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374 12 Carbon Capture and Storage

12.6.3.2 Carbonate-Based Chemical Absorption
Limestone reacts with CO 2 and water as follows:

CO 2 þ H 2 O ! H 2 CO 3 ð12:55Þ

CaCO 3 þ H 2 CO 3 ! CaðHCO 3 Þ ð12:56Þ
2
These reactions take place in the processes that are used for power plant or
cement plant flue gas cleaning. Usually crushed limestone is packed in a reactor and
wetted by a continuous flow of water. The CO 2 -laden flue gas is pumped through
the reactor to enable the above chemical reactions.
Example 12.3:
How much water and CaCO 3 is needed to capture 1,000 kg of CO 2 from the flue

ð
gas? How much calcium bicarbonate Ca HCO 3 Þ is produced?
2
Solution
By considering the following chemical reactions,

CO 2 þ H 2 O ! H 2 CO 3
CaCO 3 þ H 2 CO 3 ! CaðHCO 3 Þ
2
the overall reaction formula is

CO 2 þ H 2 O þ CaCO 3 ! CaðHCO 3 Þ
2

It indicates that the mole ratio of the compound in this reaction is 1:1:1:1.
Therefore, in order to capture 1,000 kg of CO 2 , which is (1000/44) kmole, the same
mole amounts of CaCO 3 and water are consumed to produce the same mole amount
of Ca HCO 3 Þ .
ð
2
By considering their molar weight, the corresponding mass can be determined
as: 2,300 kg of CaCO 3 400 kg of water and 3,700 kg Ca HCO 3 Þ .
ð
2
Unlike other processes, the bicarbonate-rich effluent stream can be disposed in
the ocean instead of regeneration. It is environmentally beneficial in that it can
counteract ocean acidification. The challenges are the (fresh) water consumption
and the costs for transportation related to the feedstock and product disposal.
Alternatively, potassium carbonate (K 2 CO 3 ) can be used as a regenerable
chemical absorbent. The corresponding CO 2 absorption reactions are

K 2 CO 3 þ H 2 O ! KOH þ KHCO 3 ð12:57Þ

KOH þ CO 2 ! KHCO 3 ð12:58Þ