POWER June 2021 - 30

COMBINED HEAT & POWER
Power Plant Repowering with
District Heating Supply
District heating systems offer a great way to improve fuel utilization at power plants.
Two projects-one in the U.S. and one in Russia-that involved adding gas turbines
and heat recovery steam generators to existing facilities demonstrate the benefits
repowering can provide.
Ishai Oliker, PhD, PE
his article describes two power
plant repowering projects, which
resulted in improvements to heat
rate and district heat supply.
T
Jamestown, New York
The Carlson Generating Station of the
City of Jamestown, New York, offers
one example of an old, inefficient plant
that underwent a repowering project.
Originally the power plant included four
coal-fired boilers and two steam-driven
turbine-generator units with total electric
capacity of 50 MW (Units 5 and 6),
both with General Electric non-reheat
turbines.
Unit 6 was modified for district heating
(combined heat and power [CHP])
operation considering its relative ease
of retrofit to cogeneration. This turbine
(a 25-MW, 3,600 rpm, 15-stage, singleflow
condensing unit) was designed to
operate at 850 psig steam pressure,
900F temperature, and 3.5 inches of Hg
condenser pressure.
The turbine had one blanked-off extraction
point at the 11th stage from which
steam was extracted for a new district
heat exchanger (Figure 1). Higher loads
were served with additional steam from
the auxiliary steam header and used in
the existing auxiliary heat exchanger that
was arranged in series with the new district
heat exchanger.
Single-purpose efficiency for Unit 6
was 26.9%. After unit conversion to
district heating operation, the efficiency
increased to 32.5%, a 20% improvement.
Modifications to the turbine were
not required because the redistribution
of extraction flows was minimal and all
existing feedwater heaters remained in
service without modification.
Based on this modification a hot water
district heating system was developed
(Figure 2). During peak heat load
operation, the return water temperature
is 160F with water supply of 250F. The
30
1. Diagram of Carlson Generating Station Unit 6. Courtesy: Joseph Technology Corp.
district heating water flowrate during
peak load conditions is 498,000 lb/hr. The
maximum extraction flow available from
the turbine's 11th stage provides heating
for 379,000 lb/hr of district circulating
water to 210F. At the maximum heat
load conditions, 119,000 lb/hr of district
heating water bypasses the district and
auxiliary heat exchangers.
F effluent water from the district heat
exchanger is passed through the auxiliary
heat exchanger. This increases its
temperature to above 250F. Thus, when
mixed with the 119,000 lb/hr of bypassed
water at 210F, a total flowrate of 498,000
lb/hr at 250F is produced.
The district heat exchanger operates
throughout the year, providing hot water
for both space heating and domestic
use. The auxiliary heat exchanger operates
about one-third of the year. The
maximum operating pressure of the
district heat exchanger is about 20 psia;
the maximum operating pressure of the
auxiliary heat exchanger is 60 psia with
a maximum steam flow of 18,900 lb/hr.
Later, the power plant was converted
into a more efficient and technologically
advanced combined cycle plant. A
GE LM6000 gas turbine coupled with a
Deltak heat recovery steam generator
(HRSG) was installed at the plant. In simple
cycle operation, the gas turbine unit
generates 43 MW of electricity. With the
use of the HRSG and the older steam turbines
5 and 6, the plant can be operated
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in combined cycle configuration producing
about 80 MW of electric power and
80 MMBtu/hr of thermal energy.
Since the combustion process of
natural gas occurs at much higher temperatures
than normal boiler operation,
it allows for the production of steam using
the high-temperature exhaust gases
from the gas turbine exit. This removes
dependency on old, inefficient boilers to
produce the steam, and allows for much
higher fuel utilization. At current peak
conditions, the plant operates with 36%
electric efficiency. The addition of the gas
turbine and HRSG improves the overall
efficiency to about 56% when district
heating is included.
The Deltak HRSG also includes a coil
for district heating purposes. A portion
of the 170F condensate leaving turbine
6 enters the coil and is heated to approximately
375F. This water then pass2.
This diagram shows the district heating
piping network in the City of Jamestown,
New York. Courtesy: Joseph Technology Corp.
POWER | June 2021
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POWER June 2021

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