POWER October 2013 - 66

Emissions
For most operating coal-fired power plants, a pre-combustion CCS
isn't practical or feasible. (See this issue's cover story on the Edwardsport
plant for one demonstration of the economic hurdles.) The vast
majority of utility coal power generation utilizes some form of pulverized
coal combustion, and those systems would need to be converted
to fire a non-carbon-based fuel. At issue is that coal as a fuel source
consists largely of carbon molecules. If the carbon molecules in coal
are removed prior to combustion, there is little energy remaining for
thermal power production.
This is why facilities such as NRG's W.A. Parish plant are testing
the effectiveness of large-scale post-combustion CCS. W.A. Parish
is expecting to implement a commercial-scale post-combustion CCS
project using a 250-MW slipstream for carbon capture. It is anticipated
that 90% of the CO2
in a 1.65 million ton annual reduction in CO2
To achieve this large reduction in CO2
ing an amine technology designed to remove the GHG gas from the
flue gas. The amine will absorb CO2
in the slipstream will be captured, resulting
emissions.
emissions, the plant is testfrom
the flue gas and then move
it to a stripping process to sequester the captured gas and regenerate
the amine. One caveat for the technology is that the flue gas needs to
be free from flyash, sulfur, and nitrogen to be effective.
The captured CO2
will be compressed and used for EOR at the
nearby West Ranch Oil Field in Jackson County, Texas. The U.S. Department
of Energy issued its final National Environmental Policy
Act Environmental Impact Statement in February 2013 and also issued
a Record of Decision in May 2013 giving the project a potential
operational timeframe of the first quarter of 2015.
A second large-scale demonstration of CCS technology is the repowering
of Ameren's Meredosia Unit 4 using oxyfuel combustion.
That $1.65 billion project will convert the existing 200-MW unit to a
167-MW oxy-combustion unit (also known as FutureGen 2.0) with a
90% CO2
capture rate. The first phase of the project, primarily consisting
of the project's power purchase agreement, was completed in
February 2013; the project has now entered its second phase, consisting
of final permitting activities and engineering and design. The second
phase is expected to continue into summer 2014 with commercial
operations expected to begin in 2017.
For domestic power generation, this will be the first major CCS
project that does not use the sequestered CO2
reducing the amount of virgin sorbent required. Methods of contact
(between sorbent and flue gas) that are ideal for regeneration of the
sorbent include fixed, moving, and fluidized beds.
Membrane-based CO2
capture methods consist of permeable or
semi-permeable membranes that allow for selective separation of CO2
from the flue gas stream. Although these systems are ideal in precombustion
high-pressure systems, advancements are being made for
selective membranes compatible with low-partial-pressure CO2
environments
(typical of a flue gas stream). Membrane systems offer a potential
operating cost savings because there is no need for regeneration,
as is typical with solvent and sorbent capture systems.
CO2 Capture Is Only Half the Battle
Capturing CO2
emissions from a flue gas stream constitutes half of the
CCS process; the other half involves sequestering all the captured gas.
Long-term storage presents significant challenges to existing coal-fired
power plants, as their geographical location may not be suitable for
long-term storage of CO2
. FutureGen 2.0 and W.A. Parish are utilizing
the two most common types of storage in geological formations. Typically,
the captured gas is injected in a supercritical state into the desired
geological formation and held underground by impermeable caprock or
a geochemical trapping mechanism. Other forms of geological storage
include mature gas fields and un-mineable coal seams.
Other storage mechanisms have been proposed, but they have not
been tested for their long-term viability. Mineral storage is a concept
that utilizes the process of carbonate formation (reaction of CO2
metal oxides) as a mechanism for stable, long-term storage of CO2
nature this process occurs over several years and results in a significant
portion of surface limestone. A pilot plant based in Newcastle,
Australia (expected to be operational this year) is currently studying
the economics of this mineral storage-based mechanism.
There are also several pilot plants worldwide that have studied
the potential of using sequestered CO2
questered CO2
for EOR. Instead, the
CO2 will be transported and injected below ground into a geological
formation. In this region, the Mount Simon Sandstone is located
below an impermeable caprock and contains a saline aquifer. The porosity
and depth of the formation is expected to be an ideal setting for
long-term storage of CO2
.
There is hope that the EPA will be flexible about the timeframe
for enforcing regulations for existing power plants-allowing new
technologies to emerge and be applied in time to meet compliance
deadlines. CO2
they are new for the power generation industry.
Post-combustion CO2
removal methods currently under development
can be broken down into three main removal categories:
solvents, sorbents, and membranes. (A few other very early-stage
approaches were covered in the August issue feature " R&D Projects
Target Cheaper Carbon Capture, Use, and Storage. " ) Solventbased
CO2
the GHG gas from flue gas into the liquid carrier. Using thermal
energy, CO2
stream, thus regenerating the solvent and sequestering the CO2
is liberated from the solvent outside of the flue gas
from
the combustion gases.
Solid particles referred to as sorbents are similar to solvent-based
post-combustion CO2
physically adsorb CO2
vents, sorbents may be regenerated by use of thermal energy, thus
66
capture systems in that they chemically or
from the flue gas stream. Also similar to solremoval
technologies are not new processes; however,
Limited Flexibility
It is unclear at this time what the regulation for CO2
and
. In
for biomass generation. Seis
utilized in bioreactors that provide ideal growing
environments for algae. The algae produced is harvested and used to
either produce fuels such as biodiesel or to generate animal feed.
emissions will
look like for new and existing power plants, but it is certain that increased
regulations will be proposed. Although the EPA will have
flexibility with implementing carbon regulations, the fact of the matter
remains that CO2
is becoming increasingly regulated. With an
already volatile regulatory environment consisting of promulgated
regulations (Mercury and Air Toxics Standards), regulations expected
to be released shortly (316(b) Cooling Water Intake Structures and
Coal Combustion Residuals), and the potential reinstatement of overturned
regulations (Cross-State Air Pollution Rule), the potential of
new regulations further complicates the future of coal-fired power
generation.
Technologies to address the capture and storage of CO2
are availcapture
involves the chemical or physical absorption of
able but have yet to been proven on a large-scale basis. In addition
to being unproven, CCS projects are capital- and energy-intensive.
Natural gas remains at a low price point and, from a regulatory
standpoint, retains a favorable position as the " fossil fuel of choice "
for future power generation. However, over 40% of current U.S.
power is still being produced by coal-fired power plants. It is uncertain
what overall effect the expansion of CO2
regulations will have
on fossil fuel power generation-and grid reliability-but the trend
of retiring coal-fired power generation units due to existing or future
regulations is likely to continue. ■
-Brandon Bell, PE is a project manager at Valdes Engineering
and a POWER contributing editor.
www.powermag.com
POWER | October 2013
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POWER October 2013

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