Multi-Physics Modeling of Auto-Thermal Diesel Surrogate Reforming

Abundant supply and existing infrastructure makes diesel a favorable candidate for portable energy supply. Diesel generators are widely used in locations where there is no reliable electric grid, but they are noisy, have emission issues, and often have poor fuel efficiency in practical operations. Because of these drawbacks, there is interest in developing Solid Oxide Fuel Cell (SOFC) systems which use diesel fuel, but which deliver electricity quietly and with lower emissions and higher efficiency.

The current study is part of a larger effort aimed at developing 1-5 kW diesel fed solid oxide fuel cells at SOFC-Canada and is mainly focused on understanding and deconvoluting the mechanism of diesel surrogate auto-thermal reforming in an experimental packed bed reactor. Gas phase kinetics play an important role during auto-thermal reforming and this study employs a detailed kinetic model developed using the automated Reaction Mechanism Generator (RMG) software. The generated model has about 9500 reactions and 450 species incorporating updated parameters from experiment and theory. The model has been validated against ignition delay data at different equivalence ratios and was found to perform reasonably well. The model has also been validated against the packed bed reactor steady state concentration data at different operating conditions. Coupling the fluid dynamics and heat transfer effects defined above with the large number of reactions and species was found to be very difficult using currently available commercial software such as Fluent and COMSOL. Hence an iterative approach was used in which simplified packed bed plug flow reactor model with heat transfer was solved using a finite element solver while the kinetics equations were solved using the CHEMKIN plug flow solver. The generated model shows the importance of entrance region effects for auto-thermal reformer design. Gas phase oxidation/pyrolysis consumes a large part of the hydrocarbon leading to lower molecular weight products that reach the catalyst surface and participate primarily in steam reforming reactions dominant on the surface of the catalyst.