Question

name two techniques whereby carbon dioxide could be stripped from power-plant emission gases, and two tecniques that are more appropriare to more concentrated sources of the gas. for each technique specify whether a chemical or a physical process is involved

Answer 1

Carbon capture and storage (CCS) is the process of capturing waste carbon dioxide from large point sources, such as biomass or fossil fuel power plant.

Three different types of technologies for capture exist: post-combustion, pre-combustion, and oxyfuel combustion.

Post Combustion: the CO2 is removed after combustion of the fossil fuel. This is the scheme that would be applied to fossil-fuel burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources. The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station.

Pre-Combustion: in this technique the fossil fuel is partially oxidized, for instance in a gasifier. The CO from the resulting syngas (CO and H₂) reacts with added steam (H₂O) and is shifted into CO2 and H₂. The resulting CO2 can be captured from a relatively pure exhaust stream. The H₂ can now be used as fuel; the carbon dioxide is removed before combustion takes place. This scheme is applied to new fossil fuel burning power plants, or to existing plants where re-powering is an option.

Oxy-fuel Combustion:the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is recirculated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapour, the latter of which is condensed through cooling. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO
2 stored is not a fraction removed from the flue gas stream (as in the cases of pre- and post-combustion capture) but the flue gas stream itself.

CO2 Separation technique

Carbon dioxide can be separated out of air or flue gas with absorption, adsorption, or membrane gas separation technologies. Absorption, or carbon scrubbing, with amines is currently the dominant capture technology. Membrane and adsorption technologies are still in the developmental research stages, initiating primary pilot plants in the near future. Metal–organic frameworks (MOFs) are a novel class of materials that offer promise for carbon capture using adsorption technologies. Carbon dioxide adsorbs to a MOF through physisorption or chemisorption based on the porosity and selectivity of the MOF. The carbon dioxide is then stripped off the MOF using temperature swing adsorption (TSA) or pressure swing adsorption (PSA) so the MOF can be reused. Adsorbents and absorbents require regeneration steps where the CO2 is removed from the sorbent or solution that collected it out of the flue gas in order for the sorbent or solution to be reused. Monoethanolamine (MEA) solutions, the leading amine for capturing CO2, have a heat capacity between 3–4 J/g K since they are mostly water. Higher heat capacities add to the energy penalty in the solvent regeneration step. Thus, to optimize a MOF for carbon capture, low heat capacities and heats of adsorption are desired.

Another method is Direct air capture,which refers to the process of removing CO2 directly from the ambient air.