Melting Behavior of Typical Ash Particles in Reducing Atmosphere

May 24, 2012 - Two coal ashes (one with high-melting temperature and one with low-melting temperature) obtained at 600 °C in air were pressed into pe...
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Melting Behavior of Typical Ash Particles in Reducing Atmosphere Haigang Wang, Penghua Qiu,* Shijun Wu, Yun Zhu, Yaoqiang Li, and Guangbo Zhao School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang, 150001, People’s Republic of China ABSTRACT: Two coal ashes (one with high-melting temperature and one with low-melting temperature) obtained at 600 °C in air were pressed into pellets and further treated for 1 h at different temperatures from 700 to 1200 °C at the interval of 100 °C in reducing atmosphere (mole ratio CO:CO2 = 60:40) to investigate the melting behavior of typical ash particles. The final structures were characterized by XRD and SEM-EDX. The results showed that most Na, K, Ca, and Fe took the form of aluminosilicates. Although the low-melting particles (Na−K-enriched aluminosilicate particles and Ca−Fe-enriched aluminosilicate particles) were small in number percentage, these particles were already melted at 1000 °C. It is possible that Fe was enriched more readily in particles than Ca. The extent of fragmentation of calcite particle was closely related to the calcite type. The calcite particles with layered textures fragmented more. Gaussian distribution successfully simulated the calcite particle size distribution after calcite particle fragmentation. The detailed mechanism analysis showed that the Fe−O particle dissolution was primarily controlled by diffusion. The Fe−O crystals from pyrite and siderite showed similar patterns of crystal growth.

1. INTRODUCTION Gasification of coal is attracting considerable interest worldwide. However, an important factor in the successful design and operation of coal gasification systems is the ability to control and mitigate ash-related problems.1 For example, during the first commercial operating year of the Wabash River Coal Gasification Repowering Project designed with E-GAS technology, deposition occurring in the second stage gasifier and the syngas cooler brought difficulties in maintaining operation and extended scheduled shutdowns because of the need to remove the deposits.2 The Polk Power Station IGCC Project with using Texaco Technology also experienced several convective syngas cooler pluggings.3 To ensure that the slag flows down continuously along the gasifier walls, some researchers have investigated ash or slag properties, such as the viscosity,4−6 the amount of fluxes required,5−7 and the temperature of critical viscosity,4−6 and developed various viscosity models to accurately predict the viscosity of slag.8−10 Recently, FactSage software has been used to improve predictive capabilities for the behavior of minerals. Shannon et al.11 studied ash composition distribution by particle size and density to investigate its effect on mineral phase type formed at various temperatures in reducing atmosphere by FactSage modeling. van Dyk et al.12 studied the transformation of minerals at high temperatures using FactSage software and found that the mineral species that are formed contained a high number of oxygen molecules. Over recent years, some authors have focused on studying melting characteristics of coal ash in reducing atmosphere. Huffman et al.13 investigated the high temperature behavior of coal ash in both reducing and oxidizing atmospheres and found that the ash melting behavior was controlled by Fe-rich minerals in reducing atmosphere. Ninomiya et al.14 concluded that CaCO3 additives are an efficient flux for the control of ash melting. The residence time considerably influenced the ash behavior at high temperature in reducing atmosphere.15 © 2012 American Chemical Society

Ash formation mechanisms have also been investigated by some authors. The slag formation mechanism by minerals from Fe-rich coals was investigated by ten Brink et al.,16and they found that the slags from the pyrite-rich and iron oxide-rich samples both had low viscosity. The combustion stoichiometry had a great influence on iron mineral transformations.17 From the standpoint of pressure, Wu and Wall et al.18,19 found that the ash generated at high pressure was greatly different from the ash generated at low pressure. For example, the much finer ash was found at high pressure. In fact, we more focus on some low-melting ash particles. Once these ash particles hit on the heat exchange surface, deposits can easily form. Brooker20 found that the major phases of the deposits from the tubes and membranes in the syngas coolers used in the Texaco gasifier at Southern California Edison’s Cool Water power station in Daggett, CA, were Ca- and/or Feenriched minerals. Na- and/or K-enriched minerals also readily formed deposits.21 Therefore, it is essential to study the melting behavior of these typical mineral particles in reducing atmosphere. In this study, the resulting samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) to obtain semiquantitative analysis.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Two Chinese bituminous coals, denoted as LM coal and HM coal, respectively, were selected in this study, with one coal’s ash having a low-melting temperature and the other having a high-melting temperature. The coals were received as pulverized fuel, and subsequently sized by sieving to less than 97 μm in diameter. Instead of the traditional ashing condition of 815 °C, raw ashes were prepared by ashing the two coals in air in a muffle furnace at 600 °C for 24 h. This was to ensure that no sodium transformation, Na−Ca eutectics transformation, and volatilization of organically bound metals (Ca, Na, K, etc.) Received: February 11, 2012 Revised: May 22, 2012 Published: May 24, 2012 3527

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the furnace temperature reached 700, 800, 900, 1000, 1100, or 1200 °C from room temperature at a heating rate of 7 °C/min. After heat treatment with a residence time of 1 h, the ash pellet was extracted and quenched into liquid nitrogen. The pellets after heat treatment are shown in Figure 2.

took place. A similar method can be found in ref 22. The properties of raw ashes are given in Table 1. To observe the surface of the ashes

Table 1. Coal Ash Analyses coal AFT, °C, reducing atmosphere DT ST HT FT ash composition, wt % SiO2 Al2O3 CaO Fe2O3 MgO K2O Na2O TiO2 P2O5 SO3 total

LM 1130 1157 1162 1197 42.95 14.27 16.28 10.49 0.96 1.4 1.94 0.84 0.03 6.65 95.81

HM 1330 1418 1425 1453 54.01 18.39 3.16 13.83 0.91 0.34 0.60 1.85 0.26 2.43 99.01

Figure 2. The appearance of the ash pellets treated at variable temperatures.

and the change of ash pellet dimension before and after heat treatment clearly, loose ash powder was pressed into cylindrical pellets in a diameter of 13 mm and a height of about 2 mm by a small pressure of 0.05 MPa prior to further heat treatment. The small pressure of 0.05 MPa ensured that the pressing process did not affect ash formation, by processes such as fragmentation and mass diffusion. 2.2. Experimental System and Procedures. A schematic diagram of the complete experimental setup is shown in Figure 1. The ash pellet was placed in a ceramic crucible and then suddenly moved into the tube furnace in reducing atmosphere (mole ratio CO:CO2 = 60:40) when

2.3. Analytical Method. The main minerals in the powdered samples (less than 97 μm) were identified by X-ray diffractometry with Cu Kα radiation from 18° ≤ 2θ ≤ 70° at a scanning speed of 4°/min, using a diffractometer (D/MAX 2200, Rigaku, Japan) operating at 30 mA and 40 kV. Semiquantitative analysis was obtained by measuring the diffraction peak intensities of the identified minerals. The method assumes that the diffraction peak intensity of mineral is proportional to the corresponding

Figure 1. Schematic diagram of the experimental setup. 3528

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smooth and flat surface, because these molten particles always try to flow and fill the pore network around them. EDX analyses show that these low-melting particles are rich in Na and K relative to the bulk compositions in Table 1, but the mass ratios of SiO2/ Al2O3 for those particles are different. This indicates that their melting does not depend on the SiO2/Al2O3 mass ratio but on the Na2O and K2O contents. Figures 5 (the LM coal) and 6 (the HM coal) show XRD patterns of the raw ashes and their ashes treated at different temperatures. In this study, all of the minerals identified by XRD are marked by numerals and summarized in Table 2. From Figures 5 and 6, the main minerals containing K and Na are Illite+muscovite, and with increasing temperature, the diffraction peak of Illite+muscovite gradually decreases. At 1000 °C, their diffraction peak disappears. This means that Illite+muscovite formed the molten phase at 1000 °C. It is suggested that Na- and K-enriched aluminosilicate particles shown in Figure 3 are probably from the melting of Illite+muscovite. Area scanning analysis of an arbitrary zone on the surface of the ash pellet is carried out by SEM-EDX to obtain further information on the Na- and K-enriched particles. By area scanning analysis, element area distribution image can be obtained. In an element area distribution image, the brighter positions have the higher element content, which helps to find some positions where selected element content is high, so we can roughly calculate the amount and contents of some particles containing selected element, such as for those Na- and K-enriched aluminosilicate particles. The original element area distribution image from SEM-EDX is an RGB image. For better observation, the RGB image is transformed into the corresponding intensity image. Figure 7 gives the Na or K area distribution, where the white points represent pixels containing Na or K. Positions marked in circles in Figure 7 have the brighter pixel points where the Na or K content is higher. This indicates that the Na and K distributions on the surface of the ash pellets are nonuniform in content. The gray level of the element x at a pixel point is noted as g(x). In this study, the size of the image is 200 × 256 pixels. N is the number of pixel points containing the element. The parameter n(conditions) represents the number of pixel points satisfying certain conditions such as g(x) > 0. The number percentage of pixel points whose gray level satisfies certain conditions can be expressed as:

mineral volume. A similar method can be found in ref 23. As a result, the volume fraction of the amorphous phase can be obtained: Va =

Ia × 100% Ia + ∑ Ic, i

(1)

where Ic,i is the diffraction intensity of the mineral, and Ia is the diffraction intensity of the amorphous phase. The microstructure of the ashes after heat treatment was examined by scanning electron microscopy using equipment (Quanta200, FEI, America) operating at 20 kV. A linked energy dispersive X-ray spectroscopy-using detector (Genesis2000, EDAX, America) was used for semiquantitative analysis.

3. RESULTS AND DISCUSSION 3.1. Bulk Characteristics of Raw Ashes and Ash Pellets. From Table 1, the LM coal contains much higher (CaO + Fe2O3) than does the HM coal. This results in a lower melting temperature for the LM coal. The prediction can also be verified from Figure 2, which displays the appearance of ash pellets after heat-treatment at variable temperatures. From Figure 2, the ash pellet did not keep the original shape at 1200 °C for the LM coal due to the flow of the molten phase, but the ash pellet of the HM coal contracted into a ball. The CaO content exceeds the Fe2O3 content slightly in the LM coal. However, for the HM coal, the Fe2O3 content is much higher than the CaO content. This will result in a great difference in the amount of the main minerals formed during heat-treatment for the two coals. Another obvious characteristic is that there was a small change in the diameters of the ash pellets treated at less than 1000 °C, but their diameters started to decrease obviously at 1000−1100 °C. This can be attributed to the occurrence of a large amount of molten phase, facilitating densification (see Figure 3). Moreover, the volume

P=

n(conditions) × 100 N

(2)

The average gray level of element x in an intensity image is written as: g (x)ave =

Figure 3. Change of the amorphous phase volume percentage with increasing temperature.

∑ g (x ) n(g (x) > 0)

(3)

The results under some conditions 1−7 are shown in Figure 8. From Figure 8, most Na and K coalesce with Si and Al. Although Na- and K-enriched aluminosilicate particles are low in number, these particles, that is, those described in Figure 4, easily form deposits due to their low-melting temperatures. 3.3. Ca and Fe Aluminosilicate Particles. Some big pores are observed on the surface of the ash pellets treated at 1000 °C in Figure 9. Typical pores from Figure 9 are magnified and shown in Figure 10. The particles that stuck together verify the formation of molten phase occurring around and within the pores. EDX analyses in Figure 10 provide the evidence of ash particles rich in Fe, and with a high SiO2/Al2O3 mass ratio.

fraction of the amorphous phase in Figure 3 had a jump at 1000− 1100 °C. The above analyses indicate that 1000 °C is a key temperature controlling the amount of the molten phase. Generally, the low-melting minerals have already melted prior to the occurrence of a large amount of the molten phases. Therefore, 1000 °C is selected as a special temperature to identify the low-melting mineral particles in sections 3.2 and 3.3 of this study. 3.2. Na- and K-Enriched Aluminosilicate Particles. In Figure 4, typical low-melting particles are characterized by a 3529

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Figure 4. Typical Na- and K-enriched aluminosilicate particles in the ashes treated at 1000 °C.

In SiO2−CaO−FeO, SiO2−Al2O3−FeO, and SiO2−Al2O3− CaO−FeO phase diagrams,25,26 their lower eutectic temperatures are 1083 or 1093 °C, respectively. The impurities of the ash (Na2O and K2O) will further reduce the melting temperature. This means that Fe- and Ca-enriched aluminosilicate particles can form a molten phase at 1000 °C. As compared to the phase diagrams containing FeO,25 there is a high lower eutectic temperature (1170 °C) in the SiO2−Al2O3−CaO phase diagram. Therefore, the existence of FeO can reduce the melting temperature of ash. An analysis is carried out to further understand the role of FeO in reducing the melting temperature. According to the phase diagrams mentioned above, the lower eutectic points in the phase diagrams containing FeO are surrounded by Fe aluminosilicate or silicate phase fields. Therefore, the formation of the lower eutectic point partly depends on whether or not Fe aluminosilicate or silicate phases can form at a temperature simultaneously. From Figures 5 and 6, obvious diffraction peaks for anorthite and gehlenite are observed at less than 1000 °C, but Fe aluminosilicate or silicate phases are not observed until the temperature reaches 1000 °C.

Because Fe−O has higher density than other compositions and the molten phase has a low viscosity, the molten phase flows down and then the pores are formed. The molten slag flowing horizontally around the pore causes a difference in concentration of Fe and Ca between points 1 and 2. Small Ca and Fe aluminosilicate particles are common in the ash pellets shown in Figure 11. The characteristics of these particles are small size (typically 0) is about 28%. The number percentage of the pixel points containing Fe, Si, and Al under the condition 3 (g(Fe) > 0, g(Si) > 0, g(Al) > 0) is about 23%. This means that 82% (23%/28%) of the pixel points containing Fe also contains Si and Al. For Ca, a similar trend is found. It is suggested that most Ca or Fe takes the form of aluminosilicates. However, a small percentage of Ca- or Fe-enriched aluminosilicate particles (g(Ca) > g(Ca)ave or g(Fe) > g(Fe)ave) is found (conditions 5 and 6). As expected, the Ca- and Fe-enriched aluminosilicate particles (g(Ca) > g(Ca)ave and g(Fe) > g(Fe)ave) occupy a low number percentage relative to the Ca- or Fe-enriched aluminosilicate particles (condition 7). The above results also can be obtained for the HM coal (Figure 12b). Comparison of Figure 12a and b shows that under some conditions (1, 3, and 6),

CaO + 2SiO2 + Al 2O3 → anorthite(CaO·Al 2O3·2SiO2 ) (4)

2CaO + 2SiO2 + Al 2O3 → gehlenite(2CaO·Al 2O3·2SiO2 ) (5)

Gehlenite can also transform into anorthite:

27

gehlenite(2CaO ·Al 2O3·2SiO2 ) + 3SiO2 + Al 2O3 → anorthite(CaO·Al 2O3·2SiO2 )

(6)

Metakaolinite possibly provides reaction compounds, containing Al2O3 and free SiO2. In reactions 4 and 5, CaO should be from: anhydrite(CaO· SO3) → CaO + SO3

(7)

calcite(CaO ·CO2 ) → CaO + CO2

(8)

At 1000 °C, the main Fe minerals are hedenbergite and fayalite. The following reactions are suggested: 2FeO + 2SiO2 → fayalite(2FeO ·SiO2 )

(11)

(9)

CaO + FeO + 2SiO2 → hedenbergite(CaO·FeO·2SiO2 ) (10)

Reference 28 provides another path for the formation of hedenbergite: 3531

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Table 2. Minerals Marked by Numerals numerals

minerals

numerals

minerals

numerals

minerals

1 2 3 4 5 6 7

quartz calcite pyrite siderite Illite+muscovite anhydrite anorthite

8 9 10 11 12 13 14

gehlenite oldhamite hematite magnetite wuestite lime fayalite

15 16 17 18 19

almandite hedenbergite grossular ferrian skiagite aluminian sekaninaite

Figure 9. SEM images of the ash pellets treated at 1000 °C.

(shown in Table 3) can be calculated on the basis of the area scanning analyses. The values of M(Ca)/M(Fe) are 1.30 and 0.57 for the LM and the HM coal, respectively. The values of N(Ca)/N(Fe) are 2.31 and 1.57 for these two coals, respectively. The values of M(Ca)/M(Fe) are larger than the values of N(Ca)/N(Fe) for the two coals. Therefore, it is possible that Fe is enriched more readily than Ca. From Table 3, there is higher value of M(Fe)/N(g(Fe)) for the LM coal than for the HM coal. This result is coincident with that of the EDX analyses in Figure 12; for the common Ca and Fe aluminosilicate particles, there is a higher content of Fe in the LM coal relative to the HM coal. In the Buggenum IGCC power station in Netherlands, at the top of the gasifier, the hot syngas is quenched to 900 °C with cold recycled syngas to avoid fouling problems in the syngas coolers by molten or sticky ash particles entrained in the raw syngas. In the Wabash River IGCC station in the U.S., the hot syngas is quenched to 1040 °C. In this study, the low-melting particles (Na−K-enriched aluminosilicate particles and Ca−Fe-enriched aluminosilicate particles) were already melted at 1000 °C. It is

Figure 7. Na and K area distributions.

the number percentage of pixel points in the HM coal is higher than that in the LM coal. However, Figure 13 shows that the number of pixel points in the HM coal is larger than that in the LM coal under corresponding conditions. This result indicates that the average Fe or Ca content in each pixel point for the HM coal may be less than that for the LM coal. To obtain a deeper analysis, schematic diagrams of Fe and Ca distributions from area scanning analyses are adopted and shown in Figure 14. The definitions of M(Ca), M(Fe), N(Ca), and N(Fe) are presented in Figure 14. Some parameters

Figure 8. The number percentage of the pixel points containing Na and K under conditions 1−7. 3532

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Figure 10. SEM-EDX analyses of typical pores found in Figure 9.

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Figure 11. Characterization of small Ca and Fe aluminosilicate particles in the ash pellets treated at 1000 °C.

Figure 12. The number percentage of the pixel points containing Ca and Fe under conditions 1−7.

Figure 16b, the particle shape resembling natural calcite is nearly rhombic. ten Brink et al.29 found that calcite minerals were easily fragmented during heating. Therefore, Ca−S−O particle should be from calcite particle. In Figure 5, after ashing at 600 °C, the diffraction peaks of lime and oldhamite are also not found, but the obvious diffraction peak of anhydrite can be identified in the

suggested that in real IGCC systems, the inlet temperature of the syngas coolers should be controlled at less than 1000 °C. 3.4. Ca−S−O Particles. In Figure 15, there is not any gypsum in the parent coal according to XRD analysis. This indicates that Ca−S−O particles shown in Figure 16 are formed during heating at 600, 800, or 900 °C. In Figure 15, the obvious diffraction peak of calcite is identified in the parent coal, and in 3534

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Table 3. Values of Some Parameters Calculated from the Area Scanning Analyses mass ratio of Ca and Fe (M(Ca)/ M(Fe))

number ratio of pixel points containing Ca and Fe (N(Ca)/N(Fe))

mass of Fe per pixel (M(Fe)/ N(Fe)

1.3

2.31

0.00052

0.57

1.57

0.000419

LM coal HM coal

Figure 13. The number of the pixel points containing Ca and Fe under conditions 1−7.

raw ash. Therefore, the following reaction may take place during ashing: 1 CaCO3 + O2 + SO2 = CaSO4 + CO2 (12) 2 It is specialized that because of the low ashing temperature, the above reaction takes places only on the surface of calcite particle. Calcite still exists within the calcite particle. At 800 and 900 °C, the diffraction peak of oldhamite can be found by XRD in Figure 5. Therefore, the following reaction may take place on the surface of the calcite particle: CaSO4 + 4CO = CaS + CO2

Figure 15. XED pattern of the LM coal.

To obtain the calcite particle size distribution after calcite fragmentation, some procedures were taken, and the results are shown in Figure 17. Figure 17a is from the surface of the calcite particle shown in Figure 16b-1. From Figure 17b to c,d, open operation was used to obtain the calcite particle size distribution. The details of open operation can be found in ref 30. In Figure 17c and d, the unit of horizontal axis is pixel pitch. One pixel pitch is about 0.6 μm. From Figure 17d, the average size after calcite fragmentation is about 7 pixel pitch size (about 4 μm). Therefore, it is suggested that in the real IGCC system, most calcite fragments tend to reach heat-transfer surfaces by thermophoresis or Brownian motion due to their small sizes. In Figure 18, a Gaussian distribution was used to fit the calcite particle size distribution after its fragmentation. It is found that Gaussian distribution can simulate the size distribution successfully.

(13)

CaSO4 and CaS may coexist on the surface of the calcite particle. When the ash pellet was suddenly exposed to 900 °C, evolution of CO2 from calcite decomposition within the calcite particle resulted in the particle breaking into the fragments. It is expected that a calcite particle is fragmented into more, smaller particles at higher temperature. Fragmentation of calcite into a number of smaller particles is important as it influences the size distribution of the ash particle, which has been proved to affect ash transport behavior to a great extent. Large ash particles tend to impact heattransfer surfaces by inertia, whereas fine ash particles tend to reach wall surfaces by thermophoresis or Brownian motion.

Figure 14. Schematic diagrams of Fe and Ca distributions from the area scanning analyses. 3535

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Figure 16. Ca−S−O particles from calcite particles (type I) in the LM coal and its ashes after heat treatment.

An image of the Ca−S−O particle in Figure 16b at higher magnification is shown in Figure 16c. As compared to the EDX

analysis at point 2 inside the particle, the sulfur content at point 1 is higher. The existence of a concentration gradient of sulfur 3536

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Figure 17. The procedure taken to obtain the calcite particle size distribution after excluded calcite particle fragmentation.

found that the different physical properties of limestones resulted in the different morphology and microstructure of the calcined limestones.31,32 According to the XRD pattern of the HM coal in Figure 20, there is a small calcite content. Therefore, it is difficult to find the calcite particles by SEM-EDX. 3.5. Fe−O Particles. As is seen clearly in Figure 21a, some Fe−O particles form a sunflower-like structure on the surface of the melt in the LM coal treated at 1200 °C. The typical Fe−O particle is shown in Figure 21b. The partial molten phases in the middle of the sunflower-like structure consist of FeS−FeO phase and SiO2−Al2O3−CaO−FeO phase as shown in Figure 21e. According to the results of ten Brink et al.29 and Helble et al.,33 in coals, the minerals that easily fragmented during heating were pyrite and calcite. The EDX analysis shows that point 2 in the middle of the sunflower-like structure has high sulfur and Fe contents with small Ca, Na, Si, and Al contents. The XRD results in Figure 15 also show that pyrite is the major Fe-containing mineral in the LM coal. This indicates that the Fe−O particles should be from pyrite particles. It is specialized that because of the low ashing temperature of 600 °C, pyrite of the outer layer of pyrite particle was oxidized into Fe−O during ashing. Explosive fragmentation forming the sunflower-like structure occurs because of the evolution of sulfur gas from pyrite decomposition inside the Fe−O particle when the raw ash sample is suddenly exposed to the temperature of 1200 °C. As compared to the results of Srinivasachar et al.,34 a much greater extent of fragmentation is observed in this study at a similar temperature. This can be explained as follows. In Figure 21b, the pyrite particle seems to be encased in the melt. Therefore, a large amount of sulfur gas is released from one side of the Fe−O particle because of the great diffusive resistance on the other side. As a result, one side of the Fe−O particle has a much greater extent of fragmentation. The sizes of the inverted pyramid-like structure pores

Figure 18. Gauss fit of calcite particle size distribution after excluded calcite particle fragmentation.

indicates that reactions 12 and 13 are controlled by diffusion. In Figure 16b, some parts of the Ca−S−O particle, for example, the bottom area, are characterized by a layered texture. The same texture is found on the particle in Figure 16a. Thus, the Ca−S−O particle is produced from a calcite particle with layered texture. This type of calcite particle is referred to as type I. Another Ca−S−O particle is shown in Figure 19a. The unfragmented parts of the surface of this particle are smooth at 900 °C. Therefore, it is possible that the Ca−S−O particle is from the calcite particle with smooth surface in Figure 19b. The calcite particle is referred to as type II. Although it is less fragmented than the type I particle, the fissures on the type II particle matrix are obvious. A possible explanation for the difference in fragmentation extent is their different physical properties such as porosity, pore volume, and purity. Some authors 3537

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Figure 19. Ca−S−O particles from calcite particles (type II) in the LM coal and its ashes after heat treatment.

particle is unlikely to exist in crystalline form. ten Brink et al.16 investigated the mechanism of slag formation by using minerals from pyrite-rich coals. They found that a certain time is required for dissolution of the deposited Fe−O particles into the slag. To determine the dissolution time of Fe−O particles, what matters most is to understand the dissolution mechanism of Fe−O particle. Therefore, it is necessary to discuss the dissolution mechanism of Fe−O particle in detail. Because the SiO2/Al2O3 mass ratio in the LM coal is about 3 (see Table 1), a quaternary SiO2−Al2O3−CaO−FeO system with SiO2/Al2O3 mass ratio = 2.726 was chosen to discuss the dissolution mechanism of Fe−O particle. In Figure 22a, the composition of the LM coal ash is at point P. From N to P, the compositions pass through a phase field with liquidus temperature greater than 1200 °C, then a phase field with the liquidus temperature less than 1200 °C, and the Fe content gradually decreases. Accordingly, in Figure 22b, from O to P, the compositions go through an Fe-enriched field and the melting field with less Fe content than the Fe-enriched field in turn. On the basis of the above analyses, if the Fe−O particle tends to dissolve into the molten phase, Fe2+ must diffuse away from the particle to decrease the local Fe2+ concentration. This means that the Fe−O particle dissolution is primarily controlled by diffusion. Although most crystals form the molten phase at 1200 °C, the molten phase still has a high viscosity. According to the Eyring equation, both the high viscosity and the low temperature result in a small ion diffusion coefficient.35

Figure 20. XED pattern of the HM coal.

surrounded by crystals in area 1 are larger than those in area 2. The variation of the pore sizes is closely related to the crystal size. The larger the crystal size is in area 1, the larger is the size of the pores formed by the crystal growing upward. At point 3, the crystals surrounding the Fe−O particle are Fe-enriched crystals in Figure 21b and g. In the outer region of the Fe-enriched crystals such as point 4, there is the molten phase. Its compositions are similar to the bulk compositions in Table 1. The melting temperature of the LM coal ash is 1197 °C. Therefore, the Fe−O 3538

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Figure 21. Fe−O particles in the LM coal ashes treated at 1200 °C.

siderite did not occur at any heating rates.29 Therefore, the Fe−O particle cannot form a sunflower-like structure. However, in Figures 21 and 23, the Fe−O crystals have similar patterns of growth. In Figure 23b, some Fe−O crystals are forming

As shown in Figure 23, the Fe−O particle in the HM coal is different in shape from the Fe−O particle with the sunflower-like structure in Figure 21b. From Figure 20, in the HM coal, the main Fe contained minerals are siderite. The fragmentation of 3539

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Figure 22. Schematic of the dissolution mechanism of Fe−O particle.

Figure 23. Fe−O particles in the HM coal ashes treated at 1200 °C.

particles have already melted at 1000 °C. It is most possibly that Fe is enriched more readily in particles than Ca. (2) Two types of calcite particle, with different surface textures, are found. The extent of fragmentation of calcite particle is dependent upon the calcite type; the particles with layered textures fragment more. A Gaussian distribution can be applied to simulate calcite particle size distribution after calcite particle fragmentation.

triangular pores like the pores in Figure 21. High melting temperatures with the high viscosity for the HM coal may result in a low dissolution rate.

4. CONCLUSIONS (1) Most Na, K, Ca, and Fe take the form of aluminosilicates. Although the low-melting particles (Na−K-enriched aluminosilicate particles and Ca−Fe-enriched aluminosilicate particles) are small in number percentage, these 3540

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(3) The detailed mechanism analysis shows that the Fe−O particle dissolution is primarily controlled by diffusion. The Fe−O crystals from pyrite and siderite have similar crystal growth.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-451-8641-2618, ext 804. Fax: +86-451-8641-2528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This was supported by National High Technology Research and Development Program of China (2007AA05Z246) and Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51121004).

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REFERENCES

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dx.doi.org/10.1021/ef300247y | Energy Fuels 2012, 26, 3527−3541