POWER June 2021 - 31

COMBINED HEAT & POWER
AMBIENT CONDITIONS
Ambient
conditions
C
-26
-20
-15
-10
-5
5
8
15
20
25
Totals
Hours of
occurrence
Cumulative
duration
Hours Hours
21
59
220
480
780
1,440
1,520
860
1,000
1,010
1,010
21
80
300
780
1,560
3,000
4,520
5,380
6,380
7,390
8,400
Heat load
PEAK
Unit electric
output
311.2
309.4
308.0
306.5
305.1
303.7
301.8
300.7
263.5
261.4
259.7
Fuel input
Gcal/hr MW Gcal/hr
421.60
421.60
421.60
421.60
421.60
421.60
421.60
421.60
119.83
119.83
119.83
751.6
751.6
751.6
751.6
751.6
751.6
751.6
751.6
605.7
605.7
605.7
Heat
supply
Gcal
8,854
24,874
92,752
202,368
328,848
607,104
640,832
362,576
119,834
121,032
121,032
2,630,106
Table 1. Energy balance of repowered Russian unit. Source: Joseph Technology Corp.
es through a new heat exchanger and
provides the district heating water with
about 20 MMBtu/hr of heat, or it can
pass through a flash separator and turn
into low-temperature steam to provide
the deaerators and decrease the amount
of steam needed to be extracted from
the turbines.
This new system represents the possibility
for substantial savings in electrical
production because less steam
extracted from the turbines results in an
increase in electrical production. However,
the amount of steam available to the
deaerators from the flash separator is inherently
dependent on the required flow
of water through the new district heat
exchanger. The lower the flow required,
the more flow will be available to the
flash separator. In order to reduce this
flow, the temperature differential of the
district water must increase. This may
be accomplished by lowering the return
temperature of the district heating water
so that more heat can be extracted from
the power plant.
St. Petersburg, Russia
For a number of years, the electric utility
in St. Petersburg has been improving the
efficiency of its electric generation. The
repowering of its 250-MW CHP generating
unit with a 52.9-MW gas turbine at
the Southern Plant is described below.
The Southern CHP plant has an installed
electric capacity of 750 MW and
heating capacity of 2,326 MWth. It generates
electricity and provides district heating
to the adjacent southern city district.
Unit 3 of the power plant was selectJune
2021 | POWER
3. Flow diagram of the Southern Plant's repowered cycle arrangement. Courtesy: Joseph Technology
Corp.
www.powermag.com
31
ed for repowering. The unit's electric
capacity was 250 MW in the district
heating mode and 300 MW in the condensing
mode. The unit is equipped
with a supercritical steam generator
producing steam at 3,700 psig and
1,013F with a capacity of 2,204,000 lb/
hr. It was designed for firing both natural
gas and oil. The steam turbine is an
extraction/condensing machine with
two controlled district heating extractions.
The steam condenser of each turbine
is equipped with built-in bundles
to preheat the district heating makeup
water. The district heating system of
the plant provides a significant quantity
of makeup water.
In order to prevent internal corrosion
in the underground district heating piping,
the makeup water in the system
must be deaerated. The makeup water
is passed through the preheating section
in the steam turbine condenser and then
goes to a vacuum deaerator. To prevent
losses of steam from the plant cycle,
district heating supply water at a temperature
of 230F is flashed into steam,
which is used as a heating medium in
the vacuum deaerator.
An ABB/Alstom gas turbine was selected
for repowering. The electric output
of the gas turbine at International
Standards Organization (ISO) conditions
is 52.9 MW. The gas turbine hot exhaust
is delivered to the boiler burners and
partially replaces the stoichiometric air
needed for the combustion process. The
oxygen content in the exhaust is about
15% versus 21% in the air.
The gas turbine with a fixed exhaust
ANNUAL
Unit electric
generation
MWh
6,535
18,255
67,749
147,120
237,978
Fuel input
Gcal
15,784
44,344
165,352
360,768
586,248
437,328 1,082,304
458,736 1,142,432
258,568
263,536
264,050
262,283
646,376
605,745
611,802
611,802
2,422,138 5,593,292
Fuel
utilization
%
91.69
91.48
91.32
91.15
90.99
90.83
90.61
90.48
57.19
56.89
56.64
84.25

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POWER June 2021

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