The Impact of Alternative Fuels on Combustion Kinetics

Abstract

The research targets the development of detailed kinetic models to quantitatively characterize the impact of alternative fuels on the performance of Navy turbines and diesel engines. Such impacts include kinetic properties such as cetane number, flame speed, and emissions as well as physical properties such as the impact of boiling point distributions on fuel vaporization and mixing. The primary focus will be Fischer-Tropsch liquids made from natural gas, coal or biomass. The models will include both the effects of operation with these alternative fuels as well as blends of these fuels with conventional petroleum-based fuels. The team will develop the requisite kinetic rules for specific reaction types and incorporate these into detailed kinetic mechanisms to predict the combustion performance of neat alternative fuels as well as blends of these fuels with conventional fuels. Reduced kinetic models will be then developed to allow solution of the coupled kinetics/transport problems. This is a collaboration between the Colorado School of Mines (CSM) and the Lawrence Livermore National Laboratory (LLNL). The CSM/LLNL team plans to build on the substantial progress made in recent years in developing accurate detailed chemical mechanisms for the oxidation and pyrolysis of conventional fuels. Particular emphasis will be placed uponmore » reactions of the isoalkanes and the daughter radicals, especially tertiary radicals, formed by abstraction from the isoalkanes. The various components of the program are described. We have been developing the kinetic models for two iso-dodecane molecules, using the same kinetic modeling formalisms that were developed for the gasoline and diesel primary reference fuels. These mechanisms, and the thermochemical and transport coefficient submodels for them, are very close to completion at the time of this report, and we expect them to be available for kinetic simulations early in the coming year. They will provide a basis for prediction and selection of desirable F-T molecules for use in jet engine simulations, where we should be able to predict the ignition, combustion and emissions characteristics of proposed fuel components. These mechanisms include the reactions and chemical species needed to describe high temperature phenomena such as shock tube ignition and flammability behavior, and they will also include low temperature kinetics to describe other ignition phenomena such as compression ignition and knocking. During the past years, our hydrocarbon kinetics modeling group at LLNL has focused a great deal on fuels typical of gasoline and diesel fuel. About 10 years ago, we developed kinetic models for the fuel octane primary reference fuels, n-heptane [1] and iso-octane [2], which have 7 and 8 carbon atoms and are therefore representative of typical gasoline fuels. N-heptane represents the low limit of knock resistance with an octane number of 0, while iso-octane is very knock resistant with an octane number of 100. High knock resistance in iso-octane was attributed largely to the large fraction of primary C-H bonds in the molecule, including 15 of the 18 C-H bonds, and the high bond energy of these primary bonds plays a large role in this knock resistance. In contrast, in the much more ignitable n-heptane, 10 of its 16 C-H bonds are much less strongly bound secondary C-H bonds, leading to its very low octane number. All of these factors, as well as a similarly complex kinetic description of the equally important role of the transition state rings that transfer H atoms within the reacting fuel molecules, were quantified and collected into large kinetic reaction mechanisms that are used by many researchers in the fuel chemistry world.« less