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Burning Biofuels

The biofuels industry has had to confront some baggage lately. Crops for fuels might require land that would otherwise be used to grow food, and take up precious water and petroleum-derived fertilizers. When biomass is burned in order to convert, say tropical forest into a biofuel crop, such as oil palm, the CO2 balance can get out of hand.

Corn cobs to be used for cellulosic ethanol production. Credit: Poet

Nevertheless, optimal examples of biofuels such as cellulosic ethanol (especially when derived from agricultural wastes) and high energy biobutanol are generally considered to have large climate advantages over petroleum fuels.

Now a research team led by Katharina Kohse-Höinghaus, of the Department of Chemistry, Bielefeld University in Germany, have published a review paper in Angewandte Chemie

to explore what is known about what happens when we burn biofuels.

Of course there is no such thing as a free lunch – when we burn biofuels in a car we are still engaging in combustion, and the products of that combustion — other than heat and getting the car moving — are going to go into the air. Common biofuels are oxygenated, which suggests they will burn differently compared to fossil fuels. Kohse-Höinghaus points out that 1) we should  study what the products of biofuels combustion (and resulting pollution) really are and 2) we should use this knowledge along with other more well-understood criteria to decide which biofuels should receive policy support.

The authors review the available research and provide some broad-brush insights (in addition to many specifics). Biofuels can produce polycyclic aromatic hydrocarbons (PAHs) and soot, just like conventional hydrocarbons, though often in lower amounts. But that benefit is paid for by increased output of carbonyl compounds, including formaldehyde, acetaldehyde, acetone, and higher aldehydes and ketones.

A  wild card for biofuel combustion is possible emissions of NOx molecules, depending on how much nitrogen is present in the biofuel. Another unknown is what happens with mixtures of fossil fuels and biofuels, such as petroleum diesel and biodiesel.

Recently, the controversial concept of indirect land use was applied to the renewable fuels standard in the U.S. (now in its second edition). Perhaps when RFS3 is being debated, we’ll have a firmer grasp on which biofuels burn cleanest.

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