Absorption - Introduction to Carbon Capture and Sequestration - page 207

new materials or processes. If you have just synthesized two new materi-
als for carbon capture, which is the best material? Eventually it will be the
one with the lowest capital and operating costs. We need guidance as to
how to estimate these costs.
One important reason carbon capture is so expensive is that the
process requires a large amount of energy. One way of expressing the
energy costs is to determine the parasitic energy of the carbon capture
process. Minimizing this parasitic energy is a guiding principle which we
will use to compare different materials. We fi nd that this parasitic energy
is based on purely physical properties of our materials, and therefore
provides a good metric to compare materials. In addition, minimizing the
parasitic energy will contribute to a more effi cient use of fossil fuels, a
concern of many researchers.
The main components for determining parasitic energy are the
energy required for the separation and the energy needed for compres-
sion of CO 2 for geological storage. In Section 4.2, we used a thermody-
namic analysis to estimate the minimal energy from the ideal entropy of
mixing. In this section, we compare this energy with our estimates of the
energy cost of an absorption process.
Parasitic energy of absorption
To estimate the parasitic energy of the absorption process, we need to
make some simplifi cations that allow us to focus on the performance of
a solvent. We will focus on the energy required to absorb and desorb
CO 2 , ignoring, for example, the energy costs to transport the fl uids, fl ue
gas, and other operations that cost energy. As a result, our estimate will
be lower than the real energy costs. For an amine solution (MEA), for
example, we know the actual energy costs are 30% higher than our cal-
culations for a complete design [5.11]. Similarly, we assume that other
solvents will add around the same percentage of energy burden onto our
calculations. But when we are interested in comparing different solvents
for our design, the parasitic energy remains a useful metric.
The simplest absorption process is shown in Figure 5.6.1. The CO 2
from the fl ue gas is absorbed in the solvent, which is later regenerated in
the stripper. As the absorption of CO 2 releases heat, the absorption pro-
cess will not cost any energy. In the stripper we need to heat the solvent
so that the CO 2 is released. This heat has two contributions: the sensible