Investigation of the Evolution of the Chemical Structure of Bituminous Coals and Lignite during Pyrolysis

Abstract

This paper aims to investigate the evolution of the chemical structure of coal char during pyrolysis. Two bituminous coals (coals A and B) and one lignite (coal C) were pyrolyzed in a fixed bed in N₂ from 600 °C to 1100 °C. The chemical structure of coal char was characterized by Raman spectroscopy and X-ray diffraction (XRD). The carbon and oxygen functionalities of coal char were analyzed by X-ray photoelectron spectroscopy (XPS). The Raman spectroscopic parameters A_D/A_G (A_D1/A_G, A_D2/A_G, and A_D3/A_G) increased from 600 °C to 900 °C and then decreased after 900 °C, indicating that the degree of order of coal char first decreased and then increased with increasing pyrolysis temperatures (600–1100 °C). The content of graphite-like microcrystalline carbon decreased and then increased with an increase in temperature. Prominent diffraction peaks of microcrystalline carbon for coal chars A and B were observed, but only minerals were shown in diffraction patterns of coal char C since the ash content of coal chars A and B is much lower that that of coal char C. The lateral size of the crystallite plane (L_a) generally increased between 600 °C and 1100 °C. The relative content of C=O and COOH in coal chars A and B generally decreased as the temperature increased, suggesting an increase in the degree of order at higher temperatures. The oxygen functionalities of coal char were composed of organic oxygen and oxygen-containing bonds of minerals.

1. Introduction

China is the world’s first largest coal producer and consumer. In 2020, coal accounted for about 57% of primary energy consumption in China. Coal plays a very important role in China’s social and economic development [1]. Due to limited oil and natural gas reserves, China’s demand for coal resources is increasing with the advancement of industrial modernization and social urbanization [2].

Over the last few decades, a number of studies have reported on the chemical structures of coal char [3,4,5,6,7]. At the molecular level, coal char mainly consists of a carbon skeleton including O-containing functional groups, methyl, and methylene, attached and cross-linked [8]. Coal pyrolysis is a key intermediate step for determining the physiochemical properties of the newly formed char [5,7]. The change in the structure and surface chemical properties of the coal char will significantly affect the subsequent gasification or combustion reactivity [9,10,11]. Many factors such as pyrolysis temperature, pyrolysis rate and time, minerals, and coal rank will affect the physical and chemical structure of coal char. Xu et al. [12] found that high temperatures led to an ordered crystallite structure in coal chars and the increase of the fraction of organized carbon. The lateral size of the basic lattice unit of carbon in coal char increased with the increase in the pyrolysis temperature, indicating that high temperatures can promote crystallite lateral growth [6]. The high pyrolysis rate resulted in the increase of the content of organic oxygen-containing functional groups and active sites on the surface of low-rank coal char [10,13]. The stacking height (L_c) of coal chars increased with the increase in pyrolysis time, resulting in the significant development of graphitization for coal chars [14,15]. Inherent minerals of low-rank coals had strong catalytic effects on pyrolysis [2]. Minerals of K, Ca, Na, Mg, Ni, Al, and Fe had a certain inhibition effect on the graphitization of coal chars, leading to a decrease in the structural stability of coal chars [16,17,18]. The aromatic carbon content and structure of coal chars depends on the coal rank; coal chars prepared from high-rank coals have a high content of aromatic carbon [19,20].

A variety of analytical techniques, such as transmission electron microscopy (TEM) [21], FT-IR spectroscopy [22], X-ray diffraction (XRD) [23], Raman spectroscopy [24], and X-ray photoelectron spectroscopy (XPS) [25,26], have been developed and improved for characterizing the char structure. There is a correlation between different techniques of characterization [6,12,27,28]. Zhang et al. [29] used Raman spectroscopy and XRD to investigate the effect of catalysts on the reactivity and structural evolution of coal chars during pyrolysis and the results showed that they went through two stages of graphitization and aromatization. The Raman spectroscopy and XPS results indicated an obvious loss of O-containing functional groups after the coal char was treated at 900 °C [30]. Based on the FT-IR spectroscopy results, the content of O-containing functional groups in bituminous coal was greater than in lignite [10]. However, the degree of order of coal char during pyrolysis remains unclear.

The purpose of this paper is to study the influence of temperature on the evolution of the chemical structure and surface functional groups of coal char since they significantly affect the combustion and gasification reactivity. Pyrolysis experiments were performed in a quartz-tube furnace to obtain coal char samples. The carbonaceous structure of coal char was characterized by Raman spectroscopy and XRD. C and O surface functional groups were revealed by XPS. The changes in chemical structure are discussed in detail based on these techniques.

2. Experimental

2.1. Materials and Preparation

Two bituminous coals (coals A and B) and one lignite (coal C) came from the Inner Mongolia autonomous region, China. Coals A and B were from Shandong coal mine and coal C was from Shenli coal mine. The proximate analysis was performed according to the Chinese Standard Method (GB/T 30732-2014). The ultimate analysis was conducted on an elementary analyzer (VARIO EL III, Elementar Analysensysteme GmbH, Langenselbold, Germany) according to Chinese Standard Methods (GB/T 30733-2014 and GB/T 214-2007). The vitrinite reflectance was measured based on the Chinese Standard Method (GB/T 6948-2008). The results of proximate and ultimate analyses and vitrinite reflectance are presented in

Table 1. The proximate and ultimate analyses of raw coals.

Sample	Proximate Analysis (ad, wt %)				Ro/%	Ultimate Analysis (ad, wt %)
	M	A	V	FC		C	H	O a	N	S
Coal A	8.04	2.89	36.42	52.65	0.52	69.54	7.72	8.00	1.69	2.12
Coal B	6.76	1.59	31.72	59.93	0.56	77.14	4.23	7.68	1.49	1.11
Coal C	3.71	19.21	34.59	42.49	0.35	58.10	3.40	14.03	0.83	0.72

Notes: ad: air-dried basis; M: Moisture; A: Ash; V: Volatile; FC: Fixed carbon; R_o: Vitrinite reflectance; ^a: By difference.

. The ash compositions were analyzed by a PANalytical Axios (EA Almelo, The Netherlands) X-ray fluorescence (XRF) spectrometer and the results are given in

Table 2. The ash compositions (wt %) of raw coals.

Sample	SiO2	Al2O3	CaO	Fe2O3	K2O	Na2O	MgO	TiO2	SO3	P2O5
Coal ash A	18.69	9.01	38.08	13.19	0.07	0.58	3.44	1.19	15.09	0.22
Coal ash B	18.96	11.35	37.05	6.27	0.16	1.13	3.92	0.62	18.56	0.30
Coal ash C	58.43	14.02	8.47	8.63	1.18	0.72	0.80	0.97	5.25	0.92

.

2.2. Pyrolysis Experiments

The coal samples were ground and sieved to 120 μm. The raw coals were pyrolyzed in a N₂ atmosphere between 600 °C and 1100 °C in a quartz-tube furnace to obtain char samples. About 3 g of coal samples was fed into the middle of the quartz tube. Prior to the pyrolysis experiment, nitrogen (99.99%, 600 mL min⁻¹) was introduced into the reactor for 30 min to completely exhaust the air. The raw coal samples were heated to the desired pyrolysis temperature at a heating rate of 10 °C/min, and then held at this temperature for 2 h. The coal char samples were collected for analysis after the pyrolysis experiments.

2.3. Characterization of Carbon Structure

Raman spectroscopy is considered the optimal technique to determine the carbon structure of carbonaceous materials, owing to its high sensitivity to crystalline and amorphous structures. A laser micro-Raman spectrometer (LabRAM ARAMIS, Tokyo, Japan) was used to acquire information on the carbon structure of the coal char samples in this study. An Ar+ laser (λ = 532 nm) was used as the excitation source. Raman spectra were obtained in the spectral range of 800–2000 cm⁻¹. The incident laser power on the coal char samples was about 2 mW. For each sample, three different spots on the surface of the sample were randomly measured. The baseline was corrected by LabSpec 5.0 spectroscopic software package. Curve-fitting and quantitative analysis were conducted by Peakfit 4.12 (Framingham, MA, USA) software.

The mineral phase and crystalline structure of char samples was determined by D/max 2550 X-ray diffractometer (XRD). Cu Ka radiation (40 kV, 30 mA) was used as the X-ray source. Char samples were scanned at a velocity of 5°/min and a step size of 0.2° over the 2θ angular range of 5–80°. To examine the mineral phrases of char samples, the standard diffraction data were analyzed by Jade 6.5 (Livermore, CA, USA) software.

An XPS analysis of the surface functionalities of the coal char samples was performed on an ESCALAB Xi+ X-ray Photoelectron Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) with an excitation of Al Kα (12.5 kV, 16 mA). Each analysis started with a survey scan from 0 to 1352 eV at steps of 0.05 eV with a residence time of 40–50 ms. The pass energy for the survey spectra and narrow spectra was 100 and 20 eV, respectively. The spectra were charge corrected using the advantageous C1s signal at 284.80 eV. Curve fitting of the C1s and O1s bands was performed by XPS Peak4.12 software.

3. Results and Discussion

3.1. Raman Spectroscopic Analysis

The typical Raman spectra of coal chars produced in the range of 600–1100 °C are shown in Figure 1. Raman spectra of coal chars were dominated by two prominent bands, D and G bands, corresponding to a disordered or ordered structure, respectively. It can be seen that the relative intensity of the peak height decreased with an increase in pyrolysis temperature, indicating that the signal intensity decreased at higher temperatures. This showed that the O-containing structure in coal char was gradually reduced because O-containing structures tend to have a high Raman scattering ability [31].

The Raman spectra of coal char can be resolved into multiple individual bands. According to [31], four Lorentzian bands (G, D1, D2, and D4) and one Gaussian band (D3) were used to fit the Raman spectra of coal char; the typical curve-fitting results are shown in Figure 2. The initial peak positions of the D4, D1, D3, G, and D2 bands were located at 1150, 1350, 1520, 1590, and 1620 cm⁻¹, respectively. The G band is attributed to the ideal graphitic lattice (E_2g symmetry) stretching mode [32]. The D1 band is associated with the graphene layer edges vibration mode (A_1g symmetry) [33]. The D2 band is related to the surface graphene layers vibration mode (E_2g symmetry). The D3 band is associated with amorphous carbon (organic molecules, fragments, and functional groups) [32]. The D4 band is assigned to disordered graphite lattice vibrations (A_1g symmetry), sp²–sp³ hybridized carbon bonds, C–C and C=C bonds of polyenes, and ionic impurities [34]. In addition, the intensity of the G band is weaker than that of D1 (see Figure 2), implying the poor order of coal chars [35]. The intensity of the D3 and D4 bands was relatively weak, but the full width at half maximum (FWHM) was relatively large, suggesting that a considerable amount of amorphous carbon is present in coal chars.

Since the relative intensity of Raman spectra cannot fully represent the evolution of the structure, the band area of resolved peaks and band area ratios were used as Raman spectra parameters to characterize the change in carbonaceous structure in the study [35,36]. The various band area ratios and total band area are shown in Figure 3. It can be observed from Figure 3 that the values of A_Total decreased significantly from 600 °C to 900 °C and then decreased slightly from 900 °C to 1100 °C. This shows that the macromolecules in coal chars went through condensation polymerization with the increasing temperatures. The band area ratios of A_D1/A_G and A_G/A_Total are usually used to characterize the degree of order of coal chars [31,36]. These A_D/A_G parameters (A_D1/A_G, A_D2/A_G, and A_D3/A_G) increased from 600 °C to 900 °C and then decreased from 900 °C to 1100 °C, as shown in Figure 3b–d. The dehydrogenation of hydroaromatics, the growth of aromatic rings, and the formation of a large number of defects and amorphous structures resulted in an increase in A_D/A_G [3,31]. The decrease in A_D/A_G parameters from 900 °C to 1100 °C indicated further ring condensation and the formation of additional large aromatic ring systems [37]. The variations of A_D/A_G parameters also indicated that the degree of order first decreased and then increased with increasing pyrolysis temperatures [35,38,39]. In contrast, the value of A_G/A_Total decreased from 600 °C to 900 °C and then increased from 900 °C to 1100 °C, as shown in Figure 3e. This suggests that the intercalation compounds and some polar groups inside the char were rapidly vaporized or broken down [40] and the macromolecules in coal chars went through subsequent graphitization with the increasing temperature [31].

In general, the evolution of the carbon structure can be attributed to competition between the consumption of amorphous carbon and the formation of amorphous carbon [11]. It can be seen that the content of graphite-like microcrystalline carbon decreased and then increased with the increase in temperature, whereas the content of disordered carbon showed the opposite trend [40]. Positions of the D1 band of coal char shifted to lower frequencies from 600 °C to 800 °C and then shifted to higher frequencies in the range 800–1100 °C, as shown in Figure 3f. However, as the temperature increased from 800 °C to 1100 °C, the D1 band FWHM generally decreased, resulting in a significant development of graphitization for coal chars [32,35,41]. Furthermore, the G band FWHM rose slowly between 600 °C and 1000 °C, indicating an increase in a variety of ring orders and dimensions. However, the G band FWHM decreased from 1000 °C to 1100 °C, indicating an increase in the structural order [29,42]. In addition, as the temperature rose, coal char C showed higher values of A_D1/A_G, A_D2/A_G, A_D3/A_G, D1 band FWHM, and G band FWHM, but a lower value of A_G/A_Total in comparison with coal chars A and B, indicating that coal char C has a higher degree of order, a larger crystallite size, and a higher content of graphite-like crystal structures [35,43].

3.2. XRD Analysis

The XRD patterns of three different coal chars are shown in Figure 4. For coal chars A and B, two wide peaks in the ranges of 13–36° and 38–50° can be observed in Figure 4a,b, corresponding to the (002) and (100) peaks of microcrystalline graphite, respectively. In contrast, the microcrystalline (002) and (100) peaks were not visible in Figure 4c since the diffraction of microcrystalline carbon was covered by a high content of mineral matter (see Table 1). The (002) peak was ascribed to the stacking of the graphitic basal plans of char crystallites, while the (100) peak was associated with the graphite-like atomic order with a single plane [44]. As shown in Figure 4, the (002) and (100) peaks of coal char A and B were broad, indicating the presence of a large amount of turbostratic carbon. The characteristic diffraction peaks of minerals are also shown in Figure 4. The presence of quartz was revealed in all char samples. Oldhamite (CaS) and magnetite (Fe₃O₄) were identified in coal char A900. The formation of oldhamite was due to the reaction of calcium oxide (CaO) with H₂S, produced during the coal pyrolysis [45]. Magnetite was derived from the thermal decomposition of pyrite [46]. Diffraction peaks of both magnetite and hematite were observed in coal char A1000 and hematite was derived from the further thermal transformation of magnetite. Magnesioferrite (Fe₂MgO₄) was found in coal char A1100 due to the reaction of iron oxides with magnesium oxide [47]. For coal char B, the peak intensity of quartz was weaker than for the other two coal chars because the ash content of coal B was lower than that of coals A and C. Oldhamite and hematite were not found until 900 °C and 1000 °C, respectively. As shown in Figure 4c, the major mineral phases present in coal char C prepared below 900 °C were quartz and illite. However, the coal char C1000 was free of illite due to its thermal transformation into quartz at 1000 °C. The presence of oldhamite and hematite was not found until the temperature was raised to 900 °C.

To further evaluate the graphitization degree of coal chars, the apparent (002) peak was resolved into two individual Gaussian bands, i.e., γ band (ca. 20°) and (002) band (ca. 26°). The γ band was related to the packing of the saturated structure, such as aliphatic side chains, leading to asymmetry of the (002) peak [43]. The typical XRD spectra of coal char A600 was curve-fitted in Figure 5. The microstructure parameters could be calculated using the equations of Bragg and Scherrer [12,29,43,44]:

$$d_{002}=\frac{\lambda }{2\sin {\theta }_{002}}$$

(1)

$$L_a=\frac{1.84\lambda }{{\mathsf{\beta }}_{100}\cos {\theta }_{100}}$$

(2)

$$L_{\mathrm{c}}=\frac{0.9\lambda }{{\mathsf{\beta }}_{002}\cos {\theta }_{002}},$$

(3)

where d₀₀₂ represents the interplanar spacing of two aromatic layers of the microcrystalline carbon, Å; λ is the X-ray wavelength (λ = 0.1541 nm); L_c is the stacking height of the crystallites, Å; L_a is the lateral size of the crystallite plane, Å; θ₀₀₂ and θ₁₀₀ are the diffraction angles of the (002) and (100) peaks, respectively, °; β₍₀₀₂₎ and β₍₁₀₀₎ are the full width at half maximum of the (002) and (100) peaks, respectively, °; and N is the average layer number estimated from the equation N = L_c/d₀₀₂ [12,29].

The variations in d₀₀₂, L_c, L_a, and N with pyrolysis temperature are shown in Figure 6. It can be observed that the variation trend of L_c and N was generally the same, but different from the variation trend of d₀₀₂ and L_a. The d₀₀₂ decreased with an increase in temperature due to the development of structural order [14,48], as shown in Figure 6a. However, the value of L_a increased from 600 °C and 1000 °C but decreased after 1000 °C. The L_c and N for coal char B changed slightly from 600 °C to 1100 °C. The L_c and N for coal char A did not change significantly until the temperature rose from 900 °C to 1000 °C. The results showed that the extension of aromatic layers and the partial collapse of stacking structure happened at 900 °C, and a stacking structure again developed toward the graphitic structure to a varying degree at 1000 °C [49,50]. The results are also consistent with the literature [12,31,50,51,52]. The large value of d₀₀₂ and the low value of L_c mean poor crystallinity, and the low value of N implies graphitization is difficult [12]. Generally, coal char A has less d₀₀₂ but greater values of L_a, L_c, and N than coal char B. The trend of L_c and N demonstrated that the crystallinity of coal char B was lower than that of coal char A [12,53]. The degree of order for coal char A was higher than that for coal char B, which was in accordance with the results of Raman spectroscopy. In addition, as the temperature increased, the trend of L_c and N indicated that the vertical and horizontal growth of the basic lattice unit in coal char A developed gradually but only horizontal growth occurred in coal char B [6]. However, the growth of the coal char crystallite structure reduced char activity and hindered the further reaction of the char [54]. Char B had higher activity at high temperatures than char A. The ash composition in Table 2 showed that the proportion of alkaline metal in coal A was relatively low compared to coal B. The alkaline metal inhibited the graphitization of coal char B, resulting in a less ordered crystalline carbon structure [55]. The L_c for coal char A increased very slowly from 600 °C to 900 °C but then increased rapidly between 900 °C and 1100 °C, which was consistent with the literature [56,57].

3.3. XPS Analysis

The XPS survey spectra of three coal chars prepared at 600 °C are shown in Figure 7. The XPS spectra showed distinct carbon and oxygen peaks, representing the major constituents of the coal char surface. The C1s spectra were resolved into four individual peaks representing different carbon-containing functional groups, and the typical deconvolution of the C1s spectra is shown in Figure 8. These peaks correspond to aromatic carbon (C–C/C–H, 284.8 eV), phenol or ether carbons (C–O, 286.1 eV), carbonyl carbons (C=O, 287.3 eV), and carboxyl acidic groups (–COOH, 288. 9 eV) [37,58,59]. The relative content of various carbon-containing functional groups determined from the deconvolution of C1s spectra is summarized in

Table 3. Normalized relative content (%) of carbon functionalities of C1s XPS spectra.

Sample	Normalized Relative Content (%) of Carbon Functionalities
	C–C/C–H (284.6 eV)	C–O (286.1 eV)	C=O (287.3 eV)	COOH (288.9 eV)
A600	73.93	17.45	2.82	5.80
A700	67.63	27.11	–	5.25
A800	65.62	29.23	1.03	4.11
A900	59.47	31.79	3.46	5.27
A1000	65.88	33.08	0.60	0.44
A1100	63.00	36.07	0.16	0.77
B600	76.36	18.31	2.33	3.00
B700	68.05	18.18	6.49	7.28
B800	70.85	21.04	3.89	4.22
B900	66.59	24.85	3.31	5.25
B1000	70.62	19.50	4.87	5.01
B1100	63.23	30.46	2.63	3.69
C600	69.44	17.30	7.86	5.40
C700	71.47	15.42	7.07	6.04
C800	69.94	16.25	7.91	5.90
C900	67.61	18.16	8.34	5.89
C1000	71.19	15.84	7.38	5.59
C1100	70.18	17.31	7.02	5.49

.

As shown in Table 3, there was a decreasing trend in the relative content of C–C/C–H groups in coal chars A and B as the temperature increased. The relative intensity of the C–C functional group strongly increased with an increase in pyrolysis temperature, while the relative intensity of C–H decreased significantly, indicating the growth of aromatic rings and the formation of a large number of defects and amorphous structures [30]. This trend of the content of C–C/C–H groups showed that the macromolecules in coal chars went through condensation polymerization with increasing temperatures. As the temperature increased, the relative content of the C–O functional group generally increased while the relative content of the C=O functional group demonstrated the opposite trend. The disrupted C=O generated phenolic hydroxyl and oxygen bonds. These combined with the aromatic ring to form a stable conjugated structure, resulting in an increase in the C–O content [60]. The relative content of the COOH functional group of coal char A generally decreased in the range of 600–1100 °C due to the decomposition of the COOH group [61]. The relative content of C=O and COOH in coal chars A and B generally decreased as the temperature increased, implying an increase in the degree order [21]. Meanwhile, pyrolysis produced coal char with few oxygen-containing functional groups, improving the gasification reactivity of coal char and the degree of char gasification [62]. However, the relative content of C–C/C–H, C–O, C=O, and COOH of coal char C changed only slightly from 600 °C to 1100 °C, as shown in Table 3. The ash composition in Table 2 showed that the proportion of alkaline earth metals in coal C was much lower than om coals A and B. One possible reason is that the lower content of alkaline earth metal led to a lower content of C–O and higher contents of C=O and COOH during pyrolysis as the temperature increased [6,37,63].

The oxygen in coal char includes organic oxygen-containing functional groups and oxygen-containing bonds in minerals. According to the literature [37,64,65] and the XRD results, the Ols XPS spectra of coal char were deconvoluted into multiple peaks. These individual peaks were located at 529.9, 530.1, 531.6, 531.7, 531.3~531.9, 532.4, and 533.1~533.8 eV, corresponding to hematite, magnetite or magnesioferrite, albite, illite, carbonyl carbons (C=O), quartz, and phenol or ether carbons (C–O), respectively. The O1s spectra of coal char A prepared from 600 °C to 800 °C were resolved into three peaks (C–O, quartz, and C=O) since the main mineral in these chars is quartz (see Figure 4) and the typical curve fitting for A600 is shown in Figure 9a. The O1s spectra of A900 and A1000 were deconvoluted into four components (C–O, quartz, C=O, and magnetite) and the typical curve fitting for A900 is as shown in Figure 9b. The O1s spectra of A1000 were composed of C–O, quartz, C=O, and magnesioferrite, as shown in Figure 9c. The O1s spectra of coal char B and C were resolved into multiple peaks based on their mineral components; the typical results are shown in Figure 9d–h and the curve fitting results for other chars are shown in Figure S1 in the Supplementary Materials.

4. Conclusions

The following conclusions can be drawn from this study. The Raman scattering intensity decreased with increasing temperature due to the loss of O-containing structures. The results of Raman spectroscopy demonstrated that the degree of order of coal char first decreased and then increased with an increase in temperature. The XRD results showed that high temperatures led to the graphitization of coal char, but the alkali metal of coal char will inhibit the graphitization of coal char. The XPS results indicated that the relative content of C=O and COOH in coal chars A and B generally decreased as the temperature increased, suggesting an increase in the degree of order at higher temperatures.