"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," says
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 ultra clean 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 non-sulfur aromatics based on toluene and naphthalene, compounds that fuel producers would like to remain in the fuel. When sulfur is removed 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, called SARS for selective adsorption for removing sulfur, 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 on to a metal species," Dr. Xiaoliang Ma, research associate, Penn State Energy Institute, told attendees today (April 8) at the spring meeting of the American Chemical Society in Orlando, Fla. "This method will not adsorb the coexisting aromatic compounds like benzene and naphthalene."
Diesel fuel and gasoline contain 20 to 30 percent aromatics but less than 1 percent 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 is placed on a porous, non-reactive 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," says 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 can be further processed.
The researchers hope that refineries can employ the process to remove sulfur and meet future ultra clean fuel requirements and that those providing fuel for fuel cells can use the process to produce ultra clean fuel.
"Fuel cells need essentially zero sulfur fuel to operate," says Song. "Small adsorption sulfur removal systems might be used at gas stations on special clean fuel pumps for fuel cell vehicles to ensure that all sulfur is removed from the fuel. This SARS concept can also be used for on-board removal of sulfur from fuel for fuel cell system use."
The researchers would like to create a sulfur removal system for refineries that can be continuously regenerated.
The U.S. Department of Energy supported this work through the DOE National Energy Technology Laboratory UCR program and the researchers have filed a provisional patent application.