Low-temperature Oxidation Characteristics Research Articles

Classification of Composite Hazards of Gas and Coal Spontaneous Combustion in Gob

ABSTRACT To prevent the occurrence of gas and coal spontaneous combustion composite disasters in gob areas during coal mining, this study considers the gob area of the Longfeng Coal Mine as the experimental background. By analyzing the pressure values of the hydraulic props in the isolation wall, the composition of gases in the gob area, and the low-temperature oxidation characteristics of coal, it was found that the pressure values of the hydraulic props were correlated with the concentration of gases in the gob area, temperature, and the g-factor and free radical concentration of coal. The absolute values of the correlation coefficients were ranked as T > O2 > CH4 > CO2 > g-values > Ng > N2, and all correlation coefficients were greater than 0.85. When the isolation wall was located in the stress concentration area, the wall deformed and cracked, leading to frequent gas exchange between the gob-side entry retaining (GSER) and the gob area, which increased the risk of gas and coal spontaneous combustion composite disasters in the gob area. According to the safety risk level, the areas adjacent to the gob area were divided into high-risk areas (0 ~ 100 m, 300 ~ 600 m, 700 ~ 900 m, 1050 ~ 1200 m), moderate risk areas (200 ~ 300 m, 900 ~ 1050 m), general risk areas (600 ~ 700 m), and low-risk areas (100 ~ 200 m), the positions at 500 ~ 550 m and 700 ~ 750 m were located in the gas and coal spontaneous combustion composite disaster area. The study suggests that for high-risk areas, the injection amount of fire extinguishing materials should be increased to strengthen safety assurance, whereas for low-risk areas, the injection can be moderately adjusted and reduced to achieve the optimal balance between economic benefits and safety benefits.

Classification of Composite Hazards of Gas and Coal Spontaneous Combustion in Gob

ABSTRACT To prevent the occurrence of gas and coal spontaneous combustion composite disasters in gob areas during coal mining, this study considers the gob area of the Longfeng Coal Mine as the experimental background. By analyzing the pressure values of the hydraulic props in the isolation wall, the composition of gases in the gob area, and the low-temperature oxidation characteristics of coal, it was found that the pressure values of the hydraulic props were correlated with the concentration of gases in the gob area, temperature, and the g-factor and free radical concentration of coal. The absolute values of the correlation coefficients were ranked as T > O2 > CH4 > CO2 > g-values > Ng > N2, and all correlation coefficients were greater than 0.85. When the isolation wall was located in the stress concentration area, the wall deformed and cracked, leading to frequent gas exchange between the gob-side entry retaining (GSER) and the gob area, which increased the risk of gas and coal spontaneous combustion composite disasters in the gob area. According to the safety risk level, the areas adjacent to the gob area were divided into high-risk areas (0 ~ 100 m, 300 ~ 600 m, 700 ~ 900 m, 1050 ~ 1200 m), moderate risk areas (200 ~ 300 m, 900 ~ 1050 m), general risk areas (600 ~ 700 m), and low-risk areas (100 ~ 200 m), the positions at 500 ~ 550 m and 700 ~ 750 m were located in the gas and coal spontaneous combustion composite disaster area. The study suggests that for high-risk areas, the injection amount of fire extinguishing materials should be increased to strengthen safety assurance, whereas for low-risk areas, the injection can be moderately adjusted and reduced to achieve the optimal balance between economic benefits and safety benefits.

PH-responsive HMDS terminated m-Cresol for inhibiting the spontaneous combustion in different-rank coals

In this study, pH responsiveness of 1, 1, 1, 3, 3, 3-hexamethyldisilazane terminated m-Cresol (HTMC) in different-rank coals was investigated, and its inhibition performance regarding the spontaneous combustion of coal was explored. First, color changes in the deprotection reaction of HTMC under different acidic conditions were analyzed. Then, surface microstructures of these coals and pH values of their filtrates were tested. Subsequently, experiments were performed on different coal samples for grasping their low-temperature oxidation characteristics, weight loss rates, pore characteristics, and free radical variations. The experimental results show that HTMC can effectively protect phenolic hydroxyl groups in inhibitors, and the acidity of coal can promote the conversion rate of its deprotection reaction. For instance, in XAM coal (pH ≈ 6), sufficient H+ stimulates HTMC to release massive m-Cresol which contains highly reactive phenolic hydroxyl, thereby effectively suppressing increases of the concentration and type of free radicals. The crossing point temperature (177.2 °C) and inhibition rate at 110 °C (75.9 %) of the HTMC-inhibited XAM sample are 33.1 °C and 17.9 % higher than those of the m-Cresol-inhibited one, respectively. This study confirms the applicability of pH-responsive HTMC to different-rank coals and may lay a theoretical foundation for the design of efficient reactive inhibitors.

Study on the kinetic characteristics and control steps of gas production in coal spontaneous combustion under the oxidation path

The low-temperature oxidation of coal involves multiple gas production paths including desorption, pyrolysis and oxidation. These paths interfere with each other, resulting in unclear kinetic characteristics of CO and CO2 production in coal oxidation and incomplete mechanism of gas production. Therefore, this paper proposed a method that can effectively exclude the effects of desorption and pyrolysis on gas production and adopted this method to explore the kinetic characteristics of CO and CO2 production under different paths. Besides, with the aid of microscopic characterization methods, the kinetic steps controlling gas production and the mechanism of gas production were analyzed. The experimental results demonstrate that the proposed method can effectively eliminate the effects of desorption and pyrolysis, and can obtain the kinetic characteristics of gas production under the oxidation path that can reflect the intrinsic characteristics of low-temperature oxidation of coal. Based on the difference method, the activation energies of CO and CO2 produced under the oxidation path are 59.49 kJ/mol and 60.09 kJ/mol, which activation energies are almost the same, suggesting that CO and CO2 are likely to be produced from the same precursor. The gas production in low-temperature oxidation is a process in which methylene groups in coal react with oxygen to generate oxygen-containing intermediates which then decompose to produce CO and CO2. The reaction of methylene groups with oxygen is the control step of gas production kinetics in coal spontaneous combustion, resulting in the same apparent activation energies of CO and CO2 production in the stable phase under the oxidation path.

Comprehensive evaluation of low-temperature oxidation characteristics of low-rank bituminous coal and oil shale

The coexistence of coal and oil shale mining conditions exacerbates the complexity of coal spontaneous combustion (CSC) prevention, so conducting in-depth research on the spontaneous combustion characteristics of coal, oil shale, and their mixed samples is necessary. A programmed heating system simulated the low-temperature oxidation process of bituminous coal and oil shale under different air volumes. It was found that CO concentration, C2H4 concentration, and Graham's fire coefficient (R1, R2) can be used as indicator gases to predict the CSC process. Compared with oil shale and mixed samples, bituminous coal has a higher CO production rate, oxygen consumption rate, heat release intensity, and lower apparent activation energy during the slow oxidation stage. Meanwhile, bituminous coal also has lower peak values of minimum float coal thickness, lower limit oxygen concentration, and maximal air leakage intensity, which means that bituminous coal in the goaf is more prone to oxidation. However, the abundant free hydrogen bonds in oil shale can stabilize the hydrogen-deficient functional groups of bituminous coal, which weakens the oxidation ability of the mixed sample during the accelerated oxidation stage. In addition, the increased air volume fraction increases the oxygen adsorption efficiency of the mixed sample throughout the low-temperature oxidation process, which means that the M70 sample has a higher CSC risk. The results have enriched the research on the spontaneous combustion characteristics of coal, oil shale, and mixed samples, which may strengthen the management and control of CSC in coal and oil shale co-mining mines.

Experimental investigation on microstructure fractal characteristics of low-temperature oxidation of gas-bearing coal

Experimental investigation on microstructure fractal characteristics of low-temperature oxidation of gas-bearing coal

Experimental and simulation study of inert gas mixture inhibiting coal spontaneous combustion

To explore the mechanism of inhibiting spontaneous combustion of coal by mixed gases, the low-temperature oxidation characteristics of coal under different components of mixed gases were analyzed. ESR and FTIR experiments were used to investigate the effects of different gas mixtures on the activity of coal during low-temperature oxidation and the oxidation reaction of coal surface functional groups. The mechanism of chemical oxygen inhibition of mixed gas was studied by density functional theory. The results show that the larger the CO2 component in the mixed gas, the higher the ability to inhibit coal oxidation. The concentration of free radicals in coal under dry air condition is higher than that under inert mixed gas condition during oxidation heating at 30–230 °C. The oxidation ability of –CH3, –OH and oxygen-containing functional groups in the mixed gas reaction is inhibited. Through quantum chemistry calculation, it is found that the mixed gas increases the activation energy of free radicals and reduces the heat release of the reaction. This study provides theoretical reference for coal mine thermal disaster.

Experimental Study on Low-Temperature Oxidation Characteristics and Ignition Boundary Conditions of Gasoline/Hydrogenated Catalytic Biodiesel

Article Experimental Study on Low-Temperature Oxidation Characteristics and Ignition Boundary Conditions of Gasoline/Hydrogenated Catalytic Biodiesel Sicheng Lai 1, Wenjun Zhong 1,*, Tamilselvan Pachiannan 2,3, Zhixia He 1,2, and Qian Wang 1 1 School of Energy Power Engineering, Jiangsu University, Zhenjiang 212013, China 2 Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China 3 School of Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China * Correspondence: wj_zhong@ujs.edu.cn Received: 25 September 2023 Accepted: 8 December 2023 Published: 14 December 2023 Abstract: This study investigates the ignition characteristics of Hydrogenated Catalytic Biodiesel (HCB) with pure gasoline, and diesel. The experiment aims to enhance the ignition characteristics of gasoline fuel by blending it with high-reactivity HCB. It provides experimental data for the fuel blends and offers dependable support for gasoline compression ignition mode applications. To achieve this, the ignition characteristics of various fuels were studied on a variable compression ratio test bench. Experiments were conducted by varying the equivalence ratio and intake temperature of the fuel blends. By analyzing CO emissions and the maximum in-cylinder temperature, we investigated the low-temperature oxidation characteristics of the fuels. Simultaneously, we identified the critical compression ratio and critical temperature as indicators of the ignition boundaries. Finally, this study examined the heat release behavior of the fuels at low temperatures and discussed their combustion characteristics under high-temperature conditions through the heat release rate analysis. The study indicates that with the increase of HCB in the blend, the low-temperature oxidation characteristics are significantly enhanced. However, the ignition boundaries are lowered, and more pronounced secondary heat release combustion behavior is observed. When the blending ratio reaches 50% namely G50H50, it exhibits low-temperature oxidation characteristics and a secondary heat release rate similar to diesel. Their critical compression ratios are 6.8 and 6.5, and the critical temperatures are 838.74 K and 881.41 K respectively. Hence the G50H50 blend holds the potential to serve as a substitute for diesel fuel in compression ignition engines. The increase in intake temperature and equivalence ratio can also enhance low-temperature oxidation characteristics and lower the ignition boundaries. Simultaneously, they exert a certain influence on the peak and phase of the heat release rate.

Low-temperature oxidation characteristics and reaction pathways of crude oil within tight shale during air injection

The investigation of low-temperature oxidation (LTO) of crude oil within tight shale holds significant importance due to its implications for subsequent oxidation reactions and enhanced oil recovery in the process of air injection. In this study, the tight shale sample underwent oxidation at various LTO temperatures, followed by an analysis of the resulting gas composition. Furthermore, the oxidized oil was separated from the tight shale and subjected to characterization using electron paramagnetic resonance, nuclear magnetic resonance, and negative ion electrospray Fourier transform-ion cyclotron resonance mass spectrometry techniques. The primary focus was on examining the distinct LTO reaction pathways observable across different temperature ranges. The findings demonstrated a correlation between LTO temperature and the concentration of free radicals, which predominantly resided on aromatic hydrocarbons, alkanes, and oxygen atoms. Additionally, the proton count of polycyclic aromatic hydrocarbons exhibited a continuous increase from 83 to 350 °C, suggesting intensified aromatization and condensation reactions involving aliphatic and aromatic compounds. With rising LTO temperature, the molecular structure of O2 compounds underwent significant transformations, characterized by increased condensation degree and a decrease in low carbon number molecular structures, while higher equivalent double bonds and carbon number molecular structures became more prevalent. The influence of cycle path 1 diminished at temperatures ranging from 83 to 150 °C and 250 to 350 °C, whereas the significance of cycle paths 2 and 3 increased, resulting in an overall escalation of the oxidation rate with temperature elevation. It was observed that the shale oil LTO process exhibited a negative temperature coefficient within the temperature range of 150-250 °C, emphasizing the criticality of overcoming the energy barrier in this region to achieve stable combustion. This comprehensive investigation provides valuable insights into the mechanisms underlying LTO in crude oil confined within tight shale.

CO-based early warning threshold for coal spontaneous combustion

Few studies have been involved in the CO-based early warning threshold, although carbon monoxide (CO) is a vital gas indicator for predicting coal spontaneous combustion. In this paper, based on the dependency relationship between CO concentration and coal temperature, a novel approach is proposed to determine the critical value of CO. Firstly, the low-temperature oxidation characteristics of coal samples were investigated through a temperature-programmed system, and the critical temperature of coal samples was analyzed. Furthermore, the functional relationship between CO emissions and coal temperature was investigated based on on-site measurements. Accordingly, the critical value of CO corresponding to the critical temperature was determined. Finally, the migration and generation behavior of CO was studied by combining with air seepage, component transport, thermal transfer, and exothermic reaction. The results show that when the coal temperature in goaf reaches the critical temperature of 53 °C, the CO concentration will reach 110.84 ppm. This concentration is the critical value of CO. The numerical simulation analysis of CO gas migration in goaf shows that when CO concentration reaches the critical value, the CO gas concentration in the corner of the upper corner, the return air flow, and extraction pipeline are 10.69 ppm, 7.91 ppm and 67.26 ppm, respectively. This concentration is the critical value of CO at the corresponding position. The approach established can meet the requirements of identification of spontaneous combustion potential of coal, and ensure the safety of mine production.

Reaction heat effect and change characteristics of key groups in coal-oxygen intrinsic reaction path.

The intrinsic reaction of coal with oxygen in the process of low-temperature oxidation is the main reaction path leading to self-heating and spontaneous combustion of coal. Most of the existing studies regard the coal oxidation as an overall reaction, ignoring the multi-path characteristics of coal low-temperature oxidation, and it is difficult to accurately explore the intrinsic reaction characteristics between coal with oxygen. Therefore, the low-temperature oxidation process of coal was studied by using a C80 microcalorimeter and in situ FTIR technology from the macro and micro levels. The "profile subtraction method" was used to study the coal-oxygen intrinsic reaction process, and the reaction heat effect and the change characteristics of key functional groups in the process were analyzed. Furthermore, the gray correlation analysis method was used to study the relevant characteristic parameters in the reaction process and grasp the essential structure-activity relationship. The experimental results show that, compared with the overall reaction process in air atmosphere, the change in the heat release of the coal-oxygen intrinsic reaction path has changed to different degrees, and the change in the slow oxidation stage is the most significant (the heat absorption decreases by 70.1-90.9%). In addition, the characteristic temperature points show different degrees of advance, of which the initial exothermic temperature point is the largest (about 21-46 °C), which directly leads to a significant shortening of the slow oxidation stage (30.1-47.4%). The changes of functional groups in the intrinsic reaction path are more regular. With the increase of temperature, the oxygen-containing functional groups -C=O and the aliphatic hydrocarbon functional groups -CH2- and -CH3 showed a fluctuating trend of increasing and decreasing, respectively. The oxidation heat-contributing functional groups of coal are mainly related to the degree of metamorphism and the functional group reaction characteristics during the reaction. With the deepening of coalification degree, the main heat-contributing functional groups as a whole showed the change rule of oxygen-containing functional groups → aliphatic hydrocarbon functional groups → aromatic hydrocarbon functional groups. In addition, the change of -OH content in the three coal samples has a high correlation with the change of the total heat release of coal.

Effect of rare-earth-containing inhibitors on the low-temperature oxidation characteristics and thermodynamic properties of coal

The frequent occurrence of coal spontaneous combustion (CSC) disasters poses a serious threat to green and safe coal production. Currently, inhibitor application is a crucial method for preventing CSC. In this study, layered double hydroxides (LDHs) were modified using the rare-earth element La3+ to prepare inhibitors (La/LDHs) with varying La3+ content. The prepared La/LDHs were characterized through X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Through a temperature-programmed method and thermogravimetry–differential scanning calorimetry, the effect of the prepared La/LDHs on the low-temperature (30–170 °C) oxidation characteristics and thermodynamic properties of coal were investigated, and the CSC inhibition mechanism of La/LDHs was determined. The results indicated that five La/LDH inhibitors were successfully prepared. The CO concentration, CO generation rate, and oxygen absorption rate of coal samples decreased after the addition of La/LDHs. Moreover, the CSC inhibition effect of the La/LDHs was the strongest at oxidation temperatures of 110–130 °C. The composite coal samples containing La/LDHs exhibited higher adsorption temperature (T0), critical temperature (T1), stagnation temperature (T2), growth-rate temperature (T3), and burnout temperature (T7) but lower mass-maxima temperature (T4), kindling-point temperature (T5), and maximum-weight-loss-rate temperature (T6). The La/LDHs shortened the heat release process of coal and considerably reduced the quantity of heat released, with a maximum reduction of 474 J/g. Moreover, the apparent activation energy increased in the mass gain stage and the thermal decomposition stage, with the highest increases of 148.7 and 233.2 kJ/mol, respectively. Among the prepared samples, La/LDH-3 and La/LDH-5 exhibited the strongest CSC inhibition effects. These research results have important practical implications for the prevention and control of CSC during coal production and storage, and for improving environmental protection and safety in coal production.

An experimental and kinetic modeling study on low-temperature oxidation of gasoline and diesel/polyoxymethylene dimethyl ether (OME3) blends

In this work, the low-temperature oxidation of polyoxymethylene dimethyl ether (OME3) with multi-component surrogates for diesel (DS:60%n-heptane/20%cyclohexane/20%toluene) and gasoline (GS:20%n-heptane/60%iso-octane/20%toluene) blends was investigated respectively using a jet-stirred reactor (JSR) with a temperature range of 500–950 K, constant fuel fraction of 0.008, residence time (τ) of 2 s and near atmospheric pressure. Two detailed multi-component kinetic mechanisms were established to describe the low-temperature oxidation characteristics and the interaction of blended fuels. Generally, the oxidation process of highly-reactive OME3 and n-heptane is inhibited, while that of inactive fuels, including iso-octane, cyclohexane and toluene, are triggered and enhanced in various degrees in the blends. The OH radical acts as a bridge between high and low reactivity components. It is of interest that n-heptane reveals a quite different low-temperature oxidation behavior in diesel (DS) and gasoline (GS) surrogate respectively. In GS, the low-temperature oxidation of n-heptane is significantly inhibited, while in DS, the inhibition is mild, mainly due to the much different low-temperature oxidation characteristics of iso-octane and cyclohexane. As for OME3, it mainly improves the reactivity of the blends in the negative temperature coefficient (NTC) process. As for low-reactivity fuels of iso-octane and toluene, their low-temperature oxidations are completely triggered and dominated by highly-reactive fuels due to their weak OH build-up ability. On the contrary, when the low-temperature oxidation of cyclohexane is triggered, its self-enhancement also plays a crucial role in addition to the promoting effect from highly-reactive fuels. Therefore, cyclohexane reveals a promoting-ignition effect on the blended fuels, while iso-octane and toluene act as an inhibiting-ignition factor in blends.

Experimental research on spontaneous oxidization combustion characteristics of coal after pre-adsorption CO2

ABSTRACT CO2 is injected into coal bodies in advance so that CO2 is fully adsorbed in the crack pores of the coal. It can inhibit the combination of coal and oxygen, so as to prevent and control the natural combustion of coal left in goaf. However, domestic and foreign scholars have little research on the low temperature oxidation characteristics and functional group changes of coal after CO2 pre-injection. In this article, a temperature programmed experimental device is used to analyze the formation of the macro-combustion index gas; a Fourier transform infrared spectroscopy experimental analyzer was determined the content of active functional groups of each component. The results showed that when the coal-sample pre-adsorbed CO2, the generation trend of CO2 formed an approximate “V” shape, the oxygen consumption decreased by about 4%. The initial generation temperature of CO and alkane gases is lagged, and the lag temperature is 20 ~ 30°C. CO2 occupied the surface of the coal surface, and inhibiting the oxidation response of oxygen-containing functional groups. The content of hydroxyl group increases, the oxidative decomposition rate of aliphatic hydrocarbon slows down, and the content of aromatic hydrocarbon increases, which can effectively slow down the course of coal oxidation and spontaneous combustion.

New insights on low temperature oxidation characteristics and possibility of auto-ignition in light oil reservoir

High pressure air injection (HPAI) can improve oil recovery by 10%–20% due to its unique oxidation heat effect. The high temperature and pressure thermostatic device is used to study the low temperature oxidation characteristics and oxidation kinetic parameters of light oil. Then a numerical model based on the oxidation kinetic parameters is established to study the possibility of auto-ignition of low temperature oxidation of light oil. Temperature rises due to the heat release of low temperature oxidation. Higher temperature makes low temperature oxidation more intense, so the reaction rate increases. When temperature is relatively low, the oxygen addition reaction dominant, the content of oxygen-containing functional groups increases continuously, and aromatic hydrocarbon is converted into resin. With the progress of oxidation, resin is further oxidized into asphaltene. When temperature rises, carbon bond stripping reaction begins to occur, which makes macromolecular oxygenated hydrocarbons in oil decompose into small molecules of hydrocarbons and CO2. After considering gas (mainly CO2 generated by oxidation and natural gas) dissolution into oil, the viscosity of underground oil after low temperature oxidation is lower than that of original oil, which improves the mobility of oil. The activation energy of oxygen addition reaction is 49.0 kJ/mol and pre-exponential factor is 8.72 × 103s−1. The activation energy of carbon bond stripping reaction is 68.9 kJ/mol and pre-exponential factor 1.6 × 106s−1. The static low temperature oxidation experiments are simulated by CMG-STARS module based on experimental results. The simulation results show that decreasing the heat loss of experiments makes the temperature rise rapidly, which indicates that the light oil with little asphaltene also has high oxidation activity, and auto-ignition can occur in the light oil reservoir for HAPI. The conclusions of the research can provide references for field applications of HPAI in light oil reservoir.

Evolutions of Oxidation Products and Free Radicals for Coal-Gangue Samples with Various Gangue Contents

ABSTRACT To investigate the influence of gangue left in goafs on the low-temperature oxidation characteristics of coal, the variations of gas products and free radical parameters of coal-gangue mixed samples with different gangue contents were tested. The results show the concentration of gas products is directly proportional to temperature and inversely proportional to gangue content (except the sample with 20% gangue content (G(20))). In addition, the oxygen consumption rate and heat release intensity follow a variation trend similar to that of gas product. Pure gangue corresponds to the highest apparent activation energy, 57.6 kJ/mol, and G(20) has the lowest value, 48.8 kJ/mol. With the rise of temperature, the free radical concentration of the mixed samples decreased slowly when it was lower than 90°C and higher than 210°C, and increased between 90°C and 210°C. Compared with the theoretical calculation results, the mixed samples all possess higher gas product, oxygen consumption rates, heat release intensities, free radical concentrations and lower apparent activation energy in the experiments, which indicates that gangue can remarkably promote the reaction of coal during low-temperature oxidation. As a result, the gangue left in goafs may promote the risk of coal spontaneous combustion.

Study on the inhibitory mechanism of dehydrogenated antioxidants on coal spontaneous combustion

In order to comprehensively and systematically analyze the reasons why antioxidant inhibitors can scavenge free radicals in coal and inhibit coal spontaneous combustion, this paper studies the effects of VC, TBHQ, EGCG and BHT on coal spontaneous combustion by means of coal spontaneous combustion characteristics experiments and quantum chemical simulation methods. The low-temperature oxidation characteristics of coal were studied through temperature-programmed experiments. The results showed that the CO emission of coal samples with antioxidants was significantly lower than that of raw coal. At 170°C, the maximum decrease was 37.74%. Fourier infrared test showed that compared with the coal samples without antioxidant treatment, the adsorption strength of hydroxyl structure and oxygen-containing functional groups of the treated coal samples was significantly reduced. The area percentages of hydroxyl and methylene changed significantly, decreased by 7.14% and 6.46%, respectively. Subsequently, molecular models of four antioxidants were constructed using quantum chemical theory, and their Mulliken charges, BDE values ​and frontier orbitals were calculated according to density functional theory (DFT), and the active sites and inhibition mechanisms of antioxidants were discussed. The results showed that H9 of VC, H33 of EGCG, H1 of TBHQ and H40 of BHT all had strong ability to scavenge oxygen-containing free radicals, and their order of strength was TBHQ > BHT > EGCG > VC. Antioxidant inhibitors mainly reduce the number of active free radicals by removing the peroxide groups in the initial stage of the coal oxygen reaction, and remove the hydroxyl groups to prevent the further spontaneous combustion of coal and inhibit the low temperature oxidation process of coal.

Study on the Influence of the Mechanical Crushing Process on Low-Temperature Oxidation Characteristics of Coal Samples

ABSTRACT In order to study the influence of the mechanical crushing process on the low-temperature oxidation characteristics of coal, the programmed heating experiments of coal samples with different particle sizes were carried out using a programmed heating device. By comparing and analyzing the changes in CO and CO2 emission, cross-point temperature, heat release intensity, and apparent activation energy of coal samples with different particle sizes during temperature heating, the influence of particle sizes on the spontaneous combustion characteristics of coal samples was obtained. On the basis of the infrared spectrum experiment and ESR (electron paramagnetic resonance) experiment of coal samples with different particle sizes, the changes of functional groups and free radicals were further compared, and the deep-seated reasons that the mechanical crushing process affected the spontaneous combustion characteristics of coal were explored. The results showed that with the decrease in particle size, more alkyl side chains are dehydrogenated to produce alkyl radicals during mechanical crushing, and the alkyl radicals are oxidized with oxygen to form more oxygen-containing functional groups. These oxygen-containing functional groups can accelerate the low-temperature oxidation process of coal in the subsequent secondary low-temperature oxidation process, release more heat, and increase the heat release intensity. In the process of mechanical crushing, the coal sample not only changes in physical structure but also changes in chemical structure. With the decrease in particle size of coal samples, more CO and CO2 are produced in the process of low-temperature oxidation of coal, and the time to reach the cross-point temperature is advanced. However, the cross-point temperature and the apparent activation energy decreases, and the spontaneous combustion characteristics of coal are enhanced.

An experimental and kinetic modeling investigation on low‐temperature oxidation chemistry of 1,3,5‐trimethylcyclohexane in a jet‐stirred reactor

Very scarce studies aiming at the low-temperature chemistry of multi-alkylated cycloalkanes were reported until now, which widely exist in real transportation fuels. Thus, this work presents the first study on the low-temperature oxidation characteristics of 1,3,5-trimethylcyclohexane (T135MCH) in an atmospheric jet-stirred reactor (JSR) combined with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0) and fuel-rich (ϕ = 1.5) conditions. Abundant quantitative information of species is acquired including reactants, major products, reactive hydroperoxides, fuel-specific cycloalkenes and other C1–C5 oxygenated intermediates. Pronounced three-stage oxidation behavior was observed including two fuel decomposition processes and one negative-temperature-coefficient (NTC) window. A detailed model incorporating both high and low-temperature reaction scheme was proposed and validated against the measured data with reasonable agreement. Representative large oxygenated intermediates were detected in this work, such as cyclic ether (CE), cyclohexylhydroperoxide (ROOH), ketohydroperoxide (KHP), olefinic hydroperoxide (OFHP) and C9 acyclic/cyclic bi-carbonyl species whose signal profiles were further compared with the simulated mole fraction profiles justifying the occurrence of specific low-temperature chemistry. Based on the modeling analysis, the competition between the chain-branching processes forming OH and chain-propagating processes forming less reactive HO2 or CE was found to determine the distinct oxidation behavior at different temperature regions. As temperature rises, chain-branching processes are inhibited and the accumulation of HO2 is accelerated, which lead to the occurrence of self-combination forming H2O2, a chain-termination process, resulting in the NTC behavior. With the arrival of high-temperature region, H2O2 becomes thermally unstable and tends to dissociate to double OH via a chain-branching process, therefore triggering the second fuel decomposition via H-atom abstractions.

An experimental and modeling study on polyoxymethylene dimethyl ether 3 (PODE3) oxidation in a jet stirred reactor

Polyoxymethylene dimethyl ether (PODEn, n ≥ 1) is a class of oxygenated fuels containing unique carbon-oxygen chain structure and a promising alternative fuel for diesel engines. In this study, low-temperature oxidation characteristics of PODE3 were studied experimentally and numerically. Experiments were performed in a jet-stirred reactor (JSR) at equivalence ratios of 0.5, 1.0 and 2.0, in the temperature range of 500 to 950 K, and at atmospheric pressure. Mole fractions of PODE3, O2, H2, CO, CO2, CH3OH and C1-C2 hydrocarbons were measured by gas chromatograph (GC). Experimental measurements were compared with the simulation results based on two literature low-temperature oxidation models, denoted as the He model and the Cai model, respectively. Good agreement was obtained between the measured and simulated fuel consumption profiles, while a deviation was observed between the experimental and simulation results on the mole fractions of O2 and intermediate products at medium temperatures. Reaction pathway analyses based on the two models were performed, revealing that the second O2-addition reaction pathway is more significant in the prediction by the Cai model than that by the He model. Sensitivity analyses pointed out that the most important reactions affecting fuel consumption are the H-abstraction reactions of PODE3, and the decomposition of H2O2 and the consumption of CH2O become more sensitive at medium temperatures.