Capturing CO2 from ambient air - David Keith

Draft type Height
[m]
Crosssectional
area
[m 2 ]
cost per unit
[$millions]
cost with mods
[$millions]
cost per
cross-section
[$/m 2 ]
Natural 1 90 5100 36 41 8000
Forced 1 40 3000 18 18 6000
Natural 2 120 7900 25–75 31–81 4000–10000
Forced 2 50 280 0.5–1 0.5–1 1800–3500
Table 3.2: Capital cost of cooling towers. Costs represent complete installed costs. EPA costs are from
1996, adjusted to 2006 dollars using the Construction Building Index. Upper bounds of ranges reflect
towers with plume and noise abatement and unusually high site-specific costs.
1 EPA (2002)
2 Mykyntyn (2006)
and foundation. We take the cost of this addition as $4 million, typical in the wind industry. For the
demister, our prototype demonstrated that a wire mesh filter constructed manually of stainless steel wool
can be effective with an acceptable pressure drop. However, in the full-scale system a more sophisticated
system is warranted. We obtained a price quote and product specifications from a commercial particle
trap manufacturer (Amistico, 2006) and apply those directly, ignoring the substantial bulk discount that
is likely for a project as large as even a single air capture tower. We get 500 $/m 2 -demister. With a
downward flow contactor, a reasonable placement of the demister would be as an annulus around the base
of the tower. The total area of the demister can be adjusted by the height of the annulus. Demisters of this
type collect drops more efficiently at higher air velocity but the pressure drop increases with air velocity.
We expect that a total area of the demister of one half that of the tower cross section (demister velocity
twice the tower velocity) makes a reasonable trade-off between these competing effects. This is what we
assume for capital cost calculations.
Forced-draft cooling towers are more directly adaptable to air capture since they already have fans and
demisters. They are smaller, however – typically 20–50 m high and arrayed in square cells 10–20 m on
a side. They can be constructed of concrete or fiberglass for similar costs. Again, the liquid flowrate in
an air capture version would be about a tenth that of a conventional version. Also, forced draft towers
have some “splash fill” material which we will not require. Air flow velocities are similar. We will apply
contactor costs for forced draft cooling towers directly to an equivalently-dimensioned contactor.
Table 3.2 shows capital cost estimates for power plant cooling towers and includes the cost of modifications
to natural draft towers described above. Personal communication with industry experts and
estimating documents from the EPA (2000, 2002) were used to arrive at the estimates. Cost is usually
given per unit liquid flow, which is the primary figure for which cooling towers are generally sized. Typical
flow rates and sizes were then used to calculate the implied cost per unit cross-sectional area and cost
per physical structure.
The costs span a fairly wide range. This may reflect the quality of the data sources more than actual
uncertainty in cooling tower construction; cost information is proprietary and industry tends to be loathe to
share it. Some of the variation is due to inclusion versus exclusion of components, particularly noise and
plume abatement. We will use EPA costs as the base case, and consider the lowest costs in the sensitivity
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