BBC NEWS | Science/Nature | Synthetic trees could purify air

Synthetic trees could purify air

By Molly Bentley



The invention is confined to paper so far
A scientist has invented an artificial tree designed to do the job
of plants.
But the synthetic tree proposed by Dr Klaus Lackner does not much
resemble the leafy variety.
"It looks like a goal post with Venetian blinds," said the Columbia
University physicist, referring to his sketch at the annual meeting
of the American Association for the Advancement of Science in
Denver, Colorado.
But the synthetic tree would do the job of a real tree, he said. It
would draw carbon dioxide out of the air, as plants do during
photosynthesis, but retain the carbon and not release oxygen.
If built to scale, according to Dr Lackner, synthetic trees could
help clean up an atmosphere grown heavy with carbon dioxide, the
most abundant gas produced by humans and implicated in climate
warming.
He predicts that one synthetic tree could remove 90,000 tonnes of
CO2 in a year - the emissions equivalent of 15,000 cars.
"You can be a thousand times better than a living tree," he said.
Carbon sinks
For now, the synthetic tree is still a paper idea. But Dr Lackner is
serious about developing a working model. His efforts suggest the
wide net of ideas cast by scientists as they face the challenge of
mitigating climate change.
Dr Lackner believes that carbon sequestration technology must be
part of the long-term solution. Global reliance on fossil fuels
would not decrease any time soon, he said, and developing countries
cannot be expected to wait until alternatives are available.
The technology calls for two things: seizing carbon and then storing
it. Direct capture of CO2, from power plants for example, is the
simplest, according to Dr Lackner. But this doesn't work for all
polluters. A car can't capture and store its carbon dioxide
on-board; the storage tank would be too large.
"It's simply a question of weight," he said. "For every 14 grams of
gasoline you use, you are going to have 44 grams of CO2."
The alternative is to capture emissions from the wind. In this case,
a synthetic tree would act like a filter. An absorbent coating, such
as limewater, on its slats or "leaves" would seize carbon dioxide
and retain the carbon.
Dr Lackner predicts that the biggest expense would be in recycling
the absorber material.
"We have to keep the absorbent surfaces refreshed because they will
very rapidly fill up with carbon dioxide," he said. If an alkaline
solution such as limewater were used, the resulting coat of
limestone would need to be removed.
Dr Lackner is considering other less-alkaline solutions to prevent
carbonate precipitation.
"There are a number of engineering issues which need to be worked
out," he said.
Home use
A synthetic tree could be planted anywhere. A small one could sit
like a TV on the lawn to balance out the CO2 emitted by one person
or family.
But more practically, said Dr Lackner, a device the size of a barn
would sit in the open air, near repositories for easy transportation
and storage of carbon.
He estimated that 250,000 synthetic trees worldwide would be needed
to soak up the 22 billion tonnes of CO2 produced annually.
But not everyone is rooted to the idea. Massachusetts Institute of
Technology engineer Howard Herzog thinks Dr Lackner's design will
not hold together on the scale he proposes.
He said you would expend more energy in capturing the CO2 - in
keeping the slats coated in absorbent and disposing of it - than you
would save.
"Once the solvent captures the CO2, it holds it on tight," said Dr
Herzog, "and it's going to take a lot of energy to break those
bonds."
He said that much more research was needed on the technology.
"The idea of air capture is seductive and would really be great to
have," said Dr Herzog, "but it's important to separate out the
concept from the technical details."
'Early days'
Meanwhile, Dr Lackner is pursuing his idea for carbon storage. While
he was at the US Department of Energy's Los Alamos National
Laboratory, his team worked on a storage method based on a natural
chemical process known as rock weathering.
When CO2 binds with magnesium, it creates carbonate rocks which,
according to Dr Lackner, retain carbon permanently and safely.
Currently, he said, the process is still too expensive to develop on
a large scale.
But Dr Lackner is optimistic that the costs for carbon capture and
storage will come down.
"This is still the early days of climate solutions," he said.