The Mineral Transformation of Huainan Coal Ashes in Reducing Atmospheres

Jun.2006J. China Univ. of Mining& Tech( English Edition)Vol 16 No. 2The mineral transformation of huainanCoal ashes in reducing AtmospheresLI Han-xu', Yoshihiko Ninomiya, DONG Zhong-bing', ZHANG Ming-xu'Department of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, ChinaDepartment of Applied Chemistry, Chubu University, Kasugai, Aichi 487-8501, Japan'Department of Material Science& Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, ChinaAbstract: By using the advanced instrumentation of a Computer Controlled Scanning Electron Microscope(CCSEM)X-ray diffraction(XRD)and X-ray fluorescence(XRF), the ash composition and the mineral components of six typicalHuainan coals of different origins were studied. The transformation of mineral matter at high temperatures was trackedby XRD in reducing conditions. The quartz phase decreased sharply and the anorthite content tended to increase at firstand then decreased with increasing temperatures. The formed mullite phase reached a maximum at 1250 C but showeda tendency of slow decline when the temperature was over 1250 C. The mullite formed in the heating process orthitemain reasonf the high ash melting temperature of Huainan coals. Differences in peak intensity of mullite and anorthitereflected differences in phase concentration of the quenched slag fractions, which contributed to the differences in ashmelting temperatures. The differences in the location of an amorphous hump maximum indicated differences of glasstypes which may affect ash melting temperatures. For Huainan coal samples with relatively high ash melting tempera-ures, the intensity of the diffraction lines for mullite under reducing condition is high while for the samples with relatively low ash melting temperature the intensity for anorthite is highKey words: crystalline component; XRD; Huainan coal; ash meltingCLC number: TQ 5361 Introductionboiler. Why does coal behavior change so rapidly anddramatically? The answers lie in the microscopicMineral phases play an important role in the characteristics of the coals themselves. Because stan-commercial gasification and combustion processes,dard ash analyses provide only the average chemistrysuch as in gasifier and boiler erosion, ash formation of the coal, they provide no information on those miand slag tapping. Understanding the properties of croscopic characteristics which have such a large ef-mineral and ash of Huainan coals will be helpful in fect on ash behavior during combustion and gasificaproviding a better understanding of the factors that tion processescontrol their performance in gasifiers. It is essential toThe behavior of minerals in heating coal hasknow the proportion of each mineral matter, since it been widely studied for some coals(-. In Novembercontrols the melting and crystallization behavior in 1990, a cooperative project entitled Coal Ash Behav-coal utilization. To know only the elemental or oxide ior in Reducing Environment, CABRE, was launchedmpositions of mineral matter in coalsby the following institutions and companies: theient for a complete understanding of coal utilization EERC, Dow, Texaco, Shell Development Company,processes. Several parameters have been used to clas- Electric Power Research Institute(EPRI), the Neth-sify the ash behavior at high temperature by using the erlands Energy Research Foundation(ECN, joined inASTM and the JIS ash cone melting methods. These November of 1991)and the U.S. Department of En-standard test methods provide the basis for under- ergy (DOE). Although significant achievements andstanding the ash characteristics in coal gasification advances have been made in each of these objectivesand combustion. However. these tests have limited much remains unknown about the behavior of inor-usefulness in predicting the behavior of coal ash at gantin gasificationsystemshigh temperatures. The standard ash analyses often中国煤化工 consist of particlesdo not provide any indications of when ash problems rangICNMHG to sand-sizedwill occur. Often two coals with very similar ash evenguav chemistry of theseanalyses will act very differently in the gasifier and mineral particles control where the ash will end up inReceived 21 September 2005; accepted 25 October 2005jects 2003001 supported by the key project of Huainan city, 2004kj125 by the Science Funding of Department of Education of Anhui Provinceor.Tel:+86-554-6668448:E-mailaddress:hxli@@aust.edu.cnLI Han-xu et alThe Mineral Transformation of Huainan Coal Ashes in Reducing Atmospherea gasification and combustion system. XRD tech- desired temperatures ranging from 1050 C to 1450Cnique has been used widely in the study of coalsen quenchedd in water. Total quenching timebut detailed investigation of the composition and was normally 5 s. Quenched samples were examinedcrystalline phases of Huainan coal ashes at different by XRD and CCSEM analysis. XRD anal-ysis wastemperatures and in different atmospheres has not used to study the mineral transformation in the highbeen carried out to date. Hence, a study of the crys- temperature residuestalline phases in coal ashes will also help in understanding the ash behavior during coal gasification and2 CCSEM analysiscombustion processes. The present research is an ini-The ground and dried coal was mounted in waxtial step to study the mineral matter and the crystal- and allowed to harden. The mount was crosssectionedline phase transformation of some Huainan coal ashes and polished, coated with a carbon layer to eliminatein reducing conditions by using X-ray powder dif- electrostatic effects and placed in the CCSEM forfraction profilesanalysis. Six typical coal samples were analyzed byJEM-5600 with CDU-LEAP SEM-EDX and CCSEMIn the CCSEM analyses, three mag-nifications, 150for the 22.0-211.0 um, 250 for the 4.6-22.0 um and2.1 Coal samples and ash melting test800 for the 0.5-4.6 um were used to obtain thebackscattered images of samples. The CCSEM isSix typical coal samples, selected from the used to measurethe size composition and abundandHuainan coal basin in Anhui Province of China, were of mineral grains in the coalground to less than 250 mesh. All of the sampleswere ashed and the bulk ash compositions(mineral2.3 XRD analysisanalysis)were determined. Coal samples are ashed inAll ash samples were analysed by using Rigakuair at 815 C according to the Japanese standard for RINT XRD. The XRD patterns were collected at adetermination of the ash content in coalsThe ash melting apparatus is shown in Fig. Ivoltage of 40 kV and at a current of 35 mA.Theresults were compared with standards in the databasefor identification of crystalline ash components2. 4 The chemical composition of ash samples by5, Water seal bathXRFThe ash content of coal samples, the chemicalcompositions of the ash samples detected by RigakX-ray fluorescence and the melting temperatures ofthe ash samples are presented in Table 1As shown in Table 1, the ash composition ofHuaian coal samples is rich in SiO2 and Al2O3(>70%). The content of TiO2 is higher than 1% on average, the Na20 lower than 0.5% and MgO lowerFig. 1 Modified ash melting test apparatusthan 1%.The content of Cao and Fe2O3 variesgreatly from about 1% to 10% in the coal samplesAbout 1 g of ash samples was heated in an ash The results of a melting point test show that most ofmelting apparatus, which was modified to allow rapid the ashes have a FT higher than 1500 C, while onlyeating and quenching of ash samples. The sample two samples, XM and HN115 coal ashes, were foundwas dropped in the furnace and reacted for 5 min at with a FT less than 1400 CTable 1 Content, chemical composition and melting temperature of ash samplesWt(%)alA Sio2 AlO Fe20 Cao Mgo Na 0 KO SO P2Os TiO2HN0610.273984189.191.130.360242290.710.203.35>1500>1500>1500HN13116037140.74648910.54026中国煤化工149215001500CNMHHN1197504203693.21993040.3714341451KLI123447.13534.725.670.750.26332.220.1919614501500>1500XM23.1543.827710.76850.140001.537000.15250Note: The result is expressed as wt% equivalent oxide, dry basis.J.China Univof Mining&Tech(English Edition)Vol 16 No. 23 Results and Discussion0.73% in HN113 to 12.25% in HN115. The variationof pyrite content in Huainan coals indicates the geo-3.1 Mineral matter compositionlogical condition differences during the coali-fica-tionprocess. as shown in Fig. 3, several other miner-The composition of mineral matter in Huainan als and minor mineral matters are found in the coalscoals can be seen from Fig. 2. The mineral chemicat a low level of occurrence(<1%, total <4%); pericomposition of Huainan coals includes the following clase, Na Al-silicate garnet, apatite and aluminosili-main groups: kaolinite, montmorillonite, quartz, py- cate and so on form ash particles with distinctiverite,calcite,dolomite, unknown groups and other chemical compositions, but do not have great influminor mineral matter. The most abundant mineralence on ash behavior. the key difference betweenmatter in Huainan coal is that of aluminosilicate clayHN115. XM coals and other Huainan coals with aith quartz. They account for more than high ash fusion temperature is the presence of the60% of the total mineral matter in coal. Quartz in the clay minerals pyrite and calcite. Low levels of kaolinform of mineral matter in Huainan coal ranges fro1% to about 7%. The main carbonate mineral in the hn115 and XM coals are beneficial to ash meltHuainan coal is calcite [CaCO3], but dolomite [Caing. It is suggested that the higher the kaolinite inMg(CO3)] and ankerite [Ca(Mg, Fe, Mn)(CO3)2] arehuainan coals, the higher the ash fusion temperaturefrequently associated with calcite. The content of Therefore, the ash behavior of Huainan coal in gasi-calcite and dolomite ranges from 0. 16% in HN106 to fication will be controlled by the mineral groupin HNI19. The content of siderite composition, mainly by kaolinite, pyrite and calcite,[FeCo] is very low in Huainan coals, although other rather than by the properties of the average chemicalcarbonate minerals may also be present. Another im- composition of the coal ashportant non-silicate mineral, pyrite, ranges fromFig. 2 The main mineralogical composition in six huainan coals区Ca-RichaSi-RichgYpsum/Al-silicateg NaClOXidized pyrrhotite田 Ca aluminatee Ca silicateu Fe silicate中国煤化工 monosilicatesilicateXM HNI 15 HNI 19 KLICNMHGhainan coal samplesFig 3 The minor mineralogical composition in six huainan coalsxu et alThe Mineral Transformation of Huainan Coal Ashes in Reducing Atmosphere3.2 X-ray powder diffraction testnents are the principal phases(see Fig. 4). As theXRD results are shown in Fig. 4 for six typical temperature increases, thermal decomposition, trans-I ash samples XM, HN115, HN119, KLl, HN106formation and interaction and phase change occurHN113, with the ash melting temperatures of 1320C. among the components. When the temperature in-1400℃,1480℃,>1500℃,>1500℃,>1500℃ creases to 1150℃, hematite,lime, potassium metarespectively. Diffraction patterns of six coal ashes disappear and quartz, anhydrite decrease, althoughmullite increases. Mullite, which is in a high fusionillustrate the similarity among each ash fraction. Differences in peak intensity reflect differences in phase emperature phase, is derived from kaolinite.Theconcentration of the ash fractions. The main mineralreason for anorthite and gehlenite formation is thatwhich varykaolinite(clay) reacted with alkali and iron com-somewhat with coal samples, generally include quartz, pounds From 1150 C to 1350 C, the quartz phasehematite,anhydrite, lime, potassium meta, rutile and decreased sharply and the anorthite content shows anon-crystalline components or glass, implying comtendency of first increasing rapidly and then decreas-plex behavior of ashes although they were collecteding. The formed mullite phase reached a maximum atfrom the same Huainan coal deposits. The noncrys1250C and showed a tendency to decline slowlytalline component, or"glass", is comprised of alumi-when the temperature was over 1250 C. As the tem-nosilicate glassy materials which are modified by the perature increased to over 1350 C, the major miner-nclusion of Na2O, KO, MgO, CaO and FeO. Thals identified were mullite and the non-crystallineglass content is reflected by a broad"hump"in the phase. The content of mullite formed in the heatingdiffraction patternprocess was the main reason for the high ash meltingtemperature of Huainan coals40007000MAI MM HNI1S-1450LLMCE HH H HNXMQQQHN5-1350HNIl5-12501000HNI5-11500102030405060708090岭m28(°Fig. 4 XRD results of different Huainan coal ashesFig. 5 The XRD patterns of HNI15 coal ashQ Quartz, A Anhydrite, H Hematite, L lime, Pm. Potassium mica, R RutileQ Quartz, M Mullite, AN Anorthite, G. GchleniteIn light of their high ash melting temperatures, 3.4 Mineral transformation of Huainan coalHuainan coal samples are characteristic for the highashes at 1350 C under reducing conditionintensity of the diffraction lines of non-crystallinecomponents in contrast to the high quartz, hematiteUnder reducing condition, three typical Huaand anhydrite containing coals with relatively low ash inan coal ashes, HN115 with FT 1400 C, hnllwith Fr 1480 C and HN106 with Ft more than3.3 Mineral transformation of ashes at high tem1500C, were put into the furnace at the intendedtemperature of 1350 C for 5min. The XRD patternsobtained from the quenched ash samples for threeFig. 5 shows that the main mineral phaseschange when HN115 coal ash is at different tempera-The中国煤化工dHNl9 re multures and in a simulated gasification environmentCNMHGStalline. The main(60%CO, 40%CO2). The behavior of the ash sam- phaseare mule and non-crystallineles is fairly complex at high temperatures, when components. Differences in peak intensity of mullitekept in the furnace for 5 min and then quenched in a and anorthite reflect differences in phase concentra-water bath. At 815 C, quartz, hematite, anhydrite, tion of the quenched slag fractions, which contributelimesium meta and non-crystalline compo- to the difference of ash melting temperatures. Fig. 6J. China Univ. of Mining& Tech. English Edition)Vol 16 No. 2also exhibits an amorphous hump maximum in the reducing atmospheric conditions and has provided a22-2328 region for HN106 slag sample and in the new powerful tool for the study of coal mineral mat24-250 28 region for HN115 and HN119 slag sam- ter reactions at high temperaturesples. The differences in the location of the amorphous2)In light of high ash melting temperatures,hump maximum indicate the difference of glass types, Huainan coal samples are characterized by high inwhich may influence the ash melting temperature. For tensity diffraction lines for non-crystalline compo-Huainan coal samples with relatively high ash melt- nents in contrast to the high intensity of quartz,ing temperatures, the intensity of the diffraction lines hematite and anhydrite in the relatively low ashfor mullite under reducing condition is high while for melting temperatureshe samples with relatively low ash melting tempera3)The transformation of mineral matter atture that for anorthite is highperatures higher than 1150C was tracked by XRDanalyses of three typical Huainan coals in reducingconditions.As temperature increases, the quartzphase decreases sharply and the anorthite contentshows a tendency of rapid increase at first and thendecreases. Differences in peak intensity of mulliteand anorthite reflect differences in phase concentra-tion of the quenched slag fractions, which contributeto the difference of ash melting temperatures. Thedifferences in the location of an amorphous humpn An An MMHNIImaximum indicate the differences of glass typewhich may influence the ash melting temperatures“啊心HN9For Huainan coal samples with relatively high ashmelting temperatures, the intensity of the diffractionlines for mullite under reducing condition is highwhile for the samples with relatively low ash meltingtemperatures the intensity for anorthite is also-highFig 6 XRD results of three Huainan coal ashes at 1350CAcknowledgementsQ Quartz. M. Mullite, AN AnorthiteThe authors acknowledge the contributions made4 Conclusionsby the members of Ninomiya's Lab of Chubu Uni-versity and the ash Chemistry Lab of Anhui Univer1)X-ray diffraction has been an invaluable tool sity of Science and Technologyfor the characterization of coal mineral matter underReferences[1] Ward C R. Coal Geology and Coal Technology. Melbourne: Blackwell Publisher, 1984[2] Gupta R, Wall T F, Baxter L A. 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