 mm for the 1573, 1523, and 1473 K cases, respectively. This suggests that low

temperatures can facilitate an increase in deposit thickness. This may be because a high furnace temperature gives rise to more viscous liquid slag in the deposit, which impedes the deposit particles to heap up. The corresponding morphological feature of the three slags in the stable stage is shown in Figure 9. 3.3. Chemical Composition of Slagging Deposits. After the experiments, the epoxy resin was used to embed the collected ash deposit samples. To obtain a smooth crosssection, the samples embedding in the resin were cut, ground, and buffed. Subsequently, the samples were sprayed with gold and observed by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDX) to obtain chemical composition of the slags. The EDX results in oxide form are given in Table 5, and they were normalized to 100%. Table 5 shows that the dominant elements in the deposits were Si, Al, Ca, and Fe. Nevertheless, K, Na, Ti, and Mg have a low content. In addition, the contents of Si, Na, Mg, K, and Fe decreased with the thickness direction from layer 1 to layer 3 in the 1473 K case. In particular, the content of K changed significantly along the deposit growth direction. This may be because potassium generally ends up with KCl during coal combustion. This volatile alkaline species can condense homogeneously on the probe or heterogeneously 5761

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Figure 9. Morphological characteristic of the three slags in the stable stage taken by the CCD camera.

the study by Zbogar et al.,8 volatilized KCl can condense on the tube surface. Then, the condensates can react with sulfurbearing compounds (probably SO3 or SO2) to form K2SO4 in the innermost layer. Furthermore, along the direction from the first layer to the second layer, Al and Ca concentrations increased. As known to all, the deposit growth will result in the increase in the surface temperature. In consequence, the condensed alkali-bearing materials will be viscous, which will promote the deposit surface to collect the Al−Ca-containing fly ashes. Therefore, the contents of Al and Ca increase with the deposit growth direction. Meanwhile, it indicates that Al and Ca are very important to the deposit growth. In the 1523 K case, the contents of Na, Mg, and Si declined with the deposit growth direction. According to the study by Robinson et al.,27 sodium species in the coal generally end up with NaCl when the coal combusts in the boilers. Madhiyanon et al. showed that the NaCl vapors deposited on the tube principally through the homogeneous condensation in the flue gas and form sub-micrometer particulate.28 On the other hand, it could heterogeneously condense on fly ashes prior to reaching the tube. However, the deposit surface temperature increases with its growth. The condensation of NaCl vapors will be hindered by higher temperatures. Other elements did not exhibit an obvious regular tendency with thickness. However, in the 1573 K case, Na, Mg, and Fe concentrations decreased with the deposit growth direction from the first layer to the second layer. Consequently, it can be concluded that Na concentrations appear with similar tendency with the deposit thickness direction in all three cases. This suggested that Na is very important in initial layer formation. The concentration of Fe in the initial layer is largest. This may be caused by the erosion of the deposition probe.

Figure 8. Heat flux through the deposition probe as a function of the deposit thickness for different cases.

on the fly ash. This viscous condensate will promote the Si− Ca-bearing fly ashes to adhere to the deposit. Therefore, the level of K in the initial layer is the highest. However, the deposit growth will give rise to the increase in the surface temperature of the deposit. This phenomenon impedes the volatile alkaline species from condensing on the deposit surface. According to 5762

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Table 5. Chemical Compositions of the Slags Collected at Different Furnace Temperatures Determined by SEM−EDX case 1473 K

1523 K

1573 K

layer layer layer layer layer layer layer layer layer

1 2 3 1 2 3 1 2 3

Fe2O3

K2O

MgO

CaO

Na2O

Al2O3

SiO2

TiO2

9.54 6.12 8.13 6.86 7.58 6.00 8.15 6.70 2.69

0.94 0.49 0.08 0.81 0.26 0.29 0.35 0.87 0.12

0.97 0.70 0.66 0.70 0.64 0.49 0.94 0.45 0.42

14.77 19.99 17.89 17.123 20.72 18.98 16.08 15.62 16.44

0.83 0.56 0.51 0.46 0.37 0.30 0.62 0.49 0.44

15.67 19.49 22.43 18.79 16.90 23.97 19.02 21.63 28.71

55.15 52.31 48.66 53.75 51.69 48.66 53.77 52.87 50.83

2.13 1.35 1.65 1.52 1.83 1.32 1.08 1.37 0.34

3.4. Calculation of Slagging Deposit Viscosity. As is well-known, the furnace temperature significantly influences the molten fraction of ash deposits. The presence of this viscous liquid strongly determines deposit viscosity, which affects deposit strength and the sticking probability of fly ashes on the deposit surface. Consequently, acquisition of ash deposit viscosity at different furnace temperatures would be very helpful in obtaining further insight into the influence of the furnace temperature on the ash deposit growth mechanism. Slag deposits in the furnace are generally composed of a suspension of mineral phases and homogeneous liquid. These liquid−solid mixtures are referred to as non-Newtonian fluids. In this study, mixture viscosities were calculated using the Einstein−Roscoe model29 ηs = ηo(1 − cχ )−5/2

(3)

where ηs is the viscosity of the heterogeneous slag, ηo is the homogeneous liquid viscosity, c represents a constant, with a value of 1.35, and χ represents the volume fraction of solid particles. Moreover, the viscosity of the homogeneous melting slag was determined by the Urbain model.30 The volume fraction of the solid mineral phases and the constituents of the homogeneous liquid slag were obtained by FactSage. Figure 10a illustrates the variation of the liquid slag fractions in ash with the temperature. The ash started to melt at 1090 K, and the liquid slag fraction of the ash increased significantly in the range from 1300 to 1460 K. Subsequently, the growth rate of the molten slag fraction appeared with a moderate increase. Figure 10b shows the corresponding chemical compositions of the molten slag at three different temperatures where the deposition probe was located. It can be seen that SiO2, CaO, and Al2O3 were the dominant components of the molten slag. In addition, SiO2 and CaO concentrations decreased with the temperature. However, the contents of Al2O3 and Fe2O3 showed the opposite tendency. The other constituents did not change evidently with the temperature. The corresponding concentrations of crystal phases in the overall mineral phases are illustrated in Figure 10c. The major crystal phases in the overall mineral phases were anorthite and hematite. In particular, the anorthite content could be as high as 20−30 wt % of the overall slag. It is apparent that the concentrations of anorthite, hematite, and leucite increased with the temperature. Figure 11 illustrates the calculated viscosity of the ash deposit versus the temperature. Obviously, it can be found that the viscosity of the slag declined with increasing the temperature. The value did not change significantly until the temperature rose above 1650 K. In addition, ash deposit viscosities at the temperature where the deposition probe was located are also given in the figure. Clearly, the viscosity of the ash deposit that

Figure 10. (a) Calculated results of the liquid slag fraction in ash, (b) chemcial compositions in the molten slag phase at different temperatures, and (c) crystalline phases in the overall slag phase. 5763

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temperature. This may be a reason that some crystalline phases melt into the amorphous phase.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LZ12E06002) and the Key Technologies R&D Program of China (2011BAA04B01).

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Figure 11. Viscosity of the ash deposit versus the temperature.

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was formed at 1573 K was the lowest among the three deposits. This result can explain why shedding occurred in the 1573 K case. In this case, the viscous force could not counteract surface tension and gravity once the ash deposit had grown to a certain extent. Furthermore, the temperature of critical viscosity (TCV) was illustrated in Figure 11. TCV was defined as the temperature at which abrupt variation occurred in the viscosity−temperature diagram.29 It can be seen that the softening temperature (ST) is approximately 100 K lower than TCV, as shown in Figure 11. This is similar to the measured results investigated by Yuan et al.31

4. CONCLUSION The purpose of this study is to research the characteristics of ash deposits for SM coal at different furnace temperatures through digital image techniques. The following conclusions can be drawn: (1) The ash deposit growth process in the 1573 K case was significantly different from those in the 1523 and 1473 K cases. According to digital image techniques, the slag growth rates of the six stages in the 1573 K case were 0.075, 0.138, 0.062, 0.076, 0.065, and 0 mm/min, respectively. The deposit growth rates of the 1523 K case were 0.061, 0.095, 0.047, and 0 mm/min, corresponding to stages 1, 2, 3, and 4, respectively. Deposit growth rates for the four stages in the 1473 K case were 0.037, 0.106, 0.058, and 0 mm/min, respectively. (2) The stable thicknesses for the three cases were 10.05−10.17, 10.47−10.56, and 14.62−14.76 mm for the 1573, 1523, and 1473 K cases, respectively. This indicates that a low temperature can facilitate an increase in deposit thickness. Meanwhile, all three deposits were characterized by the layer structure with different colors and hardness. In addition, it can be found that the width of the deposit decreases with increasing the temperature. This may be result from more liquid-phase formation in the higher temperatures. (3) The SEM−EDX results showed that the major elements of the three deposits were Al, Si, Ca, and Fe. Meanwhile, it revealed that the contents of K and Na declined with the deposit thickness direction from the first layer to the third layer. This may be because the deposit growth will result in the increase in the deposit surface temperature. Then, this higher temperature will impede the condensation of alkali vapors. (4) The calculated results for ash deposit viscosity reveal that the viscosity of ash deposits decreased with the temperature. Additionally, the calculation results reveal that the major crystalline phases of the deposits are anorthite and hematite. They all decrease with the furnace 5764

dx.doi.org/10.1021/ef501656f | Energy Fuels 2014, 28, 5756−5765

Energy & Fuels

Article

(27) Robinson, A.; Junker, H.; Baxter, L. Energy Fuels 2002, 16, 343− 355. (28) Madhiyanon, T.; Sathitruangsak, P.; Sungworagarn, S.; Pipatmanomai, S.; Tia, S. Fuel Process. Technol. 2012, 96, 250−264. (29) Vargas, S.; Frandsen, F. J.; Johanse, K. D. Prog. Energy Combust. Sci. 2001, 27, 237−429. (30) Urbain, G.; Cambier, F.; Deletter, M.; Anseau, M. R. Trans. J. Br. Ceram. Soc. 1981, 80, 139−141. (31) Yuan, H.; Liang, Q.; Gong, X. Energy Fuels 2012, 26, 3717− 3722.

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dx.doi.org/10.1021/ef501656f | Energy Fuels 2014, 28, 5756−5765