Search

Ozone addition is not only a promising method for combustion control and enhancement but also provides a reliable platform to benchmark low-temperature oxidation chemistry. Studies of ozone-doped combustion provide additional insight into combustion chemistry at lower temperatures. This paper studies the ozone-assisted low-temperature oxidation of propane in an atmospheric jet-stirred reactor (JSR) from 350 to 770 K. More than twenty species are measured and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The experimental results show that even at temperatures as low as 450 K, the reaction of propane is initiated by Ö atoms that originate from the thermal decomposition of ozone. The reactivity of propane reaches a maximum at 650 K, decreases in the temperature range of 650 – 740 K, and then slightly increases from 740 to 770 K. The propane oxidation behavior observed in this study is effectively captured by NUIGMech1.3 combined with a Princeton ozone sub-mechanism. However, the mole fractions of certain species such as, CO, H 2 O, and H 2 O 2 are over-estimated by the mechanism at temperatures above 700 K. Kinetics analyses indicate that fuel oxidation and intermediate species formation are sensitive to the reaction temperature which determines the competing reactions involving propyl-peroxy radicals and the formation of ȮH and HȮ 2 radicals. These findings are significant in terms of improving the core mechanism and acquiring a deeper comprehension of the ozone-assisted low-temperature oxidation chemistry of alkanes. [ABSTRACT FROM AUTHOR]

Low-temperature (low-T) oxidation experiments of n -heptane –with and without NO addition– were experimentally and numerically investigated at stoichiometric conditions in a jet-stirred reactor. Experiments were performed at atmospheric pressure over a temperature range of 500–800 K. Reactants, intermediates, and products, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to gain insight into the chemical effect of NO on low-T oxidation chemistry of n -heptane. Taking 650 K as the transition temperature, the results revealed that NO addition exhibited an inhibiting effect on fuel reactivity below 650 K and a promoting effect above 650 K. The reactions of ROO + NO = RO + NO 2 and HO 2 + NO = OH + NO 2 at different temperature regions were responsible for the inhibition and promotion effects, respectively. Evidence gathered from both experimental measurements and kinetic model predictions indicated that NO addition had a significant inhibitory effect on the formation of cool flame species during the low-T oxidation process. NO suppressed low-T oxidation via the reaction of ROO + NO = RO + NO 2 , which impeded the subsequent isomerization, O 2 addition, OH-, and HO 2 -elimination reaction, and influenced product distribution of the cool flame species. The experimental observations provided detailed information about these reactive intermediates, which offered new insights into low-T oxidation phenomena and clarified the importance of NO reactions which prevent the formation of cool flame products during low-T oxidation. [ABSTRACT FROM AUTHOR]

Oxidation of pure ammonia or ammonia/dimethyl ether (NH 3 /DME) mixtures in O 2 /Ar in a jet-stirred reactor (JSR) at atmospheric pressure was experimentally studied using a synchrotron vacuum ultraviolet photoionization mass spectrometer (SVUV-PIMS) and a gas chromatograph (GC) to analyse the reaction intermediates and product species. Particular attention was given to the effect of DME addition on NH 3 → NH 2 , a critical step that initiates NH 3 oxidation and subsequent formation of nitrogen oxides. The initial mole fraction of NH 3 was set to 0.01, and for the NH 3 /DME mixture, the NH 3 /DME molar ratio was set to 1. The experiments were performed at two equivalence ratios of ϕ = 1.0 and 0.2 in the temperature range of 500 – 1100 K. A kinetic model for NH 3 /DME oxidation was developed and kinetic analysis was performed. It was found that the addition of DME significantly promotes the NH 3 to NH 2 pathway and the mechanisms of this promotion effect are different at low and high temperatures. At a low temperature (∼ 600 K), NH 3 → NH 2 is mainly promoted by OH radicals produced from the DME low-temperature chain-branching reactions, while at a high temperature (∼ 1000 K), mainly by OH radicals produced from the reactions consuming HO 2 and H radicals, which were formed from DME high temperature oxidation. At T > 800 K with ϕ = 0.2, the addition of DME promotes the formation of nitrogen oxides, however, at a low temperature of 600 K, the addition of DME only promotes the formation of N 2 O and NO 2 but not NO. A reaction pathway analysis indicates that the oxidation of NO into NO 2 at low temperature is promoted by HO 2 and CH 3 OCH 2 O 2 , the intermediate products from the low temperature oxidation of DME. [ABSTRACT FROM AUTHOR]

n -Dodecane is a potential diesel and jet fuel surrogate. The combustion chemistry of n -dodecane has long been a research focal point, but its long carbon-chain length makes it challenging to obtain detailed information on cool flame intermediates and products. Determining the detailed formation mechanism of combustion intermediates is important to validate possible reaction pathways, develop detailed reaction mechanisms, and improve kinetic models. Reports on its low-temperature (low-T) oxidation, including detailed speciation in the oxidation process, are scarce. In this work, low-T oxidation of n -dodecane was investigated in a jet-stirred reactor coupled with synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. Experimental conditions were maintained at atmospheric pressure with an initial fuel concentration of 0.4%, equivalence ratio of 0.5, and residence time of 2 s. A large number of cool flame intermediates and products, including alkenes, dienes, aldehyde/keto compounds, olefinic aldehyde/keto compounds, diones, cyclic ethers, peroxides, acids, and alcohols/ethers, were identified and classified. Particular attention is focused on understanding these intermediates and products. Detailed structures and the most likely formation channels for these species are proposed and discussed herein. These results provide a solid database for the low-T oxidation of n -dodecane, which is important for the development of future kinetic models. [ABSTRACT FROM AUTHOR]

Nitrates are a potential class of substances as cetane improvers of diesel fuel and are critical raw materials for some military explosives. Thus, an in-depth investigation of the thermal decomposition mechanism of nitrates is important for further improving their effects. Herein, a combination of experimental and computational studies was performed for the pyrolysis of n -pentyl nitrate in the temperature range of 400‒900 K. Using synchrotron radiation vacuum ultraviolet photoionization, some intermediates and products, such as ethylene, formaldehyde, propene, ethenol, pentanal, NO 2 , and HONO, were identified according to their photoionization efficiency curves. Quantum chemical calculations at the CBS-QB3 level were conducted to elucidate the overall reaction mechanism of n -pentyl nitrate pyrolysis. Along the direct bond fission of n -pentyl nitrate, primary products, C 5 H 11 O• radical and NO 2 , and butyl radical and CH 2 ONO 2 , can further dissociate into many products, in which formaldehyde is the most dominant. In addition, two hydrogen-atom migration channels followed by decomposition are verified for the decomposition of n -pentyl nitrate according to the observation of trans -HONO, pentanal, and 1-pentene. These evidences highlight that both bond fission and hydrogen migration play comparable roles as potential driving forces in the pyrolysis of n -pentyl nitrate at low pressure. This thermal decomposition mechanism of n- pentyl nitrate provides some useful clues for developing combustion models of long chain nitrates in future. [ABSTRACT FROM AUTHOR]

The low temperature oxidation of alkenes exhibits some distinct reaction pathways that may form different kinds of keto-hydroperoxides to start the chain-branching by further decomposition. This is mainly due to the existence of the double bond that the OH radical could add to it. This work explores that phenomenon by investigating the low temperature oxidation of three alkenes (1-hexene, 1-heptene and 1-decene) in a jet-stirred reactor (JSR) at atmospheric pressure. The synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments combined with hydrogen-deuterium (H-D) exchange reactions were used to evaluate the chain branching pathways. Alkenyl keto-hydroperoxide (AnKHP) and hydroxyl keto-hydroperoxide (HyKHP) are identified. They represented two different chain branching reaction networks that are initiated from the H-abstraction of the primary and secondary C Hs and via OH addition to the double bond. The experimental estimated and model predicted ratios of AnKHP to HyKHP are compared, and the result indicates that the literature model underpredicts the chain branching reactions via the formation of AnKHP, but overpredicts the chain branching reactions via HyKHP. [ABSTRACT FROM AUTHOR]

Ammonia is a promising carbon-free fuel and has attracted massive attention nowadays. However, there are still large uncertainties in the mechanism of ammonia oxidation that motivate further fundamental investigations. In this work, oxidation experiments of ammonia were carried out in a jet-stirred reactor (JSR) equipped with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry and gas chromatography. Ammonia/O 2 /Ar mixtures were tested under wide-range operating conditions covering an oxidation temperature of 700–1200 K and an equivalence ratio of ϕ = 0.1–1.0. Important intermediates and products (e.g., NO, NO 2 , N 2 O, N 2 , H 2 , and H 2 O) were detected and quantified. A chemical kinetic model for ammonia oxidation was proposed by integrating the rate constants of critical reactions from different models. The results show that ammonia oxidation reactivity is quite strong at ultra-lean conditions, manifesting lower oxidization temperatures and greater oxidization rates. However, this behavior becomes weakened rapidly with the elevation of equivalence ratio, such that ammonia oxidation characteristics are highly similar between ϕ = 0.5 and 1.0. Meanwhile, important intermediates like NO 2 and N 2 O are only detected under ultra-lean conditions. Reaction pathway analysis for equivalence ratios shows that high oxygen concentration is beneficial to the formation of HO 2 , which leads to a huge amount of OH and promotes the dehydrogenation of ammonia. With the elevation of equivalence ratio, the dehydrogenation of ammonia becomes difficult and reactions NO+HO 2 =NO 2 +OH and NH 2 +NO 2 =N 2 O+H 2 O are suppressed, which answers for the vanishing of NO 2 and N 2 O at ϕ = 0.5 and 1.0. Reaction pathway analysis for oxidation temperatures also emphasizes the significance of HO 2 and its conversion pathway to OH, but they are also strongly influenced by equivalence ratios. The current work provides useful insights into ammonia oxidation and a reliable experimental database for modeling. [ABSTRACT FROM AUTHOR]

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 H 2 O 2 , 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. [ABSTRACT FROM AUTHOR]

Furan-based derivatives are promising biomass fuels that may be used directly in internal combustion engines as alternative fuels or as additives to fossil fuels. In this work, the experimental studies of 2-acetylfuran (AF2) pyrolysis were carried out in a jet-stirred reactor in the temperature range of 770 – 1130 K and at the pressure of 760 Torr, using synchrotron vacuum ultraviolet photo-ionization mass spectrometry, coupled with gas chromatography. Key pyrolysis intermediates such as methane, ethylene, acetylene, ketene, vinylacetylene, cyclopentadiene, ethynylketene, 2-methylfuran, furfural, carbon monoxide and isomers such as allene/propyne, furan/vinylketene, benzene/fulvene, etc., were identified and quantified. The potential energy surfaces of AF2 unimolecular decomposition reactions were calculated at CBS-QB3 level. The temperature- and pressure-dependent rate constants of the relevant reactions were calculated by solving the master equation based on Rice-Ramsperger-Kassel-Marcus theory. A detailed pyrolysis kinetics of AF2 was developed based on the previous combustion models of 2-methylfuran and was validated against the current experimental results. Rate of production and sensitivity analysis showed that the main consumption pathways of AF2 pyrolysis under atmospheric pressure are unimolecular decomposition reactions, leading to 2-furyl + acetyl and furoyl + methyl, and hydrogen atom-addition reaction to C(5), leading to 2,3-dihydro-5-acetyl-3-furyl, accounting at 1070 K for 25.1%, 13.2% and 20.7% of AF2 consumption, respectively (the remaining 41.0% are due to minor reaction channels contributing < 10% each). At this temperature, the total AF2 consumption is 73%. The contributions of ipso -substitution reactions by methyl and by hydrogen atom to AF2 consumption were comparable. [ABSTRACT FROM AUTHOR]

Comprehensive analysis of the low-temperature oxidation intermediates, including the reactive intermediates and isomers, is crucial to develop the low-temperature oxidation models of the fuels. In this aim, the complementary analysis by different analytic methods is needed. Furthermore, the cross check by different analytic methods increases the fidelity of the experiment data. In this work, we developed a jet-stirred reactor (JSR) system that coupled to the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) analysis. The JSR was designed to couple the SVUV-PIMS and GC, and the temperature homogeneity and mixing of the JSR was examined by a CFD simulation. The SVUV-PIMS system was developed to have a mass resolution of ca. 5000 at m/z 100. The GC system included a flame ionization detector (FID), thermal conductivity detector (TCD), and a mass spectrometer (MS). This newly developed setup was validated by repeating the oxidation data of n -heptane in the literature. A good agreement was observed between the two datasets. The low-temperature oxidation of n -butane was then studied to show the advantage of this setup, i.e., the simultaneous and comprehensive measurement of the low-temperature oxidation products. Specifically, the SVUV-PIMS enables the separation of the C/H/O composition of the intermediates, the measurement of formaldehyde and the peroxide intermediate, while the GC analysis gives a better separation of the oxygenated isomers and the quantification of the species that the SVUV-PIMS has difficult in quantifying. The experimental method developed in this work is valuable to study the fuel low-temperature oxidation chemistry, to reveal the species pool, and to provide reliable mole fraction data for the validation and development of combustion models. [ABSTRACT FROM AUTHOR]

The pyrolysis of 1-heptene was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry at 0.04 and 1 atm and in a jet-stirred reactor using gas chromatography at 1 atm. Flow reactor pyrolysis products, including the allyl radical, cycloalkenes and aromatics, were identified and quantified. Alkenes are found to be the dominant product family, among which ethylene is the most abundant product. A detailed intermediate-to-high temperature model of 1-heptene was developed and validated against the new pyrolysis data in this work, as well as previous data of 1-heptene combustion in literature over a wide range of pressures, temperatures and equivalence ratios. Rate of production analysis and sensitivity analysis were performed to reveal the key pathways in fuel decomposition and product formation. The allylic C C bond dissociation reaction is concluded as the most important pathway in 1-heptene decomposition. Reactions of allyl, propargyl and cyclopentadienyl radicals play important roles in the formation of cycloalkenes and aromatics. Furthermore, comparative pyrolysis experiments of 1-hexene and n-heptane were also performed in the jet-stirred reactor at 1 atm using gas chromatography to explore fuel molecular structure effects on pyrolysis reactivity and product distributions among 1-alkene and n-alkane fuels. The comparison between 1-heptene and 1-hexene pyrolysis demonstrates that similar fuel molecular structure results in the similarities in primary fuel decomposition pathways and pyrolysis reactivity. Ethylene is the most abundant product in both 1-alkene pyrolysis, and the feature in 1-heptene molecular structure leads to enhanced formation of ethylene in its pyrolysis. The abundant formation of ethyl and methyl radicals leads to higher production of 1-pentene and 1-butene in 1-heptene and 1-hexene pyrolysis, respectively. The comparison between 1-heptene and n-heptane pyrolysis reveals that the existence of C C double bond enhances the pyrolysis reactivity of 1-heptene. Different from 1-heptene consumption, n-heptane consumption is dominantly controlled by H abstraction reactions instead of unimolecular decomposition reactions. Propene and 1-butene are prone to be produced in 1-heptene pyrolysis, while 1-hexene has higher mole fractions in n-heptane pyrolysis. [ABSTRACT FROM AUTHOR]

Biodiesels are promising renewable fuels that can aid in the transition to carbon neutrality. The high molecular weight and complex composition of real biodiesel fuels complicate development of compact kinetic models needed for engine simulations. Our group previously proposed the functional group approach (FGMech) to model real-fuel combustion based on the identification of intrinsic relationships between fuel molecular structure and model parameters. Establishing these relationships requires a database consisting of the model parameters of pure fuels for training. In this work, we selected five fatty acid methyl esters (FAMEs) as target fuels, including methyl pentanoate (MPE), methyl hexanoate (MHX), methyl heptanoate (MHP), methyl octanoate (MO) and methyl nonanoate (MN). To facilitate development of an FGMech reaction scheme, a decoupling model approach is adopted here for model construction. Lumped reaction mechanisms are developed to describe the (oxidative) pyrolysis of fuels while a detailed model is used for describing the conversion of pyrolysis intermediates. To validate the present model, pyrolysis experiments for these FAMEs are conducted in a jet-stirred reactor (JSR) at 1 atm and over 790–1120 K. Both the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC)/GC–MS are applied for measuring pyrolysis intermediates. The fuels, primary hydrocarbon and oxygenated products, secondary products including various aromatic compounds, are identified and quantified for model validation. The present model well-predicts the temperature window of fuel decomposition, and reasonably predicts the yields of most pyrolysis products under both present atmospheric conditions and high pressure conditions in literature. The agreement between the measured and predicted results indicates that the present decoupling methodology can accurately describe fuel decomposition and the evolution of intermediates under pyrolysis conditions. In addition, it is found that increasing alkyl CH 2 groups in C 6 to C 10 FAMEs has little influence on the yields of primary oxygenated products; however, increasing yields of hydrocarbon products with increasing alkyl CH 2 groups indicates that alkane chemistry becomes more important moving from MPE to MN. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H 2 O 2) plays a key role in the autoxidation of fuels related to ignition in compression engines, and in the degradation of volatile organic compounds (VOC) in the atmospheric autoxidation. Advanced synchrotron vacuum ultraviolet radiation photoionization mass spectrometry (SVUV-PIMS) is widely used for in-situ analysis of H 2 O 2 in the gas phase autoxidation process. However, considerable uncertainty has been observed in the quantification of H 2 O 2 by SVUV-PIMS. This work improves the quantification of H 2 O 2 by re-evaluating its photoionization cross section (PICS). Spray vaporization, combined with a low pressure flow reactor, was used to minimize decomposition and wall loss of H 2 O 2 in the PICS measurement. It was found that the PICS of H 2 O 2 obtained here was lower than that reported in the literature by a factor of two. Adopting this newly obtained PICS significantly improved quantification of H 2 O 2 , and will prove valuable in evaluating the reaction kinetics of H 2 O 2 and the development of combustion and atmospheric autoxidation models. [ABSTRACT FROM AUTHOR]

As an important component of transportation fuels, toluene has little reactivity in the low temperature regime. However, the low temperature reactivity of toluene may be enhanced by the reaction of other reactive components (e.g., n-heptane) in fuel mixtures. This work examines low temperature oxidation of toluene in jet stirred reactor oxidation of an n-heptane/toluene mixture (1:1 in mole, 500–800 K, ϕ=0.5, τ=2.0 s, p = 1 bar). Two measurement techniques, time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet radiation as the photon ionization source and gas chromatography mass spectrometry, were applied to identify and measure 32 species, including four polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (OPAH). Numerical simulations using the latest kinetic model from Lawrence Livermore National Laboratory predicted the mole fractions of fuel molecules and small intermediates well, but under-predicted the mole fractions of oxygenated aromatics (phenol, benzyl alcohol, and cresol). The identification of benzyl peroxide–an important intermediate–supported the proposed formation pathways for the identified aromatics. Model analysis highlighted the influence of H-atom abstraction, OH/H radical ipso substitution, and OH addition reactions of toluene on the formation of phenol, benzyl alcohol, and cresol, which may further grow to OPAH molecules by the addition of benzyl radical from H-atom abstraction of toluene. [ABSTRACT FROM AUTHOR]

n -Butanol is a new generation of liquid biofuel. A detailed understanding of the low-temperature combustion chemistry of n -butanol is instructive for its practical application in advanced low-temperature engines. The high-temperature combustion chemistry of n -butanol has been extensively studied, but its low-temperature combustion chemistry remains underexplored, especially the detailed species distribution. In this work, ozone addition was used to enhance the low-temperature oxidation reaction activity to achieve the oxidation of n -butanol in a jet-stirred reactor (JSR) at atmospheric pressure and in the temperature range of 400–800 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for detailed measurements of species distribution. The identification of some intermediate species contributed to elucidating and verifying the low-temperature oxidation reaction pathways of n -butanol, such as butanal, C 4 hydroxyalkyl hydroperoxides, C 4 hydroxyl cyclic ether, and C 4 keto-hydroperoxide. Furthermore, the formation of several elusive intermediates was also identified (e.g., C 2 H 4 O 2 , C 3 H 6 O 2 , C 4 H 6 O n (n = 1∼3), and C 4 H 8 O 2), highlighting the necessity of further kinetic studies on the low-temperature oxidation of n -butanol. Based on detailed experimental measurements, the n -butanol model was further developed to improve the model predictions by updating the rate constants for the H-atom abstraction reactions of n -butanol by the ȮH radical, adding the detailed butanal sub-mechanism, etc. Especially, the premature formation of CO 2 in the present experiment was satisfactorily explained and predicted by the addition of a generally accepted reaction class (i.e., RȮ 2 + RȮ 2 = RȮ + RȮ + O 2 of butanal). Overall, this updated model satisfactorily predicts both the ignition delay time reported in the literature and the species distribution obtained in this work. [ABSTRACT FROM AUTHOR]

The pyrolysis of 2-ethylfuran (EF2) in a flow reactor was studied by using synchrotron vacuum ultraviolet photoionization mass spectrometry at 846 – 1319 K and at 30 and 760 Torr. Over 20 pyrolysis products were detected and measured. A detailed kinetic model was constructed by extending the reported comprehensive 2,5-dimethylfuran model to describe the pyrolysis of EF2 and validated against the current experimental results, including methyl, 2-furylmethyl, propargyl, acetylene, ethylene, propene, ketene, 2-vinylfuran, furan/vinylketene and benzene, etc. Rate of production analysis and sensitivity analysis indicate that the main reaction pathways of EF2 pyrolysis are H-abstraction reactions forming 1-(2-furyl)ethyl and 2-(2-furyl)ethyl, unimolecular decomposition reactions forming 2-furylmethyl, and H-addition reactions forming furan and vinylketene. The corresponding products, 2-vinylfuran, 2-furylmethyl and vinylketene, etc., are indicated to be the most key products in EF2 pyrolysis. Comparisons of the formation of aromatic hydrocarbons in the pyrolysis of EF2, 2-methylfuran and 2,5-dimethylfuran show that EF2 had a lower tendency to produce aromatic soot precursors. [ABSTRACT FROM AUTHOR]

1-butene is an important intermediate in combustion of various hydrocarbon fuels and oxygenated biofuels (e.g., butanol). Understanding its oxidation chemistry can help improve ignition and combustion process in advanced engines and provide better emission control. This work addresses a discrepancy between experiments and simulations in 1-butene oxidation at low temperatures, wherein simulations with AramcoMech 3.0 model show greater fuel reactivity than experiments. To further explore 1-butene low temperature reaction pathways from 550 to 910 K, experiments were conducted in three jet-stirred reactors: two coupled to time-of-flight molecular beam mass spectrometers with synchrotron vacuum ultraviolet radiation as the photoionization source, and one coupled to gas chromatography mass spectrometer. Isomeric structure identification, comprehensive species datasets, and reactor cross examinations are provided by the combination of three experiments. The identified isomer-resolved species provide evidence of various 1-butene low temperature reaction pathways. For example, the identification of propanal confirms the Waddington reaction pathway. The kinetic model over-predicts fuel reactivity in the low temperature regime (550–700 K). Updating the rate coefficients of key reactions in the Waddington pathways, e.g., forward and reverse isomerization of hydroxyl-butyl-peroxide to butoxyl-peroxide and Waddington decomposition of butoxyl-peroxide reduces the discrepancies. The role of rate constant updates in each step of the Waddington pathway is evaluated and discussed to provide directions for future model development. [ABSTRACT FROM AUTHOR]

The low-temperature oxidation of propanol isomers (n -propanol and iso -propanol) initiated by ozone addition was carried out in a jet-stirred reactor at an equivalence ratio of 0.5, in a temperature range of 400–800 K and at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to follow the temperature dependence of key intermediates, including aldehydes, ketones, hydroperoxides, alcohols, alkanes, and alkenes, to explore their low-temperature oxidation reaction pathways. n -Propanol and iso -propanol presented different intermediate distributions during low-temperature oxidation, due to the different position of the hydroxyl group in the molecular structure. The elucidation of the distribution of these intermediate species contributed to the understanding of the low-temperature oxidation reaction network of n -propanol and iso -propanol. Furthermore, a detailed kinetic model of propanol isomers was tentatively developed by systematically updating the sub-mechanism of β-hydroxypropyl radical and by adding the four reaction channels for CO 2 formation. These four reactions involve the reactions of the Ö/ȮH atom/radical with formaldehyde/acetaldehyde. The rate of production analysis reveals the crucial roles of Ö/ȮH atoms/radicals on the whole reaction process, highlighting the controlling effects of reactions directly related to Ö/ȮH atoms/radicals. Especially, the reaction between HȮ 2 radicals and ozone dominates ȮH radical formation at lower temperatures of ca. 400–475 K, while Waddington reactions, the cyclization reactions of hydroperoxyl hydroxyalkyl radicals and the H-atom abstraction reactions of fuels/intermediates are dominantly responsible for the formation of ȮH radicals at elevated temperatures of ca. > 500 K. [ABSTRACT FROM AUTHOR]

Pyrolysis experiments of dimethyl methylphosphonate (DMMP) were carried out in a jet stirred reactor (JSR) coupled to synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 1 atm, from 900 to 1200 K, to quantitatively measure the mole fraction profiles of DMMP and its pyrolysis products. Furthermore, theoretical calculations were carried out. Geometry optimization and vibrational frequency analysis were conducted for DMMP unimolecular and isomerization reactions at the MN15/6-311+G(2df,2p) level of theory, and kinetic calculations were performed using the kinetic code of MESS. The results show that DMMP mainly undergoes isomerization channels to form its isomers before decomposing into pyrolysis products. This is different from the DMMP decomposition channels adopted in the literature models. The kinetic model of DMMP in the literature was modified and improved by adding newly calculated reactions and their rate constants and updating the rate constants of unimolecular reactions for DMMP as well based on computations. Some rate tuning of reactions by 5 or 10 factors has been tried in model development to better describe the experimental measurements. Finally, the model developed by this work showed much better prediction of DMMP consumption and major species production than the prediction by literature models. In addition, the updated model was validated against various previous experimental data (CO mole fraction distributions in the shock tube, the flame speed and species mole fraction distributions in the premixed flame), and the satisfactory validation results indicate the reliability of the DMMP model that was improved in this work. This preliminary work laid a foundation to further study the combustion properties and to develop the kinetic model of organophosphorus compounds. [ABSTRACT FROM AUTHOR]

As an important component of organic solid wastes, disposal of plastic wastes (PWs) has always been a thorny issue. Thermochemical conversion is an efficient method. In-depth research on the influence of structural differences on their pyrolysis behavior is beneficial to promote the development of thermal treatment and conversion technology of PWs. The differences in the chemical structure of different polyolefin plastics at the molecule level are mainly reflected in their side chains. Herein, a series of typical polyolefin plastics with similar backbone structure and different side chain structures, including high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyvinyl alcohol (PVA) and ethylene-vinyl acetate copolymer (EVA), were selected for comprehensive pyrolysis characterization in this work. Advanced experimental analysis methods such as in-situ DRIFT and 2D-PCIS were adopted to investigate the evolution of two weak oxygen-containing side chains during pyrolysis of PVA and EVA, respectively. Besides, typical product distribution of six samples were systematically studied by combining SVUV-PIMS and GC/MS. Spectral analysis showed that OH side chain in macromolecular structure of PVA had the weakest thermal stability, and its oxidation reaction occurred earlier than dissociation and dehydration, while the acetoxy side chain in EVA polymers could not only be directly dissociated via deacetoxylation, but also be transformed into OH side chain via deacetylation that occurred simultaneously with deacetoxylation. Further combined with SVUV-PIMS and GC/MS, the distribution characteristics of pyrolysis products of polyolefin plastics and the rules of generation and release of typical products with temperature were obtained in detail. It was found that the release of typical oxygen-containing products was earlier or slightly earlier than the release of hydrocarbons in pyrolysis of PVA or EVA. However, the release of all hydrocarbons was almost simultaneous during the respective pyrolysis process of HDPE, PP and PS, suggesting that the evolution of oxygen-containing side chains with weak thermal stability occurred earlier under the drive of temperature. Based on the characterization results, the main evolution pathways of different side chains were proposed to explain the influence mechanism of different side chain structures on the pyrolysis behavior of polyolefin plastics. The cleavage of C C bonds mainly occurred in HDPE, PP and PS, while the evolution of side chains occurred first in PVC, PVA and EVA, and exerted different influence on the degradation of carbon skeleton. [ABSTRACT FROM AUTHOR]

N-methyl pyrrole (NMP) is an important nitrogenous heterocyclic compound in bio-oils produced from biomass conversion. NMP can be used as surrogate model compound to clarify the combustion chemistry of nitrogenous fuels and NO X formation mechanism. The low- to moderate-temperature oxidation experiments of NMP are carried out in atmospheric pressure jet-stirred reactor for 0.5% NMP/oxygen/Argon mixtures at different equivalence ratios (0.5, 1.0, and 2.0) over the temperature range of 760 – 1030 K. More than twenty species, including reactants, nitrogen-containing species, and oxygenated products, are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed and comprehensive kinetic model for NMP oxidation is developed for the first time based on the pyrrole oxidation model developed by Pelucchi et al. and Chen et al.. Simulated results show that the present model can reasonably reproduce the experimental results. Rate of production analysis indicates that the H-abstraction reactions on the methyl group dominate in NMP oxidation at all equivalence ratios. NMP oxidation produces C4 and C5 nitrogen-containing compounds including pyridine and pyrrole. These species are found to be products of the primary reactions of NMP, and their formation pathways can be explained satisfactorily by the present model. The present work provides a basis to further understand the oxidation characteristics of nitrogen-containing heterocyclic fuels. [ABSTRACT FROM AUTHOR]

Pyrrole is a typical fuel nitrogen model compound of coal and it is the precursor of fuel-NO x. The mechanism and kinetics of pyrrole pyrolysis were studied in a jet-stirred reactor coupled with a synchrotron vacuum ultraviolet molecular beam mass spectrometer and a gas chromometer over the temperature range of 940–1240 K at atmospheric pressure. Two sets of new data of pyrrole pyrolysis were provided. Based on the photoionization efficiency analysis, nitrogenous products are mainly produced in the form of nitriles and dinitriles. Of the pyrrole pyrolysis products, six nitriles were newly identified including methyl isocyanide (C 2 H 3 N), cyanogen (NCCN), isocyanogen (CNCN), propionitrile (C 3 H 5 N), propanedinitrile (C 3 H 2 N 2), butanedinitrile (C 4 H 4 N 2). Mole fractions of pyridine (C 5 H 5 N), C 3 H 2 N 2 , cyanovinylacetylene (C 5 H 3 N), and benzonitrile (C 7 H 5 N) were also quantified. HCN is the dominant nitrogenous product of pyrrole pyrolysis. C 2 H 2 and CH 4 are the major carbonaceous products. An updated pyrrole kinetic model was proposed comprising 196 species and 2939 reactions. The production of pyridine and several nitriles during pyrrole pyrolysis could be reasonably predicted using the current model. C 3 H 2 N 2 is mainly produced by the reaction between CH 2 CN and other nitriles, and it is converted into CH 2 CN and CH 3 CN. The reaction between CH 3 and C 3 H 2 N 2 is the most sensitive reaction for promoting the consumption of C 3 H 2 N 2. Results of this work confirm the importance of dinitriles and nitriles during the pyrolysis of pyrrole as an important fuel-NO x model compound of coal and other nitrogen-containing fuels. [ABSTRACT FROM AUTHOR]

2-Furfuryl alcohol, derived from non-edible biomass, is the significant component of bio-oil in the pyrolysis of lignocellulose biomass and a potential alternative fuel or fuel additive. To better optimize the pyrolysis model of lignocellulose to improve the properties of pyrolysis bio-oil and further understand the combustion characteristics of 2-furfuryl alcohol, the pyrolysis experiments were performed in a flow reactor at 30 Torr for T = 1006 – 1339 K and 760 Torr for T = 850 – 1131 K, and in a jet-stirred reactor at 760 Torr for T = 750 – 1050 K. The pyrolysis products, including radicals (methyl, propargyl, allyl, and cyclopentadienyl), isomers (furfural/2-ethyl furan, furan/vinyl ketene, and allene/propyne), and aromatics (benzene, toluene, indene, etc.) were identified and measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A comprehensive kinetic model for 2-furfuryl alcohol pyrolysis was developed and was validated against the experimental data in the present and previous work. Rate of production analysis indicated that the C–O bond dissociation reaction, H-abstraction reactions on the hydroxymethyl group, and the H-addition reactions on the furan-ring controlled the consumption of 2-furfuryl alcohol and the formation of primary pyrolysis products like 2-methyl furan, 2-ethyl furan, furfural, and furan. In addition, the OH-addition reaction on the furan-ring had a certain contribution to the consumption of 2-furfuryl alcohol, while the contribution of H-shift reactions was negligible. The present work systematically compared the effects of methyl, ethyl, and hydroxymethyl groups on the decomposition of furanic fuels and the formation of major products. The results showed that the substituent groups can reduce the decomposition temperature of the fuels, especially the ethyl and hydroxymethyl groups. The alkyl substituent group can promote the formation of small hydrocarbons, of which the mole fraction increased with the length of the carbon chain increasing. The hydroxymethyl group facilitated the formation of oxygenated products. [ABSTRACT FROM AUTHOR]

The pyrolysis of isopropylamine (IPA) was studied in a jet-stirred reactor at 1 atm within 670–1100 K. Pyrolysis products and intermediates were identified and quantified with both synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography analysis. Propiolonitrile, propanenitrile, 1-methylethenylamine, 3-butenenitrile, pyridine and benzonitrile were newly identified compared with previous studies of amines. HCN and 1-methylethenylamine were found to be the dominant N-containing species in IPA pyrolysis. The peak mole fractions and temperatures of C 1 -C 2 hydrocarbons are generally higher than C 3 -C 4 ones. The peak temperatures tend to rise with the increase of degree of unsaturation for hydrocarbons with the same C numbers. Based on the observations, a detailed kinetic model consisting of 429 species and 2407 reactions was developed with reasonable predictions against the measured data. Rate-of-production analysis indicates that IPA is mainly consumed by the H-abstraction at the Cα atom: (CH 3) 2 CHNH 2 + H = (CH 3) 2 CNH 2 +H 2. Sensitivity analysis shows that the unimolecular decomposition via C C bond cleavage and H-abstraction at the Cα atom play promoting roles. The H-abstraction of propene by H-atom yielding allyl radical and self-combination of methyl radicals forming ethane exhibit inhibiting effects. Compared to the pyrolysis of n-propylamine (NPA), more amine- or imine-like species were generated during NPA pyrolysis while more nitriles were detected in IPA pyrolysis. Moreover, the peak temperatures of hydrocarbons in both NPA and IPA pyrolysis shift to higher values with the increase of degree of unsaturation. The results will enrich the understanding of IPA for further combustion kinetic studies of low sooting fuels. [ABSTRACT FROM AUTHOR]

Pyridine is an important model compound of fuel-nitrogen during the coal combustion process. The lean oxidation of pyridine was studied over 800–1100 K at atmospheric pressure with the equivalence ratio of 0.4 in a jet-stirred reactor. Pyridine and its oxidation products were measured by tunable synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry and gas chromatography analyzer. Three-stage consumption phenomenon of pyridine and O 2 was observed including a fast consumption stage at 910–920 K, a moderate consumption stage at 920–1000 K, and a slow consumption stage at 1000–1100 K. HCN, N 2, N 2 O, and HNCO are the major nitrogenous oxidation products of pyridine. CO, CO 2 , HCN, and HNCO are the dominant carbonaceous products. HNCO is uniquely detected in the lean oxidation of pyridine compared with rich oxidation. A new pyridine LTO 2.0 model comprising 210 species and 1349 reactions was proposed, providing a good prediction of pyridine and its major oxidation products against experimental data. N 2 O emission is larger than NO at 1100 K. HCN is the dominant nitrogenous intermediate under both fuel-lean and fuel-rich conditions. Results of this work provide a detailed analysis of the nitrogen evolution mechanism of pyridine oxidation under the fuel-lean condition, which is close to the real industrial low-temperature coal combustion processes including the MILD combustion, fluidized bed combustion, etc. [ABSTRACT FROM AUTHOR]

Iso -octane is the dominant component of primary reference fuel (PRF), with high knock resistance during the combustion process. Understanding the low temperature oxidation chemistry of iso -octane aids in clarifying the auto-ignition properties of gasoline surrogates and developing their kinetic models. However, its low temperature oxidation performance has only been observed under high pressure conditions; no report in the literature includes detailed speciation, hindering a comprehensive examination of the iso -octane low temperature oxidation mechanism. In this work, low temperature oxidation behavior of iso -octane was clearly observed in jet-stirred reactors (JSRs) under an initial fuel mole fraction of 0.01, residence time of 2 s, equivalence ratio of 0.25, and pressure of 1 bar. The low temperature oxidation reaction networks of iso -octane were explored by measuring low temperature oxidation intermediates in three experimental approaches: synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC), and cw -cavity ring-down spectroscopy (cw -CRDS). More than 80 intermediates were qualitatively analyzed in combination with the photoionization efficiency (PIE) spectrum, quantum chemistry calculation of the ionization energies, and GC analysis. Hydroperoxides derived from iso -octane low temperature oxidation were reported experimentally for the first time, including keto-hydroperoxides (KHP) and/or hydroperoxyl cyclic ethers (HPCE). Approximately 40 products were quantified, including aldehydes, ketones, olefins, and carboxylic acids, etc. Kinetic models in the literature have been examined using the datasets obtained from this work and reaction pathways for the important intermediates discussed. Comparison indicated that the iso -octane models in the literature must be improved to accurately capture the performance of iso -octane low temperature oxidation. [ABSTRACT FROM AUTHOR]

The reaction between phenyl radical and phenylacetylene is a prototype for the reactions of aryl radicals and alkynyl peri‑condensed aromatic hydrocarbons (PCAHs), which may help explain the dimerization of PCAHs by covalent bond with acetylene assistance. In this work, we have experimentally and theoretically investigated reaction kinetics of phenyl radical and phenylacetylene. Gas chromatography-mass spectrometry (GC–MS) is applied to separate and identify molecular structures of reaction products in a jet-stirred reactor (JSR). Diphenylacetylene is observed as the major product of this reaction, which shows dominant concentration near 1000 K. Formation of phenanthrene increases with temperature, while 9-methylene-fluorene is a minor product, and 2-ethynyl-biphenyl is negligible. The potential energy surface and rate coefficients of phenyl and phenylacetylene reaction are calculated by quantum chemistry and transition state theory. 1,2-Diphenylvinyl is the only resonance-stabilized radical (RSR) among the adducts formed via all six possible addition reaction channels. Its formation reaction channel has a much lower energy barrier and deeper potential well and, therefore, it wins the competition in the reaction of phenyl and phenylacetylene at combustion-relevant intermediate temperatures. [ABSTRACT FROM AUTHOR]

Conformational effect is an important feature of the low-temperature (low-T) oxidation chemistry of cycloalkanes. In this work, both theoretical calculation and conformational analysis were performed for the first oxygen addition in methylcyclohexane (MCH) oxidation to better understand the influence of the spatial orientation and the steric hindrance of substituted functional groups. Based on synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) measurements and adiabatic ionization energy calculations, reactive hydroperoxides, highly oxygenated molecules and stable products were identified in the jet-stirred reactor (JSR) oxidation of 1.0% and 0.5% MCH at 1.04 bar, 500–900 K and the equivalence ratio of 0.25. The abundant formation of 1-methylcyclohexene and the observation of alkenylhydroperoxides and their decomposition products verify the existence of conformation-dependent pathways in first and second oxygen addition in low-T MCH oxidation, respectively. A MCH oxidation model incorporated with conformation-dependent pathways was constructed to consider the conformational effects of cyclohexane ring and sidechain, and was validated against experimental data over a wide range of conditions in this work and literature. The consideration of conformational effects of cyclohexane ring and sidechain can generally better predict the measured species profiles and ignition delay time under low-T conditions. Based on modeling analysis, conformational effects of cyclohexane ring were found to play important roles in first oxygen addition and chain-branching in low-T MCH oxidation. Compared with the situation in low-T cyclohexane oxidation, the methyl substitution on cyclohexane ring reduces its conformational effects on chain-branching process. [ABSTRACT FROM AUTHOR]

Benefitting from the rapid development of instrumental analysis methods, intermediate products that were difficult to probe in the past can now be measured and quantified in complex reaction systems. To understand low temperature reactions of interest for combustion applications, and reduce the deviations between model predictions and experimental measurements, constant advancement in understanding low temperature oxidation process is necessary. This work examines the oxidation of n -heptane in jet-stirred reactors at atmospheric pressure, with an initial n -heptane mole fraction of 0.005, equivalence ratio of 0.5, a residence time of 1s, and over a temperature range of 500-800 K. Reaction products were analyzed using synchrotron ultra-violet photoionization mass spectrometry, gas chromatography, and Fourier-transform infrared spectroscopy. Ignition delay times of n -heptane/O 2 /CO 2 mixture were measured in a rapid compression machine at 20 and 40 bar over a 600-673 K temperature range. Based on the experimental results, a comprehensive kinetic model of n -heptane low temperature oxidation was developed by considering the sub-mechanisms of keto-hydroperoxide, cyclic ether, heptene isomers, and the third O 2 addition reaction, and by updating the rate constants of keto-hydroperoxide decomposition and second oxygen addition reactions. The combination of reaction mechanism development and evaluation of the rate constants of key reactions enabled the model to effectively predict the species concentrations and ignition delay times of n -heptane low temperature oxidation, providing additional insight into alkane low temperature oxidation chemistry. [ABSTRACT FROM AUTHOR]

Pyrolysis of norbornadiene (NBD) was investigated in a jet-stirred reactor at 760 torr and within the temperature range of 560 to 1100 K. 37 products and intermediates ranging from m/z = 26 to 178 were identified and quantified by using synchrotron radiation photoionization molecular-beam mass spectrometry accompanied by gas chromatography. The C 7 H 8 isomers, including NBD, cycloheptatriene (CHT) and toluene (A1CH 3), were differentiated. Acetylene (C 2 H 2), cyclopentadiene (C 5 H 6), C 7 H 8 isomers and other aromatics are abundant during the pyrolysis process. A detailed kinetic model was developed with reasonable predictions against the measured results, involving 264 species and 1259 reactions, which is helpful to understand the kinetic and heat features of initial consumption as well as the synergistic effect of C 2 H 2 , C 5 H 6 and A1CH 3 to the formation of aromatics. Rate-of-production analysis shows that the pyrolysis of NBD experiences two stages. The first stage involves the isomerization and decomposition of NBD, yielding A1CH 3 , CHT, C 5 H 6 and C 2 H 2. Branching ratio analysis shows that C 5 H 6 and C 2 H 2 are dominant products in the first stage. Heat release analysis indicates that the first stage is endothermic. The second stage involves the pyrolysis and interactions of first-stage products to produce light hydrocarbons and polycyclic aromatic hydrocarbons like indene, naphthalene, acenaphthylene, fluorene and phenanthrene. Three routes dominate the formation of indene, including C 5 H 6 + C 5 H 5 , benzyl radical + C 2 H 2 and cycloheptatrienyl radical + C 2 H 2 , the last one of which has been scarcely considered in the previous investigation. Sensitivity analysis indicates the reactions in the first stage exhibit strong promoting effects for the consumption of NBD, while the second-stage reactions play minor roles in NBD consumption. This work would enrich the understanding of NBD consumption, PAH formation and heat release. [ABSTRACT FROM AUTHOR]

This paper explores the low-temperature (low-T) oxidation chemistry of 2,4,4-trimethyl-1-pentene (IC8D4, diisobutylene) by using jet-stirred reactor (JSR) experiments of both IC8D4/dimethyl ether (DME) mixture and pure IC8D4 at near atmospheric pressure and low temperatures. Oxidation species are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC) and GC combined with mass spectrometry (GC/MS). It is found that the oxidation of pure IC8D4 at atmospheric pressure presents negligible low-T reactivity and negative temperature coefficient (NTC) behavior, and that the oxidation reactivity of IC8D4/DME mixture is lower than that of butene/DME mixtures previously studied. A kinetic model for low-T IC8D4/DME oxidation is developed from recent oxidation models of IC8D4 and DME. Thermodynamic data of IC8D4 and key species in its sub-mechanism are obtained from theoretical calculations in this work, while rate constants of critical reactions are updated from recent theoretical calculation studies in literature. Based on the modeling analysis, four main pathways are found to be responsible for the consumption of IC8D4 at low temperatures. Among them, the first three pathways initiated by H addition, OH addition and H abstraction on allylic carbon sites are similar to those in 1-butene/DME and isobutene/DME oxidation. These three pathways are mainly responsible for promoting or retaining OH formation. The fourth pathway initiated by H abstraction on the alkyl carbon site differs from the other three in that it is only important in IC8D4/DME oxidation while not important in butenes/DME oxidation. This fourth pathway incorporates stepwise O 2 addition and cycloaddition reaction sequences, which can promote and inhibit OH formation, respectively. The increasing contribution of this fourth pathway in IC8D4/DME oxidation reduces its reactivity compared to that of butenes/DME oxidation. [ABSTRACT FROM AUTHOR]