Energy and Electricity - Introduction to Carbon Capture and Sequestration - page 20

Section 3
Making dreamium™

Storing CO 2 in geological formations almost looks like a waste of
resources. Why is conversion of CO 2 into a useful product not the main
topic of this topic?
To answer this question we first look at the thermodynamics of car-
bon, shown in Figure 1.3.1 . We see that by burning carbon we decrease
its energy. However, CO 2 is not the lowest energy thermodynamic state;
the lowest energy state is the mineral carbonate (e.g., limestone). Indeed,
as this diagram implies, most of the carbon on earth is in the form of
limestone . The famous white cliffs of Dover actually contain much more
carbon than we can find in the atmosphere!
Unfortunately the kinetics of converting CO 2 into limestone are
extremely slow, so converting CO 2 to limestone is not possible on a short
time scale. (In our chapters on sequestration, we will show that the kinetics
are very slow because the formation of limestone (CaCO 3 ) requires one
atom of calcium for each molecule of CO 2 . The availability of vast quanti-
ties of calcium is controlled by the slow dissolution rate of calcium-contain-
ing silicate minerals.)
Figure 1.3.2 illustrates the current uses of CO 2 . The bulk of CO 2 is used
in enhanced oil recovery, where CO 2 is injected into an oil field to reduce
the viscosity of crude oil and thus facilitate its transport and pumping to the

Carbon

400 kJ/mol

Carbon dioxide

60-180 kJ/mol

Carbonate

Figure 1.3.1 Energy states of carbon
The range 60-180 kJ/mol reflects the different carbonates. Figure adapted from a pres-
entation by Klaus Lackner.