Did you know that petrol and diesel now contain a minimum of 2.5% biofuels? Thanks to the Renewable Transport Fuels Obligation, this requirement will rise to 5% by 2010. While motorists won't notice any difference when filling up, this important change is expected to prevent the emission of millions of tonnes of carbon dioxide over the next few years.
But might biofuels be helping to solve one major environmental issue simply to create another? For Valerie Dupont, of Leeds University, the increasing use of biofuels means a sticky problem. For every tonne of biodiesel made from vegetable oil, 100 kilograms of thick, viscous glycerol is produced as a byproduct. The annual 6.8bn litres biodiesel production in the European Union yields around 0.68m tonnes of crude glycerol. Although some of the sweet-tasting liquid can be purified for pharmaceutical or food applications, the rest ends up as waste.
Lake of sludge
Dupont, who has a PhD in fuel and energy, now hopes to turn this growing lake of low-grade sludge into high-value hydrogen gas. Produced from vegetable oils and methanol, biodiesel is a renewable alternative to ordinary diesel. But what green-minded motorists don't realise is that glycerol is creating a big problem.
"Glycerol is thick, viscous, full of oxygen and you cannot burn it easily," says Dupont. "Nobody knows what to do with all this glycerol from biodiesel. There is no real outlet."
Most waste glycerol is currently disposed of by incineration, a less than ideal arrangement. Burning the glycerol in a power station might seem an option but, says Dupont, poor energy conversion and inefficient combustion produces pollutants.
Glycerol - C3H5(OH)3 - is a molecule of three carbon atoms with eight hydrogen and three oxygen atoms. Unlock the hydrogen, and you'd have a rich source of fuel from renewable resources. At the moment, the world's hydrogen mostly comes from the steam reforming of natural gas - methane (CH4) - which produces hydrogen and carbon monoxide.
"Since glycerol has a high hydrogen content compared to methane, we reckon that converting crude glycerol to hydrogen is a valid alternative route," says Dupont.
Based on earlier research work, Dupont and her co-investigators are developing a viable process to release pure hydrogen (H2) and carbon dioxide (CO2) from glycerol.
The 18-month £270,000 Engineering and Physical Sciences Research Council project involves mixing glycerol with steam over a catalyst at a controlled temperature and pressure. A reusable CO2 adsorbent ensures the carbon monoxide (CO) produced reacts fully with the steam, making even more hydrogen and CO2.
"Our process is a clean, renewable alternative to conventional methods. It produces something with high value from a low grade by-product," says Dupont. "In addition, it's a near carbon-neutral process, since the CO2 generated is not derived from the use of fossil fuels." The project is using a prototype chemical reactor which will quickly answer many practical questions including the effects of impurities. Dupont is also taking a green engineering approach, aiming for a high-purity hydrogen product that would be ideal for fuel cells.
"If everything goes well, we can look at scaling up and maybe even scaling down," says Dupont. "If we had a reactor which could extract the hydrogen from glycerol it would be very interesting for distributed power generation."
While hydrogen and fuel cells go nicely together, the gas is already heavily used for fertilisers, chemical plants and food production. However, making hydrogen using natural gas or even water electrolysis is expensive and unsustainable. Finding a new source makes sense.
"Hydrogen has been identified as a key future fuel for low carbon energy systems such as power generation in fuel cells and as a transport fuel," says Dupont.
Professor Graham Hutchings of Cardiff University has other possibilities in mind. He's working on a government-funded research project involving Imperial College and Cambridge University to find different uses for waste glycerol.
"There is a glycerol problem, so people are looking for opportunities to do anything other than burn it. Turning it into hydrogen is a neat idea," says Hutchings.
His project is therefore seeking "high tonnage" answers by turning glycerol into valuable monomers for plastics production, biodegradable solvents and even fragrances. There are several research threads, currently confidential, being worked on.
"We're looking for things that have real application," says Hutchings. This could include biodegradable polymers for plastic bags or perhaps solvents for paints. If such uses are developed, the glycerol glut might well become a welcome bonus.