|Fig. 1: Carbon Capture and Storage (Source: (Wikimedia Commons)|
The carbon inventory in the atmosphere is primarily composed of CO2 and totals 850 Gigatonnes (8.5 × 1011 tonnes) of carbon. For comparison, the carbon inventory in soil is roughly 2,000 Gigatonnes, in oceans is roughly 39,000 Gigatonnes, and in sedimentary rocks is 50 million Gigatonnes.  The atmospheric, terrestrial, oceanic, and lithospheric carbon inventories are in constant flux as a result of natural processes as well as human activities. Natural carbon exchanges, such as soil respiration and photosynthesis, are approximately 15 times larger than current emissions from fossil fuel combustion. However, natural sources operate on a much larger timescale than anthropogenic sources (i.e. hundreds of millions instead of hundreds of years). 
Today, about 0.04% of the atmosphere is composed of CO2, which represents a 40% rise since pre-industrial times. Increased emissions from fossil fuel combustion as well as land-use and land-management changes have contributed to this imbalance in the carbon inventories, with an annual increase of 4 Gigatonnes in the atmosphere. As a major greenhouse gas, atmospheric CO2 influences the balance of incoming and outgoing energy in the Earth-atmosphere system. With less energy escaping than entering the Earth due to the greenhouse effect, temperatures are increased and global changes to the climate are induced. 
In December 2015, 195 countries adopted the new Paris Agreement under the United Nations Framework Convention on Climate Change, aimed to keep warming below 2°C above pre-industrial levels.  To prevent exceeding this limit, 200 Gigatonnes of carbon emissions must be avoided by 2050 and over 1000 Gigatonnes by 2100. Mitigation options include de-carbonization of transport fuels, advanced-efficiency technologies, and carbon capture and storage.
Nearly 40% of global anthropogenic CO2 emissions come from power plants, many of which utilize fossil fuel and biomass-powered steam drive turbines.  The flue gas from combustion of natural gas and coal are approximately 3% and 15% CO2, respectively.  Fossil-fueled power plants account for nearly 50% of the total CO2 emissions from fossil fuel combustion. Since power plants are large point sources of carbon emissions into the atmosphere, application of carbon capture and storage can be an attractive mitigation strategy.
There are three main strategies for eliminating carbon from power plant emissions:
De-carbonization of the fuel prior to combustion, which requires gasification of the fuel to produce H2 via partial combustion, followed by several steps to separate CO2 from H2.
Separation of CO2 from N2 and other products in the flue gas.
Using O2 instead of air in the combustion process to produce pure CO2 as the combustion product.
The current commercially available options for CO2 storage is injection into water-bearing formations or oil and gas reservoirs. Injection into oil and gas reservoirs has potential economic advantages since it could enhance hydrocarbon recovery. Current carbon capture and storage technologies have throughputs ranging from 1 Kilotonne of CO2/year to 3 Megatonnes of CO2/year.  To meet the goal of mitigating over 1000 Gigatonnes of carbon by 2100, significant investment and urgency must be directed toward research and technology development to increase the current carbon capture and storage capacity.
As with several other carbon mitigation strategies, carbon capture and storage techniques reduce emissions, but increase the energy intensity of the overall process (e.g.e energy required to regenerate the solvents). Therefore, successful application of these technologies requires global policy measures to create the enabling conditions for large-scale deployment to meet the goals set out by Paris Agreement.
© Sadaf Sobhani. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 S. A. Rackley, Carbon Capture and Storage (Butterworth-Heinemann, 2009).
 "Paris Agreement," United Nations, 2015.