Effect of Coal Ash Composition on Ash Fusion Temperatures †

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  • 182r 2009 American Chemical Society pubs.acs.org/EF

    Energy Fuels 2010, 24, 182189 : DOI:10.1021/ef900537mPublished on Web 08/11/2009

    Effect of Coal Ash Composition on Ash Fusion Temperatures

    Wen J. Song, Li H. Tang, Xue D. Zhu, Yong Q. Wu, Zi B. Zhu,*, and Shuntarou Koyama

    Engineering Research Center of Large Scale Reactor Engineering and Technology, East China University of Science andTechnology, Shanghai 200237, PR China, and Electric Powder Development Corporation Ltd., Tokyo 167-0023, Japan

    Received May 26, 2009. Revised Manuscript Received July 12, 2009

    The ash fusion temperatures (AFTs) of coal mineral matter at high temperature are important parametersfor all gasifiers. Experiments have been conducted in which mixtures of selected coal ashes and SiO2,Al2O3, CaO, Fe2O3, andMgOwere subjected to the standard test for ash fusibility. The computer softwarepackage FactSage has been used to calculate the liquidus temperatures of coal ash samples and theproportions of the various phases present as a function of temperature. The results show that the AFTs ofcoal ash samples first decrease with increasing CaO, Fe2O3, and MgO contents, then reach a minimumvalue, before increasing oncemore.However, for the effect of S/A ratio, itsAFTs are always increasedwithincreasing S/A ratios. The measured AFTs all show variations with mixture composition that correlatedclosely with liquidus temperatures for the appropriate pseudoternary phase diagrams. The liquidus andAFTs generally showed parallel compositional trends but are displaced from each other because of theinfluence of additional basic components in the coal ash. The liquidus temperatures of coal ash samples arealways higher than its AFTs.

    Introduction

    For all gasifiers, the ash fusion temperature (AFT) is animportant variable for all gasifiers.1,2 For fluid-bed gasifiers,these properties govern the upper operating temperature atwhich agglomeration of the ash is initiated. For entrained-flow gasifiers, the operating temperature should be above theflow temperature (FT) of coal ash to enable continuous slagtapping.3Thus, there is a need to study theAFTsof coal ashes.

    Many researchers have used different methods to test andpredict the AFTs of coal ash.4-15 Some investigations have

    attempted to relate the AFT to the coal ash composition, andfairly detailed relations, both statistical and empirical, havebeen established.16-21 Some researchers studied the effect ofsomeoxides on theAFTsof coal ash.For example,Huggins etal.22 used the ternary equilibrium phase diagrams to study theeffects of Fe2O3, CaO, and K2CO3 on the AFTs of coal ash.Gray et al.23 studied the effects of acid and basic fluxes on theAFTs of coal ash. Vassilev et al.24 studied the influence ofmineral and chemical composition of coal ashes on theirfusibility. Song et al.25,26 applied the thermodynamic compu-ter package FactSage to study the effect of CaO as purecompounds on the AFTs of coal ash. Wall et al. studied thethermomechanical analysis (TMA) fusibility of laboratoryash, combustion ash, and deposits formed from anAustralianthermal coal. However, to the best of our knowledge, littlework has been published regarding systematic research on theeffect of coal ash composition on the AFT of coal ashes.

    In this work, we havemeasured the AFTs of 33mixtures ofcoal ashes with SiO2, Al2O3, CaO, Fe2O3, andMgOadditives.The computer software package FactSage has been used tocalculate the liquidus temperatures of coal ash samples andthe proportions of the various phases present as a function of

    Presented at the 2009 Sino-Australian Symposium on Advanced Coaland Biomass Utilisation Technologies.

    *Towhom correspondence should be addressed. Telephone:86-21-64252309. Fax: 86-21-64253626. E-mail: zhuzb@ecust.edu.cn andwj.song@mail.ecust.edu.cn.(1) Wall, T. F.; Creelman,R.A.; Gupta, R. P.; Gupta, S.K.; Coin, C.;

    Lowe, A. Prog. Energy Combust. Sci. 1998, 24, 345353.(2) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29120.(3) Hurst, H. J.; Novak, F.; Patterson, J. H. Energy Fuels 1996, 10,

    12151219.(4) Kahraman, H.; Bos, F.; Reifenstein, A.; Coin, C. D. A. Fuel 1998,

    77, 10051011.(5) Gupta, S. K.; Wall, T. F.; Creelman, R. A.; Gupta, R. P. Fuel

    Process. Technol. 1998, 56, 3343.(6) Yin, C. G.; Luo, Z. Y.; Ni, M. J.; Cen, K. F. Fuel 1998, 77, 1777

    1782.(7) Kahraman, H.; Reifenstein, A. P.; Coin, C. D. A. Fuel 1999, 78,

    14631471.(8) Bryant, G. W.; Browning, G. J.; Emanuel, H.; Gupta, S. K.;

    Gupta, R. P.; Lucas, J. A.; Wall, T. F. Energy Fuels 2000, 14, 316325.(9) Goni, C.; Helle, S.; Garcia, X.; Gordon, A.; Parra, R.; Kelm, U.;

    Jimenez, R.; Alfaro, G. Fuel 2003, 82, 20872095.(10) van Dyk, J. C.; Baxter, L. L.; van Heerden, J. H. P.; Coetzer, R.

    L. J. Fuel 2005, 84, 17681777.(11) van Dyk, J. C.; Melzer, S.; Sobiecki, A. Miner. Eng. 2006, 19,

    11261135.(12) Li, H.; Ninomiya, Y.; Dong, Z.; Zhang, M. Chin. J. Chem. Eng.

    2006, 14, 784789.(13) Aineto,M.;Acosta,A.;Rincon, J.M.;Romero,M.Fuel 2006, 85,

    23522358.(14) van Dyk, J. C.; Waanders, F. B. Fuel 2007, 86, 27282735.(15) Yun, Y.; Yoo, Y. D.; Chung, S. W. Fuel Process. Technol. 2007,

    88, 107116.

    (16) Winegartner, B. C.; Rhodes, B. T. J. Trans. ASMEJ. Eng. Power1975, 97, 395401.

    (17) Lloyd,W. G.; Riley, J. T.; Zhon, S.; Risen,M. A.; Tibbitts, R. L.Energy Fuels 1993, 7, 490494.

    (18) Seggiani, M. Fuel 1999, 78, 11211125.(19) Jak, E. Fuel 2002, 81, 16551668.(20) Seggiani, M.; Pannocchia, G. Ind. Eng. Chem. Res. 2003, 42,

    49194926.(21) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Zhu, Z. B.;

    Koyama, S. Energy Fuels 2009, 23, 19901997.(22) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60,

    577584.(23) Gray, V. R. Fuel 1987, 66, 12301239.(24) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Fuel Process.

    Technol. 1995, 45, 2751.(25) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Rong, Y. Q.;

    Zhu, Z. B.; Koyama, S. Fuel 2009, 88, 297304.(26) Dyk, J. C. V. Miner. Eng. 2006, 19, 280286.

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    temperature. The relation between AFTs and pseudoternaryphase equilibrium diagrams has been examined by preparingthe mixtures of coal ashes with various oxides. The metallur-gical microscopy has been used to analyze the effects of theseoxides on the mineral.

    Experimental Section

    Coal Ash Samples. Four representative Chinese coal sampleswere used in the study. The ash samples were prepared in amuffle furnace at 815 C for 24 h according to the Chinesestandard GB/T 1574-1995. Chemical analysis of the sampleswas carried out using X-ray fluorescence (XRF). To study theeffects of SiO2, Al2O3, CaO, Fe2O3, and MgO on the AFTs ofcoal ashes, we added the ash to proper Sinopharm ChemicalReagent Corp. laboratory regent silicon oxide, alumina oxide,ferric oxide, calcium oxide, andmagnesium oxide. The chemicalcomposition of the 33 mixtures of coal ashes with SiO2, Al2O3,CaO, Fe2O3, and MgO additives is given in Table 1.

    Fusion Temperature Test.We performed the fusion tempera-ture tests by following the Chinese standard procedures (GB/T219-1996) in a registered independent laboratory. This testinvolves heating a sample cone of specified geometry at a rateof 5 k/min in an Ar atmosphere. The following temperatures arerecorded for each sample, corresponding to specific shapes of

    the ash cones: initial deformation temperature (IDT), softeningtemperature (ST), hemispherical temperature (HT), and flowtemperature (FT).

    Thermodynamic Equilibrium Calculations. The thermody-namic software package FactSage is the fusion of two well-known software packages in computational thermochemistry:Fact-Win and ChemSage.28 FactSage consists of a series ofinformation, database, calculation, and manipulation modulesthat enable one to access and manipulate pure substances andsolution databases. FactSage allows calculating and predict-ing multiphase equilibria, liquidus temperatures, and the pro-portions of the liquid and solid phases in a specified atmospherefor a multicomponent system.

    FactSage was used in this study to calculate the correspond-ing temperatures with a different proportion of liquid phase aswell as the equilibrium product distributions for simplified coalash systems. Phase formation data for these oxides and theircombinations were selected from the FToxid database. Calcula-tions were carried out between the solid temperature andliquidus temperature in Ar atmosphere at 1 atm pressure. Thecalculation method of FactSage is based on Gibbs energyminimization for each sample at a given temperature andcomposition range. Phases formed at concentrations below

    Table 1. Composition of Coal Ash Samples

    composition (wt. %)

    number SiO2 Al2O3 CaO Fe2O3 MgO TiO2 Na2O K2O

    Shanxi shuangliu coal ash --- CaO mixtures1 52.73 35.03 5.00 2.96 0.41 1.93 0.27 1.672 49.95 33.18 10.00 2.81 0.39 1.83 0.26 1.583 47.18 31.34 15.00 2.65 0.37 1.73 0.25 1.504 44.40 29.50 20.00 2.50 0.34 1.63 0.23 1.415 41.63 27.65 25.00 2.34 0.32 1.52 0.22 1.326 38.85 25.81 30.00 2.18 0.30 1.42 0.20 1.237 36.08 23.97 35.00 2.03 0.28 1.32 0.19 1.148 33.30 22.12 40.00 1.87 0.26 1.21 0.17 1.069 27.75 18.44 50.00 1.56 0.21 1.02 0.14 0.88

    Henan yima coal ash --- Fe2O3 mixtures1 56.15 30.28 4.91 4.02 1.45 1.29 1.87 0.592 52.65 28.39 4.60 10.00 1.36 1.21 1.75 0.553 49.73 26.82 4.34 15.00 1.28 1.14 1.65 0.524 46.80 25.24 4.09 20.00 1.21 1.07 1.56 0.495 43.88 23.66 3.83 25.00 1.13 1.01 1.46 0.466 40.95 22.08 3.58 30.00 1.05 0.94 1.36 0.437 38.03 20.51 3.32 35.00 0.98 0.87 1.26 0.408 35.01 18.93 3.07 40.00 0.90 0.81 1.17 0.37

    Shanxi guojiawan coal ash --- MgO mixtures1 53.93 30.76 5.68 4.48 1.52 1.29 1.65 0.672 53.16 30.32 5.60 4.42 3.00 1.27 1.63 0.593 52.06 29.69 5.49 4.33 5.00 1.25 1.60 0.584 50.97 29.07 5.37 4.24 7.00 1.22 1.56 0.565 49.87 28.44 5.26 4.15 9.00 1.19 1.53 0.566 48.77 27.82 5.14 4.06 11.00 1.17 1.49 0.547 47.68 27.19 5.03 3.97 13.00 1.14 1.46 0.538 46.58 26.57 4.91 3.88 15.00 1.12 1.43 0.52

    Shandong yanzhou coal ash --- S/A mixtures1 27.17 16.98 15.36 38.22 1.16 0.42 0.19 0.512 30.70 16.17 14.62 36.37 1.11 0.40 0.18 0.493 33.90 15.41 13.93 34.69 1.05 0.38 0.17 0.464 36.82 14.72 13.32 33.15 1.00 0.36 0.16 0.445 39.50 14.11 12.76 31.75 0.96 0.35 0.16 0.436 42.73 13.35 12.07 30.05 0.91 0.33 0.15 0.407 45.63 12.68 11.47 28.53 0.86 0.31 0.14 0.388 48.25 12.06 10.91 27.15 0.82 0.30 0.13 0.36

    (27) Liu, Y.; Gupta, R.; Elliott, L.; Wall, T.; Fujimori, T. FuelProcess. Technol. 2007, 88, 10991107.

    (28) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack,K.; Mahfoud, R. B.; Melancon, J.; Pelton, A. D.; Petersen, S. Calphad2002, 26, 189228.

    (29) Jak, E.; Degterov, S.; Hayes, P. C.; Pelton, A. D. Fuel 1998, 77,7784.

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    0.01 wt % were ignored. Because of the complexity of thethermodynamic models (quasi-chemical, sublattice) which re-presents the interaction of the components for phase formation,the convergence of the algorithms is slow and sensitive. In thisstudy, we used the method of Jak et al.,28 which permits theapproximation to start from lower-order subsystems, thenfinally reach the real or complete system. Once the phases havebeen determined, a total mass balance verifies the consistencyof the system.

    Results and Discussion

    Effect of CaO Content. CaO is a common additive that isused to decrease the AFTs of coal ash.30 In our experiments,CaO contents between 5% and 50% have been added tocover the range of CaO contents of most Chinese coal ashsamples.

    Figure 1 shows the plots of fusion temperatures againstCaO content for Shanxi shuangliu coal ash samples. Fusiontemperatures of coal ash samples drop as CaO contentincreases until the CaO content reaches 35%; at higherCaO content, the fusion temperatures of coal ash samplesincrease quickly. Also shown in Figure 1 is a curve represent-ing the change in liquidus temperature with CaO. It can beseen that the experimental AFT curves would closely parallelthe liquidus temperatures.

    Figure 2 illustrates a pseudoternary section constructionfor the SiO2-Al2O3-CaO-Fe2O3 system that can be used tovisually express liquidus temperatures of synthetic slag sam-ples with a SiO2/Al2O3 (S/A) ratio of 2.56 as a function ofCaO content. In Figure 2, the lines of the same colorrepresent all compositions having a given liquidus tempera-ture. The red point indicates that the SiO2-Al2O3-CaO-Fe2O3 system composition varies with changes of CaOcontent. As shown in Figure 2, the liquidus temperature ofthe samplewith aCaO content of 5% is predicted to be above1400 C; however, the sample with a CaO content of 35% isin the low melting temperature composition region with aliquidus temperature below 1300 C. This trend is similar tothat seen in theAFTs of the coal ash samples with the changein CaO content.

    To illustrate in detail the crystalline minerals and theirrelative contents, the phase assemblage of synthetic slagsamples for three different CaO content levels of 5%, 20%,35%, and 50% as a function of temperature was calculated

    by FactSage (Figure 3). Observations indicate that thesubliquidus phase changes from high-melting mullite intolow-melting gehlenite as the CaO content is increased from5% to 35%. When the CaO content is further increased to50%, the subliquidus phase changes to high-melting alphaagain, which may account for the fact that the AFTs of coalash samples with CaO contents of 5%, 20%, 35%, and 45%first decrease and then increase as the CaO content isincreased.

    In the course of our study, when the furnace temperaturewas above the FT of the sample, the sample was slowlycooled (because rapid cooling has a deleterious effect onfurnace life) and was then used to observe themicrostructureand crystallized particles by using aDMM-300metallurgicalmicroscope. The maximum objective magnification is 100,and the minimum image field is 117 90 (m). For themicrographs of the cooled coal ash samples, if the color isblack and connected, the samples are said to be in-moltensiliceous liquid slag phase. Meanwhile, the white and redcolored discrete-like particles are the crystallized particles.

    Figure 4a-d presents micrographs of cooled coal ashsamples with CaO contents of 5%, 20%, 25%, and 45%. Itcan be seen that the crystallized phase consists mainly ofwhite crystalline particles. For most of the coal ash samples,when the temperatures reached FT, most of the particles hadmelted and dissolved, thus forming a siliceous liquid slagphase (black section in Figure 4). As a result, the whiteparticles were crystallized out of the melt. According to theresults calculated by FactSage (Figure 3), we can deduce thatthe white crystalline particles seen in Figure 4a,b were mostprobably composed of amixture of leucite andmullite, whilethe white crystalline particles seen in Figure 4c were mostlikely composed of gehlenite. Furthermore, the particle sizeof crystalline particles from the synthetic slag with a CaOcontent of 35% is seen to be larger than that of the sampleswith CaO contents of 5%, 20%, and 50%.

    Effect of Fe2O3 Content. In coal, iron is predominantly inthe form ferric iron in oxidizing and inert atmospheres31 and

    Figure 1. Effect of CaO on ash fusion temperature and liquidustemperature.

    Figure 2. Calculated liqudius temperatures in the SiO2-Al2O3-CaO-Fe2O3 system on the pseudoternary section with a SiO2/Al2O3 weight ratio of 2.56.

    (30) Seggiani, M.; Pannocchia, G. Ind. Eng. Chem. Res. 2003, 42,49194926.

    (31) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R. Fuel 1981, 60,585597.

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    forms ferrous iron and even metallic iron in reducing atmo-sphere. In our work, the Fe2O3 content was varied from4.02% to 40%,which also covers the range of Fe2O3 contentsof typical Chinese coal ash samples.

    Figure 5 is a plot of the AFTs against percent Fe2O3. Thetemperatures drops as Fe2O3 content is about 35%, and athigher concentrations of Fe2O3, the AFTs remain constantor increase slightly. Also shown in Figure 5 is a curverepresenting the change in liquidus temperature with theincreasing Fe2O3 content. This trend is similar to that dis-played by the AFTs of coal ash samples as the Fe2O3 contentis increased, which also give rise to a parabolic curve. Theliquidus temperatures of coal ash samples are higher thanAFTs of coal ash samples.

    The pseudoternary phase diagram (Figure 6) displays therange of composition and liquidus temperatures of synthetic

    slag samples with different Fe2O3 contents. In Figure 6, thelines of the same color represent all compositions having agiven liquidus temperature. The red point indicates that theSiO2-Al2O3-CaO-Fe2O3 system composition varies withchanges of Fe2O3 content. From inspection of Figure 4, itcan be seen that the synthetic slag sample with an Fe2O3content of 5% is located in the high melting temperaturecomposition region. The samples gradually move into thelower melting temperature composition region as the Fe2O3content is increased up to 35%. Finally, the samples moveback into the high melting temperature composition. Thistrend is similar to the changes in the AFTs of the coal ashsamples as the Fe2O3 content is increased.

    Figure 7 shows the relative mineral content of coal ashsamples with Fe2O3 contents of 4.02%, 15%, 25%, and 40%between the solid and liquidus temperatures. It can be seenthat the subliquidus crystallized mineral changed from mul-lite to cristobalite. Analysis of Figure 7 indicates the effect ofFe2O3 content on the sensitivity of phase equilibria of coalash samples to changes in temperature by calculating theproportions of the various phases present as a function of

    Figure 3. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different CaO contents.

    Figure 4. Micrographs of slowly cooled ash-fusion cones withdifferent CaO contents.

    Figure 5. Effect of Fe2O3 on ash fusion temperature and liquidustemperature.

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    temperature. For example, the proportion of the liquid phaseof this system with the Fe2O3 contents of 4.02%, 15%, and25% increases rapidly with decreasing temperature. Forexample, the proportion of the liquid phase of this systemwith the Fe2O3 contents of 4.02% decreases from 100% to0% with a temperature decrease of 720 C. However, theFe2O3 content increased up to 30%, and the proportion ofthe liquid phase of those systemswithFe2O3 contents of 25%increase rapidly with decreasing temperature, which mayprovide an explanation for the change in theAFTs of the coalash samples giving rise to a parabolic curve as the Fe2O3content is increased.

    Figure 8 presents the microstructures and crystallinephases of quenched synthetic melt slag samples with Fe2O3

    contents of 4.02%, 15%, 25%, and 40%. It can be seen thatthe crystalline phases consist mainly of white particles whenthe Fe2O3 content is 4.02% (Figure 8a). The number of thesewhite particles clearly decreases, while the number of redcrystalline particles increases concomitantly, when theFe2O3content is increased to 25% (Figure 8b,c). When the Fe2O3content is further increased to 40%, there are a number of redand white crystalline particles, some of which appear on thesurface of the samples (Figure 8d). According to the calcula-tion results by FactSage (Figure 7), we surmise that the whitecrystalline particle in Figure 8a most probably consisted ofmullite, that the red and white crystalline particles inFigure 8b,c were likely to consist of a mixture of mulliteand iron oxide, and that the red and white crystallineparticles in Figure 8d were probably composed of a mixture

    Figure 6. Calculated liqudius temperatures in the SiO2-Al2O3-CaO-Fe2O3 system on the pseudoternary section with a SiO2/Al2O3 weight ratio of 3.16.

    Figure 7. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different Fe2O3 contents.

    Figure 8. Micrographs of slowly cooled ash-fusion cones withdifferent Fe2O3 contents.

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    of cristobalite and iron oxide. The surfaces of the quenchedmelt slag samples show uniform porosity because on heatingthe samples the open pore network is transformed to one ofmicropores, resulting in a minimum porosity level.

    Effect of MgO Content.MgO is classified as a basic oxideand can decrease the ash fusion temperatures and improvethe flow behavior of coal ash slag samples.32 We measuredthe AFTs of five guojiawan coal ash-MgO mixtures, inwhich the MgO content was varied between 1% and 15%,which covers the range of MgO contents of most Chinesecoal ash samples.

    Figure 9 presents the liquidus temperatures and the AFTsof coal ash samples as a function of the MgO content. Frominspection of Figure 9, it can be seen that the AFTs of thecoal ash samples decrease as theMgO content is increased upto 3%. At higher MgO contents, the AFTs increase oncemore. The trends of liquidus andAFTs show similarities, butnot a close parallelism.

    Figure 10 shows the coal ash composition with differentMgO contents and the liquidus temperatures calculated usingFactSage for an S/A ratio of 2.98 and a CaO/Fe2O3 ratio of3.6. In Figure 10, the lines of the same color represent allcompositions having a given liquidus temperature. The redpoint indicates that the SiO2-Al2O3-CaO-Fe2O3-MgO

    system composition varies with changes of MgO content.From inspection of Figure 10, it can be seen that the coal ashsamples with an MgO content of 1.52% are located in thehigh melting temperature composition region. The samplesgradually move into the lower melting temperature compo-sition region as the MgO content is increased up to 5%.Finally, the samples move back into the high melting tem-perature composition. This trend is similar to the changesin the AFTs of coal ash samples as the MgO content isincreased.

    Figure 11 presents the phase assemblage curves for thecoal ash samples with differentMgO contents. It can be seenthat the subliquidus crystallizedmineral does not change andis still mullite as the MgO content is increased from 1% to7%.When theMgO content is further increased to 11%, thesubliquidus crystallized mineral changes to spinel.

    Figure 12 presents surface micrographs of quenched meltcoal ash samples with MgO contents of 1.52%, 7%, 11%,and 15%. It can clearly be seen that the morphology changesas the MgO content is increased. The crystalline phaseconsists mainly of white agglomerated particles, when theMgO content is 1.52% (Figure 12a). When theMgO contentis further increased to 11% and 15%, respectively, thecrystalline phase consists of yellow particles (Figure 12c,d).According to the calculation results by FactSage, we sur-mise that the white circular crystalline particles in Figure 12amost probably consisted of mullite, that the small whitecrystalline particles in Figure 12b were likely to consist ofa mixture of leucite and mullite, and that the yellow crystal-line particles in Figure 12c,d probably consisted of spinelcontaining Mg.

    Effect of S/A Content. S/A is an important parameter thataffects the flow properties of coal ash slag. In our experi-ments, the S/A ratio was varied from 1.6 to 4.0, which coversthe range of S/A ratios found in typical Chinese coal ashsamples.

    Figure 13 presents the liquidus temperatures and theAFTsof coal ash samples as a function of the S/A ratio. It can beseen that the AFTs of coal ash samples increase withincreasing S/A ratio until the S/A ratio reaches 4.0. Thistrend is similar to that displayed by the liquidus temperaturesas S/A ratio increases.

    Figure 14 shows the projection of the liquidus surface andthe composition of coal ash samples with different S/A ratioson to the pseudoternary section (SiO2-Al2O3-(CaO Fe2O3)) at a CaO/Fe2O3 weight ratio of 1.15. From inspec-tion of Figure 14, it can be seen that the coal ash samples withan S/A ratio of 1.6 are located in the lowmelting temperaturecomposition region. The samples gradually move into thehigher melting temperature composition region as the S/Aratio is increased up to 4.0 This trend is similar to the changesin the AFTs of coal ash samples as the S/A ratio is increased.

    To illustrate in detail the crystallized minerals and theirrelative content, the phase assemblage of synthetic slagsamples for three different S/A ratio levels of 1.6, 2.2, 3.2,and 4.0 as a function of temperature was calculated byFactSage (Figure 15). Observations indicate that the sub-liquidus phase changes from low-melting anorthite intohigh-melting mullite as the S/A ratio is increased from 1.6to 2.2, and when the S/A ratio is further increased to 4.0, thesubliquidus phase reverts to high-melting corundum, whichmay account for the fact that the AFTs of coal ash sampleswith S/A ratios of 1.6, 2.2, 3.2, and 4.0 always increase as theS/A ratio is increased.

    Figure 9. Effect of MgO on ash fusion temperature and liquidustemperature.

    Figure 10. Liqudius for the system Si-Al-Ca-Fe-Mg-O withSiO2/Al2O3 weight ratio of 2.98 andCaO/ Fe2O3 weight ratio of 3.6.

    (32) Benson, S. A. Inorganic transformations and ash deposition duringcombustion; American Society of Mechanical Engineers: New York, 1992.

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    Figure 16 presents surface micrographs of quenchedmelt slag samples with S/A ratios of 1.6, 2.2, 3.2, and 4.0.It can be seen that there are clear differences in morpho-logy as the S/A ratio is increased. The crystallized phaseconsists mainly of red and white particles as the S/A ratiois 1.6 (Figure 16a). The extent of the liquid phase regionclearly decreases and the number of crystalline par-ticles increases when the S/A ratio is 2.2 (Figure 16b).Whenthe S/A ratios are further increased to 4.0, the crystal-lized phase consists of white particles (Figure 16d). Accord-ing to the calculation results by FactSage, we can deducethat the white crystalline particles seen in Figure 16awere most probably composed of a mixture of leuciteand anorthite, while the white crystalline particles seenin Figure 16b were most likely composed of a mixture

    Figure 11. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different MgO contents.

    Figure 12. Micrographs of slowly cooled ash-fusion cones withdifferent MgO contents.

    Figure 13. Effect of S/A ratio on ash fusion temperature andliquidus temperature.

    Figure 14. Liqudius for the system SiO2-Al2O3-CaO-Fe2O3 witha CaO/Fe2O3 weight ratio of 1.15.

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    of mullite and leucite. The white crystalline particles inFigure 16c,d are likely to consist of corundum containingleucite.

    Conclusion

    In our work, we have measured the AFTs of mixtures ofselected coal ashes and SiO2, Al2O3, CaO, Fe2O3, and MgOand then have studied the effect of these oxides on the AFTs.The computer software package FactSage has been used tocalculate the liquidus temperatures of coal ash samples andthe proportions of the various phases present as a function oftemperature.

    The AFTs of coal ash samples decrease with increasingCaO,Fe2O3, andMgOcontents, then reach aminimumvalue,before increasing once more, thereby resulting in parabola-type curves. For the effect of S/A, the AFTs are alwaysincreased as the S/A ratios are increased. Liquidus tempera-tures calculated by FactSage and liquidus surfaces inphase equilibrium diagrams for the pseudoternary systemshave been found to correlate well with the trends of AFTsfor coal ash additive (SiO2, Al2O3, CaO, Fe2O3, and MgO)mixtures.

    Acknowledgment. The authors acknowledge the financialsupport provided by National Basic Research Program of China(20576040).

    Figure 15. Phase assemblage-temperature curves for SiO2-Al2O3-CaO-Fe2O3-MgO-TiO2-Na2O-K2O with different S/A ratios.

    Figure 16. Micrographs of slowly cooled ash-fusion cones withdifferent MgO contents.

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