Gas and Steam Turbine Power Plants
New Procedure for Increase of Efficiency & Power
developed by TPT

Combined Cycles Power Plant CCPP

Today the CCPP is the most used power plants. CCPPs use a combination of two thermodynamic cycles: the gas turbine cycle (Brayton cycle) operating in a high-temperature and the steam turbine cycle (Rankine cycle) in a low-temperature range by using steam production in a heat recovery steam generator (HRSG). The combined cycle concept exploit the high-temperature potential of modern gas turbines and the low-temperature (cold end) of the steam cycle.

The combined cycle power plant offers high thermal efficiency, low emissions, low installed cost, flexibility in fuel selection and low operation and maintenance cost.
CCPPs are suitable for daily cycling operation due to short start-up times and for continuous base load operation. Part load efficiencies are also high due to the control of the gas turbine inlet mass flow using inlet adjustable vanes.
Figure 045a shows a standard HRSG with a triple water-steam pressure.

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Welcome to TPT Company

CCPP can be cooled by a cooling tower, a direct-cooling system or air-cooled condensers ensuring a wide range of applications. Where water is scarce, CCPP are advantageous because the cooling requirement is low due to the fact that the main coling requirement applies only to the steam process (33% of the supplied heat flow or 57% of total output).

The fuel flexibility of CCPP in limited to gases and some oils. The fuels that can be fired are those which are widely available in most parts of the world.

Main differences between combined cycle steam turbines and conventional steam turbines; in combined cycle steam turbin:
- fewer or even no steam extractions for the feed water heating
- shorter start-up times
- lower live-steam pressures, 100 to 160 bar (160 to 300 bar by STPP)

Today the net thermal efficiency of the combined cycles power plants lie between 0.56 and 0.58. The losses by the exhaust gases and condensation of the exhaust steam are still relative high and lie between 42% and 44% of the supplied heat flow.

The typical gas and steam temperature profiles in a HRSG consisting of a superheater, evaporator, and economizer operating at a single pressure. Because the gas temperature entering the HRSG is low (480 - 565 °C), the steam generation will also be lower than in conventional steam generators for the same gas flow. The economizer duty in the HRSG will also be low, leading to a high exit gas temperature. Also the effect of steam pressure is significant - the higher the steam pressure, the higher the exit gas temperature from the evaporator and the lower the steam generation rate, leading to a smaller duty in the economizer and a higher exit gas temperature. This is the reason for considering multiple-pressure units.


 Applicant: Thermal PowerTec Ltd., Zürich

The invention relates to a method for simplifying the water steam cycle and the Heat Recovery Steam Generator in combined cycle power stations. In a triple steam pressure cycle, the IP and LP evaporator heating surfaces, their associated IP and LP water steam drums and circulation pumps are eliminated and replaced by a water heating surface and two expansion vessels.

A water recycle stream from the feed water tank is conveyed by a water circulation pump, heated by a feedwater heating surface, and expanded through a throttle valve in an IP expansion vessel . The resulting vapor stream is introduced after overheating in the reheater in the IP steam turbine (2) and the condensate stream is expanded by a throttle valve in the LP expansion vessel, the resulting vapor stream is fed into the LP steam turbine (2) and the condensate stream is expanded by a throttle valve in the feedwater tank.

This process allows a continuous course of temperature difference between gas and feed water and simplifies the classification without interruptions of the feedwater heating surface in the steam generator. By eliminating the IP and LP evaporator heating surface and the reduction of the pumps from 6 to 4 large savings in construction costs of water steam circuit in combined power plants are achieved.
Figure 050 shows an execution of the new HRSG with IP and LP expansion vessels.

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Welcome to TPT Company

The new HRSG can be build in two constructions:
- Vertical HRSG with horizontal heat transfer tubes and vertical gas flow
- Horizontal HRSG with vertical heat transfer tubes and horizontal gas flow

With the new HRSG, the LP and IP evaporators are eliminated.
In the case of new vertical HRSG, only one circulation pump is needed for the HP evaporator.
The vertical new HRSG can be built more frequently in the future due to the removal of IP & LP evaporator and the minimum space requirement.

Gas side pressure drop:
The gas-side pressure drop between the feed water inlet and the HP evaporator in the standard HRSG consists of the pressure drop over HP-Eco's, IP-Eco-Eva and LP-Eco-Eva.
The gas-side pressure drops over the IP-Eva and LP-Eco-Eva are not available in the new HRSG.
The gas-side pressure drop over tube bundle with converging HP-Eco and IP-Eco tube bundle of the new HRSG is smaller than over the tube bundle with sequence sections of HP, IP and LP Eco-Eva tube bundle of the standart HRSG.

Gas Turbine Cooling The increasing of the hot gas temperature on the inlet of the gas turbine increases the efficiency of the gas turbine and thus of the CCPP. The problem of the gas turbine is particularly in the high temperature of the turbine blades. In order to bear the usual temperatures of 1000 to 1200°C, apart from the use of developed materials the turbine components are cooled additionally from the inside, by compressed and cooled cooling air.

Open Loop Air Cooling

For this open cooling a part of the compressed air is removed on different pressure levels from the compressor and used for the cooling of combustion chamber and turbine blades. The extracted cooling air must be cooled down, before it used as cooling air in the turbine. After cooling of turbine components the cooling air is mixed in the turbine with the main gas flow.
Several concepts are available for matching the cooling requirements to the CCPP.
Open cooling concepts can be used for cooling the compressed cooling air:
- Water injection (quench cooler)
- Steam injection
- Water-Steam cooling in heat exchangers

For avoiding the loss of demineralised make-up water and the loss of evaporating heat energy in the exhaust gas, the use of water-steam-cycle for the air cooling in CCPP is the most economical solution.

GT-Air Cooler (Air/Water-Steam Heat Exchanger):

The air/water-steam cooler works as a steam generator. The cooling water is supplied from the feed water of the steam turbine cycle. In the gas turbine cooler the water is evaporated, superheated and return to the steam cycle.

The air/water-steam cooler has limitations of operation range. For a given bundle geometry the limitations depends on ambient temperature and inlet temperature/pressure of cooling water.

For more information on Air/Water-Steam GT-Cooler see:

 Patent WO 2004/072544 A1: GT Air Cooler

Closed-loop steam cooling

This steam cooling system permits the higher firing temperatures required for increased efficiency. Gas turbine cooling steam is supplied from the steam turbine cycle. The steam cools the gas turbine and return to the steam cycle. The closed loop steam cooling is in the development.
GE Power Systems developed the closed loop steam cooling (H System). The GE's H System permits the higher firing temperatures and designed with the capability to achieve 60% thermal efficiency.
The gas turbine cooling system is integrated with the steam cycle. The supply of cooling steam is from HP steam turbine exhaust. The steam is delivered to the gas turbine stationary parts through casing connections and to the rotor through a conventional gland connection. The cooling steam is returned to the steam cycle at the reheat line.

If the gas turbine steam cooling (H System) of GE Power Systems and the gas turbine vacuum expansion of TPT used in a combined cycle plant, then a net thermal efficiency of over 62% is expected.

Important tasks take place in the CCPP, which are especially handled by TPT and are at research and development.

The following aspects can be handled by TPT:
- Optimization of thermodynamic design of GT air coolers
- Equalization of the water flow distribution in the parallel evaporator tubes
- Flow stability in the parallel evaporator tubes
- Behavior and performance of GT air coolers at different loads
- Failure analysis & reliability/availability for improvement of existing heat exchangers