New Questions about Toxic By-Products of Biofuel Combustion

by Alyson Kenward, Climate Central

The use of biofuels to supplement gasoline is on the rise in the US, thanks in part to US EPA guidelines that promote the biofuel content of transport fuels — especially from corn and cellulosic ethanol. The increasing use of biofuels has come under close scrutiny in recent years from researchers who say these alternatives don’t provide the environmental benefits of displacing fossil fuel use, thereby reducing emissions of greenhouse gases such as carbon dioxide (CO2).

Now scientists are raising another concern about the surge in biofuel consumption, this time centering on how each type of biofuel — from liquid ethanol to solid biomass — breaks down while burning.

Biofuel combustion processes are not well understood, and researchers are trying to determine how toxins released during combustion compare to those coming from fossil fuel burning.

In the May 10 issue of the German journal Angewandte Chemie, chemists from Sandia and Lawrence Livermore National Labs in Livermore, CA, along with German and Chinese collaborators, summarize a series of recent studies examining what exactly is coming out a biofuel tailpipe. They found that while biofuel combustion produces many of the same chemicals released during fossil fuel burning, it also generates a complicated mixture of additional chemicals that are potentially harmful to humans and the environment.

Since every biofuel has a unique chemical makeup, each one will give off a different combination of combustion products. In order to better understand which crops will make the best and safest choice for large-scale deployment, researchers have been trying to track the combustion pathways of them all. “Intimate knowledge of the chemical reaction network involved is a prerequisite to determin[ing] the value of a biofuel with respect to emissions,” the study states.

Identifying the products of biofuel combustion helps analysts assemble another piece of the complicated puzzle of how alternative fuels should best be incorporated into our energy supply. Yes, it appears that a car run on a blend of biofuels is going to emit less soot and fewer harmful particulates than a vehicle burning pure gasoline or diesel. But the alternative fuels have their own emissions signatures, each with their own implications for human health and climate change.

Biofuels, such as ethanol, contain oxygen in addition to the hydrocarbon core found in traditional fossil fuels. So, while gasoline and ethanol combustion both give off energy by tearing apart carbon-hydrogen bonds, biofuels also generate a number of other combustion products that gasoline and diesel don’t. Furthermore, nitrogen and phosphorus from fertilizers, which are used to grow biofuel crops, can remain in biofuels. The study found that the presence of these chemicals introduce an even broader spectrum of possible chemicals into the burning process.

For example, burning corn ethanol — currently the most widely used biofuel in North America — produces CO2 and small quantities of carbon monoxide, soot and other so-called “particulates,” which are also given off by fossil fuel combustion. According to recent research the amount of these chemicals coming from burning ethanol is less than from fossil fuels.

On the other hand, the presence of oxygen in ethanol opens a pathway for a myriad other combustion products, including formaldehyde and acetaldehyde. If inhaled in small quantities, these chemicals can irritate the eyes and lungs, whereas more significant exposure to these and other particulates is associated with asthma, allergies and even some cancers.

In the case of heavier biodiesel made from vegetable and soybean oils, the higher oxygen content and residual nitrogen from fertilizers further increases the complexity of combustion products. The study notes that burning biodiesel produces less of the noxious particulates associated with fossil fuels, but any advantage is lost because it also generates a mix of other toxins that don’t form from burning pure petroleum.

It remains to be seen how these new factors will be considered alongside other biofuels policy considerations — such as how affordable they are, which types offer a true carbon advantage, and how much agricultural land will be sacrificed to keep our cars running — but they should help inform which of the many alternative fuel options is going to be the safest.

(Republished with permission from Climate Central)