Chemical Engineering April 2013 - 54

Feature Report
Rectifier
Condenser
The parasitic power load for a
mechanical compressor to provide
similar chilling ranges from
1.6 to 3.2 MW. During one summer
day in 2011 at a plant in the
southwestern U.S., an ARCTIC
system maintained an inlet air
temperature of 48°F (5°C) when
the ambient temperature was
107°F (42°C). An added feature
of this process, and one that is
gaining interest in arid climates,
is that cooling coils may be
placed downstream of the HRVG
to recover water from the fluegas.
As is well known, the combustion
process, and particularly
that from gas turbines, produces
a significant quantity of water due to
the reaction of hydrogen in the methane
fuel with oxygen.
CH4 + 2O2 → CO2 + 2H2O
Solution
heat
exchanger
Refrigerant
Heat
exchanger
Air
TIAC
C
Compressor
T
Generator
Turbine
Figure 5. This generic flow diagram shows the operation of the ARCTIC system
Recovery of fluegas moisture can potentially
turn some plants from fresh
water consumers to water producers.
As an example, consider a combustion
turbine fired with natural gas at a fuel
flowrate of 87,000 lb/h. We will use
three simplifying assumptions:
* Natural gas composition is 100%
methane (CH4)
* Complete combustion is achieved in
the turbine
* The process recovers 100% of the
water produced by combustion
Stoichiometric calculations show a
water recovery rate of 390 gal/min.
Given that natural gas is typically a
very clean fuel, the recovered water
could easily be returned to the inlet of
the plant makeup-water system or to
other processes. An obviously important
issue with fluegas water recovery
is the effect that chilling would have
on the buoyancy of the fluegas stream,
and in turn, how this might influence
fan design and air-permitting issues.
In addition to inlet air cooling, other
techniques are available for maximizing
power during peak-load conditions.
One method, whose primary purpose is
to control of oxides of nitrogen (NOx)
formation, is injection of demineralized
water into the turbine. The demineralized
water lowers the combustion temperature,
which is the driving force for
NOx formation. But, the water adds
mass to the combustion gases and
improves capacity, albeit at a bit of
an efficiency penalty. Another option
that I have seen in a few recent power
applications is steam injection to the
combustion turbine. This process is
primarily utilized for power augmentation
(PAG) when an extra few megawatts
are needed from the plant. One
drawback to PAG and water injection
for NOx control is that extra makeup
water is necessary to produce the fluids
for turbine injection. This requirement
can increase the size of makeup-water
treatment equipment or demineralized
water storage capacity. The reader will
note that I have emphasized demineralized
water for these applications, as
anything of lower quality injected into
the combustion turbine would cause
fouling and corrosion.
Don't neglect HRSG chemistry
In 2008, I discussed the importance
of maintaining proper chemistry in
CPI steam generators during regular
operation and times of shutdown [2].
These principles also apply to heat
recovery steam generators. The following
list sums up many of the most
important details of steam generator
chemistry that have evolved over the
past decades, and particularly within
the past 20 years:
* Even seemingly minor contaminant
in-leakage to condensates can result
in major problems, most notably
steam-generator tube failures that
cause unit shutdowns
* For systems with water-cooled
steam condensers, the condenser
can be the most dramatic source of
52 ChemiCal engineering www.Che.Com april 2013
contaminant in-leakage. Impurity
introduction via a condenser-tube
leak has been known to cause boiler
tube failures within weeks, and
sometimes even days or hours
* Too often at CPI or other manufacturing
plants, insufficient attention
is given to condensate return from
process equipment to the steam generator.
The impurities introduced by
contaminated condensate may be
as harmful as those introduced by
condenser tube leaks. Condensate
recovery can be quite cost effective,
but only if it is properly treated to
prevent boiler contamination. Treatment
may include filtration, ion exchange,
or other techniques
* Unless the condensate/feedwater
system of a high-pressure steam
generator contains copper alloys, the
use of oxygen scavengers is highly
discouraged. These are now known
to propagate flow-accelerated corrosion
(FAC) in feedwater systems,
economizers, and the low-pressure
evaporator of HRSGs, among other
locations (for more, see Chem. Eng.;
March 2013, pp. 38-40). FAC-induced
failures have caused a number of fatalities
at power plants in the last 25
years. The same phenomenon will
occur in CPI steam generators
* The most common treatment for
IP and HP circuits in HRSGs is
the program known as phosphate
continuum, and most commonly at
the low end of a 1 to 10 parts-permillion
(ppm) phosphate range.
This program was developed by
the Electric Power Research InstiAbsorber
Fuel
Spray
receiver
HRVG
Exhaust
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Chemical Engineering April 2013

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