Increase the Output of Kombined Power Plant
with Supplementary Firing
The aim of the Supplementary Firing is to use the excess air in the gas turbine
flow throug a supplementary firing system in order to increase steam generation
and plant output.
The well-known supplementary firing system is implemented by installing
a burner in the exhaust gas duct of the gas turbine or in the inlet duct of the
The Supplementary firing is put into operation in two cases:
1. Due to the oxygen content in the exhaust gases, the temperature of the GT
exhaust gas can be increased by Duct firing (because of the stability of the
superheater material) to a maximum of approx. 780 °C.
2. In the event of failure of the gas turbine or failure of a second unit, a
Supplementary firing can be put into operation by the frish air firing (FA) in
order to generate a steamflow.
Excess air ratio in GT exhaust gas:
Due to the high excess of air in the combustion chamber, there is a higher oxygen
content in the exhaust gas of the gas turbine. In the case of a gas turbine with
a natural gas fuel, the oxygen content in the GT exhaust gas is 12 to 15 Vol-%.
i.e. the oxygen content used in combustion chamber = 9% to 6% of the air volume flow.
The reason for the remaining oxygen in the GT exhaust gas is, on the one hand,
the limited heating of approx. 900 °K (between approx. 500 °C at the compressor outlet
and approx. 1400 °C at the combustion chamber outlet) and, on the other hand, the
cooling air flow that flows through the turbine blade and bypasses the combustion
The remaining oxygen content at the gas turbine outlet is sufficient to burn a
supplementary fuel in the exhaust gas duct and to increase the temperature of the
gas turbine exhaust gases.
1. Supplementary Duct-Firing in a Combined Power Plant
The supplementary firing is an effective measure for utilizing the residual oxygen
content of the gas turbine exhaust gases. The well-known supplementary duct firing
is implemented In Fig.DuctFire.
In Fig.060, a simple gas and steam circuit of a combined Power Plant with
two-steam pressure circuits is shown. A HRSG (5) is connected downstream of the gas
turbine group. The steam generator (5) consists of an LP and HP feed water preheater,
an LP and HP evaporator,a HP superheater (21) and a reheater.
The air is compressed by the compressor (1) and a GT cooling air flow (31) is tapped
from the compressor and cooled by a cooling air cooler (30) (or two coolers in the
case of sequential combustion). The cooled cooling air flow (31) is fed into the gas
turbine to cool the GT parts. Part of the cooling air flow flows through the GT blade,
passes the burner and retains an oxygen content. To cool the GT cooling air, a water
flow (32) is tapped from the preheated HP feed water and passed through the cooling
air cooler (30), evaporated and superheated. The generated steam flow (33) is added
to the HP steam flow at the corresponding point in the superheater heating surface (21).
In the case of a gas turbine with sequential combustion (i.e. with a high-pressure
combustion chamber, high-pressure turbine, Medium-pressure combustion chamber and
Medium-pressure turbine), high-pressure and Medium-pressure cooling air coolers (30) are
Fig. 060 shows the supplementary duct firing (44 & 45) between the gas turbine and
the steam generator (5)in combined plands.
Problems of the well-known supplementary duct-firing (SDF):
- The re-firing takes place at a low exhaust gas pressure and low oxygen content and thus
the combustion behavior of the duct burner can be incomplete, so that soot can form on
- With increasing CO content in the exhaust gas of the SDF, the stability of the flame
decreases, so that soot formation on adjacent walls and flame extinction can take place
- Compliance with the CO limit values is the main problem
- The duct burner increases the NOx level and has a negative impact on the environment
- Special alloys must be required in the superheater and reheater so that they can withstand
the increased exhaust gas temperatures (from approx. 600 °C to approx. 780 °C or more)
- The large inlet duct must be long enough to ensure complete combustion and to avoid direct
flame contact on the heat transfer surfaces
- It is not possible or not a simple solution to build an Supplementary firing for existing
systems that are not designed for canal firing.
2. New Supplementary Pre-Firing FOR Combined POWER PLANTS
Inventor: YOUSSEF MUSTAFA (CH)
Applicant: Thermal PowerTec Ltd., Zürich
(see Pre-Firing in VGB Powertech)
The invention relates to a method for Supplementary pre-firing (APF) in gas and steam
turbine systems with simple or sequential combustion of the gas turbines. An airflow (35)
is tapped from a pressure stage of the compressor (1) and cooled by two air coolers (30
and 34). The cooled airflow (31) is fed to the gas turbine for cooling the GT parts, and
the cooled airflow (35b) is heated through a pre-burner duct (41). The gasflow is cooled
by the gas cooler (42) and fed to the relevant pressure stage of the gas turbine. For air
and gas cooling in the air and gas cooler, a water flow (32) is tapped from the preheated
feed water and evaporated by the cooler, superheated and inserted into the HP superheater
(21) Fig 061.
The usually used an Supplementary or duct-firing (DF) is achieved by installing a burner in
the exhaust duct of the gas turbine or in the inlet duct of the HRSG. The duct firing is
therefore an Supplementary post-firing and is an effective measure for utilizing the residual
oxygen content of the gas turbine exhaust gases. This duct-firing is a frequently used
method to increase the output in combination systems on hot summer days when the output
of the gas turbine decreases significantly.
With duct-firing in the inlet duct of the HRSG, the GT exhaust gas temperature can be
increased up to 780 °C. However, the use of duct-firing does not increase the efficiency
of the combined plant.
If the exhaust gas temperature rises above 780 °C, a fresh airflow must be supplied for
the afterburning (fresh air firing FAF). This Supplementary air increases the exhaust gas
losses and leads to a loss in the efficiency of the system.
The Supplementary firing (SF) is divided as follows:
- Supplementary Duct-firing (SDF) (state of the art)
- Fresh air firing (FAF) (state of the art)
- New Supplementary pre-firing (SPF) (according to the invention)
Fig. 061 shows an example for the implementation of the invention in gas and steam turbine
systems with simple combustion in the gas turbine and a new Supplementary pre-firing APF.
Fig. 062 shows a variant for carrying out the invention with the new Supplementary pre-firing
(ZVF) and a gas turbine with a single burner (3). An air stream ML (35) is tapped at the
compressor outlet and cooled by the cooling air cooler (30). A cooled partial airflow
(31) is fed into the gas turbine to cool the GT blade. The residual airflow (35b) is
heated by the slide (43) and pre-burner duct (41) to a temperature between 500 ° and
600 °C., for example, and cooled by a gas cooler (34). The cooled gas stream (36) is
introduced into the GT combustion chamber for combustion and for cooling the combustion
Advantages of the new Pre-Firing:
- The new Supplementary pre-firing SPF takes place through the pre-burner duct (41) at
high compression pressure and oxygen content and thus the combustion behavior of the
pre-burner duct can take place completely so that soot cannot form
- As a result of the better combustion through the pre-burner duct and when the oxygen
content limit of the gases at the GT burner outlet is adhered to or complete combustion,
no increase in the CO content in the exhaust gas and no soot formation in the HRSG can
- No increase in the NOx level or less negative impact on the environment
- Due to the pre-firing SPF, no special alloy is required for the superheater and
reheater, since the gas temperature at the inlet of the HRSG does not exceed the
temperature of 650 °C
- The pre-burner duct (41) is small, has an air temperature of up to approx. 600 °C
and is cooled by cooled air at the inlet
- When using new SPF and SDF in GT operation, the Supplementary heat output can be
increased more than with the usual post-firing (up to approx. Special alloys in the
superheater and reheater are necessary because of FAF
- It is also possible to build a pre-firing SPF for existing systems that are not
designed for canal firing.
2 Gas Turbine
3 GT Burner
35 Tapped airflow from the compressor 1
31 cooled cooling airflow
35b Airflow into air cooler 34 or into pre-burner chamber 41
30 Air cooler, Cooling air cooler
32 cooling water flow
34 Air cooler
36 Cooled gas introduced into the GT
41 Pre-burner chamber, Pre-combustion chamber
42 Gas cooler
44 duct of an Supplementary firing
45 Afterburner of Supplementary firing
SF Supplementary firing
SDF Supplementary Duct firing
FAF Fresh air firing
SPF Supplementary pre-firing