Coupled transport and kinetics in the mixing region of a hydrocarbon reformer

Abstract

Reformed liquid hydrocarbon fuels provide high energy densities for mobile power applications, such as solid-oxide fuel cells in auxiliary power units. However, numerous challenges exist in developing compact high-performance reformers. Mixture preparation prior to reaction on a reforming catalyst is certainly one im-portant challenge, as incomplete mixing with air can cause unacceptable temperature overshoots in the cata-lyst due to over-oxidation, and/or deleterious deposit formation. There is a high potential for ethylene pro-duction from gas-phase reactions in the mixing region: ethylene is known as a potential precursor. Ex-perimental and numerical studies on mixture preparation for hydrocarbon reforming have shown that the conditions in the mixing region play a critical role in the hydrocarbon reforming process. However, none of these studies provide a coherent explanation of the reported results. Consequently, a coupled transport-kinetics model was developed and simulated in this work to analyze the phenomena in the mixing region. Isolated kinetics and fluid dynamics studies were conducted prior to performing the coupled transport-kinetics calculations in order to describe the separate phenomena. The isolated kinetic study provided essential information on the temperature and residence time operating window needed to suppress substantial ethylene production. A reaction mechanism for n-alkanes that was developed by Lawrence Livermore National Laboratory was chosen. The fuel conversion and ethylene production rates are major concerns; thus, two parameters were used to quantify the ethylene production and the overall fuel conversion: the time required for 5% fuel consumption (\tau_5%) and the time required for the ethylene concentration to reach 0.1 mol% (\tau_t). The most important prediction was the unusual NTC behavior of hydrocarbon oxidation. The presence of the NTC region suggests that one way of minimizing undesirable gas-phase reactions is through ...