Hydrogen Generation for Fuel Cells in Auxiliary Power Systems

Abstract: Heavy-duty trucks are in idle operation during long periods of time, providing the vehicles with electricity via the alternator at standstill. Idling trucks contribute to large amounts of emissions and high fuel consumption as a result of the low efficiency from fuel to electricity. Auxiliary power units, which operate independently of the main engine, are promising alternatives for supplying trucks with electricity. Fuel cell-based auxiliary power units could offer high efficiencies and low noise. The hydrogen required for the fuel cell could be generated in an onboard fuel reformer using the existing truck fuel. The work presented in this thesis concerns hydrogen generation from transportation fuels by autothermal reforming focusing on the application of fuel cell auxiliary power units. Diesel and dimethyl ether have been the fuels of main focus. The work includes reactor design aspects, preparation and testing of reforming catalysts including characterization studies and evaluation of operating conditions. The thesis is a summary of five scientific papers.Major issues for succeeding with diesel reforming are fuel injection, reactant mixing and achieving fuel cell quality reformate. The results obtained in this work contribute to the continued research and development of diesel reforming catalysts and processes. A diesel reformer, designed to generate hydrogen to feed a 5 kWe polymer electrolyte fuel cell has been evaluated for autothermal reforming of commercial diesel fuel. The operational results show the feasibility of the design to generate hydrogen-rich gases from complex diesel fuel mixtures and have, together with CFD calculations, been supportive in the development of a new improved reformer design. In addition to diesel, the reforming reactor design was shown to run satisfactorily with other hydrocarbon mixtures, such as gasoline and E85. Rh-based catalysts were used in the studies and exhibit high performance during diesel reforming without coke formation on the catalyst surface. An interesting finding is that the addition of Mn to Rh catalysts appears to improve activity during diesel reforming. Therefore, Mn could be considered to be used to decrease the noble metal loading, and thereby the cost, of diesel reforming catalysts.Dimethyl ether is a potential diesel fuel alternative and has lately been considered as hydrogen carrier for fuel cells in truck auxiliary power units. The studies related to dimethyl ether have been focused on the evaluation of Pd-based catalysts and the influence of operating parameters for autothermal reforming. PdZn-based catalysts were found to be very promising for DME reforming, generating product gases with high selectivity to hydrogen and carbon dioxide. The high product selectivity is correlated to PdZn interactions, leading to decreased activity of decomposition reactions. Auxiliary power systems fueled with DME could, therefore, make possible fuel processors with very low complexity compared to diesel-fueled systems.The work presented in this thesis has enhanced our understanding of diesel and DME reforming and will serve as basis for future studies.