Effects of Parent Coal Properties on the Pyrolytic Char Chemical Structure: Insights from Micro-Raman Spectroscopy Based on 32 Kinds of Chinese Coals

: The chemical structures of pyrolytic chars prepared from 32 kinds of Chinese coals were investigated with micro-Raman spectroscopy in this study. Both ﬁrst-order and second-order Raman spectra of the chars were curve-ﬁtted and analyzed. The effects of the parent coal properties, including coal rank, volatile, ﬁxed carbon, and ash content, on the pyrolytic char structures were detailed discussed and the correlations between these coal properties and pyrolytic char chemical structures were set up. Multiple-factor analysis was done to propose a comprehensive coal property index that relates well to the pyrolytic char chemical structure. The results indicate that the aromatization degree is the key distinguishable structure of pyrolytic chars prepared from coals with various rank, and the alkyl C − H and aryl C − H structures have no signiﬁcant difference. The aromatization degree of pyrolytic char decreases with the increase of coal rank, while it increases with the increase of the ﬁxed carbon content in parent coals. The high content of moisture in parent coal can induce condensation of the pyrolytic char, but the inorganic composition probably prevents the condensation of the char. Limited correlations between the coal rank, ﬁxed carbon, moisture and ash content, and the aromatization degree of pyrolytic chars were found. A comprehensive coal property index: (ﬁxed carbon content + moisture content)/(volatile content + ash content) (in air dry basis) combining the coal properties together relates well to the aromatization degree of pyrolytic char and can act as a good indicator for the pyrolytic char chemical structure. This study reveals the effects of the parent coal properties, including coal rank, ﬁxed carbon, moisture, and ash content, on the pyrolytic char chemical structure, and provides a new comprehensive coal property index to predict the pyrolytic char chemical structure. The chars of type C are mainly from the parent coals of LY, ZC, XM, XWJ, and SX, all of which mainly have a high ash content (A ad > 30%). It reveals that apart from the inﬂuence of moisture and ash in the parent coal, the aromatization degree of pyrolytic char can be related to the parent coal rank better. It further indicates that the moisture and ash components in parent coals may have an inﬂuence on the aromatic ring structures in pyrolytic char, which deserves further deep investigation.


Introduction
It is widely accepted that coal will still play a crucial role in the world's energy mix in the near future, especially in developing countries, such as China, India, and South Africa [1][2][3]. Nowadays, pyrolysis, gasification, and combustion are the main ways for coal utilization [2,3]. In all these processes, coal pyrolysis is the first step, releasing volatiles and forming pyrolytic char, and then the pyrolytic char undergoes further conversion reactions [3][4][5][6]. Because the rate of char conversion reaction is significantly slower than that of devolatilization, the char conversion process is the rate-limited step during coal the chemical structures of raw coal. They can be obtained with a standardized quantitative process, and are suitable as indicators to forecast the pyrolytic char chemical structure. Therefore, establishing effective quantitative relationships between coal properties and pyrolytic char chemical structures is very meaningful. It requires a comprehensive study about the effects of coal properties on the pyrolytic char chemical structures and a combination of coal properties to predict the char structures based on a certain amount of coal. Unfortunately, most of the previous studies have only focused on the influence of coal rank, ignoring the influences of other coal properties, such as moisture, fixed carbon, or ash content, and no comprehensive coal property index that relates well to the pyrolytic char chemical structures has been proposed [1,4].
In the past decades, many useful techniques, including Fourier transform infrared spectrometry (FT-IR), XRD, nuclear magnetic resonance (NMR), and Raman spectroscopy, have been developed to characterize coal/char chemical structures [1,5,6,9,11,20,21]. Among these technologies, Raman spectroscopy is being increasingly widely used to study the char structures due to its high sensitivity to not only well-ordered carbon structures but also amorphous carbon structures [2,5,22]. Besides, the detection of coal or chars by micro-Raman spectroscopy is very fast, which is beneficial for the application of Raman spectroscopy in on-line coal/char analysis. Our recent studies indicated that the Raman spectrum in the first-order range could be validly interpreted to clarify the aromatic ring structures of coals and chars, and the Raman spectrum in the second-order range could also be interpreted to reveal useful structure information about aryl C−H and C−H of methyl or methylene [23]. Further investigation of the pyrolytic char chemical structures using Raman spectroscopy and clarification of the roles coal properties play during coal pyrolysis are not only meaningful for revealing the fundamental formation mechanism of pyrolytic char structures, but also for rapid assessment of the pyrolytic char structures in industrial processes.
In this study, chars were prepared from 32 kinds of Chinese coals. The micro-Raman spectrometer was used to characterize the pyrolytic char chemical structures. Both firstorder and second-order Raman spectrum of chars were interpreted to reveal the detailed chemical structures. The property parameters of parent coals, including moisture, volatiles, fixed carbon, and ash content, were related to the char structures to reveal the effects of the raw coal properties on the pyrolytic char chemical structures and to propose a comprehensive coal property parameter for the prediction of pyrolytic char chemical structures.

Experiments and Methods
In total, 32 kinds of Chinese coals with a volatile content in an air-dried basis (V ad ) of about 5% to 35%, moisture content (M ad ) of about 1% to 25%, fixed carbon content (FC ad ) of about 30% to 70%, and ash content (A ad ) of about 4% to 55% were selected as the raw coals. The properties of these coals are shown in Table S1 in the Supplementary Materials, and were also described in detail in our previous study [2].
The pyrolytic chars were prepared in a fixed-bed reaction system, described in our previous study [17]. Before an experiment, the temperature and gas flow rate in the reactor was respectively calibrated by a K-type thermocouple (Ω Company, American) and a digital bubble flow meter (Agilent Optiflow 650, American). During an experiment, the furnace temperature was first heated to 1173 K. N 2 with a flow rate of 2.00 L/min was then sent into the quartz tube reactor, removing the air to maintain a non-oxygen atmosphere. For each run, 1.0 ± 0.1 g of raw coal loaded by an alumina sample holder was sent to the constant temperature zone of the reactor rapidly and held for 7 min. After, the char was pushed to the end of the reactor and cooled down in an N 2 atmosphere. In order to study the effects of moisture in the parent coal on the pyrolytic char structures, five coals with a high moisture content were selected and dried in a furnace at 378 K for 24 h. The chars of dried coals were prepared in the same condition as those of raw coals. It must be pointed out that the chars were prepared in this condition mainly because the volatiles in coal were almost released and the pyrolysis process had occurred to a large extent [5], which is Processes 2021, 9, 1575 4 of 13 beneficial for studying the effects of coal properties on pyrolytic char structures. Besides, it is similar to the test condition for the volatile content according to Chinese National Standard GB2132-2001, facilitating the set up of universality laws for the industry.
The Raman spectra of pyrolytic chars were tested in a micro-Raman spectrometer (Jobin Yvon Lab RAM HR800, Horiba Jobin Yvon Company, Paris, France) following the process described in our previous studies [2,6,23]. The laser was an Nd-YAG laser and the wavenumber was 532 nm. An Olympus BX41 optical microscope (50 × objective lens, Olympus Company, Tokyo, Japanese) was employed to focus the laser on the sample surface. The beam diameter focusing on the sample surface was about 2 µm. The laser power was filtered and controlled at about 5 mW to reduce the thermal damage. All the Raman spectra were recorded in the range of 800 to 3400 cm −1 with an acquisition time of 10 s. In total, 10 to 15 particles were randomly selected and tested to improve the accuracy of the results as pyrolytic chars are heterogeneous [2,6,23].
The first-order Raman spectrum from 800 to 1800 cm −1 and second-order Raman spectrum from 2200 to 3400 cm −1 were curve-fitted. The first-order Raman spectra of the chars were curve-fitted by 10 Gaussian bands according to our previous studies [2,6] and Li et al. [20,22]. Among these 10 bands, only the main bands: G band, D band, and the three bands G R , V L , V R between G and D bands, are briefly described. The G band located at about 1590 cm −1 is generally attributed to aromatic rings and E 2 2g graphite vibration. In view of the limited degree of graphitization of coal char structures at 1173 K and the low Raman intensity of the limited graphitic structures, the G band is mainly attributed to the aromatic ring structures of pyrolytic chars in this study. The D band at about 1330 cm −1 mainly represents the large aromatic rings (not less than 6 rings) in the chars [24], and the three bands G R , V L , and V R together represent the small aromatic rings in the chars. The Raman spectral parameter A (GR+VL+VR) /A D (the ratio of the (G R +V L +V R ) bands area to the D band area) represents the ratio of small aromatic rings to large ones in pyrolytic chars, reflecting the aromatization degree of chars [9]. The second-order Raman spectra of chars were curve-fitted by 8 Gaussian bands according to our previous study [23]. The 2G and 2D band at 3180 cm −1 and 2670 cm −1 are the overtone of the G and D band in the first-order Raman spectrum, respectively, and the D+G band at 2925 cm −1 is the combination band of the D and G band [23]. They can mainly be attributed to the aromatic rings structure in the pyrolytic char. The (2D) L band at 2810 cm −1 can mainly be attributed to the C−H stretch in methyl and methylene, and the (2G) R band at 3060 cm −1 can mainly be attributed to the aryl C−H stretch vibration [23,25,26]. The Raman band area ratio A (2D)L /S 2 and A (2G)R /S 2 (the ratio of (2D) L band area (A (2D)L ) and the (2G) R band area (A (2G)R ) to the total Raman band area of the second-order Raman spectrum (S 2 )) respectively reflects the relative amount of the alkyl C−H structure and aryl C−H structure in the pyrolytic chars [23,25,26]. A sample of the spectrum curve-fitting is shown in Figure 1, and the band attributions are listed in Table 1.  Band Name Band Position, cm −1 Description R 960-800 C-C on alkanes and cyclic alkanes; C-H on aromatic rings

The C-H Structures in Pyrolytic Chars
It is known that coal rank is the most important coal property index [1][2][3][4]8,11]. Many studies have reported that parent coal rank can affect the pyrolytic char structures [1,4]. Generally, the volatile content in dry-ash-free basis (V daf ) in raw coal can be used to reflect the coal rank [4,5]. Therefore, the correlations between the coal rank represented by V daf and the pyrolytic char chemical structures are firstly discussed. Figure 2 shows the change trend of A (2G)R /S 2 and A (2D)L /S 2 of pyrolytic chars with V daf in raw coals. It can be seen that both A (2G)R /S 2 and A (2D)L /S 2 just slightly increase with the increase of V daf and there is a great fluctuation. When linearly curve-fitting the points, R 2 is 0.05 and 0.3 for the relations between A (2G)R /S 2 and A (2D)L /S 2 and V daf , respectively. It indicates that there are no reasonable correlations between A (2G)R /S 2 and A (2D)L /S 2 of pyrolytic char and V daf of the parent coal. It is known that A (2D)L /S 2 and A (2G)R /S 2 can respectively reflect the relative amount of alkyl C−H and aryl C−H structures in pyrolytic chars [23]. Thus, the results reveal that there is no significant relationship between the amount of alkyl C−H and aryl C−H structures in pyrolytic chars and the parent coal rank. Besides, from Figure 2, it can be seen that A (2G)R /S 2 changes from about 0.09 to 0.105, and A (2D)L /S 2 changes from about 0.13 to 0.145. The relative change percentage of A (2G)R /S 2 and A (2D)L /S 2 is about 15% and 10%, respectively, which is obviously lower than that of raw coals, with a value of about 50% and 17% shown in our previous study [23]. It reveals that the relative amount of alkyl C−H and aryl C−H structures in pyrolytic chars has no significant difference for the Processes 2021, 9, 1575 6 of 13 wide rank coals investigated in this study. It is different from the results of raw coals, where it was found that the relative amount of alkyl C−H and aryl C−H structures relates well to the coal rank. This is mainly because the alkyl C−H and aryl C−H structures in raw coal are significantly released along with the release of volatiles during coal pyrolysis [29]. Thus, the amount of alkyl C−H and aryl C−H structures is very limited and has no obvious difference for the pyrolytic chars from coals with different ranks when the volatiles are almost released after pyrolysis [6,29]. no significant relationship between the amount of alkyl C−H and aryl C−H structures in pyrolytic chars and the parent coal rank. Besides, from Figure 2, it can be seen that A(2G)R/S2 changes from about 0.09 to 0.105, and A(2D)L/S2 changes from about 0.13 to 0.145. The relative change percentage of A(2G)R/S2 and A(2D)L/S2 is about 15% and 10%, respectively, which is obviously lower than that of raw coals, with a value of about 50% and 17% shown in our previous study [23]. It reveals that the relative amount of alkyl C−H and aryl C−H structures in pyrolytic chars has no significant difference for the wide rank coals investigated in this study. It is different from the results of raw coals, where it was found that the relative amount of alkyl C−H and aryl C−H structures relates well to the coal rank. This is mainly because the alkyl C−H and aryl C−H structures in raw coal are significantly released along with the release of volatiles during coal pyrolysis [29]. Thus, the amount of alkyl C−H and aryl C−H structures is very limited and has no obvious difference for the pyrolytic chars from coals with different ranks when the volatiles are almost released after pyrolysis [6,29].  Figure 3 shows the correlation between A(GR+VL+VR)/AD (the ratio of (GR+VL+VR) bands area to the D band area) and Vdaf in parent coals. From Figure 3, it can be seen that A(GR+VL+VR)/AD of the pyrolytic char changes from about 0.8 to 1.15. The calculated relative change percentage of A(GR+VL+VR)/AD for all the chars is about 30%, which is obviously lager than that of A(2G)R/S2 and A(2D)L/S2. It is known that the Raman spectral parameter A(GR+VL+VR)/AD mainly represents the relative ratio of small aromatic rings to large ones in char, reflecting the char aromatization degree [2,6,20]. Therefore, the aromatic ring structures have a more significant difference than the C-H structures for pyrolytic chars prepared from coals with various ranks. It means that the aromatic rings' rearrangement and the aromatization degree is the key difference for pyrolytic chars from coals with various  Figure 3 shows the correlation between A (GR+VL+VR) /A D (the ratio of (G R +V L +V R ) bands area to the D band area) and V daf in parent coals. From Figure 3, it can be seen that A (GR+VL+VR) /A D of the pyrolytic char changes from about 0.8 to 1.15. The calculated relative change percentage of A (GR+VL+VR) /A D for all the chars is about 30%, which is obviously lager than that of A (2G)R /S 2 and A (2D)L /S 2 . It is known that the Raman spectral parameter A (GR+VL+VR) /A D mainly represents the relative ratio of small aromatic rings to large ones in char, reflecting the char aromatization degree [2,6,20]. Therefore, the aromatic ring structures have a more significant difference than the C-H structures for pyrolytic chars prepared from coals with various ranks. It means that the aromatic rings' rearrangement and the aromatization degree is the key difference for pyrolytic chars from coals with various ranks. Besides, it can be seen that A (GR+VL+VR) /A D of pyrolytic char increases with the increase of V daf in the parent coals on the whole. It indicates that a coal with a lower rank generally results in a char with a lower aromatization degree, which is in accordance with the literature [1,4]. However, when fitting the correlation between A (GR+VL+VR) /A D and V daf with a linear equation, R 2 is just about 0.4, which is not high due to the significant fluctuations of some chars. From the distribution of data points, it can be seen that A (GR+VL+VR) /A D fluctuates significantly with V daf , especially for the chars from the parent coals with V daf larger than 25%. It indicates that the aromatization degree of pyrolytic chars is not just related to the parent coal rank. Only using the coal rank or V daf to predict the characteristic of the aromatization degree of pyrolytic chars has limited accuracy.

The Aromatic Rings in Pyrolytic Chars
In order to clarify the reasons for the fluctuation of A (GR+VL+VR) /A D with V daf , the distribution of data points was further analyzed by combining the detailed parent coal properties. It was found that the chars can mainly be divided into three types (type A, B, and C), as shown in Figure 3. It is interesting to point out that when fitting the correlations between A (GR+VL+VR) /A D and V daf for chars of type A, B, and C with a quadratic equation, R 2 can be as high as 0.90, 0.81, and 0.82, respectively. This indicates that strong correlations exist between the aromatization degree of pyrolytic chars and V daf of parent coals for these three types of coal. From Table S1, it is known that relative to chars of type A, chars of type B are mainly from the parent coals of HSQ, SF, WCW, XLT, and HLH, all of which have common characteristics with a low ash content (A ad < 10%) and relatively high moisture. The chars of type C are mainly from the parent coals of LY, ZC, XM, XWJ, and SX, all of which mainly have a high ash content (A ad > 30%). It reveals that apart from the influence of moisture and ash in the parent coal, the aromatization degree of pyrolytic char can be related to the parent coal rank better. It further indicates that the moisture and ash components in parent coals may have an influence on the aromatic ring structures in pyrolytic char, which deserves further deep investigation. as shown in Figure 3. It is interesting to point out that when fitting the correlations between A(GR+VL+VR)/AD and Vdaf for chars of type A, B, and C with a quadratic equation, R 2 can be as high as 0.90, 0.81, and 0.82, respectively. This indicates that strong correlations exist between the aromatization degree of pyrolytic chars and Vdaf of parent coals for these three types of coal. From Table S1, it is known that relative to chars of type A, chars of type B are mainly from the parent coals of HSQ, SF, WCW, XLT, and HLH, all of which have common characteristics with a low ash content (Aad<10%) and relatively high moisture. The chars of type C are mainly from the parent coals of LY, ZC, XM, XWJ, and SX, all of which mainly have a high ash content (Aad>30%). It reveals that apart from the influence of moisture and ash in the parent coal, the aromatization degree of pyrolytic char can be related to the parent coal rank better. It further indicates that the moisture and ash components in parent coals may have an influence on the aromatic ring structures in pyrolytic char, which deserves further deep investigation. Figure 3. The correlation between Vdaf in parent coal and A(GR+VL+VR)/AD of its pyrolytic char. The chars of type A, B, and C were from coals with 10% < Aad < 30%, Aad < 10%, and Aad > 30%, respectively. Figure 4 shows the correlation between Mad in parent coal and A(GR+VL+VR)/AD of its pyrolytic char. It can be seen that there is no obvious relationship between Mad and the aromatic ring structures in its pyrolytic char overall. When dividing the chars into three types (type D, E, and F), it seems that a coal with higher Mad can result in a more condensed char. Among them, chars of type D were mainly from the coals with a moisture content lower than 5%. Chars of type E were mainly from the coals with a relatively high moisture content (>5%) and relatively low ash content (<10%). Chars of type F were mainly from the coals with a relatively high moisture content (>5%) and relatively high ash content (>10%). In this division, both the effects of the moisture content and the ash content were Figure 3. The correlation between V daf in parent coal and A (GR+VL+VR) /A D of its pyrolytic char. The chars of type A, B, and C were from coals with 10% < A ad < 30%, A ad < 10%, and A ad > 30%, respectively. Figure 4 shows the correlation between M ad in parent coal and A (GR+VL+VR) /A D of its pyrolytic char. It can be seen that there is no obvious relationship between M ad and the aromatic ring structures in its pyrolytic char overall. When dividing the chars into three types (type D, E, and F), it seems that a coal with higher M ad can result in a more condensed char. Among them, chars of type D were mainly from the coals with a moisture content lower than 5%. Chars of type E were mainly from the coals with a relatively high moisture content (>5%) and relatively low ash content (<10%). Chars of type F were mainly from the coals with a relatively high moisture content (>5%) and relatively high ash content (>10%). In this division, both the effects of the moisture content and the ash content were considered. In order to confirm the detailed effects of moisture in parent coal on the pyrolytic char structures, the structures of chars obtained from the dried coals were further analyzed. Figure 5 shows A (GR+VL+VR) /A D of pyrolytic chars from selected raw coals with a high moisture content and their dried coals. It can be seen that A (GR+VL+VR) /A D of pyrolytic chars from dried coals is slightly larger than the value from raw coals. It indicates that there are more small aromatic rings and less large aromatic rings for the chars prepared from dried coals than the chars from raw coals. In this study, the moisture in raw coals was dried at 105°C for 24 h, which has very limited effects on the chemical structures of coals [14]. Therefore, the difference is mainly due to the effects of moisture in the parent coal during the coal pyrolysis process. Thus, it directly confirms that the moisture in parent coals can increase the aromatization degree of pyrolytic chars. It is known that the moisture in raw coal is mainly released in the early stage of coal pyrolysis [14]. When the heating rate is a medium to high heating rate, the volatiles in coal can also release along with the release of moisture. The temperature of char also increases rapidly and reaches a high temperature, in which the steam gasification reaction between the steam and char can take place [6,14]. Besides, the steam in the atmospheres can react with the active components in the gaseous phase, forming some free H radicals, which can induce the condensation of aromatic rings in char [6,30,31]. Therefore, during the pyrolysis process, the moisture in parent coals may participate in the following two processes. One is that the steam generated form moisture in the parent coal can react with the volatiles releasing from coals in the early stage, forming some free H radicals, which can induce the condensation of aromatic rings in char or secondary condensation polymerization between the small aromatic rings or aliphatic hydrocarbon in gas and the aromatic rings in char [6]. Another one is that the steam gasification reaction can take place between the steam and the pyrolytic chars. The small aromatic rings can be selectively consumed during the steam gasification reaction, resulting in a more condensed char [6,30,31]. Both processes can result in more condensed pyrolytic chars, but these two processes have an opposite influence on the char weight change.

Effects of Moisture in Parent Coal on the Aromatic Rings' Structure
high temperature, in which the steam gasification reaction between the steam and char can take place [6,14]. Besides, the steam in the atmospheres can react with the active components in the gaseous phase, forming some free H radicals, which can induce the condensation of aromatic rings in char [6,30,31]. Therefore, during the pyrolysis process, the moisture in parent coals may participate in the following two processes. One is that the steam generated form moisture in the parent coal can react with the volatiles releasing from coals in the early stage, forming some free H radicals, which can induce the condensation of aromatic rings in char or secondary condensation polymerization between the small aromatic rings or aliphatic hydrocarbon in gas and the aromatic rings in char [6]. Another one is that the steam gasification reaction can take place between the steam and the pyrolytic chars. The small aromatic rings can be selectively consumed during the steam gasification reaction, resulting in a more condensed char [6,30,31]. Both processes can result in more condensed pyrolytic chars, but these two processes have an opposite influence on the char weight change.  After accurate calculation and repeated experiments, the pyrolytic char yields of the raw coals and dried coals (in a dry basis) were obtained and are shown in Figure 6. It can be seen that the char yield of dried coals is 1% to 2% lower than that of raw coals. It means that the moisture in the parent coal results in char with greater mass. Therefore, the moisture in raw coal probably transforms into steam and then induces secondary polycondensation between the light component of volatiles released from the coal and the aromatic rings in pyrolytic char, resulting in an increase of the yield and aromatization degree of pyrolytic char. It also demonstrates that the effects of a high moisture content in parent coals on the chemical structures of pyrolytic char during coal pyrolysis cannot be ignored. After accurate calculation and repeated experiments, the pyrolytic char yields of the raw coals and dried coals (in a dry basis) were obtained and are shown in Figure 6. It can be seen that the char yield of dried coals is 1% to 2% lower than that of raw coals. It means that the moisture in the parent coal results in char with greater mass. Therefore, the moisture in raw coal probably transforms into steam and then induces secondary polycondensation between the light component of volatiles released from the coal and the aromatic rings in pyrolytic char, resulting in an increase of the yield and aromatization degree of pyrolytic char. It also demonstrates that the effects of a high moisture content in parent coals on the chemical structures of pyrolytic char during coal pyrolysis cannot be ignored.

Effects of Ash in Parent Coal on the Aromatic Rings' Structure
The correlation between A ad and A (GR+VL+VR) /A D of pyrolytic char was set up and is shown in Figure 7. be seen that the char yield of dried coals is 1% to 2% lower than that of raw coals. It means that the moisture in the parent coal results in char with greater mass. Therefore, the moisture in raw coal probably transforms into steam and then induces secondary polycondensation between the light component of volatiles released from the coal and the aromatic rings in pyrolytic char, resulting in an increase of the yield and aromatization degree of pyrolytic char. It also demonstrates that the effects of a high moisture content in parent coals on the chemical structures of pyrolytic char during coal pyrolysis cannot be ignored.

Effects of Ash in Parent Coal on the Aromatic Rings' Structure
The correlation between Aad and A(GR+VL+VR)/AD of pyrolytic char was set up and is shown in Figure 7. rings in pyrolytic char, resulting in an increase of the yield and aromatization degree of pyrolytic char. It also demonstrates that the effects of a high moisture content in parent coals on the chemical structures of pyrolytic char during coal pyrolysis cannot be ignored.

Effects of Ash in Parent Coal on the Aromatic Rings' Structure
The correlation between Aad and A(GR+VL+VR)/AD of pyrolytic char was set up and is shown in Figure 7. When analyzing the results for all chars, it can be seen that A (GR+VL+VR) /A D of the char generally increases with the increase of A ad , but the correlation is just partial and not strong. The parent coal properties of the chars were further classified. It was found that when dividing the chars into type M and N by the parent coal rank, the correlations between A ad and A (GR+VL+VR) /A D improved. The chars of type N were mainly from the coals LY, JP, NCP, XQ, JM, ZC, SC, MAS, XWJ, and XM, which all have a volatile content lower than 30%, and the chars of type M were mainly from the other coals with V daf > 30%. When using the quadratic equation to curve-fit the points, R 2 of the correlations for chars of type M was as high as 0.82. It indicates that except for the influence of V daf , there is also a reasonable correlation between the ash content in parent coals and the aromatization degree of their pyrolytic chars, revealing that the ash content in the parent coal can also affect the pyrolytic char structures. A coal with a greater ash content results in a less condensed char. Our previous study indicated that the inorganic components in these series coals are mainly silicium and aluminum, which mainly exist as aluminosilicate in raw coals [32]. During coal pyrolysis, the aluminosilicate, on the one hand, can reduce the heat transfer in the char and thus reduce the thermal deactivation of the pyrolytic char. On the other hand, it can encase the catalytic components, such as K and Na, in coal, inhibiting their catalysis during pyrolysis and reducing the condensation of pyrolytic chars [6]. Therefore, a coal with a higher ash content probably results in a less condensed char.

Effects of the Fixed Carbon Content in Parent Coal on the Aromatic Rings' Structure
It is known that the volatiles and moisture in raw coal are released during coal pyrolysis, forming char containing a fixed carbon and ash content. The fixed carbon content is one parameter directly reflecting the organic content in pyrolytic chars. Therefore, it is Processes 2021, 9, 1575 10 of 13 also very important to study the effects of fixed carbon on the pyrolytic char structures. The correlation between FC ad and A (GR+VL+VR) /A D was set up and is shown in Figure 8. a coal with a higher ash content probably results in a less condensed char.

Effects of the Fixed Carbon Content in Parent Coal on the Aromatic Rings' Structure
It is known that the volatiles and moisture in raw coal are released during coal pyrolysis, forming char containing a fixed carbon and ash content. The fixed carbon content is one parameter directly reflecting the organic content in pyrolytic chars. Therefore, it is also very important to study the effects of fixed carbon on the pyrolytic char structures. The correlation between FCad and A(GR+VL+VR)/AD was set up and is shown in Figure 8. It can be seen that A(GR+VL+VR)/AD generally decreases with the increase of FCad. It indicates that the aromatization degree of the chars increases with the increase of FCad in the parent coal, which means a coal with higher FCad results in a char with a higher aromatization degree. In order to evaluate the correlation between A(GR+VL+VR)/AD and FCad, a quadratic equation was used to curve-fit the points. The result is: It can be seen that A (GR+VL+VR) /A D generally decreases with the increase of FC ad . It indicates that the aromatization degree of the chars increases with the increase of FC ad in the parent coal, which means a coal with higher FC ad results in a char with a higher aromatization degree. In order to evaluate the correlation between A (GR+VL+VR) /A D and FC ad , a quadratic equation was used to curve-fit the points. The result is: R 2 is about 0.67. The results in our previous study showed that the volatiles content in raw coals relates well to the C-H structures and aromatization degree of raw coals and the fixed carbon in raw coals is more related to the ordered structures, such as large aromatic rings [2,23]. During pyrolysis, the volatiles in coal are nearly all released, and the relatively stable structure leaves and undergoes a subsequent series pyrolysis reaction, forming the pyrolytic char finally. So, the fixed carbon content in the parent coal relates to the aromatization degree of pyrolytic char better. The results also reveals that a coal with a higher fixed carbon content results in a more condensed pyrolytic char.

Relationship between the Comprehensive Coal Property Index and Pyrolytic Char Structures
From the above discussion, it is known that the coal properties are all related to the aromatic structure of pyrolytic chars, but the correlations between the coal property parameters and the aromatic rings structure parameter are not good enough. Multiple-factor analysis was further done to search for a comprehensive coal property index, combining them together to reflect the pyrolytic char structures better [33]. A (GR+VL+VR) /A D of the pyrolytic chars was set as the dependent variable, and the moisture, volatile, fixed carbon, and ash content were set as the independent variables. Multiple linear regression was applied. The results indicate that the comprehensive coal property parameter (−0.001*M ad +0.008*V ad +0.007*A ad ) is related to A (GR+VL+VR) /A D best, and R 2 is 0.83. The results further demonstrate that the moisture, volatile, and ash content in parent coal all influence the pyrolytic char structures, and using only one of them to reflect the pyrolytic char structures is limited. It needs to be pointed out that, as there is a quantitative correlation between the four coal property parameters: M ad , V ad , FC ad , and A ad , during the multiple-factor analysis, one of them would be eliminated in the comprehensive coal property index. In view of the fact that FC ad was obtained indirectly during proximate analysis, the comprehensive coal property index (−0.001*M ad +0.008*V ad +0.007*A ad ) without FC ad is shown and discussed above.
In order to optimize the results to obtain an easy-to-use index, the comprehensive coal property index was further simplified as (FC ad + M ad )/(V ad + A ad ). The correlation between (FC ad + M ad )/(V ad + A ad ) and A (GR+VL+VR) /A D was set up and is shown in Figure 9. It can be seen that A (GR+VL+VR) /A D nearly linearly decreases with the increase of (FC ad + M ad )/(V ad + A ad ). The correlation is: R 2 is about 0.89, indicating a high relationship between A (GR+VL+VR) /A D and (FC ad + M ad )/ (V ad + A ad ). In fact, in the above analysis, it is known that coal with a higher fixed carbon and moisture content will result in a pyrolytic char with a higher aromatization degree, while coal with a higher ash and volatile content will result in a pyrolytic char with a lower aromatization degree. The comprehensive coal property index (FC ad + M ad )/(V ad + A ad ) combines these four parameters together, and thus it relates well to the aromatization degree of the pyrolytic chars. The comprehensive coal property index (FC ad + M ad )/(V ad + A ad ) can act as a good indicator of the aromatization degree of pyrolytic char from coals with various ranks.
tween the four coal property parameters: Mad, Vad, FCad, and Aad, during the multiplefactor analysis, one of them would be eliminated in the comprehensive coal property index. In view of the fact that FCad was obtained indirectly during proximate analysis, the comprehensive coal property index (−0.001*Mad+0.008*Vad+0.007*Aad) without FCad is shown and discussed above. In order to optimize the results to obtain an easy-to-use index, the comprehensive coal property index was further simplified as (FCad+Mad)/(Vad+Aad). The correlation between (FCad+Mad)/(Vad+Aad) and A(GR+VL+VR)/AD was set up and is shown in Figure 9. It can be seen that A(GR+VL+VR)/AD nearly linearly decreases with the increase of (FCad+Mad)/(Vad+Aad). The correlation is: R 2 is about 0.89, indicating a high relationship between A(GR+VL+VR)/AD and (FCad+Mad)/(Vad+Aad). In fact, in the above analysis, it is known that coal with a higher fixed carbon and moisture content will result in a pyrolytic char with a higher aromatization degree, while coal with a higher ash and volatile content will result in a pyrolytic char with a lower aromatization degree. The comprehensive coal property index (FCad+Mad)/(Vad+Aad) combines these four parameters together, and thus it relates well to the aromatization degree of the pyrolytic chars. The comprehensive coal property index (FCad+Mad)/(Vad+Aad) can act as a good indicator of the aromatization degree of pyrolytic char from coals with various ranks.

Conclusions
The chemical structures of pyrolytic chars prepared from 32 kinds of Chinese coals were investigated by micro-Raman spectroscopy in this study. The effects of the parent coal properties on the pyrolytic char chemical structure were discussed in detail. Multiple-factor analysis was performed, and a novel comprehensive coal property index that reflects the pyrolytic char chemical structure was proposed. The following conclusions can be drawn: (1) The change of the chemical structure of pyrolytic chars with a coal rank is smaller than that of raw coals. For the pyrolytic chars from coals with various ranks, the alkyl C−H and aryl C−H structures have no significant diversity, and the aromatization degree is the key difference.
(2) The high moisture content in parent coal can induce condensation of pyrolytic char, but the ash components probably to inhibit the condensation of pyrolytic char.
(3) The correlation between the volatile, fixed carbon, moisture, and ash content and the aromatization degree of pyrolytic chars is limited. Not only the coal rank but also the fixed carbon, moisture, and ash components can influence the pyrolytic char chemical structures.
(4) The comprehensive coal property index (FC ad + M ad )/(V ad + A ad ) combining the coal properties together is relates well to the aromatization degree of pyrolytic chars and can act as a good indicator for the aromatization degree of pyrolytic chars from coals with various ranks.
(5) More factors that affect the chemical structure of the pyrolysis chars can be considered and stronger correlations can be further set up in future research. Besides, the influence mechanism of the detailed mineral components in the raw coal on the chemical structures of pyrolysis chars should be further clarified.