New PNNL Small-Scale Hydrodesulfurizer/Steam Reformer System Lets Portable Fuel Cells Use JP-8 or Diesel
|JP-8 steam reformer (left) and a compact hydrodesulfurization system (right). Source: PNNL. Click to enlarge.|
A new system developed by Pacific Northwest National Laboratory allows portable fuel cells to operate using JP-8—a common fuel used worldwide in military applications with and sulfur levels that can vary considerably from region to region—or road diesel.
The development of fuel cell power systems supplied by liquid hydrocarbon fuels such as JP-8 or diesel has continued to be challenged by the difficulty in cleanly reforming these fuels without catalyst deterioration. One of the major sources of catalyst deterioration and resulting low conversion activity has been the presence of sulfur in these fuels.
Various approaches to dealing with sulfur have included: the development of sulfur-tolerant catalysts; use of sulfur specific and regenerable adsorbents; selective oxidation of sulfur species followed by their adsorptive removal; and removal by catalytic hydrodesulfurization (HDS).
All have their challenges when applied to small and potentially portable fuel cell systems. The only commercially proven technology is hydrodesulfurization (HDS), which has generally been considered impractical for operation in small-scale applications. The requirement for available high pressure hydrogen has provided a major hurdle.
To address these issues, the PNNL team has developed a small-scale hydrodesulfurization units that is coupled with a steam reformer operating at 15-20 atm. Rather than requiring hydrogen, steam reformate can be utilized for the HDS unit. A fraction of the total reformate is diverted to the HDS unit, with the remainder treated as needed and provided to the fuel cell.
In a paper presented at the 21st North American Meeting (21st NAM) of the Catalysis Society in San Francisco, PNNL scientist David King described extended tests integrating the HDS unit with a microchannel steam reformer, in which actual steam reformate feeds the HDS unit and the clean fuel as processed by the HDS unit is fed to the reformer.
Results presented at the conference showed HDS of the JP-8 fuel to less than 500 ppbw (parts per billion by weight). For the steam reformer, a modest excess of catalyst was utilized over that required for sulfur-free fuel in anticipation of possible catalyst deactivation even at 500 ppbw S. With that modification, the steam reformer maintained target performance over the course of 300 hours. For HDS of road diesel, the PNNL team achieved 0.8 ppmw S, starting with 4 ppmw in the fuel. The team believes that with implementation of improved catalysts, less than 0.5 ppmw S is achievable.
The production of clean hydrogen or syngas for portable fuel cell systems operating on JP-8 fuel is possible when employing an integrated HDS/steam reformer system. Extension to non-military applications, including road diesel fuel, also appears possible. This provides a large step forward in enabling fuel cells to operate with a range of heavy hydrocarbon fuels.—King et al.