Combined cycle power plants improve efficiency by capturing waste heat from gas turbine exhaust to generate steam and power a steam turbine. They can achieve over 60% efficiency compared to 33-25% for simple gas turbine plants. The steam is generated by a heat recovery steam generator (HRSG) that uses heat exchangers to capture energy from gas turbine exhaust to heat water into steam. This steam then drives a steam turbine which generates additional electricity. Combined cycle plants have higher efficiency and lower emissions than simple gas turbine or coal plants.

1. Combined Cycle Power
Plants

3. COMBINED CYCLE POWER PLANTS AND
COMPARISON
• Simple cycle gas turbine (GTs) plants using natural gas and fuel oil operate at around 33 percent and 25 per
cent efficiency, respectively, major part of the heat being wasted as thermal energy in the hot exhaust gases.
• Combining of multiple thermodynamic cycles to generate power, overall plant efficiency can be increased up
to 60 per cent by using a heat recovery steam generator (HRSG).
• The HSRG captures heat from high-temperature exhaust gases to generate steam, which is then supplied to a
steam turbine to generate additional electric power.
• The cycle working under this principle is known as combined cycle. In most of the instance a combined cycle
power plant utilizes gas turbines in conjunction with a steam turbine and is called a combined cycle gas
turbine (CCGT) plant.
• Different configurations of CCGT power plants are in use where each GT has its own associated HRSG, and
multiple HRSGs supplying steam to one or more steam turbines.
• For example, in a plant with a 3  1 configuration, three GT/HRSG trains supply to one steam turbine.
Similarly, there can be 1  1, 2  1 or 4  1 arrangements also. The steam turbine capacity is decided to match
the number and capacity of supplying GTs/HRSGs. These types of power plants are being installed in
increasing numbers round the world where there is access to substantial quantities of natural gas.

4. Combined Cycle Principles of Operation
• The HRSG is basically a heat exchanger popularly known as a boiler, or comprises a series of heat
exchangers.
• It generates steam for the steam turbine exchanging heat from the hot exhaust gas flow from a gas
turbine through heat exchanger tube banks.
• The tubes are arranged in sections, or modules also known as economizers, evaporators,
superheater/reheater and preheaters.
• The fluid circulation in HRSG could be either natural or forced circulation using pumps.
• Saturated steam from the steam drums or once-through system is passed through superheater or
reheater tubes to superheat the steam.
• The superheated steam produced by the HRSG is supplied to the steam turbine where it expands
through the turbine blades, imparting rotation to the turbine shaft.
• The energy delivered to the generator coupled to the drive shaft is converted into electricity.
• After exiting the steam turbine, the steam condenses in a condenser and fed back to the HRSG.

5. Figure shows a schematic diagram of a combined cycle gas turbine plant

6. • Depending on the exhaust gas characteristics HSRGs are designed and
configurations, steam requirements, etc., are decided later.
• Due to the high temperature of exhaust gases available at the gas turbine exit
(600°C), GTs are designed to produce steam at multiple pressure levels (high-
pressure steam in a large CCGT plant can reach up to 110 bar) to optimize energy
recovery.
• Generally, three sets of heat exchanger modules – one each for high pressure (HP)
steam, intermediate pressure (IP) steam, and low pressure (LP) steam – are used for
this purpose.
• The HRSGs present operational constraints on the CCGT power plant owing to
their location, directly downstream of the gas turbines. Due to the changes in
temperature and pressure of the exhaust gases thermal and mechanical stresses are
set up.
• When CCGT power plants are operated under fluctuating load conditions,
characterized by frequent starts up and shut downs or when operating under part-
load conditions, thermal stresses developed could cause damage to some
components of the HRSG. The HP steam drum and superheater headers are
subjected to the highest exhaust gas temperatures.

7. • When CCGT power plants are operated under fluctuating load conditions,
characterized by frequent starts up and shut downs or when operating under
part-load conditions, thermal stresses developed could cause damage to some
components of the HRSG. The HP steam drum and superheater headers are
subjected to the highest exhaust gas temperatures, and hence are more prone
to reduced mechanical life. Some important design and operating
considerations are the following:
(i) Temperatures of the gas and steam that the module materials can
withstand
(ii) Mechanical stability for turbulent exhaust flow
(iii) Corrosion of HRSG tubes
(iv) Steam pressures to decide drum thickness

8. Coupled Cycle – GT–ST Plant Operation
In this system, an open-circuit gas turbine has a compressor, a combustor and a
turbine. For this type of cycle, the input temperature to turbine and the output
temperature of flue gases are very high. This high-temperature flue gases have
heat energy high enough to provide heat for a second cycle that uses steam as
the working medium, that is, thermal power plant. Figure shows the working
principle of combined cycle gas turbine (CCGT) plant. Different components
of the plant are discussed further in detail.

10. 1. Air Inlet system
• Air is drawn though the large air inlet section where it is cleaned, cooled and controlled.
• Majority of all heavy-duty gas turbines are designed to operate under a wide variety of climatic and
environmental conditions due to inlet air filtration systems.
• These filtration systems are specifically designed to suit the plant location.
• Under normal operating conditions, the inlet system has the capability to process the air by removing
contaminants to levels below those that are harmful to the compressor and turbine.
• In general, the incoming air may have contaminants in solid, liquid and gaseous states in addition to
corrosive components. Gaseous contaminants include ammonia, chlorine, hydrocarbon gases, sulfur in the
form of H2S, SO2, discharge from oil cooler vents, etc. Some of the liquid contaminants include chloride
salts dissolved in water (sodium, potassium), nitrates, sulfates and hydrocarbons, etc.
• Solid contaminants may include sand, alumina and silica, rust, dust particles, alumina and silica, calcium
sulfate, ammonia compounds from fertilizer and animal feed operations, airborne seeds, etc. In addition to
these, some corrosive agents such as chlorides, nitrates and sulfates may also deposit on compressor blades
inducing stress corrosion attack and/or cause corrosion pitting.
• Alkali metals such as sodium and potassium may also combine with sulfur to form a highly corrosive agent
that may attack portions of the hot gas path. These contaminants are removed by passing through various
types of filters. Gas phase contaminants such as ammonia or sulfur that cannot be removed by filtration are
removed based on special methods.

11. 2. Turbine system
• The purified air is then compressed and mixed with natural
gas and ignited in the combustion chamber after injecting the
fuel.
• The high-pressure gas stream generated in the combustion
chamber expands in the turbine and spins the turbine rotor
and a generator, producing electricity.
• Heat of the gas turbine’s exhaust is further passed through the
heat recovery steam generator (HRSG), where live steam at
temperature between 420°C and 580°C is generated, which is
used as a working fluid in the secondary circuit.

12. 3. Heat recovery steam generator
• In heat recovery steam generator (HRSG), heat exchange takes place
between the highly purified water flowing in tubes and the hot flue gases
surrounding them, generating steam. As explained earlier, steam expands in
the turbine to run the turbine rotor and hence a coupled generator, to produce
electricity. The hot gases leave the HRSG at around 140°C before being
discharged into the atmosphere. The steam condensing and water system in
this circuit are similar to that in a typical thermal power plant. The HRSG
takes more time to warm up from cold conditions compared to hot
conditions.

13. Configuration of CCGT Plants
• The combined cycle system may have single-shaft and multi-shaft configurations.
• A single-shaft system consists of a gas turbine, a steam turbine, a generator and a
heat recovery steam generator (HRSG). The gas turbine and steam turbine are
coupled to the single generator on a single shaft.
• In a multi-shaft system, one or more gas turbine-generators and HRSGs are used
to supply steam through a common header to a separate single-steam turbine
generator. Overall investment on a multi-shaft system is about 5 per cent higher
compared to a single-shaft system. The major disadvantage of multi-shaft system
is that the number of steam turbines, condensers and condensate systems, cooling
towers and circulating water systems also proportionately increases to match the
number of gas turbines.

14. Efficiency of CCGT Plant
• Roughly, the steam turbine cycle produces one-third of the power and
gas turbine cycle produces two-thirds of the power output of the
CCGT. By combining both gas and steam cycles, high input
temperatures and low output temperatures can be achieved. The
efficiency of the cycles adds because they are powered by the same
fuel source.
• To increase the power system efficiency, it is necessary that the HRSG
be optimized, which serves as the critical link between the gas turbine
cycle and the steam turbine cycle with the objective of increasing the
steam turbine output.

15. • Overall efficiency of the combined cycle power plant
depends on the performance of HRSG. There are instances of
electric efficiency of a new combined cycle power station
reaching 58 percent at continuous output. In addition,
combined cycle units may be utilized to deliver low
temperature heat energy for industrial processes, district
heating and other uses. This is called cogeneration, and such
power plants are of then referred to as a combined heat and
power (CHP) plant.

16. • The efficiency of CCGT is increased by supplementary firing and
blade cooling. A gas turbine cooling air (TCA) cooler, with a heat
exchanger using feed water, and a fuel gas heater (FGH) were also
used to further enhance the efficiency of the plant in Kawasaki
Thermal Power Station. This minimized the loss of heat energy
allowing the plant to achieve higher thermal efficiency, outperforming
conventional combined cycle plants. In this plant, supplementary
firing was arranged at HRSG and in gas turbine a part of the
compressed air flow bypasses to cool the turbine blades.

17. Fuels
• The turbines used in combined cycle plants are
commonly fuelled with natural gas, and it is more
versatile than coal or oil and can be used in 90 per cent
of energy applications. Combined cycle plants are
usually powered by natural gas, although fuel oil,
synthesis gas or other fuels can be used.

18. Emission Control
• In order to control the emission, different tools and techniques are used, namely, a
selective catalytic reduction (SCR) system, aqueous ammonia solution, a low NOx
combustor and a dry NOx removal apparatus, as discussed below:
• 1. Selective catalytic reduction
• To control the emissions in the exhaust gas so that it remains within permitted levels as it
enters the atmosphere, the exhaust gas passes though two catalysts located in the HRSG.
One catalyst controls carbon monoxide (CO) emissions, whereas the other catalyst
controls oxides of nitrogen, (NOx) emissions.
• (i) Aqueous ammonia: Apart from the SCR, a mixture of 22 per cent ammonia and 78 per
cent water, known as aqueous ammonia is also injected into system to further reduce Nox
levels.
• (ii) To reduce NOx emissions that increase with the combustor exit gas temperature
(1500°C), a low NOx combustor can also be used.
• (iii) A dry NOx removal apparatus can be built in the heat recovery steam generator to
operate the plant confirming environmental regulation limits.

19. Advantages and Disadvantages of CCGT Plants
• Fuel efficiency:
1. As against the conventional power plants turbines fuel-conversion efficiency of 33 per cent,
turbines in combined cycle power plant have a fuel-conversion efficiency of 50 per cent or more.
This means the CCGT plant use only 50 per cent of fuel compared to a conventional plant to generate
same amount of electricity.
2. Low capital costs: The capital cost for building a combined cycle unit is two-thirds the capital cost
of a comparable coal-based power plant.
3. Ability to handle variety of fuel sources: The turbines used in combined cycle plants are fueled
with natural gas, which is more versatile than a conventional coal or oil. To meet the energy demand,
the plant can also be designed to run on alternative fuels such as agriculture-based bio gas.
4. Lower emission and fuel consumption: The specific fuel consumption of combined cycle plants is
less (fuel per kilowatt hour) and produces fewer emissions than conventional thermal power plants.
This reduces the environmental damage caused by electricity production. The fuel used in CCPT is
much cleaner when compared to coal-fired power plant.
5. Commercial availability: Combined cycle units are commercially available from all parts of the
world. They can be easily manufactured, shipped, transported and commissioned.

20. Disadvantages
• 1. The gas turbine can only use natural gas or high-grade oils such as
diesel fuel.
• 2. Its operation is location specific because the combined cycle can be
operated only in locations where these fuels are available and cost
effective.