Chemical Engineering April 2013 - 53

Deaerator (DA)
LP
drum
LP SH
steam
LP
drum
IP SH
steam
LP
drum
HP IP
LP
Steam turbine
Condenser
Reheater
FW
DA
pump
LP
pump
IP
HP
blowdown blowdown blowdown
Steam
from IP
(if used)
Condensate
polisher
(if used)
LEGEND
Preheater Economizer Superheat
or reheat
CT = Combustion
turbine
DA = Deaerator
FW = Feedwater
HP = High pressure
Condensate
pump
IP = Intermediate
pressure
LP = Low pressure
SH = Superheated
Attemperation
water
Combustion
turbine
Condensate
storage
Makeup
system
CT
exhaust
HP SH steam
IP SH
steam
Steam injection
to CT
(if used)
Figure 3. The most common type of HRSG is the multi-pressure drum type. This schematic is
for a three-pressure HRSG
Figure 4. This
graph shows the
degradation of
aeroderivative
combustion-turbine
efficiency with
increasing ambient
air temperature
above 48°F
25
30
35
40
45
50
55
-40
-20
20
40
60
Ambient temperature, °F
taken from any of the circuits, or, as is
most efficient, from a non-condensing
turbine. A less complex scenario that
may be better for cogeneration applications
is a combustion turbine with a
single-pressure HRSG perhaps without
a steam turbine, where the HRSG
operation is less complex than with
multi-pressure units. For example, a
colleague and I recently prepared the
water balance for a proposed power
project in which one scenario called
for two combined-cycle units, one with
a CT and a three-pressure HRSG, and
the other with a CT and a single-pressure
HRSG that partly supplies steam
for power augmentation. This example
illustrates the flexibility that is possible
with combined-cycle generation.
Net efficiencies of combined-cycle
units for power production have closed
in on 60%, while up to 80% efficiency
is possible for co-generation. However,
capacity and efficiency are greatly
influenced by inlet air conditions -
temperature foremost - but also humidity.
Maximizing
capacity output
Figure 4 illustrates what happens to
aeroderivative combustion-turbine
capacity with increasing ambient air
temperatures. Similar effects occur in
larger frame units, albeit without a
subcooling loss of efficiency.
To this point in time, the two most
80 100 120
common methods for turbine
inlet-air cooling have been
mechanical chilling or water-fed
evaporative cooling.
With the former, parasitic
power load may be in the 1
to 3 MW range. Evaporative
coolers on the other hand, as
their name implies, chill inlet
air by evaporation of water
spread upon an inlet media.
Water requirements for these
coolers may range from 30 to
80 gal/min. Also, evaporative
coolers can only provide chilling
down to a fixed approach
to wet-bulb temperature.
A third method is gaining
interest, absorption refrigeration
turbine inlet air conditioning
(ARCTIC), which
mitigates or eliminates both
parasitic power consumption
and water requirement
issues, while offering precise
control over inlet air temperatures.
Such control can be quite important
for plants based in hot or humid environments.
ARCTIC technology uses
the extremely well-known process of
closed-cycle, ammonia-based absorption
refrigeration, with turbine exhaust
heat as the energy source.
Figure 5 outlines the general flow
schematic for ARCTIC technology. The
process relies upon classic thermodynamic
refrigeration. The turbine exhaust
warms an aqueous ammonia solution
in heat exchanger coils (HRVG
on the diagram) located within the exhaust-gas
path. The ammonia is then
separated in the rectifier to produce a
nearly pure vapor, which is condensed,
reduced in pressure, and then allowed
to expand within turbine inlet air-cooling
coils (TIAC) located in the inlet air
stream. The pure ammonia discharge
from the cooling coils is blended with
the aqueous ammonia bottoms product
from the rectifier (a process that
requires additional heat exchangers
due to the exothermic reaction), and is
re-pressurized for return to the HRVG.
Thus, the process is a closed loop, with
all equipment but the TIAC and HRVG
coils located on a single skid.
A standard skid-mounted cooling
unit can provide over 2,000 tons of
chilling at 220 kW of auxiliary load.
ChemiCal engineering www.Che.Com april 2013 51
Generator output, MW
http://www.Che.Com
Chemical Engineering April 2013

Table of Contents for the Digital Edition of Chemical Engineering April 2013
