Carbon Sequestration

MANY countries THAT attended the United Nations Framework Convention on Climate Change in Kyoto promised to learn how to mitigate the problem of climate change by managing the global carbon cycle. This resolve shows the significance of carbon sequestration for alleviating the global warming problem. Carbon sequestration refers to the provision of long-term storage of carbon in the terrestrial biosphere, underground, or in the oceans, so that the carbon dioxide (CO2) buildup in the atmosphere will decrease or slow.

CO2 makes up approximately 47 percent of greenhouse gases, making it a primary contributor to global warming. The level of CO2 in the atmosphere has risen from the last century (pre-industrial) level of 280 parts per million (ppm) to the present level of

375 ppm. Carbon sequestration is intended to reduce the atmospheric CO2 concentration, which is predicted to exponentially rise because of higher global energy use and extensive deforestation in the 21st century. Carbon sequestration can be accomplished by maintaining or enhancing natural processes such as managing forest ecosystems and storing carbon in biomass and soil, or by artificially sequestering carbon in underground geologic repositories, enhancing net oceanic carbon uptakes, and sequencing the genomes of micro-organisms for carbon management.

The Kyoto Protocol recognizes forestry and land-use change activities as carbon sinks and sources. When plants grow, they absorb carbon dioxide from the atmosphere as part of photosynthesis. This is known as carbon sequestration, because carbon is removed from the atmosphere. Therefore, a living plant helps carbon sink. Forest decay does generate carbon into the atmosphere, but sustainable use of forest biomass in biofuel production reduces this amount. Biofuel also reduces the use of fossil fuels, which are major contributors to greenhouse gas production. By the beginning of the 2010s, most of the countries in the world will mandate a minimum of 10 percent biofuel use. Scientists also agree that world forests have the greatest long-term potential to sequester atmospheric carbon by protecting forested lands, slowing deforestation, encouraging reforestation, and agroforestry. Deforestation, harvesting, and forest degradation contribute 1.8 Gt carbon per year.

Soil is a major reservoir of terrestrial carbon. It is about 3.3 times the size of the atmospheric pool. Therefore, if the soil is enriched through sequestration of atmospheric carbon, global warming can be managed significantly. This will also ensure global food security, because enhancement of soil carbon enhances the soil health for sustainable soil production. No-till farming, residue mulching, cover-cropping, and crop rotation are some of the methods that are employed to sequester carbon into soil. Using pyrolysis technique, half of the carbon in biomass can be reduced to charcoal and, thus, decrease the potential to act as a carbon source. The charcoal is later deposited in soil to increase its carbon content.

Atmospheric carbon can be reduced by enhancing the net oceanic uptake from the atmosphere by fertilization of phytoplankton with nutrients and injecting CO2 to ocean depths greater than 3,281 ft. (1,000

m.), or into deep geologic formations. This reduction can also be achieved by carbon capture and storage (CCS); for example, collecting CO2 at point sources (such as power plants) and injecting it directly into the deep ocean. If CCS is applied to modern conventional power plants, CO2 emission into the atmosphere could be reduced by 80-90 percent. However, this injection into the deep sea might influence sea creatures negatively (due to the decrease in pH of sea water). Therefore, carbon storing in deep geologic formations has the greatest potential. Oil fields, gas fields, saline formations, unminable coal seams, and saline-filled basalt formations are suggested storage sites. Mineral storage of CO2 is another potential means of carbon sequestration. In this process, CO2 is exothermically reacted with abundantly available metal (Mg and Ca) oxides, which produce stable carbonates.

More advanced CO2 capture techniques are being developed as part of carbon sequestration. Presently, there are three techniques in use: post-combustion, pre-combustion, and oxyfuel combustion. In postcombustion processes, CO2 is captured from flue gases (the gas that exits to the atmosphere through a flue or pipe) at power stations. The pre-combustion technique is widely applied in fertilizer, chemical, gaseous fuel (hydrogen and methane), and power production. In these cases, the fossil fuel is gasified and the resulting CO2 is easily captured from a relatively pure exhaust stream.

In the third type (oxyfuel combustion) the lignite is burned in oxygen instead of air and produces a flue gas consisting only of CO2 and water vapor. This is cooled and condensed to a pure CO2 stream that can be transported to the sequestration site and stored. According to scientists, this technique is very promising, but a lot of energy is needed for the initial air separation. Other ambitious techniques under development are the genetic manipulation of plants and trees to enhance CO2 sequestering potential and sequencing the genomes of microbes that produce fuels such as methane and hydrogen or aid in carbon sequestration.

SEE ALSO: Biomass; Carbon Cycle; Carbon Emissions; Carbon Sinks; Forests; Oceanic Changes.

BIBLIOGRApHY. R. Lal, M. Griffin, and J. Apt, "Managing Soil Carbon," Science (v.304, 2004); M. Maroto and M. Valer, eds., Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century (Kluwer Academic/Plenum Publishers, 2002); W.G. Ormerod, P. Ferund, and A. Smith, Ocean Storage of CO2 (International Energy Agency Greenhouse Gas R&D Program, 2002); R.T. Watson et al., eds., Land Use, Land Use Change, and Forestry (Cambridge University Press, 2000).

SUDHANSHU SEKHAR PANDA Gainesville State College

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