Wayne K. Metcalfe

Laminar Flame Speed and Ignition Delay Time Data for the Kinetic Modeling of Hydrogen and Syngas Fuel Blends

Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10atm and initial temperatures up to 443K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2-O2 chemical kinetics at standard temperature and pressures up to 10atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 ratio of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios, where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetics model. A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures. [DOI: 10.1115/1.4007737]

Ab Initio Study of the Decomposition of 2,5-Dimethylfuran

The initial steps in the thermal decomposition of 2,5-dimethylfuran are identified as scission of the C-H bond in the methyl side chain and formation of β- and α-carbenes via 3,2-H and 2,3-methyl shifts, respectively. A variety of channels are explored which prise the aromatic ring open and lead to a number of intermediates whose basic properties are essentially unknown. Once the furan ring is opened demethylation to yield highly unsaturated species such as allenylketenes appears to be a feature of this chemistry. The energetics of H abstraction by the hydroxyl radical (and other abstracting species) from a number of mono- and disubstituted methyl furans has been studied. H-atom addition to 2,5-dimethylfuran followed by methyl elimination is shown to be the most important route to formation of the less reactive 2-methylfuran. Identification of 2-ethenylfuran as an C(6)H(6)O intermediate in 2,5-dimethylfuran flames is probably not correct and is more likely the isomeric 2,5-dimethylene-2,5-dihydrofuran for which credible formation channels exist.

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice-Ramsperger-Kassel-Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1-C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

Ab Initio Chemical Kinetics of Methyl Formate Decomposition: The Simplest Model Biodiesel

The energetics and kinetics of methyl formate decomposition have been investigated by high-level ab initio calculations with rate constant predictions. The paucity of reliable experimental data for methyl formate has been circumvented by studying a very similar system, namely, the decarboxylation of acetic acid, in order to help validate the theoretical calculations. Our study shows that methyl formate decomposes to methanol and carbon monoxide, almost exclusively, with a high pressure limit rate constant of k(1)(infinity) = 2.128 x 10(12)T(0.735) exp(-34,535/T) s(-1), and the decomposition of acetic acid to methane and carbon dioxide proceeds with a rate constant, k(4)(infinity), of 1.668 x 10(10)T(1.079) exp -35,541/T s(-1). Experimental values for the formation enthalpy of methyl formate are discussed, and it is shown that these can be reconciled with our computed value for DeltaH(f) (298.15 K) of -360.1 +/- 2.2 kJ mol(-1). In turn, bond dissociation energies for all single bonds in the molecule are presented.

Ignition Delay Time and Laminar Flame Speed Calculations for Natural Gas/Hydrogen Blends at Elevated Pressures

Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio. The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.

LAMINAR FLAME SPEED MEASUREMENTS AND MODELING OF ALKANE BLENDS AT ELEVATED PRESSURES WITH VARIOUS DILUENTS

Laminar flame speeds at elevated pressure for methane-based fuel blends are important for refining the chemical kinetics that are relevant at engine conditions. The present paper builds on earlier measurements and modeling by the authors by extending the validity of a chemical kinetics mechanism to laminar flame speed measurements obtained in mixtures containing significant levels of helium. Such mixtures increase the stability of the experimental flames at elevated pressures and extend the range of laminar flame speeds. Two experimental techniques were utilized, namely a Bunsen burner method and an expanding spherical flame method. Pressures up to 10 atm were studied, and the mixtures ranged from pure methane to binary blends of CH4 /C2 H6 and CH4 /C3 H8 . In the Bunsen flames, the data include elevated initial temperatures up to 650 K. There is generally good agreement between model and experiment, although some discrepancies still exist with respect to equivalence ratio for certain cases. A significant result of the present study is that the effect of mixture composition on flame speed is well captured by the mechanism over the extreme ranges of initial pressure and temperature covered herein. Similarly, the mechanism does an excellent job at modeling the effect of initial temperature for methane-based mixtures up to at least 650 K.Copyright

Numerical Study on the Effect of Real Syngas Compositions on Ignition Delay Times and Laminar Flame Speeds at Gas Turbine Conditions

Depending on the feedstock and the production method, the composition of syngas can include (in addition to H2 and CO) small hydrocarbons, diluents (CO2, water, and N2), and impurities (H2S, NH3, NOx, etc.). Despite this fact, most of the studies on syngas combustion do not include hydrocarbons or impurities and in some cases not even diluents in the fuel mixture composition. Hence, studies with realistic syngas composition are necessary to help in designing gas turbines. The aim of this work was to investigate numerically the effect of the variation in the syngas composition on some fundamental combustion properties of premixed systems such as laminar flame speed and ignition delay time at realistic engine operating conditions. Several pressures, temperatures, and equivalence ratios were investigated for the ignition delay times, namely 1, 10, and 35 atm, 900–1400 K, and ϕ = 0.5 and 1.0. For laminar flame speed, temperatures of 300 and 500 K were studied at pressures of 1 atm and 15 atm. Results showed that the addition of hydrocarbons generally reduces the reactivity of the mixture (longer ignition delay time, slower flame speed) due to chemical kinetic effects. The amplitude of this effect is, however, dependent on the nature and concentration of the hydrocarbon as well as the initial condition (pressure, temperature, and equivalence ratio).

Ignition Delay Time Measurements and Modeling of n- Pentane and iso-Pentane at Elevated Pressures

Ignition delay times of n-pentane and iso-pentane in real fuel-air mixtures with oxygen and nitrogen were measured in a shock tube and a rapid compression machine (RCM). Data were then used to improve the current kinetics model. All results were obtained at an equivalence ratio of 1.0 and at pressures near 1, 10, and 20 atm for the shock tube and 10 and 20 atm for the RCM. Experimental temperatures ranged from 641 to 1410 K. When compared to the chemical kinetics model, in general, ignition delay times were well predicted for n-pentane, while iso-pentane predictions were longer than experimental values in the negative temperature coefficient region. To the authors’ knowledge, this study covers conditions not yet present in the literature and will expand fundamental knowledge of npentane and iso-pentane combustion kinetics.

IGNITION DELAY TIME EXPERIMENTS FOR NATURAL GAS/HYDROGEN BLENDS AT ELEVATED PRESSURES

Applications of natural gases that contain high levels of hydrogen have become a primary interest in the gas turbine market. While the ignition delay times of hydrogen and of the individual hydrocarbons in natural gases can be considered well known, there have been few previous experimental studies into the effects of different levels of hydrogen on the ignition delay times of natural gases at gas turbine conditions. To examine the effects of hydrogen content at gas turbine conditions, shocktube experiments were performed on nine mixtures of an L9 matrix. The L9 matrix was developed by varying four factors: natural gas higher-order hydrocarbon content of 0, 18.75, or 37.5%; hydrogen content of the total fuel mixture of 30, 60, or 80%; equivalence ratios of 0.3, 0.5, or 1; and pressures of 1, 10, or 30 atm. Temperatures ranged from 1092 K to 1722 K, and all mixtures were diluted in 90% Ar. Correlations for each mixture were developed from the ignition delay times and, using these correlations, a factor sensitivity analysis was performed. It was found that hydrogen played the most significant role in the ignition delay times of a mixture. Pressure was almost as important as hydrogen content, especially as temperature increased. Equivalence ratio was slightly more important than hydrocarbon content of the natural gas, but both were less important than pressure or hydrogen content. Comparison with a modern chemical kinetic model demonstrated that the model captures well the relative impacts of H2 content, temperature, and pressure, but some improvements are still needed in terms of absolute ignition delay times.

Ignition and Oxidation of Ethylene-Air Mixtures at Elevated Pressures

Shock-tube experiments have been performed to determine ignition delay times of undiluted ethylene-air mixtures over a wide range of temperatures (1003 ≤ T (K) ≤ 1401), stoichiometries (0.5 ≤ φ ≤ 2.0), and pressures (1.1 ≤ P (atm) ≤ 24.9). The results of this work are compared to other experimental data available in the literature and to a chemical kinetics model that has been developed over the past few years using high-pressure, lower order hydrocarbon ignition delay times. The present agreement between model and data is fair, and the experiments show good agreement with the only other high-pressure data set available in literature for ethylene.