FIELD OF INVENTION:
The invention relates to a combined cycle power plant with fuel gas compressors
and fuel gas heating system. In particular, the invention teaches about
performance improvement of Gas based combined Cycle Plants with gas booster
compressors by modification of fuel gas heater drain system.
BACKGROUND OF THE INVENTION:
Gas Turbine based combined cycle power plants are clean and efficient source of
power and several such plants are establishing in India. A gas turbine based
combined cycle power plant operates on two thermodynamic principles viz.
Brayton Cycle for Gas Turbine (also called topping cycle) and Rankine Cycle for
Steam Turbine (also called bottoming cycle).
ln a gas turbine, air is compressed using a compressor and the compressed air
and fuel (Natural Gas/ High Speed Diesel/ Naphtha etc.) are burnt in combustion
chamber. Combustion products so produced are then expanded in a turbine to
produce power. Air Compressor and Turbine have common shaft and part of work
produced in Turbine is used to run the compressor.
Exhaust, gases from turbine, which have high exhaust temperatures, then passes
through a heat exchanger called Heat Recovery Steam Generator (HRSG) or Waste
Heat Recovery Boiler (WHRB) to produce steam before being exhausted to the
atmosphere. Steam produced in HRSG/WHRB is used to run a steam turbine to
produce work.
At some stations, fuel gas supply pressure is not sufficient for direct use in Gas
Turbines. As the fuel is required to be mixed with compressed air, fuel supply
pressure should be higher than gas turbine compressor discharge pressure. To
increase the pressure of available low pressure fuel gas, Gas Booster Compressors
(GBC) are used.
These compressors increase the fuel gas pressure to the extent that the gas can
be injected in the combustion chamber.

In order to improve the CCPP efficiency further, fuel gas is pre-heated through
external sources before entering combustion chamber.
Generally, the fuel gas is heated by hot feed water tapped-off from HRSG,
preferably from IP Feed Water header in a shell and tube type heat exchanger
called performance Fuel Gas Healer to transfer heat. Hot fuel gas is used in gas
turbine and cold feed water is drained to condenser.
In CCPP plants where GBC are envisaged for the gas turbine, the temperature of
fuel gas at the exit of GBC also increases with increase in gas pressure. In
performance fuel gas heater, feed water drain temperature increases due to higher
inlet fuel gas temperature. This high temperature drain is then led to the
condenser. This high temperature drain to condenser causes increase in the
condenser heat load and hence results in poor back pressure and lower steam
turbine power. Hence, it is proposed that the drain from performance fuel gas
heater may be diverted to deaerator in place of condenser. This will result in
reduced heat load on condenser and higher STG output.
Additionally, as the drain is not being pumped through Condensate Extraction
Pump (CEP), it will result in reduced auxiliary power of CEP.
OBJECTS OF THE INVENTION:
An object of the invention is to is to utilize the energy of high temperature drain
for increasing the plant power output.
SUMMARY OF THE INVENTION:
A modified gas turbine based combined cycle system having GBC and fuel gas
heating system to utilize the residual heat of fuel gas heater drain, the
improvement is characterized in that: the heat energy from fuel gas heater drain is
diverted to deaerator.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1 shows fuel Gas heating and drain system in accordance to the prior art.
Fig 2 shows fuel Gas heating and drain system in accordance to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Fig 1 shows fuel Gas heating and drain system, it is indicated that fuel gas is
heated before entering Gas Turbine. The heating fluid is feed water from IP
Economiser. After fuel gas heater, feed wafer is drained to condenser.
Fig 2 shows fuel Gas heating and drain system, it is indicated that fuel gas is
heated before entering Gas Turbine. The heating fluid is feed water from IP
Economiser. After fuel gas heater, feed water is drained to deaerator.
To improve the Gas Turbine efficiency, fuel gas is pre-heated through external
sources before entering Gas Turbine. Generally, the fuel gas is heated by hot feed
water tapped-off from HRSG, through Performance Fuel Gas Heater. Hot fuel gas
is used in gas turbine and cold feed water is returned to condenser.
ln CCPP plants with gas booster compressors, feed water drain temperature
increases due to higher inlet fuel gas temperature. It is proposed to divert this
high temperature drain to deaerator in place of present system of draining it to
condenser. This will result in reduced heat load on condenser, higher STG output,
and reduced auxiliary power of CEP. Accordingly, plant efficiency and heat rate
will improve.
EXAMPLES / PREFERRED EMBODIMENTS:
For a Frame 9FA Gas Turbine based CCPP with one GTG, one HRSG and one STG
configuration, the analysis with existing arrangement was carried out and
comparative results are as below:



The proposed arrangement is only possible, if the plant is designed considering
standalone deaerator. As most of the CCPPs are designed without standalone
deaerator and instead deaerating condenser is envisaged, the proposed
arrangement is unique.

WE CLAIM:
1. A modified gas turbine based combined cycle system having GBC and fuel gas
heating system to utilize the residual heat of fuel gas heater drain, the
improvement being characterized in that:
- the heat energy from fuel gas heater drain is diverted to deaerator;
wherein the heat load on a condenser is reduced yielding a higher STG output.
2. A modified gas turbine based combined cycle system having GBC and fuel gas
heating system to utilize the residual heat of fuel gas heater drain, as
substantially described and illustrated herein with reference to the accompanying
drawings.
3. A modified gas turbine based combined cycle system having GBC and fuel gas
heating system to utilize the residual heat of fuel gas heater drain, as claimed in
claim 1, wherein the auxiliary power of Condensate Extraction Pump (CEP) is
reduced.

ABSTRACT
TITLE: A COMBINED CYCLE POWER PLANT WITH FUEL GAS
COMPRESSORS AND FUEL GAS HEATING SYSTEM
A modified gas turbine based combined cycle system having GBC and fuel gas
heating system to utilize the residual heat of fuel gas heater drain, the
improvement being characterized in that: the heat energy from fuel gas heater
drain is diverted to deaerator; wherein the heat load on a condenser is reduced
yielding a higher STG output.