Relationship between coal ash composition and ash fusion temperatures

Relationship between coal ash composition and ash fusion temperatures

Fuel 105 (2013) 293–300 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Relationship between...

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Fuel 105 (2013) 293–300

Contents lists available at SciVerse ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Relationship between coal ash composition and ash fusion temperatures Bo Liu, Qihui He, Zihao Jiang, Renfu Xu, Baixing Hu ⇑ School of Chemistry and Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing, Jiangsu 210093, PR China

h i g h l i g h t s " This study is based on a five-component SiO2–Al2O3–CaO–Fe2O3–K2O system. " This study uses a combination of XRD and SEM techniques. " The effects of CaO, Fe2O3, K2O and silica-to-alumina ratio on AFTs are studied. " Conclusions found from synthetic ashes are applicable to real coal ashes.

a r t i c l e

i n f o

Article history: Received 8 February 2012 Received in revised form 6 June 2012 Accepted 7 June 2012 Available online 23 June 2012 Keywords: Ash fusion temperatures X-ray diffraction Scanning electron microscope X-ray photoelectron spectrometer Synthetic ash samples

a b s t r a c t Coal ash fusion characterization and the ash fusion temperatures (AFTs) are important parameters for coal industry. Previous research has proved that the AFTs are correlated with the chemical composition of coal ash samples. In this paper, a five-component SiO2–Al2O3–CaO–Fe2O3–K2O system is set. The AFTs of 34 synthetic ashes were measured in a carbon atmosphere, and the trends of the AFTs analyzed. Different from other studies, X-ray diffraction (XRD) and scanning electron microscope (SEM) were combined to explore the relationships between the measured AFTs and the mineral composition, morphology and microstructure of ash samples. Furthermore, the effect of the element valence state is also taken into account and X-ray photoelectron spectrometer (XPS) is used to analyze it firstly. The results show that the AFTs decrease with the increasing Fe2O3 content and silica-to-alumina (S/A) ratio. However, for the effect of CaO content, the AFTs reach a minimum value and then increase once again. Furthermore, the AFTs show no significant change as the K2O content varies. At last, all the conclusions found from the synthetic samples are also applicable for the seventeen bituminous coal ashes collected locally and abroad. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The coal ash fusion temperature (AFT) is an important parameter on coal ash fouling and slagging properties. It determines the behavior of coal ash in the processes of coal combustion, gasification, liquefaction and ash utilization [1–3]. It can cause slagging problems on combustion chamber and pipe surfaces and decline heat transfer efficiency [4,5]. It is said that the operating temperature should be above the fluid temperature (FT) [6]. Thus, it is quite necessary to study the AFTs for not only theoretically, but for practical application. There have been a number of studies carried out to calculate or predict the AFTs [7–9]. Jak used the thermodynamic computer package FactSage to study the relationship between AFTs and equilibrium phase diagram to develop a new AFT prediction method [10]. Seggiani developed the partial least-squares regression method to predict the AFTs [11]. Yin developed a back propagation ⇑ Corresponding author. Tel./fax: +86 25 83592878. E-mail address: [email protected] (B. Hu). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.046

neural network model to predict coal AFTs [12]. Some researchers also related the AFTs to the coal ash composition [13–15]. Gray et al. imported a multiple and stepwise regression analysis to investigate the correlation between ash composition and their AFTs [16]. Song changed the content of the primary oxides to study the effect of coal ash composition on AFTs [17]. These methods could not obtain highly precise results or could only be used in a limited range of conditions because of the complex chemical and mineralogical composition of coal ashes [18]. Coal ash is a mixture of oxides, so the properties of coal ashes at high temperature might be similar with the synthetic ashes composed by primary oxides [19]. Beside, the chemical composition of the synthetic ashes can be easily controlled. In order to simplify the experimental procedure and set up a model for application guidance, we decided to carry out research using synthetic ashes. We assembled 34 synthetic ashes mixed with SiO2, Al2O3, CaO, and Fe2O3 to study the effects of CaO, Fe2O3 and silica-to-alumina ratio on the AFTs respectively. Besides, in order to consider the problem more rigorously, we considered to add one oxide, which had a low content in ashes to the system. It was said that initial

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deformation temperature (IDT) was mainly related to substantial melting of minerals containing K2O in these ashes [20], thus, K2CO3 were added to the mixtures.

Table 2 Chemical composition of synthetic ashes. No.

Composition of synthetic ashes (wt.%) SiO2

2. Experimental section 2.1. Coal ash samples Seventeen typical coal samples were collected locally and abroad in this work. All the coal samples were ashed in air according to Chinese standard GB/T 1574-1995. All these samples are analyzed using X-ray fluorescence (XRF). The chemical compositions were presented in Table 1. The compositions of collected coal samples are used to determine the formula of synthetic ash samples. 2.2. Synthetic ash samples Thirty-four synthetic ash samples whose chemical composition were based on the seventeen collected coal ash samples were prepared using SiO2, Al2O3, Fe2O3, CaO and K2O. The individual content is presented in Table 2. 2.3. Ash fusion temperatures The AFT test supplies four temperatures which describes the softening and melting behavior of ash when it is heated: initial deformation temperature (IDT), spherical temperature (ST), hemispherical temperature (HT), and fluid temperature (FT) [21]. In this work, the AFT test was carried out in a carbon atmosphere which could be defined as weak reducing atmosphere according to Chinese standard GB/T 219-1996. In order to achieve a carbon atmosphere, we filled the testing corundum boat with enough graphite powder and made the furnace inclosed to ensure that ash cones are in protection of carbon during the whole testing procedure [22]. The AFTs results are presented in Table 1. After reaching the FT, the coal cones were natural cooled at 10 K/min and collected for instrumental analysis. 2.4. Instrumental analysis The mineral composition of ash samples was determined by X-ray diffraction (XRD-600, SHIMADZU, Japan). The XRD curves were analyzed by the computer software package MDI Jade 5.0.

Al2O3

Fe2O3

CaO

K2O

S/A

9.4 8.9 8.5 7.9 7.5 7.0 6.5 6.0

5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

– – – – – – – –

1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8

1.00 5.00 10.0 15.00 20.0 25.00 30.00

16.8 16.0 15.2 14.4 13.5 12.7 11.8

– – – – – – –

1.8 1.8 1.8 1.8 1.8 1.8 1.8

Ashes of different S/A ratios 16 14.3 23.9 17 19.0 21.2 18 23.0 19.2 19 26.7 17.8 20 29.8 16.5 21 33.1 15.7 22 35.4 14.8 23 38.3 14.2 24 40.9 13.6 25 43.5 13.2 26 46.0 12.8 27 48.6 12.4

21.7 21.0 20.3 19.5 18.9 18.0 17.5 16.7 16.0 15.2 14.5 13.7

40.1 38.8 37.5 36.0 34.8 33.2 32.3 30.8 29.5 28.1 26.7 25.3

– – – – – – – – – – – –

0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9

Ashes of different K2O content 28 48.9 27.0 29 48.7 26.9 30 48.5 26.7 31 48.3 26.6 32 48.1 26.5 33 47.8 26.4 34 47.6 26.3

8.4 8.3 8.3 8.3 8.2 8.2 8.1

15.5 15.4 15.3 15.2 15.2 15.1 15.0

0.2 0.5 0.8 1.1 1.4 1.7 2.0

1.8 1.8 1.8 1.8 1.8 1.8 1.8

Ashes of different CaO content 1 55.2 30.4 2 52.3 28.8 3 49.3 27.2 4 46.5 25.6 5 43.5 24.0 6 40.6 22.4 7 37.7 20.8 8 34.8 19.2 Ashes of different Fe2O3 9 53.0 10 50.9 11 48.2 12 45.5 13 42.9 14 40.2 15 37.5

content 29.2 28.1 26.6 25.1 23.6 22.1 20.7

The morphology and microstructure of an individual ash particle was analyzed using scanning electron microscope (SSX-550, SHIMADZU, Japan). The element valence state in ash samples were identified using X-ray photoelectron spectrometer (PHI5000 VersaProbe, ULVAC-PHI, Japan).

Table 1 Chemical composition and AFTs of coal ash samples. No.

Ash samples

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Shanxi Datong Heilongjiang Jixi Shandong Xinwen Anhui Huainan Neimenggu lijiata Liaoning Fuxin Shanxi Huating Shandong Zaozhuang Shandong yanzhou Shanxi gujiao Sofia Bulgaria Montana US Miike Japan New Hope Australia Mafty Russia Coal Mountain Canada Donbass Ukraine

The AFTs (oC)

Content of oxides (wt.%) SiO2

Al2O3

S/A

Fe2O3

CaO

MgO

K2O

Na2O

SO3

TiO2

P2O5

IDT

ST

HT

FT

56.6 64.1 52.9 50.8 41.7 52.7 39.7 34.9 23.0 24.2 32.0 44.5 47.7 62.5 60.6 39.9 53.8

25.4 27.2 31.9 36.0 23.0 33.5 18.2 16.5 34.0 27.1 11.3 20.3 20.8 30.0 21.9 27.3 20.4

2.2 2.4 1.7 1.4 1.8 1.6 2.2 2.1 0.7 0.9 2.8 2.2 2.3 2.1 2.8 1.5 2.6

6.5 2.9 4.9 6.1 7.1 5.9 17.1 8.0 22.1 25.2 10.6 1.6 10.2 2.6 5.1 2.5 15.1

3.7 0.9 2.7 1.9 13.1 1.8 17.5 37.1 17.3 18.0 27.7 15.7 9.7 0.9 5.0 22.1 2.8

1.4 0.5 1.2 0.6 2.1 1.1 0.9 2.0 1.7 0.7 2.8 3.5 1.5 0.4 1.9 3.2 1.3

1.4 1.6 1.7 1.3 0.8 0.7 0.3 0.2 0.5 0.8 1.1 1.1 1.0 0.5 1.1 0.6 1.0

0.6 0.2 0.7 0.3 0.9 0.3 0.2 0.1 0.3 0.4 0.2 1.3 2.1 0.9 0.5 0.7 0.6

2.9 0.9 2.2 1.4 10.0 1.4 4.5 0.6 0.6 1.1 3.2 1.1 1.7 0.6 0.7 2.0 2.1

0.9 1.5 1.6 1.3 0.9 1.5 1.2 0.3 0.4 0.4 0.7 1.8 1.2 1.2 0.7 0.5 0.8

0.5 0.1 0.1 0.2 0.2 1.0 0.2 0.1 0.1 0.3 0.2 1.3 0.1 0.1 0.5 0.7 0.6

1239 >1450 >1450 >1450 1141 >1450 1160 1180 1155 1220 1100 1125 1165 >1450 1170 1200 1150

1294 >1450 >1450 >1450 1158 >1450 1200 1201 1186 1247 1120 1150 1180 >1450 1190 1240 1175

1308 >1450 >1450 >1450 1165 >1450 1220 1208 1198 1253 1215 1200 1300 >1450 1335 1380 1355

1352 >1450 >1450 >1450 1177 >1450 1300 1230 1225 1280 1225 1205 1320 >1450 1360 1450 1370

IDT, initial deformation temperature; ST, spherical temperature; HT, hemispherical temperature; FT, fluid temperature.

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content increased until it reached 30% and increased quickly at a higher CaO content. The probable reasons might be as follows: CaO could form eutectic with other minerals under an appropriate content at high temperature, it decreases the AFTs. As the CaO content increased constantly, excessive CaO exited as CaO monomer form, its high melting point leads to a rapid increase of the AFTs. The species of minerals in coal ashes affect the melting behavior to a large extent [23]. XRD analyses were conducted to compare the different mineral composition of the synthetic ash samples. The XRD patterns of ash samples with CaO content of 5%, 30% and 40% are shown in Fig. 2. It obviously indicates that the main species of mineral changes from high-melting mullite into lowmelting anorthite while the CaO content increased from 5% to 30%. However, as the CaO content increases, it changes into highmelting gehlenite and Lime.

Fig. 1. Effect of CaO content on the AFTs.

3. Results and discussion 3.1. Effect on CaO content CaO is a fluxing agent which is commonly used to reduce the AFTs of coal ashes. In this study, the CaO content was varied between 5% and 40%, which covered a range of all the collected coal samples. Fig. 1 presents the experimental AFTs of the synthetic ash samples as a function of CaO content. As shown in Fig. 1, the AFTs of synthetic ash samples prepared in this study dropped as CaO

Fig. 2. XRD analysis of synthetic ashes with different CaO content of 5% (a), 30% (b), 40% (c). M: Mullite; A: Anorthite; C: Corundum; G: Gehlenite; L: Lime; W: Wollastonite.

Fig. 3. SEM images of synthetic ashes with different CaO content of 5% (a), 30% (b) and 40% (c).

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Fig. 3 showed the microstructure of synthetic ash samples with CaO content of 5%, 30% and 40%. The melted particles present a dense layer structure and have a smooth surface in parts a and c of Fig. 3, whereas particles in parts b of Fig. 3 present a loose structure and have a rough surface with lots of irregularly shaped particles adsorbed. Particles with dense microstructure make their physical structure firmer and not easy to deform, which present higher ATFs [24]. From the analysis above, it is shown that the SEM photos match the curve in Fig. 2. 3.2. Effect on Fe2O3 content Iron is commonly found in coal ash, the difference is it appears predominantly in the ferric iron under oxidative and inert atmosphere while in the ferrous iron or even metallic iron under reductive atmosphere. The valence state of iron affect the melting behavior of ashes to a great extent. Fig. 4 showed the effect of the Fe2O3 content on experimental AFTs. It could be seen that the AFTs of synthetic ash samples drops as the Fe2O3 content increases in the entire content range. This might be caused by the valence state of iron. The presence of iron was verified by XPS analysis (Fig. 5). Both of the binding energies for Fe 2p and O 1s in the spectrum were calibrated to C 1s (284.8 eV) of adventitious carbon as a Ref. [25]. As shown in these two spectrum, the peak positions of Fe 2p3/2 and Fe 2p1/2 were 710.4 and 724.2 eV respectively with Fe2O3 content of 1%. As the Fe2O3 content is increased to 15%, the two peak values moved to 709.5 and 722.8 eV respectively. Thus, we could infer that more Fe (III) transforms to Fe(II) during heating in a carbon atmosphere, as the Fe2O3 content increases [26]. Actually, the decrease of the AFTs is caused by the increase in FeO, rather than Fe2O3. Fe(II) reduces the AFTs efficiently on two ways: Fe(II) and unsaturated O2 form a new chemical bond, that affects the original structure of ashes. On the other hand, FeO forms eutectic compounds with other oxides at high temperatures. Mineralogical analysis was carried out by XRD. Fig. 6 displays the main species of mineral in synthetic ash samples with Fe2O3 content of 1% and 30%. It could be seen that the high-melting mullite disappears while the low-melting cristobalite is formed. Fig. 7 presents the microstructure of ash samples with different Fe2O3 content of 1%, 15% and 30%. It could obviously find that the particles in Fig. 7a have a slippery and smooth surface, they present a dense layer structure. As the Fe2O3 content increases, the dense structure is changed into a loose structure and its surface is covered with more microparticles. This may lead to a decrease of the AFTs.

Fig. 5. XPS patterns of synthetic ashes with Fe2O3 content of 1% and 15%.

Fig. 6. XRD analysis of synthetic ashes with different Fe2O3 content of 1% (a) and 30% (b). M: Mullite; A: Anorthite; C: Cristobalite; H: Hematite.

3.3. Effect on S/A ratio

Fig. 4. Effect of Fe2O3 content on the AFTs.

Silica-to-alumina ratio (S/A) has been proved to be an important factor associated with coal ash melting properties [27]. Fig. 8 displayed the effect of the S/A ratio on experimental AFTs. As shown

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Fig. 8. Effect of S/A ratio on the AFTs.

Fig. 7. SEM images of synthetic ashes with different Fe2O3 content of 1% (a), 15% (b) and 30% (c).

in Fig. 8, the AFTs of synthetic ash samples decreases as the S/A ratio increases until it reaches 1.5. At higher S/A ratio, the AFTs decreases slightly or even remains constant. The phenomena supposed to be mostly connected with the variety of Al2O3 content. At low S/A ratios, the samples have a high Al2O3 content, its ionic crystal and high melting point increases the AFTs [28]. As the SiO2 content increases (at high S/A ratios), SiO2 exits in the amorphous form in coal ashes, SiO2 content makes little contribution to the variety of AFTs. As a result, the AFTs decreased slightly or even remains constant. Fig. 9 presents the mineral composition of ash samples with S/A ratio of 0.6, 1.5 and 3.9. It can be seen that the main species of mineral is high-melting corundum at low S/A ratios. As the S/A ratio increases, the main species all changed into low-melting anorthite and gehlenite.

Fig. 9. XRD patterns of synthetic ashes with different S/A ratio of 0.6 (a), 1.5 (b), 3.9 0 (c). A: Anorthite; C: Corundum; G: Gehlenite; A : Andradite; H: Hematite.

Fig. 10 shows the SEM images of ash samples with different S/A ratio of 0.6, 1.5, 2.7 and 3.9. It is found in Fig. 10a that the micromorphology of the particles shows a firm and dense structure corresponding to a high AFTs. In Fig. 10b, the firm physical structure is changed, which leads to a decrease of the AFTs. Furthermore, the microstructures of particles in part b, c and d in Fig. 10 change little, which may be one reason why AFTs decreases slightly or even remains constant. 3.4. Effect on K2O content As presented in Table 1, K2O has a low level in seventeen collected coal ash samples. However, there are several studies

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Fig. 11. Effect of K2O content on the AFTs.

Fig. 12. XRD patterns of synthetic ashes with different K2O content of 0.2% (a), 1.1% (b), and 2.0% (c). I: Illite; A: Anorthite.

Fig. 10. SEM images of synthetic ashes with different S/A ratio of 0.6 (a), 1.5 (b), 2.7 (c) and 3.9 (d).

performed on the effect of potassium oxide on fusibility behavior of coal ashes [29–31]. Fig. 11 shows the effect of K2O content on experimental AFTs. It can find that the AFTs presents a minor change in the entire content range. Most of the K2O in the samples

was found to be illite, instead of the free potassium state, which was proved by the XRD analysis (Fig. 12). As a result, the decrease of the AFTs caused by K2O becoming much smaller. Fig. 12 presents the mineral composition of ash samples with different K2O content of 0.2%, 1.1% and 2.0%. It could be seen that after heating, all these three samples formed a similar mineral composition of illite and anorthite, which leads little change on the AFTs. SEM analysis was also carried out to obtain the microstructure of ash samples with K2O content of 0.2%, 1.1% and 2.0%. From the SEM patterns (Fig. 13), all the particles were found to have a

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4. Conclusions In this work, seventeen typical coal samples were collected locally and abroad, the compositions of their ashes after heating were analyzed using XRF. On the basis of this knowledge, thirtyfour synthetic ashes were prepared as a mixture of SiO2, Al2O3, CaO, Fe2O3 and K2O. Then we measured their AFTs in a carbon atmosphere to study the effect of these oxides on AFTs. From the experimental results, the AFTs decreases with increasing Fe2O3 content and S/A ratio. For the effect of CaO content, the AFTs reaches a minimum value when CaO content approaches 30% and then increases once again. Besides, the AFTs shows no significant change as the K2O content varies. XRD analysis and SEM images are used to determine the mineral composition and microstructure of ashes respectively. Compared the experimental results with the AFTs of seventeen real coal samples, we found all the conclusions obtained from the synthetic samples were also applicable for the real bituminous ashes listed in this study. Acknowledgments The authors acknowledge the financial support provided by Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing University. References

Fig. 13. SEM images of synthetic ashes with different K2O content of 0.2% (a), 1.1% (b) and 2.0% (c).

smooth surface and a tight structure. They correspond to the higher AFTs showed in Fig. 11. In Table 1, we found samples with AFTs higher than 1450 °C all have low CaO content (<5%), low Fe2O3 content (<2%) and low S/A ratio (<1.5). However, samples show lower AFTs have high CaO content (>25%), high Fe2O3 content (>15%), or high S/A ratio (>2.0). The phenomenon matched the experimental results. It proved in this study that the properties of coal ashes at high temperature were similar to the synthetic ashes and all the conclusions found from the synthetic samples are also applicable to the seventeen collected bituminous coal ashes.

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