POWER May 2015 - 47

Table 1. Levelized cost of electricity (LCOE) for new generation (operating
in 2019). This table is abridged from the version appearing in the 2014 Annual Energy
Outlook. Source: Energy Information Administration
Option
Integrated gasification
combined cycle with CCS
Advanced combined
cycle with CCS
Advanced nuclear
Wind (onshore)
Wind (offshore)
Solar photovoltaic
Solar thermal
Capacity
factor (%)
85
87
90
35
37
25
20
geology. Others include the need for:
■ Convergence of technology to minimize
technical risk and cost-a dominant technology
and several deep-pocket, bankable
suppliers competing with value engineering
and evolutionary cost reductions.
■ Understanding the impact on grid reliability
and operational and maintenance
costs.
■ Insurance companies willing to provide
coverage.
■ Banks willing to finance debt service.
■ Public utility commissions allowing cost
recovery.
Costs Are " In Line "
The U.S. Energy Information Administration
(EIA) released a report on levelized
costs of electricity (LCOE) for new generation
resources in its Annual Energy Outlook
last year (Table 1). The critical point is that
coal with CCS is not economically disadvantaged
when you consider that a generating
facility that operates, at best, one-third
or one-fourth of the time (wind and solar),
and unpredictably at that, is different from
one that hums closer to 24/7/365. Renewable-sourced
generation can be made to
resemble baseload when paired with other
resources (see " Leveraging Generation
Synergies with Hybrid Plants " in the April
issue), but storage, for example, would
add significantly to the LCOE. Other cost
evaluations of generation options show
that coal with CCS can compete, once it is
proven and commercial.
Retrofits Are Problematic
Prospects for retrofitting CCS to existing plants
are not straightforward. According to analyses
conducted by Dale Simbeck, SFA Pacific Inc.,
the following issues present themselves:
■ Many coal units are old and inefficient.
Adding CCS would reduce output by 20%
May 2015 | POWER
Levelized capital
cost (2012 $/MWh)
$97.80
$30.30
$71.40
$64.10
$175.40
$114.50
$195.00
Transmission cost
(2012 $/MWh)
$1.20
$1.20
$1.10
$3.20
$5.80
$4.10
$6.00
Total system LCOE
(2012 $/MWh)
$147.40
$91.30
$96.10
$80.30
$204.10
$130.00
$243.10
to 30% (because of CCS's parasitic needs)
and result in overall fuel-to-electricity efficiencies
of 24% to 25%.
■ Space is severely constrained because of
the plot area already taken up by the addition
of particulate, SO2
and NOx
processes over several decades.
■ Distance to major CO2
removal
storage or reuse
sites is prohibitive, especially for older
power plants near urban areas.
■ Permitting long CO2
pipelines may be difficult
or impossible.
Simbeck argues that simply replacing
older coal plants with gas-fired combined
cycles (CCs), with or without CCS, is a better
long-term strategy for carbon mitigation
than retrofitting coal plants with CCS. His
analysis shows that natural gas prices have
to surpass $13/MMBtu before it makes more
sense to replace older coal units with new
coal equipped with CCS.
Because of the large energy penalties associated
with CCS, Simbeck and others in
the industry prefer to evaluate systems based
on CO2
avoided rather than CO2
captured.
The cost per avoided ton is higher than the
cost per captured ton, because it takes into
account the significantly reduced efficiency
and output of a unit equipped with CCS,
which lead to higher emissions per increment
of electricity generated.
Another knotty issue with CCS retrofits is
that many techniques to minimize the efficiency
and output penalties involve extensive thermal
integration of the process with the power
generation unit. In other words, according to
a report by Worley Parsons, Carbon Capture
Overview, retrofitting older units means more
than " bolting on " the CCS equipment.
Three Carbon Capture Pathways
The three accepted generic pathways for carbon
capture are:
■ Pre-combustion: Coal is gasified to prowww.powermag.com
CARBON
CAPTURE AND SEQUESTRATION
duce hydrogen and CO2
, and the latter is
stripped out of the syngas before the fuel
is burned, usually in a gas turbine.
■ Post-combustion: CO2
is stripped out of
the flue gas exhaust from a conventional
boiler or gas turbine.
■ Oxy-combustion: Coal is " burned " in a
pressure vessel using pure oxygen instead
of air.
Both the pre-combustion and oxy-combustion
options result in a more concentrated
CO2
stream, which reduces the cost and complexity
of separation.
The tradeoffs among the three are very intricate.
For example, the CO2
operation may be smaller and less complex,
but a costly air-separation plant is required.
Post-combustion CO2
stripping unit
stripping is most
familiar to the power industry-AES has
operated a 30-MW equivalent post-combustion
CO2
Point (Oklahoma) circulating fluidized bed
facility for almost three decades-however,
contaminants in the flue gas can impair the
process. To illustrate: According to the Worley
Parsons report, even a power plant with
a state-of-the-art SO2
removal process at its Shady
scrubber would require
an SO2 polishing step before the flue gas
enters the CCS process. Traditionally, utilities
overwhelmingly prefer post-combustion
emissions control systems because they can
be " turned off " if there are operating issues,
without jeopardizing electricity output.
On the other hand, pre-combustion, or
syngas stripping, enjoys an 80-year legacy of
technical optimization and commercial practice
in the petrochemical industry, where CO2
removal is called " gas sweetening. " However,
coal is far more heterogeneous than oil or
natural gas, and the constituents of coal flue
gas or syngas-such as ash, O2
, ammonia,
trace metals, and sulfur-all aggravate the
CO2
stripping process and are typically present
in much larger quantities than in petrochemical
applications.
Regardless of technology familiarity,
scale remains a challenge. As the Worley
Parsons report notes, between 1978 and
2000, at least 12 commercial CO2
capture
plants have been commissioned and range in
size from 90 to 1,200 tons/day; yet, a 500MW
coal-fired unit will require CO2
capture
more like 5,500 tons/day. Also illuminating
is that all of these units employ the same
basic process (mono-ethanol amine, MEA)
with variations in the strength of the solvent
and additives, such as inhibitors, used to optimize
the process (Figure 1).
Whether retrofit or new, the Worley Parsons
report concedes that while " there are no
clear long-term [CCS] winners, post-combustion
is the only proven technology com47
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