CARB study shows clean-diesel beats CNG on PM, 'toxics'; CNG monopoly shaken

Diesel Fuel News, April 29, 2002 by Jack Peckham

A California Air Resources Board (CARB) study shows that buses equipped with diesel particulate filters (DPFs) running on ultra-low sulfur diesel (ULSD) actually produce less particulate matter (PM) emissions and "toxics" than a bus running on compressed natural gas (ONG).

CNG produces exhaust species up to eight times more mutagenic than clean-diesel exhaust, and CNG produces higher pollutant emissions in eight of 11 categories, the study shows. As a result, "green diesel" bus maker International, Diesel Technology Forum and other clean-diesel defenders hailed the study as proof that public agencies cannot defend "green" policies requiring CNG monopolies. They urge agencies instead to allow clean-diesel to compete freely with CNG or other "green" technologies.

The CARB findings match those of several other recent studies showing that clean-diesel clearly beats CNG on virtually all "toxic" emissions (see Diesel Fuel News 3/18/2 002, p15; 4/16/01, p12,. 9/11/00, pl2, 7/17/00, p1).

The study immediately threw the pro-CNG, ban-diesel "green" lobby into a tizzy, rushing out with press releases claiming that CNG still can be "cleaner." The groups -- Natural Resources Defense Council, Sierra Club, Coalition for Clean Air, American Lung Association of California -- offered neither proof nor any new data to back this claim, however.

* Same 'Diesel Kills' Tactics In LA, DC

Some of these groups (NRDC, Sierra Club) passed out leaflets saying "Diesel Kills, Buy CNG" and used the same language in a raucous demonstration outside a Washington (DC) Metro Area Transit Authority (WMATA) board meeting this month. The board voted to purchase CNG buses exclusively, as WMATA chair Chris Zimmerman claimed that "a CNG bus emits less NOx and particulate matter than a diesel bus" as justification for giving CNG a monopoly.

Meantime, in California, CNG's "green" monopoly lobbying ally -- California Natural Gas Vehicle Coalition (CNGVC) -- likewise issued a statement claiming that CNG is "still the yardstick" for ultra-clean standards.

CNGVC claimed that unlike the CNG bus in the new CARB study, latest-generation CNG buses will use oxidation catalysts to control PM and toxics emissions further. Problem: It offered no data showing that these are greatly different than the PM/toxic emissions from the latest clean-diesel buses, and failed to mention that no DPF technology exists for a CNG vehicle, unlike diesel vehicles.

Bottom line: Evolving clean-diesel (using DPF and ULSD) and CNG technologies seem heading toward the point of trivialization on PM and "toxics," with CNG only enjoying a small edge in nitrogen oxides (NOx) emissions, at least until low-NOx diesel engines start to appear later this year and again in 2006.

"Both the CNG bus and the trap-equipped diesel bus produced lower emissions compared to the conventional diesel bus," CARB Chairman Allan Lloyd said. "We are working now to obtain better performance from CNG buses when they are equipped with state-of-the-art aftertreatment equipment."

However, other CARB studies show that replacing old, high-NOx diesel buses with new, lowNOx diesel buses (with DPFs) still delivers more total emissions reduction on a dollar-for-dollar basis, given the huge cost penalty with CNG technology (including CNG refueling infrastructure). The point: Many more clean-diesel vehicles can be put on the road for the same money spent on CNG.

California Chamber of Commerce (CCC) and Californians for a Sound Fuel Strategy (including refiners, fleets and school districts) said the new CARB study shows the wisdom of fuel-neutral public policy, rather than CNG monopolies pushed by the gas lobby and "green" groups.

"This research validates the notion of a fuel-neutral approach by regulators and lawmakers," said CCC's Jeanne Cain, VP-government relations.

"It was a naive assumption that CNG emitted fewer particulates than diesel," said former Arco toxicologist Charles Lapin, now a consultant to diesel engine makers. Lapin pointed to earlier studies-ignored by the green/gas lobby -- that showed an "appalling" disregard for CNG toxics issues (see Diesel Fuel News 4/16/01, p12).

"The blind rush by some government agencies away from modem clean diesel to CNG-fueled public transit may carry the same harmful effects" as the mistaken push for MTBE oxygen mandates in gasoline, said Diesel Technology Forum executive director Allen Schaeffer.

"The comparison reaffirms the decision made by 21 California transit agencies last year that chose a clean diesel future over CNG," Schaeffer added.

CARB's study chart (below) shows what CARB found in its side-by-side tests of a model year 2000 New Flyer 40-foot bus with a DDC Series 50G CNG engine, compared to an older, 1998 model year New Flyer 40-foot bus with a DDC Series 50 diesel engine with a catalyzed DPF, running on a 15ppm sulfur ULSD from BP-Arco.

CARB points out that the findings, while internally reviewed for data quality, still await further analysis and peer review.

The mutagenicity findings "cannot be used to quantify cancer risk," the CARB report says. Ironically, South Coast Air Quality Management District (metro Los Angeles) brushed aside such cancer-risk calculation cautions when it decided that only diesel emissions have significant air toxics cancer risks. This finding was used to justify a CNG monopoly on most public fleet vehicle purchases, including transit buses.