Oxidation Characteristics Of Coal Research Articles

Response of Macroscopic Coal Oxidation and Pyrolysis Parameters to Different Extractants

ABSTRACT The active structure of coal molecules is one of the key factors affecting the characteristics of coal spontaneous combustion (CSC). By actively stripping or reducing the key active structure of coal molecules, and then studying the influence of different structures on the oxidation characteristics of coal, it is helpful to accurately and efficiently inhibit CSC. Herein, using coal samples from Caojiatan Mine in Shaanxi Province, China, the CSC characteristics of 10 coal groups treated with different solvents and the response of the macroscopic coal oxidation and pyrolysis parameters to each extractant were investigated. The extractant type considerably affected CSC, and the oxidization of coal was most responsive to biomass extraction agents, which had the best weakening effect on CSC. The order of CSC characteristics of extracted coal from strong to weak is AE> YM> CYC> AOT> CYH> EG> MT> TP> THF> CYC + AOT + AE> EG + TP. In general, all extraction reagents in each group have a certain weakening effect on the CSC activity of coal samples. The extraction of aromatic hydrocarbon structures can effectively delay the oxidation of coal. The study provides new insights for the development of CSC mitigation technology and have important practical significance for active prevention and control of CSC risks.

Evolution of pore structure and changes in oxidation characteristics of coal after heating freezing treatment

To study the spontaneous combustion characteristics of preheated coal treated by liquid nitrogen (LN2), surface structure test, thermal analysis experiment and chemical characteristic analysis experiment were used to study the change characteristics of surface structure, oxidation characteristic and chemical structure of coal under the action of LN2 quenching. The results showed that with the increase of preheating temperature, the fractal dimension, specific surface area, pore volume and average pore diameter of the LN2 treated coal (quenched-coal) increase. With the increase of preheating temperature, under the oxidation condition, the CO and CO2 produced at ST of 230 ℃ are 1.19 times and 1.21 times higher than those at ST of 70 ℃. The CO/CO2 emission of quenched-coal is significantly increased, and the secondary oxidation capacity is stronger. Due to the more developed pore cracks of coal under pyrolysis and quenching conditions, the natural characteristics of coal are stronger. The oxygen adsorption capacity and apparent activation energy of pyrolysis coal in the stageⅠ affected by LN2 are stronger. Oxidized coal shows stronger oxidation and heat release ability in the stageⅡ under the action of LN2, resulting in higher apparent activation energy in this endothermic stage. The addition of LN2 blocked the oxidation reaction of preheated coal and sealed the structure of -C = O, –COOH and –OH. The bridge bond –CH2- structure of pyrolysis coal is further destroyed, and the ratio of –CH2/–CH3 is reduced. The increase of alkyl free radicals in pyrolysis coal and alkyl oxygen free radicals in oxidation coal accelerates the spontaneous combustion oxidation of coal. The research results have important guiding significance for the occurrence and prevention of coal reignition disaster after LN2 fire quenching.

Synergistic Effect of the Moisture Content and pH Value of Aqueous Solutions on Oxidation Characteristics of Coal.

In order to study the synergistic effect of the moisture content and pH value of aqueous solutions on coal oxidation, coal samples with different moistures were prepared by using aqueous solutions with different pH values (3, 5, 7, and 8). The CO, C2H4 production, oxygen consumption rate, and crossing point temperature parameters in the process of low-temperature oxidation of the prepared coal and raw coal samples were studied by using a low-temperature oxidation device. Thermogravimetry-derivative thermogravimetry (TG-DTG) curves of coal samples with different moistures were obtained on the basis of thermogravimetric analysis of coal by using a synchronous thermal analyzer. According to the characteristic temperature, the process of coal spontaneous combustion (CSC) can be divided into the water evaporation and desorption stage, oxygen absorption and weight gain stage, thermal decomposition and combustion stage, and burnout stage. The Coats-Redfern integral method was used to select the appropriate reaction mechanism function to calculate the apparent activation energy at different stages of the coal reaction process. The results show that the activation energy of the oxidation reaction of the coal sample with a moisture of 15% is the smallest, and the oxidation reaction is the most easy to occur. When the coal samples with a moisture of 15% were oxidized at low temperature, the CO and C2H4 emissions and oxygen consumption rates were the highest. The pH value of the aqueous solution has dual effects on CSC. When the moisture is 15%, the higher the pH value of the aqueous solution, the weaker the promotion effect on the coal oxidation process is and the lower the pH value of the aqueous solution is, the higher the production of CO and C2H4 and the oxygen consumption rate is and the earlier the crossing point temperature is reached during low-temperature oxidation of the coal sample.

Study on the Difference in Oxidation Characteristics of Coal and Gangue from the Same Coal Seam

ABSTRACT The gangue hills and underground backfill mining gob where crushed coal and coal gangue coexist face severe risks of spontaneous combustion. Studying the differences in oxidation characteristics between coal and gangue from the same coal seam is beneficial for spontaneous combustion. This paper tested the changes in radicals, functional groups, and gas products during the oxidation of raw coal, raw gangue, and washed gangue and analyzed their differences. Finally, gray correlation analysis was used to calculate the correlation between active groups and gas. The results showed that raw coal contained a more significant initial radical concentration and g factor value than raw and washed gangue. Its aliphatic hydrocarbons and oxygen-containing functional groups content was also significantly higher. The growth rate of radical concentration in the early oxidation stage of washed gangue was larger than that of raw coal and raw gangue due to the high sulfur content. At the same time, the temperature at which the g factor started to increase rapidly and the temperature at which the aliphatic hydrocarbons decreased rapidly was lower than that of raw coal and raw gangue. Gray correlation analysis showed that the active groups closely related to oxygen consumption were CH3 and CH2, while the groups closely associated with CO generation were C=O, COOH, Ng, and g factor.

Stage changes in the oxidizing properties of long-term water-soaked coal and analysis of key reactive groups

To elucidate the impact of prolonged water immersion on the evolution of key active functional groups during the low-temperature oxidation of coal, the programmed temperature ramping method, in-situ Fourier transform infrared spectroscopy (in-situ FTIR), microcalorimetry (C80) experiments, thermodynamic analysis, and gray and Pearson correlation analysis were performed. The analysis involved coal samples exposed to different immersion durations (3, 6, 9, and 12 months). The results reveal that long-term water immersion exerts a stage-wise influence on the oxidation characteristics of coal, with the critical time point being 6 months. Before this period, the gradual accumulation of active sites progressively enhances the oxidation characteristics. However, after 6 months, the generation of active sites is suppressed, resulting in a gradual weakening of the coal's oxidation behavior. Correlation analysis revealed that the correlation between free hydroxyl groups and gaseous products is strongest in coal immersed for 6 months (A6), with a correlation coefficient of 0.91. During the accelerated oxidation stage, the heat released from the soaked coal is significantly greater than that in other stages, which promotes the formation of C=O and –COOH functional groups and further reduces the apparent activation energy. According to the Coats–Redfern method, A6 exhibits the smallest apparent activation energy—only 30 % of the original value of coal. The correlation between free hydroxyl groups and apparent activation energy is strongest at this stage, with a correlation coefficient of 0.92. This indicates that free hydroxyl groups and the accelerated oxidation stage are, respectively, the key active functional groups and critical phases in the coal oxidation process.

Characteristics of coal oxidation and spontaneous combustion in Baishihu Mine, Xinjiang, China

The characteristics of oxidized spontaneous combustion of coal can reflect the performance of coal, and an appropriate structural model can reflect it more intuitively. In this study, samples from Baishihu Coal Mine were used to study the macromolecular structure, microcrystal structure, and oxidation process of coal by X-ray photoelectron spectroscopy (XPS), carbon nuclear magnetic resonance (13C-NMR), and Fourier infrared spectroscopy (FTIR). The molecular formula C198H164O40N2 and the molecular structure model were obtained. ChemDraw and Materials Studio were used for the experimental data, and high-resolution transmission electron microscopy (HRTEM) was used to verify the aromatic ring structure built to make the constructed structural model more accurate. In the water evaporation stage, the high ring aromatic layer is converted into the low ring number. Furthermore, in the high-temperature stage, the low ring aromatic layer is transformed due to the coking and condensation reaction of the coal sample. The C element in the coal sample mainly participates in the reaction in C–C and C–H forms. The spacing and effective number of aromatic layers are relatively stable. The aggregation state of coal is a macromolecular group formed between molecules with different aromatic structures and fat structures, which is formed by the interaction of internal defects and pores of molecular groups. With the increase in the treatment temperature, water loss is heavier, oxygen absorption and weight gain are perplexing, and the value of the burnout temperature is higher. The apparent activation energy of the coal–oxygen reaction increases, and the reaction is more intricate to achieve. This study furthers the understanding of coal spontaneous combustion in this mining area, provides a reference for the prevention and control of coal spontaneous combustion.

Investigation on thermal analysis and FTIR microscopic characteristics of artificially-oxidized coal and chronic naturally-oxidized coal during secondary oxidation

To reduce mine fire risk and ensure efficient resource utilization, an in-depth research must be conducted on the secondary oxidation of coal. Accordingly, the macroscopic oxidation characteristics and microscopic composition of newly-mined coal, artificially oxidized coal (the newly-mined coal oxidized at 70 °C and 180 °C) and repeated-mined coal (oxidized naturally over 10 years) were evaluated in this study. And these coal samples were labeled N, N-70, N-180, and R, respectively. The parameters indicating the oxidation characteristics of coal were analyzed by simultaneous thermal analysis-Fourier transform infrared spectroscopy (STA-FTIR) and Shimazu Fourier transform infrared spectrometry (FTIR-8400) from the perspective of coal thermal reaction and functional groups. The experimental results suggest that the temperature T2 required by N-70, N-180, and R to reach the oxidation stage was lower than that of N. In addition, CO and CO2 gases were released earlier from these coals than from N. The same release was earlier for N-180 and N-70 compared to that for R. Compared with N, the content of hydroxyl and aliphatic hydrocarbons in N-70, N-180 and R after the initial oxidation decreases, and the decrease of N-70, N-180 was greater than that of R. The content of oxygen-containing functional groups (-COOH, C=O, -COO-) increased significantly, and the increase for N-70, N-180 was greater than that of R. These results indicate that coal was more prone to secondary oxidization after primary oxidation. This is because the coal absorbed heat released during the primary oxidation and subsequently had a higher activity. The artificially-oxidized coal in the laboratory absorbed more heat and generated more oxygen-containing functional groups (-COOH, C=O, -COO-) than the chronic naturally-oxidized coal in the mine face, resulting in a higher risk of spontaneous combustion during secondary oxidation.

Influence of High Temperature Thermal Radiation on the Transition Characteristics of Coal Oxidation and Spontaneous Combustion

In the goaf of the coal mine, there will be some high-temperature points before or during the fire. Under certain conditions, these high-temperature points will radiate heat to the surrounding coal in the form of thermal radiation, which, in turn, may also ignite the coal. Taking this situation into consideration, this study aims to investigate the influence of high-temperature thermal radiation on the transformation characteristics of coal oxidation and spontaneous combustion using the high-temperature thermal radiation method. The results show that an increase in thermal radiation value reduces the ignition time of coal gradually. The peak heat release rate, total heat release, peak smoke release rate, and total smoke release gradually increase. Additionally, the total carbon monoxide release reduces gradually, and the peak carbon dioxide production rate increases gradually. It is worth noting that as the heat radiation value increases, the peak value of CO production rate of lignite and bituminous coal is noted to decrease gradually, whereas that of anthracite increases gradually. The total carbon dioxide emissions of bituminous coal and anthracite increased gradually, whereas the total carbon dioxide emissions of lignite increased firstly and then decreased. This work proposes a novel method to study the coal oxidation and spontaneous combustion by a widely-recognized combustion apparatus.

Investigating the effect of coal particle size on spontaneous combustion and oxidation characteristics of coal.

As a key parameter, the particle size of residual coal contributes greatly to its oxidation characteristics, so it is a significant and far-reaching topic to explore the role of different particle sizes in coal spontaneous combustion disaster. In this work, temperature-programmed system (TPS) was applied to analyze the oxygen consumption rate and CO and C2H4 production rules of six groups of coal samples with different particle sizes in the process of oxidation heating. The critical temperature (CT) and xerochasy temperature (XT) of different coal samples were obtained, and the coal oxidation process was divided into three stages (S1, slow oxidation stage; S2, fast oxidation stage; and S3, combustion stage). Then, the apparent activation energy (E) and pre-exponential factor (A) in three stages were regressed combined with Arrhenius formula. The results show that the smaller the coal particle size is, the larger the specific surface area is, the stronger the adsorption capacity of coal molecules and oxygen molecules is, resulting in the larger oxygen consumption rate. The values of CT and XT with particle size of 0.125-0.18mm and 2-4mm are the smallest and largest. For coal samples with the same particle size, the maximum values of E and A occur in stage S3 and the minimum values appear in stage S1. This is mainly due to the higher temperature of stage S3, which allows the activation of functional groups with higher apparent activation energy, stronger collisions between activated molecules, and more intense oxidation reactions.

Inhibitory effect and mechanism of l-ascorbic acid combined with tea polyphenols on coal spontaneous combustion

The inhibitory effects of compound antioxidants (l-ascorbic acid (VC) and tea polyphenols (TP)) on coal oxidation were studied. The characteristics of coal oxidation was investigated by temperature-programmed experiments. The changes of active groups in coal were confirmed by Fourier-transform infrared spectroscopy. Quantum chemical theory was used to explore the reaction mechanism. Experimental results showed that compound antioxidants exhibit significant synergistic inhibition effect on coal oxidation. The apparent activation energy of coal oxidation increases by 16.23 kJ/mol for the addition of 6 wt% compound antioxidants (TP:VC = 2:1). The content of aliphatic hydrocarbons, –OH, CO and COOH in the antioxidant-treated coal decreases, while that of ether bond and aromatic bond increases. H15 of VC or H48 of epigallocatechin gallate (the main component of TP) can readily scavenge oxygen-containing radicals and inhibit the production of active groups in coal. Water-soluble antioxidants can be evenly dispersed on the coal surface, which provides an environment-friendly and efficient approach to preventing coal spontaneous combustion.

Erosion effect on microstructure change and oxidation behavior of long-flame coal under different pH conditions

In order to study the effect of aqueous solutions with different pH values on coal oxidation behavior, coal samples were eroded in aqueous solution of various pH values (4, 5, 7 and 8) for 30 days and to study its microstructure change and oxidation behaviors. The pore structure and functional groups were characterized by low-temperature N2 adsorption and Fourier transform infrared spectroscopy. The oxidation characteristics of coal were investigated by temperature-programmed experiments. Compared with raw coal, larger average pore diameter and pore volume, more active functional groups, such as methyl, methylene, hydroxy and ether, were found in the eroded coal. Among them, the microstructure of coal eroded in acid water changes most obviously. Eroded coal samples have more pores with diameter > 20 nm and an obvious increase in particle surface roughness, which facilitates the adsorption and transportation of O2 in coal body. The O2 consumption and CO production of dried eroded coal, especially eroded in aqueous solution with pH = 4, are more than those of raw coal, and its apparent activation energy decreases both in the slow oxidation stage and the fast oxidation stage, which indicates that the eroded coal is more prone to spontaneous combustion.

Effect of Igneous Intrusions on Low-temperature Oxidation Characteristics of Coal in Daxing Mine, China

ABSTRACT In order to investigate the effect of igneous intrusions on low-temperature oxidation characteristics of coal, metamorphic coal samples affected by igneous intrusions and normal coal samples were collected from Daxing Mine. The thermal effect of coal at a low temperature was studied by Thermogravimetric analysis and Differential scanning calorimetry (TG-DSC). The oxidation kinetics experiment was used to analyze the oxygen consumption, the index gas, the crossing point temperature (CPT) and apparent activation energy of coal. Compared with normal coal, the metamorphic coal had lower weight loss rate and higher oxygen absorption rate due to igneous intrusions. At the same time, the metamorphic coal absorbed less heat at low-temperature stage and entered the exothermic stage first, and the total heat release in the whole stage was larger. The author also found that the initial exothermic temperature of coal decreased obviously due to igneous intrusions. In addition, metamorphic coal had lower apparent activation energy because of igneous intrusions, which indicated that the molecular structure of coal became easy to be activated, and the energy needed for activation became lower. Fourier transform infrared spectroscopy (FTIR) analysis suggested igneous intrusions changed the chemical structure of coal and increased the branching degree and short aliphatic chains. Moreover, metamorphic coal consumed more oxygen and produced more carbon monoxide, ethylene and ethane, with lower crossing point temperatures, which concluded that the igneous intrusions accelerated the oxidation reaction of coal at low-temperature stage and increased the spontaneous combustion risk of Daxing coal.

Low-temperature oxidation characteristics of bituminous coal with different gas contents: a case study

ABSTRACT The present study tests the low-temperature oxidation characteristics of coal with different gas contents through the temperature-programmed experiment system. On the basis of the experimental data, the crossing-point temperature (CPT), law of coal heating, and coal consumption in the experimental process are obtained. Then, the energy conservation formula and coal oxygen kinetics formula of the low-temperature coal oxidation process are established, and the apparent activation energy and the reaction enthalpy of O2 for coal with different gas contents are calculated. Results show that when the gas content of coal increases, the CPT increases, the apparent activation energy increases, and the reaction enthalpy of O2 decreases.

Changes in spontaneous combustion characteristics of low-rank coal through pre-oxidation at low temperatures

This study investigated the changes in spontaneous combustion susceptibility of low-rank coal through preoxidation processing at low temperatures. The pre-oxidation processing on low-rank coal was conducted for a certain time at 60–150 °C in normal atmospheric conditions. The oxidation characteristics of coal at low temperature were investigated by measuring the temperature of coal and consumption of O2 gas during the pre-oxidation processing. Physical properties of coal and changes in crossing-point temperature (CPT) caused by the pre-oxidation processing were also analyzed. Higher the temperature for pre-oxidation, the more consumption of O2 gas in coal, and larger increase in temperature of the coal was observed. There were no significant changes in the weight of coal samples and calorific value in pre-oxidation processing upto 130 °C. It was found, from Fourier Transform Infrared Spectroscopy (FTIR) analysis, that the coal which underwent pre-oxidation processing upto 80 °C showed no significant difference from raw coal in terms of content. However, higher the temperature for preoxidation, larger decrease in aliphatic hydrocarbon and ether in the coal. As a result of CPT measurement, higher the temperature for pre-oxidation, greater the increase in CTP value of the coal. Therefore, it is expected to reduce the risk of spontaneous combustion susceptibility through the pre-oxidation method. From these results, it was confirmed that the spontaneous combustion susceptibility of the coal can be suppressed without a significant reduction in weight and calories through the preoxidation processing of low-rank coal under the proper conditions.

Kinetic Study of the Changes in Element Occurrence During Low-Temperature Oxidation of Coal

Complex macromolecular matrix of coal is divided into the elements of C, O, H, S and N, which are all involved into the concerned reaction. Based on the changes in element occurrence during coal oxidation at low temperature, the pseudo-first-order reaction kinetics was used to evaluate the kinetic characteristics of coal oxidation. Coats and Refern's as well as Horowiz and Metzger's models were also used to model the changes in element occurrence. The activation energies obtained by the pseudo-first order kinetic have been found to be almost similar to those calculated by applying the Coats and Redfern's equation and the energies are higher when Horowitz and Metzger's equation is applied. The release of element H shows the highest rate constants (K) and lowest activation energy value (E-a) when compared with these values for C and N. A kinetic compensation effect between K and E-a was observed for the changes in element occurrence.