The phenomena of secondary weight loss in high-temperature coal

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The phenomena of secondary weight loss in high-temperature coal pyrolysis Xiongwei Zeng, Shu Zheng, and Huaichun Zhou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01613 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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The phenomena of secondary weight loss in hightemperature coal pyrolysis Xiongwei Zeng †, Shu Zheng **, †, Huaichun Zhou *, † † Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China KEYWORDS: Coal pyrolysis; Secondary weight loss; TG; XRD; SiC; AlN

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Abstract. Studies on coal pyrolysis via Thermo-gravimetric Analyzer (TGA) are typically performed under a variety of temperature increases, beginning at ambient temperature. A main limitation in these procedures is that the reaction temperatures (generally lower than 1000 °C) are typically lower than those met in pulverized coal (PC) boilers and the ash flow temperatures (AFTs, generally higher than 1100 °C). In this paper, five Chinese coals are clarified to low- or mid-AFT coals as their AFTs increase. They were then prepared in TGA at several temperatures below or above their AFTs under N2 atmosphere for about 30 min after a heating procedure at a rate of 80 °C/min. The coal and residual char samples were collected and analyzed via the X-Ray Diffraction (XRD) and compared to the chemical thermodynamic calculation. The secondary weight loss appeared between 1200-1450°C in the TG curves of four mid-AFT coals, but not the low-AFT coal. The results showed that this secondary weight loss was caused by the carbothermal reduction and carbothermal reduction nitridation reaction between minerals and carbon, which led to the appearance of silicon carbide, aluminum nitride, and silicon nitride. The low concentration of SiO2 and Al2O3 and the high concentration of Fe2O3 and CaO inhibited the secondary weight loss process in high-temperature coal pyrolysis. The reason why the secondary weight loss was not present in low-AFT coal was possibly due to low-melting eutectic noncrystal matters consisting of calcium oxide and ferrum oxide. These fused low-melting eutectic non-crystal matters stopped the carbothermal reaction in high pyrolysis temperautres and inhibited the production of SiC, AlN, and Si3N4.


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1

INTRODUCTION

The coal pyrolysis1-3 is an fundamental part of coal combustion, as it is the basis of coal gasification4-6 and distillation7. There has been much research performed, although most 8-13 focus on the detailed mechanism of low-temperature (< 1100K) coal pyrolysis. Solomon10 proposed a nine-step reaction of volatile matters evolution in the coal pyrolysis process. With increasing exudation temperature, the products, gas and moisture absorbed in coal, crystal moisture, CO and CO2, tar, CH4 and unsaturated hydrocarbon, as well as saturated hydrocarbons, were orderly devolatilized. High-temperature (> 1100K) coal pyrolysis has noticeably less research 14-18. According to the study by Marsh14, coal reactivity rapidly decreased as temperatures increased from 1100K to 2000K. The aromatic and hydroaromatic structural units were bound to each other by short aliphatic and ether bridges, which formed a macromolecular network15. Wei et al.2, 16 measured the reactivity of coal pyrolysis in a Thermo-gravimetry analysis (TGA)19 where there were two obvious mass loss processes at temperatures ranging from 673-1100K and 1100-1500K. They suggested that the mass loss at 1200-1500K was due to the broken of side chain and aromatic structural units, but their study provided no evidence to substantiate their claims. These studies focused on the structural change of coal molecular in high temperature pyrolysis. Potential reactions between the mineral and char were ignored, including the carbothermalreduction reactions of SiO2 with coal char20-24 and the carbothermic nitridation of Al2O3 with N2 and coal char24, 25. Devečerski et al.20 pioneered the carbothermal-reduction reaction of fibrous magnesium silicate (Mg6Si4O10(OH)8) with carbon, where the formation of SiC was confirmed by X-Ray Diffraction (XRD)26-29 and SEM/EDS analysis30, 31. Our former work22 investigated the char burnout characteristics below and above their ash flow temperatures (AFT), results 3 ACS Paragon Plus Environment

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indicated that the melted ash softened and wrapped the coal particle, and the coal burnout was decreased as the combustion temperature was beyond the AFT. Wang et al.24 separately blended the quartz and kaolinite with the de-ashing coal char, and examined the reactions of blended samples at temperatures up to 1600ºC in Ar and N2 atmospheres via TGA, off-gas analysis, and XRD. The results indicated that the carbothermal reactions of quartz and kaolinite occurred at 1200ºC and 1150ºC. In the N2 atmosphere, the carbothermic nitridation occurred for both quartz and kaolinite together, forming SiC, Si3N4, and AlN. To the best of our knowledge, the effect of melted mineral and carbothermal-reduction reactions of minerals with carbonaceous constituent on coal high-temperature pyrolysis has not been studied. The following study addressed the conditions where the pyrolysis temperature is higher than the AFT. The AFT variability within coal types is one of the key indexes of ash fusion characteristic32-38. The difference between the AFT and pyrolysis temperature would affect the high-temperature coal pyrolysis. In this paper, the five Chinese char samples were prepared from coals at temperatures below and above their AFTs in a N2 atmosphere. At first, the coal samples were quantitatively characterized by Thermo-gravimetry (TG), Differential thermal gravity (DTG), and Differential Scanning Calorimetry (DSC)39. The residual char samples were analyzed via XRD and paired with the chemical thermodynamic calculation40, 41. The precise mass information of subjects was received in pyrolysis process. The production gained by XRD would verify the results of chemical thermodynamic calculation in the high-temperature pyrolysis of coal. 2 2.1

EXPERIMENTAL SETUP Coal properties and Apparatus


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Five coals were utilized in the present work, including the Hulunbeier lignite, Xinjing bituminous, Tangzhai bituminous, Yishang mean, and Hanyu mean coals. The average sizes of the coal samples were 76 µm, with all samples less than 200 µm. The coal characteristics analysis, ash components, and fusion characteristic temperatures are presented in Table 1 and Table 2. As shown in Table 2, the five coals could be classified as whether low- or mid-AFT coals as their AFTs increased. Only Hulunbeier coal belonged to the low-AFT coal (AFT < 1200 ºC), with the rest belonging to mid-AFT coals (1200 ºC < AFT < 1400 ºC). A TGA from Beijing Henven Scientific Instrument Factory was employed. The minimum sensitivity was 0.1 µg and the data was collected every second in a temperature range of 25ºC to 1450ºC. The XRD was located in Tsinghua Univercity, and was utilized to identify the reactions of carbon with minerals in high-temperature coal pyrolysis. Results were verified the chemical thermodynamic calculation. 2.2

Experimental method

All coal samples were measured at 20 mg, dewatered at 100 ºC for 30 min, heated at a fixed heating rate of 80 ºC/min starting at 100 ºC and rising to the temperature TH with N2 atmosphere (200 mL/min), and then held at TH for 30 min to complete volatile devolatilization. The residual char samples were then cooled to ambient temperature (25 ºC). Table 2 showed that the AFT (1392 ºC) of Xinjing coal was higher than the AFT (1160 ºC) of Hulunbeier coal. The TH I in mode I was set as high as 1450 ºC to ensure the chars reach temperatures above their AFTs. The TH II in mode II was set to 1000 ºC for Hulunbeier coal and 1200 ºC for the others. The error analysis in TGA measurements and FFT smoothed treatment were detailed in our former study42.


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In addition, the gas products were collected and analyzed after the secondary weight loss in the high-temperature coal pyrolysis process. 3

CHEMICAL THERMODYNAMIC CALCULATION

The potential reactions of carbon and minerals were investigated using the chemical thermodynamic calculation when pyrolysis temperature exceeded the AFT. The Gibbs free energy43 of each reaction was calculated using the van’t hoff equation44:

Π(

pj θ

)

uj

p (1) pi ui Π( θ ) p ∆G ∆ Gθ (T ) (J/mol)was Where r m (J/mol) was Gibbs free energy change per mole of reaction, r m ∆ r Gm = ∆ r Gmθ (T ) + RT ln Q f = ∆ r Gmθ (T ) + RT ln

the Gibbs free energy change per mole of reaction for unmixed reactants and products at standard conditions, R was the gas constant=8.31 J/mol·K, T (K) was the absolute temperature, Qf (J/mol) was the reaction quotient,

pj

(atm) was partial pressure of gas product j,

uj

was

θ p coefficient of gas product j, p (atm) was the standard partial pressure of reaction, i (atm) was

partial pressure of gas reactant i, and

ui

was coefficient of gas reactant i.

The law, represented the spontaneity of the reaction:

∆ r Gm < 0

Favored reaction (Spontaneous)

∆ r Gm = 0

Neither the forward nor the reverse reaction prevails (Equilibrium)

∆ r Gm > 0

Disfavored reaction (Nonspontaneous)


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According to the law, the potential reactions of carbon and minerals would be obtained when the pyrolysis temperature was either below or above AFT. 4

RESULTS AND DISCUSSION

4.1 Pyrolysis characteristics of low-AFT coal Fig. 1 depicted the obtained temperature, TG, DTG, and DSC profiles for Hulunbeier lignite coal. According to the DSC23 thermogram, the first stage was endothermic de-watering process (before 30 min). The second was exothermic volatile rapid evolution (between 30 to 40 min), followed by an endothermic process (between 40 to 60 min) in mode I. This suggested that endothermic events occurred with the exothermic volatile slow evolution according to the DSC profile in mode II. Minerals melted in this endothermic procedure when the temperature increased beyond the AFT. The thermoplastic properties25 varied fast as viscosity, porosity, and char fluxility changed. These factors caused the varying DSC profiles in both modes between 40 to 60 min. Note, the pyrolysis mass loss weights of both modes were similar.

4.2 Pyrolysis characteristics of mid-AFT coal The obtained temperature, TG, DTG, and DSC profiles for Yishang coal were shown in Fig. 2. Compared to the Hulunbeier curves, endothermic events occurred with the exothermic volatile evolution in mode I for Xinjing coal, which were confirmed by the DSC and DTG profiles between 45 to 60 min. As shown in Table 3, a secondary weight loss (ΔmV2) of about 4.5-16.5% of the initial mass appeared in mode I of the four mid-AFT coals. Wei et al.2, 16 discovered a similar phenomenon of secondary weight loss, which was attributed to the broken of side chain and aromatic structural 7 ACS Paragon Plus Environment

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units but no evidences were provided in their study. The following chemical thermodynamic calculation and XRD analysis were utilized to investigate the secondary weight loss process.

4.3 Coal chemical thermodynamic calculation The major components of coal ash were SiO2, Al2O3, Fe2O3, CaO, MgO, and CuO (see Table 2). This suggested that the potential reactions during the secondary weight loss contained the carbothermal-reduction reactions of metallic oxide with carbon or CO, the carbothermic nitridation of metallic oxide with N2 and carbon, and the reduction of metallic oxide with pure metal. According to Eq. (1), the standard Gibbs free energy of each potential reaction varied with the temperature. Part of the reactions and results were listed in Table 4. When the secondary weight loss occurred in coal high-temperature pyrolysis the gas productions were collected in sealed bags and analyzed in gas chromatography (GC)45. The results of the detected gas products were presented in Table 5. Combined Table 4 with Table 5, the actual Gibbs free energy of each potential reaction varied by the temperature was obtained in Eq. (1). According to the law of reaction spontaneity,

∆ r Gm < 0

,

the reacted temperature ranges were obtained first. The reactions were then identified during the secondary weight loss as shown in Fig. 3. The identified reactions were reactions (8), (12), (13), and (17). Assuming that the SiO2 and Al2O3 in coals were totally reacted with carbon in reactions (8) and (12), the possible maximum weight loss at m2-m3 can be estimated from the mass balance: ∆mmax = ∆mmax, SiO2 + ∆mmax,Al2 O3 =

mSiO2 60

× 2 × 28 + (

mAl2 O3 102

× 3 × 28 −

mAl2 O3 102

× 28)

(18)

with 8 ACS Paragon Plus Environment

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∆mmax

the possible maximum weight loss at m2-m3 (mg)

∆mmax,SiO2

the possible maximum weight loss caused by SiO2 (mg)

∆mmax, Al2 O3

the possible maximum weight loss caused by Al2O3 (mg)

The following ratio was used for the comparison of possible maximum weight loss the actual weight loss

∆mactual

∆mmax

with

at m2-m3:

ϕ=

∆mactual ∆mmax

(19)

with

ϕ

the percentage of actual weight loss with the possible maximum weight loss (%)

∆mactual

the actual weight loss, m2-m3 (mg)

The computation results were shown in Table 6. The actual weight loss of each mid-AFT coals was less than the possible maximum weight loss, which indicated that the contents of SiO2 and Al2O3 in coal were full enough to support the secondary weight loss at m2-m3. The mass fraction of metallic oxides in four mid-AFT ashes varied with ratio ϕ was shown in Fig. 4. As the ratio ϕ increased from 40.9 to 85.7%, the sum of mass fraction of SiO2 and Al2O3 increased while the sum of mass fraction of Fe2O3 and CaO decreased. It was shown that the low concentration of SiO2 and Al2O3 and the high concentration of Fe2O3 and CaO inhibited the secondary weight loss process.

4.4 XRD analysis The mineralogical analysis of coals and chars with varying TH were compared in Fig. 5. The minerals in coal (Fig. 5(a)) consisted mainly of SiO2, kaolinite (Al2O3·2SiO2·2H2O), gypsum 9 ACS Paragon Plus Environment

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(CaSO4·2H2O), FeS2, anorthite (CaO·Al2O3·2SiO2), and CaCO3. The mineral in the charII (Fig. 5(b)) consisted mainly of anorthite, SiO2, Al2O3, and mulite (3Al2O3·SiO2). From Fig. 5(b) and Fig. 5(a), it is suggested that when the pyrolysis temperature increased from 27ºC to 1200ºC, kaolinite, gypsum, and CaCO3 decomposed. SiO2 and Al2O3 decomposed from kaolinite and combined with CaO from CaCO3, contributing to the appearance of anorthite and mulite. When the tempearature was maintained at 1200ºC, the carbothermal-reduction reactions of SiO2 and Al2O3 with coal char began, complimenting with the low intensity of SiC, Si3N4, and AlN in Fig. 5(b). As shown in Fig. 5(c), charI in mode I (above AFT) contained SiC, Si3N4, and AlN, which confirmed the reactions (8), (12), (13), and (17). These occurred when the pyrolysis temperature exceeded the AFT. Fig. 6 showed the XRD patterns of Hanyu samples of coal (a), charII in mode II (b), and charI in mode I (c). By comparing Fig. 6 with Fig. 5, it was noted that the major crystal peaks in Hanyu coal were kaolinite, gypsum, quartz, and CaCO3. When the docomposition temperature increased to 1200ºC, the crystal peaks of kaolinite, gypsum, and CaCO3 dissappered and the new characteristic peaks of SiO2, Al2O3, CaS, and mulite were formed as shwon in Fig. 6(b). When the pyrolysis temperature further increased to 1450ºC, the crystal peaks of SiC, Si3N4, and AlN were revealed in Hanyu charI. This also indicated that the carbothermal-reduction reactions and the formation of SiC, Si3N4, and AlN were in consistence with the observations on the second weight loss of TG and the endothermic peak of DSC in Fig. 2. Fig. 7 depicted the XRD patterns of Hulunbeier samples of coal (a), charII in mode II (b), and charI in mode I (c). The major minerals presented in Hulunbeier coal were SiO2, kaolinite, gypsum, CaCO3, and FeS2. The intensity peaks of minerals in XRD patterns of Hulunbeier coal 10 ACS Paragon Plus Environment

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were lower than those in Xinjing coal. Apart from the peaks caused by SiO2, CaCO3, and CaS, the salient peaks seen in Fig. 7(b) and Fig. 7(c) could be attributed to anorthite. No other products were confirmed. It seems that a part of calcium oxide and ferrum oxide associated with aluminum oxide and silicon oxide to form anorthite with high AFT after combining the high content of calcium and ferrum in Hulunbeier ash, see Table 2. Large parts were formed with lowmelting eutectic non-crystal matters. These low-melting eutectic non-crystal matters were fused and exhibited inert reactivity with carbon in high pyrolysis temperatures, which could not be identified via XRD as shown in Fig. 7(b) and Fig. 7(c). The fused minerals wrapped the carbon inside the char particles42 and inhibited the production of silicon carbide, aluminum nitride, and silicon nitride. It seems this caused absence of the secondary weight loss in Hulunbeier coal but the detialed mechanism of chemical and physical reactions is still unknown. 5

CONCLUSIONS

The five Chinese coals (Hulunbeier lignite, Xinjing bituminous, Tangzhai bituminous, Yishang mean, and Hanyu mean coal) in order of increasing AFT, were utilized to investigate the pyrolysis characteristics below and above their AFTs. By comparing the TGA profiles, coal chemical thermodynamic calculation, and XRD patterns of the char samples, the conclusions were obtained as followed: (1) A phenomenon of secondary weight loss was seen in high-temperature pyrolysis of the four mid-AFT coals, but was not present in the low-AFT coal. (2) Based on the combination of TGA, chemical thermodynamic calculation, and XRD analysis, the secondary weight loss appeared in high-temperautre pyrolysis was primarily


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caused by the carbothermal-reduction reactions of metallic oxides with carbon and the carbothermic nitridation of metallic oxides with N2 and carbon:

SiO 2 + 3C  → SiC + 2CO(g)

Al 2 O3 + 3C + N 2 (g)  → 2AlN + 3CO(g)

3SiO 2 + 6C + 2N 2 (g)  → Si 3 N 4 + 6CO(g)

Si3 N 4 + 3C  → 3SiC + 2N 2 (g) (3) According to the TGA results and chemical thermodynamic calculation, the low concentration of SiO2 and Al2O3 and the high concentration of Fe2O3 and CaO inhibited the secondary weight loss process in coal high-temperature pyrolysis. (4) A part of calcium oxide and ferrum oxide reacted with aluminum oxide and silicon oxide to form anorthite with high AFT. This is likely due to the high content of calcium and ferrum in Hulunbeier ash. The ash also formed low-melting eutectic non-crystal matters. These low-melting eutectic non-crystal matters were fused and exhibited inert reactivity with carbon in high pyrolysis temperatures. The fused minerals covered the carbon inside the char particles and inhibited the produce of silicon carbide, aluminum nitride, and silicon nitride, resulting in the absence of the secondary weight loss in low-AFT coal but the detialed mechanism of chemical and physical reactions is still unknown.

ACKNOWLEDGEMENTS


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This study was supported by the National Key Research Development Program of China (No. 2017YFB0601900), the National Natural Science Foundation of China (Nos. 51406095 and 51025622), and the Fundamental Research Funds for the Central Universities (Nos. 2017ZZD005).


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FIGURES

Fig. 1. The obtained temperature, TG, DTG, and DSC profiles for Hulunbeier coal in mode I (TH I = 1450 ºC higher than AFT) and mode II (TH II = 1000 ºC lower than AFT).


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Fig. 2. The temperature, TG, DTG, and DSC profiles for Yishang coal obtained in mode I (TH I = 1450 ºC higher than AFT) and mode II (TH II = 1200 ºC lower than AFT).


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Fig. 3. The actual Gibbs free energy of identified reactions during the secondary weight loss


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Fig. 4. The mass fraction of metallic oxides in four mid-AFT ashes varied with ratio ϕ (mf,SiO2-mass fraction of SiO2, mf,Al2O3-mass fraction of Al2O3, mf,Fe2O3-mass fraction of Fe2O3, mf,CaO-mass fraction of CaO in ash)


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Fig. 5. XRD patterns of Xinjing samples: (a) coal; (b) charII in mode II (TH II =1200 °C) and (c) charI in mode I (TH I =1450 °C)


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Fig. 6. XRD patterns of Hanyu samples: (a) coal; (b) charII in mode II (TH II =1200 °C) and (c) charI in mode I (TH I =1450 °C)


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Fig. 7. XRD patterns of Hulunbeier samples: (a) coal; (b) charII in mode II (TH II =1200 °C) and (c) charI in mode I (TH I =1450 °C)


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TABLES Table 1: Ultimate and proximate analysis of coal samples (as received basis, wt.% ). Sample

Proximate analysis %(w/w ar)

Ultimate analysis %(w/w ar)

Fixed carbon

Volatile matter

Ash

Moisture

C

H

O

N

S

Lower heating value Q net, ar (MJ/kg)

Hulunbeier

44.86

35.44

11.05

8.65

59.46

3.35

16.44

0.80

0.25

25.6

Xinjing

36.59

23.21

38.99

1.21

34.29

0.63

22.84

0.76

1.29

17.6

Tangzhai

50.65

24.87

22.58

1.90

51.29

2.51

19.23

1.10

1.39

24.3

Yishang

62.20

19.11

17.11

1.59

52.32

1.85

23.88

0.59

2.67

25.7

Hanyu

57.63

16.56

25.22

0.59

43.66

2.23

25.51

1.00

1.78

24.0


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Table 2: Ash components and fusion characteristic temperature analysis of the coals Fusion characteristic temperature(℃ ℃)

Ash compositions (wt. %) Coal

a

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

P2 O 5

K2 O

Na2O

DT a

ST b

HT c

FT d

Hulunberer

55.63

11.51

14.22

11.24

2.14

0.76

3.02

0.06

0.48

0.94

1090

1100

1110

1160

Xinjing

55.41

23.10

6.79

5.54

2.47

1.02

3.61

0.22

0.96

0.88

1370

1386

1389

1392

Tangzhai

61.78

22.1

5.31

2.8

0.53

1.21

4.04

0.39

1.25

0.59

1276

1307

1310

1331

Yishang

57.88

18.57

8.26

7.09

2.38

0.93

3.31

0.12

0.54

0.92

1200

1344

1350

1358

Hanyu

55.58

22.88

6.73

6.25

2.45

1.01

3.58

0.19

0.44

0.89

1364

1381

1384

1390

Deformation Temperature. b Softening Temperature. c Hemispherical Temperature. d Flow Temperature.


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Energy & Fuels

Table 3: The TGA results for five coals in both modes Coal

Mode

ΔmM

ΔmV1

ΔmV2

m0-m1

m1-m2

m2-m3

1.21 1.50

0.6

3.2

1.25

2.7 0.0

13.7 0.0

1.33 1.19

19.1 16.1

0.9 0.3

4.5 1.4

1.42 1.54

16.0 23.5

2.0 0.0

10.1 0.0

1.27 1.32

0.2 1.0 4.4 22.0 3.3 16.5 mode I Low-AFT coal b Mid-AFT coal c mode I (TH I =1450 °C, higher than AFT for all coals) d mode II (TH II =1000°C for Hulunbeier coal and 1200 °C for the rest, lower than AFT)

1.76

Tangzhai b Yishang b Hanyu b Xinjing b

% 8.6 8.7

mg 4.8 4.8

% 23.9 23.8

mg 0.0 0.0

0.3

1.7

4.6

22.9

mode I mode II

0.4 0.3

1.9 1.5

4.6 3.8

23.0 19.2

mode I mode II

0.3 0.1

1.6 0.5

3.8 3.2

mode I

0.1 0.2

0.6 1.0

3.2 4.7

%

% 0.0 0.0

Hulunbeier a

mg 1.7 1.7

Residual Error

d

mode II

c

mode I mode II

mode II

a


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Table 4: The standard Gibbs free energy of some potential reactions The standard Gibbs free energy varied with temperature θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m θ m

Reactions

∆ r G = 1184331.71 − 584.69T(25 ~ 2000°C)

Al 2 O3 + 3C  → 2Al + 3CO(g)

(2)

∆ r G = 588161.51 − 352.58T(25 ~ 2000°C)

SiO 2 + 2C  → Si + 2CO(g)

(3)

∆ r G = 108687.78 − 149.67T(25 ~ 2000°C)

FeO + C  → Fe + CO(g)

(4)

∆ r G = 471926 − 196.45T(25 ~ 2000°C)

CaO + C  → Ca + CO(g)

(5)

∆ r G = 437825.67 − 200.12T(25 ~ 2000°C)

MgO + C  → Mg + CO(g)

(6)

∆ r G = 2148330.93 − 1085.83T(25 ~ 2000°C)

2Al2 O3 + 9C  → Al4 C3 + 6CO(g)

(7)

∆ r G = 514401.75 − 337.44T(25 ~ 2000°C)

SiO 2 + 3C  → SiC + 2CO(g)

(8)

∆ r G = 338719.83 − 459.02T(25 ~ 2000°C)

3FeO + 4C  → Fe3C + 3CO(g)

(9)

∆ r G = 407463.35 − 223.3T(25 ~ 2000°C)

CaO + 3C  → CaC 2 + CO(g)

(10)

∆ r G = 945711.41 − 416.11T(25 ~ 2000°C)

2MgO + 5C  → Mg 2 C3 + 2CO(g)

(11)

∆ r G = 600400.41 − 355.82T(25 ~ 2000°C)

Al 2 O3 + 3C + N 2 (g)  → 2AlN + 3CO(g)

(12)

∆ r G = 1032444.1 − 660.91T(25 ~ 2000°C)

3SiO 2 + 6C + 2N 2 (g)  → Si 3 N 4 + 6CO(g) (13)

∆ r G = 855773.69 − 1049.76T(25 ~ 2000°C)

8FeO + 8C + N 2 (g)  → 2Fe 4 N + 8CO(g)

(14)

∆ r G = 1032359.69 − 380.75T(25 ~ 2000°C)

3CaO + 3C + N 2 (g)  → Ca 3 N 2 + 3CO(g)

(15)

∆ r G = 898815.71 − 373.21T(25 ~ 2000°C)

3MgO + 3C + N 2 (g)  → Mg 3 N 2 + 3CO(g) (16)

∆ r G = 516530.64 − 352.32T(25 ~ 2000°C)

Si3 N 4 + 3C  → 3SiC + 2N 2 (g)

(17)


Page 25 of 32

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Energy & Fuels

Table 5: The GC results of gas productions during the secondary weight loss Coal Xinjing (%) Tangzhai (%) Yishang (%) Hanyu (%) Average concentration (%) Average partial pressure (atm) Gas productions in yield (%)

CO2 1.05 1.03 1.09 0.98 l.04 0.01 25.87

CO 2.25 1.83 2.06 1.98 2.03 0.02 50.50

SO2 1.05 0.88 1.07 0.81 0.95 0.01 23.63

N2 95.65 96.26 95.78 96.23 95.98 0.96 -


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Page 26 of 32

Table 6: the quantitative computation results of the weight loss at m2-m3

mSiO2

mAl2 O3

mFe2O3

mCaO

∆mmax

∆mactual

ϕ

mg

mg

mg

mg

mg

mg

%

Hulunbeier

1.23

0.25

0.31

0.25

1.29

0

0

Xinjing

4.32

1.80

0.53

0.43

5.02

3.30

65.7

Tangzhai

2.79

1.00

0.24

0.13

3.15

2.70

85.7

Yishang

1.98

0.64

0.28

0.24

2.20

0.90

40.9

Hanyu

2.80

1.15

0.34

0.32

3.25

2.00

61.5

Coal


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Energy & Fuels

AUTHOR INFORMATION

Corresponding Authors * Huaichun Zhou. Phone: (+86) 010-61771512. Email: [email protected] ** Shu Zheng. Phone: (+86) 010-61771514. Email: [email protected]

Present Addresses † Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China

Author Contributions Xiongwei Zeng: Experiments and data analysis Shu Zheng: Proposed and supported the research Huaichun Zhou: Analysis principle and explanation of results

Notes The authors declare no competing financial interest.


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Page 28 of 32

ABBREVIATIONS PC, pulverized coal TGA, thermo-gravimetric analyzer FC, fixed carbon VM, volatile matter A, ash M, moisture ar, as received basis TG, Thermo-gravimetry DTG, Differential thermal gravity DSC, Differential Scanning Calorimetry DT, ash deformation temperature (°C) ST, ash softening temperature (°C) HT, ash hemispherical temperature (°C) AFT, ash flow temperature FFT, Fast Fourier Transformation

TH, the highest temperature during the experiment (°C) Q, lower heating value (MJ/kg)


Page 29 of 32

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Energy & Fuels

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The phenomena of secondary weight loss in high-temperature coal

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