Capturing CO2 from ambient air - David Keith

chemistry.
To capture a large fraction of CO2 emissions, for instance all of the emissions from the transportation
sector (40% of total), it is natural to seek a sorbent which can capture CO2 and then be regenerated and
reused in a cycle. Most likely, this would be coupled with deep geological sequestration of CO2. The
regeneration can be accomplished with a temperature swing, such as when amine solvents are heated, a
pressure swing, such as a solid sorbent which releases CO2 when exposed to a vacuum, or with a chemical
reaction.
An ideal sorbent would have a binding energy with CO2 just larger than the 20 kJ/mol required to
pull it from the atmosphere, would be inexpensive, abundant, and non-hazardous. Research on new and
novel sorbents with these characteristics is ongoing. However, two well-known sorbents which, while
not optimal on the 1st condition, satisfy the latter three well, are aqueous solutions of calcium hydroxide
(Ca(OH)2) and sodium hydroxide (NaOH).
1.4.4 Metal hydroxide sorbents
In order to support the feasibility of air capture, example systems with metal hydroxide sorbents have
been proposed (Lackner et al., 2001; Dubey et al., 2002; Zeman and Lackner, 2004; Keith et al., 2006;
Baciocchi et al., 2006). The proposals generally include a sodium hydroxide (NaOH) or calcium hydroxide
(Ca(OH)2) solution which absorbs CO2 and is regenerated by “calcination” (see next chapter). In most
of these proposals, the specific form of the system has remained vague.
A system using Ca(OH)2 solution is outlined by Dubey et al. (2002). It consists of pools of water
saturated with Ca(OH)2 which absorb CO2 as wind blows across the surface. Solid CaCO3 is periodically
collected from the bottom of the pool and calcined (see Chapter 2). Dubey et. al. estimate a cost of about
20 $/t-CO2 for this scheme.
A related proposal is to use NaOH solution. Ca(OH)2 is still used in the regeneration of the NaOH, and
the end product – CaCO3 to be calcined – is the same. However NaOH solution has the advantages over
Ca(OH)2 that it can have lower vapor pressure to prevent water loss, it can contain higher concentrations
of hydroxide for more efficient contacting systems, and is less prone to cause scaling (accumulation of
solid carbonate minerals on surfaces). Zeman and Lackner (2004) have claimed the cost of operating a
NaOH-based capture system can be as low as 7–20 $/t-CO2 , though it has been disputed whether the
system could be operated at any cost and energy requirement low enough to make it feasible (Herzog,
2003). The component of the system which makes contact with air to extract CO2 (which we term the
“contactor”) remains a particular point of contention, with suggested forms including large convection
towers (Lackner et al., 2001), open, stagnant pools (Dubey et al., 2002), falling film channels (Zeman,
2006), and packed scrubbing towers (Herzog, 2003; Baciocchi et al., 2006). Herzog estimates that the
energy requirement of a packed tower contactor similar to the ones used to capture CO2 from power plant
flue gas would be 6–12 times the energy produced when the fossil fuel was initially burned.
In addition to energy requirements, a natural concern for any large-scale aqueous contacting system
is the quantity of water lost by evaporation. Water loss may be particularly large in an air capture system
since the low concentration of CO2 in the atmosphere requires a large amount of interaction between the
gas and liquid phases. Water loss in proposed systems has not been previously calculated or addressed.
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