Carbon Dioxide Capture and Storage
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Carbon Dioxide Capture and Storage Sally M. Benson (Stanford University, USA) and Franklin M. Orr, Jr. (Stanford University, USA)
Reducing CO2 emissions from the use of fossil fuel is the primary purpose of carbon dioxide capture and storage (CCS). Two basic approaches to CCS are available.1,2 In one approach, CO2 is captured directly from the industrial source, concentrated into a nearly pure form, and then pumped deep underground for long-term storage (see Figure 1). As an alternative to storage in underground geological formations, it has also been suggested that CO2 could be stored in the ocean. This could be done either by dissolving it in the mid-depth ocean (1–3 km) or by forming pools of CO2 on the sea bottom where the ocean is deeper than 3 km and, consequently, CO2 is denser than seawater. The second approach to CCS captures CO2 directly from the atmosphere by enhancing natural biological processes that sequester CO2 in plants, soils, and marine sediments. All of these options for CCS have been investigated over the past decade, their potential to mitigate CO2 emissions has been evaluated,1 and several summaries are available.1,3,4 With over 60% of worldwide CO2 emissions coming from point sources that are potentially amenable to CO2 capture and a minimum of 2,000 Gt (billion metric tonnes) of storage capacity in deep geological formations, the prospects for CCS to make a large contribution to reducing CO2 emissions are great.1 Technical and economic assessments suggest that, over the coming century, CCS could contribute up to 20% of needed CO2 emission reductions, on par with expected reductions from efficiency improvements and large-scale deployment of renewable energy resources.5
Scientists and engineers are working both to lower costs and to increase the efficiency of post-combustion capture. Research opportunities include more efficient and robust chemical solvents and membranes for separating CO2 from N2, as well as materials to reduce capital costs of the large separation vessels and contactors needed for industrial-scale capture. New materials that can withstand higher temperatures and pressures could also improve the efficiency of power generation with CO2 capture. Pre-combustion capture might offer lower costs and higher efficiency. Here, the fossil fuel is first gasified to produce syngas, a mixture of H2 and CO. In the process of gasification, a nearly pure stream of CO2 is produced. If all of the CO is further converted to CO2 by the water–gas shift reaction, a pure stream of hydrogen is produced that emits only water after combustion. Gasification is a well-established technology in the chemical manufacturing and refining industries, but there is only limited experience with gasification combined with power generation. A number of projects to demonstrate electricity production with pre-combustion capture are underway today, using a technology called integrated gasification combined cycle (IGCC). Cost for pre-combustion CO2 capture are estimated to be about $20 per tonne,3 but more experience
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