Capturing CO2 from ambient air - David Keith

As a bound on the effects of coalescence, we will consider contactors where S = 1 m 2 /m 3 in a single fluid
system, and S = 2 in an air-assist system.
3.4 Contactor Cost
Estimating the cost of a contactor is a dual problem. On the one hand, we try to estimate the capital and
operating costs of a device to be built in the future to which no complete analogue currently exists. On the
other hand, we must assume that the future engineers of the device will optimize the design to minimize
costs, and they will have considerable leeway to do so. So the two sides of the problem, specifying the
design and estimating the costs, feed back on each other.
Air capture only makes sense in a very large scale deployment, so we can expect that engineers designing
such devices will not be limited to off-the-shelf technology or process experience from other industries,
especially since here we are not concerned with the cost of early air capture plants, but of the average or
“nth plant” cost. In order to estimate that cost today we have to make some informed judgment about what
the optimal system will look like. We must do so under considerable uncertainty about some parameters –
uncertainty that would be resolved for those future engineers. In particular, our uncertainty about the full
effects of coalescence and breakup, and about the technical potential and costs of relevant technologies,
makes specifying the contactor difficult.
Ideally we would like to build all of the relevant parameters and functions into a cost model and
perform a nonlinear optimization to find the system which minimizes cost per unit CO2 captured, as the
engineers and operators of a real plant are likely to do. However, we do not have sufficient knowledge of
the functional relationship between, for example, capital cost and contactor height, or spray nozzle type
and operating cost, to complete such a model. Instead we will choose several scenarios, and for each
scenario choose a set of reasonable (but not optimal) parameters to use in a simple cost model. The goal
will be to choose parameters such that they are on the order of the likely optimums while being well
withing the practical capabilities of known technology.
We know our model overestimates the rate of coalescence, so we will bound the effect of coalescence
by considering no-coalescence cases and cases based on the model results. We will also consider systems
using single fluid nozzles and using air-assist nozzles.
3.4.1 Mass transfer
We define the CO2 absorption rate constant, kspray, such that
dC
dt = SksprayC(t) (3.14)
where C is the CO2 concentration in air in mol/m 3 . Then kspray is the absorption rate per unit CO2 in air
per unit drop surface area, with units:
kspray ∼ ( mol
s
1 m3 m
)(
m2)( ) =
mol s
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