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362 12 Carbon Capture and Storage

conversion to CO 2 and H 2 O is quantified using the gas yield; which is defined as
the fraction of the fuel oxidized to CO 2 or H 2 O. Similar to the approaches in
oxyfuel combustion, water vapor is removed from highly concentrated CO 2 stream
that can be compressed for transport and storage.

12.4.3 Post-combustion Carbon Capture

Post-combustion CO 2 capture is mainly achieved by flue gas cleaning for stationary
sources such as a power plant and an industrial facility. Unlike pre-combustion and
oxyfuel combustion approaches, post-combustion carbon capture technologies are
more complex due to more air powered combustion-related air contaminants.
Capture of most of the contaminants has been introduced in the other chapters of
this book.
CO 2 separation is enabled after all the post-combustion air cleaning processes
introduced in the chapter for post-combustion air emission control. As shown in
Table 12.3, the cleaned flue gas usually contains 10–15 % of CO 2 that is diluted by
nitrogen gas. Therefore, an effective post-combustion CO 2 separation technology
should be able to cope with low CO 2 partial pressure. In general, physical sorption
is not effective for separation of gases with low partial pressure. Therefore, we
should focus on chemisorption technologies for post-combustion CO 2 capture.
If we consider low CO 2 partial pressure as a disadvantage of post-combustion
CO 2 capture, this disadvantage is compensated with its flexibility in retrofitting an
existing facility without major modifications. Capture ambient CO 2 has been pro-
posed as well. It can be considered as an alternative CO 2 capture process, but at
temperatures lower than those of combustion flue gases before discharge.
Despite the differences in pre- and post-combustion approaches, CO 2 must be
separated from the carrier gas. They share the fundamentals introduced in Chap. 5,
primarily the separation of gas (CO 2 specifically) from the mixture. However, at the
time of writing this book, all the CCS projects are under pilot tests. No single large-
scale plant with CCS is known to be operational continuously.
The major barrier to the commercialization of CCS process at large scale is the
high costs associated with carbon separation, transport, and injection. Currently, the
most widely used technology for CO 2 separation is based on amine-solvents, for
example, monoethanolamine (MEA). A large amount of energy is required for
solvent regeneration. The energy consumption rate, assuming a 30 wt% MEA
(aqueous) solution and 90 % of removal efficiency, was estimated to be in the range
of 2.5–3.6 GJ per ton of CO 2 . Additional energy consumption for compressing the
captured CO 2 to the required pressure of 150 bar for transportation and storage is
0.42 GJ/ton CO 2 . These numbers can be transferred to extra costs of $50–150 per
ton CO 2 removed using amines [18]. The existing CCS technologies are far from
being cost-effective and unattractive for large-scale applications. Much more
research is needed for design of low-cost sorbents and optimized process design
CO 2 capture. Our focus that follows is on the sorbent and related process design.