(535c) Experimental and Modeling Analysis of C6 Isomers: Implications for Solid-Oxide Fuel Cell Operation

The need for a cleaner and more efficient energy supply has driven research efforts in the direction of fuel cell technology. Solid Oxide Fuel Cells (SOFCs) have the potential to bridge the gap between traditional fossil fuel combustion processes and a renewable energy economy. SOFCs can operate directly on hydrocarbon fuels with much higher efficiencies and lower costs to the environment than traditional combustion processes. Because of the high operating temperatures of SOFCs, there is the potential for gas-phase chemistry to occur in addition to catalytic and electrochemistry. In order to further the advancement of SOFC technology, it is necessary to understand all three chemistries taking place in the fuel cell. A combined experimental and modeling analysis has been performed on the pyrolysis of C6 isomers over the temperature range of 550-700 C in an effort to understand the gas-phase kinetics expected to occur in SOFCs operating on liquid hydrocarbon fuels. Electronic structure theory has become an integral part of mechanism development. Rate coefficient estimates can be made confidently where there is a lack of experimental data by applying electronic structure calculations. These results can be incorporated with Transition State Theory (TST) to estimate rate coefficients. The H-abstraction reactions and alkyl isomerization chemistry of an existing C6 kinetic mechanism has been updated based on rate rules determined from CBS-QB3 electronic structure calculations. The mechanism was also updated to more realistically define pressure dependence for the dissociation of hydrocarbons. This mechanism was used to model the gas-phase kinetics of n-hexane and compared to data collected over a temperature range that permitted the kinetics to be observed over a wide conversion range. The model captured the product selectivity and fuel conversion observed experimentally (see Fig. 1). The pyrolysis of 2,3 dimethylbutane was also characterized under similar conditions. Unlike n-hexane, the carbon balance was not maintained at high temperatures, suggesting the occurrence of molecular weight growth reactions that could lead to deposits. The model's predictions of conversion and major product distributions were satisfactory. However, C5 species were overestimated and the expected heavies were underestimated. Several new radical addition and radical recombination channels were examined as potential ways to allow the C5 species to form deposit precursors. The experiments and model were extended to cyclohexane pyrolysis. The rate rules that successfully modeled the linear and branched C6 systems proved to be unsuccessful for the cyclic system. Instead, electronic structure calculations and TST were used to determine the high pressure rate coefficients for the system. A comparison of these rate coefficients to those based on the non-cyclic rules exposed reasons why the cyclic system does not conform to the non-cyclic rules. In retrospect, these different rate rules can be rationalized in terms of the complexities introduced when rings are present. The new cyclic mechanism successfully captured the observed trends in major product distributions as a function of temperature. Cyclohexane was similar to 2,3 dimethylbutane in that missing carbon suggests heavier species are being formed. Finally, this validated kinetic model was used to predict the kinetics of a composite fuel consisting of a mixture of the C6 isomers at 800 C, a typical SOFC operating temperature. Under these conditions there will be high conversions of liquid hydrocarbons at very short residence times and temperature, residence time, and fuel structure will greatly influence the major product distributions and the formation of deposit precursors. The products are likely to consist of smaller alkanes, alkenes, and H2. Therefore the catalyst will see species that are far removed from the parent fuel, and the catalytic chemistry will be influenced greatly by the gas-phase kinetics. Furthermore, the pyrolysis systems are far from equilibrium, and therefore detailed kinetic mechanisms are needed to account for the gas-phase chemistry in SOFC operation.