Report: Biodiesel Cuts Some Emissions, Raises Others

A 20% biofuel blend can reduce a diesel engine's smoke and regulated emissions. But it also increases the output of toxic but unregulated substances such as formaldehyde, according to a study from the University of Michigan.

The U-M team says its findings show that biofuels can alter the composition and sometimes significantly raise the toxicity of diesel exhaust.

The group also found significant differences depending on engine loading and whether a catalytic converter is used. The team reports its findings in the current issue of Energy & Fuels.

The researchers looked at emissions during idle and load conditions for two engines: a 2002 vintage 1.7-liter Isuzu diesel for passenger cars and a 2008 6.4-liter Ford diesel for medium-duty trucks. Neither engine uses modern "clean" diesel technology, which might produce different results.

The UM team tested each engine with U.S. standard ultra-low-sulfur diesel fuel and with B20, a blend of the same fuel and 20% soy methyl ester biodiesel.

Burning B20 in the small engine under load reduced emissions of particulates, elemental carbon, hydrocarbons, formaldehyde and most volatile organic compounds. But the researchers say adding a catalytic converter to the small engine resulted in higher HC, oxides of nitrogen, formaldehyde, naphthalene and other VOC emissions.

In the large engine under load, using B20 reduced most emissions but increased NOx.

At idle, burning B20 boosted emissions from both engines of hydrocarbons, particulates, elemental carbon, formaldehyde, benzene and other VOCs. Particulates zoomed 60% for the large engine, and benzene emissions jumped 40% for the small engine, the researchers say. They speculate that the increases may be a result of incomplete combustion.