To perform their analyses, Maravelias, Vilas Distinguished Achievement and Paul A. Elfers Professor of Chemical and Biological Engineering; and Fasahati, a postdoctoral researcher working with Maravelias, compared two processes for producing biofuels from corn stover: biological conversion to ethanol (BCE) and catalytic conversion to alkenes (CCA). Alkenes are long carbon chains containing at least one double-bond; those produced by the CCA process are used to make diesel.

Their results show that all three parameters – carbon efficiency, process energy requirements, and fuel efficiency – are important, and that the most carbon efficient strategy may not automatically be the most effective strategy overall.

“For example, BCE, which produces ethanol, has very high carbon efficiency, but the fuel itself has lower energy content,” says Fasahati. “With CCA, less of the original carbon ends up in the fuel produced, but the fuel has a higher efficiency when you burn it in a car.”

Fasahati and Maravelias also factored electricity generation into their calculation of production energy efficiency for each strategy. Electricity is a byproduct of converting biomass to fuel in a biorefinery: the waste (mostly lignin and conversion residue) is burned, resulting in heat which powers the conversion process and creates electricity which can be sold to the grid.

Fasahati and Maravelias recognized that when electricity generation was incorporated into their analyses, both strategies had the potential to produce more total mechanical energy for transportation, because the electricity produced could be used to power electric cars while the fuel itself could power traditional internal-combustion vehicles.