Kinetics Of Low-temperature Oxidation Research Articles

Combined experimental and kinetic modeling study on the low-temperature oxidation of OME3/n-heptane blends

Experimental and kinetic studies on the blending of ethers or diesters with larger alkanes at low temperatures are enormously scarce. In this paper, the low-temperature oxidation (LTO) kinetics model of polyoxymethylene dimethyl ether 3 (OME3)/n-heptane (500 K∼950 K) was established by using the jet-stirred reactor (JSR) to conduct the LTO experiments of OME3/n-heptane with different blending ratios. GC/MS was used to detect and quantify 17 kinds of stable products in the LTO process of OME3/n-heptane. The detailed OME3/n-heptane kinetic model was fully verified experimentally. The interaction mechanism of OME3/n-heptane was discussed based on the experiments and corresponding chemical kinetic analysis. Results show that the oxidative decomposition processes of OME3 and n-heptane were significantly different, the LTO and NTC process of OME3 were significantly weaker than those of n-heptane, but the high-temperature reactivity of OME3 was much stronger. In addition, n-heptane significantly promoted the intermediate species CH3OCHO and COCOC*O of OME3, mainly because n-heptane significantly prolonged the LTO of OME3 in the fuel blends. And the NTC process of the fuel blends was mainly dominated by OME3, while the LTO process was more influenced by n-heptane, which resulted in the weakened NTC of n-heptane and the significantly enhanced LTO process of OME3.

Isothermal low-temperature oxidation behavior and kinetics of heavy oil

In this study, the isothermal low-temperature oxidation (LTO) behavior of heavy oil was investigated by thermogravimetry combined with Fourier transform infrared spectroscopy. Furthermore, the kinetics analysis was conducted using general reaction rate models. The findings revealed that the LTO process of heavy oil could be divided into three distinct phases. The first phase, occurring between 50 and 100 °C, exhibited minimal mass loss and low absorbance of gas-phase products. In the second phase (100–250 °C) the produced gas predominantly consisted of saturated hydrocarbons and substances containing C=O groups. Lastly, in the third phase (250–300 °C), the signals of CO2 and CO in the produced gas were significantly higher compared to the 50–250 °C range, indicating a shift in the oxidation mode of heavy oil from oxygenation to bond cleavage for temperatures higher than 250 °C. Kinetics analysis demonstrated an initial increase in reaction order and reaction rate from 50 to 100 °C, followed by a decrease from 100 to 250 °C, and a final increase from 250 to 300 °C. The negative activation energy observed in the second temperature interval was attributed to the negative temperature coefficient effect. This study provided valuable insights into the isothermal LTO reactions of heavy oil.

Elucidating the low temperature chemistry of o-xylene enhanced by the reaction pool of dimethyl ether

The branched aromatic hydrocarbon with short side chains, such as o-xylene, has weak low temperature oxidation reactivity, while it could proceed low temperature chain recycling reactions by blending a fuel component with higher oxidation reactivity. However, the study of low temperature oxidation kinetics of o-xylene is very scarce. Therefore, this work investigated the low temperature oxidation process of o-xylene in an atmospheric jet stirred reactor by adding a biofuel with higher low temperature oxidation reactivity, i.e. dimethyl ether (DME). A series of characteristic low temperature intermediates, such as phenyl hydroperoxides and ketohydroperoxide from the oxidation of o-xylene, were detected by using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The chemical structures of representative low temperature intermediates were determined by comparing the ionization thresholds from measured photoionization efficiency curves with calculated ionization energies. A kinetic model for o-xylene and DME mixture was developed and verified by literature and present experimental data. Rate of production (ROP) and sensitivity analyses were performed, which indicate that the low temperature oxidation chemistry of o-xylene, such as the negative temperature coefficient behavior, is enhanced by the reaction pool of DME. According to the classical low temperature oxidation mechanism of n-alkanes, the low temperature reactions of o-xylene were proposed. In particular, phenyl hydroperoxides and ketohydroperoxides were revealed to be key indicators for the occurrence of first and second O2 addition reactions during the oxidation of o-xylene, respectively. In addition, the sensitivity analysis uncovers the interaction kinetics between the reaction pools of o-xylene and DME, as well as their effects on fuel consumption and products formation. In summary, present evidence elucidates the low temperature chemistry of a representative branched aromatic, which could contribute to the development of future engines and control of combustion pollutants.

Theoretical study on low-temperature oxidation kinetics of methyl pentanoate

In order to develop reliable combustion models of biodiesel, the low-temperature oxidation kinetics of methyl pentanoate (MP), the smallest methyl ester with the negative temperature coefficient (NTC), is theoretically investigated. We use the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ and CASPT2/CBS(T-Q)//CASSCF(7e,5o)/cc-pVTZ methods to explore the potential energy surfaces (PESs) of the decomposition of different MP peroxy radicals, including dissociation, isomerization, and concerted HO2 elimination. Comparisons among the PESs of MP and other methyl esters reveal that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. Based on the determined PESs, the pressure- and temperature-dependent rate constants for the low-temperature oxidation of MP are computed at 298−1200 K and 0.01−100 atm. The kinetic calculations suggest that the dissociation rate constants of different methyl ester peroxy radicals are similar at the same active C site, and the channels that lead to tri-molecular products are significant during the low-temperature oxidation of large methyl esters. The literature combustion model of MP is revised with our calculations. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.Novelty and Significance statementMethyl pentanoate (MP) is the smallest methyl ester with the negative temperature coefficient (NTC), but the low-temperature oxidation kinetics in the available combustion model of MP is still insufficient to interpret its NTC behavior. To develop a reliable combustion model for MP, the potential energy surfaces (PESs) of the decompositions of different MP peroxy radicals are investigated. Results show that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. With our calculated rate constants, the literature combustion model is revised. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.

Revealing the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene sensitized by dimethyl ether

Low to moderate temperature oxidation has been a vital topic in the combustion chemistry due to its critical role in controlling the combustion characteristics such as autoignition and pollutant emissions. Low temperature oxidation kinetics of alkanes and oxygenated biofuels have been widely explored, while that of branched aromatics is poorly understood. This work aims to investigate the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene (TMB, C9H12), which is a multi-branched alkylbenzene with weak low temperature oxidation reactivity. To enhance the low temperature oxidation reactivity of TMB, dimethyl ether (DME) was added to efficiently provide chain initiators. The oxidation experiment of TMB/DME mixture was performed with an atmospheric jet stirred reactor coupled with synchrotron vacuum ultraviolet radiation photoionization mass spectrometry. The negative temperature coefficient (NTC) behavior, as well as featured aromatic intermediates, especially phenyl hydroperoxides and unexpected highly oxygenated molecules (HOMs), were observed in the experiment. The detection of these fingerprint aromatic intermediates provide crucial evidence for revealing the low to moderate oxidation kinetics of TMB. In addition, a kinetic model was developed to provide in-depth kinetic analysis. The results demonstrate that the active chain initiators produced from DME oxidation trigger the low temperature chain-branching reactions of TMB. The chain recycling process is further sustained due to the regeneration of active intermediates such as phenyl hydroperoxides and •OH radicals. Aromatic intermediates with chemical compositions of C9H12Ox and C9H10Ox were measured and identified to be key indicators for the low temperature chemistry of TMB. In particular, the aromatic HOMs (C9H12O6 and C9H10O5) are fingerprint intermediates produced from the third O2 addition and subsequent reactions of TMB. Besides, the sensitivity analysis indicates that the interaction kinetics between TMB and DME by sharing the highly sensitive reactions of hydroperoxides and •OH radicals. The present work extends the conceptual reaction schemes proposed in alkanes towards branched aromatics, especially sequential O2 addition reactions that contributing to the formation of phenyl hydroperoxides and HOMs.

Modelling Self-Heating and Self-Ignition Processes during Biomass Storage

A mathematical model was developed to predict the self-heating and self-ignition processes of relatively dry biomass during storage, considering in detail the effects of moisture exchange behaviour, low-temperature oxidation reaction and associated heat and mass transfer. Basket heating tests on fir pellets and powder at temperatures of 180–200 °C were conducted to observe the heating process and determine the kinetics of low-temperature chemical oxidation for model validation. As a result, it was demonstrated that the developed model could reasonably represent the self-heating and spontaneous combustion processes of biomass storage. Furthermore, the numerical study and model sensitivity analysis revealed that reasonably describing the low-temperature oxidation and associated heat and mass transfer process with reliable estimations of kinetic and thermophysical parameters of the biomass material is necessary for predicting the self-ignition, considering the effect of water exchange behaviour is essential to predict the self-heating process even for relatively dry biomass, such as pellets, with the moisture content up to 15–20%.

Plasma-assisted low-temperature oxidation of n-butane: A synchrotron photoionization mass spectrometry and kinetic modeling study

The chemical kinetics of plasma-assisted low-temperature oxidation of n-butane (n-C4H10, 340 K, 30 Torr) is investigated by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and kinetic modeling. Species measurements are conducted in a flow reactor activated by a nanosecond discharge using SVUV-PIMS. Detailed species identification and quantification are reported in the present work. Fuel-specific oxygenated species including 2-butenal (2-C3H5CHO), methyl vinyl ketone (CH3COC2H3), butanone (CH3COC2H5), butanal (n-C3H7CHO), 2-methyloxetane (-CH(CH3)CH2CH2O-), 1,2-epoxybutane (-CH(C2H5)CH2O-), tetrahydrofuran (-CH2CH2CH2CH2O-), 1-/2-butanol (1-/2-C4H9OH), and butyl hydroperoxide (C4H9O2H) are detected. A kinetic model for n-C4H10 plasma is developed, which well predicts the mole fractions of most species observed in experiments. Kinetic modeling reveals that electron-impact reactions of n-C4H10 are the main sources for various C1-C4 alkenes, alkynes, and radicals. Therefore, these reactions have a significant effect on the fuel consumption pathways and species pool formation. Reactions involving alkyl peroxy (RO2), especially the self- and cross-reactions of RO2, are essential for RO2 depletion and the formation of alcohols, ketones, and aldehydes. On the other hand, the misprediction of the species detected in the system, such as C4 cyclic ethers, ethyl formate (HCOOC2H5), and acrolein (C2H3CHO), indicates that their formation pathways in the current mechanism are incompletely described. Further experimental and numerical research on these subjects is desired for the development of the kinetic models.

Characterization of cool flame products during the low temperature oxidation of n-pentylbenzene

Radical chain recycling reactions represent a general topic in the chemistry of combustion, atmosphere, polymerization and so on. The nature and amount of OH radicals that generated by hydroperoxides during the oxidation of alkanes have been proven to drive the overall reactivity by enhancing the chain recycling steps. While low temperature oxidation kinetics of alkanes have been widely investigated, the comprehensive intermediates distribution during the low temperature oxidation of n-alkylbenzenes remains unexplored. This work selects n-pentylbenzene as a simple representative to investigate the low temperature oxidation kinetics of n-alkylbenzenes, especially intermediates and reactions that control the reactivity. The experiments were performed with a jet-stirred reactor coupled with the synchrotron vacuum ultraviolet radiation photoionization mass spectrometry (SVUV-PIMS). Around 70 intermediates, including phenyl hydroperoxides and highly oxygenated molecules (HOMs) were detected. By combining theoretical calculations and kinetic analysis, potential compositions and structures of key intermediates were identified and categorized into groups by their characteristic molecular formulae and chemical functionalities, such as C11H16On, C11H14On, C11H10On. Fingerprint hydroperoxides, including phenyl alkyl hydroperoxides, ketohydroperoxides, olefinic hydroperoxides, ketodihydroperoxides, were revealed to be formed from the characteristic first, second and third O2 addition and sequential reactions. These hydroperoxides play a dominant role in driving the low temperature oxidation reactivity by generating active OH radicals. The present experimental evidence, including the negative temperature coefficient behavior of n-alkylbenzenes as well as featured HOMs from the third O2 addition, extend the conceptual reaction schemes proposed in alkanes towards the oxidation of n-alkylbenzenes.

Isothermal low-temperature oxidation kinetics of heavy crude oil and its oxidized oils

A large amount of heavy oil inevitably encountered different degrees of isothermal low-temperature oxidation (LTO) reactions in the processes of in-situ combustion, storage, and transportation, so it was of vital importance to study the isothermal LTO kinetics of heavy oil that was still less well-understood. In this study, a thermogravimetry was used to study the isothermal LTO characteristics of heavy crude oil and its oils oxidized at 120, 160, and 200 °C, and four LTO temperatures of 125, 200, 275, and 350 °C with 1 h heating treatment were taken into account. The isothermal kinetic models were then established based on the n-order reactions. Subsequently, the comparisons of conversion degree between experiments and predictions were conducted to ensure the reliability of the established kinetics. The results showed that the mass losses for all the oil samples were almost linearly proportional to the LTO duration at 125 °C, indicating that the LTO time generated a significant effect on the thermal deposition. The shift rightward of the LTO peak with the elevated LTO temperature verified that the structures of all the oil samples were affected by LTO to a certain extent. After thermal degradation at 350 °C, 49.5% of LTO residue remained for the oxidized at 200 °C, obviously more than that for the other three oil samples. This gave a hint that the oils oxidized at higher LTO temperatures could be transformed into more fuel serving for combustion reactions. The appropriate values of reaction order of crude oil and its oils oxidized at 120, 160, and 200 °C were determined to be 4, 5, 5, and 7, respectively. There was no apparent difference in the values of activation energy, indicating that all the oil samples had close mass loss rates that determined the entire mass loss during isothermal LTO. The oil oxidized at 200 °C had fairly higher frequency factor than the other three oil samples, due primarily to its stronger polar molecular structure that resulted in higher molecular collision frequency. The prediction lines matched well with the experimental ones for all the oil samples, demonstrating that the developed isothermal kinetics was a useful tool to predict the LTO behavior of heavy oils.

Probing the low temperature oxidation chemistry of RP-3 kerosene: Experimental and kinetic modeling investigation

Elucidating the formation of intermediates, especially active species like peroxy radicals and hydroperoxides, is crucial to explore the low temperature reaction network and validate the kinetic model. While low temperature oxidation kinetics of single hydrocarbon has been widely studied, the comprehensive speciation study on the transportation fuels is still lacking and challenging due to the complex compositions of real fuels. This work aims to investigate the speciation pool and develop predictable kinetic model for the low temperature oxidation of RP-3 kerosene. An atmospheric jet stirred reactor combined with synchrotron vacuum ultraviolet photoionization mass spectrometry was used to investigate the RP-3 kerosene oxidation in the temperature range of 480–770 K and equivalence ratios of 0.5, 1.0 and 2.0. Major products and key intermediates were quantified and categorized into several groups, including peroxy radicals, hydroperoxides, alkenes, alcohols, aldehydes, acids and aromatics. Furthermore, a three-component surrogate fuel consisting of 60.0% n-dodecane, 31.5% ethylcyclohexane, and 8.5% n-butylbenzene (by mole) was formulated to represent the RP-3 kerosene. A kinetic model for the three-component surrogate fuel was developed based on the comprehensively validated kinetic model of single components. This model was validated against the low temperature oxidation speciation data, ignition delay times and laminar flame speeds of RP-3 over wide range of conditions. Rate of production and sensitivity analysis were performed to explore the low temperature oxidation reaction network. The results indicate that the negative temperature coefficient behavior of RP-3 is dominated by the reactions of n-alkane, which is easier to proceed low temperature chain-propagation reactions leading to the production active OH radicals. The reaction network of each fuel component is effectively coupled from the view of OH regeneration and consumption, and finally affects the distribution of speciation pool during RP-3 low temperature oxidation. Regarding the characteristic speciation pool, it is found that the formation and consumption of peroxy radicals, hydroperoxides, alkene, aldehyde, and alcohol intermediates are controlled by the reaction network of alkane. While the formation and depletion of aromatic intermediates are affected by the interaction kinetics of fuel components with different chemical functionalities. These results improve the current understanding of the speciation pool and interaction kinetics during RP-3 low temperature oxidation. The kinetic model of RP-3 kerosene developed and validated in this work could be applied for numerical simulation of aero-engine.

Experimental and kinetic modeling study of the low- temperature and high-pressure combustion chemistry of straight chain pentanol isomers: 1-, 2- and 3-Pentanol

Pentanols have received significant attention as a potential alternative fuel or fuel additive owing to their high energy densities and low vapor pressure. The development of robust chemical kinetic models for alternative fuels which can provide accurate and efficient predictions of combustion performance across a wide range of engine relevant conditions is important in developing cleaner, more efficient combustors. Although the high temperature oxidation kinetics of pentanol isomers has been researched considerably, their low temperature combustion chemistry needs further investigation. While previously proposed low temperature mechanisms for 1-pentanol based on analogy and rate rules need further refinement, the low temperature oxidation kinetics of 2-pentanol and 3-pentanol has not been studied previously by any means, experimentally or theoretically. A newly developed kinetic mechanism is presented in this work for the three straight chain pentanol isomers: 1-, 2- and 3-pentanol. Low temperature kinetics is based on a recent study by Lockwood et al., 2022 [20] involving theoretical calculations at the CCSD(T)/cc-pV∞Z level of theory for the oxidation pathways involving alcohol peroxy radicals. Rate of production analyses performed in this study highlight the importance of the newly added pressure-dependent reactions of the α-alcohol peroxy radical forming an RȮ2 adduct. While the α-alcohol fuel radical reacts with O2 to directly decompose via a chemically activated pathway at low pressures, the formation of the RȮ2 adduct is favored at high pressures. The detailed model is comprehensively validated against new ignition experiments at low temperature and high pressure, together with the wide range of data available in the literature. Both qualitative and quantitative predictions of the experimental data using the proposed kinetic model are satisfactory for all three pentanol isomers studied here.

Diethyl ether oxidation: Revisiting the kinetics of the intramolecular hydrogen abstraction reactions of its primary and secondary peroxy radicals

• Reaction kinetics of intramolecular hydrogen abstraction reactions of peroxy radicals yielded by diethyl ether are revisited with high level ab initio calculations. • The individual role of variational, tunneling and multistructural torsional anharmonicity effect in the calculation of rate constant is analyzed in detail. • The special effects of diastereomers and conformers generated via pseudo-rotations within the saddle point ring are emphasized. • The prediction performances of several representative chemical kinetic models of diethyl ether updated with our calculated rate constants are tested. • 1 Yaozong Duan, M. Monge-Palacios and E. Grajales-Gonzalez contributed equally. Diethyl ether is an attractive alternative to diesel fuel for its high autoignition propensity, high energy density, and potential to reduce pollutant emissions. Low-temperature oxidation kinetics play an important role governing fuel autoignition in engines. Peroxy radical intramolecular hydrogen atom abstraction reactions are key isomerization steps in the chain branching reaction sequence at low temperatures. In this study, the kinetics of intramolecular hydrogen atom abstraction reactions of the primary and secondary peroxy radicals were revisited using the multistructural torsional variational transition state theory with small curvature tunneling corrections. High-pressure limit rate constants within a broad temperature range were obtained, which differ from those previously reported for the investigated reactive systems due to the differences in the estimated barrier heights, the role played by multistructural torsional anharmonicity, and the effects of tunneling. We also observed that some higher energy conformers of the saddle point species, some of which with diastereomers, play a surprisingly large role. Similarly, tunneling is pronounced at low temperature combustion conditions, claiming for robust methods to estimate this effect. Different kinetic models for diethyl ether were used to test our calculated rate constants and NASA polynomials for both diethyl ether peroxy radicals, and the results showed an overall slightly better performance in the prediction of ignition delay times at high pressures.

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.

Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.

Kinetic insights into plasma-assisted low-temperature oxidation of propane with synchrotron photoionization mass spectrometry

The present study investigates the chemical kinetics of low-temperature oxidation of propane activated by nanosecond pulsed dielectric barrier discharge in a flow reactor. Molecular beam mass spectrometry with tunable synchrotron vacuum ultraviolet photoionization (SVUV-PIMS) is employed for components identification and quantification. A series of hydrocarbons and oxygenated intermediates are observed, including fuel-specific species reported for the first time in this system like propyl hydroperoxide (C3H7OOH), acrolein (C3H4O2), oxetane (-CH2CH2CH2O-), methyloxirane (-CH(CH3)CH2O-), and propanol (n-/i-C3H7OH), which provide original kinetic information in the plasma-assisted low-temperature oxidation of propane. The detection of alkyl hydroperoxide (ROOH) demonstrates the existence of alkyl peroxy (RO2) formed through oxygen addition to alkyl (R) and the detection of cyclic ethers provides evidence for the presence of hydroperoxyl alkyl (QOOH). A kinetic model incorporating plasma chemistry and combustion chemistry is developed for plasma-assisted oxidation of propane. Path fluxes of fuel consumption and oxygenated intermediates formation are provided based on model analysis. Besides H abstractions by the OH radical, reactions activated by plasma involving electron impact dissociation, O(1D)/Ar* quenching dissociation, and ionization contribute much to fuel consumption. The formed propyl (n-/i-C3H7) are exclusively consumed through oxygen addition producing propyl peroxy (n-/i-C3H7O2). Propyl peroxy can participate in self- and cross-reactions yielding various oxygenated intermediates such as aldehydes, ketones, alcohols, and propoxy (n-/i-C3H7O). Discrepancies between simulations and experiments suggest that further kinetic studies on low-temperature mechanisms, including propyl peroxy related reactions for plasma-assisted combustion modeling, are still desired.

Evaluation of oxygen concentration on low-temperature oxidation kinetics of long-flame coal

Oxygen concentration is the vital environmental factor affecting the evolution of coal spontaneous combustion. Long-flame coal was heated from 30 to 800 °C at different oxygen concentrations (3, 5, 9, 13, 17, and 21 mass%) and heating rates (2.0, 5.0, 10.0, and 15.0 °C/min). The thermal reaction process was explored and analyzed. The effects of oxygen concentration on characteristic temperature and related parameters were assessed. The effects of heating rate and oxygen concentration on characteristic temperature and related parameters were evaluated. Under the condition of oxygen deficiency, the oxidation rate of coal was low, it took more time to consume the active groups in a given quality coal, and the increase of oxygen concentration had a strong positive effect on early and late combustion. The apparent activation energy (Ea) increased and decreased the conversion degree of oxygen absorption (mass gain) and high-temperature combustion stage, respectively. With the decrease of oxygen concentration, Ea in each stage decreased. These results contribute to the understanding of the evolution of coal fires in oxygen-lean environments.

Isothermal Low-Temperature Oxidation Kinetics of Heavy Crude Oil and its Oxidized Oils

Isothermal Low-Temperature Oxidation Kinetics of Heavy Crude Oil and its Oxidized Oils

Diastereomers and Low-Temperature Oxidation

Diastereomers have historically been ignored when building kinetic mechanisms for combustion. Low-temperature oxidation kinetics, which continues to gain interest in both combustion and atmospheric communities, may be affected by the inclusion of diastereomers in radical chain-branching pathways. In this work, key intermediates and transition states lacking stereochemical specification in an existing diethyl ether low-temperature oxidation mechanism were replaced with their diastereomeric counterparts. Rate coefficients for reactions involving diastereomers were computed with ab initio transition state theory master equation calculations. The presence of diastereomers increased rate coefficients by factors of 1.2-1.6 across various temperatures and pressures. Ignition delay simulations incorporating these revised rate coefficients indicate that the diastereomers enhanced the overall reactivity of the mechanism by almost 15% and increased the peak ketohydroperoxide concentration by 30% in the negative temperature coefficient region at combustion-relevant pressures. These results provide an illustrative indication of the important role of stereomeric effects in oxidation kinetics.

Kinetic study of plasma-assisted n-dodecane/O2/N2 pyrolysis and oxidation in a nanosecond-pulsed discharge

The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O2/N2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NOx chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N2* plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO2 and NO2-fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N2/O2 mixtures.

Effects of non-equilibrium excitation on methane oxidation in a low-temperature RF discharge

The kinetic effects of non-equilibrium excitation by direct electron impact on low-temperature oxidation of CH4 were investigated by experiment and simulation. We focused on the vibrational-electronic-chemistry coupling of methane and oxygen molecules under conditions of immediate reduced electric field strengths of 30–100 Td in an RF dielectric barrier discharge. A detailed plasma chemistry mechanism governing the oxidation processes in an He/CH4/O2 combustible mixture was proposed and studied by including a set of electron impact reactions, dissociative recombination reactions, reactions involving vibrationally- and electronically- excited species, and important three-body recombination reactions. A linear increase in reactant consumption with an increase in plasma power was observed experimentally. This suggested the presence of decoupling between the molecular excitation by plasma and the low-temperature chemistry. However, CO formation showed a non-linear trend, with its formation increasing with lower energy inputs and decreasing at higher energy inputs. By modelling the chemical kinetic sensitivity and reaction pathways, we found that the formation of radicals via the chain propagation reactions CH4 + O(1D) → CH3 + OH, and O2(a1Δg) + H → O + OH was mainly accelerated by the electronically excited species O(1D) and O2(a1Δg). The numerical simulation also revealed that under conditions of incomplete relaxation, the vibrational species CH4(v) and O2(v) enhanced chain propagating reactions, such as CH4(v) + O → CH3 + OH, CH4(v) + OH → CH3 + H2O, O2(v) + H → O + OH, thus stimulating the production of active radicals and final products. Specifically, for an E/N value of 68.2 Td in a stoichiometric mixture (0.05 CH4/0.1 O2/0.85 He), O(1D), CH4(v13), and O2(v) were estimated to contribute to 12.7%, 3.6%, and 3.8% of the production of OH radicals respectively. The reaction channel CH4(v13) + OH → H2O + CH3 was estimated to be responsible for 1.6% of the H2O formation. These results highlight the strong roles of vibrational states in a complex plasma chemistry system and provide new insights into the roles of excited species in the low-temperature oxidation kinetics of methane.