Capturing CO2 from ambient air - David Keith

7Na2CO3(l)+5Na2Ti3O7(s) ⇋ 4Na8Ti5O14(s)+7CO2(g)
ΔH = 63 kJ/mol-CO2
4Na8Ti5O14(s)+7H2O → 14NaOH(aq)+5Na2Ti3O7(s)
ΔH = 7.6 kJ/mol-NaOH
Table 2.3: Chemistry of caustic recovery with titanates. From Richards et al. (2002).
Figure 2.1: Example air capture system. CO2 from the atmosphere is absorbed into NaOH solution in
the contactor. The resulting Na2CO3 solution is regenerated to NaOH in a series of reaction vessels by
addition of CaO in the causticizer. The resulting CaCO3 solids are in turn regenerated to CaO by heating
in the calciner. CO2 in the calciner flue gas is captured and compressed by conventional means.
(Lindberg and Backman, 2004).
On the one hand, this process relies on a reaction (2) that has about the same heat of reaction as
calcination (Richards et al., 2002) and must be run at about the same temperature, so this process does not
appear to offer an advantage. On the other hand, it may be possible to keep all compounds in a liquid
or aqueous phase throughout the reaction cycle. This would simplify caustic recovery by avoiding solids
handling and dewatering steps. Also, the liquid phase allows for very efficient heat exchange; perhaps the
heat released from reactions 3 and 4 can be recovered at a usefully high temperature to compensate for
the demands of 1 and 2. Additionally, if the boiler is fired indirectly or with oxyfuel, it may be possible
to extract CO2 from the boiler in a pure stream. Perhaps we can run the reaction at positive pressure,
saving energy in the final compression step. Industry experience demonstrates that autocaustization works,
though at this point it is speculation whether the process would be favorable for air capture.
Another possible caustic recovery mechanism is by addition of sodium titanates. The chemistry of
caustic recovery with titanates is shown in Table 2.3. The promising point in this case is that the heat of
reactions is about a third of that in calcination, suggesting this process offers significant energy savings
compared with calcium-based systems. Titanate-based caustic recovery is in early stages of development.
2.3 Example air capture system
2.3.1 Overview
We have now briefly discussed all the components necessary to construct an air capture system. To illustrate
a complete system that could be built with available technology we will describe an example with a
spray-based contactor and kraft recovery plant. The appeal of this system is that the chemicals involved
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