Capturing CO2 from ambient air - David Keith

Chapter 4
Cost of air capture
Although a handful of researchers have described example air capture systems and two other researches
have estimated the energy requirements of a system (those results are reproduced in Table 2.4), serious
end-to-end cost estimates of a well-specified system are scarce. Such an estimate is attempted here. We
limit ourselves to calcination-based systems. Clearly the cost of air capture with such a system cannot fall
below the cost of calcination alone in a well-optimized, large scale industrial system. Thus in the Section
4.1, we calculate a lower bound cost based on industrial lime manufacturing.
We also expect that the cost of air capture at the time it is deployed, likely decades in the future, would
not exceed the cost of a system that could be built today with well-known technology. And so in Section
4.2 we estimate the cost of an air capture system using current technology. We also make an estimate
using some newer technology that is likely to be available in the near term, like an oxyfuel fluidized-bed
calciner and pellet reactor for the caustization reaction.
4.1 Lower bound
Theoretically, the energy demand of the proposed system is dominated by the Calciner, where CaCO3 is
heated to release the captured CO2, Reaction 3 in Table 2.1 (this dominance is borne out by cost estimates
of the components of the total system). This reaction has a large thermodynamic energy requirement which
must be overcome in even the most advanced calcining system, and significant capital requirements in
terms of a large, high-temperature reactor which must be precisely tuned to maintain calcination efficiency.
The calcining operation in its simplest form is performed by the lime manufacturing industry at large
scale and using long-established technology. In the production of high-calcium quicklime (CaO) in particular,
crushed Calcite (CaCO3) is heated in a kiln to form the product. Through the industry’s long
experience, it has developed a very efficient process, with energy requirements close to the thermodynamic
limit (≈ 85%). One can make arguments for how the costs of the other components of the air
capture system can be greatly reduced by advances in design or economies of scale, but it is unlikely that a
calcination-based air capture system could be less expensive than the current industrial calcining process,
which has been optimized through decades of industrial experience. 1
1 A possibility which might be argued as a way to reduce the energy requirements below the requirement of Reaction 4 (and
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