Millions of tonnes of carbon dioxide could be prevented from entering the atmosphere following the discovery of a way to turn coal, grass or municipal waste more efficiently into clean fuels.

Scientists have adapted a process called "gasification" which is already used to clean up dirty materials before they are used to generate electricity or to make renewable fuels. The technique involves heating organic matter to produce a mixture of hydrogen and carbon monoxide, called syngas.

However gasification is very energy-intensive, requiring high-temperature air, steam or oxygen to react with the organic material. Heating this up leads to the release of large amounts of carbon dioxide. In addition, gasification is often inefficient, leaving behind significant amounts of solid waste at the end of the process.

To find out how to make the process more efficient, researchers led by Marco Castaldi, at the department of earth and environmental engineering at Columbia University, tried varying the atmosphere in the gasifier. They found that, by adding CO 2 into the steam atmosphere of a gasifier, significantly more of the biomass or coal was turned into useful syngas.

The technique has a double benefit for the environment: it provides a use for CO 2 that would otherwise escape into the atmosphere and, after the hydrogen is siphoned off from the syngas, the remaining carbon monoxide can be buried safely underground.

Castaldi's results will be published this week in the Journal of Environmental Science & Technology. His team calculated that using CO 2 during gasification of a biomass fuel such as beechgrass, in order to make enough biofuel for a fifth of the world's transport demands, would use up 437m tonnes of the greenhouse gas. Preventing that from entering the atmosphere would be equivalent to removing 308m vehicles from the road.

Replacing 30% of the steam atmosphere of a gasifier with CO 2 ensured that all the solid fuel was turned into syngas. Castaldi's process reduces the amount of water that needs to be heated in the gasifier, thereby saving energy, and is 10 to 30% more efficient than standard gasification.

"You take a solid fuel like a biomass or a coal or even municipal waste and typically what you do is gasify it using steam, air or oxygen," said Castaldi. "In that typical oxidation process, the air reacts very quickly and forms a very recalcitrant carbon char that takes very high temperatures to get converted into gases.

"When you use steam, the problem is that it's not as reactive as oxygen but it's a little too slow." He added: "CO 2 is a little more reactive than steam but not as reactive as oxygen. The CO 2 , as it's converting a solid fuel to a gas, also has the ability to react with the carbon char that is forming."

Working at the same temperature as a normal gasifier, using CO 2 means a better conversion of solid fuel into syngas.

"If I operate at 1,000C and don't use CO 2 I'll have some residual carbon left over, which could be a fuel – that's an efficiency penalty," said Castaldi. "Using about 30% CO 2 , for that same 1,000C you get the complete gasification of the carbon into the syngas."

Applied to a modern IGCC (integrated gasification combined cycle) power station, which gasifies coal, this can lead to an efficiency gain of up to 4%.