Joel M. Hall

Ignition of Lean Methane-Based Fuel Blends at Gas Turbine Pressures

Shock-tube experiments and chemical kinetics modeling were performed to further understand the ignition and oxidation kinetics of lean methane-based fuel blends at gas turbine pressures. Such data are required because the likelihood of gas turbine engines operating on CH 4 -based fuel blends with significant (>10%) amounts of hydrogen, ethane, and other hydrocarbons is very high. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of CH 4 , CH 4 /H 2 , CH 4 /C 2 H 6 , and CH 4 /C 3 H 8 in ratios ranging from 90/10% to 60/40%. Lean fuel/air equivalence ratios (Φ=0.5) were utilized, and the test pressures ranged from 0.54 to 30.0 atm. The test temperatures were from 1090 K to 2001 K. Significant reductions in ignition delay time were seen with the fuel blends relative to the CH 4 -only mixtures at all conditions. However, the temperature dependence (i.e., activation energy) of the ignition times was little affected by the additives for the range of mixtures and temperatures of this study. In general, the activation energy of ignition for all mixtures except the CH 4 /C 3 H 8 one was smaller at temperatures below approximately 1300 K (∼27 kcal/mol) than at temperatures above this value (∼41 kcal/mol). A methane/hydrocarbon-oxidation chemical kinetics mechanism developed in a recent study was able to reproduce the high-pressure, fuel-lean data for the fuel/air mixtures. The results herein extend the ignition delay time database for lean methane blends to higher pressures (30 atm) and lower temperatures (1100 K) than considered previously and represent a major step toward understanding the oxidation chemistry of such mixtures at gas turbine pressures. Extrapolation of the results to gas turbine premixer conditions at temperatures less than 800 K should be avoided however because the temperature dependence of the ignition time may change dramatically from that obtained herein.

Comparison of characteristic time diagnostics for ignition and oxidation of fuel/oxidizer mixtures behind reflected shock waves

ABSTRACT Various methods for determining characteristic times of shock-tube ignition and oxidation are compared. Onset and peak times were obtained from time histories for four different species (CH, CH*, OH, OH*) as predicted by a modern detailed kinetics mechanism. Appropriate submechanisms for CH* and OH* formation and quenching were added to the existing mechanism to differentiate the excited-state species from the ground-state molecules. The modeling focused on mixtures of acetylene or ethane with oxygen highly diluted in argon at high temperatures (1200–2050 K) and nearly atmospheric pressures. Using a detailed mechanism known to accurately simulate the shock-tube chemistry, emphasis was placed on cohesion of characteristic times among the species and the extent to which one may be used to predict another. Generally, ignition onset times were found to be more consistent than peak times, with OH peaking at times least typical of the group. Onset time versus inverse temperature curves based on any one species agree with those of the other three species to within 25% for the hydrocarbon mixtures and given mechanism utilized herein. Results suggest that ignition onset time should be used for greater consistency, and kinetics modeling of excited-state species such as OH* and CH* should be included if comparing to data obtained using chemiluminescence diagnostics.

Ignition and Oxidation of Ethylene-Oxygen-Diluent Mixtures with and Without Silane

Several dilute mixtures of varying concentrations and equivalence ratios (Φ = 0.5, 1.0) of C 2 H 4 /O 2 /Ar/SiH 4 were studied between 1115-1900 K and 0.9-3.3 atm. Argon dilution ranged from 96-98% with total concentrations between 0.67 and 3.2 x 10 - 5 mol/cm 3 . Reaction progress was monitored using chemiluminescence emission from the hydroxyl radical near 307 nm. For SiH 4 concentrations less than 10% of the ethylene in the mixture by volume, the ignition delay time was reduced by approximately 30% to greater than 50%. The addition of SiH 4 had a small effect on ignition activation energy, indicating the chain branching mechanism for C 2 H 4 ignition is sped up but not altered greatly by the silane at higher temperatures. After adding an appropriate OH* submechanism, several modern kinetics mechanisms containing high-temperature ethylene chemistry were compared to the data without SiH4. Most of the mechanisms captured the ignition activation energy quite well, but only the mechanism of Wang and Laskin (1998) was typically within 10% of the absolute experimental ignition times over the entire range of conditions. The basic formation and quenching characteristics of the OH* profiles were reproduced by most mechanisms, but each requires some improvement to match all features.

Kinetics of OH Chemiluminescence in the Presence of Hydrocarbons

Shock-tube experiments and chemical kinetics simulations have been used to optimize a kinetics model for the electronically excited OH radical (OH*) in a hydrocarbon environment at high temperatures (1420 < T < 2335 K) and at pressures between 0.8 and 3.0 atm. Ultraviolet emission near 307 nm was recorded from the shock-tube sidewall, and absolute concentrations were used to obtain a rate expression for the elementary reaction primarily responsible for formation of OH*, i.e. CH + O2 ⇆ OH* + CO. Absolute concentrations were deduced from a previously established calibration employing the relatively well-known kinetics of the H2/O2 system. To isolate the phenomena of interest, experiments were performed with mixtures of CH4 and H2 with O2, highly diluted in Argon, and elementary reactions accounting for formation and quenching of OH* were added to the GRI-Mech 3.0 detailed mechanism. Sensitivity analyses indicate that the maximum OH* concentration is most strongly sensitive to the formation path through CH + O2 so that this reaction rate could be found by varying its coefficient to match the experimental concentration. In this manner, the rate coefficient was found to be k1 = (4 ± 3)×10 cm mol s, independent of temperature and pressure. Uncertainties in this rate due to the calibration reaction and to scatter in the data are considered, and results are compared to values previously available in the literature. The end result is an optimized diagnostic accurate over a broad range of conditions encountered in combustion and propulsion applications.

Development of a Chemical Kinetics Mechanism for CH 4 /H 2 /Air Ignition at Elevated Pressures

As part of an on-going research project to characterize the combustion properties of natural gas fuel blends of interest to the power-generation industry, a detailed chemical kinetics mechanism for the ignition of CH4/H2 fuel blends has been developed and validated using shock-tube data. Emphasis has been placed on refining the hydrogen and methane oxidation kinetics for application to gas turbine operating conditions, namely high pressures (10 – 30 atm) and low-to-intermediate temperatures (800 – 1500 K). Ultimate goals of the research project include incorporation of the combustion chemistry into reacting-flow CFD codes for engineering design and analysis; this paper documents the process of developing an accurate kinetics model for the fuels and conditions of interest. This work will be followed by future efforts to reduce the results for use in CFD modeling. The new model proposed herein is based on the well-known methane oxidation kinetics of GRI-Mech 3.0 with additional reactions added to account for pathways that become important at high pressures and/or low temperatures. Sensitivity analyses were used to identity the elementary reactions governing the calculated ignition times; these reaction rates were subsequently varied, constrained within the range of experimental measurements, to best-fit shock-tube data taken from previous publications by the authors. The end result is a combustion model that is accurate over the range of conditions for which data are currently available, and which can be reduced for use in practical CFD calculations for the design and analysis of gas turbine combustors burning methane-based fuel blends.

A Shock-Tube Study of the Oxidation of C2H6/O2/Ar and C 2 H 6 /SiH 4 /O 2 /Ar Mixtures

*† ‡ § ** Ethane ignition and oxidation behind reflected shock waves with and without silane addition were studied using several dilute mixtures of varying concentrations and equivalence ratios (0.5 < φ < 2.0). The C2H6/SiH4/O2/Ar mixtures were studied at temperatures and pressures between 1230-1862 K and 0.9-3.0 atm, respectively. Argon dilution ranged from 91-98%. The reaction process was studied by monitoring the A 2 Σ + → X 2 Π chemiluminescence emission from the hydroxyl radical near 307 nm. In this study and in several previous studies, there have been different ways of obtaining the ignition delay time both in terms of diagnostics and in definition. A summary of the different techniques is given and used to make fair comparisons with other studies. A correlation of ignition delay time with temperature and concentration is proposed and compared with previous studies. This correlation has an r 2 value of over 0.98, mainly due to the inclusion of an argon concentration dependency. The overall activation energy for ethane ignition was found to be 36.0 kcal/mol over the range of conditions studied. Consistency of the data with previous ignition experiments is discussed. The experimental results indicate that silane addition to an ethane mixture at levels as low as 20% of the fuel can create up to a 50% reduction in ignition delay time for fuel-lean mixtures at high temperatures. It also created a reduction of about 22% for stoichiometric mixtures. Comparison of the present ethane-only ignition data with results from the literature highlights some of the differences in ignition definition, diagnostics, and range of conditions amongst the different studies.

A High-Pressure Kinetics Model for the Ignition of Natural-Gas Fuel Blends

*† Shock-tube data and chemical kinetics simulations have been used to optimize a combustion mechanism for the ignition of methane-based fuel blends. This work is part of an on-going project to measure the combustion characteristics of natural-gas fuel blends at practical conditions. The model was assembled from various literature sources based on the well-known methane chemistry of GRI-Mech 3.0. Additional compounds and reactions were added to account for oxidation pathways that become important at gas-turbine operating pressures, i.e. 20 – 30 atm, and to model C2- and C3-compounds. The starting mechanism was updated with recent data, especially the H2/O2 mechanism and the enthalpy of formation of OH. Sensitivity and pathway analyses were used to identify the important elementary reactions, whose rates were then modified, within their experimental uncertainty, to best-fit experimental data collected at pressures from 10 – 50 atm and temperatures from 1000 – 1500 K. Results are presented for mixtures of H2/O2/Ar, CH4/O2/Ar, CH4/Air, CH4/H2/Air, and CH4/C2H6/Air. The new model is able to reproduce the experimental ignition times for all mixtures over the range of conditions studied. While further work is planned to measure a broader range of fuels and blending, the present work documents the modeling process that will be used throughout and provides excellent performance at the conditions studied thus far. This model has practical applications in the design and operation of gas-turbine power generation systems and of propulsion systems operating at similar conditions.

Ignition Delay Time Measurements of C2HX Fuels and Comparison to Several Detailed Kinetics Mechanisms

Recent results from experiments and modeling by the authors are reviewed for the ignition of acetylene, ethylene, and ethane in oxygen/argon mixtures at temperatures between 1000 and 2300 K and pressures near 1 atm. The ignition measurements were obtained behind reflected shock waves using emission from electronically excited OH and CH radicals to monitor the reaction progress. While many discrepancies exist amongst previous studies for these lower-order hydrocarbons, the accuracy afforded by the present experiments provides conclusive evidence verifying the trends seen in certain studies from the literature. Several modern, detailed chemical kinetics mechanisms were compared to the new results with some models showing quite good agreement with both ignition delay times and species profiles, particularly for stoichiometric mixtures. However, improvement is still required to match the entire range of fuel concentrations, temperatures, and mixture ratios, particularly for fuel-rich mixtures.Copyright