Combustion of biofuels

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2011

Analytical and computational techniques examine the process of biodiesel combustion pathways.

KEY CONCEPTS
• Biofuel combustion differs from hydrocarbon-based fuels in that oxygenated derivatives such as alcohols, aldehydes, ethers and ketones are formed.
• Biofuel combustion pathways are studied through complementary analytical and computational techniques.
• Different toxic species derived from the oxygen in biofuels are produced during combustion compared to larger amounts of purely hydrocarbon pollutants detected with petroleum-based fuels.

A better understanding of the positive and negative characteristics of biofuels such as biodiesel and ethanol is essential in order to maximize their effectiveness. Biofuels are composed of different organic molecules than their petroleum counterparts. This means there is every expectation they also will exhibit different performance properties.

In a previous TLT article, the inherent tendency of biodiesel derived from soybean oil to oxidize was discussed (1). Soybean oil contains a large concentration of the polyunsaturated fatty acid linoleic acid (approximately 55%). Linoleic acid readily oxidizes, which can have a long-term impact on the viability of biodiesel derived from soybean oil. The use of a proprietary antioxidant at low treat rates has been found to retard the oxidation of biodiesel.

In contrast, biodiesel produced from canola oil has much lower levels of linoleic acid and higher levels of oleic acid, which is more stable than linoleic acid. Canola oil is the largest source of biodiesel fuel in Europe, while soybean oil is the largest source in the United States.

Fuel combustion is an important area because of the growing challenge to reduce emissions. Hydrocarbon-based fuels generate such pollutants as hydrocarbons, carbon monoxide, particulates and NO_x.

With the greater use of biofuels, greater insight is needed to learn more about the combustion pathways of these fuels and how they differ from hydrocarbons. A discussion was held with Drs. Nils Hansen, principal member, technical staff at Sandia National Laboratories in Livermore, Calif., and Charles Westbrook, physicist at Lawrence Livermore National Laboratory in Livermore, Calif. Both individuals are actively involved in learning more about the combustion pathways of biofuels and are co-authors of a recent review (2).

OXYGEN IS THE DIFFERENCE
Westbrook indicates that the biggest difference between the combustion of hydrocarbon- based fuels and biofuels is the presence of oxygen inside the actual fuel molecules. He says, “Biofuels are derived from natural materials that are mainly based on ester chemistry. They differ from hydrocarbons in that the biofuels will undertake combustion to form other oxygenated derivatives such as alcohols, aldehydes, ethers and ketones (see Figure 3).”


Figure 3. The presence of oxygenated species in biofuels leads to different combustion pathways than are seen with hydrocarbons. (Courtesy of Sandia National Laboratories)

The researchers have taken two different approaches to understanding biofuel combustion that are complementary. Hansen says, “Analysis of general combustion is very difficult due to the active chemistry and fluid dynamics that is occurring. We chose an experimental approach in which the combustion process is simplified by premixing known biofuels and even simpler organic molecules with the oxidizer before the flame front.”

Two major analytical techniques were used by Hansen to better understand the species formed during combustion. He says, “We used laser diagnostics and mass spectrometry to better understand the molecules that are formed during biofuel combustion and how they differ from molecules formed during combustion of conventional diesel fuels.”

Westbrook has taken a computational approach through the use of simulations to analyze combustion pathways. He says, “We compare our results with experiments run by Hansen and use kinetic chemistry to better understand how the chemical composition is changing in the laboratory- based model flames. Then we will pass our results and questions back to Hansen so that he can design additional experiments.”

Biodiesel is an interesting case because its combustion pathways are relatively simple. Westbrook says, “Biodiesel consists of five or six different methyl ester species ranging in size from C14 to C25 which undergo combustion to eventually produce carbon dioxide and water. Simulation studies with specific methyl ester chains have been very helpful in determining the combustion pathway for biodiesel.”

One of the reasons for this finding is that the molecules in biodiesel are entirely straight-chain organics. There are no aromatics present in biodiesel. In contrast, combustion of petroleum-derived diesel is much more complex. Diesel contains not only straight-chain and branched hydrocarbons, but also aromatics.

Hansen says, “Diesel contains thousands of chemical species, and its combustion pathway is much more complicated. The variability of diesel also does not make it any easier to determine the mechanism of the combustion process. To make matters even more difficult, we can go down to the local fuel station and the composition of the diesel fuel will vary on a daily basis. Analysis of diesel is very much similar to shooting at a moving target.”

Another challenge facing the researchers is how to evaluate the combustion pathways of blends of petroleum-derived fuels with biofuels. One example is the use of up to 15% biodiesel in petroleum diesel.

Westbrook comments, “The oxygen that is present in biodiesel fuel actually reduces some toxic emissions from diesel engines. For example, we know that when oxygen is added to diesel fuel, less soot is produced in diesel combustion, so oxygenated diesel fuels reduce soot emissions.”

The combustion of biofuels does not eliminate the formation of toxic species. Westbrook says, “With biofuels, significant levels of oxygenated toxic species are generated during combustion that are derived from oxygen in the fuel. Examples include aldehydes such as formaldehyde and acetaldehyde, as well as other species such as methanol and methyl ethyl ketone. Petroleum-based fuels produce smaller levels of these toxic oxygenated species and larger amounts of purely hydrocarbons pollutants such as 1,3-butadiene.”

One other factor under consideration with biofuels is the presence in diesel fuel of nitrogen-containing species. The researchers indicate that nitrogen species must be examined because they will generate NO_x during combustion.

Future work for the researchers will involve looking further at butanol and biodiesel. Hansen says, “Butanol has become a more popular biofuel due to the potential for algae and bacteria to produce it in large quantities.”

Westbrook will be looking at developing better models for the combustion of biodiesel. He says, “Biodiesel can be produced from such different feedstocks as palm oil and beef tallow oil. We are now looking to use computer simulations to determine how a specific raw material will affect the combustion of biodiesel.”

Additional information on the work being done to examine the combustion of biofuels can be obtained by contacting mike Janes of Sandia National Laboratories at mejanes@sandia.gov. 

REFERENCES
1. Canter, N. (2009),“examining Biodiesel oxidation,”TLT, 65 (3), pp. 18–19.
2. Hoinghaus, K., Oswald P., Cool, T., Kasper, T., Hansen, N., Qi, F., Westbrook, C. and Westmoreland, P. (2010),“Biofuel Combustion Chemistry: From Ethanol to Biodiesel,” Angewandte Chemie International Edition, 49 (21), pp.3572–3597.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.