Steam Power Plants
Special Cycles for Steam Power Plants
Repowering of Existing Plants
Steam Power Plants
In the steam cycle of a power plant the condensate from hotwel pumped to LP pressure, heated
in LP feedwater heaters 1 to 4 and deaerated in the direct contact heater/deaerator 5.
The feedwater pumped to high pressure, heated in HP heaters 6 and 7 before it enters the boiler
where superheated steam is produced. The steam is superheated in the boiler to 540°C. The
superheated steam is sent to the steam turbine where the steam expands to low pressure providing
the energy to drive a generator. The exhaust steam from the low pressure turbine has to be
condensated in the condenser in order to complete the steam cycle.
The exhaust steam enters condenser-tube bundles that have cooling water circulating through the tubes.
The cooling water causes the steam to condense at a temperature of about 32–38°C
and that creates an absolute pressure in the condenser of about 5–7 kPa,
a vacuum of about 95 kPa relative to atmospheric pressure. The condenser creates the low pressure
required to increase the efficiency of the turbines. The limiting factor is the temperature of the
cooling water and that is limited by the prevailing average climatic conditions at the power
plant's location.
Optimization of the feedwater heaters and the water-steam cycle improved the profitability
and availability of the steam power plant.
Feedwater heating
The feedwater used in the steam boiler is a means of transferring heat energy from the burning
fuel to the mechanical energy of the spinning steam turbine. The total feedwater consists of
recirculated condensed steam, referred to as condensate, from the steam turbines plus purified
makeup water. Because the metallic materials it contacts are subject to corrosion at high
temperatures and pressures, the makeup water is highly purified before use. A system of water
softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes
an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter.
The makeup water in a 500 MWe plant amounts to perhaps 1.25 L/s to offset the small losses from
steam leaks in the system.
The feedwater cycle begins with condensate water being pumped out of the condenser after
travelling through the steam turbines. The condensate flow rate at full load in a 500 MWe
plant is about 0.38 m³/s. The water flows through a series of six or seven intermediate feedwater
heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and
gaining temperature at each stage. Typically, the condensate plus the makeup water then flows
though a deaerator that removes dissolved air from the water, further purifying and reducing its
corrosivity. The water may be dosed following this point with hydrazine, a chemical that removes
the remaining oxygen in the water to below 5 parts per billion (ppb). It is also dosed with pH
control agents such as ammonia or morpholine to keep the residual acidity low and thus
non-corrosive.
Today the net thermal efficiency of the steam power plants lie between 0.42 and 0.47.
The loss by the condensation of the exhaust steam is high and lie between 43% and 48% of the
supplied heat flow.
Special Cycles for Steam Power Plants:
Feedwater Circuit with Steam Desuperheater to the Deaerator:
Patent EP 0972911
LP Direct Contact Feedwater Heater Cycle:
Patent DE 19524216
Circuit for LP Feedwater Heaters:
Patent EP 1041251
separate desuperheater cycles:
In a commonly used heater circuit for the feed water heating by turbine bleed steam,
a separate desuperheater is switched. The new desuperheater can be switched in different
forms in connection with the extraction steam from the IP turbine (Fig. 1, 2 and 3).
According to a circuit in Fig. 1 the separate IP desuperheater for desuperheating of
extraction steam for the direct contact heater DCH is switched. The desuperheater is fed
at the tube side with condensate from the first high-pressure feedwater heater by
a circulation pump and increased condensate pressure. By the desuperheating of the extraction
steam in the desuperheater a heating and partial vaporization of the condensate flow is achieved.
For the purpose of a feed water heating, the condensate is fed into the steam space of the last
high pressure heater.
According to Fig..2, the second separate IP desuperheater for desuperheating the bleed steam of
the first HP preheater 7 is selected, as replacement of the integrated desuperheater of HP
heater 7. The inventive desuperheater increased the thermal efficiency by +0.0005 and the
turbine output +1.8 MW.
The new third circuit of IP desuperheater shows Fig.3. According to this circuit, the separate
IP desuperheater for desuperheating the bleed steam of the LP heater 5 is selected. The
superheated steam of the LP heater is taken from the steam line at the outlet of the IP turbine.
The desuperheater is fed with saturated condensate at the tube side from the last LP heater by
a condensate forward pump. The subcooled condensate after the pump flows through the desuperheater
tubes, warmed and partially vaporated and fed into the vapor space of the first LP heater 7.
The desuperheater increases the thermal efficiency of +0.0003 and the turbine output to +0.5 MW.
As the temperature of the condensate in the desuperheater tubes above the saturation temperature
of the Bleed steam, in the new desuperheaters is no risk of condensation and the superheating of
the Bleed steam can be fully utilized.
The conventional desuperheater circuit used to feed water preheating are shown in Fig.4 and 5.
Fig.4 shows a cross connected desuperheater which is connected to the first HP heater.
Fig.5 shows a HP desuperheater for the direct contact heater DCH which is connected after the
feedwater pump as published in patent document:
Feedwater Circuit with Steam Desuperheater to the Deaerator:
Patent EP 0972911
The following table shows the effects of different desuperheater circuits on the base power
plant with a power output of 800 MW without separate desuperheater and with a supercritical
steam pressure:
Through the desuperheater for the direct contact heater 6 the heat consumption is reduced, the
thermal efficiency increased by approximately 0.00075 (for example from 0.47 to 0.47075)
and the turbine output of the power plant will be increased by approximately 0.38% (3.2 MW).
According to Fig.4 the known cross-connected HP desuperheater to the first HP heater increase
the thermal efficiency by approximately 0.00078, however, reduces the heat input in the steam
generator and reheater by about -1% (-15.4 MW). This result a reduction in the output of the
steam turbine by about -0.81% (-7 MW).
According to the circuit in Fig.5 the desuperheater is connected to the water side between the
feedwater pump and the first HP heater. As a result, the thermal efficiency of the steam power
plant is increased by about 0.0003.
The invented separate IP desuperheater can be addittionally installed to an existing steam
turbine plant either it contains or not a cross-connected desuperheater. In addiction the
invented desuperheater can be instoled as well if the HP heaters of tube sheet or header type
design.
For a HP feedwater heater system that already has a cross-connected desuperheater to the first
HP heater, desuperheater can be supplementary installed for the direct contact heater and for
the last LP heater according to the Fig. 1 and 3.
Repowering of Existing Steam Power Plants
Repowering an existing steam power plant can be achieved by combining it, in
whole or in part, with a gas turbine into a combined cycle plant. Repowering is ideal for
plants in which the steam turbines, after many years of operation, still have considerable service-
live expectancy, but the boilers are ready for replacement. The boilers are normally replaced or
supplemented with gas turbines and HRSG. Some plants are repowered purely in order to benefit from
the efficiency increase even though they are far from the end of their design life.
Repowering increases the output and efficiency of the power plant while improving plant
reliability and decreasing plant emission.
The equipment to repowering will vary from case to case and depends on technical and economic
criteria: Building and foundations, steam turbine and generator, condenser and cooling system.
The size relationship between the steam turbine and gas turbine is a main efficiency driver in
a repowering application. It is important to have a good fit between the size of the gas turbine
and the steam turbine.
There are three main options available when deciding to repower:
1. Heat recovery combined cycle repowering
The existing fired boiler is replaced with one (or two) efficient GT and one heat recovery
steam generator (HRSG) by changing a part of the water/steam cycle.
2. Hot wind box combined cycle repowering (HWBR)
Steam power plants with reheat steam turbine can be repowered using the concept of "hot wind box".
One (or more) gas turbine (GT) is installed and the high temperature GT exhaust gas flows first
through the windbox of the present fired boiler to utilising the existing water/steam cycle and
steam turbine (ST) and then through a waste-heat recovery heater used for most of the feedwater
preheating. The rest of the preheating is done using the existing preheaters and steam turbine
extractions.
HWBR has a high degree of technical complexity.
3. Heat recovery and fired combined cycle repowering (Hybrid PP)
A new gas turbine and new HRSG are installed in parallel to the conventional boiler to
provide a second source of HP live steam for the steam turbine. During normal load the existing
fired boiler can operate the steam turbine. During high load the GT and HRSG of the combined cycle
process can be utilized. This concept fits especially for large steam plants and offers
more flexibility than other options due to the fired boiler and HRSG for meeting the load needs.
The exhaust heat at the cold end of the HRSG is used for preheating of partial feedwater. The
existing steam extractions and feed heaters are used for preheating of the part of feedwater flow,
allowing steam turbine extraction flows to be reduced and increasing steam turbine output.
Three operating modes are possible:
- Original mode without the gas turbine and HRSG in operation
- Hybrid mode, where the coventional cycle, gas turbine and HRSG are in operation
- Combined cycle mode, where the GT, HRSG and ST are in opration without the conventional boiler.
The highest overall efficiencies are obtained with pure combined cycle mode and the highest output
with hybrid mode.
TPT can help you in following questions:
- Performance analysis for economical evaluation of plant options
- Optimization of thermodynamic design of FW Heaters and Deaerator
- Optimization of thermodynamic cycles improves of the efficiency of power plants
- Static & Dynamic behavior of systems: Transients after disturbance, start-up/shut down
- Using of exist heat exchangers in the repowered power plant
- Failure analysis & reliability/availability for improvement of existing heat exchangers