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Independent Professional: Experienced educator and management consultant for engineering educational institutions, researcher, trainer, technical consultant on sustainable technologies, related to cement manufacturing and characterisation, using industrial and agricultural wastes in cement and concrete, durability of concrete and fuel cell power.

Saturday, March 21, 2009

Controling Climate Change: Carbon Sequestering

Carbon Sequestering:

The biosphere pumps out 550 gigatonnes of carbon yearly; anthropogenic carbon is only 30 gigatonnes. Ninety-nine per cent of the carbon that is fixed by plants is released back into the atmosphere, within a year or so by consumers like bacteria, nematodes and worms. Any method to remove excess carbon dioxide must do six things:

(i) Move hundreds of megatons of carbon
(ii) Sequester that carbon for thousands of years.
(iii) The method should be repeatable for centuries.
(iv) It should be something that can be implemented immediately using methods already at hand.
(v) It should not cause unacceptable environmental damage.
(vi) It should be economical.

Lenton and Vaughan [1] analyzed 17 schemes for cooling the planet. Roughly half involve changing the reflectivity of the atmosphere or the ground, employing human made pollution, machines to alter clouds or schemes to lighten deserts or city roofs with plastic sheets or white paint. The other half would involve altering earth's carbon cycle to draw in CO2, either by growing massive amounts of new trees, boosting the growth of carbon-sucking algae at sea or creating machines that draw down the atmospheric carbon and store it underground.

Controling Climate Change: Land or the Oceans as the Carbon Sink

One effective method of sequestering carbon is through the massive burial of charcoal. It would mean farmers turning all their agricultural waste - which contains carbon that the plants sequestered through photosynthesis - into non-biodegradable charcoal and burying it in the soil. Then you can start shifting really hefty quantities of carbon out of the system and pull the CO2 down quite fast. Farmers burn their crop waste at very low oxygen levels to turn it into charcoal, which is then then ploughed into the field. A little CO2 is released but the bulk of it gets converted to carbon. The farmer gets a few per cent of biofuel as a by-product of the combustion process, which he can sell. This scheme needs no subsidy as the farmer makes a profit.

Another way is making bales of the crop residue – the stalks and such left after harvesting – and then sinking the bales into the deep ocean. With 30 percent of global crop residues, the build up of global carbon dioxide in the atmosphere could reduce by up to 15 percent a year, according to one calculation [2]. The crop residue would be baled with existing equipment and transported by trucks, barges or trains to ports, just as crops are. The bales would be barged to where the ocean is 1,500 meters deep and then the bales would be weighted with rock and sunk. The ocean waters below 1,500 meters do not mix significantly with the upper waters. In the deep ocean it is cold, oxygen is limited and there are few marine organisms that can break down crop residue. That means what is put there will stay there for thousands of years.

Strand and Benford [2] carefully tallied how much carbon would be released during the harvest, transportation and sinking of 30 percent of U.S. crop residues and compared that to how much carbon could be sequestered. They say the process would be 92 percent efficient. That's more efficient than any other use of crop residue he considered, including simply leaving crop residue in the field, which is 14 percent efficient at sequestering carbon or using crop residue to produce ethanol, which avoids the use fossil fuels, but is only 32 percent efficient. Sequestering crop residue biomass in the deep ocean is essentially recycling atmospheric carbon back into deep sediments.

Conroling Climate Chage: Carbon dioxide capture and storage (CCS)

40 percent of man-made CO2 emissions come from burning coal. One approach that is gaining currency among environmental scientists is carbon dioxide capture and storage (CCS), a form of carbon sequestration in which CO2 is removed from the waste gas of power plants, typically by absorbing it in a liquid, and subsequently burying it deep underground.

Indeed, CCS technology is already in use. Since 1996, the Norwegian company Statoil has been stripping about a million tons of CO2 a year out of natural gas from the Sleipner West field under the North Sea and injecting it at high pressure into a saline aquifer. Avoiding Norway's tax on CO2 emissions balanced most of the costs of the project. An even larger project, begun in 2000, takes CO2 from North Dakota and sinks it into an oil field in Weyburn, Saskatchewan, Canada. Here, selling the extra oil that the injected C02 squeezes out largely offsets the costs. These efforts and others show that the sequestered CO2 can be monitored, and that it largely stays confined underground. These cases notwithstanding, CCS is expensive. Capturing even a modest part of the 25 billion tons of CO2 emitted each year, experts say, will require economic incentives.The technology and technical know already exists. Whether it's adding chemical scrubbers to existing coal plants or building entirely new ones that gasify the coal before burning it (allowing the CO2 to be separated out).

At present, there are two such power plants in the world—one in China that is under construction and a small one in Germany that separates out the CO2 to store it in an abandoned natural gas field. Adding such CCS technology to coal plants adds roughly $ 65 per metric tonne of CO2 to the cost of electricity (2009 estimate), according to Howard Herzog, a research engineer at the Massachusetts Institute of Technology (MIT)

1. Tim Lenton, Nem Vaughan, "The radiative forcing potential of different climate geoengineering options", Atmospheric Chemistry and Physics Discussions, Vol. 9, pp. 2559-2608
2. Stuart E. Strand, Gregory Benford, "Ocean Sequestration of Crop Residue Carbon: Recycling Fossil Fuel Carbon Back to Deep Sediments", Environ. Sci. Technol., 2009, 43 (4), pp 1000–1007