Pure hydrogen production by reforming reactions through Pd-based membranes

Hydrogen can be obtained from multiple sources, renewable and fossil fuels: research and technological innovations related to production from renewables (especially biomass) have increased in the last decades. However, at present, the most diffused and the less expensive source for hydrogen production remains the reforming of hydrocarbons, in particular methane. The hydrogen produced can be used in electrochemical devices, for example fuel cells, for the production of electrical energy with only water vapor and heat as byproducts. The use of hydrogen as an alternative to traditional energy sources could cover stationary applications of electric energy production (housing, industrial plants), mobile applications (cars and motorcycles), as well as small electronic devices. Nevertheless, the adoption of particular fuel cells requires high purity of hydrogen: in fact, the carbon monoxide, even at levels of a few tens of ppm, can poison the cell resulting in a drastic reduction of the performance of the same. On the other hand, the carbon monoxide is one of the byproducts of the reforming reactions of hydrocarbons and is therefore always present (up to 10-15% of the product stream, depending on the operating conditions and catalysts used). Therefore, appropriate processes must be considered to purify the hydrogen produced by reforming. The membranes for the separation of ultra-pure hydrogen are based on metals selectively permeable to hydrogen: these devices can be implemented in small or medium scale applications. The Pd-Ag membranes, which are well known to exhibit infinite hydrogen selectivity, however, are expensive. Furthermore, the uploading of hydrogenation into the Pd-Ag system can compromise the selectivity and the lifetime of the permeator. As widely reported on literature, the issue of the cost can be approached with two strategies: the reduction of the Pd content (i.e. decrease the thickness of the Pd layer) or investigation of material alternative to noble metals. An experimental setup manufactured at ENEA Frascati laboratories and consisting of a traditional high temperature reformer coupled to a multi-membrane device (19 membrane with length about 250 mm, wall thickness 0.150 mm, diameter 10 mm) has been operated for the production of pure hydrogen by reforming reactions. Experimental test campaigns, methane steam reforming and auto-thermal reforming of methane have been carried out by varying the operative conditions such as the reforming temperature, the reaction (lumen) pressure and the feed flow rate. Up to about 3 NL min-1 of ultra-pure hydrogen have been produced, thus demonstrating that the membrane setup is capable to feed a proton exchange membrane fuel cell of some hundreds watt. Finally, a further test campaign has been performed in the same two-step process in order to investigate the effect of use diluted ethanol in the methane steam reforming. The combined methane and ethanol reforming can improve the hydrogen yield also when the ethanol concentration in the liquid phase is very low (typical, for example, of the bioethanol). The results of this work have demonstrated the applicability of the membrane process studied to produce high purity hydrogen from reforming of methane and/or biomass. Particularly, they can suggest the use of membrane reformers coupled to Polymer Electrolyte Membrane fuel cells for small- medium scale (5-10 kW) stationary or vehicular systems.