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Mineral Transformation in Char and Its Effect on Coal Char Gasification Reactivity at High Temperatures, Part 1: Mineral Transformation in Char Zhibin Ma,†,‡ Jin Bai,*,† Wen Li,† Zongqing Bai,† and Lingxue Kong†,‡ †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

ABSTRACT: The mineral transformation in the ash, slow pyrolysis char, and partially gasified char at high temperatures were investigated in this part by XRD and SEM-EDX. All the mineral transformations possibly affecting gasification were considered. The results show that the mineral transformation in slow pyrolysis char greatly differs from that in high temperature ash (950− 1500 °C). The predominant minerals in slow pyrolysis char and high temperature ash are oldhamite and gehlenite, respectively. The crystalline mineral disappears in ash above 1300 °C while it still exists in the char even at 1500 °C. In the char, anhydrite is reduced by carbon to oldhamite at high temperature and increasing pyrolysis temperature is beneficial for the formation of oldhamite. It is verified that the presence of char impedes the formation of aluminosilicates and thus the catalytic minerals remain in char even at 1500 °C. However, the oldhamite in char also transforms to gehlenite during char gasification with the presence of CO2. The carbon thermal reaction is proved to be feasible only under inert atmosphere rather than reducing atmosphere in this study. Besides, morphology of minerals in char at high temperature is characterized. Considering the interaction between minerals and coal matrix, it is not adequate to evaluate the mineral transformation during gasification by only using ash.

1. INTRODUCTION Coal gasification is one of the prominent methods for coal clean utilization. The syngas obtained from coal gasification could be converted into value-added chemical products and liquid fuels.1 In recent years, the operating temperature of the gasifier kept being raised in order to improve carbon conversion efficiency. For example, the gasification temperatures in Texaco and Shell gasifiers are up to 1700 °C, higher than the flow temperature of coal ash for slagging.2,3 Understanding the reactivity of coals at high temperature, and its variation with the progress of gasification reaction, is fundamental to the design and operation of gasifiers.2 The correlation of coal characteristic properties such as carbon content, volatile matter, maceral content, and rank with coal gasification reactivity has been widely discussed in the literature.1−4 On the other hand, inorganic matter in coal plays an important role during hightemperature gasification and mineral transformations obviously affect the coal gasification reactivity. The interaction between mineral matter and char at high temperature may catalyze gasification reaction or impede the gasification reaction. However, the behavior of inorganic matter in coal varies with the elevated temperature. The minerals in coal may interact with each other to form new crystalline minerals at specific temperatures, which would melt into amorphous phase above the melting temperature of the corresponding ash. Many works2,5,6 in the open literature try to explain the complicated transformation of mineral matter at high temperature. Bai et al2,5 applied the most common methods to understand mineral matter transformation. Coal is first placed in a muffle furnace at 815 °C to obtain ash. The ash is then heated to the preset temperature under various atmospheres to obtain high temperature ash samples, which are analyzed by XRD, SEMEDX, etc. In situ analysis method is also applied for mineral © 2013 American Chemical Society

Table 1. Proximate and Ultimate Analysis of XLT Coal proximate analysis (wt%, d)

a

ultimate analysis (wt%, daf)

A

V

FC

C

H

N

S

Oa

11.30

42.79

45.91

66.41

3.07

1.39

1.81

27.32

By difference.

transformation. Van Dyk et al.6 studied the mineral transformation in coal ash from 500 to 1400 °C under Ar atmosphere by HT-XRD and FactSage modeling. The studies mentioned above only considered interactions among inorganic minerals. However, the interaction between coal matrix and mineral matter also influences the transformation. Vessileva et al.7 investigated the mineral transformation from 100 °C to the ash fusion temperature under air by using coal samples directly. Grigore et al.8 found that oldhamite was generated easier than anhydrite in coke, and the presence of carbon influenced the formation of anhydrite. It is believed that the phase transformations and reactions observed through heating preliminary ash samples could not represent the actual reactions with organic matter. In addition, the minerals react with char to form carbides at high temperature. It is found that quartz reacted with coal char to form SiC above 1200 °C, and calcium oxide was gradually reduced by coal char between 950 and 1450 °C and then carbided above 1450 °C.9−11 When the CaO was mixed with quartz or meta-kaolinite, the carbon thermal reduction of CaO by coal char was inhibited, as CaO reacted with SiO2 or meta-kaolinite to form silicates or Received: March 8, 2013 Revised: July 9, 2013 Published: August 5, 2013 4545

dx.doi.org/10.1021/ef4010626 | Energy Fuels 2013, 27, 4545−4554

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Table 2. Main Compositions and Melting Temperatures of XLT Raw Asha main compositions (wt, %)

a

melting temperatures (°C)

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

DT

ST

HT

FT

33.97

10.88

11.67

27.9

2.8

9.72

1243

1271

1287

1299

DT, deformation temperature; ST, soften temperature; HT, hemisphere temperature; FT, flow temperature.

Figure 1. XRD patterns of major minerals in raw ash and HTAs at different temperatures. Q, quartz; Ah, anhydrite; Gh, gehlenite; H, hematite; Mg, magnetite; O, oldhamite.

aluminosilicates. Liu et al.12 studied the reactions among mineral matters and their morphology during char gasification at elevated temperatures. It was found that SiC was formed in char only at 1500 °C and vanished away during gasification. Obviously, the atmosphere also influences the carbon thermal reaction, but the previous work by Wang et al.9,10 and Wu et al.11 was performed only under inert atmosphere. Above all, mineral transformation with char and without char is lack of comparison and full investigation, especially under gasification conditions. Carbon thermal reaction is well understood under inert atmosphere, but the effect of atmosphere is still unknown.

Further investigation on mineral transformation during coal gasification is necessary to better understand the effect of ash on char gasification. In this part, mineral transformation was investigated sufficiently by two methods. One was the conventional method used by Bai et al.2,5 For comparison, a different method was also applied for understanding the interaction between minerals and char matrix under different atmospheres. Slow pyrolysis chars were obtained in horizontal tube furnace under reducing atmosphere and inert atmosphere from 950 to 1500 °C. The chars were ashed at 120−150 °C using a radio frequency 4546


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defined as raw ash in this paper according to the traditional expression in previous studies.2,5,6,8 The melting temperatures of the raw ash were determined following GB/T 219-2008. The compositions and melting temperatures of the raw ash were presented in Table 2. 2.3. Mineral Transformation in Raw Ash. To investigate the mineral transformation in the ash, the high temperature ash (denoted as HTA) was prepared according to the method in the ref 5. Briefly, the raw ash was heated at 950, 1100, 1300, 1400, and 1500 °C and kept for 30 min under reducing atmosphere (CO:CO2 = 6:4, gas flow is 300 mL/min) in a horizontal tube furnace. HTAs were taken out and immersed into an ice−water bath immediately. The HTAs made at different temperatures were denoted as HTA-950, HTA-1100, HTA-1300, HTA-1400, and HTA-1500, respectively. 2.4. Preparation of Char and Low Temperature Ash. A set of slow pyrolysis char samples were prepared with XLT coal at 950, 1100, 1300, 1400, and 1500 °C under reducing atmosphere (CO:Ar = 6:4, gas flow is 300 mL/min) in a horizontal tube furnace and denoted as CR-950, CR-1100, CR-1300, CR-1400, and CR-1500, respectively, where C stands for char, R means reducing atmosphere, and X is coal pyrolysis temperature. The particle size of raw coal was below 193 μm. The coal sample was heated to final temperature at the rate of 6 °C/min and maintained for 30 min. To ensure that the minerals in char did not change during cooling, the corundum crucible containing the char sample was taken out and immersed into an enclosed small steel container immediately when the treatment finished. The upper oxidized layer in the crucible was discarded. To analyze the minerals in char accurately, the organic matters in these char samples were removed by oxygen plasma oxidation in a K1050X plasma furnace (Quorum Technologies Ltd.) and the char samples were ashed until mass fluctuation is below 0.001 g. The oxygen plasma oxidation is a reliable method because it has minimal effect on the mineral species, and it has been well described in the literature by Frazier and Belcher.13 The acquired LTAs were denoted as LTA-CR-950, LTA-CR-1100, etc. For comparison, another set of slow pyrolysis chars were prepared from 1300 to 1500 °C under inert atmosphere (argon, flow rate: 300 mL/min) to explore the effect of atmosphere on the carbon thermal reaction among minerals and char. The corresponding LTAs were labeled as LTA-CA-1300, LTA-CA1400, and LTA-CA-1500, where A means argon atmosphere. The partially gasified char and the gasification residue were also prepared from XLT coal in the horizontal tube furnace. The heating procedure was similar with mentioned above. Argon atmosphere was switched to CO2 when furnace temperature reached preset temperature, the flow rate of gas was 300 mL/min, and the total residence

Figure 2. Content of major minerals in HTAs at different temperatures.

oxygen plasma furnace. Low temperature ash (LTA) was obtained by removing the carbon or organic matters with minimal alteration of the mineral species,13,14 and mineral phases in LTAs were determined by XRD and SIROQUANT. Furthermore, the mineral transformation in char during gasification at around melting temperature of coal ash was studied by comparing the minerals in raw char, partially gasified char and ash after total gasification. The morphological changes of minerals in char at high temperature were also studied by SEM-EDX.

2. EXPERIMENTAL SECTION 2.1. Coal Sample. A typical Chinese coal from Yunnan, Xiao-longtan (denoted as XLT) lignite was employed in this work. The proximate and ultimate analysis of XLT coal was listed in Table 1. 2.2. Raw Ash. The ash prepared at 815 °C in a muffle furnace following the procedures of Chinese Standard (GB/T 1574-2001) was

Table 3. Reactions among Minerals at High Temperature and Gibbs Free Energy ΔG (kJ) reactions

CaSO4 = CaO + SO3

(1)

CaSO4 = CaO + SO2 + 1/2O2

(2)

CaSO4 + CO = CaO + SO2 + CO2

(3)

2CaO + Al 2O3 + SiO2 = Ca 2(Al 2SiO7 ) CaSO4 + 4CO = CaS + 4CO2

2FeO + SiO2 = Fe2SiO4

1100 °C

1300 °C

1500 °C

181.42

157.35

126.43

97.83

165.83

128.03

78.91

−10.67

−35.60

−67.68

32.22

−179.33

−177.69

−174.66

−169.69

−265.85

−315.91

−381.26

−444.05

−385.03

−484.56

−581.23

−97.465

(4)

(5)

CaO + Al 2O3 + 2SiO2 = Ca(Al 2Si 2O8)

950 °C

(6)

(7)

3FeO + Al 2O3 + 3SiO2 = Fe3Al 2Si3O12

(8)

2FeO + 2Al 2O3 + 4SiO2 = FeAl4Si4O18

(9)

CaSO4 + 2C = CaS + 2CO2

(10)

CaSO4 + 3C = CaS + CO2 + 2CO

(11)

−309.11

CaS + 3CO2 = CaO + SO2 + 3CO

(12)

168.66

142.09

106.97

72.22

179.33

177.69

174.66

169.69

−360.83

−348.81

−332.81

−316.83

CaS + 4CO2 = CaSO4 + 4CO

CaS + 3/2O2 = CaO + SO2

(13) (14)

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Figure 3. XRD patterns of major minerals in LTAs from the chars prepared under reducing atmosphere at different temperatures. Q, quartz; Ah, anhydrite; Ca, calcium sulfate hydrate; Gh, gehlenite; H, hematite; Mg, magnetite, O, oldhamite; Mh, maghemite. time was 30 min. The carbon conversion of partially gasified char was adjusted by gasification time. The corundum crucible containing char sample was taken out and immersed into an enclosed small steel container immediately when the procedure finished. The carbon or organic matter in partially gasified char was also removed at low temperature by oxygen plasma oxidation. The gasification residue was prepared by sufficient gasification of XLT coal with CO2 at 1100 °C for 30 min. 2.5. Analysis. The XRD patterns of the HTAs and LTAs were recorded using a Rigaku Miniflex II DESKTOP X-ray diffractometer with Cu Kα radiation. A step size of 0.02° at the speed of 4°(2θ)/min over 10−80° was applied. The minerals in samples were quantified by SIROQUANT technique developed by Taylor.15 The software uses the full-profile Rietveld method of refining the shape of a calculated XRD pattern against the profile of a measured pattern. The total calculated pattern is the sum of the individual phase calculated patterns.13 To determine the content of amorphous

in samples, two different spiking materials could be used including a synthetic corundum powder and a crystalline zinc oxide.16 Zinc oxide was added into samples to determine the content of amorphous in this work. The ash mixed zinc oxide was ground in an agate pestle and mortar until a homogeneous color and texture was achieved. The thermodynamic software package FactSage is the integration of two well-known software packages in computational thermochemistry: Fact-Win and ChemSage.17 The isothermal reaction properties (ΔH, ΔS, ΔG, ΔV, and ΔA) of a stoichiometric reaction were determined by the Reaction module in FactSage. The ΔG was used to evaluate the possibility of mineral reactions under different conditions. Besides, the Equilib module was also used for verifying the occurrence of carbon thermal reactions in gasification. A high resolution scanning electron microscope (JSM-6700F) with an energy dispersive spectrometer (EDX) was employed for investigating the morphological change of mineral in high-temperature char. 4548


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3. RESULTS AND DISCUSSION 3.1. Mineral Transformation in Raw Ash without Char. XRD patterns for HTAs and the content of major minerals in HTAs are shown in Figures 1 and 2, respectively. The XRD pattern of HTA-1500 is not shown here because it is very similar to that of HTA-1400. The possible reactions among minerals are listed in Table 3, and ΔG of some reactions was also calculated by FactSage. In Figure 2, the content of anhydrite (CaSO4) decreases monotonously with increasing temperature, and it disappears above 1100 °C. It is clear that the selfdecomposition of anhydrite as eqs 1 and 2 in Table 3 is impossible below 1700 °C for thermodynamic limitation. However, the existence of CO impels the decomposition of anhydrite between 950 and 1100 °C. The decomposition temperature of anhydrite drops dramatically under reducing atmosphere.12 Furthermore, the generated CaO reacts with SiO2 and Al2O3 to form gehlenite (Ca2Al2SiO7) easily at high temperature (eq 4), which also accelerates the decomposition of anhydrite. In addition, the oldhamite (CaS) is found in the HTAs at 950 and 1000 °C for the reaction in eq 5, and the calculation results by Factsage prove that the reaction could occur at 950 °C. The content of gehlenite increases sharply with the elevated temperature so that a large amount of CaO is consumed. Therefore, most anhydritre decomposes to form CaO (eq 3) and the oldhamite is not formed above 1100 °C. A large amount of gehlenite forms in the HTAs from 950 to 1300 °C by eq 3. The content of gehlenite first increases with elevated temperature and reaches maximum at 1200 °C and then decreases above 1200 °C due to the formation of the eutectic mixture. The gehlenite melts at 1400 °C into vitreous matters in HTAs. Meanwhile, the crystalline minerals vanish away above 1400 °C. The major Fe-containing species in HTAs are hematite (Fe2O3) and magnetite (Fe3O4) according to Figure 1. Hercynite (FeAl2O4) and fayalite (Fe2SiO4) are not found in this experiment, which may be due to the low content of SiO2 and Al2O3. Wang et al.18 found that anorthite (CaAl2Si2O8) and gehlenite were formed more easily than Fe and Ca−Fe aluminosilicates because the ΔG of eqs 4 and 6 are lower than those of eqs 7−9. The hematite and magnetite disappear at 1300 °C, which is attributed to the formation of eutectic mixture with SiO2 and aluminosilicates. The content of amorphous phase in the XLT raw ash is up to 40.9%. Some of them exist originally in the coal, and the other is from the decomposition of minerals in coal during ashing. The content of amorphous phase first decreases from 950 to 1100 °C due to the fact that the oxides in amorphous phase participate in the formation of gehlenite and then increases above 1200 °C because of the formation of the eutectic mixture. 3.2. Mineral Transformation in Char. Figure 3 shows XRD patterns for LTAs from raw coal and slow pyrolysis chars prepared under reducing atmosphere from 950 to 1500 °C, and the quantitative results of minerals are shown in Figure 4. The major minerals in XLT coal are quartz (SiO2), calcium sulfate hydrate (CaSO4·H2O), and pyrite (FeS2). Quartz disappears above 1100 °C because it reacts with CaO and Al2O3 to form aluminosilicates. The calcium sulfate hydrate loses crystal water easily and transforms to anhydrite with increasing temperature. The pyrite is oxidized to hematite and magnetite during pyrolysis. The major crystalline minerals in the chars prepared under reducing atmosphere at high temperature are oldhamite, gehlenite, hematite, and magnetite.

Figure 4. Content of major minerals in the chars prepared under reducing atmosphere at different temperatures.

Figure 5. Comparison of gehlenite content in HTAs and LTAs.

3.3. Effect of Char on Mineral Transformation. The minerals in the chars largely differ with those in HTAs at the same temperature range. The presence of char influences the mineral transformation at high temperature dramatically. Only a small amount of anhydrite in chars transforms to gehlenite compared with HTAs, while most anhydrite in chars transforms to oldhamite at high temperature. The content of oldhamite in chars is even higher than that of gehlenite, and increasing temperature is beneficial to the formation of oldhamite in chars. Moreover, anhydrite still exists in the chars even at 1500 °C, although its content is quite low. The oldhamite forms in HTAs at 950 and 1000 °C due to eq 5, but it is not found in the HTAs above 1100 °C. Hence, the formation of a large amount of oldhamite may be influenced by the presence of chars. Results by Factsage indicate that the eqs 10 and 11 could occur between 950 and 1500 °C. In addition, the ΔG of eq 11 is much lower than those of eqs 5 and 10 at the same temperature. Therefore, the eq 11 should be responsible for the formation of a large amount of oldhamite in the char at high temperature. Similar with the HTAs, the major Fe-containing minerals in the char are hematite and magnetite. There are no crystalline minerals in the HTAs above 1300 °C, and almost all the 4549


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Figure 6. XRD patterns of LTAs from the chars prepared under Ar atmosphere between 1300 and 1500 °C. Mo, moissanite 3C, syn(SiC).

minerals transform to vitreous matters. By contrast, the crystalline minerals such as oldhamite, gehlenite, and iron oxide still exist in the char at 1400 and 1500 °C. The eutectic mixtures could not form largely in the char due to the interaction with char matrix. Figure 5 illustrates the comparison about the content of gehlenite in HTAs and LTAs. The content of gehlenite in HTAs is much higher than that in XLT chars at the same temperature. The crystalline minerals in HTAs vanish away above 1300 °C, while they still exist in the char at 1500 °C. The predominant mineral in HTA and LTA is gehlenite and oldhamite, respectively. These results indicate that the presence of carbon in the char influences seriously on the minerals transformation at high temperature. At first, most anhydrite in char transforms to oldhamite rather than gehlenite due to the reduction action of carbon. Furthermore, the minerals disperse in the char and partially contact with each other closely and thus the probability for reaction to form aluminosilicates drops obviously. In hence, catalytic minerals, beneficial to the char gasification, could remain in the char at high temperature. 3.4. Effect of Atmosphere on the Carbon Thermal Reaction. Wang et al.9,10 found that SiO2 and CaO reacted with char to form SiC and CaC2 at high temperature, and Liu12 et al. also found that SiO2 reacted with carbon in slow pyrolysis char to generate SiC at 1500 °C. The mechanism which is generally accepted is as follows.10 SiO2 + C → SiO(g) + CO

(15)

SiO(g) + 2C → SiC + CO

(16)

However, Figure 3 illustrates that no carbide is found in XLT char between 950 and 1500 °C. Experiments in previous studies were performed under inert atmosphere instead of gasification atmosphere. To clarify the effect of atmosphere on carbide formation, the slow pyrolysis chars were also prepared under argon with the same procedure as mentioned in

Figure 7. Content of SiO2 (a) and SiO (b) in carbon thermal carbon reaction under different atmospheres.

Section 2.2. LTAs of the slow pyrolysis chars were analyzed by XRD as shown in Figure 6. 4550


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Figure 8. XRD patterns of LTAs from (a) raw char (carbon conversion XC = 0), (b) partially gasified char (XC = 47%), and (c) gasification residue (XC = 100%) at 1100 °C. Q, quartz; Ah, anhydrite; Gh, gehlenite; H, hematite; Mg, magnetite; O, oldhamite; L, lime.

The moissanite (SiC) was found in XLT char prepared under Ar atmosphere between 1300 and 1500 °C, which indicates that the carbon thermal reaction between SiO2 and char occurs under inert atmosphere. The thermodynamic results by FactSage in Figure 7a also indicate that SiO2 transforms to SiC completely at 1220 °C under Ar atmosphere. With the ratio of CO in atmosphere increasing, the reaction of SiO2 and C becomes more difficult due to the restrain by CO under reducing atmosphere according to chemical equilibrium. Both reactions in eqs 15 and 16 are restrained by increasing ratio of CO because both the formation temperature and the content of SiO increased as shown in Figure 7b. The initial reaction temperature for SiO2 and C is 1472 °C when the concentration of CO is 60% in the atmosphere according to Figure 7, but when the char is gasified with mixture gas (CO:CO2 = 6:4), the concentration of CO during gasification should be far more than 60%. Thus, the formation temperature of SiC during gasification should be at least higher than 1500 °C. The investigation in this work is only performed from 950 to 1500 °C, so it is reasonable that no SiC found in the char under reducing atmosphere. Hence, the carbon thermal reaction during gasification should occur at the temperature which is high enough to overcome the restrain effect by CO from char gasification. Other carbides such as CaC2, FexCy, and so on are not found in the char, which may be associated with the kind of minerals in char. Except SiC, the major minerals in chars prepared under Ar atmosphere still are oldhamite, gehlenite, hematite, and magnetite, which are similar to the chars prepared under reducing atmosphere. 3.5. Mineral Transformation during Char Gasification. The partially gasified chars at 1100 °C were prepared to explore the mineral transformation during char gasification. XRD patterns of chars with different carbon conversion at 1100 °C and the contents of typical minerals in chars are shown in Figures 8 and 9, respectively. Obviously, the content of

Figure 9. Contents of gehlenite and oldhamite in chars with different carbon conversion at 1100 °C.

oldhamite decreases monotonously with proceeding of the gasification reaction and finally disappears. Meanwhile, the content of gehlenite increases monotonously with the increasing carbon conversion (Figure 8). The anhydrite vanishes away in the gasification residue, which proves that anhydrite decomposes easier in the ash than in the char. Garciá et al.19 found that CaS was oxidized by O2 to generate CaSO4 during coal combustion. Obviously, this reaction cannot occur during char gasification as anhydrite trends to decompose in this process. As shown in Table 3, the oldhamite cannot be oxidized by CO2, although it disappears after the feeding of CO2 because eqs 12−13 cannot occur between 950 and 1500 °C for thermodynamic limitation reasons. However, the char gasification proceeds with the migration of the oxygen atom. The oldhamite may be oxidized by the oxygen atom in coal to form CaO, and the calculation results of ΔG show that eq 14 could occur at this temperature range. Lime is found in the gasification 4551


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Figure 10. SEM images and corresponding EDX analysis of XLT chars at different temperatures.

to form CaO. The predominant mineral in gasification residue becomes gehlenite, which is similar with the HTAs. 3.6. Morphological Changes of Minerals in Char. Not only does the mineral transformation take place at high temperature but also the morphology of the mineral changes in char. The morphological transformation of minerals in char at high temperature also has a notable effect on the char gasification

residue, and it should be the product of eq 14. Similar observations are also obtained for the partially gasified chars at 950 and 1300 °C. Furthermore, the generated CaO reacts with SiO2 and Al2O3 to form gehlenite, and this reaction could promote the proceeding of eq 14. Hence, the oldhamite decreases and even disappears during gasification because it is oxidized by oxygen atom generated in the process of gasification 4552


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reactivity because the catalytic action of minerals is associated with their contact area with carbon particles in char and the aggregation of minerals reduces the contact area at high temperature. The morphology of minerals in char between 950 and 1500 °C were investigated by SEM-EDX. The SEM images and corresponding EDX analysis of the char at 950, 1100, 1300, and 1500 °C were illustrated in Figure 10. In Figure 10a, the minerals disperse homogeneously on the surface of the char at 950 °C, and almost no aggregation was found among the mineral particles. With temperature increases, the inorganic minerals begin to aggregate on the char surface at 1100 °C (Figure 10b). In Figure 10b,b′, the areas 1 and 3 contain more inorganic minerals than area 2. In area 3, it is mainly Ca-containing minerals. The minerals aggregate seriously and develop to smooth sphere adhered to the surface of char at 1300 °C (Figure 10c). The EDX analysis (Figure 10c′) shows that the content of C in the spheres lessens greatly, and the spheres contain more inorganic minerals. This implies that the minerals separate out from char to some extent. Larger spheres form at 1500 °C (Figure 10d), and the spheres bond together on the surface of char. The EDX analysis at 1500 °C (Figure 10 d′) shows that much more inorganic minerals separate out from chars and they adhere to the surface of char. Comparing the morphological change of char at different temperatures, it is found that more and more minerals aggregate on the surface of char with the elevated temperature. Although the sphere surface became smooth above 1300 °C, EDX analysis indicates that the spheres still contain a certain amount of carbon. In other words, the minerals aggregate at high temperature but they cannot be separated wholly from carbon. The whole process mentioned above about morphological transform of minerals in char at high temperature could be described as Figure 11. The aggregation of

leads to the formation of oldhamite and the dilution effect for minerals by char. (2) The carbon thermal reaction is impeded by CO in the atmosphere. Moissanite (SiC) is only found under inert atmosphere above 1300 °C but not under gasification atmosphere due to the impeded effect by CO. (3) The minerals in char aggregate to form smooth spheres adhered to the surface of char above ash flow temperature. The aggregation of minerals at high temperature not only lessens the effective contact area of catalytic minerals with carbon but also reduces the contact of gasification agent with char.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-0351-4048967. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (no. 2010CB227003), National Natural Science Foundation of China (no. 21006121), Shanxi Province Science Foundation for Youths (2010021008-2), and the Joint Foundation of Natural Science Foundation of China and Shenhua Group Corporation Ltd. (U1261209).

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Figure 11. The morphological transform of minerals in char at high temperature.

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minerals on the surface of carbon reduces the effective contact area of catalytic minerals with carbon, which weakens the catalytic action of minerals on the gasification reaction. Moreover, the presence of spheres on the surface of char hinders the contact of gasification agent with carbon, which decreases the gasification efficiency.

ABBREVIATIONS USED HTA = high temperature ash LTA = low temperature ash HTA-X = (X represents the heat treatment temperature of ash, X = 950, 1100, 1300, 1400, 1500) the high temperature ash prepared from the raw ash at X °C CR-X = (C stands for char, R means reducing atmosphere, and X is coal pyrolysis temperature, X = 950, 1100, 1300, 1400, 1500) the slow pyrolysis char prepared from XLT coal under reducing atmosphere (VCO:VAr = 6:4) at X °C LTA-CR-X = (X = 950, 1100, 1300, 1400, 1500) the low temperature ash from the char sample CR-X LTA-CA-X = (A means argon atmosphere, X = 1300, 1400, 1500) the low temperature ash from the char prepared under argon atmosphere at X °C REFERENCES

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4. CONCLUSIONS The mineral transformation in XLT ash and XLT pyrolysis char under reducing atmosphere between 950 and 1500 °C were investigated by XRD and SIROQUANT. The influences of char on mineral transformation, the effect of atmosphere on carbon thermal reaction and mineral transformation were discussed in detail but concisely. The main conclusions could be drawn from this work: (1) The presence of char impedes the formation of aluminosilicates, such as gehlenite, because the competition reaction 4553


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