Feasibility of the direct generation of hydrogen for fuel-cell-powered vehicles by on-board steam reforming of naphtha

Abstract

A process flow sheet for the production of hydrogen to run a 50 kW fuel-cell-powered-vehicle by steam reforming of naphtha is presented. The major units in the flow sheet involve a desulfurization unit, a steam reformer, a low temperature (LT) shift reactor, a methanation reactor, and a membrane separator unit. The flow sheet is simulated using HYSYS (a steady state simulator) and the material and energy flows for each stream are obtained. For the peak load of 50 kW, it is found that 14 l/h naphtha is needed, which means that a 70 l fuel tank in the vehicle is sufficient for 5 h drive. The amount of water needed is not a critical factor, since it is generated in the fuel cell and quantities of water-makeup can be kept at the minimum level. Catalytic processes involved are briefly reviewed and commercial catalysts used are indicated. The amount of catalyst required in each reactive unit is computed by employing the design parameters (temperature, pressure, and space velocities) reported in the literature. In the desulfurization step, it is found that about 1.6 l of a bed of ZnO is capable of handling a stream of naphtha with 1500 ppm of sulfur for 45 h of continuous operation before regeneration or replacement of the bed becomes necessary. This, however, is based on operation at 10 atm. Operation at lower pressure level will increase the desulfurization catalyst requirements, maybe to a prohibitive level. Over the reformer Liquid-Hourly Space-Velocity range of 1–4 h −1, the amount of the supported nickel catalyst varies from 14 to 4 l, respectively. For the LT shift reactor the amount of catalyst required ranges from 4 to 60 l on going from 3×10 2 to 4×10 3 h −1 typical Gas-Hourly Space-Velocity. The catalyst here is CuO–ZnO supported on Al 2O 3. The last methanation step to remove traces of poisonous CO requires about 3.5 l of nickel supported by various oxides. To selectively separate hydrogen, it is suggested to use a palladium–silver membrane, which is reported to give ultra-pure hydrogen.