This January 20, 2018 story is corrected to reflect that LCFS changes would be in line with carbon neutrality goals. It also adds a note regarding the statistical significance of NOx increases in study.

By Laura Sanicola

(Reuters] – While renewable diesel is often praised as cleaner burning fuel, a study recently found that the fuel does not reduce emissions from truck engines. California regulators should be concerned about this finding.

As the nation’s largest car market, the state has taken aggressive steps to reduce fossil fuel emission from all cars and encourage the production of renewable diesel. This is key in reducing the emissions from hard-to-electrify sources such as trucking.

These efforts form part of California’s Low Carbon Fuel Standard, (LCFS), a regulation that aims to reduce the state’s carbon intensity for transportation fuels.

Renewable diesel has a lower greenhouse gas emission than petroleum-based diesel. Renewable diesel has been promoted for its ability to lower the emission of oxides de nitrogen (NOx), which contributes towards ozone loss and can cause respiratory problems.

A study by California Air Resources Board, (CARB), in November found that engines with more recent production emit higher levels of NOx when using renewable diesel with 35% to more biodiesel.

CARB discovered that these new technology diesel engines (or NTDEs) run on 100% renewable diesel had NOx emission increases which were statistically not significant compared to CARB diesel.

This could impact the manner regulators review the LCFS. It was this that sparked investment in renewable diesel made from vegetable oils and fats.

The state regulators have been looking into changes to LCFS to align them with an update in 2022 on ways to make the state carbon neutral by 2045. According to the study, regulators may have to assess whether renewable diesel raises emissions in poorer areas.

CARB stated that it had “identified many questions regarding the study results”, and would be open to public comments until January.

The request for comments was not answered by regulators.

Heavy-duty trucks are among the biggest contributors of NOx emissions. This is because they act as a precursor for ozone or particulate matter production. The U.S. Environmental Protection Agency reports that NOx emissions have fallen by 60% in the past 20 years thanks to improved emission control technology.

Companies and regulators claimed that renewable diesel reduces NOx emissions 10%. They cited earlier research that looked at the fuel’s performance with older engines.

The study found that trucks equipped with renewable diesel-powered engines didn’t have significant lower NOx emission levels. While these NTDEs 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.

“CARB threw caution to the wind and opened the door to renewable diesel’s unlimited use without having properly studied NOx emissions impact in NTDEs,” said Pat McDuff, chief executive officer at Glendale-based California Fueling, in a public comment submitted in January.

California regulators should reverse the ban on his company selling additives for fuel that reduce NOx emissions from biodiesel, he said.

The state is trying bring 19 states into compliance with the 2015 air quality standards. Two regions, the South Coast and San Joaquin Valley Air Basin, have been targeted by CARB to reduce NOx emissions. A new regulation was adopted by regulators in 2020 to cut NOx emissions at 90% by 2027.

According to Tristan Brown (associate professor of energy resource economics and New York City’s Climate Action Council advisor), renewable diesel reduces greenhouse gas emissions.

Brown pointed out that biodiesel blends in the United States are typically 20% to 20%. Brown explained that it is unclear how much NOx is emitted at NTDE engine volumes of between 10% to 20% for biodiesel blends. The study does not report this information.