Effect of Coal Ash Composition on Ash Fusion Temperatures †

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<ul><li><p>182r 2009 American Chemical Society pubs.acs.org/EF</p><p>Energy Fuels 2010, 24, 182189 : DOI:10.1021/ef900537mPublished on Web 08/11/2009</p><p>Effect of Coal Ash Composition on Ash Fusion Temperatures</p><p>Wen J. Song, Li H. Tang, Xue D. Zhu, Yong Q. Wu, Zi B. Zhu,*, and Shuntarou Koyama</p><p>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</p><p>Received May 26, 2009. Revised Manuscript Received July 12, 2009</p><p>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.</p><p>Introduction</p><p>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.</p><p>Many researchers have used different methods to test andpredict the AFTs of coal ash.4-15 Some investigations have</p><p>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.</p><p>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</p><p> Presented at the 2009 Sino-Australian Symposium on Advanced Coaland Biomass Utilisation Technologies.</p><p>*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.;</p><p>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,</p><p>12151219.(4) Kahraman, H.; Bos, F.; Reifenstein, A.; Coin, C. D. A. Fuel 1998,</p><p>77, 10051011.(5) Gupta, S. K.; Wall, T. F.; Creelman, R. A.; Gupta, R. P. Fuel</p><p>Process. Technol. 1998, 56, 3343.(6) Yin, C. G.; Luo, Z. Y.; Ni, M. J.; Cen, K. F. Fuel 1998, 77, 1777</p><p>1782.(7) Kahraman, H.; Reifenstein, A. P.; Coin, C. D. A. Fuel 1999, 78,</p><p>14631471.(8) Bryant, G. W.; Browning, G. J.; Emanuel, H.; Gupta, S. K.;</p><p>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.;</p><p>Jimenez, R.; Alfaro, G. Fuel 2003, 82, 20872095.(10) van Dyk, J. C.; Baxter, L. L.; van Heerden, J. H. P.; Coetzer, R.</p><p>L. J. Fuel 2005, 84, 17681777.(11) van Dyk, J. C.; Melzer, S.; Sobiecki, A. Miner. Eng. 2006, 19,</p><p>11261135.(12) Li, H.; Ninomiya, Y.; Dong, Z.; Zhang, M. Chin. J. Chem. Eng.</p><p>2006, 14, 784789.(13) Aineto,M.;Acosta,A.;Rincon, J.M.;Romero,M.Fuel 2006, 85,</p><p>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,</p><p>88, 107116.</p><p>(16) Winegartner, B. C.; Rhodes, B. T. J. Trans. ASMEJ. Eng. Power1975, 97, 395401.</p><p>(17) Lloyd,W. G.; Riley, J. T.; Zhon, S.; Risen,M. A.; Tibbitts, R. L.Energy Fuels 1993, 7, 490494.</p><p>(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,</p><p>49194926.(21) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Zhu, Z. B.;</p><p>Koyama, S. Energy Fuels 2009, 23, 19901997.(22) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P. Fuel 1981, 60,</p><p>577584.(23) Gray, V. R. Fuel 1987, 66, 12301239.(24) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Fuel Process.</p><p>Technol. 1995, 45, 2751.(25) Song, W. J.; Tang, L. H.; Zhu, X. D.; Wu, Y. Q.; Rong, Y. Q.;</p><p>Zhu, Z. B.; Koyama, S. Fuel 2009, 88, 297304.(26) Dyk, J. C. V. Miner. Eng. 2006, 19, 280286.</p></li><li><p>183</p><p>Energy Fuels 2010, 24, 182189 : DOI:10.1021/ef900537m Song et al.</p><p>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.</p><p>Experimental Section</p><p>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.</p><p>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</p><p>the ash cones: initial deformation temperature (IDT), softeningtemperature (ST), hemispherical temperature (HT), and flowtemperature (FT).</p><p>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.</p><p>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</p><p>Table 1. Composition of Coal Ash Samples</p><p>composition (wt. %)</p><p>number SiO2 Al2O3 CaO Fe2O3 MgO TiO2 Na2O K2O</p><p>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</p><p>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</p><p>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</p><p>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</p><p>(27) Liu, Y.; Gupta, R.; Elliott, L.; Wall, T.; Fujimori, T. FuelProcess. Technol. 2007, 88, 10991107.</p><p>(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.</p><p>(29) Jak, E.; Degterov, S.; Hayes, P. C.; Pelton, A. D. Fuel 1998, 77,7784.</p></li><li><p>184</p><p>Energy Fuels 2010, 24, 182189 : DOI:10.1021/ef900537m Song et al.</p><p>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.</p><p>Results and Discussion</p><p>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.</p><p>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.</p><p>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.</p><p>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</p><p>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.</p><p>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.</p><p>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%.</p><p>Effect of...</p></li></ul>

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