CNG BUS EMISSIONS SIGNIFICANTLY REDUCED BY CONTROL DEVICE

Transit buses powered by low-emission compressed natural gas can be made even cleaner with oxidation catalyst controls, according to a study recently released by the California Air Resources Board.

"This study confirms that overall emissions from CNG buses continue to be cleaner than diesel engines that use low sulfur fuel and particulate traps," said Barry Wallerstein, executive officer of the South Coast Air Quality Management District.

"Further development of emissions controls for CNG buses should make them cleaner yet," he said. "We are on the right track in promoting the use of CNG and other alternative fuel vehicles in our region to reduce air pollution."

In tests conducted on two CNG transit buses equipped with oxidation catalysts, CARB scientists found their nitrogen oxide emissions to be about 50 percent less than diesel buses fueled with low-sulfur diesel and equipped with a particulate trap.

Nitrogen oxide emissions reductions are critical in Southern California since they contribute to the formation of both ground-level ozone and fine particulate, or PM₁₀. Those are the region’s two worst pollutants, associated with a range of health effects including increased hospitalizations, worsened asthma symptoms and premature deaths.

In addition, unlike diesel buses, emissions from CNG buses with catalysts do not have a high ratio of nitrogen dioxide (NO₂) to total oxides of nitrogen (NOx). A high NO₂/NOx ratio results in a higher potential for ozone and nitric acid formation.

The CNG bus had comparable emissions to the diesel bus for particulate matter mass and total number of "ultrafine" particles. PM₁₀ particles are smaller than 10 microns (one micron is one millionth of one meter) or about one-seventh the diameter of a human hair. Ultrafine particles typically are 5 to 100 nanometers in diameter (one nanometer is one billionth of one meter), and can be inhaled deeply into the lungs.

CNG buses, either with or without oxidation catalysts, have lower emissions of carbon dioxide than diesel buses. Carbon dioxide is a greenhouse gas that contributes to global warming.

The study also found that compared to CNG buses with no emission controls, the buses with an oxidation catalyst:

• Reduced 1,3 butadiene to levels so low they could not be detected;
• Reduced formaldehyde emissions by more than 95 percent;
• Reduced non-methane hydrocarbon emissions by more than 88 percent; and
• Reduced total particulate matter, carbon monoxide, total hydrocarbons and total number of ultrafine particles.

Earlier this year, CARB conducted tests comparing emissions from a diesel bus with a particulate trap and a CNG bus with no emission controls. AQMD officials urged CARB to conduct additional testing on CNG buses with emission controls and supplied the oxidation catalysts for the tests.

Compared to a diesel bus, the CNG bus did have marginally higher emissions of aldehydes, a class of compounds including formaldehyde, but the test methodology used is not adequate to correctly measure this low level of concentration, Wallerstein said. Test results for mutagenicity and PAH levels have not been completed yet.

The two CNG buses tested had engines and catalysts made by different manufacturers. In the carbon monoxide emissions test, one CNG bus had higher emissions and the other roughly equivalent emissions as the diesel bus, indicating that a catalyst can be "tuned" to reduce carbon monoxide emissions to very low levels.

AQMD is now sponsoring research with major engine manufacturers to further reduce emissions from natural gas engines, and to further optimize the emissions reduction performance of oxidation catalysts on CNG vehicles. In the near future, AQMD plans to sponsor research to develop particulate traps as well as cleaner lubrication oils for CNG vehicles.

For more information see CARB’s study on the web.

AQMD is the air pollution control agency for Orange County and major portions of Los Angeles, San Bernardino and Riverside counties.