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Climate Change

In this section of the site we'll look at three aspects of climate change:
  • where it's heading
  • answers to questions
  • opportunities in technology

We'll build these up into three separate sections. First today, some promising technology for the future.

Climate change technology air capture of carbon dioxide

It's becoming increasingly clear that we're not going to be able to cut down the flow of greenhouse gases into the atmosphere in the near future that would require greater political will and international cooperation than we've yet seen.

So the focus is likely to grow on the challenge of getting the greenhouse gases back out again.

Trees do it, in an ideal way, capturing carbon dioxide and turning it into oxygen which we breathe.

Can we find mechanical or chemical ways of 'scrubbing' carbon dioxide out of the air?

An article in the 12 January issue of the magazine New Scientist entitled 'Can technology clear the air?' looks at some ideas that are showing promise.

Absorbing carbon dioxide

There's two orthodox chemical processes for absorbing carbon dioxide. One is to use sodium hydroxide solution (caustic soda).

Sodium hydroxide is an alkali, and carbon dioxide is an acid gas, and the two react to form sodium carbonate (washing soda).

A second process is to use calcium oxide. It combines with carbon dioxide to form calcium carbonate. The reaction needs some heat to drive it.

One of the people working with a sodium hydroxide absorber is Klaus Lackner of Columbia University in New York City. He and his colleague Allen Wright use an ion exchange resin a polymer impregnated with sodium hydroxide. The plastic is set up in vertical sheets, the air blows through it, and the gas gets absorbed in the resin.

The sodium hydroxide in the resin has the sodium ions at one end firmly attached to the polymer, and the hydroxide ions in a looser state, so that they are easily displaced by carbon dioxide. It binds to the sodium to produce sodium bicarbonate the familiar bicarbonate of soda.

The dry resin has this affinity for CO2, so freely absorbs it. But when wet, the attraction for CO2 diminishes. So when the resin has absorbed as much of the atmospheric gas as it can, moisture is added, and it releases the gas which can then be collected.

The two researchers have been granted a patent on the process and say that with their small laboratory model they're collecting a few tens of kilograms of CO2 a day. With two years' work and around $20 million of venture capital, they say, they could build a device that would remove a tonne a day and fit into a standard shipping container.

Caustic at Calgary

At the University of Calgary in Alberta, Canada, they also use sodium hydroxide as an absorbent. 

The sodium hydroxide is sprayed in a fine shower onto air that's blown in to the top of a 4-metre-high cylinder of heavy-duty cardboard lined with PVC. The CO2 in the air gets absorbed into the spray and reacts, and the droplets of sodium carbonate sink down to the bottom of the cylinder.

The Calgary team of David Keith and his student Joshuah Stolaroff have drawn up an outline of how an industrial version of this process would look.

It would be like an aircraft hangar, says the New Scientist article 'with fans at one end blowing air through a mist of sodium hydroxide from nozzles in the ceiling. Drains in the floor would collect the sodium carbonate solution.'

A snag with a scrubber

There is a snag with ths process. It takes a great deal of energy.  To get the carbon dioxide out of the sodium carbonate, and recycle it back to caustic soda, requires the use of a kiln heated to 900 degrees C.

David Keith says that he thinks he can get energy costs down to $100 per tonne of CO2, which is several times more than what polluters currently pay under the European Union emissions control scheme. But he says, 'You still might end up doing it because you get into crisis mode.'

In other words, we may yet get to the stage where we simply have to get the carbon dioxide out of the atmosphere, with the main criterion being the output of the process rather than the cost.

A solar option

Meanwhile at the Swiss Federal Institute of Technology (ETH) in Zurich, they're using calcium oxide as the absorber and a solar energy source. The method of getting solar power is based on an existing system tracking the sun with a field of mirrors and focusing the rays on a target.

The ETH target for the sunlight is a transparent tube filled with pellets of calcium oxide. The solar rays heat tube and contents to 400 degrees C. Air mixed with a small amount of steam is pumped in at the bottom and rises up through the hot pellets. At this temperature, the calcium oxide reactc with the CO2 in the air to form calcium carbonate.

'By the time the air leaves, there is no CO2,' says solar researcher Aldo Steinfeld of ETH. 'We go from 385 parts per million to practically zero.'

The conversion process takes less than 15 minutes. The operator then clsoes the intake valve and inttensifies the light to push the temperature to 800 degrees C. This drives off the CO2 as a stream of pure gas, which can be taken away. And then we are back with the pellets in calcium oxide form ready to go round the cycle again.

They've got the system working and the next step is to scale it up, but they're not yet at the stage of knowing the cost per tonne.

Carbon into cash

So there's three options for new technology here. Absorb the CO2 in a a sodium hydroxide resin at Columbia University in New York City.  Or absorb the gas in a sodium hydroxide spray at Calgary. Or react it with sun-heated calcium oxide pellets at ETH.

The New Scientist article compares the three. What makes the article particularly interesting is the identity of one of the authors. He is Wallace Broecker, one of the world's leading climate scientists. So he knows all too well the scale of the global problem and the importance of finding solutions. His co-author is writer Robert Kunzig.

They like the look of the resin process that Klaus Lackner has developed at Columbia. One great advantage, they point out, is that it doesn't need much energy for recycling. Just 40 degrees will do it to release the CO2 from the resin, as against temperatures of 800 and 900 degress for the other two.

They note that Lackner hasn't yet got a cost figure, but they say that he thinks it will be cheap enough to open up commercial opportunities for his technology.

'Fruit and vegetable growers routinely enrich the air in their greenhouses with extra CO2 and can pay as much as $300 a tonne for the stuff. Lackner reckons an air scrubber attached directly to the greenhouse could beat that price.'

It turns out that he has already built a demonstration model for this, a mini-greenhouse around a metre long.

'Attached to one end is a plastic tube containing the ion exchange resin, which absorbs CO2 from the air. When most of the sodium hydroxide in the resin has been converted to sodium bicarbonate, Lackner evacuates the tube and then allows it to refill with moist air from the greenhouse. This releases the CO2, which can then be flushed out into the greenhouse. The device generates around a kilogram of CO2 per day, which is converted into biomass by tomato plants inside the greenhouse.'

The article notes that another possible use for the CO2 is for the oil industry, to flush oil out of ageing fields.

'Lackner reckons that an air scrubber capable of capturing a tonne of CO2 a day could become commercially viable both for this market and for horticulture, even in the absence of government caps on carbon emisssions.'

And if that happens, the writers say, another market would open up, which could be vast: combining air-captured CO2 with hydrogen to make methanol, a fuel for motor engines.

Some possibilities

This technology has some interesting possibilities for the north of Scotland. It needs energy to drive it and we're going at some stage in the future to be producing a big surplus of energy from renewable sources, which will require expensive cabling for its export.

So could an alternative for some of that energy be to be to channel it into carbon dioxide scrubbing? Could the scrubbers be a means for coping with variations in output, with a big enough bank of them to have capacity to absorb peaks?

In other words, it may be more cost-effective to make local use of the output from some of the renewables sources, and export instead the added-value product carbon dioxide for greenhouses and oil wells.

And if efficient processes can be devised for combining carbon dioxide with hydrogen to produce methanol, then could this be done locally as well, the hydrogen being produced by renewable energy sources?

We'll look forward to following closely the progress of this technology.




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