The influences of mineral behaviour on blended coal ash fusioncharacteristics

J.-R. Qiu* , F. Li, Y. Zheng, C.-G. Zheng, H.-C. Zhou

National Coal Combustion Laboratory, Huazhong University of Science and Technology Wuhan, Hubei 430074, People’s Republic of China

Received 6 January 1998; received in revised form 23 June 1998; accepted 1 December 1998

Abstract

The ash fusion characteristics and mineral behaviour of three kinds of individual coal ash and a series of two-component blended asheswere studied. The ash fusibility and chemical composition were measured and analysed. The samples were heated progressively from 8008Cto the initial deformation temperature (IT) at 1008C intervals in reducing atmosphere. Mineral composition and type at each temperatureinterval were determined by X-ray diffraction (XRD) analysis. The results show that blended ash softening temperatures do not changelinearly with blending ratios. It can lie between or lower or higher than that of the individual parent coals. Some combinations of componentcoal’s mineral produce low-melting eutectic minerals at high temperature and this is the main reason causing the non-arithmetic averaging ofsoftening temperature of blended ash. It is consistent with the results from the ternary system phase diagram.q 1999 Elsevier Science Ltd.All rights reserved.

Keywords:Blended coal ash; Mineral matter; Ternary system phase diagram

1. Introduction

Boiler slagging has been recognised as one of the mosttroublesome operational problems associated withpulverised coal combustion. Especially in China, coal-fired boilers were expected to remain the main energysource for a long time. In recent years, many power stationsburn blended coal to the meet different requirements such assolving transportation problem, reducing slagging and SOx

emission. However, this practice may result in serious slag-ging problems if the design coal and the substitute coals in ablend do not behave similarly. A number of articles on ashdeposition have been presented [1–4]. The behaviour ofminerals in coal ash has been widely studied for individualcoals [3–7], and also analysed by ternary phase diagrams[7,8]. Little has been done on the slagging characteristics ofcoal blends [9–12]. This research concentrated mainly onash deposition changes with blending. A complete under-standing on fusion characteristics of blended coal ash andmineral matter behaviour is far from being realised. Thepresent article describes a study for the ash fusibility andmineral matter composition at different temperatures forthe three kinds of typical Chinese coal ash and a series of

two-component blended ashes with the objective of eluci-dating the reasons for the non-linearity of ash fusiontemperature of coal blend. The results are analysed anddiscussed by the phase diagram of the ternary systemSiO2–Al2O3–CaO. In addition, the non-averaging of ashfusion temperature of blended coal ash was analysed fromthe mineralogy point of view. The correlation between fusi-bility and mineral behaviour during coal heating isdiscussed.

2. Experimental

Three coals, I, J and K, were selected on the basis thatthese coals provide the blend components for two Chinesepower plants. The rank of the coal spanned the range fromanthracite to bituminous coal. Particle size range of 150–250 mesh was used. Coal particles were crushed into apowered form in a small test miller. All particles are passthrough a 150-mesh sieve.

Proximate analysis of carbon, hydrogen, nitrogen andsulphur were determined by standard procedures.

The fusion temperature in reducing atmosphere wasmeasured by ash fusion temperature auto-analyser accord-ing to the GB219-74 standard. Chemical methodology wasused to analyse the ash composition.

The analysis data are given in Table 1.

Fuel 78 (1999) 963–969

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(99)00005-8

* Corresponding author. Tel.: 0086-27-8754-5526; fax: 0086-27-8754-5526.

E-mail address:jrqiu@public.wh.hb.cn (J.-R. Qiu)

In order to explain ash fusion behaviour, X-ray diffraction(XRD) was used to study mineral transformations forblended coal ash in a reducing atmosphere at differenttemperatures. For temperatures lower than 8008C, thereare no chemical reactions occurring amongst the mineralsof coal ashes other than decomposition [5]. Therefore, theblended-ashes were prepared by hand-mixing J coal ashwith I or K coal ash obtained at 8008C. The content of theJ coal ash in each blend was varied between 20 and 80 wt.%.Then, the individual coal ashes and blended ashes wereheated in a reducing atmosphere progressively from 8008Cto the initial deformation temperature (IT) at intervals of

1008C. The samples were quenched by water to avoid thecrystallinity changes. Mineral composition and type at eachtemperature interval were determined by X-ray powderdiffraction using Cu target, Ni ray filter, 30 mA tube currentand 30 kV tube voltage. Mineral structure and the changeswere measured by SEM.

3. Results and discussion

3.1. Ash fusion characteristics

Ash fusibility can be used as a tool for measuring thedeposit build-up and slagging performance of coalsalthough it is widely recognised that the test is far removedfrom the actual time–temperature history of ash particles ina power station boiler.

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Table 1Coal analyses (air-dried basis) and ash composition

Samples I J K

Proximate(wt.%)Moisture 3.8 2.7 1.8Ash 26.5 16.2 4.2VM 11.8 27.6 21.4Ultimate (wt.%)C 62.5 63.8 56.5H 2.1 3.7 3.3N 6.3 1.4 1.0S 2.1 0.6 0.3Ash chemical composition(wt.%)SiO2 49.2 48.4 38.2Al 2O3 37.7 25.5 25.7Fe2O3 4.2 3.5 10.8CaO 1.04 2.41 13.50MgO 1.27 5.54 6.43Na2O 0.93 0.45 0.96K2O 0.99 2.00 1.35TiO2 1.21 0.71 0.68

Table 2Fusion characteristic temperature of samples

Samples Initialdeformationtemperature(8C)

Softeningtemperature(8C)

Fusiontemperature(8C)

I . 1500J 1320 1350 1380K 1200 1380 140020% I180% J 1320 1380 141040% I160% J 1430 1460 151060% I140% J . 150080% I120% J . 150020% K180% J 1290 1330 137040% K160% J 1230 1280 133060% K140% J 1170 1200 122080% K120% J 1170 1210 1230

Fig. 1. Softening temperature for J–I blends and J–K blends.

Table 2 shows the fusion temperature of the three basecoal ashes and the blended ashes. The softening tempera-tures of the blended ash are shown in Fig. 1.

The ash softening temperatures for various percentages ofdifferent coal blended ash illustrate the non-arithmeticaveraging.

Fig. 1 shows that the ashes obtained from blending I coalashes into J coal ash have higher softening temperature thanthe J coal ash itself. Although the blended ash softeningtemperature increased with the increase of I coal content,the softening temperatures did not change linearly with theblending ratio.

The ashes from blending J coal ash with 40–80 wt.% Kcoal ash have lower ash softening points than those of theindividual coals.

3.2. Mineral matter transformation of blended ashes

In order to explain the non-linear relation of softeningtemperature of blended coals, X-ray diffraction analysis

was used to investigate the mineral behaviour from theview point of the dependence of ash fusibility on its mineralmatter. As the technique was limited to the detection ofcrystalline phases, the characteristics of amorphous phaseswill be obtained by analysis of the base line height of XRDcurve (the higher the base curve, the more the amorphousphases) incorporated with SEM examination and ternaryphase diagram prediction.

Fig. 2 presents the XRD patterns of 8008C ashsamples for the individual coals. The results show that themain mineral matter for I coal ash at 8008C were quartz,anhydrite and haematite and, for J and K coal ash werequartz, calcite, anhydrite and haematite. The main mineralmatter for blended coal ash containing 60 wt.% K coal ashand 40 wt.% J coal ash was quartz and anhydrite.

As the temperature increased, oxidation, thermal decom-position, transformation and reaction between componentsoccurred.

Fig. 3 shows the mineral diffraction intensity peak valuesof the mixtures of 20 wt.% I coal ash mixed with 80 wt.% Jcoal ash. Fig. 4 represents the diffraction intensity peakvalue of 60 wt.% K coal has blended with 40 wt.% J coalash.

It can be seen that the diffraction peak of quartz inblended ash decreases with increasing temperature. For20 wt.% I180 wt.% J blended ash, mullite (Al2O3·2SiO2)and anorthite (CaO·Al2O3·2SiO2) are formed because ofthe interaction of quartz with Al2O3 and CaO at about10008C. Moreover, the quartz XRD peak diminishes andbecomes a glass phase at about 13008C based on the SEManalysis. The anhydrite XRD peak disappeared at about11008C. As no gypsum exists in the base coal, anhydritein the blended ashes can be considered as a product ofcalcite and sulphur dioxide. Mullite and anorthite coexistedin blended ash at the IT (13208C) temperature. However,only mullite can be found in I coal at the IT temperature(15008C), and only anorthite was found in J coal (ITtemperature of 13208C).

For the blended ashes mixed with 60 wt.% K and 40 wt.%J coal, quartz reacts with aluminium oxide and calciumoxide to produce gehlenite (2CaO·Al2O3·SiO2), anorthite

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Fig. 2. XRD patterns of individual ash samples at 8008C.

Fig. 3. XRD intensity-temperature curve of 20 wt.% I180 wt.% J blendedashes (reducing atmosphere).

and calcium aluminium acid (CaO·Al2O3) at 8008C, 9008Cand 10008C, respectively. Quartz became amorphous at13008C, anhydrite disappeared at 10008C and anorthite,which has a significant effect on the fusion characteristics,diminished at softening temperature (12208C). Mulliteformed at about 10008C, and disappeared at about 14008C.The main mineral matter in blended ashes at IT temperature(11708C) are gehlenite, anorthite and calcium aluminiumacid. However, the mineral matter of K coal at IT tempera-ture (12008C) is gehlenite and in J coal at IT temperature(13208C) is anorthite. Gehlenite, quartz, anorthite andcalcium aluminium acid coexisting in 60 wt.%K140 wt.% J ash blends will produce a low-melting eutec-tic mixture which reduces the ash softening temperature.This is the main reason for the lower ash melting tempera-ture of blended coal ashes.

Fig. 5 shows the results of the SEM analysis of20 wt.% I180 wt.% J blended ash and 60 wt.%K140 wt.% J blended at 9008C and at the IT tempera-ture. For I1J blended ash, the main mineral matter at9008C is quartz, anhydrite, montmorillonite and amor-phousa -Fe2O3. However, as the temperature increasedto the IT (13208C), the main mineral matter wasmullite, amorphous and some glass phase matter. Thebase line of the XRD curve is higher at IT temperaturethan that at 9008C. For K1J blended ash, the mainmineral matter at 9008C is quartz, anhydrite, feldspar,a -Fe2O3 and amorphous. As the temperature increasedto the IT (11708C), the mineral species are melted andglass phase matter and Fe2O3 are formed. It is the resultof gehlenite, quartz, anorthite and calcium aluminiumacid coexisting in 60 wt.% K140 wt.% J ash that blendsforming a low-melting eutectic mixture. The XRD base

line is higher than that at 9008C. It indicates that theliquid phase is formed at a temperature lower than thesoftening temperature in a reducing atmosphere.

In addition, it shows that the mineral matters having lowmelting temperature and producing the low-temperatureeutectic easily melted and formed the glassy type at the ITtemperature.

3.3. The changes of mineral species with blending ratio

For coal blended ash with different proportions at theadditional coal ash, the mineral matter transformation isdifferent.

Fig. 6 shows the mineral composition and diffractionintensity peak of I coal, J coal and their blended ashes.Fig. 7 shows the mineral composition and diffraction inten-sity peak of J coal, K coal and their blended ashes at the ITtemperature.

It can be seen from Fig. 6 that only anorthite (its softeningtemperature is 15528C) can be found in J coal ash at the ITtemperature, and only mullite (its softening temperature is18508C) was found in I coal ash at the IT temperature. In theblended ashes that mixed I coal ash into J coal ash, anorthiteXRD peak diminishes gradually with the increasing contentof I coal ash. However, the mullite XRD peak increases withan increase in I coal content. Moreover, the softeningtemperature increases with increase of I coal content. There-fore, as the softening temperature of blended ashes increase,an anorthite XRD peak reduces and mullite XRD peakheightens.

Fig. 7 shows that only gehlenite (softening tempera-ture is 15938C) can be found in K coal ash at the ITtemperature and only anorthite can be found in J coal at

J.-R. Qiu et al. / Fuel 78 (1999) 963–969966

Fig. 4. XRD intensity-temperature curves of 60 wt.% K140 wt.% J blended ashes (reducing atmosphere).

the IT temperature. However, for blended ashes, themineral composition changed at the IT temperature.Several kinds of mineral matter such as gehlenite,calcium aluminium acid and quartz coexisted in theblend ashes when K coal ash content reached 60–80 wt.% in the blended ashes. These minerals interact

with each other to form low-melting eutectic that causedthe softening temperature of some blended ashes to decreasemarkedly. As K coal ash content increases over 80 wt.% inthe blended ash, anorthite diminishes gradually, and onlygehlenite can be found in the blended ash at the ITtemperature.

J.-R. Qiu et al. / Fuel 78 (1999) 963–969 967

Fig. 5. SEM analysis results of blended ashes: (a) SEM analysis of 20 wt.% I180 wt.% J blended ash at 9008C; (b) SEM analysis of 60 wt.% K140 wt.% Jblended ash at 9008C; (c) 20 wt.% I180 wt.% J at IT temperature (13208C); (d) 60 wt.% K140 wt.% J at IT temperature (11708C).

J.-R. Qiu et al. / Fuel 78 (1999) 963–969968

Fig. 6. XRD intensity of minerals in I–J blended coal ashes at IT.Fig. 7. XRD intensity of minerals in K–J blended ashes at IT.

Fig. 8. SiO2–Al2O3–CaO ternary phase diagram.

4. Analysis by ternary equilibrium phase diagram

Fig. 8 is the ternary system diagram of SiO2–Al2O3–CaO.I, J and K represent the three investigated coal’s ash chemi-cal composition. Mineral matter in the region includingpoint I, corresponding to I coal ash, is mullite; mineralmatter in the region of the J point corresponding to J coalash, is anorthite and mineral matter in the K point region,corresponding to K coal ash is, gehlenite. It is consistentwith the results from Figs. 6 and 7 at the IT temperaturewhere E and F represent two points of low-melting eutecticin low-temperature melting region marked by etching. It canbe seen that the points representative of I1J blended ash isfar away from the low-temperature eutectic region and thefusion temperature changes are limited. However, thetemperature curve between I and J is non-linear becausethe temperature distribution in the ternary system phasediagram is non-linear. This result is consistent with experi-mental results. For the J1K blended ash, the connection linebetween J and K crosses the low-temperature eutecticregion. It implies that some of the blended ashes lie in thisregion. Therefore for some J1K blended ashes the softeningtemperatures are lower than those for the individual base.These results are consistent with experimental results shownin Fig. 1. Mineral compositions in these points are gehleniteand anorthite, and it is comparable with the results analysedby XRD at the IT temperature as shown in Fig. 7.

5. Conclusion

For the power station coal blends investigated the blendedash softening temperatures are not linearly related to theblending ratio of the coal ashes.

For different blended ashes, the mineral composition andtransformation during heating are different. Temperaturesand the blending ratio have significant effects on the mineralbehaviour. Low-temperature eutectics in blended ash

minerals at high temperature are responsible for non-linearroles between blended ash-melting temperature and blend-ing ratios. Blended ash mineral compositions at IT areconsistent with the ternary system phase diagram SiO2–Al2O3–CaO.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (59406003)

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