Mineral transformation and morphological change during pyrolysis

Article Cite This: Energy Fuels 2019, 33, 6134−6147

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Mineral Transformation and Morphological Change during Pyrolysis and Gasification of Victorian Brown Coals in an Entrained Flow Reactor Tao Xu* and Sankar Bhattacharya

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Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia ABSTRACT: Mineral transformations and morphological changes of Victorian brown coals were investigated in a two-stage process, coal pyrolysis followed by gasification of char in CO2, using an entrained flow reactor. The mineral transformations during coal pyrolysis and char gasification were examined over a wide range of temperatures between 700 and 1400 °C by X-ray diffraction and secondary scanning electron microscopy with energy-dispersive X-ray spectroscopy. In general, mineral transformations of Victorian brown coals were found to happen at high temperatures (1000−1400 °C), not at low temperatures. During coal pyrolysis, Fe2O3 from the oxidation of Fe3O4 was formed in Yallourn (YL) samples, but Fe3O4 from the reduction of Fe2O3 was formed in Maddingley (MD) samples. During char gasification, reduction of Fe2O3 by CO and decomposition of CaSO4 by CO were found in YL and MD samples. Furthermore, CaO from CaSO4 decomposition was transformed to Ca2SiO4 in YL and was transformed to CaMgSiO4 in MD samples. YL and MD also showed similar morphological changes during gasification. Mineral constituents with high Fe content were first found at 1000 °C because of reduction of Fe2O3. Melting of sulfates like MgSO4 and ablite was then found at 1200 °C. However, Loy Yang coal with a high percentage of SiO2 seemed to be thermochemically stable in terms of the behavior of mineral matters. Mineral transformations during char gasification of YL and MD tended to decrease the ash fusion temperature and slag viscosity, and enhance the gasification rate in CO2. inorganic species.12 Accordingly, it is crucial to understand the behavior of the minerals during gasification for the design and operation of entrained flow gasifiers. Extensive research on mineral transformation during combustion of coal has been conducted in the past through Xray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), gas adsorption analyses, etc.13−15 Combustion happens in an oxidizing atmosphere, but gasification usually operates in a reducing atmosphere (H2 + CO). Because of this, mineral behavior during gasification can be distinctly different from that during combustion. Mineral transformation during coal gasification has been investigated in a thermogravimetric analyzer (TGA) and fixed bed. Mei et al. investigated the Na-containing mineral transformation during CO2 gasification and found that Na2CO3 was transformed into sodium aluminum silicates at 700 °C, which contributes to the deactivation of the catalyst.16 Wu et al. found that the formation of eutectic mixtures in the FeO−SiO2−Al2O3 and CaO−SiO2−Al2O3 accelerated the ash melting during gasification.17 Ma et al. found anhydrite reduced by carbon to form oldhamite, which also transformed to gehlenite during char-CO2 gasification.18 However, most previous research was conducted at low temperatures below 1100 °C, which may not be applicable for high-temperature entrained flow gasification (1200−1400 °C). Moreover, previous research using fixed bed or TGA, which underwent

1. INTRODUCTION Australia has the second largest brown coal reserves, 36.2 billion tonnes, in the world,1,2 and approximately 97% of Australian brown coal resources are located in Victoria. In Victoria, brown coal supplies more than 80% of the electricity generation.3 Victorian brown coal is generally used in mine-mouth power plants with low efficiency and great greenhouse emission. Therefore, the development of reduced air pollution and more efficient technologies for such vast resources is being driven by government and industry.4 Coal gasification is one clean coal utilization technology which turns coal to high-value liquid fuels and chemicals.5 Gasification is a heterogeneous reaction which is generally divided into coal pyrolysis and the subsequent char gasification. There are three main gasifier types: fixed-bed gasifiers, fluidizedbed gasifiers, and entrained flow gasifiers. Currently, entrained flow gasifiers are the most commonly used gasification technology for large-scale integrated gasification combined cycles (IGCC) and coal-to-products applications, as they are suitable for any rank coal in the form of a slurry or dry.6 Entrained flow gasifiers achieve high throughput and conversion for a wide range of feedstocks by operating at high temperature (up to 1700 °C) and low residence time (usually several seconds residence time).6−8 It is widely acknowledged in the literature that the behavior of the inorganic minerals of lignites during gasification is just as importantperhaps even more importantas the entrained flow gasification performance and efficiency, especially for the commercial application of lignites in gasification.9−11 In entrained flow gasification applications, the behavior of minerals in coal is directly related to heat transfer, ash fusion, and slagging reactions and release of the © 2019 American Chemical Society

Received: March 26, 2019 Revised: June 25, 2019 Published: June 27, 2019 6134

DOI: 10.1021/acs.energyfuels.9b00924 Energy Fuels 2019, 33, 6134−6147

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Energy & Fuels

which can be heated up to 1000 °C. This reactor is of 2 m length and 50 mm diameter and is described in detail elsewhere.5,20 Using this EFR, low-temperature chars and gasified chars were prepared at 700−900 °C. Typically, the reactor was electrically heated to the desired temperature. Coal samples were fed from the top and pyrolyzed in the reactor under an N2 gas flow rate of 5 L/min. The pyrolyzed char samples were then gasified at 20% CO2 in N2 at the same temperature range and total gas flow rate of 5 L/min to generate gasified chars. The residence time for pyrolysis and gasification process was around 7 s, calculated based on gas flow rate and particle velocity. The second EFR is a high-temperature one and can be heated up to 1650 °C. This EFR is of 3.8 m length and 89 mm diameter, and the detail is described elsewhere.7 The high-temperature chars and gasified chars at 1000−1400 °C were generated by using this reactor. A typical experimental procedure is as follows: the reactor was preheated to the desired temperature at least 3 h before coal feeding. The nitrogen gas was introduced and flushed in the reactor for at least 1 h to remove the oxygen. When the oxygen level in the system detected by the micro-GC was below 0.5%, the coal samples were fed by the screw feeder from the top at a feeding rate of around 1 g/min. The coal particles were entrained by N2 (16 L/min) into the reactor and were pyrolyzed in the reactor with a residence time of around 7 s to generate the chars. The pyrolyzed chars were then gasified at 20% CO2 in N2 under a similar feeding rate of around 1 g/min, the same temperature, and the same total gas flow rate of 16 L/min to generate gasified chars. The residence time for gasification procedure was around 7 s. 2.3. Sample Analysis. The samples of raw coals, chars, and gasified chars were burned at 550 °C for 6 h in a muffle furnace. The ashing method used in the study, especially for raw coal, is to maximize the fixation of sulfur in the ash which retained as sodium and calcium sulfates. Some research has shown that the amount of sulfur fixed during ashing of Victorian brown raw coal is independent of the temperature at 370−800 °C.21 The non-mineral iron, magnesium, and aluminum form individual oxides and complex mixed oxide species. Any pyrite is oxidized to hematite (Fe2O3). The other mineral species, quartz and clay are not affected under the conditions of ashing procedure. The influence of ashing on char should be limited, and minerals in chars are stable at 550 °C in air. Fe2O3 and Fe3O4 are stable at temperature below 570 °C in air.22 After gasification, gasified chars still contain a large amount of carbon content. For example, YL gasified ash has ∼70% carbon content at 1000 °C and has around 26% carbon content at 1200 °C. Because of the high content of carbon, the ashing procedure becomes necessary for gasified chars to determine the mineral phases by XRD. The XRD patterns of their ash samples were then recorded using a Rigaku MiniFlex 600 XRD instrument with copper K radiation (30 kV, 30 mA). A step size of 0.2° at the speed of 1° (2θ) over the angular range of 5 to 100° was applied. The minerals in the samples were identified and qualified by the XRD analysis software MDI Jade 6.5. This study investigated the major minerals (i.e., Al2O3, CaO, Fe2O3, MgO, SiO2 and silicates) that are (1) found in char and (2) crystalline in nature which can be detected by XRD. Alkali and alkaline earth metals (normally vaporize at studied temperatures) and few other minerals have not been investigated here. The morphological change of minerals in chars and gasified chars were investigated using a high-resolution scanning microscopy (JEOL JSM-7001F FEGSEM) which had secondary electron microscopy (SEM), backscattered electron microscopy (BSEM), and energy dispersive X-ray spectroscopy (EDX). In this study, The SEM was used to identify the sample surface morphology, the BSEM was used to detect the distribution of inorganic matters in the sample, and the EDX was used to provide the detailed information on the molten part and mineral-rich part. The Factsage 6.4, a well-known thermodynamic software package, was used to predict the reaction and transformation of inorganic matters during CO2 gasification of coal. The minerals under different conditions were predicted by the Equilib module in Factsage. The isothermal reaction properties (ΔG and ΔH) of a stoichiometric reaction were determined by the Reaction module in Factsage. The ΔG

low gas throughput and low heating rate, is not consistent with entrained flow conditions (high gas throughput and high heating rate). Investigations of mineral transformation during entrained flow gasification are sparse. Liu et al. studied mineral reaction and morphological during CO2 gasification at elevated temperatures. It was found that mineral reactions were quite different at low temperatures compared to high temperatures.12 Only one study has been conducted for CO2 gasification of the Victorian brown coal.19 It was found that thermal decomposition of CaSO4 and retention of CaCO3 took place at low temperature; the formation of Ca2Fe2O5 and Ca2SiO4 occurred at high temperature. Nonetheless, the study is limited to one Victorian brown coal: Morwell coal. Victorian brown coals are all low-rank coal with high as-mined moisture, but their ash contents are markedly distinct because of the mine locations. Even if modes of mineral occurrence are likewise similar, the amount and proportion of inorganic mineral components are varied widely. The main objective of this study is to investigate mineral transformations and morphological changes of three typical Victorian brown coalsYallourn, Maddingley, and Loy Yang coalduring entrained flow pyrolysis and gasification under conditions of industrial interest. The parent coals were pyrolyzed at 700−1400 °C in nitrogen, and then the pyrolysis chars were gasified at a corresponding temperature with CO2 in the entrained flow reactor. The coals, pyrolyzed chars, and gasified chars were characterized by SEM/EDX to investigate morphological changes during entrained flow pyrolysis and gasification process. Their ashes were characterized by XRD to investigate the mineral transformation. Factsage modeling was also conducted to predict mineral reactions and transformation of the minerals during coal gasification. This study offers a better understanding of the mineral behavior during entrained flow gasification of Victorian brown coals.

2. EXPERIMENTAL SECTION 2.1. Coal Sample. Three Victorian brown coals were employed in this study: Yallourn (YL) and high-ash Loy Yang (LY) coal from the Latrobe Valley, and Maddingley (MD) coal from the Maddingley mine. Coal samples were air-dried and sieved to a particle size range of 90− 106 μm, within the particle size range for industrial entrained flow gasifiers. The prepared coals were oven-dried at 105 °C for 5 h, and the moisture content was measured immediately before the experiments. The properties of coal samples are listed in Table 1. 2.2. Experimental Procedure for Char and Gasified Char Preparation. Two entrained flow reactors (EFRs) were used in the experiments of this study. The first EFR is a low-temperature reactor

Table 1. Properties of Yallourn, Maddingley, and Loy Yang Coal Yallourn moisture (oven-dried, wt %) 0.64−2.72 proximate analysis (dry basis, wt %) volatile 48.18 fixed carbon 51.82 ash 2.32 ultimate analysis (dry basis, wt %) carbon 62.99 hydrogen 4.98 nitrogen 0.54 sulfur 0.40 oxygen (by difference) 24.97

Maddingley

Loy Yang

1.01−1.50

0.49−1.50

47.50 37.26 15.24

48.00 43.58 8.42

50.91 4.56 0.51 3.34 21.21

60.01 4.68 0.58 0.70 20.74 6135


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Energy & Fuels was used to evaluate the possibility of mineral reactions under different conditions.

According to the results of Table 3, YL and LY coal ashes are not expected to melt under the maximum temperature chosen for this study, 1400 °C. As shown in Table 3, flow temperature of YL and LY >1560 °C was higher than that of MD by at least 390 °C. One of the common indices to correlate the AFT to the ash composition is the alkali index determined by E1.23 In general, the higher the alkali index, the higher the AFT.

3. RESULTS AND DISCUSSION 3.1. Mineral Transformation during Coal Pyrolysis. This section investigated the transformation and reaction of inorganic components including mineral and non-mineral matters during pyrolysis of Victorian brown coals. The low temperature pyrolysis, 700−900 °C, was conducted in N2 by using the low-temperature entrained flow reactor, and the hightemperature pyrolysis, 1000−1400 °C, was carried out in N2 by using the high-temperature entrained flow reactor. The effect of the temperature has been investigated and discussed in the following sections. 3.1.1. Coal Characterization. Three Victorian brown coals, Yallourn, Maddingley, and Loy Yang, were used in this study. The proximate analysis and ultimate analysis of these coals are presented in Table 1. As evident, among Victorian brown coals with 0.5−12.5% ash content, Yallourn coal is low ash coal (2.32% ash content), Loy Yang coal is medium ash coal (8.34% ash content), and Maddingley coal is high ash coal (15.24% ash content). The raw coal ash was prepared at 550 °C after 6 h combustion in a muffle furnace. The ash fusion temperature of the coal was measured by SGS Australia according to ISO 540. The results of these X-ray fluorescence (XRF) measurements are generally accurate to less than 0.01%. The elemental composition and ash fusion temperature (AFT) of three coals are shown in Tables 2

alkali index = ash (%) × Fe2O3 + CaO + MgO + Na 2O + Fe2O SiO2 + Al 2O3 (E1)

For the fuels investigated, YL (0.62) had a high alkali index which explains the high AFT of YL. By contrast, the index of MD (0.21) was approximately ten times greater than that of LY (0.023), but the AFT results of MD and LY showed opposite trend. Therefore, the alkali index is not sufficient to explain the significant difference in AFT of MD and LY in this case, despite the difference in composition being confirmed. Vassilev et al. suggested that the relative influence of the oxides for increasing hemispherical temperature is in the order TiO2 > Al2O3 > SiO2 and decreasing hemispherical temperature is in the order of SO3 > CaO > MgO > Fe2O3 ≥ Na2O; K2O shows intermediate behavior.24,25 The larger presence of SiO2 and Al2O3 in LY samples may explain the higher AFT in LY and the lower AFT in MD may be due to the larger presence of SO3 and Fe2O3. 3.1.2. Mineral Reaction. The mineral transformation in chars was examined by XRD. Figure 1 shows the XRD patterns of YL, MD, and LY ash samples from raw coal and chars prepared at different temperatures. According to Bhattacharya and Harttig,9 the minerals detected in the XRD patterns can be classified as the following by the intensity of the X-ray peak: dominant >60%; co-dominant >50%; sub-dominant, 20−50%; minor, 5−20%. The mineralogical composition of coal ash and char ash of YL, MD, and LY is presented in Tables 4−6. All the possible mineral reactions are listed in Table 7 and were evaluated by the thermodynamic software (Factsage) in terms of Gibbs energy (ΔG) and enthalpy (ΔH). ΔG was used to evaluate the possibility of mineral reactions under different conditions. The major mineral phases in YL chars were found to be quartz (SiO2), anhydrite (CaSO4), and magnetite (Fe3O4). After coal pyrolysis, the intensity of SiO2 peaks increased with increasing temperature, and SiO2 became the dominant phase in char at 800−1000 °C. However, when the temperature increased from 1200 to 1400 °C, the intensity of SiO2 peaks significantly decreased while the intensity of Al2SiO5 peaks increased. This change may be a result of Al2O3 reacting with the SiO2 (R1). The Fe3O4 peaks were found in YL chars at 700−1400 °C. Interestingly, the Fe2O3 peaks were found at 1200−1400 °C, and the intensity increased with temperature, which was a result of the oxidation of Fe3O4 (R2). Regarding the oxygen source for R2, it is clear from Table 7 that it cannot be from the decomposition of Fe3O4 (R3) because of the thermodynamical limitation (ΔG > 0). This shows that the source of oxygen for R3 could be derived from coal pyrolysis. For MD coal, the CaSO4 and SiO2 were found to be the major phases in chars. In contrast to YL with only Fe3O4 in the coal and low temperature chars (700−1000 °C), both the Fe2O3 and Fe3O4 peak were observed in MD coal and chars (700−1400 °C). At 1200−1400 °C, with increasing temperature, the intensity of Fe2O3 peaks decreased but the intensity of Fe3O4

Table 2. Chemical Compositions (%) of Yallourn, Maddingley, and Loy Yang Coal Ashes Al2O3 BaO CaO Fe2O3 K2O MgO Na2O SiO2 SO3 TiO2 total

Yallourn

Maddingley

Loy Yang

1.75 0.28 8.29 51.59 0.18 17.69 5.78 1.36 12.55 0.54

15.24 0.02 9.24 20.55 0.08 7.58 8.92 19.02 19.02 0.34

22.11 0.01 2.05 4.11 0.61 5.51 7.87 47.16 7.41 3.17

100

100

100

Table 3. Ash Fusion Temperatures (°C) at Reduction Atmosphere deformation sphere hemisphere flow

Yallourn

Maddingley

Loy Yang

>1560 >1560 >1560 >1560

1120 1150 1160 1170

1250 1400 1440 >1560

and 3. As can be seen, three coals differed widely in their elemental composition of ash. Yallourn (YL) coal was Fe-rich with 51.59% Fe2O3 in coal ash; Maddingley (MD) coal was rich of Fe, Si and S with around 20% Fe2O3, SiO2, and SO3, respectively; and Loy Yang (LY) coal was Si-rich with 47.16% SiO2. 6136


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Table 4. Mineralogical Compositions of YL Coal Ash and Char Ash Samples by XRDa mineralogical composition sample

dominant phase(s)

co-dominant phase(s)

sub-dominant phase(s)

coal ash 700 °C char ash

Fe3O4 CaSO4

800 °C char ash 900 °C char ash 1000 °C char ash 1200 °C char ash

SiO2

CaSO4

CaSO4 SiO2 Fe3O4 Fe3O4

SiO2

CaSO4

Fe3O4

SiO2

CaSO4

Fe3O4

CaSO4

Fe3O4

1400 °C char ash

CaSO4

Fe3O4

Fe2O3 Al2SiO5 Fe2O3 Al2SiO5

a

minor phase(s)

MgAl2O4 MgAl2O4

Listed in decreasing order of X-ray peak intensity.

Table 5. Mineralogical Compositions of MD Coal Ash and Char Ash Samples by XRDa mineralogical composition dominant phase(s)



coal ash

CaSO4

SiO2

700 °C char ash

SiO2

Fe3O4 Fe2O3 CaSO4

800 °C char ash

SiO2

CaSO4

900 °C char ash

SiO2

CaSO4

1000 °C char ash

SiO2

CaSO4

1200 °C char ash

SiO2

CaSO4

1400 °C char ash

CaSO4

sample

a

Fe3O4

SiO2

minor phases(s)

Fe2O3 Fe3O4 Fe2O3 Fe3O4 Fe2O3 Fe3O4 Fe2O3 Fe3O4 Fe3O4 Fe2O3 Fe2O3 Al2Si2O5


Table 6. Mineralogical Compositions of LY Coal Ash and Char Ash Samples by XRDa mineralogical composition sample

Figure 1. XRD pattern of char pyrolyzed at various temperatures: (A) YL, (B) MD, and (C) LY.

peaks increased. Fe3O4 is most frequently formed at high temperature, which was first identified as major mineral phase in MD chars at 1400 °C. Fe3O4 can form from the thermal decomposition of Fe2O3 at 1400 °C.26−28 It also can be formed by the reduction of Fe2O3 with the presence of CO at around 1000 °C.26,29 During pyrolysis of MD coal, a significant amount of CO (around 5%) was generated at high temperatures.30 Hence, the increased intensity of Fe3O4 can mainly be associated with the reduction of Fe2O3 by CO (R4) at 1000−1400 °C. Before the full transformation from Fe2O3 to Fe3O4, the coexistence of Fe2O3 and Fe3O4 happened in MD chars at 1400 °C. The same results were found in combustion ash of the Beulah coals and Spanish brown coals at high temperatures.26,28 Similar

dominant phase(s)

coal ash

SiO2

1000 °C char ash

SiO2

1200 °C char ash

SiO2

1400 °C char ash

SiO2

a



trace phase(s) CaSO4 Fe2O3 CaSO4 CaAl4O7 Fe2O3 CaSO4 CaAl4O7 Fe2O3 CaSO4 CaAl4O7 Fe2O3


to YL, the intensity of SiO2 peaks in MD chars increased with increasing temperature, and SiO2 became the dominant phase in 6137


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Energy & Fuels Table 7. Gibbs Energy and Enthalpy for Reactions among Minerals at High Temperatures ΔG (kJ) reaction

ΔH (kJ)

1000 °C

1200 °C

1400 °C

1000 °C

1200 °C

1400 °C

−6.23

−5.43

SiO2 + Al 2O3 → Al 2SiO5

(R1)

−2.48

−1.86

−1.32

−6.59

2Fe3O4 + 0.5O2 → 3Fe2O3

(R2)

−70.02

−42.66

−14.89

−243.31

Fe3O4 → 3FeO + 0.5O2

(R3)

162.21

140.91

117.73

297.66

298.32

394.66

3Fe2O3 + CO → 2Fe3O4 + CO2

(R4)

−102.17

−112.42

−123.22

−38.37

−34.97

−30.54

Fe2O3 + MgO → MgFe2O4

(R5)

9.07

22.95

−33.07

−35.48

−36.49

CaSO4 → CaO + SO2 + 0.5O2

(R6)

153.09

103.27

54.95

475.12

464.99

448.86

CaSO4 + CO → CaO + SO2 + CO2

(R7)

−19.09

−51.82

−83.17

193.61

184.42

169.29

15.9

−245.6

−249.03

2CaO + SiO2 → Ca 2SiO4

(R8)

−131.24

−137.19

−143.32

−93.91

−92.71

−91.37

CaO + MgO + SiO2 → CaMgSiO4

(R9)

−107.61

−107.26

−107.03

−110.24

−109.42

−108.35

CaO + Al 2O3 + 2SiO2 → CaAl 2Si 2O8

(R10)

−131.64

−135.62

−139.86

−106.98

−105.39

−103.18

Table 8. Major Crystalline Minerals in YL, MD, and LY Chars and Mineral Reaction during Coal Pyrolysis YL

MD

LY

Major Minerals SiO2 (quartz) CaSO4 (anhydrite)

SiO2 (quartz) CaSO4 (anhydrite) Fe3O4 (magnetite)

SiO2 (quartz)

Mineral Reactions 1000 − 1400 ° C

1200 − 1400 ° C

3Fe2O3 + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2Fe3O4 + CO2

Fe3O4 + 0.5O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 3Fe2O3

1400 ° C

n.d.a

6Fe2O3 ⎯⎯⎯⎯⎯⎯⎯→ 4Fe3O4 + O2

1000 − 1400 ° C

SiO2 + Al 2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Al 2SiO5

1000 − 1400 ° C

SiO2 + Al 2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Al 2SiO5 a

n.d.: not detected.

Table 9. Properties of Chars Pyrolyzed at Various Temperatures on a Dry Basis, wt % fuel

pyrolysis temp (°C)

VM

FC

C

H

N

S

Oa

ash

YL

700 800 900 1000 1200 1400

17.30 8.25 5.17 11.50 6.14 5.14

71.05 83.24 84.25 80.60 86.28 89.01

73.04 72.68 77.10 83.17 88.62 89.28

2.46 2.63 1.89 1.42 1.24 0.82

0.87 0.82 0.71 0.61 0.78 0.66

0.35 0.29 0.29 0.39 0.34 0.30

11.63 15.07 9.43 6.51 1.44 3.09

11.65 8.51 10.58 7.90 7.58 5.85

MD

700 800 900 1000 1200 1400

16.25 13.08 10.90 19.56 17.68 8.62

49.53 52.27 54.44 51.84 50.13 61.99

57.31 53.46 52.35 57.90 55.71 62.88

1.26 1.59 1.32 1.28 0.98 1.28

0.57 0.62 0.61 0.41 0.41 0.45

0.26 3.67 4.07 4.37 5.03 3.77

6.38 6.01 6.99 7.44 5.68 2.23

34.22 34.65 34.66 28.60 32.19 29.39

LY

1000 1200 1400

8.92 7.47 5.74

79.93 77.34 76.95

70.94 72.22 74.46

1.38 1.24 0.87

0.68 0.56 0.59

0.71 0.57 0.43

15.14 10.22 6.34

11.15 15.19 17.31

a

By difference.

char at 800−1200 °C. At 1400 °C, the intensity of SiO2 peaks dramatically decreased while the intensity of Al2SiO5 peaks increased most likely due to the reaction R1 between SiO2 and Al2O3. The CaSO4 was the major phase in chars of YL and MD. The intensity of CaSO4 peaks in chars increased with the temperature and became the dominant phase at high temperature as 1400 °C due to the decrease of the SiO2 intensity.

For LY samples, it was found that the SiO2 is the only major and dominant phase in raw coal and chars pyrolyzed at 1000− 1400 °C as the LY coal ash is Si-rich (47.16%). The SiO2 intensity of LY samples is so high that other minerals are trace phases which are undetectable by XRD. Hence, the LY sample is thermally stable and no mineral reactions are found during pyrolysis. 6138


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Energy & Fuels The results indicated that the mineral transformation of Victorian brown coals during pyrolysis mainly happened at a high temperature between 1000 and 1400 °C, and no mineral reaction was observed below 1000 °C. The behavior of mineral matters during coal pyrolysis also widely differed in each coal due to the significant difference of mineral components among coals. The major crystalline minerals in chars and mineral reaction during coal pyrolysis are summarized in Table 8. Overall, YL and MD coal ash had high Fe content (51% and 21%, respectively) which works as a catalyst for gasification and improves coal/char reactivity.31 At 1000−1400 °C, mineral reactions of Fe containing minerals in YL and MD samples are quite different during coal pyrolysis. Fe2O3 from the oxidation of Fe3O4 was observed in YL samples, but Fe3O4 from the reduction of Fe2O3 was observed in MD samples. According to the finding of Suzuki et al., a lower oxidation state of iron, Fe3O4 instead of Fe2O3, seems to be more active in gasification of YL.32 Mineral transformation during pyrolysis may decrease the gasification reactivity of YL char, but increase the gasification reactivity of MD char. However, no significant mineral transformation was found in LY during coal pyrolysis as LY coal ash had high Si content (∼48%) which has no catalytic effect on gasification.33 3.2. Mineral Transformation during Char Gasification. This section presents the mineral transformation during char gasification in CO2. The gasified chars at 700−1400 °C were examined by the XRD and are discussed in the following sections. 3.2.1. Char Characterization. The chars of YL, MD, and LY were prepared by using two entrained flow reactors. The low temperature chars (700−900 °C) were generated in N2 using the low temperature entrained flow gasifier. The high temperature chars (1000−1400 °C) were generated in N2 using the high temperature entrained flow gasifier. The ultimate analysis and proximate analysis of chars prepared at various temperatures are presented in Table 9. The gasified chars generated from entrained flow gasification (700−1400 °C) were burned at 550 °C for 6 h in a muffle furnace to prepare ash samples for the XRD analysis. The results of XRD analysis are presented and discussed in the following section. 3.2.2. Mineral Transformation. The mineral transformations in gasified chars were examined by the XRD. Figure 2 shows the XRD patterns of YL, MD, and LY ash samples from raw coal, char prepared at 1200 °C, and gasified chars generated at different temperatures. The mineralogical compositions of coal ash and gasified char ash of YL, MD, and LY are presented in Tables 10−13. The mineral reactions occurring in the Victorian brown coals and their ashes are described below. Anhydrite (CaSO4) was a major mineral phase in YL and MD ashes from coal and gasified chars at 700−1200 °C. Anhydrite was found to be chemically stable from 700 to 1000 °C in gasified char ashes of YL and MD. However, above 1000 °C, with the increasing temperature, the intensity of CaSO4 peaks decreased. It is evident from Table 7 the thermal decomposition of CaSO4 (R6) cannot take place because of the thermodynamical limitation. During the CO2 gasification of Victorian brown coals, a significant amount of CO (∼10%) was generated at high temperatures.30 Hence, the decrease of CaSO4 intensity could rise from the reaction R7 between CaSO4 and CO from the CO2 gasification. At 1200−1400 °C, the calcium made by above reactions reacted with other available species to form

Figure 2. XRD pattern of sample series at various temperatures: (A) YL, (B) MD, and (C) LY.

bredigite (Ca2SiO4) in YL gasified char ashes (R8), form monticellite (CaMgSiO4) in MD samples (R9), and form anorthite (CaAl2Si2O8) in LY samples (R10). The results agreed with the findings of the American lignites that the calcium oxide made by decomposition of CaSO4 reacted with other available species such as silicates or magnesium oxides or alumina silicates to form different minerals such as bredigite and members of the gehlenite−akermanite solid solutions.27 Quartz (SiO2) was a dominant mineral phase in gasified char ashes generated at 700−1200 °C from YL, MD, and LY. Moreover, quartz was the only major and dominant mineral phase for LY ashes from coal, char, and gasified chars, because of 6139


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Energy & Fuels Table 10. Mineralogical Compositions of YL Coal Ash and Gasified Char Ash Samples by XRDa

Table 12. Mineralogical Compositions of LY Coal Ash and Gasified Char Ash Samples by XRDa

mineralogical composition sample coal ash 700 °C ash

dominant phase(s) Fe3O4/ MgFe2O4 SiO2

800 °C ash

SiO2

900 °C ash

SiO2

1000 °C ash

SiO2

1200 °C ash

SiO2

1400 °C ash

Fe3O4/ MgFe2O4



mineralogical composition minor phases(s)

sample coal ash

SiO2

MgSO4

1000 °C ash

SiO2

Na2SO4 MgSO4 Na2SO4

1200 °C ash

SiO2

1400 °C ash

SiO2

CaSO4 Fe3O4/ MgFe2O4 CaSO4 CaSO4 Fe3O4/ MgFe2O4 CaSO4 Fe3O4/ MgFe2O4 MgSO4 CaSO4 Fe3O4/ MgFe2O4 Fe3O4/ MgFe2O4 CaSO4

Na2SO4

a

Ca2SiO4

Ca2SiO4 CaSO4

Listed in decreasing order of X-ray peak intensity

Table 11. Mineralogical Compositions of MD Coal Ash and Gasified Char Ash Samples by XRDa mineralogical composition sample coal ash 700 °C ash

CaSO4 SiO2

800 °C ash

SiO2

900 °C ash

SiO2

1000 °C ash

SiO2

1200 °C ash

SiO2

1400 °C ash

SiO2

co-dominant phase(s) SiO2

CaSO4


minor phases(s)

Fe2O3

Fe3O4/ MgFe2O4 Fe2O3 CaSO4 Fe3O4/ MgFe2O4 Fe2O3 Fe3O4/ MgFe2O4 Fe2O3



trace phases(s) CaSO4 Fe2O3 NaAlSi3O8 CaAl2Si2O8 NaAlSi3O8 CaAl2Si2O8 NaAlSi3O8 CaAl2Si2O8


Magnetite, Fe3O4, and magnesioferrite, MgFe2O4, have very similar X-ray pattern, which makes them difficult to distinguish. Since YL and MD coal ash were Fe-rich, containing 51.6% and 20.6% Fe, respectively, members of magnetite−magnesioferrite series were the major mineral phase in all YL ash samples from coal and gasified chars at 700−1400 °C, and also presented in MD ashes from gasified chars at 1000−1400 °C. It was found that the intensity of Fe3O4/MgFe2O4 peaks in YL and MD ashes increased with increasing temperature at 1000−1400 °C. The results indicated that Fe3O4/MgFe2O4 could be formed at high temperatures. They can form from the thermal decomposition of Fe2O3 and reduction of hematite (Fe2O3) by CO, which is discussed in the following content. Hematite (Fe2O3) was presented in MD gasified chars at 700−1400 °C and became a major mineral phase at 1000−1400 °C. The Fe2O3 was chemically stable at low temperatures, 700− 900 °C. However, with the increasing temperature, the intensity of Fe2O3 peaks significantly decreased and the intensity of Fe3O4/MgFe2O4 peaks, by contrast, increased. The thermal decomposition of Fe2O3 to Fe3O4 happened at 1400 °C. The reduction of Fe2O3 to Fe3O4 by reacting with CO (R4) has also been observed during the gasification of lignite.27,29 Hence, in our char gasification, in the presence of an atmosphere (∼10% CO) and high temperature, the decrease of Fe2O3 intensity can be rose from the thermal decomposition and reduction of Fe2O3 for Fe3O4 or the reaction (R5) between MgO and Fe2O3 for MgFe2O4. However, R5 cannot occur at high temperature due to the thermodynamical limitation (ΔG > 0) as shown in Table 7. Accordingly, more Fe3O4, not MgFe2O4, was formed with increasing temperature due to the thermal decomposition and R4 which are thermodynamically favored. Bredigite (Ca2SiO4) forms at 1000−1200 °C. It formed in ashes of YL samples which had low aluminum content (1.75%), but not in MD and LY ashes with greater than 15% Al. By contrast, Ca−Al silicates and Ca−Mg silicates were preferably formed and appeared in MD and LY samples, with access to calcium. In the high Al samples of MD and YL, bredigite either was not present owing to lack of access of calcium or only present as a trace mineral phase not detectable by XRD. Some research found that lignite ashes, such as Beulah-Zap lignite and Morwell coal, formed Ca2SiO4 at high temperatures.23,26,27 Overall, it was found that mineral reactions during gasification of Victorian brown coals happened at high temperatures between 1000 and 1400 °C, and major mineral phases were chemically stable at low temperatures. Evidently, there were significant differences for different coals in the behavior of mineral matters because of the wide difference in minerals. The

Na2SO4

a

dominant phase(s)

dominant phase(s)

CaSO4 Fe2O3 CaSO4 Fe2O3 CaSO4 Fe2O3 CaMgSiO4

CaMgSiO4

CaSO4 CaMgSiO4

a


the abundance of Si in LY coal ash (around 50% SiO2). Quartz will undergo structure transformation with increasing temperature, converting from trigonal α-quartz to hexagonal β-quartz at 573 °C, to hexagonal β-tridymite at 870 °C, and to cubic βcristobalite at 1470 °C. Despite the thermal structure change, quartz is generally chemically stable to at least 1000 °C.27 In the YL and MD gasified ash samples, quartz was stable at 700−1000 °C and still presented at 1200 °C in all samples. However, at high temperature, >1000 °C, the intensity of the SiO2 peaks decreased with increasing temperature, and the intensity of calcium silicates peaks increased. Two recent researches have found the same change of SiO2 at high temperature in Morwell and Loy Yang coal during gasification in CO2.7,23 6140


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Energy & Fuels Table 13. Mineral Reactions during Char Gasification of YL, MD, and LY YL

MD

1000−1400 ° C

1000−1400 ° C

CaSO4 + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO + SO2 + CO2

LY

CaSO4 + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO + SO2 + CO2 1000−1400 ° C

1200−1400 ° C

1000−1400 ° C

CaO + MgO + SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaMgSiO4

2CaO + SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2SiO4 1000−1400 ° C

CaO + Al 2O3 + 2SiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaAl 2Si 2O8

1000−1400 ° C

3Fe2O3 + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2Fe3O4 + CO2

3Fe2O3 + CO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2Fe3O4 + CO2

Table 14. Comparison of Mineral Transformations during Lignite Gasification and Combustion lignite

conditions

major mineral transformations 1000−1400 ° C

SiO2

1000−1400 ° C

SiO2 + MgO

1000−1400 ° C

Yallourn

gasification

CaSO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯→ Ca 2SiO4 ,

Maddingley

gasification

CaSO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaMgSiO4 ,

Loy Yang

gasification

CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaAl 2Si 2O8

Morwell19

gasification

CaSO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯→ Ca 2SiO4 ,

Rhenish19

gasification

CaSO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2MgSi2O7

Indian head29

gasification

Fe2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe

Fe2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe3O4 1000−1400 ° C

Fe2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe3O4

Al2O3 +SiO2 (1000−1400 ° C)

>1200 ° C

CaCO3 (>1200 ° C)

SiO2

Fe3O4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2Fe2O5

SiO2 + MgO

1000−1400 ° C

1200−1400 ° C

CaS (850° C)

SiO2 (850−1450 ° C)

CaSO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2SiO4 Al2O3 +SiO2

CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2Al 2SiO7 , Beulah-Zap26

combustion

500 ° C

MgO + SiO (850−1450 ° C)2

CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2MgSi2O7

800−1200 ° C

FeS2 ⎯⎯⎯⎯⎯⎯→ Fe2O3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe3O4 300 ° C

SiO2 (880 ° C)

NaNO3 ⎯⎯⎯⎯⎯⎯→ Na 2O → Na 2SO4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Na 2Si 2O5 + Na 2SiO10 + Na 2SiO3 Xinzhuangzi34

combustion

Spanish brown coals28

combustion

250−900 ° C

FeS2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe2O3 + Fe3O4 , 600−750 ° C

> 850 ° C

> 850 ° C

CaCO3 ⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯→ CaSO4

1400 ° C

FeS2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe2O3 ⎯⎯⎯⎯⎯⎯⎯→ Fe3O4 350−650 ° C

> 650 ° C

1060 ° C

Al2O3 +SiO2

CaCO3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯→ CaSO4 ⎯⎯⎯⎯⎯⎯⎯→ CaO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca 2Al 2Si 2O8

Table 15. Predicted Inorganic Compositions after YL Coal Gasification composition (wt %) phase

inorganic matter

700 °C

800 °C

900 °C

1000 °C

1200 °C

1400 °C

magnesioferrite anhydrite sodium sulfate magnesium sulfate hematite forsterite spinel periclase calcium iron oxide perovskite-A merwinite

MgFe2O4 CaSO4 Na2SO4 MgSO4 Fe2O3 Mg2SiO4 MgAl2O4 MgO CaFe2O4 CaTiO3 Ca3MgSi2O8

49.26b 17.61b 11.53d 9.32d 5.81 2.78 2.14

55.64b 18.09b 11.77d 6.55d 1.95 2.86 2.2

60.85c 18.95c

67.97c 21.16c

58.04c −c

52.32a −d

3.15 10.23 27.16 0.87

3.02 9.92 28.99 0.86 4.58

3.05d

0.08

2.99 2.3 0.76

3.34 2.57 1.96

a Dominant mineral phase found in XRD measurements. bCo-dominant mineral phase found in XRD measurements. cSub-dominant mineral phase found in XRD measurements. dMinor mineral phase found in XRD measurements.

decomposition of CaSO4 at around 1000 °C both happens during coal gasification and combustion. The calcium from the decomposition may then form Ca−Al silicates or calcium silicates. During coal gasification, in the presence of CO and H2, the phase change from Fe2O3 to Fe3O4 or Fe occurred. The

mineral reactions during CO2 gasification of YL, MD, and LY chars are summarized in Table 13. The behavior of mineral matters of this study is compared with the results under gasification and combustion condition from the literature, as seen in Table 14. As seen, the 6141


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Energy & Fuels Table 16. Predicted Inorganic Compositions after MD Coal Gasification composition (wt %) phase

inorganic matter

700 °C

cordierite anhydrite hematite sodium sulfate sapphirine forsterite nepheline spinel monticellite(s) merwinite magnetite

Mg2Al4Si5O18 CaSO4 Fe2O3 Na2SO4 Mg4Al10Si2O23 Mg2SiO4 NaAlSiO4 MgAl2O4 CaMgSiO4 Ca3MgSi2O8 Fe3O4

27.29 21.29c 19.51c 18.79 7.05 3.49

800 °C

900 °C

1000 °C

1200 °C

1400 °C

23.1c 21.16c 5.01 6.36 11.36 32.08

23.04c 21.28c

22.87a 21.51b

−b 25.48b

−c −b

30.58 11.06 20.89c 7.18 −b

6.61 32.21 11.63 −c 22.98 25.94b

6.09 11.22 33.04

11.59 35.35 4.55 −c −b

a

b

Co-dominant mineral phase found in XRD measurements. Sub-dominant mineral phase found in XRD measurements. cMinor mineral phase found in XRD measurements.

other predicted minerals which were not found in XRD measurements. This could be attributed to three reasons: (1) non-crystalline structure of the predicted mineral phases which was not detected by the XRD;7 (2) thermal change of coal minerals during ashing procedure or inaccurate mineral;27 or (3) incorrect mineral input for equilibrium calculations where some metal element may exist in the form of organic matters, not minerals. 3.4. Morphological Change. The morphological change in gasified chars of YL, MD, and LY between 800 and 1400 °C was examined by using a field-emission scanning electron microscope (FE-SEM) (Hitachi SU8010) which had secondary electron microscopy (SEM), backscattered electron microscopy (BSEM), and energy dispersive X-ray spectroscopy (EDX). In this study, The SEM was used to identify the sample surface morphology, the BSEM was used to detect the distribution of inorganic matters in the sample, and the EDX was used to provide the detailed information on the molten part and mineral-rich part. 3.4.1. SEM Analysis. It can be seen from SEM images in Figures 3−5, with the increasing temperature and carbon conversion, the particle size of YL and MD gasified chars significantly decreased to ∼20 μm and ∼50 μm respectively at 1400 °C (Figures 3O and 4O). By contrast, the particle size of LY samples did not significantly change with temperature, and its size was ∼100 μm at 1400 °C, seen in Figure 5K. This does not mean that no size change of LY char particles occurs during gasification at high temperatures. Under high temperature and high conversion, the majority of the char particles (>95%) have been converted to fine particles. The SEM results of LY samples show that few particles are not fully converted and their size almost remains the same at high temperatures. The change of particle size during char gasification indicated that reaction mode of the sample, which provides useful information for the reaction model selection in kinetic studies.35 The final particle morphology suggested that the Victorian brown coal chars may be suitable for the volumetric model and modified volumetric model in which it is assumed that the sample reacts homogeneously with the gasifying agent. The morphology of mineral matters in gasified chars of YL and MD differed at different temperatures. At a low temperature below 1000 °C, no morphological change was observed in minerals. The mineral constituents started to appear and were observed on the sample surface at 1000 °C, seen in Figures 3G and 4G. With temperature increased, minerals aggregated and

Fe3O4 also can be formed during the combustion, in an oxidizing environment, by the decomposition of Fe2O3. 3.3. Modeling of Mineral Transformation during Coal−CO2 Gasification. The Factsage 6.4, a well-known thermodynamic software package, was used to predict the reaction and transformation of inorganic matters during coal gasification at various temperatures and 20% CO2. Based on the experimental coal feeding rate (1 g/min) and inlet gas flow rate (3.2 L/min CO2 and 12.8 L/min N2), the weight of coal elements, determined by ultimate analysis and ash composition, and the weight of the reactant gas, CO2, were input into the Reaction module in the Factsage. The composition of inorganic matters after coal gasification at 1 atm and various temperatures was then determined by the Equilib module in Factsage. The predicted inorganic compositions of YL, MD, and LY after coal gasification at various temperatures are presented in Tables 15−17. Table 17. Predicted Composition of Inorganic Matters after LY Coal Gasification composition (wt %) phase

inorganic matter

1000 °C

1200 °C

1400 °C

high-albite anorthite nepheline sapphirine forsterite ilmenite leucite (RHF)-B ulvospinel cordierite

NaAlSi3O8 CaAl2Si2O8 NaAlSiO4 Mg4Al10Si2O23 Mg2SiO4 (FeO)(TiO2) KAlSi2O6 (FeO)2(TiO2) Mg2Al4Si5O18

52.37a 10.98a 10.68 8.18 7.54 4.68 3.03 2.74

53.79a 10.98a 8.91 8.84 7.28 4.85 2.67 2.5

45.06a 11.55a 1.41 2.64 6.68 0.29 32.59

a

Minor mineral phase found in XRD measurements

Compared with the XRD patterns in Figure 2, it was found that the equilibrium predictions for YL and MD samples reasonably agreed with the experimental results. The major minerals in YL and MD samples were successfully predicted by Facsage modeling. However, one major deficiency of the Facsage modeling was found to be that it could not predict the actual change of SiO2 which was thermal stable below 1000 °C and the dominant mineral phase, in most cases, in gasified chars. Because of this deficiency, predictions for Si-rich LY samples turned out to disagree with the XRD results. There was 6142


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Energy & Fuels

Figure 3. SEM, BSEM, and EDX analysis of YL gasified chars at 800−1400 °C and 20% CO2.

like albite (melting point: 1100 °C) contributed to the morphological change of mineral melting at 1200 °C. By contrast, the LY samples showed different morphological change. Unlike YL and MD samples, few mineral constituents were observed at a higher temperature as 1200 °C (Figure 5G). There was also no melting of minerals observed on the particles at high temperatures. The carbon conversion X% at different temperatures has been calculated in our previous papers,5,30 which is presented in Figure 3−5. As seen, at 1400 °C and carbon conversion of 98%, minerals aggregated and developed to small spheres which coated the whole large particle, seen in Figure 5M. The EDX analysis (Figure 3E,J,O) showed high presence of Si which was expected to present in the form of SiO2 based on XRD results. A high percentage of SiO2 in LY samples, which is thermal chemically stable, may explain the consistency in the morphology of LY samples. Overall, with the increasing temperature, carbon conversion increased, and less carbon and more minerals were left in the particles. At carbon conversion of 99%, almost only mineral spheres can be observed in the samples of YL and MD. The morphological results demonstrated that the mineral transformation happened at high temperatures between 1000 and 1400 °C. With the increasing temperature and carbon conversion, mineral constituents with high Fe content were first found in YL and MD samples at 1000 °C, followed by melting of minerals like sulfates and albite at 1200 °C. Compared to high Fe content YL and MD samples, high Sicontent LY samples seemed to be more stable in particle

melted seriously, and developed a smooth surface of char at 1200 °C, seen in Figures 3L and 4L. At 1400 °C, the minerals developed to smooth sphere particles and aggregated with each other to form larger particles, seen in Figures 3P and 4P, resulting in the increase of the particle size. 3.4.2. BSEM and EDX Analysis. As can be seen in the BSEM images, the distribution of mineral matters also changed with the temperature and carbon conversion. In the BSEM images, the mineral-rich area generally looks brighter than the other area because of more metal elements. The EDX analysis of YL, MD, and LY samples showed and indicated that the bright area in the particle contained less carbon and more inorganic minerals than the dark area. The YL and MD gasified chars had a very similar morphological change in mineral matters at various temperatures. It was found that minerals disperse homogeneously in the gasified char at 800 °C, and no evidence of mineral aggregation was found in particles, seen in Figures 3C and 4C. As the temperature and carbon conversion increased, inorganic minerals stated to aggregate and adhered to the char surface, seen in Figures 3H and 4H. At 1200 °C, the minerals aggregated and melted seriously, and covered the majority of the particle surface (Figures 3M and 4M). The EDX analysis showed initial mineral constituents of YL and MD at 1000 °C contained a high value of Fe, demonstrating the reduction of Fe2O3 happened above 1000 °C. Moreover, The EDX analysis (Figure 3N and 4N) showed high presence of Mg, Na, and S in melting part of samples at 1200 °C, indicating the presence of sulfates like MgSO4 (melting point: 1124 °C) and Na-containing minerals 6143


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Figure 4. SEM, BSEM, and EDX analysis of MD gasified chars at 800−1400 °C and 20% CO2.

Figure 5. SEM, BSEM, and EDX analysis of LY gasified chars at 1000−1400 °C and 20% CO2.

morphology with respect to particle size and behavior of mineral matters. 3.5. Implications on Ash Fusibility, Slag Viscosity, and Gasification Reactivity. The behavior of mineral matters

during char gasification strongly affects the ash fusion, slag behavior, and gasification reactivity.36 The detected mineral phases during char gasification are summarized in Table 18. It is evident that all the YL, MD, and LY char samples consisted of 6144


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Energy & Fuels Table 18. Melting Point of the Detected Mineral Phasesa temperature predicted (°C) phase

chemical formula

albite anhydrite anorthite hematite larnite magnesium ferrite magnesium sulfate magnetite monticellite quartz sodium sulfate

NaAlSi3O8 CaSO4 CaAl2Si2O8 Fe2O3 Ca2SiO4 MgFe2O4 MgSO4 Fe3O4 CaMgSiO4 SiO2 Na2SO4

coal

700

800

900

1,2,3

1,2

1,2

1,2

2,3

2

2

2

1

1 1 1

1 1 1

1 1 1

1,2 1

1,2 1

1,2 1

1 2,3

1000

1200

1400

melting point (°C)

3 1,2 3 2

3 1,2 3 2 1,2

3 1,2 3 2 1 1,2

1,2 2 1,2,3

1,2 2 2,3

1100 1450 1274 1538 1540 171337 1124 1590 1503 1670 884

1,2 1,2 2 1,2,3 1

a

1: YL; 2: MD; 3: LY.

greatly enhanced the gasification rate.4,32 Therefore, the transformations of Ca-containing and Fe-containing minerals in YL and MD would both have positive effect on char-CO2 gasification rate. According to our previous study, at high temperature, the gasification reactivity of YL was highest, followed by MD and then LY.42 Certainly, ash content and mineral reactions are two of the most important factors accounting for the different gasification behavior of Victorian brown coals.

various minerals with a wide range of melting points. While the YL and MD ashes contained large percentage of iron oxides, silicates, and anhydrite, the LY ash contained predominantly quartz and few silicates. The low melting minerals such as sodium sulfate and magnesium sulfate in YL would not cause problems as they are expected to form the slag in the operating temperature range of entrained flow gasifiers. Additionally, the high melting minerals like quartz and silicates would not contribute to slag formation. During gasification, the mineral transformation, from high melting point SiO2 (1670 °C) to low melting point silicates such as Ca2SiO4 (1540 °C), CaMgSiO4 (1503 °C), and CaAl2Si2O8 (1274 °C), led to decrease ash fusion temperature. By contrast, the formation of high melting point Fe3O4 (1590 °C) from Fe2O3 reduction at 1000−1400 °C increased ash fusion temperature. However, the interaction and behavior of the different minerals results in the formation of secondary reaction products and eutectic mixtures whose melting temperature is dependent on the composition, which needs to be further investigated. Apart from ash fusibility and melting temperatures, mineral matters in ash is directly related to slag viscosity which is crucial for slagging gasifiers to determine the operating temperature.38 For entrained flow gasifiers, the slag viscosity is optimal around 15 Pa·s and acceptable until 25 Pa·s to ensure reliable continuous slag tapping.38 Nowok found that reduction of iron under reducing conditions decreased slag viscosity,39 while Xu et al. found that slag viscosity decreased when CaO was added to coal.40 This indicates that the mineral transformations during gasification, Fe3O4 formation by Fe2O3 reduction and CaO formation by CaSO4 decomposition, may decrease slag viscosity. Alkali and alkaline-earth metals (AAME) like Ca and some of the transition metals (e.g., Fe) in coal have shown catalytic activity for coal gasification.4 During char gasification, the decomposition of CaSO4 to CaO was found in YL and MD at 1000−1400 °C. As CaO showed more activity than CaSO4 in gasification,41 lime formed by anhydrite decomposition would enhance gasification rate. CaO further reacts with SiO2 or MgO or Al2O3 to form Ca2SiO4, as long as CaMgSiO4, CaAl2Si2O8 at high temperatures, which results in no observation of CaO in gasified chars. Meanwhile, the Fe3O4 formed by reduction of Fe2O3 was found in the two fuels. Suzuki et al. found that a lower oxidation state of iron, Fe3O4 instead of Fe2O3, was more active in gasification of YL.32 As a result, the formation of Fe3O4 during gasification may increase the gasification reactivity of char in CO2. Moreover, when the additive like Ca added with iron, it

4. CONCLUSIONS This work investigated the mineral behavior of YL, MD, and LY during entrained flow pyrolysis and CO2 gasification between 700 and 1400 °C by XRD and SEM-EDX. The effect of temperature on mineral transformations and morphological changes during the two-stage entrained flow gasification was discussed in detail. The mineral reactions during char gasification happened at high temperatures between 1000 and 1400 °C, not low temperatures. There were significant differences for parent coals in mineral reactions during coal pyrolysis and char gasification. During coal pyrolysis, the formation of Al2SiO5 was found in both YL and MD chars at 1000−1400 °C. An increase of the Fe2O3 intensity upon the oxidation of Fe3O4 was found in YL chars at 1200−1400 °C, which would decrease char reactivity in CO2. However, the MD chars showed the opposite change in Fe2O3 because of the reduction of Fe2O3 by CO, which may increase char reactivity. During char gasification, a decomposition of CaSO4 by CO and an increase in Fe3O4 intensity upon thermal decomposition and reduction of Fe2O3 were found in both YL and MD gasified chars at 1000−1400 °C. The CaO made by CaSO 4 decomposition further reacted with SiO2 to form the Ca2SiO4 in YL samples and form CaMgSiO4 in MD samples at 1200− 1400 °C. Mineral transformations of YL and MD tended to increase gasification reactivity of char in CO2. By contrast, SiO2 was the only major mineral phase in YL chars, and gasified chars, so few mineral reactions were observed during coal pyrolysis and char gasification. YL and MD samples showed very similar morphological changes in gasified chars at 1000−1400 °C. The mineral constituents with high Fe content, generated by the reduction of Fe2O3, started to be found on the surface at 1000 °C, and then sulfates like MgSO4 and Na-containing minerals like albite melted at 1200 °C. At 1400 °C, the particles developed as smooth spheres and aggregated with each other. By contrast, 6145


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high Si content LY samples showed more stable morphology regarding particle size and the behavior of mineral matters. Mineral transformation during gasification may decrease ash fusion temperature and slag viscosity, and increase gasification reactivity of char.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Xu: 0000-0001-9023-5134 Sankar Bhattacharya: 0000-0002-7590-6814 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the China Scholarship Council, Brown Coal Innovation Australia (BCIA), and Monash University for financial assistance. The authors also acknowledge the Monash Centre for Electron Microscopy for the use of their facilities.

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Mineral transformation and morphological change during pyrolysis

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