What are the different methods of carbon capture and storage (CCS) currently being used or developed?

The main methods of carbon capture and storage (CCS) that are currently being used or developed include post-combustion capture, pre-combustion capture, and oxy-fuel combustion. Post-combustion capture involves removing carbon dioxide (CO2) from the flue gases emitted by power plants and industrial processes. Pre-combustion capture focuses on separating CO2 before fuel combustion, typically using a combination of gasification and water-gas shift processes. Oxy-fuel combustion involves burning fuel with pure oxygen, resulting in a higher concentration of CO2 for easier capture. Other emerging techniques include direct air capture (DAC), which removes CO2 directly from ambient air, and mineralization, which involves converting CO2 into stable carbonate minerals.

Long answer

1. Post-combustion Capture: This method involves capturing CO2 from the flue gases emitted during power generation or industrial processes. Various technologies are being deployed for this purpose including absorption using solvents like amine compounds or chilled ammonia-based systems, adsorption on solid sorbents like zeolites or activated carbon, and membrane separation techniques. The captured CO2 is then compressed and transported to the storage site for secure long-term storage underground.

2. Pre-combustion Capture: Pre-combustion capture aims to separate CO2 before the fuel combustion process. This is mainly employed in Integrated Gasification Combined Cycle (IGCC) power plants where fossil fuels like coal or natural gas are converted into syngas through gasification. The syngas is then shifted to generate hydrogen-rich gas while producing separate streams of hydrogen (to be used as clean fuel) and CO2 for storage.

3. Oxy-fuel Combustion: In this approach, fuel is burned using pure oxygen instead of air, resulting in a flue gas stream consisting mainly of water vapor and concentrated CO2. This makes CO2 capture easier as it does not need complicated separation steps. The captured CO2 is compressed and transported for storage, usually in depleted oil and gas fields or deep saline aquifers.

4. Direct Air Capture (DAC): DAC technology directly removes CO2 from the ambient air, irrespective of its source. It employs sorbents or solvents to selectively adsorb CO2, which is then separated and purified for storage. Although still in early stages of development, DAC offers the potential to remove historical carbon emissions along with ongoing sources.

5. Mineralization: This method involves converting captured CO2 into mineral forms such as carbonate rocks or stable bicarbonates through various chemical reactions with specific minerals or waste materials like steel slags. By enhancing natural carbonation processes, mineralization allows long-term and secure storage of CO2.

It is worth mentioning that while these CCS methods are being actively explored and demonstrated, their large-scale deployment still faces technical, economic, and regulatory challenges that need to be overcome for widespread implementation in order to mitigate climate change effectively.