Jaap de Vries

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.

Ignition and Flame Speed Kinetics of Two Natural Gas Blends With High Levels of Heavier Hydrocarbons

High-pressure experiments and chemical kinetics modeling were performed to generate a database and a chemical kinetic model that can characterize the combustion chemistry of methane-based fuel blends containing significant levels of heavy hydrocarbons (up to 37.5% by volume). Ignition delay times were measured in two different shock tubes and in a rapid compression machine at pressures up to 34 atm and temperatures from 740 K to 1660 K. Laminar flame speeds were also measured at pressures up to 4 atm using a high-pressure vessel with optical access. Two different fuel blends containing ethane, propane, n-butane, and n-pentane added to methane were studied at equivalence ratios varying from lean (0.3) to rich (2.0). This paper represents the most comprehensive set of experimental ignition and laminar flame speed data available in the open literature for CH 4 /C 2 H 6 /C 3 H 8 /C 4 H 10 /C 5 H 12 fuel blends with significant levels of C2 + hydrocarbons. Using these data, a detailed chemical kinetics model based on current and recent work by the authors was compiled and refined. The predictions of the model are very good over the entire range of ignition delay times, considering the fact that the data set is so thorough. Nonetheless, some improvements to the model can still be made with respect to ignition times at the lowest temperatures and for the laminar flame speeds at pressures above 1 atm and at rich conditions.

Effect of Higher-Order Hydrocarbons on Methane-Based Fuel Chemistry at Gas Turbine Pressures

A chemical kinetics mechanism designed for the oxidation of methane-hydrocarbon blends at elevated pressures was used to study the effect of higher-order hydrocarbons on ignition delay time and flame speed at gas turbine conditions. The mechanism was developed from recent data and modeling conducted by the authors, including pressures above 30 atm, temperatures as low as 700 K, and alkane additives from C2 H6 through C5 H12 . Calculations focused on three target natural gas mixtures containing CH4 mole fractions from 62.5 to 98%. The results show the effects that pressure, temperature, and hydrocarbon content have on the combustion chemistry of the fuel-air mixtures. For example, autoignition times exhibit nonlinear trends with increasing pressure and decreasing temperature. Experiments in the authors’ laboratories are ongoing, and an overview of the related facilities is provided.Copyright

Design and Validation of a Reduced Test Matrix for the Autoignition of Gas Turbine Fuel Blends

Changes in fuel composition for both aero-engine as well as power generation applications is a topic of concern since fuel variability can have a great impact on the reliability and performance of the burner design. Autoignition experiments for a wide range of likely fuel blends containing CH4 mixed with combinations of C2 H6 , C3 H8 , C4 H10 , C5 H12 , and H2 are planned in the authors’ shock-tube laboratory. However, testing every possible fuel blend and interaction is not feasible within a reasonable time and cost. To predict the surface response over the complete mixture domain, a special experimental design has been developed reducing the amount of ‘trials’ needed significantly from 243 to only 41 using the Box-Behnkin factorial design methodology. Kinetics modeling was used to obtain numerical results for this matrix of fuel blends when applied to autoignition at a temperature of 800 K and pressure of 17 atm. A further attempt was made to reduce the 41-test matrix to a 21-test matrix. This was done using special mixture experimental techniques, and the kinetics model was used to compare the smaller matrix to the expected results of the larger one. The new 21-Test matrix produced a numerical correlation that agreed well with the results from the 41-test matrix, indicating that the smaller matrix will provide the same autoignition information as the larger one with acceptable precision.Copyright

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.