A Numerical Evaluation of the Design of an Autothermal Reformer for the Onboard Production of Hydrogen From Iso-Octane
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A numerical study was carried out to improve the design of an autothermal reformer for the onboard production of hydrogen to be used in fuel-cell- powered auxiliary power units (APU) to provide heating and electricity in long haul trucks when they are at rest. The development of these auxiliary power units is based upon the use of power generated by solid oxide fuel cell (SOFC) system, instead of from a conventional gasoline engine. The present work was undertaken to improve the design of a prototype autothermal fuel reformer that had been developed by the Fuel Cell Research Centre (FCRC) at Queen’s University to convert liquid hydrocarbon truck fuel to a hydrogen rich product gas. In this development work and in the previous work iso-octane (C8H18) has been used as a surrogate fuel. Using this surrogate of gasoline, the reformer was simulated using various inlet steam/carbon (H2O/C), oxygen/carbon (O/C) molar ratios and gas-hourly-space-velocity (GHSV). In the reformer considered the reforming process is carried out in a compact tubular reactor with a centerline thermocouple tube using a 2% Pt-ZrCe based catalyst with a local porosity of 0.6. During the initial simulations, it was observed that near the start of the catalyst region there were large temperature gradients due to an exothermic partial oxidation reaction. In order to reduce the temperature gradients and facilitate heat transfer by conduction along the reformer, the central thermocouple tube was replaced with a central solid rod. The effects of variations in the thermal conductivity of central solid rod, of the reactor wall, of the catalyst bed, of the inert porous material near the inlet and the outlet of the catalyst bed, of the gas hourly space velocity, of the effectiveness factor of the chemical reaction mechanism on the performance of the reactor were studied. The results so obtained were analyzed to determine potential design improvements that would increase the hydrogen output. The results were compared with the previous numerical and experimental results obtained in the previous studies of the reformer and found to be in good agreement with the general trends of the temperature profiles as well as the outlet molar concentrations of product species. After the analysis and evaluation of all the results, it was found that by replacement of central thermocouple tube with central solid rod made of high conductivity material and by using material for inert porous region at the outlet that had a thermal conductivity equal to that of the catalyst bed led to more even temperature profiles within the catalyst region. It was also found that the hydrogen molar percentage output could be increased by approximately more than 25% and that the length of the reactor could be reduced by 20mm by incorporating these changes in the reformer design.