In addition to the capture and recovery chemistry, there are problems in moving the great quantity of air which must be processed. Consider a facility that captures 1 MtCO2/yr. This value has been used in our work for the nominal full-scale plant in part to match the scale of some unit operations in current industrial practice, and because a large capacity is needed to make progress against total global emissions. Current commodity chemical plants for products such as ethylene/propylene, ammonia and methanol (as well as coal-fired power plants) are already being built at this scale. Assuming 400 ppm CO2, 50% capture, and 90% annual availability of the capture system, the system would have to process 500 million kg/hr of air, or 6.5 million m3/min, or a cubic kilometer of air at standard conditions in about 2.5 hours.
This amount of air could be moved by either natural draft or forced draft using low-head fans. In either case, in the absence of an ambient wind of greater velocity than the flows into or out of the air capture plant, there would be a tendency for the air capture facility to recirculate low-CO2 effluent air to its intake. This condition sets the facility scale. Assuming a typical wind velocity of 5 m/s, about 20 000 m2 of intake area - or a square 150 meters on a side - is needed to collect the necessary amount of air. It is plausible to imagine a facility comparable in size to an open-roof sports stadium or, more likely, a number of separate smaller air contacting units all feeding a CO2-rich sorbent liquid to a central CO2 recovery facility.
Restricting air-side pressure drops to the stagnation pressure p V2/2 obtainable without external energy input corresponds to operation at a pressure drop in the range 50-150 Pa. We can imagine developing this pressure difference through a combination of near-stagnation pressure at the inlet and a reduced pressure in a venturi or aerofoil system at the exhaust point, or we may consider it to be an initial estimate of the head to be developed by whatever type of auxiliary (i.e. wind-augmenting) fan system might be used.
While this pressure drop is low, it is in the range of other large industrial systems. Chemical plant distillation towers designed for vacuum conditions use low-pressure-drop packing to minimize the pressure and temperature at the bottom of the column and therefore the degradation of thermally sensitive chemicals. Evaporative cooling towers perform a mass- (and heat-) transfer function similar to a CO2 absorber and have been built at very large scales. Windmills demonstrate that several megawatts of useful work can be recovered from a moderate wind from a swept area similar to the intake area of the air-capture plant. This energy is available to move air through the contacting system and is probably best tapped as the original kinetic energy rather than as converted electricity. This value also suggests the magnitude of fan work that would be needed to maintain production on windless days.
The large amount of air to be handled also affects the amount of sorbent liquid which must be circulated and brought into contact with the air. If a conventional countercurrent contacting system were to be used, there would be a significant energy penalty for lifting to the top of that contactor an amount of liquid which is of similar order of magnitude as the mass of air to be processed. That lifting work cannot be recovered because the liquid remains at one atmosphere pressure (i.e. does not develop any elevation head) as it falls freely down over a packing material which spreads it into thin layers and streams with a large surface area (note that the mixing of these layers as they flow moderates a decline in the absorption rate as the surface liquid becomes saturated with CO2). In our exploration of contactor operation we have found that continuous liquid circulation is not necessary, and that periodic pulsed addition of sodium hydroxide sorbent solution to the top of the contactor is sufficient (Section 6.5).
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