Introduction to Carbon Capture - Introduction to Carbon Capture and Sequestration - page 153

composition of the fl ue gas ( x fl ue ) because these streams come from the
power plant. We can, however, specify the concentrations in the capture
stream ( x cap ) and in the exhaust stream ( x exh ). The corresponding follows
from the mass balances:

( )

( )

x − x

x − x

flue cap

flue exh

n = n and n = n

( ) ( )

cap flue exh flue

x x x x

− −

cap exh exh cap

We can now compute the entropy per mole of fl ue gas as a function of
our two design variables, the compositions of the exhaust ( x exh ) and the
capture stream ( x cap ):

( )

sep

x − x

∆ S

( )

sep flue exh mix

∆= s = s x

( )

cap

n

x − x

flue

cap exh

( )

x − x

flue cap

( ) ( )

mix mix

+ s x − s x

( )

exh flue

x − x

exh cap

This yields for the given fl ow the minimum work per mole of CO 2 per
second:

( )

sep

x − x

TS ∆ T

( )

flue exh

mix

w = = s x

( )

min cap

xn xn

x − x

flue flue flue flue

cap exh

( )

x − x

flue cap

mix

( )

mix

( )

+ s x − s x

( )

exh flue

x − x

exh cap

Figure 4.2.2 shows the effect of the initial concentration of CO 2 in the
fl ue gas: the lower the concentration, the higher the minimum work per kg
CO 2 [4.4]. This relationship explains why it is not a great idea to try to cap-
ture CO 2 directly from the air. Capturing CO 2 from the atmosphere would
require up to fi ve times the energy input compared to capturing CO 2
directly at a point source, such as a power plant (see Figure 4.2.2 ). Even
among different types of power plants, there are differences in effi ciency.
Flue gasses from coal-fi red power plants have a higher concentration of
CO 2 (10-15%) compared to those from natural gas-fi red power plants