Effect of Residence Time on Coal Ash Behavior at High Temperatures in Reducing Atmosphere

Published: October 31, 2011

r 2011 American Chemical Society 5594 dx.doi.org/10.1021/ef201245x | Energy Fuels 2011, 25, 5594–5604

ARTICLE

pubs.acs.org/EF

Effect of Residence Time on Coal Ash Behavior at High Temperaturesin Reducing AtmosphereHaigang Wang,* Penghua Qiu, Yun Zhu, Shijun Wu, Wenhua Zhao, and Shaohua Wu

School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang, 150001,P.R. China

ABSTRACT: Two coals (one coal with high-melting temperature ash and one with low-melting temperature ash) were chosen toinvestigate the effect of residence time at high temperatures in a reducing atmosphere on coal ash behavior. Scanning electronmicroscopy linked with energy dispersive X-ray analysis (SEM-EDX) and X-ray diffraction (XRD) were used to characterize the ashbehavior. The residence time has a great influence on the amount and composition of minerals, especially for the high-meltingtemperature ash. The diffraction peak intensities for most minerals formed in the high-melting temperature coal ash after heattreatment reach their maximum value after 2 min, and then, they decrease. This trend is not apparent for the low-meltingtemperature ash. The high-melting temperature ash also has a higher mineral content than the low-melting temperature ash for thesame residence time. Formation of a large amount of mullite in the high-melting temperature ash after heat treatment indicates thatmullite formation is rapid, but a high (Na2O + K2O + CaO) content can inhibit the formation of mullite. Different mineralcompositions have been found in the ash samples under nonthermal equilibrium and thermal equilibrium conditions. Most mineralsformed under thermodynamic equilibrium conditions are not observed under the nonequilibrium conditions. This is attributed tothe low stability of minerals at high temperatures. The kinetic limitations of the dissolution of FeO particles must also be considered,especially for larger FeO particles.

1. INTRODUCTION

Integrated gasification combined cycle (IGCC) power plantsoffer improved power generation efficiency, as well as loweremissions of greenhouse gases and particulates into atmosphere,compared with conventional generation technologies. In en-trained flow gasifiers, coal particles are combusted, gasified,and entrained, and the minerals in the coal transform into ash.Some ash particles become trapped on the wall of the gasificationchamber. This series of processes takes a few seconds.1

Figure 1 shows slag deposit formation. The ash particles reachthe surface of the liquid slag layer first, and then the ash particlesmay be trapped at different positions such as positions 1 or 2, oreven position 3 deep in the layer, and eventually dissolve into theslag layer. It is possible that thermodynamic equilibrium is notreached immediately between trapped ash particles and the slag.As a result, the ash particles do not have enough time to dissolveinto the liquid slag. Whether or not the ash particles dissolve intothe tapped slag significantly influences the slag properties, such asfusibility of ash, physical properties, and viscosity. These proper-ties determine effective operation of the gasifiers.

To ensure that the slag flows continuously down the gasifierwalls, on the basis of phase equilibrium, some researchers haveconcentrated on characterizing slag properties, such as viscosity,2�4

the amount of fluxing material required,3�5 and the temperature ofcritical viscosity,2�4 and have developed various viscositymodels toaccurately predict the viscosity of slag.6�8

The use of thermodynamic computer software (FactSage) hasenabled significant improvements in predictive capabilities forthe behavior of minerals. Shannon et al.9 studied ash compositiondistribution by particle size and density to investigate its effect onmineral phase type formed at various temperatures in a reducingatmosphere by FactSage modeling. In an investigation targeting

the fixed bed gasification process, van Dyk et al.10 studied thetransformation of minerals at high temperatures, using FactSagesoftware, and found that the mineral species that are formedcontain a high number of oxygen molecules.

Some authors examined the melting behavior of coal ash inreducing atmosphere in a modified ash melting apparatus.Huffman et al.11 found that the atmosphere (reducing vs oxidizing)had considerable influence on the behavior of coal ash at hightemperatures and the mineral reactions in the oxidizing atmo-sphere were somewhat simpler than those in the reducingatmosphere. Ninomiya et al.12 pointed out that CaCO3 additivesare an efficient fluxing element for the control of ash melting,more particularly for the melting of the A12O3-rich ash.

Thermomechanical analysis (TMA) has emerged as a tech-nique that can characterize coal ash and slag behavior. Bryantet al.13 quantified calcium flux (CaO) levels necessary for theviscosity required for coal fired slagging gasifiers (15�25 Pa s)using TMA. Buhre et al.14 demonstrated that the technique couldrapidly provide an indication of the viscosity of slags at hightemperatures and could prove to be an alternative, cost-effectivetechnique, compared to current high-temperature ash sampleviscosity measurement techniques.

Most of the investigators mentioned above were devoted instudying ash or slag behavior under the thermodynamic equilib-rium conditions and gained some valuable conclusions. However,few investigations have focused on the effect of the residence timein a reducing atmosphere on coal ash behavior. The aim of thisstudy is to analyze towhat extent the residence time influences coal

Received: August 16, 2011Revised: October 26, 2011

5595 dx.doi.org/10.1021/ef201245x |Energy Fuels 2011, 25, 5594–5604

Energy & Fuels ARTICLE

ash behavior at high temperatures in a reducing atmosphere (moleratio CO/CO2 = 1.5). Scanning electron microscopy linked withenergy dispersive X-ray analysis (SEM-EDX) and X-ray diffraction(XRD) are used to characterize the resulting ash samples.

2. EXPERIMENTAL SECTION

2.1. Preparation and Analysis of Ash Samples. Two repre-sentative bituminous coal samples were selected for this study: one coalash was A-poor, and the other was A-rich (A = Na2O + K2O + CaO).Both coal samples were crushed into particles less than 97 μm indiameter. A-poor coal ash has a high-melting temperature compared tothat of the A-rich coal ash. The ash samples were prepared by ashing the

two coal samples in air in a muffle furnace at 600 �C for 24 h. Ashanalyses of the two coal samples are given in Table 1.2.2. Experimental Procedures. A schematic diagram of the

experimental setup is shown in Figure 2. The temperature at position1 rises to 1400 �C from room temperature at heating rate of 7 �C/min ina reducing atmosphere (mole ratio CO/CO2 = 1.5), and then, a ceramiccrucible loaded with ash sample is quickly placed at position 1. Theresidence time is set as 1, 2, 5, or 20 min. After that, the ceramic cruciblewas quickly extracted from position 1 and quenched in liquid nitrogen.

Some investigations15�17 found that a residence time of 30 min wassufficient to achieve equilibrium in the ash�slag system at fixed tempera-tures. In the present study, to investigate the differences between ashsamples heat-treated under non-thermodynamic equilibrium and thermo-dynamic equilibrium conditions, a longer residence time of 60 min wasused to allow the ash samples to reach thermodynamic equilibrium.The detailed procedure is as follows. The temperature at position 1rises to 1100, 1200, 1300, or 1400 �C from room temperature at heatingrate of 7 �C/min, and then, a ceramic crucible loaded with ash sample isquickly placed at position 1. The residence time was set as 60 min. Afterthat, the ceramic crucible was quickly extracted from position 1 andquenched into liquid nitrogen.

3. RESULTS AND DISCUSSION

3.1. Analysis of Melting Characteristics of Coal Ash. XRDpatterns of two coal samples are presented in Figure 3 and show

Figure 1. Slag deposit formation.

Table 1. Coal Ash Analyses

A-poor coal A-rich coal

ash content, % ad 3.05 7.69

AFT, �C, Reducing AtmosphereDT 1158 1130

ST 1311 1157

HT 1315 1162

FT 1316 1197

Ash Composition, wt%

SiO2 54.92 42.95

Al2O3 21.28 14.27

CaO 3.68 16.28

Fe2O3 13.31 10.49

MgO 1.12 0.96

K2O 0.26 1.4

Na2O 0.19 1.94

TiO2 2.18 0.84

P2O5 0.07 0.03

SO3 2 6.65

Total 99.01 95.81

SiO2/ Al2O3 2.6 3.0

Figure 2. Schematic diagram of experimental setup.

Figure 3. XRD patterns of the two coal samples. Minerals marked bynumerals 1�27 are given in Table 2.



that the two coal samples have significant differences in mineralcontent and composition. The minerals marked by numerals1�27 are given in Table 2. The A-rich coal contains more calciteand illite than the A-poor coal, which is consistent with the higherbulk (Na2O + K2O + CaO) content of its ash (Table 1). Therelatively high pyrite (Figure 3) content indicates that inorganicsulfur is mainly bound to Fe to form pyrite, while the main Fe-containing mineral in the A-poor coal is siderite. The obviousdiffraction peaks from anhydrite and hematite in the XRDpatterns of the two ash samples (Figure 4) show that thetransformations of calcite, pyrite, and siderite during the ashingprocess can be described by the following reactions:

8CaCO3 þ 4FeS2 þ 7O2 ¼ 8CaSO4 þ 2Fe2O3 ð1Þ

4FeCO3 þ O2 ¼ 2Fe2O3 þ 4CO2 ð2Þ

The XRD patterns of the two coal ash samples heat-treated at1400 �C for different residence times are given in Figures 5 (A-poor) and 6 (A-rich). The main minerals in the A-poor coal ashsample heat-treated at 1400 �C are mullite, sillimanite, alman-dine, (almandine + fayalite), hedengergite, and ferrosilite. Theeffects of the residence time on the intensities of main mineraldiffraction peak are shown in Figure 7. Great variations inintensity from 1 to 20 min indicate that the residence time hasa considerable influence on the amount of the minerals and alsoshow that, at 1400 �C, the A-poor coal ash sample takes morethan 20 min to reach thermodynamic equilibrium.As the residence time increases, the peak intensities for the

main minerals (except sillimanite) reach their maximum after2 min, and then, they decrease. In Figure 5 the occurrence of alarge amount of mullite after 1 min indicates that mullite

Table 2. Minerals Marked by Numerals 1-27

numerals minerals numerals minerals numerals minerals

1 quartz 10 mullite 19 cyclowollastonite

SiO2 3Al2O3 3 2SiO2 CaSiO3

2 calcite 11 almandite 20 grossular ferrian

CaCO3 3FeO 3Al2O3 3 3SiO2 Ca3(Al1.3325Fe0.6675)Si3O12

3 kolinite 12 hedengergite 21 fayalite

Al2Si2O(OH)2 Ca1.5Fe0.5(SiO3)2 2FeO 3 SiO2

4 illite 13 ferrosilite 22 trdymite

KAl4(SiAl)8O10(OH)4!4H2O FeSiO3 SiO2

5 pyrite 14 sillimanite 23 albite, ordered

FeS2 Al2O3 3 SiO2 NaAlSi3O8

6 siderite 15 anorthite 24 nepheline, potassian

FeCO3 CaAl2Si2O8 (K,Na)AlSiO4

7 anhydrite 16 hercynite 25 clinoferrosilite

CaSO4 FeO 3Al2O3 FeSiO3

8 hematite 17 sekaninatie 26 hedenbergite

Fe2O3 Fe2Al4Si5O18 CaFe (SiO3)29 wuestite 18 wollastonite-1A, ferrian 27 skiagite calcian

Fe0.942O CaFe(SiO)3 (Ca1.08Fe1.92)Fe2(SiO4)3

Figure 4. XRD patterns of coal ash samples prepared at 600 �C in air.Minerals marked by numerals 1�27 are given in Table 2.

Figure 5. XRD patterns of A-poor coal ash samples after heat treatmentat 1400 �C for different residence times. Minerals marked by numerals1�27 are given in Table 2.



formation is fast. The crystallization kinetics of mullite formationin the system Al2O3�SiO2 has been investigated by severalgroups.18�20 Their results show that mullite formation has a highactivation energy, but significant differences exist between thepublished results, which have been attributed to the different rawmaterials used to produce mullite. A-poor coal ash samplecontains higher flux contents than the raw materials (Al2O3 +SiO2 = 100%) used by these groups. The higher flux contents canreduce the activation energy of mullite formation. Mullite is thestable phase at high temperature.21 Pure mullite cannot melt at1400 �C unless it forms eutectic fields with other minerals.Therefore, the formation rate of the mullite eutectic fieldsdetermines the melting rate of mullite.The nonuniform spatial distribution of ash components can be

confirmed by comparing different regions of the SEM images onthe left of Figure 8. The white regions in the left-hand SEMimages in Figure 8 are made up of small crystals. The highermagnification views of these regions are shown on the right ofFigure 8. According to EDX analysis, these crystals are Fe�Caaluminosilicates, and their Fe content exceeds their Ca content.According to crystallization kinetics, the total rate of crystalformation is a product of nucleation rate and growth rate inFigure 9.22 Tg is the temperature of the maximum crystal growthrate. The temperature at Tg helps the growth of crystals. Tn is thetemperature of the maximum crystal nucleation rate. In thisstudy, at 1400 �C, the ash sample is most likely located in the hightemperature metastable area. Therefore, small sized crystalsare found.On the basis of the above information, the formation rate of

the mullite eutectic fields is possibly controlled by ion diffusion.These ions are from the minerals that can form eutectic fieldswith mullite. The ion diffusion rate depends on the viscosity ofthe high temperature ash sample and the concentration gradientsof the ions. Even if the A-poor coal ash sample is fully liquid, thelow content of fluxes (Na2O + K2O + CaO + FeO) means that itcould still have high viscosity; furthermore, for the short resi-dence times, large amounts of suspended crystalline phases alsoincrease its viscosity at high temperatures. The influence of thesuspended crystalline phase on the viscosity has been previouslydiscussed.23,24

Figure 10 shows some eutectic fields in the relevant phasediagrams.25,26 The main minerals that can form eutectic fieldswith mullite are anorthite, hercynite, and sekaninaite. However,

only small amounts of anorthite, hercynite, and sekaninaite wereidentified in Figure 5. The low concentrations of these mineralsmay lead to their small ion concentration gradients. The smallconcentration gradients result in the low overall ion transporta-tion rate. These factors slow down the diffusion of the ions. Withincreasing residence time, the formation rate of the mulliteeutectic fields gradually increases, and then, mullite turns intothe aluminosilicate melt.The intensity variations with increasing residence time for

almandine, ferrosilite, hedenbergite, and mullite are quite similar(Figure 7). However, there are no primary phase fields for theseminerals in the phase diagrams in Figure 10. This means that themelting of these minerals does not primarily depend on theformation of associated eutectic fields but on their own meltingcharacteristics. Therefore, a high content of mullite and mineralsthat cannot form eutectic fields causes the high melting tem-perature of the A-poor coal ash. At high temperatures, sillimaniteis not stable and can convert into mullite.27 As can be seen fromFigure 5, the residence time has obvious effects on the mineralcomposition of the A-poor coal ash sample. For example, newdiffraction peaks occur at 2θ = 46.95� and 69.87� after 2 and5 min, respectively.The XRD patterns of Figures 5 and 6 are very different,

indicating that the content and compositions of minerals formedin the heat-treated A-rich coal ash samples differ greatly fromthose formed in the A-poor coal ash samples under the sameheat-treated conditions. Overall, a lower content of minerals isfound in the A-rich coal ash sample compared with the A-poorcoal ash sample heat-treated under the same conditions. Becauseof the high content of (Na2O + K2O + CaO), no mullite isidentified in the A-rich coal ash sample. Alkaline metal andalkaline earth cations are able to enter the mullite structure.20,28

The following reactions may proceed:

3Al2O3 3 2SiO2 þ R2OðNa2O, K2OÞ

f

Na2O 3Al2O3 3 2SiO2

K2O 3Al2O3 3 2SiO2

K2O 3Al2O3 3 4SiO2

8>><>>:

9>>=>>; þ 2Al2O3 ð3Þ

3Al2O3 3 2SiO2 þ CaO f CaO 3Al2O3 3 2SiO2 þ 2Al2O3

ð4ÞTherefore, the main minerals found in the A-rich coal ash heat-treated at 1400 �C for 1 min are anorthite, nepheline, and albite.Similarly, Bai et al.29 also did not observe the formation ofmullite in the ash samples heat-treated at 1300 �C for 3, 5, 10,15, 20, and 25 min. The content of (Na2O + K2O + CaO) in theash sample used in their study was more than 36% by mass.From Figure 6, the minor minerals are clinoferrosilite, heden-bergite, and skiagite calcian, and most minerals have turned intoaluminosilicate melt with 2 min; however, peaks from smallamounts of newminerals can be found at 2θ = 19.90, 33.51, and69.38�. A shorter residence time will be needed to investigatethe influence of residence time on the A-rich coal ash behaviorat high temperatures.The XRD patterns of the two coal ash samples heat-treated with

long residence times (thermodynamic equilibrium states) and atshort residence times (nonequilibrium states) are collected inFigures 11 and 12. Figure 11 shows that, for the A-poor coal ashsample,manyminerals formedunder the thermodynamic equilibrium

Figure 6. XRD patterns of A-rich coal ash samples after heat treatmentat 1400 �C for different residence times Minerals marked by numerals1�27 are given in Table 2.



conditions are almost absent in the A-poor coal ash sample heat-treated at 1400 �C under the nonthermodynamic equilibriumconditions: anorthite (2θ = 21.80�), almandite (2θ = 43.20�),and hercynite (2θ = 58.84�), for example. This effect is also seenin the A-rich coal ash sample (Figure 12), with anorthite (2θ =23.53�), Skiagite, aluminian (2θ = 42.11�), and fayalite (2θ =49.40�) present in the thermodynamic equilibrium state. Thisindicates that these minerals are not stable at high temperatures,even for a short residence time. That is to say, the melt rate ofthese minerals is far greater than their formation rate. For theA-poor coal ash sample, a few minerals, such as grossular, ferrian

and wollastonite-1A, ferrian, which formed under the nonequili-brium conditions, were not identified under the equilibriumconditions. Grossular, wollastonite, and anorthite can be con-verted to one another.30,31 Generally, wollastonite is a transientphase.32 Occurrence of grossular may be because of the nonuni-form spatial distribution of components.33 However, for theA-rich coal ash sample, this effect is less than for the A-poor coalash sample.3.2. Estimation of Dissolution Time of an FeO Particle.

Examples of FeO particles formed under equilibrium conditionsand nonequilibrium conditions are shown in Figures 13�15.

Figure 7. XRD intensities vs residence time for different minerals.



These FeO particles are likely derived from FeS2 particles because asmall amountof sulfurwas identifiedbyEDX inFigures 13�15. Someof theFeS2 in theFeS2 particlewas not oxidized intoFe2O3 during theashing process. When the ash sample was suddenly exposed to hightemperature, severe escape of sulfur gas from the decomposition ofFeS2 inside theFe2O3particle resulted in the formationof a pit or porein the middle of the FeO particle. Srinivasachar et al.34 found thatcompletely decomposed pyrite particles exhibited massive fissure,facilitating the possibility of fragmentation.Fe�S has very low solubility in aluminosilicate melt, and Fe�S

must oxidize to FeO before its coalescence with aluminosilicates.35

Therefore, themelting of pyrite into aluminosilicate melt is roughlydivided into two steps. First, pyrite is oxidized to FeOn. Second,FeOn melts into the aluminosilicate melt. ten Brink et al.36

investigated the mechanism of slag formation by using mineralsfrom pyrite-rich coals. They found that a certain time is required fordissolution of the deposited particles into the slag. Therefore, it is

important to determine the dissolution time of FeOn. The classicalshrinking core model37 is used to estimate the dissolution time ofthe FeO particle. A schematic of particle dissolution into the liquidphase in the case of boundary layer and surface reaction control isshown in Figure 16. Using this model, the dissolution kinetics forsurface reaction and boundary layer diffusion in the liquid phasein the Stokes regime can be given by

RR0

� �� 1 ¼ kCFeObulk

R0FFeOt ð5Þ

RR0

� �2

� 1 ¼ 2bDðCFeObulk � CFeOinterf ÞR20FFeO

t ð6Þ

where R is the radius of particle (m), R0 is the initial radius ofparticle (m), k is the surface reaction rate constant (m/s),D is the

Figure 8. SEM images of the spatially nonuniform crystal distributionfor A-poor coal ash samples heat-treated at 1400 �C for differentresidence times.

Figure 9. Relationship among temperatures, nucleation rate, andgrowth rate.22

Figure 11. XRD patterns of A-poor coal ash samples heat-treated forlonger times (thermodynamic equilibrium states) and short times(nonequilibrium states).

Figure 10. Some eutectic fields in relevant phase diagrams.



diffusion coefficient of FeO in the liquid phase (m2/s), FFeO is theparticle density (kg/m3), t is time (s), and b is the stoichiometriccoefficient. (CFeObulk � CFeOinterf) is the difference between theconcentrations of FeO in the bulk ash sample and at the particle�boundary layer interface (kg/m3).In Figures 13 and 14, SEM-EDX analysis shows that the crystals

surrounding the FeO particle are richer in Fe relative to the bulk ashcomposition in Table 1. Occurrence of Fe-rich crystals indicates thatthe local liquid phase has already reached saturation. For the FeOparticle to dissolve into the liquid phase, Fe2+ must diffuse awayfrom the particle to decrease the local Fe2+ concentration. Thisindicates that the FeO particle dissolution is primarily controlled

by diffusion at high temperatures. Some researchers38�40 foundthat the rate controlling mechanism of the dissolution of otheroxide particles (MgO, CaO, and Al2O3) was boundary layerdiffusion in the molten phase at high temperatures. Therefore,the boundary layer dissolution mechanism (eq 6) is employed tocalculate the dissolution time of the FeO particle. According toeq 6, the dissolution time in seconds is

t ¼ R20FFeO

2bDðCFeOinterf � CFeObulkÞ ð7Þ

Some investigators41�43 have found a good correspondencebetween the predictions of the Eyring equation and experimen-tally derived diffusivities in silicate melts. Therefore, the diffusioncoefficient may be obtained by the Eyring relation:

D ¼ kTηλ

ð8Þ

where k is the Boltzmann constant (K/J), T is temperature in K,η is bulk viscosity (pa s), and λ is the jump distance (m).Generally, λ is approximated by the diameter of the diffusion ion(Fe2+). The density and viscosity of the liquid phase areestimated by the method proposed by Mills et al.44 and Kalma-novitch et al.45 According to the XRD patterns in Figure 12, themain Fe-bearing minerals are hedengergite (CaFe(SiO3)2).Therefore, the following dissolution process for FeO particlesis suggested:

FeO þ CaO þ 2SiO2 ¼ ½CaFeðSiO3Þ2�crystal ð9Þ

½CaFeðSiO3Þ2�crystal ¼ ½Ca2þ þ Fe2þ þ 2SiO34þ�slag

ð10Þ

Figure 13. Typical FeO particle found in the ash sample treated at1100 �C for 60 min.

Figure 14. Typical FeO particle found in the ash sample treated at 1200�C for 60 min.

Figure 12. XRD patterns of A-rich coal ash samples heat-treated forlonger times (thermodynamic equilibrium states) and short times(nonequilibrium states).



The FeO dissolution reaction can be represented in simpleterms by

FeO ¼ ½FeO�slag ð11ÞTherefore, the stoichiometric coefficient, b, for the FeO

particle dissolution reaction as written is 1 in eq 7.Under the assumption of local equilibrium, the interfacial

concentration (CFeOinterf) at the particle�boundary layerinterface is determined using the quaternary SiO2�Al2O3�CaO-FeO phase diagram. Table 1 shows that the SiO2/Al2O3

mass ratio in the A-rich coal ash sample is about 3. Therefore,in this study, a quaternary SiO2�Al2O3�CaO-FeO systemwith SiO2/Al2O3 mass ratio = 2.726 was chosen to determineinterfacial composition. A schematic of the determination of

the interfacial composition between the FeO particle andthe ash�slag system at a certain temperature is shown inFigure 17. In Figure 17, the composition of the A-rich coal ashsample is at point P. The point of intersection of the isothermand the isopleth representing composition with iso-((SiO2+Al2O3)/CaO) mass ratio is the saturation composition be-tween the FeO particle and the ash�slag system at a certaintemperature: for example, points Q and M. This saturationcomposition corresponds to the interfacial composition at the

Figure 15. SEM-EDX analysis of a typical FeO particle in an ash sample treated at 1400 �C for 1 min.

Figure 17. Schematic of the determination of the melt saturationcomposition for the dissolution of FeO in the bulk melt.

Figure 16. Schematic overview of diffusion control dissolution regimeand surface reaction control regime.



particle�boundary layer interface, which is used to calculateinterfacial concentration.The heat treatment temperature of 1400 �C is a little higher

than themelting point of crystalline FeO. In Figure 15, small FeOcrystals are merging to form large FeO crystals. This will reducethe total particle boundary area and the associated free energy,46

which indicates that the FeO crystals are stable over a shortresidence time. In Figure 17, point N (the FeO corner) isconsidered as the interfacial composition at the particle�bound-ary layer interface at 1400 �C. That is to say, the interfacialconcentration (CFeOinterf) at the particle�boundary layer inter-face is equal to its density value (FFeO). Equation 7 is expressed as

t ¼ R20FFeO

2DðFFeO � CFeObulkÞ ð12Þ

The diameter of the FeO particle is easily determined from theSEM image in Figure 15a. Its diameter is about 140 μm, which islarger than the maximum diameter of the coal particles (97 μm).This indicates that the fragmentation caused by severe escape ofsulfur gas results in an increase in FeO particle bulk volume andtherefore a decrease in FeO particle density. However, it is

difficult to determine the diameter of the FeO particle beforefragmentation.Therefore, the FeO particle described in Figure 15a is a result

of fragmentation of an FeO particle of unknown diameter (seeFigure 18 for some possibilities). The dissolution times of theFeOparticle before and after fragmentation and expansion to 140μm are shown in Table 3. From Table 3, when an FeO particle with70 μm diameter is fragmented to 140 μm diameter, its dissolutiontime exceeds 20 min. At 1400 �C, although the FeO crystals existfor short residence times, that is, those described in Figure 15, forlonger times, the FeO crystals possibly form liquid FeO. Therefore,it is difficult to identify liquid FeO by XRD or SEM-EDX. Toidentify whether or not the FeO exists in the ash sample heat-treated at 1400 �C for 20 min, a crystallization experiment wascarried out to make FeO crystallize from the liquid FeO that hadnot been incorporated into the aluminosilicate melt.The following experimental procedure was designed. The

temperature at position 1 rises to 1400 �C from room tempera-ture at heating rate of 7 �C/min, and then, the temperature fieldwas measured to find the position at which the temperature was1200 �C, denoted as position 2. Then, the ceramic crucibleloaded with ash sample is quickly placed at the position 1(1400 �C) for 20 min. After that, the ceramic crucible wasquickly moved to the position 2 (1200 �C) for an additional30 min. Finally, the ceramic crucible with ash sample was quenchedin liquid nitrogen. The SEM-EDX analysis of the resultingsample is shown in Figure 19. It demonstrates the existence ofFeO in the ash sample heat-treated at 1400 �C for 20 min.In actual gasifiers, the size of an FeO particle should corre-

spond to that of an FeO fragment. The FeO fragments are farsmaller than the FeO particles described in Figures 13�15, andthe FeO fragments may impact slag film at a significant velocity.Under the action of certain forces (capillary force, buoyancy, dragforce, and fluid-addedmass force) the FeO fragment may travel adistance into the slag layer and then stay at a specific position inthe slag layer, where the slag may be partly crystallized, asdescribed in Figure 1. Therefore, the dissolution time of theFeO particles with small size in slags at different temperaturesmust be calculated to investigate the effect of these factors on thedissolution time. The modified Urbain model is applicable forcompletely liquid slag. The effect of a solid phase suspended inthe liquid phase can be accounted as an effective viscosity. Theeffective viscosity, ηe, is a function of the viscosity of the pureliquid, η, and volume fraction of solid particles (crystallineparticles), θ.

ηe ¼ ηð1 þ cθ þ dθ2Þ ð13Þ

Table 3. Comparison of the Dissolution Times of FeOParticles before Fragmentation and Expansion and the Cor-responding FeO Particles Once Fragmented and Expanded to140 μm

diam before

frag (μm)

dissoln time before

frag (min)

dissoln time after

frag (min)

60 2.5 45.1

70 3.5 23.7

80 4.5 18.7

97 6.7 15.8

Figure 19. SEM-EDX analysis of the ash sample heated-treated incrystallization experiment.

Figure 18. FeO particle expansion upon fragmentation and diameterincrease to 140 μm.



where c and d are constants. The values of c and d were derivedfrom following theory: c = 2.5 and d = 7.349.44 Some referenceshave discussed accuracy of the viscosity calculation.47,48

The calculated results are shown in Figure 20. As can be seenfrom Figure 20a, the dissolution time of the FeO fragment is farless than that of the FeO particle described in Figure 15, but thedissolution time increases sharply with increasing particle size.For siderite (FeCO3)-rich coal, the fragmentation of siderite didnot occur at any heating rates.49 Therefore, an FeO particlederived from a siderite particle is similar in size to the parentsiderite particle. It is inevitable that the dissolution time of anFeO particle derived from a siderite particle is far longer than thatfor an FeO particle derived from a pyrite particle under the sameconditions. This indicates that kinetic limitations should beconsidered when studying melting characteristics and viscosity�temperature characteristics of coal ash, especially for siderite-richcoal. In Figure 20b, the great reduction in the dissolution time ofthe FeO particle with increasing the heat treatment temperatureis mainly attributed to a decrease in the viscosity and an increasein the interfacial concentration.

4. CONCLUSIONS

The main results from this study are summarized below:1. The residence time has a great influence on the amount and

the composition of minerals, especially for the high-melting

temperature coal ash. For most minerals, the amountformed during the heat-treatment of high-melting tempera-ture coal ash increases to a maximum and then drops down.This trend is not obvious in the low-melting temperaturecoal ash. The high melting coal ash sample has a highermineral content than the low-melting temperature ashsample for the same residence time.

2. The formation rate of mullite is fast at high temperatures,but high (Na2O + K2O + CaO) content can inhibit theformation of mullite. At high temperatures, the high-melting temperature ash requires a longer time than the low-melting temperature ash sample to reach thermodynamicequilibrium.

3. The mineral compositions formed under the thermody-namic equilibrium and nonequilibrium conditions are verydifferent. Most minerals formed under the thermodynamicequilibrium conditions were not identified under the none-quilibrium conditions. This was attributed to the poorstability of many minerals at high temperatures.

4. The experimental results together with calculations basedon the classical shrinking core model show that the kineticsof the dissolution of the FeO particles must be considered,especially for FeO particles with large sizes.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +86-451-86412618, ext 859. Fax: +86-451-86412528.E-mail: [email protected].

’ACKNOWLEDGMENT

This research was supported by National High TechnologyResearch andDevelopment Programof China (2007AA05Z246).

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Effect of Residence Time on Coal Ash Behavior at High Temperatures in Reducing Atmosphere