Capturing CO2 from ambient air - David Keith

technologies already operating. The status of those technologies is instructive. If we just knew the
reactions in Table 2.1, we wouldn’t necessarily be convinced that a system based on that chemistry is
practical at a large scale. However, knowing it is practiced in the pulp and paper industry indicates it
is. And the argument can go in the other direction: knowing that the kraft process is used in the pulp
and paper industry and that they do not recover heat from the slaking reaction begs the question:
why not? It may indicate that heat recovery from slaking would also not be possible in other caustic
recovery applications, like air capture.
4. Consider the state of the art. How much of the system can be assembled with available components?
How much does this cost? This is the hard part. Given that we have come this far, we may
be able to make an estimate of performance based on known technology (or near-term technology
assessed for other applications) that gives a meaningful answer to the decision maker’s questions.
Note that there is a fundamental asymmetry here: an estimate of performance that reaches “good
enough” on some measure supports the feasibility of the technology because real future performance
is likely to be better than a present-technology version. But an estimate of very low performance
is not necessarily meaningful. It could be that the system used for analysis was poorly chosen or
that a key supporting technology doesn’t exist but may still be developed. Herzog (2003) falls into
this trap when attempting a cost estimate for air capture. He uses a current-technology version of
a contactor and finds the energy costs to be two orders of magnitude higher than what we found in
this paper, from there concluding that the concept is not feasible. He uses an apparently-plausible
industrial analogy, but fails to optimize it sufficiently for the new purpose. This is related to what
was referred to in Section 3.4 as the “dual problem” of assessing a system and developing a version
of the system for assessment. It is difficult to do the latter without actually engineering the system
in a detailed way and attempting to optimize it. However this is not the goal of a technology assessment,
so some middle ground must be found. Some degree of cleverness must be used to perform
the analysis on a favorable version of the technology.
5. What technological gains must be made for the technology to work? If we have reached this
step with no definitive answer, than we must start thinking in terms of likelihood rather than absolute
bounds. The question is, “given the performance calculated in Step 4”, or, “given the need for
supporting technology X”, “how much innovation is necessary for this technology to be useful?”
And then consider the likelihood of that innovation. Note that supporting technologies may come
from outside the field, as has all of the technology we have discussed for air capture. If uncertainty
about some physical process, or about the structure of the market, or about some other fundamental
factor is impeding the analysis, we might ask what answer, when the uncertainty is resolved, would
kill the technology. And then suggest further research on those points.
6. Consider the positive and negative effects this technology would have, if realized. Is support
of the technology worthwhile? If it became technically and economically viable, what direct and
indirect consequences would arise? This step is probably under-emphasized in technology assessment.
When considering the history of technology and unintended consequences, its importance is
obvious.
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