Capturing CO2 from ambient air - David Keith

egeneration at a high enough concentration (> 90%) for compression. The process is termed “postcombustion
capture”. One can also remove CO2 from the fuel before combustion (“pre-combustion capture”)
by converting it to hydrogen and CO2 by means of a water-gas shift reaction. CO2 is collected after
the shift then hydrogen is burned to generate electricity. The third option is to feed the plant with pure
oxygen instead of air, an “oxyfuel” system. This way, the flue gas has a high enough CO2 concentration to
be compressed directly. Energy is saved by avoiding the solvent absorption and regenerating but expended
for separation of oxygen from air.
Once compressed to a liquid, the CO2 is ready for transportation to a sequestration site, which generally
occurs via a pipeline like those currently used for oil and gas. Sequestration sites may include spent oil
fields and unmineable coal seams, where the cost of storage can be offset by enhanced recovery of oil
or extraction of methane, respectively. CO2 can also be pumped into the ocean, though this may have
harmful side-effects. The most secure type of sites are deep saline aquifers, which also have the largest
estimated storage capacity: at least 40 years of CO2 emitted at the current rate, and probably much more
(IPCC, 2005).
There are three currently operating industrial scale CCS projects, with many more planned. IPCC
expects that CCS will be widely deployed when the cost of CO2 emissions (value of emissions reduction)
reaches 25–30 $/t-CO2, and that CCS will account for a substantial share of carbon mitigation in future
scenarios with carbon restrictions.
In the form described, CCS can only work with CO2 from points sources, which currently account
for about 40% of total emissions (and many of those point sources would be small, inaccessible by CO2
pipelines, or otherwise not amenable to carbon capture). CCS can facilitate reductions in other sectors if
vehicles, home heating, and other distributed sources are made to run on hydrogen which in turn is produced
in facilities equipped with CCS. Uncertainty remains about the feasibility and cost of switching to
this “hydrogen economy”, and it may be high; Keith and Farrell (2003) calculate that the cost of switching
to hydrogen fuel cell cars could be 300 $/t-CO2.
Even with a very successful CCS program and aggressive deployment of alternative energy sources, atmospheric
concentrations of CO2 will continue increasing throughout the century (IPCC, 2000a). Because
CO2 is long-lived in the atmosphere, cutting emissions does not reduce the concentration, but rather slows
the rate of increase. If emissions are cut to zero, it would still take many centuries for natural removal
mechanisms to bring CO2 close to pre-industrial levels.
1.2 Air capture
Though nearly all current research on CCS focuses on capture from large point sources, it is also possible
to capture CO2 directly from the atmosphere, a process we call “air capture”. Compared with point source
capture, air capture has several advantages. CO2 emissions from any sector can be captured, including
emissions from diffuse sources such as automobiles, airplanes, agriculture, and home heating. The capture
unit can be located at a favorable sequestration site, avoiding the need for extensive CO2-transportation
infrastructure. Consider a future climate scenario where society has been slow to adopt mitigation measures,
but a sudden shift in the climate system dramatically raises concern and demand for action. Because
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