Experimental Study on the Low-Temperature Ignition Behavior of Gas Turbine Fuel Blends

摘要

There is an increasing demand for fuel flexibility in power-generation gas turbines. Changes in fuel composition can dramatically change the reliability and performance of a combustor. Natural gas used for the power-generation industry consists predominately of methane. However, geographical location and seasonal behavior causes the likelihood of gas-turbine engines to operate on CH_4-based fuel blends with significant (> 10%) amounts of hydrogen and other hydrocarbons. One of the problems occurring is the likelihood of pre ignition or autoignition in the premix circuitry used in power generation gas turbines. This is caused by the reduction in reaction time due to the higher-order hydrocarbon or hydrogen additives to the fuel mixture. To study the likelihood of auto ignition occurring in the pre-mixer, autoignition experiments for combinations of CH_4, C_2H_6, C_3H_8, C_4H_(10), C_5H_(12) and H_2 have been performed in the authors' shock-tube laboratory. These experiments were performed under realistic gas turbine pressures (20 atm) and temperatures (800 K). Due to the many different variables present in this study, testing every possible combination of fuel components was not feasible within reasonable time and cost. Therefore, a design of experiment method was utilized to reduce the experiments needed for this study. A Box-Behnkin factorial design method was used to obtain a 41 trial test matrix. Successively another approach called the simplex lattice design was used resulting in a 21 trial test matrix. Both these experimental design matrices where validated to capture the important chemical kinetics behavior using a chemical kinetics mechanism from the Lawrence Livermore National Laboratories. It was found that the numerical correlation produced by the 21-trial test matrix agreed well with the simulated data from the chemical kinetics model. Subsequent to creating the experimental scheme, the actual experiments were performed in the authors' shock-tube laboratory and are presented in this paper. The results from the physical experiments show strong non-linear ignition behavior when plotted in Arrhenius form, which is a similar trend as seen with the model. However, the experiments show much shorter ignition delay times than the model, proving that these fuel blends are more prone to autoignition behavior than initially predicted by the model.