Modelling and optimization of a bioethanol steam reformer for fuel cell systems

JAVIER A. FRANCESCONI; MIGUEL C. MUSSATI; NICOLÁS J. SCENNA; PIO A. AGUIRRE

Rosario (Argentina)

Workshop; Workshop on Mathematical Modelling of energy and mass transfer processes, and applications; 2005

Fuel cell power systems for transportation applications have received increased attention in last years because of their potential for high fuel efficiency and lower emissions. The operation conditions of automotive vehicles require short times of start-up, light equipment, volumes reduced and efficient operation at different conditions. Thus, small size process units and specific designs are required. A model-based reactor optimization permits to obtain both designs for reducing volumes and optimal operation conditions as temperature and pressure profiles. Many research and development projects have been conducting in both the fuel cell itself and the fuel processors for generating hydrogen. There exist several routes for hydrogen production from the primary fuels. Ethanol presents several advantages related to natural availability, storage and handling safety, ethanol can be produced renewably from several biomass sources. Besides the bioethanol-to-hydrogen system has the significant advantage of being nearly CO2 neutral, since the produced carbon dioxide is consumed for biomass growth, thus offering a nearly closed carbon loop . It is the goal of this communication to investigate the reformer reactor design of a bioethanol processor for proton exchange membrane (PEM) fuel cells. This works combines well-known models for the chemistry in the fixed-bed tube with energy modelling of the furnace chamber. A one-dimensional heterogeneous model of catalytic fixed-bed reactor located in a furnace chamber is developed and implemented for steam reforming of bioethanol into hydrogen for fuel cell applications. Differential-algebraic equations (DAEs) describing conservation laws for mass, energy and radiation are solved. Two concepts of steam reforming with different configuration of the furnace chamber are evaluated. The reforming process is endothermic and requires an external heat supply from hydrocarbon combustion. Then, a design for improving the heat transfer and satisfying the production levels required and the restrictions of admissible pressure drop is needed. By modelling the combustion chamber coupled to the reformer allowed optimizing the design variables to reduce the total equipment volume. The model computes the exigencies required for the constructive materials, such as maximum operation temperature for steel, refractory and insulating materials. An optimization problem was formulated to optimize the operative and design variables that minimize the system volume or weight. The optimization problem determines the optimal value for reactor length, reactor diameter, catalyst particle diameter, furnace thickness, refractory and insulation thickness. The DAEs system was implemented in gPROMS software (general Process Modelling System).  The solver used in gPROMS is based on variable time step/variable order Backward Differentiation Formulae. The gPROMS optimization solver implements a “single-shooting” optimization algorithm. The developed model is useful for estimating the minimum and relative sizes of the reactor components and, more importantly, identifying the optimization opportunities for an improved global system performance