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The effect of ethanol on n-heptane oxidation was studied using the reactive molecular dynamics (RMD) method. With different ethanol concentrations, simulations were performed under fuel-rich condit...

The extent of intersystem crossing in the O(3P) + C6H6 reaction, a prototypical system for spin-forbidden reactions in oxygenated aromatic molecules, is theoretically evaluated for the first time. Calculations are performed using nonadiabatic transition-state theory coupled with stochastic master equation simulations and Landau-Zener theory. It is found that the dominant intersystem crossing pathways connect the T2 and S0 potential energy surfaces through at least two distinct minimum-energy crossing points. The calculated channel-specific rate constants and intersystem crossing branching fractions differ from previous literature estimates and provide valuable kinetic data for the investigation of benzene and polycyclic aromatic hydrocarbons oxidation in interstellar, atmospheric, and combustion conditions. The theoretical results are supported by crossed molecular beam experiments with electron ionization mass-spectrometric detection and time-of-flight analysis at 8.2 kcal/mol collision energy. This system is a suitable benchmark for theoretical and experimental studies of intersystem crossing in aromatic species.

The potential of dimethyl ether (DME) and dimethoxymethane (DMM), representatives of the attractive oxymethylene ether (OME) alternative fuel family, are explored here as reactivity enhancers for methane-fueled polygeneration processes. Typically, such processes that can flexibly generate power, heat, or chemicals, operate under fuel-rich conditions in gas turbines or internal combustion engines. To provide a consistent basis for the underlying reaction mechanisms, it is recognized that speciation data for the DME/CH4 fuel combination are available for such conditions while such information for the DMM/CH4 system is largely lacking. In addition, it should be noted that a detailed speciation study in flames, i.e., combustion systems involving chemistry and transport processes over a large temperature range, is still missing in spite of the potential of such systems to provide extended species information. In a systematic approach using speciation with electron ionization molecular-beam mass spectrometry (EI-MBMS), we thus report, as a first step, investigation of six fuel-rich premixed flames of DME and DMM and their blends with methane with special attention on interesting chemicals. Secondly, a comprehensive but compact DME/DMM/CH4 model (PolyMech2.1) is developed based on these data. This model is then examined against available experimental data under conditions from various facilities, focusing preferentially on elevated pressure and fuel-rich conditions. Comparison with existing literature models is also included in this evaluation. Thirdly, an analysis is given on this basis, via the extensively tested PolyMech2.1 model, for assumed polygeneration conditions in a homogeneous charge compression ignition (HCCI) engine environment. The main interest of this model-assisted exploration is to evaluate whether addition of DME or DMM in a polygeneration process can lead to potentially useful conditions for the production of syngas or other chemicals, along with work and heat. The flame results show that high syngas yields, i.e., up to similar to 78% for CO and similar to 35% for H-2, can be obtained in their burnt gases. From the large number of intermediates detected, predominantly acetylene, ethylene, ethane, and formaldehyde show yields of 2.1-4.4% (C-2 hydrocarbons) and 3.4-8.7% (CH2O), respectively. Also, methanol and methyl formate show comparably high yields of up to 0.6-6.7% in the flames with DMM, which is 1-2 orders of magnitude higher than in those with DME as the additive. In the modeling-assisted exploration of the engine process, the PolyMech2.1 model is seen to perform at significantly reduced computational costs compared to a recently validated model without sacrificing the prediction performance. Promising conditions for the assumed polygeneration process using fuel combinations in the DME/DMM/CH4 system are identified with attractive syngas yields of up to 77% together with work and heat output at exergetic efficiencies of up to 89% with DME. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The boundaries between phthalocyanine synthesis and combustion chemistry were broken through to achieve molecular fireproofing via in situ phthalocyanine (Pcs) synthesis during combustion. Furthermore in situ Pcs chemistry achieves a flame-retarding organic thermoplastic polymer, showing state-of-the-art fire-safety performance and an ultra-low fire hazard.

The identification of the primary products and the determination of their branching ratios as a function of translational energy (temperature) for multi-channel elementary (bimolecular) reactions of importance in combustion flames and plasma-assisted combustion still represent a challenge for traditional kinetics experiments. On the other hand, this kind of information is central for the detailed modeling of combustion/plasma systems. In this short review, the significant contribution provided in this area by the crossed molecular beam (CMB) scattering method with “universal” mass-spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB technique empowered with “soft” electron-impact ionization as recently implemented in our laboratory, and report on its application to the study of the multi-channel elementary reactions of ground state atomic oxygen, O(3P), with unsaturated hydrocarbons containing two carbon atoms (acetylene and ethylene), three carbon atoms (propyne, propene, and allene), and also four carbon atoms (1-butene, 1,2-butadiene, and 1,3-butadiene), which are of paramount interest in combustion flames and plasma-assisted combustion of hydrocarbons. These studies are usually complemented in a synergistic manner by high-level electronic structure calculations of the underlying potential energy surfaces and related statistical (and dynamical when feasible) calculations of product branching ratios. The complementarity to kinetics studies and the implications of the dynamics results for the modeling of combustion/plasma chemistry will be commented on.

Reliable modeling of hydrocarbon oxidation relies critically on knowledge of the branching fractions (BFs) as a function of temperature (T) and pressure (p) for the products of the reaction of the hydrocarbon with atomic oxygen in its ground state, O(3P). During the past decade, we have performed in-depth investigations of the reactions of O(3P) with a variety of small unsaturated hydrocarbons using the crossed molecular beam (CMB) technique with universal mass spectrometric (MS) detection and time-of-flight (TOF) analysis, combined with synergistic theoretical calculations of the relevant potential energy surfaces (PESs) and statistical computations of product BFs, including intersystem crossing (ISC). This has allowed us to determine the primary products, their BFs, and extent of ISC to ultimately provide theoretical channel-specific rate constants as a function of T and p. In this work, we have extended this approach to the oxidation of one of the most important species involved in the combustion of aromatics: the benzene (C6H6) molecule. Despite extensive experimental and theoretical studies on the kinetics and dynamics of the O(3P) + C6H6 reaction, the relative importance of the C6H5O (phenoxy) + H open-shell products and of the spin-forbidden C5H6 (cyclopentadiene) + CO and phenol adduct closed-shell products are still open issues, which have hampered the development of reliable benzene combustion models. With the CMB technique, we have investigated the reaction dynamics of O(3P) + benzene at a collision energy (Ec) of 8.2 kcal/mol, focusing on the occurrence of the phenoxy + H and spin-forbidden C5H6 + CO and phenol channels in order to shed further light on the dynamics of this complex and important reaction, including the role of ISC. Concurrently, we have also investigated the reaction dynamics of O(1D) + benzene at the same Ec. Synergistic high-level electronic structure calculations of the underlying triplet/singlet PESs, including nonadiabatic couplings, have been performed to complement and assist the interpretation of the experimental results. Statistical (RRKM)/master equation (ME) computations of the product distribution and BFs on these PESs, with inclusion of ISC, have been performed and compared to experiment. In light of the reasonable agreement between the CMB experiment, literature kinetic experimental results, and theoretical predictions for the O(3P) + benzene reaction, the so-validated computational methodology has been used to predict (i) the BF between the C6H5O + H and C5H6 + CO channels as a function of collision energy and temperature (at 0.1 and 1 bar), showing that their increase progressively favors radical (phenoxy + H)-forming over molecule (C5H6 + CO and phenol stabilization)-forming channels, and (ii) channel-specific rate constants as a function of T and p, which are expected to be useful for improved combustion models.

The present work quantifies the impact of fuel chemistry on burning modes using premixed dimethyl ether (DME), ethanol (EtOH) and methane flames in a back-to-burnt opposed jet configuration. The study considers equivalence ratios 0 ≤ Φ ≤ 1, resulting in a Damkohler (Da) number range 0.06 ≤ Da ≤ 5.1. Multi-scale turbulence (Re ≃ 19,550 and Ret ≃ 360) is generated by means of a cross fractal grid and kept constant along with the enthalpy of the hot combustion products (THCP = 1700 K) of the counterflow stream. The mean turbulent rate of strain exceeds the laminar extinction rate for all flames. Simultaneous Mie scattering, OH-PLIF and PIV are used to identify reactants, mixing, weakly reacting, strongly reacting and product fluids. The relative balance between conventional flame propagation and auto-ignition based combustion is highlighted using suitably defined Da numbers and a more rapid transition towards self-sustained (e.g. flamelet type) combustion is observed for DME. The strain rate distribution on the reactant fluid surface for methane remains similar to the (non-reactive) mixing layer ( Φ = 0 ), while DME and EtOH flames gradually detach from the stagnation plane with increasing Φ leading to stabilisation in regions with lower compressive rates of strain. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours.

We present in this short communication a modification to our previous ethanol reduced combustion chemistry (Millán-Merino, 2018) that eliminates nonphysical values of the species concentrations which we discovered in applying the mechanism to the combustion of an isolated ethanol droplet. This unsteady test is reported here to check the multipurpose character of the reduced mechanism for a problem that combines non-homogeneous autoignition, rich and lean premixed-flame propagation, and the development of a diffusion flame, as well as a the presence of a cold moving boundary at the droplet surface. During the computations, production and consumption rates of the alfa-hydroxyethyl (CH3CHOH) intermediary radical became unbalanced, invalidating its steady-state hypothesis, which was used during the derivation of the reduced scheme. This difficulty is removed here by taking CH3CHOH out of steady state, thereby augmenting slightly the reduced mechanism. This work was supported by the project ENE2015-65852-C2-1-R (MINECO/FEDER,UE).

We have measured the kinetics and thermochemistry of the reaction of 3-methylpropargyl radical (but-2-yn-1-yl) with molecular oxygen over temperature (223–681 K) and bath gas density ( 1.2 − 15.0 × 10 16 cm − 3 ) ranges employing photoionization mass-spectrometry. At low temperatures (223–304 K), the reaction proceeds overwhelmingly by a simple addition reaction to the − CH 2 end of the radical, and the measured CH 3 CCCH 2 • + O 2 reaction rate coefficient shows negative temperature dependence and depends on bath gas density. At intermediate temperatures (340–395 K), the addition reaction equilibrates and the equilibrium constant was determined at different temperatures. At high temperatures (465–681 K), the kinetics is governed by O2 addition to the third carbon atom of the radical, and rate coefficient measurements were again possible. The high temperature CH 3 CCCH 2 • + O 2 rate coefficient is much smaller than at low T, shows positive temperature dependence, and is independent of bath gas density. In the intermediate and high temperature ranges, we observe a formation signal for ketene (ethenone). The reaction was further investigated by combining the experimental results with quantum chemical calculations and master equation modeling. By making small adjustments ( 2 − 3 kJ mol − 1 ) to the energies of two key transition states, the model reproduces the experimental results within uncertainties. The experimentally constrained master equation model was used to simulate the CH 3 CCCH 2 • + O 2 reaction system at temperatures and pressures relevant to combustion.

To further refine the existing high-temperature combustion chemistry mechanisms of Sarin surrogates or to develop new ones, the ignition delay times of mixtures containing various Sarin surrogates have been studied in the authors’ laboratory, namely dimethyl-methylphosphonate, diethyl-methylphosphonate (DEMP), and triethylphosphate, and the results are compared for the first time herein. They were each measured in a heated shock tube, with the DEMP-related ignition delay times being the new data reported in this paper. The Sarin surrogates were studied in neat mixtures with oxygen or seeded to baseline mixtures of hydrogen or methane at around 1.5 atm. Noticeable differences were observed between the ignition delay times of the three simulants, whether sole or mixed with a fuel. Comparisons of OH* time histories obtained from each surrogate highlight the similarities and differences in chemical structure among the different compounds. In mixtures with oxygen and Ar, the three surrogates present similar ignition delay times below 1380 K, whereas the ignition delay time results rapidly diverge above this temperature. When the surrogates were added into $$\hbox {H}_{2}/\hbox {O}_{2}$$ or $$\hbox {CH}_{4}/\hbox {O}_{2}$$ mixtures, large changes in the reactivity of the mixtures were observed. These changes in reactivity are however dependent on the surrogate, for each fuel investigated.

In this study, a series of ignition delay time (IDT) experiments of 1-butene were performed in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) under conditions of relevance to practical combustors. This is the first 1-butene IDT data taken at engine relevant conditions, and the combination of HPST and RCM results greatly expands the range of data available for the oxidation of 1-butene to higher pressures (10-50 atm), lower temperatures (670-1350 K) and to a wide range of equivalence ratios (0.5-2.0).A comprehensive chemical kinetic mechanism to describe the combustion of 1-butene has simultaneously been applied. It has been validated using the IDT data measured here in addition to a large variety of literature data: IDTs, speciation data from jet-stirred reactor (JSR), premixed flame, and flow reactor, and laminar flame speed data. Important reactions have been identified via flux and sensitivity analyses including: (a) H-atom abstraction from 1-butene by hydroxyl radicals and molecular oxygen from different carbon sites; (b) addition reactions, including hydrogen atom and hydroxyl radical addition to 1-butene; (c) allylic radical chemistry, including the addition reactions with methyl radical, hydroperoxy radical and self-recombination; (d) vinylic radical chemistry, including the addition reaction with molecular oxygen; (e) alcohol radical chemistry, including the Waddington type propagating reaction pathways and alkyl radical low-temperature branching chemical pathways. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved. The authors thank the entire group members at Combustion Chemistry Centre for helpful discussions. This work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. peer-reviewed 2019-04-13

Bioalcohols are a promising family of biofuels. Among them, 1-butanol has a strong potential as a substitute for petrol. In this manuscript, we report on a theoretical and experimental characterization of 1-butanol thermal decomposition, a very important process in the 1-butanol combustion at high temperatures. Advantage has been taken of a flash pyrolysis experimental set-up with mass spectrometric detection, in which the brief residence time of the pyrolyzing mixture inside a short, resistively heated SiC tube allows the identification of the primary products of the decomposing species, limiting secondary processes. Dedicated electronic structure calculations of the relevant potential energy surface have also been performed and RRKM estimates of the rate coefficients and product branching ratios up to 2,000 K are provided. Both electronic structure and RRKM calculations are in line with previous determinations. According to the present study, the H2O elimination channel leading to 1-butene is more important than previously believed. In addition to that, we provide experimental evidence that butanal formation by H2 elimination is not a primary decomposition route. Finally, we have experimental evidence of a small yield of the CH3 elimination channel.

The hydroxyl radical reactions are of extremely significant in combustion chemistry. Direct dynamics simulations are used to study the atomic-level mechanisms of OH(H2O)n + CH4 (n = 0,1,2) reactions starting from the [(H2O)nOH---H---CH3] central barrier, which is the important step in the combustion of CH4 in an O2/H2O environment. The simulation results show the propensity for the solvated products due to the thermodynamic preference and the stability of water cluster in products. Although there is a deep minimum (H2O)nH2O---CH3 complex in the product exit channel on potential energy surface, the majority of the trajectories avoided this well and instead directly dissociated to products, showing a non-IRC behavior. Significantly, with the addition of two water molecules, the reaction probability, and thus the reaction rate constant increases compared to the one hydrated reaction. This study provides an understanding of CH4 combustion in O2/H2O atmosphere.

Butenes are intermediates ubiquitously formed by decomposition and oxidation of larger hydrocarbons (e.g. alkanes) or alcohols present in conventional or reformulated fuels. In this study, a series of novel igni-tion delay time (IDT) experiments of trans-2-butene were performed in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) under conditions of relevance to practical combustors. This is the first IDT data of trans-2-butene taken at engine relevant conditions, and the combination of HPST and RCM results greatly expands the range of data available for the oxidation of trans-2-butene to higher pressures (10-50 atm), lower temperatures (670-1350 K) and a wide range of equivalence ratios (0.5-2.0). A comprehensive chemical kinetic mechanism has simultaneously been developed to describe the combustion of trans-2-butene. It has been validated using the IDT data measured here in addition to a large variety of literature data: jet-stirred reactor (JSR) speciation data, premixed flame speciation data, flow reactor speci-ation data and laminar flame speed data. Moreover, the reactivity of trans-2-butene is compared to that of the other two isomers, 1-butene and isobutene, and these comparisons are discussed. Important reactions are highlighted via flux and sensitivity analyses and help explain the differences in reactivity among the butene isomers. (C) 2016 by The Combustion Institute. Published by Elsevier Inc. The authors thank the entire group members at Combustion Chemistry Centre for helpful discussions. This work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. peer-reviewed 2018-06-16

Theoretical aspects of the development of a chemical kinetic model for the pyrolysis and combustion of a cyclic ketone, cyclopentanone, are considered. Calculated thermodynamic and kinetic data are presented for the first time for the principal species including 2- and 3-oxo-cyclopentyl radicals, which are in reasonable agreement with the literature. These radicals can be formed via H atom abstraction reactions by (H) over dot and O atoms and OH, HO2, and CH3. radicals, the rate constants of which have been calculated. Abstraction from the beta-hydrogen atom is the dominant process when OH is involved, but the reverse holds true for H(O) over dot(2) radicals. The subsequent beta-scission of the radicals formed is also determined, and it is shown that recent tunable VUV photoionization mass spectrometry experiments can be interpreted in this light. The bulk of the calculations used the composite model chemistry G4, which was benchmarked in the simplest case with a coupled cluster treatment, CCSD(T), in the complete basis set limit. The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. The work at LLNL was conducted as part of the Co-Optimization of Fuels & Engines (Co-Optima) project sponsored by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies and Vehicle Technologies Offices and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Computational resources were provided by the Irish Centre for HighEnd Computing, ICHEC. peer-reviewed

This contribution combines results of experiments with kinetic modelling to probe the unusual behaviour of carbonyl sulfide (COS), a sulfur species that frequently arises in fuel systems. The experiments identified CO and SO2 as the primary oxidation products, with no formation of CO2. The low ignition temperature (

The combustion chemistry of formaldehyde in fuel-rich flames has been studied by numerical modeling and sensitivity analysis. It has been shown that the wide flammability limits of CH2O/air mixtures are due to features of the combustion chemistry of formaldehyde at high equivalence ratios rather than to the superadiabatic temperature effect. In this case, the thermal decomposition reaction of hydrogen peroxide H2O2 plays a key role in the conventional branching reactions.

Products formed in the reaction of C 2 H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C 6 H 6 . The photoionization spectrum of the C 6 H 6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.

The present work focuses on new measurement of laminar burning velocities of ethanol-air mixtures at 1 atm pressure and elevated mixture temperatures using an externally heated meso-scale diverging channel technique. The burning velocity measurements were carried out for a temperature range of 350–620 K and equivalence ratio range of 0.7–1.3. Various detailed kinetic models available in literature were used for assessment and comparison with experimental results. The experimental results show a good match at lower mixture temperatures across all equivalence ratios. However, at higher temperatures, the difference between the measurements and predictions of different kinetic models is considerably higher, particularly for rich mixture regime. The effect of mixture temperature on laminar burning velocity was assessed using power law correlation, S u = S u , 0 ( T u / T u , 0 ) α . The variation of temperature exponent, α with equivalence ratio, Φ showed a minimum value for slightly rich mixtures. This variation of the measured laminar burning velocity and temperature exponent at elevated mixture temperatures and predictions using various kinetic mechanisms shows a good match for lean mixtures.

This paper presents a detailed investigation of 2-methylbutanol combustion chemistry in low-pressure premixed flames. This chemistry is of particular interest to study because this compound is potentially a lignocellulosic-based, next-generation biofuel. The detailed chemical structure of a stoichiometric low-pressure (25 Torr) flame was determined using flame-sampling molecular-beam mass spectrometry. A total of 55 species were identified and subsequently quantitative mole fraction profiles as function of distance from the burner surface were determined. In an independent effort, a detailed flame chemistry model for 2-methylbutanol was assembled based on recent knowledge gained from combustion chemistry studies for butanol isomers ([Sarathy et al. Combust. Flame 159 (6) (2012) 2028-2055]) and iso- pentanol (3-methylbutanol) [Sarathy et al. Combust. Flame 160 (12) (2013) 2712-2728]. Experimentally determined and modeled mole fraction profiles were compared to demonstrate the model’s capabilities. Examples of individual mole fraction profiles are discussed together with the most significant fuel consumption pathways to highlight the combustion chemistry of 2-methylbutanol. Discrepancies between experimental and modeling results are used to suggest areas where improvement of the kinetic model would be needed.

The effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for a B100 biodiesel are investigated. The surrogate is a mixture of 50% n-decane/50% methyl-octanoate (molar) to represent methyl-oleate. The combustion chemistry and soot formation are solved using a mechanism with 288 species and 2073 reactions coupled with a sectional soot model, respectively. Soot volume fraction (fv) and temperature profiles are compared to the experimentally measured values for this biodiesel surrogate. In addition, the effects of the ester moiety on soot formation and flame chemistry are studied by numerically comparing the biodiesel surrogate flame with a pure n-decane flame. The model predicts both temperature and fv profiles with a good accuracy. Some discrepancies for fv on the flame centerline are observed between the model and the experiments; it is suggested that these discrepancies are because the model and the experiment cannot distinguish nascent transparent soot from mature soot and because the mechanism under-predicts PAH formation rates. Both n-decane and B100 surrogate flames have similar fv and temperature profiles when both flames have the same energy input. This suggests that the ester moiety does not have a major impact on soot formation. In addition, early production of CO and higher concentrations of some oxygenated species such as formaldehyde are observed in the predicted concentration contour plots of the B100 surrogate flame when they are compared to the n-decane flame. Reaction pathway analysis reveals that the higher peak concentrations of formaldehyde and the early production of CO from CH2CO and CH3CO2 that come directly from the ester moiety in the B100 surrogate are much more pronounced than other species in the B100 surrogate flame and are recognized as the main differentiating characteristics of the B100 surrogate flame from the n-decane flame.

A detailed numerical investigation of combustion of unsupported and fiber supported n-heptane droplets in reduced gravity is presented. The primary focus is on the effects of support fibers on the droplet burning rates and flame structure. A 21-step n-heptane reaction mechanism consisting of 20 species is employed to model the combustion chemistry. The volume-of-fluid (VOF) method is employed to capture the liquid-gas interface while allowing for time-dependent two-phase multidimensional flows. Computed burning rates and flame stand-off ratios are compared with the experimental results of Jackson. Predicted flame structures are also validated with the experimental results of Mikami. The present computational results agree well with the experimental results. The results indicate that the support fibers can have significant impact on droplet burning rates and flame structure.

Hydrogen abstraction reactions from hydrofluorocarbons are important reaction steps in both atmospheric and combustion chemistry. In this study, kinetics of the H-abstraction reactions from methane...

Brominated flame retardants (BFRs) are organobromine compounds with an inhibitory effect on combustion chemistry tending to reduce the flammability of products. Concerns about health effects and environmental threats have led to phase-out or restrictions in the use of Penta-, Octa- and Deca-BDE technical formulations, increasing the demand for Novel BFRs (NBFRs) as replacements for the banned formulations. This study examined the occurrence of legacy and NBFRs in the dust from the interior of private cars in Thessaloniki, Greece, aged from 1 to 19 years with variable origin and characteristics. The determinants included 20 Polybrominated Diphenyl Ethers (PBDEs) (Di-to Deca-BDEs), four NBFRs such as Decabromodiphenylethane (DBDPE), 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE), 2-ethylhexyl2,3,4,5-tetrabromobenzoate (TBB), and bis(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBPH), three isomers of hexabromocyclododecane (HBCD), and tetrabromobisphenol A (TBBPA). The concentrations of Sigma(20)1313DE ranged from 132 to 54,666 ng g(-1) being dominated by BDE-209. The concentrations of Sigma(4)NBFRs ranged from 48 to 7626 ng g(-1) and were dominated by DBDPE, the major substitute of BDE-209. HBCDs ranged between

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states.

This paper presents a computational fluid dynamics (CFD) modelling study to investigate Victorian brown coal combustion in a 550 MW utility boiler under the air-fired (standard) and three oxy-fuel-fired cases. The standard case was modelled based on the real operating conditions of Loy Yang A power plant located in the state of Victoria, Australia. A level of confidence of the present CFD model was achieved validating four parameters of the standard combustion case, as well as the previous preliminary CFD studies which were conducted on a lab-scale (100 kW) unit firing lignite and propane under oxy-fuel-fired scenarios. The oxy-fuel combustion cases are known as OF25 (25vol. % O2 concentration), OF27 (27vol. % O2 concentration), and OF29 (29vol. % O2 concentration). The predictions of OF29 combustion case were considerably similar to the standard firing results in terms of gas temperature levels and radiative heat transfer compared with OF25 and OF27 combustion scenarios. This similarity was because of increasing the residence time of pulverised coal (PC) in the combustion zone and O2 concentration in feed oxidizer gases. Furthermore, a significant increase in the CO2 concentrations and a noticeable decrease in the nitric oxides (NOx) formation were noted under all oxy-fuel combustion conditions. This numerical study of oxy-fuel combustion in a full-scale tangentially-fired PC boiler is important prior to its execution in real-life power plants.

We report on the determination of primary products and their branching ratios for the combustion relevant O(3P)+allene reaction by the crossed molecular beams method with soft electron-ionization mass-spectrometric detection at a collision energy of 39.3 kJ/mol. We have explored the reaction dynamics of the open channels leading to C2H4+CO, C2H2+H2CO, C2H3+HCO, CH2CCHO+H, and CH2CO+CH2. Because some of the observed products can only be formed via intersystem crossing (ISC) from triplet to singlet potential energy surfaces, from the product branching ratios we have inferred the extent of ISC. The conclusion is that the O(3P)+allene reaction proceeds mostly (>90%) via ISC. This observation poses the question of how important it is to consider nonadiabatic effects for this and other similar systems involved in combustion chemistry. Another important conclusion is that the interaction of atomic oxygen with allene breaks apart the three-carbon atom chain, mostly producing CO and ethylene.

Although the propene + OH reaction has been in the center of interest of numerous experimental and theoretical studies, rate coefficients have never been determined experimentally between ∼600 and ∼750 K, where the reaction is governed by the complex interaction of addition, back-dissociation and abstraction. In this work OH time-profiles are measured in two independent laboratories over a wide temperature region (200–950 K) and are analyzed incorporating recent theoretical results. The datasets are consistent both with each other and with the calculated rate coefficients. We present a simplified set of reactions validated over a broad temperature and pressure range, that can be used in smaller combustion models for propene + OH. In addition, the experimentally observed kinetic isotope effect for the abstraction is rationalized using ab initio calculations and variational transition-state theory. We recommend the following approximate description of the OH + C3H6 reaction C3H6 + OH ⇆ C3H6OH (R1a,R-1a) C3H6 + OH → C3H5 + H2O (R1b) k 1a (200 K ≤ T ≤ 950 K; 1 bar ≤ P) = 1.45 × 10-11 (T/K)-0.18 e 460 K/T cm3 molecule-1 s-1 k -1a (200 K ≤ T ≤ 950 K; 1 bar ≤ P) = 5.74 × 1012 e -12690 K/T s-1 k 1b (200 K ≤ T ≤ 950 K) = 1.63 × 10-18 (T/K)2.36 e -725 K/T cm3 molecule-1 s-1

The pyrolysis and oxidation of benzene occupies a critical role in the combustion chemistry of practical fuels. Despite numerous experimental and numerical investigations, uncertainties still exist...

Combustion Chemistry The chemical origin of soot is a persistent puzzle. It is clear that small hydrocarbon fragments formed in flames must aggregate into larger particles, but the initial driving force for aggregation remains a mystery. Johansson et al. combined theory and mass spectrometry to suggest a solution based on resonance-stabilized radicals (see the Perspective by Thomson and Mitra). Aromatics such as cyclopentadiene have a characteristically weak C–H bond because their cleavage produces radicals with extended spans of π-electron conjugation. Clusters thus build up through successive coupling reactions that extend conjugation in stabilized radicals of larger and larger size. Science , this issue p. [997][1]; see also p. [978][2] [1]: /lookup/doi/10.1126/science.aat3417 [2]: /lookup/doi/10.1126/science.aau5941

There is growing interest in the use of furans, a class of alternative fuels derived from biomass, as transportation fuels. This paper reviews recent progress in the characterization of its combustion properties. It reviews their production processes, theoretical kinetic explorations and fundamental combustion properties. The theoretical efforts are focused on the mechanistic pathways for furan decomposition and oxidation, as well as the development of detailed chemical kinetic models. The experiments reviewed are mostly concerned with the temporal evolutions of homogeneous reactors and the propagation of laminar flames. The main thrust in homogeneous reactors is to determine global chemical time scales such as ignition delay times. Some studies have adopted a comparative approach to bring out reactivity differences. Chemical kinetic models with varying degrees of predictive success have been established. Experiments have revealed the relative behavior of their combustion. The growing body of literature in this area of combustion chemistry of alternative fuels shows a great potential for these fuels in terms of sustainable production and engine performance. However, these studies raise further questions regarding the chemical interactions of furans with other hydrocarbons. There are also open questions about the toxicity of the byproducts of combustion.

A theoretical study on combustion of JA2, RPD-380, and RPD-351, which are modified double-base propellants composed primarily of three identical nitrate ester ingredients, is presented. A one-dimensional, two-phase model was used [M.S. Miller, W.R. Anderson, in: V. Yang, T.B. Brill, W.Z. Ren (Eds.), Solid Propellant Combustion Chemistry, Combustion and Motor Interior Ballistics, Progress in Astronautics and Aeronautics, vol. 185, AIAA, Reston, VA, 2000, pp. 501–531, (a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. This approach has been shown to give good agreement between predicted and experimental results for several nitrate ester propellants, including JA2 [(a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. Extension of the model to the two RPD variants yields results in good agreement with existing experimental data. Comparisons of the response of predicted burning rates to experimental formulation changes at gun pressures, and to the initial propellant temperature are particularly encouraging. Our results show the burning rate ordering of these propellants is JA2

The rate constants and product branching ratios for the reactions of phosphorus oxychloride anions, POxCly− for x = 1–2 and y = 1–3, with O2(a 1Δg), Cl2, and H2O have been measured in a selected ion flow tube (SIFT) at 298 K. A mixture of O2(a 1Δg) in O2 has been produced using a recently designed chemical singlet oxygen generator (sparger) with an emission detection scheme adopted previously in our laboratory. The experiments continue a series of investigations into the oxidation reactions of POxCly− ions, searching for pathways to the terminal PO2− and PO3− ions observed in combustion chemistry with POCl3 present. None of the POxCly− ions react with H2O or O2(a 1Δg). The O2(a 1Δg) rate constants have a limit of

We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O((3)P) + propyne and O((3)P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene ((1)CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O((3)P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O((3)P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O((3)P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected (1)CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

© 2015 American Chemical Society. Substituted furans, including furanic ethers, derived from nonedible biomass have been proposed as second-generation biofuels. In order to use these molecules as fuels, it is important to understand how they break apart thermally. In this work, a series of experiments were conducted to study the unimolecular and low-pressure bimolecular decomposition mechanisms of the smallest furanic ether, 2-methoxyfuran. Electronic structure (CBS-QB3) calculations indicate this substituted furan has an unusually weak O - CH3 bond, approximately 190 kJ mol-1 (45 kcal mol-1); thus, the primary decomposition pathway is through bond scission resulting in CH3 and 2-furanyloxy (O - C4H3O) radicals. Final products from the ring opening of the furanyloxy radical include 2 CO, HC - CH, and H. The decomposition of methoxyfuran is studied over a range of concentrations (0.0025-0.1%) in helium or argon in a heated silicon carbide (SiC) microtubular flow reactor (0.66-1 mm i.d., 2.5-3.5 cm long) with reactor wall temperatures from 300 to 1300 K. Inlet pressures to the reactor are 150-1500 Torr, and the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products formed at early pyrolysis times (100 μs) are detected by 118.2 nm (10.487 eV) photoionization mass spectrometry (PIMS), tunable synchrotron VUV PIMS, and matrix infrared absorption spectroscopy. Secondary products resulting from H or CH3 addition to the parent and reaction with 2-furanyloxy were also observed and include CH2=CH-CHO, CH3-CH=CH-CHO, CH3=CO-CH=CH2, and furanones; under the conditions in the reactor, we estimate these reactions contribute to at most 1-3% of total methoxyfuran decomposition. This work also includes observation and characterization of an allylic lactone radical, 2-furanyloxy (O-C4H3O), with the assignment of several intense vibrational bands in an Ar matrix, an estimate of the ionization threshold, and photoionization efficiency. A pressure-dependent kinetic mechanism is also developed to model the decomposition behavior of methoxyfuran and provide pathways for the minor bimolecular reaction channels that are observed experimentally.

To study combustion chemistry at low temperatures in a shock tube, it is of great importance to increase experimental test times, and this can be done by tailoring the interface between the driver and driven gases. Using unconventional driver-gas tailoring with the assistance of tailoring curves, shock-tube test times were increased from 1 to 15 ms for reflected-shock temperatures below 1,000 K. Provided in this paper is the introduction of tailoring curves, produced from a one-dimensional perfect gas model for a wide range of driver gases and the production and demonstration of successful driver mixtures containing helium combined with either propane or carbon dioxide. The He/CO2 and He/C3H8 driver mixtures provide a unique way to produce a tailored interface and, hence, longer test times, when facility modification is not an option. The tailoring curves can be used to guide future applications of this technique to other configurations. Nonreacting validation experiments using driver mixtures identified from the tailoring curves were performed over a range of reflected-shock temperatures from approximately 800 to 1,400 K, and some examples of ignition-time experiments that could not have otherwise been erformed are presented.

Although the combustion chemistry of aliphatic hydrocarbons has been extensively documented, the oxidation of cyclic hydrocarbons has been studied to a much lesser extent. To provide a deeper understanding of the combustion chemistry of naphthenes, the oxidation of methylcyclohexane was studied in a series of high-temperature shock tube experiments. Ignition delay times for a series of mixtures, of varying methylcyclohexane/oxygen equivalence ratios (phi=0.5, 1.0, 2.0), were measured over reflected shock temperatures of 1200-2100 K and reflected shock pressures of 1.0, 2.0, and 4.0 atm. A detailed chemical kinetic mechanism has been assembled to simulate the shock tube results and flow reactor experiments, with good agreement observed.

There is much interest in the combustion mechanism of organophosphorus compounds (OPCs) due to their role as potential halon replacements in fire suppression. A continuing investigation of the inhibition activity of organophosphorus compounds under a range of equivalence ratios was performed experimentally and computationally, as measured by the burning velocity. Updates to a previous mechanism were made by the addition and modification of reactions in the mechanism for a more complete description of the inhibition reactions. Reaction pathways for HOPO 2 + H and HOPO + H are analyzed using the BAC-G2 approach. A new reaction pathway for HOPO 2 + H = PO 2 + H 2 O has been identified which results in a higher rate constant than that reported in the literature. In this work, the laminar flame speed is measured experimentally and calculated numerically for a premixed propane/air flame at 1 atm, under a range of equivalence ratios, undoped and doped with dimethyl methylphosphonate (DMMP). A detailed investigation of the catalytic cycles involved in the recombination of key flame radicals is made for two equivalence ratios, fuel lean and fuel rich. From this, the importance of different catalytic cycles involved in the lean versus rich case is discussed. The chemical kinetic model indicates that the HOPO 2 ⇔ PO 2 inhibition cycle is more important in the lean flame than the rich. The OPCs are similarly effective across the range, demonstrating the robustness of OPCs as flame suppressants. In addition, it is shown that the phosphorus compounds are most active in the high-temperature region of the flame. This may, in part, explain their high level of inhibition effectiveness.

The flame structure of a fuel-rich (ϕ = 2.4), laminar premixed, and lightly sooting acetylene flame at 40 mbar and the influence of ethanol addition on the species pool was investigated. Special emphasis was put on the analysis of important soot precursors like propargyl, benzene, and the polyynes. The mole fractions of more than 50 stable and radical species up to m/z = 170 are obtained experimentally in the flames by molecular-beam mass spectrometry (MBMS) in combination with single-photon ionization (SPI) by vacuum ultraviolet (VUV) radiation from the Advanced Light Source (ALS) in Berkeley, CA, USA. For the neat acetylene flame, successful measurements were performed with a combination of MBMS and imaging photoelectron photoion coincidence spectrometry (iPEPICO) at the VUV beamline at the Swiss Light Source (SLS) in Villigen, Switzerland and adding additional species information to the data set. Some interesting isomers (C3H2, C4H5, C4H2O) can be clearly identified by comparison of measured photoionization efficiency (PIE) curves or threshold photoelectron (TPE) spectra with Franck–Condon simulations or literature spectra, respectively. Because of apparatus improvements, the chemical resolution in this study goes beyond prior work and provides a high-quality data set for the development of reaction mechanisms at fuel-rich, low-pressure conditions.

Because of their importance in atmospheric and combustion chemistry, the rate coefficients and mechanisms of gas-phase reactions of the OH radical have been studied extensively, and the kinetic dat...

A novel experimental technique was examined whose goal was the ejection of radical species into the gas phase from a platform (film) of cold non-reactive material. The underlying principle was one of photo-initiated heat release in a stratum that lies below a layer of CO2 or a layer of amorphous solid water (ASW) and CO2. A molecular precursor to the radical species of interest is deposited near or on the film's surface, where it can be photo-dissociated. It proved unfeasible to avoid the rampant formation of fissures, as opposed to large "flakes." This led to many interesting results, but resulted in our aborting the scheme as a means of launching cold C2H radical into the gas phase. A journal article resulted that is germane to astrophysics but not combustion chemistry.

International audience; This is Part II of a series of three papers which jointly address the combustion chemistry of furan and its alkylated derivatives 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) under premixed low-pressure flame conditions. Some of them are considered to be promising biofuels. With furan as a common basis studied in Part I of this series, the present paper addresses two laminar premixed low-pressure (20 and 40 mbar) flat argon-diluted (50%) flames of MF which were studied with electron ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) for equivalence ratios phi = 1.0 and 1.7, identical conditions to those for the previously reported furan flames. Mole fractions of reactants, products as well as stable and reactive intermediates were measured as a function of the distance above the burner. Kinetic modeling was performed using a comprehensive reaction mechanism for all three fuels given in Part I and described in the three parts of this series. A comparison of the experimental results and the simulation shows reasonable agreement, as also seen for the furan flames in Part I before. This set of experiments is thus considered to be a valuable additional basis for the validation of the model. The main reaction pathways of MF consumption have been derived from reaction flow analyses, and differences to furan combustion chemistry under the same conditions are discussed.

The journal of physical chemistry / A, Molecules, spectroscopy, kinetics, environment & general theory 117(31), 6757-6770 (2013). doi:10.1021/jp4063675, Published by American Chemical Society, Washington, DC

Premixed low-pressure tetrahydrofuran/oxygen/argon flames are investigated by photoionization molecular-beam mass spectrometry using vacuum-ultraviolet synchrotron radiation. For two equivalence ratios (φ = 1.00 and 1.75), mole fractions are measured as a function of distance from the burner for almost 60 intermediates with molar masses ranging from 2 (H2) to 88 (C4H6O2), providing a broad database for flame modeling studies. The isomeric composition is resolved by comparisons between experimental photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. Special emphasis is put on the resolution of the first reaction steps in the fuel destruction. The photoionization experiments are complemented by electron-ionization molecular-beam mass-spectrometry measurements that provide data with high mass resolution. For three additional flames with intermediate equivalence ratios (φ = 1.20, 1.40 and 1.60), mole fractions of major species and photoionization efficiency spectra of intermediate species are reported, extending the database for the development of chemical kinetic models.

In the case of several formaldehyde derivatives, with importance in atmospheric and combustion chemistry, the currently available thermochemical values suffer from considerably large uncertainties. In this study a high-accuracy theoretical model chemistry has been used to provide accurate thermochemical data including heats of formation at 0 and 298 K and standard molar entropies at 298 K for CF(2)O, FCO, HFCO, HClCO, FClCO, HOCO, and NH(2)CO. For most of the thermochemical quantities studied here, this investigation delivers the best available estimate.