rand

nItaa

nghai, 200093, Chinaniversity

a r t i c l e i n f o

Article history:Received 29 January 2010Received in revised form 4 June 2010Accepted 23 June 2010

Keywords:Blended coal ashAsh fusion characteristics

length (B.L.) of bond Si (70)O (86) to become longer and unstable.

Fuel Processing Technology 91 (2010) 15911600

Contents lists available at ScienceDirect

Fuel Processing

e ls 2010 Elsevier B.V. All rights reserved.

1. Introduction

China is the world's largest producer and user of coal, the demandof coal has been rapidly growing in the past 10 years [1]. Entrainedow gasier has been greatly improved as one of the most importantclean coal technologies in China, since it not only can satisfy thegrowing demand of primary energy but also can supply electricity,liquid fuels, hydrogen and other chemical materials if needed, withlow pollution levels in China [2,3]. For prevailing slagging entrained

well as high AFT (FTN1673 K). High AFT coals account for about 55% ofthe annual output and 57% of the retained coal reserves in China [5].As a result, those high AFT coals would be expected to require eitheradding ux agents, such as limestone, Fe2O3 and K2CO3 etc. orblending with low AFT coals. As compared with the method of addingux agents, coal blending is a good method for high AFT coal toprovide a consistent feedstock of fuel for slagging gasier becausethere is no increase of ash content and corresponding oxygenconsumption, which is necessary to melt added ux agents [6,7].ow gasiers, such as Shell, Texaco, Preno, Gtemperature) of feeding coal must be lowerperature (1673 K) for safe operation with a mcoals usually have relatively high total ash con

Corresponding author. Tel.: +86 13501618961; faxE-mail address: zhzhx222@163.com (Z. Zhang).

0378-3820/$ see front matter 2010 Elsevier B.V. Aldoi:10.1016/j.fuproc.2010.06.007anorthite by the effect of Ca , and the entered Ca is located in the center of [SiO4] -tetrahedron ring inthe anorthite crystal lattice. Taking the [SiO4]4-tetrahedron, which is composed of Si (70), O (78), O (48),O (91), O (86) as an example, the Ca2+ can capture the partial electronics of O (86) and cause the bondMineral transitionQuantum chemistrya b s t r a c t

The main mineral melting behavior and mineral reaction mechanism at molecular level of Chinese blendedcoal ash under gasication condition (30% H2, 66% CO, 4% CO2) from 1073 K to 1573 K were studied throughthe ASTM test, X-ray diffraction (XRD), ternary phase diagram system and quantum chemistry calculationwith ab-initio calculations. The results show that with increasing blending mass fraction of low ash fusiontemperature (AFT) ash (ash B), the location of blended ash in ternary systems is transferred from the mulliteregion to the anorthite region, as the dominant crystal mineral of blended ash at around DT (XRD analysis) isalso transferred from mullite to anorthite. The calcium-bearing minerals, such as anhydrite, calcite etc., canreact with mullite and the precursors of mullite (metakaolinite etc.), which is one of the main refractoryminerals in high AFT ash (ash A), and is converted into low-melting minerals (anorthite, gehlenite, andfayalite etc.) in the temperature range between 1273 K and 1403 K. The reaction between mullite and CaO toform anorthite plays a signicant role in decreasing AFTs of blended coal ash A/B. It is because the chemicalactivity of the highest occupied molecular orbits (HOMO) in mullite cluster is stronger than that of thelowest unoccupied molecular orbits (LUMO) in mullite cluster, the Ca2+ as electron acceptor can easily enterinto the crystal lattice of mullite mainly through O (7) and O (12) and cause the rupture of bonds Al (1)O(13) (in the [AlO6]9-octahedron) and Al (8)O (13) (in the [AlO4]5-tetrahedron), which are weaker thanany other bonds in crystal lattice of mullite. Finally, the entrance of Ca2+ can force mullite to transform to

2+ 2+ 4SP etc., the AFT (FT, owthan the operating tem-olten slag [3,4]. Chinesetent, around 2728%, as

Thus, it is necessmelting mechanfor a continuousslagging gasierorganic materialhigh temperaturand the mineralundergo chemic

: +86 21 34207274.

l rights reserved.d Department of Chemical Engineering, Nagoya U , Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, JapanShanghai Electric Power Generation Group, Shanghai 201100, Chinac Department of Power Engineering, University of Shanghai for Science & Technology, ShaMain mineral melting behavior and mineof blended coal ash under gasication co

Xiaojiang Wu a,b, Zhongxiao Zhang a,, Yushuang CheNobusuke Kobayashi d, Shigekatsu Mori d, Yoshinori,a School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, Chinb

j ourna l homepage: www.l reaction mechanism at molecular levelitionc, Tuo Zhou c, Junjie Fan c, Guilin Piao d,ya d

Technology

ev ie r.com/ locate / fuprocary to clarify the melting behavior and fundamentalism of blended coal ash under gasication conditionssuccessful safe operation of slagging combustors ands when gasifying blended Chinese coal. In gasiers,s in coal are completely combusted and gasied ate (14731873 K) and high pressure (3.04.5 MPa),matters in coal would transform into ash and alsoal reaction, and morphology changes; and eventually

respectively. When temperature reached the predetermined tempera-ture, each samplewas taken out and quenched inwater quickly to avoidthe crystal phase changes of ashes A, B and their blended ash A/B [30].Some ash samples at various temperatures were dried and ground intoa powder of required particle size in a carnelian bowl for XRD analysis(D/Max PC 2550, Japan).

2.3. Quantum chemistry calculation

In order to provide an insight into themineral transition process inthe conversion of mullite into anorthite when heating blended ash A/B under gasication condition, the quantum chemistry calculationwith ab-initial density function theory B3LYP method [31] wasemployed to calculate the surface chemical characteristics of mullite

1592 X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600fusion at high temperature [8,9]. The relationships between AFT andthe chemical and mineral compositions of coals and coal ashes havebeen extensively studied through ASTM test, chemical analysis,scanning electron microscope (SEM), energy dispersive X-ray ana-lyzer (EDX), X-ray diffraction (XRD) and phase diagrams theory etc.[1014]. Previous studies show that the mineral matters in ash andtheir melting behavior plays a signicant role during ash meltingbehavior at high temperature. In general, themain refractorymineralsin coals and coal ashes are quartz (SiO2), metakaolinite (Al2O32SiO2),mullite (3Al2O32SiO2) and rutile (TiO2) etc., while the main uxingminerals are anhydrite (CaSO4), anorthite (CaOAl2O32SiO2), gehle-nite (2CaOAl2O32SiO2), K-feldspars (KAlSi3O8), Ca silicates andfayalite (2FeOSiO2), almandite (3FeOAl2O33SiO2) etc. [15,16].Especially for calcium- and iron-bearing minerals, such as anhydrite,calcite (CaO), hematite (Fe2O3) and wustite (FeO) etc., they would actas the key factors in uxing aluminosilicates and lowering MP(melting point) of ash during coal combustion and gasication[16,17]. Furthermore, various mineral matters in ash will also reactwith each other complexly to form some eutectic mixtures that startto melt and cause ash melting mechanism of blended coal ash tobecome more complicated at high temperature [16,17]. The meltingbehaviour of coal mineral is a complex process: the blending of coalsinevitably makes it even more complex.

Recently, with the development of quantum chemistry theory andcomputer technology, the semi-experimental and ab-initio computa-tional methods have become a powerful tool to investigate themolecular surface chemical characteristics and chemical reactionmechanism of various minerals at a molecular level [18,19]. In thearea of fuel science, the computational program has been applied tosimulate the molecular structure of coal [20,21], which has a guidingsignicance to provide an insight into the interaction mechanismbetween gas molecule and coal or char molecule in coal liquefaction[22,23]. In the eld of mineral matters, atomistic simulation tech-niques were used to investigate the interaction mechanism betweenmineral and absorber, such as heavy metal ion [24,25]. The mainatomic orbital populations of some frontier molecular orbits, covalentbond level of main atoms and other properties of minerals or mineralsafter absorption of heavy metal ion can be obtained directly throughab-initio density functional calculation method [2628]. Li J. et al [29]has used ab-initio density functional calculation method to guide theselection of appropriate uxing agent to reduce AFT of coal ash.Therefore, it seems that it is possible to investigate the complicatedinteraction mechanism of minerals in blended coal ash at molecularlevel through ab-initio density functional calculation method.Through quantum calculation of main minerals (refractory or uxingminerals) in ash, some important information on mineral reactionprocess during heating, such as reaction activity sites, atomic netcharge, a particular molecular orbital energy level etc., can beobtained to provide an insight into the main interaction mechanismof minerals in ash. In order to clarify the fundamental mineraltransition mechanism of blended ash under gasication condition,blended coal ashes of one typical Chinese high AFT coal ash and onetypical Chinese low AFT coal ash with different blending ratios atvarious temperature levels under gasication condition (30% H2, 66%CO, 4% CO2) were obtained and their corresponding mineral com-positions were analyzed by XRD analysis in detail. Furthermore, themolecular chemical characteristics of main minerals, such as mullite,anorthite etc., were also studied by quantum chemistry calculation toprovide an insight into the main mineral reaction mechanism ofblended ash at molecular level under gasication conditions. For thecurrent research, only the molecular chemical characteristics ofmullite and anorthite have been studied. Further calculation onother minerals such as genhlenite, wustite, hercynite (FeOAl2O3),fayalite etc., would take place to completely interpret the fundamen-tal interaction mechanism of minerals in blended ash during heating

under gasication condition.2. Material and methods

2.1. Coal sample

Two typical Chinese bituminous coals A and B with different AFTswere selected for use in this work. One is high AFT coal and the other islow AFT coal. Coal sampleswere ground to less than 75 m(under 200meshes) in diameter. Laboratory ash samples (1088 K/1 h, under air)were prepared from crushed coal samples and an ash oxide anal-ysis was determined in accordance with Chinese national standards(GB-210). The AFT test, which can provide four temperatures (DT, ST,HT and FT) that characterize the fusibility behavior of laboratoryash was performed in both oxidizing and a simulated gasicationenvironment (30% H2, 66% CO, 4% CO2) according to Chinese nationalstandard (GB/T219-1996). The ash chemical composition of test coalsis illustrated in Table 1. A number of blended ash samples wereprepared on mass basis with a similar SiO2/Al2O3 ratio and differentFeOn or CaO levels in blended ashes A/B. The normalized compositionof ashes A, B and their blendedmixtures are respectively shown on theappropriate position in the CaOSiO2Al2O3 and FeOSiO2Al2O3ternary phase diagram system (Fig. 1 (a) and (b)). The initial mineralcomposition of ashes A and B at 1073 K is also shown in Fig. 2.

2.2. Experimental details

The blended ash samples A/Bwith various blendingmass ratios (20,40, 60 and 80 wt.%) of ash B were prepared and mixed well beforecarrying out the experiment. TheAFT test of ashes A, B and their blendedashes A/B was performed based on Chinese national standard underboth oxidizing (air) and a simulated gasication environment (30% H2,66% CO, 4% CO2) in a HR-4 ash fusion testing apparatus (HR-4, HenanProvince). Furthermore, in order to investigate the mineral transitionbehavior of blended ashA/Bunder gasication condition, around1 g ashsamples (ashes A, B and blended ashes A/B) were spread out in a layer(about 5 mm thickness) and heated to reach various temperatures(1073, 1173, 1273, 1373 1473 and 1573 K) in an electric furnace (Fig. 3)under a simulated gasication environmentwith 20 K/min heating rate,

Table 1Ash composition of coal ashes A and B.

Ash composition, wt.% A B

SiO2 50.62 43.41Al2O3 39.3 12.23Fe2O3 3.70 16.54CaO 0.70 12.08MgO 0.20 2.56SO3 0.10 7.82TiO2 1.12 0.65K2O 0.64 0.54Na2O 0.27 0.74P2O5 0.36 0.16and anorthite at molecular level. In practice, the KohnSham theory

Fig. 1. Representation of ashes A, B, C and their corresponding blended ashes on the ternary phase diagram. (a) Ternary phase diagram CaOSiO2Al2O3 system. (b) Ternary phasediagram FeOSiO2Al2O3 system.

1593X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600

1594 X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600can be applied in several distinct ways depending on what is beinginvestigated. In the chemistry eld, one popular functional is knownas BLYP (from the name Becke for the exchange part and Lee, Yangand Parr for the correlation part). Even more widely used is B3LYPwhich is a hybrid functional in which the exchange energy, in this casefrom Becke's exchange functional, is combined with the exact ener-gy from the HartreeFock theory [31]. The Gaussian 03 package(631 G (d, p) basis set) [32] was used in this study. Through quantumchemical calculation, some important information on surface chemicalcharacteristics of minerals at molecular level, such as reaction activity

Fig. 2. The XRD pattern

Fig. 3. Schematic diagram of experimental system. 1gas bottle; 2owmeter; 3mix room(blended ash); 8samples(at different temperatures).sites, bond order (B.O.), the highest occupiedmolecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) etc., can beobtained. Mullite belongs to the orthorhombic system, with celldimensions: a=7.60 , b=7.68 , c=2.88 and the space group isPbam. The primitive cells of mullite with the lattice parameters arechosen from the study of Takayuki [33]. Anorthite belongs to the triclinicsystem, with cell dimensions: a=8.19 , b=1.29 , c=1.42 and thespace group is P1. The primitive cells of anorthite with the latticeparameters are chosen from the study of F. F. Franklin and R. P. Donald[34]. In order to avoid higher false charge, the O atoms, which are on the

s of ashes A and B.

; 4value; 5electric furnace (~1873 K); 6temperature control box; 7ash sample/

cell edge of mullite/anorthite were saturated by H atoms to obtainelectric neutrality [35,36]. The atomic cluster of mullite and anorthitecan be written as [SiAl4O10H9] and Ca8[Al16Si16O64H14] respectively.

3. Results and discussion

3.1. Fusibility of blended coal ash A/B during heating under gasicationconditions

Lines AB on the ternary phase diagrams (CaOSiO2Al2O3 andFeOSiO2Al2O3) represent the range of blended ash A/B composi-tion (Fig. 1 (a) and (b)). As it is can be seen from Fig. 1, the initial bulkash composition of ash A (high AFTs, DTN1773 K) is located in themullite region of both CaOSiO2Al2O3 and FeOSiO2Al2O3 phasediagrams, while the anorthite region of CaOSiO2Al2O3 and thetridymite region of FeOSiO2Al2O3 are ash B's locations. Along withthe increasing mass fraction of ash B in blended ash A/B, the bulk ash

chemical composition of blended ash A/B changes along with thestraight line AB from the mullite region to the anorthite region in theCaOSiO2Al2O3 system and from the mullite region to the tridy-mite region in the FeOSiO2Al2O3 system. The relationshipbetween AFTs of blended ash A/B and their corresponding liq-uidus temperatures from the phase diagrams (CaOSiO2Al2O3and FeOSiO2Al2O3) under both oxidizing and gasication con-ditions are shown in Fig. 4. As it can be seen from Fig. 4, theexperimental AFT curves of blended ashes A/B would closely parallelthe liquidus temperatures from the CaOSiO2Al2O3 phase diagramunder oxidizing atmosphere, as well as the liquidus temperaturesfrom the FeOSiO2Al2O3 phase diagram under gasication condi-tions. The AFTs of those blended coal ashes A/B whose normalizedchemical composition is located near the boundaries betweentwo mineral regions or triple points are lower and changed moreevidently than that of those blended ashes A/B whose normalizedchemical compositions are far away from the boundaries or triple

1595X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600Fig. 4. Relationship between AFTs of blended ashes and liquidus from phase diagrams. (a) Oxidizing atmosphere. (b) Reducing atmosphere.

points in the ternary phase system. Therefore, in order to decreasethe AFT of high AFT coal, the optimum mixture ratio of blended ashis near the boundaries or triple points in the CaOSiO2Al2O3 or FeOSiO2Al2O3 phase diagram.

3.2. Main mineral transition behavior of ash A and B during heatingunder gasication conditions

The main mineral transitions in ashes A, B and their blended ashesA/B under gasication conditions from 1073 K to 1573 K are discussedbased on Fig. 5 and the 3D graphical visualization of XRD results,which are given in Fig. 6. As it can be seen from Fig. 2, when comparedwith the initial identied minerals in ash B at 1073 K, ash A is mainlycomposed of quartz and muscovite (KAl2AlSi3O10(OH)2) and a smallamount of rutile (TiO2). Also, there are no identied lowMPminerals,such as calcite, rankinite, anhydrite, anorthite, dioposide and augite(Ca(Mg,Fe)Si2O6), but these are identied in ash B. The main mineraltransitions that occurred during heating between 1073 K and 1573 Kin ash A are mullite, quartz, wustite and hercynite etc. (Figs. 5 and 6(a)). With increasing temperature, mullite is formed and increaseddue to the decomposition of metakaolinite (Al2O3SiO2) when the

meanwhile the decomposition of anhydrite also occurred at around11731373 K [37]. Anorthite becomes stable at around 1273 K, while,the XRD peak of anorthite decreases with further increase oftemperature above 12731373 K probably due to partial melting ofthe phase assemblage [37]. Above 1573 K ash B is completely molten(Fig. 6 (d)). The decrease in intensity of the mineral matters as afunction of temperature is not only a decomposition of some mineralphases, but also the formation of liquid (slag) because of someminerals with low-melting point in ash B, such as anorthite, rankinite,fayalite, almandite etc.

3.3. Main mineral transition behavior of blended ash A/B during heatingunder gasication conditions

For blended ashes A/B, The Ca-, Fe-, Na- and Mg-bearing mineralsin ash B, such as anhydrite, wustite, albite and augite etc., can reactwith quartz and mullite, which are mainly refractory minerals in ashA, to form some lowMPminerals such as anorthite, hercynite, fayaliteetc., at above 1373 K [37]. Therefore, with increasing blending ratio ofash B in blended ash A/B, the peak value of mullite at varioustemperatures decreases and new low MP minerals, such as anorthite,

1596 X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600temperature is higher than 1273 K [37]. When temperature rises to1573 K, mullite is the dominant and single crystalline phase in ash Abecause of the high MP of mullite (MP, 2123 K Fig. 6 (a)). Theother minerals in ash A, such as quartz, wustite and hercynite etc.,decreased and disappeared with the increase of temperature (Figs. 5and 6 (a)). Quartz is also one of themain refractoryminerals with highMP (MP, 1983 K) in coal and coal ash. However, under reducingconditions, the refractory nature of quartz or its polymorphs can becompromised because of lime, wustite and other calcium or iron-richminerals (anhydrite, pyrite, siderite (FeCO3)) which can react stronglywith quartz to a yield mixture of anorthite, wollastonite(CaOSiO2),rankinite, fayalite and silicate melts in ash [38]. Under reducingcondition, wustite is formed at around 1173 K mainly due to thedecomposition of hematite and siderite [39]. With further increase oftemperature, wustite starts to react with mullite and quartz etc., toform fayalite and hercynite etc., at around 12731373 K. As it can beseen from Figs. 5 and 6, the XRD peak of wustite in both ashes A and Bdecreases at around 1173 K, and meanwhile the XRD peak of fayalite,almandite and hercynite increases. For low AFT coal ash B, the mainmineral transition that occurred during heating between 1073 K and1573 K are anorthite, quartz, anhydrite etc. (Figs. 5 and 6 (d)). As itcan be seen from Figs. 5 and 6, anorthite starts to form as the productsfrom anhydrite, alumina and silica at around 11731373 K, andFig. 5. Main minerals change of ashes A, B and blended ash A/Bfayalite etc., are formed and increased in the temperature range of12731573 K when compared with the main mineral composition ofash A at various temperatures (Figs. 5 and 6). When the blending ratioof ash B reaches 40 wt.%, it is because the bulk ash chemical com-position of blended ash A/B still is located in themullite region of boththe CaOSiO2Al2O3 and FeOSiO2Al2O3 phase systems, the maincrystal mineral at high temperature (1573 K) is still mullite except forthe lower peak value of mullite than that of mullite in ash A at 1573 K(Fig. 6 (a) and (b)). Though some portions of the initial active meltssuch as anorthite, hecynite, fayalite etc., with low viscosity are alsoformed in blended ash A/B (40% B+60% A) in the temperature rangefrom 1423 to 1468 K (Figs. 5 and 6 (b)), those melts do not dissolvethe refractory mineral phases completely such as mullite, cristobalite,cordierite etc. In that case (40% B+60% A), mullite and anorthitecoexist in the temperature range of 14231463 K (Fig. 6 (b)). Whenthe blending ratio of ash B reaches 80 wt.%, it is because the location ofblended ash A/B in CaOSiO2Al2O3 phase diagram has been changedfrom the mullite region to the anorthite region, the mullite crystallinephase disappeared and anorthite, rankinite, fayalite etc., become thedominant crystal minerals of blended ash A/B in the temperaturerange of 12681468 K (Figs. 5 and 6 (c)). Along with increasing massfraction of ash B in blended ash A/B, the content of calcium- or iron-bearing minerals will increase and react with some main refractoryas a function of temperature under gasication conditions.

1597echnology 91 (2010) 15911600X. Wu et al. / Fuel Processing Tminerals in ash A, such ash mullite and quartz etc., to form low MPminerals (anorthite, wollastonite, fayalite, hercynite etc.). The mainmineral reactions of ashes A, B and their blended ash A/B duringheating at gasicaiotn condition are shown as follows:

hematiteFe2O3e

1173K

FeO + O2 reducingcondition 1

anhydriteCaSO4N 1273K

CaO + SO3 2

mullite3Al2O3 2SiO2 + CaOe

1403K

anorthiteCaO Al2O3 2SiO23

anorthiteCaO Al2O3 2SiO2 + CaO gehlenite2CaO Al2O3 2SiO24

quartzSiO2 + CaO wollastoniteCaO SiO2 5

wollastoniteCaO SiO2 + CaOAlite3CaO SiO2 6

quartzSiO2 + FeO1173

e

1273K

clinoferrosiliteFeO SiO2 7

Fig. 6. Integral intensities of main minerals of ashes A, B aclinoferrosiliteFeO SiO2 + FeO1173

e

1273K

fayalite2FeO SiO28

mullite3Al2O3 2SiO2 + FeO1273

e

1473K

fayalite2FeO SiO2+ hercyniteFeO Al2O3 9

anorthiteCaOAl2O32SiO2 + FeO1273

e

1473K

fayalite2FeOSiO2+ hercyniteFeOAl2O3 + almandite3FeOAl2O33SiO2 10

Among these main mineral reactions, the mineral reaction (3) isespecially noteworthy because it will play a signicant role in mullite(melting point, MP=2133 K) transformed to anorthite (MP=1826 K). Therefore, for current research, the molecular surfacechemical characteristics of mullite and anorthite were rstly focusedon to analyze the fundamental mineral reaction mechanism ofblended ash A/B. Further calculation on other minerals in ash wouldtake place to complete the database for fully understanding meltingmechanism of coal ash/blended ash at high temperature.

3.4. Reaction activity of mullite cluster

The HOMO and LUMOmaps of mullite are shown in Fig. 7. As it canbe seen from Fig. 7, the HOMO of mullite is almost constituted of

nd their blended ash A/B as function of temperature.

oxygen atoms, especially the oxygen atoms O (7) and O (12), whichconnect with Si (16). The LUMO of mullite is mostly constituted of Al(1), Al (8) and corresponding connected oxygen atoms, such as O (3),O (9), O (13) and O (14). Based on the frontier orbital theory, thefrontier orbits in the HOMO or LUMO have more chemical reactivitythan any other orbits and play a key role in the chemical reaction ofminerals on the molecular level [40]. In order to clarify the frontierorbits in the mullite molecular structure, the bond order (B.O.),population (Pop.), net charge (N.C.) and bond length (B.L.) of HOMOand LUMO in themullite molecules are shown in Table 2. According tothe B.O.(P(A)) and B. L. (R(A)) in mullite, the B.O. of bond Al (1)O(13) is lower than that of Al (8)O (13), Si (16)O (7), Al (8)O (14)and Si (16)O (12), and the B.L. of bond Si (16)O (12) is shorter thanthat of Al(8)O(14), Si (16)O (7), Al (8)O (13) and Al (1)O (13).Therefore, the covalent bond of Si (16)O (12) is the strongest inmullite, because of the highest B.O. (0.6650) and the shortest B.L.(0.16673 ), whereas the bond of Al (1)O (13) is the weakest in

Ca2+, Na+ etc. can easily enter into the crystal lattice of mullite fromthe HOMO atoms, such as Si (16), O (5), O (7), and O (12) and causebond Al (1)O (13) to break.

3.5. Reaction activity of anorthite cluster

The HOMO and LUMO maps of anorthite are shown in Fig. 8. As itcan be seen from Fig. 8, the HOMOof anorthite is almost constituted ofoxygen atoms, especially the oxygen atoms O (31), O (42) and O(101), which connect with Al (15). The LUMO of anorthite is mostlyconstituted of Ca (55), Al (19), Al (57), Al (59) and correspondingconnected oxygen atoms, such as O (35), O (91), O (97), and O (99).Based on the frontier orbital theory, the frontier orbits in the HOMOorLUMO have more chemical reactivity than any other orbits and play akey role in the chemical reaction of minerals on the molecular level[40]. In order to clarify the frontier orbits in anorthite molecularstructure, the B.O., Pop., N.C., and B.L. of HOMO in anorthite moleculesare shown in Table 2. According to the B.O.(P(A)) and B. L. (R(A)) inanorthite, the B.O. of bond Al (15)O (31) is lower than that ofAl (15)O (42) and Al (15)O (101), and the B.L. of bond Al (15)O(101) is shorter than that of Al (15)O (42) and Al (15)O (31).Therefore, the covalent bond of Al (15)O (101) is the strongest inanorthite, because of the highest B.O. (0.5657) and the shortest B.L.(1.74023 ), whereas the bond of Al (15)O (31) is the weakest inanorthite because of the lowest B.O. (0.3719) and the longest B.L.(1.84206 ). It is because the O (31) has a low net charge (absolutevalue) relative to that of O (42) and O (101) (see Table 2), therelatively large electronic cloud are mainly located around O (31) andthe relatively large overlapping electronic cloud density is located on

1598 X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600Fig. 7. HOMO and LUMO maps of mullite surface structure. (a) HOMO map of mullite

surface structure. (b) LUMO map of mullite surface structure.mullite because of the lowest B.O. (0.2630) and the longest B.L.(1.9752 ). It is because the O (13) has a relatively low net charge(absolute value) than that of O (12), O (14) and O (7) (Table 2), therelatively large electronic cloud is mainly located around O (13) andthe relatively large overlapping electronic cloud density is locatedon bond Al (1)O (13). Therefore, cations as electron acceptors, such

Table 2The bond order (B.O.), population (Pop.), net charge (N.C.) and bond length (B.L.) ofmullite and anorthite.

Atom Population Net charge Bond Bond order Bond length(Pop.) (N.C.) (B.O.) ()

MulliteSi (16) 1.5410 1.6930 O (7) 0.9930 1.0960 Si(16)O(7) 0.6540 0.17183O (12) 1.3320 1.3080 Si(16)O(12) 0.6650 0.16673Al (1) 2.1200 2.0420 Al(1)O(13) 0.2630 1.9752Al (8) 1.9690 2.0080 O (3) 1.3590 1.2960 Al(8)O(3) 0.5750 0.17009O (9) 1.1190 1.1870 Al(8)O(9) 0.6025 0.16734O (13) 0.7060 0.7690 Al(8)O(13) 0.4530 0.18386O (14) 1.1470 1.1980 Al(8)O(14) 0.6620 0.16843

AnorthiteSi (70) 1.2399 1.3646 O (48) 0.6535 0.7888 Si(70)O(48) 0.8001 1.6389O (78) 0.6160 0.7153 Si(70)O(78) 0.7706 1.6762O (86) 0.6135 0.7923 Si(70)O(86) 0.7366 1.6890O (91) 0.6641 0.7734 Si(70)O(91) 0.9646 1.6113Al (17) 1.041261 1.17357 O (33) 0.713516 0.83238 Al(17)O(33) 0.6715 1.69097O (40) 0.654882 0.77044 Al(17)O(40) 0.3041 1.99228O (99) 0.681562 0.90540 Al(17)O(99) 0.6545 1.70259Al (15) 1.231309 1.33275 O (31) 0.572465 0.64076 Al(15)O(31) 0.3719 1.84206O (42) 0.675449 0.85678 Al(15)O(42) 0.4536 1.80488O (101) 0.532719 0.68454 Al(15)(101) 0.5657 1.74023bond Al (15)O (31). Therefore, cations as electron acceptors, such

1599X. Wu et al. / Fuel Processing Technology 91 (2010) 15911600Ca2+, Na+ etc. can easily enter into the crystal lattice of anorthite fromthe HOMO atoms, such as Al (15), O (31), O (42), and O (101) andcause the bond Al (15)O (31) to break.

3.6. The stability of mullite and anorthite cluster

The optimized structure of mullite crystal is shown in Fig. 7. It canbe seen that the mullite molecule cluster mainly consisted of one[SiO4]4-tetrahedral, one [AlO6]9-octahedron and three [AlO4]5-tetrahedrals. The Ca2+ as electron acceptor can easily enter into thelattice of mullite from oxygen atoms O (7), O (12), which connectwith Si (16) and cause a transformation from mullite to anorthite.Therefore, when mullite reacts with other minerals, which containCa2+ cations, the electrons that connect with oxygen in the HOMO ofmullite are prone to lean to the Ca2+ to reach the electric-charge

Fig. 8.HOMO and LUMOmaps of anorthite surface structure. (a) HOMOmap of anothitesurface structure. (b) LUMO map of anorthite surface structure.with Ca2+ in the [SiO4]4-tetrahedron is the weakest bond. Accordingto the mulliken atomic populations [41] in [SiO4]4-tetrahedron, theorder of that is O (91)bO (48)bO (78)bO (86), the atom of O (86),which connects with Ca2+ has the highest value of mulliken pop-ulation. The Ca2+ can capture the electron, which composes the Sorbital of bond ObrSi and causes the unstable state of anorthite.

4. Conclusions

(1) Coal ash melting temperature can be decreased by ashblending effectively. The phase equilibrium diagram can be used toexplain the phase transformation of ash mineral from mullite toanorthite. In order to decrease the AFT of high AFT coal, the optimummixture ratio of blended ash is near the boundaries or the triple pointsin CaOSiO2Al2O3 and FeOSiO2Al2O3 phase diagram.

(2)With increasing blending mass fraction of ash B, the location ofblended ash A/B in the ternary systems is transferred from the mulliteregion to the anorthite region, and the dominant crystal mineral ofblended ash A/B at around DT temperature is also transferred frommullite to anorthite. The calcium-bearing minerals, such as anhydrite,calcite etc., can react with mullite, which is a major refractory mineralin ash A, to form low-melting minerals (anorthite, fayalite andgehlenite etc.) in the temperature range from 1273 K to 1403 K andcause the AFT of blended ash A/B to become lower.When the blendingmass ratio of ash B reaches 80%, all of the mullite in ash A reacts withcalcium-bearing minerals in ash B and forms anorthite, In this case(80% ash B+20% ash A), the AFT of blended ash A/B is decreasedsharply.

(3) Based on quantum chemical calculation of the mullite andanorthite molecular structure, some fundamental information onmineral reaction between mullite and Ca2+ has been obtained. TheCa2+ as electron acceptor can easily enter into the crystal lattice ofmullite from O (7) and O (12), which connect with Si (16) and causethe rupture of bond Al (1)O (13) and Al (8)O (13). Finally, the Ca2+,which entered the mullite is located in the center of [SiO4]4-tetrahedron ring in anorthite. Taking the [SiO4]4-tetrahedron, whichis composed of Si (70), O (78), O (48), O (91), and O (86) as anexample, the Ca2+ can capture the partial electronics of O (86) andcause the B.L of bond Si (70)O (86) to become longer and unstable.

NomenclatureAFT Ash fusion temperatureASTM American society for testing and materialsB.L. Bond lengthB.O. Bond orderDT Deformation temperatureEDX Energy dispersive X-ray analyzerFT Flow temperatureHOMO Highest occupied molecular orbitsHT Hemispherical temperatureLUMO Lowest unoccupied molecular orbitsMP Melting pointN.C. Net chargebalance and nally cause the change of mullite crystal properties toform anorthite by the entrance of Ca2+.

As it can be seen from Fig. 7, the anorthite molecules mainlyconsisted of eight-membered [SiO4]4-tetrahedral rings and eight[AlO4]4-tetrahedrals. The Ca2+ is mainly located in the cavity of theeight-membered [SiO4]4-tetrahedral rings. Taking the [SiO4]4-tetrahedron, which is composed of Si (70), O (78), O (48), O (91),and O (86) (see Fig. 4) as an example, the B.O. of bond Si (70)O (86)in the [SiO4]4-tetrahedron is lower than that of Si (70)O (78),Si (70)O (48) and Si (70)O (91), as well as the B.L. of bond Si (70)O(86) is longer than that of Si(70)O(78), Si(70)O(48) and Si(70)O(91) (Table 2). Therefore, the bond of Si (70)O (86), which connectsPop. Population

SEM Scanning electron microscopyST Softening temperatureXRD X-ray diffraction

Acknowledgements

This research was partially supported by the National NaturalScience Foundation of China (50906055) and China PostdoctoralScience Foundation (20090450571) in China. The authors alsoacknowledge the project members and may people relevant to thisproject.

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Main mineral melting behavior and mineral reaction mechanism at molecular level of blended coal ash under gasification cond...IntroductionMaterial and methodsCoal sampleExperimental detailsQuantum chemistry calculation

Results and discussionFusibility of blended coal ash A/B during heating under gasification conditionsMain mineral transition behavior of ash A and B during heating under gasification conditionsMain mineral transition behavior of blended ash A/B during heating under gasification conditionsReaction activity of mullite clusterReaction activity of anorthite clusterThe stability of mullite and anorthite cluster

ConclusionsNomenclatureAcknowledgementsReferences