Derivative approaches to fuelling glycerol use

The ideal way to deal with the glut of glycerol generated because it’s the main byproduct of biodiesel production, amounting to around 1.25 million tonnes each year, would be to use it as a fuel. Although scientists have investigated several ways for doing this, such as converting it into syngas or ethanol (see Busting the glut), these approaches have so far not proved efficient enough to be economically viable. Not to be dissuaded, two research groups have now investigated a couple of new approaches to this dilemma.

One of these approaches involves utilizing glycerol as a fuel additive rather than as a fuel, because glycerol is an oxygen-rich compound and thus should help fuels such as biodiesel to burn cleaner, reducing sooty emissions. When this has been tried, however, glycerol hasn’t proved particularly effective, due to its high viscosity, high melting point and high auto-ignition temperature. So a team of scientists and engineers from Hungary and Saudi Arabia, led by Aamir Farooq at the King Abdullah University of Science and Technology (KAUST), decided to investigate whether glycerol carbonate, a derivative of glycerol, might fare better.

Glycerol carbonate is just as oxygen-rich as as glycerol, but it is less viscous and has a lower melting point, although it has an even higher auto-ignition temperature. Glycerol can be converted into glycerol carbonate at high yields by simply reacting it with dimethyl carbonate over a zeolite catalyst. To determine its potential as a fuel additive, Farooq and his team mathematically modelled how it would burn in an engine.

As they report in a paper in Sustainable Energy & Fuels, this modelling revealed that, at the high temperatures in a vehicle engine, glycerol carbonate decomposes almost completely to carbon dioxide and 3-hydroxypropanal. This is a promising finding, because aldehydes such as 3-hydroxypropanal are known to be very effective at reducing soot production.

“Our results show that glycerol carbonate has great potential to promote cleaner combustion as a fuel additive,” concludes team member Binod Giri, a research scientist at KAUST.

Rather than use glycerol in an engine, Michael Stockenhuber and his colleagues at the University of Newcastle in Australia investigated using it to generate electricity in a solid oxide fuel cell (SOFC). This kind of fuel cell can work with hydrocarbons such as methane and ethanol, which are broken down at the anode to release electrons that flow around an external circuit to the cathode, generating electricity. But no one had tested whether a SOFC would work with glycerol as well.

In addition to glycerol, Stockenhuber and his colleagues also tested two derivatives of glycerol, acrolein and allyl alcohol, which can be produced by heating glycerol over a catalyst. These tests were conducted on a SOCF with an anode made from a nickel alloy that doesn’t tend to become coated with the carbon compounds produced by the breakdown of hydrocarbons.

As Stockenhuber and his colleagues report in a paper in Energy Technology, allyl alcohol performed better than acrolein, both in terms of generating a larger current and not deactivating the nickel alloy anode. Glycerol also worked well, generating electricity for over 90 hours. However, when Stockenhuber and his colleagues added salts, which are common contaminants in the glycerol generated as a biodiesel byproduct, they tended to deactivate the anode. This was due to the salts reacting with the surface of the anode to form products such as nickel chloride.

So in both novel approaches, specific glycerol derivatives seem to be a better bet than glycerol itself. But the effect on the glut should, hopefully, be exactly the same.

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.