Multi-method analysis of the nonlinear influence of moisture content on the surface structure and oxidation characteristics of bituminous coal

2.1. SEM-EDS

A Quanta 450 Field Emission Scanning Electron Microscope (FEG-SEM) from FEI (USA) coupled with a Bruker XFlash 6|30 EDS spectrometer was used. The operational procedure included sample preparation and fixation, vacuum mode selection and adjustment, electron beam parameter adjustment, imaging condition optimization, target area selection, and EDS acquisition parameter setting and spectrum collection.

2.2. Synchronous thermal analysis experiment

A NETZSCH STA 449F5-QMS 403D synchronous thermal analyzer was used for synchronous thermal analysis. Each coal sample weighed 10 mg, with temperatures ranging from 30℃ to 800℃, a heating rate of 10℃·min^-1, and a gas flow rate of 50 mL·min^-1 (N₂:O₂ = 4:1).

2.3. Gas adsorption experiment

The oxygen adsorption characteristics of coal samples were studied using a Quantachrome Instruments H-Sorb 2600 gas adsorption analyzer. The experimental conditions were set at 25℃ and 0.1 MPa. Prior to the experiment, coal samples with different moisture contents were dried under a vacuum at 105℃ for 4 hours.

2.4. Closed-coal sample oxidation experiment

A self-developed closed-coal oxidation device was employed to study the oxygen consumption changes over time during low-temperature oxidation of bituminous coal [25-27], The relationship between the oxygen consumption rate and oxygen content of the coal samples was analyzed.

2.5. Molecular system pyrolysis simulation

Using MestReNova software, the Wiser coal molecular model was constructed and optimized in the Forcite module of Material Studio [30]. Reaction systems containing oxygen, coal molecules, and varying numbers of H₂O molecules were constructed. The AC module was used to optimize density, and the lowest energy system model was selected after 30 annealing optimizations in the Forcite module.

Molecular reaction kinetics calculations were performed using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software with the ReaxFF force field. The simulation conditions included an microcanonical ensemble (NVE) ensemble, Berendsen temperature control, a time step of 0.2 fs, a heating rate of 5 K/ps, a temperature range of 300–1000 K, and recording reaction trajectories every 200 K.

3.1. SEM-EDS test spectra

The SEM-EDS experimental results are shown in Figure 1. EDS spectral scanning revealed that the carbon content peaked highest at 92.57% with 25% moisture content and its lowest at 87.32% with 5% moisture content, possibly due to fluctuations in organic matter content and coal structure. The oxygen content was highest at 9.03% with 5% moisture content and lowest at 4.51% with 25% moisture content.

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Figure 1.

Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) test results of coal samples with different moisture contents.

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This variation is closely related to the presence of moisture, as higher moisture content may lead to restructuring of the coal surface, altering the exposure degree of surface elements and thus affecting the oxygen content detected by SEM-EDS. In addition, higher moisture content may dissolve more water-soluble oxygen-containing compounds in the coal, resulting in decreased oxygen content. Nitrogen content fluctuated irregularly with changes in moisture content, peaking at 15% (3.87%) and reaching its lowest at 10% and 25% (2.65%), which is related to the distribution of mineral matter and organic nitrogen compounds in the coal samples. The sulfur content was highest at 15% moisture content (1.16%) and lowest at 20% (0.13%), with its variation possibly related to the formation and decomposition of sulfides and sulfates in the coal samples. Numerous scholars’ research has shown that higher O element content in coal correlates with increased self-ignition propensity [31,32], indicating that the coal sample with 5% moisture content has the strongest self-ignition tendency.

The SEM analysis of three regions from each coal sample highlighted notable differences in pore structure and surface morphology, which directly influence their self-ignition properties. The coal sample with 1.3% moisture content exhibited a relatively rough surface and large pores in all three regions, with these pores being unevenly distributed and partially interconnected, facilitating oxygen entry and diffusion and increasing the risk of self-ignition. Compared with other samples, the 5% moisture content sample showed the roughest surface, with large and unevenly distributed pores and numerous interconnected pores, making it the most prone to self-ignition among all samples. In contrast to the 5% moisture content sample, the 10.0% moisture content sample had a smoother surface but medium-sized pores with uniform distribution and good interconnectivity between pores, also presenting a high self-ignition risk.

The 15.0% moisture content sample showed characteristics intermediate between other samples in the three regions, with a relatively rough surface and medium-sized pores uniformly distributed. Both the 25.0% and 20.0% moisture content samples exhibited relatively flat surfaces with smaller, densely packed pores in all three regions. In particular, the 25.0% moisture content sample had a complex and compact pore structure, greatly limiting oxygen diffusion and making it the most difficult to self-ignite. Although the 20.0% moisture content sample had dense pores, its surface was slightly flatter compared to the 25.0% moisture content sample.

3.2. Synchronous thermal analysis results

The purpose of synchronous thermal analysis experiments was to study the oxidation properties of coal samples in their moist state. Therefore, samples were not dried before the experiments. The results of synchronous thermal analysis experiments are shown in Figure 2.

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Figure 2.

(a-f) Results of synchronous thermal analysis experiments.

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Characteristic temperature points for coal samples with different moisture contents were determined according to the method in reference [33,34]: initial temperature (T₀), the endpoint of water loss and weight loss (T₁), maximum oxygen absorption weight gain temperature (T₂), ignition point temperature (T₃), maximum weight loss rate point temperature (T₄), and burnout temperature (T₅). The thermal parameters obtained from the DSC curves in Figure 2(c) are shown in Table 1. The ignition point temperature of the LS coal sample was lowest at 5% moisture content, about 9℃ lower than that of the raw coal (1.3% moisture content).

The peak values and peak areas in the DSC curves (Figure 2(c)) from the synchronous thermal analysis experiments of coal samples can be used to evaluate the thermal stability of coal samples and related thermodynamic information. Table 1 shows that in the water evaporation phase of LS coal samples, the peak temperatures were relatively stable, ranging from 79.3 to 83.3°C. This suggests that moisture in coal samples with varying water content primarily exists as free water and is minimally influenced by the overall moisture content. The endothermic amount increased slightly with increasing moisture content, but the change was not significant. In the oxidative exothermic stage, the peak temperatures did not differ greatly, around 380°C, but the oxidative exothermic amount was highest at 528.48 J·g^-1 for the coal sample with 5% moisture content.

Based on the TG curves in Figure 2(a), the reaction process of coal can be roughly divided into three: T₀-T₁ water loss and weight loss stage, T₁-T₂ oxidation weight gain stage (oxygen absorption weight gain stage), and T₂-T₅ combustion stage. According to the Arrhenius formula, the corresponding activation energies were calculated using the TG curve data for each stage: water loss activation energy, ignition apparent activation energy, and combustion activation energy.

Based on the references [35,36] and describing the coal oxidation process as a first-order reaction, the calculation formula for the activation energy of coal at each stage can be simplified as Eqs. (1) and (2):

Where β is the heating rate, ℃·min^-1; A is the pre-exponential factor, s^-1; E is the activation energy, KJ·mol^-1; R is the molar gas constant, 8.314 J·(mol·k)^-1; α is the conversion rate of coal oxidation at each stage (calculated by Eq. (2)), %; m₀ is the initial weight of the coal sample at each stage, mg; m_∞ is the final weight of the coal sample at each stage, mg; m is the weight of unreacted coal sample at t, mg.

Defining the left side of Eq. (1) as the dependent variable (Y-axis coordinate) and (R·T)^-1 as the independent variable (X-axis coordinate), the relationships between ln(G(a)·T^-2) and (R·T)^-1 for different stages of coal samples are shown in Figure 2(d), 2(e), and 2(f), obtained from the TG curves. By linear fitting of the experimental results in Figure 2, the reaction activation energies of coal samples at different stages were obtained, as shown in Table 2.

From the activation energy of the T₀-T₁ water loss and weight loss stage, when the moisture content is 5% and 10%, the water loss activation energy is lower than that of raw coal and other moisture-content coal samples. This indicates that when the coal sample is at 5–10% moisture content, the water in the coal is more easily evaporated. When the moisture content of coal is in this range of 5–10%, although the presence of water in coal occupies a certain number of adsorption sites, forming hydrogen bonds stronger than intermolecular interactions, it increases the porosity in coal during the heating and evaporation process, making it easier for adsorbed gas molecules to desorb and leave the coal body. When there is more water in the coal and water molecules occupy all pores, the hydrogen bonding between coal and water molecules, as well as between water molecules, along with intermolecular forces and cohesive forces, make it more difficult for water molecules to evaporate completely.

The activation energy of the T₁-T₂ oxidation weight gain stage, known as the ignition apparent activation energy [37], indicates the minimum energy required for chemical reactions between coal macromolecules and oxygen. By comparing this value, the self-ignition tendency of different coal samples can be assessed. As shown in Table 2, the ignition apparent activation energy of the LS coal sample at 5% moisture content is 88.80 KJ/mol, lower than that of raw coal at 89.50 KJ/mol, indicating a higher propensity for self-ignition in the 5% moisture content sample.

3.3. Gas adsorption experiment results

Gas adsorption experiments were conducted to examine how coal’s oxygen adsorption behavior varies with different moisture contents under standard temperature and pressure, simulating environmental conditions in goaf areas. Figure 3 illustrates the oxygen adsorption curves for six groups of LS raw coal and coal samples with moisture contents of 5%, 10%, 15%, 20%, and 25%.

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Figure 3.

Experimental curves of oxygen adsorption of coal samples with different moisture content.

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After drying, coal samples with varying moisture contents demonstrated different oxygen adsorption capacities under standard conditions. There was no clear positive or negative correlation between their adsorption capacities and moisture contents. The coal sample with 5% moisture content exhibited the fastest oxygen adsorption rate and the highest amount of adsorbed oxygen. With increasing moisture content, the rates and amounts of oxygen adsorption displayed varied trends compared with raw coal. Specifically, samples with 25% and 10% moisture content had similarly lower oxygen adsorption amounts compared to those with 15% and 20% moisture content. However, the oxygen adsorption rate for the 25% sample exceeded that of the 10%, 15%, and 20% samples but was still lower than that of raw coal and the 5% sample. This behavior is likely due to higher moisture content leading to more hydrogen bonds between water and coal functional groups (such as carbonyl and hydroxyl), which drying alone cannot completely remove, leaving residual moisture that decreases oxygen affinity and increases resistance to gas diffusion.

Combined with the SEM test results, when the moisture content is below the critical value, water has little effect on the coal morphology, neither significantly increasing the surface roughness and the size and number of internal pores, nor can it interact with coal to form more adsorption sites and oxygen-philic groups on the coal surface. When the moisture content is at the critical value and after water evaporation, the coal surface becomes roughest, with increased fissures and pores, providing more active sites for gas adsorption.

When the moisture content exceeds the critical value, the effect of water on coal adsorption performance is mainly negative. As moisture content increases, water forms hydrogen bonds with functional groups in coal, thereby reducing the affinity between coal and gas and lowering the adsorption amount and rate of coal. In addition, the presence of water hinders gas diffusion, limiting its adsorption rate. After exceeding the critical moisture content, a further increase in water has a gradually weakening effect on coal adsorption performance. At this point, the coal pores are basically occupied by water, and further increase in moisture content has little effect on porosity and specific surface area.

3.4. Closed-coal sample oxidation test

This study utilized a self-developed closed-coal oxidation test device to monitor the oxygen volume fraction in a closed circulation pipeline for Linsheng Mine coal samples with different moisture contents under standard temperature and pressure. The relationship between oxygen consumption rate and oxygen volume fraction was calculated based on these values. The test device, principles, and results are shown in Figure 4. As can be seen from Figure 4, the oxygen volume fraction decreases exponentially with increasing test time. Due to the increase in the relative content of other gases, competitive adsorption occurs among different gas molecules, causing active sites on the coal molecule surface to be occupied. Although coal can still react with oxygen in this state, the reaction rate is very slow, and this state is called the “asymptotic oxygen equilibrium” state. The calculated oxygen consumption rate of coal under standard temperature and pressure is positively correlated with the oxygen volume fraction, and there is a linear function relationship between the two.

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Figure 4.

Closed-coal oxidation test device and test results.

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The calculation principles of the test parameters are as follows [27]:

Assuming that the oxygen volume fraction C(τ) in the sealed container approximately follows a negative exponential function distribution, i.e.,

Where C₀ is the oxygen volume fraction value in the coal sample container before the closed-coal oxidation test begins; λ_c is the decay rate of the oxygen volume fraction in the coal sample container; C_b is the oxygen volume fraction value in the coal sample container when the test reaches the “asymptotic oxygen equilibrium” state; τ is the test time, min.

Taking the logarithm of Eq. (3) yields an equation suitable for linear regression, and regression analysis determines λ_c. Eq. (4) can be derived.

The rate of change in molar mass of oxygen consumption for the coal sample in the container:

Where $\gamma$ is the volumetric oxygen consumption rate of the coal sample in the closed-coal oxidation test, mol·(cm³·s)^-1.

Substitute Eq. (3) into Eq. (5) to obtain $\gamma$ for oxygen volume fraction C(τ):

Eq. (6) indicates that the volumetric oxygen consumption rate of the coal sample container is linearly proportional to the oxygen volume fraction. It suggests that for loosely packed coal bodies, the oxygen consumption rate directly relates to the oxygen volume fraction.

When examining the influence of moisture on closed-coal oxidation, the 5% moisture content sample consumed the most oxygen under standard conditions, resulting in the lowest oxygen volume fraction (approximately 9.2%) at equilibrium, with the highest oxygen consumption rate. Conversely, the 25% moisture content sample had the highest oxygen volume fraction (about 15%) at equilibrium, with the lowest oxygen consumption rate. The oxygen volume fractions for raw coal and the 10% moisture content sample at equilibrium were 10.7% and 10.1%, respectively, with minimal differences. However, the oxygen consumption rate for raw coal was initially higher than that for the 10% sample, but as oxidation continued, the rate for the 10% sample surpassed that of raw coal. This might be due to the decrease in moisture content and the increase in reactive sites over time for the 10% sample.

From the results of the closed-coal oxidation test, the effect of moisture on the oxygen consumption characteristics of coal samples under standard temperature and pressure is nonlinear. When oxidizing under standard temperature and pressure, coal samples with 5% moisture content may be in a range where moisture promotes the oxidation reaction rate. The presence of moisture may promote certain hydrolysis reactions or other water-related reactions that consume oxygen. At the same time, moisture may provide active sites during the reaction process, promoting the oxidation reaction. Therefore, coal samples with this moisture content may exhibit higher reaction rates and greater oxygen consumption.

Furthermore, different moisture contents may lead to the formation of different products in the oxidation reaction. At 5% moisture content, coal may form some highly reactive intermediate products during oxidation, leading to faster oxygen consumption. However, when the moisture content exceeds a certain threshold, the products formed may consume less oxygen, resulting in slower oxygen consumption. When the moisture content exceeds 5%, this moisture-induced oxygen consumption gradually surpasses the moisture-promoted reaction, leading to reduced oxygen consumption.

When the moisture content is very low, there may not be enough moisture to participate in the oxidation reaction effectively. Under low-temperature conditions, moisture may take a longer time to be released and participate in the reaction. Therefore, coal samples with low moisture content may require more time to form active sites, resulting in slower reaction rates and less oxygen consumption. Under low moisture content conditions, coal oxidation may choose some other reaction pathways, which are slower compared to the pathways at higher moisture content. This may lead to reduced reaction rates and decreased oxygen consumption.

3.5. Molecular system pyrolysis simulation

In simulation studies on the oxidation and adsorption characteristics of bituminous coal molecules, scholars often use the Wiser coal molecular model [38-41]. In this study, the bituminous coal molecular model and coal-oxygen reaction systems with different water molecule contents were used to perform pyrolysis simulations under the ReaxFF force field. The changes in the total number of molecules in the reaction system and the number of coal self-ignition indicator gas molecules (CO₂, CO, C₂H₂, C₂H₄) generated during the pyrolysis process were recorded as a function of calculation time [35,42]. The molecular modeling and simulation results are shown in Figure 5.

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Figure 5.

Modeling and pyrolysis simulation of coal-oxygen molecular reaction systems with different moisture contents. ReaxFF: Reactive force field

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The results in Figure 5 show that there is little difference in the total number of molecules during the pyrolysis process of reaction systems with different moisture contents. The changes in the number of coal self-ignition indicator gas molecules such as CO₂, CO, C₂H₂, and C₂H₄ recorded in the pyrolysis simulation results have the following characteristics:

1. When the water molecule content in the reaction system is different, there are certain differences in the number of C₂H₂ molecules generated during the coal sample pyrolysis process and their initial generation time. The coal-oxygen reaction system with 10% moisture content was the earliest to show C₂H₂ molecules during the pyrolysis calculation process, followed by the coal-oxygen reaction system with 5% moisture content, while the coal-oxygen reaction system with 20% moisture content generated C₂H₂ molecules the latest. In terms of the amount of C₂H₂ molecules generated, the coal-oxygen reaction system of raw coal had the highest peak content during pyrolysis, and the stable amount of C₂H₂ molecules was also the highest in the later stages of calculation.

2. In terms of the generation of C₂H₄ molecules, the coal-oxygen reaction system with 5% moisture content was the earliest to show ethylene (C₂H₄) molecules during the pyrolysis calculation process. Under this moisture content condition, the existence time of ethylene molecules in the reaction system was the longest, but the peak content of generated C₂H₄ molecules was relatively small (close to that of raw coal).

3. In the pyrolysis process of the coal-oxygen reaction system, the generation of CO molecules for the 5% moisture content sample occurred slightly later than for raw coal but earlier than for the 25% and 20% moisture content systems. The amount of CO gas produced during pyrolysis was roughly the same across different moisture content systems.

4. The coal-oxygen reaction system with raw coal moisture content generated CO₂ molecules slightly earlier than the other groups and the early generation amount of CO₂ molecules was relatively large. The final number of CO₂ molecules generated at the end of the reaction was also the highest.

The reasons for the earlier generation of ethylene and acetylene in coal samples with 5% and 10% moisture content lie in the balancing effect of moisture in the coal pyrolysis process, which both facilitates the cooling of the coal sample and provides suitable conditions for the decomposition of organic matter, causing the generation of volatile gases to occur earlier than under other moisture content conditions. Specifically:

1. Moderate moisture promotes coal pyrolysis: In the moisture content range of 5∼10%, the presence of water helps disperse and absorb heat from the coal sample. The cooling effect of moisture on the coal heating process inhibits rapid temperature rise, thereby slowing down the coal pyrolysis rate. However, moderate moisture does not excessively inhibit the pyrolysis of organic matter but provides appropriate temperature and medium conditions for the decomposition of organic substances.

2. Pyrolysis leads to the release of volatile gases: Coal pyrolysis refers to the decomposition of organic matter in coal into volatile gases and coke under high-temperature conditions. While heating with moderate moisture content, volatile organic matter in the coal sample is relatively easily released. These volatile organic compounds include ethylene and acetylene, which are important indicator gases in the coal self-ignition process.

3. Balanced influence of moisture on gas generation: With moderate moisture content, while water aids in cooling the coal, it also provides sufficient activity to promote the decomposition of organic matter. This balance results in a higher rate of gas generation under moderate moisture conditions. Ethylene and acetylene are generated during the high-temperature pyrolysis process of coal; hence, moderate moisture helps promote the earlier generation of these gases.

4. Synergistic effect of pyrolysis rate and gas generation: Moderate moisture content results in a moderate rate of organic matter pyrolysis in coal under high-temperature conditions while maintaining sufficient reaction activity. This synergistic effect causes coal to release gases such as ethylene and acetylene earlier under moderate moisture content conditions.

High moisture content delayed the generation of gases such as ethylene and acetylene, primarily due to moisture’s impact on coal pyrolysis and gas production processes. The main reasons include the following:

1. Moisture inhibits the coal pyrolysis process: When the moisture content of the coal sample is high, the heat absorption effect of water causes the temperature to rise slowly during the heating process. However, moisture simultaneously inhibits the pyrolysis of organic matter. Coal pyrolysis refers to the process of organic matter decomposing into volatile gases at high temperatures, but the presence of moisture may absorb some heat, causing the coal to reach the decomposition temperature of volatile organic matter more slowly.

2. Moisture reduces the coal sample’s temperature: The heat absorption effect leads to a slower temperature rise and a relatively lower final temperature of the coal. During heating, high moisture content coal samples experience slow temperature increases due to water evaporation and heat absorption, resulting in a relatively lower final temperature.

3. Inhibition of volatile gas release: Due to the inhibitory effect of moisture on the pyrolysis of organic matter, high moisture content may lead to a slower decomposition rate of volatile organic matter, slowing down the release of gases such as ethylene and acetylene. Under high moisture content conditions, volatile organic matter requires a higher temperature to be released, and the release rate is also relatively slow.