Absorption - Introduction to Carbon Capture and Sequestration - page 210

exhaust capture

Heat
exchange

steam

Q in

Q out

flue gas

Figure 5.7.1 Simple absorption process with heat exchange
The most simple absorption process requires large amounts of energy to heat the sol-
vent in the regeneration step. By using a heat exchanger, a signifi cant amount of energy
can be recovered.

Optimizing amine scrubbing

We learned in Section 4.2 that the minimum theoretical energy requirement
for separating CO 2 from fl ue gas and compressing it to 150 bar is about
7% of the power plant's output. Let us borrow some typical numbers from
Professor Rochelle's lecture to see how these energy requirements trans-
late to costs. An energy cost of 20-30% of the power plant's output trans-
lates to a carbon capture cost of about $20 per tonne of CO 2 (2011
numbers). The energy cost is not the only cost associated with adding a
carbon capture process to a power plant. The second major cost is the
capital equipment cost for CO 2 capture, which for an 800 MW power plant
will be about a billion dollars. About a third of that cost would be for the
absorber, a third for the stripper, and a third for the compressor. This costs
us an additional $20 to $40 per tonne CO 2 . The third major cost associated
with amine scrubbing results from the amine degradation. Amines can
oxidize and thermally degrade, so we have to replace the solvent periodi-
cally, which adds about $1 to $5 per tonne CO 2 . Environmentally, the costs
are higher because these degraded amines need to be treated.

Irreversibility

The minimum energy required for a separation is an absolute thermody-
namic minimum, independent of the process that is chosen. For a given