CARB study confirms oxi-cat-equipped CNG no better than clean-diesel on PM, toxics

Diesel Fuel News, March 17, 2003 by Jack Peckham

A California Air Resources Board (CARB) study confirms that in side-by-side tests of a compressed natural gas (CNG) bus with an oxidation catalyst compared to a diesel bus with a particulate matter (PM) filter, the diesel bus still beats CNG on PM and "toxics."

The finding contradicts claims by certain "green" groups and the CNG lobby that CNG would beat clean-diesel if fitted with state-of-art exhaust cleanup technology.

In the CARB study, CNG bus with oxi-cat produced 20.7 milligrams/mile of PM, versus 14.2 mg/mi PM on the clean-diesel bus. CNG had about half the nitrogen oxides (NOx) emissions -13.9 mg/mile versus 31.1 mg/mi on clean-diesel.

Methane and total hydrocarbon emissions (THC) on clean-diesel were non-detect, whereas CNG had 13.7 grams/mile methane and 14.1 g/mi THC. Diesel had slightly higher [CO.sub.2] emissions per mile (25.13 g/mi versus 19.87 g/mi CNG) but methane is 20 times worse than [CO.sub.2] on "greenhouse gas" emissions.

Toxic carbonyl emissions were six times worse on CNG with oxi-cat than clean-diesel. The relatively more toxic heavy polyaromatic hydrocarbon (PAH) emissions were lower on clean-diesel, but the less-toxic light PAH's were five times higher on diesel than CNG.

The findings support CARB's decision to allow either clean-diesel or CNG buses to compete in the California marketplace, CARB told the World Truck Conference (organized by California a Trucking Association) this month.