CO2 capture

[Dr Mordrid]

Capture or convert?

It doesn't have to capture at all, just convert the CO2 into something useful elsewhere or at least less harmful.

ex: in auto catalytic converters pollutants are combined with atmospheric oxygen to make other compounds without sequestration.

The goal of the synthetic photosynthesis researchers is conversion of CO2 into compounds that can be converted into alcohol, presumably for use as a fuel source.

CO2 in, E100 out.

Converters of this type need very little of the actual catalyst, just enough to very thinly coat the active surface of the supporting matrix.

[Brian Ellis]

Maybe, but to convert CO2 to CH3(CH2)nOH requires you to put in more energy than you get out from burning it. Where is that energy coming from? 2nd law of thermodynamics, please! And don't say the sun, as even nature cannot produce sufficient photosynthesis to convert the CO2 we spew out of our chimneys and exhaust pipes into cellulose etc. with an estimated 90,000,000 km² of greenery in the world. Remember we are talking about BILLIONS of TONNES of CO2 to make a difference, not a few grams in a laboratory test tube.

Traditional CH3CH2OH (ethanol) production has CO2 as a by-product. You are trying to do the opposite.

In any case, nitride adsorption does not actually do any conversion in the initial stage. It may be able to fix the gas prior to a secondary reaction.

I don't believe you are keeping a sense of proportion.

Edit: Your comparison with a catalytic converter is not good. In a catpot, the thousands of Pt-coated ceramic balls reach temps of ~800°C as a result of combustion of C, CO, HCs and other organic compounds, including unburnt fuel. This oxidation reaction results in all the carbon-containing molecules being reacted to CO2. The catalytic action of the Pt in the presence of HCs pushes the temp up to the operating temp, causing a release of energy which actually comes from the chemical energy contained within the molecules. It is not something for nothing (2nd law, again). You cannot reverse the action without replacing the energy released plus some more (100% efficiency is a pipe dream).

[Dr Mordrid]

I was only describing the physical layout that allows a minimal amount of catalyst to be used with a large surface area, not the chemistry which in this case would differ greatly.

BTW they don't use the Pt coated ceramic balls anymore. It's a ceramic or stainless steel foil honeycomb coated with silicon or aluminum then the catalyst; Platinum, Palladium, Rhodium, Iron, Cerium, Manganese etc.

[KRSESQ]

Maybe trying to imitate photosynthesis is the wrong approach. Perhaps a "reverse fermentation" is better suited. Feed in CO2 and ethanol and get out sugar water which can be used to ferment more ethanol and the circle of life is complete.

@Brian: still grappling with those numbers. Damn, I wish I was better at this!

Kevin

[Dr Mordrid]

A New Scientist article on the catalyst here

Catalyst could help turn CO2 into fuel

A new catalyst that can split carbon dioxide gas could allow us to use carbon from the atmosphere as a fuel source in a similar way to plants.

"Breaking open the very stable bonds in CO2 is one of the biggest challenges in synthetic chemistry," says Frederic Goettmann, a chemist at the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany. "But plants have been doing it for millions of years."

Plants use the energy of sunlight to cleave the relatively stable chemical bonds between the carbon and oxygen atoms in a carbon dioxide molecule. In photosynthesis, the CO2 molecule is initially bonded to nitrogen atoms, making reactive compounds called carbamates. These less stable compounds can then be broken down, allowing the carbon to be used in the synthesis of other plant products, such as sugars and proteins.

In an attempt to emulate this natural process, Goettmann and colleagues Arne Thomas and Markus Antonietti developed their own nitrogen-based catalyst that can produce carbamates. The graphite-like compound is made from flat layers of carbon and nitrogen atoms arranged in hexagons.

The team heated a mixture of CO2 and benzene with the catalyst to a temperature of 150 ºC, at about three times atmospheric pressure. In a first step, the catalyst enabled the CO2 to form a reactive carbamate, like that made in plants.

Oxygen grab

The catalyst's next useful step was to enable the benzene molecules to grab the oxygen atom from the CO2 in the carbamate, producing phenol and a reactive carbon monoxide (CO) species.

"Carbon monoxide can be used to build new carbon-carbon bonds," explains Goettmann. "We have taken the first step towards using carbon dioxide from the atmosphere as a source for chemical synthesis."

Future refinements could allow chemists to reduce their dependence on fossil fuels as sources for making chemicals. Liquid fuel could also be made from CO split from CO2, says Goettmann. "It was common in Second World War Germany and in South Africa in the 1980s to make fuel from CO derived from coal," he adds.

The researchers are now trying to bring their method even closer to photosynthesis. "The benzene reaction currently supplies the energy that splits the CO2," Goettmann says, "but in plants it is light." The new catalyst absorbs ultraviolet radiation, so the team is experimenting to see if light can provide the energy instead.

Recycled carbon

Joe Wood, a chemical engineer at Birmingham University in the UK, is also researching ways of fixing CO2. "There's growing interest in using it as a recycled input into the chemical industry," he says.

The Max Planck technique has only been demonstrated on a small scale and it has a low yield of 20%, he points out. "But it looks quite promising," he adds. "The catalyst can be made cheaply and it works at a relatively low temperature."

The products of the technique are well suited to making drugs or herbicides, says Wood, "so hopefully they can improve the efficiency and scale it up."

Reference: Angewandte Chemie (vol 46, p 1) DOI:10.1002/anie.200603478

[Brian Ellis]

"The benzene reaction currently supplies the energy that splits the CO2," Goettmann says, "but in plants it is light." The new catalyst absorbs ultraviolet radiation, so the team is experimenting to see if light can provide the energy instead

There is the nub. How many billions of tonnes of benzene (containing 6 carbon atoms/molecule) will be required and where will it come from?

[Dr Mordrid]

You glossed over the term "currently" which strongly implies they're pursuing alternative reactions. This is bolstered in the next sentences reference to using UV.

[NetSnake]

a slightly different approach...

http://www.nature.com/news/2007/070312/full/070312-7.html

Published online: 15 March 2007; | doi:10.1038/news070312-7
What's the future of coal?

A US study calls for more investment and focus on carbon capture and storage to make for cleaner coal. News@nature finds out what's taking so long.
Emma Marris


After two years of work, an interdisciplinary group of Massachusetts Institute of Technology (MIT) professors released their report on The Future of Coal yesterday in Washington DC, making recommendations about how the United States should use coal for energy.

Isn't coal the dirtiest fuel there is?

It makes the most carbon dioxide, yes. Coal is mostly carbon, after all. In the United States in 2005, 1 kilowatt-hour of electricity made from coal produced about 1 kilogram of carbon dioxide emissions. The same amount of electricity made from natural gas emits half as much, whereas nuclear, wind and solar emit no CO2.

But if some of those coal emissions could be captured and pumped underground, then coal could clean up its act. As challenging as this may be, it may be easier than replacing coal altogether; coal currently provides a quarter of world energy and half of electrical power in the United States. China is building an average of two 500-megawatt coal plants each week, the report says.

How hard is it to get the emissions underground?

Hard. Here is the process as it would work with the most common type of power plant: carbon on its way out of the plant passes upwards through a tower in which a solution of amines is flowing downwards. The CO2 binds to the solvent, which flows out the bottom. The CO2 is then stripped from the solvent so that the liquid can be re-used. This is done with heat, usually some of the steam that would otherwise be turning turbines and making power. The upshot is that the efficiency of the power plant goes down. The CO2 then has to be compressed into a supercritical fluid and piped deep into the earth, into porous rocks or saltwater aquifers where it will stay — one hopes — for millennia.

How much does all this cost?

The report estimates that capture, pressurization, transport and storage would cost about $30 per tonne of CO2. Plants that have all these capabilities are more complex and so also cost more to build.

Could existing plants be modified to do this?

Yes, but retrofitting plants is more complicated than strapping on additional equipment. One of the co-chairs of the report's authoring panel, Ernest Moniz, likened the process to "major surgery". The report's authors believe that retrofits are unlikely to happen for the most common type of plants.

If you're going to build an entirely new plant, better cost returns can be found - at the moment — by turning coal into a gas before burning it, and then capturing the carbon. However, any plant with carbon capture will always be less efficient than one without it.

Why would a power company build a more expensive plant that will produce less energy?

They won't, until it makes business sense for them to do so. If it costs $30 a tonne to sequester the carbon, they won't do it until it costs that much to emit a tonne of carbon. That is why the report calls for carbon regulation in the United States as soon as possible.

And they won't do it until the technology is proven, which most people say it isn't. There are three small-scale demonstration sequestration plants around the world. But they aren't big enough, and aren't collecting enough data, according to the report, which calls for the United States to build three to five more demonstration plants, each pumping on the order of 1 million tones a year into the ground.

This sounds like it will take a while.

Yes, and conventional coal plants are likely to be built in the meantime. Interestingly, NASA policy guru Jim Hansen and some US politicians are talking about moratoriums on building new plants while the new technology is developed. The report does not go this far, merely suggesting they be built as efficiently as possible.

What about all the coal plants in China? Are they going to change?



The short answer is 'not soon'. Economies in China and India are rapidly growing and, particularly in China, that growth is being fuelled with coal. The two will probably account for 70% of new coal demand through 2030. But central governmental control of all this growth is weak.

The MIT group conducted several case studies in China and concluded that the government probably can't force a technology on its populace if it isn't economically feasible. But they end the section with a caution against being too critical: "We would be unwise to expect of the Chinese what we do not expect of ourselves."

[Marshmallowman]

Pumping it under ground is just pushing the problem into the future, and the losses in efficency by doing this make the alternatives that much more viable.

The oz governement is going through a "clean coal"(that is a contradiction in terms) phase right now, i think we should just cut to the chase and just go renewable..becasue we are going to have to do at some point, whether it is 10 or 100 years time.

I have been thinking a bit about CO2 cracking stuff, how does a kind of reverse laser systems sound. Instead of exciting CO2 to a metstable state and allowing it to deacy and produce light, we do the reverse we use light(solar) to excite a metastable state and then use some secondary process to bump a bit more energy to allow the dissassociation to occur. Of course chances are you will probably just be producing CO and O2 to start with, but by reducing the threshold energy required for a reaction you might be able to find a better reaction path.

Of course you plants do it a lot better than we can, and the produce quite tasty fuel as well, not to mention intoxicating (ethanol biofuels)

[Brian Ellis]

There's nothing new about stripping CO2 with monoethanolamine (MEA) then sequestering it underground. However, compressing it to supercriticality requires enormous pressure, nearly 80 kg/cm² and this would require a lot of energy. The article does not mention the energy requirement for the technique, but I suspect it would be very high.

However, I've used MEA a lot for industrial saponification. To explain what it is like, let's start with an ammonia molecule NH3 and substitute an ethanol molecule for one of the hydrogen atoms, fixing it in place by the methyl group H2NCH2-CH2-OH. It is caustically alkaline (I've had severe desquamation of the skin of the hand from accidental contact with a 5% aqueous solution), it is quite toxic with pronounced suppression of the CNS, and it smells like rotten fish that has been urinated on. It will capture CO2 from flue gas in a cold scrubber (e.g., the gas has to be cooled to ~25-30°C). The bad part: in order to prevent release of the MEA to the chimney, it requires a weak acid scrubber then a plain water one after it. To release the CO2, the carbonated solution needs to be heated by a flue gas heat exchanger (which also serves to cool the gas partially). Unfortunately, MEA solution is very corrosive. Even Mo-stainless steel (18-8-2-0.5) will be slightly attacked at high temps over time, so all the hot side of the plant is made from titanium, explaining the cost.

However, the biggest concern is how long will the high-pressure CO2 stay down the hole, remembering both coal mines and oil wells are drilled into porous substrates (limestones, dolomites, sandstones). Why go to the enormous expense and energy if the stuff is going to beetle out over a year or three? This is still largely unproven at an industrial scale.

Then there is the problem of quantities. About an estimated 8 billion tonnes of CO2 come out of electrical power-generating station smokestacks each year. If it were decided to do this on all of them, where are you going to put 8 billion tonnes of the stuff each year?

Yes, it may be feasible on a small scale, but...

I re-iterate this link!