Capturing CO2 from ambient air - David Keith

analysis.
3.4.3 Operating cost
The physical operating and maintenance cost of a cooling tower is typically estimated at 4% of capital
per year. There is some argument that the fraction is 5% for smaller cooling towers and drops to 2% for
the largest towers (EPA, 2002), but the flat 4% is a widely accepted rule of thumb. It includes purchase
of makeup water at typical industrial prices ($0.13/m 3 ) – much more water than is likely to be used in
an air capture system. It also includes periodic replacement of pumps of which an air capture system
would require 1/5 as many. So this operating cost may be an overestimate, but neither water nor pump
maintenance makes up the majority of operating cost, so it is retained.
Electricity for running pumps and fans is the other important operating cost. Electricity is used by
pumps to lift solution up the height of the tower and to overcome the pressure drop across the nozzle,
ΔPnozzle. The fans use energy to overcome the pressure drop across the tower, which is dominated by
the particle trap, and to accelerate the air. Frictional losses are considered comparatively small and are
neglected. The total rate of energy use is then given by:
˙E = F
(ΔPnozzle + ρsolngH)+
εpump
A
(ΔPairvair + ρairv
ε f an
2 air) (3.17)
where ρsoln and ρair are the densities of air and of the solution and εpump and ε f an are the mechanical
efficiencies of the pumps and fans, respectively.
For the cost calculations we assume ΔPnozzle = 350 kPa for the single fluid case (a reasonable value,
though smaller pressure-drops are possible). In the air-assist nozzle case, we use a nozzle pressure of 280
kPa and air/liquid volume ratio of 5. The latter figure is on the small side of typical values, so there has
been some optimization of nozzle choice. We take the efficiency of the pump as 80% and the efficiency of
the fan as 70%. We purchase carbon-neutral electricity at the price, pelec, of $0.07/kW-hr, or $19/GJ. The
reflects roughly the cost of base-load electricity from nuclear or CCS plants. In practice, the electricity
could be generated on-site, potentially with excess sold to the grid. However, using a simple market rate
for electricity allows us to decouple the costs of generator and contactor.
3.4.4 Scenarios
As previously mentioned, we consider scenarios based on both mechanical draft and forced-draft cooling
towers, with mass transfer cases with and without coalescence, with the intent to bound the contactor cost.
They are as follows:
1. Favorable conditions. In this scenario, we do not choose all parameters to be as favorable as possible
or make any unrealistically hopeful assumptions, however several important parameters are set at
the favorable side of their reasonable range. The low end of capital cost is assumed for the larger
natural draft cooling tower. Dual fluid nozzles are assumed, and no coalescence takes place.
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