Methanol oxidation up to 100 atm in a supercritical pressure jet-stirred reactor

Methanol (CH₃OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. The experimental results show that the onset temperature of CH₃OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH₃OH oxidation from the literature and added some new low-temperature reaction pathways such as CH₂O + HO₂ = HOCH₂O₂ (RO₂), RO₂ + RO₂ = HOCH₂O (RO) + HOCH₂O (RO) + O₂, and CH₃OH + RO₂ = CH₂OH + HOCH₂O₂H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO₂ chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO₂ chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO₂ formation.     

Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550–700 K) oxidation chemistry of isobutanol and tert-butanol

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4–5.1 Torr) experiments at 550 and 700 K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O₂ by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the α-hydroxyalkyl radical with O₂ is predominantly linked to chain-terminating formation of HO₂. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O₂ with β-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO₂ elimination is not possible in the β-hydroxyalkyl + O₂ reaction because of the absence of a beta C–H bond; this channel is available in the β-hydroxyalkyl + O₂ reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K.     

A kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O₂ addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.     

Formation pathways of HO2 and OH changing as a function of temperature in photolytically initiated oxidation of dimethyl ether

The time resolved product formation in oxidation of dimethyl ether (DME) has been studied between 298-625 K and 20-90 torr total pressure. Near-infrared frequency modulation spectroscopy (FMS) with Herriott type multi pass optics and UV absorption spectroscopy (UV) were conducted in the same cell. The reaction was initiated by pulsed photolysis in a mixture of Cl₂, O₂, and DME via CH₃OCH₂ radical formation. The reaction process was investigated through FMS measurement of HO₂ and OH, and UV measurement of CH₃OCH₂O₂. The yields of HO₂ and OH are obtained by comparison with reference mixtures, Cl₂, O₂, and CH₃OH for HO₂, and Cl₂, O₂, CH₃OH, and NO for OH, which convert 100% of initial Cl to HO₂ and OH. The CH₃OCH₂O₂ yield is also obtained. It was found that the HO₂ yield increases sharply over 500 K mainly with a longer time constant than that of R + O₂ reaction, while a prompt component exists throughout the temperature range at a few percent yield. OH was found to be produced promptly at a yield considerably larger than that known for the simplest alkanes. The CH₃OCH₂O₂ profile has a prompt rise followed by a gradual decay whose rate is consistent with the slow HO₂ formation. The species profiles were successfully predicted with a model constructed by modifying the existing one to suit the reduced pressure condition. After modification, it was inferred that the HO₂ formation over 500 K is secondary from HCHO + OH and HCO + O₂ and a part of HCO is formed directly from the O₂ adduct, whereas the HO₂ formation below 500 K is governed by CH₃OCH₂O₂ chemistry. The HCO forming pathway via isomerization-decomposition of the O₂ adduct, which was not included in the former models, was supported by our quantum-chemical calculations.     

Insights on keto-hydroperoxide formation from O2 addition to the beta-tetrahydrofuran radical

Cyclic ethers provide an interesting case study of low-temperature oxidation chemistry, especially in relevance to biofuels. A recent experimental study (Hansen et al., 2019) revealed new questions regarding the low-temperature oxidation mechanism of tetrahydrofuran (THF) concerning the formation of keto-hydroperoxides. In particular, keto-hydroperoxides originating from the THF-β radical were not captured accurately by current literature models, motivating this work to calculate the energetics of the first and second O₂-addition pathways for THF radicals. Electronic structure calculations at the CCSD(T)/cc-pV∞Z//M06-2X/cc-pVTZ level of theory were used to generate potential energy surfaces for the α-C and β-C THF radicals and subsequent pathways to the formation of the keto-hydroperoxide isomers. These are the first theoretical calculations of the second O₂-addition radical pathways for the THF-β radical. Results from the theorical work provided further insight into the low-temperature oxidation of THF. This included identifying the pathways most likely to form the keto-hydroperoxide isomers observed in prior experimental work; and detecting that the shortcomings in prior models are likely due to uncertainties in R + THF abstraction reaction rates. These conclusions will motivate future work for accurate THF kinetic model development.     

环己烷在射流搅拌反应器中的低温氧化动力学初探:实验和动力学模型研究

We report the investigation on the low-temperature oxidation of cyclohexane in a jet-stirred reactor over 500-742 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for identifying and quantifying the oxidation species. Major products, cyclic olefins, and oxygenated products including reactive hydroperoxides and high oxygen compounds were detected. Compared with n-alkanes, a narrow low-temperature window (~80 K) was observed in the low-temperature oxidation of cyclohexane. Besides, a kinetic model for cyclohexane oxidation was developed based on the CNRS model[Combust. Flame 160, 2319 (2013)], which can better capture the experimental results than previous models. Based on the modeling analysis, the 1,5-H shift dominates the crucial isomerization steps of the first and second O₂ addition products in the low-temperature chain branching process of cyclohexane. The negative temperature coefficient behavior of cyclohexane oxidation results from the reduced chain branching due to the competition from chain inhibition and propagation reactions, i.e. the reaction between cyclohexyl radical and O₂ and the decomposition of cyclohexylperoxy radical, both producing cyclohexene and HO₂ radical, as well as the decomposition of cyclohexylhydroperoxy radical producing hex-5-en-1-al and OH radical.     

Exploring the oxidation chemistry of diisopropyl ether: Jet-stirred reactor experiments and kinetic modeling

Diisopropyl ether (DIPE) is considered as a promising gasoline additive due to the favorable blending Reid vapor pressure and the low water solubility. To get a good understanding of the DIPE oxidation chemistry, oxidation experiments of a stoichiometric mixture of DIPE/O₂/Ar/Kr were performed in a jet-stirred reactor (JSR) at atmospheric pressure over the temperature range of 525–900 K in this work. About 30 intermediates and products were identified and quantified using a photoionization molecular-beam mass spectrometer (PI-MBMS). Furthermore, a detailed kinetic model was proposed for DIPE oxidation, which showed satisfactory performances in predicting the species concentration profiles in this work as well as those in literature. For DIPE oxidation, the fuel consumption was observed only above 750 K, even though DIPE has two tertiary hydrogen atoms that are easy to be abstracted so that low-temperature oxidation reactivity is expected. The low oxidation reactivity at low temperature is because the formed OOQOOH radical mostly dissociates back to QOOH+O₂, instead of undergoing intramolecular isomerization which leads to the low-temperature chain-branching. At higher temperature, DIPE is mainly consumed by hydrogen abstraction reactions from the carbon atoms adjacent to the oxygen atom, producing dominantly the IC₃H₇OC(CH₃)₂ fuel radical, which then decomposes rapidly via CO bond β-scission instead of combining with O₂. In contrast, the minor fuel radical IC₃H₇OCH(CH₃)CH₂ tends to go through the O₂ addition reaction and the subsequent chain branching reactions, as confirmed by the detection of cyclic ether intermediates. Propylene and acetone are the most abundant intermediates in DIPE oxidation, both of which predominantly come from the initial fuel decomposition steps. Other intermediates are mainly formed via the consumption of these two species.     

Chemical kinetic interactions of NO with a multi-component gasoline surrogate: Experiments and modeling

This work reports an experimental and modeling study on the chemical kinetic interactions of NO with a multi-component gasoline surrogate, namely PACE-20, using a twin-piston rapid compression machine at a stochiometric fuel loading with 20% EGR (exhaust gas recirculation) by mass, pressures of 20 and 40 bar, and temperatures from 700 to 930 K. Five NO concentrations are investigated, namely 0, 20, 50, 70 and 150 ppm, where NO addition effects are characterized through changes in PACE-20 ignition reactivity and heat release characteristics. Experiments indicate that within the low-temperature regime, NO promotes low-temperature heat release rate and main ignition reactivity at low addition levels, with saturation or even inhibiting effects observed at >50 ppm NO addition, while within the NTC/intermediate-temperature regime, adding NO only promotes reactivity. A recently updated, detailed chemical kinetic model with chemistry specific to NOx/hydrocarbons interaction incorporated is used to simulate the experiments, and reasonable agreement is obtained. In-depth sensitivity and rate of production analyses are further performed. The results indicate that NO interacts with PACE-20 via two types of interaction: (a) direct interactions between NO and PACE-20 derivatives, primarily through NO+HO₂↔NO₂+OH and RO₂+NO↔RO+NO₂, and (b) indirect interactions between PACE-20 derivatives and NO₂ produced from the direct interactions, primarily through R+NO₂↔RO+NO. The observed NO inhibiting effect at low temperatures and 150 ppm NO addition is attributed to the lack of HO₂ radicals to sustain NO consumption via NO+HO₂↔NO₂+OH, and the take-up of inhibiting pathways via RO₂+NO↔RO+NO₂. The results also indicate that even with the presence of multiple fuel components, NOx/hydrocarbons interactions are highly selective, and are mainly initiated by the interactions between NO and RO₂ radicals from cyclopentane and ethanol, as well as between NO₂ and R radicals from toluene, 1,2,4-trimethylbenzene and 1-hexene. Further studies on these interactive reactions are therefore highly recommended.     

Vibrational non-equilibrium in the hydrogen–oxygen reaction. Comparison with experiment

A theoretical model is proposed for the chemical and vibrational kinetics of hydrogen oxidation based on consistent accounting of the vibrational non-equilibrium of the HO₂ radical that forms as a result of the bimolecular recombination H+O₂ → HO₂. In the proposed model, the chain branching H+O₂ = O+OH and inhibiting H+O₂+M = HO₂+M formal reactions are treated (in the terms of elementary processes) as a single multi-channel process of forming, intramolecular energy redistribution between modes, relaxation, and unimolecular decay of the comparatively long-lived vibrationally excited HO₂ radical, which is able to react and exchange energy with the other components of the mixture. The model takes into account the vibrational non-equilibrium of the starting (primary) H₂ and O₂ molecules, as well as the most important molecular intermediates HO₂, OH, O₂(¹Δ), and the main reaction product H₂O. It is shown that the hydrogen–oxygen reaction proceeds in the absence of vibrational equilibrium, and the vibrationally excited HO₂(v) radical acts as a key intermediate in a fundamentally important chain branching process and in the generation of electronically excited species O₂(¹Δ), O(¹D), and OH(²Σ⁺). The calculated results are compared with the shock tube experimental data for strongly diluted H₂–O₂ mixtures at 1000 < T < 2500 K, 0.5 < p < 4 atm. It is demonstrated that this approach is promising from the standpoint of reconciling the predictions of the theoretical model with experimental data obtained by different authors for various compositions and conditions using different methods. For T < 1500 K, the nature of the hydrogen–oxygen reaction is especially non-equilibrium, and the vibrational non-equilibrium of the HO₂ radical is the essence of this process. The quantitative estimation of the vibrational relaxation characteristic time of the HO₂ radical in its collisions with H₂ molecules has been obtained as a result of the comparison of different experimental data on induction time measurements with the relevant calculations.     

Oxidation of ethyl methyl ether: Jet-stirred reactor experiments and kinetic modeling

Larger ethers such as diethyl ether (DEE) and di-n-propyl ether (DPE) have different oxidation behavior (double-NTC behavior) compared to the simplest dimethyl ether (DME). Such phenomena are interpreted with different reactions and processes in different ether kinetic models, which also predict different formation pathways of oxidation intermediates such as acids. To gain further insights into the oxidation kinetics of linear ethers, ethyl methyl ether (EME), which has a nonsymmetrical structure, was studied in this work. Oxidation experiments of 1% of EME were performed in a jet-stirred reactor at 1 atm, a residence time of 2 s, an equivalence ratio of 1, and over a temperature range of 375–850 K. The intermediates were analyzed with photoionization molecular-beam mass spectrometry. To explain the oxidation behavior of EME, a detailed kinetic model was also constructed. The oxidation of EME spans a wider temperature range than DME, but no obvious double-NTC behavior was observed as DEE. Based on the model analysis and profiles of critical intermediates such as ketohydroperoxides (KHPs) and CH₃O₂H, the low-temperature oxidation behavior of EME was explained by the chain-branching reactions of the fuel itself and the oxidation intermediates. Abundant species such as aldehydes, acids, esters, and fuel-specific dione species were detected and could be well reproduced by the current model. In particular, acids are produced by the decomposition of KHPs and subsequent reactions of the intermediate CH₃CHO. Esters and dione species are mainly formed via fuel-related pathways.     

A kinetic investigation on low-temperature ignition of propane with ozone addition in an RCM

To extend the temperature for propane ignition to a lower region (< 680 K), ozone (O₃) was used as an ignition promoter to investigate the low-temperature chemistry of propane. Ignition delay times for propane containing varying concentrations of O₃ (0, 100, and 1000 ppm) were measured at 25 bar, 654–882 K, and equivalence ratios of 0.5 and 1.0 in a rapid compression machine (RCM). Species profiles during propane ignition with varying O₃ concentrations were recorded using a fast sampling system combined with a gas chromatograph (GC). A kinetic model for propane ignition with O₃ was developed. O₃ shortened ignition delay times of propane significantly, and the NTC behavior was weakened. O atoms released from O₃ reacted with propane through hydrogen abstraction reactions, which led to the fast production of OH radicals. The following oxidation of fuel radicals generated additional OH radicals. Consequently, the inhibition caused by the slow chemistry of hydrogen peroxide (H₂O₂) in the NTC region was weakened in the presence of O₃. Experimental results with O₃ addition can provide extra constraints on the low-temperature chemistry of propane. Species profiles during propane ignition at 730 K with 1000 ppm O₃ addition showed the production of propanal (C₂H₅CHO), acetone (CH₃COCH₃), and acetaldehyde (CH₃CHO) was promoted significantly. Model analyses indicated that O₃ shifted the oxidation temperature of propane to a lower region, in which reactions of ROO radicals (NC₃H₇O₂ and IC₃H₇O₂) tend to generate RO radicals (NC₃H₇O and IC₃H₇O). The promotion of RO radicals led to the fast production of C₂H₅CHO, CH₃COCH₃, and CH₃CHO. The corresponding species profile highlighted the reaction relevant to ROO and RO radicals (NC₃H₇O + O₂ = C₂H₅CHO + HO₂ and 2 IC₃H₇O₂ = 2 IC₃H₇O + O₂). Rate constants of these reactions were updated, which can potentially improve the performance of the core mechanism under lower temperatures and provide references for model development of larger hydrocarbons.     

Studies of high-pressure n-butane oxidation with CO2 dilution up to 100 atm using a supercritical-pressure jet-stirred reactor

A novel supercritical-pressure jet stirred reactor (SP-JSR) is developed to operate up to 200 atm. The SP-JSR provides a unique platform to conduct kinetic studies at low and intermediate temperatures at extreme pressures under uniform temperature distribution and a short flow residence time. n-Butane oxidations with varying levels of CO₂ dilutions at pressures of 10 and 100 atm and over a temperature range of 500-900 K were conducted using the SP-JSR. The experiment showed that at 100 atm, a weak NTC behavior is observed and the intermediate temperature oxidation is shifted to lower temperatures. Furthermore, the results showed that CO₂ addition at supercritical conditions slows down the fuel oxidation at intermediate temperature while has little effect on the low temperature oxidation. The Healy model under-predicts the NTC behavior and shows little sensitivity of the effect of CO₂ addition on the n-butane oxidation. Reaction pathway and sensitivity analyses exhibit that both the low and intermediate temperature chemistries are controlled by RO₂ consumption pathways. In addition, the reactions of CH₃CO (+ M) and CH₃CO + O₂ become important at 100 atm. The results also revealed that fuel oxidation kinetics is insensitive to the third body effect of CO₂. The kinetic effect of supercritical CO₂ addition may come from the reactions involving H₂O₂, CO, CH₂O, and CH₃CHO, especially for the reactions of CO₂ + H and CO₂ + OH.     

Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions

The resonantly-stabilized cyclopentadienyl radical (C₅H₅) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C₅H₅ reaction with the hydroperoxyl radical (HO₂) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO₂ + C₅H₅ reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO₂ + C₅H₅ reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C₅H₅O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C₅H₅O radical is the main C₅H₅O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C₅H₅O with OH and O₂ radicals is included in the kinetic mechanisms based on analogy rules. As a result, C₅H₅O mostly reacts with O₂ producing smaller C₃/C₄ species and large amounts of C₅H₄O, suggesting that further investigations of the reactivity of both C₅H₅O and C₅H₄O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO₂ + C₅H₅ reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation.     

High pressure ignition delay times of H2/CO mixture in carbon dioxide and argon diluent

Ignition Delay Time (IDT) plays a significant role in combustion process of advanced power cycles such as direct-fired supercritical carbon dioxide (sCO₂) cycle. In this cycle, fuel and oxidizer are heavily diluted with carbon dioxide (CO₂) and autoignite at a combustor inlet pressure range of 10–30 MPa and a temperature range of 900–1500 K. A fuel candidate for sCO₂ power cycle applications is syngas (H₂/CO mixture); however, its ignition properties at these conditions are not studied. Moreover, the existing chemical kinetics models have not been evaluated for H₂/CO mixtures applications relevant to elevated pressure conditions and under large dilution levels of CO₂. Therefore, two tasks are performed in this study. First, IDTs of a H₂/CO=95:5 mixture at stoichiometric and rich (Φ=2) conditions are measured in a high-pressure shock tube under 95.5% CO₂ dilution level and at 10 MPa and 20 MPa for a temperature range of 1161–1365 K. For the experimental conditions considered in this work, Aramco 2.0, FFCM-1, HP-Mech and USC Mech II kinetic models are capable of capturing IDT data. Second, similar experiments are conducted by replacing the CO₂ dilute gas with Argon (Ar) to understand the chemical effect of CO₂ on IDT globally. Sensitivity analysis results reveal that for both diluents, reaction H + O₂(+M)=HO₂(+M) is the most important reaction in controlling ignition. Further, a rate of production analysis shows that CO₂ has a competing effect on OH radical production. On one hand, CO₂ accelerates the consumption of H radicals through H + O₂+CO₂→HO₂+CO₂ therefore hindering HO₂+HOH+OH reaction for OH production. On the other hand, CO₂ is shown to enhance OH production through H₂O₂+M=OH+OH+M. These kinetic effects from CO₂ cancel out, therefore CO₂ does not significantly alter the IDT globally when compared to the Ar bath case. This is confirmed by both experimental results and simulation.     

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 detailed kinetic modeling study of the high pressure oxidation of methanol sensitized by nitric oxide and nitrogen dioxide

The perturbation of the combustion by NO_x is important in several practical systems (recent NO_x-reduction strategies, combustion with exhaust-gas recirculation in diesel and HCCI engines and for mild combustion). New experimental results were obtained for the oxidation of methanol in absence and in presence of NO or NO₂ in a fused silica jet-stirred reactor operating at 10 atm, over the temperature range 700-1100 K. Probe sampling followed by on-line FTIR analyses and off-line GC-TCD/FID analyses permitted to measure the concentration profiles of the reactants, stable intermediates and the final products. A detailed chemical kinetic modeling of the present experiments was performed. An overall good agreement between the present data and this modeling was obtained. The oxidation of methanol is significantly sensitized by NO₂, whereas the effect of NO is more limited. According to the proposed model, the mutual sensitization of the oxidation of methanol and NO proceeds through the NO to NO₂ conversion by HO₂. The increased production of OH resulting from the oxidation of NO by HO₂ promotes the oxidation of the fuel. A simplified reaction scheme can be proposed for the NO-seeded oxidation of methanol: NO + HO₂ ⇒ NO₂ + OH followed by OH + CH₃OH ⇒ H₂O + CH₂OH and CH₃O. The enhanced oxidation of methanol by addition of NO₂ is also due to additional OH production through: NO₂ + HO₂ ⇒ HONO + O₂, NO₂ + H ⇒ NO + OH and HONO ⇒ NO + OH followed by OH + CH₃OH ⇒ CH₂OH and CH₃O. The further reactions CH₂OH + O₂ ⇒ CH₂O + HO₂; CH₃O ⇒ CH₂O + H; CH₂O + OH ⇒ HCO; HCO + O₂ ⇒ HO₂ and H + O₂ ⇒ HO₂ complete the sequence whether NO or NO₂ is added.     

Measurements of H2O2 in low temperature dimethyl ether oxidation

H₂O₂ is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H₂O₂ and CH₃OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750 K, using molecular beam electron-ionization mass spectrometry (MBMS). H₂O₂ measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H + DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H₂O₂. Peroxide, as well as O₂, DME, CO_, and CH₃OCHO profiles are reasonably well predicted, though profile predictions for H₂/CO₂ and CH₂O are poor above and below ～625 K, respectively. The effect of the collisional efficiencies for the H + O₂ + M = HO₂ + M reaction on DME oxidation was investigated by replacing 20% He with 20% CO₂. Observed changes in measured H₂O₂ concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H₂O₂, CH₂O and CH₃OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH₂OCO and HCOOH directly to CO₂ may be responsible for model under-prediction of CO₂. The H abstraction reactions for DME and/or CH₂O and the unimolecular decomposition of HOCH₂O merit further scrutiny towards improving the prediction of H₂ formation.     

Acetaldehyde oxidation at elevated pressure

A detailed chemical kinetic model for oxidation of CH₃CHO at intermediate to high temperature and elevated pressure has been developed and evaluated by comparing predictions to novel high-pressure flow reactor experiments as well as shock tube ignition delay measurements and jet-stirred reactor data from literature. The flow reactor experiments were conducted with a slightly lean CH₃CHO/O₂ mixture highly diluted in N₂ at 600–900 K and pressures of 25 and 100 bar. At the highest pressure, the oxidation of CH₃CHO was in the NTC regime, controlled to a large extent by the thermal stability and reactions of peroxide species such as HO₂, CH₃OO, and CH₃C(O)OO. Model predictions were generally in good agreement with the experimental data, even though the predicted temperature for onset of reaction was overpredicted at 100 bar. This discrepancy was attributed mainly to uncertainties in the CH₃C(O)OO reaction subset. Predictions of ignition delays in shock tubes and species profiles in JSR experiments were also satisfactory. At temperatures above the NTC regime, acetaldehyde ignition and oxidation is affected mainly by the competition between dissociation of CH₃CHO and reaction with the radical pool, and by reactions in the methane subset.     

Study of the Characteristics of Elementary Processes in a Chain Hydrogen Burning Reaction in Oxygen

The characteristics of possible chain explosive hydrogen burning reactions in an oxidizing medium are calculated on the potential energy surface. Specifically, reactions H₂ + O₂ → H₂O + O, H₂ + O₂ → HO₂ + H, and H₂ + O₂ → OH + OH are considered. Special attention is devoted to the production of a pair of fast highly reactive OH radicals. Because of the high activation threshold, this reaction is often excluded from the known kinetic scheme of hydrogen burning. However, a spread in estimates of kinetic characteristics and a disagreement between theoretical predictions with experimental results suggest that the kinetic scheme should be refined.     

Global Pathway Analysis: a hierarchical framework to understand complex chemical kinetics

An automated hierarchical framework, Global Pathway Analysis (GPA), is presented to understand complex chemical kinetics. The behaviour of the reacting system at macro level is bridged to the elementary reaction level by Global Pathways, which are the chemical pathways from an initial reactant species to a final product species. For each Global Pathway, its dominancy and effect on the system, such as those on the production or consumption of radicals, are quantified to understand its contribution to the system. Four examples are presented as demonstration: First, the classical second explosion limit of hydrogen is found to be resulted from the change of dominancy of a pressure-dependent Global Pathway, which consumes radical via H?+?O₂?+?M?=?HO₂?+?M reaction. Next, it is found that the negative temperature coefficient (NTC) regime of n-heptane is resulted from the competition between a low-temperature Global Pathway and a high-temperature Global Pathway. Third, a non-monotonic relation between autoignition delays and toluene ratio in toluene/n-decane mixture is analysed. This automated framework has been placed in public domain. Reduced kinetic models can be generated based on Global Pathways too. Finally, this methodology is demonstrated using DNS simulation results of the extinction and re-ignition of a turbulent non-premixed flame. The differences between simulation results are investigated using two different kinetics models via the analysis of global pathways.