Study on the Formation Mechanism of Acetaldehyde during the Low-Temperature Oxidation of Coal

Abstract

The threshold dilution ratio of acetaldehyde is much larger than those of other odor compounds generated during the spontaneous combustion process and so it is the most important odorant. Studying the mechanism by which acetaldehyde is generated can provide the necessary theoretical support for acetaldehyde-based odor analysis. In the present work, the release of acetaldehyde was monitored while heating lignite, long-flame coal, and coking coal specimens under either air or nitrogen. The data show that acetaldehyde was primarily produced by the oxidation of active sites in the coal rather than by the pyrolysis of oxygen-containing functional groups. Based on quantum chemistry and coal–oxygen reaction theory, the transition state approach was used to further study the formation of acetaldehyde during the low-temperature oxidation of coal. Using density functional theory, three different coal molecule structures were modeled and optimized structures for acetaldehyde formation and the energies, bond lengths, and virtual frequencies of each reaction stagnation point were obtained at the B3LYP-D3/6-311G** and M062X-D3/Def2-TZVP levels. The results indicate that the low-temperature oxidation of coal to generate acetaldehyde involves the capture of H atoms from aliphatic side chains to generate peroxy radicals. These radicals then attack unsaturated C atoms through complex inversions to generate peroxides. In the third step of this process, the O–O single bonds in the peroxides break in response to thermal energy to form carbonyl groups. Finally, specific C–C or C–O bonds on the aliphatic side chains are thermally cleaved to generate acetaldehyde.

1. Introduction

The spontaneous combustion of coal is one of the primary issues associated with coal mine safety. This phenomenon is especially dangerous because it can involve long incubation periods, is difficult to detect, proceeds over long time spans, and can cause significant damage.¹ Spontaneous combustion destroys valuable coal resources while also producing large amounts of toxic gases and CO₂ that may affect the drive toward carbon neutrality.² The key to the prevention and control of coal spontaneous combustion is the accurate identification of gases generated in the early stage of the process. Thus, there has been much research concerning variations in the types and concentrations of gases released in the early stage of the low-temperature oxidation of coal. Presently, three main methods are commonly used to predict the spontaneous combustion of coal, based on monitoring gases, temperature, or odor.³ The odor analysis method has been gradually developed in recent years and is predicated on the capture and assessment of aldehydes, alkanes, and volatile aromatic compounds released during the low-temperature oxidation of coal in real-time.⁴ This technique can identify slight changes in substances released in the early stage of coal oxidation and therefore predict an increase in temperature to 50 °C and can also ascertain which substances are undergoing combustion. Compared with other prediction methods, this process shows exceptional performance and can recognize combustion at temperatures 30–40 °C below those detected by conventional gas analysis. This early warning capacity provides valuable time for the prevention and extinguishing of combustion such that efficiency is greatly improved.

Aldehydes are the main volatile odorants produced during the spontaneous combustion of coal via low-temperature oxidation and many studies have examined the formation of these compounds. Wang et al. found that the α-methylene groups in coal molecules are initially attacked by oxygen and readily decomposed by heating to generate reactive oxygen-containing functional groups such as aldehydes and carboxylic acids.⁵ Xu et al. reported that carbonyl and aldehyde groups are important reactive sites during the low-temperature oxidation of coal and that the concentrations of these groups gradually increase with the increase in coal temperature.⁶⁻⁸ On this basis, acetaldehyde generated by the spontaneous combustion of coal has been examined as a possible indicator gas. Zhao et al. demonstrated that the concentration of acetaldehyde produced by the low-temperature oxidation of coal is 2.5 times those of other aldehydes.⁹ Miao et al. studied the evolution of high-molecular-weight gaseous products during the low-temperature oxidation of coal and found that bituminous coal released 22 different volatile compounds, with acetaldehyde accounting for the largest proportion of all aldehydes.¹⁰ Wang et al. studied the release of acetaldehyde from different coal types with varying particle sizes and at different oxygen concentrations. Their work divided the spontaneous combustion of coal into slow, accelerated, and severe oxidation stages together with a vigorous oxidative combustion stage.¹¹ Although many studies have examined the release of acetaldehyde during coal oxidation and the factors affecting the amount of acetaldehyde released, the microscopic phenomena associated with this process have not been investigated. This has resulted in a lack of progress in the development of odor analysis methods.

In recent years, the rapid development of computer technology has enabled the exploration of the mechanism of coal spontaneous combustion from a microscopic perspective with the help of quantum chemical computing tools.¹²⁻¹⁷ Wang et al. examined a series of cyclic chain reactions involved in the low-temperature oxidation of coal and the reaction paths of various free radicals through quantum chemical calculation methods. Their work established that peroxy radicals are essential to this process and that hydrocarbon radicals obtained from aliphatic hydrocarbon side chains on coal molecules are attacked by oxygen molecules to produce peroxides that are subsequently decomposed by heat to generate new reactive radicals.¹⁸ Zhang et al. selected coal molecules with different free radical structures to study the oxidation reaction pathways of various free radicals. Their work determined that the free radicals generated by the dangling bonds of aromatic hydrocarbons and alkyl groups during the low-temperature oxidation of coal are most easily oxidized to form peroxy radicals.¹⁹ Chen et al. analyzed the oxidation products obtained from coal and found that, during the coal–oxygen recombination reactions, hydrogen radicals generated from hydroxyl groups initially combined with O atoms to generate HO₂ radicals that subsequently reacted to produce peroxide and water.²⁰⁻²² Wang et al. examined the main active sites during coal spontaneous combustion and devised a chain reaction model. This group also calculated the bond lengths and thermodynamic data for the structures involved in the combustion reactions to provide support for the analysis of the coal low-temperature oxidation mechanism.²³

As noted, acetaldehyde is the most critical odorant released during the low-temperature oxidation of coal. As such, the study of the acetaldehyde formation mechanism provides important theoretical support for the prediction of the spontaneous combustion of coal. The present work studied the release of acetaldehyde during the heating of lignite, long-flame coal, and coking coal under either air or nitrogen to determine the processes responsible for acetaldehyde generation during the low-temperature oxidation and pyrolysis of coal. Using quantum chemical calculations, models of coal molecules having −CH₂–CH₃, −CH₂–CH₂–ĊH₂, and −O–CH₂–CH₃ structures were established. Using these models, the low-temperature oxidation of coal was simulated at the B3LYP-D3/6-311G** and M062X-D3/Def2-TZVP levels. The optimized structures involved in the reactions associated with the low-temperature oxidation of coal molecules that generate acetaldehyde were investigated as a means of elucidating the acetaldehyde formation mechanism.

2. Experiment

2.1. Experimental Samples and Devices

Three coal samples with different degrees of metamorphism were employed in this work: Ximeng lignite (XM), Shendong long-flame coal (SD), and Xishan coking coal (XS). In each case, coal samples comprising large lumps were selected from the mining face then sealed and transported to the laboratory where they were crushed to obtain particles in the 0.25–1.70 mm size range. The approximate and ultimate analyses of the three coal samples are shown in Table 1.

Table 1. Proximate and Ultimate Analyses of Coal Used in Experiments^a.

	proximate analysis, wt %			ultimate analysis, wt %, daf
coal sample	Mad	Ad	Vdaf	C	H	O	S	N
XM coal	32.4	10.77	46.35	67.44	3.72	25.26	1.19	3.29
SD coal	9.28	7.62	27.81	75.92	4.36	17.14	0.96	1.49
XS coal	1.39	6.54	36.71	82.17	5.16	10.32	1.94	1.28

^aAd, air-drying base; d, dry base; daf, dry ash-free base.

According to the results of proximate and ultimate analysis, the moisture (M_ad), ash (A_d), volatile matter (V_adf), and O elemental content in coal showed a gradual decrease with increasing metamorphism from the XM coal to the XS coal, while the C and H elemental contents showed a gradual increase with the increasing metamorphism. This fully reflects that these three coal samples are very different, which in turn justifies the selection of these three coal samples with different degrees of metamorphism.

A specially designed temperature-programmed experimental device was used to simulate the low-temperature oxidation of coal, as shown in Figure 1. This system consisted of a gas supply cylinder, muffle furnace, gas chromatograph, 2,4-dinitrophenylhydrazine (DNPH) gas sampling tube, and other equipment. A flow chart summarizing the acetaldehyde detection apparatus and the process is presented in Figure 2. This system comprised an acetonitrile solution, volatile organic compound sampling tube, high-performance liquid chromatograph, and other equipment.

Figure 1.

Temperature-programmed experimental setup: ① air cylinder, ② nitrogen cylinder, ③ rubber hose, ④ flowmeter, ⑤ muffle furnace, ⑥ drying tube, ⑦ DNPH sampling tube, ⑧ gas chromatograph, ⑨ host, and display.

Figure 2.

Acetaldehyde detection flow chart: ① acetonitrile solution, ② grading cylinder, ③ plastic tip dropper, ④ DNPH sampling tube, ⑤ VOC sampling tube, ⑥ high-performance liquid chromatography detector, ⑦ host, and ⑧ display.

2.2. Experimental Process

In each trial, a 4 kg sample of sieved coal was transferred into the muffle furnace, which was connected to the gas stream, and the heating rate and gas flow were set to 1 °C/min and 1.2 L/min, respectively. Considering the early nature of acetaldehyde generation during the low-temperature oxidation of coal¹¹ and the purpose of the study is to make early prediction forecasts of coal spontaneous combustion, the experimental temperature range was set at 30–180 °C. When the temperature of the coal sample reached 30 °C, the concentration of oxygen in the muffle furnace was detected with gas chromatography. At the same time, the DNPH sampling tube was removed and the acetaldehyde collected in the tube was eluted with 5 mL of acetonitrile solution. The eluted phenylhydrazone derivatives were transferred onto the volatile organic compounds sampling tube and then analyzed by high-performance liquid chromatography to determine the concentration of acetaldehyde released from the coal body at 30 °C. Throughout these trials, the acetaldehyde concentration released from the coal body and the oxygen concentration in the muffle were determined at each 30 °C increase in temperature. Each experiment was carried out twice, injecting either nitrogen or air into the muffle furnace to study the effects of the atmosphere on the formation of acetaldehyde.

2.3. Experimental Results

Coal is an amorphous substance with a macromolecular structure containing many different active sites and oxygen-based functional groups. As the coal temperature is increased, acetaldehyde may be formed via two pathways. In pathway 1, carbonyl groups initially present in the coal undergo pyrolysis, while in pathway 2, active sites in the coal react with oxygen to generate peroxides or perhydrides. These species are subsequently pyrolyzed to generate a series of intermediate products and new functional groups with the eventual formation of acetaldehyde. These pathways are examined below.

2.3.1. Formation of Acetaldehyde during Coal Pyrolysis

The formation of acetaldehyde during coal pyrolysis was assessed by heating the coal in a nitrogen environment while monitoring the release of this compound at different temperatures from the three different metamorphic coal samples. Because these trials were performed under nitrogen, the acetaldehyde generated during the heating process can be considered to have been produced as a result of pyrolysis. Table 2 summarizes the data regarding acetaldehyde formation under nitrogen.

Table 2. Released Concentration of Acetaldehyde under a Nitrogen Environment.

	acetaldehyde release concentration (mg/m3)
coal sample	30 °C	60 °C	90 °C	120 °C	150 °C	180 °C
XM coal	0	2.72	0.32	0.13	0	0
SD coal	0	0.58	0	0	0	0
XS coal	0	0	0	0	0	0

As can be seen from Table 1, all three coal samples produced minimal acetaldehyde via pyrolysis. In particular, the XS coal (with a relatively high degree of metamorphism) generated no acetaldehyde over the temperature range of 30–180 °C. The acetaldehyde amounts released by the XM and SD coals gradually decreased as the coal temperature was increased. The acetaldehyde concentrations obtained from these specimens were 2.72 and 0.58 mg/m³ at 60 °C, but decreased to 0.13 and 0 mg/m³ at 120 °C, respectively. It can be concluded that, under nitrogen, the coal released only small amounts of acetaldehyde on heating and so the pyrolysis of coal generated minimal amounts of this compound.

2.3.2. Formation of Acetaldehyde during Coal Oxidation

Figure 3 summarizes the concentrations of acetaldehyde that were released along with the oxygen concentrations observed during the low-temperature oxidation of the three coal samples. Each of these materials evidently produced a large amount of acetaldehyde when heated under air, and increasing the temperature rapidly increased the acetaldehyde release from the XM and XS coal specimens. The acetaldehyde concentrations obtained from the XM coal at 90, 120, 150, and 180 °C were 194.23, 256.13, 250.01, and 240.21 mg/m³, representing increases by factors of 7.86, 10.36, 10.12, and 9.72 relative to the concentration at 60 °C, respectively. Compared with the value of 1.72 mg/m³ at 60 °C, the acetaldehyde releases from the XS coal were also increased by factors of 1.87, 4.12, 36.26, and 75.30. At 60 °C, the acetaldehyde concentration obtained from the SD coal was 8.76 mg/m³ but increased to 163.73 mg/m³ at 90 °C. The acetaldehyde concentration generated by this coal was maintained in the range of 163.73–231.04 mg/m³ with further increases in temperature.

Figure 3.

Release concentration of acetaldehyde and the proportion of oxygen concentration during the low-temperature oxidation of coal.

It can be seen from these data that the production of acetaldehyde exhibited a significant positive correlation with the consumption of oxygen. As an example, at 60 °C, the oxygen concentration measured in a trial with the XM coal was 19.88% and the acetaldehyde concentration was 24.71 mg/m³ while these values changed to 11.24% and 256.13 mg/m³ at 120 °C. The acetaldehyde release pattern of SD coal remains the same as that of XM coal, and the degree of metamorphism of XS coal is higher than that of XM and SD coal, which makes the acetaldehyde release pattern different from that of XM and SD coal, showing only two stages of a slow rise and a rapid rise. The variation pattern of oxygen concentration for SD and XS coals is consistent with that of XM coals, and both show a continuous decreasing trend.

Based on the above analysis, it is evident that acetaldehyde was primarily generated via the second pathway during the low-temperature oxidation of coal. Following contact between the coal and oxygen, oxygen molecules initially attacked active groups in the coal to form unstable intermediate products, such as peroxides or perhydrides. As the reactions progressed, unstable bonds in these intermediates were broken to form new reactive groups that transitioned to carbonyl groups. These sites were then degraded at high temperatures to produce acetaldehyde.

3. Quantum Chemical Calculations

3.1. Calculation Method

The structures of all reactants (R), intermediates (IM), and transition states (TS) were optimized at the B3LYP-D3/6-311G** level. The correctness of the transition states was ascertained by calculating the harmonic vibrational frequencies. The energies of all structures were calculated at the M062X-D3/Def2-TZVP level.

3.2. Coal Molecular Structure Model

According to previous studies,²⁴⁻²⁶ the quantity of aromatic rings in the molecular structure of coal has no effect on the formation of gaseous products. The coal–oxygen recombination reaction occurs between active groups and oxygen molecules and the main active sites in coal are located within aliphatic groups.²⁷ By reviewing the literature with similar research directions in this paper, it was found that Wang et al. established a coal molecule model with a −CH₂–CH₂–CH₂–ĊH₂ structure and studied various elementary reactions involved in the low-temperature oxidation process,¹⁸ while Shi et al. used a −O–CH₃ structure to examine the oxidation mechanism.²⁸ In addition, based on the coal–oxygen composite theory and the coal–oxygen reaction mechanism, it is conjectured that the generation of acetaldehyde gas is probably divided into two major steps. The first step is the attack of the aliphatic hydrocarbon side chain by oxygen to produce a carbonyl group. The second step is the breakage of the aliphatic hydrocarbon side chain by heat, which eventually produces acetaldehyde.

Based on previous studies and the structural characteristics of the acetaldehyde molecule, the calculation time and accuracy of the results were balanced using three structural models comprising −CH₂–CH₃, −CH₂–CH₂–ĊH₂, and −O–CH₂–CH₃ side chains (Table 3).²⁹ On this basis, the low-temperature oxidation mechanism and acetaldehyde generation pathways were theoretically studied.

Table 3. Three Kinds of Coal Molecular Structure Models^a.

^aNote: dark gray ball is a C atom, light gray ball is an H atom, red ball is an O atom.

3.3. Reaction Characteristics of Coal Molecular Structure

3.3.1. Frontier Orbit Analysis

According to molecular orbital theory, frontier orbitals and nearby molecular orbitals have the greatest effect on the reactivity of substances, and electrons in the frontier orbitals are the most active and the first to react.³⁰ The frontier orbitals can be divided into lowest occupied molecular orbitals (LUMOs) and highest occupied molecular orbitals (HOMOs). LUMOs preferentially accept electrons, and the associated atoms tend to participate in nucleophilic reactions. In contrast, HOMOs tend to supply electrons and the associated atoms are active sites for electrophilic reactions. During the low-temperature oxidative combustion of coal, oxygen acting as a nucleophile will first attack the sites having HOMOs with the highest electron densities. The LUMO and HOMO distributions determined for the three coal structures are shown in Figures 4–6, in which the green and red regions in these diagrams represent the negative and positive phases of the orbital wave functions, respectively.

Figure 4.

Frontier orbital distribution of the −CH₂–CH₃ coal molecular structure.

Figure 6.

Frontier orbital distribution of the −O–CH₂–CH₃ coal molecular structure.

Figure 5.

Frontier orbital distribution of the −CH₂–CH₂–ĊH₂ coal molecular structure.

The energy gap (ΔE_gap) is an important parameter determining the reactivity of the coal, and a smaller gap indicates a less stable structure with greater reactivity. The energy gaps were calculated according to eq 1 and the resulting activity parameters for the three coal structures are provided in Table 4.

1

Table 4. Coal Molecular Structure Activity Parameters.

category	ELUMO (eV)	EHOMO (eV)	ΔEgap (eV)
–CH2–CH3 structure	0.02284	–0.29967	0.32244
–CH2–CH2–ĊH2 structure	0.02348	–0.25799	0.28147
–O–CH2–CH3 structure	0.02012	–0.28179	0.30191

It can be seen from Table 3 that the energy gap values of the coal molecules of −CH₂–CH₃, −CH₂–CH₂–ĊH₂, and −O–CH₂–CH₃ structures are 0.32244, 0.28147, and 0.30191, respectively. The energy gaps of all three types of coal were determined to be small, indicating that these coal specimens would be expected to exhibit low stability and vigorous reactivity with oxygen.

3.3.2. Electrostatic Potential Analysis

Based on quantum chemical theory, the electrostatic potentials of the three coal models were calculated and projection diagrams are shown in Figure 7. Here, the red and blue areas represent regions where the electrostatic potential is negative and positive, respectively, with the electrons being at higher and lower energy levels, the number of electrons being larger and smaller, and the electron density being higher and lower, respectively.³¹ The regions around the H atoms are blue, while those near the O atoms are red, indicating that these sites would tend to undergo nucleophilic and electrophilic reactions, respectively.

Figure 7.

Projection diagram of the electrostatic potential of coal molecular structure.

In Figure 7, the methylene H atoms in the −CH₂–CH₃ and −O–CH₂–CH₃ structures are associated with the darkest color, while, in the −CH₂–CH₂–ĊH₂ model, the H atoms on the unsaturated C atom are darkest. These regions would therefore be more prone to nucleophilic attack by oxygen.

4. Results and Discussion

4.1. Carbonyl Generation Pathway

4.1.1. Carbonyl Generation with the −CH₂–CH₃ Structure

The optimized structures involved in each step of the low-temperature oxidation of coal molecules with the −CH₂–CH₃ structure to generate carbonyl groups are shown in Figure 8. The associated energy values, bond lengths, and virtual frequencies for each reaction stagnation point are summarized in Table 5.

Figure 8.

–CH₂–CH₃ structural model to generate carbonyl reaction pathway.

Table 5. Virtual Frequency Energy and Bond Length Data of Each Reaction Stagnation Point in the Oxidation Process of the −CH₂–CH₃ Structure.

structure name	virtual frequency (cm–1)	energy (Hartree)	atomic relationship	bond length (Å)
R	none	–461.0647	R(3,12)	1.509
			R(12,15)	1.533
TS1	–48.34	–461.0559	R(3,12)	1.509
			R(12,15)	1.530
IM1	none	–461.0377	R(3,12)	1.511
			R(12,15)	1.526
IM2	none	–461.0753	R(3,12)	1.514
			R(12,15)	1.520
TS2	–140.76	–461.0182	R(3,12)	1.516
			R(12,15)	1.518
IM3	none	–461.1236	R(3,12)	1.495
			R(12,15)	1.506

The TS has only one virtual frequency, while the IMs have no virtual frequencies, suggesting that these are the optimized structures for the low-temperature oxidation of coal to form carbonyl groups.

The present analysis of the coal–oxygen reaction mechanism and the quantum chemical calculation results (Figure 8) indicate that the O₂₀ atom in an oxygen molecule will initially attack an H₁₄ in a methylene group on an aliphatic side chain. As the oxygen molecule continues to approach H₁₄, the oxygen–oxygen double bond in the oxygen molecule is continuously elongated and gradually transitions to a single bond, and the C₁₂–H₁₄ single bond is broken and TS1 is formed. Following this, the oxygen molecule abstracts the H atom from the methylene group to generate a peroxy radical serving as IM1. This radical then undergoes a complex series of movements and there is a strong interaction between the unsaturated atoms O₁₉ and C₁₂ such that a carbon–oxygen single bond is formed between these two to generate a peroxide (IM2). As the reaction progresses and energy is accumulated, the oxygen–oxygen single bond in IM2 is broken to generate a free hydroxyl group and form TS2. In the TS2 structure, the O₁₉ atom connected to the C₁₂ atom does not have a full valence shell and the energy of the C₁₂–H₁₃ single bond is low such that this bond is readily cleaved. As the temperature continues to rise, the molecular structure of the coal is therefore rearranged and the single bond between C₁₂ and O₁₉ becomes a double bond, while a free hydroxyl group combines with the H₁₃ atom to form water molecules, thus generating IM3.

4.1.2. Carbonyl Generation with the −CH₂–CH₂–ĊH₂ Structure

The optimized structures involved in each step of the reaction path during the low-temperature oxidation of coal molecules with the −CH₂–CH₂–ĊH₂ structure to generate carbonyl groups are presented in Figure 9. The energies, bond lengths, and virtual frequency data for each reaction stagnation point are provided in Table 6.

Figure 9.

–CH₂–CH₂–ĊH₂ structural model to generate the carbonyl reaction pathway.

Table 6. Virtual Frequency Energy and Bond Length Data of Each Reaction Stagnation Point in the Oxidation Process of the −CH₂–CH₂–ĊH₂ Structure.

structure name	virtual frequency (cm–1)	energy (Hartree)	atomic relationship	bond length (Å)
R	none	–499.6885	R(3,12)	1.501
			R(12,15)	1.537
			R(15,18)	1.489
TS1	–111.33	–499.6593	R(3,12)	1.511
			R(12,15)	1.559
			R(15,18)	1.513
TS2	–26.53	–499.6505	R(3,12)	1.508
			R(12,15)	1.551
			R(15,18)	1.488
IM1	none	–499.7622	R(3,12)	1.510
			R(12,15)	1.561
			R(15,18)	1.513

It can be seen from Table 6 that there was only one virtual frequency for both TS1 and TS2 and no virtual frequencies for R and IM1. These results indicate that the optimized structures involved in each step of the reaction were correct.

It is apparent from Figure 9 that C₁₈ in R loses an electron after which it is preferentially attacked by an oxygen molecule. During this process, the O₂₁–O₂₂ bond length changes from 1.161 to 1.294 Å and the oxygen–oxygen double bond becomes a single bond. The O₂₂ in the coal connects with the unsaturated C atom C₁₈ to generate TS1 and form a peroxide. The oxygen–oxygen single bond in this structure is longer than normal and thus is weakened and will break. At the same time, the unsaturated O₂₁ attacks the H₂₀ atom on C₁₈, causing the C₁₈–H₂₀ bond to break while the free H₂₀ combines with O₂₁ to form a free hydroxyl group and produce TS2. As the temperature continues to increase, H₁₉ is separated from the γ C atom in the TS2 structure to form a free H atom, which in turn combines with a hydroxyl group to form a free water molecule. The breakage of the oxygen–oxygen single bond also causes the O₂₂ attached to the γ-carbon to become an unsaturated O atom. Finally, the carbon–oxygen single bond becomes a double bond, eventually forming IM1 and producing the carbonyl group.

4.1.3. Carbonyl Generation with the −O–CH₂–CH₃ Structure

The optimized structures involved in the low-temperature oxidation of coal molecules with −O–CH₂–CH₃ side chains to form carbonyl groups are shown in Figure 10, while the energies, bond lengths, and virtual frequencies are summarized in Table 7. No virtual frequencies were identified for IM1 and IM2 and only one virtual frequency for TS1 and TS2 (−54.31 and −246.04 cm^–1, respectively), thus ensuring the correctness of the reaction process.

Figure 10.

–O–CH₂–CH₃ structural model to generate carbonyl reaction pathway.

Table 7. Virtual Frequency Energy and Bond Length Data of Each Reaction Stagnation Point in the Oxidation Process of the −O–CH₂–CH₃ Structure.

structure name	virtual frequency (cm–1)	energy (Hartree)	atomic relationship	bond length (Å)
R	none	–536.2723	R(3,12)	1.357
			R(12,13)	1.426
			R(13,16)	1.512
TS1	–54.31	–536.2649	R(3,12)	1.359
			R(12,13)	1.422
			R(13,16)	1.514
IM1	none	–536.2573	R(3,12)	1.364
			R(12,13)	1.412
			R(13,16)	1.519
IM2	none	–536.2811	R(3,12)	1.371
			R(12,13)	1.406
			R(13,16)	1.527
TS2	–246.04	–536.2320	R(3,12)	1.369
			R(12,13)	1.406
			R(13,16)	1.522
IM3	none	–536.3617	R(3,12)	1.362
			R(12,13)	1.391
			R(13,16)	1.497

The electrostatic potential projection diagram indicates that this structure has the most reactive and least stable methylene groups on the aliphatic side chains. As such, these groups readily react with oxygen molecules. Therefore, during coal oxidation at low temperatures, oxygen molecules will preferentially attack the H atoms on the methylene sites. As O₂₁ gradually approaches H₁₄, the O₂₁–O₂₂ bond is elongated and transitions to a single bond, and the C₁₃–H₁₄ single bond is broken to generate TS1. The O₂₁ attaches to H₁₄ to generate a peroxy radical and form IM1. This radical subsequently undergoes a series of complex inversions and mutual interactions with unsaturated atoms, such that O₂₀ and C₁₃ form a single bond to produce IM2. Because the coal–oxygen recombination reaction is exothermic, the continuous accumulation of heat causes the O–O bond in the IM2 structure to rupture and so TS2 is generated. As a consequence of van der Waals forces, the C₁₃–O₂₀ bond in the TS2 structure changes to a double bond, while the C₁₃–H₁₅ bond is broken to form a free H₁₅ atom. This free H₁₅ combines with a free hydroxyl group to generate a free water molecule and IM3 is formed.

These quantum chemical calculations demonstrated that the oxidation of the aliphatic side chains involves three processes. First, an oxygen molecule attacks an H atom on the most active C atom on a side chain to generate a peroxy radical. Second, this peroxy radical undergoes a complex series of movements and combines with an unsaturated C atom to produce a peroxide. Third, the oxygen–oxygen single bond in the peroxide breaks, generating unsaturated O atoms and highly reactive hydroxyl groups. As the reaction continues, a C–H bond breaks and the C–O bond gradually changes from a single to a double bond to form a carbonyl group.

4.2. Acetaldehyde Generation Mechanism

Figure 11 presents a diagram summarizing the formation of carbonyl groups during the low-temperature oxidation of coal molecules having −CH₂–CH₃, −CH₂–CH₂–ĊH₂, and −O–CH₂–CH₃ structures. Table 4 indicates that the C₃–C₁₂ bond length in the −CH₂–CH₃ model is 1.495 Å, while the C₁₂–C₁₅ bond length is longer at 1.506 Å. With the progress of the reaction, the C₁₂–C₁₅ bond shown in Figure 11a ruptures to generate a free methyl group and an unsaturated C₁₂ atom. Therefore, acetaldehyde could not be generated during the low-temperature oxidation of coal molecules with a −CH₂–CH₃ structure. Compared with the C₃–C₁₂, C₁₂–C₁₅, and C₁₅–C₁₈ bond lengths in the R structure in Figure 9, the C₁₂–C₁₅ bond length in the −CH₂–CH₂–ĊH₂ structure underwent the greatest elongation, from 1.537 Å in the reactant structure to 1.561 Å in the carbonyl structure. With the progress of the reaction, the coal temperature gradually increased, the C₁₂–C₁₅ single bond shown in Figure 11b was broken, and the unsaturated C atoms were combined with free H atoms to generate acetaldehyde.

Figure 11.

Carbonyl structures generated during the low-temperature oxidation of coal molecules with three structures.

Table 6 indicates that the C₃–O₁₂ bond length in the −O–CH₂–CH₃ structure was 1.362 Å, while the O₁₂–C₁₃ and C₁₃–C₁₅ bond lengths were 1.391 and 1.497 Å, respectively. The carbon–oxygen single bond is asymmetric, while the carbon–carbon single bond is symmetric. The stability of the asymmetric chemical bond will be lower and so the carbon–oxygen single bond would be expected to be highly susceptible to breakage and the C₃–O₁₂ bond length in the resulting carbonyl structure would be less than the O₁₂–C₁₃ bond length. Therefore, during oxidative heating, the O₁₂–C₁₃ bond shown in Figure 11c will be broken first and the unsaturated C₁₃ will combine with free H atoms to form acetaldehyde.

5. Conclusions

The release of acetaldehyde during the heating of different coal types under either air or nitrogen and the associated mechanism were studied. The following conclusions were made.

• (1)When heated under nitrogen, all three coal samples released minimal acetaldehyde. In contrast, heating in air generated large amounts of acetaldehyde and the production of acetaldehyde was positively correlated with the consumption of oxygen. Thus, acetaldehyde released during the low-temperature oxidation of coal is not the result of pyrolysis but rather the product of reactions between coal and oxygen.
• (2)Modeling of −CH₂–CH₂–ĊH₂ and −O–CH₂–CH₃ structures as aliphatic side chains indicated that the most active methylene groups were not directly connected to the aromatic rings and that, during the low-temperature oxidation process, these sites were oxidized to carbonyl groups. The C–C and C–O bonds were determined to have low energy values and so were readily ruptured when heated so that −CH₂–Ċ=O moieties separated to produce acetaldehyde. However, in the case of −CH₂–CH₃ groups directly connected to aromatic rings, −CH₂–Ċ=O moieties could not separate to form acetaldehyde during low-temperature oxidation.
• (3)The formation of acetaldehyde during low-temperature oxidation comprises four steps. First, an oxygen molecule captures an H atom on the most active C of an aliphatic side chain to generate an −O–O–H free radical, after which this radical connects with the unsaturated C atom of the side chain through a complex process such as inversion to generate a very unstable peroxide. In the third step, the oxygen–oxygen single bond in the peroxide is broken and a carbonyl structure is formed on the aliphatic side chain.
• (4)By studying the mechanism of acetaldehyde generation during the low-temperature oxidation of coal, we found that acetaldehyde is mainly a product of the complex reaction of coal–oxygen, and the generation of acetaldehyde goes through four steps. This provides a theoretical basis for the future development of an odor sensor specifically for the detection of acetaldehyde gas. By combining odor sensors and the characteristics of the early generation of acetaldehyde during the spontaneous combustion of coal, we can realize the early prediction of the spontaneous combustion of coal in an accurate and smart way.

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (52074188).

The authors declare no competing financial interest.