Influence of Fe2O3 and Atmosphere on Crystallization Characteristics

Dec 11, 2014 - Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Salt Lake City, Utah 84112, United States...
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Influence of Fe2O3 and Atmosphere on Crystallization Characteristics of Synthetic Coal Slags Weiwei Xuan,† Kevin J. Whitty,‡ Qingliang Guan,† Dapeng Bi,† Zhonghua Zhan,‡ and Jiansheng Zhang*,† †

Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 10084, People’s Republic of China ‡ Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Salt Lake City, Utah 84112, United States ABSTRACT: Crystallization in the molten slag of entrained-flow coal gasifiers can increase its viscosity and affect the flow of the slag layer along the wall. The chemical composition of the slag plays an important role in crystallization behavior and crystal characteristics. The aim of the work reported here is to study the influence of Fe2O3 on crystallization tendency. Six synthetic slags with different Fe2O3 concentrations, ranging from 5 to 30 wt %, were investigated using differential scanning calorimetry (DSC) and the single hot thermocouple technique (SHTT). In addition, three of the slags were reduced in a 60:40 CO/CO2 atmosphere, and their crystallization characteristics were compared to the original slags. The time−temperature−transformation (TTT) diagrams for each slag were constructed. The Kissinger method was used to calculate the activation energy based on DSC results under cooling conditions, and the Johnson−Mehl−Avrami (JMA) equation was applied to calculate crystallization mechanisms under isothermal conditions. As the iron oxide concentration increased, a higher crystallization temperature and lower activation energy were observed, which indicates a stronger crystallization tendency. However, when the slags were in reducing conditions, the crystallization tendency declined slightly with a higher activation energy and lower crystallization temperature. The thermodynamic modeling program FactSage was applied to predict the equilibrium composition of the system. The modeling results indicate that reduced slags have a lower liquidus temperature and less species of solids, which provided further evidence of the comparative weakness of crystallization in a reduced atmosphere. The upward shift of the crystallization temperature caused by iron oxides will significantly influence the temperature of critical viscosity (Tcv) of crystalline slag.

1. INTRODUCTION During high-temperature entrained-flow gasification of coal, the minerals in the ash form a molten slag layer along the side walls of the gasification chamber. It is important to understand and control the fluidity of this slag to ensure normal operation of the gasifier. Slagging gasifiers typically operate at temperatures between 1200 and 1600 °C,1,2 and crystallization of some phases inside the slag is possible in this temperature range. The typical feature of crystalline slag is the temperature of critical viscosity (Tcv),3−5 at which a rapid change of viscosity occurs on the viscosity−temperature curve. If the fluidity is not good enough, slag blockage at the gasification chamber exit can occur, which, in some cases, results in the shutdown of the gasifier. The chemical composition of coal ash is an important factor affecting ash fusibility and slag viscosity, which are the most critical parameters in determining the oxygen consumption and, thus, the economical feasibility of gasifiers. Slag with a high iron content and low alkali, such as the slag of Pittsburgh No. 8 coal, has been reported to be typical of crystalline slags and has been found to form dendritic hercynites and corundum.3 Iron is a heterovalent component in coal ash, and slag shows significant variation under different oxidation−reduction conditions. Variations in chemical and structural properties as well as crystallization characteristics are expected with the change of gas composition. Under reducing conditions, the melting of slag is usually most affected by iron, which can be seen in the iron corner of the FeO−Al2O3−SiO2 phase diagram, and slag © 2014 American Chemical Society

may form iron-bearing minerals, such as wustite (FeO), fayalite (Fe2SiO4), or hercynite (FeAl2O4).6,7 Some studies have addressed the influence of chemical components on ash fusility or viscosity,8−10 and the redox kinetics of iron and the oxygen transfer in silicate melts have been studied in metallurgy or magmatic systems.11−15 However, not much data is available on crystallization characteristics for synthetic or real coal slags. Conditions inside a gasifier and the high concentrations of hydrogen and carbon monoxide maintain reducing conditions, which control the form of iron. Even metallic iron may be observed in strongly reducing atmospheres, such as N2/CO or CO2/H2, but little or no reduction of FeO occurs in environments with milder reducing gas, such as CO/CO2, even when there is an excess of CO.16−18 Reduced Fe2+ and oxidized Fe3+ have been reported to affect crystallization differently. Fe3+ can be amphoteric based on its relative concentration. Fe2+, in contrast, is always a network modifier, which can decrease the polymerization and viscosity.19−21 In the study presented here, the influence of iron oxide concentrations and gas atmosphere was studied experimentally using differential scanning calorimetry (DSC) and the single hot thermocouple technique (SHTT), and the thermodynamic modeling package FactSage was used to evaluate phase compositions. Six synthetic slags of different iron oxide Received: September 15, 2014 Revised: December 8, 2014 Published: December 11, 2014 405

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To prepare reduced slag samples, slag was placed in a 30 μL alumina crucible, which was put on an alumina support controlled by a handle outside and on top of the furnace. Under the furnace, a stainless-steel box contained water that would quench the sample. Before heating, the furnace was evacuated with a vacuum pump to make sure that the sealing was effective. Nitrogen was introduced into the furnace for about 20 min, and then the gas was switched to the 60:40 CO/CO2 mixed gas at about 500 °C. When the sample had been exposed to reducing conditions for the desired amount of time, the insulation above the box was removed and the handle was rotated, causing the crucible to drop swiftly into the water. The quenching time was rapid, taking less than 1 s, so that the phases were solidified at a high temperature without crystals being formed in the quenching process. Finally, nitrogen was kept for 20 min to expel the reducing gas. The three slags with 10, 20, and 30% iron content were reduced in this furnace. They were heated to 1500 °C and maintained for 1 h before being quickly quenched in the water. Then, the quenched samples were crushed into smaller particles and ground in an agate mortar to less than 200 μm. The reduced samples were sealed in an inert atmosphere and ready for analysis. 2.2. Experimental Methods. 2.2.1. DSC and Kinetics. The sample was measured for its heat release during the continuous cooling period in Mettler Toledo TGA/DSC, which has six pairs of thermocouples beneath the crucible to ensure an accurate heat signal. The furnace has a water-cooling system, so that it can achieve good temperature stability and relatively high cooling rates. A sample of approximately 20 mg was placed in a Pt crucible with an identical crucible as a reference. Samples were heated to 1500 °C and kept at that temperature for 20 min to ensure that they were melted. Then, they were cooled to 900 °C at different cooling rates, ranging from 10 to 40 °C/min to allow for determination of crystallization kinetics. The activation energy of each sample was calculated to compare the crystallization behavior for different slag compositions. Several methods have been proposed to determine the kinetics of the nonisothermal crystallization process from DSC/differential thermal analysis (DTA) data.28−33 The Kissinger method28 is commonly used to calculate kinetics. It is derived from the basic rate equation

concentrations, varying from 5 to 30 wt %, were prepared to study their crystallization characteristics. To figure out the influence of the atmosphere, three individual reduced slags in a 60:40 CO/CO2 (molar ratio) atmosphere were melted at 1500 °C in a high-temperature oven and then rapidly waterquenched. The ratio of CO/CO2 was fixed according the reducing atmosphere in measurement of ash fusion temperature (AFT).22 The crystallization characteristics of reduced slags were compared to the non-reduced slags.

2. EXPERIMENTAL SECTION 2.1. Synthetic Slag Sample Preparation. 2.1.1. Slags with Oxidized Iron. Because there are some impurities and complex phases in real coal ash, previous investigators have attempted to simplify the system using surrogate materials.23−25 In this study, synthetic slags were prepared with five pure oxides, as shown in Table 1, which

Table 1. Chemical Composition of the Six Synthetic Slags composition (wt %)

SiO2

Al2O3

CaO

Fe2O3

MgO

base/acid ratio (R)

1 2 3 4 5 6

45.06 42.68 40.31 37.94 35.57 33.20

17.43 16.51 15.59 14.68 13.76 12.84

24.47 23.18 21.90 20.61 19.32 18.03

5.00 10.00 15.00 20.00 25.00 30.00

8.05 7.62 7.20 6.78 6.35 5.93

0.61 0.70 0.80 0.92 1.05 1.63

constituted more than 95% of the components in real ash. We previously investigated the influence of the temperature and cooling rate on crystallization characteristics of a slag with composition identical to slag 4.26 For the synthetic slags used in this study, the concentration of Fe2O3 varied from 5 to 30 wt %, which approximated the iron content range in Chinese coal ashes. The other ratios of oxides were kept constant. Five analytical reagents were mixed in pure ethyl alcohol and dried at 100 °C for more than 10 h. The dried mineral mixture was then ground in an agate mortar to less than 200 μm to promote better homogeneity. The base/acid ratio R, which is defined as (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3), increased from 0.61 to 1.63 as the iron oxide content increased, a range of which was reported to have a slagging potential.27 2.1.2. Reduced Slags. Figure 1 shows a schematic of a sealed furnace (60 mm diameter × 190 mm) equipped with a CO2−CO gasmixing line. The furnace can operate at temperatures as high as 1600 °C. Cooling water circulating on both the top and bottom of the furnace maintains a low temperature for the O-rings inside the flanges.

⎛ −E ⎞ dα ⎟f (α) = A exp⎜ ⎝ RT ⎠ dt where α is the extent of conversion, t is the time, A is the preexponential factor, and f(α) is the reaction model. Assuming that the crystallization is a first-order reaction, the Kissinger method can be expressed as

ln(β /Tp2) = − E/RTp + constant β = dT /dt where Tp represents the peak temperature on the DSC curve, R is the gas constant, and β is the cooling rate. Here, the calculated result E indicates the apparent activation energy of crystallization. 2.2.2. SHTT. In the SHTT, a small amount of slag mineral mixture is placed in a narrow U-shaped bend of a sensitive high-temperature thermocouple, which is placed in a high-temperature chamber to cause the slag to melt. Surface tension keeps the molten slag on the thermocouple. The slag and thermocouple are cooled, while a video camera focused through a scope records the surface of the slag. The outstanding advantage of SHTT is its high cooling rate up to 100 °C/ s, which is significantly higher than the critical cooling rate for glass formation in a number of similar oxide systems.34−36 Thus, the isothermal crystallization process can be studied by in situ observation, enabling us to construct a time−temperature−transformation (TTT) diagram. Because of the low magnification of the scope, some of the quenched slags on the thermocouple were analyzed by scanning electron microscopy (SEM). Image analyses were performed using a program designed to quantitatively evaluate the volume fraction of crystals. The isothermal kinetics of crystallization were calculated using the well-known

Figure 1. Schematic diagram of the quenching furnace. 406

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Johnson−Mehl−Avrami (JMA) equation. Details of this method and calculation are described elsewhere.26 2.2.3. Thermodynamic Equilibrium Calculations. FactSage, a thermodynamic computer package, can be used to make predictions of multiphase equilibria, liquidus temperature, and the proportion of liquid and solid phases for a multicomponent system.8,37 In this study, FactSage was used to predict the equilibrium phases for the synthetic slags in a temperature range from 800 to 1600 °C using a 20 °C interval. Phase formation data for the primary oxides and their combinations were selected from the FToxid and FactPS databases. The results were used as a reference to analyze the crystallization tendency.

that the iron oxides promote the formation of crystals, shifting crystallization to a high temperature. By plotting ln(β/Tp2) against 1/Tp, on the basis of the Kissinger equation, linear relationships for the six samples were obtained. The resulting estimated activation energies are also shown in Figure 3. The activation energy is an indication of the energy barrier that must be overcome for crystals to form. Generally speaking, the lower the activation energy, the higher the tendency to crystallize. The crystallization was so weak for sample 1 that the crystallization peak could not be exactly determined. Thus, the activation energy cannot be calculated. The energy barrier decreases as more iron oxides are added, indicating higher crystallization tendency. 3.1.2. In Situ Observation of SHTT. 3.1.2.1. Isothermal Conditions. Isothermal SHTT experiments were carried out in a wide temperature range, between 1000 and 1350 °C, for each slag. After melting, the slag was rapidly quenched to the desired temperature, which was maintained as the sample was observed and recorded by video. Crystals did not form immediately but began to appear after a period of time. The time before crystal growth began, known as the incubation time, is the time required to initiate nucleation at a fixed temperature28 and is an important parameter for characterizing the crystallization property. The TTT diagrams in Figure 4 illustrate the process. Crystallization of sample 1 with only 5 wt % iron oxide is too weak to observe the crystals. The first curve on the left is the starting point when crystals began to appear, and the second curve represents when no more apparent crystal growth was observed. To make a convenient comparison, both the x and y coordinate axes were set in the same range. The crystallization temperature under isothermal conditions shifts to a higher temperature region as the iron oxide content of the slag increases. When the content of iron oxide is less than 15%, crystallization could only be found when the temperature was under 1200 °C. As the concentration of Fe2O3 increased to 30%, the crystallization range became wider and the crystallization temperature rose to 1350 °C. This observation is in agreement with the results of DSC, which suggests that, with an increase of iron oxides, crystallization occurred at higher temperatures. To better understand the influence of Fe 2 O 3 , the crystallization characteristics and isothermal kinetics of different samples at 1100 °C were studied. Figure 5 shows the final crystals of different slags at 1100 °C. The slag with 5% iron oxide solidified solely in glass form with no apparent crystal growth. As the Fe2O3 content increased, more crystals precipitated from the melt. The degree of crystallization can be expressed in terms of a crystallization ratio, X, which is the fraction of total observable surface area occupied by crystals. When the content of Fe2O3 reached 30 wt %, the crystallization ratio observed visually was small, not as much as expected. In fact, close visual inspection of recordings of the crystallization process reveals that the crystals in the slag with 30 wt % iron oxides would form a filmy white layer on the surface that later sank to the inside of the slag. Song et al.8 observed that, with a high content of Fe2O3, up to 40%, red crystalline particles, predicted as iron oxide, could be observed. In our experiment, the final quenched slag with 30% Fe2O3 was analyzed by SEM, as shown in Figure 6, from which we could better identify the details of the crystals. Small crystals formed on both the surface and inside of the slag in the left image, and different layers of crystals are apparent in the right image. This morphology explains the contrasting results of the slag with 30% Fe2O3 that

3. RESULTS AND DISCUSSION 3.1. Influence of Ferric Iron Oxide. 3.1.1. DSC Results. Figure 2 shows the exothermic peaks on DSC curves of the six

Figure 2. DSC results of slags with different ratios of iron oxide at 10 °C/min.

original slags under the same 10 °C/min cooling rate. The exothermic peak of the 1 sample (bottom curve) is gentle and covers a wide temperature range with little heat release. The change of the peak temperature and normalized heat are drawn in Figure 3. The normalized heat, as a reflection of the crystallization degree, is the heat generated divided by mass, so that the crystallization tendency can be compared between samples. As iron oxides increase, a larger exothermic peak and more released normalized heat indicate a stronger crystallization tendency. In addition, as iron oxides increase, the exothermic peak occurs earlier during cooling, which means

Figure 3. Influence of the iron oxide concentration on the DSC peak temperature, heat release, and activation energy. 407

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Figure 4. TTT diagrams of the considered slags with different Fe2O3 ratios.

Table 2. Crystallization Mechanisms of Different Contents of Fe2O3 at 1100 °C concentration of Fe2O3 (wt %)

n

K

10 15 20 25 30

4.98 1.50 1.83 1.53 1.43

0.01 0.12 0.15 0.26 0.24

around 4 indicates the interface reaction of three-dimensional growth, while the others should be one-/two-dimensional growth. Figure 7 shows the crystallization ratio with time, in Figure 5. Crystallization of different samples under an isothermal temperature of 1100 °C.

Figure 6. SEM of quenched slag with 30% Fe2O3.

Figure 7. Crystallization ratio X of different contents of Fe2O3 with time at 1100 °C.

more heat was detected in DSC, but only a small proportion of crystals could be observed in the in situ image. Because iron in the slag leads to translucence at high temperatures, the crystals inside the slag or crystals with a similar color to the slag were not able to be recognized, which is a limitation of the SHTT method. The dynamic crystallization kinetics were calculated using the JMA equation. The results are listed in Table 2. The crystallization mechanism of the slag with 10% Fe2O3 is different from the other samples. The Avrami exponent n

which the scatter is the experimental result and the solid line is the Avrami equation with calculated results, shown in Table 2. The model and experimental result agree well. At a temperature of 1100 °C, when Fe2O3 increased to 15%, the growth rate increased rapidly and crystallized in a short time. On the basis of the enlarged figure, we can see that, with a higher content of iron oxide, the crystallization rate also increased. 3.1.2.2. Continuous Cooling Conditions. The continuous cooling characteristics of the five samples were studied under in situ observation. Figure 8 shows the morphology of crystals 408

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Figure 10. DSC results of slags with oxide and reduced iron at 10 °C/ min.

Figure 8. Crystallization of different samples under a continuous cooling rate of 20 °C/min.

under a cooling rate of 20 °C/min. In comparison to the morphology at a constant temperature of 1100 °C in Figure 5, the larger crystals imply better growth in the high-temperature region. The extent of crystallization versus temperature during cooling at 20 °C/min is shown in Figure 9. These results

Figure 11. Influence of the gas atmosphere on the activation energy and normalized heat of crystallization.

may be attributed to the presence of alkaline earth metals Mg and Ca, which reduces the effect of iron on the melt structure.21,40 3.2.2. In Situ Observation of SHTT. The three reduced samples were investigated under equivalent isothermal conditions as those used for the original slags with oxidized iron. The TTT diagrams were also constructed, as shown in Figure 12. The reduced and oxidized samples are marked with open and solid points, respectively. The influence of the atmosphere lies mainly in the high-temperature region and shows that the reduced samples require a longer incubation time and a higher undercooling to crystallize. The crystallization image of reduced slags with different contents of Fe2O3 at 1100 °C was recorded in Figure 13. The in situ observation during the heating and cooling process shows that the fluidity of the reduced slags was not as good as that of the oxidized slags. The melting films seemed more dense and thick, which might make the crystals difficult to form. 3.2.3. FactSage Prediction of the Equilibrium Condition. The equilibrium phases and solid fractions of the three groups of slags were calculated using FactSage, as shown in Figure 14. The reduced slags were predicted under excess CO with the constant ratio of 60:40 CO/CO2. There is an evident increase of the liquidus temperature with more spinel formed as the iron oxides increase, which is in agreement with the results described earlier. Previous studies suggest that iron increased the formation of hercynite spinel from melts, depleting the slag from Fe, Mg, and Al.7 The primary precipitated spinels can act as nucleation sites that promote the subsequent crystallization process. However, the formation of spinel in this experiment can only be predicted through FactSage, with no definitive

Figure 9. Crystallization ratio with the temperature of slags with different Fe2O3 contents at 20 °C/min.

provide further evidence that, with increased iron oxide, the crystallization temperature becomes higher. This may indicate that the high content of iron oxide can lead to a lower viscosity in a high-temperature region and, thus, promote the crystallization. This finding is in agreement with previous studies that showed that, in high-temperature melts, the addition of Fe2O3 can reduce viscosity and result in more effective nucleation and crystallization38 and that the addition of iron oxides leads to lower viscosity values and higher Tcv.39 3.2. Influence of the Atmosphere. 3.2.1. DSC Results. Figure 10 compares the exothermic peaks of three groups of slags with oxidized and reduced iron at 10 °C/min cooling rate. The normalized heat and the calculated activation energy are shown in Figure 11. Under a reducing atmosphere, the crystallization of slags was weakened. The peak temperature shifted slightly toward a lower temperature, and the normalized heat of crystallization was greatly reduced. However, the shape of the exothermic peaks was similar. The solid lines in Figure 11 represent the reduced slags, while the dash lines below are for the original slags. The crystallization energy barrier became higher after the slags were reduced. Both the slight increase of the energy barrier and less crystallization heat in the DSC results are indications that crystallization became more difficult after reduction. However, the effect is not strong even for the slag with the highest content (30%) of Fe2O3. The weakness 409

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Figure 12. TTT diagrams showing the influence of the iron content and gas atmosphere.

lower Tcv; thus, the fluidity is more inclined to display Newtonian behavior in a high-temperature region. The disparity caused by the different valences of iron can also be found in a previous high-iron slag, which showed Newtonian behavior in a reducing atmosphere and a sharp critical viscosity in an oxidation state. Figure 13. Crystallization of reduced slags with different concentrations of Fe2O3 at a constant temperature of 1100 °C.

4. CONCLUSION To study the influence of iron on slag crystallization characteristics, six synthetic slags with different concentrations of iron oxide, ranging from 5 to 30%, were prepared. In addition, three of the samples with 10, 20, and 30% Fe2O3 were reduced under the 60:40 CO/CO2 atmosphere in a sealed hightemperature furnace. DSC and in situ observation methods were combined to study the crystallization under continuous cooling and isothermal conditions. The enhancement of crystallization by iron oxide is significant. With a higher ratio of Fe2O3, more crystallization heat was released and the crystallization shifted to a higher temperature, potentially leading to a higher Tcv in viscosity. The kinetics under isothermal 1100 °C show that the growth rate of crystals increases with the addition of iron oxide. The influence of the atmosphere can be attributed to the change of the valence state of iron in the slag. When the slags are reduced to contain more Fe2+, there is a decline in the crystallization temperature, which can be seen in the TTT

experimental evidence of its existence by in situ observation. The thermodynamic data package FactSage can predict equilibrium slag compositions, but the experiments performed in this study did not reach equilibrium; therefore, the predicted and observed results did not match well. Another possibility may be that the spinel crystals are particularly small or that their color is not easily distinguished from the reddish melt. Evaluating the influence of the atmosphere, we can see that the equilibrium phases of reduced slags are more concise, with less solid types, and that the liquidus temperature is lower than for slags in an inert environment. The prediction of reduced samples provides further evidence that the crystallization shifts to a lower temperature when Fe3+ changed to Fe2+. Less spinel was formed, and most iron behaved like Ca2+ or Mg2+ as a modifier, which led to the lower crystallization temperature. The reduction of the influence of crystals in the hightemperature region will lead to a decrease in viscosity and a

Figure 14. FactSage prediction of the solid fraction with different iron oxides under different atmospheres. Top panels are in an inert (N2) environment, and bottom panels are in a CO/CO2 environment. 410

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(17) Wei, Y.; Li, H.; Yamada, N.; Sato, A.; Ninomiya, Y.; Honma, K.; Tanosaki, T. A microscopic study of the precipitation of metallic iron in slag from iron-rich coal during high temperature gasification. Fuel 2013, 103, 101−110. (18) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. A. Mechanistic study of the formation of slags from iron-rich coals. Fuel 1996, 75 (8), 952−958. (19) Mysen, B. O.; Virgo, D.; Seifert, F. A. Redox equilibria of iron in alkaline earth silicate melts: relationships between melt structure, oxygen fugacity, temperature and properties of iron-bearing silicate liquids. Am. Mineral. 1984, 69, 834−847. (20) Bouhifd, M. A.; Richet, P.; Besson, P.; Roskosz, M.; Ingrin, J. Redox state, microstructure and viscosity of a partially crystallized basalt melt. Earth Planet. Sci. Lett. 2004, 218 (1 − 2), 31−44. (21) Chevrel, M. O.; Giordano, D.; Potuzak, M.; Courtial, P.; Dingwell, D. B. Physical properties of CaAl2Si2O8−CaMgSi2O6− FeO−Fe2O3 melts: Analogues for extra-terrestrial basalt. Chem. Geol. 2013, 346, 93−105. (22) American Society for Testing and Materials (ASTM). Annual Book of ASTM Standards, Part 26; ASTM: Philadelphia, PA, 1976; pp 263−268. (23) Kalmanovitch, D. P.; Williamson, J. Crystallization of coal ash melts. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society (ACS): Washington, D.C., 1986; ACS Symposium Series, Vol. 301, Chapter 18, pp 234−255. (24) Barrett, E. P. The fusion, flow and clinkering of coal ash. A survey of the chemical background. Chem. Coal Util. 1945, 1 (15), 496−576. (25) Vorres, K. S.; Greenberg, S.; Poeppel, R. Viscosity of synthetic coal ash slags. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; American Chemical Society (ACS): Washington, D.C., 1986; ACS Symposium Series, Vol. 301, Chapter 12, pp 156−169. (26) Xuan, W.; Whitty, K. J.; Guan, Q.; Bi, D.; Zhang, Z. Influence of isothermal temperature and cooling rates on crystallization characteristics of a synthetic coal slag. Fuel 2014, 137 (1), 193−199. (27) Miller, B. G. Coal Energy Systems; Elsevier Academic Press: Burlington, MA, 2005; pp 232−236. (28) Kissinger, H. E. Variation of peak temperature with heating rate in differential thermal analysis. J. Res. Natl. Bur. Stand. 1956, 57 (4), 217−221. (29) Kissinger, H. E. Reaction kinetics in differential thermal analysi. Anal. Chem. 1957, 29 (11), 1702−1706. (30) Ozawa, T. Kinetics of non-isothermal crystallization. Polymer 1971, 12 (3), 150−158. (31) Matusita, K.; Sakka, S.; Matsui, Y. Determination of activation energy for crystal growth by differential thermal analysis. J. Mater. Sci. 1975, 10 (6), 961−966. (32) Matusita, K.; Sakka, S. Kinetic study of the crystallization of glass by differential scanning calorimetry. Phys. Chem. Glasses 1979, 20 (4), 81−84. (33) Matusita, K.; Komatsu, T.; Yokota, R. Kinetics of nonisothermal crystallization process and activation energy for crystal growth in amorphous materials. J. Mater. Sci. 1984, 19 (1), 291−296. (34) Asayama, E.; Takebe, H.; Morinaga, K. Critical cooling rates for the formation of glass for silicate melts. ISIJ Int. 1993, 33 (1), 233− 238. (35) Scholze, H. Glass: Nature, Structure, and Properties; SpringerVerlag: Berlin, Germany, 1991. (36) Kashiwaya, Y.; Cicutti, C. E.; Cramb, A. W.; Ishii, K. Development of double and single hot thermocouple technique for in situ observation and measurement of mold slag crystallization. ISIJ Int. 1998, 38 (4), 348−356. (37) Jak, E. Prediction of coal ash fusion temperatures with the F*A*C*T thermodynamic computer package. Fuel 2002, 81 (13), 1655−1668. (38) Rezvani, M.; Eftekhari-Yekta, B.; Solati-Hashjin, M.; Marghussian, V. K. Effect of Cr2O3, Fe2O3 and TiO2 nucleants on the crystallization behaviour of SiO2−Al2O3−CaO−MgO(R2O) glassceramics. Ceram. Int. 2005, 31 (1), 75−80.

diagrams and FactSage predictions. The different valence of iron can cause reduced slags to display Newtonian behavior. A better understanding of slag crystallization properties could be achieved through a further study of the crystal phases, such as individual samples prepared for X-ray diffraction (XRD), because the amount of slag produced in the SHTT experiments is not enough. These experiments will be performed later.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +11-86-10-62795930. E-mail: zhang-jsh@ tsinghua.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors kindly acknowledge the support of the National Natural Science Foundation of China (NSFC) for this work (51176097).

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REFERENCES

(1) Collot, A. Matching gasification technologies to coal properties. Int. J. Coal Geol. 2006, 65 (3−4), 191−212. (2) Minchener, A. J. Coal gasification for advanced power generation. Fuel 2005, 84 (17), 2222−2235. (3) Oh, M. S.; Brooker, D. D.; de Paz, E. F.; Brady, J. J.; Decker, T. R. Effect of crystalline phase formation on coal slag viscosity. Fuel Process. Technol. 1995, 44 (1−3), 191−199. (4) Nowok, J. W. Viscosity and phase transformation in coal ash slags near and below the temperature of critical viscosity. Energy Fuels 1994, 8 (6), 1324−1336. (5) Yuan, H.; Liang, Q.; Gong, X. Crystallization of coal ash slags at high temperatures and effects on the viscosity. Energy Fuels 2012, 26 (6), 3717−3722. (6) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres. Fuel 1981, 60 (7), 585−597. (7) Pontikes, Y.; Machiels, L.; Onisei, S.; Pandelaers, L.; Geysen, D.; Jones, P. T.; Blanpain, B. Slags with a high Al and Fe content as precursors for inorganic polymers. Appl. Clay Sci. 2013, 73, 93−102. (8) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Zhu, Z. B.; Koyama, S. Effect of coal ash composition on ash fusion temperatures. Energy Fuels 2010, 24, 182−189. (9) Higman, C.; van der Burgt, M. Gasification, 2nd ed.; Elsevier: Amsterdam, Netherlands, 2008. (10) Ilyushechkin, A. Y.; Hla, S. S.; Roberts, D. G.; Kinaev, N. N. The effect of solids and phase compositions on viscosity behaviour and Tcv of slags from Australian bituminous coals. J. Non-Cryst. Solids 2011, 357 (3), 893−902. (11) Semykina, A.; Nakano, J.; Sridhar, S.; Shatokha, V.; Seetharaman, S. Confocal microscopic studies on evolution of crystals during oxidation of the FeO−CaO−SiO2−MnO Slags. Metall. Mater. Trans. B 2010, 41 (5), 940−945. (12) Max, W. Fe in magmaAn overview. Ann. Geophys. 2005, 48, 609−617. (13) Dingwell, D. B. Redox viscometry of some Fe-bearing silicate melts. Am. Mineral. 1991, 76, 1560−1562. (14) Mysen, B. Relations between Structure, Redox Equilibria of Iron, and Properties of Magmatic Liquids; Springer: New York, 1991; pp 41− 98. (15) Mysen, B.; Virgo, D. Iron-bearing silicate melts: Relations between pressure and redox equilibria. Phys. Chem. Miner. 1985, 12 (4), 191−200. (16) Lim, S.; Oh, M. Prediction of coal slag foaming under gasification conditions by thermodynamic equilibrium calculations. Korean J. Chem. Eng. 2007, 24 (5), 911−916. 411

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Energy & Fuels

Article

(39) Hurst, H. J.; Novak, F.; Patterson, J. H. Viscosity measurements and empirical predictions for some model gasifier slags. Fuel 1999, 78 (4), 439−444. (40) Toplis, M. J.; Dingwell, D. B.; Libourel, G. The effect of phosphorus on the iron redox ratio, viscosity, and density of an evolved ferro-basalt. Contrib. Mineral. Petrol. 1994, 117 (3), 293−304.

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