Sept. 10, 2002
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.
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