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This study investigates changes in the concentration and types of free radicals in the process of coal heating, first rising and then falling. Hailar lignite, Panjiang bituminous coal, and Yangquan anthracite were selected as coal test samples. The results show that the lignite’s concentration of free radical changes during heating is greater than that of bituminous coal or anthracite. It clearly shows that lignite is more prone to spontaneous combustion. In the heating and cooling portion of the experiment, the concentration of free radicals during the cooling process was much more than that of free radicals at the same temperature during the heating process. These results obtained from this research study can provide a reference for the prevention and control of the spontaneous combustion of coal with changes in temperature. This study provides a theoretical basis for the prevention and control of spontaneous combustion of coal and the selection of retarding agents and methods in the process of flame retarding by testing the free radical changes of coal at different temperatures. Also, it provides a reference for preventing and controlling coal oxidation with the change in temperature, first rising and then falling.

Injecting nitrogen into goaf has been widely adopted for preventing fire hazards in coal mines. In this paper, the coupling relation between different locations of negative pressure of cross-cut drainage and nitrogen injection was investigated. The minefield data collection was carried out by an in situ beam tube system on the intake airway and return airway of the mine goaf. The validated Computational Fluid Dynamics (CFD) model that was secondarily modified by on-site collected data was applied for further research. It is demonstrated that the area of the spontaneous combustion zone generally shows a sharp decline first, then tends to stabilize, and finally has a slight drop and rise with the increasing nitrogen injection time. It is obvious that the location of the negative pressure of cross-cut exerts a significant influence on the optimal nitrogen injection location and time. When the cross-cut is located in the center of the air leakage zone, spontaneous combustion zone, and asphyxiation zone of goaf, the optimal nitrogen injection location and time correspond to the P2 (25 m, 1200 min), P3 (30 m, 120 min), and P4 (35m, 1800 min), respectively. According to the simulation result, the specific relation between the optimal nitrogen injection point N(x) and the distance from the working distance of the cross-cut (x) by Newton interpolation polynomial analysis was figured out and verified that N(x) = 24.70808 + 0.293356x – 0.001436x2. It is hoped that the result can provide scientific guidance for coal mine fire prevention and control with nitrogen injection.

To further study the mechanism of coal self-heating, the reaction sequences and thermodynamic properties between sulfur-containing groups and oxygen during coal self-heating were analyzed. The benzyl mercaptan and diphenyl sulfide were selected as typical sulfur-containing structures existing in coal. Their structural parameters, frontier orbital characteristics, and thermodynamic parameters were analyzed through quantum chemistry calculation and their detailed reaction sequences with oxygen were proposed. The results indicate that the thiol structure in coal can easily react with oxygen at low temperatures and release large amounts of heat (146.70 kJ/mol) during coal self-heating, providing active free radicals and energy for subsequent chain reactions of coal spontaneous combustion. The oxidation reaction between the thioether structure and oxygen cannot occur at room temperature. With the accumulation of heat, thioether gradually becomes active and reacts with oxygen to form sulfoxide and release an enormous amount of heat (248.09 kJ/mol), which can be further oxidized to sulfone with an increase in temperature. The reaction models of thiol and thioether groups during coal self-heating were proposed, which involves eight main reaction sequences (R1∼R8). It indicates that the reactions of thiol and thioether groups play crucial roles during the evolution of coal self-heating, with a slow oxidation stage at low temperatures and an accelerated oxidation stage at high temperatures.

Coal spontaneous combustion can cause serious coal mine fire and gas explosion. In order to overcome the problem that the effect of existing materials to inhibit coal spontaneous combustion is reduced at high temperature and some environmental protection problems of materials, this study developed an effective, green and environmental protection antioxidant foamed gel (AFG). Modified sodium polyacrylate, konjac glucomannan, sodium dodecyl sulfate, sodium alpha-olefin sulfonate, modified silicone polyether microemulsion, montmorillonite, and tert-butyl hydroquinone were selected as raw materials. A single factor experiment was conducted to determine the appropriate adding proportions for each component. Then, the optimal proportion for the five components were fitted and verified by response surface analysis. The structures of the samples were characterized, and their properties were compared and tested. The fluidity, water loss, and thermogravimetry experiments demonstrated that the effect of AFG was excellent. In the experiment of in situ diffuse reflectance Fourier transform infrared spectroscopy, the AFG inhibition reaction was verified. Finally, combined with the structural characteristics of the AFG, the following inhibiting mechanisms were confirmed: AFG adhered to the coal surface and blocked the coal–oxygen contact; the antioxidant component slowed down chemisorption and the chemical reactions; AFG interrupted the free radical chain reaction.

In order to further understand the mechanism of coal self-heating in the initial stage, the aldehyde group was analyzed by using the quantum chemistry methods. The charge distribution, structural parameters, and molecular orbital were analyzed to determine the active sites existing in the structure of aldehyde group. Then, a chemical reaction model including five elementary reaction sequences was established. In elementary reaction E1, the hydrogen of the aldehyde group is captured by hydroxyl to form the aldehyde radical, which provides the reactant and accumulates heat for the subsequent reaction. In elementary reaction E2, the aldehyde radical further reacts to form a carbon-free radical (R·) and CO, which is the main source for CO generation during coal spontaneous combustion. In elementary reaction E3, the aldehyde radical is oxidized to a carboxyl radical, providing the reactant for elementary reaction E4, which is directly related to CO2 generation during coal spontaneous combustion. The thermodynamic parameters of the elementary reactions were further analyzed and confirmed by quantum chemistry methods. The results are helpful for further understanding the pathways of CO generation in the initial stage of coal spontaneous combustion, which provides theoretical support for prediction of coal spontaneous combustion.

Coal spontaneous combustion is essentially the reaction of active groups, which usually occurs in low oxygen atmospheres. This paper studied the real-time changes of active groups during coal reaction at low oxygen concentrations by in situ FTIR. The results show that the active groups have different reactive activities and changing trends, i.e. continuously increase, continuously decrease, decrease at first and then increase. Both the quantity and changing trends are influenced by oxygen supply. The oxygen-containing groups having small reactive activity increase with temperature rise. The active groups having large reactive activity have two vibrational types. The bending vibration groups change with temperature rise as the same style, while the stretching vibration ones change in different styles. For the same group, the quantity and reactive activity of stretching vibration ones are greater than bending vibration ones. The active groups still can react with oxygen even under an oxygen atmosphere of 5%, but pyrolysis is the main reaction form under this condition. It will provide active groups or sites for coal self-heating. Therefore, it's better to control the oxygen concentration below 5% for fire prevention. The results will be helpful for further understanding the mechanism of coal self-heating.

Coalfield fire area reburning is one of the serious disasters in fire prevention and safety production. In this study, a synchronous thermal analyzer was used to conduct isothermal pyrolysis of jet coal at different temperatures, and the reaction characteristic parameters of different pyrolysis residual structures were analyzed. FTIR was used to measure group contents in raw coal and different pyrolysis residues. Programmed oxidation thermogravimetric experiments were carried out on the residues to obtain their oxidation characteristic parameters. The results demonstrated that the reaction characteristic parameters of the residual structures changed at 450 °C. The pyrolysis reaction mainly affected the variation of hydroxyl, aliphatic hydrocarbon, and aromatic hydrocarbon groups. The increase in pyrolysis temperature resulted in the decline in hydroxyl and aliphatic hydrocarbon groups as well as the increase in the aromatic hydrocarbon group. After pyrolysis, the ignition point temperature of the coal sample decreased, which causes the coal more likely to spontaneously ignite. It indicated that the pyrolysis residue at 450 °C is most likely to reburn. Compared with raw coal, the maximum combustion intensity of the pyrolysis residue was greatly increased, which reached the peak at 500 °C.

Large-scale coalmine accidents have occurred occasionally with great impact on society. To assess the characteristics of large-scale coalmine accidents, a database involving 782 Chinese accidents from 1950 to 2016 was built and analysed. The historical change of large-scale coalmine accidents was divided into four stages according to the accident number per year. Explosions and fires were the primary causes and deemed to be thermodynamically driven accidents due to their identical thermochemical essence of combustion, interchangeability and producing similar gaseous hazards. By constructing an analysis framework, some useful features were derived from the database. About 55% of the gas explosions occurred in coalmines with low methane-gas emission rates. Coal dust explosions were responsible for about 59% of the fires and explosions with over 100 casualties. The interchangeability of fires and explosions led to 79% of the large-scale coalmine fires and explosions in recent years. As ignition sources, blasting work, self-heating and friction and impact caused 86% of the fires and explosions from 2007 to 2016. About 44% of fires and explosions happened in coalmines with central ventilation systems. About 85% of fires and explosions concentrated on the locations of coalface, heading face and roadway.

Recent developments in the in-situ transformation of coal resources have heightened the possibility of exploring the oxidized coal regasification in coalfield fires. For further understanding the regasification characteristics of oxidized lignite residues in coalfield fire, Shengli lignite was selected for conducting ultra-lean oxygen pre-oxidation under different isothermal temperatures and CO2 regasification experiments. Fourier transform infrared spectroscopy results indicate that higher temperature leads to the increase of aromatic hydrocarbon and reduction of aliphatic hydrocarbon side chains on the coal structure. It is found that the pre-oxidation of under 1% oxygen concentration and 300 °C will increase the specific surface area and pore volume, which would be beneficial for the following regasification. The analysis of gasification parameters suggests that the gasification quality and performance of lignite residues decline with the increasing oxygen concentration and rising isothermal temperatures in pre-oxidation. The kinetic analysis demonstrates that when the oxygen concentration ranges from 1% to 5%, the activation energy of gasification increases by 16.744 kJ/mol. The findings suggest that the lignite pre-oxidation under ultra-lean oxygen conditions will inevitably increase the activation energy of CO2 gasification, which causes the coal more difficult to be utilized effectively.

A green, halogen-free, efficient, stable, and low-cost intumescent nanogel (ING) was prepared by combining an inorganic nanoparticle flame-retardant system with a chemical intumescent flame-retardant system. Polyacrylamide (PAM) gel was prepared by solution polymerization, then the modified nano-silica (MNS) and intumescent flame retardant were added to the PAM gel successively to prepare the ING. The preparation mechanism was analyzed, and response surface analysis showed that the optimal addition ratio of acrylamide, MNS, and potassium persulfate was 10:0.4:0.07, and the optimal reaction temperature was 75 ℃. The modification of nano-silica and the surface morphology of the composite gel were observed by scanning electron microscopy. In the electron spin resonance experiment, the free radical concentration of 5.70131 × 1017/g of the ING inhibited coal was significantly lower than other coal samples. The content of aliphatic hydrocarbon, aromatic hydrocarbon, and the oxygen-containing functional group in the in situ diffuse reflection Fourier transform infrared spectroscopy experiments all indicated that the ING achieved an excellent inhibition effect. Finally, the inhibition mechanism of the ING was analyzed for its physical, chemical, and composite effects. The PAM gel, MNS, and the intumescent flame retardant system demonstrated a triple inhibition effect, which indicates that ING is an ideal material for preventing the spontaneous combustion of coal.

To further understand the development of coal self-heating, the reaction sequences and thermal properties of aliphatic hydrocarbon groups during coal self-heating were analyzed. The structural parameters, frontier orbital characteristics, molecular orbital, and perturbation energy of aliphatic hydrocarbons and oxygen were analyzed by the quantum chemistry method. Then, the reaction pathways of aliphatic hydrocarbon groups and the corresponding reaction model were proposed. The results indicate that the reactions of aliphatic hydrocarbon groups include three kinds, i.e., the hydrogen capture by oxygen, reaction between aliphatic hydrocarbon radicals and the hydroxyl radical, and reaction between aliphatic hydrocarbon radicals and oxygen. The main reactions include the reaction between carbon free radicals and oxygen (E1), the reaction between aliphatic hydrocarbon and the hydroxyl radical (E2), the reaction between methyne and oxygen (E3), and other spontaneous reactions caused by E1 and E3 (E2). There is ...

Organic sulfur functional groups in coal not only have certain oxidation characteristics at low temperature and but also can break to produce free radicals, which is bound to have a considerable effect on coal spontaneous combustion. However, this area has not been reported yet. There are relatively few studies on variation laws of organic sulfur functional groups in coal in the coal spontaneous combustion process (especially low-temperature oxidation stage), and the release of SO2, the gaseous product of organic sulfur during the low-temperature oxidation, is also rarely reported. Therefore, it is necessary to systematically obtain the change laws of organic sulfur forms in coal during low-temperature oxidation. In this paper, the morphological changes of organic sulfur in coal during low-temperature oxidation were studied first, and then the changes of organic sulfur forms in coal before and after low-temperature oxidation and the release of the low-temperature oxidation product SO2 were studied by using the X-ray photon spectroscopy (XPS) technique and the XLZ-1090 infrared gas analyzer. The XPS analysis results show that the relative contents of mercaptan and sulfide are reduced; the content of thiophene sulfur decreases slightly or remains unchanged; and the contents of sulfone, sulfonate and sulfate all increase. This verifies that the low-temperature oxidation process of active organic sulfur functional groups in coal is: sulfide → sulfoxide → sulfone, mercaptan → disulphide → sulfoxide → sulfone → sulfonic acid. The study reveals that the release of gaseous SO2 is not detected during the low-temperature oxidation of the raw coal samples, whereas it is detected during the low-temperature oxidation of the acid-treated coal samples. This is because the acid treatment can remove alkaline minerals in coal, weakens the inhibitory effect of alkaline minerals on SO2 and enable SO2 to escape almost completely. In addition, the study also discovers that the time and amount of SO2 gas release during the low-temperature oxidation process are directly related to the metamorphic degree and organic sulfur content of coal.

Studying the effect of oxygen in coal oxidation is very important for understanding and controlling coal spontaneous combustion. However, the oxygen effect is not very easy to determine clearly due...

Reactions of free radicals play essential roles in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways and details during this process have not been revealed by far. This paper established reaction pathways of free radicals during coal spontaneous combustion based on typical active free radicals by quantum chemistry method. The specific structural parameters and thermodynamic data for reactions (R1 ~ R9) are calculated to reveal the reaction process. The cyclic model of free radicals during coal self-heating has been proposed for the first time. The results indicate that most of the reactions involved in free radicals are exothermic reactions with heat release ranging from 7.568 ~ 167.113 kJ/mol. The initial reactions (R1 and R2) can accumulate heat and generate active peroxide free radical (R-O-O ) as well as hydroxyl radical ( OH). Both of them can occur easily at the normal temperature. The heat release increases the ambient temperature, which causes non-spontaneous reactions to proceed successively. The decomposition of peroxide free radicals triggers the reaction pathways of the cyclic model. The hydroxyl radical can lead to the reaction with active sites in coal, which can accumulate plenty of heat and supply the R for initiating reaction R(1) in the cycle. Both hydroxyl radicals and hydrocarbon radicals play vital roles in consuming active groups in coal and oxygen continuously. The results demonstrate that to fundamentally prevent coal spontaneous combustion, it is necessary to eliminate the key active free radicals (R , R-O-O , and OH) to terminate the cyclic chain reaction.

R3-CH structure is a typical active group during coal self-heating. In this paper, Ph-CH(CH3)–CH2-Ph was selected to analyze reaction activity as well as mechanism between R3-CH structure and oxygen in coal self-heating by Multiwfn and Gaussian calculation. The results demonstrate that R3-CH structure especially the H atom is the most active site in coal self-heating with oxygen. The oxygen attacks R3-CH site in the Ph-CH(CH3)–CH2-Ph, which results in the breaking of C–H bond and formation of O–H bond. The free energy barrier of this oxidation reaction is 8.28 kJ/mol and the heat release of the process is 86.89 kJ/mol, which indicates that it can easily occur at room temperature. From the perspective of products, the aromatic hydrocarbon radicals and hydroxyl radicals related to the initial spontaneous combustion of coal are generated, which is of great significance for the further development of coal self-heating. Subsequently, the carbon radical is combined with a single oxygen atom to generate oxygen-containing free radical which plays vital roles in initial coal spontaneous combustion.

During coal self-heating, reactions of carboxyl groups feature in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways during this process still have not been revealed. This paper selected the Ar–CH2–COOH as a typical carboxyl group containing structure for the analysis of the reaction pathways and enhancement effect on the coal self-heating process by quantum chemistry calculations. The results indicate that the hydrogen atoms in carboxyl groups are the active sites, which undergo the oxidation process and self-reaction process during coal self-heating. They both have two elementary reactions, namely (i) the hydrogen abstraction of –COOH by oxygen and the decarboxylation of the –COO· free radical and (ii) the hydrogen abstraction of –COOH and its pyrolysis. The total enthalpy change and activation energy of the oxidation process are 76.93 kJ/mol and 127.85 kJ/mol, respectively, which indicate that this process is endothermic and will occur at medium temperatures. For the hydrogen abstraction of –COOH by hydrocarbon free radicals, the thermal parameters are 53.53 kJ/mol and 56.13 kJ/mol, respectively, which has the same thermodynamic properties as the oxidation process. However, for the pyrolysis, the thermal parameters are –42.53 kJ/mol and 493.68 kJ/mol, respectively, and is thus exothermic and would not occur until the coal reaches high temperatures. They affect heat accumulation greatly, generate carbon dioxide, and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

In order to further understand the characteristics of coal reaction under low oxygen concentration atmosphere, this study tested the thermodynamic characteristics during reaction processes under low oxygen concentrations. The kinetic factors were also analyzed based on testing results. The results show that there are some characteristic temperatures during coal spontaneous combustion and the influences of oxygen concentration on these temperatures are different. The influences of oxygen concentration on the mechanism function and kinetic parameters depend on the coal ranks and the reaction stages. They are different at different reaction stages for the same coal, and at the same reaction stage for different coals. This phenomenon is essentially resulted from the different influences of oxygen concentration on the micro reaction sequences during coal spontaneous combustion. The study will be helpful for further understanding of the actual development process of coal spontaneous combustion.

For further understanding spontaneous combustion of coal in fire suspectable zone, this paper analyzed coal pyrolysis in low-oxygen conditions by thermogravimetric analysis and online FTIR. The results indicate that the oxidation and pyrolyzation of candle coal occur simultaneously at 3 %∼5 % oxygen concentration, the intensity of oxidation reaction can still promote the coal self-heating. With the decrease of oxygen concentration and temperature, the C C on benzene ring decreases while the oxygen-containing groups increase. With oxygen concentration decreasing continuously, the oxygen-containing groups begin to appear surplus. When the temperature rises to 500 °C, the content of −COOH and −OH still remains 0.3 % under 3 % oxygen concentration, but disappears at 5 % oxygen concentration. It indicates that 3 % oxygen supply cannot provide necessary oxidation conditions for candle coal. Consequently, in order to fully suppress the development and re-burning of underground coal fire, the oxygen concentration should be controlled below 3 %.

Hydroxyl groups are one of the key factors for the development of coal self-heating, although their detailed reaction pathways are still unclear. This study investigated the reaction pathways in coal self-heating by the method of quantum chemistry calculation. The Ar–CH2–CH(CH3)–OH was selected as a typical structure unit for the calculation. The results indicate that the hydrogen atoms in hydroxyl groups and R3–CH are the active sites. For the hydrogen atoms in hydroxyl groups, they are directly abstracted by oxygen. For hydrogen atoms in R3–CH, they are abstracted by oxygen at first and generate peroxy-hydroxyl free radicals, which abstract the hydrogen atoms in hydroxyl groups later. The reaction of R3–CH contains three elementary reactions, i.e., the hydrogen abstraction of R3–CH by oxygen, the conjugation reaction between the R3C■ and oxygen atom, and the hydrogen abstraction of –OH by hydroxyl free radicals. Then, the microstructure parameters, IRC pathways, and reaction dynamic parameters were respectively analyzed for the four reactions. For the hydrogen abstraction of –OH by oxygen, the enthalpy change and activation energy are 137.63 and 334.44 kJ/mol, respectively, which will occur at medium temperatures and the corresponding heat effect is great. For the reaction of R3–CH, the enthalpy change and the activation energy are −3.45 and 55.79 kJ/mol, respectively, which will occur at low temperatures while the corresponding heat influence is weak. They both affect heat accumulation and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

The existing coal self-heating inhibitors usually have the shortcomings of short effective time and low inhibition effect. This study proposed a kind of controlled-release inhibitor and analyzed their differences in inhibition effects compared with existing inhibitors on coal self-heating. The controlled-release inhibitor is made from the synthesis of halogen inhibitors, catechin, copolymer, solvent and surfactant. The controlled-release inhibitor will not break down quickly until coal temperature reaches a value about 70 °C and can continually inhibit the process of coal self-heating after a longer time. The crossing point temperature, propensity to spontaneous combustion and CO generation of coal samples processed by different inhibition methods were tested separately based on an oxidation dynamic method. The results show that the controlled-release inhibitor can inhibit the coal self-heating more effectively for a longer time than existing halogen inhibitors. This study provides a new method for more efficient prevention of coal spontaneous combustion.

Organic chemical reactions cause the temperature rising during coal oxidation; however, because of the complex structure of coal, it is difficult to analyze and characterize the reactions involved in coal low-temperature oxidation. To date, a main reaction pathway describing the heating progress during coal oxidation has not been proposed. Here, a series of cyclic chain reactions is presented to describe the oxidation mechanism during coal spontaneous combustion using quantum chemistry calculations. Main active sites and their molecular models were built. Three interactive modes of active orbitals and detailed reaction sequences of coal oxidation are proposed. The structural parameters and thermodynamic data were calculated and the orders of reactions for transformations between functional groups were identified based on their activation energies. The reaction pathway was constructed based on functional transformation relationships and the order of reactions. The results show that main reactions occurring during coal oxidation can be defined as the reactions of oxygen and hydroxide free radicals reacting with coal active sites. Methyne and carbon free radicals reacting with oxygen is the initial reaction during coal oxidation. The decomposition of peroxides linking the reaction pathway form cyclic chain. Hydroxyl and aliphatic hydrocarbon radicals as key of chain reactions consumes coal active sites and oxygen continuously. Aliphatic hydrocarbons appear to contribute more to heat release during coal oxidation due to greater heat release and lower activation energy of their reactions. Limited spontaneous reactions maintain constant apparent activation energy for the oxidation until the chain reactions are generated; the apparent activation energy then increases. Low-ranking coals have higher apparent activation energies during oxidation due to more oxygen-containing groups and side chains contain more reactions with higher activation energy. Results from this study can improve understanding of mechanism of coal oxidation and provide a guide to forecasting and preventing spontaneous combustion of coal in underground coal mines or coal stockpiles.

The reburning of pyrolysis residual is very important in coal fire control and preventing fire resurgence. This paper researched the pyrolysis and reoxidation characteristics of lignite using thermogravimetric and infrared analysis. The macroscopic mass characteristics and microstructure composition characteristics of coal pyrolysis residues were analyzed. The critical temperature for lignite pyrolysis and pyrolysis residues reburning. The results show that a significant turning point before 450 °C such as the variation of pyrolysis residual weight, the variation of constant temperature residual weight, and the pyrolysis residual combustible component weight. The aliphatic hydrocarbons are gradually reduced due to the side chain cracking in the coal, and the aromatic hydrocarbons increase with the degree of aromatization of the coal structure in lignite pyrolysis. The pyrolysis residue at ~400 °C is the easiest to reburning, and the pyrolysis residue at ~450 °C had the strongest reburning intensity.

The longwall gob (mined-out) area is one of the main places that are prone to coal spontaneous combustion and most of the residual coal in it is in oxygen-depleted air as it is a semi-enclosed space. A DSC (Differential scanning calorimetry) test system was designed to accurately test the heat generation of coal oxidation in different oxygen concentration atmosphere, based on which the kinetics parameters (activation energy and pre-exponential factor) of coal low-temperature oxidation in oxygen-depleted air were solved out. The results show that the kinetics parameters present obvious stage features and the lower the oxygen concentration is, the smaller is the difference of the kinetics parameters that in different oxidation stages. When the oxygen concentration is lower than 5% and 3% for jet coal and meagre coal respectively, the kinetics parameters of slow oxidation start to be greater than that of rapid oxidation. Both in the slow oxidation and rapid oxidation stage, with the decrease of oxygen concentration, kinetics parameters present significant decline on the whole while in different oxygen concentration range, the decline rate is different. It's concluded that when assessing the residual coal's self-heating risk, we need to use the corresponding kinetics parameters of coal oxidation in the oxygen concentration of the location where the residual coal is and the safety factor will be greater to only use the kinetics parameters of coal oxidation in slow oxidation stage. This study is of great significance for the assessment and control of the self-heating risk of coal that in different oxygen concentration atmospheres of the longwall gob areas.

A comparison of in-situ series diffuse reflection Fourier transform infrared and non-series Fourier transform infrared in studying the conversion of functional groups in the process of coal oxidation is studied. It is found that non-series Fourier transform infrared could not monitor the changes of functional groups precisely while in-situ Fourier transform infrared can. In-situ Fourier transform infrared was also used to monitor the oxidation processes of low ranks of coal at low temperatures (30–220°C) and a series data was obtained. The variation regularity of six main functional groups was discussed and found that in-situ Fourier transform infrared would make it possible to reveal the mechanism of coal oxidation.

Coal spontaneous combustion is one of the major natural disasters faced in coal mines. The accurate prediction of the thermal risk of coal is of great importance. However, there isn't a widely accepted approach to get the oxidation process of coal that under adiabatic condition or in a specific environment under mine at present. To demonstrate whether the advanced kinetics simulation method could be employed to obtain the accurate oxidation process of coal for determining the coal's thermal risk in the mine design phase and mining phase, DSC experiments were conducted by C80 micro-calorimeter to get the heat behavior of coal, based on which the kinetic parameters can be solved and the oxidation process of coal can be predicted. The results showed that the kinetics based simulation method was successfully used to predict the adiabatic temperature rise process of coal for risk prediction. The deviation between the predicted curve and tested curve that obtained by adiabatic test is small enough to be accepted. Kinetics based simulation method is a promising candidate, instead of adiabatic test, to assess the propensity of coal to spontaneous combustion, which can play an important role in the design phase of the mine and mining area. Moreover, through establishing the heat balance equation of residual coal and with the aid of kinetics based simulation method, the oxidation process of coal that in the suffocation zone of the gob was also accurately predicted. According to the index t 70 (the time required for coal to reach 70 °C) and v min (the lower limit of the advancing speed of the working face) obtained from the predicted curve, the thermal risk of coal was predicted to guide the further adjustment of the advancing speed of the working face, the amount of the injected mud and the determination that whether to add other fire prevention measures. Kinetics based simulation method, be of great practical importance in risk prediction of coal that in the gob, can be also used as a convenient tool to guide the safe production in the actual mining process.

The remarkable differences between the infrared spectra of oxygen adsorption and nitrogen desorption in coal have been experimentally and theoretically investigated. Density functional theory calculations were performed to better explain the mechanism of oxygen adsorption using six different molecular models of coal. In addition, the remarkable differences of infrared spectra between oxygen adsorption and nitrogen desorption was defined as the index V, which was used to classify the spontaneous combustion tendency of coal. The experimental data indicated that the spectra in the 4000–2000 cm−1 and 1250–1050 cm−1 regions exhibited significant changes. These results suggest that the mechanism of oxygen adsorption is the alteration and transfer of charge density around the activated sites, which leads to the observed changes of the infrared spectra. The V, which is related to the alteration of spectral intensity, is found to decrease with the increase of adiabatic oxidation time and the relative spon...

This study details an in situ Fourier transform infrared spectroscopy analytical system that was employed to follow chemical variations in the functional groups on coal surface during the oxidation process at low temperatures. In the reported in situ Fourier transform infrared spectroscopy system, a special chamber was used to contain the coal powders, and a gas inlet tube and a programmable heater were used to simulate different reaction atmospheres and temperatures. The comparisons between in situ and ex situ Fourier transform infrared spectroscopy spectra indicate that the in situ Fourier transform infrared spectroscopy data offer a more accurate reflection of changes in the functional groups. The real-time changes of aliphatic hydrocarbon groups and oxygen-containing groups in a lignite coal sample were analyzed from 30°C to 220°C using in situ Fourier transform infrared spectroscopy. The experimental results indicate that the chemical variations in the functional groups are affected by their...

The extinction coefficient differs for different functional groups making the quantitative analysis of their infrared spectra problematic. Although the coefficients of some functional groups have been researched previously, they are inaccurate because of deviations in techniques used and linear fitting. The distribution and structural characteristics of functional groups from coal infrared spectra is therefore inaccurate. In this study, quantum chemistry methods are used for the quantification of coal infrared spectra based on the Beer–Lambert law. The effect of the extinction coefficients of different functional groups is eliminated. The experimental infrared intensities, unit absorption intensities and their ratios are obtained to calculate the percentage composition of coal functional groups and molecular structure parameters. Accurate distribution and structural characteristics of coal functional groups are obtained. A linear relationship between the distribution of some functional groups and structural parameters is obtained. Multiple structural parameters are used to determine the grade and maturity of coal samples. The ethylene in coal oxidation experiments below 200 °C is consistent with the hydrocarbon-generating potential parameters. These results will improve the accuracy of the quantitative analysis of infrared spectra for determining coal structural features and provide an index for the utilization and prevention of the spontaneous combustion of coal.

A rapid, simple method was developed for the determination of R70 self-heating rate of coal. The coal samples were tested under adiabatic and programmable temperature conditions, separately. It indicates that it is a linear relationship between the differential value of oxygen concentration at 70 °C under a programmable heating condition ( Δ C O 2 , 70 ) and the adiabatic R70 self-heating rate. This linear relationship can be described by a fitted trendline equation R 70 = 0.68504 Δ C O 2 , 70 + 0.12098 . So the R70 can be determined by testing the value of Δ C O 2 , 70 . For most coals, the testing processes of Δ C O 2 , 70 need a time less than 1.5 h. A new evaluation method for the propensity of coal to spontaneous combustion was also proposed from previous R70 method. Based on the value of Δ C O 2 , 70 , a coal with a Δ C O 2 , 70 value below 0.55% is considered to be low risk, 0.55–1% medium risk, higher than 1% high risk. The study will be helpful for more widely and convenient use of R70 self-heating rate.

The distribution of functional groups in two different rank coal samples and their real-time changes during the oxygen-free reaction of coal were tested. An in situ FTIR system was designed to test...

In order to understand the characteristic of coal-oxygen reaction, oxygen consumption of coal at low temperatures is tested. The results show that the change of oxygen consumption with the rise of temperature is a non-liner process. Oxygen consumption decreases with the rising of coal temperature from 30 °C to 50 °C and reaches the minimum at 50 °C. Then, it increases steadily and rapidly upon 50 °C. This characteristic is more obvious for high rank coals. The difference of oxygen consumption at the same temperature is significant for different rank coals. The test shows the difference of the coal samples tested in the paper reaches 78.6% at 100 °C. Based on the theory of coal-oxygen reaction, the phenomenon above is analyzed in the physical and chemical character, and the appearance of coal-oxygen is complex.

In order to further understand the self-heating of coal, a chemical reaction model was proposed based on the previous model including two parallel reaction sequences. There are three different reaction sequences in this model. In comparison with the two parallel reaction sequences, the third reaction sequence is the self-reaction of initial active groups in coal. The reaction products of seven different ranks of coal samples were tested under inert atmosphere. The coal samples were particles ranging from 0.18 mm to 0.38 mm; the flow rate of nitrogen flowing into coal sample was 100 mL/min; the heating rate of reaction surrounding was 0.8 °C/min; the tests were carried out at temperatures starting from 30 °C and reaching 180 °C. The generation of the reaction products was analyzed every 10 °C rise of coal temperature from 30 °C. The results show all the coal samples tested generate CO, CO 2 , CH 4 and C 2 H 6 at low temperatures; only some special coal samples can generate C 2 H 4 and C 3 H 8 and their generation amount were small; no coal sample generate C 2 H 2 . Based on the analysis, it indicates the self reaction of initial active groups exists in the self-heating of coal at low temperatures. The model including three reaction sequences can present the self-heating of coal comprehensively.