Influences of mineral matter on high temperature gasification of coal char

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 37, Issue 2, April 2009 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2009, 37(2), 134138

RESEARCH PAPER

Influences of mineral matter on high temperature gasification of coal char BAI Jin1,*, LI Wen1, LI Chun-zhu2, BAI Zong-qing1, LI Bao-qing1 1

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

2

Department of Chemical Engineering, Monash University, VIC 3800, Australia

Abstract: X-ray diffraction (XRD) was performed to analyze the mineral matter in coal ash from 1100 to 1500qC, with an interval of 50qC. Relative Intensity Ratio (RIR) method was used to calculate the content of each crystalline in coal ash. The amount of mullite increases with elevated temperature but that of SiO2 decreases. The transformation of mullite and SiO2 accords well with the binary phase diagram of SiO2–Al2O3. At high temperature, the amorphous mineral matters, mainly, aluminosilicates melt above ash softening temperature. During CO2 gasification at high temperature, the carbon conversion is hindered by the aluminosilicates melts, which cover the surface and block the pores of coal particles. Because of the short and ordered sequence of melts, FT-IR analysis was taken for the melts in coal ash after gasification. The structure alteration of aluminosilicates influences the surface tension of melts, which determines the interaction between coal particles and aluminosilicates. Keywords:

aluminosilicate; melts; high temperature gasification

Coal gasification is the prominent clean method for coal utilization, and large scale entrained flow gasification technique is the key and promising technology for gasification. The operation temperature, which is important for the gasification characteristic and process in entrained flow gasifier is up to 1700qC, which is far above the ash fusion temperature. However, the previous works on mineral matters were focusing on the transformations at approximately 1000qC and few works were performed above ash fusion temperature[1–3]. Efficient gasification of a particular coal depends sensitively on the high-temperature behavior of mineral matters in coal char. The traditional parameters used to classify coals for gasification applications are the ash soften temperature (ST) and fluid temperature (FT). Deformation temperature (DT), ST and FT, together with various empirical formulae based on ash chemistry, have been widely used to evaluate the influences of mineral matters on high temperature coal gasification. Although the ash-fusion test and ash chemistry provide useful guidelines, they are often unreliable in predicting the behavior of minerals in real gasification processes. Alkaline and earth alkaline in coal ash are catalyst for coal gasification reaction, but most of them volatilize when the

temperature is reaching 1000qC[4]. A few alkaline and earth alkaline metals still in the ash were in the form of aluminosilicates, which have little catalytic effect on gasification. Tang et al studied char gasification with CO2 from 900 to 1500qC, and concluded that the molten ash hindered the gasification reaction at high temperature[5]. Sun et al related characteristics of molten ash to the structures of melts and figured out the method to estimate the structures of melts with chemical composition of ash[6]. Li et al investigated the structure of aluminosilicates with IR and considered that the shift of IR bands was caused by the alteration of melts structure[4]. The influence of aluminosilicates on gasification was determined by the contact between the melts and char particles, and the surface tension and viscosity of melts decided the contact angle. In this work, the high-temperature (1100–1500°C) transformations and reactions undergone by the mineral-derived phases in Huainan coal ash samples under reducing conditions were investigated by X-ray diffraction, and the CO2 gasification of chars prepared at different conditions were also taken according to GB/T 220-2001 to investigate influences of mineral matter on high temperature char gasification. The ashes separated from the gasification

Received: 13- Sep -2008; Revised: 28- Nov -2008 * Corresponding author. 86-351-4048967, E-mail: [email protected] Foundation item: Supported by the Major State Basic Research Development Program of China (973 Program, 2004CB217704-3) and International Joint Project of MOST (2005DFA60220). Copyright ” 2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

BAI Jin et al. / Journal of Fuel Chemistry and Technology, 2009, 37(2): 134138

residues were studied by FT-IR to decipher the mineral transformation during gasification.

1 1.1

Experimental Samples

Huainan (HN) coal was selected primarily with the aim of representing high FT coal in China. The chars were prepared with 80 meshes coal in a muffle furnace. The slow pyrolysis chars were heated with 6qC/min to 950, 1200, and 1400qC, respectively, and kept for 20 min. The slow pyrolysis chars are marked as HM950, HM1200, and HM1400. The rapid pyrolysis chars were prepared in a drop tube furnace and the temperatures were 950, 1200, and 1400qC, respectively, with the residence time of approximately 2 s. The rapid pyrolysis chars are denoted as HK950, HK1200, and HK1400. The de-ash chars were prepared based on GB/T7560-2001. Raw ash samples for the investigation of minerals transformation were prepared at 815ºC based on GB/T 1574-2001. Briefly, the temperature rises to 500qC within 30 min, and then, holds for another 30 min. After that the temperature rises to 815qC and then keeps for approximately 60 min. The ultimate and proximate analyses of chars and HN coal are given in Table 1. The chemical compositions and melting temperatures of coal ash prepared at 815qC according to GB/T1574-1995 are listed in Tables 2 and 3. 1.2

Experimental procedure

In the ash-fusion test, a graphite crucible was used to rapidly quench the ash sample from the high temperature at reducing atmosphere (CO2 : CO=6 : 4) into ice water. Total quenching time was normally from 5 to 10 s, and ash samples were treated at the interval of 50qC from 1100 to 1500qC. All quenched ash samples were collected and examined by X-ray diffraction (XRD). Table 1 Sample

HN coal chars with the particle size of 80 meshes, including rapid pyrolysis chars (HK), slow pyrolysis chars (HM), and ash free slow pyrolysis chars (HMD), were loaded in a bench scale fixed bed reactor at 50°C intervals from 1100 to 1500°C to test their CO2 gasification behaviors. The atmosphere was the mixture of 60% CO2 and 40% Ar. The gas flow rate was 300 mL/min to reduce the internal and external diffusions. The residues after gasification were separated by hand and the ash particles were collected and analyzed by FT-IR. 1.3

Analysis

The XRD data were collected using an ARL diffractometer. Relative Intensity Ratio(RIR) method was performed for quantitative analysis[7]: Crystallinity [%] = 100 × Ȉ Inet. / (Ȉ Itot. – Bgr.const.), where Inet: crystal intensity; Bgr.const: background intensity; Itot: total intensity. RIR values determined from PDF-2 Date Base were used for this study. Calibration constants (RIRs) can vary significantly depending on actual phase compositions. The NICOLET NEXUS 470 FTIR was used for mineral analysis from 4000 cm–1 to 400 cm–1.

2 2.1

Results and discussion Mineral transformation

The mineral compositions in HN coal ash at 815°C were determined by XRD[8]. As shown in Fig. 1, quartz, iron oxide, and grossite are the major crystal phases. The humps between d=4.6162~3.4776 indicate that the amorphous phases are quartz and grossite. The following reactions are possible at 815°C among minerals in coal: Al2O3·2SiO2·2H2Oĺ2H2O+2SiO2+Al2O3 3CaO+ Al2O3ĺCa3Al2O6 CaO+ 2Al2O3ĺCaAl4O7 4FeCO3+O2ĺ2Fe2O3+4CO2

Proximate and ultimate analyses of HN coal and chars

Proximate analysis w/%

Ultimate analysis wdaf /%

Ad

Vdaf

FCdaf

C

H

St,d

N

HN coal

18.30

44.07

55.93

84.12

HK950

24.29

13.24

62.47

67.44

6.07

1.54

0.44

2.35

0.380

HK1200

26.22

6.42

67.36

1.87

69.45

1.13

0.343

1.81

HK1400

26.97

5.09

HM950

26.14

1.68

67.94

70.18

0.66

0.303

1.60

72.18

74.08

0.24

0.312

0.40

HM1200

26.35

HM1400

26.55

1.42

72.23

74.54

0.14

0.304

0.13

1.08

72.37

75.71

0.10

0.214

0.09

BAI Jin et al. / Journal of Fuel Chemistry and Technology, 2009, 37(2): 134138 Table 2

Chemical composition of HN coal ash

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

K2O

Na2O

55.60

33.50

3.83

2.15

0.68

1.41

0.46

0.54

Table 3

Melting temperatures of HN coal ash

DT*

ST*

FT*

1250qC

1510qC

1600qC

DT*: Deformation temperature, ST*: Soften temperature, FT*: Fluid temperature

Fig. 1

XRD patterns of HN coal ash at 815°C

XRD results of different mineral phases observed in quenched ash samples (60% CO and 40% CO2) are summarized and compared in Fig. 2. The major components are SiO2, mullite (3Al2O3·2SiO2), and sillimate (Al2SiO5). The amount of SiO2 decreases greatly from 1100 to 1500°C. Mullite is the main component at high temperature, and its content increases with the rising temperature. The amount of sillimate shows a maximal value at 1300°C, which is the concrete evidence of solid-state reactions between different mineral components at high temperature: CaAl4O7ĺCaO+2Al2O3

Fig. 2

Minerals transformation for HN coal ash at different

Al2O3+SiO2ĺAl2O3·SiO2 Al2O3·SiO2+Al2O3+ SiO2ĺ3Al2O3·2SiO2 Fayalite, a layer-silicate mineral rich in iron and silicate, appears to transform sometimes to glassy or poorly crystalline phases and sometimes to ferrite phases when the temperature is above 1350°C[2]. However, the ferrite phases are instable because Fe2+ and Fe3+ have strong ionic polarization, which might be the reason without ferrite phase detected by XRD. The occurrence mode of minerals at high temperatures is not only in crystal phase but also in amorphous phase. The ratio of them calculated from XRD is shown in Fig. 3. Most mullite and SiO2 are in crystal phase for their refractory behavior. The minerals in amorphous phase have great influences on the polymerization degree and surface tension of the molten fluids. Surface tension influenced by polymerization is the essence of the contact between aluminosilicate and char particles. Because of the shortage of accurate method to analyze the amorphous phase at present, FT-IR is a potential method to determine the alteration of melts structure with its sensitivity to short range order structure. 2.2 2.2.1

Chars gasification Rapid pyrolysis char

Gasification characteristics of HN coal chars are shown in Fig. 4. The initial conversions are all quite high for rapid pyrolysis chars. The initial conversions of HK950 and HK950-d are higher than those of HK1200, HK1400, HK1200-d, and HK 1400-d, which represent that the gasification conversion of char decreases with the increasing temperatures of char preparation. When the gasification temperature is approximately 1350°C, the conversions of rapid pyrolysis chars have a decrease tendency.

Fig. 3

Ratio of non-crystal and crystal at different

temperatures

temperatures

: SiO2; : sillimate; : mullite; 1: fayalite

: noncrystal/crystal

BAI Jin et al. / Journal of Fuel Chemistry and Technology, 2009, 37(2): 134138

Fig. 4

Gasification characteristics of rapid HN coal chars

(a): rapid pyrolysis char; (b): de-ash rapid pyrolysis char : HK950; : HK1200; : HK1400

However, the conversions of de-ash rapid pyrolysis chars still increase at approximately 1350°C. The difference gasification behavior of raw coal chars and de-ash coal chars is caused by the structure changes of aluminosilicates. Molten aluminosilicates at high temperature can block the reactions between char and CO2. When the temperature is above 1350°C, the conversion sequence of rapid pyrolysis char was HK1400>HK950>HK1200. Melts with different structures have different polymerization degree, which leads to different surface tension, and therefore, the contact between chars and melts is discrepant. Figure 5 shows the FT-IR spectra of ashes in rapid pyrolysis char gasification residues. As shown in Fig. 5, the absorption band at 1500 cm–1 is caused by C=C variation, and Si–O band is approximately 1000 cm–1, Si–O–Al band is under 550 cm–1[9], and bands at approximatey 450 cm–1 are assigned to Si–O–Si. The shifts of Si–O, Si–O–Si, and Si–O–Al bands can represent the polymerizaiton degree of aluminosilicates[10]. The surface tension determined by the polymerization degree has great influence on the spread of molten minerals on the char surface. Table 4 shows variation of peaks in IR of mineral matters in HK chars. In Table 4, the Si–O–Si asymmetric stretching band moves to low wave number and the Si–O symmetric vibration band moves to high wave number. These results illustrate that the polymerization degree increases and the surface tension of melts increases. Thus, the chances for melts spreading on the char surface and blocking char pores are reduced. However, the change of asymmetric vibration of Si–O–Al does not follow any rules. In aluminosilicates, Al can replace Si to be the former of frameworks and the alkaline cations act as valence balance of the structure. When Al3+ is in the tetrahedron structure, it can enhance the polymerization of aluminosilicates. In contrast, Al3+ can also be a frameworks modifier in the valence vacancy of octahedron structure and in this condition it can reduce the

polymerization of aluminosilicates. The bands of Si–O–Al are sensitive to cations[10], so the band alterations of Si–O–Al are not available for judging the polymerization degree. On the basis of the alteration of wave numbers, it is deduced that the surface tension sequence is HK1400>HK950>HK1200. Considering the gasification characteristic of rapid chars, this sequence can explain the gasification reactivity of rapid chars well. 2.2.2

Slow pyrolysis char

Figure 6 shows the gasification characteristics of slow HN coal chars.

Fig. 5

FT-IR spectra of ashes in rapid pyrolysis char gasification residues

Table 4

Variation of peak in IR of mineral matters in HK chars (cm–1)

Sample

Si–O–Si

Si–O

Si–O–Al

Asymmetric

Symmetric

Asymmetric

stretching

stretching

vibration

HK950

1100

812

553

HK1200

1090

820

555

HK1400

1080

829

553

BAI Jin et al. / Journal of Fuel Chemistry and Technology, 2009, 37(2): 134138

Fig. 6

Gasification characteristics of slow HN coal chars

(a): slow pyrolysis char; (b): de-ash slow pyrolysis char : HM950; : HM1200; : HM1400

Fig.7

FT-IR spectra of ashes in slow pyrolysis char gasification residues

Table 5

Variation of peak in IR of mineral matters in HM chars (cm–1) Si–O–Si

Si–O

Si–O–Al

Asymmetric

Symmetric

Asymmetric

stretching

stretching

vibration

HM950

1090

816

552

HM1200

1080

798

555

HM1400

1090

798

553

Sample

The initial gasification conversions of char decrease with the temperature increase of char preparation because of the graphitization of char structure. The gasification behaviors of raw char and de-ash char are similar because the mineral matters in coal are already agglomerated during the char preparation process. Melt minerals have little effect on the specific surface area of chars. When the gasification temperature is above the char preparation temperature, the aluminosilicates will re-melt and suppress the gasification reaction, which results in the decrease of char conversions of HM1200 and HM1400 above 1400°C. The initial conversions of de-ash slow pyrolysis chars are lower than

those of raw slow pyrolysis chars, which indicates that de-ash process might have little pore expansion effect on de-ash slow chars or some minerals might have catalytic gasification effect on raw coal chars. FT-IR was also used to analyze the ash separated from slow pyrolysis char gasification residues. The FT-IR spectra of ashes in slow pyrolysis char gasification residues are shown in Fig. 7, and the alterations of the wave number are listed in Table 5. The wave numbers of Si–O–Si asymmetric stretching band are in the following order: HM1200
3

Conclusions

The major crystal phases in HN ash are mullite and SiO2. The content of mullite increases with temperature because mullite is thermodynamically stable at high temperature. However, the content of SiO2 decreases because of the reactions between SiO2 and Al2O3. Both mullite and SiO2 are melted at very high temperature, so the molten mineral is amorphous aluminosilicates. Using FT-IR method, the degree of aluminosilicate polymerization can be determined. During gasification process, higher degree of aluminosilicate polymerization has less impact on the gasification reaction.

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