POWER June 2010 - 44

water management
Case Study 2: Brackish
Water Used at a Zero-
Liquid Discharge Plant
A 1,560-MW combined-cycle plant in
the U.S. has been operating since 2006
using well water as its cooling source.
In this plant, severe water quality issues
were experienced, including high
total dissolved solids levels, high calcium
and magnesium deposits, and high
total silica. The plant uses a zero-liquid
discharge (ZLD) design, where no water
is discharged offsite, and the poor-quality
effluent water was made suitable for
use in upstream plant applications.
The tower and circulating water system
were designed to negate the effects of the
poor water quality. A lime softener was
used to treat makeup water having high
calcium and magnesium concentrations.
After being treated in the lime softener,
the makeup water was recirculated up
to 15-16 cycles of concentration in the
cooling tower, creating highly concentrated
blowdown. Increasing the cycles
of concentration minimizes makeup water
requirements. The need to combat the severe
water quality led plant engineers to
implement the ZLD policy. All spent cooling
water was sent to the site evaporation
pond rather than to a natural water body.
One of the key decisions made during
design of the cooling tower involved the
materials for the hardware used to assemble
the tower. In a typical cooling tower
design with acceptable water quality,
the hardware selected is typically a costeffective,
reliable, and low-maintenance
grade of stainless steel. Using stainless
steel in the cooling tower at this plant
would have resulted in a high amount of
corrosion if the water were not treated
regularly. Even with regular treatment,
maintenance costs would have been too
high to offset any savings associated
with using stainless steel in lieu of more
expensive alloys. A high-grade hardware
was required to combat degradation, decay,
and biological growth in the tower.
The plant opted for silicon or aluminum
bronze as the metal of choice. In addition
to being exceptionally corrosion resistant
compared to stainless steel, both
silicon and aluminum bronze are also
known for their biostatic effects, which
limit colonization of harmful bacteria.
44
vapor pressure and specific heat. Therefore,
for a given heat load, salt water towers are
larger than freshwater towers because more
heat exchange area is needed to compensate
for the salt-related performance loss.
A larger cooling tower demands higher fan
horsepower to move the required air volume
through the larger facility.
The use of seawater not only affects the
physical properties of water but also results
in higher scaling, corrosion, and biological
fouling. Calcium carbonate (CaCO3
) scale
is observed in most cooling applications.
The presence of scale should be avoided
on any heat exchange surface because a
CaCO3
circulating water system to make up for
evaporation, blowdown, drift, and any
other losses, which deplete the recirculating
water inventory and/or concentrate the
solids in this water. The required makeup
rate to hold a given solids concentration
ratio may then be computed based on an
assumed cooling tower cycles of concentration.
For details on the calculation procedure,
see " Strategies to Reduce Sulfuric
Acid Usage in Evaporative Cooling Water
Systems " (POWER, March 2010).
thickness of as little as 0.1 inch can
reduce heat transfer by up to 40%. Scale
inhibitors are designed to prevent the deposition
of CaCO3
. Typical inhibitors are
phosphonates and polymers. Higher salinity
levels make scale inhibitors less effective,
and the operation of seawater cooling
towers requires advanced scaling control.
Proper acid addition can help to minimize
scale inhibitor requirements.
Certain water characteristics can also
contribute to increased rates of corrosion.
Water temperature, chemistry, halogen residuals,
and dissolved oxygen are some
of the factors that affect corrosion rates.
Corrosion inhibitors such as azoles, phosphates,
and zinc can be used to lessen the
risk of steel corrosion.
Bacteria and algae that are present can
form micro slimes and films that impede
heat
Component Selection
Considerations
The quality of makeup water can potentially
impact every part of a wet cooling
tower. Figure 5 depicts components in both
a cross-flow and counter-flow mechanical
draft cooling tower configuration. These
components can be selected specifically to
accommodate the use of poor quality water.
Optimization studies must be performed to
determine the best balance of water consumption,
tower materials, tower cost and
size, and output for the site conditions of a
particular plant. For example:
■ Larger motors may be required to offset
the poor heat transfer properties of seawater.
Larger motors may also be required to
offset increased pressure drops associated
with high-efficiency drift eliminators.
transfer. Aquatic species including
barnacles, bryozoa, oysters, and mussels
can contribute to macro-fouling. Some of
the chlorines, bromine, and nonoxidizing
biocides can control both micro- and macro-biological
fouling. Chlorine discharge
levels are often regulated by permit requirements.
Adding the proper amount of sodium
bisulfite can eliminate excess chlorine.
Sufficient water must be added to the
■ The level of total suspended solids (TSS)
in the water source must be considered for
proper selection of the heat exchange surface
in a wet cooling tower (the fill).
■ High-efficiency drift eliminators can be
incorporated to minimize emissions.
■ More expensive hardware material may
be considered. Copper alloys can easily
erode by contact with suspended solids.
Other alloys, such as titanium, are free of
corrosion products and reduce the number
of sites for potential TSS entrapment
5. The cross-flow and counter-flow cooling tower. Source: Bechtel Power Corp.
Air
out
Air
out
Fan
Drift eliminators
Hot
water
Fill
Air in
Inlet louvers
Water basin
Cold water
www.powermag.com
Rain zone
Wi
Cold water
POWER | June 2010
Fan
Sprays
Hot
water
http://www.powermag.com
POWER June 2010

Table of Contents for the Digital Edition of POWER June 2010
