1 June 2002
Ultraclean fuels show promise for fuel cells
University Park, Pa.—A process that removes organic sulfur from liquid fuels at low temperatures and ambient pressure, without using hydrogen, may help refiners provide fuels for fuel cells, said researchers at Penn State.
"Currently, the U.S. Environmental Protection Agency allows 500 parts per million sulfur in diesel fuel and 350 parts per million in gasoline, but by 2006, the regulations will require only 15 parts per million sulfur in diesel and 30 parts per million in gasoline," said Dr. Chunsan Song, associate professor of fuel science and program coordinator, Clean Fuels and Catalysis, Penn State Energy Institute. "Long before that, however, we will need ultraclean fuels for fuel cells."
Removing organic sulfur from hydrocarbon fuels is difficult because the sulfur is usually bound to aromatic compounds that exist together with nonsulfur aromatics based on toluene and naphthalene-compounds fuel producers would like to remain in the fuel, Song said. When refiners remove sulfur with the aromatic compounds, further treatment of the sulfur rich fraction becomes difficult.
Current methods of removing sulfur from liquid fuels use high temperatures and pressure and hydrogen gas.
The new Penn State process, selective adsorption for removing sulfur (SARS), goes at low temperatures and pressure and does not use hydrogen or other reactive gases.
"We have developed a process that selectively adsorbs organic sulfur onto a metal species," said Dr. Xiaoliang Ma, research associate, Penn State Energy Institute. "This method will not adsorb the coexisting aromatic compounds like benzene and naphthalene."
Diesel fuel and gasoline contain 20% to 30% aromatics but less than 1% sulfur, so removing the sulfur without removing the aromatics is difficult. The transition metals or transition metal alloys used in the process selectively grab the sulfur. The active adsorbent goes on a porous, nonreactive substrate that allows the greatest surface area for adsorption. Adsorption occurs when the sulfur molecules attach to the transition metals on the substrate and remain there separate from the fuel.
"The absorbent transition metals can clean 10 times their volume of fuel, but eventually the system becomes saturated with sulfur," said Michael Sprague, graduate student in fuel science. "Solvent regeneration can restore activity."
Initially, there is an activation step to activate the absorbent materials, but after that, adsorption and regeneration of the absorbent are all that they need. The solvent can be reclaimed for future use while the sulfur undergoes further processing.
The researchers hope refineries can employ the process to remove sulfur and meet future ultraclean fuel requirements and that those providing fuel for fuel cells can use the process to produce ultraclean fuel.
"Fuel cells need essentially zero sulfur fuel to operate," said Song.
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