By Paul Rincon
BBC News Online science staff
Engineers are trying to build a system to remove the greenhouse gas carbon dioxide from the atmosphere in an effort to reduce climate warming.
Power plants will rely on fossil fuels for some time to come
Two companies based in Tucson, Arizona, in the US are involved in the project, which aims to complete the first phase of a working unit by early 2005.
Scientists have discussed so-called "wind-scrubbers" in principle but this is the first attempt to build one.
Details of the research are published in Chemistry & Industry magazine.
Allen Wright of Global Research Technologies and his brother Burton Wright of Kelly, Wright Associates decided to use the combined expertise of their firms to build a structure that can capture and store carbon dioxide.
The system would process large volumes of air with low carbon dioxide (CO2) concentrations.
Unlike current systems to capture plant flue gas from power stations - which remove CO2 at high concentrations - the wind scrubbers would not need to be located near the source of the emissions.
Some researchers believe the principle could be a key element of future frameworks for reducing atmospheric concentrations of the greenhouse gas, mitigating the effects of climate change.
The project is still in its early stages, however, and the design of the scrubber has yet to be formalised. They have specified only that the structure should present a 10-sq-metre area to the wind.
Proof of concept
"The main purpose of the project is to demonstrate that it's technically and economically feasible to do this," Allen Wright told BBC News Online.
He added that the stored CO2 could be supplied to the oil industry for use in the process of enhanced oil recovery.
In this process, CO2 is pumped into depleted oil wells where it acts as a solvent, dissolving residual oil into a solution. This solution is then removed at the well head and the CO2 separated off.
"While that is not a solution to all global CO2 emissions, it is enough to warrant development of the industry to get this technology going," Allen added.
Some say scrubbers could contribute to an overall reduction in CO2
The systems might also find a commercial application with companies involved in emissions trading schemes.
The system will consist of a filter assembly that draws air through it and into contact with a hydroxide solution - probably sodium hydroxide - to remove the CO2.
The hydroxide reacts strongly with carbon dioxide producing carbonate. However, large amounts of energy are needed to sequester CO2 from carbonate, so a commercially viable version of the technology might need to be based upon another reaction.
But, said Allen Wright, "if we can prove it's economically viable using that process, things can only get better".
Not everyone is so enthusiastic about the idea, though.
"I don't think it will work," chemical engineer Howard J Herzog, of the Massachusetts Institute of Technology, US, told BBC News Online.
He explained that his own calculations - based on ideas for the project he'd seen so far - suggested the process would not break even in terms of its energy cost.
"You have to put more energy in to clean up the CO2 than you do to make it in the first place," said Mr Herzog. He added that one of the crucial issues was maximising contact between the airstream and the absorbent - in this case the hydroxide.
The team behind the project regard this as a key challenge, but not an insurmountable one. They said they needed to determine the best way to take advantage of natural wind and convection to drive air through the filter.
"It could have plates like a Venetian blind, or spines. Alternatively, it could be a screen with different-sized holes or with shapes cut into it," said Donovan Kelly of Kelly, Wright Associates.
Mr Kelly added that volumes of the hydroxide could also be different at separate levels in the system.
"The chemistry and the theory involved in this project show it can be done. It is obviously carried out at point emission control in power plants. We're just trying to do it at lower concentrations," Burton Wright explained.
Some place faith in the technology because global reliance on fossil fuels is unlikely to decrease in the near future. Allen Wright said he was strongly in favour of developing alternative energy sources.
But in the meantime wind scrubbers would be a more economically viable alternative to retro-fitting old power stations with systems to capture flue gas, he added.