Increase the Output of Kombined Power Plant
with Supplementary Firing

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 HRSG.

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 chamber.
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 switched.
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 adjacent walls
- 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)

SUMMARY
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 chamber wall.

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 take place.
- 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.


1    Compressor
2    Gas Turbine
3    GT Burner
5    HRSG
21  Supperheater
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