Construction of macromolecular structure in KunNing coal and analysis of Macro-Micro oxidation characteristics

2.1 Coal sample preparation and proximate/ultimate analyses

The raw bituminous coal collected from KunNing Mine in JinZhong City, Shanxi Province was sealed and transported to the laboratory. It was then crushed and sieved below 200 mesh under nitrogen atmosphere to obtain the coal sample (named KN) for subsequent experiments. The KN coal sample was analyzed for proximate and ultimate analyses following Chinese National Standards GB/T 212–2008 and GB/T 31391–2015, using 5E-MAG6600B industrial analyzer manufactured by Jiaozuo Hua coal Mining Equipment Co., Ltd. in China and Vario EL elemental analyzer produced by Elementar Group in Germany, respectively. The results are shown in Table 1. Based on the elemental analysis results in Table 1, the atomic ratios referenced to carbon were calculated as shown in Table 2.

2.2 XPS analysis

X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher Scientific ESCALAB Xi + spectrometer with Al Ka radiation (hv = 1486.6 eV) as the excitation source under ultra-high vacuum around 8 × 10^-10 Pa. The signal was accumulated for 10 cycles. The passing energy was 100 eV for survey scan and 30 eV for narrow scan with a step size of 0.05 eV and dwell time of 40–50 ms. All binding energies were referenced to C 1 s at 284.80 eV.

2.3 ¹³C NMR analysis

A 400 MHz Bruker nuclear magnetic resonance spectrometer with a resolution of 4.0 mm was used for ¹³C NMR measurements, equipped with a double resonance MAS probe with a rotor spinning speed of 10 kHz. The pulse width was 4 μs with a pulse delay time of 1 s and contact time of 2 ms. The number of scans was 10000. Carbon atomic information was probed following SY/T 5777–1995.

2.4 FTIR analysis

Fourier transform infrared spectra were collected on a Thermo Fisher Scientific Nicolet iS10 spectrometer with a resolution of 0.5–1 cm⁻¹ over a scanning range of 4000–400 cm⁻¹ and 32 scans. The coal sample was mixed and ground thoroughly with KBr at 1:100 prior to measurement.

2.5 Enclosed coal oxidation experiments

The enclosed coal oxidation experimental setup was independently developed and designed by Professor Li Zongxiang from Liaoning Technical University (Li et al., 2017; Hu and Li, 2022). The schematic diagram of the experimental assembly is illustrated in Fig. 1, which includes: 1) computer, 2) gas pump, 3) gas concentration sensor, 4) data collector, 5) flow meter, 6) condensing tube, 7) collecting bottle, 8) drying tube, 9) preheating pipeline, 10) coal container, and 11) thermostatic oven.

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Fig. 1

Schematic of sealed oxygen consumption experimental device.

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The experimental procedure is as follows: 2.0 kg of crushed coal with a particle size of 0.4–2.4 mm was loaded into the coal container. Gas lines were disconnected 10 mins prior to the test for flushing to eliminate interferences. After reconnecting the setup and ensuring no gas leaks, the thermostatic oven was run to set temperatures of 25, 30, 35, 40, 45, 50, 55, 60, 65, 70 °C for 10 tests. O₂, CO and CO₂ concentration data were recorded by data acquisition software until the O₂ curve flattened.

2.6 Coal molecular modeling and ReaxFF pyrolysis simulation

Coal macromolecule construction. Based on elemental analysis, FTIR, XPS and ¹³C NMR results, the coal molecular structure was built in Material Studio and geometry optimized in the Dreiding force field using the Forcite module with Fine quality.

Construction of coal-oxygen mixed systems. The mixed systems containing coal molecules and O₂ were constructed in Amorphous Cell module under 0.5 g/m³ density (Feng et al., 2019; Zhang, 2022). NPT (constant pressure/temperature) ensemble was employed for density optimization with 30 frames. The lowest energy frame was taken and annealed 30 cycles from 300 to 1000 K under NVE (microcanonical ensemble) using the Anneal module in Forcite to get the optimized lowest energy model.

ReaxFF pyrolysis simulation. The constructed mixed systems were simulated using ReaxFF reactive force field in LAMMPS package. The dynamics calculations were performed under 3D periodic boundary conditions using force field parameters containing C/H/O/N/S/Na elements. The simulation was carried out in the NVE ensemble using the REAX/C computational module, with a time step of 0.2 fs and heating rate of 5 K/ps from 300 to 5000 K (Liu et al.,).

3.1 XPS analysis

X-ray photoelectron spectroscopy (XPS) can accurately determine the characteristics of organic functional groups in the 2–5 nm surface molecular layer of coal by measuring the binding energies of photoelectrons from different elements and valence states. The survey spectrum along with C 1 s, N 1 s, O 1 s, S 2p deconvoluted peaks of the KN coal sample are displayed in Fig. 2. The attribution summary of binding energies for quantitative analysis of functional group types and quantities is listed in Table 3.

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Fig. 2

XPS spectra of the KN coal sample (a - survey; b - C 1 s; c - N 1 s; d - O 1 s; e - S 2p).

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As shown in Fig. 2 and Table 1, the main components of the coal sample are C and O-containing functional groups and structures, while N and S-containing groups are relatively less. In the C 1 s binding energy region (Fig. 2b), there are 4 fitted peaks attributed to C–H/C–C, C-O, C = O and COO–, among which C–H/C–C and C-O are predominant with area percentages of 64.50 % and 26.64 %, respectively. C = O and COO– contents are lower, consistent with the O 1 s deconvolution results in Fig. 2d, where O atoms mainly exist as C-O, C = O and COO–, with C-O accounting for 74.90 %. As displayed in Fig. 2c, the N 1 s region contains 3 peaks, attributed to Pyrrole and Pyridine, with Pyrrole as the major form. Fig. 2e shows 2 peaks for S 2p, with peak 1 (61.48 %) belonging to thiophene organic sulfur structure and the other peak assigned to inorganic sulfates.

3.2 ¹³C NMR analysis

¹³C NMR spectroscopy can sensitively determine the carbon existence forms in coal molecules, thereby reflecting the approximate structure of coal molecular backbone. The ¹³C NMR spectrum is divided into four chemical shift regions based on the types of chemical functionalities: 0–60 ppm - aliphatic carbon, 60–90 ppm - ether oxygen, 90–165 ppm - aromatic carbon, and ∼ 200 ppm - carbonyl peak. Fitting of the processed data yielded 13 peaks in the ¹³C NMR spectrum of the KN coal sample, as displayed in Fig. 3, with detailed parameters listed in Table 4.

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Fig. 3

Experimental and fitted ¹³C NMR spectra of the coal sample.

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Protonated aromatic carbons account for 47.57 % of the total peak area, indicating the predominant basic structure of protonated aromatic carbons in KN coal molecular skeleton, while other carbon types (like bridgehead aromatic, branched aromatic, aromatic methyl, oxygen-substituted aromatic carbons, etc.) are present in smaller quantities.

According to chemical shift regions and analysis of Table 4, the structural parameters involving 12 carbon species in the molecular backbone were obtained as shown in Table 5 (Yao et al., 2003; Solum et al., 1989).

€The mean aromatic condensation degree X_BP was calculated using Eq. (1).

The obtained X_BP value for KN coal sample is 0.08, which can be used to estimate the number and condensation degree of benzene rings as well as the aromatic ring size.

3.3 FTIR analysis

FTIR spectroscopy is an important technique to determine the connectivity modes of aliphatic structures, oxygen-containing functionalities, aromatic structures and other cycloalkane structures in coal. Fig. 4 illustrates the fitted FTIR spectrum of the KN coal sample, which can be primarily categorized into 4 regions (Gao et al., 2018; Gao, 2021; Zhang, 2022): 900–700 cm⁻¹; 1800–1000 cm⁻¹, 3000–2800 cm⁻¹ and 3600–3000 cm⁻¹. The peaks from 900 − 700 cm⁻¹ and 1800–1000 cm⁻¹ correspond to various aromatic hydrocarbon structures, including 1,2-disubstituted, 1,2,4/1,2,3,4-trisubstituted aromatics and C–C vibrations in aromatics.

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Fig. 4

Fitted FTIR spectrum of the coal sample (a is the full spectrum curve).

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The vibration of carbonyl groups (C = O) in the 1800–1000 cm⁻¹ range likely indicates the presence of some carbonyl-containing compounds. The 3000–2800 cm⁻¹ region represents aliphatic CH and CH₂ vibrations, suggesting aliphatic compounds in coal. C-O vibrations in 1800–1000 cm⁻¹ imply coal may contain phenols, alcohols, ethers, esters and other functionalities. Further FTIR analysis parameters of the KN coal sample are listed in Table 6.

3.4 Construction of coal macromolecular models

Elucidating coal macromolecular structures is crucial for investigating the physicochemical properties of coal at the microscopic level, providing access to transition states, free radicals, bond energies and many other results hard to obtain from macroscopic experiments via quantum mechanical molecular simulations.

According to the elemental ratios in Table 2, assuming the number of C atoms in the coal molecule is x, the molecular formula of KN coal sample can be written as C_xH_0.5109xO_0.1001xN_0.0128xS_0.0109x. Relevant research shows the molecular weight of coal is around 2000–3000, and the atomic numbers should be integers. Hence, the molecular formula of KN coal was determined using Eq. (2) by taking x  = 149, giving C₁₄₉H₇₆O₁₅N₂S₂ and M = 2204.

The types and numbers of carbon atoms were calculated based on the ¹³C NMR results (Gao, 2021), including protonated aromatic (F_a^H), bridgehead aromatic (F_a^B), branched aromatic (F_a^S), oxygen-substituted aromatic (F_a^P), total aromatic (F_a^’), carbonyl (F_a^C) and aliphatic (F_Al) carbons, as listed in Table 7.

3.4.4

3.4.4 Coal macromolecular model

Based on the above analysis and Table 7, the molecular structure of KN coal is primarily composed of benzene and naphthalene units, with negligible anthracene/phenanthrene contents, as well as small amounts of pyrrole and thiophene structures. The aromatic unit types and numbers are listed in Table 9.

Table 9 Aromatic structural units in the coal sample.

Structure type	XBP	Number	Structure type	XBP	Number
	0	0		0	4
	0	2		0.25	5
	0	2

The coal molecular structure was constructed using MS software. The ¹³C NMR spectrum of this macromolecular model was calculated using MestReNova and compared with the experimental spectrum in Fig. 2. The molecular structure was adjusted to match the experimental NMR results as much as possible, as shown in Fig. 5. Geometry optimization was then performed in the Dreiding force field using the Forcite module in MS. The final optimized macromolecular structure model of KN coal is displayed in Fig. 6.

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Fig. 5

Comparison of experimental and model ¹³C NMR spectra.

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Fig. 6

Molecular structure model of KN coal.

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3.5 Coal oxidation characteristics analysis

3.5.1

3.5.1 Enclosed coal oxidation test results

The air leakage rate in the loose coal bed inside the enclosed coal oxidation test setup can be regarded as a constant. Thus, according to mass transfer principles, the oxygen consumption rate λ₁, and CO and CO₂ generation rates λ₂ and λ₃ at the same temperature under different oxygen volume fractions can be expressed as:

(3)

$${\lambda }_1=-\frac{d{f}_1(C_1(\tau ))}{d\tau }·\frac{1}{22400}$$

(4)

$${\lambda }_2=-\frac{d{f}_2(C_2(\tau ))}{d\tau }·\frac{1}{22400}$$

(5)

$${\lambda }_3=-\frac{d{f}_3(C_3(\tau ))}{d\tau }·\frac{1}{22400}$$ Where f₁(C₁(τ)), f₂(C₂(τ)), f₃(C₃(τ)) are functions corresponding to O₂, CO and CO₂ volume fractions in the enclosed coal oxidation tests, respectively; C₁(τ), C₂(τ), C₃(τ) represent O₂, CO and CO₂ volume fractions at time τ, %; λ₁ is the volumetric oxygen consumption rate, mol·(cm³·s)^-1; λ₂ is the CO volumetric release rate, mol·(cm³·s)^-1; λ₃ is the CO₂ volumetric release rate, mol·(cm³·s)^-1. τ is time, s.

The maximum temperature in the enclosed oxidation experiments is 70 °C. Hence, it can be assumed that oxygen is only converted to CO and CO₂ during the coal oxidation reactions inside the test setup, without considering other intermediates. The oxidation heat release intensity of coal can then be estimated using the bond energy balance method (Li et al., 2023; Chen et al., 2005; Chu et al., 2008):

(6)

$$Q=({\lambda }_1-{\lambda }_2-{\lambda }_3)\mathrm{\Delta }{h}_1+{\lambda }_2\mathrm{\Delta }{h}_2+{\lambda }_3\mathrm{\Delta }{h}_3$$

Where Q is the oxidation heat release intensity, J·(cm³·s)^-1; Δh₁ is the chemical adsorption heat of coal oxidation, 58.80 KJ·mol⁻¹; Δh₂ is the heat for CO formation, 110.54 KJ·mol⁻¹; Δh₃ is the heat for CO₂ formation, 393.51 KJ·mol⁻¹.

Combining Equations 3–6, the oxygen consumption rate, CO and CO₂ generation rates, and oxidation heat release intensity of the coal samples under 20 % oxygen volume fraction at different temperatures were calculated and plotted in Fig. 7.

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Fig. 7

Enclosed coal oxidation test results.

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Fig. 7a and 7b show the variation of the three gases with time during the enclosed oxidation test at 25 °C. Fig. 7c-f illustrate that the oxygen consumption rate, CO and CO₂ generation rates, and oxidation heat release intensity of the coal sample all increase exponentially with rising temperature. Compared to 25 °C, the oxygen consumption rate, oxidation heat release intensity at 70 °C are enhanced by about 800 % and 700 %, respectively.

3.5.2

3.5.2 ReaxFF pyrolysis simulation of coal macromolecules

The enclosed coal oxidation experiments in this study focused on investigating the oxidation characteristics below 70 °C. ReaxFF simulation of coal has significant advantages in studying the chemical reactions, intermolecular interactions, structural evolution and product formation during coal high temperature reactions and combustion from the atomic level. The constructed mixed systems containing coal molecules and O₂ with different oxygen amounts are shown in Fig. 8. Assuming that all atoms in the coal molecule react completely to form stable structures (CO₂, H₂O, N₂, SO₂) through pyrolysis (Zhang, 2022). In this case, the number of oxygen molecules that need to be added to the reaction system is approximately 900, and the oxygen content of this reaction system is defined as 100 %. Therefore, the oxygen contents in Fig. 8 are sequentially: 0 %, 25 %, 50 %, 75 %, 100 %, and 125 %.

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Fig. 8

KN coal reaction systems with different oxygen content.

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Fig. 9 illustrates the generation behaviors of indicator gases during the ReaxFF pyrolysis kinetics simulation of the KN coal sample, where Fig. 9a and 9b present the CO₂ and CO content changes along the reaction process. With increasing oxygen content in the reaction systems, the CO₂ and CO amounts also gradually rise. However, in the later reaction stage (250000 simulation steps), the CO content starts to decline while CO₂ increases, consistent with the trend exhibited in Fig. 7b. As displayed in Fig. 9c and 9d, the existence durations of C₂H₂ and C₂H₄ increase with higher oxygen content, and their content variation trends are similar.

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Fig. 9

Simulated generation patterns of indicator gases for KN coal sample.

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In the reaction system with 1000 oxygen molecules, the CO₂, CO, C₂H₂ and C₂H₄ gas molecules were labeled and traced back to determine the initial positions of atoms in coal macromolecules that generate these four indicator gases. This allows identification of structural units in the KN coal molecule responsible for the oxidative formation of CO₂, CO, C₂H₂ and C₂H₄, thereby inferring their potential major reaction pathways, as depicted in Fig. 10.

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Fig. 10

Positions and reaction pathways of indicator gas molecules generated from KN coal.

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The main generation pathways of CO₂ include:

(1) The C–H bond in the aldehyde is first broken to generate an aldehyde free radical, which then reacts with the oxygen molecule in the air to form an oxidized intermediate product, and finally CO₂ is generated through a series of reaction steps (including generation of ketone intermediates). (2) The carbon-hydrogen bond (C–H bond) in the phenol is broken to form a phenol free radical or ion. The phenol free radical reacts with the oxygen molecule in the air to generate an aldehyde and water. The generated aldehyde continues to oxidize and eventually forms carbon dioxide. (3) Carboxyl generation of CO₂ reaction process: the carboxyl group connected to the aromatic structure removes a hydrogen radical, and then detaches from the aromatic structure to form CO₂. (4) Methoxy (CH₃O•) reacts with oxygen to produce formaldehyde (CH₂O) and a hydrogen peroxide free radical (HO₂•). Formaldehyde (CH₂O) is further oxidized to CO₂ and H₂O.

The main generation pathways of CO include:

(1) The hydrolysis reaction of esters generates carboxyl groups, followed by decarboxylation of carboxylic acids to generate CO. (2) Phenols undergo oxidative decarboxylation reactions to generate CO. (3) Carbonyl compounds generate CO by breaking the carbon–oxygen double bond in the carbonyl. (4) Ethers directly thermally decompose to generate carbon-centered free radicals and CO.

The main generation pathways of C₂H₄ include:

(1) In structures connected to other structures, the cyclic alkane structure opens the ring and loses two connected carbon-containing structures that are not connected to other structures, which directly form a double bond to form a C₂H₄ molecule. (2) Alcohols undergo dehydration to generate carbocations. The carbocations are deprotonated to generate carbanions. Two carbanions are coupled to generate ethylene.

The main generation pathways of C₂H₂ include:

(1) In cyclic alkane structures connected to other structures, two connected carbon-containing structures that are not connected to other structures are released after ring opening, and then each carbon atom loses a hydrogen radical to form a carbon–carbon triple bond and form C₂H₂. (2) In the structure where the benzene ring and cyclic alkane are connected, the cyclic alkane is broken, and the carbon atom on the cyclic alkane connecting the ethyl group is released along with the ethyl group. After the released part loses a methyl group as a whole, the remaining part forms a carbon–carbon triple bond to form a C₂H₂ molecule.