Low-sulfur gasoline and diesel: The key to lower vehicle emissions
It is impossible to reduce air pollution from the transportation sector without getting sulfur out of fuels. Sulfur is a pollutant itself, but more importantly it interferes with and eventually disables control technologies for all other air pollutants. No significant air pollution reduction strategy can work without reducing sulfur to near-zero levels.
This paper addresses the need to reduce sulfur in transportation fuels and the benefits that can be realized in terms of total pollutant emissions. Sulfur fouls conventional and advanced technologies to control vehicle emissions, including carbon monoxide (CO), particulate matter (PM), nitrogen oxides (NOx) and hydrocarbons (HC). Low-sulfur fuels are the key to reducing emissions from existing vehicles and enabling advanced control technologies and fuel-efficient designs for new vehicles.
Sulfur is a naturally occurring component of crude oil and is found in both gasoline and diesel. When those fuels are burned, sulfur is emitted as sulfur dioxide (SO2) or sulfate particulate matter. Any reduction in fuel sulfur immediately reduces these sulfur compounds and, as sulfur levels decline past a certain point, the benefits increase to include total pollutant emissions.
Reduced sulfur fuel (~150 ppm) makes existing vehicles cleaner, decreasing emissions of CO, HC, and NOx from catalyst-equipped gasoline vehicles and PM emissions from diesels, with and without oxidation catalysts. These benefits increase as vehicles are designed to meet higher emissions standards and sulfur levels are reduced further. Low sulfur fuel (~50 ppm) allows for advanced control technologies for diesel vehicles. Near-zero sulfur fuel (~10 ppm) allows for the use of NOx adsorbers, increasing NOx control to over 90% in both diesel and gasoline vehicles. This enables more fuel-efficient engine designs that are incompatible with current emissions control systems. Particulate filters achieve the maximum efficiency with near-zero sulfur fuels, approaching 100% control of PM.
The technologies required to reduce sulfur to near-zero levels are in use in many areas of the world. Current costs are reasonable and the refining industry continues to make progress in developing more active catalysts and novel processes for removal of sulfur, reducing costs even further.
Studies show the benefits of sulfur reduction far outweigh the costs, even though required refinery investments continue to be significant. The U.S. EPA found human health and environmental benefits due to sulfur reduction were ten times higher than the costs. (This study assumed stricter emissions standards contingent on low-sulfur fuels.) Furthermore, a European study showed that near-zero sulfur fuels significantly reduce total fuel costs by increasing fuel economy. The considerable potential for greenhouse gas emission reductions adds further to the health, environmental, and social benefits of sulfur reduction.