Combined Cycle Gas Turbine Power Plant Part 1

POWER PLANT
Anurak ATTHASIT, Ph.D.
1
a Power & Energy Collection| Bangkok July 2015
The presentation has made for general information and does not comprise comment or any
recommendation of any kind. Readers should consider their own circumstances and rely on their own
enquiries in relation to matters contained in this handout.
Combined Cycle Gas Turbine Introduction

A. ATTHASIT a Power & Energy Collection
HRSG Heat Recovery Steam Generators 2
Bloomberg New Energy Finance (renewables), EIA (coal, gas, liquids), PRIS (nuclear). Nuclear
capacity includes only operational plants, not those defined by the IAEA as being in long-
term shutdown. Note: net generation for ‘central producers’ as defined by the EIA
Global nameplate installed electricity
capacity versus net generation, 2011
| World Energy Council 2013 Cost of Energy Technologies | www.ecn.nl | www.enea.it | www.etsap.org
Fossil-Fuel generation capacity is foreseen to
glow continuously, although its relative
contribution will fall from 67% in 2012 to 40–
45% by 2030.
Approximately 21% of the world’s electricity
production is based on natural gas. The
global gas-fired generation capacity
amounts to 1168 GWe (2007)
Combined-cycle gas turbines are much cheaper and easier
to build than coal plants and are considerably cleaner.
It is one of the dominant options for both intermediate-load
(2000 to 5000 hrs/yr) or base-load (>5000 hrs/yr) electricity
generation

Cogeneration Systems 3
 Cogeneration plants are
often classified according to
their prime movers.
 most commonly used
cogeneration systems are gas
turbines and steam turbines.
 The primary system is the gas
turbine which produces
useful energy and rejects
waste heat to the heat
recovery boiler, and the
steam turbine being the
secondary system recovers
the waste heat to produce
electrical energy and process
heat.
Source: http://www.mge.com//
Efficiencies of the cogeneration power plants are higher than those of the
conventional power plant due to the recovery of waste heat from the exhaust
gas from the individual plant.

Combined Cycle Power Plant 4
 Combined cycle power plant
– most is the combination
between gas and steam
technologies resulting
significant improvements in
thermal efficiency over
conventional single cycle
power plant.
 In a CCGT plant the thermal
efficiency is extended to
approximately 50-60 per
cent, by piping the exhaust
gas from the gas turbine into
a heat recovery steam
generator.
Source: FGE Power, LLC | www.pennenergy.com//

Brayton Cycle – Thermodynamics 5
 Brayton Cycle gas turbine
utilizes air as working fluids.
 Brayton Cycle has high
source temperature and
rejects heat at a temperature
that is conveniently used as
the energy source for the
Rankine Cycle.
 Process C-D – Adiabatic,
quasi-static (or reversible)
expansion in the turbine to
drive the compressor and the
remaining work out to turn a
generator for electrical
power generation
0
0
1500
Exhaust gas flow and exhaust temperature versus compressor inlet air temperature
Gas Turbine – Gas Flow Path | Source: imgkid.com/
Atmospheric Pressure
Enthalpy
Entropy
Pressure
Specific Volume
C
D
C
D
A
A
B
B
| to note: Adiabatic flame temperature is 1950 °C for methane

Brayton Cycle – Variation of Cycle Efficiency 6
 GT efficiency is increasing
with the firing temperature for
a given pressure ratio
 The hot gas coming out of
the combustor chamber can
reach very high temperature
(adiabatic flame
temperature is 1950 ‰for
methane)
 Turbine material melt
temperature is approximately
1400 °C
 Blade cooling system have
been developed to increase
high temperature tolerance,
and even higher than the
turbine material melt
temperature
40:1
30:1
20:1
10:1
1000°C
1100°C
1200°C
1300°C
1400°C
1500°C
55%
50%
45%
40%
35%
30%
200 250 300 350 400 450 500 550 600
Specific Output (kJ/kg)
GTCycleEfficiency(%)

Brayton Cycle – Variation of Cycle Efficiency 7
 Ideal Simple Brayton Cycle
the cycle efficiency is
determined by the pressure
ratio across the compressor
 Industrial gas turbines are
designed for stationary
applications and have lower
pressure ratios – typically up
to 18:1
 Aircraft Engine Pressure Ratio
- large increase in cycle
pressure ratio to date
 Aeroderivative gas turbines
are lighter weight compact
engines adapted from
aircraft jet engine design
which operate at higher
compression ratios – up to
30:1
The ideal simple Brayton cycle efficiency for monatomic and diatomic gases
80%
60%
40%
20%
Cycle
Efficiency
Pressure
Ratio
5 10 15
K=1.4 (diatomic)
K=1.67 (monatomic)
Variation of cycle efficiency
with engine pressure ratio for
both a diatomic gas such as air
and a monatomic gas such as
helium, argon, or neon.
𝞰 = 1 −
1
𝞹
𝞬−1
𝞬
CF6-6
CF6-50E
CF6-80E
CFM56-2
CFM56-3C
CFM56-5C4
GE90
40
30
20
10
1960 1970 1980 1990 2000
Year of Certification
OverallPressureRatio(OPR),T/O
Trent 775
Trent 890
Gas turbine aeroengine pressure ratio trends

Brayton Cycle – Configuration Options 8
 Compressor Intercooler -
the modification of Brayton
Cycle by combining a
number of stages of
compression in series with
coolers (called intercoolers)
between each stage.
 Reheat - Likewise, the
expansion process is
staged with the gas being
reheated between each
stage.
 Regeneration - the last
turbine stage and the last
compressor outlet is also
used
The highest-efficiency Brayton cycles are regenerative cycles with low pressure ratios
Entropy
1
2
3
4
5
6
7
8
9
10
AB
Temperature
Cooler
Regenerator
Heater Heater
Compressor Compressor Turbine Turbine
Exhaust
Powernet

Brayton Cycle - Gas Turbine Performance 9
 In Heavy-Duty GT, the
flue-gas, with a mass flow
rate between 350 t/h and
560 t/h, have a
temperature ranging from
550 to 590 °C.
 Efficiency of most of gas
turbines are between 30%
and 40%
 About 50% unused power
pass through the exhaust
gas (mass flux, high level
enthalpy)
GT Efficiency of Gas Turbines (1996-2001)
Industrial
Heavy
Duty
Aero-
derivatives
0 50 100 150 200 250 300 350
25
30
35
40
45
Efficiency(%)
GT Net Plant Output (MW)
Combustor
Compressor Turbine
Exhaust
Powernet 100 MW
4.5 kg/s, 225 MW
Air, 211.81 kg/s
T 541 °C, CO2 12.86 kg/s, H2O 10.12 kg/s
O2 29.08 kg/s, N2 164.76 kg/s
𝞰c=88% 𝞰t=93%
P=30 bar, TiT=1400 C
| Some manufacturers have adapted aeroderative gas turbine to
industrial needs. They have lower efficiency since their are not designed to
work continuously.
| In opposition to the
aeroderative gas
turbines, their are mainly
used at nominal
conditions. Their ability
to reach full load in a
few minutes makes
them fit to peak
consumption
requirements.
| Their main characteristics is their small dimension and
weight. Moreover, their are built to work on a very large
domain around the nominal conditions.

Combination of Gas Turbine and Steam Power Cycle 10
 Heat of the gas turbine's
exhaust is used to generate
steam by passing it through
a heat recovery steam
generator (HRSG) with a
live steam temperature
between 420 and 580 °C.
 Combining both gas and
steam cycles, high input
temperatures and low
output temperatures can
be achieved.
 The efficiency of the cycles
add, because they are
powered by the same fuel
source
Brayton Cycle
Rankine
(Steam) Cycle
Temperature
Entropy
Heat
Transfer
Exhaust Gas
to Steam
Simplified CCGT Process

Combined Power Cycle 11
Above 60% LHV efﬁciencies reached today by
combined cycle power plants are the result of
integration into a single production unit of two
complementary technologies in terms of
temperature levels:
1,300 °C hot gas enters in an
aero-derivative expansion
turbine and come out
around 500°C
450°C and 30°C operation
range of steam plant from
superheated vapour to the
cooled condensate water

CCGT Plant Configuration 12
 CCGT can be single shaft
or multi shaft
 Single shaft combined
cycle plant comprises a
gas turbine and a steam
turbine driving a common
generator, requires less
equipment, less controls
and less machinery and
thus less investment
 In a multi-shaft combined
cycle plant, each gas
turbine and each steam
turbine has its own
generator
Source: www.c2es.org/publications/leveraging-natural-gas-reduce-greenhouse-gas-
emissions
Multi-Shaft design enables two or more gas turbines to operate in conjunction with a
single steam turbine, which can be more economical during operation than a number
of single shaft units.

CCGT – Heat & Mass Balance 13
Synoptic View of Single Pressure CCGT Exergy Balance of Single Pressure CCGT
Renaud G. | Energy Systems: A New Approach to Engineering Thermodynamics |www.crcpress.com - www.thermoptim.org

Exergy Loss in a CCGT process 14
 Improving the electrical
efficiency of the gas turbine
power plant by using the
exhaust gas heat in a steam
cycle
 More than four percent of
the energy in the steam
process can be recovered to
add a total of 48 % to 50%
efficiency output of the
combined Brayton and
Rankine cycle
 Major losses are primarily via
the exhaust gas to ambient,
cooling water and
condensate at the steam
process
Electrical Power
Generator
Loss
Auxiliary
Power
Electrical Power
Electrical
Power
Auxiliary Steam
Heat
from
Exhaust
Gas
Main Cooling
Water
Aux Cooling Water
Exhaust Gas to Ambient
Gland Seal and Internal Cooling
100% Fuel in Brayton Cycle
Rankine
Cycle
Source: www.es.mw.tum.de | www.sankey-diagrams.com/tag/efficiency/

Exergy Destruction Rate 15
 The stack gas exergy and
the exergy destruction
due to heat transfer
decrease with the
increase in number of
pressure levels of steam
generation
 The increase of the
number of pressure levels
of steam generation
increases the total and
specific investment cost
of the plant but as it leads
to increase the efficiency
and power output of the
plant
ExergyDestructionRate(MW)
0
10
20
30
Dual Pressure Triple Pressure w/o Reheat Triple Pressure Reheat
Mohammad T.M. et al | Exergetic and economic evaluation of the
effect of HRSG configurations on the performance of combined cycle
power plants | Energy Conversion and Management | 2011
HRSG
ST
Condenser

HRSG Heat Recovery Steam Generators 16
Alstom attributes some of its HRSG’s operational flexibility to the use of multiple
desuperheaters—one between the final two stages of superheating and one between the HP
outlet and the steam turbine. Also to an integrated plant design that allows owners to
manage fast-start GT exhaust conditions to meet steam-turbine ramp-rate limits.
www.alstom.com
Credits | 30 min for a hot start (maximum shutdown of about eight hours); 100 min for a warm start (maximum shutdown
of about 60 hours); 150 min for a cold start (shutdown of more than 120 hours)
HRSG for each pressure level, consists of three
heat exchangers: Economizers are used to
heat water close to saturation, Evaporators to
produce saturated steam and Superheaters
to produce superheated steam.
The most important parameters of
HRSGs are pinch point, approach
point and gas side pressure drop
through the heat recovery
system, which affect the
effectiveness of heat exchange.
High exhaust-gas mass flows
associated with large gas turbines
impact superheater and reheater
designs as the design of the inlet
duct. Gas velocity is of concern
because of its possible effects on
tube vibration and heat absorption

HRSG Double vs. Triple Pressure 17
 Efficiency
Improvements due to
Increasing the Number
of Pressure Stages
 Designing the HRSG is a
complex optimization
problem and remains
challenge, which does
not arise in
conventional boilers
40% and 43% of the supplied heat flow goes to losses by the exhaust gases and
condensation of the exhaust steam
Source: Siemens

Key Parameters Influence CCGT Performance 18
 Topping Cycle (Gas
Turbine): Ambient
Temperature;
Compressor pressure
ratio; Turbine inlet
temperature
 Bottom Cycle (HRSG &
ST): In order to maximize
the GTCC efficiency, the
optimization of the HRSG
operating parameters is
considered, i.e. the
minimization of exergy
losses; Pinch point;
Pressure levels
www.turbinesinfo.com
Much effort to increase the firing temperature is done by gas
turbine manufactures - more efficiently at higher combustion
gas temperatures | www.sciencemag.org
The evolution of allowable gas temperature at the entry to the
gas turbine and the contribution of superalloy development,
film cooling technology, thermal barrier coatings and (in the
future) ceramic matrix composite (CMC) air foils and perhaps
novel cooling concepts. | http://www.virginia.edu/

Discussion 19
http://pca-invest.com/ | Energy Technology Systems Analysis Programme | IEA ETSAP – Technology Brief E02
110 GW per year as per IEA Baseline scenario (IEA, 2008)
estimates a need for new CCGT power plant capacity
for the period 2005-2050.
CCGT plant with a 1700°C class
gas-turbine may attain an
electrical efficiency of 62–65%
(LHV) the CCGT efficiency
is expected to increase from
today’s 52%–60% to a maximum of
64% by 2020.
High exhaust-gas mass flows
associated with large gas turbines
impact superheater and reheater
designs as the design of the inlet
duct. Gas velocity is of concern
because of its possible effects on
tube vibration and heat absorption

THANK YOU
20
The presentation has made for general information and does not comprise comment or any
recommendation of any kind. Readers should consider their own circumstances and rely on their own
enquiries in relation to matters contained in this handout.

Combined Cycle Gas Turbine Power Plant Part 1

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