Desulfurization of Jet Fuel for Fuel Cell-based APU Systems in Aircraft

Summary To prevent the catalysts in fuel cell systems fr om poisoning by sulfur containing substances the fuel to be used must be desulfurized to a maximum of 10 ppmw of sulfur. Since the conventional hydrodesulfurization process employed in the refinery industry is not suitable for mobile fuel cell applications (e.g. auxiliary power units, APUs), the present study aims at developing an alternative process and determining it s technical feasibility. A large number of processes were assessed with respect to their application in fuel cell APUs. The results revealed that a two-step process combining pervaporation and adsorption is a suitable process for the on-board desulfurization of jet fuel. The investigations to evaluate this process are presented in this paper. Seven different membrane materials and ten sorbent materials were screened to choose the most suitable candidates. Further laboratory experiments were conducted to optimize the operating conditions and to collect data for a pilot plant design. Different jet fuel qualities with up to 1650 ppmw of sulfur can be desulfurized to a level of 10 ppmw. 1 Introduction Fuel cells are well suited for on-board power supply in aircraft, ships and heavy duty vehicles. The use of fuel cell systems in aircraft offers the possibility to simplify the aircraft layout. Important systems in aircraft, i.e. the gas turbine powered auxiliary power unit (APU) for electricity supply, the fuel tank inerting system and the water tank, can be substituted by one single system, the fuel cell system. The waste heat of the fuel cell system can be used for ice protection. These measures reduce the consumption of jet fuel, increase aircraft efficiency and allow operation with low emissions. Additionally, the costs for aircraft related investments, for aircraft maintenance and operation can be reduced. APUs driven by conventional gas turbines operate at an efficiency of about 15% on the ground [1], while an APU based on autothermal reforming of diesel or gasoline in combination with a Polymer Electrolyte Fuel Cell (PEFC) can achieve a system efficiency of up to 36–37% [2, 3]. To operate fuel cells with the fuel available on board, the fuel is converted into a hydrogen-rich gas by a process of catalytic reforming. Since both the catalysts in the reformer and in the fuel cell can be deactivated by the sulfur compounds contained in the fuel, the liquid fuel must be desulfurized to a level with a maximum of 10 ppmw (parts per million by weight) of sulfur [4, 5, 6]. Whereas diesel fuel for road vehicles within the EU is already desulfurized at the refinery, jet fuel is permitted to have up to 3000 ppmw of sulfur worldwide [7, 8]. An analysis of fuel samples showed that jet fuel with a total sulfur content ranging from 300 ppmw to 700 ppmw has been marketed in Europe. On-board desulfurization is therefore required for the use of fuel cell auxiliary power units (APUs) in aircraft.