Exhaustive research on emissions technologies

In a time when regulators are demanding reduced emissions from the transportation sector, Pacific Northwest National Laboratory is conducting fundamental scientific research that could help vehicles clean up their act.

Vehicles contribute heavily to four of the air pollutants monitored and regulated by the Environmental Protection Agency hydrocarbons, carbon monoxide, oxides of nitrogen and particulate matter. The EPA's aggressive requirements for diesel engines in heavy-duty trucks include a 90 percent reduction in particulate matter emissions by 2007 and a 90 percent reduction of oxides of nitrogen, also known as NOx, by 2010. NOx reacts with the hydrocarbons in the atmosphere to form ozone, a major component of smog.

Industry is considering several approaches to meeting the stringent requirements because no widely applicable technology exists to solve the problem. "Industry is constantly changing requirements and preferences. But interestingly enough, the science doesn't change," said George Muntean who oversees the Exhaust Emission Science Laboratory, a program that integrates emissions research at PNNL. "Our research is applicable regardless of which emission treatment technology comes out on top."

Many technological approaches to reducing emissions rely upon delivering a "reductant" to the exhaust stream that chemically reacts with harmful NOx molecules. NOx forms when harmless oxygen and nitrogen in the air are exposed to the high temperatures in combustion engines. A reductant is a molecule that aids in converting NOx back into components of clean air nitrogen, oxygen and water.

"There are a lot of applications built around this principle," Muntean said, "but we're looking at them from the same general perspective." Researchers are striving to improve the selectivity of the reductant, which would make it more effective. They also are applying their expertise in surface science to understand what happens on the surface of the catalyst where the reaction between NOx and the reductant takes place.

"Think about it," Muntean said. "There are no moving parts. The gas goes in dirty, flows over a large surface area material, and comes out clean. It's the surface that affects performance." Researchers need to know how molecules lay on the surface, how they interact with the surface and how the surface changes. With this understanding, they can go about preventing things that reduce the catalyst's effectiveness, such as sulfur in the exhaust blocking active reaction sites or high temperatures causing the surface to deteriorate.

The Laboratory is working with key industry partners on different aspects of these challenges. For example, through a cooperative research and development agreement (CRADA) with Caterpillar, researchers are developing a sulfur trap that potentially could be used with heavy-duty diesel engines. The sulfur trap removes the sulfur in exhaust that poisons the catalyst used to reduce NOx.

A CRADA project with Cummins focuses on NOx adsorbers, which act like a sponge to adsorb and then dispose of NOx. PNNL is working on preventing the adsorbers from deactivating in the presence of sulfur.

Teaming with Detroit Diesel, PNNL is investigating another exhaust after treatment alternative called urea selective catalytic reduction. In this approach, urea is broken down into ammonia, which is used as the reductant to strip oxygen from the NOx molecules. While this technology has been used in stationary power production plants, it hasn't made its way to mobile applications such as heavy-duty trucks.

To address particulate matter emissions, researchers also are studying fundamental processes that affect how particulate matter, or soot, gets captured on the surface of a filter or particulate matter "trap." After the particulate matter in diesel exhaust is captured, it is burned to renew the filter surface. Researchers are addressing technical hurdles, such as preventing the filter from plugging, cracking and melting as well as improving size, cost and performance.

"At the end of the day, the device doesn't matter, because our part is the same," Muntean said. "The different approaches may seem like they have nothing in common, while in reality, the same science is behind them all."