Effects of Syngas Addition on Combustion Characteristics of Gasoline Surrogate Fuel

Syngas has the potential to become an alternative fuel for internal combustion engines. In this work, a detailed mechanism containing 1389 species and 5942 reactions was developed to examine the combustion of syngas/gasoline blends. The influence of syngas addition on the ignition delay time (IDT) and laminar flame speed of gasoline fuel was studied. Two influencing factors were considered: the mixing ratio of syngas and the H₂/CO ratio in syngas. The changes in heat release, free radical concentrations, and emissions were also studied. Syngas can boost the system's reaction activity and promote ignition in the high-temperature area over 1000 K. However, the diluting effect is visible at low temperatures below 1000 K, leading to an IDT lag. The effect of the H₂/CO ratio on the IDT was not as pronounced as expected. The addition of syngas can inhibit the knock combustion of the engine to a certain extent, but it will also lead to a violent exothermic process and a decrease in the total release of heat. Syngas addition increases the concentration of small molecule radicals and promotes the laminar flame speed. At higher temperatures and pressure levels, the trend of syngas/gasoline laminar flame speed is more dependent on changes in OH radical concentrations. The addition of syngas favors the promotion of complete combustion and the reduction of HC emissions but also results in an additional increase in CO. Combustion at lower temperatures has lower CO and HC emissions.

Heterogeneous and Combined Heterogeneous/Homogeneous Combustion Modeling of SOFC Off-Gases

The heterogeneous (catalytic) and the hetero-/homogeneous (catalytic and gas-phase) combustion processes of solid oxide fuel cell (SOFC) off-gases with compositions typical of a high cell utilization rate are investigated with high-fidelity 2D simulations in a platinum-coated planar channel using detailed hetero-/homogeneous chemistry. The pressures are 1-8 bar; the reactant streams have volumetric H2 and CO contents 0.7-1.5 and 5.3-9.7%, respectively; H2O and CO2 dilutions are ∼40 and ∼50%, respectively; and the global fuel/air equivalence ratio is 0.90. Water inhibits chemically the catalytic oxidation of H2, as it leads to high H(s) surface coverage that favors the recombinative desorption of H(s) to H2. On the other hand, H2O promotes chemically the catalytic oxidation of CO by creating high OH(s) coverage that in turn accelerates the CO consumption. Strong flames are established at the highest H2 content cases and for pressures p ≥ 3 bar. For all cases with vigorous homogeneous combustion, the catalytic and gas-phase reaction pathways coexist and compete with each other for the consumption of H2 and CO. The large H2O content leads to gas-phase production of H2 via the reaction H2O + H = H2 + OH. However, the gas-phase produced H2 is subsequently consumed by the catalytic pathway, such that nearly complete H2 conversion is attained at the reactor outlet. Gaseous chemistry does not affect the reactor lengths required for complete H2 conversion but substantially reduces the corresponding lengths for CO conversion. The H2 emissions decrease with increasing pressure and are in the range 8-110 ppmv, while the CO emissions increase with rising pressure and span the range 0.3-52 ppmv, thus leading to corrected CO emissions (at 15% O2) of less than 15 ppmv. Finally, the peak wall temperatures are largely acceptable in terms of catalyst thermal stability.

Supercritical water co-oxidation behavior in the different monohydric alcohol-ammonia reaction environment

The degradation of ammonia is a key rate-limiting step during the supercritical water oxidation of nitrogen-containing organics. This paper studied the co-oxidation behavior between different ammonia-alcohol environments, including the influence of reaction parameters and the co-oxidation mechanism. The results showed that increasing temperature, oxidation coefficient, residence time, and alcohol concentration significantly promoted the degradation of NH₃-N and TOC, while rising the ammonia concentration enhanced the NH₃-N destruction but inhibited the TOC degradation. Alcohols were oxidized first in the co-oxidation system to produce more OH* and HO₂* radicals. Ethanol generated the highest concentration of HO₂* in the shortest time, leading to more significant ammonia removal than isopropanol and methanol; however, the produced intermediate products like aldehydes and ketones reacted with residual ammonia to generate a small amount of organics at lower temperatures, inhibiting the degradation of alcohols slightly, and combined catalyst or nitrate in the batch reactor or used continuous supercritical water oxidation or supercritical hydrothermal combustion system without controlling the exotherm of fuels could improve this.

Experimental and Numerical Study of the Laminar Burning Velocity and Pollutant Emissions of the Mixture Gas of Methane and Carbon Dioxide

This paper presents the experimental and numerical study of the laminar burning velocity and pollutant emissions of the mixture gas of methane and carbon dioxide. Compared to previous research, a wider range of experimental conditions was realized in this paper: CO₂ dilution level up to 60% (volume fraction) and equivalence ratio of 0.7–1.3. The burning velocities were measured using the heat flux method. The CO and NO emissions after premixed combustion were measured by a gas analyzer placed 20 cm downstream of the flame. The one-dimensional free flames were simulated using the in-house laminar flame code CHEM1D. Four chemical kinetic mechanisms, GRI-Mech 3.0, San Diego, Konnov, and USC Mech II were used in Chem1D. The results showed that, for laminar burning velocity, the simulation results are all lower than the experimental results. GRI Mech 3.0 showed the best agreement when the CO₂ content was below 20%. USC Mech II showed the best consistency when the CO₂ content was between 40 and 60%. For CO emission, these four mechanisms all showed a small error compared with the experiments. When CO₂ content is higher than 40%, the deviation between simulation and experiment becomes bigger. When the CO₂ ratio is more than 20%, the proportion of CO₂ does not affect CO emission so much. For NO emission, when the CO₂ content is 40%, the results from simulation and experiment showed a good agreement. As the proportion of CO₂ increases, the difference in NO emissions decreases.

Discriminating between the dissociative photoionization and thermal decomposition products of ethylene glycol by synchrotron VUV photoionization mass spectrometry and theoretical calculations

Landscape of dissociative photoionization and thermal decompositions of ethylene glycol.

Numerical Simulation of the Effect of CH4/CO Concentration on Combustion Characteristics of Low Calorific Value Syngas

The composition of low calorific value synthesis gas varies greatly depending on the raw material and processing technology, which makes the combustion extremely complicated. The three mechanisms of the GRI-Mech 3.0, Li-Model, and FFCM-Mech are used to numerically simulate CH₄/CO/H₂/N₂ air premixed combustion by using ANSYS CHEMKIN-PRO. The numerical simulation is the calculation of laminar flame velocity and adiabatic flame temperature at an initial temperature of 298 K, an equivalence ratio of 0.6-1.4, and an initial pressure of 0.1-0.5 MPa, discussing through thermodynamics and chemical kinetics. The formation of NO _X , H, and OH radicals by fuel composition was analyzed. The result shows that the concentrations of H, O, and OH radicals have a positive effect on laminar flame velocity. The combustion reaction of H₂ is higher than that of CH₄ and CO; with the increase of N₂ content, the priority is higher. The thermal diffusivity of flame under different equivalence ratios is affected by inert gas, which affects adiabatic combustion temperature and laminar combustion velocity. In thermal kinetics and chemical kinetics, CH₄ has more influence on combustion temperature than CO, while laminar flame velocity is relatively low. Under the change of initial pressure, the laminar combustion flux increases to the initial pressure and the laminar combustion velocity decreases to the increase in pressure. Reactions H + O₂ = O + OH, HO₂ + H = 2OH, and CH₃ + HO₂ = OH + CH₃O are mainly due to change in the concentration of O, H, and OH radicals.

Numerical and Experimental Study of a Jet-and-Recirculation Stabilized Low Calorific Combustor for a Hybrid Power Plant

An atmospheric prototype burner is studied with numerical and experimental tools. The burner system is designed for operation in a hybrid power plant for decentralized energy conversion. In order to realize such a coupled system, a reliable combustion system has to be established. Numerical and experimental findings in the presented study demonstrate the capabilities of the novel burner system in suitable operation conditions. In this system, a solid oxide fuel cell (SOFC) is mounted upstream of the burner in the gas turbine system. The combination of both realizes a large operational flexibility with comparably high overall efficiency. Since the combustor is operated with SOFC off-gas, several challenges arise. Low calorific combustion needs careful burner design and numerical modeling, since the heat-loss mechanisms occur to be in the order of magnitude of thermal power output. Thus, different modeling strategies are discussed in the paper. The numerical studies are compared with experimental results and high-quality simulation results complement limited measured findings with easy-to-use low fidelity RANS models. A priori measurements are employed for the selection of investigation points. It is shown that the presented combustor system is able to cover low-calorific combustion over a large range of operation conditions, despite major heat-loss effects, which are characterized by means of numerical CFD (Computational Fluid Dynamics) modeling.

Accurate Rate Constants for the Forward and Reverse H + CO ↔ HCO Reactions at the High-Pressure Limit

The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius' equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol^-1, while similar fittings provided, respectively, 1.96 and 3.22 kcal mol^-1, considering now forward rate constants from a temperature range of 298-373 K. This last activation energy determination (3.22 kcal mol^-1) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol^-1, also obtained from traditional fittings in the range 298-373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol^-1). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 10⁴ atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures.

Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes

Supercritical water oxidation (SCWO) is a promising wastewater treatment technology owing to its various advantages such as rapid reactions and non-polluting products. However, problems like corrosion and salt decomposition set obstacles to its commercialization. To address these problems, researchers have been developing the optimal reactor design and strengthening measures based on sufficient understandings of the degradation kinetics. The essence of the SCWO process and the roles of oxygen and hydrogen peroxide are summarized in this work. Then, the research status and progress of empirical models, semi-empirical models, and detailed chemical kinetic models (DCKMs) are systematically reviewed. Additionally, this paper is the first to summarize the research progress of quantum chemistry and molecular dynamics simulation. The challenge and further development of kinetics models for the optimization of reactors and the directional transformation of pollutants are pointed out.

Review on an Advanced Combustion Technology: Supercritical Hydrothermal Combustion

Supercritical hydrothermal combustion, a new and promising homogeneous combustion technology with a wide range of application scenarios and broad development prospects, provides creative ideas and means for the enhanced degradation of organic wastes, hydrothermal spallation drilling, thermal recovery of heavy oil, etc. This technology is elaborated upon in five parts: (1) introducing the main devices including semi-batch reactor and continuous reactor to study the hydrothermal flame in accordance with research institutions, (2) presenting the research status of related numerical simulation from the angles of reaction kinetics and flow-reaction, (3) summarizing the characteristics of hydrothermal flame and combustion by five key parameters, (4) dividing up ignition process and explaining ignition mechanism from the perspectives of critical physical properties of water and heat transfer and mixing conditions, (5) discussing and forecasting its industrial applications including hydrothermal spallation drilling, the thermal recovery of heavy oil, the clean conversion and utilization of coal-based fuel, and the harmless treatment of pollutants. By and large, this paper analyzed in detail everything from experimental equipment to industrial applications, from combustion characteristics to ignition mechanisms, and from summary conclusions to prospect prediction. In the end, herein is summarized a couple of existing paramount scientific and technical obstacles in hydrothermal combustion. Further significant studies in the future should include excellent reactors, advanced monitoring techniques, and powerful computational fluid dynamics.

Dual Fuel Reaction Mechanism 2.0 including NOx Formation and Laminar Flame Speed Calculations Using Methane/Propane/n-Heptane Fuel Blends

This study presents the further development of the TU Wien dual fuel mechanism, which was optimized for simulating ignition and combustion in a rapid compression expansion machine (RCEM) in dual fuel mode using diesel and natural gas at pressures higher than 60 bar at the start of injection. The mechanism is based on the Complete San Diego mechanism with n-heptane extension and was attuned to the RCEM measurements to achieve high agreement between experiments and simulation. This resulted in a specific application area. To obtain a mechanism for a wider parameter range, the Arrhenius parameter changes performed were analyzed and updated. Furthermore, the San Diego nitrogen sub-mechanism was added to consider NOx formation. The ignition delay time-reducing effect of propane addition to methane was closely examined and improved. To investigate the propagation of the flame front, the laminar flame speed of methane–air mixtures was simulated and compared with measured values from literature. Deviations at stoichiometric and fuel-rich conditions were found and by further mechanism optimization reduced significantly. To be able to justify the parameter changes performed, the resulting reaction rate coefficients were compared with data from the National Institute of Standards and Technology chemical kinetics database.

Diagnosis and Improvement of Combustion Characteristics of Methanol Miniature Reciprocating Piston Internal Combustion Engine

A micro-reciprocating piston internal combustion engine with liquid hydrocarbon fuel has the potential to supply ultrahigh density energy to micro electro mechanical system because of its high-density energy, simple structure, and mature energy conversion principle. However, the diagnostic test of the combustion characteristics of the micro reciprocating piston internal combustion engine shows that its combustion characteristics are poor, and the combustion rate was lower with the combustion duration of more than 50 °CA. The mean indicated pressure (Pmi) value was only 0.137 MPa, the combustion stability was very poor, and the cycle variation rate of the Pmi was up to 60%. To improve its combustion performance, the method to enhance combustion in micro-space is explored then. Mechanism studies have shown that the pyrolysis reaction of nitromethane and hydrogen peroxide can produce amounts of free radicals OH, with the possibility of improving the combustion of methanol. Therefore, a method for adjusting the composition of methanol fuel to enhance combustion is proposed, and the method is theoretically confirmed. Finally, based on this method, the test was carried out. The results showed that the combustion rate increased and the combustion duration decreased by 6% after adding nitromethane. The power performance was enhanced, and the Pmi value was increased by 30%. The combustion stability was enhanced, and the cycle variation rate of the Pmi was reduced to 16.9%. Nitromethane has a significant effect on improving the combustion characteristics of methanol, and the enhancement of the latter was mainly reflected in the ignition phase of the combustion process. This study indicates that exploring the fuel additive that can increase the concentration of OH radical in the reaction is an effective method to improve the micro-space combustion, which will facilitate the development of micro-piston internal combustion engine to supply energy to a micro electro mechanical system.

Molecular dynamics of combustion reactions in supercritical carbon dioxide. Part 4: boxed MD study of formyl radical dissociation and recombination

Fossil fuel oxy-combustion is an emergent technology where habitual nitrogen diluent is replaced by high pressure (supercritical) carbon dioxide. The supercritical state of CO₂ increases the efficiency of the energy conversion and the absence of nitrogen from the reaction mixture reduces pollution by NO_x. However, the effects of a supercritical environment on elementary reactions kinetics are not well understood at present. We used boxed molecular dynamics simulations at the QM/MM theory level to predict the kinetics of dissociation/recombination reaction HCO^• + [M] ↔ H^• + CO + [M], an important elementary step in many combustion processes. A wide range of temperatures (400-1600 K) and pressures (0.3-1000 atm) were studied. Potentials of mean force were plotted and used to predict activation free energies and rate constants. Based on the data obtained, extended Arrhenius equation parameters were fitted and tabulated. The apparent activation energy for the recombination reaction becomes negative above 30 atm. As the temperature increased, the pressure effect on the rate constant decreased. While at 400 K the pressure increase from 0.3 atm to 300 atm accelerated the dissociation reaction by a factor of 250, at 1600 K the same pressure increase accelerated this reaction by a factor of 100. Graphical abstract Formyl radical surrounded by carbon dioxide molecules.

H2/CO/air premixed and partially premixed flame structure at different pressures based on reaction limit analysis

Premixed and partially premixed flames (PPFs) of H₂/CO/air syngas are studied numerically to investigate the effect of pressure on syngas PPF structure. Chemical characteristics of the syngas flame at different pressures are investigated based on reaction limit analysis using a one-dimensional configuration. The results show that CO affects the syngas reaction limits through both physical effects that consist mainly in dilution and chemical effects that are related to both R23 (CO + OH = CO₂ + H) and HCO pathway. In particular, the HCO pathway weakens the flame at low pressures due to the chain-terminating effect of R25 (HCO + O₂ = CO + HO₂) and R26 (HCO + H = CO + H₂), and enhances the flame at high pressures because of the contribution of R25 to the HO₂ chain-branching process. These CO chemical characteristics are also observed in the premixed zone of 50% H₂ + 50% CO syngas PPFs whereas only R23 is important in the non-premixed zone.

Pressure-Dependent Rate Constant Predictions Utilizing the Inverse Laplace Transform: A Victim of Deficient Input Data

k(E) can be calculated either from the Rice-Ramsperger-Kassel-Marcus theory or by inverting macroscopic rate constants k(T). Here, we elaborate the inverse Laplace transform approach for k(E) reconstruction by examining the impact of k(T) data fitting accuracy. For this approach, any inaccuracy in the reconstructed k(E) results from inaccurate/incomplete k(T) description. Therefore, we demonstrate how an improved mathematical description of k(T) data leads to accurate k(E) data. Refitting inaccurate/incomplete k(T), hence, allows for recapturing k(T) information that yields more accurate k(E) reconstructions. The present work suggests that accurate representation of experimental and theoretical k(T) data in a broad temperature range could be used to obtain k(T,p). Thus, purely temperature-dependent kinetic models could be converted into fully temperature- and pressure-dependent kinetic models.

Uncertainty Quantification in Chemical Modeling

A module of PrIMe automated data-centric infrastructure, Bound-to-Bound Data Collaboration (B2BDC), was used for the analysis of systematic uncertainty and data consistency of the H2/CO reaction model (73/17). In order to achieve this purpose, a dataset of 167 experimental targets (ignition delay time and laminar flame speed) and 55 active model parameters (pre-exponent factors in the Arrhenius form of the reaction rate coefficients) was constructed. Consistency analysis of experimental data from the composed dataset revealed disagreement between models and data. Two consistency measures were applied to identify the quality of experimental targets (Quantities of Interest, QoI): scalar consistency measure, which quantifies the tightening index of the constraints while still ensuring the existence of a set of the model parameter values whose associated modeling output predicts the experimental QoIs within the uncertainty bounds; and a newly-developed method of computing the vector consistency measure (VCM), which determines the minimal bound changes for QoIs initially identified as inconsistent, each bound by its own extent, while still ensuring the existence of a set of the model parameter values whose associated modeling output predicts the experimental QoIs within the uncertainty bounds. The consistency analysis suggested that elimination of 45 experimental targets, 8 of which were self- inconsistent, would lead to a consistent dataset. After that the feasible parameter set was constructed through decrease uncertainty parameters for several reaction rate coefficients. This dataset was subjected for the B2BDC framework model optimization and analysis on. Forth methods of parameter optimization were applied, including those unique in the B2BDC framework. The optimized models showed improved agreement with experimental values, as compared to the initially-assembled model. Moreover, predictions for experiments not included in the initial dataset were investigated. The results demonstrate benefits of applying the B2BDC methodology for development of predictive kinetic models.

Sub-Doppler infrared spectroscopy of CH2OH radical in a slit supersonic jet: Vibration-rotation-tunneling dynamics in the symmetric CH stretch manifold

The sub-Doppler CH-symmetric stretch (ν₃) infrared absorption spectrum of a hydroxymethyl (CH₂OH) radical is observed and analyzed with the radical formed in a slit-jet supersonic discharge expansion (T_rot = 18 K) via Cl atom mediated H atom abstraction from methanol. The high sensitivity of the spectrometer and reduced spectral congestion associated with the cooled expansion enable first infrared spectroscopic observation of hydroxymethyl transitions from both ± symmetry tunneling states resulting from large amplitude COH torsional motion. Nuclear spin statistics due to exchange of the two methyl H-atoms aid in unambiguous rovibrational assignment of two A-type K_a = 0 ← 0 and K_a = 1 ← 1 bands out of each ± tunneling state, with additional spectral information obtained from spin-rotation splittings in P, Q, and R branch K_a = 1 ← 1 transitions that become resolved at low N. A high level ab initio potential surface (CCSD(T)-f12b/cc-pvnzf12 (n = 2,3)/CBS) is calculated in the large amplitude COH torsional and CH₂ wag coordinates, which in the adiabatic approximation and with zero point correction predicts ground state tunneling splittings in good qualitative agreement with experiment. Of particular astrochemical interest, a combined fit of the present infrared ground state combination differences with recently reported millimeter-wave frequencies permits the determination of improved accuracy rotational constants for the ground vibrational state, which will facilitate ongoing millimeter/microwave searches for a hydroxymethyl radical in the interstellar medium.

Computational Kinetics by Variational Transition-State Theory with Semiclassical Multidimensional Tunneling: Direct Dynamics Rate Constants for the Abstraction of H from CH3OH by Triplet Oxygen Atoms

Rate constants and the product branching ratio for hydrogen abstraction from CH3OH by O(3P) were computed with multistructural variational transition-state theory including microcanonically optimized multidimensional tunneling. Benchmark calculations of the forward and reverse classical barrier heights and the reaction energetics have been carried out by using coupled cluster theory and multireference calculations to select the most reliable density functional method for direct dynamics computations of the rate constants. The dynamics calculations included the anharmonicity of the zero-point energies and partition functions, with specific-reaction-parameter scaling factors for reactants and transition states, and multistructural torsional anharmonicity was included for the torsion around the C-O bond in methanol and in the transition states. The resulting rate constants are presented over a wider range than they are available from experiment, but in the temperature range where experiments are available, they agree well with experimental values, which is encouraging for their reliability over the wider temperature range and for future computations of oxygen atom reaction rates. In contrast to a previous computational prediction, the branching ratio predicted by the present work shows that the formation of CH2OH + OH is the dominant channel over the whole range of temperature from 250 to 2000 K.

Development of a Joint Hydrogen and Syngas Combustion Mechanism Based on an Optimization Approach

A comprehensive and hierarchical optimization of a joint hydrogen and syngas combustion mechanism has been carried out. The Kéromnès et al. (Combust Flame, 2013, 160, 995-1011) mechanism for syngas combustion was updated with our recently optimized hydrogen combustion mechanism (Varga et al., Proc Combust Inst, 2015, 35, 589-596) and optimized using a comprehensive set of direct and indirect experimental data relevant to hydrogen and syngas combustion. The collection of experimental data consisted of ignition measurements in shock tubes and rapid compression machines, burning velocity measurements, and species profiles measured using shock tubes, flow reactors, and jet-stirred reactors. The experimental conditions covered wide ranges of temperatures (800-2500 K), pressures (0.5-50 bar), equivalence ratios (ϕ = 0.3-5.0), and C/H ratios (0-3). In total, 48 Arrhenius parameters and 5 third-body collision efficiency parameters of 18 elementary reactions were optimized using these experimental data. A large number of directly measured rate coefficient values belonging to 15 of the reaction steps were also utilized. The optimization has resulted in a H2/CO combustion mechanism, which is applicable to a wide range of conditions. Moreover, new recommended rate parameters with their covariance matrix and temperature-dependent uncertainty ranges of the optimized rate coefficients are provided. The optimized mechanism was compared to 19 recent hydrogen and syngas combustion mechanisms and is shown to provide the best reproduction of the experimental data.

Weakly Bound Free Radicals in Combustion: "Prompt" Dissociation of Formyl Radicals and Its Effect on Laminar Flame Speeds

Weakly bound free radicals have low-dissociation thresholds such that at high temperatures, time scales for dissociation and collisional relaxation become comparable, leading to significant dissociation during the vibrational-rotational relaxation process. Here we characterize this "prompt" dissociation of formyl (HCO), an important combustion radical, using direct dynamics calculations for OH + CH2O and H + CH2O (key HCO-forming reactions). For all other HCO-forming reactions, presumption of a thermal incipient HCO distribution was used to derive prompt dissociation fractions. Inclusion of these theoretically derived HCO prompt dissociation fractions into combustion kinetics models provides an additional source for H-atoms that feeds chain-branching reactions. Simulations using these updated combustion models are therefore shown to enhance flame propagation in 1,3,5-trioxane and acetylene. The present results suggest that HCO prompt dissociation should be included when simulating flames of hydrocarbons and oxygenated molecules and that prompt dissociations of other weakly bound radicals may also impact combustion simulations.

Pyrolysis Pathways of the Furanic Ether 2-Methoxyfuran

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.

Quartic force fields for excited electronic states: rovibronic reference data for the 1 (2)A' and 1 (2)A″ states of the isoformyl radical, HOC

Quartic force fields (QFFs) have been shown to be an effective, accurate, and relatively compact means of computing rovibrational spectroscopic data for numerous molecules with numerous applications. However, excited states have been nearly excluded from the this approach since most accurate QFFs are based on the "gold standard" coupled cluster singles, doubles, and perturbative triples [CCSD(T)] method which is not readily extended to excited states. In this work, rovibronic spectroscopic data is provided for the isoformyl radical, a molecule of significance in combustion and astrochemistry, both through the traditional means of variational access to excited states with CCSD(T) and in the novel extension of QFFs routinely to treat electronically excited states through the standard coupled cluster excited state approach, equation of motion (EOM) CCSD. It is shown here that the new EOM-based QFF provides structural parameters and rotational constants that are quite close to those from a related CCSD(T)-based QFF for the 1 (2)A(″) excited state of HOC. The anharmonic vibrational frequency percent differences between the two QFFs are less than 0.4% for the O-H stretch, less than 1.9% for the C-O stretch, and around 3.0% for the bend. Even so, the pure excited state EOM-QFF anharmonic frequencies are still very good abinitio representations that may be applied to systems where electronically excited states are not variationally accessible. Additionally, rovibrational spectroscopic data is provided for the 1 (2)A(') ground state of HOC and for both the ground and excited state of DOC.