Influence of coal blending on mineral transformation at high temperatures

MININGScienceDirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 19(2009)0300-0305wwwelsevier.com/locate/jcumtInfluence of coal blending on mineral transformationat high temperaturesBAIJin, LI Wen', LI Chun-zhu, BAI Zong-qing, LI Bao-qingState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science,Taiyuan, Shanxi 030001, china2Department of Chemical Engineering, Monash University, Monash, Vic 3800, australiaAbstract: Transformation of mineral matter is important for coal utilization at high temperatures. This is especially true for blendedcoal. XRD and FTiR were employed together to study the transformation of mineral matter at high temperature in blended coals. Itwas found that the concentration of catalytic minerals, namely iron oxides, increases with an increasing ratio of Shenfu coal, whichcould improve coal gasification. The transformation characteristics of the minerals in blended coals are not exactly predictable fromthe blend ratio. This was proved by comparing the iron oxide content to the blending ratio. The results from FTIR are comparablewith those from XRD. FTIR is an effective method for examining variation in mineral matterKeywords: coal blending; high temperature; mineral matter transformation1 Introductiontional utilization of other poor quality coals havinghigh ash content and high ash melting tempera-The utilization ratio of poor quality coal is in-reasing in China every year. The large scale develHuggins et al. studied the relationship between ashopment of coal resources has been under way for melting temperature and mineral transformation inseveral decades. Coal blending is one feasible method coal. The behavior of mineral matter under reducingfor efficient coal utilization. It is difficult to predict and oxidizing atmospheres was describeds. TG anthe characteristic of blended coals particularly since DTG methods were utilized to study the transformathe mineral matter in the blended coal does not be- tion of mineral matter under combustion conditionshave in simple proportion to the blending ratio Slag by Tomeczek-, who deduced that mineral matterand fouling problems in boiler and gasification fur- transformation in blends could be predicted from thenaces, which are the key factors influencing their characteristics of the separate minerals. Li et alsteady operation, are serious when using blended compared the characteristics of mineral matter at thecoals in Chinadeformation temperature(DT), the softening temThe prospective reserves of Huainan coal are about perature (ST)and the flowing temperature(FT)44.4 billion tons, and proved reserves are approxi- However, their results were less useful for gasificamately 15.3 billion tons. However, most Huainan tion and combustion work due to the limited tem-coals are of poor quality, having both high ash con- perature range in those experimentsLi Han-xu ettent and high ash-fusion temperatures Flux agents, or al. studied the influence of fluxing agents on the ashcoals with a low ash-melting temperature, must be melting behavior of Huainan coal and proposed optiadded to Huainan coals during their utilization. Coal mized methods for altering the ash melting behaviorblending is more effective than flux additives because of those coals -. In general, the way mineral matterthe latter will add a heat load to the furnace and lower behaves in a coal blend at high temperature is notits efficiency. The study of mineral matter transfor- fully understood at present. The aim of this workmation in Huainan blended coals is important for ef- the study of mineral transformation in blended coalsficient combustion and gasification of Huainan coal. using XRD and FTiR techniques.This information will also give guidance for the ra-中国煤化工Received 15 December 2008: accepted 20 February 2009Projects 2005CB217701-03 supported by the National Basic Research Program of ChYHCNMHGScience and technology oforresponding author. Tel: +86-351-4048967: E-mail address: stone@ sxiccaccnBAI Jin et alInfluence of coal blending on mineral transformation at high temperatu2 Experimental2.3 AnalysisThe XRd data were collected using an arl dif-2.1 Samplesfractometer, CuKa radiation, scanned from 10 to 800Two Chinese coals Shenfu and Huainan coal were at a rate of 1 /4 min. The relative intensity ratio(RIR)used in this work. Their properties are listed in Tablesmethod was used for the quantitative analysiss).1 and 2. Shenfu and Huainan coals of 80-mesh wereCrystallinity=100x2Inet /2 Itot.-Bgr constblended in the ratios of 1: 4. 2: 3, 3: 2 and 4: 1. Thesewhere Inet. is the crystal intensity; Bgr. const. thesamples were labeled S2H8, S4H6, S6H4 and S8H2, background intensity; and, Itot. the total intensityrespectively. The properties of the blended coal are RIR values determined from the pDF-2 databasepresented in Tables 2 and 3. Raw ash samples for in- were used for this study. Calibration constants(RIRsvestigation of mineral transformation were prepared can vary significantly depending on the actual phaseat 815C following the procedures of GB/T 1574- compositions. A NICOLET NEXUS 470 FTIR was2001. Briefly, the temperature rises to 500%C within used for mineral analysis, scanning the range from30 min and then holds there for another 30 min after 4000 to 400 cm at a resolution of i cmwhich time the temperature rises to 815%C and is heldthere for about 60 min3 Results and discussionTable 1 Chemical composition of Shenfu andHuainan coal ashes(96) 3.1 Composition of low temperature ashmple SiO2 Al2O Fe20, Cao MgO TiO2 Na20 KO SOyThe composition of the low temperature ashesSF coa25291l.2612.8934.623.910901.600.7174from the different blended coals was determined byHN coal556033.503.832.150681.410.460.540.95460. 0.95 XRD(Fig. 1). The main minerals, determined byTable 2 Ash fusion temperatures of Shenfu, Huainancomparison to the PDF-2 database and quantitatiyand blended coalsRIR analysis, are listed in Table 4. The compdepended on the blending ratio but the reactions be-tween the various minerals did not simply follow the1500ratio of minerals present in the un-blended coals. The$4H61220composition could not be exactly predicted from thebehavior of the single coals because of the interac170tions among the various mineral matters at high tem-SF coalpeTable 3 Chemical analysis of ash from blended coal (%SampleSiO,AlgO3Fe O3S2H8S6H4374120.16S8H22.2 ProcedureThe ash-fusion test was modified to allow rapidS6H4uenching of the ash samples from a high tempera-ture and reducing atmosphere(CO2: CO=6: 4 )by im-mersion into ice water. a graphite crucible containingthe ash sample was withdrawn from the oven anduenched into water. Ash samples were treated at 50S2H&C intervals over the range from 1100 to 1500C.102030405060The total quenching time was normally from 5 to 10 s260°)All quenched ash samples were collected and examFig. 1 XRD patterns of mineral matter inined by X-ray diffraction(XRD)and FTIRblended coals(815°CTable 4 Composition of mineral matter in ash prepared at 815CCaSO4(3%),Fe2O3(8%),SiO2(58%),中国煤化工CaSO (5%), Fe2O(10%), SiO2(48%), CaAloCNMHG.Al20J(49%)CaSO4(11%),Fe2O(11%),siO2(32%),Ca12A1Oy59), NaAl, Ou (2%), Al O3 (3%)CaSO4(12%),Fe2Of(11%),SiO2(30%),Ca12Al4Oy(7%),CaAl4Of11%),Al2O(1%)Mining sAdding Shenfu coal increased the calcium and ironcontent and decreased the amounts of silicon andMullitealuminum. From the decrease in the peak at→ Anorthite→CaAl2O620=23.68 it may be deduced that the silicon oxidecontent decreased. The peak at 20-18 reflects thecontent of CajAl,4O33 and increased as more Shenfucoal was added to the blend. The iron oxide andCaALO, contents increased: notice that the peak at2=33°and34° becomes stronger.The20=27.5 implies the existence of NaAl3 Si3O11. Itsconcentration first increased and then decreased withfurther addition of the shenfu coal. This observation105011501250135014501550may be explained as follows. The sodium contentTemperature (C)increases with the amount of Shenfu coal. So then theFig 3 Content of mineral matter in S2H8 prepared atcontent of NaAl3Si3On first increases. However, asdifferent temperaturesthe content of calcium increases in the blendCa12Al14O33 is generated. This results in a decrease inThe major components in ash S2H8 were mulliteNaAl3SigO1 content as further Shenfu coal is added. (3Al2O3 2SiO2), SiO, sillimanite(Al2O3.,)andThe following reactions are assumed to occur:anorthite (AlO3 SiO), which was close to the ashcompositionNayo+Al2O3.2SiO2-NaAl3Si3O1calcium, which is caused by adding Shenfu coal, ledCaSO4→CaO+SO3to the formation of anorthite. As the ashing temperaCao+Al2O3-Caj2Al14033ture increased from 1100 to 1500oC the content ofSiO2 decreased, which is attributed to the reaction3.2 Mineral transformations in S2H8between mullite and anorthite. when the temperaturewas above 1400C, the peaks from SiOz disappearedKRD patterns( Fig. 2)can explain the mineral The following reactions are postulated to occur 2-4.91transformations in the S2H8 blend. Mineral matterCa12Al14O33-Cao+Al2O3produced at different temperatures was determined bythe RIR method and comparison to the PDF2 data-Cao+ SiO+Al2O3-CaAl2Si2Osbase, as shown in Fig. 3SiOz+ALO3++Al2O3 SiO2S2HEThe content of CaAl2SizOg increased at first andthen decreased as the temperature was raised. Near1300C the major components were sillimanite andS2H81450mullite, which greatly increased in concentration atelevated temperatures. When the temperature waS2H81400around 1400C, Ca3 AlO6 was generated due to itsstabilization by the existence of Na2O. It has beens2H81350reported that when the content of Na2O is about 4%,Ca3A1,Os is stable even at 1500Cl5. Io. At higherS2H81300temperatures Fe203 gradually turned into Fe3O4 andsome Fe295Sio. sO4 was formed. They are catalogeds iron oxides, which decreased with increasing temperature, and the content of Fe2AlgSisO18 increasedS2H81200dually. when thethe temperature was above 1400Cthe Fe2ALSisO18 content was greatly decreased be-S2H81150cause the iron oxides turned into FeAl2O4 followingthe Feo-SiO2-Al2O3 ternary phase diagram. FeAl2O4S2H81 100was not detected by XRD because of its poor crystallinty. A large amount of amorphous phase was gen-erated when the temperature was over 1250C. Acomparison between crystalline peak and hump areasFig 2 XRD patterns of mineral matter in S2H8 ashfrom中国煤化工9128(R= Amorphousprepared at different temperaturesIncreCNMHre in the followingnfluence of coal blending on mineral transformation at high temperaturesR1250(35%)