(35d) Combined Pre-Reforming-Desulfurization of High-Sulfur Diesel for Hydrogen Fueling Station Applications

Considerable research and development efforts are being conducted, worldwide, to substitute hydrogen for gasoline and diesel in transportation sectors. A major challenge facing future Hydrogen Economy is the issue of hydrogen fuel delivery to dispensing stations. In the near term, it may be necessary to deliver diesel fuel directly to the fueling station wherein it is reformed to hydrogen, on demand. This approach has the following advantages: (1) Existing fuel delivery infrastructure can be utilized, (2) More energy is delivered per trip when the tanker is filled with diesel instead of liquid or compressed hydrogen, and (3) The fueling station would be able to service both internal combustion and fuel cell powered vehicles concurrently. Reforming high-sulfur hydrocarbon fuels (e.g., diesel, JP-8, etc.) is particularly challenging due to rapid deactivation of conventional reforming catalysts by sulfurous compounds.

Florida Solar Energy Center (FSEC), in collaboration with the Chevron Technology Ventures (CTV), has developed a new on-demand hydrogen production technology for fueling station applications. In this process, the diesel fuel is first catalytically pre-reformed to shorter chain hydrocarbons (C1-C6) before being fed to the main reformer, where it is converted to high-purity hydrogen gas. Here, we report on the catalytic pre-reforming of high-sulfur diesel to C1-C6 alkanes in a bench scale unit (in this work, 3180-5240 ppmw of thiophene was added to commercial diesel). In the pre-reformer, most sulfurous species present in the fuel were converted to predominantly hydrogen sulfide (H2S). The sulfur removal from the pre-reformate gas was required in order to avoid deactivation of the catalyst (mostly, Ni-based catalysts) in the main reformer. Desulfurization of the pre-reformate gas was carried out in a special regenerative redox system developed at FSEC, which includes a Fe-based scrubber coupled with an electrolyzer. In particular, ferrous/ferric (Fe2+/Fe3+) redox couple was used for oxidizing H2S to elemental sulfur as follows:

4Fe3+(aq) +2H2S(g) = 4Fe2+(aq) + 2S(s) + 4H+(aq)

We have shown that ferrous ion can be electrochemically oxidized back to the ferric state readily, in a closed process. Desulfurization occurred optimally at the following process conditions: 0.1 M iron (total) concentration, pH of 1.7, set by addition of sulfuric acid, and 50oC electrolyte temperature. The integrated pre-reformer and sulfur-scrubbing unit operated successfully for 100 hours meeting the required desulfurization target of 95% removal.