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Spontaneous combustion of coal gangue (CG) hills has caused varieties of secondary disasters that seriously endanger the ecological environment of the world. The emission law of index gases and their oxidation kinetics during the secondary oxidation process of CG with different ranks of oxidation were studied by using the temperature programmed device and online mass spectrometer (MS). Fourier transform infrared spectroscopy (FTIR) was used to reveal the changes of the CG internal active functional groups. The results showed that the energy required for the combustion of CG with low rank of pre-oxidation was significantly lower than that of the raw sample. However, as the oxidation rank increased, due to amounts of volatile components were released in the process of oxidation reaction, the CG in the combustion process of the emission of index gases and its oxygen consumption rate gradually reduced; the rapid oxidation stage shifted to the direction of the high temperature. In this study, the risk of spontaneous combustion of CG after oxidation at 80 ℃ under 3% oxygen concentration was the strongest. The results of this paper are of great guiding significance for exploring the spontaneous combustion characteristics of CG hills and their prevention by traditional covering method. [ABSTRACT FROM AUTHOR]

High-temperature coal oxidation generates a substantial amount of CO and CO2, especially CO, rapidly, which is a primary factor leading to casualties. The oxygen concentration is the predominant factor affecting the high-temperature oxidation of coal, particularly in the generation of CO and CO2. Understanding the generation patterns and relationship characteristics of CO and CO2 under various oxygen concentrations is important for predicting high-temperature coal oxidation and ensuring personnel safety. The paper used TG–MS and FTIR to study the formation characteristics of isothermal oxidation of CO and CO2 in long-flame coal under different oxygen concentrations. The results indicate that there exists an optimal threshold for CO production when the oxygen concentration is about 9% during the isothermal temperature oxidation process at 200.00–350.00 °C. When the oxygen concentration is less than 13%, the influence of oxygen on CO2 production is weakened. As the temperature increases, the pattern of CO + CO2 production and CO/CO2 is reversed, with CO + CO2 increasing and CO/CO2 decreasing. The relationship between the total production of CO and CO2 and the loss of coal mass follows the function y = a + bx at the isothermal temperatures of 300.00 °C and 350.00 °C. An increase in oxygen concentration does not alter the linear relationship. In the isothermal oxidation process of coal, carbonyl and carboxyl groups are the key groups for CO and CO2 production. At 200.00 °C and 250.00 °C, carbonyl and carboxyl groups increase, and other groups are being converted to carbonyl and carboxyl groups while decomposing into CO and CO2. At 300.00 °C and 350.00 °C, the relative content of carboxyl groups decreases. The research findings play a fundamental role in coal mine disaster prevention and control. [ABSTRACT FROM AUTHOR]

The evolution of the oxidation‐pyrolysis competitive mechanism during low‐rank coal combustion at lean‐oxygen conditions determines the development of the coalfield fires and the burning loss rate of the coal resource. In the present work, the mass increments of oxygen‐chemisorption were measured and calculated firstly. An abstract shrinking core model was built for distinguishing the oxidation and pyrolysis pathways. On this basis, the consumption rates of oxidation and pyrolysis were respectively calculated with the TGA data obtained under the lean‐oxygen gradient of 21–0 %. The results demonstrate that the oxidation path gradually delays in temperature while the pyrolysis exhibits great temperature stability as the oxygen concentration decreases. Besides, the significant transition of the competitive consumption mechanism exists below 9 % for the oxidation phase before ignition and 5 % for the combustion phase after ignition, respectively. Two temperature indicators of TC1 and TC2 for guiding the optimal fire extinguishing schemes based on the actual oxygen concentrations and temperatures are proposed. Furthermore, 1 % is identified as the limiting oxygen concentration for the development of the coalfield fires where the ignition mechanisms of both the HST and WD coal transform into the homogeneous ignition. The WD lignite, which always maintains the coke state as the burnout mechanism, holds higher reactivity but wider dispersion degrees of combustion compared with the HST sub‐bituminous coal. [ABSTRACT FROM AUTHOR]

The wetting properties of ALES (Ammonium lauryl ether sulfate), SLES (sodium lauryl ether sulfate), TD (dodecyl triethanolamine salfate) and SDS (sodium dodecyl sulfate) solutions and their adsorption capacities on coal dust surface were evaluated by surface tension, contact angle, sink time, wetting rate, and changes in the micromorphology and functional groups on coal surface. The results showed that the four surfactants share similar adhesion and spreading wettability, while they differ notably in immersion wetting. A water-surfactant-low rank coal (LRC) system was established using Materials Studio software and the Wender coal model; then quantum chemical calculations and molecular dynamics (MD) simulations were conducted. The results showed that ALES is of the widest relative concentration distribution range (25.25–60 Å), the largest overlap range (25 Å) and the largest diffusion coefficient (D = 0.318); N H 4 + can easily penetrate the surfactant layer, which proves the strong modification ability of ALES to LRC. ALES/LRC/H 2 O has the lowest interaction energy and the most H-bonds, indicating that ALES is of a strong adsorption capacity. Based on the experimental data and simulation results, the integrated wettabilities of the four surfactants follow ALES＞SLES＞TD＞SDS, partially contributed by the hydrolytic cations and EO groups. • For different wetting types, the ability of surfactants to wet coal dust is different. • Hydrolytic cations play an important role in improving the wetting ability of surfactant solutions. • ALES, SLES and TD are more easily adsorbed on the LRC surface than SDS. • The binding of TD, SDS, and Na + to H 2 O is stronger than that of ALES, SLES, NH 4 +, and TEA. • The comprehensive wetting ability is ranked as ALES＞SLES＞TD＞SDS. [ABSTRACT FROM AUTHOR]

In this paper, a novel type of composite inhibitor (PAA/SA-CC) was developed and characterized with the aim of achieving sustained and highly effective inhibition on spontaneous combustion of low-rank coal. A concept of realizing the caged-wrapping and sustained-releasing function of polymer as regard to high-activity chemical inhibitor was proposed, basing on which the composite inhibitor was prepared with poly(acrylic acid)/sodium alginate super absorbent and (+)-catechin. Subsequently, synchronous thermal analysis was carried out to investigate the influence of inhibitors on the tendency of coal to spontaneous combustion, which was characterized by four characteristic temperatures and three other characteristic parameters marked. Furthermore, hot-surface ignition tests were conducted to validate the inhibition on coal ignition behavior. Finally, non-isothermal isoconversional methods were performed by Starink model and Friedman-Reich-Levi model, respectively. The results demonstrate that PAA/SA-CC synergizes the inhibiting characteristics of the two distinctive inhibitors, especially exerts a continuously high-efficiency inhibition on the coal-oxygen chemisorption, which performs better than the case of mechanically blending. Besides, both the two kinetic analysis approaches agree on the results that the incorporation of 3 wt.% PAA/SA-CC shifts the apparent activation energy-temperature evolution to higher levels. [ABSTRACT FROM AUTHOR]

Abstract: The effects of various combinations of catechin and poly(ethylene glycol) (PEG) concentrations on the inhibition of coal oxidation were studied. The oxidation behaviors of coal samples both with and without the additives were examined, based on measurements of cross point temperature, oxygen consumption during low temperature oxidation and carbon monoxide emission rates. The results demonstrate that an additive consisting of a combination of both catechin and PEG 200 is capable of inhibiting the oxidation process. In situ monitoring of the surfaces of coal samples during oxidation also indicated that these additives suppress coal oxidation by accelerating the formation of ether bonds. Accordingly, a suppression mechanism is proposed. [Copyright &y& Elsevier]

Highlights: [•] Distribution of functional groups at coal surface has been analyzed. [•] Change concentrations of functional groups at coal surface have been analyzed. [•] Efficient experimental approach to selecting chemical inhibitors has been proposed. [Copyright &y& Elsevier]

Inert gases can effectively inhibit coal spontaneous combustion. In this paper, the inhibition effect of inert gases (N2 and CO2) on coal spontaneous combustion was studied. In the low-temperature oxidation stage, the constant-temperature heat release and apparent activation energy of coal sample were measured and calculated by the C80 micro-calorimeter. In the high-temperature combustion stage, the critical temperature, maximum peak temperature, ignition temperature, and burn-out temperature of coal samples were analyzed by the synchronous thermal analyzer. The results demonstrate that with the decrease of O2 concentration, the oxidation heat release of coal samples drops gradually while the apparent activation energy increases gradually. In the N2 and CO2 atmospheres, as the O2 concentration is reduced to 1.5% and 3%, respectively, the value of apparent activation energy changes from negative to positive, and the spontaneous reaction transits to a nonspontaneous reaction. The TG-DTG (thermogravimetric-derivative thermogravimetric) curve of coal sample in the high-temperature combustion stage indicates that the critical temperature exhibits a W-shaped trend with the decrease of O2 concentration, which also leads to gradual increases of maximum peak temperature, ignition temperature, and burn-out temperature. The above results signify that increasing the inert gas concentration can gradually reduce the oxidation and combustion rate and improve the inhibition effect on coal spontaneous combustion. In addition, when the O2 concentration is the same, the inhibition effect of CO2 on coal spontaneous combustion is superior to that of N2. [ABSTRACT FROM AUTHOR]

In this study, an anion-cation compound foaming agent (ACFA) was proposed based on the synergistic mechanism, so as to realize efficient production of low-concentration foam for preventing coal spontaneous combustion. The foaming and coal spontaneous combustion prevention properties of ACFA and anion compound foaming agent (AFA) were investigated by using Foamscan and the coal spontaneous combustion testing system, respectively. The results show that foaming speeds of the two agents both increase with the rise of concentration. The critical micelle concentrations of ACFA and AFA are 1‰ and 4‰, respectively. Compared with AFA, ACFA can reach the required foam volume faster. Besides, the foam produced by ACFA can stabilize 115% longer and maintain its effect longer, because ACFA contains more liquid than AFA at the critical micelle concentration. Moreover, the densely and uniformly distributed foam produced by ACFA can more effectively prevent oxygen from feeding the coal. Since coal samples treated with ACFA and AFA both boast high inhibition rates, they can serve as foam fire-extinguishing agents to prevent coal spontaneous combustion in underground mines, but ACFA is more efficient and economical. [ABSTRACT FROM AUTHOR]

The lean-oxygen combustion of incompletely pyrolyzed char dominated at the combustion front of coalfield fires. Functional group distributions and carbon micro-crystal dimensions were characterized for developing the molecular model (C 184 H 124 N 2 O 16 S) of pyrolyzed coal char at 400 °C (TLG P400). The overall combustion process of TLG P400 under various oxygen/fuel ratios (Ω) were simulated in an intuitive approach with ReaxFF. The findings indicated that the primary carbon-containing intermediates transformed from large pieces of aromatic fragments C 40+ with self-similarity to heavy components of C 15 –C 40 in initial combustion stage, then to light components of C 5 –C 14 in late stage. With Ω increasing from 1.0 to 5.0, the final carbon conversion was found to rise from 78.50% to 96.32%, while the CO/CO 2 ratio in the products sharply decreased from 2.575 to 0.213. The apparent activation energy was calculated as 194.49 kJ/mol for TLG P400 combustion at Ω = 1.0. Higher oxygen concentration enhanced the adsorption proportion of edge carbon, resulting in the generation of unstable intermediates featuring meta - or para -semiquinone. These transformations promoted the detachment of edge carbon and weakened the bottle-neck effect of the single-benzene ring on rapid oxidation. This study provides valuable insights into combustion mechanism and monitoring approaches for controlling coalfield fires. • A molecular model of coal char is developed based on measured structural components. • Product and structural evolutions of coal char burning are elucidated by ReaxFF MD. • Effect of oxygen-supply on reburning reactivity of pyrolyzed coal char is clarified. • Single-ring residues contribute to the delayed lean-oxygen combustion of coal char. [ABSTRACT FROM AUTHOR]

• Pore evolution during lean-oxygen reburning of pyrolyzed semicoke was clarified. • Oxygen diffusion mechanism transforms from Knudsen to transition diffusion at 650 °C. • Warning temperature for accelerating spread of coal fire was determined as 650 °C. The lean-oxygen combustion of incompletely pyrolyzed char dominates at the combustion front of coalfield fires. Using non-local-density functional theory (NLDFT) and Barrette-Joynere-Halenda (BJH) analysis based on N 2 and CO 2 composited adsorption isotherms, the pore (0.4–300 nm) structure characteristics of pyrolyzed char during lean-oxygen combustion were obtained. The results indicate that the main pore morphology changes from irregular ink-bottle shape of raw coal to cylindrical shape of pyrolyzed char, then to slit-shaped and wedge-shaped pores under the effect of oxidative expansion, and finally coalesce into amorphous macropores during combustion. For the weak-caking coal, softened colloids diffuse and block the mesopores, leading to the reverse development of mesopores into smaller-sized pores. The pore development changes from the generation of secondary micropores in pyrolysis stage to the progressive expansion mode of mesopores in initial combustion stage, then to the free coalescence mode of macropores in late combustion stage. The dominant internal oxygen diffusion mechanism transforms from Knudsen diffusion to transition diffusion at 650 °C, and the carbon-oxygen reaction interfaces shift from the inner surface of deep micropores to the shallow mesopores and macropores. Finally, 650 °C is determined as the early-warning temperature for the accelerating spread of coalfield fires. [ABSTRACT FROM AUTHOR]

• Leap-migration oxygen concentration of low-rank coal combustion is 5 %–3 %. • The change of oxygen concentration (5 %–1 %) is more sensitive to coal rank. • Below 3 % oxygen concentration, the E α begins to decrease continuously. • Oxidation consumption reaction is completely limited at 1 % oxygen concentration. • The critical oxygen concentration for low-rank coal is 3 %. Effectively preventing the re-ignition of fire zones and safely unsealing enclosed fire zones are challenging. To address these issues, a thermogravimetric analysis technique was used to explore the leap-migration mechanism of the characteristic parameters (burnout index (H), comprehensive combustion performance parameters (C b , S , and H F), and maximum combustion mass loss rate ( dW max ))/temperatures (maximum pyrolysis mass loss rate point temperature (T Vmax ), pyrolysis endpoint temperature (T d), thermal polycondensation endpoint temperature (T p), maximum combustion mass loss rate point temperature (T Wmax ), burnout temperature (T h), and combustion half-peak width (Δ T 1 / 2 )) of low-rank coal oxygen-lean combustion under different time-scale effects (TSEs). The sensitivity of characteristic parameters/temperatures to changes in oxygen concentration under coal rank transition was analyzed. Simultaneously, the kinetic real-time transformation mechanism during the oxygen-lean combustion stage progression was investigated. The results revealed that because of the competition between oxidation and pyrolysis, the leap-migration interval of characteristic parameters/temperatures for oxygen-lean combustion of low-rank coals is in the range of 5 %–3% oxygen concentration. This feature was less affected by the coal rank but is not affected by the TSE. In the low oxygen-lean range (5 %-1%), a change of oxygen concentration was more sensitive to the coal rank, and the high sensitivity of the oxygen concentration change reduced the influence of the coal rank on coal combustion performance. In addition, the apparent activation energy (E α) curves during the early stage of combustion were relatively concentrated with the change of oxygen concentration, but relatively dispersed during the late stage of combustion. At 3 % oxygen concentration, the E α values of different coals began to continuously decrease with increasing conversion rate, approaching the spontaneous reaction activation energy value of 40 kJ/mol. At oxygen concentration of 1 % reflected the complete limitation of the oxidation consumption reaction. Therefore, the critical oxygen concentration of typical low-rank coal oxygen-lean combustion was determined below 3 % oxygen concentration. The results of this study can provide scientific guidance for the unsealing of fire zones, and the effective prevention and control of the formation of new fire zone disasters. [ABSTRACT FROM AUTHOR]

The utilization of Polyurethane foaming materials (PUF) to seal coal fissures presents a significant challenge due to the substantial heat generated during the reaction process, potentially accelerating fires. In order to study this issue, we propose a novel low-heat polymerization mechanism by incorporating a hydrated salt phase change composite that efficiently absorbs polymerization heat while encapsulating liquid water using expanded graphite (EG). Our findings demonstrate that integrating 10 % EG-6 leak-free phase change material effectively reduces the reaction temperature to a safer 91.4 °C, ensuring minimal impact on the inherent properties of the material. Thorough analyses via TGA and in-situ IR experiments reveal a noteworthy 17 °C elevation in the modified PUF's thermodynamic characteristic temperature point. Additionally, we developed a mixed combustion model of PUF and coal to investigate the gas generation pattern of polyurethane in the mine-filled state where fire occurs. Specifically, the introduction of PUF reduced O 2 consumption and CO production while increasing CO 2 and C 2 H 4 production, which is consistent with the reality of increased carbon hydrocarbon gases being monitored downhole. These findings suggest a synergistic, mutually beneficial relationship between the two during the low-temperature oxidation stage. This research offers perspectives for the development of polymer materials for coal mines and the safe application of actual filling. [Display omitted] • Influence of different leakage degree of hydrated salt phase change materials on the properties of PUF. • A novel PUF and coal mixing and combustion model to emulate the actual filling state of polyurethane within underground coal mines. [ABSTRACT FROM AUTHOR]

Highlights: [•] It is a linear relationship between and the adiabatic R 70 self-heating rate. [•] A rapid method was developed to determine R 70, which need a time shorter than 1.5h. [•] The was used as an index to classify the spontaneous combustion tendency of coal. [ABSTRACT FROM AUTHOR]

Composites of physical (superabsorbent hydrogel) and chemical (ascorbic acid) inhibitors were investigated to ascertain their ability to prevent the spontaneous combustion of coal. CO emissions from the oxidation of the composite inhibitor-treated coal were markedly lower than those for tests with raw coal. Less oxygen was also consumed during low-temperature oxidation, while the crossing point temperature of the inhibitor-treated coals was higher than that of raw coal. Together, these results demonstrate that the composite inhibitors can effectively mitigate coal oxidation. However, variation in the effectiveness was observed depending on the inhibitor-loading and the blend of the individual inhibitors in the composite. The composite inhibitor was shown to more effectively prevent oxidation than use of an individual inhibitor. The network structure of the composite inhibitor, revealed by SEM analysis, was suggested to explain the suppression of the coal oxidation. [ABSTRACT FROM AUTHOR]

This work deals with the preparation and characterization of fly ash (FA) supported aqueous three-phase foams containing N 2 for coal spontaneous combustion prevention and control. Uniform and dense three-phase foams featuring FA contents between 20 wt.% and 33 wt.% are prepared in batch mode using a home-made foam generating column with an internal Venturi tube. The effect of FA on the foamability, static and dynamic stability of the three-phase foams is discussed. Compared with two phase foams, the FA supported three-phase foams manifest higher static and dynamic stability, which increases with the FA content. Base on the excellent static and dynamic stability, a structure model to attach FA particles at the N 2 –water interfaces of the three-phase foams was proposed to elucidate the observed behaviors. The as prepared FA supported three-phase foams, exhibiting excellent coal fire extinguishing characteristic, can be used as a promising fire extinguishing material, especially for preventing the large coal area spontaneous combustion or extinguishing fire in high and unknown locations in goaf. [ABSTRACT FROM AUTHOR]