Comprehensive H2/O2 kinetic model for high‐pressure combustion

An updated H₂/O₂ kinetic model based on that of Li et al. (Int J Chem Kinet 36, 2004, 566–575) is presented and tested against a wide range of combustion targets. The primary motivations of the model revision are to incorporate recent improvements in rate constant treatment and resolve discrepancies between experimental data and predictions using recently published kinetic models in dilute, high‐pressure flames. Attempts are made to identify major remaining sources of uncertainties, in both the reaction rate parameters and the assumptions of the kinetic model, affecting predictions of relevant combustion behavior. With regard to model parameters, present uncertainties in the temperature and pressure dependence of rate constants for HO₂ formation and consumption reactions are demonstrated to substantially affect predictive capabilities at high‐pressure, low‐temperature conditions. With regard to model assumptions, calculations are performed to investigate several reactions/processes that have not received much attention previously. Results from ab initio calculations and modeling studies imply that inclusion of H + HO₂ = H₂O + O in the kinetic model might be warranted, though further studies are necessary to ascertain its role in combustion modeling. In addition, it appears that characterization of nonlinear bath‐gas mixture rule behavior for H + O₂(+ M) = HO₂(+ M) in multicomponent bath gases might be necessary to predict high‐pressure flame speeds within ～15%. The updated model is tested against all of the previous validation targets considered by Li et al. as well as new targets from a number of recent studies. Special attention is devoted to establishing a context for evaluating model performance against experimental data by careful consideration of uncertainties in measurements, initial conditions, and physical model assumptions. For example, ignition delay times in shock tubes are shown to be sensitive to potential impurity effects, which have been suggested to accelerate early radical pool growth in shock tube speciation studies. In addition, speciation predictions in burner‐stabilized flames are found to be more sensitive to uncertainties in experimental boundary conditions than to uncertainties in kinetics and transport. Predictions using the present model adequately reproduce previous validation targets and show substantially improved agreement against recent high‐pressure flame speed and shock tube speciation measurements. Comparisons of predictions of several other kinetic models with the experimental data for nearly the entire validation set used here are also provided in the Supporting Information. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 444–474, 2012     

Kinetics of rich ammonia flames

We report laser absorption measurements of NH₃ decay within the flame front region of rich, atmospheric pressure ammonia flames. These data are combined with earlier OH, NH, and NH₂ measurements to obtain new estimates for the oscillator strength of NH₂. This value, f_i = 6.4 × 10^?5 for the ^PQ_1,7 line in the (0,9,0) ← (0,0.0) vibrational band of the A²A₁ ← X²B₁ transition, suggests ΔH(NH) ? 87 kcal/mol. The ammonia profiles were also combined with previous data on NO, NH, NH₂, and OH to provide an extensive database at fuel equivalence ratios (ø) of 1.28, 1.50, and 1.81 for comparison to our kinetic model predictions. This modeling used a one-dimensional flame code which explicitly accounts for the diffusional component in our flame experiments. Modeling results using a conventional mechanism predicted concentration profiles which deviated markedly from our observations. It was possible to obtain much more satisfactory fits by postulating reactions between various NH_i (i = 1, 2) species to form N—N bonds. The N₂H_j (j = 1–3) species could then lose H atoms via dissociation to ultimately form N₂. Inclusion of these reactions in the mechanism allowed us to predict concentration—distance profiles for five different species at three different equivalence ratios that are in good agreement with experiment. The most important component of this mechanism is the recognition that the NH_i + NH_i reactions dominate the kinetics in rich flames. A most satisfying aspect of these calculations is that the key rate constants in the NH_i + NH_i sequence were estimated using simple RRK theory.     

Homogeneous Fermi liquid with ‘artificial’ repulsive inverse square law interparticle potential energy

A good deal is known by now on the so-called jellium model of the homogeneous electron liquid. However, much of the quantitative progress at experimentally realizable densities has come from quantal computer simulation. Therefore, we here consider a homogeneous Fermion liquid with ‘artificial’ repulsive interaction λ/(r_ij )² between Fermions i and j at separation r_ij . We discuss first of all the way the static structure function S(q), essentially the Fourier transform of the pair correlation function, is changed because of non-zero λ from the ‘Fermi hole’ form due entirely to Pauli principle effects between parallel spin Fermions. Unlike jellium with e ²/r_ij repulsive interactions, S(q) is proportional to q at long wavelengths, whereas the plasmon in jellium annulls the q term and S(q) is quadratic in q as q tends to zero. However for λ/(r_ij )² interactions, the coefficient of q appearing in the Fermi hole structure factor, is renormalized by particle repulsions. Then some discussion is given of Fermion quasiparticle lifetimes τ as the Fermi surface is approached. Arguments are presented that τ^?1 is proportional to |E???E _F| as E tends to the Fermi energy. This is already interesting, in fact, in connection with the jellium model and therefore an approximate analytic form of τ is finally derived.     

Hylleraas method for many‐electron atoms. I. The Hamiltonian

A general expression for the nonrelativistic Hamiltonian for n‐electron atoms with the fixed nucleus approximation is derived in a straightforward manner using the chain rule. The kinetic energy part is transformed into the mutually independent distance coordinates r_i, r_ij, and the polar angles θ_i, and φ_i. This form of the Hamiltonian is very appropriate for calculating integrals using Slater orbitals, not only of states of S symmetry, but also of states with higher angular momentum, as P states. As a first step in a study of the Hylleraas method for five‐electron systems, variational calculations on the ²P ground state of boron atom are performed without any interelectronic distance. The orbital exponents are optimized. The single‐term reference wave function leads to an energy of ?24.498369 atomic units (a.u.) with a virial factor of η = 2.0000000009, which coincides with the Hartree–Fock energy ?24.498369 a.u. A 150‐term wave function expansion leads to an energy of ?24.541246 a.u., with a factor of η = 1.9999999912, which represents 28% of the correlation energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Detection of Polyatomic Species in Non-Premixed Flames Using Tunable Diode Laser Absorption Spectroscopy

Quantitative determinations of polyatomic species in laboratory-scale hydrocarbon diffusion flames using tunable diode laser absorption spectroscopy is discussed. The diode laser used in these studies pumped the ν₄ fundamental of methane and the ν₄ + ν₅ combination band of acetylene in the 1260-1290 cm⁻¹ range in two different burner systems which supported both methane/air and ethylene/air non-premixed flames. For both molecules, the sensitivity of the measurement was a strong function of the local flame temperature, falling off rapidly as temperatures approached 2000 K. The detection limits for acetylene were lower than those for methane because of lower spectral line density in the former experiments.     

Correlation between thermal conductivity and bond length alternation in carbon nanotubes: A combined reverse nonequilibrium molecular dynamics—Crystal orbital analysis

The thermal conductivity (λ) of carbon nanotubes (CNTs) with chirality indices (5,0), (10,0), (5,5), and (10,10) has been studied by reverse nonequilibrium molecular dynamics (RNEMD) simulations as a function of different bond length alternation patterns (Δr_i). The Δr_i dependence of the bond force constant (k_rx) in the molecular dynamics force field has been modeled with the help of an electronic band structure approach. These calculations show that the Δr_i dependence of k_rx in tubes with not too small a diameter can be mapped by a simple linear bond length–bond order correlation. A bond length alternation with an overall reduction in the length of the nanotube causes an enhancement of λ, whereas an alternation scheme leading to an elongation of the tube is coupled to a decrease of the thermal conductivity. This effect is more pronounced in carbon nanotubes with larger diameters. The formation of a polyene‐like structure in the direction of the longitudinal axis has a negligible influence on λ. A comparative analysis of the RNEMD and crystal orbital results indicates that Δr_i‐dependent modifications of λ and the electrical conductivity are uncorrelated. This behavior is in‐line with a heat transfer that is not carried by electrons. Modifications of λ as a function of the bond alternation in the (10,10) nanotube are explained with the help of power spectra, which provide access to the density of vibrational states. We have suggested longitudinal low‐energy modes in the spectra that might be responsible for the Δr_i dependence of λ. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Carbon-13 NMR spectra of coumarin and methoxycoumarins—a reinvestigation of charge density/Chemical Shift Relations

The carbon-13 NMR spectra of coumarin, 6-, 7-, 8-methoxycoumarin, and 5,7-, 7,8-, 5,8- and 6,7-dimethoxycoumarin have been measured and assigned. It is shown that substituent induced chemical shifts S(δ) in the mono- and disubstituted systems correlate well with the HMO atom-atom polarisibilities π_ij of the parent compound: S_i(δ_i) = 80.13 π_ij with a standard deviation of 1.42 ppm and a correlation factor of 0.994. Correlations between δ(¹³C) values and charge densities calculated by various semi-empirical methods are less successful.     

Absolute analysis by flame atomic absorption spectroscopy: Present status and some problems

Recent progress and main problems encountered in the theoretical interpretation of some stages in the spectrochemical analysis by flame AAS with a slot burner and a sharp-line source are reviewed. The effect of “narrowing” of the aerosol stream as compared with the gas stream above the flame front was established theoretically and confirmed experimentally. The resulting theory describing the analyte distribution across the flame permitted to explain many features of these flames, in particular, the effect of sensitivity enhancement in the presence of an excess of matrix. A simple method is proposed for the determination of atomic diffusion coefficients.The results of calculations of the composition and temperatures of flames employed in analytical practice, obtained for a wide range of the fuel—oxidant ratio, were used to determine the capabilities of these flames as to the dissociation of monoxides. Practically total dissociation of almost all elements of the Periodic Table was proved theoretically and confirmed experimentally to occur in the nitrous oxide—acetylene flame. The formation of low-volatile lithium and tin carbides in the presence of carbon was established. This effect accounts for “anomalies” in the behavior of these elements in low-temperature flames.The line shifts Δν_s in flames were measured by interferometric scans of line profiles from a hollow-cathode lamp and flame. The existence of a theoretical relationship between Δν_s and the Lorentz line width Δν_L was confirmed. Calculations of line absorption were generalized to take into account the shift and hyperfine structure of the lines. Systematic errors in these calculations do not exceed 10%.A discussion is given of the major difficulties facing absolute measurements based on this analytical technique.     

Study of combustion intermediates in fuel‐rich methyl methacrylate flame with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

A fuel‐rich premixed laminar methyl methacrylate (MMA)/O₂/Ar flame at low pressure (30 Torr) with the equivalence ratio (?) of 1.60 is studied in this work. Synchrotron vacuum ultraviolet photoionization combined with molecular beam mass spectrometry is employed to identify the combustion intermediates including isomeric intermediates. The observed combustion intermediates can be classified as four types: radicals, non‐cyclic hydrocarbons, cyclic hydrocarbons and oxygenates. Benzene is the unique aromatic hydrocarbon detected in this work, and several oxygenates with two oxygen atoms are identified. Mole fraction profiles of most intermediates are evaluated, which can help understand the MMA combustion mechanism under fuel‐rich conditions. The similarities among rich flames of MMA and other oxygenated fuels, as well as the characteristics of rich MMA flame, are also discussed. The results show that combustion of MMA not only reduces soot emissions, but also has low concentrations of some potential toxic by‐products. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of Hydrogen Enhancement in LPG/Air Premixed Flame

A numerical study of hydrogen-enhanced liquefied petroleum gas (LPG) + air flames was presented. The variations of the adiabatic burning velocity in different conditions of combustion (?=0.7-1.4) were studied extensively. The hydrogen content in the fuel was varied from 0% to 45% and the dilution factor was from 21% to 16%. Since the major components of LPG are butane and propane, an appropriate chemical kinetic model must be chosen to solve the chemical reaction of C3 and C4 species. Validation of the chemical kinetic model against the fundamental combustion data was performed to insure accuracy. In addition, independent simulations were conducted in the opposed-jet, symmetric, twin-flame configuration. The effects of fluid mechanics, as manifested by the induced strain rate, were also considered. The effects of extinction strain rate on flame temperature and the flammability limits were calculated and the results showed that hydrogen-enhanced LPG/air premixed flames were more stable at high flame strain. The lean flammability limits were extended by the H₂ addition.     

Intermolecular potential parameters and combining rules determined from viscosity data

The law of corresponding states has been demonstrated for a number of pure substances and binary mixtures and provides evidence that the transport properties viscosity and diffusion can be determined from a molecular shape function, often taken to be a Lennard–Jones 12‐6 potential, that requires two scaling parameters: a well depth ε_ij and a collision diameter σ_ij, both of which depend on the interacting species i and j. We obtain estimates for ε_ij and σ_ij of interacting species by finding the values that provide the best fit to viscosity data for binary mixtures and compare these to calculated parameters using several “combining rules” that have been suggested for determining parameter values for binary collisions from parameter values that describe collisions of like molecules. Different combining rules give different values for σ_ij and ε_ij, and for some mixtures the differences between these values and the best‐fit parameter values are rather large. There is a curve in (ε_ij, σ_ij) space such that parameter values on the curve generate a calculated viscosity in good agreement with measurements for a pure gas or a binary mixture. The various combining rules produce couples of parameters ε_ij, σ_ij that lie close to the curve and, therefore, generate predicted mixture viscosities in satisfactory agreement with experiment. Although the combining rules were found to underpredict the viscosity in most of the cases, Kong's rule was found to work better than the others, but none of the combining rules consistently yields parameter values near the best‐fit values, suggesting that improved rules could be developed. © 2010 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 42: 713–723, 2010     

A comprehensive kinetic mechanism for CO,CH2O,and CH3OH combustion

New experimental profiles of stable species concentrations are reported for formaldehyde oxidation in a variable pressure flow reactor at initial temperatures of 850–950 K and at constant pressures ranging from 1.5 to 6.0 atm. These data, along with other data published in the literature and a previous comprehensive chemical kinetic model for methanol oxidation, are used to hierarchically develop an updated mechanism for CO/H₂O/H₂/O₂, CH₂O, and CH₃OH oxidation. Important modifications include recent revisions for the hydrogen–oxygen submechanism (Li et al., Int J Chem Kinet 2004, 36, 565), an updated submechanism for methanol reactions, and kinetic and thermochemical parameter modifications based upon recently published information. New rate constant correlations are recommended for CO + OH = CO₂ + H ( R23 ) and HCO + M = H + CO + M ( R24 ), motivated by a new identification of the temperatures over which these rate constants most affect laminar flame speed predictions (Zhao et al., Int J Chem Kinet 2005, 37, 282). The new weighted least‐squares fit of literature experimental data for ( R23 ) yields k₂₃ = 2.23 × 10⁵T^1.89exp(583/T) cm³/mol/s and reflects significantly lower rate constant values at low and intermediate temperatures in comparison to another recently recommended correlation and theoretical predictions. The weighted least‐squares fit of literature results for ( R24 ) yields k₂₄ = 4.75 × 10¹¹T^0.66exp(?7485/T) cm³/mol/s, which predicts values within uncertainties of both prior and new (Friedrichs et al., Phys Chem Chem Phys 2002, 4, 5778; DeSain et al., Chem Phys Lett 2001, 347, 79) measurements. Use of either of the data correlations reported in Friedrichs et al. (2002) and DeSain et al. (2001) for this reaction significantly degrades laminar flame speed predictions for oxygenated fuels as well as for other hydrocarbons. The present C₁/O₂ mechanism compares favorably against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay, and flow reactor species time history data at each level of hierarchical development. Very good agreement of the model predictions with all of the experimental measurements is demonstrated. © 2007 Wiley Periodicals, Inc. 39: 109–136, 2007     

Orbital functional theory of linear response and excitation

Orbital functional theory (OFT) is based on a rule that determines a single‐determinant reference state Φ for any exact N‐electron eigenstate Ψ. An OFT model postulates an explicit correlation energy functional E_c of occupied orbital functions {?_i} and occupation numbers {n_i}. The orbital Euler–Lagrange equations are analogous to Kohn–Sham equations, but do not in general contain local potential functions. Time‐dependent Hartree–Fock theory is generalized in OFT to a formally exact linear response theory that includes electronic correlation. In the exchange‐only limit, the theory reduces to the random‐phase approximation of many‐body theory. The formalism determines excitation energies. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

The effect of the catalytic recombination reactions of H and OH^? involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H₂/O₂/N₂ flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH^? radicals with PO, PO₂, HOPO, and HOPO₂ play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (?) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H₂/O₂/N₂ flames, as distinct from that of hydrocarbon flames, increases as ? is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.     

An identity in chemical thermodynamics

In this work we consider the equilibrium state of a multicomponent system in which chemical reactions occur, and its sensitivity to the change of any of the thermodynamic parameters influencing the equilibrium. A general expression is obtained for the sensitivity coefficientn _i/Y, wheren _i is the equilibrium amount of theith chemical species in the system, andY is a thermodynamic parameter such as temperature, pressure, initial amount of a species, chemical potential of a species, etc. We demonstrate that the sensitivity coefficients are linear combinations of contributions originating from certain special reactions, called response reactions, which are defined in the paper.     

Experimental and numerical simulation of effects of CO2/N2 concentration and initial temperature on combustion characteristics of biomass syngas

Biomass syngas is a form of renewable energy with very broad application prospects, and it has different combustion characteristics according to the fuel composition and processing technology of biomass syngas. The influence of combustion composition, diluent and temperature variation on combustion characteristics were studied in this paper. The FFCM-1 mechanism was used to investigate the combustion characteristics of CO/CH₄/H₂ under varied diluents CO₂/N₂ and temperature by using spherical expansion flame method and ANSYS CHEMKIN-PRO. The experimental laminar burning velocity was compared with the simulation results of FFCM-1 mechanism. The results reveal that the experimental data are in good agreement with the simulation results, which are somewhat different under the condition of rich fuel. The laminar burning velocity decreases significantly with the increase of diluent CO₂/N₂, with the effect of diluent CO₂ being more significant. The laminar burning velocity increase dramatically with the increase of initial temperature, and the adiabatic flame temperature also decreases with the increase of diluent. The reduction caused by diluent CO₂ is much larger than that caused by diluent N₂. The change of initial temperature also affects the adiabatic flame temperature, but the range of variation is not as pronounced as that of diluent. Not only was the interaction between the combustion characteristics of CO/CH₄/H₂ under different diluents and temperature changes explored in this paper, but the influence mechanism was also revealed in depth.     

A Chemical Kinetic Mechanism for 2‐Bromo‐3,3,3‐trifluoropropene (2‐BTP) Flame Inhibition

In this work, we report a detailed chemical kinetic mechanism to describe the flame inhibition chemistry of the fire‐suppressant 2‐bromo‐3,3,3‐trifluoropropene (2‐BTP), under consideration as a replacement for CF₃Br. Under some conditions, the effectiveness of 2‐BTP is similar to that of CF₃Br; however, like other potential halon replacements, it failed an U.S. Federal Aviation Authority (FAA) qualifying test for its use in cargo bays. Large overpressures are observed in that test and indicate an exothermic reaction of the agent under those conditions. The kinetic model reported herein lays the groundwork to understand the seemingly conflicting behavior on a fundamental basis. The present mechanism and parameters are based on an extensive literature review supplemented with new quantum chemical calculations. The first part of the present article documents the information considered and provides traceability with respect to the reaction set, species thermochemistry, and kinetic parameters. In additional work, presented more fully elsewhere, we have combined the 2‐BTP chemical kinetic mechanism developed here with several other submodels from the literature and then used the combined mechanism to simulate premixed flames over a range of fuel/air stoichiometries and agent loadings. Overall, the modeling results qualitatively predicted observations found in cup‐burner tests and FAA Aerosol Can Tests, including the extinguishing concentrations required and the lean‐to‐rich dependence of mixtures. With these data in hand, in a second phase of the present work, we perform a reaction path analysis of major species under several modeled conditions. This analysis leads to a qualitative understanding of the ability of 2‐BTP to act as both an inhibitor and a fuel, depending on the conditions and suggests areas of the kinetic model that should be further investigated and refined.     

On the role of transport in the combustion kinetics of a steady-state premixed laminar CO + H2 + O2 flame

The role of mechanistic steps, diffusion, and their interrelation is explored in a steady-state premixed laminar CO + H₂ + O₂ flame using a numerical model. Sensitivity coefficients and Green's functions calculated for this system offer systematic characterization of the role of diffusion and exothermicity in carbon monoxide oxidation kinetics. The results reveal that the uncertainties in transport parameters are as important to the model predictions as those in the kinetic steps. The rate controlling steps of the CO + H₂ + O₂ reaction are found to be different for adiabatic and nonadiabatic premixed flames, and also for systems with and without transport. In particular, the reactions of the hydroperoxyl radical with hydrogen, oxygen, and hydroxyl radicals are found to be important at all temperatures in the fuel lean (40 torr) adiabatic flame studied here. The diffusive mixing of chemical species from the low and the high temperature portions of the flame and the larger heats of reaction associated with the hydroperoxyl radicals are found to be responsible for the increased importance of these reactions. © 1994 John Wiley & Sons, Inc.     

ɛ‐caprolactone and lactide polymerization promoted by an ionic titanium(IV)/iron(III) polynuclear halo‐alkoxide

The ionic [Ti₃(µ₃‐OPrⁱ)₂(µ‐OPrⁱ)₃(OPrⁱ)₆][FeCl₄] halo‐alkoxide ( A ) was investigated for its activity towards the bulk polymerization of rac‐lactide (rac‐LA) and ?‐caprolactone (?‐CL) in various temperatures, monomer/ A molar proportions, and reaction times. The reactivity of A in the ring‐opening polymerization (ROP) of both monomers is mainly due to the cationic [Ti₃(OPrⁱ)₁₁]⁺ unity and proceeds through the coordination–insertion mechanism. Molecular weights ranging from 6,379 to 13,950 g mol^?1 and PDI values varying from 1.22 to 1.52 were obtained. Results of ROP kinetic studies for both ?‐CL and rac‐LA confirm that the reaction rates are first‐order with respect to monomers. The production of poly(?‐caprolactone) shows a higher sensitivity of the reaction rate to temperature, while the polymerization of rac‐LA is slower and more dependent on the thermal stability of the active species during the propagation step. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2509–2517