|Innovation: Cool Steam Reforming|
GEI COOL STEAM REFORMING
Carbon based fuel processor cost for traditional PEM fuel cell systems is driven by stack contamination requirements. For example:
• The fuel reformat hydrogen sulfide (H2S) tolerance for traditional low temperature PEM stacks is 100 PPB (parts per billion). Conversely, for high temperature PEM stacks the H2S tolerance is 10 PPM (parts per million) or 100 times more tolerate.
• The fuel reformat CO (Carbon Monoxide) tolerance for low temperature PEM level is 10 PPM while for the high temperature PEM the limit is 3-5% by volume, or 50,000 PPM.
As such, low temperature PEM stacks require expensive preferential oxidation and hydrogen membrane purification clean-up equipment to reduce H2S and CO fuel contamination to acceptable levels. The Equilibrium Concentration plot shows the %CO as a function of temperature and provides the rationale for cool steam reforming.
Equilibrium Concentration vs. Temperature
The percent CO increases with reforming temperature while the percent of un-reacted methane decreases with reforming temperature. There exist a delicate balance between CO composition, reaction temperature, unused fuel cell anode exhaust components CH4 (methane) and H2, and system efficiency which are impacted by the anode tail gas burner and fuel cell stack stoichmetric ratios.
The GEI fuel processing strategy employs a "cool" steam low temperature (550C-600C) steam reformer as shown below in the comparison for low temperature and high temperature PEM fuel cell stacks.
The high temperature PEM not only is more tolerate of H2S and CO, but provides high temperature heat for the steam generation required for fuel reforming. Additionally,the excess unused anode fuel exhaust can be re-cycled to the combustor (i.e. anode tail gas burner) to minimize the external fuel required. This strategy results in a very efficient system while reducing cost, size, complexity, and parasitic losses. Current cost savings estimates are 40%-60% depending upon volume with 30% less hardware.
Methane Reforming Flow % vs. Temperature
As discussed, traditional low-temperature PEM cells require nearly pure hydrogen which significantly impacts both the fuel processing equipment cost and size. The drawing to the right compares the cylindrical high temperature PEM fuel cell reformer (shown in blue) compared with the additional H2 membrane purifier required for low temperature PEM fuel cells. This additional equipment increases the space volume by 42%, increases the weight by 100%, and increases cost by 50%. Our reformer partner provides a patented micro channel design for efficient steam reforming of gaseous and liquid logistic fuels. When coupled with high temperature fuel cells, which do not require expensive preferential oxidation reactors and membrane purifiers, the reformer is compact and efficient. Additionally, special reforming catalyst technology allows for a broad diversity in fuel selection such as diesel, propane, natural gas, methane, methanol, ethanol, JP8, and bio-renewable fuels.