Capturing carbon from the air

Materials World magazine
,
1 Jan 2009
Artist's impression of the facility

A facility to absorb carbon dioxide from ambient air could help in the battle against global warming.

Professor David Keith of the University of Calgary, Canada, aims to enable absorption of carbon dioxide emissions from sources such as aircraft and automotives.

Keith says, ‘At first, capturing carbon dioxide from air, where it is at a concentration of 0.04%, seems absurd when we are just starting to do cost-effective capture at power plants where carbon dioxide is at a concentration of more than 10%. But the thermo-dynamics suggest that air capture might only be a bit harder. We are trying to turn that theory into engineering reality’.

The custom-built pilot plant tower has obtained the equivalent of about 20t/yr of CO2 on a single square metre of scrubbing material – the average amount of emissions that one person
produces each year in North America, says Keith.

There are two key stages of the process. First, carbon dioxide is extracted from ambient air in a contactor. Air is fanned over structured chemical process packing, through which sodium hydroxide is flowing. This reacts with carbon dioxide to make sodium carbonate. The sodium hydroxide in this mix is regenerated in a titanate cycle, which is traditionally used to remove caustic in the paper and pulp industry. This generates a stream of pure carbon dioxide for storage in an underground repository, such as an exhausted oil field.

The regeneration cycle requires such high-grade heat that it would have to be powered by gas, coal or nuclear power. However, researchers say that the facility will capture 10 times as much carbon dioxide as would be created during power generation to run the process.

The pilot plant captures carbon dioxide directly from the air using less than 100kW-hours of electricity per tonne of the gas. The team is now working to scale up the process, which could be installed wherever costs are lowest.

The installations are described as having ‘cross-flow slab geometry’. Each would measure around 200m long and 300m high, and there would be several per facility. Keith says, ‘Because this is really heavy duty industrial chemistry, it will only work if it is big. We are working to design systems that would operate in the range of one to 10 mega tonnes of carbon dioxide per year – very big, billions of dollars systems’.

Keith sees designing the regeneration kiln as being the biggest challenge to ensure all the chemical engineering steps work properly, and that it is energy efficient and economically viable.

He says, ‘Our goal is that, in about two to three years from now, we will have an integrated design that gives some sense of the feasibility’.

John Oakey, Head of the Energy Technology Centre at Cranfield University, UK, welcomes the initiative, with the proviso that it proves to be economic. ‘This could be another one of the tools we’ve got. My only concern would be that there might only be niche opportunities for it, where you have the right combination of circumstances to make it economic’.