Low-temperature Oxidation Experiments Research Articles

Study on Secondary Oxidation Risk of Coal with Variable Soaking Time under the Influence of Oxygen Concentration.

Primary and secondary low-temperature oxidation experiments were conducted on lignite immersed in water for short (50 days) and long (200 days) periods under three different oxygen concentrations (8, 18, and 21%), which provided a theoretical basis for the identification and risk judgment of a spontaneous combustion zone in goaf. The results indicate that in both the short-term water-immersed coal (STWIC) and long-term water-immersed coal (LTWIC), the apparent activation energy (Ea) for the three stages of secondary oxidation is lower than that for primary oxidation under 18% oxygen concentration, suggesting a greater risk of spontaneous combustion. The STWIC and LTWIC that have experienced a primary oxidation in the goaf are more prone to secondary oxidation spontaneous combustion when the oxygen concentration is 8 and 18%, while the risk of secondary oxidation spontaneous combustion is lower than that of primary oxidation at 21%. During the experimentation, a temperature inflection point was observed, which decreased with an increase in the oxygen concentration conditions. Beyond this temperature inflection point, the oxygen consumption rate and heat release intensity during the primary oxidation of STWIC surpassed those during secondary oxidation. Moreover, the generation rates of CO and CO2 during the secondary low-temperature oxidation process of LTWIC were lower than those of STWIC.

Experimental and modeling investigation of the oxidation of n-hexylbenzene: Insight into the formation of low-temperature intermediates

As vital components in transportation fuels, an in-depth understanding of the oxidation mechanism of n-alkylbenzenes is pivotal for minimizing pollutant emissions during combustion processes. However, previous research was mostly focused on high-temperature oxidation, while investigations on low-temperature oxidation of long-chain n-alkylbenzenes remains scarce. In this work, n-hexylbenzene (C12H18) was selected to conduct low-temperature oxidation experiments in a jet-stirred reactor. A series of crucial intermediates and products were detected and then quantified by the synchrotron vacuum ultraviolet radiation photoionization mass spectrometry (SVUV-PIMS). In particular, characteristic aromatic oxygenated intermediates with compositions of C12H18Oy, C12H16Oy, C12H14Oy and C12H12Oy (0 ≤ y ≤ 3) were measured, which reveal the occurrence of first O2 addition, second O2 addition, and sequential oxidation reactions. To disclose the crucial reactions that control the fuel decomposition and formation of oxygenated intermediates, a detailed kinetic model of n-hexylbenzene was developed. This model was further utilized to explore the oxidation behaviors of C6 to C12n-alkylbenzenes. It was revealed that n-hexylbenzene exhibits the strongest low-temperature oxidation reactivity, while C6C10n-alkylbenzenes display little reactivity under present conditions. In conclusion, the reasonable performance of this model in predicting the oxidation behaviors of n-alkylbenzenes indicates its potential application in elucidating the low-temperature oxidation chemistry of heavier n-alkylbenzenes.

Experimental and kinetic modeling study of 1,3,5-trimethylbenzene oxidation at elevated pressure

This work presents the experimental and modeling results of 1,3,5-trimethylbenzene (T135MB) oxidation in a jet-stirred reactor at equivalence ratios (Φ) of 0.4 and 2.0, temperature range of 660–1020 K and pressure of 12.0 atm. 5 aromatic compounds, 5 oxygenated species and 6 light hydrocarbons were identified and quantified using GC and GC-MS. Compared with previous oxidation studies, 1-ethyl-3,5-dimethylbenzene, 3,5-dimethylbenzaldehyde, 3,5-dimethylphenol, ethylene and acetylene were newly detected. A detailed chemical kinetic model involving 911 species and 5370 reactions was developed and validated against experimental results of T135MB oxidation speciation at jet-stirred reactor and shock tube, laminar burning velocities and ignition delay times. Rate-of-production analysis shows that the dominant consumption channel of T135MB is H-abstraction by OH radicals on methyl sites, and 3,5-dimethylbenzaldehyde is a key intermediate in the oxidation of T135MB. Compared with atmospheric pressure, sharp consumption of T135MB was observed (the fuel conversion rate changes from 25 % at 838 K to 95 % at 839 K) under fuel-lean condition at 12 atm, which results from the rapid growth of OH radicals controlled by H2O2(+M) = 2OH(+M). Sensitivity analysis reveals that H-abstraction reactions of T135MB by OH and H on methyl sites are the most inhibiting reactions at Φ = 0.4 and 2.0, respectively, when T135MB conversion rate is 25 %. However, these two reactions become promoting at 95 % T135MB conversion. The H-abstraction reaction of T135MB with O2 on methyl sites is the most promoting at 25 % T135MB conversion, shifting to an inhibiting reaction at 95 % conversion.Novelty and significance statement: The novelty of this research is that high-pressure and low-temperature oxidation experiment of T135MB covering both fuel-lean and fuel-rich conditions was first reported in this work with abundant intermediates information. Rapid consumption of T135MB under fuel-lean condition was observed, and new intermediates such as 3,5-dimethylbenzaldehyde were detected which plays an important role in the fuel conversion. Additionally, a comprehensive kinetic model for T135MB was developed, which well predicts speciation, ignition delay times and laminar flame speeds under wide range of conditions. It is significant because the high pressure data is very important for the model validation since it is more close to the real engine-operating conditions. Moreover, the model provides insight on the high pressure chemical kinetics of the aviation surrogate fuel consumption and deepens the understanding of low-temperature oxidation of alkyl aromatics.

Verification of the oxygen consumption rate of the remaining coal in the gob by research and experiment

ABSTRACTOxygen consumption rate is an important index to evaluate the oxidation performance of coal, and it usually has an exponential relationship with temperature. However, during the temperature-rising oxidation experiment, it was found that as the temperature increased, the oxygen concentration at the outlet of the reaction tank would gradually decrease to a certain value, which caused the oxygen consumption rate to gradually approach a certain constant and thus be unable to accurately express the oxygen consumption characteristics of coal. To solve this problem, we introduce a new parameter and propose a new calculation model. We conducted low-temperature oxidation experiments on coal samples from Yang Chang Wan Mine (YCW) and An Ze (AZ) Mines under different oxygen concentrations and different gas flow conditions. Experimental results show that the new model has an exponential relationship with temperature for the same coal sample. This relationship depends on the inherent properties of the coal and is independent of the oxygen concentration and flow rate. Through exponential fitting, the determination coefficients of the new models for the two coal samples under different oxygen concentrations are increased by 7.55% and 3.55%, respectively, compared with the old model; under different flow conditions, the coefficients of determination are increased by 2.93% and 6.08%, respectively. These results indicate that the new model can more accurately express the oxidation characteristics of coal compared to the old model, providing a more accurate theoretical basis for future studies on the spontaneous combustion of coal.

Study on Highly Stable Biomass Gel Foam with Ultra-Strong Film-Forming Performance Based on the Double Network Structure to Inhibit Coal Spontaneous Combustion

ABSTRACT To further enhance gel foam stability, an ultra-strong film-forming stabilized biomass gel foam (PE/SA-Ca) with the double network structure was achieved by using pectin (PE), sodium alginate (SA), calcium L-lactate (Ca-L) and biomass foaming systems (BS). The results showed that the double network gel foam had ultra-strong film-forming and stability performances than the single network gel foam. Specifically, when the PE:SA ratio of the double network gel matrix was 5:5, the film thickness of PE/SA-Ca increased from 1 to 4.25 mm and the half-life increased from 3 to 22 days compared to the single network gel foam. In addition, PE/SA-Ca could maintain film integrity at high temperature up to 120°C. The water retention rate of PE/SA-Ca was increased by 79.5% compared to pure foam. The inhibition rate of PE/SA-Ca in the coal low-temperature oxidation experiment was as high as 78.06% at 180°C. Fire extinguishing experiment showed that PE/SA-Ca could rapidly reduce the coal fire temperature from 960 to 68.9°C within 20 min and effectively prevent prevention of coal re-ignition. This research provides a technical method for the development of highly stable, ultra-strong film-forming and eco-friendly gel foam.

Exploring the Chemical Kinetics of the First Oxygen Addition to Di-n-Propyl Ether Radicals at Low Temperatures.

Linear ethers are promising biomass fuels with high volume energy density and high cetane number that may be used directly as alternative fuels or fuel additives in internal combustion engines. In this work, the first O2 addition reaction of the di-n-propyl ether radical was investigated using high-level quantum chemical calculations combined with the Rice-Ramsperger-Kassel-Marcus theory to solve the master equation. The potential energy surfaces of di-n-propyl ether radicals (C6H13O) with O2 were constructed at the QCISD(T)/CBS//M062X/6-311++G(d, p) level, and the rate constants were calculated for the pressure range of 0.1-100 atm and the temperature range of 300-1500 K and fitted by a modified Arrhenius formula. The calculations show that the consumption of di-n-propyl ether peroxyl radicals (C6H13O3) via the five-membered ring or six-membered ring transition-state reaction channel is most favorable. In addition, the low-temperature oxidation experiments of di-n-propyl ether were validated based on the calculations of the current work, and the results showed that the calculations were a good predictor of the experimental results.

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.

Experimental Study on the Inhibitory Effect of Organic Phosphonic Acid Compound on Coal Spontaneous Combustion

ABSTRACT The minerals in coal contain a certain amount of metal ions, among which transition metal ions have a catalytic effect on coal spontaneous combustion (CSC). Therefore, the addition of metal ion chelating agent to coal will weaken this catalytic effect, thereby inhibiting CSC. As a class of highly effective metal ion chelating agent, two kinds of organic phosphonic acid compounds (OPAC), 1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP) and 2-Hydroxyphosphonoacetic Acid (HPAA), were investigated for their inhibitory effect on CSC. First, the low-temperature oxidation experiment was performed on coal samples before and after OPAC treatment. The experimental results show that HEDP and HPAA could reduce CO generation during spontaneous combustion of coal with different metamorphic degrees. Besides, the inhibiting efficiency of HEDP on CSC is higher than that of HPAA. Then, the effects of HEDP and HPAA on the thermal properties of CSC were investigated using synchronous thermal analysis experiment. The experimental results show that HEDP and HPAA could significantly weaken the oxygen adsorption, mass loss, and heat release during the high-temperature oxidation stage of CSC and exhibit the same result that HEDP is better than HPAA. Finally, the reasons for the difference in the inhibitory effects of HEDP and HPAA were analyzed and the inhibition mechanism of them on CSC was revealed via quantum chemical calculation. The results can provide new ideas for preventing CSC and developing new inhibitors.

Experimental and kinetic modeling studies on the interaction of DMM3-isooctane blends during the low-temperature oxidation

Reactivity controlled compression ignition (RCCI) engines have been extensively investigated in the past decade due to its good performance in balancing the NOx-soot trade off relationship and controlling the heat release rate. However, studies on the interaction between the reactive and inert fuels are scarce, which is critical for regulating the RCCI combustion process. In this study, the interaction mechanism between polyoxymethylene dimethyl ethers (DMM3) and isooctane during the low-temperature oxidation was investigated through experiments and simulations. The low-temperature oxidation experiments were conducted in a jet-stirred reactor (JSR) at near atmospheric pressure with different blending ratios. A detailed DMM3-isooctane mechanism containing 2896 species and 9676 reactions was developed and fully validated with the experimental results. In addition, an impact factor fw was proposed to quantitatively analyze the influence degree of interaction between fuel and intermediate species. The results indicate that the low-temperature oxidation of isooctane is triggered and significantly enhanced in the blending fuels. The OH radicals generated by DMM3 are crucial to trigger the low-temperature reaction of isooctane and always play a dominant role in the whole isooctane oxidation process. In addition, isooctane reveals an intense inhibition on the formation of two key intermediate species of DMM3, i.e., methyl formate and methoxymethyl formate. This inhibitory effect ranges from the beginning of low-temperature oxidation of DMM3 to the end of the NTC process (580 K ∼ 750 K). When the temperature is higher than 800 K, the inhibitory effect of isooctane on methyl formate and methoxymethyl formate is significantly weakened and negligible. In addition, the blending of inactive isooctane will result in an oxygen-rich environment for DMM3 during the low-temperature oxidation, which will accelerate the formation of methyl formate and methoxymethyl formate. However, this promoting effect is limited, compared with inhibiting effect of isooctane.

Experimental preparation and mechanism analysis of a neotype composite gel for coal spontaneous combustion prevention and coal-fire extinguishment

Spontaneous coal combustion (SCC) seriously threatens the safety production of coal mines. A neoteric irreversibly thermo-sensitive composite gel based on konjac glucomannan (KGM) and expandable graphite (EG) was elaborately synthesized for preventing SCC disasters and coal fires. Firstly, the primary properties such as the gelation time, the water retention rate, the rheological law, the microstructure characteristics, and the thermal decomposition properties concerning the composite gel were explored systematically. The results demonstrated that composite gel is non-Newton fluids with excellent self-repairing capability, and the gel exhibited controllable gelation properties, favorable seepage capacity, and strong water retention characteristics. Besides, SEM observations and thermogravimetry analysis suggested that the interspersed network structures of KGM embedded by EG exhibits higher denseness, greater physical strength, superior structural stability, and more remarkable thermal stability, and EG was closely adhered to the KGM hydrogel network but distributed inhomogeneously. Furthermore, the high-temperature pyrolysis tests showed that the decomposition rate accelerated to 0.234 %/°C and 0.225 %/°C at the temperature of 266.5 °C and 275.1 °C, respectively. Meanwhile, both the low-temperature oxidation experiments and fire-extinguishing experiments showed a favorable inhibition effect of the composite KGM@EG gel, indicating that KGM@EG gel is applicable to inhibit the high-temperature pyrolysis rate of coal, and could effectively inhibit the heat buildup, O2 consumption, and CO formation over the oxidation and combustion process of coal, which would lower the possibility of SCC and coal fires. Therefore, KGM@EG composite gel can play an efficient SCC-preventing and fire-extinguishing role in the safety production of coal mines. Finally, the underlying mechanism for SCC preventing and fire-extinguishing concerning the thermo-sensitive gel was revealed and proposed.

Pathway exploration in low-temperature oxidation of a new-generation bio-hybrid fuel 1,3-dioxane

The joint and flexible utilization of renewable electricity, ligno-cellulosic biomass, and/or CO2 point sources to produce so-called bio-hybrid fuels is a promising solution to achieve carbon neutrality while still meeting the energy demand of the transportation sector. One of the new-generation bio-hybrid fuels is 1,3-dioxane. It has a special chemical structure with two oxygen atoms in a six-membered ring. In this work, the low-temperature oxidation of 1,3-dioxane was studied theoretically and experimentally. Potential energy surfaces of the products of the O2 recombination with the three radicals formed from the H-atom abstraction of 1,3-dioxane were calculated at the DLPNO-CCSD(T)/CBS//B2PLYP-D3/cc-pVTZ level. The reaction rate coefficients were calculated with the RRKM/master equation method (T = 500–2000 K, p = 0.01–100 atm). To validate the proposed pathways, low-temperature oxidation experiments of 1,3-dioxane were performed in a jet stirred reactor (JSR) coupled with a synchrotron photon ionization time of flight molecular beam mass spectrometer (T = 590 K, p = 1 bar). Key intermediates in the investigated pathways were captured and identified by the combination of measured photon ionization efficiency curves and calculated ionization energies. Compared to cyclohexane, which has no oxygen in the six-membered ring, 1,3-dioxane has much weaker C-H bonds for the carbon between the two oxygen atoms, thus enabling faster internal H-atom migration from ROO to QOOH. Furthermore, oxidation of 1,3-dioxane tends to favor cyclic ethers + OH (chain propagation) instead of alkenes + HO2 (chain termination), explaining its high reactivity in the low-temperature regime.

Experimental and kinetic modeling studies on low-temperature oxidation of Polyoxymethylene Dimethyl Ether (DMM1-3) in a jet-stirred reactor

In this paper, based on the pyrolysis kinetics model of DMM1-3 previously constructed, the low-temperature oxidation kinetics model of DMM1-3 (500 K∼950 K) was established by using the jet-stirred reactor (JSR) to conduct low-temperature oxidation experiments of DMM1-3 with different equivalence ratios. The model was fully verified experimentally, and the simulation results can well reproduce the trend of fuel and important intermediate specie concentrations in the temperature range of 500 K ∼ 950 K. 9 species were identified and quantified by GC/MS in the low-temperature oxidation process of DMM1-3. Results show that DMM1 has an oxidative decomposition process after dehydrogenation, while both DMM2 and DMM3 have strong low-temperature oxidation reaction and weak NTC process, but there are certain differences in the specific oxidation process. CH3OCHO and COCOC*O are important intermediates in the low-temperature oxidation process of DMM1-3, and their peak concentrations in the high-temperature region are not affected by equivalence ratios. And the peak concentrations of CH3OCHO and COCOC*O in the low-temperature oxidation experiments of DMM2 and DMM3 increased with decreasing equivalence ratios. The current detailed model and corresponding experimental results can be helpful for the construction of reliable reduced DMMn mechanisms.

Effect of Water Content on Oxidative Combustion and Pyrolysis Characteristics of Gas Coal

ABSTRACT The oxidative combustion and pyrolysis characteristics of gas coal with different water content (2.86%, 9.06%, 15.81%, 22.83%, and 30.12%) were systemically investigated by thermogravimetric – differential scanning calorimetry and low-temperature oxidation experiments. Results show that the mass loss and heat absorption-exothermic pattern are inconsistent in oxidative combustion and pyrolysis processes of gas coal containing water. The behavior of internal water dissipation in gas coal is consistent in oxidative combustion and pyrolysis processes, but the influence on them is significantly different in high-temperature stage (450–600 ℃), the internal water dissipation behavior makes the coal pyrolysis reaction faster and makes the coal oxygen reaction slower.

Study on the Reaction Path of -CH3 and -CHO Functional Groups during Coal Spontaneous Combustion: Quantum Chemistry and Experimental Research

Coal spontaneous combustion (CSC) is a disaster that seriously threatens safe production in coal mines. Revealing the mechanism of CSC can provide a theoretical basis for its prevention and control. Compared with experimental research is limited by the complexity of coal molecular structure, the quantum chemical calculation method can simplify the complex molecular structure and realize the exploration of the mechanism of CSC from the micro level. In this study, toluene and phenylacetaldehyde were used as model compounds, and the quantum chemical calculation method was adopted. The reaction processes of the methyl and aldehyde groups with oxygen were investigated with the aid of the Gaussian 09 software, using the B3LYP functional and the 6-311 + G(d,p) basis set and including the D3 dispersion correction. On this basis, the generation mechanisms of CO and CO2, two important indicator gases in the process of CSC, were explored. The calculation results show that the Gibbs free energy changes and enthalpy changes in the two reaction systems are both of negative values. Accordingly, it is judged that the reactions belong to spontaneous exothermic reactions. In the reaction processes, the activation energy of CO is less than that of CO2, indicating that CO is formed more easily in the above-two reaction processes. In addition, the variations in concentrations of important oxidation products (CO and CO2) and main active functional groups (such as methyl, carboxyl and carbonyl) with temperature were revealed through a low-temperature oxidation experiment. The experimental results verify the accuracy of the above quantum chemical reaction path. Moreover, it is also found that the generation mechanisms of CO and CO2 in coal samples with different metamorphic degrees are different. To be specific, for low-rank coal (HYH), CO and CO2 mainly come from the oxidation of alkyl side chains; for high-rank coal (CQ), CO is produced by the oxidation of alkyl side chains, and CO2 is attributed to the inherent oxygen-containing structure.

Variable pressure JSR study of low temperature oxidation chemistry of n-heptane by synchrotron photoionization mass spectrometry

Low temperature oxidation chemistry is crucial in the auto-ignition process of internal combustion engines. The development of laboratory based reactors and diagnostic systems promotes understanding of the low temperature oxidation mechanism. This work develops a variable pressure jet-stirred reactor (VP-JSR) platform working from one to ten bar. Compared to previous high pressure studies which routinely use gas chromatography to analyze low temperature oxidation products, the difference in this work is the possibility of probing intermediates by synchrotron vacuum ultraviolet photoionization mass spectrometry via molecular beam sampling. The setup was validated by repeating n-heptane low temperature oxidation experiments at one and ten bar, respectively. Good agreement was observed between the data in this work and the data in the literature. A much more detailed species pool was probed compared to the literature study, including H2O2, carbonyl acids, alkylhydroperoxides, and keto-hydroperoxides. As a first demonstration of the usefulness of this design, the low temperature oxidation of n-heptane with initial fuel mole fraction of 0.005, residence time of 2 s, and equivalence ratio of 1.0 was studied at one, five, and ten bar. The preliminary results, including reactants, final products, and the initial low temperature oxidation intermediates, are discussed. The VP-JSR system developed in this work is valuable for study of the fuel chemistry from one to ten bar, to develop and examine chemical kinetic models, and to guide the development of the reaction system at even higher pressures, such as those in engine function and supercritical conditions.

Compound effects of water immersion and pyritic sulfur on the microstructure and spontaneous combustion of non-caking coal

In order to reveal the compound effects of water immersion and pyrite on the microstructure and spontaneous combustion characteristics of non-caking coal, three coal samples from two neighboring mines were selected and prepared. The SEM observation, CO2 and N2 adsorption, XRD, XRF, FTIR tests and low temperature oxidation experiment were carried out on raw coal and soaked coal (S30). Showing that a large number of pores and fissures developed after waterlogging, especially M2 sample, the oxidation of its pyrite content (FeS2) leads to the promotion of pore development in coal matrix. Simultaneously, the presence of pyrite was more favorable for the generation of various functional groups and the maintenance of long chain aliphatic hydrocarbons during soaking. In conclusion, the presence of pyrite contributes to the micro-physical and chemical changes in the coal body during the water immersion, which further promotes the propensity for spontaneous combustion (combining data on oxygen consumption, gas products and CPT changes) of soaked coal.

Reactive adsorption mechanism of O2 onto coal vitrinite during the low temperature oxidation process

Here, incorporating with the experiments of low-temperature oxidation, the adsorption and diffusion of O2 molecules onto macromolecular model of coal vitrinite was simulated via the MD (molecular dynamics) and GCMC (grand canonical monte carlo). The energy variation and the dominated adsorption configuration of O2 were clarified, as well as the RDF (radial distribution function) and the self-diffusion coefficients of various functional groups. The isosteric heat of O2 adsorption lies within 16.40–16.57 kJ/mol, which decreases linearly with the increasing temperature. The GCMC and RDF analysis indicated that the adsorption affinity of various functional groups for O2 follows the order of aromatic > pyrrole > pyridine and the O2 molecules prefer to absorb onto the coal vitrinite at the L orientation (OO bond perpendicular to coal vitrinite plane) for aromatics and at the V orientation (OO bond parallel to coal vitrinite plane) for pyridine rings. The self-diffusion coefficients of O2 molecules are in the order of magnitude of 10−6-10−5 m2/s. Thus, both the adsorption affinity and diffusion ability of O2 molecules differ distinctly among various function groups, providing the energy and materials for the gradual self-activation reaction of spontaneous combustion of coal. The low-temperature oxidation experiments suggest that the oxidation reaction of different structures in the low-temperature oxidation process of coal occurs at different oxidation temperatures, which also proves that the different oxidation activity of structures in coal. The pyrrole, pyridine, and aromatic are dominant adsorption sites for O2 molecules and also serve as the first order reaction zone of coal spontaneous combustion, producing more heat than the locations of the aliphatic chains and oxygen functional groups due to their wide distribution. The simulation and experimental results indicated two functions of the physical adsorption for coal spontaneous combustion. One is to delivery oxygen for oxidation reaction and the other is to release heat as the triggering mechanism of the coal spontaneous combustion.

Experimental and kinetic modeling studies of the low-temperature oxidation of 2-methylfuran in a jet-stirred reactor

2-Methylfuran (MF), a promising biofuel, produced from non-edible biomass, is desirable as an alternative fuel or fuel additive to internal combustion engines. For a better understanding of the ignition process in engines, the low-temperature oxidation experiments of MF at the temperature range of 600 – 925 K and atmospheric pressure with different equivalence ratios (0.5, 1.0, and 2.0), were performed in a jet-stirred reactor. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to identify and measure the intermediates and products in the oxidation process, especially for the isomers (furfural/2-ethylfuran and furan/vinyl ketene), species with identical mass-to-charge ratio (methanol/oxygen and aldehyde/carbon dioxide), and the other oxygenated and hydrocarbon products. The typical negative-temperature-coefficient behavior was not observed in the low-temperature oxidation of MF. The dominant consumption pathways of MF in low-temperature oxidation are the hydroxyl radical/hydrogen atom-addition reactions on the ring and hydrogen atom-abstraction reactions on the side methyl group by hydroxyl radical/hydroperoxyl radical/hydrogen atom, while the contribution of unimolecular decomposition reactions is almost negligible. 2-Furylmethyl radical is a key intermediate in the low-temperature oxidation of MF. Furfural and 2-ethylfuran, as the subsequent products from 2-furylmethyl low-temperature oxidation, were identified as the early-stage products. Besides, the important chemistry of other important species, especially acrolein and methyl vinyl ketone, were also discussed at different equivalence ratios.

Study on the effect of organic sulfur on coal spontaneous combustion based on model compounds

The composition and structure of organic sulfur in coal are rather complicated. Thus, no workable method has been adopted to accurately and quantitatively study organic sulfur in coal so far. Coal spontaneous combustion (CSC) is a severe disaster occurring widely in coal industry. It is an extremely complex physical and chemical process as well as a major scientific and technical problem that has always puzzled researchers. Studying the complex chemical reaction of coal by model compounds is a reasonable method extensively recognized and used in the international coal chemistry research field. Model compounds represent certain kinds of specific functional groups in coal. Through an investigation on the reaction mechanism of model compounds, the complex mechanism of coal molecules can be revealed. In this study, in order to better investigate the mechanism of organic sulfur during CSC, representative organic sulfur model compounds containing different organic sulfur functional groups were selected for the low-temperature oxidation experiment and the thermal analysis. Their oxygen consumption amounts and oxidized products concentrations were determined by gas chromatography and infrared analyzer. Besides, the weights and endothermic/exothermic rates of these model compounds during low-temperature oxidation were measured using the thermogravimetry/differential scanning calorimetry (TG/DSC) method. In this way, their oxidative and thermodynamic characteristics were revealed. On this basis, the organic sulfur model compounds were uniformly mixed with the coal samples for the low-temperature oxidation experiment in the hope of exploring its effect on oxygen consumption amounts and indicative gas products during CSC. Finally, the mechanism of the interactions between organic sulfur and other reactive groups in coal during low-temperature oxidization were obtained.

Influence of methane on the prediction index gases of coal spontaneous combustion: A case study in Xishan coalfield, China

Lean-oxygen environment caused by methane in high gassy coal mine goaf has a significant influence on the prediction index gases of residual coal spontaneous combustion. The coal sample from Xishan coalfield in China was used to conduct the low-temperature oxidation experiments under different lean-oxygen environments. The experimental results show that different lean-oxygen environments had a significant delay effect on the formation of CO, CO2, H2 and C2H6, and the lower the O2 concentration, the more obvious the delay effect, and the delay effect of methane was greater than that of N2. A lean-oxygen environment caused by methane also had a significant influence on the values of CO/ΔO2 ratio and CO2/ΔO2 ratio. However, when the temperature was < 170 °C, the change of O2 concentration had no significant influence on the value of CO/CO2 ratio, and the relationship between the value of CO/CO2 ratio and temperature T can be accurately expressed by exponential function. The absolute amount of CO, CO2 and C2H6 generated by 1kg residual coal (CCO, CCO2 and CC2H6), which can reduce the influence of air leakage and residual coal mass on prediction index gases, were introduced to predict the spontaneous combustion in high gassy coal mine goaf. Then, the distribution diagrams of CCO, CCO2 and CC2H6 with temperature and O2 concentration were obtained, so as to accurately predict the degree of spontaneous combustion in high gassy coal mine goaf. The research results are of great significance to the prevention of spontaneous combustion in goaf of high gassy mine.