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.
To find out how to make the process more efficient, researchers led by Marco Castaldi, at Columbia University, tried varying the atmosphere in the gasifier. They found that, by adding CO2 to 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 CO2 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 CO2 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 437m tonnes of the greenhouse gas. Preventing that entering the atmosphere would equate to taking 308m vehicles off the road.
Replacing 30% of the steam atmosphere of a gasifier with CO2 ensured that all the solid fuel was turned into syngas. Castaldi's process reduces the amount of water that needs to be heated, thereby saving energy, and is 10 to 30% more efficient than standard gasification.
"If I operate at 1,000C and don't use CO2 I'll have some residual carbon left over, which could be a fuel – that's an efficiency penalty," said Castaldi. "Using about 30% CO2, 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%. "While that may not sound like much, for a power plant producing 500 megawatts of energy, it is significant," said Castaldi.