SACRAMENTO, Calif. — A transit bus equipped with a particulate filter running on low sulfur diesel fuel produced lower emissions than a compressed natural gas bus, according to a study released April 18 by the California Air Resources Board.
The particulate-filtered bus released lower amounts of particulate matter (PM) and toxic organic compounds, but produced an increase in nitrogen oxide (NOx) emitted as nitrogen dioxide (NO2) compared to the CNG bus, CARB said.
However, while the study, which also examined a low sulfur fuel bus without a diesel particulate filter (DPF), said both CNG and DPF buses are “significantly superior” to conventional diesel buses, “no single technology is clearly superior to the others.”
“Both technologies are proven and offer a verified benefit for reduction of total PM mass emissions,” the report said.
CARB studied emissions from a CNG 40-passenger transit bus; a diesel 40-passenger transit bus running on low-sulfur ECD-1 Arco diesel fuel; and the same diesel bus retrofitted with a Johnson Matthey Continuously Regenerating Technology DPF and running on ECD-1.
The DPF bus performed better in eight of 11 emission categories tested. The CNG bus only recorded lower levels of NOx, NO2 and CO2. Furthermore, the report said the mutagenicity findings suggest that “CNG PM is not inert and may pose a toxic risk.” Mutagenicity is the measure of mutations in an organism and evidence of substance’s potential toxicity. The report cautioned, however, that the mutagenic numbers cannot be used directly to determine cancer risk by inhalation.
The CNG NOx emissions were approximately a third lower than diesel vehicle emissions, but, the report said, the “modest” increase in NO2 emissions in DPFs was offset by the significant beneficial reductions in PM and non-methane hydrocarbon.
According to CARB, the data also suggested levels of some toxic pollutants, such as benzene, in CNG exhaust “require further study and may warrant additional control.”
The CNG bus, however, was not equipped with a particulate filter or other after-treatment equipment, such as an oxidation catalyst. The agency plans to conduct additional tests with the CNG bus refitted with an oxidation catalyst and a new CNG bus equipped with an installed oxidation catalyst. Results are expected in mid-2002.
Officials from the CNG and low sulfur fuel camps both reacted positively to the study, though for obvious different reasons.
“CARB’s research suggests the possibility of significant adverse health impact from widespread use of natural gas vehicles,” said Dr. William B. Bunn, Vice President, health, safety and productivity of International Truck and Engine Corporation, which manufacture’s a low sulfur fuel-compatible bus engine. “Assertions that natural gas exhaust is not toxic are not credible. In fact, as this new study corroborates, low-emitting diesel exhaust may include fewer pollutants than natural gas exhaust.”
CNG officials took a different tack, saying the research shows that yesterday’s natural gas technology compares with tomorrow’s diesel technology.
“This comparison might be valid if the natural gas engine producers were not already equipping engines with pollution controls, but the fact is-they are,” said Michael Eaves, chairman of the California Natural Gas Vehicle Coalition. “The natural gas industry is committed to being the cleanest transportation fuel, and we are working continually to improve our engines so that all Californians can benefit.”
Source: School Transportation News, May 2002.