Introduction to Carbon Capture - Introduction to Carbon Capture and Sequestration - page 154

20

CO 2 in air (380-580 ppm)

15

10

CO 2 in NGCC (5-8%)
CO 2 in PCC (10-15%)

5

CO 2 in IGCC (40-60%)

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Mole fraction of CO 2

Figure 4.2.2 Minimum work to capture CO 2 as a function of the initial concentration of
CO 2 in the fl ue gas
The fi gure illustrates the differences in minimum work to capture 100% of the CO 2 for fl ue
gasses from different processes: IGCC (integrated gasifi cation combined cycle), PCC
(coal), NGCC (natural gas), and capture directly from air. Figure based on data from [4.4].

(5-8%). Because of this difference, carbon capture from gas-fi red power
plants tends to be relatively more expensive than capture from coal-fi red
power plants. This explains why most CCS research to date has focused
on coal-fi red power plants; they produce more CO 2 and are therefore bet-
ter entities to fi rst consider for capturing CO 2 more effi ciently.
Another interesting way to look at the minimum energy requirement
is to express it as a fraction of the total energy produced [4.5]. The aver-
age USA coal-fi red power plant generates 3.43 GJ net electricity per
tonne of CO 2 emitted. Figure 4.2.3 shows that if we capture 100% of the
CO 2 with a fl ue gas of 15% CO 2 , the minimum energy needed to capture
is 5.12% of the electrical energy generated by the power plant. If we
capture 90% of the CO 2 , this number reduces to 4.22% of the electrical
energy generated by the power plant. This calculation shows that it costs
20% more energy to separate the last 10% of CO 2 from the fl ue gas — a
disproportionate amount of energy for the outcome. For this reason most
regulations do not require 100% capture, but a much more sensible 90%.
For a typical reference value for a coal-fi red power plant (12% CO 2 ), the
minimum energy for a 90% separation is about 158 kJ/kg CO 2 [4.6].