POWER May 2011 - 42

THE FUTURE OF COAL
cesses, usually those being developed at a
university or by a small firm, and has nurtured
them along for a few years until they
were ready to compete for DOE funding.
The new processes generally fall within
five categories:
■ Absorption: Research focuses on developing
novel solvent processes such as complexed
ionic liquids, oligomers, and phase
separation.
■ Adsorption: Novel designs of packed
and fluidized beds are being investigated
in parallel with new sorbent materials,
such as those based on carbon
or metal organic frameworks with the
desired thermodynamic properties and
robustness.
■ Biological: Research focuses on quantifying
the carbon-mitigating potential of
various utility-connected algae systems,
with emphasis on life-cycle analyses for
net CO2
accounting.
■ Membranes: Developers are working on
polymer membranes that remove CO2
from
flue gas with both high selectivity and permeability.
Although they are still largely
untested for this application, membranes
have potential to lower the energy penalty.
■ Mineralization: Flue gas is scrubbed by
a base solution to form a solid product,
thereby capturing CO2
without requiring
compression or underground storage.
Obtaining the base solution at low
energy demand remains a challenge.
EPRI supports the early stages of R&D
on new capture processes through its Technology
Innovation program, performing due
diligence, process simulations, materials development,
and lab testing. Through industrial
collaboration, EPRI also provides support
and funding through bench tests, pilot-scale
projects, and larger-scale demonstrations of
capture technology (see sidebar).
CO2 Compression, Transport,
and Storage
Although as much as 80% of the cost of CCS
is attributable to the capture process, most
of the uncertainties surround the means of
permanently storing CO2
public need to be confident that CO2
. The industry and
can be
safely injected and stored in underground
formations over very long periods without
undesirable side effects. Beyond the scientific
investigations related to geologic sites and
injection technologies, there are regulatory,
legal, and long-term liability issues that some
analysts believe will be the biggest obstacles
to widespread CCS commercialization.
The process of transporting CO2
via pipelines
is commercially established. However,
42
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POWER | May 2011
energy requirements for CO2
compression
are substantial. Compression is estimated
to account for one-third to one-fourth of the
total energy demand of a CCS system on a
power plant. Southwest Research Institute is
investigating two novel compression systems
with the potential to reduce CO2
compression
power requirements by 35%. One concept is
a semi-isothermal compression process in
which the CO2
is continually cooled using an
internal cooling jacket (intra-stage cooling)
rather than conventional inter-stage cooling.
The other concept involves the use of refrigeration
to liquefy the CO2
can be increased using a cryogenic pump,
rather than a compressor.
The DOE/NETL is supporting development
by Ramgen Power Systems of a supersonic
shock wave compression technology,
similar to an aircraft ram-jet engine. The
compressor design, known as Rampressor,
features a rotating disk that operates at high
peripheral speeds to generate shock waves
that compress CO2
with higher efficiency
than conventional technologies. (See " Capturing
CO2
: Gas Compression vs. Liquefaction, "
June 2009.)
Around the world, geologic sequestration
is being demonstrated by numerous
of CO2
small-scale projects. In Canada, Norway,
and Algeria, large-scale projects sequester a
combined total of ~5 million metric tons of
CO2
per year-the approximate output of a
baseload 750-MWe coal-fired power plant.
The most promising formations for
geologic storage of CO2
include depleted
oil- and gas-bearing formations, saline formations,
and deep, unminable coal seams. To
adequately qualify a site for geologic sequestration
of CO2
.
In the U.S., the seven regional partner,
characterization studies must
confirm the site's storage capacity and ability
to safely store CO2
, so that its pressure
ships of DOE's nationwide Regional Carbon
Sequestration Partnerships program are conducting
pilot-scale CO2
tests in differing geologic formations. These
pilot-scale tests are to be followed by largervolume
tests involving storage of ~1 million
metric tons of CO2
or more, along with postinjection
monitoring to track migration of
the CO2
. EPRI is responsible for most of the
field work at the southeastern regional project
(pilot-scale and larger volume injections)
and also for the drilling and testing of one or
two characterization wells in Arizona.
EPRI is engaged in a multiyear research
and information exchange program to determine
how pure a CO2
stream must be when
delivered to an injection formation. The objective
is to understand the purity that enables
a target injection formation to realize
its maximum injectivity and capacity, while
avoiding excessively high costs in the capture
system due to unnecessarily tight purity requirements.
In
2010, EPRI initiated a field study to
evaluate potential impacts of dissolved CO2
on groundwater quality. By injecting carbonated
groundwater into a shallow aquifer
system and observing the effects, the field
study simulates a hypothetical CO2
leak from
a deep geologic storage reservoir into an underground
source of drinking water. Results
from this study will help improve our fundamental
understanding of the geochemical
processes that lead to the introduction into
groundwater of CO2
leakage path from the injection formation
to an underground source of potable
drinking water.
EPRI also is teaming with industry collaborators
to study the integration of larger-scale
carbon capture systems with storage technologies.
Projects include AEP's MountainOperating
Flexibility Required
A recent EPRI study explored the potential
economic impact, on a coal-fired plant
with carbon capture and sequestration
(CCS), of temporarily bypassing the CO2
capture system during periods of peak
power demand to increase the plant's economic
performance. Findings showed this
mode of operation would be profitable if
the value of the increased power sales and
lower per-unit fuel cost outweighed the
cost of higher CO2 emissions.
A related study investigated the value
of operating flexibility for plants with CCS
when bidding into the ancillary services
market. Results indicate that being able to
temporarily turn off CO2 capture systems
during times of high power demand could
generate enough revenue via the sale of
" 10-minute reserve " and " 1-hour reserve "
credits to offset 15% to 19% of the initial
capital cost of a greenfield integrated gasification
combined-cycle plant and almost
45% of the cost of retrofitting an existing
pulverized coal plant with CCS. However,
the ability to quickly shut off capture systems
needs to be proven, and it is unclear
if regulators will allow that kind of CCS
system operating flexibility.
injection validation
-induced mobilization of
heavy metals and organics along a potential
CO2
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