Page 396 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

12.6 CO 2 Separation by Absorption 375

These reactions are slow at low CO 2 partial pressure, although the regeneration
of the absorbent can be achieved at relatively low desorption energy of about
0.9–1.6 MJ/kg-CO 2 . Therefore, it is not recommended for post-combustion CO 2
capture.

12.6.3.3 Aqueous Ammonia-Based Chemical Absorption

An aqueous ammonia solution reacts with water and CO 2 to form ammonium
carbonate and ammonium bicarbonate. The overall reaction can be described as
follows:

NH 3 þ CO 2 þ H 2 O ! NH 4 HCO 3 ð12:59Þ

Ammonia based chemical absorption/scrubbing also can capture SO 2 and NO 2
from a flue gas. While it allows multiple air emission control, it is challenging to
produce pure acidic gases. The best option is to use the ammonium salts as fertilizer
feedstock.
A typical ammonia-based CO 2 capture process is as follows. In a counter flow
absorption tower flue gas flows upward with aqueous ammonia downward. The
absorption process operates at near-freezing conditions (0–10 °C) with the flue gas
cooled by the upstream de-SO 2 process. The low temperature allows high
absorption capacity and reduces ammonia evaporation, which is also called
“ammonia slip.”
Downstream the absorption tower, ammonia slip is further reduced by cold-
water washing [39]. The effluent gas contains mainly nitrogen, oxygen and low-
concentration penetrated CO 2 . The solvent is regenerated at a temperatures of
>120 °C pressures of >2 MPa. Cold-water washing is also employed to reduce
ammonia slip in the generation process.

12.6.4 Ionic Liquids as CO 2 Solvents

Ionic liquids (ILs) have been developed for the physical and chemical absorption of
CO 2 . ILs are melting organic slats with unique properties. They comprise a large
organic cation and a small inorganic anion. The use of ILs as CO 2 solvents is
believed to have many advantages over conventional amine-based solvents, such as
potentially lower energy consumption in the solvent regeneration step, lower vol-
atility, lower vapor pressure, non-flammability, more thermally stable, and easier
recycling. However, all these features are subjected to further R&D evaluation, and,
they come with an unusually high manufacturing cost [9].
A great number of ILs have been developed and tested in laboratory. Among
these ILs, the imidazolium class is the most widely investigated and reported. ILs