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372 12 Carbon Capture and Storage

high volatility of the solvent. And its corrosivity and energy intensive regeneration
result in high costs in capital and operation.
Improved amines, such as the secondary amines (e.g., DEA) and tertiary amines
(e.g., MDEA) have been considered as an alternative for MEA. Primary and sec-
ondary amines react with CO 2 quickly to form carbamate through the zwitterion
mechanism. For MEA, the corresponding heat of absorption is Q ¼ DH R ¼
2:0 MJ/kg CO 2 . The reaction of CO 2 with secondary amines has a lower enthalpy
of reaction [54], which favors regeneration of the solvent by stripping.
Tertiary amines react with CO 2 following a base-catalyzed hydration mecha-
nism, which is different from the zwitterion mechanism to form bicarbonate instead
of carbamate. The overall reaction indicates a theoretical CO 2 loading capacity of
1 mol of CO 2 per mole of tertiary amine. However, the reactivity of tertiary amines
with respect to CO 2 is lower than that of primary or secondary amines. The cor-
responding enthalpy of reaction for the bicarbonate formation is lower than that for
the carbamate formation. This means lower energy consumption for solvent
regeneration.
Piperazine (PZ) has been used in blended systems as an additive to increase the
rate of absorption of CO 2 in systems with low absorption rates but otherwise
attractive solvent characteristics [23]. Adding Piperazine into the solvent can
improve the rate and capacity of CO 2 absorption by the following reactions.

KHCO 3 ! K þ HCO ð12:46Þ
þ
3

PZ þ HCO $ PZCOO þ H 2 O ð12:47Þ

3

PZCOO þ HCO $ PZ COOð Þ þH 2 O ð12:48Þ
3 2
Reactions (12.47)and (12.48) also require low energy for regeneration. On the
other hand, thermal degradation of Piperazine in CO 2 capture is a technical chal-
lenge that deserves further investigation.
An example of an optimized process for post-combustion CO 2 capture by amine
scrubbing is available in the DOE-NETL report [19]. The CO 2 absorption rate of
Piperazine (PZ) doubles that of 30 wt% MEA with 1.8 times the intrinsic working
capacity. The incoming flue gas is cleaned by water spray to remove fine partic-
ulates and cooled down to 40–60 °C. Filtered lean solvent enters the amine
scrubber, usually packed bed from upper level and flows downward by gravity to
absorb CO 2 from the counter flow flue gas. Cleaned flue gas is further cold-water
washed to recover the penetrating solvent droplets or vapor before discharging to
the atmosphere through the stack. The washing water joins the lean solvent to form
the rich solvent exiting the scrubber at the bottom of the tower. In addition to the
main absorber, the process may also need SO 2 removal before the PZ absorber.
Recently, sterically hindered amines have also attracted considerable attention
for their low regeneration costs. A sterically hindered amine is a primary amine
where the amino group is attached to a tertiary carbon atom; it can also be a
secondary amine where the amino group is attached to a secondary or tertiary