Tuesday, January 26, 2010

Ethanol for fuel cell

There has been considerable interest in direct ethanol fuel cells (DEFC). Acta-nanotech , a European firm manufacturing hydrogen, alcohol, ethanol and methanol fuel cell batteries, is developing non-precious metal catalysts for a variety of fuels including ethanol. The U.S. Army held a workshop on DEFC a few years ago. However there are some formidable technical challenges in doing direct ethanol, mainly related to the existence of the carbon-carbon bond in the molecule. Even if direct ethanol is proven to be technically feasible, it would likely be limited to fuel cell systems with rather limited power, say 1 kW or less. It will not be possible to use ethanol in fuel cell systems required for or aviation. This market would not provide a significant increase in the total ethanol usage in the range of interest to a transport bioethanol producer, though it might provide advantages for small, portable fuel cells in view of the wider availability, lower toxicity and somewhat greater energy density of ethanol compared to methanol.

In contrast to the cell reaction of direct methanol fuel cells (DMFC), other carbon-containing molecules are produced in addition to CO2.The catalyst has to break C-C bonds, but not C-O bonds and then oxidize the C fragments before they form coke. Therefore the catalysts that can best break C-C bonds are not likely going to be stable in the fuel cell.

In the steam reforming of ethanol, ethanol is reacted with steam to produce hydrogen and CO2. The ethanol is needed to be fairly dilute for optimal performance. The energy per unit volume is too low to make on-board reforming work for fuel cell application.

If you disregard that and assume for a moment that hydrogen storage is economically feasible, the steam reforming of ethanol could work, although no perfect catalyst has been developed as yet. The biggest issue is the stability of the catalyst. Rhodium is the most stable of the ethanol steam reforming catalysts but it is highly expensive. Cobalt is an alternative but its stability is the subject of research.

M/s CellTech have developed the Liquid Tin Anode Fuel Cell (LTA) which is a type of Solid Oxide Fuel Cell (SOFC) capable of operating directly and efficiently on virtually any hydrocarbon fuel including diesel, JP-8 (Jet propellant 8 is a jet fuel) or ethanol. Power ranges from 50 Watts to megawatts are feasible. The company is developing a 500 Watt LTA stack which will operate on JP-8 for military portable power applications. The Liquid Tin Anode is not affected by sulfur in fuel. The catalyst is liquid and therefore there is no microstructure to be attacked as with nickel or precious metal anode catalysts. Sulfur is rejected as H2S and/or SO2 depending upon operating conditions.


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