Combustion inside diesel engine cylinder

Up in smoke - solutions to burning biofuels

New information on how biofuels burn is paving the way to greener transport fuels, reports E&T.

While biofuels such as bio-ethanol and bio-diesel can offer a clean alternative to petroleum-based transport fuels, anxieties on risks to biodiversity or depleted food supplies have tarnished their green image. But amid the very public debate, the combustion chemistry of biofuels has been forgotten.

What happens when these fuels burn, what pollutants are formed and are the fuels as environmentally-friendly as we think? The answers to such fundamental questions have been long known for hydrocarbon fuels, but when it comes to any biofuel, the level of understanding is a lot less. And this lack of knowledge threatens to slow industry take-up.

'With hydrocarbon fuels, the transport community can, for example, design new engines based on models that use computation fluid dynamics and combustion chemistry,' says Professor Philip Westmoreland, executive director of the North Carolina State University for Computational Science and Engineering, US. 'Colleagues at [US conglomerate] United Technologies told me they used to have to make 50 jet engine [prototypes] for every new engine they developed as each time they wanted to make a change they had to build a new engine. Today, they do much more modelling and take about two attempts to get it right. You can't do this with biofuels yet.'

So why not? Compared to a petroleum-based fuel, the combustion chemistry of a biofuel is diverse and more complex. While traditional fuels have been studied for decades using so-called molecular beam mass spectrometry (MBMS), the complex biofuel requires more powerful analytical techniques.

This situation is changing thanks to a method pioneered by Westmoreland and colleagues eight years ago. Photoionisation MBMS makes use of a synchrotron light source to produce bright white light, filtered to specific energies, to study flames and identify chemicals much more effectively. Worldwide the technique has provided precious information on the emissions and pollutants that form when biofuels burn.

Ethanol, the most widely used biofuel to date, will produce more of the hazardous air pollutant formaldehyde than the equivalent amount of hydrocarbon, when burned under certain conditions. In contrast, adding ethanol to a fuel can reduce particulate emissions (see 'Sweet alternative?').

Additional studies have revealed that some biofuels release up to five times more nitrogen oxide when burned than previously thought. Rapeseed biofuel, which represents more than 80 per cent of worldwide biodiesel, has a particularly high nitrogen content due to heavy use of nitrogen fertilisers. In contrast, maize and sugar cane, widely used in bio-ethanol production, have less than half the nitrogen content of rapeseed oil.

Know your biofuel

While such a discovery sounds alarming given the global warming potential of nitrogen oxide is almost 300 times higher than carbon dioxide, Westmoreland is unruffled. Admitting the scientific community was 'caught a little bit by surprise', he asserts the information has first indicated that the nitrogen content of a biomass should be very relevant when selecting a biofuel source, and second, that using a biofuel without making the necessary technological adaptations to an engine could cause undesirable consequences.

'You can seldom fix something unless you know how it's happening,' he adds. 'I think there is a tendency among the general public to want to turn to something else when problems arise in new technologies. The reality is for engineers, things can be fixed rather than discarded.'

So given the recent insights into the combustion chemistries and pollutants formed when burning different biofuels, is Westmoreland in a position to advise on which biofuel is best to reduce greenhouse gas emissions? No. While the US professor asserts that understanding the key elements of a biofuel's combustion chemistry is an important step toward the 'intelligent selection' of next-generation alternative fuels, no fuel is best.

'Our research provides a means [of identifying] which fuels will perform better in certain types of systems. Some perform better in gasoline engines and others in diesel-type engines,' says Westmoreland.

'Right now ethanol is prominent as we have the technologies to make it and understand how to use it with our existing fuel distribution infrastructure. However, you're not going to get a final recommendation of 'place all your bets on this fuel', which many people would love to know.'

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