Effects of Vitrinite in Low-Rank Coal on the Structure and Combustion Reactivity of Pyrolysis Chars.

Pyrolysis is a highly promising technology for the efficient utilization of low-rank coal. The structure of coal plays an important role in its utilization. In this paper, the evolution of the char structure during heat treatment (200-800 °C) of Naomaohu coal and its different vitrinite-rich fractions was studied. The functional group structure, aromatic ring structure, and crystallite size of chars were determined by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD) spectroscopy, respectively. The results indicated that minerals inhibit the condensation reaction of aromatic rings during pyrolysis. The high vitrinite content in coal is conducive to the formation of larger char crystallite average sizes (L a). The relationship between L a (1.69-3.10 nm) and the Raman band area ratio A (GR+VL+VR)/A D or A D/A all was established. In addition, the combustion performance and kinetics of chars were also investigated. The results showed that the char from high contents of the liptinite fraction has lower combustion reactivity, and demineralization treatment has significantly reduced the combustion reactivity of char.

Introduction

Low-rank coal has an abundance of reserves, a high moisture content, and a low calorific value.[1,2] The direct combustion of low-rank coal generates a low utilization rate and environmental pollution.[3,4] The development of clean coal technology is of great significance to improve the utilization efficiency of low-rank coal and reduce environmental pollution. Pyrolysis is a highly promising technology in clean coal technology as it can produce high-value chemicals from coal.[5] In addition, coal pyrolysis is an essential step in coal conversion technologies, such as combustion, gasification, and liquefaction.[6] In recent years, scholars thoroughly examined coal pyrolysis under different conditions. The coal pyrolysis reaction and product distribution are significantly affected by certain coal properties such as the coal grade,[7] coal moisture content,[8] and inorganic matter content,[9−11] as well as pyrolysis conditions such as the pyrolysis temperature,[12,13] heating time,[12,13] atmosphere,[1,12] and pressure.[14] Nevertheless, a complete understanding of the coal pyrolysis reaction has yet to be accomplished.

Coal consists of organic microscopic components and a small amount of inorganic matter. Coal organic microscopic components are divided into three groups: vitrinite, inertinite, and liptinite.[15] Many studies have examined the macerals of coal and their effect on coal pyrolysis given that macerals are the basic components of coal. The study of the pyrolysis behavior of macerals can provide a basis for understanding the structure of coal and then establish a relationship between the pyrolysis behavior and the structural characteristics of coal. The difference in the maceral structure would affect the structure of coal char during pyrolysis. Sun et al.[16] studied the char structure after pyrolysis of vitrinite and inertinite that were separated from Chinese Shenmu bituminous coal. It revealed that under the same pyrolysis temperature, inertinite chars had higher carbon content, higher aromaticity, and lower hydrogen content than vitrinite chars. Roberts et al.[17] reported that the chemical structure of pyrolysis chars resulted from the inertinite-rich and vitrinite-rich coals at 700–1000 °C was similar, but the physical structure was quite different. Compared with vitrinite-rich chars, inertinite-rich chars were more denser, had lower isotropy, and had larger crystallite average size (La). However, Zhao et al.[18] found that the differences in the elemental compositions of vitrinite-rich chars and inertinite-rich chars decreased with an increase of pyrolysis temperature and they had similar element compositions and structure characteristics at 650 °C by ultimate and X-ray diffraction (XRD) analyses. Previous studies indicated that the pyrolysis performance of organic macerals in coal was different, although no consistent conclusion on the influence of coal macerals on coal pyrolysis and the difference between individual maceral pyrolysis systems was established. This may be due to differences in the types of coal or experimental methods, which also illustrates the complexity and heterogeneity of the coal structure, which requires further research.

Char is the main product of coal pyrolysis, and combustion is one of the major applications of char that can generate significant amounts of energy.[19,20] Studying the combustion performance of char can play an important role in improving the combustion efficiency of char, reducing environmental pollution, and achieving its efficient and clean utilization.[21] Cai et al.[22] found that the combustion reactivity of char decreased rapidly with an increase of inertinite content in coal. Louw et al.[23] found that the burnout temperature of inertinite-rich coal char was 680 °C, which was higher than 650 °C of vitrinite-rich coal char by the burnout curve of coal char. At present, there are limited research studies on the combustion reactivity of pyrolysis char from different maceral concentrates. There is a more consistent finding that char from vitrinite-rich has higher reactivity than char from inertinite-rich. However, there are a few reports on the influence of inherent minerals and liptinite groups in coal on the combustion reactivity of pyrolysis coal char.

Coal in the Naomaohu mining area (Xinjiang Province, China) is a typical low-rank coal with an abundance of reserves. The effects of the Naomaohu coal vitrinite content on the structure and characteristics of char obtained by Naomaohu coal pyrolysis were studied. In addition, the combustion reactivity and reaction kinetics of chars were also examined. Analyses on the different maceral content influences on pyrolysis chars aimed to provide a theoretical basis for the development of advanced coal pyrolysis technology.

Results and Discussion

Thermogravimetry–Derivative Thermogravimetry (TG–DTG) Analysis of the Coal Samples

Figure presents the total mass loss of the Naomaohu raw coal (NMH-R), demineralized Naomaohu sample (NMH-D), vitrinite-rich fraction (NMH-V), high content of inertinite fraction relative to raw coal (NMH-I), and high content of liptinite fraction relative to raw coal (NMH-L), specifically 45.1, 48.3, 46.4, 42.9, and 54.2%, respectively. To minimize any external influence of the sample ash content on the weight-loss rate, the mass losses of the ash-free basis NMH-R, NMH-D, NMH-V, NMH-I, and NMH-L samples were calculated to be 47.7, 48.7, 48.3, 56.0, and 56.7%, respectively. Among all of the coal samples, the ash-free basis NMH-L coal sample exhibited the highest mass-loss rate because the content of the liptinite group in the coal sample was as high as 20.4%. The liptinite group exhibited the highest volatile matter content as compared to vitrinite and inertinite.[24] The ash-free basis NMH-I exhibited a mass-loss rate as high as 56.0%, which was much higher than that of ash-free basis NMH-R, NMH-D, and NMH-V. These results were mainly because the NMH-I coal sample exhibited an ash content of 23.3%, which was much higher than that of the other samples and promoted the release of volatiles.[25] However, there was no significant difference in mass loss between NMH-R and NMH-D. On the one hand, low ash content (5.4%) of NMH-R had no significant effect on the release of volatiles. On the other hand, the pore structure of NMH-D changed during the demineralization process,[26] which was conducive to the release of volatile matter. The five samples exhibited similar TG–DTG curve trends. The weight of the coal samples rapidly decreased with temperature increasing from 400 to 500 °C, and the mass-loss rate reached a maximum at approximately 450 °C. The mass-loss rate of the coal samples decreased as the temperature exceeded 500 °C. According to the TG analysis, the pyrolysis temperature was selected as 200, 300, 350, 400, 420, 440, 460, 480, 500, 550, 600, 700, and 800 °C in the subsequent preparation of chars. The mass loss of the coal samples was mainly concentrated within the temperature range of 400–500 °C. As a result, we reduced the heating rate when the temperature reached the range of 350–550 °C.

Figure 1

TG–DTG analysis curves of five NMH coal samples: (a) air-dried basis coal by experiments and (b) ash-free basis coal by calculations.

Fourier Transform Infrared (FTIR) Spectra of Char

The FTIR spectra of coal samples are presented in Figure , wherein the coal samples consisted primarily of aromatic nuclear groups (1600 cm–1), aliphatic side chains (1370, 1440, and 2800–3000 cm–1), and some oxygen-containing groups (1100–1400 and 1700 cm–1). Notably, NMH-I contained a significant amount of clay minerals (1038, 1013, and 912 cm–1) as compared to the other samples and was consistent with the industrial analysis of its ash content, which reached as high as 23.3%.

Figure 2

FTIR spectra of the five NMH coal samples.

The FTIR spectra of NMH-L chars are shown in Figure a, wherein 2920 and 2850 cm–1 correspond to the asymmetric and symmetric stretching vibrations of CH2 on the aliphatic structure.[27−29] In addition, the absorption intensity exhibited a gradual decrease with increasing pyrolysis temperature until it completely disappeared at 480 °C. A strong absorbance was observed at 1600 cm–1 due to the aromatic C=C ring stretching vibration,[7,27,28] which completely disappeared when the temperature increased to 800 °C. The absorption peak at 1700 cm–1 mainly correlated to the stretching vibration of the aromatic carbonyl/carboxyl C=O groups.[27,29] In addition, the studied samples exhibited shoulder peaks on the 1600 cm–1 peak, which completely disappeared when the temperature increased to 500 °C. The absorption of the symmetrical aromatic ether C–O–C stretching vibration, asymmetric aromatic ether C–O–C stretching vibration, asymmetric aliphatic ether C–O–C stretching vibration was also observed at 1024, 1270, and 1161 cm–1, respectively.[29] These bands were relatively stable at lower temperatures and remained stable when the pyrolysis temperature was below 500 °C. The observed bands gradually decreased with an increase in the pyrolysis temperature above 500 °C and disappeared completely at 650 °C. The FTIR spectra of NMH-R, NMH-D, and NMH-V char did not exhibit a clear difference against the FTIR spectra of NMH-L char (Figures S1–S3). Figure b shows the FTIR spectra of NMH-I chars at different pyrolysis temperatures. The strong absorbance in the range 1000–1100 and 912 cm–1 represented the clay minerals due to Si–O.[30−32] The absorption peaks at 2920 and 2850 cm–1 completely disappeared as the temperature increased to 440 °C; the aromatic C=C peak at 1600 cm–1 disappeared completely until 800 °C; and the C=O absorption peak at 1700 cm–1 effectively diminished at 480 °C. The absorbances at 1038 and 912 cm–1 were stable at lower temperatures. In addition, the absorption intensity remained relatively constant as the pyrolysis temperature decreased to temperatures below 480 °C. At temperatures above 480 °C, the absorption peak gradually decreased as the pyrolysis temperature increased.

Figure 3

FTIR spectra of chars obtained from (a) NMH-L and (b) NMH-I pyrolysis.

Raman Spectra of Char

The Raman spectrum of char exhibited a D band that corresponded to the structures of the large aromatic ring systems (≥6 rings) as well as GR, VL, and VR bands that represented the relatively small aromatic ring systems (3–5 rings).[10,33,34] The band area ratio of the D band to all of the Raman bands (AD/Aall) characterized the content of large aromatic rings in char. Therefore, the band area ratio of the sum of GR, VL, and VR bands to D band (A(G/AD) can be taken as a brief measure of the ratio between the smaller aromatic ring systems typically found in amorphous carbon (3–5 rings) and large aromatic ring systems (≥6 rings).[10]Figure shows the Raman band area ratio A(G/AD and AD/Aall of chars as a function of the pyrolysis temperature. All five coal samples exhibited congruent pyrolysis temperature trends. A(G/AD and AD/Aall were mainly affected by the pyrolysis temperature. Specifically, A(G/AD decreased from 1.03–1.15 to 0.56–0.61 and AD/Aall increased from 26–27 to 39–42% when the pyrolysis temperature was increased from 200 to 800 °C. The A(G/AD ratio slowly changed at temperatures below 500 °C. When samples were heated to a temperature above 500 °C, the A(G/AD ratio rapidly decreased with the increase in temperature. The decrease in A(G/AD was due to the release or condensation of the small aromatic ring system (<5 rings). AD/Aall did not exhibit any obvious changes at pyrolysis temperatures below 400 °C. The AD/Aall of chars rapidly increased as the pyrolysis temperature increased within the range of 400–800 °C. The symbols indicate the mean values of 10 measurements for each sample and the error bars denote the standard deviations in Figure .

Figure 4

Raman band area ratio (a) A(G/AD and (b) AD/Aall of chars as a function of pyrolysis temperatures.

XRD of Chars

Figure shows the XRD profiles and crystalline structure parameters of chars that were prepared at different temperatures (400–800 °C) from NMH-R coal. The char crystalline structure parameters, specifically La, average height of crystallites (Lc), and interlayer spacing (d002), were calculated according to the Bragg formula.[28] Some scholars have found another diffraction peak (γ, 20°) to the left of the 002 peak, which is related to an aliphatic structure.[35] Origin 8.0 software was used to deconvolve the diffraction pattern of 2θ in the range of 10–34° to obtain the position and area of the 002 and γ peaks. The areas of the γ and 002 peaks were equal to the number of aliphatic carbon atom (Cal) and aromatic carbon atom (Car), respectively. Therefore, the coal aromaticity (fa) was defined as fa = Car/(Car + Cal) = A002/(A002+Aγ).[28,32,36]

Figure 5

(a) XRD patterns and (b) crystalline structure parameters of chars prepared at different temperatures (400–800 °C).

According to Figure a, each XRD profile exhibited two distinct peaks in the ranges of 15–30 and 35–50°, corresponding to the 002 and 100 peaks. Narrower and higher 002 and 100 peaks indicated better orientation and larger size coal char aromatic layers, respectively. As the pyrolysis temperature increased, the position of the 002 peak moved to a higher angle and then remained stable at temperatures above 500 °C. The 100 peak position gradually shifted to a higher angle with an increase in pyrolysis temperature, such that the morphology of the peak became narrower. In addition, small inorganic minerals were observed in chars, mainly including quartz, calcite, and lime.[28,37,38]Figure b indicates that d002 and Lc remained relatively stable as the pyrolysis temperature increased. In addition, the larger La of char gradually increased from 1.69 to 2.99 nm, indicating that the diameter of the aromatic ring in char gradually increased, which follows the Raman spectrum analysis results. When the pyrolysis temperature increased from 400 to 800 °C, the fa of char increased from 0.50 to 0.75 because of the rapid decrease of aliphatic compounds due to the decomposition of the aliphatic chains in coal into volatile matter.[39,40]

To study the effect of the maceral content on the crystalline structure of char, XRD analysis was performed on chars obtained from the pyrolysis of NMH-R, NMH-D, NMH-V, NMH-I, and NMH-L coals at 800 °C. Figure presents the XRD patterns and crystalline structure parameters of the chars. The char pyrolysis of the different macerals samples all exhibited obvious 002 and 100 peaks (Figure a). NMH-I char exhibited the widest and lowest 002 and 100 peaks as compared to the other chars, as well as more inorganic minerals due to the higher ash content. NMH-I char exhibited the smallest La and d002, indicating that the diameter and distance of the aromatic ring in NMH-I char were the smallest. These results were combined with the Raman spectroscopy analysis results of coal char, which indicated that the NMH-I char exhibited the largest A(G/AD and smallest AD/Aall, specifying that the significant presence of minerals inhibited the coal char condensation reaction.[9] NMH-I char exhibited the largest fa, whereas NMH-D char presented the smallest fa, indicating that the minerals in coal were conducive to improvements in the char aromaticity. NMH-V char exhibited the largest La value followed by NMH-R char, indicating that higher vitrinite contents resulted in larger aromatic char ring sizes.

Figure 6

(a) XRD patterns and (b) crystalline structure parameters of chars prepared at 800 °C by different coal samples. Q, quarts; C, calcite; and L, lime.

Tuinstra and Koenig proposed that the intensity ratio (ID/IG) from the Raman spectrum was linearly related to the inverse of the average lateral sizes (1/La) obtained by XRD.[41] In 1989, Knight and White (KW) studied a variety of carbon materials and obtained an empirical formula La (nm) = 4.4(ID/IG)−1 for calculating La, hereinafter referred to as the KW formula.[42] Cançado et al. systematically studied the relationship between the ID/IG and La of the Raman spectra at different excitation wavelengths (λ1 = 647.0, 568.0, 514.5, 488.0, and 457.9 nm) and obtained the empirical formula La (nm) = (2.4 × 10–10)λ14 (ID/IG)−1.[43][43] Baldan et al. calculated the La of vitreous carbon using the KW, Cançado, and Bragg formulas.[44] The KW and Bragg analytical results were well in agreement, whereas the Cançado and Bragg formulas generated quite different La values. Zickler et al. found that the KW formula was not applicable when La was less than 2 nm.[45] Maslova et al. found a linear relationship between full width at half-maximum (FWHM-G) and crystallite size La (FWHM-G = 14 + 430/La), where the crystallite diameter range was applicable for La > 15 nm.[46]

This study analyzed the crystalline structure based on the Raman spectroscopy of chars, estimated La using the KW and Cançado formulas, and compared the results with the XRD analysis results. The calculated La value from the Raman spectroscopy results was greater than the La value calculated from the XRD analysis results. Therefore, we analyzed the relationship between the Raman spectral parameters and the XRD-calculated La value (Figure ). A correlation was observed between the diameter of the aromatic ring layer La and the Raman spectral parameters A(G/AD, AD/Aall. A(G/AD rapidly decreased as La gradually increased. In addition, A(G/AD gently changed when La increased to 2.80 nm. As such, the correlation equations between A(G/AD and La were obtained by fitting A(G/AD = 4.01–2.35La + 0.40La2, to which the results generated a correlation coefficient of R2 = 0.9755. AD/Aall linearly increased from 26 to 42% as La gradually increases from 1.69 to 3.10 nm. The correlation fitting equation AD/Aall = 11.47 La + 8.76 was obtained, with a correlation coefficient of R2 = 0.8638.

Figure 7

Correlation between the average lateral sizes (La) and (a) A(G/AD and (b) AD/Aall.

Combustion Reactivity of Char

Figure a–e shows the TG–DTG curves of chars obtained from the pyrolysis of different coal samples at 400, 500, 550, 600, and 650 °C. The DTG curves of NMH-R char, NMH-V char, NMH-I char, and NMH-L char exhibited an obvious weight-loss peak within the temperature range of 285 and 489 °C due to the release of volatile matter during char combustion, thereby reaching the ignition point and igniting fixed carbon combustion. Another weight-loss peak was observed at the temperature range of 489–569 °C, representing the slow combustion of the remaining char because the ash generated during combustion covered the surface of the combustible material, thus hindering contact between oxygen and the combustible material.[23] Therefore, NMH-D char only exhibited one weight-loss peak in its combustion curves. In addition, the TG curves of NMH-R char, NMH-V char, and NMH-L char exhibited a slight weight-loss peak near 650 °C according to the XRD patterns of the coal chars (Figure a). In addition, NMH-R char, NMH-V char, and NMH-L char exhibited a CaCO3 diffraction peak at 2θ of 29°, indicating that the weight-loss peak was due to the decomposition of CaCO3 in coal char.[37]

Figure 8

TG–DTG curves of chars that pyrolysis from (a) NMH-R, (b) NMH-D, (c) NMH-V, (d) NMH-I, and (e) NMH-L.

The combustion activation energies (E, kJ/mol) of pyrolysis char at different temperatures were calculated based on the random pore model (RPM), unreacted shrinking core model (URSCM), and volume model (VM) results (Table S1). These three models exhibited different calculated char combustion kinetic parameters. Specifically, the VM and the RPM generated the highest and lowest calculated activation energies, respectively. The calculation results of the three models all exhibited good correlations, and their R2 values were all generally greater than 0.9763. Among them, the RPM presented the best correlation coefficients. Consequently, the apparent activation energy of char combustion was analyzed by the RPM (Figure ). The combustion activation energies of NMH-R char, NMH-I char, and NMH-L char did not significantly change when the pyrolysis temperature was below 500 °C. The combustion performance of char is mainly affected by its chemical and pore structure.[47] With the increase of pyrolysis temperature, the volatile content of char decreases, causing the reduction of combustion performance. On the other hand, in the early stage of pyrolysis, with the release of water and volatile matter in the coal sample, a large number of micropores are formed. With the continuous increase of pyrolysis temperature, char undergoes polycondensation, the micropores in char cracks form mesopores or macropores. The specific surface area and micropores of char decrease, which is not conducive to the contact of combustible and oxygen in the combustion process.[48] From the Raman analysis results (Figure ), when the pyrolysis temperature was higher than 500 °C, the polycondensation reaction of char increased. Except for NMH-D char, char obtained from the pyrolysis of other coal samples at 550 or 600 °C had the highest weight-loss peak (Figure ), indicated that the combustion of char was the most intense. The reason may be that at this pyrolysis temperature, char had the most micropore structure and the largest specific surface area while having more combustible materials. However, the combustion reactivity of the NMH-D char decreased with the increase of pyrolysis temperature, which is because the pore structure of the coal sample was changed by the demineralization process.[26] When the pyrolysis temperature was increased from 500 to 650 °C, the combustion activation energy of NMH-R char slightly increased but the combustion activation energy of NMH-I char and NMH-L char increased significantly. NMH-V char exhibited a more stable combustion activation energy within the pyrolysis temperature range of 400–550 °C. In addition, a smaller increase was observed from 550 to 600 °C, reflecting the better combustion performance of NMH-V char from pyrolysis at 550 °C. The combustion kinetic parameters of NMH-V pyrolysis char at 550 °C were obtained by nonlinear fitting E = 34.00 kJ/mol, A0 = 2.18 min–1, ψ = 53.75, and the combustion kinetic equation was obtained from eq

Figure 9

Activation energy of chars during combustion, as calculated by the RPM.

Although the vitrinite was the main component of the five samples and the content was more than 70%, there was a large difference in the combustion reactivity of chars from different vitrinite-rich fractions (Figure ). Under the same pyrolysis conditions, the combustion activation energy of NMH-V char was the lowest, indicated that vitrinite char had high combustion activity, which was consistent with previous reports.[23] The combustion activation energy of NMH-D char was much higher than the activation energy of other chars pyrolyzed under the same conditions, which indicated that demineralization decreased the combustion reactivity of pyrolysis char. Zhang et al.’s research[49] showed that the internal minerals in coal strongly promoted the combustion of char. The activation energy of NMH-I char was lower than that of NMH-D char, but it was much higher than that of NMH-V char, NMH-R char, and NMH-L char. The reason is that NMH-I char contains a lot of internal minerals, which form a barrier on the surface of the char during the combustion process, hinders the contact between the combustible substance and air,[23] thus reduces the combustion reactivity of the char. The combustion activation energy of NMH-L chars was higher than that of NMH-R and NMH-V chars, indicating that char from high contents of the liptinite fraction has lower reactivity than char from high contents of the vitrinite fraction. The order of the combustion apparent activation energy of chars obtained under the same heat treatment conditions can be summarized as follows: ENMH-D char > ENMH-I char > ENMH-L char > ENMH-R char > ENMH-V char.

Conclusions

The present study demonstrated the influence of different maceral contents in Naomaohu coal on the char structure of coal pyrolysis. The NMH-I sample contained a large amount of minerals that inhibited the condensation reaction of aromatic rings during coal pyrolysis. Coal samples with higher vitrinite contents exhibited larger char aromatic ring La sizes when pyrolyzed at 800 °C. The La (1.69–3.10 nm) of chars was well in agreement with the Raman spectral parameters A(G/AD and AD/Aall. These data provided clear evidence that the maceral content of the coal samples significantly affected the combustion reactivity of coal char. The RPM model better reflected the char combustion kinetics as compared to the VM and USCM models. The combustion apparent activation energy of chars obtained under the same heat treatment conditions (400–650 °C) can be ordered as follows: ENMH-D char > ENMH-I char > ENMH-L char > ENMH-R char > ENMH-V char.

Experimental Methods

Materials and TG–DTG Analyses

NMH-R was collected from the Naomaohu coal field in the Xinjiang Province, China. Coal was ground to pass a 100 mesh screen. NMH-V, NMH-I, and NMH-L samples were obtained by hand separation. The maceral content, proximate analysis, and ultimate analysis of NMH-R, NMH-V, NMH-I, and NMH-L are shown in Table . The NMH-D was obtained by demineralizing the NMH-R sample following the demineralization procedure of Gong et al.[9] TG–DTG analysis was investigated using a TG analyzer (STA 409 PC/PG, NETZSCH, Selb, Germany). Approximately 8 mg of coal sample was placed in a ceramic crucible and heated from 25 to 900 °C at a heating rate of 10 °C/min using Ar as the carrier gas at a constant flow rate of 35 mL/min.

Table 1

Maceral Content, Proximate Analysis, and Ultimate Analysis of the Coal Samples

	maceral content (vol %)			proximate analysis (wt %)			ultimate analysis (wt %, daf)
sample	vitrinite	inertinite	liptinite	Mad	Ad	Vdaf	C	H	N	S	Oa
NMH-R	90.1	0.8	8.3	3.5	5.4	52.2	71.6	6.0	0.9	0.4	21.1
NMH-V	93.0	0.4	4.2	5.7	4.0	50.0	72.7	6.0	0.9	0.2	20.2
NMH-I	82.0	5.0	6.4	2.8	23.3	57.9	68.6	6.5	0.7	0.9	23.5
NMH-L	77.8	1.0	20.4	3.5	4.4	58.1	71.8	6.5	0.8	0.2	20.7

By difference.

Char Preparation

The five coal samples were pyrolyzed under different temperatures (200–800 °C) in a heating stage (HRTS-1000, Huitong, Shanghai, China). Approximately 12 mg of coal samples was placed in the heating stage and heated to a preset temperature under an Ar pyrolysis atmosphere. The heating rate was set at 10 °C/min from 25 to 350 °C, 5 °C/min from 350 to 550 °C, and 10 °C/min again from 550 to 800 °C.

Characterization of Coal Char

The FTIR spectra of chars were measured using an IRTracer-100 (IRTracer-100, Shimadzu, Kyoto, Japan) using the attenuated total reflectance technique. The spectra were recorded from 4000 to 400 cm–1 at a resolution of 4 cm–1. XRD analysis of chars was measured using a Rigaku Ultimate IV diffractometer with Cu Kα radiation (40 kV/40 mA) at a scanning speed of 2 °/min in the scan range of angle 10–80°. The Raman spectra of chars were measured using a JY/Horiba LabRam HR Raman system (Horiba Jobin Yvon, Villeneuve d’Ascq, France); the system was equipped with 531.95 nm (frequency-doubled Nd:YAG) laser excitation and a 600 grooves/mm grating with a spectral resolution of approximately 1 cm–1. An approximately 1.2 mW laser light was focused on the sample to acquire the spectra within the range 1800–800 cm–1. The integration time was set at 30 s, with two accumulations per spectrum. The error bars in the figures denote the standard deviation of the measurements of the different particles of each char sample. The Raman spectra were curve-fitted with an Origin 8.0/Peak Fitting Module. Each spectrum was resolved into 10 Gaussian bands following Li et al., and the assignment of 10 bands also has been described in their study.[10,33,34]

The optimum temperature range for the liquid products from coal pyrolysis was set within the range of 400–650 °C, and the combustion performance and kinetics of chars obtained from coal pyrolysis at 400, 500, 550, 600, and 650 °C were investigated. Char combustion reactivity was obtained in an STA 409 PC/PG using nonisothermal thermogravimetric analysis, which has been widely used for evaluating char reactivity.[1,13,37,38,50] For each test, approximately 6 mg of sample was placed in a pan and heated from room temperature to 105 °C, at which the temperature was held for 15 min to drive off the moisture. The sample was then heated at 10 °C/min to 900 °C in air with a flow rate of 50 mL/min.

Reaction Kinetics

Based on the thermogravimetric data, the combustion conversion (α) of char was calculated by the following formulawhere α is the conversion and m0, mt, and mf are the masses at the initial time, reaction time t, and final experimental time, respectively.

The gas–solid reaction at constant pressure was generally kinetically expressed as followswhere k is the apparent reaction rate constant (s–1) and f(α) is the kinetic mechanism function of the oxidative combustion reaction. The activation energy of the gas–solid reaction was also kinetically determined based on the following formulawhere A is the pre-exponential factor (min–1), R is the gas constant, and T is the temperature (K).

Three kinds of gas–solid reaction kinetic models were used to study the combustion kinetics of char, including RPM,[51−55] URSCM,[52−55] and VM.[54−56] The mechanism equation expressions of the three models arewhere ψ is the parameter of reaction particle structure; kRPM, kURSCM, and kVM are the reaction rate constants of three combustion reaction models, respectively.

In the combustion reaction, the heating rate β (K/min) and t can be correlated as followsEquations and eq 1 were substituted into eq based on the Coats–Redfern integral method,[28,57] thereby obtaining E and A using the calculation method detailed by Peng et al.[56] Similarly, E and A were calculated using URSCM. Equations , Equations 4, and eq 8 were combined to generate the following formulawhere A0 is related to the reaction rate constant (min–1), and the nonlinear fitting method was used to obtain the E and A0 of the RPM.