Anmin Wang,Jiawei Huang,Meng Zhao,Yue Liu,Daiyong Cao,Yingchun Wei,Langting Wei
Effects of Functional Groups in Coal with Different Vitrinite/Inertinite Ratios on Pyrolysis Products
Maceral composition is one of the key factors affecting the liquefaction and gasification of coal, which has attracted extensive attention of researchers working on coal chemical industry. To elucidate the impact of vitrinite and inertinite in coal on pyrolysis products, vitrinite and inertinite were extracted from a single coal sample and mixed to create six samples with varying vitrinite/inertinite ratios. The samples were subjected to thermogravimetry coupled online with mass spectrometry (TG–MS) experiments, and the Fourier transform infrared spectrometry (FITR) experiment was used to determine the macromolecular structures before and after the TG–MS experiments. The results show that the maximum mass loss rate is proportional to the vitrinite content and inversely proportional to the inertinite content, and increased vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to low temperatures. Based on FTIR experiments, the sample’s CH2/CH3 content, representing the length of aliphatic side chains, decreases significantly after pyrolysis, and the greater the loss of CH2/CH3, the greater the intensity of organic molecule production, indicating that aliphatic side chains are likely to yield organic molecule products. The aromatic degree (I) of samples rises sharply and steadily with increasing inertinite content. After high-temperature pyrolysis, the polycondensation degree of aromatic rings (DOC) and relative abundance of aromatic and aliphatic hydrogen (Har/Hal) within the sample increased significantly, indicating the thermal degradation rate of aromatic hydrogen content is much lower than that of aliphatic hydrogen. When the pyrolysis temperature is lower than 400 °C, the higher the inertinite content, the easier it is to produce CO2, whereas an increase in vitrinite leads to an increase in CO production. At this stage, the “–C–O–” functional group is pyrolyzed to produce CO and CO2. When the temperature exceeds 400 °C, the CO2 output intensity of vitrinite-rich samples is much higher than that of inertinite-rich samples, while the CO output intensity of vitrinite-rich samples is lower, and the higher the vitrinite content, the higher the peak temperature of CO gas production of samples, indicating that when the temperature exceeds 400 °C, the increase of vitrinite inhibits CO production and promotes CO2 production. At this stage, the reduction of each sample’s “–C–O–” functional group after pyrolysis is positively correlated with the maximum CO gas production intensity, and the reduction of each sample’s “–C=O” functional group after pyrolysis is positively correlated with the maximum CO2 gas production intensity. As a result, the “–C–O–” functional group is more likely to produce CO, whereas the “–C=O” functional group is more likely to be pyrolyzed to CO2. Hydrogen is primarily produced during the polycondensation and aromatization processes, and its production is proportional to the dynamic DOC values after pyrolysis. The higher the I value after pyrolysis, the lower the maximum gas production peak intensity of CH4 and C2H6, which indicates that increasing the aromatic proportion is detrimental to CH4 and C2H6 production. This work is expected to provide theoretical support of the liquefaction and gasification of coal with different vitrinite/inertinite ratios.
Effects of Functional Groups in Coal with Different Vitrinite/Inertinite Ratios on Pyrolysis Products

Maceral composition is one of the key factors affecting the liquefaction and gasification of coal, which has attracted extensive attention of researchers working on coal chemical industry. To elucidate the impact of vitrinite and inertinite in coal on pyrolysis products, vitrinite and inertinite were extracted from a single coal sample and mixed to create six samples with varying vitrinite/inertinite ratios. The samples were subjected to thermogravimetry coupled online with mass spectrometry (TG–MS) experiments, and the Fourier transform infrared spectrometry (FITR) experiment was used to determine the macromolecular structures before and after the TG–MS experiments. The results show that the maximum mass loss rate is proportional to the vitrinite content and inversely proportional to the inertinite content, and increased vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to low temperatures. Based on FTIR experiments, the sample’s CH2/CH3 content, representing the length of aliphatic side chains, decreases significantly after pyrolysis, and the greater the loss of CH2/CH3, the greater the intensity of organic molecule production, indicating that aliphatic side chains are likely to yield organic molecule products. The aromatic degree (I) of samples rises sharply and steadily with increasing inertinite content. After high-temperature pyrolysis, the polycondensation degree of aromatic rings (DOC) and relative abundance of aromatic and aliphatic hydrogen (Har/Hal) within the sample increased significantly, indicating the thermal degradation rate of aromatic hydrogen content is much lower than that of aliphatic hydrogen. When the pyrolysis temperature is lower than 400 °C, the higher the inertinite content, the easier it is to produce CO2, whereas an increase in vitrinite leads to an increase in CO production. At this stage, the “–C–O–” functional group is pyrolyzed to produce CO and CO2. When the temperature exceeds 400 °C, the CO2 output intensity of vitrinite-rich samples is much higher than that of inertinite-rich samples, while the CO output intensity of vitrinite-rich samples is lower, and the higher the vitrinite content, the higher the peak temperature of CO gas production of samples, indicating that when the temperature exceeds 400 °C, the increase of vitrinite inhibits CO production and promotes CO2 production. At this stage, the reduction of each sample’s “–C–O–” functional group after pyrolysis is positively correlated with the maximum CO gas production intensity, and the reduction of each sample’s “–C=O” functional group after pyrolysis is positively correlated with the maximum CO2 gas production intensity. As a result, the “–C–O–” functional group is more likely to produce CO, whereas the “–C=O” functional group is more likely to be pyrolyzed to CO2. Hydrogen is primarily produced during the polycondensation and aromatization processes, and its production is proportional to the dynamic DOC values after pyrolysis. The higher the I value after pyrolysis, the lower the maximum gas production peak intensity of CH4 and C2H6, which indicates that increasing the aromatic proportion is detrimental to CH4 and C2H6 production. This work is expected to provide theoretical support of the liquefaction and gasification of coal with different vitrinite/inertinite ratios.

The study of coal pyrolysis attracts a large number of experts in the fields of coal combustion, clean utilization of coal, coal gas, and pyrolysis behaviors; moreover, evolution characteristics of pyrolysis products and coal properties are usually the main research topics in the pyrolysis experiment. Experimental studies on the pyrolysis of coal are generally conducted due to the significant practicability of coal pyrolysis experiments, and the thermogravimetric experiment was a conventional method to study the coal pyrolysis process. However, research on the pyrolysis properties of different macerals in coal has not been extensively conducted. China’s requirements for clean coal utilization have gradually improved in recent years. Due to the varying physical and chemical properties of different macerals, researchers are increasingly focusing on developing new theories and methods to separate macerals for various applications. As a result, the pyrolysis properties and macromolecular structures of various macerals have become research topics of great interest in the fields of coal geology, coal chemical industry, and coal-derived gas industry. Vitrinite and inertinite have very different macromolecular structures. Vitrinite contains higher concentrations of hydrocarbon, nitrogen, and sulfur elements than inertinite, while inertinite contains higher oxygen content, resulting in significant differences in pyrolysis products. Researchers discovered that coal with a higher inertinite content is less reactive during low-temperature pyrolysis, and vitrinite and inertinite exhibit distinct differences during pyrolysis. At the same pyrolysis temperature, vitrinite contains higher H and lower C contents, higher aliphatic C–H contents, hydrogen bonding, and lower aromaticity than inertinite. According to Li et al., each maceral in coal has different thermal stability from others. It was also discovered that vitrinite has a higher pyrolysis reactivity than inertinite, and the number of broken covalent bonds in vitrinite after pyrolysis is far greater than those in inertinite. Inertinite has a higher condensation index of aromatic rings and graphitizes faster than vitrinite during pyrolysis. As a result, it is discovered that the molecular evolution of vitrinite differs significantly from that of inertinite during pyrolysis, which can result in different products when heating coals with different vitrinite to inertinite ratios. Coal pyrolysis is divided into three stages: evaporation of small molecules, decomposition of oxygen-containing groups, and devolatilization of aliphatic groups. Low molecular compounds embedded in coal have little effect on thermal mass loss behavior but affect coal carbon structure distribution. Vitrinite is the main hydrocarbon-generating maceral in coal, and its pyrolysis gas (CH4, H2O, H2, CO2, and CO) production capacity is proportional to the heating rate. The higher the heating rate, the higher the gas production peak temperature. It was demonstrated that different macerals generated different pyrolysis products when the coal was heated, vitrinite softened to form a porous mosaic and filled the structured inertinite or cement with unstructured inertinite, and alkyl radicals generated in the vitrinite were transferred to the inertinite. Wang et al. discovered that the mixture of macerals improved the peak temperature of CH4 yield significantly based on a copyrolysis experiment of semicoke and bituminous coal. It was also demonstrated that petroleum formation in coals is attributable to intermolecular interactions. According to Wang et al., the total amount of emissions from vitrinite, including CO2, CO, CH4, and H2, was greater than that from inertinite. Based on the observation of a dramatic increase in viscosity along with the retention of volatile matter in the copyrolysis experiment of vitrinite and inertinite, Aziz et al. opined that both inertinite and vitrinite affected pyrolysis products in multiple ways. According to Cheng et al. findings in 2021, the difference of the pyrolysis products between vitrinite and inertinite was caused by the differences in carbon distribution. Therefore, it is evident that the pyrolysis products of vitrinite and inertinite are different; however, there is little study on the relationship between the pyrolysis products and functional groups of the mixture with different vitrinite/inertinite ratios, which is important to the cleaning utilization of coal. To better understand the effects of different vitrinite/inertinite ratios on the pyrolysis products due to the different functional groups, as well as the heating temperatures, copyrolysis with different ratios of vitrinite and inertinite was performed using thermogravimetry coupled mass spectrometry (TG–MS) experiments. Furthermore, the Fourier transform infrared spectrometry (FITR) experiment was employed to analyze the samples before and after the TG–MS experiment in order to determine their macromolecular structures. Finally, the relationship between the pyrolysis products and the functional groups was investigated.

The low-rank coal sample was collected from the southern Junggar Basin in NW China that weighed approximately 4 kg, with banded vitrain and fusain to allow easy stripping. The sample has a vitrinite reflectance (Ro) of 0.73%, as well as moisture (Mad), ash yield, volatile yield, and fixed carbon content of 2.88, 10.57, 30.2, and 56.35%, respectively. The vitrain and fusain were stripped off and mixed into six different samples labeled samples 1–6. The six samples were then demineralized using the following procedures: samples were crushed into 200 meshes (75 μm), and 15 g of powder sample was mixed with 80 mL of HCl (36% by mass) solution in a plastic beaker. After stirring the mixture for 4 h in a constant temperature water bath at 60 °C, the HCl solution was suction filtered out. Subsequently, 80 mL of 40% HF solution was added to the coal sample, followed by the same water bath and acid washing treatment, and finally, cleaning and filtering the residual coal sample with ultrapure water until the filtrate was tested to be free of precipitation with AgNO3 solution. Finally, we filtered out the acid-washed coal sample using filter paper and vacuum-dried it in an oven for 24 h at 60 °C to obtain the sample after deashing. After deashing, a portion of the sample was prepared to petrographic analysis (using China National Standards GB/T 8899-2013), proximate analysis (using China National Standards GB/T 212-2008), and ultimate analysis (using China National Standards GB/T 31391-2015), and the results are shown in Table 1.

The thermo plus EV2/thermo mass photometer (Japan) was used for the TG–MS experiment. After drying 20 mg of each sample, the TG–MS experiment was carried out with helium as the test atmosphere. The heating temperature ranged from room temperature to 1000 °C, with a heating rate of 10 °C/min. The pyrolytic gas was subjected to an MS detection test at a 300 mL/min flow rate. The mass-to-charge ratio (M/Z) ranged from 1 to 200, with a mass spectrum (MS) signal acquisition time of 1 s.

With the instrument of Thermo Scientific Nicolet iS20, both samples before and after the TG–MS experiment were analyzed using the FTIR experiment to examine their macromolecular structures. The samples were mixed with potassium bromide at a mass ratio of 1:200. After pressing, they were dried and placed in the sample holder of an infrared spectrometer’s sample room for scanning. To obtain the Fourier transform infrared spectrum (FTIR) curve, 32 scans were collected for each sample with a spectral resolution of 4 cm–1. Previous studies on the peak assignment of different bands of the infrared spectrum report the infrared spectrum curve in this experiment was divided into six ranges, namely, 3200–3600, 2800–3000, 1560–1625, 1350–1460, 1000–1060, and 700–900 cm–1 (Table 2). Additionally, curve fitting is a widely used method for FTIR spectra analysis, in which the curves within different ranges can be fitted by several regular and symmetrical peaks, as shown in Figure 1, and the functional groups of samples can be obtained based on the peak area calculation. Therefore, curve fitting analysis was conducted for the curves of each band. The macromolecular structure of coal analyzed by FTIR can be represented by the following parameters: (1) CH2/CH3 = A (2940–2900 cm–1)/A (2940–3000 cm–1), representing the length of aliphatic side chains; (2) I = A (900–700 cm–1)/A (3000–2800 cm–1), representing the aromatic degree; (3) DOC = A (900–700 cm–1)/A (1600 cm–1), representing the polycondensation degree of aromatic rings; (4) Har/Hal = A (1650–1520 cm–1)/A (3000–2800 cm–1), representing the relative abundance of aromatic to aliphatic hydrogen. Here, A denotes the integrated areas of different wavenumbers, with the ratios having different denotative meanings: the larger the value of the CH2/H3, the longer the side chains and the lower the branching degree; the higher the I value, the higher the aromaticity; similarly, the higher the DOC, the higher the polycondensation degree of the aromatic rings; the higher the Har/Hal, the higher the relative abundance of aromatic to aliphatic hydrogen.

The obtained TG data include remained mass, temperature, and time (min), so the thermogravimetric mass with the temperature can be calculated by the remained mass data, and the differential thermogravimetric curves, which consist of mass loss rate data, can be calculated by the mass loss data, time, and temperature, which is automatically calculated by the computer based on the recording data during the experiment. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of the samples are shown in Figure 2. The sample pyrolysis process can be divided into four stages: (1) the sample mass gradually decreases from 30 to 400 °C (Figure 2a), and the DTG curve reveals that the rate of mass loss varies slightly (Figure 2b). (2) Samples have suffered violent pyrolytic reaction at high temperatures ranging from 400 to 600 °C, and the mass loss rate increases dramatically (Figure 2b). The maximum mass loss rate and corresponding temperature of samples 1 to 6 are −0.252%/°C with 448.7 °C, −0.276%/°C with 447.9 °C, −0.180%/°C with 448.5 °C, −0.198%/°C with 449.4 °C, −0.151%/°C with 449.2 °C, and −0.002%/°C with 451.2 °C, respectively. The pyrolysis rate of sample 6 is relatively stable, and its maximum pyrolysis rate is more than 50 times lower than that of the other samples. The maximum pyrolysis temperature and maximum mass loss rate of the samples differ, demonstrating that even within the same coal sample, the difference in macerals is sufficient to affect the pyrolysis process. (3) After 600 °C, the samples’ pyrolysis rate gradually decreases and is less than 0.025%/°C after 800 °C. Mass loss decreases gradually from sample 1 to sample 6 during the pyrolysis process. Sample 1 has a maximum mass loss of 38.25%, while sample 6 has a maximum mass loss of 29.05%. The correlation analysis between the maceral content of the sample and the maximum mass loss rate of the sample shows that the maximum mass loss rate is proportional to the vitrinite content and inversely proportional to the inertinite content (Figure 3a), indicating that the vitrinite is primarily pyrolyzed during pyrolysis, whereas the inertinite is difficult to pyrolyze. Furthermore, a correlation analysis of the sample’s maceral content with its maximum pyrolysis rate and peak pyrolysis temperature reveals that the maximum pyrolysis temperature has a weak negative correlation with the vitrinite content and a strong positive correlation with the inertinite content (Figure 3b), in which the weak correlation may be caused by the limited maximum pyrolysis temperature range which only varies from 447.9 to 451.2 °C, and the weak correlation indicates that the maceral has limited influence on the maximum pyrolysis temperature. The maximum rate correlates positively with vitrinite content and negatively with inertinite content. As a result, increasing the vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to low temperatures, whereas increasing the inertinite content slows the pyrolysis process and shifts the pyrolysis peak to high temperatures.

During pyrolysis, the intensity of methane (CH4), ethane (C2H6), carbon monoxide (CO), carbon dioxide (CO2), vapor (H2O), and hydrogen (H2) generation is primarily investigated. Figure 3 depicts the pyrolysis results. The methane generation intensity curve has a single peak, and from samples 1– to 6, the methane generation intensity gradually increases (Figure 4a), with apparent regularity, and the peak temperature of gas generation is between 500 and 630 °C. Ethane gas generation intensity curves of samples 4, 5, and 6 exhibit multiple peaks with poor regularity (Figure 4b), whereas those of samples 1, 2, and 3 exhibit single peaks, with the maximum gas production intensity decreasing from sample 1 to sample 3, with the temperatures of 477.1, 471.4, and 470.9 °C corresponding to the maximum gas production rates. The samples’ CO production intensity curves have multiple peaks (Figure 4c), with a wave peak representing the maximum gas production intensity. The CO production intensity gradually increases from sample 1 to sample 4, while gradually decreasing from sample 4 to sample 6 (Figure 4c). Interestingly, CO production peak temperatures gradually decrease from sample 1 to 6, with values of 653.8, 578.0, 480.0, 478.5, 477.3, and 479.4 °C, respectively. The samples’ CO production intensity curves exhibit bimodal characteristics (Figure 4d), with corresponding temperatures of around 460 and 600 °C, respectively. The gas production intensity curve of water vapor has a unimodal feature (Figure 4e), and only samples 1, 4, 5, and 6 produced water vapors, whereas samples 2 and 3 did not. The hydrogen production intensities of the samples decrease from samples 1 to 6 (Figure 4f), where the temperature range corresponding to the maximum gas production intensities is 742.4–754.3 °C. Considering the relationship between the content of macerals and products, the vitrinite content is positively correlated with the intensity of methane gas production (Figure 5a), indicating that vitrinite is the main gas-generating component of methane production and that increasing inertinite content is not conducive to methane generation. The temperature corresponding to the maximum CO gas production intensity has a positive correlation with the vitrinite content but a negative correlation with the inertinite content (Figure 5b), indicating that increasing the inertinite content can lower the temperature corresponding to the maximum CO production intensity. Since inertinite has a higher oxygen content than vitrinite, this phenomenon appears to have a material basis. The maximum yield of hydrogen increases with increasing vitrinite content (Figure 5c), and the correlation coefficient is 0.7401, which is because the H2 maximum intensity suddenly increases when the vitrinite content exceeds 50%. As a result, it can be found that increasing vitrinite content is conducive to hydrogen production, which is determined by the high hydrogen content in vitrinite.

Figure 6 depicts the FTIR curves obtained before and after the pyrolysis experiment, and Table 3 depicts the calculated molecular structure parameters. In terms of the difference between different samples before the experiment, the 3200–3600 cm–1 section of samples is relatively flat except Sample 1, indicating that Sample 1 has more hydroxyl groups than other samples. The peak of aliphatic CH (2800–3000 cm–1) tends to decrease from samples 1 to 6. From samples 1 to 6, the peaks (1350–1460 cm–1) represent aliphatic CH2 and CH3 transition from a sharp to gentle slope. The peaks of aromatic functional groups in different samples show no discernible difference. From samples 1 to 6, the content of inertinite increases and that of vitrinite decreases, and the CH2/CH3 decreases (Figure 7a), and it is found that the CH2/CH3 value is negatively correlated with the content of inertinite (Figure 7b), demonstrating that vitrinite contains more aliphatic functional groups and aliphatic side chains than inertinite. After pyrolysis, the sample underwent high-temperature thermal decomposition, and the wave peak representing aliphatic groups (2800–3000 cm–1) virtually disappeared (Figure 6), indicating that high-temperature pyrolysis decomposes most aliphatic groups. Because of the high vitrinite content in sample 1, the wave peaks (1350–1460 cm–1) representing aliphatic CH2 and CH3 become much smoother, and only sample 1 has obvious wave peaks. Furthermore, the peaks representing aromatic C=C, C–O–C, and aromatic CH are observable but flatten after pyrolysis, indicating that aromatic groups and ether bonds are more stable than aliphatic groups and are decomposed into gaseous products. In terms of molecular structure changes before and after pyrolysis, all samples’ CH2/CH3 decrease significantly after high-temperature pyrolysis (Figure 7a), indicating that the aliphatic molecular side chain in coal decreases sharply and that before pyrolysis, CH2/CH3 is negatively correlated with the inertinite content; however, after pyrolysis, CH2/CH3 is positively correlated with the inertinite content (Figure 7b), and that is because the content of inertinite can decelerate the pyrolysis rate, as shown in Figure 2b; the more the content of inertinite is, the more aliphatic side chains in vitrinite remain after pyrolysis. The I values of the samples before pyrolysis are consistent (Figure 7c), whereas after pyrolysis, they increase sharply and show an increasing trend with increasing inertinite content (Figure 7d), attributable to the fact that aliphatic functional groups are much easier to decompose than aromatic functional groups, as shown in Figure 6. The samples’ DOC values increase significantly after pyrolysis (Figure 7e), but with no significant correlation between DOC increase and inertinite content (Figure 7f). The samples’ Har/Hal ratios increase significantly after high-temperature pyrolysis (Figure 7g), indicating that the thermal degradation rates of the aromatic hydrogen contents are much lower than that of the aliphatic hydrogen contents. Furthermore, the Har/Hal of the samples after pyrolysis is positively correlated with the content of inertinite (Figure 7h) because the Har/Hal increases sharply when the inertinite content exceeds 50%, and the data are limited, so the correlation coefficient is relatively weak with a value being 0.5455. As a result, it is found that the aromatic hydrogen content of inertinite is difficult to pyrolyze.

As shown in Table 2, the main oxygen-containing functional groups in the samples’ molecular structure are −OH, −C=O, and–C–O–, whereas the oxygen-containing molecules in the products are primarily H2O, CO, and CO2. Thus, there must be some correlations between oxygenated functional groups in the samples and oxygenated molecular products. It is very interesting that only sample 6 (rich in inertinite) has obvious CO2 output when the temperature is less than 100 °C (Figure 4), and only samples 4, 5, and 6 have obvious CO output with the intensity order of sample 6 < sample 5 < sample 4 at 100–200 °C. As a result, at low temperatures (less than 400 °C), the higher the content of the inertinite group, the easier it is to produce CO2, and an increase in vitrinite can lead to more CO production in the coal samples. When the temperature rises above 400 °C, the CO2 production intensity of vitrinite-rich samples (samples 1, 2, and 3) is much higher than that of inertinite-rich samples (samples 4, 5, and 6), while the CO production intensity of vitrinite-rich samples (samples 1, 2, and 3) is lower. The higher the vitrinite content, the higher the sample’s CO production peak temperature (Figure 5b), indicating that at high temperatures (greater than 400 °C), increase in vitrinite inhibits CO production while promoting CO2 production. Furthermore, when the temperature exceeds 400 °C, H2O production is initiated, and only samples 1, 4, 5, and 6 produce water vapor, and the intensity of water production in samples 4, 5, and 6 is much higher than that in sample 1, while samples 2 and 3 do not produce obvious water vapor. As a result, the increase in inertinite promotes the yield of H2O. Therefore, the effects of vitrinite and inertinite on oxygen-containing molecular products differ at different temperature stages, and the analysis of the effects of oxygen-containing functional groups must be performed at both low and high temperatures. In theory, −OH can be easily converted to H2O. The “–OH” functional group content of samples 1 to 6 after pyrolysis decreases by 49.62, 14.66, 44.19, 43.27, 93.66, and 94.25%, respectively (Figure 8b), but there is no obvious correlation with the H2O output intensity. CO and CO2 are produced by oxygen-containing functional groups such as “–C=O” and “–C–O–.” Every sample reduces the percentage of “–C–O–” functional groups by more than 95% and the percentage of “–C=O” functional groups by 82.67–97.02%. After pyrolysis, the reduction of each sample’s “–C–O–” functional group shows a positive correlation with the maximum CO production intensity (Figure 8a) but no obvious correlation with the maximum CO2 production intensity. Moreover, the reduction of each sample’s “–C=O” functional group after pyrolysis shows a positive correlation with the maximum CO2 gas production intensity (Figure 8c) but no obvious correlation with the maximum CO production intensity. Therefore, the “–C–O–” functional group is more likely to produce CO, whereas the “–C=O” functional group is more likely to be pyrolyzed to CO2.

Aliphatic groups primarily have a spectral range of 3000–2800 cm–1. Previous research indicates that the peaks in the center at 2954, 2921, and 2890 cm–1, respectively, represent aliphatic CH3 asymmetric stretching, aliphatic CH2 asymmetric stretching, and aliphatic C–H symmetric stretching. Calculating the aliphatic hydrocarbon content quantitatively can provide an essential parameter for studying the chemical structure of coal: CH2/CH3 represents the length of aliphatic side chains. This parameter can reflect not only the degree of metamorphism of coal but also the main skeleton of coal’s macromolecular structure. Aromatic C=C ring stretching (1650–1520 cm–1) and CH aromatic out-of-plane deformation (700–900 cm–1) dominate the aromatic functional groups in coal. According to Section 3.3, the terms I, DOC, and Har/Hal are used to describe the aromatic structure’s characteristics. Other pyrolysis products, besides the oxygenated products, include CH4, C2H6, and H2, with peak pyrolysis temperatures ranging from 500.0 to 622.8 °C, 452.0 to 749.0 °C, and 742.4 to 754.3 °C, respectively. The gas production temperature ranges of CH4 and C2H6 overlap significantly and are lower than the gas production temperature of H2. Small organic molecules, such as CH4 and C2H6, are expected to be produced by the aliphatic side chain molecules. Section 3.3 shows that after coal pyrolysis, CH2/CH3 increased while I, DOC, and Har/Hal decreased, indicating that vitrinite and inertinite have undergone strong aromatization and condensation, resulting in the production of organic small molecules and H2. As a result, the ratios of CH2/CH3, I, DOC, and Har/Hal after pyrolysis to before pyrolysis, namely, Δ(CH2/CH3), Δ(I), Δ(DOC), and Δ(Har/Hal), are plotted with the peak intensities of CH4, C2H6, and H2. It demonstrates that the Δ(CH2/CH3) has a significant negative correlation with the maximum gas production intensities of CH4, C2H6, and H2 (Figure 9a), indicating that the more aliphatic side chains fall off (i.e., the smaller the Δ(CH2/CH3), the higher the gas production intensity, confirming that the main products of aliphatic side chains are CH4, C2H6, and H2. The Δ(Har/Hal) ratio has a negative correlation with the maximum gas production intensities of CH4 and C2H6 (Figure 9b), indicating that the aromatic hydrogen content to aliphatic hydrogen content varies significantly (i.e., the lower the value of Δ(Har/Hal), the easier it is to produce C2H6 and H2). Only a positive correlation exists between the Δ(DOC) and the peak intensity of H2 gas production (Figure 9c). Because the hydrogen content of aromatic and aliphatic groups in vitrinite after pyrolysis decreases significantly, this indicates that hydrogen is produced in large quantities during polycondensation and aromatization. The Δ(I) has a negative correlation with the maximum peak intensity of CH4 and C2H6 production (Figure 9d), indicating that a higher aromatic degree (i.e., a higher Δ(I) value) is not conducive for CH4 and C2H6 production.

In this study, vitrinite and inertinite are extracted from the same coal sample, and then, TG–MS experiments with different vitrinite and inertinite mixture ratios are performed. The FTIR experiment examines the molecular structure of samples before and after pyrolysis experiments, as well as the effects of functional groups on pyrolysis products. The following are the findings. (1) The maximum mass loss rate obtained from TG–MS experiments is proportional to the vitrinite content and inversely proportional to the inertinite content. Increased vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to a low temperature, whereas increased inertinite content slows the pyrolysis process and shifts the pyrolysis peak to a high temperature. (2) Increasing inertinite content is not conducive to methane generation but reduces the maximum CO production intensity. The maximum yield of hydrogen increases with the increase of the vitrinite content, thus indicating that the increase in vitrinite content facilitates the production of hydrogen. (3) After pyrolysis, the sample’s CH2/CH3 decreases significantly while its I, DOC, and Har/Hal increase significantly, indicating that the aromatic group’s thermal degradation rate is much lower than that of the aliphatic group. Furthermore, the Har/Hal of the sample after pyrolysis is positively correlated with the inertinite content, indicating that the hydrogen in inertinite is difficult to pyrolyze. (4) When the pyrolysis temperature is less than 400 °C, the higher the inertinite content, the easier it is to produce CO2, whereas an increase in vitrinite leads to an increase in CO production in coal samples. When the temperature rises above 400 °C, the CO2 production intensity of vitrinite-rich samples is much higher than that of inertinite-rich samples, while the CO production intensity of vitrinite-rich samples is lower. (5) The “–C–O–” is first pyrolyzed to generate CO and CO2 at temperatures lower than 400 °C. When the temperature exceeds 400 °C, “–C–O–” functional groups are more likely to produce CO, whereas “–C=O” functional groups are more likely to pyrolyze to CO2. (6) The main products of aliphatic side chains are CH4 and C2H6, and large amounts of hydrogen are produced during polycondensation and aromatization, and a higher aromatic degree is not conducive to CH4 and C2H6 production.