Carbon Capture and Storage

Hannu Jääskeläinen

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• Introduction
• Energy Requirements
• CO₂ Capture and Separation
• CO₂ Transport
• CO₂ Utilization and Storage
• Applications

Introduction

Carbon capture and storage (CCS) or carbon capture utilization and storage (CCUS) technologies involve the capture of CO₂, primarily from large point sources like power generation. Application to more dispersed sources such as ships and vehicles as well as direct air capture have also been proposed. The captured CO₂ could be utilized in a range of applications or else injected into deep geological formations such as depleted oil and gas reservoirs or saline aquifers. Some climate policy centers consider a wide-scale deployment of CCS/CCUS technologies to be necessary to achieve ‘net zero’ GHG emission goals and to decarbonize sectors with hard-to-abate emissions [5968].

Carbon capture technologies still require much work to be deployed on a wider scale. As CCS processes require high energy input, they tend to have a significant negative impact on the efficiency and the economics of the associated industrial process. Experience shows that CCS technologies also present a range of technical and scalability challenges [5753]. In the United States, CCS demonstration projects resulted in varying levels of success, despite sizable government subsidies [5969].

Energy Requirements

To separate a compound such as CO₂ from a gas stream containing other compounds, the second law of thermodynamics requires that work be carried out to perform the separation. The theoretical minimum amount of work of separation that is required can be directly determined from the Gibbs free energy of the mixture from which CO₂ is to be separated. The lower the concentration of CO₂ in the mixture, the higher the work of separation. The minimum work of separation also increases as the purity of the captured CO₂ increases, Figure 1 [6536].

Additionally, the efficiency of a given separation process (i.e., the ratio of the theoretical minimum work of separation to the real work of separation) decreases with increasing dilution of CO₂ concentration because the amount of unwanted or inert gas (mostly nitrogen) that needs to be processed increases [6537] [6538]. However, different processes can have significantly different second law efficiencies. Some of this difficulty can be overcome by using processes that are more efficient for separation of lower concentration CO₂ mixtures. The second law efficiency of CO₂ separation from several industrial processes where the CO₂ concentration prior to separation ranges from 3.6% to 90% ranges from 11% to 25% [6539]. For capture of CO₂ from the ambient air (direct air capture) where the CO₂ concentration is about 400 ppm, the second law efficiency for many processes ranges from 5-10% [6540].

The work of separation only accounts for the energy required to separate CO₂ from a gas stream. Additional energy will be required to move the source gas through the separation plant and prepare the separated CO₂ for transport through compression and/or liquefaction.

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