Land Use and Geo-Engineering - Introduction to Carbon Capture and Sequestration - page 534

can adsorb at very low partial pressures. At present, two types of
processes are proposed:

• Adsorption into highly alkaline solutions (see Box 11.3.3 ) [11.10]

• Adsorption onto solids [11.11,11.12]

The design of fl ue gas capture processes requires capture of 90% of
the inlet CO 2 ; Direct Air Capture processes, however, are not subject to

Box 11.3. 3 Carbon capture with sodium hydroxide

In Chapter 5, we saw that adding a base to water will signifi cantly enhance the CO 2
solubility in the mixture because of the reaction of CO 2 with the hydroxide:

CO 2 + NaOH ↔ Na 2 CO 3 + H 2 O

This is a very exothermic reaction that is effi cient in capturing very small concentra-
tions of CO 2 . The idea is to use this process to capture CO 2 directly from the air.
Similar to capturing CO 2 from fl ue gasses using absorption, we need to regenerate
the solvent and compress the CO 2 for, say, geological sequestration.
The regeneration of sodium carbonate into sodium hydroxide is called caus-
ticization and is a well-known industrial process. The process involves three steps.
First, Na 2 CO 3 is reacted with lime, Ca(OH) 2 , to form NaOH and lime mud (CaCO 3 ):

Na 2 CO 3 + Ca(OH) 2 ↔ 2NaOH + CaCO 3

∆ H 100°C = − 5.3 kJ/mol CO 2

The calcium carbonate precipitates in this reaction and is subsequently calcined
(heated with air) to recover the CO 2 :

CaCO 3 ↔ CaO(s) + CO 2

∆ H 900°C = 179 kJ/mol CO 2

The CaO is converted back to Ca(OH) 2 through quicklime hydration:

CaO(s) + H 2 O ↔ Ca(OH) 2 ,

∆ H 100°C = −65 kJ/mol CO 2

The enthalpy of absorption of CO 2 from air into sodium hydroxide solution for a 1 M
solution at standard conditions is -109.4 kJ/mol CO 2 . Hence, the minimum energy
to recover the CO 2 is also 109.4 kJ/mol CO 2 . We see that the calcination step
already takes 179 kJ/mol CO 2 , which indicates that the required energy for conven-
tional causticization is far beyond the thermodynamic minimum. Therefore, this
process consumes a large amount of energy.