# Analysis of Technologies for Carbon Dioxide Capture from the Air
## 2 DAC Technologies
### Absorption  
In absorption technology for DAC, water solutions of strong bases containing hydroxide sorbents with a strong CO\({}_{2}\) affinity, such as sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)\({}_{2}\)), are used [3]. The absorption with Ca(OH)\({}_{2}\) was proposed by [PERSON] et al. [12] and it follows the reactions described below [18] (see Equations (2)-(4)):  
\[Ca(OH)_{2}+CO_{2}\ \to CaCO_{3}+H_{2}O\ \ \ \ \ \Delta H_{25\ ^{\circ}{\rm C}}=-109 \ \rm{kJ/mol} \tag{2}\]  
\[CaCO_{3}\ \to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{3}\]  
\[CaO+H_{2}O\ \to Ca(OH)_{2}\ \ \ \ \Delta H^{\circ}_{100\ ^{\circ}{\rm C}}=-64.5\ \rm{kJ/mol} \tag{4}\]  
The reaction starts with calcium hydroxide and CO\({}_{2}\) to form calcium carbonate (CaCO\({}_{3}\)). The obtained calcium carbonate is precipitated, accumulated, separated from water, dried and calcined above 900 \({}^{\circ}\)C. In this step, calcium oxide (CaO) and CO\({}_{2}\) (concentrated stream) are obtained. Afterwards, calcium oxide is hydrated to produce calcium hydroxide. This reaction scheme allows a closed cycle. However, there are some disadvantages to this process, such as the great amount of lost water, high temperatures and high required energies for the calcination step (179.2 kJ/mol), and the low solubility of calcium hydroxide in water, which limits the hydroxide concentration for CO\({}_{2}\) capture [3,18]. The use of NaOH water solution was proposed by [PERSON] et al. [37] and has a similarity to the Kraft process used in the paper industry to extract cellulose from wood. Compared to Ca(OH)\({}_{2}\), NaOH solution allows a strong binding of CO\({}_{2}\) but with a higher solubility of carbonate in water. In this process, there are two cycles working simultaneously. In the first, CO\({}_{2}\) is captured by NaOH to produce sodium carbonate (Na\({}_{2}\)CO\({}_{3}\)) in the absorber through an exothermic reaction evolving at ambient temperature and pressure (see Equation (5)). In the second cycle, known as the regeneration cycle, NaOH is regenerated in the causticizer/precipitator: Na\({}_{2}\)CO\({}_{3}\) reacts with Ca(OH)\({}_{2}\) to produce NaOH and CaCO\({}_{3}\) (see Equation (6)). From calcium carbonate, CO\({}_{2}\) (concentrated stream) is recovered in the calciner/kiln (see Equation (7)) and receives oxygen (O\({}_{2}\)) from an air separation unit. As mentioned above, this step requires a lot of energy, with temperatures above 900 \({}^{\circ}\)C to allow the removal of water and decomposition of CaCO\({}_{3}\). Afterwards, CaO is hydrated to Ca(OH)\({}_{2}\) in the slaker (see Equation (8)) so that it can be sent to the precipitator to be used again [18]. Figure 3 shows the scheme of this process. \[Absorber\ 2 Na(OH)+CO_{2}\ \to Na_{2}CO_{3}+H_{2}O\ \ \ \ \ \Delta H_{25\ ^{\circ}{\rm C}}=-109.4\ \rm{kJ/mol} \tag{5}\]  
\[Causticizer\ Na_{2}CO_{3}+Ca(OH)_{2}\ \to 2 Na(OH)+CaCO_{3}\ \ \ \ \Delta H_{100\ ^{\circ}{\rm C}}=-5.3\ \rm{kJ/mol} \tag{6}\]  
\[Calciner\ CaCO_{3}\to CaO+CO_{2}\ \ \ \ \Delta H_{900\ ^{\circ}{\rm C}}=+179.2\ \rm{kJ/mol} \tag{7}\]  
\[Slaker\ CaO+H_{2}O\ \to Ca(OH)_{2}\ \ \ \ \Delta H_{100\ ^{\circ}{\rm C}}=-64.5\ \rm{kJ/mol} \tag{8}\]An energy calculation showed that the energy for the calcination (179.2 kJ/mol) is well above the calculated thermodynamic minimum for the absorption stage (\(-\)109.4 kJ/mol): a high energy requirement is still present [41]. In fact, strong binds are present within the CaCO\({}_{3}\) molecules [3]. In addition to this high energy requirement, the water loss (water before calcination in the calciner must be removed) is an additional disadvantage of the process. [PERSON] [42] estimated that 90 gH\({}_{2}\)O/gCO\({}_{2}\) captured are lost. On the other hand, [PERSON] et al.