Modification of ash fusion behavior of coal with high ash fusion temperature by red mud addition

Modification of ash fusion behavior of coal with high ash fusion temperature by red mud addition

Fuel 192 (2017) 121–127 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Modifica...

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Fuel 192 (2017) 121–127

Contents lists available at ScienceDirect

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

Full Length Article

Modification of ash fusion behavior of coal with high ash fusion temperature by red mud addition Huixia Xiao a,b, Fenghai Li a,b,c,⇑, Quanrun Liu b, Shaohua Ji c, Hongli Fan b, Meiling Xu b, Qianqian Guo b, Mingjie Ma a, Xiuwei Ma a,b a b c

College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454003, China Department of Chemistry and Chemical Engineering, Heze University, Heze 274015, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

a r t i c l e

i n f o

Article history: Received 1 September 2016 Received in revised form 14 October 2016 Accepted 7 December 2016 Available online 18 December 2016 Keywords: High ash fusion temperature (AFT) coals Red mud Ash fusion behavior Modification

a b s t r a c t The influences of two red mud (RM) samples (ZZ and ZM) on fusion behaviors of three high ash fusion temperature (AFT) coals (FZ, JC and JZ) were investigated. It was evaluated by determining AFTs of coals, RMs, and the mixtures of coal ash with RM, as well as their compositions of ashes and mineral. The results showed that the contents of CaO, Fe2O3, and Na2O in RMs were higher than those in coal ashes, which caused RMs’ AFTs lower than those of coals. The formation of high melting-point mullite might be the main factor causing coal AFTs higher. With the increase in RM mass ratio, the increasing amount of low-melting eutectic (anorthite (CaAl2Si2O8), fayalite (Fe2SiO4), and anorthite (Na-rich) (Ca,Na)(Si, Al)4O8)) and amorphous matter made AFTs lower. ZZ reduced AFTs obviously than ZM because the content of Na2O in ZZ was higher than that in ZM. Owing to lower ion potential of Na+ than those of Ca2+ and Fe2+, Na+ is easier to enter into the lattice of mullite and results in the reduction of AFTs. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Coal, approximately accounting for 80% global total fossil fuels, is the most important resource for power generation worldwide [1]. Coal directly combustion for power generation has led to series of problems (the global warming and environmental pollution). In order to solve these problems, the power plants combined with Integrated Coal Gasification Combined Cycle (IGCC) with the characteristics of coal clean utilization and wide flexibility in feedstock (coal, biomass, garbage, etc.) has become a hot issue worldwide, e.g., Europe in Buggenum (NL) [2] and Puertollano (ES) [3], which both employing entrained-flow gasification. Entrained-flow gasification is gaining important attentions due to the advantages of wide adaptability to coal and high efficiency [4]. Ash fusion temperature (AFT) is one of the crucial parameters for the design of gasifier and the selecting of its operational conditions. It is generally used to predict the fusibility behavior of coal ash during coal gasification [5]. In a typical entrainedflow gasifier, the optimal flow temperature (FT) of coal is approximately ranges over from 1340 °C to 1400 °C [6], and slag viscosity ⇑ Corresponding author at: Department of Chemistry and Chemical Engineering, Heze University, Heze 274015, China. E-mail address: [email protected] (F. Li). http://dx.doi.org/10.1016/j.fuel.2016.12.012 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

is approximately 25 Pas between 1300 °C and 1500 °C for its tapping smoothly [7]. However, >57% of coals have high AFT (FT > 1400 °C) in China. These high AFT coals gasify directly in the entrained-flow gasifier might reduce gasification efficiency due to ash accumulation and slag (ash deposition in the radiative section of gasifier [8]) blocking, which make the shutdown of gasification system further [9]. Consequently, it is necessary to reduce coal AFT to meet the requirement of entrained-flow gasifier. Ash deposition phenomena are influenced by coal type (ash composition, fusion temperature, and the distribution of mineral matter), reaction atmosphere, particle temperature, and so forth [10]. The adjustment of AFT is generally achieved by adding flux [11,12] and coal blending [13]. In the aspect of adding flux, Jing et al. found that the formation of low melting eutectic because of increasing iron, sodium and calcium content resulted in the decrease of AFTs [14]. Vassilev et al. pointed out that the alkaline cation was the donor of oxygen, which could terminate the aggregation of polymer and reduce its viscosity [15]. Akiyama et al. found that MgO played a role in decreasing the molten slag fraction due to MgO involved the formation of solid-phase aluminosilicates [16]. A Na-based fluxing agent Na2O and composite fluxing agent (CaO:Fe2O3 = 3:1) were used to decrease the AFTs of Dongshan coal and Xishan coal [17]. In the aspect of coal blending, coal blending experiments of Pingsuo coal and Shenhua coal, Tianci

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coal and Xiaotun coal indicated the variation in mullite and anorthite contents were the main reason leading to the AFT change of mixed coal [18]. Although adding flux and blending coal both can reduce AFTs of coal, they also have disadvantages: the former is easy to make AFT change rapidly; the latter is greatly restricted by high transport cost. Therefore, it is necessary to find cheap complex fluxes to reduce the AFTs of high ash-melting coals. Red mud (RM) is solid waste from producing aluminum oxide. Every year, nearly 70–120 million tons of RMs are produced worldwide [19]. It is a hazardous material due to its high alkalinity and radioactivity (contain large amounts of alkaline (e.g., iron, calcium, sodium) and small amount of radioactive elements (cesium, strontium, uranium, etc.)) [20,21]. A large amount long-time stack RM not only pollutes the atmospheric environment, also leads to soil alkalization and groundwater contamination, which pose a security risk to the people’s safety [22–24]. Therefore, the comprehensive utilization of RM has become a hot spot worldwide. RM has been used in the fields of building materials [25], adsorbent [26], catalyst or catalyst carrier, rare earth metal extraction, and so on [27,28]. In the field of coal conversion, RM was used as a kind of catalyst for liquefaction of coal and biomass [29,30], and hydrogenating anthracene oil [31], as well as a desulfurization agent for sulfur removal from high-temperature gas [32]. However, the application of RM to reduce coal AFT is rarely reported. The aim of this paper was to investigate the effects of RM on coal AFT and its mechanism. It is expected to provide references to adjust the ash fusion characteristics during coal conversion.

2. Materials and methods

2.2. Ash preparation Laboratory ashes of coal samples were prepared according to Chinese standard procedures (GB/T1574-2001). The samples were placed in a muffle furnace. The temperature was increased to 500 °C within 30 min, held at 500 °C for another 30 min, then raised to 815 °C, and kept at this temperature for 2 h. Finally, the ashes were removed and cooled to room temperature. Sample mixtures at different temperatures were prepared as follows. RMs were added into laboratory ashes with different mass ratios of 5, 10, 15, 20, 25, and 30%, respectively. The ash-RM mixtures were mixed until reached uniformity. Mixed ash samples at 1200 °C and the coal ash samples at 815, 900, 1000, 1100, 1200 and 1300 °C were prepared in an ALHR-2 AFT analyzer (Ao-lian Co. Ltd., China) to keep the same condition as their AFTs testing [33]. The samples were taken out at pre-setting temperature, and were quenched immediately by ice water to prevent phase variation and crystal segregation. The quenched samples were placed in a vacuum oven at 105 °C for 36 h, and then were stored in a cabinet dryer before using.

2.3. AFT measurements of samples AFT measurements were conducted on the ALHR-2 AFT analyzer. AFT tests strictly followed Chinese standard test method (GB/219-2008). The ash cone was first heated to 900 °C at 20 °C/min, and then increased at 5 °C/min in reducing atmosphere (1:1, H2/CO2, volume ratio). During the heating process, the deformation temperature (DT), softening temperature (ST), hemisphere temperature (HT), and FT of ashes were obtained based on the variation of ash cone shape.

2.1. Raw materials The three air-dried coals (Fangzheng coal, Jiaozuo coal, and Jincheng coal respectively referred to as FZ, JC and JZ) were provided by Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS). Two kinds of RM on air-dried basis (Zhongzhou and Zhongmei, respectively referred to as ZZ and ZM) were provided by College of Chemistry and Chemical Engineering, Henan Polytechnic University. The five samples were crushed to <0.198 mm, dried at 105 °C for 24 h in a nitrogen (N2) atmosphere, and stored in a cabinet dryer before use. Proximate and ultimate analyses determined by Chinese standard (GB/T476-2001 and GB/T212-2012) are shown in Table 1, respectively. All coals had low water content (1.51–3.88%) and the total sulfur content (0.59–1.76%), and high fixed carbon content (53.93–76.76%). Among the three coals, the volatile matter content of FZ was the highest, but the fixed carbon content was the lowest.

2.4. Ash analytical methods Sample compositions conducted on an X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Japan) with a Rh target X-ray tube (50 kV, 40 mA) are shown in Table 2. The mineral composition was performed on an X-ray diffraction (XRD) using a D/max 2400X power diffract meter (Rignaku co., Japan) with the characteristic Cu Ka radiation. Operating conditions were 40 kV, Ka1 = 0.15408 nm and 100 mA. Scanning speed was 5° 2h/min with the step size of 0.01° 2h in the range of 5° and 80°. The mineral compositions were analyzed by the computer software package MDI Jade 5.0.

3. Results and discussion 3.1. Analysis of the basic properties of five samples

Table 1 Proximate and ultimate analysis of coal samples. Sample

FZ

JC

JZ

Proximate analysis on air-dried basis (wt.%) Moisture 3.88 Volatile matter 29.89 Ash 12.30 Fixed carbon 53.93

2.35 7.76 15.60 74.79

1.51 8.38 13.35 76.76

Ultimate analysis on dry ash free basis (wt.%) Carbon 78.30 Hydrogen 4.14 Oxygen 2.40 Nitrogen 1.10 Sulfur 1.76

76.73 2.80 3.20 0.90 0.77

80.65 2.85 1.39 1.17 0.59

AFTs and the ash compositions of five samples are shown in Table 2. The AFTs of RMs are lower than those of coals. It is clear that the contents of CaO, Fe2O3 and Na2O in RMs are obviously higher than those of coal ashes. It was found that the ionic potential of Ca2+, Na+ and Fe3+ was lower (20.20 nm 1, 10.53 nm 1 and 39.47 nm 1, respectively), which prevented polymer formation [34]. They also reacted with other minerals to form low temperature eutectics under reducing atmosphere, thereby decreasing the AFTs [34]. A strong relationship between the AFT and the acid/base (A/B = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O + SO3)) ratio was found [35]. AFTs increases with increasing A/B ratios [36]. A/B values increase in the order ZZ < ZM < FZ < JC < JZ, which are also shown in Table 2. To a certain extent, this reflects increase order of AFTs of five samples.

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H. Xiao et al. / Fuel 192 (2017) 121–127 Table 2 AFTs and compositions of coals ash and RMs. Sample

FZ

JC

JZ

ZZ

ZM

AFTs of five samples (°C) DT ST HT FT

1497 1500 1500 1500

1500 1500 1500 1500

1500 1500 1500 1500

1138 1145 1149 1173

1207 1212 1218 1248

Ash compositions SiO2 Al2O3 Fe2O3 CaO MgO K2O SO3 Na2O TiO2 P2O5 A/B

45.34 34.90 9.34 6.34 0.98 0.32 1.24 0.41 1.10 0.07 4.37

49.08 33.84 4.52 3.97 0.85 1.08 2.06 1.04 1.39 0.39 6.24

47.60 35.72 2.02 6.29 0.74 0.19 2.43 0.53 1.48 0.50 6.95

20.06 24.03 14.87 15.98 0.86 1.19 1.38 7.64 2.16 0.21 1.10

16.73 23.67 15.28 16.58 1.84 1.82 0.46 1.66 4.18 0.88 1.18

A/B value = (SiO2 + Al2O3 + TiO2)/(Fe2O3 + CaO + MgO + Na2O + K2O + SO3); DT–deformation temperature, ST–softening temperature, HT–hemisphere temperature, FT–flow temperature.

3.2. Variation in the coal AFT with different mass ratios of RM 3.2.1. Variation in the coal AFT with different mass ratios of ZM The influence of ZM on AFTs of three coals is presented in Fig. 1. The effect of ZM on FZ coal AFTs presents an obvious difference from those on JC and JZ. The FZ AFTs decreased relatively evenly with increasing ZM mass ratio, while for JC and JZ, the variation of AFTs of the mixed sample initially was basically unchanged (0–10%), then rapidly decreased (10–20%), finally decreased slowly (20–30%) with increasing ZM mass ratio. Ca2+ and Fe3+ contents were rich in ZM, they entered the mullite crystal to from more active AOA as electron donor easily, which led to the fracture of weak chemical stability of AlAO covalent bond [37], finally resulted in the transition from mullite to anorthite, fayalite, and rich in sodium calcium, thereby reducing AFTs. The FT of the FZ mixture decreased to 1285 °C when ZM mass ratio reached 30%, whereas for JC and JZ, the FTs only reduced to 1337 °C and 1345 °C, respectively. This is consistent with the values of A/B ratios increase order for three ash samples. 3.2.2. Variation in the coal AFT with different mass ratios of ZZ The effects of ZZ addition on AFTs of three coals are shown in Fig. 2. AFTs of three kinds of blends of ZZ show the same tendency as ZM. It can be known from Fig. 2, for FZ and JZ, the effect of AFT presents a similar trend to Fig. 1. In order to compare the AFT (ST) decrease for three coals through the addition of ZZ and ZM, the original ST of three coals and the mixed ashes as well as their differences are shown in Table 3. As can be seen from Table 3 that ZZ decrease AFTs a little more than that of ZM for three coals, respectively. Although more contents of Fe2O3 and CaO in ZM, when ZZ mass ratio was the same of that of ZM, the former decreased the AFT more obviously than the latter. This is related to high Na2O content in ZZ (7.64%), which can react with mullite and quartz, resulting in the generation of low melting-point (MP) eutectics of nepheline (NaAlSiO4) and albite (NaAlSi3O8) [35]. What’s more, according to frontier molecular orbital theory, DE of albite (DE = 2.317 eV) is far less than that of anorthite (DE = 4.832 eV), albite formation needs much less energy than that of anorthite generation [38]. On the other hand, Na+ intends to combine with the oxygen atoms with larger electro negativity and forms the structure AOAMAOA because it has lower ionic potential than that of Ca+ and Fe2+ [39]. Simultaneously Na+ was easier to destroy the OASi covalent bond, which broke the network structure of the silicon aluminum compound, and make the correlation of [SiO4]4

and [A1O4]5 reduce [37]. Thus, the content of mullite decreased obviously with the addition of ZZ than that of ZM, which led to the variation difference in the AFT. 3.3. Mineral behaviors during ash fusion The AFTs not only related to its chemical composition, but also closely associated with the mineral composition. To explore the relationship between the mineral evolution and the ash characteristics during coal gasification, the mineral compositions of ashes at different temperatures are analyzed by XRD, the results are shown in Fig. 3. The diffraction intensity indicated that the mineral content decreased with increasing temperature. Mineral crystals of FZ ashes at 815 °C and 900 °C were mainly in the form of quartz (SiO2, MP: 1723 °C), hematite (Fe2O3, MP: 1565 °C), anhydrite (CaSO4), and muscovite-2M1 (KAl3Si3O10(OH)2). At 1000 °C, sanidine (KAlSi3O8) was resulted from the decomposition of muscovite-2M1 [36]. Mullite (Al6Si2O13, MP: 1860 °C, the transformation from metakaolin (Al2Si2O7)), fayalite (Fe2SiO4, MP: 618 °C, the interaction between mullite and wustite (FeO)), and anorthite (CaAl2Si2O8, MP: 1590 °C, reactions of mullite and calcium oxide (CaO)) were detected at 1100 °C. The amorphous matter is transformed from fayalite and anorthite at the temperatures above 1200 °C [40]. Compared with FZ, halloysite (Al2Si2H12O13) was found in JZ and JC ashes at 815 °C and 900 °C. Gehlenite (Ca2Al2SiO7, MP: 1170 °C) was discovered in JZ ashes at 1000 °C, which came from these actions: anhydrite (CaSO4) ? calcium oxide (CaO) + sulfur dioxide (SO2); calcium oxide (CaO) + mullite (Al6Si2O13) ? anorthite (CaAl2Si2O8); anorthite (CaAl2Si2O8) + calcium oxide (CaO) ? gehlenite (Ca2Al2SiO7) [39]. Mullite was found in the three ashes, but the content in JC and FZ (1000 °C) was lower than that in JZ (1170 °C), which resulted from the transformation of halloysite: halloysite (Al2Si2H12O13) ? metakaolin (Al2Si2O7) + steam (H2O); metakaolin (Al2Si2O7) ? mullite (Al6Si2O13) [36]. Mullite replaced quartz as the skeleton of the silicate melt polymer in high temperature ashes when the temperature was above 1200 °C, which resulted in the high AFTs. 3.4. Mineral transformations during mixed RM 3.4.1. Mineral transformations during mixed ZM Fig. 4 illustrates the XRD patterns of the three mixed ashes with different mass ratios of ZM at 1200 °C. At 1200 °C, FZ, JC and JZ

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FZ

1500

1400 1350 1300

DT ST HT FT

1450

Temperature/

Temperature/

1450

FZ

1500

DT ST HT FT

1400 1350 1300 1250

1250

1200 0

5

10

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0

30

5

ZM mass ratio/%

JC

1500

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DT ST HT FT

1450

Temperature/

1400

JC

1500

DT ST HT FT

1450

Temperature/

10

ZZ mass ratio/%

1400 1350 1300

1300

1250 1250 0

5

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JZ

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DT ST HT FT

1450

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1400

JZ

1500

DT ST HT FT

1450

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15

ZZ mass ratio/%

ZM mass ratio/%

1500

10

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1300

1300 0

5

10

15

20

25

30

ZM mass ratio/%

0

5

10

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30

ZZ mass ratio/%

Fig. 1. Variation of the coal AFT with different mass ratios of ZM.

Fig. 2. Variation of the coal AFT with different mass ratios of ZZ.

ashes were mainly composed of the high-MP mullite and quartz. For FZ ashes, low-MP hercynite appeared in the mixed ashes when ZM mass ratio was 5%. Cordierite (Fe-rich) ((Mg,Fe)2Al4Si5O8) was found in the mixture with ZM mass ratio at 10%. The quartz and

mullite content decreased markedly, while anorthite increased greatly based on their diffraction intensities. The formation of low-MP eutectic of anorthite and cordierite decreased AFTs [33]. When the ZM mass ratio was 25%, anorthite (Na-rich)

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H. Xiao et al. / Fuel 192 (2017) 121–127 Table 3 The original ST of three coals and the mixed ashes as well as their differences. Ash samples A

B

C

ST/°C

FZ

DTS

FZ 70%FZ + 30%ZZ 70%FZ + 30%ZM

1500 1245 1260

255 240

JC 70%JC + 30%ZZ 70%JC + 30%ZM

1500 1271 1290

229 210

JZ 70%JZ + 30%ZZ 70%JZ + 30%ZM

1500 1311 1318

189 182

((Ca,Na)(Si,Al)4O8, the reaction between anorthite and albite [38,41]), and magnesiofrrite (MgFe3O4, the decomposition of cordierite or the reaction between magnesium and wustite [42]) appeared in the mixed ashes, which decreased rapidly AFTs of FZ. With the further increase of ZM, the diffraction peaks of high-MP mullite disappeared and the formation of low-MP eutectic of anorthite (Na-rich) and fayalite led to AFTs of the mixed ashes decrease. For JC and JZ ashes, when the ZM mass ratio increased from 5 to 10%, the high-MP mullite content decreased and the low-MP anorthite content increased, but the latter diffraction peak intensity was still lower than that of the former, which makes their AFTs still higher than 1500 °C. When the ZM mass ratio was higher than 10%, the diffraction peak intensities of anorthite and fayalite (Fe2SiO4) significantly enhanced. Mullite content gradually reduced until it disappeared, thereby rapidly decreasing the AFTs when the amount of ZZ ranged from 10 to 20%. The iron content in the ZM was higher than that of ZZ, when ZM mass ratio was less, wustite and other minerals interactions generate low-MP minerals such as hercynite, magnesiofrrite, ringwoodite, and fayalite, which decrease their AFTs. However, iron was transferred to high-MP magnetite (MP > 1540 °C) with increasing ZM mass ratio, which make slow down the speed of decrease AFTs.

6

66

6+1 8 66 1 7 6 1 26 8 6 1 3 2 1 3 5 2 1 3 2 1 2 4 3 1 3 2 2 41 3 6

10

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6 6

6

6 6 6 1 6 6+1 8 666 8 1

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6 6

3.4.2. Mineral transformations during mixed ZZ Fig. 5 illustrates the XRD patterns of the three mixed ashes with different mass ratios of ZZ at 1200 °C. The mineral diffraction intensities revealed that the total content of mineral decreased as the ZZ mass ratio increased. For FZ, when the ZZ mass ratio was 5%, low-MP magnesiofrrite emerged, and anorthite (Na-rich) appeared at 15% ZZ mixed ashes. Owing to the high sodium oxide content of ZZ (7.64%) and the reaction between sodium oxide and calcium oxide, the anorthite (Na-rich) content increased, leading to eutectic formation [43]. When ZZ was added into ash, Na+ as the electron acceptor easily entered into the lattice of mullite, causing the mullite structure transforms into nepheline structure. The interactions of nepheline and anorthite led to low-MP eutectic formation, which reduced AFTs [44]. Therefore, the AFTs of the mixed ashes decreased as the ZZ mass ratio increased. Due to the low iron content in JZ and JC, this did not lead to the formation of ringwoodite but also caused hercynite appear in the mixed ash sample with high ZZ mass ratio. Fayalite appeared in JC mixed ashes when the ZZ ratios were 20%, and hercynite emerged at 25% ZZ mixed ash. With the increasing mass ratio of ZZ, the intensity of the diffraction peak of low-MP minerals (anorthite, hercynite and anorthite (Na-rich)) increased, and the diffraction peak of high-MP (quartz and mullite) disappeared, which causing AFTs decrease. When the ZZ mass ratio was 20%, for JC and JZ, high-MP magnetite was accompanied by the emergence of magnesiofrrite. This might explain the variation in the AFTs of the JZ and JC mixture.

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2-Theta/° Fig. 3. The different heat treatment temperature of various mineral diffraction intensity patterns 1–Quartz (SiO2), 2–Anhydrite (CaSO4), 3–Hematite (Fe2O3), 4–Muscovite-2M1 (KAl3Si3O10(OH)2), 5–Sanidine (KAlSi3O8), 6–Mulite (Al6Si2O13), 7–Fayalite (Fe2SiO4), 8–Anorthite (CaAl2Si2O8), 9–Halloysite (Al2Si2O76H2O), 10–Gehlenite (Ca2Al2SiO7).

4. Conclusion RM addition provides a promising way to reduce AFTs to meet the requirement of entrained-flow gasification. The main results of this work are summarized as follows:

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3+5 3+5 3 3 3+5 3+5 3 3 1 3

FZ

78 78

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5+3 5+3 5 5 5 378 5 5+3 3+5 3 9 5 3 78 1 3 3+5 1 3+57+86 1 3 33 3+5 3 1 7 7+86 1 3 3 1 1 14 1 3 1 1 1

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Fig. 4. Mixed ash XRD patterns of three coal ash and ZM at 1200 °C 1–Mulite (Al6Si2O13), 2–Quartz (SiO2), 3–Anorthite (CaAl2Si2O8), 4–Cordierite, Fe-rich ((Mg, Fe)2Al4Si5O8), 5–Anorthite, Na-rich((Ca,Na)(Si,Al)4O8), 6–Hercynite (Fe2Al2O5), 7–Magnesiofrrite (MgFe3O4), 8–Ringwoodite, Fe-rich ((Mg,Fe)2SiO4), 9–Mgnetite (Fe3O4), 10–Fayalite (Fe2SiO4).

(1) AFTs of coals are higher than those of RMs because the contents of CaO, Fe2O3 and Na2O in coal ashes are lower than those in RMs. (2) With increasing RMs mass ratio, the amount of low-melting eutectic matters (CaAl2Si2O8, Fe2SiO4 and (Ca,Na)(Si,Al)4O8) and amorphous matter increases at a high temperature, leading to the decrease AFTs.

3+5 1 1+2

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3

1

11+7

1

70

2-Theta/°

3+5 3+5 5 3 3+5 33 2

1

0%ZM

50

3+5 3+5 5 5

30%ZM

3

1 1

30

1

2-Theta/°

3+5

3+5 3 5 3 7+9 5 3 3 3 3+5 3+5 3 3 5 3 7+9 5 3 3 6 3+5 33 7+9 1 3 3 2 1 1 3 11+7

20%ZZ

40

2-Theta/° JZ

1

2-Theta/° JC

76

25%ZZ

13 111 4 11 1+2 1 3 1 1 1 3 1 1 2

70

3+5

3+5 3 3 3 3 3+5 3+5 3 3 3

1

23

2-Theta/° JC

30%ZZ

1

40

20%ZZ 1 15%ZZ 1 10%ZZ 1

5%ZZ

1

1 0%ZZ

50

60

70

2-Theta/° Fig. 5. Mixed ash XRD patterns of three coal ash and ZZ at 1200 °C 1–Mulite (Al6Si2O13), 2–Quartz (SiO2), 3–Anorthite (CaAl2Si2O8), 4–Cordierite, Fe-rich ((Mg, Fe)2Al4Si5O8), 5–Anorthite, Na-rich((Ca,Na)(Si,Al)4O8), 6–Hercynite (Fe2Al2O5), 7–Magnesiofrrite (MgFe3O4), 8–Ringwoodite, Fe-rich ((Mg,Fe)2SiO4), 9–Mgnetite (Fe3O4), 10–Fayalite (Fe2SiO4).

(3) The content of Na2O content of ZZ is higher than that of ZM. Na+ is easier to enter into the lattice of mullite because the ion potential of Na+ is lower than that of Ca2+ and Fe2+. This result in the effect of ZZ on reducing AFTs is more obviously than that of ZM.

H. Xiao et al. / Fuel 192 (2017) 121–127

Acknowledgments This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07050103), the Natural Science Foundation of Shandong Province, China (ZR2014BM014), and Youth Natural Science Foundation of Shanxi Province, China (Y5SJ1A1121).

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