Spontaneous Combustion Research Articles

The development of exothermic surface reaction between coal and oxygen affected by methane during coal spontaneous combustion in gob

Coal spontaneous combustion (CSC) derives from the exothermic coal-oxygen reaction occurring at coal's surface. However, methane's influence on this process is still unclear in gob environments. To this end, non-isothermal experiments were performed to elucidate methane's impact on the exothermic effect, pore structure evolvement, and surface composite variation in CSC. Our findings indicated that methane inhibited the exothermic effect; however, low methane (<42.9 %) did not significantly reduce the danger of CSC, but increased methane explosion risk. Methane's influence on pore structure can be elevated by a higher temperature, leading to larger pores, greater surface area, and higher pore volumes. X-ray photoelectron spectroscopic analysis revealed an increase in C1s, C-C and C-H bonds with rising methane and a decrease in O1s and oxygen-containing groups. C-O groups were the main structures affected by methane. O1s and oxygen-containing groups remained stable at low (<20 %) and high (>50 %) methane concentrations, because coal lay in oxygen-rich and fuel-rich oxidation states, respectively. Methane's influence on exothermic reaction originates from competitive adsorption. Methane competes with oxygen and other gases for adsorption sites. This changes the surface properties and affects the competitive adsorption between methane and other gases. This study can provide valuable insights for mitigating CSC risk.

Inhibitory Kinetics and Mechanism of the Low Temperature Oxidation of Coal by Polyethylene Glycol-Citric Acid

ABSTRACT To address the issues of low inhibition rate, high cost, and insufficient research on physicochemical synergistic inhibition mechanisms, this study examined the inhibitory effect and mechanism of Polyethylene Glycol-Citric Acid (PEG-CA) on low-temperature oxidation of coal. Differential scanning calorimetry (DSC) was used to analyze the inhibition and thermal behavior at various inhibitor ratios. Fourier transform infrared spectroscopy was used to experimentally assess changes in the functional groups of the coal samples pre- and post-inhibition. Principal component analysis identified the key mechanism of PEG-CA in inhibiting the low-temperature oxidation of coal. It showed that PEG-CA increased the heat absorption of the coal and delayed the average thermal equilibrium temperature of the coal oxidation process by more than 70°C. The addition of PEG-CA increased the maximum heat absorption rate of the coal sample by 2.4 times and the activation energy of coal oxidation by 1.6–3.9 times. The primary inhibition mechanism is that PEG-CA-forming chelates with the metal ions of coal, preventing C-H bond activation and reducing the content of this functional group. Additionally, PEG-CA etherifies with the hydroxyl groups of the coal, creating relatively stable ether bonds. This study offers a cost-effective and eco-friendly solution for the coal mining industry and provides a new theoretical foundation for developing composite corrosion inhibitors and controlling spontaneous coal combustion.

Free Radical Reactivity Characteristic in the Secondary Oxidation of Oxidized Coal

ABSTRACT To investigate the reactivity of free radicals during the secondary oxidation phase, the coal surface’s ability to adsorb oxygen and its free radical reactivity were researched. The study uncovered the free radical process behind the spontaneous combustion of oxidized coal. The results indicate that increased active adsorption sites form on the surface of oxidized coal, enhancing the absorption of oxygen. The higher the level of oxidation, the greater the capacity for oxygen adsorption. Coal 1, Coal 2, and Coal 3 represent lignite, gas coal and fat coal, in that order, with their level of metamorphism increasing sequentially. Low-rank Coal 1 contains numerous side chains in its molecular structure, with side chain covalent bonds breaking at a lower temperature of 80°C, leading to the rapid generation of free radicals. During primary oxidation, the free radical structure of low-rank Coal 1 transitions to a relatively stable state, requiring a higher temperature (120°C) for secondary oxidation to trigger chain reactions and transfers. The free radical reaction in oxidized coal is more vigorous compared to raw coal, with increased reactivity in secondary oxidation. The intensity of the free radical reaction in Coal 1–120°C, Coal 2–80°C, and Coal 3–80°C is higher during secondary oxidation.

Self-Healing fluorinated polymer deep eutectic electrolytes for stable lithium metal batteries

Lithium metal batteries (LMB) have attracted great attention in the last two decades for their high theoretical capacity and ultralow redox potential. However, LMB commercial applications still face safety risks such as electrolyte leakage, short circuits and even spontaneous combustion. Fluorinated polymer electrolytes have the advantages of high thermal stability and wide electrochemical window, endowing them an ideal alternative to liquid electrolytes. This work draws inspiration from the self-healing mechanism by introducing 2-ureido-4-pyrimidinone (UPy) units and successfully demonstrates that the fluorinated polymer deep eutectic electrolytes (FPDE) show high electrochemical performances in LMB. Self-healing structural units can alleviate the volumetric expansion of lithium anode during the charging and discharging process, while the fluorinated group endow the electrolyte wide electrochemical window, high migration number and superior interface stability. The joint effect of deep eutectic electrolytes and fluoroalkane makes FPDE show excellent flame-retardant performance with 0 s self-extinguish time. Symmetric Li||Li cell using the FPDE shows stable performance up to 6000 h. The full cell with Li||FPDE||LFP provides 80.4 % capacity retention after 600 cycles. This work might show some potential for development high safety LMBs.

Effects of alcohol fuels on SACI engine and analyze of prediction of SACI engine performance by artificial neural networks

The combustion characteristics of spark assistant compression ignition mode under different loads and fuels were explored, as well as by constructing artificial neural networks(ANN) model, the prediction ability of different algorithms for engine performance was discussed. Burning methanol and n-butanol can help to improve IMEP and induce earlier spontaneous combustion. The knock intensity(KI) of engine fueled with methanol was highest followed by n-butanol and gasoline, but maximum amplitude of filtered pressure oscillation(MAPO) shows the opposite trend. KI showed great positive liner correlation with ignition timing. Methanol showed the most outstanding tolerance on the compression ignition state. Fueled with methanol can decreased equivalent brake specific fuel consumption(ESFC) up to 52.9 % compared with the initial SI gasoline engine. N-butanol can improve BSNOx, BSTHC and BSCO concurrently, however fueled with methanol will worsen BSNOx. ANN model for engine combustion, economy and emissions performance was built. The prediction accuracy of Bayesian regularization algorithm for engine performance predicting was highest, but it have no advantage in the calculating times when the amount of data was large while the Levenberg-Marquardt algorithm would be the ideal efficient. The mean square error of ESFC and emissions parameters under scaled conjugate gradients algorithm always poorest.

Research on the Experiment Analysis of Spontaneous Combustion Characteristics of the15# Coal Seam in Fusheng Coal Mine

According to the spontaneous combustion characteristics of 15# coal seam in the fusheng mine, the programming temperature rising test with the coal sample from fusheng coal mine was conducted, which researched the concerntration and the corresponding pyrolysis termperature of index gas during programming temperature rising of the coal sample. The index gases included the CO, CO2, CH4, C2H4, C2H2 and so on.

Dynamic correlation between surface carbon response and underlying emissions from spontaneous combustion goaf: field study of an abandoned coal mine

Abandoned coal mine goaf is affected by air leakages and prone to spontaneous combustion, resulting in environmental pollution and geological disasters. Haizhou Open-pit Mine adopts both underground and open-pit mining methods. During the long-term mining process, the original stable stratum structure is constantly destroyed, and the slope slides, increasing cracks and severe air leakage around the goaf and roadway. The spontaneous combustion of coal is particularly prominent after the mine shut down. At present, there is no suitable indirect monitoring method to effectively explore the spontaneous combustion area in goaf. The study developed an all-weather monitoring plan and conducted multi-point continuous long-term measurements of the spontaneous combustion state in one abandoned coal mine goaf located in the eastern part of the Haizhou Open-pit Mine. We evaluated the dynamic correlation between surface CO2 flux (SCF) and changes in the underground fire areas, determined the scope and evolution trend of the fire areas, and identified the distribution and change laws of SCF. The results show a significant positive correlation between SCF and soil temperature; moreover, the SCF value was found to reflect the CO2 emission intensity of the goaf. The high SCF in the test area showed month-wise expansion and increase, while the CO2 emission gradually increased monthly, and the calculated annual total emission was approximately 7017 t. Hence, the study can further provide guidance for the monitoring of spontaneous combustion in shallow coal seams, goaf and the assessment of CO2 emissions from underground coal fires through the on-site monitoring and analysis results.

Solid Metal Chemical and Thermal Injury Management.

Solid metals may create a variety of injuries. White phosphorous (WP) is a metal that causes both caustic and thermal injuries. Because of its broad use in munitions and smoke screens during conflicts and wars, all military clinicians should be competent at WP injury identification and acute therapy, as well as long-term consequence recognition. English-language manuscripts addressing WP injuries were curated from PubMed and Medline from inception to January 31, 2024. Data regarding WP injury identification, management, and sequelae were abstracted to construct a Scale for the Assessment of Narrative Review Articles guideline-consistent narrative review. White phosphorous appears to be ubiquitous in military conflicts. White phosphorous creates a characteristic wound appearance accompanied by smoke, a garlic aroma, and spontaneous combustion on contact with air. Decontamination and burning prevention or cessation are key and may rely on aqueous irrigation and submersion or immersion in substances that prevent air contact. Topical cooling is a key aspect of preventing spontaneous ignition as well. Disposal of all contaminated clothing and gear is essential to prevent additional injury, especially to rescuers. Long-term sequelae relate to phosphorous absorption and may lead to death. Chronic or repeated exposure may induce jaw osteonecrosis. Tactical Combat Casualty Care recommendations do not currently address WP injury management. Education and management regarding WP acute injury and late sequelae is essential for acute battlefield and definitive facility care. Resource-replete and resource-limited settings may use related approaches for acute management and ignition prevention. Current burn wound management recommendations should incorporate specific WP management principles and actions for military clinicians at every level of skill and environment.

Adsorption simulation of gas released during oxidation of bituminous coal

ABSTRACT The adsorption of gases released during coal oxidation have a significant impact on the accuracy of CSC prediction. However, there are few research on the adsorption of C2H4 and C2H6 by coal. Therefore, the adsorption characteristics of N2/CO2/C2H4/C2H6 released during the spontaneous combustion of bituminous coal were investigated by programmed heating experiment and molecular simulation. The results showed that the low-volatile coal exhibited a higher adsorption capacity for CO2, N2, and C2H4 compared to high-volatile coal. The sequence of adsorption capacity for both low-volatile coal and High-volatile coal was CO2 > C2H6. The electrostatic potential of oxygen functional groups was ranked as follows: Ph-OH > Ph-COOH > R-COOH > R-OH. The interaction energy of gases at the -COOH position was greater than that at the -OH position, indicating that -COOH is more likely to provide adsorption sites for gases.

Investigating how oxygen levels and particle size impact the thermodynamics of low temperature coal oxidation: A case study using weakly caking coal

Oxygen concentration and particle size play critical roles in the combustion of coal. Notably, the low temperature oxidation stage is considered pivotal for coal spontaneous combustion (CSC). In this study, we examined the exothermic oxidation of low-temperature weakly caking coal. We used a C80 microcalorimeter to assess the impact of different oxygen concentrations and particle sizes. For weakly caking coal during low-temperature oxidation, the heat flow (HF) curve decreases as oxygen concentration drops. The characteristic temperature increases, and the HF curve shows a noticeable lag. Under identical external conditions, decreasing the particle size of coal results in a reduced minimum floating coal thickness, lowers the oxygen concentration needed for CSC, increases the upper-limit of air leakage intensity, and makes the coal more susceptible to CSC. Reducing the particle size significantly increases the exothermic heat release during low-temperature oxidation, leading to a higher risk of CSC. These research results not only deepen the understanding of the law of coal spontaneous combustion, but also provide a scientific basis for improving the technical level of coal spontaneous combustion fire prevention and control.

Preparation of fully physically crosslinked double-network gel and its fire prevention mechanism

It is important to gain an in-depth understanding of the structure of fire prevention and extinguishing materials during self-heating of coal. In this paper, an Al3+-CMC/PAM-MMT fire prevention and extinguishing gel with double-network was prepared using sodium carboxymethyl cellulose (CMC) and polyacrylamide (PAM) as raw materials, and aluminum citrate (AlCit) and sodium montmorillonite (Na-MMT) as auxiliary materials, by introducing the fully physically crosslinking effect. The optimal amounts of CMC, PAM, AlCit and MMT were determined to be 2 %, 3 %, 8 %, and 3 %, respectively, based on gelation time, stability, and resistance of the gel. The micromorphological image of the double-network gel exhibited a laminar structure of parallel-aligned fibrous networks, where the pores and channels exhibited more complex shapes and became smaller in size, highlighting the superiority of the double-network architecture. The rheological tests showed that the double-network gel had superior structural stability and viscosity under varying shear rates, making it more effective in covering the fire source, enhancing extinguishing efficiency. Additionally, its higher storage and loss moduli indicated stronger elasticity and mechanical properties compared to the single-network gel, emphasizing its advantages in energy storage and recovery. The characterization of active functional groups showed that there was an effective suppression of reactions involving hydroxyl, hydrocarbon, and carbonyl functional groups, during the spontaneous combustion process of coal, following treatment with the gel. Additionally, a proposed mechanism for the gel formation was presented. In the sealing performance test, the pressure-bearing capacity of the double-network gel was approximately twice that of the single-network gel. It indicated that the multi-level structure formed by the gel led to higher density of physical cross-linking points, which provided better sealing performance. Thermogravimetric analysis showed significant decrease in total calorific value and increase in activation energy of coal, after treatment with the gel, rendering it more difficult for spontaneous coal combustion. In the small-scale fire extinguishing performance test, the double network of Al3+-CMC/PAM-MMT gel demonstrated effective suppression of spontaneous combustion of coal and reduced the susceptibility to re-ignition. The fire prevention mechanism of the gel was further explored experimentally.

Improving the Wettability and Fire Retardancy of Three-Phase Foam by Simulated Sea Mud

ABSTRACT As coal mining depths deepen and mining intensity increases, coal spontaneous combustion has become the leading cause of coal mining accidents. Sea mud is an environmentally friendly natural material rich in inorganic salts that inhibit coal oxidation. Based on sea mud, a new method of suppressing coal spontaneous combustion has been proposed by exploring the essential characteristics of sea mud three-phase foam with different concentrations. The results demonstrated that the contact angle of the three-phase foam solution with 20 wt% of sea mud decreased by 55.4° compared with that of pure water, the cross-point temperature of coal (245.85°C) increased by 16.15°C, and the peak temperature (490.57°C) increased by 16.47°C, which proved that its wettability and thermal stability were both enhanced. In conclusion, the three-phase foam with 20 wt% sea mud has been demonstrated to exhibit a superior ability to prevent the spontaneous combustion of coal.

Mechanism study of replacing adsorbed O2 molecules in different pore sizes under different CO2/N2 inert gas conditions

Injecting inert gases into the goaf is a crucial method for inhibiting coal spontaneous combustion. Research has verified that the co-injection of CO2 and N2 provides a more effective control measure compared to a single gas, but the microscopic mechanism underlying the synergistic displacement of O2 by CO2 and N2 remains unclear. Injecting inert gases into the goaf is a crucial method for inhibiting coal spontaneous combustion. Research has verified that the co-injection of CO2 and N2 provides a more effective control measure compared to a single gas, but the microscopic mechanism underlying the synergistic displacement of O2 by CO2 and N2 remains unclear. Here, for the constructed multi-scale aperture slit model, the Grand Canonical Monte Carlo simulation (GCMC) and Molecular Dynamics (MD) simulation are used to investigate the mechanism of displacing adsorbed O2 molecules under different CO2/N2 inert gas conditions. The results indicate that CO2 and N2 aggregated on opposing sides of the coal molecules and dispersed within the slit layer, respectively. At 1 nm pore size, the relative concentration of O2 for optimal displacement was 1.054. The absolute value of the average binding energy is small when the CO2:N2 gas ratio is 80:20 and 40:60 at 2 nm and 4 nm apertures, respectively. This suggests that as the aperture size increases, the displacement effect on O2 shifts towards a higher proportion of N2, with a more conspicuous replacement effect as the aperture widens. The smaller pores ranging from 1-2 nm predominantly feature CO2 displacing adsorbed state O2, while larger pores exceeding 2 nm primarily N2 facilitate the transportation of free state O2.

Experimental investigation on the impact of water immersion time and particle size on the ignition characteristics of coal dust

After drying, water-immersed coal is more prone to spontaneous combustion than raw coal. However, the influence mechanisms of particle size and water immersion duration on the physicochemical properties of coal dust, as well as the explosion and fire hazards of water-immersed coal dust, have not been sufficiently investigated. In this study, two particle sizes of coal dust were selected, and experiments were conducted using the Godbert-Greenwald furnace, Hartmann tube, Muffle furnace, and Fourier Transform Infrared Spectrometer. The study investigated the impact of different water immersion times, dust particle sizes, and dust concentrations on the sensitivity of coal dust fires and explosions, analyzed the process of functional group changes in coal dust particles in water and other mechanisms of transformation. The results indicated that the water immersion process increased the number of active functional groups on the surface of coal dust, significantly reducing the Minimum Ignition Temperature (MIT) and Minimum Ignition Energy (MIE) values for different samples, and also enhancing the ignition sensitivity and peak temperature of coal dust layer fires. This study contributes to a deeper understanding of the characteristics of coal dust and is of great significance for improving the prevention and control of coal dust fires and explosions.

A novel phosphogypsum based self-produced gas expansion slurry for preventing coal spontaneous combustion

In order to solve the coal spontaneous combustion disaster in the upper layer goaf during the slicing mining process, a slurry that can expand by self-produced gas was developed using phosphogypsum and sodium bicarbonate as raw materials. The effect of different factors on the performance of phosphogypsum based self-produced gas expansion slurry (PSES) was analyzed. The optimal ratio and conditions of PSES were determined through orthogonal experiments. The thermal insulation performance and cooling and fire extinguishing performance of PSES were studied. Finally, it was applied in Luwa coal mine in Shandong province. The research results indicated that the optimal experimental conditions and ratios for PSES were phosphogypsum content of 56 wt%, sodium bicarbonate content of 21 wt%, water temperature of 25 ℃, and mixing time of 65 s. Under these conditions, the expansion performance of the slurry was good, and the setting time and compressive strength can meet the requirements for sealing the coal and rock cracks in the goaf. Under the same heat source, the thicker the slurry, the better its thermal insulation effect. Under the same thickness, the higher the heat source temperature, the shorter the effective insulation time(EIT). After the slurry was injected into the cracks of the broken coal, it wrapped around the broken coal from bottom to top. The temperatures of A-layer, B-layer, and C-layer of the coal pile decreased in sequence. After half an hour of grouting, the temperatures at all points dropped below 50 ℃ without any reignition phenomenon. After injecting PSES into the coal spontaneous combustion dangerous area in the upper layer goaf, the concentrations of CO and O2 in the monitoring borehole decreased to a safe range, and the generated CO2 concentration finally stabilized at around 4.6 %. It showed that PSES can seal the coal and rock cracks in the goaf, suppress the coal spontaneous combustion, and guarantee the lower layer working face can be mined.

Effect of humidity on moisture absorption of sodium percarbonate and its thermal hazard

Sodium percarbonate (SPC), also known as sodium carbonate–hydrogen peroxide, is a bleaching. The thermal decomposition of SPC produces oxygen, which slightly increases its bleaching ability. Nevertheless, the heat and oxygen produced pose a fire risk, potentially leading to spontaneous combustion when the SPC is stored alongside flammable materials. Previous studies have noted that water vapour affects SPC decomposition; However, the moisture absorption of SPC and the particular details of the effect have not been reported. In this study, the moisture absorption test revealed the deliquescence of SPC above 75% relative humidity (RH) and its hygroscopicity above 60% RH. Deliquescence occurred at 30 °C and 84% RH for 192 h, and the absorbed water caused its decomposition. Adiabatic calorimetry revealed only 30 wt% water, which was similar to that absorbed by SPC stored for 24 h at 30 °C and 97% RH, decreasing the thermal stability of SPC. These results indicate that the humidity surrounding the SPC is an extremely important parameter for estimating thermal hazards during the storage and manufacturing of SPC.

Study on microstructure evolution and oxidation kinetics in Coal-Oil Symbiosis

Differences in the spontaneous combustion mechanism characteristics of Coal-Oil Symbiosis (COS) significantly affect coal mines' safety management and ecological environment maintenance. Accordingly, this study aims to investigate COS's macroscopic and microstructural characteristics with different oil mass percentage using simultaneous thermal analysis, low-temperature N2 adsorption, scanning electron microscopy (SEM), and in-situ Fourier transform infrared spectroscopy (FTIR). The results showed that with the increase of oil mass percentage, the COS displayed the weakening of oxygen absorption and the advance of some characteristic temperatures, and 11.5 °C advanced the maximum weight loss temperature on average. For the 25 % oil sample, the ignition temperature was 9.5 °C lower than that of the raw coal. Additionally, the apparent activation energy of the high oil mass percentage sample was significantly reduced in the pyrolysis and combustion stages, and when the oil mass percentage was 25 %, the activation energies of the two stages decreased by 89 % and 60.65 %, respectively. Compared to raw coal, COS exhibits fewer macropores and surface pores covered by oil, which limits oxygen adsorption. Moreover, COS with higher oil mass percentage had an increase in hydroxyl and aliphatic hydrocarbon groups, and the CH3 + CH2 content of COS increased by 69.2 % on average, providing more active groups, thereby promoting spontaneous combustion. This study provides an important reference and theoretical support for further understanding the structural evolution and oxidation kinetic behavior of COS, contributing to disaster prevention and ecological environmental protection in coal-oil coexistence mining areas.

Effect of Sphingomonas polyaromaticivorans on spontaneous combustion of lignite during low-temperature oxidation

The inhibition of coal spontaneous combustion (CSC) by microorganisms presents an environmentally friendly method to reduce coal oxidation rates in mines and delay natural ignition. In this study, XRD, FTIR, and TG-DTG-DSC experiments were employed to analyze the inhibitory effect of microorganisms on CSC from both microstructural and macroscopic oxidation perspectives. The results indicated that microorganisms enhanced the ordering and graphitization of coal’s microcrystalline structure, reduced the number of functional groups, and disrupted aliphatic hydrocarbons and oxygen-containing groups. The average microcrystalline diameter and the number of effectively stacked aromatic layers decreased by 32.04 % and 27.22 %, respectively, while the total area of aliphatic hydrocarbon peaks decreased by 55.56 %. The characteristic temperature points shifted to higher zones, with the maximum weight loss rate temperature and burnout temperature exhibiting significant increases of 67.87℃ and 138.67℃, respectively. The thermal effect of coal oxidation was diminished, with total heat release decreasing by 77.8 J/mg. Additionally, the apparent activation energy (Ea) increased by 1.45 times, and the pre-exponential factor (ln A) reduced to 61.05 % of that of raw coal. This study establishes a foundation for future microbial inhibition of CSC.

Rheological Characterization of Inorganic Curing Foam for Preventing Spontaneous Combustion of Coal

ABSTRACT This study examines the rheological characteristics of inorganic cured foam (ICF). The apparent viscosity, yield stress, and modulus of elasticity of fresh cement-fly ash composite slurries with varying fly ash blending ratios at water/ash ratios of 0.4, 0.5, 0.6, and 0.7 were determined. The findings indicate that, under identical testing conditions, the apparent viscosity, yield stress, and modulus of elasticity of fresh cement-fly ash composite slurries (FCFS) gradually decrease with increasing water/ash ratio. The apparent viscosity and modulus of elasticity of inorganic curing foam (ICF) decrease as the volume expansion rate increases, while the linear viscoelastic region (LVER) broadens with higher volume expansion rates. Furthermore, the fitted apparent viscosity-shear stress curve for FCFS aligns with the power law model, whereas the apparent viscosity-shear stress curve for ICF slurries is better represented by the Carreau-Yasuda function model. The results of the study will contribute to the industrial application of inorganic curing foams for the prevention of spontaneous coal combustion.

Study on preparation and properties of steel slag based composite gel for mine fire prevention and extinguishing

A novel fire-preventing composite gel, mainly made from steel slag (SS) and silica fume (SF), was created for a coal mine’s spontaneous combustion. The gelation mechanism of the steel slag-based composite gel (SSG) was investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (SEM). The findings suggest that SSG is generated through the processes of hydration and geopolymerization involving SS and SF. And in the alkaline milieu of a 1.5 M water glass solution, SSG manifests enhanced strength and water retention capacities. Moreover, the fire prevention and extinguishing performance of the SSG was analyzed and verified using low-temperature oxidation, thermogravimetry, and low temperature nitrogen adsorption experiment (LTNA). The SSG has proven highly effective in suppressing spontaneous coal combustion by inhibiting CO production, raising the coal oxidation temperature, and reducing the contact area between oxygen and the coal body.