POWER May 2015 - 48

CARBON CAPTURE AND SEQUESTRATION
1. MEA absorption regeneration process. The MEA process involves these major
unit operations: flue gas cooling (bottom left), mixing and reaction with amine-based solvent
(top, right), and stripping of the CO2 from the amine and recycle of the solvent (right side).
Courtesy: Worley Parsons
higher in the fuel gas stream and the CO2
fraction
is not as dilute. This reduces the size and
cost of separation process vessels.
Syngas from a coal gasifier generally contains
hydrogen and carbon monoxide. The
water-gas shift reaction (in which water and
CO react to form hydrogen and CO2
ployed upstream of the CO2
) is emstripping
unit.
Other impurities (such as sulfur and particulate)
are also stripped out upstream.
The 582-MW greenfield Kemper project
uses Mississippi lignite as feedstock in a
gasification process specifically tailored for
low-rank coal, called transport integrated
gasification (TRIG) and developed by Southern
Company and the engineering firm KBR.
The process (for power plant applications)
is based on air-blown circulating fluidized
bed gasification (Figure 3). Most IGCC units
operating to date employ oxygen in an entrained
bed reactor.
The plant has experienced numerous
schedule delays and cost overruns (total
costs are running close to $6 billion), but
the combined cycle has been running on
natural gas, and the gasifier/CCS portion is
scheduled to start up by May. It is expected
that 65% of the CO2
(3 million tons annupatible
with the world's huge investment in
the fossil-fuel infrastructure. " SFA Pacific's
Simbeck reaches a similar conclusion, stating
that only the MEA and modified MEA
processes can be reasonably considered by
power plant owner/operators at this time.
Technology Contenders
The only currently operating full-scale power
generating facility with CCS is Boundary Dam
Unit 3 in Saskatchewan, Canada. The CCS
technologies used by SaskPower's Boundary
Dam and the other projects expected to start
up before 2020 have to be considered leading
contenders, having achieved reasonable scale
and significant operating time.
Boundary Dam-Shell Cansolv. The
Boundary Dam project captures and sequesters
90% of the CO2
, or 1 million tons
annually, from a 1960s vintage 150-MW
lignite-fired unit as part of enhanced oil recovery
(EOR) operation. CCS and a 66-kilometer
pipeline were added as part of a retrofit
and refurbishment program that proved to be
less expensive than installing a new gas-fired
CC, according to SaskPower. Total project
cost is close to $1.5 billion, $240 million of
which was picked up by the Canadian government.
SaskPower reported in February
that, after 130 days of commercial operating
experience, the performance of the plant was
" exceeding expectations. "
The proprietary CO2
stripping process
used at Boundary Dam is Shell Cansolv, one
48
of several amine-based chemical solvent processes
(Figure 2). Generally, amine-based
stripping involves a solvent, such as MEA,
di-ethanol amine (DEA), or methyl di-ethanol
(MDEA), often with other compounds
in the mixture to improve performance. The
solvent forms " weak " chemical bonds with
CO2
, making it easy to strip the CO2
/CO2 removal process.
in a
subsequent regeneration step, usually by applying
heat. Note that Cansolv is a combined
SO2
SaskPower reports that the parasitic energy
requirements are 21%, lower than the
typical 24% to 40% reported for other CCS
schemes. The utility has also reported that
it could shave $200 million off of a similar
project based on the lessons learned thus far.
Part of the learning process has involved a pilot
unit built and operated at Boundary Dam
and another CCS research facility at SaskPower's
Shand station.
After several years of operation, Boundary
Dam would qualify for a TRL of 7, according
to the National Coal Council report referenced
earlier. Important as it is, the Boundary
Dam project was originally planned to be
three times the size actually completed.
Kemper County Energy Facility-IGCC.
Mississippi Power Co.'s integrated gasification
combined cycle (IGCC) Kemper County
Energy Facility represents the pre-combustion
approach. Its essential advantage is that the
gasifiers operate at elevated pressures (500
to 1,000 psig) so the CO2
partial pressure is
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ally) will be captured. As at Boundary Dam,
Kemper's CO2
60 miles away. Selexol CO2
is destined for an EOR site,
stripping is
employed and is distinguished from aminebased
processes through use of a physical
solvent. Generally, physical solvent processes
require less steam to strip out the
weakly bonded CO2
The target CO2
.
emission rate from the
facility is 800 lb/MWh, significantly lower
than the EPA's projected standard of 1,100
lb/MWh. Other byproducts from the facility
will be sulfuric acid and ammonia.
Once operating for several years, Kemper
could qualify for a TRL of 8, according to the
Fossil Forward report.
Petra Nova-KM CDR. At the WA Parish
plant in Texas, one of the largest coal-fired
stations in the U.S., owner/operator NRG
Energy will capture 1.4 million tons of CO2
as a 250-MW (equivalent) slipstream (90%
CO2
removal) from a 600-MW unit and deliver
it via 82-mile pipeline to EOR fields in
West Texas. Mitsubishi Heavy Industries'
(MHI) KM CDR process (Figure 4), employing
a proprietary solvent called KS-1, will be
used to strip the CO2
from the flue gas. This
process was tested at Southern Company's
James Barry station at 25 MW equivalent.
KS-1 is described as a sterically hindered
amine solvent, essentially one where the
amine molecule has been modified to improve
its ability to react with CO2
. According
to MHI, KS-1 results in a process that
requires 68% of the regeneration energy
POWER | May 2015

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POWER May 2015

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