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EFFECT OF CHEMICAL COMPOSITION ONSINTERING BEHAVIOR OF JINCHENG COALASH UNDER GASIFICATION ATMOSPHERENijie Jing a , Qinhui Wang a , Zhongyang Luo a & Kefa Cen aa State Key Laboratory of Clean Energy Utilization, ZhejiangUniversity , Hangzhou , Zhejiang , ChinaPublished online: 02 Nov 2011.

To cite this article: Nijie Jing , Qinhui Wang , Zhongyang Luo & Kefa Cen (2012) EFFECT OF CHEMICALCOMPOSITION ON SINTERING BEHAVIOR OF JINCHENG COAL ASH UNDER GASIFICATION ATMOSPHERE,Chemical Engineering Communications, 199:2, 189-202, DOI: 10.1080/00986445.2011.582531

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Effect of Chemical Composition on SinteringBehavior of Jincheng Coal Ash Under

Gasification Atmosphere

NIJIE JING, QINHUI WANG, ZHONGYANG LUO,AND KEFA CEN

State Key Laboratory of Clean Energy Utilization, Zhejiang University,Hangzhou, Zhejiang, China

In order to obtain detailed understanding of coal ash sintering behavior, a series ofexperiments have been done on the effect of Jincheng coal ash composition on sin-tering temperature under gasification atmosphere by the pressure-drop technique.The mineral transformations within its sintering temperature range are simulatedby the thermodynamic equilibrium calculation module of FactSage to better under-stand the experiment results. These results show that the sintering temperaturedecreases initially, and then rises with increasing SiO2. On the whole, the sinteringtemperatures decrease with increase in Fe2O3, CaO, Na2O, and K2O. Their effectsare quite similar, but the degrees are different. As MgO increases, the sintering tem-perature decreases. However when MgO is increased above a certain limit, the effectof MgO on the sintering temperature is not obvious.

Keywords Ash fusibility; FactSage thermodynamic modeling; Mineraltransformation; Sintering temperature

Introduction

Coal ash characteristics (such as fusibility and sintering behavior) as a coal source–specific property provide additional information on suitability for combustion andgasification purposes. Therefore, chemical and mineral interactions must be under-stood in order to determine suitability for fluidized bed gasification purposes withregards to ash characteristics.

Numerous articles (Ninomiya and Sato, 1997; Reifenstein et al., 1999; Yanget al., 2007; Bai et al., 2008; van Dyk et al., 2009) on minerals in coal ash have beenpresented and revealed that behavior of minerals in coal ash has great influence onthe ash characteristics. However, the influence of minerals on the ash characteristicsis determined by the reactions between the principal oxides frequently found in coalash (Vassilev et al., 1995), i.e., SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, and K2O.Also, the results from previous experiments by the pressure-drop technique (PDT)(Al-Otoom et al., 2000, 2005) revealed that the composition of coal ash affected

Address correspondence to Qinhui Wang, State Key Laboratory of Clean EnergyUtilization, Zhejiang University, 310027, Zheda Rd. 38, Hangzhou, Zhejiang, China. E-mail:qhwang@zju.edu.cn

Chem. Eng. Comm., 199:189–202, 2012Copyright # Taylor & Francis Group, LLCISSN: 0098-6445 print=1563-5201 onlineDOI: 10.1080/00986445.2011.582531

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the mineral interactions and transformations. But the experimental methods areunable to predict quantitatively the mineral transformations in coal ash.

However, progress in chemical thermodynamics as well as the development ofcomputational methods, computer software, and hardware make it possible topredict the phase equilibrium in multicomponent coal ash systems (Li et al., 2006).Several studies (Jak et al., 1998, 2000; Bryant et al., 2000; Onderwater et al., 2000)on the prediction of minerals have used the thermodynamic model. For example,van Dyk et al. (2006) discussed mineral matter transformation during Sasol-Lurgifixed-bed dry bottom gasification by means of high-temperature X-ray diffraction(HT-XRD), in combination with FactSage modeling. In order to understand theash behavior of Huainan coals, Li et al. (2006) used FactSage modeling to predictthe reactions occurring between minerals, in combination with ash fusion tempera-ture (AFT) and X-ray diffraction (XRD). Jak (2002) applied the F�A�C�T softwarepackage to predict coal ash fusion temperature and discussed the mineral matterinteractions at high temperature. FactSage (Bale et al., 2002) is a powerful self-teaching aid with a series of information, database, and calculation componentsand enables one to manipulate pure substances and solution database. Also, the ther-modynamical equilibrium calculation procedure is based on Gibbs energy minimiza-tion. The computer software package FactSage can be used to calculate thetemperatures corresponding to different proportions of the liquid phase and predictthe phase equilibria of synthetic ash samples (Song et al., 2009).

Although some work has been conducted on the prediction of minerals, littlework has been systematically done on the effect of composition of Jincheng coalash on mineral phases formed in the sintering temperature range, in combinationwith FactSage modeling.

The primary purpose of this study is to investigate the effect of ash compositionon sintering temperature under gasification atmosphere by the pressure-drop tech-nique and also to study the mineral transformations with ash composition withinthe sintering temperature range through FactSage modeling based on Gibbs energyminimization.

Experimental and Procedure

Preparation of Ash Samples

As a typical anthracite in China, Jincheng coal is used as the experimental material.The ash chemical composition is shown in Table I. Coal samples were prepared byashing Jincheng coal in a muffle furnace at 820�C for 2 h according to the Chinesestandard (GB=T212-2001). Coal ash containing SiO2, Fe2O3, MgO, CaO, Na2O,and K2O was created by mixing the pure chemical samples (SiO2, Fe2O3, MgO,CaO, Na2CO3, and K2CO3) into the ash samples and then grinding the samples in

Table I. Ash composition analysis

Composition, %

Ash sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3

Jincheng 48.1 31.84 5.9 4.96 0.98 1.14 1.09 3.07

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a mortar for homogeneous mixing. After that the mixtures were heated to 815�C for1 h in the muffle furnace.

Experiments by Pressure-Drop Technique

A schematic diagram of the experimental setup is presented in Figure 1. Thepressure-drop measuring device consists of a high-temperature tube furnace, a mea-suring device, gas system, and heating system. In the experiments, the gasificationatmosphere shown in Table II is used. The mixed gas is passed into the reactor fromthe gas system at a flow rate of 5mL=min controlled by the mass flow meter. A coalash cylindrical pellet located in the quartz tube (1000mm long, 8mm ID) is heatedfrom ambient temperature at 10�C=min in a horizontal tube furnace and reacts withthe gases. This produces a pressure drop between the two ends of the ash pellet whenthe gas passes through it. Pressure drop due to the increase of reaction temperature iscontinuously recorded during the experiment. It decreases greatly when the sinteringprocess of ash particles occurs. The variation trend of pressure-drop is shown inFigure 2. The sintering temperature is designated as the point at which the pressuredrop reaches a maximum. The pressure-drop technique can measure the samplesquickly and sensitively with a reproducibility of �9�C, which is in agreement withthe literature (Al-Otoom et al., 2000).

Thermodynamic Modeling

To understand the experimental results better, the FactSage (5.2) thermodynamicalequilibrium module was used to predict proportions of mineral phase in specifiedtemperature and atmosphere. The calculation was conducted by varying the contentsof ash composition under atmospheric pressure during the sintering temperaturerange from 600 to 900�C.

The experiment parameters were used as the initial calculation conditions for theequilibrium module. The ash composition of SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O,

Figure 1. Experimental setup of pressure-drop sintering technique (PDS): 1, temperature con-troller (temperature control thermocouple); 2, measured temperature thermocouple; 3, tubefurnace; 4, quartz tube; 5, U-tube manometer; 6, flow meter.

Table II. Gas composition of gasification atmosphere

Gas composition, %vol.

Atmosphere H2 CO CH4 CO2

Gasification 40.8 36.5 1.92 20.8

Sintering Behavior of Jincheng Coal Ash 191

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K2O, and SO3 was input into the reaction table of the equilibrium module of Fact-Sage software. The atmosphere used in the calculation is the typical gasificationatmosphere (the gas composition is shown in Table II). The pressure was set atatmospheric pressure, and the initial and final temperatures were set at 600�C and900�C, respectively.

Results and Discussion

Effect of SiO2 on Sintering Temperature

Figure 3 shows that the sintering temperature varies with SiO2 content under thegasification reaction atmosphere. As seen in Figure 3, the sintering temperature

Figure 3. Effect of SiO2 on sintering temperature. (Figure provided in color online.)

Figure 2. Variation of pressure-drop with temperature. (Figure provided in color online.)

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decreases initially (from 870�C at 48.1% to 845�C at 50% SiO2), and then rises whenthe SiO2 content is higher than 60% (from 850 to 880�C). The reason can be visua-lized by means of Figure 4, where the mineral is changed in composition. Hercynite(FeAl2O4) decreases and disappears at 60% SiO2. Meanwhile, ferrocordierite(Fe2Al4Si5O18) increases when the content of SiO2 ranges from 48.1% to 60%, andlevels out at the SiO2 content >60%. These iron-based compounds (such as hercyniteand ferrocordierite) may produce low-temperature eutectics under the gasificationatmosphere (McLennan et al., 2000) and hence reduce the sintering temperature.Also, the high-temperature minerals, such as mullite and tridymite, change a littlefrom 48.1% to 60% SiO2. Therefore, the sintering temperature reduces initially.

However, the rise in the sintering temperature is due to increasing SiO2 contentabove a certain limit. This leads to a condition where there are no available oxides toreact with more free SiO2 to produce low-melting eutectics. As can be seen fromFigure 4, hercynite disappears and then ferrocordierite reaches a certain value andkeeps constant, while the tridymite phase still increases when the content of SiO2

is >60%, which increases the sintering temperature with the melting temperatureof 1680�C. Therefore, the sintering temperature rises again but only a little withthe increase in SiO2 content.

The reactions between them are as follows during the gasification process:

Fe2O3 ! FeO ð1Þ

FeOþ Al2O3 ! FeO � Al2O3ðhercyniteÞ ð2Þ

2ðFeO � Al2O3ÞðhercyniteÞ þ 5SiO2 ! Fe2Al4Si5O18ðferrocodieriteÞ ð3Þ

Figure 4. FactSage modeling results on the mineral transformation with SiO2 content. (Figureprovided in color online.)

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2FeO þ 2Al2O3 þ 5SiO2 ! Fe2Al4Si5O18ðferrocodieriteÞ ð4Þ

Effect of Fe2O3 on Sintering Temperature

The effect of Fe2O3 content on the sintering temperature is given in Figure 5.With an increase in Fe2O3 content, the sintering temperatures decrease dramati-cally. The main reasons are that Fe2O3 can be converted into FeO in the gasifica-tion atmosphere, and Fe2þ may react with other oxides to form liquid phases(Song et al., 2009), thereby decreasing the sintering temperatures. Meanwhile FeOappears to react with CaO, SiO2, and Al2O3 to produce the low-temperatureeutectics. So the sintering temperatures decrease dramatically as the Fe2O3 contentincreases.

The FactSage simulation result is shown in Figure 6. As the content of Fe2O3

increases, the hercynite phase starts to increase. Ferrocordierite (Fe2Al4Si5O18)increases at first and then decreases when Fe2O3 ranges from 15% to 35%. Mullite(Al6Si2O13) disappears at 15% Fe2O3, while fayalite (2FeO � SiO2) appears at 15%Fe2O3. The reactions between these minerals are expected to occur as shown inEquations (5) and (6). The minerals such as fayalite and hercynite are iron-basedcompounds that produce low-temperature eutectics to decrease the sintering tem-perature. As a result, the higher the content of fayalite and hercynite, the lowerthe sintering temperature. Such a trend is obvious in Figure 5.

3Al2O3 � 2SiO2ðmulliteÞ þ FeOþ SiO2 ! Fe2Al4Si5O18 þ FeO � Al2O3ðhercyniteÞð5Þ

Fe2Al4Si5O18 þ FeO ! 2FeO � SiO2ðfayaliteÞ þ FeO � Al2O3ðhercyniteÞ ð6Þ

Figure 5. Effect of Fe2O3 on sintering temperature. (Figure provided in color online.)

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Effect of CaO on Sintering Temperature

As can be seen from Figure 7, the sintering temperatures decrease under the gasifi-cation atmosphere as the CaO content increases. Cen et al. (1994) studied the vis-cosity curve trend with the CaO content and found that the viscosity of the coal

Figure 6. FactSage modeling results of the mineral transformations with Fe2O3 content.(Figure provided in color online.)

Figure 7. Effect of CaO on sintering temperature. (Figure provided in color online.)

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ash decreased as the CaO content increased, resulting in a decrease in the sinteringtemperature. According to the FactSage simulation result shown in Figure 8, mulliteand Fe2Al4Si5O18 disappear gradually, while anorthite (CaAl2Si2O8) and gehlenite(2CaO �Al2O3 � SiO2) increase with the increase in CaO content by way of the reac-tions shown in Equations (7)–(9). Anorthite and gehlenite have significant effects onthe fusion characteristics, since they will produce a low-melting eutectic mixture withquartz and calcium aluminum acid, which reduces the ash softening temperature(Qiu et al., 1999), thereby decreasing the sintering temperature.

CaOþ SiO2 þ Al2O3 ! CaO � Al2O3 � 2SiO2ðanorthiteÞ ð7Þ

3Al2O3 � 2SiO2ðmulliteÞ þ CaO ! CaO � Al2O3 � 2SiO2ðanorthiteÞ ð8Þ

CaO � Al2O3 � 2SiO2ðanorthiteÞ þ CaO ! 2CaO � Al2O3 � SiO2ðanorthiteÞ ð9Þ

Effect of MgO on Sintering Temperature

Figure 9 indicates that the sintering temperature varies with increase in MgO con-tent. It can be seen that the sintering temperature decreases as the MgO contentincreases. According to the FactSage modeling result presented in Figure 10, asthe content of MgO increases, mullite (Al6Si2O13) decreases linearly while cordierite(Mg2Al4Si5O18) increases. This is caused by the additive MgO reacting with the freeAl2O3 and SiO2 (Equation (10)). Fe2Al4Si5O18 decreases linearly and hercynite phasestarts to increase linearly accordingly. This is due to Fe2Al4Si5O18 converting tothe hercynite gradually (Equation (11)). Under the gasification atmosphere,

Figure 8. FactSage modeling results of the mineral transformations with CaO content. (Figureprovided in color online.)

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Fe2Al4Si5O18 and hercynite, as iron-based compounds, reduce the ash sinteringtemperatures.

However it is noted (in Figure 9) that when increasing the MgO content above acertain limit, the effect of MgO on the sintering temperatures is not significant. This

Figure 10. FactSage modeling results of the mineral transformations with MgO content.(Figure provided in color online.)

Figure 9. Effect of MgO on sintering temperature. (Figure provided in color online.)

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is because there is not more formation of the iron-containing minerals with increasein MgO content. Hence the sintering temperature does not vary apparently with theincrease of MgO.

2MgOþ 2Al2O3 þ 5SiO2 ! Mg2Al4Si5O18ðcordieriteÞ ð10Þ

Fe2Al4Si5O18ð2FeO � 2Al2O3 � 5SiO2Þ ! FeO � Al2O3ðhercyniteÞ ð11Þ

Effect of Na2O on Sintering Temperature

As seen in Figure 11, the sintering temperature decreases with increase in Na2O con-tents in the typical gasification atmosphere. The Na2O content affects the sinteringtemperatures significantly, since nepheline (Na2O �Al2O3 � 2SiO2), a typical basicmineral with low fusibility, is formed as a reaction product from Na2O, Al2O3,and quartz (Equation (12)). From the FactSage modeling result shown inFigure 12, mullite and Fe2Al4Si5O18 decrease smoothly with an increase in Na2Ocontent, while hercynite phase starts to increase linearly and keeps constant atNa2O content >4%. Albite (NaAlSi3O8) increases as Na2O content increases andthen decreases, whereas nepheline (Na2O �Al2O3 � 2SiO2) is present and increases lin-early. This is a result of Na2O decomposed from albite in reaction with SiO2 andAl2O3 to produce nepheline (Equation (12)).

As mentioned above, nephiline and Na2SO4 are fusible phases. Also the ash withNa silicates and alkaline sulfates has high mobility and solution activity of the melt.This active liquid absorbs and dissolves the remaining refractory minerals very rap-idly (Vassilev et al., 1995). Accordingly, the sintering temperature of the ash withnepheline and alkaline sulfates is lower. Although the Na2O content is very low, it

Figure 11. Effect of Na2O on sintering temperature. (Figure provided in color online.)

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can be seen that a change in Na2O content may have a significant influence (van Dykand Waanders 2007) on the sintering temperature.

Na2Oþ Al2O3 þ SiO2 ! Na2O � Al2O3 � 2SiO2ðnephelineÞ ð12Þ

Figure 12. FactSage modeling results of the mineral transformations with Na2O content.(Figure provided in color online.)

Figure 13. Effect of K2O on sintering temperature. (Figure provided in color online.)

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Effect of K2O on Sintering Temperature

The effect of K2O content on the sintering temperature is given in Figure 13, and itshows that the sintering temperature declines with increasing K2O. In the FactSagemodeling results (Figure 14), as the content of K2O increases, leucite (KAlSi2O6)rises linearly, and leucite has a lower melting temperature. Fe2Al4Si5O18 disappearsgradually and hercynite (FeAl2O4) increases. At the same time, albite (NaAlSi3O8)disappears and nepheline begins to appear with the increase of K2O content. Thereason is the same as above. Furthermore, the ash containing iron-based compounds(Fe2Al4Si5O18 and hercynite) and nepheline (Na2O �Al2O3 � 2SiO2) and alkaline sul-fates have lower sintering temperatures.

K2Oþ Al2O3 þ 4SiO2 ! K2O � Al2O3 � 4SiO2ðleuctiteÞ ð13Þ

Conclusions

In this study, a series of experiments has been completed on the effect of the compo-sition of Jincheng coal ash on sintering temperature under gasification atmosphereby the pressure-drop measuring device. In addition, the mineral transformationswith the composition of coal ash within its sintering temperature range are simulatedby the thermodynamic equilibrium calculation module of FactSage to better under-stand the experimental results. Also, the experimental results by the pressure-droptechnique are qualitatively consistent with the calculations by the FactSage thermo-dynamic model. Predictions on mineral types and proportions can be determined byusing FactSage results.

The main results from this work are summarized below:

1. The sintering temperature decreases at first and then rises with increasing SiO2

content. The main reason for the decrease may be that SiO2 reacts with other

Figure 14. FactSage modeling results of the mineral transformations with K2O content.(Figure provided in color online.)

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oxides to produce the low-melting eutectic minerals, leading to sintering tempera-ture drop. However, this exists up to a certain limit until there are no availableoxides to react with much freer SiO2 to produce low-melting eutectics. Therefore,the sintering temperature goes up later with increase of the SiO2 content of theash.

2. With an increase in Fe2O3 content, the sintering temperature decreases dramati-cally. This confirms that Fe2O3 can convert into FeO under the gasificationatmosphere, and Fe2þ may react with other oxides to produce low-temperatureeutectics (such as fayalite and hercynite), thereby decreasing the sintering tem-perature.

3. As the CaO content increases, the sintering temperature declines. This canpossibly be explained by the fact that anorthite and gehlenite may produce alow-melting eutectic mixture with quartz and calcium aluminum acid, reducingthe ash sintering temperature.

4. As the MgO content increases, the sintering temperature decreases under the gasi-fication atmosphere. But if increasing the MgO content exceeds a certain limit,the effect of MgO on the sintering temperatures is relatively small.

5. Increasing the Na2O and K2O contents reduces the sintering temperature signifi-cantly. The decline of the sintering temperature may be the result of the low melt-ing temperature of minerals, such as nepheline and alkaline sulfates.

Acknowledgments

This work has been supported by National Basic Research Program of China(No. 2005CB221201). The authors acknowledge the contributions made by the mem-bers of State Key Laboratory of Clean Energy Utilization.

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