Low-temperature Oxidation Stage Research Articles

Effects of water immersion on the pore structure and thermodynamic properties of coal gangue

A large amount of coal gangue (CG) produced in coal mining accumulates to form CG hills, and the combustion properties change under the effect of leaching, increasing the risk of spontaneous combustion. The study was mainly carried out by nitrogen adsorption, infrared spectroscopy and thermogravimetry on the physicochemical properties of the CG under different immersion times (30, 60, 90 days). And the activation energy of the immersion CG at different oxidation or combustion states was calculated. The results show that the pyrite contained in gangue is oxidized after immersion, and the heat released by oxidation and the acid solution formed have an obvious effect on the pore expansion of gangue. The mesoporous and macroporous increase by 2.61% and 0.95% respectively, which strengthens the physical and chemical adsorption of oxygen for CG. The immersion process also destroys the structure of the gangue, transforming stable functional groups into active ones, especially the most significant increase in hydroxyl and oxygen-containing functional groups. The characteristic temperature changed under the action of water immersion, the cracking temperature increased, and other characteristic temperatures decreased, among which the ignition temperature decreased by about 20℃. In addition, the water immersion process inhibits the low-temperature oxidation stage of CG and promotes the high-temperature combustion stage, and the risk of spontaneous combustion is the strongest after 60 days of immersion in water. The results can provide guidance for the prevention of spontaneous combustion in drenched CG hills.

Analysis of the wetting and exothermic properties of preoxidized coal and the microscopic mechanism

In the processes of coal mining and transportation, low-temperature oxidation occurs continuously, and it is accompanied by multiple wetting. The effect of water on coal spontaneous combustion (CSC) cannot be ignored. To study the wetting and exothermic properties of preoxidized coal and the mechanism of its effect on CSC, molecular dynamics simulations, microcalorimetry, scanning electron microscopy (SEM) and fourier transform infrared spectrometer (FTIR) are used to analyze the molecular dynamics mechanism, heat of wetting (Hw), surface structure and microscopic group changes. Experimental results show that the surface potential distribution of coal is uneven, which leads to the adsorption of polar water molecules; the total energy of the system continues to decrease during the adsorption process. The preoxidation treatment further develops pore fissures, increases the percentage of hydrophilic groups and evaporates the original water. Therefore, in the coal low-temperature oxidation stage, the higher the degree of preoxidation is, the greater the Hw value. After preoxidation and wetting, the loose and porous characteristics of coal are further reflected; the oxygen percolation and adsorption are enhanced, which accelerates the system heat accumulation and thus intensifies the risk of CSC. The experimental results provide an important theoretical basis for the prevention of CSC.

Thermo-oxidative behavior and kinetics analysis of light and heavy oils based on TG, DSC, and FTIR

In-situ combustion (ISC), as a time-honored replacement technology for heavy reservoir development, needs to improve its application effect urgently. The oxidation exothermic behavior and reaction activity of heavy oil directly determine the EOR effect of ISC technology during air injection. In this work, the oxidation heat effect and combustion activity of light and heavy crude oils, and the physical properties of their oxidized oils were studied and compared through thermogravimetry (TG), differential scanning calorimetry (DSC), static non-isothermal oxidation and Fourier transform infrared spectroscopy (FTIR). The results indicate that the main reaction pathway of MH light oil is distillation and volatilization under 200 °C, the mass loss is obvious and the heat release is more than that at the high temperature oxidation (HTO) stage. After the full reaction of FC heavy oil at 350 °C, there is still about 30% low temperature oxidation (LTO) left. After the LTO and fuel deposition (FD) stages, heavy oil can produce more coke. MH light oil and FC heavy oil show an obvious mass loss at the stage of LTO and HTO, which is 68.8% and 27.3% (10 °C/min) respectively, the corresponding enthalpies are 3.02 kJ/g and 15.668 kJ/g as well. Between 470 and 500 °C, the endothermic reaction of heavy oil becomes weak, which may relate to the condensation of heavy components such as resins that release heat. The LTO activation energy of light oil decreases with the conversion, and temperature increase can promote the oxidation reaction of active components. The LTO activation energy of heavy oil increases with the conversion, while that in FD and HTO stage increases obviously, which indicates that coke combustion activity is low. The content of C1–C6 in heavy oil decreases from 5.81% to 3.57% with heating rates. The oxidation reaction in the full temperature range can improve the quality of heavy crude oil. The vibration peak at 1707 cm-1 of the MH oxidized oil gets weak, the vibration peak at 1539 cm-1 disappears, and the number of carboxyls, ether groups, aromatic rings, and miscellaneous molecules in the oxidized oil declines. The vibration peak at 3460 cm-1 of the oxidized oil enhances significantly, and the number of carboxyls and hydroxyl molecules increased.

A numerical simulation investigation on low permeability reservoirs air flooding: Oxidation reaction models and factors

A precise oxidation reaction model determines the reliability of numerical simulation results and the prediction of field performance of air injection-based enhanced oil recovery (EOR). By reviewing the classical oxidation reaction model and combining with geological characteristics of a low permeability reservoir, this work creatively divides pseudo-components and temperature regions and further proposes the improved low temperature oxidation (LTO), middle temperature oxidation (MTO), and high temperature oxidation (HTO) numerical models with chemical reaction coefficients. The EOR effects of reservoir temperature, permeability, air flow rate and bottom hole pressure during LTO and full temperature regions are determined on the basis of a 3D reservoir simulation model via CMG STARS. The results show that peak oil production rates of N2 flooding and LTO are 17.70 and 29.97 m3/d, respectively. The cumulative oil production in LTO stage is about 35%, which is higher than that in N2 flooding. The oxygen addition reaction in LTO stage can release much heat and increase sweep efficiency from thermal expansion and viscosity reduction. The peak oil production rates of LTO and full temperature oxidation (FTO) reactions are 42.16 and 44.34 m3/d, respectively, and the cumulative oil production of the FTO model was about 17% higher than that of LTO. The FTO model could more truly reflect the production performance during air flooding of low permeability oil reservoir. Strong oxygen consumption capacity and significant heat release effects can be observed under higher initial temperature, permeability, air flow rate, and lower bottom hole pressure. Thermal flooding and flue gas flooding are the main EOR mechanisms in FTO process. Considering the inescapable field problem of gas channeling, thorough contact between oil and air can be promoted by artificially preheating oil reservoir and adjusting air injection rate and pressure. Thus, oil recovery can be further improved under the premise of ensuring the safety of field application of air flooding technique.

Valorization of the microalgae fixing CO2 from flue gas by co-hydrothermal liquefaction with high-protein microalgae: Denitrogenation of bio-oil by ash and high energy recovery

Microalgae have significant potential for carbon sequestration due to their high photosynthesis efficiency. In this study, co-hydrothermal liquefaction (co-HTL) of these high-ash microalgae (Scenedesmus) and the high-protein microalgae (Spirulina) was carried out to obtain the high quality and high energy recovery bio-oil. The results show that co-HTL facilitates the conversion of organic matter in microalgae to bio-oil and gas products. Compared with individual Spirulina HTL, the contents of carbon and hydrogen in bio-oil derived from 7:3 (Spirulina to Scenedesmus mass ratio, BO-7:3) co-HTL increase to 74.09% and 8.83%, and the nitrogen content is significantly reduced to 6.67%. Besides, BO-7:3 has a maximum energy recovery of 94.64%. The relative contents of nitrogen-heterocyclic compounds and amides/amines in bio-oil derived from co-HTL decrease, and that of hydrocarbons increase. The actual value of ammonium ion conversion of nitrogen in microalgae is greater than the theoretical value, indicating that ash can effectively promote the conversion of nitrogen in microalgae to the aqueous product. The DTG curve of BO-7:3 has obvious peaks in low-temperature oxidation stage and high-temperature oxidation stage. Besides, co-HTL can effectively reduce the release of nitrogen-containing gas pollutants, which is beneficial to the application of microalgae-derived bio-oil.

Microstructure changes and reaction process of cotton at low-temperature oxidation stage

Low-temperature oxidation is an important stage before spontaneous combustion of cotton. It is of great significance to explore the changes of cotton microstructure during low-temperature oxidation to prevent cotton fire. The typical long-staple cotton and unginned cotton were selected for pre-oxidation treatment. The oxidation characteristics and apparent activation energy of cottons at different oxidation temperatures were studied by TG and DSC measurement. The micro characteristics of cottons were studied by SEM, X-ray diffraction and FTIR. The results showed that low-temperature oxidation stage of cotton could be divided into initial pre-heating stage (60 °C ∼ 90 °C) and rapid oxidation stage (120 °C ∼ 210 °C). The cotton was mainly in the physical adsorption stage below 90 °C, the apparent activation energy of cotton was positive, and the content of various functional groups did not change significantly. At 90 °C ∼ 210 °C, the cotton was mainly at the stage of chemical adsorption, the apparent activation energy was negative, and the content of various functional groups changed significantly. 120 °C could be regarded as the active temperature point, which the average diameter and crystallinity of long-staple cotton and unginned cotton were the largest. At 120 °C, cellulose macromolecules began to depolymerize. The glucose ring of cellulose broke to form l-glucose, which was then broken down into CO2 and water. Further, the reaction process of cotton at low-temperature oxidation stage was proposed.

Study on oxidation kinetics of low-rank coal during the spontaneous combustion latency

Spontaneous combustion latency is the initial stage of low-temperature oxidation of coal. This stage provides the original heat for coal spontaneous combustion, which has great research value. However, the oxidation process at this stage is difficult to obtain due to the low oxidation reaction rate of coal in the spontaneous combustion latency. Therefore, we designed a closed oxygen consumption experimental device. This device can realize automatic monitoring and data acquisition of the experimental gas concentration. This device was used to carry out isothermal oxidation experiments on coal at different temperatures and oxidation times. The reaction order and reaction rate constant of the kinetics of the main stage of isothermal oxidation of coal in spontaneous combustion latency are mainly studied. The key active groups that affect the reaction rate constant are discussed by the corresponding Fourier transform infrared spectroscopy experiments. The results show that the reaction order is close to 1 at 30 °C. Then, the reaction order showed an overall downward trend with increasing temperature. The reaction rate constant increases with increasing temperature. As the oxidation times increases, the reaction rate constant decreases gradually, and aliphatic groups show a decreasing trend. Aliphatic structures are key active groups that affect the reaction rate constant during the spontaneous combustion latency. This study helps to improve the understanding of the initial stage of low-temperature oxidation of coal and guide the prevention of coal spontaneous combustion.

Thermal decomposition characteristics of the tire pyrolysis oil derived from a twin-auger reactor: Study of kinetics and evolved gases

This study presents the characterization of the thermal decomposition (oxidation and pyrolysis) behavior of the oil (TPO) derived from the pyrolysis of End-of-Life Tires (ELT) in a twin-auger pyrolyzer, by means of thermogravimetric and calorimetric analyses, coupled with Fourier transform infrared spectroscopy (TG–FTIR). TPO oxidation and pyrolysis were conducted using air and N2, respectively, at three different heating rates (5, 10, and 20 °C/min) in a temperature range between 30 and 700 °C. Along the temperature program, the evolved gases were directed to the FTIR cell, where the functional groups within species present were measured. A global kinetic analysis was performed for the TPO oxidation, using four isoconversional methods, as well as the distributed activation energy model (DAEM) developed by Miura and Maki. The results obtained suggest that the oxidation process of TPO can be divided into three different reaction stages, namely: low-temperature oxidation (LTO) (<400 °C), fuel decomposition (400–500 °C), and high-temperature oxidation (HTO) (500–700 °C). Within the LTO stage, oxygen addition reaction to produce hydroperoxides were considered dominant in the initial stages, while the decomposition of the formed hydroperoxides was more significant at the later stage. Due to the characteristics of TPO, e.g., the presence of highly volatile compounds, the evaporation of hydrocarbons played an important role within the LTO stage. In the fuel decomposition stage, the formation of coke by the oxidative cracking of LTO residue and oxygen addition were believed to be the main reactions leading to gaseous products such as CO, CO2, H2O. Finally, in the HTO stage, the oxidation of coke was considered as the main reaction, exhibiting an evident exothermic activity. The activation energy distribution shows a similar pattern among the isoconversional methods and DAEM, with fluctuations between 40 and 200 kJ/mol. The analysis presented in this work sheds new light on the thermal decomposition of oil derived from the pyrolysis of ELT, which is relevant for its use in combustion systems.

Experimental study and thermodynamic analysis of coal spontaneous combustion characteristics

Spontaneous coal combustion is one of the most common disasters in coal mine production. In order to explore the mechanism of coal spontaneous combustion more deeply, coal samples from the Yangdong wellfield of Jizhong Energy were selected for oxidative heat energy analysis experiments. A temperature-programmed experiment was selected to study the changes in characteristic parameters during the low-temperature oxidation of coal under different air supply conditions. TG-DSC experiments were conducted to study the characteristic temperature changes and thermodynamic characteristics of coal combustion processes at different heating rates. The study results show that the coal is most easily oxidised in the low-temperature oxidation stage when the air supply is 120 ml/min. The oxygen consumption rate, CO generation rate, and maximum and minimum heat release intensity are all greater at this airflow than under other conditions. The process of spontaneous combustion of coal has six characteristic temperature points and is divided into five stages. The characteristic temperature of the coal sample increased with the increase of the heating rate, and the TG/DTG curve showed a hysteresis phenomenon. DSC temperature curve shifts toward the high temperature with the increase of the heating rate, and the exothermic region is expanded. Isokinetic analysis (F-W-O and V-W) and Coats-Redfern model for calculating thermodynamic parameters. The activation energy of the samples decreased with the increase of the heating rate in the range of 2∼20°C·min−1 and showed a decreasing trend with the increase of the conversion rate.

Experimental investigation on using antioxidants to control spontaneous combustion of lignite coal

To investigate the effects of antioxidants on the microstructure and characteristic parameters of coal during the oxidation process of coal, three antioxidants (tea polyphenols, resveratrol, and β-carotene) at different mass ratios (1 wt%, 3 wt%, 5 wt%, and 7 wt%) were selected to treat raw coal samples. Fourier transform infrared spectroscopy was conducted to explore the changes in functional group content before and after antioxidant treatment of coal samples. The results show that the content of functional groups in coal had a trend of decreasing and then increasing with a rising mass fraction of antioxidants, and the optimum mass ratio of antioxidants was predicted. On this basis, the effects of antioxidants on characteristic temperature points and gas products of coal samples were determined by the thermogravimetric-infrared experiment. The test results show that the critical temperature and dry cracking temperature of the antioxidant-treated coal samples increased to different degrees compared to the raw coal during the low-temperature oxidation stage. Meanwhile, the concentration and yield of CO, CO2, and CH4 of the antioxidant-treated coal samples showed a decreasing trend compared to raw coal. The chemical kinetics method was used to determine the apparent activation energy in the process of coal oxidation. The apparent activation energies of TP, Res, and β-Car inhibited coal samples in the stage of water evaporation and gas desorption increased by 39 %, 30 %, and 26 %, respectively, compared with the raw coal, while the apparent activation energies in the oxygen absorption and weight gain stage were respectively increased 19.4 %, 12.9 %, and 14.3 % higher than raw coal. The ability of antioxidants to inhibit the spontaneous combustion of coal is TP > Res > β-Car. The results enrich the research on antioxidant inhibition of coal spontaneous combustion and provide a theoretical basis for preventing coal spontaneous combustion.

Catalytic capacity evolution of montmorillonite in in-situ combustion of heavy oil

Clay minerals such as montmorillonite are significant in petroleum storage and recovery. Montmorillonite (Mt) is a natural solid acid catalyst containing Brønsted and Lewis acid sites and can catalyze heavy oil oxidation (HOO) during in-situ combustion (ISC). However, its catalytic activity largely depends on the heating process of heavy oil. In the study, the catalytic capacity evolution of Mt in ISC of heavy oil was explored with the isoconversional method and thermal analysis. In low-temperature oxidation (LTO) and high-temperature oxidation (HTO) stages, the addition of Mt largely reduced the peak temperatures (Tpeak) of DTA and DTG and the Tpeak of H2O (LTO and HTO stages) and CO2 (HTO stage) in HOO. Notably, Mt did not always show the positive catalytic effect. In LTO stage, the thermal transformation process had a small effect on DTA Tpeak, Tpeak of H2O and CO2, and the reaction activation energy (Eα) of HOO. In HTO stage, the catalytic effect of Mt on HOO was related to its thermal transformation process, especially interlayer water (IW) dehydration and dehydroxylation. In HTO stage, the IW dehydration process of Mt showed the strongest catalytic effect on HOO, significantly reduced DTA Tpeak and Eα, and resulted in the lowest Tpeak of H2O and CO2. The dehydroxylation process of Mt also showed a certain effect, whereas the overlapping process between the dehydration (IW) reaction and partial dehydroxylation reaction of Mt had no positive catalytic effect on HOO. DTA, TG-FTIR, and non-isothermal kinetic analysis further confirmed that the catalytic capacity of Mt in HTO stage was improved by IW dehydration process of Mt and weakened by the dehydroxylation process of Mt.

Spontaneous combustion characteristics and mechanism of water-immersed and air-dried brown coal

ABSTRACT To reveal the influential laws and mechanism of water immersion on the spontaneous combustion traits of lignite in the low-temperature oxidation stage. Raw coal (HL), water-immersed for 30 days (HL-S30), 90 days (HL-S90) and 150 days (HL-S150), were taken as the research objects. The pore evolution characteristics and functional group migration law of coal were explored by Scanning Electron Microscopy (SEM), Low-temperature Nitrogen Adsorption Test (LNAT) and Fourier Transform Infrared Spectroscopy (FTIR). The low-temperature oxidation procedure was simulated by Programmed Heating Test (PHT), and the gas release characteristics, exothermicity and ultimate spontaneous combustion parameters of the relic coal were compared. These findings demonstrated that the discriminant indexes of spontaneous combustion tendency of four specimens change periodically with temperature, but generally the spontaneous combustion intensity of HL-S30 and HL-S150 is greater than HL, and HL-S90 is the least. The micro reasons for increasing oxidation activity of HL-S30 and HL-S150 are as follows: the number of micropores of water-immersed coal is almost 3 ~ 4 times that of HL, and the pores expanded with immersion which helps to enhance the adsorption of oxygen; the content of broken aromatic hydrocarbon -C = C- reached 50%, the total amount of hydroxy -OH decreased to 2/3 of HL, while the content of carboxyl -COOH and ether-oxygen bond -C-O- of active group doubled which are beneficial to accelerate the process of free radical chain reaction, and promote the gas release and the temperature of residual coal to reach the ignition point. The research findings enrich the basic theory of spontaneous combustion traits of water-impregnated coal, which help reduce the fire accidents and hidden dangers in actual production.

Effects of kaolinite and its thermal transformation on oxidation of heavy oil

Kaolinite is a common 1:1 layered clay mineral in oil reservoirs and also a natural solid acid catalyst. Some studies have shown that kaolinite has a catalytic effect on the oxidation of heavy oil. Its catalytic effect depends on the type and amount of acid sites and is closely related to its thermal transformation, particularly the dehydroxylation process. The effects of kaolinite and its thermal transformation on the oxidation of heavy oil were investigated with thermal analysis and the Ozawa-Flynn-Wall isoconversional method. The results revealed that kaolinite had significant effects on the High-temperature oxidation (HTO) and Fuel deposition (FD) stages of heavy oil oxidation, but its effect on the Low-temperature oxidation (LTO) stage was small. In the HTO stage, the temperature at which the CO2 release rate peaked decreased dramatically, whereas the temperature at which the H2O release rate peaked increased remarkably. With the increase in the content of added kaolinite, the maximum activation energy in the FD stage of the oxidation reaction of heavy oil and the average activation energy in the HTO stage gradually decreased. It was noteworthy that the transformation of kaolinite upon thermal treatment, especially the dehydroxylation process above 400 °C, resulted in a progressive decline in the peak temperature in the HTO stage of heavy oil, but had little influence on that in the LTO stage of heavy oil oxidation.

Characteristics of coal-oxygen chemisorption at the low-temperature oxidation stage: DFT and experimental study

Oxygen chemisorption on the surface of coal plays a key role in the low-temperature oxidation of coal. Due to the complex molecular structure of coal, it is difficult to fully analyse and characterize coal-oxygen chemisorption, so its mechanism remains unclear. Based on the simple coal molecule active groups, the reaction pathway, electron transfer and heat of adsorption of oxygen adsorbed by each active group in the coal molecule were calculated by density functional theory (DFT). The characteristics of chemisorption and its relationship to the molecular coal structure at different temperatures were studied experimentally. The results showed that carbon radicals (·C radicals) were the oxygen chemisorption sites, and the activation energy of free hydroxyl radicals for hydrogen extraction to produce ·C radicals was 0.1–5.02 kJ/mol, that of methyl radicals was 18.43–26.32 kJ/mol, and that of oxygen radicals was as high as 107.56–132.17 kJ/mol. The free radicals in coal play a major role in the production of chemisorption sites at room temperature. Oxygen was chemically adsorbed on the coal surface in the form of peroxide radicals (COO·), binding to the ·C radicals with a binding energy of −0.2935 eV. Then, the chemical reaction generated gas, free radicals and new ·C radicals. The coal-oxygen chemisorption reaction sequence of different temperature stages was constructed to determine the key reaction of chemisorption. This study could improve the understanding of coal oxidation at low temperatures and guide the prevention of spontaneous coal combustion.

Microstructure Changes and Reaction Process of Cotton at Low-Temperature Oxidation Stage

Microstructure Changes and Reaction Process of Cotton at Low-Temperature Oxidation Stage

Gray Correlation Analysis of CO Generation Pattern during Process of Low-temperature Spontaneous combustion of Jurassic Coal

ABSTRACT Spontaneous combustion of coal is a phenomenon that exists in all coal mining countries. The characteristics of CO production during the spontaneous combustion of coal are the focus of attention of researchers. A novel programmed oxidation heating simulation system with a small-sample size was developed to study the combustion characteristics. The device was used to study the characteristics of CO generation during low-temperature coal oxidation under different conditions (three coal ranks, six particle sizes, four air volumes, and four oxygen concentrations). The microscopic characteristics of the coal structure were tested through Fourier-transform infrared spectroscopy (FTIR), and three types of active functional groups were observed. Ten factors affecting gas production in the low-temperature oxidation stage were analyzed by gray correlation. This study found that moisture in coal had the greatest effect on CO generation, followed by O-containing functional groups. Aliphatic hydrocarbons had the least correlation with CO generation. Further, the CO concentration increased exponentially during the heating process in low-temperature oxidation. In the low-temperature stage, carboxyl, carbonyl, and side-chain C–O bonds of the benzene ring were the main sources of CO generation.

Laboratory investigation of the spontaneous combustion characteristics and mechanisms of typical vegetable oils

To effectively prevent and control the autoignition of vegetable oil, the spontaneous combustion characteristics and oxidation mechanisms of three typical vegetable oils were investigated. Microcal, TG-DSC, Raman spectroscopy and GC-MS were used to analyze the self-ignition characteristics of the samples at both macro- and micro-scales. The results showed that the initial exothermic temperatures were between 65 and 90 °C, after which vigorous oxidation began to release abundant heat. DSC data showed that the entire combustion process of the samples includes 4 typical stages: low-temperature oxidation stage, thermal decomposition stage, combustion stage, and after-combustion stage. The activation energy of the low-temperature oxidation stage reached as low as 33.13 kJ/mol, indicating that the vegetable oil is extremely prone to self-oxidation even when the temperature <200 °C. And the self-ignition tendency was in the order linseed oil > perilla seed oil > safflower seed oil. Based on analysis of the organic molecular structures, it was found that the unsaturated groups in vegetable oil are responsible for its autoignition behavior. Taking linseed oil as an example, 3 peroxide intermediates, namely 10,12-Octadecadienoicacid,9-hydroperoxy-, (10E,12Z)-;9,11-Octadecadienoicacid, 13-hydroperoxy-, (9Z,11E, 13S)-, and 9-Octadecenoic acid, 13-hydroperoxy-, (Z)-(8CI, 9CI) were marked as significant indicators to trace the low temperature oxidation reaction path of the oil samples.

An novel experimental study on the thermorunaway behavior and kinetic characteristics of oxidation coal in a low temperature reoxidation process

The coal spontaneous combustion is a great threat to the coal seam mining, personal safety and ecological environment. The coal spontaneous combustion in the underground goaf refers to a reoxidation process of oxidation coal. In this paper, the coal thermorunaway behavior is introduced in the low-temperature oxidation stage as a potential early-warning index for the spontaneous combustion prevention. A novel test platform using mesh basket is employed to reveal the kinetic characteristics of oxidation ordos lignite. Experimental results show that the thermorunaway temperature of ordos lignite is about 135 °C. The coal structure collapse occurs together with a plummet in the coal sample mass. As the coal oxidation level rises, the thermorunaway temperature and maximum mass gradient pose an advance rule, and the 80 °C oxidation coal creates a higher mass gradient than 40 °C and 60 °C. The oxidation coals possess more mesopore and macropore with the total specific surface area decline by 37.9% from raw coal to 80 °C oxidation coal. The coal hardness exhibits a reduction as the oxidation level rises that promotes the structure collapse. The peak area of –CH2 and –CH accounts for 65.3% in 60 °C oxidation coal and –CH3 plays a dominated role for 80 °C oxidation coal thermorunaway.

Influence of air supply on coal spontaneous combustion during support withdrawal in fully mechanized coal mining and its prevention

It is necessary to change the air supply rate of the working face during the withdrawal of fully mechanized mining, making it important to study the oxidation characteristics of coal samples under different air supply rates. Through a self-made temperature-programmed experimental device, our focus was on studying the change laws of indicator gases released during the low-temperature (303.15–473.15 K) oxidation stage when the air supply rates of the coal samples were 0.67, 1.33, 2, 2.67, and 3.33 mL/s. The experimental results showed that the air supply increased, the concentrations of CO, C2H6, C3H8, C2H4, and C2H2 generated by the coal sample at the same temperature decreased, and the oxidation process decelerated. The initial temperatures of the four hydrocarbon gases were delayed to varying degrees with the increase in the air volume, and C2H4 was found to be more suitable as a hydrocarbon gas for the early warning of coal spontaneous combustion. Surface fitting was applied to analyze the change law of the CO generation rate under the combined effect of temperature and air supply; the change was divided into three stages. The CO concentration model at the upper corner of the working face during the withdrawal period was deduced, and comprehensive safety measures were put forward to prevent coal spontaneous combustion during the withdrawal period.

Effect of Oxygen Concentration on the Oxidative Thermodynamics and Spontaneous Combustion of Pulverized Coal.

Spontaneous combustion of pulverized coal has become a safety topic and has been extensively researched. This study using differential scanning calorimetry investigated the exothermic characteristics and spontaneous combustion risk of three metamorphic pulverized coal samples during oxidative combustion, for oxygen concentrations of 21, 19, 17, 15, 13, 11, 9, 7, and 5 vol %. Results indicated that decreased oxygen concentrations reduced exothermic intensity and substantially increased ignition temperatures. The oxidative thermal release observed during the combustion stage was conspicuously higher than during the low-temperature oxidation stage. Thermal release during low-temperature oxidation was low during low oxygen concentrations; however, when the oxygen concentration was less than 13.0 vol.%, it had a considerable influence on exothermic combustion. When the oxygen level was lowered from 21.0 to 5.0 vol %, spontaneous combustion risk indexes lessened from 2.07 (sample A), 1.85 (sample B), and 0.81 [J/(mg min °C2)] (sample C) to 1.08 (sample A), 1.13 (sample B), and 0.40 [J/(mg min °C2)] (sample C), respectively. Both apparent activation energy and spontaneous combustion risk indexes of the samples decreased saliently as oxygen concentration decreased. Thus, reducing oxygen concentration would be an effective method of inhibiting or possibly even preventing the spontaneous combustion of pulverized coal.