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Effect of Quenching Temperatures on Melting Characteristics of Coal Ash in a Reducing Atmosphere Haigang Wang, Penghua Qiu,* Yaoqiang Li, Zongjie Han, Shijun Wu, and Gangbo 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 representative Chinese bituminous coals (one with rich Ca content, having a low melting temperature, and the other with rich Fe content, having a high melting temperature) were selected for this study to investigate the effect of quenching temperatures on melting characteristics of coal ash in a reducing atmosphere (6:4 CO/CO2). The final structures were characterized by X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray (SEM− EDX), and differential thermal analysis (DTA). The results show that, with the quenching temperature increasing, the crystalline volume percentage experiences a maximum value. However, the thermal properties (the glass transition temperature and the melting temperature) determine the maximum crystalline volume percentage temperature. There is a large variation in the phase compositions with an increase of the quenching temperatures, but the maximum diffraction peak intensities of most minerals simultaneously occur. A comparison of the coal ash samples after quench and direct heat treatment shows that there is a low crystalline volume percentage and simple phase compositions for the quenching process at corresponding temperatures, which are due to a preheat treatment at high temperatures prior to quench. Anorthite can form a better crystal shape in the coal ash sample with a low melting temperature and high Ca content for the quenching process. The diffusion controls the dissolution of the Fe−O particle, especially for the high melting temperature coal ash with a high Fe content. conditions. Bai et al.6 discussed the ash behavior at high temperature and analyzed the effect of the residence time on the compositions of coal ash in a reducing atmosphere. Ashforming reactions during lignite gasification were deduced by a computer-controlled scanning electron microscope (CCSEM).7 van Dyk et al.8 studied the transformation of minerals at high temperatures using FactSage software and found that the formed mineral species contained a high number of oxygen molecules. They also obtained an acceptable linear correlation between oxygen-capture tendencies versus CaO content. To ensure that the molten slag flows down freely along the gasifier walls, some other researchers made great efforts on the study of viscosity−temperature characteristics of slag9−13 and flux requirements14 and developed various viscosity models to accurately predict the viscosity of slag.10−12 To understand in detail why some minerals especially produce troublesome slag, the ash formation mechanism by some minerals from coals has been investigated. The pyriteand iron-oxide-rich samples both produced a low-viscosity slag.15 McLennan et al.16 further found that iron mineral transformations were greatly affected by combustion stoichiometry, and the detailed ash formation mechanisms on iron minerals were obtained. The pressure also had an effect on the ash formation mechanisms. Wu et al. and Wall et al.17,18 found that the ash generated at high pressure was much finer than the ash generated at low pressure. The physicochemical characteristics of gasifier fly ash and slag were studied to develop a special material for a wide range

1. INTRODUCTION The global electricity demand is increasing at about 3 times the rate of total energy, while the industry is expected to reduce CO2 emissions because of global warming. As a consequence, there is pressure to improve the efficiency of energy use through changes in technology and to reduce emissions of greenhouse gases. In the near and medium term, there are several options for clean and more efficient electric power generation technologies, such as OXY blown combustion with flue gas recycle (oxy/FGR), pulverized combustion and ultra-supercritical steam (PC/USC), and integrated gasification in combined cycle (IGCC). CO2 emission, efficiency, and costs of those technologies were compared in ref 1. The results of the comparison show that IGCC can promise to provide a large share of the future world energy needs in an economical, reliable, and environmentally friendly way. In entrained flow gasifiers, coal particles are combusted, gasified, and entrained, which gives rise to the gasification byproduct in the form of fly ash and slag. The fly ash accounts for about 10% of the gasification byproduct. One of the key problems on the fly ash (or slag) in an IGCC system is the deposition of the ash (or slag) onto the surface of downstream equipments. In existing IGCC power plants, i.e., Buggenum IGCC plant, Wabash River IGCC plant, and Polk Power IGCC plant, the significant effect of deposition was found on the reliability, availability, and maintainability of the system.2−4 At present, the studies on the ash (or slag) in a reducing atmosphere can be divided into four aspects according to their research purposes. Some authors reported ash melting behavior and mineral transformation during heating in a reducing atmosphere. Huffman et al.5 investigated the high-temperature behavior of coal ash in both reducing and oxidizing atmospheric © 2012 American Chemical Society

Received: January 17, 2012 Revised: February 28, 2012 Published: March 6, 2012 2204

dx.doi.org/10.1021/ef300097e | Energy Fuels 2012, 26, 2204−2219

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Figure 1. Gasification technologies with different quenching methods.

Figure 2. Simplified quenching process.

of applications. Kennedy et al.19 reported the possible use of foamed gasifier slag as low-temperature bulk thermal insulation materials. The full characteristics of gasifier fly ash and slag were investigated by Acosta et al. to explore their capabilities as the ceramics and glass to produce porous glass−ceramic materials.20,21 In fact, at the outlet of the gasifier reactor, the syngas temperature is around 1400 °C and the fly ash (or slag) is in liquid form. To protect downstream equipment from the deposition of the fly ash (or slag), a quench is needed to solidify the fly ash (or slag) and make it non-sticky. Dependent upon the gasifiers, there are different alternatives for quench in Figure 1. Quench by the recycle of cooled syngas is applied in the Shell and Prenflo gasifiers. Such quench can drop the syngas temperature from around 1400 to 900 °C. Chemical quench is the addition of a second gasification step, which uses the sensible heat in the hot syngas and not oxygen to gasify the coal feed with water. E-GAS (Figure 1c) incorporates this quenching method in their slurry feeding gasifier. After quench, the syngas temperature falls to 1030 °C. Tampa IGCC

demonstration plant adopts Texaco gasifier, and the hot syngas directly flows into a radiant syngas cooler, a dividing-wall-type heat exchanger, to drop the syngas temperature to about 700 °C. To summarize the above analysis, all four quenching flow paths were simplified and shown in Figure 2. However, for the different quenching methods, the resulting syngas temperature is different. The difference will inevitably influence the deposition of the fly ash (or slag) entained by the resulting syngas on the downstream equipment. The ash (or slag) experiencing quenching treatment and direct heat treatment also has great differences in melting characteristics. The direct heat treatment means that raw ash obtained by ashing was directly heated to a temperature. At present, few investigations have been focused on the effect of variable quenching temperatures on the melting characteristics of coal ash in a reducing atmosphere. The aim of this study is to characterize the coal ash samples treated at different quenching temperatures and to compare the melting characteristics of coal ash after quench and direct heat treatment by X-ray diffraction 2205


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than 97 μm first and then ashed in a muffle to obtain raw ashes according to the Chinese Standard GB/T1574-2007. Ash analyses of coal samples are given in Table 1. 2.2. Experimental Procedures. To create a reducing atmosphere, a blend gas flow of CO/CO2 with a molar ratio of 6:4 passed through the ash sample inside the furnace from top to bottom at a flow rate of 4 L/min, as shown in Figure 3. The dimension of the heating zone in the furnace is about 150 and 500 mm in diameter and height, respectively. When the temperature at position 1 reached 1400 °C, a temperature measurement was performed along the axis of the furnace, to mark the positions 2, 3, and 4, where the temperatures are 1200, 1000, and 800 °C, respectively. After that, a ceramic crucible loading the ash sample was placed at position 1 (1400 °C) for 30 min, and then it was quickly moved to position 2, 3, or 4 for an additional 30 min to quench the ash sample at 1200, 1000, or 800 °C, respectively. Finally, the sample was quickly extracted from position 2, 3, or 4 and quenched into liquid nitrogen. The quenching temperatures were denoted as −800, −1000, and −1200 °C. To clearly indentify of the effect of the quenching process on the ash melting characteristics, the raw ash sample was also directly heattreated at different temperatures in a reducing atmosphere. The procedure is as follows: when the temperature at position 1 rose to 1000, 1100, 1200, 1300, or 1400 °C, the raw ash sample was directly placed at position 1. After 30 min, the sample was quenched into the liquid nitrogen. 2.3. Calculation of Quenching Time. After the ash sample stayed at position 1 for 30 min and then suddenly placed to positions 2, 3, or 4, it would take a certain time to reach the temperature of the new position. During the time, the ash sample is under nonequilibrium conditions. In general, the non-equilibrium conditions have a great influence on the nucleation rate and growth rate of the crystal.22,23 Therefore, it is necessary to estimate the quenching time. The diameter and thickness of the ash sample are 25 and 2 mm, respectively, and the shape of the ash sample can be regarded as a infinite plate. Thus, the quenching time can be given by24

(XRD), scanning electron microscopy coupled with energydispersive X-ray (SEM−EDX), and differential thermal analysis (DTA).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Two representative Chinese bituminous coals were selected for this study: one with rich Ca content, having a low melting temperature, noted as LMC, and the other with rich Fe content, having a high melting temperature, noted as HMF. For this experiment, both of the two coal samples were milled to less

Table 1. Chemical Analysis and Melting Temperatures of Coal Ashes LMC coal

HMF coal

Ash Fusion Temperature (AFT) (°C, Reducing Atmosphere) deformation temperature (DT) 1130 1330 softening temperature (ST) 1157 1418 hemisphere temperature (HT) 1162 1425 flow temperature (FT) 1197 1453 Ash Composition (wt %) SiO2 42.95 54.01 Al2O3 14.27 18.39 CaO 16.28 3.16 Fe2O3 10.49 13.83 MgO 0.96 0.91 K2O 1.40 0.34 Na2O 1.94 0.60 TiO2 0.84 1.85 P2O5 0.03 0.26 SO3 6.65 2.43 SiO2 + Al2O3 57.22 72.40 SiO2/Al2O3 3.01 2.94 CaO + Fe2O3 + MgO + K2O + Na2O 40.07 18.84

2 2 sin(η1) t − t∝ = e−η1 Fo cos(η1) t0 − t∝ η1 + sin(η1)cos(η1)

(1)

where t is the surface temperature of the ash sample, t∝ is the temperature of position 2, 3, or 4, t0 is the initial temperature of the ash temperature, η1 is the characteristic value, and Fo is the Fourier number ατ Fo = 2 (2) δ where α is the thermal diffusivity, τ is the quenching time, and δ is the characteristic length. The relation between the characteristic value (η1) and Biot (Bi) is from ref 24. Bi is represented as

Bi =

hδ λ

(3)

where h is the heat-transfer coefficient and λ is the slag thermal conductivity. Because radiative heat transfer is the main heat-transfer mode, the heat-transfer coefficient (h) is obtained h=

εσ(t m 4 − t∝4) t m − t∝

(4)

where ε is the slag emissivity. The slag emissivity of 0.83 is considered as a constant value. σ is the Boltzmann constant. tm is the mean

Figure 3. Schematic diagram of the experimental setup.

Table 2. Values of the Parameters in Equations 1−8 LMC coal ash HMF coal ash

t∝ (°C)

h (W m−2 K−1)

λ (W m−1 K−1)

ρ (kg/m3)

c (J kg−1 K−1)

Fo

Bi

−1000 −1200 −1000 −1200

979.63 655.72 979.63 655.72

2.23 2.38 2.41 2.58

2788.17 2788.17 2733.43 2733.43

1775.83 1895.98 1963.23 2096.08

9.82 13.50 10.18 4.68

0.44 0.28 0.41 0.26

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Table 3. Quenching Time of the Ash Sample samples

LMC coal ash

quenching process (°C) quench time (s)

1400−1200 33

HMF coal ash 1400−1000 25

1400−1200 30

1400−1000 22

Table 4. Phases Identified in the Raw Ash Samples and Their Relative Volume Percentage phase

quartz

calcite

hematite

muscovite

anhydrite

anorthite

amorphous phase

LMC coal ash (vol %) HMF coal ash (vol %)

61.3 67.7

2.1 3.4

12.1 21.1

3.9 1.1

12.0 6.6

8.7

40.1 32.1

Figure 4. XRD patterns of the LMC coal ash samples at different quenching temperatures.

Figure 5. XRD patterns of the HMF coal ash samples at different quenching temperatures.

Figure 6. Correlation volume percentage of the crystalline phase and the quenching temperatures.

temperature. Slag thermal conductivity (λ) is calculated from the thermal diffusivity (α), specific heat (Cp), and density (ρ), using the following relationship:

The detailed calculations of the specific heat and density have been given as

λ = αC pρ

Cp =

(5) −7

ρ=

−1 25

with α = 4.5 × 10 m s . The slag density (ρ) and specific heat (Cp) are evaluated as the functions of the composition and temperature by the Bottinga26 and Mills25 methods, respectively. 2

∑ XiC pi

∑ X iM i ∑ XiVi

(6)

(7)

In eqs 6 and 7, Cpi, Xi, Vi, and Mi are the molar heat capacity, molar fraction, molar volume, and molecular weight of each component. The 2207


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Figure 7. DTA curves (with a heating rate of 10 °C/min) of the ash samples after heat treatment at 1400 °C for 30 min in a reducing atmosphere.

Figure 9. Comparison of XRD patterns of the LMC coal ash samples after direct heat treatment and quench.

content in the LMC coal ash is high. This will lower the ash melting temperature considerably.29 Anorther characteristic of the LMC coal ash sample is its higher CaO content than Fe2O3. The raw ash samples were also analyzed by XRD. The approximate volume percentage of crystalline phases was determined by the area of specific peaks that they occupied, as shown in Table 4. In comparison to the HMF coal ash, the high CaO content in the LMC coal ash helps to form anorthite. This is mainly because Ca2+ cations favor the substitution of silicon for aluminum.30 3.2. Characteristics of the Phase Structure after Quench. XRD patterns of two ash samples at different quenching temperatures are collected in Figures 4 and 5. The position of diffraction peaks were labeled in numerals. The main crystalline minerals are anorthite (41.4 vol %) and Fe-rich minerals (58.6 vol %) at −1000 °C for the LMC coal ash sample and mullite (30.1 vol %) and Fe-rich minerals (69.9 vol %) at −1200 °C for the HMF coal sample. Most Fe-rich minerals are distributed on the right of the XRD patterns, but anorthite is on the left of XRD patterns, which indicates small interplanar spacing [d (Å)] for those Fe-rich minerals. The crystal structure is closer for less interplanar spacing. However, the distribution of mullite is scattered. From the point of view of the mineral phase, mullite is considered as a refractory mineral. Its high volume content results in high ash melting temperatures.29

Figure 8. Differences in phase compositions for the LMC and HMF ash samples after the quench.

temperature dependence of the heat capacity is frequently expressed in the form C p = a + bT − cT −2

(8)

and recommended values of these constants a, b, and c for the various slag components are given in ref 27. The values of the parameters in eqs 1−8 have been given in Table 2, and the quenching time is shown in Table 3. From Table 3, the quenching time is much shorter than the residence time and the non-equilibrium process can be ignored.

3. RESULTS AND DISCUSSION 3.1. Characterization of Coal and Raw Ash Samples. Table 1 indicates a wide variation in the composition of the two raw ash samples. Bryers28 investigated the influence of the basic oxide content in ash on the fusibility in reducing conditions for different ranks of North American coals and found that the coal ash with about 40% basic oxides and SiO2/Al2O3 ≫ 1 had a low fusibility. Although the two ash samples have a similar SiO2/ Al2O3 value, the LMC coal ash is characterized by a high content of basic oxides (CaO + Fe2O3 + MgO + K2O + Na2O = 40.07%) relative to the HMF coal ash, which is reflected in its low ash melting temperatures. Meanwhile, the SO3 2208


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Figure 12. Typical SEM images of the HMF ash samples after direct heat treatment and quench.

Figure 10. Comparison of XRD patterns of the HMF coal ash samples after direct heat treatment and quench.

Figure 11. Typical SEM images of the LMC coal ash samples after direct heat treatment and quench.

Figure 6 shows that, with the increase of the quenching temperatures, the volume percentage of the crystalline phases attains a maximum and then decreases. This indicates that the high and low quenching temperatures both do not go against the formation of the crystalline phases. The kinetics of crystal growth are driven by the Gibbs free energy difference between the relevant crystalline and liquid phases. The rates are zero at

Figure 13. Crystalline phase volume percentage of the coal ash samples after direct heat treatment and quench. 2209


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Figure 14. SEM images of the LMC coal ash sample after direct heat treatment at 1000 °C.

glass (Tg) at 750 and 910 °C, respectively. The melting temperature (Tm) of the LMC ash sample is about 1195 °C. This result is consistent with the melting temperature value shown in Table 1. In fact, Tg represents the temperature at which the mobility of network-forming cations (Si and Al) becomes vanishingly small.34,35 Therefore, the high Tg may put off the maximum crystallization rate temperature for the HMF ash sample. From another perspective, the reduced glass transition temperature Tgr (Tgr = Tg/Tm) of the HMF ash sample is 0.66, which is higher than that of the LMC ash sample (0.63). The melting temperature of the HMF ash sample refers to the value shown in Table 1. Fokin et al.36 concluded that, with an increase of the reduced glass transition temperature, the maximum nucleation rate temperature increases. Therefore, it is expected that the maximum crystallization rate temperature will be high. As a result, the maximum volume percentage temperature for the HMF ash sample is higher than that for the LMC ash sample. In Figures 4 and 5, the phase compositions are most complex at −1000 °C for the LMC coal ash sample while at −1200 °C for the HMF coal ash sample, but it does not cover those

the melting temperature because of the small thermodynamic driving force and increase at a lower temperature until the rate at which atoms may be brought to the crystal−liquid interface becomes limited by rapidly decreasing ion diffusivities.31,32 Therefore, it is most likely that the volume percent of the crystalline phases first experiences an increasing process and then a decreasing process with the increase of the quenching temperatures. A reflection is similar for the XRD patterns of the ash samples at −800 and −1400 °C; namely, the structure of the liquid therefore appears “frozen” at −800 °C. However, the maximum volume percentage temperatures for the two coal ash samples are different in Figure 6. The maximum volume percent is at −1000 °C for the LMC coal ash sample and at −1200 °C for the HMF coal ash sample. This may mainly depend upon some characteristics of the ash samples, such as the melting temperature and the glass transition temperature. Figure 7 shows DTA curves of the ash samples after heat treatment at 1400 °C for 30 min in a reducing atmosphere. In Figure 7, some small fluctuations may indicate the molecular rearrangement preceding glass crystallization.33 From Figure 7, it is observed that the two ash samples exhibit a transition of 2210


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Figure 16. SEM images of the LMC ash sample after quench at −1200 °C.

relatively stable. This phenomenon is frequently observed in the study of coal ash properties on the basis of DTA.37,38 Roskosz et al.39 found that mineral compositions are highly variable as a function of the temperature but that changes are governed by strongly temperature-dependent mobilities of network-modifying and network-forming cations. In Figures 4 and 5, for the most mineral phases in the two coal ash samples after quench, their maximum diffraction peak intensities simultaneously occur, at −1000 °C for the LMC coal ash sample and at −1200 °C for the HMF coal ash sample. This means that the crystallization rate of the mineral phases does

Figure 15. SEM images of Fe-enriched regions in the LMC coal ash sample after direct heat treatment at 1000 °C.

observed at all of the quenching temperatures. For example, few new peaks like 19 are observed in Figure 4 at −1200 °C for the LMC coal ash sample and like 21 in Figure 5 at −1000 °C for the HMF coal ash sample. In Figure 7, Tc represents the crystallization peak temperature. There are not obvious crystallization peaks for the two ash samples heat-treated at 1400 °C for 30 min, which indicates that the ash samples are 2211


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results in the formation of Ca-/Na-/K-rich minerals, such as anorthite. Therefore, the low contents of CaO + Na2O + K2O help the formation of mullite labeled in numerals 1, 2, 4, 10, and 19 in Figure 8. Wollastonite is an intermediate compound and normally observed prior to the formation of anorthite.42,43 A high melting temperature for the HMF coal ash sample implies high viscosity. The high viscosity limits the mobility of the ion in order that transformation of wollastonite into anorthite may be inhabited. Figure 8 shows that the relative high amount of wollastonite exists, for example, at the positions labeled in numerals 9, 14, and 18. Almandine is readily formed in acid rocks at low pressure.44 This may be why almandine occurs at peak 16 for the HMF coal ash sample with high acid oxide contents of SiO2 + Al2O3. 3.3. Comparison of the Phase Structure between Ash Samples after Direct Heat Treatment and Quench. Comparisons of XRD patterns between the coal ash samples after direct heat treatment and quench at the corresponding temperatures are shown in Figures 9 and 10. The comparisons indicate the occurrence of a great variation in phase compositions. For the quenching process, a preheat treatment at 1400 °C favors homogenization of the melt. However, the ash samples after direct heat treatment are less homogeneous. In Figures 11 and 12, SEM images record the surfaces of the ash samples after direct heat treatment and quench. SEM images reveal considerable differences in the microstructure of the ash samples treated by these two different processes,

Figure 17. SEM images of the LMC ash sample after quench at −1000 °C.

not depend upon the mineral characteristics of itself but the bulk characteristics of the ash sample. A comparison of XRD patterns in Figure 8 shows a great variation in the phase composition for the two coal ash samples after quench. The minerals labeled in numerals represent some minerals that do not cover each other in the XRD patterns. The occurrence of anorthite at the numerals (1−4 and 6) is attributed to a high CaO content in the LMC raw ash sample. Another result from the high CaO content is the presence of quantities of hedengergite [Ca1 − xFe1 + x(SiO3)2] in Figure 4. Hedengergite [CaFe(SiO3)2] at peak 7 is richer in Ca than other compositions of hedengergite. Alkaline metal and alkaline earth cations are able to enter the mullite structure,40,41 which

Figure 18. SEM images of the HMF ash sample after quench at −1200 °C. 2212


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Figure 19. SEM images of the HMF ash sample after quench at −1000 °C.

especially at the low direct heat treatment temperature. The high porosity is clearly visible on the surface of the ash samples after direct heat treatment at the low temperature and pore size distribution unevenly dispersed in the matrix. This means a low degree of sintering. In this case, elements are located mainly in restricted corresponding regions. Some regions with a smooth surface relative to other regions also mean inhomogenity in element compositions. At the direct heat treatment temperatures of 1200 °C, element diffusion prompts the disappearance of the pores and improves the degree of homogenity. As a result, the difference in the phase compositions diminishes in Figure 9b. The preheat treatment at 1400 °C may influence the transformation of Fe2O3. At high temperatures, Fe2O3 easily decomposes into low-valence iron oxide45 and can also be rapidly reduced. In contrast, the ash samples after direct heat treatment do not experience the preheat treatment. Partial Fe2O3 is not able to be reduced. Therefore, Fe3+ minerals and Fe2O3 can still be identified in the ash samples after direct heat treatment, especially for the HMF coal ash samples in Figure 10, which complicate the phase structure. Figure 13 presents the crystalline phase volume percentage of coal ash samples after direct heat treatment and quench. The general observation to be drawn from Figure 13 is that there is a high crystalline phase volume percentage in the coal ash samples after direct heat treatment relative to that after quench at corresponding temperatures. With an increasing temperature, this difference reduces. For both coal ash samples, at 1000 °C, the high crystalline phase volume percent is reflected in quartz and anorthite. At 1000 °C, the LMC coal ash sample contains

Figure 20. SEM images of the HMF coal ash sample after the direct heat treatment at 1000 °C.

29.6% quartz and 23.9% anorthite in volume and the HMF coal ash sample contains 63.2% quartz and 16.3% anorthite in volume. At −1000 °C, no quartz crystallizes from the melt. To make a comparison to ash components shown in Table 1, Fe from EDX analysis is transformed into Fe2O3 on the basis of the Fe element balance (Fe → Fe2O3). In Figure 14, anorthite starts to appear in the crystal boundary on the surface of the partial melting matrix. According to the SiO2−Al2O3−FeO phase diagram and SiO2−CaO−FeO phase diagram,46 the 2213


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Figure 21. SEM images of the HMF coal ash sample after direct heat treatment at 1100 °C.

However, at 1200 °C, because the temperature exceeds the lower leucite temperature, most minerals, including anorthite, turn into melt. In Figure 15, according to EDX analysis, the regions where the big pores are located are rich in Fe. After homogenization by pretreatment at 1400 °C, the distribution of Fe tends to be homogenious. Consequently, the amount of anorthite is small. Although there is a similar crystalline phase volume percentage for the HMF coal ash samples at 1200 and −1200 °C, the main compositions of the crystalline phase are different because of variable valence iron. Fe2O3 still obviously exists in the HMF coal ash sample at 1200 °C in Figure 10. As a result, Fe3+ or (Fe3+ and Fe2+) minerals, such as andradite [Ca3Fe2(SiO4)3], skiagite, and

lower leucite temperature is 1083 and 1093 °C, respectively. Additionally, ash impurities, including Na2O, K2O, and MgO, can further reduce the melting temperature of ash. Therefore, in Figure 14, the partial melting matrix is mainly due to the existence of FeO. van Dyk et al.47 found that, at 1000 °C, anorthite started to crystallize, probably because of partial melting of mineral phases. A high CaO content favors the formation of anorthite.30 From microscopic perspectives, anorthite is the tectosilicate to nucleate in the melt. Fe−O can cause a breakdown of the Si−O framework. Therefore, the nucleation of the feldspars is greatly inhibited in the presence of Fe−O.48 For the LMC coal ash sample, at 1200 and −1200 °C, the main crystalline phases are still anorthite. 2214


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with the orientation of the boundaries.49 In this study, although anorthite is a well-formed shape, the high aspect ratio of about 25 indicates that its growth is still in the initial stage. Anorthite crystals formed at −1200 and −1000 °C are partly different in texture. At −1000 °C, anorthite crystals are randomly oriented with a low amount of twins. This means that high viscosity limits the arrangement of anorthite toward the lowest energy mode. In Figure 14, anorthite has nucleated but not grown up. The additional observation of Figure 14 shows that the sample contains both isolated large pores and generally interconnected small pores in large numbers as a result of incomplete densification. These pores may inhibit the growth of crystal gains.50 In addition,

calcian [(Ca1.08Fe1.92)Fe2(SiO4)3] most partly contribute to the volume percentage of the crystalline phases. 3.4. Typical Phase Texture. Figures 16 and 17 show the microstructures of typical phases after quench at 1200 and 1000 °C. SEM−EDX examination reveals that anorthite occurs as a wellformed shape. The anorthite grains found are elongated and lath-shaped with numerous growth twins. Pores are located on the grain boundary. Long and narrow pores are relatively common. The high aspect ratio (c/a) of anorthite grains indicates that their crystal growth is very anisotropic. Growth of anorthite grains was found to take place along the c direction preferentially, which is due to anisotropic surface energy varying 2215


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magnitude faster than that of Si or Al.54 In Table 1, the LMC coal ash sample has a high SiO2/Al2O3 ratio of 3.00. In anorthite, the SiO2/Al2O3 ratio is 1.18. Therefore, Al diffusion may be rate-limiting. Because of the high Fe content in the HMF coal ash sample in Table 1, it is difficult to identify anorthite by SEM−EDX. At the same heat treatment temperature of 1400 °C, because of the low content of fluxes in the HMF coal ash sample in Table 1, its homogeneity degree is low relative to the LMC coal ash sample. Therefore, there are some element-enriched regions in Figure 18a. The white regions in Figure 18a are

the crystals suspended in the melt can increase viscosity of the melt.51,52 The high viscosity does not help the ion diffusion and prevents anorthite growth. After homogenization, the nucleation of anorthite is inhibited, but nucleation is a random phenomenon. Because of the surface energy of the cluster of atoms, ions, or molecules, clusters of a critical size have a chance to survive and grow to macroscopic sizes within the supercooled systems.53 Once anorthite is nucleated, the small porosity and low content of solid phases can promote the growth of anorthite. Electrical conductivity measurements demonstrate that the mobility of calcium is 7 orders of 2216


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tightly, the EDX analysis is not affected by the surrounding glass matrix. In comparison to the crystal shape in Figures 21c, 22c, and 23c, it is also concluded that, in Figures 21c and 22c, the crystals should be Fe−O crystals. The Fe−O crystallization is most likely attributed to the local high Fe ion concentration. If Fe−O crystallization want to dissolve into the melt, the Fe ion must diffuse away to decrease the local Fe ion concentration. Even though the HMF coal ash sample experiences a heat treatment of 1400 °C, the big Fe−O particles may also exist, i.e., a Fe−O particle described in Figure 24. The dissolution of the Fe−O particle into the glass matrix must transport through a transition region. This process is controlled by ion diffusion.

4. CONCLUSTION (1) With the quenching temperature increasing, there is a maximum crystalline volume percentage. The maximum crystalline volume percentage temperature is mainly determined by thermal properties of ash. The quench temperature of 800 °C is enough to make the sample keep the structure at high temperatures. (2) The maximum diffraction peak intensities of most minerals simultaneously occur for the quenching process. This indicates that their crystallization rates do not depend upon the mineral characteristics of itself but the bulk characteristics of the ash sample. (3) A preheat treatment at high temperatures results in a low crystalline volume percentage and simple phase compositions for the quenching process relative to the direct heat treatment process at corresponding temperatures. (4) Anorthite can form a good crystal shape in the low melting temperature coal ash with a high Ca content for the quenching process. It is most likely that Al ion diffusion controls the growth of anorthite. The diffusion also controls the dissolution of the Fe−O particle, especially for the high melting temperature coal ash with a high Fe content.

Figure 24. Typical Fe−O particle in the HMF coal ash sample after quench at −1200 °C.

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made up of Fe-rich crystals relative to their Ca content, but its compositions are similar to the bulk compositions in Table 1. In comparison to the white regions, the gray glass matrix contains more SiO2 and Al2O3. Therefore, the formation of the gray glass matrix is attributed to the high viscosity. The crystals have a similar SiO2/Al2O3 ratio and SiO2 + Al2O3 content in Figures 18 and 19, but a smaller crystal size is found at −1000 °C. This is attributed to the high viscosity at the low quenching temperature. A large amount of Fe−O-enriched regions with a small amount of Si, Al, Na, and Mg elements occurs in the HMF ash sample after direct heat treatment at 1000 °C, i.e., region 1 described in Figure 20. It looks like “glue”, and Fe diffuses outward into the surrounding regions. The “glue” results from the partial melting matrix according to the SiO2−Al2O3−FeO phase diagram.46 A part of region 1 is magnified and placed on the top right corner of the SEM image. It is clear that small Fe−O crystals are crystallizing from the partly melting matrix. The Fe−O crystallization is more obvious in the HMF coal ash samples after direct heat treatment at 1100, 1200, and 1300 °C. At 1100 °C, in Figure 21b, the Fe−O particle seems to float on the glass matrix. Small Fe−O particles are growing up. The scope of the SEM−EDX point analysis is about 2 × 2 μm. Therefore, in Figures 21c and 22c, because of the effect of the surrounding glass matrix, there is high SiO2 + Al2O3 content. However, it is sure that the glass matrix that the Fe−O particles crystallize from is rich in Fe. At 1300 °C, the Fe−O crystallization phenomenon is more obvious than that at 1000, 1100, and 1200 °C. Because the Fe−O crystals arrange

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-451-86412618, ext. 804. Fax: +86-451-86412528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (2007AA05Z246).

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