POWER May 2011 - 46

THE FUTURE OF COAL
CO2 capture is included in a plant design,
water use per net MWh is expected to increase
by 30% to 90%. (See " Determining
Carbon Capture and Sequestration's Water
Demands, " Mar. 2010.)
The majority of water use in power plants
is related to process cooling (mostly lowgrade
heat rejection). Conventional cooling
is achieved through once-through cooling,
which withdraws water from a nearby water
body, pumps it to a condenser, and discharges
the heated cooling water back into the water
body. Where local water resources cannot
support once-through cooling, use of a wet
cooling tower reduces water withdrawals but
increases consumption and may impose a
heat rate penalty.
To reduce water use, research efforts are
focusing on alternative cooling technologies
and options for reducing plant cooling
loads. For example, dry cooling systems, or
air-cooled condensers (ACCs), can be used
as the heat sink for the various cooling duties
in a power plant.
A wet-surface, air-cooled heat exchanger
(WSACHE) allows use of high cycles of
concentration or low-quality makeup water.
In some plants with dry ACCs, WSACHEs
are used to remove heat from a closed-loop
auxiliary cooling system serving turbine
lube oil coolers and other small loads. Hybrid
wet-dry cooling systems can minimize
the negative impacts of dry cooling during
hot weather by using an evaporative cooling
tower to handle a portion of the cooling
duty during those periods. (See " Appraising
Our Future Cooling Water Options, "
June 2010.)
Power plants also can decrease the impact
on freshwater supplies by using nontraditional
sources of water for make-up. One
potential source of water for a power plant
is its own wastewater. One method for upgrading
and reusing wastewater in a power
plant is to include a " zero liquid discharge "
(ZLD) system. ZLD systems use evaporative
or reverse osmosis processes to concentrate
the impurities in the wastewater for disposal
while producing a water stream of suitable
purity for reuse.
EPRI's Advanced Cooling Technologies
project supports research to increase
water use efficiency and conservation at
fossil, nuclear, and renewable power plants
through engineering and economic analysis,
improved dry and hybrid cooling, reduced
water losses from cooling towers, use of degraded
water, and enhanced water resource
management and forecasting. The project
is examining cooling water availability impacts
on power plant siting, meteorological
impacts on ACCs, indirect dry cooling, hybrid
tower designs, water-recovery options,
wet-surface air coolers, advanced bottoming
cycles, and methods to preserve oncethrough
cooling options.
Conclusion
It is technically feasible to reduce CO2
emissions
from coal-fired generation, cut other
emissions to near-zero levels, and reduce
plant water use and discharge. However,
many of the necessary technologies are not
yet at the level of developmental maturity
required for affordable widespread deployment,
and time is needed to test and validate
new technologies. Achieving these goals will
require acknowledging the urgency of the
challenges and a sustained commitment to
a broad program of aggressive public- and
private-sector R&D. ■
-Dr. Andrew Maxson (amaxson@epri
.com) is a project manager for EPRI's
Industry Technology Demonstration
program, and Dr. Jeffrey Phillips (jphillip@epri.com)
is the program manager for
EPRI's CoalFleet for Tomorrow program.
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