A process that removes organic sulphur from liquid fuels at low temperatures and ambient pressure without using hydrogen, may help refiners provide fuels for fuel cells and meet the US government’s ultra-clean fuel requirements, according to Penn State researchers.
‘Currently, the US Environmental Protection Agency allows 500 parts per million sulphur in diesel fuel and 350 parts per million in gasoline,’ said Dr. Chunsan Song, associate professor of fuel science and program co-ordinator, Clean Fuels and Catalysis, Penn State Energy Institute. ‘But by 2006, the regulations will require only 15 parts per million sulphur in diesel and 30 parts per million in gasoline.’
Removing organic sulphur from hydrocarbon fuels is difficult because the sulphur is usually bound to aromatic compounds that exist together with non-sulphur aromatics based on toluene and naphthalene, compounds that fuel producers would like to remain in the fuel. When sulphur is removed with the aromatic compounds, further treatment of the sulphur rich fraction becomes difficult.
Current methods of removing sulphur from liquid fuels use high temperatures and pressure and hydrogen gas. The new Penn State process, called SARS (selective adsorption for removing sulphur) goes at low temperatures and pressure and does not use hydrogen or other reactive gases.
‘We have developed a process that selectively adsorbs organic sulphur on to a metal species,’ said Dr. Xiaoliang Ma, research associate, Penn State Energy Institute. ‘This method will not adsorb the co-existing aromatic compounds like benzene and naphthalene.’
Diesel fuel and gasoline contain 20 to 30 percent aromatics but less than one percent sulphur, so removing the sulphur without removing the aromatics is difficult.
The transition metals or transition metal alloys used in the process selectively grab the sulphur. The active adsorbent is placed on a porous, non-reactive substrate that is said to allow the greatest surface area for adsorption. Adsorption occurs when the sulphur 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 sulphur,’ 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 sulphur can be further processed.
The researchers hope that refineries can employ the process to remove sulphur 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 sulphur fuel to operate,’ said Song. ‘Small adsorption sulphur removal systems might be used at gas stations on special clean fuel pumps for fuel cell vehicles to ensure that all sulphur is removed from the fuel. This SARS concept can also be used for on-board removal of sulphur from fuel for fuel cell system use.’