California study suggests CNG emissions more toxic than diesel systems

May 6, 2002
New comparative environmental data from the California Air Resources Board (CARB) indicates that California's massive South Coast Air Quality Management

New comparative environmental data from the California Air Resources Board (CARB) indicates that California's massive South Coast Air Quality Management District in Los Angeles CA may have jumped the gun in mandating compressed natural gas (CNG) for buses and other vehicles in its jurisdiction.

The new CARB data suggests emissions from CNG buses may actually be more toxic than its alternative, modern diesel systems, according to Allen Schaeffer, executive director of the Diesel Technology Forum. "This study certainly challenges conventional thinking. It shows us that alternative fuels like CNG are not always as clean as they have been accepted to be, and more importantly, that we have not given fair consideration to the tremendous progress made in clean diesel technology."

The announcement by CARB that diesel engines with soot filters emit fewer and less-toxic compounds than "clean" CNG engines questions the objectivity and wisdom of South Coast District's two-year-old CNG-only decision, says Schaeffer. The policy, based on conjecture rather than comparative data, has seen taxpayers subsidize hundreds of millions of dollars for new CNG vehicles and fueling stations in the midst of the natural gas shortage.

"It was a naive assumption that CNG emitted fewer particulates than diesel," says toxicologist Charles Lapin. He said he was surprised that toxic formaldehyde levels in CNG emissions were so much higher than diesel, but he acknowledged that only one or two studies have looked at the toxicity of CNG emissions. "Several months ago, California determined that it didn't adequately study the adverse effects of the gasoline additive MTBE before mandating its use, and Californians are now paying the price," says Schaeffer. "The blind rush by some government agencies away from modern clean diesel to CNG-fueled public transit may carry the same harmful effects. Our message is that all fuels and technologies should be evaluated fairly and openly."

CARB, one of the world's leading advocates of "alternative" technologies, discovered in its tests with South Coast AQMD that a diesel bus with a soot trap outperformed a newer CNG bus in eight out of 11 pollution tests, according to the Diesel Technology Forum information. The results also suggest that the CNG exhaust components may be more harmful than those from low-sulfur diesel fuel.

The CARB data corroborates studies done in Sweden in 2000 but were rejected by the South Coast AQMD and many policymakers at CARB during the decision-making process in 2000 and 2001. The results also parallel a much larger California study of trucks, waste haulers, and school buses conducted by BP/ARCO along with CARB, South Coast AQMD, and the US Department of Energy, according to the Forum information.

"The comparison reaffirms the decision made by 21 California transit agencies last year that chose a clean diesel future over CNG," says Schaeffer. "And it leaves the South Coast Air Quality Management District, a co-sponsor of the study, questioning the wisdom of its policies that have advocated 'clean' alternatives with impunity. These findings only further reinforce the value of clean diesel technology in meeting air quality goals and what the majority of transit districts and school bus fleets already know: they can get more clean air for the buck with clean diesel technology."

The technology includes cleaner engines, cleaner fuel, and emissions systems that collectively reduce soot, nitrogen oxides, and other pollutants. Experience with CNG by transit districts around the country suggests that not only is it far more expensive technology to own and to operate, but it is less fuel efficient, less reliable and breaks down more often, the Form states.

"All engines produce emissions, and neither diesel nor CNG are exempt," said Schaeffer. "Technology is constantly evolving towards lowering emissions. These kinds of studies not only improve our knowledge of the science, but we also hope they promote change in the process that compares technologies and the perception and understanding about just how far we have come in diesel technology."