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. 2022 Mar 1;7(10):8547–8557. doi: 10.1021/acsomega.1c06350

Analysis of Pyrolysis Performance and Molecular Structure of Five Kinds of Low-Rank Coals in Xinjiang Based on the TG-DTG Method

Xian-Kang Shan †, Shuai-Li Zhao †, Ya-Ya Ma †, Wenlong Mo †,*, Xian-Yong Wei †,‡,*
PMCID: PMC8928551  PMID: 35309428

Abstract

graphic file with name ao1c06350_0007.jpg

Taking five coal samples (FCSs) in Xinjiang as the research object, characterizations such as proximate analysis, ultimate analysis, Fourier transform infrared (FTIR), and thermogravimetry-differential thermal analysis (TG-DTG) were carried out. The Coats–Redfern model was used to simulate pyrolysis kinetics of FCSs under different reaction orders (ROs). The results showed that except for HSBC, the R2 of the other four coal samples are all higher than 0.9, which showed a good correlation effect. FCSs present similar reaction activation energy in the same RO and temperature range. Results of FTIR showed that the hydroxyl groups of FCSs, in the range of 3100–3600 cm–1, were mainly self-associated hydroxyl hydrogen bonds and hydroxyl π bonds, and they occupied over 63%. Among them, the pyrolysis characteristic index (D) of XBC was 4.139 × 10–6, higher than those of other samples, and it showed good pyrolysis performance. Moreover, by reducing the temperature range appropriately, the fitting results showed a better correlation effect.

1. Introduction

Study of the coal structure is an important foundation for realizing efficient utilization of coal. Studies have shown that the typical chemical structure models of coal included the Fuchs model,1 Given model,2 Wiser model,3 and Shinn model.4 Because of the complexity of the coal structure and diversity of its properties and utilization, there is still a lack of specific research on coal types with different metamorphisms and coal compositions.5 The classical physical structural models of coal include the Hirsch model,6 cross-linking model,7 host–guest model,8 association model,9 and Riley model.10 Advanced characterization methods such as X-ray diffraction,11 infrared spectroscopy,12 nuclear magnetic resonance (NMR) spectroscopy,13 statistics, and structural analysis14 have been used in the structural studies of coal.

Pyrolysis of coal is the conversion to organic components of coal by thermal decomposition under anaerobic conditions.15−17 During this process, decomposition of functional groups (FGs) and fracture of macromolecular chains occur simultaneously.18 Pyrolysis coke, liquid tar, and pyrolysis gas are the primary products of this process.19−21 For low-rank coal, more alkyl side chains, heteroatom-containing groups, and higher reaction activity make it easier to realize efficient utilization through pyrolysis conversion.22,23

The Coats–Redfern model (CRm) is a commonly used approximation formula, and it is one of the well-known approximate formulas according to previous studies.24,25 Although its accuracy needs to be improved, CRm is simple to linearize the kinetic equation. More researchers have used it.26−28 Mo et al.29 studied the combustion performance for five kinds of oily sludge in Karamay by Fourier transform infrared (FTIR) and thermogravimetry-differential thermal analysis (TG-DTG) analyses, and found that the combustion performance was related to the volatile content. However, many studies30−32 on pyrolysis/combustion characteristics focused on the combustion part, but there are a few studies on the calculation of the pyrolysis index.

In this paper, five low-rank coals (including lignite, bituminous coal, and sub-bituminous coal) in Xinjiang were taken as the research object. Through FTIR and thermogravimetry tests, the pyrolysis kinetics of FCSs was simulated and analyzed based on the CRm, and the appropriate reaction order (RO) was determined by calculating the fitting parameters. At the same time, the pyrolysis characteristics and pyrolysis index were calculated.

2. Results and Discussion

2.1. Proximate and Ultimate Analyses

Table 1 shows the proximate and ultimate data of FCSs. As can be seen from Table 1, the moisture (Mad) content of XSBC, NL, and WSBC is above 10%, indicating that they have relatively high Mad. The volatile matter (Vdaf) of FCSs ranges from 31 to 53%, indicating that the content of small molecules in FCSs varies from region to region and these compounds can be released by pyrolysis. Except for HSBC coal, the ash content (Ad) of the other four samples was very small, below 10%. These studies demonstrate35,36 that Vdaf is mainly composed of CO, CO2, H2, CH4, N2, other small molecular gases, and volatile organic compounds.

Table 1. Proximate and Ultimate Analyses of FCSsa.

  proximate analysis, w (%)
 ultimate analysis, w (%)
        
sample Mad Ad Vdaf FCdaf* Cdaf Hdaf Ndaf St,d Odaf* H/C O/C LHV33 HHV34
XSBC 16.33 4.00 31.52 68.48 78.46 3.24 0.71 0.40 >17.20 0.50 0.16 22.19 23.37
WSBC 13.32 5.19 32.57 67.42 78.00 3.86 0.93 0.49 >16.73 0.59 0.16 22.56 23.73
HSBC 5.88 21.18 42.81 57.19 74.91 5.65 1.50 0.37 >17.57 0.91 0.18 18.60 19.80
XBC 2.21 3.59 43.86 56.14 78.60 5.27 1.28 0.40 >14.45 0.80 0.14 23.97 25.13
NL 10.36 9.45 52.12 47.88 70.95 3.33 0.63 0.42 >24.67 0.56 0.26 19.07 20.27
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a

*-by difference.

Ultimate analysis showed that NL had the least amount of elemental C and XBC had the most, with the amount of elemental H ranging from 3–6%. The O/C ratio of FCSs was relatively low (0.14–0.26), indicating that the organic matter in the FCSs had a high molecular mass and was composed mainly of cyclic compounds.37,38 The H/C ratio of HSBC was as high as 0.91, which indicates high saturation of organic matter.39,40

Furthermore, as can be seen from Table 1, the Vdaf of FCSs in descending order is as follows: NL > XBC > HSBC > WSBC > XSBC. Since the pyrolysis reaction mainly involves the thermal decomposition of organic matter, so we deduce that the ultimate weight loss (WL) of FCSs is NL > XBC > HSBC > WSBC > XSBC.

2.2. FTIR Analysis

2.2.1. FTIR Spectrum

Figure 1 shows the FTIR spectra of FCSs. The absorption peak (Abp) between 3200 and 3600 cm–1 is attributed to the hydroxyl group,41 and Abp at 3435 cm–1 is attributed to the OH−π bond. For this FG, XBC, and XSBC are stronger, WSBC is weaker, and the other two coal samples are almost free of this FG. The Abps at 2917 cm–1 and 2849 cm–1 are attributed to the aliphatic symmetric and asymmetric C–H stretching vibration, such as aliphatic structures −CH3, −CH2, and −CH,42 indicating that there are more aliphatic organic compounds in FCSs. Among them, XBC has the highest strength, followed by XSBC and WSBC, with NL and HSBC with a lower content. The peak at 1603 cm–1 belongs to the C=O bond, indicating that FCSs contain ketones, aldehydes, acids, and other substances containing C=O bonds, and here, the Abps of XBC and XSBC are significantly stronger than the other three samples, indicating that they contain relatively more of the three organic compounds mentioned above. At 1444 cm–1 is the aromatic ring skeleton vibration Abp, where the order for absorption intensity is XBC < XSBC/WSBC < HSBC < NL, corresponding to the order for the O/C ratio XBC < XSBC/WSBC < HSBC < NL in Table 1. An out-of-plane bending vibration Abp caused by the benzene ring is present in the FCSs at 900–720 cm–1, with the lowest intensity of NL, which is consistent with the weak vibration Abp of the aromatic ring skeleton observed at 1444 cm–1.

Figure 1.

Figure 1

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IR distribution of FCSs.

2.2.2. FTIR Semiquantitative Analysis

According to the types of FG, the IR spectra can be divided into the following four regions: 3100–3600, 2800–3000, 1000–1800, and 700–900 cm–1.43,52 The IR spectra of these regions were fitted using Peakfit software and semiquantitative analysis was performed to analyze the FCSs.44,45,52 The results of the fitting are shown in Figure 2 and Table 2 to provide a detailed understanding of the type and distribution of the organic matter structure in FCSs. The Abp of hydroxyl FGs of FCSs is mainly distributed in the range of 3100–3600 cm–1 (Figure 2) and contains mainly self-associated hydroxyl hydrogen bonds, hydroxyl π bonds, hydroxyl cyclic hydrogen bonds, and hydroxyl ether hydrogen bonds. Within this range, the positions of the IR Abps are almost the same, providing further evidence that the types of FGs in FCSs are the same. In terms of the percentage of the peak area, hydroxyl groups in the FCSs are mainly self-associated hydroxyl hydrogen bonds and hydroxyl π bonds, which account for more than 63% of the total area. Among them, the percentage of self-associated hydroxyl hydrogen bonds was the highest, over 40%.

Figure 2.

Figure 2

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FTIR curve-fitting results of FCSs.

Table 2. Change in the Contents of Groups of FCSs.
    content (%)
wave number (cm–1) functional groups XSBC WSBC HSBC XBC NL
3600–3500 OH−π 2.07 0.62 12.05 1.32 1.36
3500–3350 self-associated OH 41.56 58.71 51.27 40.21 41.46
3350–3260 OH–ether O 35.23 24.76 11.87 30.64 32.18
3260–3170 cyclic OH 28.73 15.91 11.77 27.82 25.00
2950 aliphatic −CH3 15.98 17.91 14.57 17.61 18.90
2920 asymmetric aliphatic −CH2 41.90 45.79 40.09 39.17 35.83
2890 aliphatic −CH 17.57 20.75 18.77 18.16 20.30
2850 symmetric aliphatic −CH2 24.54 30.32 26.57 25.06 24.96
1690 carboxylic acids C=O 12.11 7.64 10.69 3.20 17.21
1610 conjugated C=O 22.45 18.79 21.09 25.64 24.73
1560 aromatic C=C 8.37 2.60 2.13 4.69 13.25
1440 asymmetric CH3–, CH2– 11.58 6.87 8.77 20.58 16.25
1350 CH3-Ar, R 11.51 1.36 1.74 11.63 9.45
1245 symmetric deformation −CH3 13.77 4.26 2.98 14.44 9.30
1165 C–O phenols 13.76 25.15 24.16 14.00 5.95
1090 grease C–O 6.45 33.30 45.05 5.82 3.87
900–860 one adjacent H deformation 3.49 71.53 28.44 23.83 3.56
860–810 two adjacent H deformations 24.41 11.56 17.73 41.67 28.13
810–750 three adjacent H deformations 23.44 15.06 46.04 14.03 62.38
750–720 four adjacent H deformations 48.65 13.86 13.69 20.47 5.92
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From Figure 2, aliphatic −CHx Abps occur in the range of 2800–3000 cm–1 with four subpeaks. As can be seen in Table 2, the −CHx Abp is mainly symmetric and asymmetric aliphatic −CH2, which sum up to approximately 60–70%, with approximately 40% of asymmetric aliphatic −CH2. The other two subpeaks are smaller in area, and the content of aliphatic −CH and aliphatic −CH3 in each component is almost the same, about 19 and 17%, respectively, indicating that aliphatic hydrocarbons in FCSs are mainly in the form of long and less branched chains.46 As can be seen from Table 2, the FG peak area of FCS varies considerably in the range of 1000–1800 cm–1. The C=O bonds located at 1800–1650 cm–1 can be divided into aliphatic C=O bonds and aromatic C=O bonds. For carboxylic acid C=O appears at 1690 cm–1, NL accounted for 17.21%, and the relative content difference of other samples is very small. Conjugated C=O is found at 1610 cm–1, and the contents of FCSs almost reach more than 20%. There is an aromatic C=C FG at 1560 cm–1 with similar content distribution to that at 1690 cm–1. For the FGs at 1350–1245 cm–1, the content of HSBC and WSBC was only about 2.00% and the remaining coal samples contained about 10%. Grease C–O was present at 1090 cm–1 in 45.05 and 33.30% for HSBC and WSBC, respectively, while the other three coal samples contained only 5%.

From the peak area data in Table 2, HSBC and NL were mainly trisubstituted by benzene rings in the range of 700–900 cm–1, and their relative contents were 46.04 and 62.38%, respectively; WSBC was mainly substituted by benzene rings with relative contents of 71.53%; XBC was mainly disubstituted by benzene rings with relative contents of 41.67%; and XSBC was mainly tetrasubstituted by benzene rings, and its relative content was 48.65%.

2.3. Pyrolysis Performance of FCSs

2.3.1. Pyrolysis Characteristics

Figure 3 shows the TG-DTG curves for FCSs. From the TG diagram, the WL of WSBC is the smallest, about 30.44%; the WL of NL is the largest, about 51.69%; the WL of the other three samples was XSBC, 32.08%; HSBC, 38.77%; and XBC, 47.05% respectively. Based on proximate analysis, WSBC had a Vdaf of 32.57%; XSBC, HSBC, and XBC had a Vdaf of 31–44%; and NL had the largest Vdaf of approximately 50%. Therefore, the higher Vdaf in the sample, the greater the WL, and slightly higher the WL per sample than Vdaf. It is possible that the number of samples required for TG characterization is relatively less than for volatility testing. A smaller number of samples transfer more heat, resulting in a higher proportion of substances escaping from coal at the same temperature. According to the data calculation in Figure 3, the final WL size order of FCSs is NL > XBC > HSBC > XSBC > WSBC. The results are basically consistent with the results in Section 2.1.

Figure 3.

Figure 3

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TG-DTG of FCSs.

As shown in Figure 3, the pyrolysis process of FCSs can be divided into three stages:47−50 the first stage occurs at around 100 °C. At this stage, water and small molecule gases (N2, CO2, CH4, etc.) will be removed from the coal. The second stage occurs between 400 and 500 °C. At this stage, the pyrolysis reaction is very intense and volatiles are released in the process. The third stage occurs above 550 °C and is mainly a condensation reaction, during which the semi-coke will further decompose and precipitate a large amount of H and a small amount of CH4. In this stage, the ordering of the semi-coke will be further enhanced and eventually be completely converted into coke.

2.3.2. Conversion–Temperature Relationship

Figure 4 shows the relationship between pyrolysis conversion and temperature. When the CRm is used to perform kinetic fitting of the residue pyrolysis process, we regard it as a complete process for fitting analysis. Because of the significant differences in pyrolysis characteristics at different temperatures, the fitting results have large errors and cannot objectively reflect the pyrolysis of the residue, therefore, the pyrolysis process is divided into three stages of low-temperature, mid-temperature, and high-temperature for kinetic fitting. Combined with the TG-DTG curve and the tangent method,47,50 we divide the whole process into three temperature ranges: low-temperature section (250–360 °C), mid-temperature section (360–550 °C), and high-temperature section (550–800 °C).

Figure 4.

Figure 4

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Temperature conversion diagram of FCSs.

Table 3 gives the pyrolysis characteristic indexes of FCSs. According to the literature,51 the higher the value of D, the better the pyrolysis performance of the sample. The order of D value is XBC > XSBC > NL > HSBC > WSBC, so XBC exhibits best pyrolysis performance, which is 4.139 × 10–6.

Table 3. Pyrolysis Characteristics of FCSsa,
sample Ti (°C) Tf (°C) Mf (%) M∞ (%) Tmax (°C) ΔT1/2 WL1 WL2 WL3 D (10–7)
XSBC 337.03 701.06 66.99 33.01 445.03 289.47 2.72 27.56 11.36 2.34
WSBC 389.33 612.18 69.56 30.44 458.76 99.27 2.45 12.94 11.48 0.07
HSBC 374.06 579.02 62.12 37.88 440.06 90.01 1.85 19.81 9.53 0.19
XBC 389.33 561.54 52.87 47.13 449.24 86.82 4.35 29.04 9.04 41.39
NL 374.54 533.47 49.31 50.69 439.96 82.22 1.55 14.52 9.17 0.59
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The comprehensive pyrolysis characteristic index D extracts to evaluate the pyrolysis characteristics.

2.3.2.

where (dw/dt)max is the maximum mass loss rate (%/min); (dw/dt)mean is the average mass loss rate (%/min), M∞ is weight loss, and M∞ = 1 – Mf/100 (%); Ti is the initial temperature (°C); Tmax is the temperature according to the (dw/dt)max, and ΔT1/2 is the temperature interval when (dw/dt)/(dw/dt)max = 0.5; Mf is the pyrolysis residue mass; ML1 is the WL in the low-temperature section (250–360 °C); ML2 is the WL in the mid-temperature section (360–550 °C); and ML3 is the WL in the high-temperature section (550–800 °C).

In the low and high-temperature sections, the influence of temperature on the pyrolysis conversion is relatively small, and the change of the pyrolysis conversion curve is relatively smooth. In the mid-temperature section, the change of pyrolysis conversion was more obvious as the temperature increased, indicating that more pyrolysis products would be obtained from the decomposition of coal in the mid-temperature section. Taking n = 1, 2, 3, 4, and 5, the kinetic fitting results are shown in Figure 5 to calculate the kinetic parameters of pyrolysis of FCSs in the low and high-temperature section. The results of the calculations are shown in Tables 4–7.

Figure 5.

Figure 5

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Fitting diagram of pyrolysis kinetics of FCSs.

Table 4. Pyrolysis Kinetic Parameters of FCSs with Different Reaction Stages in the Low-Temperature Section.
sample RO y R2 E (kJ/mol) A (min–1) 2RT/E
XBC at 250–360 °C
  1 –1582.30x – 12.71 0.9924 13.156 4.79 × 10–2 0.661
2 –1673.27x – 12.52 0.9914 13.912 6.13 × 10–2 0.625
3 –1766.31x – 12.32 0.9903 14.686 7.87 × 10–2 0.592
4 –1861.42x – 12.12 0.9893 15.477 1.01 × 10–1 0.562
5 –1958.60x – 11.92 0.9882 16.285 1.31 × 10–1 0.534
XSBC at 250–360 °C
  1 –531.32x – 13.71 0.9401 4.417 5.92 × 10–3 1.970
2 –660.02x – 13.41 0.9459 5.488 9.93 × 10–3 1.585
3 –795.06x – 13.09 0.9495 6.611 1.63 × 10–2 1.316
4 –936.41x – 12.77 0.9519 7.786 2.66 × 10–2 1.117
5 –1083.98x – 12.43 0.9535 9.013 4.32 × 10–2 0.965
HSBC at 250–360 °C
  1 –462.27x – 14.24 0.8220 3.843 3.02 × 10–3 2.263
2 –543.69x – 14.05 0.8435 4.520 4.30 × 10–3 1.924
3 –627.82x – 13.85 0.8591 5.220 6.04 × 10–3 1.667
4 –714.63x – 13.65 0.8708 5.942 8.42 × 10–3 1.464
5 –804.11x – 13.44 0.8799 6.686 1.17 × 10–2 1.301
NL at 250–360 °C
  1 –743.19x – 13.31 0.9052 6.179 1.24 × 10–2 1.408
2 –894.63x – 12.96 0.9127 7.438 2.09 × 10–2 1.170
3 –1053.90x – 12.61 0.9117 8.763 3.53 × 10–2 0.993
4 –1220.92x – 12.23 0.9212 10.151 5.93 × 10–2 0.857
5 –1395.60x – 11.85 0.9237 11.604 1.00 × 10–1 0.750
WSBC at 250–360 °C
  1 –748.63x – 13.88 0.9532 6.224 7.02 × 10–3 1.398
2 –831.94x – 13.69 0.9541 6.917 9.42 × 10–3 1.258
3 –917.67x – 13.50 0.9548 7.630 1.26 × 10–2 1.140
4 –1005.82x – 13.30 0.9553 8.363 1.68 × 10–2 1.040
5 –1096.37x – 13.10 0.9556 9.116 2.25 × 10–2 0.954
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Table 7. Pyrolysis Kinetic Parameters of FCSs with Different ROs in the Original Low-Temperature Section (170–360 °C).
sample RO y R2 E (kJ/mol) A (min–1) 2RT/E
XBC at 170–360 °C
  1 –1402.64x – 13.02 0.9917 11.662 3.11 × 10–2 0.632
2 –1461.34x – 12.88 0.9902 12.150 3.71 × 10–2 0.606
3 –1521.10x – 12.74 0.9887 12.647 4.44 × 10–2 0.583
4 –1581.91x + 12.60 0.9871 13.153 5.31 × 10–2 0.560
5 –1643.78x – 12.46 0.9855 13.667 6.4 × 10–2 0.539
XSBC at 170–360 °C
  1 –459.80x – 13.83 0.9776 3.823 3.12 × 104 1.928
2 –552.54x – 13.59 0.9754 4.594 7 × 10–3 1.604
3 –649.10x – 12.74 0.9730 5.397 1 × 10–2 1.365
4 –749.48x – 13.09 0.9706 6.232 1.5 × 10–2 1.183
5 –853.64x – 12.83 0.9682 7.098 2.3 × 10–2 1.038
HSBC at 170–360 °C
  1 –158.13x – 14.76 0.4587 1.315 6.12 × 10–4 5.605
2 –210.21x – 14.62 0.5595 1.748 9.37 × 10–4 4.216
3 –263.78x – 14.48 0.6289 2.193 1.36 × 10–3 3.360
4 –318.86x – 14.33 0.6781 2.651 1.90 × 10–3 2.780
5 –375.43x – 14.18 0.7141 3.122 2.61 × 10–3 2.361
NL at 170–360 °C
  1 –256.18x – 14.14 0.4781 2.130 1.84 × 10–3 3.460
2 –345.46x – 13.91 0.5709 2.872 3 × 10–3 2.566
3 –438.78x – 113.67 0.6327 3.648 5 × 10–3 2.020
4 –536.12x – 13.41 0.6755 4.458 8 × 10–3 1.653
5 –637.43x – 13.15 0.7064 5.300 1.2 × 10–2 1.390
WSBC at 170–360 °C
  1 –1539.11x + 0.21 0.9827 4.015 1.91 × 104 1.835
2 –482.88x – 14.34 0.9085 4.470 2.87 × 10–3 1.648
3 –593.81x – 14.06 0.9113 4.937 5 × 10–3 1.493
4 –651.29x – 13.91 0.9122 5.415 6 × 10–3 1.361
5 –710.11x – 13.76 0.9127 5.904 7 × 10–3 1.248
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Table 4 shows the calculated kinetic parameters for the pyrolysis of FCSs at different ROs in the low-temperature section. As can be seen from Table 4 that with the increase of RO, except for HSBC coal, the correlation coefficient R2 of other coal samples increases, and the value of R2 → 1, which indicates that the kinetic fitting error gradually decreases. The 2RT/E value decreases with the increase of RO. Combined with the assumption that 2RT/E → 0 and R2 → 1, the fitting effect of FCSs is best under n = 5 in the low-temperature section.

In terms of activation energy (AE) at n = 5, HSBC had the lowest AE of 6.686 kJ/mol; XBC and NL had the largest AEs, both above 11 kJ/mol, indicating that HSBC was the most suitable, with the lowest energy consumption for pyrolysis in the low-temperature section at n = 5. Compared to the other samples, XBC and NL had higher energy consumption for pyrolysis and were not suitable for low-temperature pyrolysis.

Table 5 shows the calculated results of the kinetic parameters for the pyrolysis of FCSs at different ROs in the mid-temperature section. As can be seen from Table 5, for n = 1–5, the correlation coefficient R2 of FCSs has little difference, mainly concentrated above 0.965, indicating that the kinetic fitting errors are small. 2RT/E values decrease with the increase of RO. Combined with the assumption that 2RT/E → 0, the results indicate that the reaction model is more suitable for the pyrolysis process as RO increases. Considering that the fitted correlation coefficient R2 is similar, the lower the RO, the lower the AE. Therefore, the fitting effect of FCSs is best when n = 1 in the mid-temperature section.

Table 5. Pyrolysis Kinetic Parameters of FCSs with Different Reaction Stages in the Mid-Temperature Section.
sample RO y R2 E (kJ/mol) A (min–1) 2RT/E
XBC at 360–550 °C
  1 –6010.84x – 5.76 0.9682 49.977 1.89 × 102 0.170
2 –7770.73x – 3.04 0.9795 64.610 3.73 × 103 0.163
3 –9820.25x + 0.12 0.9814 81.651 1.11 × 105 0.129
4 –12120.61x + 3.66 0.9785 100.776 4.71 × 106 0.104
5 –14622.78x + 7.50 0.9737 121.581 3.64 × 108 0.087
XSBC at 360–550 °C
  1 –2766.35x – 10.19 0.9917 23.001 1.89 × 102 0.458
2 –3767.82x – 8.55 0.9933 31.328 7.27 0.336
3 –4915.91x – 6.69 0.9900 40.873 6.10 × 101 0.258
4 –6198.00x – 4.63 0.9852 51.533 6.07 × 102 0.204
5 –7596.39x – 2.38 0.9804 63.160 7.03 × 103 0.167
HSBC at 360–550 °C
  1 –4929.11x – 7.20 0.9619 40.983 3.69 × 101 0.257
2 –6464.36x – 4.79 0.9765 53.748 5.84 × 102 0.196
3 –8244.93x – 2.01 0.9815 68.552 1.11 × 104 0.154
4 –10241.70x + 1.10 0.9815 85.155 3.07 × 105 0.124
5 –12416.92x + 10.19 0.9794 103.240 1.09 × 107 0.102
NL at 360–550 °C
  1 –4454.67x – 7.43 0.9546 37.038 2.63 × 101 0.284
2 –6661.72x – 3.95 0.9739 55.389 1.28 × 103 0.190
3 –9339.26x + 0.25 0.9776 77.651 1.19 × 105 0.136
4 –12387.63x + 5.01 0.9759 102.997 1.86 × 107 0.102
5 –15700.15x + 1.68 0.9731 130.538 4.15 × 108 0.081
WSBC at 360–550 °C
  1 –4789.10x – 7.60 0.9732 39.819 2.39 × 102 0.264
2 –6038.65x – 5.65 0.9762 50.208 214 × 102 0.210
3 –7460.31x – 3.43 0.9743 62.029 2.12 × 104 0.170
4 –9039.07x – 0.98 0.9700 75.155 3.39 × 104 0.141
5 –10754.15x + 1.68 0.9650 89.415 5.76 × 108 0.118
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From the AE values for n = 1, XSBC had the smallest AE value of 31.328 kJ/mol; XBC and NL had the largest AE, both above 121 kJ/mol, indicating that XSBC was the most suitable and had the lowest energy consumption for pyrolysis in the mid-temperature section and n = 1, while XBC and NL were not suitable in this temperature range.

Table 6 shows the calculated kinetic parameters for the pyrolysis of FCSs in different reaction stages in the high-temperature section. From Table 6, it can be seen that the correlation coefficient R2 values gradually decreased with the increase of RO, except for XBC and NL, indicating that the kinetic fitting error gradually increased. 2RT/E values decreased with the increase of RO, combined with the assumption that 2RT/E → 0, it shows that with the increase of the RO, the reaction model is more suitable for the pyrolysis process, and combined with the assumption that R2 → 1, n = 2 is more suitable for the high-temperature section. In terms of AE at n = 2, WSBC had the smallest AE of 25.378 kJ/mol and the XSBC coal sample had the largest AE of 47.060 kJ/mol, indicating that WSBC was the most suitable and had the lowest energy consumption for pyrolysis in the high-temperature section and at n = 2, and XSBC coal sample was the opposite.

Table 6. Pyrolysis Kinetic Parameters of FCSs with Different Reaction Stages in the High-Temperature Section.
sample RO y R2 E (kJ/mol) A (min–1) 2RT/E
XBC at 550–800 °C
  1 –1041.20x + 12.02 0.9796 8.657 6.27 × 10–2 1.581
2 –4364.04x – 7.43 0.9932 36.285 2.58 × 101 0.377
3 –8786.35x – 1.44 0.9896 73.054 2.09 × 104 0.187
4 –13704.66x + 5.24 0.9887 113.947 2.60 × 107 0.120
5 –18792.20x + 12.20 0.9887 156.247 3.73 × 1010 0.088
XSBC at 550–800 °C
  1 –964.58x + 11.19 0.9991 16.319 2.71 × 10–1 0.839
2 –5659.93x – 6.34 0.9915 47.060 9.95 × 101 0.291
3 –10531.23x – 0.05 0.9833 87.562 9.99 × 104 0.156
4 –16003.96x + 7.01 0.9803 133.064 1.78 × 108 0.103
5 –21725.56x + 14.43 0.9796 180.637 4 × 1011 0.076
HSBC at 550–800 °C
  1 –961.58x – 12.21 0.9817 7.995 4.77 × 102 1.712
2 –3796.75x – 8.27 0.9935 31.568 9.73 0.434
3 –7545.60x – 0.05 0.9897 62.738 3.22 × 103 0.218
4 –11762.50x + 2.60 0.9885 97.799 1.58 × 106 0.140
5 –16164.60x + 8.63 0.9883 134.400 9.05 × 108 0.102
NL at 550–800 °C
  1 –508.87x – 12.45 0.9012 4.231 2.00 × 10–2 3.24
2 –3785.33x – 7.74 0.9499 31.473 1.64 × 101 0.435
3 –8170.46x – 1.57 0.9529 67.933 1.7 × 104 0.201
4 –12965.24x + 5.22 0.9557 107.799 2.41 × 107 0.127
5 –17867.86x + 12.23 0.9843 148.562 3.67 × 1010 0.092
WSBC at 550–800 °C
  1 –1359.45x – 11.89 0.9945 11.303 9.33 × 102 1.211
2 –4254.96x + 14.43 0.9935 25.378 1.48 × 101 0.387
3 –8053.73x – 2.91 0.9872 66.962 4.37 × 103 0.204
4 –12354.30x + 2.797 0.9849 102.720 2.03 × 106 0.133
5 –16877.47x + 8.826 0.9843 140.327 1.15 × 109 0.098
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From Tables 4 to 6, in the low-temperature section, 2RT/E values decrease with the increase of RO for the other four coal samples except for XBC; however, the correlation coefficient R2 decreases significantly with the increase of RO, indicating an increasingly poor fit to the pyrolysis kinetics. From the AE data, it can be found that AE increases with the increasing number of RO. Considering 2RT/E → 0 and kinetic fitting, the pyrolysis RO of the above FCSs in the low-temperature section is considered to be 5.

In the mid-temperature section, the FCSs correlation coefficient R2 is similar and the kinetic fitting errors are small. 2RT/E values decrease with the increasing number of RO. Considering the AE of the reaction, the energy consumption is lower when n = 1.

In the high-temperature section, the 2RT/E values of FCSs decrease with increasing number of RO, but when RO n = 1, the 2RT/E values were relatively large, some even greater than 1, which was inconsistent with the assumption that 2RT/E → 0, indicating that CRm was not applicable at this temperature. At RO of 2, 3, 4, and 5, the correlation coefficient R2 → 1, indicates a good fit for the kinetics of coal pyrolysis; the results show that the pyrolysis of FCSs at high-temperature is dominated by n = 2, 3, 4, and 5.

2.3.3. Comparative Analysis

It is worth mentioning that when calculating the kinetic parameters of the low-temperature section, the correlation coefficient is different because of the division of different temperature sections. Therefore, this paper simply analyzed the influence of different temperature sections on the correlation coefficient in the low-temperature section.

Table 7 shows the calculated pyrolysis kinetic parameters for FCSs with different ROs in the original low-temperature section (170–360 °C). As can be seen from Table 6, the R2 is as low as 0.4587 and 0.4781 for HSBC and NL, respectively, which showed poor correlation. To solve this problem, we adjusted the original temperature section (170–360 °C) to a new one (250–360 °C) and the problem was effectively solved. The maximum R2 for HSBC and NL increased to 0.8708 and 0.9237, respectively, and the fitted results showed good correlations. Therefore, we speculate that the differences in pyrolysis characteristics between the above two coal samples in the original temperature range are too large, resulting in large errors in the fitting results when fitting the kinetics of the coal pyrolysis process with the CRm model, which cannot objectively reflect the pyrolysis of coal samples. Therefore, on appropriately reducing the temperature range, the fitted results showed a better correlation.

3. Conclusions

The IR spectra showed that in the range of 3600–3100 cm–1, the hydroxyl groups in FCSs are mainly self-associating hydroxyl hydrogen bond, hydroxyl hydrogen bond, and hydroxyl cyclic hydrogen bond; in the range of 2800–3000 cm–1, the Abps of FCSs are mainly symmetric and asymmetric aliphatic −CH2; in the range of 1000–1800 cm–1, conjugated C=O is found at 1610 cm–1. By calculating the pyrolysis characteristic index, XBC was found to have better pyrolysis performance. TG-DTG showed that the FCSs showed good correlation at low, medium, and high temperatures; at low-temperature and n = 1, HSBC was the most suitable and had the lowest pyrolysis energy consumption. The pyrolysis energy consumption of XBC and NL coal samples is large. In the mid-temperature section and at n = 1, XSBC was the most suitable and had the lowest pyrolysis energy consumption. XBC and NL had higher pyrolysis energy consumption. In the high-temperature section, n = 2, WSBC is the most suitable and has the lowest pyrolysis energy consumption. XSBC has a higher pyrolysis energy consumption. Through the kinetic fit plots of FCSs and RO, it is found that with the increase of RO, the higher the temperature, the more obvious the effect of temperature on the conversion rate. When fitting the kinetics of the pyrolysis process of FCSs with CRm, the fitting results have large errors and could not objectively reflect the pyrolysis process of FCSs. Therefore, by appropriately reducing the temperature range, the fitted results showed a better correlation.

4. Experimental Section

4.1. Raw Materials

The raw materials used in this experiment are Xigou bituminous coal (XBC), Xiheishan sub-bituminous coal (XSBC), Hefeng sub-bituminous coal (HSBC), Naomaohu lignite (NL), and Wucaiwan sub-bituminous coal (WSBC) from Xinjiang (China). The sample preparation method is ASTM D2013-2007. Before the experiment, the coal samples were ground and passed through a 200 mesh sieve.

4.2. Analysis Methods

Proximate analysis, ultimate analysis, FTIR, and TG-DTG characterization studies were carried out on five coal samples (FCSs). A GB/T 28731-2012 was used for proximate analysis, a VarioMax elemental analyzer was used for ultimate analysis, FTIR was performed using a TENSOR-27 infrared spectrometer, and TG-DTG was performed using a SDT Q600 thermogravimetric analyzer and the system was heated from 298 K to 1253 K at a rate of 5 K/min with N2 as the carrier gas.

4.3. Kinetics Study

CRm52−54 was used to simulate the pyrolysis kinetics of FCSs and the method of CRm can refer to the previous work of Mo et al.52

Acknowledgments

The authors sincerely acknowledge the financial support from the National Key Research and Development Program of China (2018YFC1902101), the Tianchi project for introducing high-level talents to Xinjiang Uyghur Autonomous Region (China), and the open project from the Key Laboratory of Coal Processing and Efficient Utilization (China University of Mining and Technology), Ministry of Education.

The authors declare no competing financial interest.

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