Capturing CO2 from ambient air - David Keith

Chapter 5
Discussion
Overview
In this thesis, we have analyzed the potential performance and cost of technology for CO2 capture from
ambient air. We began by briefly describing the climate change problem and carbon management options
and identified the unique features of air capture that could help address this problem. We argued that the
fundamental physics of air capture is favorable and so a low-cost air capture technology can likely be
developed. Still, many energy experts are skeptical that air capture is a realistic pursuit, and so proponents
of air capture (including the author) have turned to existing technology to demonstrate the feasibility of
the process, outlining several example systems. The most convincing of these examples uses an aqueous
NaOH sorbent and calcination-based regeneration system. We developed this concept into a complete
system, based on current industrial processes, for which the capital costs, operating costs, and energy
requirements can be estimated. Because no ready analogy with cost information was available, we developed
an example contactor in detail, and built a pilot-scale prototype. Using insight from this experimental
work, we estimated the cost of a complete system.
5.1 Findings and implications
In the prototype contactor, we demonstrated the core features of a spray-based contactor, suggesting it is
feasible in some form. We identified the design constraints on a successful full scale contactor. Small
drops, low capital cost, and low spray density seem to be the routes to a low-cost contactor. On the other
hand, coalescence of drops challenges the efficient operation of a contactor and high capital costs can
drive up the overall cost substantially. Given current evidence, we can not say that contactor costs are
small compared with the total air capture system; they may make up a large fraction of the total. However,
there is enough room for improvement by redesign that we expect the contactor not to be the dominant
cost. With energy requirements for packed towers and spray towers all tending toward the range of 0.5–1.5
GJ/t-CO2, it seems the contactor would have an irreducible cost of 10–30 $/t-CO2 for the electricity.
Our analysis strongly suggests that a spray-based contactor is feasible in some form. Though there
remains considerable uncertainty in this analysis, it is not dramatically different than that one obtains in
the assessment of many future energy technologies. The cost estimate of 250 $/t-CO2, is very high. That
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