Capturing CO2 from ambient air - David Keith

2. Median assumptions, no coalescence. In this scenario, the higher EPA capital costs for a naturaldraft
tower are used. Modest, single-fluid spray conditions are assumed (parameters are not optimized
for a no-coalescence world, but rather left at low density where coalesce may not matter when
all effects are accounted for).
3. Median assumptions with coalescence. This uses EPA costs for the forced-draft tower and modest,
single-fluid spray conditions.
4. Dual-fluid nozzle with coalescence. Same as 3, but with dual-fluid nozzle conditions.
5. Coalescence with dual fluid, lower capital. This scenario uses the low capital estimate for a forced
draft tower, illustrating the strong influence of capital cost in coalescence conditions.
6. Optimized. This scenario uses the lower cost estimate for a forced draft tower, dual-fluid nozzles,
and no coalescence. Additionally, some parameters are tuned to slightly more favorable values based
on what, in the author’s subjective opinion, would be possible in an optimized system. The nozzle
pressure is dropped from 280 kPa to 180 kPa, the pump efficiency is brought from 80% to 90%, and
the spray constant is increased to 4 × 10−3 m s , reflecting a higher molarity solution.
3.4.5 Total cost
We consider three main components of the contactor cost: capital, operation and maintenance, and electricity
for operating pumps and fans. Though there are other costs, we expect these to dominate. Capital
is amortized at a 15% capital charge rate. If we denote the amortized capital cost by Cap and the yearly
operating cost, excluding electricity, by O&M, then the total cost per unit CO2 captured is:
total cost Cap+O&M + pelec ˙E
=
CO2 captured ˙M
where ˙M is calculated from equation 3.16 and ˙E is calculated from Equation 3.17.
Parameter choice
(3.18)
For each of the scenarios described in the previous section, we have a fixed geometry, spray distribution,
capital, and maintenance cost. The two remaining tunable parameters are vair and F. When these are
chosen, ˙E and S can be calculated and then ˙M. Both parameters effect the total cost in complex ways.
Together they determine the spray density by Equation 3.13 which in turn determines S. With other
parameters fixed, larger F will increase ˙M (by increasing S), but also increase ˙E. As sprays become more
dense, coalescence causes them to be less efficient and so energy cost overwhelms the total at high F. At
low F, both ˙E and ˙M are small and so capital cost overwhelms the total. For the scenarios, we choose
F such that spray densities remain low enough that the spray retains a reasonable efficiency, or, in the
no-coalescence cases, low enough that the spray could be expected to retain a reasonable efficiency if
coalescence were accounted for. It is a somewhat subjective choice. The alternative would be to perform
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