Adding oxygen to the mix

Oxygen can be both a problem and a solution for biofuels. Whereas the hydrocarbons that make up fossil fuels are just that – compounds containing hydrogen and carbon – plant biomass also contains oxygen, which tends to make its way into any biofuels derived from that biomass. 

Too much oxygen causes a problem, ruining the fuel properties of the biofuel and preventing it from being blended with fossil fuels. This is the case with the bio-oil produced by heating biomass in the absence of oxygen, requiring it to be catalytically upgraded to remove the excess oxygen. But just a little bit of oxygen can actually prove beneficial, because if there’s one thing oxygen is good at it’s combustion, and so it can improve the combustion properties of the biofuel and of any fossil fuels it is blended with. This causes the fuel to burn more efficiently and cleanly, reducing the production of pollutants such as soot.

The trick is to find those biofuels with just the right level of oxygen, and that is one of the aims of a new initiative from the US Department of Energy. Termed Co-Optimization of Fuels and Engines (Co-Optima), this initiative, which began in May 2018, is designed to develop more efficient, low-emission fuels and engines, including by finding biofuels with just the right level of oxygen.

One way to do this is to design novel biofuels with just the right level and that is the approach taken by a team led by Derek Vardon from the US National Renewable Energy Laboratory (NREL). Specifically, Vardon and his team were looking for a biofuel that could improve the combustion properties of diesel when blended with it.

"If you look at biomass, 30% of it is oxygen," said Vardon. "If we can figure out clever ways to keep it around and tailor how it's incorporated in the fuel, you can get a lot more out of biomass and improve the performance of diesel fuel."

Vardon and his team decided to focus on ethers, a class of oxygen-containing organic molecules, derived from carboxylic acids, which are naturally produced by several microbes from the fermentation of lignocellulosic sugars. As reported in a paper in the Proceedings of the National Academy of Sciences, they first used computational models to screen a whole load of candidate ethers to find the one with the best mix of properties for blending with diesel. This led them to an ether molecule called 4-butoxyheptane.

Next, they developed a simple catalytic process for producing this ether molecule from carboxylic acids and then experimentally tested it in blends with diesel. The results were impressive: compared with pure diesel, a diesel blend containing 20% 4-butoxyheptane showed a 10% improvement in ignition quality and a 20% reduction in soot production.

An alternative way is to screen existing biofuels to find those with just the right level of oxygen, which is the approach taken by Subith Vasu and his colleagues at the University of Central Florida. Other researchers working as part of the Co-Optima initiative have already performed the first step in this process, by cutting an initial list of 10,000 possible biofuels down to a shortlist of just 10 based on an initial analysis of their properties. This shortlist comprises several alcohols, the olefin di-isobutylene, the ketone cyclopentanone and methyl furan.

Vasu and his colleagues then conducted a more detailed study of the soot-producing properties of five of these compounds: ethanol, methyl acetate, di-isobutylene, cyclopentanone and methyl furan. They did this by conducting controlled combustion experiments in a shock tube, which works by firing a high-pressure gas at the fuel sample at high temperatures and pressures, and measuring the soot produced with laser absorption spectroscopy. As they report in another paper in the Proceedings of the National Academy of Sciences, they did this for blends of the five biofuels with ethylene, as a simplified hydrocarbon fuel.

This revealed that, compared with ethylene on its own, ethanol, cyclopentanone and methyl acetate all reduced soot production, with ethanol producing the greatest reductions. In contrast, diisobutylene and methyl furan actually increased soot production. So while scientists continue to  search for better biofuels, it’s good to know that we’re already benefitting from the combustion-enhancing properties of ethanol.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.