Among green innovations, powering buses with old chicken legs is one of the more inventive. The use of 100% sustainable biodiesel made from tallow (rendered animal fat) and used cooking oil has certainly caught the imagination of passengers on Stagecoach's Stewarton-Darvel service. "We've seen an increase in the number of passengers travelling on the route," says Stagecoach spokesman Steven Stewart. The six-month trial has had another green spin-off: in the first two months, 5000 containers for used cooking oil were issued to households along the route, which could then be taken to an East Ayrshire Council recycling centre in return for money-off bus vouchers. Eight tonnes of oil were collected, giving the whole enterprise a pleasingly self-sustaining nature.

The biodiesel comes from Argent Energy near Motherwell, which was the first large-scale biodiesel plant in the UK when it opened in 2005. Argent produces more than 50 million litres of biodiesel every year, which has the great advantage of helping dispose of a difficult waste product. As well as old cooking oil, it uses tallow, produced at rendering plants from abattoir waste, trimmings from butchers, and supermarket meat that is past its sell-by date.

Argent is proud of its environmental record. And this was precisely what attracted Stagecoach. What it liked about Argent's product was that "it doesn't involve destroying habitats or competing with the human food chain".

Cars can take diesel or petrol blended with small amounts of biofuel without engine modification. But Stagecoach had to fit dual fuel tanks to those buses running on pure biodiesel. Stagecoach calculates that by moving to biofuel, it reduced its carbon dioxide emissions by 69 tonnes in two months.

This sort of product is the lovable face of biofuels and has a big contribution to make to cutting greenhouse gas emissions. But there is another, less acceptable, side which is at the heart of an increasingly bad-tempered debate between government, environmental campaign groups and scientists. It focuses on the fact that, while some biofuels represent an 80% or even 90% decrease in greenhouse gas emissions compared with fossil fuels, others offer no improvement. These arguments will be aired on Thursday, when leading bio-energy experts come together at the Edinburgh International Science Festival (EISF) to discuss bio-energy.

The chorus of disapproval is reaching a crescendo because of the pending introduction of the Renewable Transport Fuels Obligation (RTFO) on April 15 - bringing in quotas for the amount of vehicle petrol and diesel that must come from biofuels (2.5% this year and 5% by 2010). The latest salvo in the debate came this week from Prof Bob Watson, chief scientific adviser at the Department of Environment, Food and Rural Affairs. He said: "If one started to use biofuels and in reality that policy led to an increase in greenhouse gases rather than a decrease, that would obviously be insane."

On the face of it, using biofuels has much to recommend it. Burning fossil fuels adds to the carbon load in the atmosphere by releasing carbon dioxide that would otherwise have remained underground. By contrast, plant-derived oil only releases carbon dioxide that's being cycled in and out of the atmosphere anyway. In 2006, the European Commission said: "Increased use of biofuels will bring numerous benefits, by reducing Europe's dependence on fossil-fuel imports, reducing greenhouse gas emissions, providing new outlets for farmers and opening up economic possibilities in developing countries."

But in the past two years there has been a backlash over the fact that not all biofuels are created equal. Most biodiesel and bioethanol (the petrol replacement) are made from virgin crops. While this can be done sustainably, many developments have been anything but. Critics highlight rainforest destruction in Malaysia and Indonesia to make way for palm oil monocultures, and the diversion of food crops such as wheat and soya, pushing up food prices. The fact that some biofuels, such as some American bioethanol, are so inefficient to produce they're hardly any better than fossil fuels, has brought a note of farce to the debate. Because of all this, Prof Watson believes that introducing the quotas before the effects of doing so were properly understood, would be wrong. He is the latest in a string of government scientists to express such views. Prof Sir David King, the government's former chief scientific officer, believes biofuel quotas should be put on hold until a government review on the economic and environmental impacts of biofuels, commissioned last month, is complete. Other dissenters include the Royal Society, which issued a report in January warning that the RTFO risked failing to deliver significant reductions in greenhouse gas emissions because it did not contain a target to reduce them.

In the wake of these stern voices have come the battle cries of the environmental lobby. A coalition of campaign groups, including Greenpeace, Friends of the Earth, Oxfam and the RSPB, has written to UK Transport Secretary Ruth Kelly urging her to put a moratorium on the quota introduction until strong sustainability criteria are in place.

The lack of mandatory criteria is one of the main criticisms of the RTFO, which is being brought in to comply with the EU biofuels directive. However, such criteria would probably be challenged under World Trade Organisation rules. Instead, the government is introducing a reporting system by which companies must report publicly on where their fuels are coming from, to encourage them to source their fuel sustainably. The government wants to be able to reward biofuels under the RTFO for making carbon savings and applying other sustainability criteria, though these rewards will have to comply with EU and WTO rules. Gordon Brown says he takes the concerns about food security and deforestation seriously and wants to introduce an effective standard.

Dr Jeremy Woods, lecturer in bio-energy at Imperial College London, who will be a panellist at the EISF debate, was on the working group that produced the Royal Society's biofuels report. "Biofuels have real potential to help with greenhouse gas issues and possibly rural development," he says.

"I think people who think the biofuels issue is going to go away through things like moratoria, are missing the point. Economically, the train has left the station. This is already roaring ahead."

He believes the public reporting system is an exciting development. If a company can't guarantee its biofuel crop has been grown sustainably, it will be forced to admit it publicly. Also, the RTFO reporting scheme "embeds a calculation on greenhouse gases and reporting on broader sustainability criteria into policy". He is concerned that if there were a moratorium on the RTFO, the momentum for sustainable biofuels production, would be lost. "I think a delay in this process is very dangerous environmentally. A moratorium, I'm afraid, is messing around and it may destroy our chance to get this methodology in place. What we need is for the RTFO to reward good practice and penalise bad practice."

Christine Raines, professor of plant biology at Essex University and another panellist in Thursday's debate, believes scientific advances may soon affect some of the underlying issues. While food crops are the focus of biofuel production at present, in future, non-food crops may become the mainstay - so-called second-generation biofuels. Bioethanol in future could be derived from cellulose-rich trees and the by-products of food production, such as straw from wheat. "At the moment, we're not good at breaking these things down," she says. "But in future you could manipulate the type of cell wall so that it was more easily degradable." One crop which is the focus of much research is miscanthus, a woody perennial grass - it already has a "superbly high yield", it requires "a very low input of nitrogen fertiliser" and is an efficient user of water "Biofuels are not instantly going to solve energy issues," she says, "though they can contribute significantly. What's really healthy is that we have a debate."


  • A Green Future in Bioenergy? panel discussion takes place on Thursday, 8pm-9pm, at the National Museum of Scotland, Chambers Street, Edinburgh.

For tickets, call 0131 524 9830 or visit sciencefestival.co.uk