Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-08-29
2004-04-27
Gartenberg, Ehud (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S727000, C060S039210
Reexamination Certificate
active
06725665
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to a method of operating gas turbines, as for example in conventional gas turbine power plants or a compressed air energy storage (CAES) system, and in particular to a method of operating gas turbines having multiple burners.
BACKGROUND OF THE INVENTION
The layout of a conventional gas turbine power plant with a compressor is generally known.
The layout of an example of a compressed air energy storage (CAES) system is shown schematically in FIG.
1
. It comprises a cavern
1
for the storage of compressed air used for in particular for the generation of power during high demand periods. The compressed air is admitted by the control of a valve arrangement
9
to a recuperator
2
where it is heated by heat transfer from exhaust from the gas turbine. A further valve arrangement
8
controls the admission of the compressed air to an air turbine
3
. A combustion chamber
4
and gas turbine
5
are arranged downstream of the air turbine
3
. An auxiliary burner
6
is arranged following the gas turbine
5
and before the recuperator
2
. In this CAES arrangement the gas turbine combustion chamber
4
comprises multiple burners. When the gas turbine
5
is operated at various combustion chamber heat loads the number of active burners is varied. An automatic activation or deactivation of individual burners or burner groups by means of a gas turbine controller requires one criterion or several criteria that define the switch points for the burners. For example, this criterion can be the gas turbine load.
For premixed combustion taking place in a gas turbine combustion chamber the combustion event can be characterized by the equivalence ratio &PHgr; given by the ratio of the mass flow rate of fuel to that of oxidizer where the oxidizer usually is air. This is expressed by equation 1:
Φ
=
afr
m
.
fuel
m
.
ox
The factor afr (abbreviation for air fraction, generally the oxidizer being air) is the ratio of oxidizer mass flow to fuel mass flow for stoichiometric combustion, i.e. for a complete chemical reaction, where neither oxidizer nor fuel residuals are present in the exhaust gas.
By definition the equivalence ratio &PHgr; can take on any value between zero and infinity. However, for technical combustion the range is given by the flame stability limits. These limits are approached when the reaction cannot release enough heat to sustain chemical reaction and the flame subsequently extinguishes. This can happen if either excessive oxidizer or fuel is present. In the first case, the stability limit is defined as the “lean extinction limit”.
The combustion emissions, in particular NOx, correlate strongly with the flame temperature. Flame temperature and hence emissions can be controlled by varying the amounts and distribution of fuel and oxidizer in the combustion chamber.
For a gas turbine having multiple burners, a single burner equivalence ratio &PHgr;
SB
is given by equation 2:
Φ
SB
=
afr
m
.
fuel
,
SB
m
.
air
,
SB
.
It is an important indicator of flame temperature, combustion stability and emissions. Furthermore, a combustion chamber equivalence ratio is defined by equation 3:
Φ
CC
=
afr
m
.
fuel
,
CC
m
.
air
,
CC
The value of &PHgr;
CC
, together with the combustion chamber air inlet temperature and the fuel temperature, determines the firing temperature of the combustion chamber.
In a similar manner, the gas turbine equivalence ratio &PHgr;
GT
is related to the gas turbine inlet mix temperature T
GT TIT
, which is an important parameter of the overall gas turbine operation. This equivalence ratio is given by equation 4:
Φ
GT
=
afr
m
.
fuel
,
GT
m
.
air
,
GT
.
The gas turbine air mass flow, the combustion chamber air mass flow and the air admission to one single burner are determined, in part, by the gas turbine design geometry. The gas turbine fuel mass flow is identical to the combustion chamber fuel flow. The ratio of one single burner's fuel flow to the combustion chamber fuel mass flow is, however, dependent on the number n and configuration of the active burners. Hence, the different equivalence ratios are closely related by
&PHgr;
CC
=ƒ(&PHgr;
GT
), and equation 5
&PHgr;
SB
=ƒ(&PHgr;
CC
,n). equation 6
In gas turbine combustion chambers with multiple burners, the burner technology is preferably geared to, but not limited to a lean combustion technology reducing emissions. The combustion chamber's burners are switched on and off individually or are arranged in separately switchable burner groups. In order to achieve stable combustion and low emissions, the number of individual burners or burner groups in operation are varied over the range of operation.
Switching burners on or off with a constant &PHgr;
GT
distributes a certain amount of fuel to the burners leading to a shift in the burner equivalence ratio &PHgr;
SB
. If the number of active burners is reduced, the burner equivalence ratio &PHgr;
SB
will increase and consequently higher flame temperatures and higher emissions will occur. On the other hand, the activation of further burners will reduce the equivalence ratio &PHgr;
SB
. If the combustion process operates too closely to the extinction limit prior to the switch, some or even all burners will extinguish.
In conventional gas turbines load changes during start-up, shutdown or load following mode are accomplished by changing the air and/or fuel mass flow. In case of a load increase, relevant changes in the combustion chamber can occur such as:
The gas turbine inlet mix temperature T
GT TIT
and/or the air mass flow increases.
The combustion chamber heat load increases (given by the product of combustion chamber air mass flow and the temperature difference between combustion chamber air inlet temperature and combustion chamber exhaust outlet temperature).
The total fuel mass flow into the combustion chamber increases with the heat load.
The combustion chamber equivalence ratio &PHgr;
CC
increases with the temperature difference between combustion chamber air inlet temperature and combustion chamber exhaust outlet temperature.
Basically the operation of the combustion chamber can respond to an increased heat load in three ways. More fuel is added with either a reduced, same, or increased number of active burners.
Whenever the combustion chamber equivalence ratio &PHgr;
CC
increases, a constant or even reduced number of burners will result in a higher burner equivalence ratio &PHgr;
SB
. Hence, if the number of burners is not increased in order to compensate for the increased &PHgr;
CC
, emissions and single burner heat load will increase. But even if &PHgr;
CC
does not change, the higher heat load may allow for burner operation closer to the lean extinction limit. Hence, switching on additional single burners when increasing the gas turbine load may be advantageous to reduce emissions and heat load of the single burners. A reduced single burner heat load will, in turn, reduce the thermal stress of single burners.
FIG. 2
shows several gas turbine temperatures that are of interest concerning the operation of a gas turbine and its combustion chamber. Herein the temperature of the premixed flame can be found at position
10
. The combustion gases are cooled down by air bypassing the main reaction zone and reentering the combustion chamber at position
20
. The so-called “firing temperature” is defined as the temperature directly upstream of the turbine's first vane row at position
30
. This temperature and the temperature upstream of the first moving blade row at position
40
are limited with respect to the vane and blade material. To ensure their mechanical integrity both vanes and blades of modern gas turbines are usually cooled with air or steam. The temperature that would be found, if all the cooling media were mixed with the combustion chamber exhaust gases, is defined as the gas turbine inlet mix temperature T
GT TIT
. This temperature cannot be measured, however it can be determined by calculation.
Criteria a
Keller-Sornig Peter
Tuschy Ilja
Alstom Technology LTD
Burns Doane Swecker & Mathis L.L.P.
Gartenberg Ehud
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