Capturing CO2 from ambient air - David Keith

Scenario # 1 2 3 4 5 6
Tower type natural natural forced forced forced forced
Capital estimate low mid mid mid low low
nozzle type dual single single dual dual dual
coalescence model no coal. no coal. coal. coal. coal. no coal.
Capital cost 9 39 226 140 37 8
O&M 2 10 60 37 10 2
Pumping electricity cost 18 18 28 21 18 9
Fan electricity cost 6 6 15 9 8 4
Total cost 40 70 300 200 70 20
Table 3.3: Cost estimates for the contactor. Costs given in $/ton-CO2.
a multivariate nonlinear optimization to find F in each scenario, but this is both computationally difficult
and not likely to yield realistic results.
The choice of F is tied to vair by the spray density relationship. But if we assume F is adjusted to
maintain constant spray density and thus constant S, we can look at the other effects of vair. Since vair
controls the residence time of air in the contactor, it controls the outlet CO2 concentration. For high vair,
the CO2 depletion problem described in Section 3.3.1 becomes unimportant, increasing ˙M slightly, but ˙E
becomes very high because of the needed fan energy. At low vair, the inverse is true. Another constraint
on vair is that very small values may not be stable because of interference from natural wind. We take 2
m/s as a minimum plausible value. Otherwise vair is set to maintain the captured fraction of CO2 between
40 and 60%. The optimal value appears to be in this range, and variation within this range does not change
costs drastically.
For example, in Scenario 2, we have a 90 m tall tower, single fluid nozzle, and no coalescence. For this
scenario we will target a spray density yielding S = 6m 2 /m 3 . The top curve in Figure 3.10 illustrates the
relationship between F and S for the no-coalescence, single-fluid case, and F is 0.3L/(s · m 2 ) where S = 6
for vair = 2m/s. Plugging the values into Equation 3.15 we find that CO2 depletion is somewhat high. It
turns out that a higher vair reduces total cost, so we move to vair = 4 and F increases proportionally to
0.6L/(s · m 2 ). We could have alternately chosen vair = 3 resulting in lower energy use but higher relative
capital for about the same total cost. Running these parameters along with the capital and maintenance
costs described in previous sections through Equations 3.16–3.18 gives the total cost.
Results
A spreadsheet model was used to calculate costs for each of the six scenarios following the procedure in
the example for Scenario 2 above. A summary is presented in Table 3.3.
We first see that even the lowest cost, for the “optimized” system, is significant on the scale of the
total system, which has been previously estimated at 150 $/t-CO2 (Keith et al., 2006). Costs are overall
large, even in the case of no coalescence. Energy costs are the majority in the two lowest-cost scenarios,
elsewhere capital cost dominates. Particularly in the scenarios with coalescence, capital cost becomes
huge so that, in two cases, the contactor costs are likely to overwhelm costs of the balance of the air
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