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COAL
The dramatic increase in cost for the IGCC case
shows that using absorption on the gas turbine
exhaust is not an effective way to capture C02.
Hence alternative schemes are based on treating
the gasifier product gas to concentrate the car
bon before combustion and take advantage of
the operating pressure. Therefore an additional
exercise looked at an IGCC system where the
fuel gas was shifted in a high and a low tempera
ture shift reactor and then cleaned in a Selexol
unit (IGCC Selexol). The H2S and C02 leave the
unit in separate streams. The cleaned fuel gas is
burned in the gas turbines. Results are shown in
the last column of Table 1 .
Physical absorption using Selexol is the most
appropriate technique to remove C02 from IGCC
fuel gases. A higher gasification pressure will
facilitate the C02 removal and increase the over
all power production efficiency. The use of more
advanced gas turbines could result in an in
crease in the overall efficiency.
Comparing the Capture Options
Processing techniques for the capture of C02 are
predominantly influenced by the concentration or
partial pressure of the gas to be captured.
Table 2 illustrates some results for several CO
Power System
PF+FGD Base Case
+ Membrane
+ Membrane & MEA
+ Absorption (MEA)
+ Cryogenics
+ Adsorption PSA
+ Adsorption TSA
TABLE 2
capture alternatives as applied to just the
PF+FGD option. It illustrates how the cost of
C02 avoided relates to cost of C02 captured i.e.,
it incorporates the extra C02 produced as a
result of generating the power required of the
capture process. The cost of C02 avoided is not
the complete story. It is only of value as a
measure when comparing capture results for the
same fuel and power generation technology.
None of the alternative capture processes
(membrane separation, cryogenic distillation,
pressure swing adsorption, and temperature
swing adsorption)
proved more economical than
monoethanolamine (MEA) absorption.
Conclusions
At the moment the conventional approach to cap
ture CO from a PF+FGD, or GTCC plant is to
"scrub"
the flue gas using absorption technology.
Currently, MEA is the absorption technology of
choice for capturing from powerplants. It is
C02
a fully proven technology bearing no technical
risk.
When analyzing short- to medium-term tech
nologies associated with IGCC, the Selexol
process itself requires relatively
CAPTURE DATA FOR THE PF+FGD SYSTEM
Efficiency
m
40
31
30
29
28
29
4-41
Power Cost
(mills/kWm
49.0
77.6
74.7
74.0
114
179
Cost C02
Avoided
($/tonne)
45.0
42.3
35.0
84
264
Emission Rate
ofC02
(oCOa/KWTi)
829
194
222
116
57
335
little energy.
THE SYNTHETIC FUELS REPORT, JANUARY 1995