Environmental Engineering Reference
In-Depth Information
Section 1
Introduction
From a technological point of view, there are different ways to capture
CO 2 . The simplest way is to use post-combustion carbon capture ,
because it can be added to an existing power plant without having to
modify the power plant itself. For this reason post-combustion capture
is often the preferred economic solution, especially for older power
plants.
Each type of capture technology involves a different technique for
gas separation. To weigh the pros and cons of each strategy, we need to
take a detailed look at how a power plant works. Figure 4.1.1 shows a
simple coal-fi red power plant [4.1]. In the direct combustion step, pulver-
ized coal is mixed with air and burned. The heat of this combustion is
recovered to produce high-pressure steam, which in turn drives an elec-
tric turbine to produce electrical power. Simplifi ed, the chemical reaction
for this process can be written as:
fuel
+
O 2
heat
+
CO 2 +
H 2 O
Unfortunately, this equation is somewhat idealized. In real life, coal
not only contains carbon and hydrogen, but other elements (N forming
NO x and S forming SO x ) and trace metals (e.g., Hg). These impurities
have to be removed before the fl ue gas is vented into the atmosphere,
and that process requires money and effort. Tacking CO 2 separation
onto the process requires even more energy or electricity, which we
assume we will take directly from the power plant itself. This energy drain
is another important barrier for large-scale adoption of CCS, because
the process will reduce the effi ciency of a power plant (see Box 4.1.1 ).
A typical post-combustion carbon capture process is shown in
Figure 4.1.2. There is a cost in energy associated with both the capture
and the compression steps. As we will discuss in detail later, carbon cap-
ture processes involve the regeneration of the capturing materials. This
regeneration is done using heat (low-pressure steam from the power
plant), which detracts from the electricity production of the plant.
Additionally, more energy will be required in order to prepare captured
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