California's Battle to Cut Emissions with Biofuels Burns in New Truck Engines

by Laura Sanicola (Reuters) Renewable diesel is touted as a cleaner-burning fuel, but a recent study has shown the fuel falls short on one measure of reducing pollution from new truck engines - giving pause to California regulators who support increased production.

Renewable diesel lowers greenhouse gas emissions compared with petroleum-based diesel. The fuel has also been promoted as a way to cut emissions of oxides of nitrogen (NOx), a harmful pollutant that contributes to ozone deterioration and causes respiratory problems.

However, engines made more recently emit more NOx when running on renewable diesel, especially when blended with 35% biodiesel or more, compared with conventional diesel, according to a study released by California Air Resources Board (CARB) in November.

That could affect the way regulators revise the LCFS, which spurred investment in renewable diesel, made from fats and vegetable oils.

CARB said it has "identified several questions about the study results" that require further evaluation, and will be accepting public comment on the study until the end of January.

Companies and regulators had previously purported that renewable diesel reduced NOx emissions by 10%, citing the results of earlier studies that examined the fuel's performance in older engines.

But trucks with newer engines that ran renewable diesel did not meaningfully lower NOx emissions, according to the study. While these new technology diesel engines, or 'NTDE' engines, are present in only 43% of the state’s commercial vehicle registrations, they account for more than 75% of the miles traveled among the state's heavy-duty fleet.

Renewable diesel generally cuts greenhouse gas emissions and other pollutants, said Tristan Brown, associate professor of energy resource economics at SUNY and advisor on New York's Climate Action Council.

Brown noted most biodiesel blending in the United States is 20% or less. "The real question is what amount of NOx is emitted by NTDE engines at volumes of 10% and 20% biodiesel blend levels, and that is not reported by the study," Brown said. READ MORE