Dig deep, June 19th 2008, from The Economist print edition

Carbon storage will be expensive at best. At worst, it may not work

EVEN in the most alternative-friendly future imaginable, coal is unlikely to go away. It is cheap, abundant and often local. So what can be done to make coal’s use more acceptable?

One much-discussed possibility is carbon capture and storage, or CCS, which involves burying the carbon dioxide deep underground. The generating companies have high hopes of it (see chart 3). There are just two problems. No one knows if it will work (in other words, if the CO2 will stay buried). But whether it works or not, it will be expensive—so much so that the alternatives start to look rather attractive. The one serious attempt to investigate its use in an actual power station, the FutureGen project, based in Illinois, was cancelled in January because the expected cost had risen from $830m to $1.8 billion.

The “capture” part is not that hard. Carbon dioxide reacts with a group of chemicals called amines. At low temperatures CO2 and amines combine. At higher temperatures they separate. Power-station exhaust can thus be purged of its CO2 by running it through an amine bath before it is vented, and the amine can be warmed to release the gas where it will do no harm. Better still, the coal can be reacted with water to produce a mixture of CO2 and hydrogen in which the carbon dioxide is much more concentrated than in normal flue gas, so it is easier to scrub out. What is then burned is pure hydrogen.

All this processing is expensive, but an experimental plant in Denmark that uses monoethanolamine as the captor has been running for two years. Alstom, a French firm, has almost finished building one in Wisconsin that uses ammonia.

It is what comes next that is the problem. The disposal of carbon dioxide needs to be permanent, so a lot of conditions have to be met. To be a successful burial site, a body of rock needs to be more than 1km underground. That depth provides enough pressure to turn CO2 into what is known as a supercritical fluid, a form in which the stuff is more likely to stay put. The rock in question also has to have enough pores and cracks in it to accommodate the CO2. Lastly, it needs to be covered with a layer of non-porous, non-cracked rock to provide a leakproof cap.

So far, only three successful CCS projects are under way. The Weyburn-Midale CO2 project is burying carbon dioxide from a coal gasification plant in North Dakota in a depleted oil field in Saskatchewan. The Salah gasfield project in Algeria, run by BP, strips CO2 from local natural gas and injects it back into the ground. And Statoil, a large Norwegian oil and gas company, performs a similar trick at two places in the North Sea. None of these projects is actually linked to generating electricity. Still, a few years ago they were touted proudly. But the touting has become more nervous, and no new projects have come on stream.

The scale of the problem is awesome. The three showcase projects each dump about a million tonnes of CO2 a year. But America’s electricity industry alone produces 1.5 billion tonnes, which would mean finding 1,500 appropriate sites, and nobody knows whether the country’s geology can oblige. Even transporting that amount of gas would be a huge task.

As to the cost, a report published last year by MIT reckons on $25 a tonne to capture CO2 and pressurise it into a superfluid, and $5 a tonne to transport it to its burial site. It therefore suggests that power stations which dump CO2 into the atmosphere should be charged $30 a tonne, a figure conveniently near both the middle of the IPCC’s suggested carbon price and the actual price in Europe. Another report, by a consultancy called Synapse Energy Economics, notes that American power companies are already starting to employ carbon prices in their internal accounting, using a range of $3-61 a tonne. Again, the middle of that range is about $30.

Such a charge, whether a tax or a system of tradable permits to pollute, would change energy economics radically. But even the most optimistic proponents of carbon capture and storage doubt it will be a serious alternative much before 2020. And by then both the physical and the political climate may look rather different.

Comments

U.Bonne, 28 Nov.'08: It was thought that by storing CO2 in liquid form, at supercritical pressure, the higher density-than-water CO2 would sink to the bottomof the ocean and stay there. However, it is still able to form water clathrates and to diffuse into the water, so that leakage over long periods of time is likely.

A more permanent repository may be provided by CO2 reaction with peridotite, a very abundant mineral in the Earth's crust. In some areas peridotite strata even come close to the surface. The rate of reaction is slow at mbient temperature, so that pumping the CO2 down to regions of higher temperature, after suitable fracturing the rock, offers some promise.

For more information, see refs.[1,2] below. However, I am still looking for info on what exactly would bind CO2. Is it that peridotite contains significant amounts of unreacted CaO, which would eagerly combine with CO2 to form CaCO3 (calcium carbonate) rock?

References 

[1]  Peter B. Kelemen1 and Jürg Matter (Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964), "In situ carbonation of peridotite for CO2 storage," PNAS vol. 105 no. 45 17295-17300 (2008) November 11,  Edited by David Walker, Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY, and approved September 22, 2008 (received for review June 17, 2008) .  Abstract. 

The rate of natural carbonation of tectonically exposed mantle peridotite during weathering and low-temperature alteration can be enhanced to develop a significant sink for atmospheric CO2. Natural carbonation of peridotite in the Samail ophiolite, an uplifted slice of oceanic crust and upper mantle in the Sultanate of Oman, is surprisingly rapid. Carbonate veins in mantle peridotite in Oman have an average 14C age of ≈26,000 years, and are not 30–95 million years old as previously believed. These data and reconnaissance mapping show that ≈104 to 105 tons per year of atmospheric CO2 are converted to solid carbonate minerals via peridotite weathering in Oman. Peridotite carbonation can be accelerated via drilling, hydraulic fracture, input of purified CO2 at elevated pressure, and, in particular, increased temperature at depth. After an initial heating step, CO2 pumped at 25 or 30 °C can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy. In situ carbonation of peridotite could consume >1 billion tons of CO2 per year in Oman alone, affording a low-cost, safe, and permanent method to capture and store atmospheric CO2.

peterk@ldeo.columbia.edu  Klaus S. Lackner 

Peter B. Kelemen and Jürg Matter (2008) In situ carbonation of peridotite for CO2 storage. PNAS doi: 10.1073/pnas.0805794105 

Conflict of interest statement: P.B.K. and J.M. have a preliminary patent filing for the technique of heating peridotite to achieve self-sustaining, rapid carbonation. 

This article contains supporting information online at www.pnas.org/cgi/content/full/0805794105/DCSupplemental. http://www.pnas.org/content/suppl/2008/11/03/0805794105.DCSupplemental/0805794105SI.pdf

[2] Scientists say peridotite rock can soak up CO2. Mon Nov 10, 2008 3:48am EST 

http://www.reuters.com/article/scienceNews/idUSTRE4A59IB20081110?feedType=RSS&feedName=scienceNews 

Geologist Peter Kelemen and geochemist Juerg Matter said the naturally occurring process can be supercharged 1 million times to grow underground minerals that can permanently store 2 billion or more of the 30 billion tons of carbon dioxide emitted by human activity every year. 

Their study will appear in the November 11 edition of the Proceedings of the Natural Academy of Sciences. 

Peridotite is the most common rock found in the Earth's mantle, or the layer directly below the crust. It also appears on the surface, particularly in Oman, which is conveniently close to a region that produces substantial amounts of carbon dioxide in the production of fossil fuels. 

"To be near all that oil and gas infrastructure is not a bad thing," Matter said in an interview. 

They also calculated the costs of mining the rock and bringing it directly to greenhouse gas emitting power plants, but determined it was too expensive.   

The scientists, who are both at Columbia University's Lamont-Doherty Earth Observatory in New York, say they could kick-start peridotite's carbon storage process by boring down and injecting it with heated water containing pressurized carbon dioxide. They have a preliminary patent filing for the technique.   

They say 4 billion to 5 billion tons a year of the gas could be stored near Oman by using peridotite in parallel with another emerging technique developed by Columbia's Klaus Lackner that uses synthetic "trees" which suck carbon dioxide out of the air.