Effect of Cooling Process on the Generation and Growth of Crystals in

May 24, 2016 - theory of crystals and glasses was applied to describe the promotion of crystal growth with longer residence time. Secondary electron i...
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Effect of Cooling Process on the Generation and Growth of Crystals in Coal Slag Zhongjie Shen, Rixiang Li, Qinfeng Liang, Jianliang Xu, and Haifeng Liu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education and Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China ABSTRACT: Slag tapping in the entrained-flow gasifier is of importance to the successful and stable operation. The aim of the paper was to study the effect of cooling process on the generation of crystals in coal slag. A high temperature resistance furnace was applied to control the cooling process of the slag combined with the FactSage prediction. Crystal type and peak intensity were detected and analyzed by X-ray diffraction (XRD). Results of diffraction intensity and peaks indicated that low cooling rates and long residence times at fixed temperatures benefited the nucleation and growth of crystals. The relaxation and diffusion theory of crystals and glasses was applied to describe the promotion of crystal growth with longer residence time. Secondary electron images and backscattered electron images by scanning electron microscope (SEM) also proved the promoted growth of the crystals with amorphous phase, crystal number, size, and shape of regularity.

1. INTRODUCTION Efficient and environmentally sustainable technologies are urgent for the utilization of energy. Coal is one of the most widely used primary energy sources in the fastest growing economies, such as China and India.1 Optimization of coal gasification technology improves generation efficiency and greenhouse gas emissions, where the gasifier has played a key role in the expansion of the integrated gasification combined cycle (IGCC).2−4 The entrained-flow coal gasification technology with fuel feedstock flexibility has become one of the leading clean coal technologies.5 For a slagging gasifier, slag tapping was one of the key factors to successful operation of gasifiers. The properties of slag (e.g., chemical composition, rheology, crystallization, and flow properties) affected tapping significantly. Laboratory and theoretical studies had focused on the formation and rheology of slag and the conversion behavior of coal to syngas via gasification reaction.5−11 Various empirical models have been developed to study the viscous behaviors of slags,12−17 which fitted on the condition that slag was homogeneous and completely molten without assumption of any mineral crystallized. However, slag viscosity was sensitive to the inner structure and compound, and models have been developed to predict the viscosities of varied slags based on the components and network structure.18−20 Groen et al.21 studied the rheology and crystallization of gasification slag and used the fractional crystallization theory to predict the observed crystalline phases. The liquid phase and slag flow characteristics were also analyzed with the viscosity model and FactSage software.22 Yuan et al.23 predicted and analyzed crystallization behaviors of four typical Chinese coal samples, and results showed that the viscosity of the molten slag correlated with the content of crystalline phases. Therefore, the crystalline phase during the cooling affected the properties of slags obviously. When slag flows along the wall of a gasifier, variation and fluctuation of the slag temperature affect the generation and growth of crystals below the initial crystallization temperature. Kim and Oh24 have investigated the effects of cooling rate on © XXXX American Chemical Society

the experimentally determined critical-viscosity temperature (Tcv) and used a synthetic slag to exhibit the crystalline slag behavior. The residence time of coal ash at high temperatures had considerable influences on the compositions of coal ash, while the amorphous phase of mineral matters increased with the increasing temperature.25 The effect of cooling process on the nucleation and crystallization behavior in molten glass had been widely studied for the glass-ceramics industry. Nucleation temperature and time affected the crystallization behavior of ZrO2/ZrSiO4 with the addition of borosilicate glass.26 The addition of MgO or FeO in the slag system affected the crystallization time and temperature, and a higher basicity in the slag sample caused a delayed crystallization in the continuous transformation.27−29 A noncrystallizing liquid vitrified with decreasing or increasing pressure, and the system could not attain an equilibrium configuration in the available time and transformed into the glassy state.30 At lower temperatures with high degrees of undercooling the crystal incubation time was shortened and the crystallization ratio increased.31 On the basis of these studies for the crystallization behaviors, the cooling process is of significant importance to the generation and growth of crystals in molten coal slag. In this study, the effects of the cooling process on the generation and growth of crystals in molten coal slag were investigated, based on the slag cooling experiments and FactSage prediction. Crystallization behaviors under different cooling rates were investigated and detected by X-ray diffraction (XRD). Varied residence temperatures below the initial crystallization temperature were also held for the growth of crystals in the molten slag to analyze the relaxation effect, according to the environment in the industrial gasifier. The effect of the residence time on the generation and growth of the crystals was also studied. Micrographs detected from scanning Received: November 6, 2015 Revised: May 11, 2016

A

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate Analysis and Ultimate Analysis of the Raw Coal Used in This Study proximate analysis (wt %) raw coal

ultimate analysis (wt %)

M

V

FC

A

C

H

S

N

8.50

26.60

57.17

7.73

82.29

3.11

0.97

1.13

Table 2. Chemical Composition (wt %) of Coal Ash Sample component

SiO2

CaO

Al2O3

Fe2O3

Na2O

MgO

TiO2

K2O

SO3

coal ash

37.04

25.73

19.37

11.07

3.18

1.44

0.52

0.40

0.38

electron microscope (SEM) were also applied to analyze the crystal generation and growth under different cooling processes.

2. EXPERIMENTAL SECTION 2.1. Experimental Material. A bituminous coal from China was used to prepare the coal ash in the N17/HR-K muffle furnace (Nabertherm Company, Lilienthal, Germany). The proximate analysis and ultimate analysis of the raw coal used in this study is shown in Table 1. In the experiment of coal ash preparation, the temperature first rose from room temperature to 500 °C within 30 min and then was held for another 30 min. This heating process was to remove the crystallization water and sulfides and also avoid the reaction between oxysulfide and CaO. Then the temperature was heated to 815 °C within 30 min and held for about 120 min for the complete reaction of coal and air to make coal ash. Table 2 shows the chemical compositions of the coal ash sample analyzed by Advant’X Intellipower 3600 X-ray fluorescence (Thermo Fisher Scientific, America). Coal ash contained a high content of CaO and Fe2O3 in this study. Alkali metal in the coal ash helped to reduce the ash fusion temperature,32 which could be noted in Table 3. The

Figure 1. FactSage prediction for the phase equilibrium of coal slag versus temperature.

Table 3. Ash Fusion Temperatures (AFT) of Coal Ash Sample coal ash

DT (°C)

ST (°C)

HT (°C)

FT (°C)

1129

1141

1146

1161

ash fusion temperatures (AFT) of the coal ash, which contained deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT),33 were analyzed by a 5E-AF4000 ash fusion point determination meter (Kaiyuan Company, Changsha, China) under a reducing atmosphere. During the ash fusion detection experiment, deformation temperature (DT) was that the top of the coal ash cone circled or began to tilt, indicating coal ash started to melt. The softening temperature (ST) mean the tip of the coal ash cone bended and touched the holder. The hemispherical temperature (HT) showed that the shape of the coal ash cone transformed to hemispherical and most of the coal ash cone has been melted. The flow temperature (FT) displayed the complete molten slag tiled on the holder, which could not distinguish between apparent shape and slag. The flow temperature of the coal ash was 1161 °C, and the temperature of the coal ash was heated to 1350 °C to fully melt in the study. 2.2. FactSage Modeling. On the basis of the Gibbs free energy minimization and empirical data, FactSage software could predict equilibrium for solid−liquid−gas phases and chemical compositions of mineral mixture. 34 Experimental data with various chemical compositions, temperatures, and pressures were imported and calculated according to the phase equilibrium. In our study, FactSage 6.4 software was applied with inner FACT and FToxid databases. Oxide components less than 1 wt % were excluded. The prediction results of FactSage software for coal slag are shown in Figure 1. 2.3. Experimental Apparatus and Method. The slag melting and cooling experiments were performed in a high temperature resistance furnace (Kaiyuan Company, Changsha, China). The schematic of the experimental apparatus is shown in Figure 2 below. The furnace consists of a MoSi2 heating element, a corundum crucible,

Figure 2. Schematic of the high temperature resistance furnace. inner/outer cooling fin, a cooling fan, and a temperature controller. The MoSi2 heating element (99.9% purity) was vertically installed on both sides of the furnace wall. The corundum crucible (99.9% purity) was placed in the center of the furnace. The furnace was sealed, and a fan on the back of the furnace controlled the cooling temperature combined with the heating element. The cooling process of the furnace was controlled by the heat transfer between cooling fin and cooling fan. The outer cooling fin was distributed as reticulate structure on the top of the furnace and connected with the inner cooling fin. In addition to the furnace door, the rest of the furnace had a good sealing property. Therefore, when the furnace was closed the heating furnace was nearly sealed, and during the experiment the furnace kept an atmospheric stability. Coal ash samples in the crucible were heated to 1350 °C within 180 min in the air atmosphere. The temperature was held at 1350 °C for 60 min to melt coal ash into molten slag. In the industrial gasifier, temperature of the molten slag layer varied with slag thickness and the inner molten slag was cooled at different cooling rates with the increase of thickness.35 Figure 3 shows the diagram of the temperature controlling process of the whole experiment. Temperature of the furnace in this study decreased to the same residence temperature under different cooling rates (e.g., 2 °C/min, 7 °C/min, and 11 °C/ min). Crystals generated and grew inside liquid slag when the slag temperature decreased below the initial crystallization temperature according to the continuous cooling transformation (CCT) and B

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Diagram of temperature controlling process of slag. temperature time transformation (TTT) experiments.31,36,37 Varied residence temperatures were set based on the FactSage software prediction in Figure 1 below or above the initial crystallization temperature (e.g., 1170, 1190, and 1220 °C). The residence time for the growth of crystal at the isothermal temperature before quenching was classified as 0 min, 10 min, 30 min, and 60 min. Finally the corundum crucible of molten slag was taken out from the furnace and quenched immediately. During the water quenching process, only the bottom and outer wall of the corundum crucible contacted the water and the slag had no direct contact with water. However, the liquid slag level inside the corundum crucible was below the water level outside. The last step of the cooling process was water quenching instead of natural cooling in case the morphology or quantity of the crystallized minerals changed during the natural cooling. The corundum crucible was longitudinal cut for the cross section of slag, and then the morphology on the slag surface and dissected section was observed and analyzed by the SU-1510 scanning electron microscope (Hitachi Company, Japan), with the combination of the energy dispersive spectrometer (EDS) to detect the chemical compositions of crystals. Finally the coal slag sample was ground to powder and detected for the intensity of the diffraction peak and crystal type by a D/MAX 2550-VB/PC X-ray powder diffractometer (PANalytical B.V, Netherlands).

Figure 4. XRD spectra of the slags with different cooling rates (residence temperature: 1170 °C, residence time: 10 min).

nucleus formation and crystal growth. Although excessive cooling rates provide a large driving force for nucleation to occur, the time needed for diffusion of atoms and subsequent relaxation may limit nucleation and growth of crystals. Therefore, at high cooling rate of 11 °C/min, the lower intensity of the crystal peak showed the crystal growth was limited and weakened for completing the process of relaxation. Low cooling rates may have lower driving forces for nucleation, but sufficient time may be provided to the system for nucleation and crystal growth to occur. Atoms remained in the molten slag and did not tend to gather and form crystal lattice at a high cooling rate in the experiment. Therefore, the relative intensity of the diffraction peak at higher cooling rate weakened, compared to the peak at lower cooling rate. 3.2. Effect of Residence Time. Residence time of molten coal slag in an industrial gasifier is an important factor for slag tapping. Literature studies had been carried out to study the ash deposition and flow properties of the slag.38−41 However, seldom did literature concentrate on crystallization behavior for the effects of residence time in a gasifier. In accordance with the experimental research about the slag flow properties, the slag flow velocity from the calculation of entrained-flow gasifier was about 0.0026−0.050 m/s.42,43 The height of an industrial gasifier was approximately from 5 to 12 meters,44−46 and thus residence time of slag was approximately estimated by an hour scale. Figures 5 and 6 present the XRD spectra of coal slags under a cooling rate of 7 °C/min with different residence times (e.g., 60 min, 30 min, 10 min, and 0 min), respectively. The number of peaks and intensity of crystals increased with the increase of residence time. In Figure 5, crystal peak intensity at residence time of 60 min also had the highest value at the crystal face indices of (211) with similar result of residence temperature at 1190 °C in Figure 6. It also could be found in Figures 5 and 6 that with decreasing residence the number of peaks at 2θ from 30° to 50° also decreased. Longer residence time provided sufficient time for atoms to rearrange themselves within the thermodynamically preferred lattice sites to form characteristic

3. RESULTS AND DISCUSSION 3.1. Effect of Cooling Rate. Figure 4 showed the results of XRD spectra for coal slag samples under different cooling rates. After the slag molten process, varied cooling rates (e.g., 2 °C/ min, 7 °C/min, and 11 °C/min) were set to decrease the slag temperature from 1350 to 1170 °C. From XRD spectra, intensity of the peak differed from the cooling rates. For instance, the intensity of the highest peak at the crystal face indices of (211) was about 1954 at the cooling rate of 2 °C/ min. The intensities of the highest peaks at the crystal face indices of (211) under cooling rates of 7 °C/min and 11 °C/ min were 1427 and 1281, respectively, which were lower than the result of 2 °C/min. From Figure 4, intensities of other peaks at the crystal face indices of (111), (201), and (312) showed similar tendency with the cooling rate increasing. Higher intensity of the peak for crystal detection means that coal slag had a better crystallinity, which indicated that low cooling rate benefited the crystal growth. When the characteristic time of molecular motions responsible for structural rearrangements became longer than the time scale of experiment, glass transition occurred on cooling or compressing a liquid.30 Structural rearrangements of the molecules (i.e., relaxation) occurred, and energy released in the liquid when the liquid phase transformed to the solid state. This needed a time span determined by the cooling rate in the experiment. The formation of crystals contained two steps: C

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. XRD spectra of the slags with different residence temperatures (residence time: 0 min, cooling rate: 7 °C/min).

Figure 5. XRD spectra of the slags with different residence time (residence temperature: 1190 °C, cooling rate: 7 °C/min).

Figure 6. XRD spectra of the slags with different residence times (residence temperature: 1170 °C, cooling rate: 7 °C/min).

Figure 8. XRD spectra of the slags with different residence temperatures (residence time: 30 min, cooling rate: 7 °C/min).

crystal structures and compensate for the nonoptimal cooling rates to reach its thermodynamic stable state.30 If high cooling rate hindered the structural rearrangement, longer residence time would counteract its influence to a certain extent. The molecules and atoms recombine during long residence time, and the nucleus generated and grew to from the crystals. 3.3. Effect of Residence Temperature. The X-ray diffraction spectrum results for the effects of residence temperature on crystallization behavior are given from Figures 7−9. It was obvious that the intensity value of the crystal peak

at the crystal face indices of (211) increased with the decrease of residence temperature. Besides at other crystal face indices the intensity increased with decreasing residence temperature. At the residence temperature of 1220 °C, crystal phase in liquid slag was low according to the FactSage prediction in Figure 1. When the temperature decreased to 1150 °C in Figure 1, the solid-phase mass in liquid slag increased dramatically. Thus, the results from the X-ray diffraction spectrum in Figure 7 showed the similar tendency that number of crystal peak increased D

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 10. SEM micrographs of the coal slag samples at different residence temperature for (a) 1150 °C, (b)1170 °C, (c) 1190 °C, and (d) 1220 °C. Figure 9. XRD spectra of the slags with different residence temperatures (residence time: 60 min, cooling rate: 7 °C/min).

obviously and the intensity also increased, which proved low residence temperature below initial crystallization temperature benefited the nucleation generation and the growth of crystal. With the residence time increasing from Figures 7 to 9, the number and intensity of the crystal peak increased and gave the similar results compared to Section 3.2. The generation of nuclear needed driving force and the crystal needed long relaxation time to grow. Below the initial crystallization temperature, the content of crystal reasonably increased with decreasing temperature and increasing residence time. Therefore, residence temperatures for the generation and growth of crystal also had an influence on the crystallization behavior, and this was similar to results in the literature.31 3.4. SEM-EDS Analysis. Figure 10a−d displays the morphology graphs of coal slag surface at different residence temperatures. From Figure 10a,b, crystals of rectangular structure were clearly formed on the coal slag surface. However, amorphous or vitreous phase was also obviously found from Figure 10c,d above the initial crystallization temperature. Below the initial crystallization temperature, crystals deposited out form the liquid slag. The decreased of residence temperature increased the number and feature of crystal from Figure 10a−d. According to the FactSage prediction, the solid-phase mass in liquid slag increased obviously below 1190 °C, which mean nucleation increased and the crystal continued to grow. The cooling rate provided a driving force for nucleation to occur and was of importance to growth of crystal during slag flow. Figure 11 gives the secondary electron images and backscattered electron images under the effects of varied cooling rates, respectively. With the increase of cooling rate form Figure 11a−d, the size of the crystal decreased and the number of crystals on the slag surface increased obviously. For instance, the size of the crystal in Figure 11a was nearly 200 μm while the size of the crystal at a cooling rate of 11 °C/min (Figure 11c) decreased to nearly 100 μm. From the backscattered electron images in Figures 11d−f, it could be clearly found that the size of crystal decreased both in length

Figure 11. SEM micrographs of crystals under the effect of different cooling rates. Secondary electron image (up): (a) 2 °C/min, (b) 7 °C/ min, and (c) 11 °C/min, backscattered electron image (down): (d) 2 °C/min, (e) 7 °C/min, and (f) 11 °C/min.

and width direction while the number of crystals inside the same region increased. These backscattered electron images were observed from the cross section of cutting slag. The black line in Figure 11d was the crack during the experiment. Results from Figure 11a−f correspond to the analyses of the XRD spectra, which also indicated that slag under lower cooling rate tended to form a better crystallinity. In addition, the long residence time benefited the generation and growth of crystals, which was also found from the morphology analysis in Figure 12. Low residence time mainly formed the amorphous phase in Figure 12a−c while long residence time increased the number and feature of crystals in Figure 12d−f. The generation ofthe nucleus and growth of crystal needed time to achieve a balance between the liquid and crystal surfaces. Higher temperature above the initial crystallization temperature broke the balance and increased the surface energy; thus, low temperature and long residence help the elements to restructure and reach a lowest surface energy. From the EDS point analysis, similar elemental compositions and contents were detected in the four points in Figure 13. The contents of Si, Ca, and Al form metal elements that were all at high levels, and it could be speculated that the crystal might E

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 12. SEM micrographs of the coal slag samples at the temperature of 1170 °C with different residence times for (a) 0 min, (b) 10 min, (c) 30 min, and (d−f) 60 min.

for the long residence time while the amorphous phase occupied the majority of the slag surface for the low residence time. Lower residence temperature below the incubation temperature of crystal increased crystallization behavior, and with increasing cooling rate the backscattered electron images indicated that the size of the crystal decreased both in length and width direction while the number of crystals inside the same area increased.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-21-64251418. E-mail: hfl[email protected]. cn (Haifeng Liu). Figure 13. Element compositions and contents of the crystals for different points.

Funding

This study was supported by the National Natural Science Foundation of China (U1402272), the Foundation of Shanghai Science and Technology Committee (14dz1200100), and the National Natural Science Foundation of China (21376082).

belong to calcium aluminum silicate. XRD analysis proved that the crystals formed in the slag were mainly gehlenite (2CaO· Al2O3·SiO2), and at this temperature gehlenite occurred in liquid slag from the prediction of FactSage in Figure 1. Therefore, the generation and growth of the crystal in slag was affected by the cooling rate, residence time, and residence temperature from the morphology analysis on the slag surface.

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) World Energy Outlook 2012; OECD/IEA: 2012. (2) Kasule, J. S.; Turton, R.; Bhattacharyya, D.; Zitney, S. E. Mathematical modeling of a single-stage, downward-firing, entrainedflow gasifier. Ind. Eng. Chem. Res. 2012, 51, 6429−6440. (3) Liu, Z.; Green, W. H. Analysis of adsorbent-based warm CO2 capture technology for intergrated gasification combined cycle (IGCC) power plants. Ind. Eng. Chem. Res. 2014, 53, 11145−11158. (4) Vandersickel, A.; Field, R. P.; Chen, W. B.; Mancini, N. D.; Mitsos, A. Cao-based energy and CO2 storage system for the flexibilization of an IGCC plant with carbon capture. Ind. Eng. Chem. Res. 2014, 53, 12032−12043. (5) Wu, S. Y.; Huang, S.; Ji, L. Y.; Wu, Y. Q.; Gao, J. S. Structure characteristics and gasification activity of residual carbon from entrained-flow coal gasification slag. Fuel 2014, 122, 67−75.

4. CONCLUSION This study investigated the effect of the cooling process on the generation and growth of crystal in slag. Based on the prediction of FactSage and the actual environment in the industrial gasifier, the variation of the cooling process obviously affected the crystallization behavior of molten slag. With the relaxation and diffusion theory in crystallization and glass, low cooling rate benefited the generation of the crystals, which showed higher peak intensity and better crystallinity from XRD spectra. Long residence time below the initial crystallization temperature promoted the generation and growth of crystal. In addition, figurate tetragonal crystals formed on the slag surface F

DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (6) 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. (7) Ilyushechkin, A. Y.; Hla, S. S.; Roberts, D. G.; Kinaev, N. N. The effect of solids and phase compositions on viscosity behavior and Tcv of slags from Australian bituminous coals. J. Non-Cryst. Solids 2011, 357, 893−902. (8) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Rong, Y. Q.; Zhu, Z. B.; Koyama, S. Fusibility and flow properties of coal ash and slag. Fuel 2009, 88, 297−304. (9) Wu, L. S.; Ek, M.; Song, M. H.; Sichen, D. The effect of solid particles on liquid viscosity. Steel Res. Int. 2011, 82, 388−397. (10) Nowok, J. W. Viscosity and structural state of iron in coal ash slags under gasification conditions. Energy Fuels 1995, 9, 534−549. (11) Li, X. Y.; Li, G. Y.; Cao, Z. D.; Xu, S. S. Research on flow characteristics of slag film in a slag tapping gasifier. Energy Fuels 2010, 24, 5109−5115. (12) Watt, J. D.; Fereday, F. Flow properties of slags formed from ashes of British coals. 1. Viscosity of homogeneous liquid slags in relation to slag composition. J. Inst. Fuel 1969, 42, 99. (13) Urbain, G.; Gambier, F.; Deleter, M.; Anseau, M. R. Viscosity of silicate melts. Trans. J. Br. Ceram. Soc. 1981, 80, 139−141. (14) Hurst, H. J.; Novak, F.; Patterson, J. H. Viscosity measurements and empirical predictions for some model gasifier slags. Fuel 1999, 78, 439−444. (15) Hurst, H. J.; Novak, F.; Patterson, J. H. Viscosity measurements and empirical predictions for fluxed Australian bituminous coal ashes. Fuel 1999, 78, 1831−1840. (16) Hurst, J. H.; Patterson, J. H.; Quintanar, A. Viscosity measurements and empirical predictions for some model gasifier slags-II. Fuel 2000, 79, 1797−1799. (17) Browning, G.; Bryant, G.; Hurst, H. J.; Lucas, J. A.; Wall, T. F. An empirical method for the prediction of coal ash slag viscosity. Energy Fuels 2003, 17, 731−737. (18) Iida, T.; Sakai, H.; Kita, Y. Extension of a viscosity equation based on the network parameter to blast furnace type slags. J. High Temp. Soc. Jpn. 1999, 25, 93−102. (19) Iida, T.; Sakai, H.; Kita, Y.; Shigeno, K. An Equation for accurate prediction of the viscosities of blast furnace type slags from chemical composition. ISIJ Int. 2000, 40, S110−S114. (20) Nakamoto, M.; Lee, J.; Tanaka, T. A model for estimation of viscosity of molten silicate slag. ISIJ Int. 2005, 45, 651−656. (21) Groen, J. C.; Brooker, D. D.; Welch, P. J.; Oh, M. S. Gasification slag rheology and crystallization in titanium-rich, iron-calciumaluminosilicate glasses. Fuel Process. Technol. 1998, 56, 103−127. (22) Kondratiev, A.; Jak, E. Predicting coal ash slag flow characteristics (viscosity model for the Al 2O3 -CaO-‘FeO’-SiO2 system). Fuel 2001, 80, 1989−2000. (23) Yuan, H. P.; Liang, Q. F.; Gong, X. Crystallization of coal ash slags at high temperatures and effects on the viscosity. Energy Fuels 2012, 26, 3717−3722. (24) Kim, Y.; Oh, M. S. Effect of cooling rate and alumina dissolution on the determination of temperature of critical viscosity of molten slag. Fuel Process. Technol. 2010, 91, 853−858. (25) Bai, J.; Li, W.; Li, B. Q. Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere. Fuel 2008, 87, 583−591. (26) Hasanuzzaman, M.; Olabi, A. G. Effect of nucleation temperature and time on crystallization behavior of Zirconia/Zircon added borosilicate glass. Presented at the ECCM15-15TH European conference on composite materials, Venice, Italy, June 2012; pp 24− 28. (27) Jung, S. S.; Sohn, I. Crystallization behavior of the CaO-Al2O3MgO system studied with a confocal laser scanning microscope. Metall. Mater. Trans. B 2012, 43, 1530−1539. (28) Jung, S. S.; Sohn, I. Effect of FeO concentration on the crystallization of high-temperature CaO-Al2O3-MgO-FeO melts. J. Am. Ceram. Soc. 2013, 96, 1309−1316.

(29) Jung, S. S.; Sohn, I. Crystallization control for remediation of a FetO-rich CaO-SiO2-Al2O3-MgO EAF waste slag. Environ. Sci. Technol. 2014, 48, 1886−1892. (30) Ngai, K. L. Relaxation and diffusion in complex systems; Springer: New York, 2011. (31) Xuan, W. W.; Whitty, K. J.; Guan, Q. L.; Bi, D. P.; Zhang, J. S. Influence of isothermal temperature and cooling rates on crystallization characteristics of a synthetic coal slag. Fuel 2014, 137, 193− 199. (32) Liu, B.; He, Q. H.; Jiang, Z. H.; Xu, R. F.; Hu, B. X. Relationship between coal ash composition and ash fusion temperatures. Fuel 2013, 105, 293−300. (33) Van Dyk, J. C.; Benson, S. A.; Laumb, M. L.; Waanders, B. Coal and coal ash characteristics to understand mineral transformations and slag formation. Fuel 2009, 88, 1057−1063. (34) Bale, C. W.; Pelton, A. D.; Thompson, W. T.; Eriksson, G.; Hack, K.; Chartrand, P. et al. FactSage. http://www.FactSage. com2009. (35) Liang, Q. F.; Guo, X. L.; Dai, Z. H.; Liu, H. F.; Gong, X. An investigation on the heat transfer behavior and slag deposition of membrane wall in pilot-scale entrained-flow gasifier. Fuel 2012, 102, 491−498. (36) Li, J.; Wang, W. L.; Wei, J.; Huang, D.; Matsuura, H. A kinetic study of the effect of Na2O on the crystallization behavior of mold fluxes for casting medium carbon steel. ISIJ Int. 2012, 52, 2220−2225. (37) Gan, L.; Zhang, C. X.; Zhou, J. C.; Shangguan, F. Q. Continuous cooling crystallization kinetics of a molten blast furnace slag. J. NonCryst. Solids 2012, 358, 20−24. (38) Akiyama, K.; Pak, H.; Tada, T.; Ueki, Y.; Yoshiie, R.; Naruse, I. Ash deposition behavior of upgraded brown coal and bituminous coal. Energy Fuels 2010, 24, 4138−4143. (39) Ai, W. G.; Kuhlman, J. M. Simulation of coal ash particle deposition experiments. Energy Fuels 2011, 25, 708−718. (40) Ni, J. J.; Yu, G. S.; Guo, Q. H.; Zhou, Z. J.; Wang, F. C. Submodel for predicting slag deposition formation in slagging gasification systems. Energy Fuels 2011, 25, 1004−1009. (41) Wang, J.; Liu, H. F.; Liang, Q. F.; Xu, J. L. Experimental and numerical study on slag deposition and growth at the slag tap hole region of Shell gasifier. Fuel Process. Technol. 2013, 106, 704−711. (42) Ni, J. J.; Zhou, Z. J.; Yu, G. S.; Liang, W. F.; Wang, F. C. Molten slag flow and phase transformation behaviors in a slagging entrainedflow coal gasifier. Ind. Eng. Chem. Res. 2010, 49, 12302−12310. (43) Ye, I.; Ryu, C. Numerical modeling of slag flow and heat transfer on the wall of an entrained coal gasifier. Fuel 2015, 150, 64−74. (44) Seggiani, M. Modelling and simulation of time varying slag flow in a Prenflo entrained-flow gasifier. Fuel 1998, 77, 1611−1621. (45) Slezak, A.; Kuhlman, J. M.; Shadle, L. J.; Spenik, J.; Shi, S. CFD simulation of entrained-flow coal gasification: Coal particle density/ size fraction effects. Powder Technol. 2010, 203, 98−108. (46) Jeong, H. J.; Seo, D. K.; Hwang, J. CFD modeling for coal size effect on coal gasification in a two-stagecommercial entrained-bed gasifier with an improved char gasification model. Appl. Energy 2014, 123, 29−36.

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DOI: 10.1021/acs.energyfuels.5b02625 Energy Fuels XXXX, XXX, XXX−XXX