Environmental Engineering Reference
In-Depth Information
CO
2
to
compression/
dehydration
Condenser
Vent gas to
reheat stack
Reflux drum
Reflux
pump
Lean amine
cooler
Storage
tank
Booster
pump
Regenerator
(stripper)
Absorber
Cross
exchanger
Reboiler
Na
2
CO
3
MEA
reclaimer
Flue gas
from
power plant
Filtration
Sludge
Figure 10.10
Flowsheet of the monoethanolamine (MEA) process for CO
2
capture.
Solvent absorption has been used for decades for producing CO
2
from flue gas. The produced
CO
2
is used for dry ice manufacturing and for carbonated drinks, and it is used in large quantities
for enhanced oil recovery at semi-depleted oil reservoirs where the injection of high-pressure CO
2
into the reservoir helps to bring forth more oil to the surface. However, for all these uses, CO
2
is
captured from the flue gas of
natural gas
combustion. The reason is that coal and oil combustion
produce copious quantities of SO
2
,NO
x
, and other contaminants that “poison” the solvent. If coal
or oil were used as the source of CO
2
, the flue gas would have to be thoroughly purified before
entering the absorption tower, or the coal would have to be gasified before combustion.
The thermal efficiency of a coal gasification combined cycle power plant with CO
2
capture
by MEA is 30-35% compared to 40-45% without capture, and the cost increment of electricity
production is around 50%. Thus, this method is less efficient and costlier than estimated for air
separation-CO
2
recycling, but the technology is well established, whereas the oxygen combustion
technology is still in the development stage.
10.4.3.3 Membrane Separation
Gas separation by membranes relies on the different permeation rates of gases through the membrane
pores. Hydrogen, in particular, because of its small molecular size, permeates faster than any other
gas through small pores. Polymer membranes are being used commercially in recovery of hydrogen
from a purge stream in ammonia synthesis, methanol synthesis, and oil refineries. Typical membrane
examples are polysulfone/silicon, cellulose acetate, polyphenyleneoxide, polyalkene, polyimide,
polydimethylsiloxane, and polyetherimide.
The membrane method could be used for capturing CO
2
from a mixture of CO
2
and H
2
, which
is the product of coal gasification and the water shift reaction (see Section 5.3.2). This gas mixture
is subjected to membrane separation. The hydrogen permeates much faster through a membrane
than does CO
2
, and only a few stages of membrane separation would be necessary for almost 100%
separation of the gases. The separated H
2
can be used as a fuel in a gas turbine or in a fuel cell for
power production. The separated CO
2
would be compressed and sequestered.
