New method could make carbon sequestration cheaper
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Friday, August 22nd, 2008
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Removing carbon dioxide from smokestacks and storing it
permanently is one of the possible solutions to global warming, but remains
expensive to do. A new technique could make carbon sequestration economical on
a large scale, while producing useful materials on the side.
Dirk Van Essendelft, a chemical engineer at Pennsylvania State
University in University
Park, described the method on August 19 in Philadelphia during a meeting of the American
Chemical Society. He proposed a new way to mix CO2 with a type of
mineral called serpentine, producing sand and another common mineral similar to
chalk.
Capturing the CO2 from smokestacks requires
energy. Van Essendelft said that, according to his calculations, a power plant
that captures its emissions for storage into serpentine would suffer only a 10
percent loss of energy. If the technique can be applied on a large scale,
storing carbon in minerals would become competitive with other proposals for
carbon sequestration, such as pumping CO2 deep underground. “It puts
mineral carbonation back in the game, as far as energy consumption,” he said.
Mercedes Maroto-Valer of the University
of Nottingham in England says that the technique
could be economical for large-scale carbon sequestration.
In his small-scale reactor, Van Essendelft grinds a
serpentine rock mixed with water and acid. The combination of grinding and
chemical action (mainly the acid) breaks down serpentine into magnesium and
silica, which is essentially sand. He then adds ammonia and pumps CO2 in.
The ammonia neutralizes the acid, allowing the CO2 to dissolve and
react with magnesium, forming magnesium carbonate.
Magnesium carbonate is similar to chalk and has several
applications. For example, Van Essendelft says, it could be used instead of
limestone to produce cement.
Previous attempts at storing carbon in magnesium carbonate
were energetically less favorable, since they required compressing CO2
before dissolving it, Van Essendelft says. His technique works at ordinary pressure.
He and others have now formed a start-up company, hoping to
develop the technique into one that can be applied economically on a large
scale. To do so, Van Essendelft says, the first step will be to build a reactor
that can process CO2 continuously, rather than in batches as the
current prototype does.
Serpentine is plentiful near the two U.S. coasts,
but expensive to transport in large quantities, points out Chris Plum, the
start-up’s president. Using it to store carbon would be economical in some
geographic locations, while other carbon-sequestration techniques should be
used in others, he says. “None of these processes is single-handedly going to
solve the problem.”
Found in: Chemistry, Environment and Science & Society
Capturing carbon
Assuming that the proponents of carbon sequestration are honest and serious, Fred Pearce's analysis of the pointlessness of their endeavours should go a long way towards convincing them that they are on the wrong track (29 March, p 36).
As they all have impeccable credentials in business, politics and even science, I wouldn't dream of questioning either their qualifications or their honesty. A nagging thought remains, however.
What if there were proponents of carbon capture who were not honest? All the arguments Pearce advances to counter it would then be seen to work in their favour.
They would not mind the decades of inaction caused by endless testing, nor would they object to the costs of this and other research - which would benefit aspects of their industry and be paid for by the state. Nor would they mind if their project failed, because these are, with few exceptions, the very people who have denied the existence of global warming, at least here in Australia where the coal industry is massively influential.
From Owen Jordan
Fred Pearce omitted two crucial points. First, carbon sequestration shares with nuclear power the indefensible moral position of forcing future generations to deal with the consequences of our greed for energy. That's the easy bit.
Secondly, Pearce perpetuates the myth that as much as two-thirds of the emissions of climate-change gases will be captured if carbon capture and storage is put in place at the power station. Dream on!
Most coal today comes from opencast workings. Emissions, primarily of methane, CO2 and carbon monoxide, start as soon as the overburden above the coal seams is stripped away. Conservative estimates suggest that these gases alone account for about twice the emissions of the burning of the coal mined.
It gets worse: 98 per cent of what is dug out in an opencast coal mine is not coal, but perhaps 25 per cent (at least 10 times the amount of coal extracted) will be shale and mudstone with a carbon content of up to 50 per cent.
This cannot be burned, because of its high ash content, but it still oxidises if exposed to air. Another conservative estimate is that this carbon source has the potential to emit three or four times as much CO2 as the mined coal. Capture of CO2 at power stations therefore amounts to 5 to 10 per cent, maximum, of the emissions from the process of working and burning coal.
So carbon capture proposals share the platform with nuclear power on a second front: our ceaseless ability to construct webs of self-delusion about what we are doing to the planet, accompanied by a queue of politicians wanting us to believe them.
Cwmllynfell, West Glamorgan, UK
Is Carbon Sequestration Completely Useless?
I have been giving Mark Jaccard and other carbon sequestration enthusiasts a hard time, but does that mean that carbon sequestration is a complete waste of time? Not necessarily, but you have to be aware of the costs and of the niches where the technology is a good fit. At the very least it is a good way to hoist the coal industry on its own petard. They say sequestration will make them as green as other fuels? Fine, you can still sell coal as a fuel as long as you reduce emissions to the level of natural gas. We're not putting you out of business, we just believe you're telling the truth about sequestration, wink, wink.
As I have mentioned before, the concentration of CO2 in flue gas is so low, and the cost of separating it out is so high both in terms of money and in terms of GHG emissions that it is not worth tackling that problem and probably will never be. It's much better value for money to just stay away from high-carbon fuels as much as possible. It's like the problem with extracting fuel from the tar sands. Right now extracting uses large quantities of natural gas as feedstock and other energy sources to move the stuff and heat it up. You could then add more energy and use it to sequester some of the carbon. But when you do all the math a much simpler solution is staring you in the face: rather than using natural gas to process the tar sand into a fuel, use the natural gas as a fuel directly and leave the tar in the ground. You get to deliver a cleaner fuel to markets, at lower cost and with much lower GHG emissions. And you avoid destroying the entire Athabaska basin. Everybody wins.
Back to carbon sequestration. There are plenty of processes where CO2 is produced in higher concentration, where the separation cost is much lower. Right now that CO2 is usually just being released into the air. There are also plenty of processes that consume CO2 and where customers are willing to pay good money to get a source of it. So much so that there is a market for the drilling of underground CO2 wells, taking naturally sequestered CO2 out of the ground to satisfy a market demand. The low-hanging fruit is to bring the two together, to make sure that CO2 in the ground stays in the ground, and then to make sure that everyone captures the easily captured CO2 and that any excess that can not be used gets buried.
Some of the easily captured sources of CO2 include ammonia production for fertilizer, fermentation, lime calcination, detergents, and natural gas wells. Oddly enough, when producing "clean energy" like fuel ethanol or natural gas, a lot of CO2 get dumped into the atmosphere. Most gas wells contain a lot of CO2. The industrial processes for preparing the gas for market does the separation of practically pure CO2 at virtually no additional cost. Don't release it, capture it and make gas even greener. If possible, sell it. If not, back in the ground it goes.
Fermentation, particularly for alcohol, produces a lot of CO2. That's why beer has bubbles. Actually, that used to be the reason. Often the CO2 produced is released into the air during fermentation, and other CO2 is pumped into the beer at the end. Remember that for every molecule of ethanol that you drink or put in your car, a molecule of CO2 escapes into the atmosphere. Catch it and use it.
Various chemical processes generate CO2. In some cases, petrochemical plants are already capturing it. There is the famous example of the ethylene glycol plant of Shell Chemicals in Scotford selling CO2 to Air Liquide, which processes it for the soft drink industry. But larger-scale processes could also capture their CO2, including the production of ammonia and the calcination of carbonates in lime kilns to make cement. Again, the gas is produced in high concentrations and is easily captured.
Various processes use CO2. Some use it and sequester it, and some use it and release it, so the same argument applies to them: catch it and recycle it. It's used in the beverage industry. Huge waste - it necessarily gets released into the air. It's used in refrigeration as dry ice or to replace freon. Well, it's better than freon anyway but it is still released into the air. Is it better in terms of CO2 to use dry ice in refrigeration rather than portable refrigeration units? Not sure. It's used in some chemical processes: urea and ethanol. It's used in enhanced oil recovery. It's pumped into the ground as a solvent. That has the potential to be sequestered, but in actual fact it gets pumped right back out once it's done its job underground and tends to be released into the air. Close but not quite there. It can also be used in greenhouses, for two purposes - as a pesticide since it is after all a poison in high concentration, and to enrich the air. Plants convert it into sugars and such things. Give them a little more in their atmosphere and it replaces some fertilizer, and allows cold countries to reduce their food imports a bit. The processes that use CO2 already get it in part from the process that produce it, and in part from CO2 wells. Let's at least put the wells out of business. In relative terms it's not a major part of our total GHG emissions that are affected, but it's a start and it's cheap to do compared to alternatives.
All of this is to say that carbon capture has a role to play in reducing greenhouse gas emissions, particularly by co-locating producers and consumers of CO2. And you can also often use the synergy to make use of waste heat. Carbon storage also has a role, when all the CO2 that is easily captured has saturated the market for industrial uses and can be buried relatively cheaply. But extracting it from flue gas to bury it? Only when through conservation and efficiency we have eliminated all the high-carbon fuel use (coal, tar sand, heavy oil, biofuels) that we can. Then when carbon taxes reach $150 a ton it becomes worthwhile to consider extracting CO2 from flue gas. But that's really a last resort. There is no good reason to ever choose carbon capture and storage over other alternatives, say the experts. Wind, solar, and conservation are a lot cheaper for the same effect.
Chris Plum
This would be an ideal fit for a coal-fired power plant. Not only could the plant dispose of CO2 with this technique, but also sulfur dioxide (as sulfuric acid). The same trains that bring in the serpentine could leave loaded with valuable cinderblocks, conduits and pavers made from on-premises magnesium cement and fly ash.
The magnesium concrete would cure especially rapidly and with greater strength if it were autoclaved in supercritical CO2 while in the mold. A continuous casting process could produce an endless "pultrusion" of fiberglass, magnesium cement and flyash cured by supercritical CO2 potentially superior to steel-reinforced aluminosilicate-concrete for conduits. Such a material could also substitute for pressure-treated wood in products ranging from railroad bed ties to backyard sundecks; could even take the place of dimensional lumber entirely in Gulf Coast housing construction where its straightness and stability as well as fire, rot and moisture resistance would make it superior to fifth-growth pine products.
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