Technology Solutions Getting the sulfur out of diesel fuel As EPA gears up to announce much lower allowable sulfur levels in diesel fuel, researchers are stepping up efforts to develop new technologies for desulfurizing it. The challenge is a little more difficult than for regular gasoline, however. There is no silver-bullet solution for removing sulfur from diesel fuel, explained Lester Wyborny, a chemical engineer with EPA's Office of Transportation Air Quality. But EPA is nonetheless hoping to propose the new rule for diesel sulfur levels "as soon as possible," ideally this spring, he said. EPA is under pressure to drastically reduce sulfur levels in diesel fuels, in line with the 90% sulfur reductions announced for regular gasoline in late December (Environ. Sci. Technol. 2000, 34 (5), 142A-143A). Besides the obvious reduction of sulfur dioxide emissions, which contribute to acid rain, lowered diesel sulfur levels can cut particulate emissions. A study published by the Department of Energy (DOE) in late January showed that, when used in conjunction with special diesel particulate filters, sulfur levels of 30 parts per million (ppm) could significandy reduce particulate emissions from big diesel trucks Cutting the fuel's sulfur level may also help prevent ozone formation according to the Ozone Transport Commission The main problem with cleaning up diesel by using hydrodesulfurization (HDS), the mediod conventionally employed to remove sulfur from regular gasoline by the oil industry, is diesel's abundance of sulfur-containing dibenzothiophene molecules. The sulfur is embedded so deeply inside these large molecules that it is difficult for the metal catalysts used in HDS to attack, Wyborny explained. Although HDS can ultimately remove sulfur from diesel, using the process becomes "quite expensive" as the amount of sulfur that must be culled from diesel goes up, said Robert Campagna, president of Refining © 2000 American Chemical Society
Process Services, Inc., an oil industry consulting firm in Cheswick, PA. Campagna nonetheless expects mat die majority of refiners will use conventional HDS technology because "refiners tend not to be risk takers."
EPA is expected to drastically reduce the amount of allowable sulfur in diesel fuel this spring, spurring some refiners to find new ways to desulfurize diesel fuel.
However, some small refiners, like Petro Star, Inc., of Anchorage, AK, have never made the $50-$80 million investment in HDS equipment. "We can't afford HDS," stated Walter Gore, manager of the project that Petro Star embarked on in 1996 "out of pure desperation" to devise a less expensive method for desulfurizing diesel fuel. To make sure the company has as many options as possible, Petro Star has also obtained a license for a biological method of removing fuel sulfur developed by Energy BioSystems Corp of Woodlands TX and it is porting work by DOE's Oak Ridge National Laboratory (ORNL) to develop a second biological method In bench-scale laboratory testing, Petro Star's conversion-extraction desulfurization process has succeeded in reducing sulfur levels to 10 ppm. The conversion step of die process, which works at relatively low pressures and temperatures, polarizes the sulfurcontaining molecules in die fuel, according to a paper presented at a National Petrochemical Refiners
Association conference at die end of March. These polarized molecules are removed by a solvent widi similar polarity in the extraction step. The company expects to have a pilot-scale process up and running by August. The research being conducted at ORNL focuses on recapturing the 7-15% of fuel that gets lost in Petro Star's extraction step, according to Abhijeet P. Borole, senior research associate at ORNL's Bioprocessing Research and Development Center. Using a strain of Pseudomonas bacteria genetically engineered to take up sulfur compounds by researchers at Spain's Consejo Superior de Investigaciones Cientfficas, Borole is trying to optimize the conditions under which the bacteria will attack the sulfur compounds Energy BioSystems has been pilottesting its biological desulfurization process since 1994, said Desulfurization Program Manager Phil DiGrazia. The Rhodococcus bacteria serving as the biocatalyst in the process are genetically engineered to prefer sulfur compounds like dibenzothiophene tiiat are hard to attack with conventional HDS. Because the bacteria cannot yet consistendy achieve sulfur reductions in line widi die 10-20 ppm range mat EPA is expected to propose, the researchers are promoting it as a complementary treatment to HDS DiGrazia said In order for any of the technologies being developed for removing sulfur from diesel to be truly marketable, mey will have to prove diemselves highly effective, Wyborny stressed. The "after-treatment" equipment that will be added to automotive engines— such as particulate traps and NO^. absorbers—to remove pollutants from diesel engines is very sensitive to sulfur, he explained. Because EPA wants to ensure diat die new diesel formulations won't foul this automotive equipment, there are fewer opportunities to provide smaller refiners widi a break in terms of lifting the sulfur limits he said KELLYN S. BETTS
APRIL 1, 2000 /ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 6 1 A
