Disconnects in Logic (2) - Can't do the Chemistry

From the wayback machine 21 February, 2003, though he's still "at it" -
Dr Klaus Lackner has the hots for "synthetic trees" to "purify air" as the Beeb has it.

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.

Sounds good - we all need the air "purified" of the nasty CO2 we breathe out, but I'm not so sure about that "not release oxygen", so let's get the details

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.

Good-o - I want to be a thousand times better than a living tree, much better than being a thousand times better than a dead one in my opinion (I don't think he meant it that way - Ed.).

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.

A "paper idea" - is that anything like a "half-baked" one? Let's hear him out before we jump to conclusions.

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."

My word - the man's a genius - it would take someone with a good knowledge of chemistry and maths to really appreciate this simple but brilliant idea. Luckily I have one I prepared earlier, several decades earlier in fact, and it's me. I've no idea why he picked 14 grams of octane (gasoline), the molecular weight is 114, but neverrmind, 114 grams produces 252 grams, and so 14 grams produces 43.2 grams of CO2. Good enough for a physicist, I suppose, and if the hydrocarbon mix is taken into account I may have to give him credit for accuracy. May, could and might are the three magic words in climate "science" - the universal cop-out. I digress, something I have a perverse liking for, but bear with me, dear reader.

Let's assume that the limewater, a solution of Ca(OH)2 (calcium hydroxide) in water is used as an absorbent. How is this magic CO2 guzzler manufactured? CaO is a constituent of cement for building mortar, and is made by roasting limestone at a very high temperature. Just a mo. - didn't he mention limestone in his last sentence? Indeed he did, so here's a summary of the process. Limestone is burnt in gas-fired rotary kilns, and the resulting calcium oxide is allowed to cool. Water is added (slaking), creating Ca(OH)2 (calcium hydroxide) in solution and releasing some of the heat absorbed in the roasting. Here it is in formulae, and this is the killer:

CaCO3 (limestone) + heat -> CaO (calcium oxide) + CO2

CaO + H2O -> Ca(OH)2

And the "magic tree" CO2 absorbing process:

Ca(OH)2 + CO2 -> CaCO3 + H2O

The net process is therefore:

burn methane (CH4, natural gas) + 3O2 (oxygen) -> heat + CO2

limestone + heat + water -> (less) heat + water + limestone + CO2 (from natural gas)

The only sufficiently large source of calcium on the planet is in the form of limestone or chalk. The growth of plankton with calcium-carbonate based shells or internal skeletons locks up CO2 for millions of years, and our physicist wants to release it to use the rest of the molecule to absorb the same amount of CO2 from the atmosphere, consuming vast amounts of otherwise useful natural gas in the process, and also oxygen from the air to burn the gas, which process itself generates CO2. The outcome is more CO2 than at the start of the entire cycle, and vast quantities of "limestone" sludge. Where to dispose of it - there's always the limestone quarries, which unfortunately won't be big enough, as the limestone now contains a lot of water. He could always dump it in the sea, which is part of the natural limestone/chalk process anyway. Then there's transport costs for CO2-belching trucks, construction costs, environmental damage..... You couldn't make it up.

BTW - if you have any suggestions for Dr Klaus for a "better tree" than his, one that isn't expensive to manufacture, doesn't consume large amounts of valuable energy resource, requires little or no maintenance, is cheap or even free to manufacture, actually removes CO2 from the atmosphere, doesn't spoil the view, and as an added bonus generates oxygen and may even have valuable by-products, please leave a comment, and I'll see if I can let him know about it. I'm still working on it.