Evolution Mechanism of Macromolecular Structure in Coal during Heat Treatment : Based on FTIR and XRD In Situ Analysis Techniques

School of Earth and Space Sciences, University of Science and Technology of China, CAS Key Laboratory of Crust-Mantle Materials and Environment, Hefei, Anhui 230026, China Research Center of Coal Resources Safe Mining and Clean Utilization, LiaoNing Technical University, Fuxin 125105, Liaoning, China Exploration Research Institute, Anhui Provincial Bureau of Coal Geology, Hefei, Anhui 23008, China


Introduction
e integrally structural arrangement of coal increases with proceeding coalification up to the formation of organic macromolecules of flat shape with carbon atoms in the central part [1][2][3].During coalification, the size of coal molecules increases as well as the degree of their ordering and macromolecules arrange themselves laterally or vertically to form a lamellar system [3].It has long been supposed that the aggregate structure of coal plays a vital role during coalification [4].erefore, it is very necessary to deeply understand the structural evolution mechanism of coal during pyrolysis [5].
In general, the pyrolysis process of coal can be divided into two main stages.One is the depolymerization or decomposition stage where gaseous (e.g., gas and water vapor) and liquid products (e.g., tar) were generated.e other is the condensation or repolymerization stage where the turbostratic lamellar system took place.Amounts of heterogeneous pyrolysis reactions can take place in the aforementioned two types of competitive processes.A better knowledge of coal structure alteration during pyrolysis could promote to comprehend the reaction process of coalification [6,7].ere have been a number of methods to provide new insights into the relationship between coal structure and its pyrolysis process.
In the past couple of decades, the ex situ XRD and FTIR analytical methods were used widely to supply important information on the changes of coal structure under heat treatment [5,8,9].Mae et al. [10] reported the relationship of the yield of pyrolysis products with coal structure.Wang et al. [11] studied the structure and pyrolysis characteristics of Chinese western coals, demonstrating the relationship between evolution of coal composition and its physicochemical structure.On the basis of X-ray, studies of coal reported by many researchers [12][13][14] show that the X-ray diffraction pattern of coal resembles a very blurred graphite pattern similar to that obtained with activated carbons and carbon black.Of note, Hirsch [15], Watanabe et al. [16], and Sonibare et al. [17] proposed that the XRD structural parameters (interlayer spacing of crystalline structure (d 002 ) and crystallite sizes (L a and L c )) can be used to evaluate the structure of coal and other carbonaceous materials with lower carbon crystallinity.ese parameters can be obtained using the Scherrer equation and fitting parameters.For the XRD study of coal structure under pyrolysis conditions, Zhang et al. [18] presented the correlation between coal element composition and coal structure and the evolution of XRD structural parameters of coal with the change of temperature.However, they did not discuss the changes of XRD structural parameters of coal with temperature but also did not study the changes of macromolecular functional groups of coal with temperature, especially oxygen-containing functional groups.Li et al. [19] characterized the structural parameters of coal at different pyrolysis temperatures using in situ XRD technology and obtained the relationship between the corresponding structural parameters and temperature.However, they did not use FTIR technology to analyze the change of functional groups of coal with temperature.
e FTIR technique focuses on determining composition on the physicochemical structures of coal.Kister et al. [20], Christy et al. [21], and Wang et al. [22] obtained that the C�O bonds were almost totally lacking in coal, which had isolated C�C bonds and C≡C bonds, and aliphatic groups CH, CH 2 , CH 3 , and aromatic ring groups were abundant.Wu et al. [23] used FTIR and TG analysis methods to study the chemical structure evolution and small-molecule gas yield of coal during pyrolysis.However, the FTIR they used is not in situ analysis technology, which is different from this study.
In conclusion, although ex situ analysis technology can also obtain information on coal structure and its reactivity, but because of the complexity of coal structure transformation during pyrolysis, this analysis method cannot characterize the instantaneous information of coal structure at a certain temperature [24].For that reason, in the present study, the in situ XRD and FTIR analytical techniques were employed at real temperature without cooling coal samples and to assist in quantitative analysis for expounding the structural evolution characteristics of coal during pyrolysis.
e purpose of this study was (1) probing deeply the structural evolution signatures of different ranked coals during pyrolysis using in situ XRD and FTIR analytical techniques; (2) a better understanding of the relationship between the evolution of coal structural parameters and pyrolysis temperature of coal; (3) the reference standard of structure parameters of coal with different ranks under realtime pyrolysis temperature was proposed.eir coal ranks are lignite for SX coal, subbituminous coal for KL coal, bituminous gas coal for HN coal, bituminous coking coal for HB coal, and anthracite for GZ coal in turn.

Experimental
Each coal sample weighed approximately 200 g.ese samples were pulverized and sieved to obtain particles of <56 μm in diameter and dried in a desiccator at 60 °C for 12 h.After that, they were demineralized to avoid the interference of mineral matter in coal before subjecting them to in situ XRD and FTIR experiments.Detailed removal process of minerals from coal and preparation of coal KBr pellets were described in previous studies [25].

Proximate and Ultimate Analyses.
e proximate and ultimate analyses of five coal samples before pyrolysis were executed according to ISO 625: 1975 (E) and ISO 333: 1983 (E), respectively.e results are presented in Table 1.

In Situ FTIR Spectroscopy Measurement.
For each set of experiments, after the reactor grasped approximately 20 g coal sample, the reactor was sealed and heated in a temperature-programmed furnace.e heat treatment experiment of coal was carried out under oxygen-free conditions.Five rounds of experiments were conducted at difference temperatures, including 100, 200, 300, 400, and 2 Journal of Spectroscopy 500 °C.e furnace temperature increased at 5 °C/min and was held for 24 hours isothermally after reaching a chosen temperature.Finally, the in situ FTIR spectrograms of coal were measured at each characteristic temperature.In this work, the infrared spectral bands selected were 2700 to 3000 cm −1 and 1300 to 1800 cm −1 .In the range of 2700 to 3000 cm −1 , two characteristic absorption peaks (2920 and 2860 cm −1 ) were obtained using fitting parameters.Overlapping peaks in 1300 to 1800 cm −1 [25-27] also were separated using the same method.Taking HB as an example, the specific fitting results are exhibited in Figure 2.
e changing features of the infrared spectral structural parameters with respect to different functional groups were previously summarized by José et al. [26].Nevertheless, the main concern is the influence of heat treatment on the structural parameters of coal in this study.Many spectral parameters based on FTIR have been used to illuminate the structural characteristics of coal [26,27].According to the selection of parameters reported in the previous studies, in this study, we focus on the intensity and area ratio of characteristic peaks.
e specific parameters and their corresponding functional groups are described as follows: (i) P 1 and P 2 represent the ratio of the integrated areas of aromatic hydrocarbon CH x stretching to aliphatic CH x stretching (ii) P 3 reveals the variation of oxygen-containing functional groups or the atomic ratio of oxygen to carbon (O/C) e in situ XRD data collection was performed by Philips X'Pert PRO X-ray powder di raction, using Ni-ltered Cu Kα radiation and a scintillation detector.e XRD pattern was recorded over a 2θ interval of 10-60 °, with a step size of 0.034 and 2 s/step counter time.e heat treatment experiment of coal was carried out under oxygen-free conditions.Five powered coal samples were heated from room temperature to 900 °C with the heating rate of 10 °C/min and were kept at constant temperature (25,100,200, 300, 400, 500, 600, 700, 800, and 900 °C) for 20 min.Meanwhile, the in situ XRD di ractograms were monitored at each characteristic temperature.Structural parameters (L a , L c , and d 002 ) were calculated according to Scherrer equations ( 1)-( 3), respectively:

4
Journal of Spectroscopy interlayer spacing: where λ refers to the wavelength of the radiation in the experiment (λ is 1.5406 Å in this study).B 100 and B 002 represent the width of (100) and (002) peaks at halfmaximum height, respectively.φ 100 and φ 002 represent the peak position of (100) and (002) peaks, respectively.e θ 002 refers to the Bragg angle of (002) peak.

Relationship between Coal Element Composition and Its
Structure.Some information about the molecular structures of coal can be obtained from an analysis for the elements carbon (C), hydrogen (H), sulfur (S), and nitrogen (N) [28].e atomic ratio of hydrogen to carbon (H/C) varies in di erent ranked coals.From Table 1, it is seen that lignite (SX), subbituminous coal (KL), and bituminous coals (HB and HN) have higher H/C ratios (from 1.03 to 0.70), whereas anthracite (GZ) has the lowest H/C ratio (0.41). e results show the H/C ratio decreases with the degree of coali cation.Furthermore, the total content of O + N + S in coal di ers appreciably.With the increase of coal rank, the O + N + S content in coal decreases signi cantly (Table 1), because the O, N, and S would be taken o during coali cation, nally forming a graphite-like structure which would have a lower atomic H/C ratio.According to Hirsch [29], in this study, SX and KL, and HN coals (<85% C daf ) have an "open" structure with lamellae rarely oriented and connected by cross-links, and HB coal (85% < C daf ≤ 91%) has a "liquid" structure with lamellae some oriented and many cross-links interrupted, and GZ coal (>91% C daf ) has an "anthracitic" structure with both lamellae and pores orientated.

FTIR Characteristics of Coal before Pyrolysis.
e infrared spectra of coal with the increase of coal rank are shown in Figure 3. Assignments of characteristic peak positions corresponding to di erent functional groups are also depicted in Figure 3 based on [26,[30][31][32][33].As shown in Figure 3, the regular change of infrared characteristic peak of coal was closely related to the increase of C daf content in coal.According to Van Krevelen [34], the absorbance of functional groups decreases considerably depending on the degree of coali cation.
e following conclusions can be drawn in the coal series lignite-subbituminous coal-bituminous coal-anthracite based on Figure 3: (1) e dashed line 2 between 2850 cm −1 and 3000 cm −1 reveals that, with the increase of coal rank, the absorbance of aliphatic CH 2 or CH 3 stretching vibration dwindles considerably, suggesting the decrease of hydrocarbon content in coal.e change in dashed 1 (aromatic hydrocarbon) was similar to that in dashed line 2, which indicates that hydrogen was gradually removed in higher-rank coal, leading to the decrease of intensity of aromatic ring stretching vibration in coal.
(2) In lignite (SX) and subbituminous coal (KL), the absorbance of aromatic carboxyl and carbonyl groups (dashed line 3) was obvious.However, it was absent in bituminous coal (KL and HN) and anthracite (GZ).According to Van Krevelen [34], it can be deduced that as coal rank increases, most of the carboxyl and carbonyl groups in coal can be gradually incorporated into the orthoquinone system structure.
(3) Because of the presence of polar substituents in aromatic rings [30,31], the absorbance of aromatic nucleus C═C groups (dashed line 4) starts to decrease from lignite (SX) to bituminous coal (HN) and then increases from bituminous coal (HB) to anthracite (GZ).(4) In the range of lignite (SX) to bituminous coal (HN), the absorbance of phenolic deformation C─O─C groups (dashed line 5) decreases obviously.Up to anthracite (GZ), it disappears progressively.(5) e absorbance of aromatic out-of-plane bending (dashed line 6) increases slightly between lignite (SX) and subbituminous coal (KL).Reaching the stage of bituminous coal (HB), it decreases moderately.Up to anthracite (GZ), it decreases to a less content.

In Situ FTIR Evolution Characteristics of Di erent
Rank Coals during Pyrolysis.As the pyrolysis temperature increases, in situ infrared spectra of ve coal samples are shown in Figure 4.In the range of 3800 to 3200 cm −1 , this absorption band is assigned to the −OH groups, which stands for the presence of phenols, ethers, and alcohols.According to Sonibare et al. [17], this absorption band can also re ect the vibration intensity of water molecules in coal.In comparison to low-ranked coal (Figures 4(a  It was attributed to the C-S stretching vibration based on the study by Angoni [13] and Sonibare et al. [17].Likewise, some peaks between 900 cm −1 and 700 cm −1 ascribed to the existence of low intensity aromatic (C-H) ar and its out-ofplane bending modes are also present in all coal samples.is result means that the existence of this characteristics band was independent on the increase of coal rank.
Especially, signi cant changes in the oxygen-containing functional groups are observed in the range of 1600 to 1800 cm −1 band (Figure 4).e curve-tting results (Figure 2) reveal the presence of three characteristics peaks at approximately 1724, 1669, and 1630 cm −1 , which were attributed to phenolic esters, carboxylic acids, and conjugate ketonic structures, respectively [35].Given that the importance of this band, we examine the changes in oxygencontaining functional groups from two perspectives.
(1) E ect of Temperature on Oxygen-Containing Functional Groups.For lignite-subbituminous coal-bituminous coalanthracite series, with the increase of temperature from 100 to 500 °C, the absorption at ∼1724 cm −1 shifts to ∼1694 cm −1 because the vibration intensity of aliphatic esters and phenol esters was reduced [34,35]; the band of carboxylic acids shifts from ∼1669 cm −1 to ∼1644 cm −1 , which re ects a progressive conversion of aliphatic carboxylic acids into aromatic carboxyls [35]; the slight excursion of conjugated structures was detected from ∼1630 to ∼1620 cm −1 .On the other hand, the intensity of the above characteristic band decreases slightly from 100 to 400 °C and drops abruptly at 500 °C.More generally, coal pyrolysis can be divided into a three-step process associated with di erent chemical reactions that take place in three temperature range intervals, <300, 300-600, and 600-800 °C.According to Arenillas et al. [30,31], the pyrolysis process of coal in the corresponding temperature ranges can be described as follows: (1) degassing; (2) depolymerization; (3) polycondensation.e full pyrolysis of coal usually takes place in "depolymerization."Based on this, it can be deemed that when the temperature was higher than 400 °C, the studied coal began to "depolymerize," leading to the decomposition of the relatively high molecular weight substances [30,31].
(2) E ect of Coal Rank at the Same Temperature on Oxygen-Containing Functional Groups.Figure 5 shows in situ infrared spectra of ve coal samples at the same temperature.As can be seen from Figure 5, under the same temperature conditions, in the range of lignite (SX) to bituminous coal (HN), the absorbance of carboxyl groups at ∼1700 cm −1 and phenolic ester groups at ∼1500 cm −1 decreases slightly.ese two characteristic bands between bituminous coal (HB) and anthracite (GZ) almost disappear.e result shows that as the coal rank increases, the intensity of decarboxylation and decarbonylation becomes deeper correspondingly [30][31][32].Interestingly, once the temperature reaches 500 °C, their absorbance can drop immediately, which indicates that the    8

Journal of Spectroscopy
Journal of Spectroscopy degree of coal pyrolysis is not related to the degree of coali cation and may be related to the bond energy of covalent bond of oxygen-containing groups in coal [33].

E ect of Temperature on Structural
Parameters. e calculated infrared spectral structural parameters are presented in Table 2.For raw coal, with the increase of coal rank, the structural parameters (P 1 , P 2 , P 3 , P 4 , and P 5 ) of coal increase accordingly, which means that high-ranked coal (GZ and HB) containing less H, less O, and more C becomes more aromatic than lower-ranked coal (HN, KL and SX).However, these parameters change in a di erent way after coal pyrolysis.
e detailed conclusions can be drawn as follows: (1) For these samples (GZ, HB, and HN), the contraction vibration intensity of the −CH 3 band (2920 cm −1 and 2860 cm −1 ) is very weak at di erent temperatures and the peak area of this band cannot be obtained so that the P 1 value cannot be calculated.For KL and SX samples, the P 1 value shows a decreasing trend with the increase of temperature at less than 500 °C.Above 500 °C, the P 1 value cannot be calculated.
is indicates that the absorption peaks of hydrogen on aliphatic hydrocarbon and naphthenic hydrocarbon groups appear at 2920 cm −1 and 2860 cm −1 , and the absorption intensity of these peaks decreases signi cantly with the deepening of the coali cation degree (after KL coal).For low-rank coals (KL and SX), the intensity of these peaks also decreased signi cantly when the temperature exceeded 400 °C; (2) with the increase of coal rank, the P 2 value of coal tends to increase to a certain extent.However, for the same coal sample, with increasing temperature, the P 2 value of coal gradually decreases.is indicates that temperature changes the molecular structure of coal and weakens the absorption of hydrogen on aliphatic and aromatic hydrocarbon groups; (3) with the increase of coal rank, P 3 value shows a certain increasing trend.For lowrank coal, there is a strong absorption peak at 1600 cm −1 , while for high-rank coal, the intensity of the absorption peak decreases.is absorption peak may be attributed to the overlap of hydrogen-bonded carbonyl group and aromatic ring C C double bond absorption, which gradually weakens with the deepening of coali cation.By analyzing the infrared spectrum of coal, it can be seen that the attenuation of 1600 cm −1 absorption peak is higher than that of 1700 cm −1 , so P3 value increases.Moreover, the reduction of P 3 value during coal pyrolysis shows that, with the increase of polymerization and decrease in the atomic O/C ratio, the amount of oxygen-containing functional groups in coal starts to decrease progressively; (4) P 4 and P 5 are both ratios re ecting the intensity of hydrogen absorption peaks on aliphatic and aromatic hydrocarbon groups.With the increase of coal rank, they all show di erent degrees of increase.For the same coal sample, as the temperature increases, they also gradually increase.
is re ects that temperature increases the vibration intensity of aromatic and aliphatic CH x groups, resulting in an increase in the number of some oxygen-containing functional groups in coal.Journal of Spectroscopy pattern of coal presents certain regularity with the increase of the degree of coalification. Figure 6(a) is an XRD pattern of five coal samples before pyrolysis using the Debye-Scherrer method [28], with coal rank increasing sequentially from top to bottom.It can be seen from the diffraction pattern that anthracite (GZ) has the uppermost diffraction peak (002) and the narrowest width (002).e diffraction width and height of bituminous coal (HB and HN) and subbituminous coal (KL) are similar, but both are lower and wider than those of anthracite.Compared with other coals, the diffraction height and width of lignite (SX) are the lowest and widest.e shape change of diffraction curves of five coal samples has certain regularity.With increasing coal rank, the diffraction height on the diffraction curves also increases correspondingly, while the diffraction width becomes narrow progressively, and the diffraction position shifts gradually to the high angle (2 sin θ/λ) region.Taking the 2θ angle values corresponding to (002) crystal mesh of five coal samples as an example, this change can be clearly observed (Table 3) that the position of (002) band shifts gradually to a high angle value (2 sin θ/λ) from 0.27 to 0.29.

XRD Investigation
As shown in Figure 6(a), the asymmetric phenomenon of the (002) band suggests the existence of another band (λ) at its left-hand side, which makes the left side of the (002) band broad and diffused [25].e diffraction height of the (λ) band at low angle becomes relatively higher due to strong scattering between low-angle particles.As coal rank increases, the height of the (002) band becomes higher but the height of the (λ) band becomes lower.
Taking the SX coal sample as an example, its XRD pattern was fitted using Gaussian function (Figure 6(b)).In conjunction with Scherrer equations ( 1)-( 3), the XRD structural parameters of five coal samples were calculated and are presented in Table 3.
Table 3 shows that the interlayer spacing of carbon hexagons (d 002 ) is 0.3567 nm in lignite (SX), 0.3571 nm in subbituminous coal (KL), 0.3561 nm in bituminous gas coal (HN), 0.3518 nm in bituminous coal (HB), and 0.3498 nm in anthracite (GZ).e results show with the increase of coal rank, the d 002 values decreases accordingly.Compared with the d 002 value (0.335 nm) of graphite, the d 002 values of five coal samples were significantly greater, which suggests a lower degree of the crystalline order in these coals.From Table 3, the crystallite height (L c ) value increases from 1.379 nm for lignite (SX) to 1.984 nm for anthracite (GZ), but the crystallite diameter (L a ) value shows the different trend relative to L c .In all coal samples, the L a remains

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Journal of Spectroscopy constant at nearly 2.979 nm. e same conclusion was also documented in the literature [36].It can be deemed that with increasing coal rank, the degree of polymerization between adjacent carbon crystals was restricted by molecular forces between side chains [36].Combining L a and L c changes, it can be speculated that the natural orientation under pressure and geothermal gradient can be direction-dependent during coali cation [8].

In Situ XRD Evolution Characteristics of Di erent Coals during Pyrolysis.
e in situ XRD patterns of ve coal samples during pyrolysis are shown in Figure 7.
As can be seen from Figure 7, the shape of the XRD pattern of ve coal samples did not change signi cantly before 500 °C.In comparison to coal before pyrolysis, the XRD patterns of all coals present a more sharp band (λ) at approximately 0.23 (2 sin θ/λ) in the range of 25 to 500 °C.When temperature is up to 900 °C, the intensity of the (λ) band decreases signi cantly.On the other hand, the position of the (002) band remains unchanged rstly between 25 °C and 500 °C, but above 500 °C, the shape of the (002) band becomes sharper and shifts to higher 2θ angle values.In addition, this band tends to be more asymmetric above 500 °C than at lower temperatures (Figures 7(c)-7(e)).

E ect of Temperature on Structural Parameters.
Taking the HB coal sample as example, Origin 8.0 software was used to t its XRD patterns in the 2θ range of 14-32 °to obtain (002) and (λ) bands and their structural parameters (d 002 , L a , and L c ). e in situ XRD curve tting results of the HB coal sample at di erent temperatures (300, 600, and 900 °C) are shown in Figure 8.
Table 3 shows in situ XRD structural parameters of ve coal samples at di erent temperatures.Figure 9 plots the correlation between temperature and the above structural parameters.Some conclusions were obtained as follows: (1) L a .Figure 9(a) shows that, with increasing temperature, the variation range of L a value is extremely small (approximately 0.09 nm), suggesting that the relationship between the L a value of coal and temperature becomes quite inappropriate.According to Hirsch [29] and Lu et al. [36], it can be deemed that, with increasing temperature, the bridge structure in coal becomes unstable as a reaction to increasing forces in the aromatic nucleus but this structure may di er in molecular weight and may contain several types of geometric and positional isomers.As a result, the change of L a for aromatic nucleus is complexity and heterogeneity.In this study, the variation of L a value is in accordance with the reported in the literature [8,36].

Journal of Spectroscopy
(2) L c . Figure 9(b) shows that, with increasing temperature, the change trend of L c value can be divided into two main stages.ere is a slight decline in the range of 25 to 500 °C.After 500 °C, the L c shows a degree of increase.is phenomenon is likely to be related to an evident change in the structure caused by the expulsion of hydrocarbons and other gases physically trapped in the inertinite network [30].
(3) d 002 .e d 002 that is considered to be a measure of the perfection in the two-dimensional turbostratic system is characteristic of the substances with two-dimensional periodicity [29].Figure 9(c) shows that, with increasing temperature, the change of d 002 values reflects two different changing processes.One stage is in the range of 25-500 °C.In this stage, the aggregate d 002 value increases obviously in lignite (SX) and bituminous coal (HN and HB), whereas it decreases slightly in anthracite (GZ).e other stage is between 500 °C and 900 °C, where the d 002 value decreases considerably.is is a good evidence to demonstrate positively such change dependence on the increase of temperature rather than coal rank.

Conclusions
e structural evolution signatures of different ranked coal during pyrolysis were deeply investigated using in situ XRD and FTIR analytical methods.e main conclusions are as follows: (1) the FTIR spectra analysis shows that when temperature reaches between 400 and 500 °C, coal structure occurs in abrupt change in the intensity of absorption peak; (2) the X-ray diffraction analysis illustrates that with the increase of temperature, two parameters (d 002 and L c ) change accordingly, but the value of L a remains unchanged; (3) combining the results of the two experiments, it can be seen that, in the temperature range of less than 500 °C, the size of aromatic lamellae of coal decreases, the distance between lamellae decreases, and the degree of bridge bond breaking, aliphatic side chain cracking, and oxygencontaining functional group cracking rises, suggesting that the degree of aromatization of coal increase.

2. 1 .
Samples and Preparation.A set of five different ranked coals having different geological ages was selected from five coal basins located on North China and South China coalbearing region (Figure 1).ese coal samples were denoted according to their location and geological age as: GZ (Guizhou province, Longtan formation in Late Permian epoch), HB (Hubei province, Chuyanken formation in Late Triassic epoch), HN (Anhui province, Upper Shihezi formation in Late Permian epoch), KL (Hebei province, Lower Shihezi formation in Early Permian epoch), and SX (Shanxi province, Taiyuan formation in Late Carboniferous epoch).

Figure 6 :
Figure 6: XRD pattern of ve coal samples before pyrolysis (a) and the curve tting result of the SX coal sample before pyrolysis as a function of Gaussian function (b).

Table 1 :
Results of proximate and ultimate analyses for five coal samples.
3.3.1.XRD Characteristics of Coal before Pyrolysis.More generally, coal is amorphous, but its aromatic structure tends to graphitize gradually during coali cation.us, the XRD

Table 3 :
Statistic results of the XRD parameters of five coal samples before and after pyrolysis.