Capturing CO2 from ambient air - David Keith

transfer rate ( ˙M). We can further break down energy use into the 3 most important quantities: lifting
energy, ˙Eli fting, nozzle head energy, ˙Enozzle, and fan energy, ˙E f an. Following Equation 3.17 (which is
general for most spray-based contactors), we can express these quantities in terms of the parameters which
system designers have substantial control over:
˙Eli fting ∝HF
˙Enozzle ∝ΔPnozzleF
˙E f an ˜∝ΔPairvair
where “ ˜∝” denotes “approximately proportional to”, because we are neglecting (relatively unimportant)
inertial effects.
Most design decisions trade off between lowering one type of energy use and raising another, or between
energy use and capital cost. For example, a very tall contactor with very low spray density would
operate very efficiently, but the capital cost per unit CO2 captured would be very large. The goal is to work
with is to work with the trade-offs to minimize total cost per ton.
We have considered designs based on power plant cooling towers because the cost of such structures is
well known. A better design can clearly be achieved by designing the system from scratch with air capture
specifically in mind, but data were not available for us perform an optimization of structural design. With a
more detailed understanding of component-wise capital costs and spray technology, and a more complete
model of drop collision and coalescence, a significantly different form may emerge. Shorter towers,
taller towers, fiberglass skin towers, counter-current designs, and many other variations are possible. A
description of some possible alternative designs follows.
Shorter tower
Most industrial spray towers are shorter than the power plant cooling towers we have considered. SO2scrubbing
towers are typically on the order of 10 m high, for instance. As we saw in Figure 3.11, coalescence
in shorter towers tends not to be as important so that the absorption by the spray per unit height
remains high: ˙M decreases but ˙Eli fting decreases proportionally. However, ˙Enozzle and ˙E f an remain unchanged
and so become relatively more important to total cost. This is the fundamental trade-off in setting
contactor height: at high H, nozzle and fan energy become less important but coalescence drives up
˙Eli fting. At low H, ˙Enozzle and ˙E f an tend to dominate. Of course, H also affects capital cost, with shorter
towers presumably being less capital-intensive. However, the ratio Cap/ ˙M is the important quantity, and
it is not clear how that relates to H.
If three conditions can be met, then short towers may offer significantly lower costs and than the
estimates for 50–120 m towers: (1) short towers can be constructed with much lower capital than tall
ones, (2) nozzles can be used with relatively low ΔPnozzle, and (3) ΔPair or vair can be adjusted to give
a sufficiently low ˙E f an. At least condition 2 appears likely from our knowledge of commercial spray
technology.
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