Influence of CaO on Crystallization Characteristics of Synthetic Coal

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Influence of CaO 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: A significant challenge for operation of entrained-flow coal gasifiers is the crystallization that occurs inside the liquid slag, which results in an increase in viscosity and slower flow of the slag along the wall. The chemical composition of slag is an intrinsic factor in determining crystallization characteristics. In this study, nine synthetic slags with various CaO concentrations (5−45%) were investigated for their crystallization tendency. Differential scanning calorimetry (DSC) and observations using the single hot thermocouple technique (SHTT) were combined to measure the influence of CaO on the crystallization temperature and crystalline morphology and to construct temperature−time transformation (TTT) diagrams. The crystalline phases were determined using scanning electron microscopy (SEM) with an energy-dispersive X-ray detector (EDX) and were compared to phases predicted by FactSage simulations. For the prepared slags, the wide range of CaO concentrations considered crossed through several stable crystalline phases. The resulting influence on crystallization behavior and degree of crystallization is consequently not linear or proportional to the CaO content. Slags with 15−35% CaO had a higher crystallization tendency and lower crystallization temperature than slags with lower CaO contents. Diopside and anorthite were the two main crystalline phases for slags with less than 35% CaO. However, when the content of CaO exceeded 40%, calcium tended to specifically combine with silica to form Ca2SiO4, which resulted in high crystallization temperatures.

1. INTRODUCTION During entrained-flow gasification of coal, the temperature inside a gasification chamber typically ranges between 1325 and 1600 °C, making the crystallization of some slag phases possible. The slag viscosity behavior is highly influenced by changes in the solid and liquid phases as well as the amount and type of solid material formed in the molten slag.1−3 A typical feature of crystalline slag is its critical viscosity temperature (Tcv),4−6 below which there is a rapid increase on the viscosity−temperature curve. If the fluidity is insufficient, slag blockage may occur at the exit chamber and can even shut down the gasifier. In addition, crystallized phases on the surface or pores of the sidewalls can influence thermodynamic stability and thermal conductivity. This effect may cause excessive wear to refractory walls or excessive heat loss through water-cooled walls. The chemical composition of coal ash is an important factor in slagging gasifiers because it affects ash fusibility, slag viscosity, and refractory life. Pulverized limestone is often used as a flux in entrained-flow gasifiers to improve flow properties because of its abundance and low cost. Numerous studies have focused on the influence of limestone on the ash fusion temperature and ash slag viscosity.7−9 At gasification temperatures, limestone releases CO2 to form CaO, which can affect viscosity of molten coal slags and make influence the sensitivity of slag viscosity to temperature.10,11 In industry vernacular, CaO “shortens the glass”, which is a reference to the pronounced effect that the temperature has on viscosity, causing a sharp viscosity increase with decreasing temperatures. An earlier study on viscosity of mold flux showed that viscosity decreased © 2014 American Chemical Society

with an increase of the CaO/SiO2 ratio over a range of 0.5− 0.9.12 Song et al. have found that the addition of CaO led to a decrease of both the ash fusion temperature (AFT) and viscosity, whereas there was an increase in AFT when the content of CaO exceeded 40%.7,9 Kong et al. observed an initial decrease in Tcv as CaO was added, up to a concentration of about 15%, after which Tcv increased.13 In this study, nine synthetic slags with CaO contents ranging from 5 to 45% were prepared to study crystallization characteristics using differential scanning calorimetry (DSC) and the single hot thermocouple technique (SHTT) under isothermal and non-isothermal conditions and compared to equilibrium compositions predicted using FactSage. DSC was used to identify the non-isothermal crystallization onset of different slags at a constant cooling rate. SHTT was performed under both isothermal and cooling conditions to study the crystallization process with in situ observation. FactSage was used to predict the equilibrium composition of slag−CaO mixtures at specific temperatures. The three methods were used to study the influence of CaO on crystallization characteristics, such as critical viscosity temperature, crystallization ratio, and crystalline phases.

2. EXPERIMENTAL SECTION 2.1. Synthetic Slag Samples. Because of the impurities and complex phases in coal ash, previous investigations have attempted to Received: May 28, 2014 Revised: September 8, 2014 Published: September 9, 2014 6627

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

SiO2

Al2O3

CaO

Fe2O3

MgO

SiO2/Al2O3

SiO2 + Al2O3

R

1 2 3 4 5 6 7 8 9

46.05 43.63 41.20 38.78 36.35 33.93 31.51 29.08 26.66

17.81 16.87 15.94 15.00 14.06 13.12 12.19 11.25 10.31

5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00

22.92 21.71 20.50 19.30 18.09 16.89 15.68 14.47 13.27

8.22 7.79 7.36 6.92 6.49 6.06 5.63 5.19 4.76

2.59 2.59 2.59 2.59 2.59 2.59 2.59 2.59 2.59

63.86 60.50 57.14 53.78 50.42 47.06 43.69 40.33 36.97

0.57 0.66 0.76 0.87 1.00 1.15 1.32 1.51 2.36

simplify the system using surrogate materials.14,15 In this study, synthetic slags were prepared with five pure oxides that constitute more than 95% of the components in actual coal ash. Table 1 shows the chemical composition of the synthetic slags. The concentration of CaO ranged from 5 to 45 wt %, which approximated the range differences for Chinese coal ashes. The other oxide ratios remained constant. Five analytical reagents were mixed in a homogeneous sample by blending them in pure ethyl alcohol and drying the samples at 100 °C for at least 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 ((Fe2O3 + CaO + MgO)/(SiO2 + Al2O3)) of the synthetic slags considered in this study ranged from 0.57 to 2.36; the ratio of SiO2/Al2O3 was kept at 2.59, while the sum of SiO2 and Al2O3 decreased with the increase of CaO. 2.2. Experimental Methods. 2.2.1. DSC and Kinetics. For the DSC tests, a sample of approximately 20 mg was placed in a Pt crucible with an identical crucible as the reference. The sample was measured for its heat release during a continuous cooling period in the Mettler Toledo thermogravimetric analysis (TGA)/DSC. This instrument has six pairs of thermocouples beneath the crucible, which provide an accurate heat signal. The furnace has a water-cooling system, enabling it to achieve better temperature stability and relatively high cooling rates. The samples were heated to 1500 °C, which melted the mineral mixture into a synthetic slag, and held at that temperature for 20 min. They were then 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 can determine the kinetics of the non-isothermal crystallization process from the DSC/differential thermal analysis (DTA) data.16−21 The Kissinger method used here is commonly used to calculate kinetics.16 It is derived from the basic rate equation

⎛ −E ⎞ dα ⎟f (α) = A exp⎜ ⎝ RT ⎠ dt

crystallization process can be studied by in situ observation. The resulting data can be used to construct temperature−time transformation (TTT) diagrams. Details of this method are described elsewhere.25 2.2.3. Thermodynamic Equilibrium Calculations. FactSage, a thermodynamic analysis software package, can be used to make predictions of multiphase equilibria, liquidus temperature, and the proportion of liquid and solid phases for a multicomponent system.7,26 In this study, FactSage was used to predict the equilibrium phases from 800 to 1600 °C with 20 °C intervals. 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 interpret the crystallization change. 2.2.4. Scanning Electron Microscopy (SEM). Because of the low magnification of SHTT, a FEI Quanta 600 FEG scanning electron microscope was used to analyze some of the quenched slags on the thermocouple for crystal morphology details. In addition, the local composition of the material was determined with an energy-dispersive X-ray detector (EDX), which could then be compared to the predictions from FactSage.

3. RESULTS AND DISCUSSION 3.1. DSC Results. Figure 1 shows the exothermic peaks of the nine original slags on the DSC curves under the same 10

(1)

where α is the extent of conversion, t is the time, A is the preexponential factor, and f(α) is the reaction model. Assuming crystallization as a first-order reaction, the Kissinger method can be expressed as

ln(β /Tp2) = − E/RTp + constant β = dT /dt

(2)

Figure 1. Comparison of DSC curves of different contents of CaO.

where Tp represents the peak temperature on a 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. The SHTT combines a hot thermocouple technique with video observation and image analysis, thereby providing direct observation of the crystallization process and subsequent analysis of the crystal fraction. The main advantage of using SHTT is the high cooling rate that can be achieved (up to 100 °C/s), which is significantly higher than the critical cooling rate for glass formation for a number of similar oxide systems.22−24 Thus, the sample can be quickly cooled to a target temperature, and the isothermal

°C/min cooling rate. The onset of the crystallization peak oscillated with an increase of CaO: a first decrease followed by an increase with the 30 wt % CaO sample standing out as having a lower crystallization temperature. The exothermic peak of the sample with 5% CaO was not readily observable, covering a wide temperature range with little heat release. With the increase of CaO, a larger exothermic peak area indicated a stronger crystallization tendency; two peaks were detected when the content of CaO reached 25%. However, when the 6628

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but the change was not significant. When the content of CaO increased to 45%, the activation energy of the low-temperature peak increased significantly, which may be attributed to the occurrence of the high-temperature peak because the precipitated calcium silicate oxide changed the proportion of the residue. Here, the energy barrier of the high-temperature peak dropped significantly, but there was only minor heat release. A previous study recorded a similar tendency for glass ceramics,27 where an increase in the CaO/SiO2 ratio resulted in a decrease of observed activation energy. The increase in CaO enhanced the fraction of non-bridging oxygen, so that less energy was needed to break the network. However, a further increase of CaO increased the value, implying that the polymerization of the glass network was improved. 3.2. In Situ Observation of SHTT. 3.2.1. Isothermal Conditions. Isothermal experiments were carried out under temperatures ranging from 1000 to 1350 °C for each slag. After melting, the slag was rapidly quenched to the desired temperature, which was maintained throughout the crystallization process. 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. Figure 3 shows the TTT diagram. The first on the left is the starting point when crystals began to appear and the second 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, except for the sample with 45% CaO, because it had a high crystallization temperature. Each TTT curve has an extremum (nose) that represents the least time required to achieve a given degree of crystallinity. In the nose region, the overall crystallization process (including both nucleation and crystal growth) is much faster than at other temperatures.29 As a whole, the TTT curves of the samples

content increased to 45%, there were two separate peaks; a small high-temperature crystallization peak was detected at around 1350 °C. Figure 2 uses a red line to show the tendency of normalized heat (generated heat divided by mass). A major increase in

Figure 2. Activation energy calculated by the Kissinger method with different contents of CaO.

normalized heat generation occurred when the content of CaO increased to 20%. However, with a further increase of calcium (higher than 35%), the normalized heat generated dropped. Linear relationships for the nine samples were obtained by plotting ln(β/Tp2) against 1/Tp based on the Kissinger equation. These linear relationships also provided an estimate for the activation energy (Figure 2). The activation energy is an indication of an energy barrier that must be overcome for crystals to form. Generally, the lower the activation energy value, the higher the tendency to crystallize. There was a slight decrease of activation energy between the 20 and 40% range,

Figure 3. TTT diagrams of the sample slags with different CaO ratios. 6629

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under in situ observation at a cooling rate of 20 °C/min. Figure 6 shows the influence of CaO on the initial crystallization

with CaO between 25 and 40% demonstrated a shift toward a lower temperature region compared to the slags with low CaO contents. When the content of CaO was increased from 10 to 40%, the nose temperature decreased from 1150 to 1000 °C and the incubation time around the nose was shortened. Figure 4 shows a comparison of the final changeless images of the

Figure 6. Crystallization temperature under SHTT with different contents of CaO.

temperature. As the content of CaO increased, the initial crystallization temperature shifted higher and then declined when the CaO content increased to 25%. However, a further increase of the CaO content beyond 40% led to a sharp rise of the crystallization temperature to 1300 °C. The tendency was in agreement with the DSC results. Figure 7 shows the crystals

Figure 4. Crystals of different samples under an isothermal temperature of 1100 °C (excludes 45% CaO at 1200 °C).

crystals after the end time of TTT at 1100 °C. When there was a small amount of CaO (5 or 10%), the crystals appeared as a thin layer on the surface of the slags. At successively higher CaO concentrations, more crystals occurred and gathered together. The resulting grains made it easier to distinguish from the liquid. When the percentage of CaO increased to 40%, however, the crystals were not obvious on the surface from the image. However, the exothermic peak of the DSC signal for the 35% CaO sample (Figure 1) showed significant crystallization. Therefore, it is assumed another new mineral generated but is difficult to distinguish in the image because of the similar color to the liquid. Crystals formed on the slag surface were observed to dissolve into the bulk molten slag when the content of CaO exceeded 40% (Figure 5). The time of dissolution was marked with a red

Figure 7. Crystals of different samples under a continuous cooling rate of 20 °C/min.

formed when cooled to a low temperature. The crystals were larger and more obvious compared to those under isothermal conditions. For continuous cooling conditions, the crystals first experienced a high-temperature region, in which viscosity was relatively low. Thus, the ions moved in an orderly manner and more easily formed large crystals. However, the position where the crystals formed and grew was almost the same for the isothermal and non-isothermal conditions, which means that the nucleation was not random and was influenced by some impurities or external factors. When cooled further, crystals grew at the foundation that had been formed because of the existing nucleus. Image analyses were performed to quantitatively evaluate the crystal fraction, defined as the volume ratio of crystals to the total volume of the melt, which is often substituted by the area

Figure 5. Dissolving process of the crystal in the slag with 40 wt % CaO at 1150 °C.

point in the TTT diagrams. At first, the crystals occurred dendritic in a specific area, but after several seconds (149 s), the crystals started to dissolve and spread into a thin layer on the surface of the slag that is not easily distinguished. This phenomenon was observed at particular isothermal temperatures between 1100 and 1200 °C. It is an indication that some crystal formed at first may change as the slag flows along the wall in the gasifier. Therefore, the viscosity can also be highly affected. 3.2.2. Continuous Cooling Conditions. The continuous cooling characteristics of the nine samples were also studied 6630

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crystallization ratios of the images. The crystallization ratios in the slags with the CaO content in the 15−35% range were higher than those with lower or higher CaO content. Also, the dissolving process of crystals also occurred for the slag with 40% CaO between 1152 and 1145 °C. 3.3. FactSage Predictions of Equilibrium Conditions. The equilibrium phases and solid fractions of the nine samples with different CaO ratios were calculated using FactSage (Figure 9). The initial crystalline phase formed upon cooling for slags with CaO less than 30% was predicted to be spinel. However, on the basis of the crystallization temperature in SHTT and peak temperature in DSC, the main crystallization range was detected between 1000 and 1300 °C, which corresponds to the range of calcic clinopyroxene [XY(Si,Al)2O6] and anorthite (CaAl2Si2O8) formation. With a low CaO content, anorthite was the main predicted mineral; it occurred at approximately 1300 °C. Diopside (CaMgSi2O6) is a monoclinic pyroxene mineral; it is the significant end member of clinopyroxene. As calcium increased, more pyroxene than anorthite was formed at relatively low temperatures (less than 1200 °C). When the content of CaO increased to more than 30%, the major calculated mineral was melilite [Ca2(Fe,Mg)-

ratio. In this way, only the crystals on the surface are counted, neglecting the internal crystals of the melt. The image analysis computer program was based on RGB decomposition of images. The crystals were easily identified after establishing the background because their G values differed greatly from those of the other melting slags. Figure 8 shows the calculated

Figure 8. Crystallization ratio with different contents of CaO under continuous cooling conditions.

Figure 9. FactSage predictions of solid fractions with different contents of CaO. 6631

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Figure 10. Morphology of the quenched slag in SHTT experiments: (a) sample 3 at 1175 °C, (b) sample 7 at 1075 °C, and (c) sample 9 at 1300 °C.

Table 2. EDX Results (Atomic Percent) of the Three Samples in Figure 7 element

Si

Al

Ca

Fe

Mg

O

crystals

sample 3 (15%) sample 7 (35%) sample 9 (45%)

17.77 12.88 12.99

5.37 5.79 2.29

8.83 11.32 21.00

6.01 6.52 2.32

4.79 4.96 3.36

57.23 58.52 58.03

Ca(Fe,Mg,Al)Si2O6 Ca2(Fe,Mg,Al)Si2O7 Ca2SiO4

Figure 11. Prediction results from FactSage: (a) crystallization temperature of different minerals and (b) solid fraction of different minerals at 1100 °C.

Si2O7]. Ca2SiO4 was also predicted, having a higher calcium ratio than the other elements. Figure 10 shows the SEM morphology of the samples after their TTT diagram experiment with the EDX point analysis of the crystals (Table 2). There is the possibility that the crystals were a combination of phases. However, it is assumed that there is only one dominant crystal, which is also verified on DSC, excluding the samples with two distinct peaks. In the image, only one kind of morphology was observed, corresponding to one mineral phase, because the results from two-point EDX analysis at different positions but with the same crystal morphology are almost the same. According to the EDX results, the crystals of the samples with 15, 35, and 45 wt % CaO displayed different shape and morphology, which corresponded to diopside, melilite, and Ca2SiO4, respectively. The formation temperature and solid fractions at 1100 °C of the four main minerals (anorthite, pyroxene, melilite, and Ca2SiO4) were recorded to perform a detailed change analysis (Figure 11). The crystallization temperature tendency was in agreement with the DSC and SHTT results; as calcium increased, the crystallization temperature first slightly increased and then declined when the CaO percentage exceeded 20%. When the calcium content increased to more than 40%, the crystallization temperature significantly increased. However, experimental results showed that most of the crystallization temperatures in the experiment were below 1300 °C (including

the 40% CaO sample), indicating that melilite was not an influence on the crystallization temperature. As for the solid fraction, the experimental results of DSC and SHTT showed that the high fraction of crystals occurred in the samples within the range of 15−35% CaO, which should be the mineral diopside (with parts of anorthite and melilite). Diopside forms variably colored (typically dull green) crystals30 with thin white streaks. However, anorthite is transparent or translucent, making it difficult to detect in the translucent reddish slag. Anorthite and diopside detected in prepared slags are often the desired phases for the MgO−CaO−Al2O3−SiO2 system, and the increase of CaO is beneficial to the formation of diopside.27 For the samples with a high CaO content, melilite was predicted to make up a high proportion of the solids; however, this was not detected in the actual experiment. Therefore, it can be inferred that melilite is relatively difficult to precipitate and that the primary solid phase of slag with high CaO should be Ca2SiO4 in high-temperature regions. However, the liquidus temperature predicted by FactSage for the sample with 45% CaO is higher than 1500 °C, which is beyond the maximum temperature of the apparatus. Therefore, in the experiments when heated to 1500 °C, some solids may still be present and impact the following crystallization process. However, there is little doubt about the high tendency to form calcium silicate oxides with high CaO and SiO2 contents according to FactSage prediction. A study of one previous similar slag showed that 6632

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temperature and the change of the crystallization ratio can significantly impact the fluidity of slags. The study of the influence of CaO on crystallization for specific coal slags could provide more information and guidance when adding limestone as a fluxing agent.

silicon was easily incorporated with calcium to form wollastonite (CaO·SiO2) at a high temperature (approximately 1300 °C).31 As for our 45% CaO slag, the final crystals at high temperature changed to 2CaO·SiO22 because of the higher ratio of calcium to silicon. The excessive addition of CaO into coal slag may cause the viscosity−temperature curve to behave differently, such that the sample is more likely to act as a crystalline slag.13 This altered behavior can be explained by the influence of calcium silicate formation at high temperatures. However, not all of the solids predicted by FactSage could be found in the experiment, and the solid fraction also disagreed. The same deviation of FactSage from experiments has been reported before, because its prediction only provides results in equilibrium, while in practice, the condition was generally nonequilibrium.32 This also explains the findings by Kong et al.,33 for which the solid amount in the liquid slag predicted by FactSage had no direct relationship with Tcv. Moreover, they concluded that the tendency of Tcv was to initially decrease with CaCO3 additions and then to increase with greater additions, which is in agreement with the experimental results of this paper.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +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) 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. (2) Wu, L.; Ek, M.; Song, M.; Sichen, D. The effect of solid particles on liquid viscosity. Steel Res. Int. 2011, 82 (4), 388−397. (3) Wright, S.; Zhang, L.; Sun, S.; Jahanshahi, S. Viscosity of a CaO− MgO−Al2O3−SiO2 melt containing spinel particles at 1646 K. Metall. Mater. Trans. B 2000, 31 (1), 97−104. (4) 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. (5) 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. (6) 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. (7) 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. (8) Lu, T.; Zhang, L.; Zhang, Y.; Feng, Y.; Li, H. X. Effect of mineral composition on coal ash fusion temperature. J. Fuel Chem. Technol. 2010, 38 (1), 23−28. (9) Song, W. J.; Tang, L.; Zhu, X.; Wu, Y.; Rong, Y. Q.; Zhu, Z. B. Fusibility and flow properties of Shell gasifier slag. CIESC J. 2009, 60 (7), 1781−1786. (10) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. Rheological properties of high-temperature melts of coal ashes and other silicates. Prog. Energy Combust. 2001, 27 (3), 237−429. (11) Lakatos, T.; Johansson, L. G.; Simmingsköld, B. Viscosity temperature relations in the glass system SiO2−Al2O3−Na2O−K2O− CaO−MgO in the composition range of technical glasses. Glass Technol. 1972, 13 (3), 88−95. (12) Tang, X.; Zhang, Z.; Guo, M.; Zhang, M.; Wang, X. Viscosities behavior of CaO−SiO2−MgO−Al2O3 slag with low mass ratio of CaO to SiO2 and wide range of Al2O3 content. J. Iron Steel Res. Int. 2011, 18 (2), 1−17. (13) Kong, L.; Bai, J.; Li, W.; Bai, Z.; Guo, Z. Effect of lime addition on slag fluidity of coal ash. J. Fuel Chem. Technol. 2011, 39 (6), 407− 411. (14) 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. (15) Barrett, E. P. The fusion, flow and clinkering of coal ash. A survey of the chemical background. In Chemistry of Coal Utilization;

4. CONCLUSION For the synthetic slags used in this study, the content of CaO ranged from 5 to 45%, resulting in complex phase diagrams that led to nonlinear trends with regard to crystallization behavior. Three analytical methods (DSC, in situ observation, and FactSage prediction) were combined to provide a comprehensive understanding of the influence of CaO on crystallization characteristics. Crystallization activation energies determined from DSC results using the Kissinger method ranged from roughly 200 to 400 kJ/mol. TTT diagrams were constructed using results from isothermal SHTT experiments. The crystallization process was also recorded in SHTT experiments under continuous cooling conditions. At low concentrations of CaO (∼5%), the crystallization was too weak to generate an obvious crystallization peak on the DSC curve. However, under in situ observations, a thin layer of crystal could be observed on the surface of the slag at 1100 °C after a relatively long incubation time. According to the FactSage prediction, the mineral in such cases should be anorthite. With the increase of CaO, crystallization of the slags became significant, especially in those with a calcium range between 15 and 35%. There were distinct crystallization peaks on the DSC curve with high normalized heat release and lower calculated energy. The crystallization temperature increased but only slightly. The higher crystallization ratio of in situ images confirmed the high crystallization tendency within this range as well. Diopside was the main crystal, because its dull color was easily recognized. The crystallization changed radically with an increase of CaO to 40% because of the occurrence of calcium silicate (Ca2SiO4), which accounts for the high-temperature peak and low-energy barrier around 1300 °C. However, the peak is not significant with small heat release. The Ca2SiO4 crystals could dissolve during the crystallization process in both isothermal and non-isothermal conditions. Although the crystalline phases were revealed from the combination of EDX and FactSage, more exact results will be determined further using individual quenching samples prepared by a rapidly quenching furnace later. Understanding the solid−liquid change inside slags is of great importance because crystallization is the fundamental reason for critical viscosity. Both the shift of the crystallization 6633

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dx.doi.org/10.1021/ef501215u | Energy Fuels 2014, 28, 6627−6634