A numerical study of an autothermal reformer for the production of hydrogen from Iso-octane
Sylvestre, Steven W. J.
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The development of an auxiliary power unit (APU) capable of providing climate control and electricity in long haul trucks is of significant interest due to the expected economic and environmental benefits. A potentially efficient and environmentally friendly APU design is one based on the use of an autothermal fuel reformer that converts on-board truck fuel to a hydrogen rich gas that directly fuels a solid oxide fuel cell unit. To assist in the development of such a unit a numerical study of the autothermal reforming of iso-octane in a compact tubular reactor has been undertaken. This was done to determine the reactor performance and the factors that potentially influence its performance. Variations in the wall thermal conductivity, the catalyst thermal conductivity, the catalyst porosity, the conditions of the inlet reactant gas, and the effectiveness factor of the chemical reaction mechanism have been studied to determine their effects on the performance of the reformer. It has been found that higher thermal conductivities of the outer wall and in the catalyst region gave increased dry hydrogen yield and fuel conversion. The study of the effects of inlet species concentration ratios indicated that maximum hydrogen yield was obtained with an oxygen-to-fuel ratio between 1.0 and 1.15 and a steam-to-fuel ratio of approximately 3.0. Results obtained with various inlet species temperatures and bed porosities showed only small changes in the reformer performance. While the results obtained here do provide useful information about the performance of the autothermal reformer, the model used has been re-assessed and it is recommended that an improved model be used in future work. In particular, the assumed effectiveness factors for all of the chemical reactions occurring in the catalyst region need to be improved. This was highlighted by the fact that a brief study indicated a very strong dependence of the reformer product gas composition on the effectiveness factor. This indicates that while the present model is able to predict trends in the reformer performance, it is limited in its accuracy due to the fact that the effectiveness factor used is only approximately known.