Water Treatment CCPP
Water Treatment CCPP
Water Treatment CCPP
plant modications.
This article will examine water treatment technologies such as membrane
separation (e.g., reverse osmosis [RO]),
electrodialysis, and electrodeionization
(EDI), and the fading, yet unique, role
that ion exchange (IX) can play. It will
also address the critical role that chemical
water treatment companies can play in
avoiding costly mistakes during power
plant design, how to best t chemical
treatment options, and what can go severely wrong when the raw water to the
plant is not critically evaluated.
Background
The authors experience with new
independent power plant projects has
shown that water treatment is a common
contributing factor to delayed start-ups,
subsequent poor plant heat rates, and a
cause of unscheduled downtime. Water
treatment also is often the orphan of plant
designers, seen as the necessary evil
on the overall projects ow schematic
and capex, and as such it often does not
receive the attention it merits.
Engineering, procurement, and construction rms (so-called EPCs) employed in-house water treatment specialists in the past that were knowledgeable
in the selection of appropriate equip-
equipment vendors use for installing zeolite softeners is that they provide a good
ltration barrier for the RO plant. True,
IX resin beds can be good particulate
lters, yet not cost-effective at doing it.
Let true lters do the ltration. There
are certain instances where the use of
zeolite softening ahead of a RO plant
is justied. With the advent of EDI as
a post-RO polishing step, the permeate
feed to the EDI needs to have a hardness
concentration of less than 1 mg/L. If the
raw water hardness exceeds 100 mg/L,
the EDI will likely receive undesirable
hardness leakage. A softener upstream
of the RO unit will solve that problem.
But even under these circumstances, it
often makes more sense to install only
partial ow softening and avoid higher
capital equipment and operating costs,
large equipment footprints, as well as
unnecessary saline softener regeneration
efuent. Resin softeners are also wellknown bacteria breeding grounds that
can cause fouling in the RO system.
Decarbonator. Avoid installing a decarbonator (a.k.a degasier) for carbon
dioxide (alkalinity) removal. These units
are normally installed downstream of the
clarication/ltration step, and ahead
of the RO, and are often accompanied
by a pre-acidication step since a low
pH is required to effectively achieve
the stripping of the gaseous alkalinity.
The most cost-effective solution is to
have a small caustic soda feed to the
RO feedwater, thus converting the CO2
to bicarbonate alkalinity (HCO3) that
is readily removed by the membranes.
One exception to this would be waters
capable of developing very high LSI
values (high tendency to form calcium
carbonate scale) where the use of caustic
would not be recommended.
Choosing The Right Equipment
Technology
Many different technologies and variations of the same technology exist in the
market place today. Plant owners are
well advised to be very involved in the
selection of the best available technology that can also provide the lowest life
cycle cost.
What is my best route? RO or IX?
This is a commonly asked question, and
4
the answer generally lies in the total dissolved solids (TDS) concentration of the
raw water. A good rule of thumb to use
is 150 mg/L TDS. If higher than this,
RO offers almost always the lowest life
cycle cost. There are, obviously, other
important criteria to consider. Although
IX operates at higher water recoveries
(i.e., less efuent volume), the TDS load
to be disposed of is much higher than that
of RO, and IX is thus considered not to be
an environmentally friendly technology.
A drawback of RO is that typically 25%
of the feedwater becomes a waste stream.
This may impact disposal strategies at
some sites, and needs to be considered.
In certain instances, RO reject or a portion thereof can be recycled to other
water systems, such as makeup to the
cooling tower, or the ash water system.
Membrane-based technologies may also
offer lower safety risks in the plant as
they often eliminate the need for strong
acid and alkali materials that are staple
consumables in IX plants.
Other technologies such as nanoltration (NF) and electrodialysis reversal
(EDR) should be carefully considered.
Both can improve project economics,
especially when dealing with more saline
feedwaters, as they act as roughing
demineralizers. NF can comfortably
handle high TDS concentrations but at
much lower operating pressures, saving
on operating costs, and allowing the
downstream RO system to assume a more
polishing role. EDR plants also operate
successfully in power plants around the
world in various roles, that include the
treatment of cooling tower blowdown
and RO concentrate, and they can play
an important role in zero discharge, and
other water reuse applications. Figure 2
shows two competing technologiesIX
and RO.
Reverse osmosis is often the technology of choice for new power plants. If
you select RO, here is some very important advice for owners: Have a bid
specication for all OEMs that covers
the following points, so that bids can
be compared on an apples to apples
basis:
1. Ensure that the RO system is designed
with a permeate ux of 12-16 gallons per
square foot per day (gfd) for raw waters
that undergoes the standard pretreatment
13, 2001).
Weed, R.H.; Tvedt, T.J.; Cotton, I.J. ErosionCorrosion in Utility Systems, Power-Gen,
Anaheim, Calif. (Dec. 5-7, 1995).
This paper was presented at ULTRAPURE WATER Asia, which was conducted Sept. 6-7, 2006,
in Singapore.
Key words:
ION EXCHANGE,
TABLE B
Recommended Essential Analyses for
the Evaluation of a Raw Water Supply
Parameter
Typically
reported as
pH
Calcium hardness
Magnesium hardness
Total silica
mg/L as CaCO3
mg/L as CaCO3
mg/L as the ion
Reactive silica
Total iron
Manganese
Total aluminum
Barium
Strontium
TDS
Conductivity
Suspended solids
Sodium
Bicarbonate
Chloride
Total sulfur, as sulfate
Fluoride
Nitrate
TOC
Total phosphate
Color
Commonly symbol
or abbreviation
CaH
MgH
SiO2
Notes/concerns
Note: The above constitutes a well-detailed representation of most water sources that are used in Power plant projects. Other species should be tested
(e.g., ammonia, phenols, oil and grease, copper, sulde, mercury) as specic needs dictate.
TABLE A
Steam Turbine Manufacturers Purity Requirements
Parameter
ABB
Cation conductivity,
mmhos/cm (S/cm)
< 0.5
Dissolved oxygen, ppb (mg/L)
Sodium, ppb (mg/L)
10
Chloride, ppb (mg/L)
Silica, ppb (mg/L)
50
Copper, ppb (mg/L)
Iron, ppb (mg/L)
Na : PO4 molar ratio
Total dissolved solids, ppb (mg/L) max.
GE
Westinghouse
< 0.2
< 0.3
< 10
<5
<5
< 10
<2
< 20
20
20
2.3 2.7
50
TABLE C
Condensate Polisher Application Guidelines.
Contaminant
Fe and Cu oxide
Iron .or hardness
TDS
Oil/Organic
Organic (3)
Form
insoluble
crud
soluble
Source
corrosion in
condensate system
condensate
corrosion, makeup
contamination
soluble
cooling water
in-leakage,
process contamination
insoluble
process, coolers
soluble
process
Sodium
Zeolite
yes
Cellulose
Precoat
yes
yes
no
no
yes
yes
no
no
no
yes (1)
yes (1)
no (2)
no
yes
no
no
no
no (2)
no
yes (2)
no
Notes: (1): These units are designed primarily as lters. Because of the large amount of water being treated, they will quickly exhaust if exposed to
high TDS.
(2): Oil and certain organics are removed by fouling the IX media. Activated carbon has been used but results are generally unsatisfactory. Each
potential contaminant must be tested.