Combustion – Flame holder having protective flame enclosing or flame...
Reexamination Certificate
2002-05-16
2004-01-06
Basichas, Alfred (Department: 3743)
Combustion
Flame holder having protective flame enclosing or flame...
C431S115000, C431S116000, C431S009000
Reexamination Certificate
active
06672863
ABSTRACT:
This application claims priority under 35 U.S.C. §§ 119 and/or 365 to Appln. No. 2001 1010/01 filed in Switzerland on Jun. 1, 2001; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a burner for a gas turbine or hot-gas generation for the combustion of liquid or gaseous fuel and to a method for operating it.
BACKGROUND OF THE INVENTION
A principal problem which has to be solved within the framework of the development of industrial premixing burners for use in gas turbines or for hot-gas generation is the stabilization of the flame primarily in the part-load operating mode. Most industrial burners of this type utilize a swirl flow for generating a backflow zone on the burner axis. In these burners, flame stabilization takes place aerodynamically, that is to say without special flame holders. In this case, the backflow zones, which occur during the breakdown of the vortex, or the outer recirculation zones are utilized. Hot exhaust gases from these zones in this case ignite the fresh fuel/air mixture.
A burner according to the prior art, in which, for example, a backflow zone of this type is formed on the axis of the burner, is described in EP 0 210 462 A1. In the dual burner, specified there, for a gas turbine, the swirl body is formed from at least two double-curved metal plates acted upon by tangential air inflow, the plates being folded so as to be widened outward in the outflow direction. During outflow into the combustion chamber, a backflow zone at the downstream end of the inner cone is formed on the axis of the burner as a result of the increasing swirl coefficient in the flow direction. The geometry of the burner is in this case selected such that the vortex flow at the center has low swirl and axial velocity excess. The increase in the swirl coefficient in the axial direction then leads to the vortex backflow zone remaining in a stable position.
Further examples of what are known as double-cone burners are found in the prior art in EP 0 321 809 B1 and in EP 0 433 790 B1. In these burners with a conical shape opening in the flow direction, in which there are two part-cone bodies which are positioned one on the other and the center axes of which run, offset to one another, in the longitudinal direction, combustion air flows through the tangential inflow slots formed as a result of the offset into the interior of the burner. Simultaneously, during inflow through these slots, fuel is admixed with the combustion air, with the result that a conical fuel/combustion-air cone is formed and, again, a backflow zone in a stable position is formed in the region of the burner mouth.
In burners of this type, a power output reduction is achieved principally by a reduction in the fuel mass flow, with the air mass flow remaining approximately constant. That is to say, in other words, that, with a decreasing power output, the fuel/air mixture becomes increasingly leaner. However, since modern premixing burners are already operated near the lean extinguishing limit for the purpose of NOx minimization, other combustion concepts have to be developed for the part-load operating mode, in order to prevent extinguishing or an unstable behavior in the case of an increasingly leaner fuel/air mixture.
The prior art discloses, as combustion concepts for the part-load operating mode, for example, what is known as burner staging, in which individual burners are switched off in a specific manner, so that the remaining burners can be operated under full load. Particularly in the case of annular combustion chambers with a plurality of mutually offset burner rings having a different radius, this concept can be employed with a certain amount of success.
On the other hand, the transition from premixing combustion to diffusion-flame-like combustion is proposed, which, as is known, has a lower extinguishing limit in relation to the temperature. Consequently, a double operation of individual burners, which is employed according to the load, to be precise a premix-like and a diffusion-like operation, is proposed, in order to prevent extinguishing in the part-load mode. The problem with this, however, is that, on the one hand, it is complicated to design a burner for two different operating modes and, on the other hand, diffusion-like combustion usually cannot be carried out optimally in terms of emissions.
EP 0 866 267 A1 discloses the mixing of fresh air with recirculated smoke gas in the mirror-symmetrically tangentially arranged feed ducts of a double-cone burner in the case of atmospheric combustion. The combustion air enriched with the recirculated exhaust gas gives rise, for example, to better evaporation of the liquid fuel fed, via a central fuel nozzle, within the premixing zone induced by the length of the premixing burner. Although a lowering of pollutant emissions can consequently advantageously be achieved, nevertheless one disadvantage in a stabilization of the burner during the starting phase is that it is necessary to have a blow-off device which is connected operatively to the air plenum and by the use of which the admission pressure in the plenum is lowered, the air mass flow through the burner is reduced and consequently the air ratio is decreased.
SUMMARY OF THE INVENTION
The object of the invention is, therefore, to make available a burner for a gas turbine or hot-gas generation for the combustion of liquid or gaseous fuel, in which burner fuel is mixed with combustion air in a burner interior, is fed to a combustion chamber and is burnt in this combustion chamber, and a method for operating a burner of this type, which makes it possible to have a stable part-load operating mode.
As already mentioned above, double-cone burners from the prior art cannot achieve the abovementioned object, since, because operation is already lean in the full-load mode, in the part-load mode the flame becomes unstable or is even extinguished.
The present invention achieves the object by the provision of means which can stabilize the flame in the part-load mode.
The subject of the invention is consequently a burner of the abovementioned type, in which means are provided which make it possible to recirculate hot exhaust gas out of the combustion chamber into the burner interior for stabilization in the part-load mode.
The essence of the invention is, therefore, that the hot exhaust gases from the combustion chamber are used to stabilize the flow behavior in the burner interior and near the burner mouth, particularly in the part-load mode, that is to say during lean operation with reduced power output. Such recirculation of exhaust gases makes it possible to use burners of this type in machines (in particular, machines with variable inlet guide vane assemblies, VIGV) in a load range 30-100%.
According to a first preferred embodiment of the invention, the means are a recirculation line which, furthermore, picks up preferably hot exhaust gas on an axial combustion chamber wall near outer backflow zones present next to the burner mouth issuing into the combustion chamber and which feeds it to the burner interior in the region of a burner tip facing away from the combustion chamber. In such recirculation of the hot exhaust gases from a backflow zone, this recirculation takes place usually passively, that is to say the flow of hot exhaust gas into the burner interior does not have to be driven.
Another embodiment of the invention is distinguished in that the burner has at least one inner backflow zone. In a burner of this type, the result of the recirculation of the hot exhaust gases is that precisely this inner central backflow zone is stabilized on the axis of the burner by these hot exhaust gases.
In a further embodiment of the invention, the burner is a double-cone burner with at least two part-cone bodies positioned one on the other and having a conical shape opening toward the combustion chamber in the flow direction, the center axes of these part-cone bodies running, offset to one another in the longitudinal direction, in such a way that tan
Doebbeling Klaus
Paikert Bettina
Paschereit Christian Oliver
Alstom Technology Ltd
Basichas Alfred
Burns Doane Swecker & Mathis L.L.P.
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