Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
1998-10-27
2001-08-07
Lazarus, Ira S. (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S173000, C431S284000, C431S354000, C060S737000, C060S748000, C239S424000, C239S405000
Reexamination Certificate
active
06270338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for operating a premix burner and to a corresponding premix burner for carrying out the method.
2. Discussion of Background
Combustion chambers with premix burners which are designed as so-called double-cone burners and in which the fuel is supplied from the outside by plug-in fuel lances have long proven suitable for stationary gas turbines in power plants. The lance is generally configured as a two-fuel lance, i.e. it is possible, as desired, to supply gaseous fuel, e.g. pilot gas, and/or liquid fuel, for example an oil/water mixture. To this end, a liquid-fuel pipe, an atomizer pipe and a pilot-gas pipe are arranged concentrically in the lance. The pipes each form a duct for the liquid fuel, the atomizer air and the pilot gas, which ducts, at the lance head, end in a central fuel nozzle. The head of the fuel lance projects into a corresponding inner pipe of the double-cone burner, so that the fuel emerging passes centrally into the burner inner chamber which adjoins the inner pipe (cf. DE 43 06 956 A1).
EP 0,321,809 B1 has also disclosed a double-cone burner which is provided for use in a combustion chamber which is connected to a gas turbine. This burner comprises two hollow part bodies which complement one another to form the double-cone burner and are arranged radially offset with respect to one another. It has a hollow-cone-shaped inner chamber which increases in size in the direction of flow and has tangential air-inlet slots. The fuel is supplied to the double-cone burner from the outside via the fuel lance which opens out into the central liquid-fuel nozzle. The latter forms a hollow-cone-shaped fuel spray, consisting of liquid fuel and air, in the burner inner chamber, in which spray most of the fuel droplets are concentrated at the outer end of the conical spray pattern.
Owing to the large injection angle of approx. 30° and the absence of an axial impulse in the center, these sprays are highly susceptible to centrifugal forces which are generated by the turbulent flow in the interior of the burner. As a result, the fuel droplets are carried relatively quickly outward by centrifugal forces, resulting, under certain operating conditions, in a not insignificant quantity of the liquid fuel striking the inner walls of the burner.
To atomize liquid fuels, inter alia so-called plain-jet atomizers are also used, which atomizers produce a conical plain jet of uniformly distributed fuel droplets. Such a solution is known from the textbook “Atomization and sprays”, by A. Lefebvre, West Lafayette, Indiana 1989, pp. 106/107, 238-241. In the case of this atomizer nozzle, the liquid fuel is ejected at high pressure from an antechamber through a small, circular injection opening of a defined guide length. As a result, the plain-jet atomizer produces a fuel jet with an injection angle of approximately 5° to 15°.
However, owing to this small injection angle and the fact that the associated atomization only takes place further downstream, such plain-jet atomizers are not used in combustion chambers of gas turbine installations which are fitted with premix burners, since they require rapid atomization of the liquid fuel. In addition, the plain-jet atomizer described is not particularly suitable for numerous combustion applications, since it has a tendency to concentrate the fuel drops in a small area directly downstream of the nozzle. Particularly under the unfavorable conditions of a low air/fuel ratio and at a low air speed, it is not possible to achieve a sufficient level of atomization.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel method for operating a premix burner which has improved operational reliability and functioning during certain types of operation. In addition, it is intended to specify a corresponding premix burner for carrying out the method.
According to the invention, this is achieved by the fact that, in a method for operating a premix burner which is designed in accordance with the preamble of claim
1
, at least one liquid fuel is injected into the inner chamber of the premix burner in a plain jet and with an injection angle &agr; of less than 10°.
For this purpose, the liquid-fuel nozzle is provided with a simple injection opening which has a guide length l and a diameter d. Owing to the influence of the opening, the liquid fuel injected through the injection opening axially into the inner chamber of the premix burner forms a plain jet, the injection angle of which is less than 10° and is therefore relatively small. The fuel jet and the combustion-air flow interact in the interior of the premix burner. Primarily as a result of the shear forces between the fuel jet and the turbulent combustion air, successful atomization is achieved in the downstream region of the premix burner, as a result of which atomization fine droplets which are suitable for combustion are produced. Owing to the small injection angle and the concentration of the axial impulse of the injected fuel in the burner axis, the influence of the angular flow on the fuel droplets is significantly reduced. As a result of the centrifugal force, the droplets are carried away from the center and for the most part mixed with the combustion air. In addition, the fuel droplets are evaporated before they reach the burner walls. In this way, it is possible for the plain jet to penetrate a substantial distance through the premix burner without the fuel droplets wetting the burner walls. Despite a considerably worse atomization quality than in conventional liquid-fuel nozzles, sufficient atomization does take place, as evidenced by the fact that there is no significant rise in the emission of pollutants.
The liquid-fuel nozzle used is particularly simple, robust and reliable, which, not unimportantly, also contributes to reducing costs. Its most important parameters are the diameter d, the guide length l and the shape of the injection opening. The degree of turbulence in the flow of fuel, which is defined primarily by the conditions upstream of the injection opening and by the abovementioned axial guide length, is also a decisive factor for the atomization.
Particularly advantageously, the injection opening has a guide length to diameter ratio of 4≦l/d≦6. Test results given in the textbook “Atomization and sprays”, which has already been mentioned above, by A. Lefebvre, West Lafayette, Indiana 1989, pp. 155-161, in particular in FIG.
5
.
4
., show the influence of the guide length to diameter ratio of the injection opening on the injection coefficient, i.e. on the ratio of the current flow rate to the theoretical flow rate through the injection opening. In that study, l/d quotients of up to 10 were examined and it was established that the greatest injection coefficient is achieved at an l/d quotient of approx. 2. In contrast to this teaching, the premix burner according to the invention has been equipped with a liquid-fuel nozzle, the injection opening of which has a guide length to diameter ratio of 4≦l/d≦6 and consequently has an injection coefficient which lies significantly below the maximum. Nevertheless, the use of a liquid-fuel nozzle designed in this way has made it possible, in a premix burner, to achieve a compact liquid-fuel spray with the desired injection angle and the necessary impulse.
Owing to this compact liquid-fuel spray, such an atomizer nozzle, or a correspondingly equipped premix burner, still does not have a completely prepared fuel mixture present at the burner head. For this reason, a pulse-free operation is achieved over a broad load range and also with a different quantity of water. In addition, the compact liquid-fuel spray does not strike the burner walls, so that overheating of the premix burner and the combustion chamber can also be prevented, as can coking inside the premix burner. A further advantage, which can be attributed to the liquid-fuel spray being situated exclusively inside the combustion-air flow, is the successful ignition an
Eroglu Adnan
Hellat Jaan
Keller Jakob
McMillan Robin
Suter Roger
Asea Brown Boveri AG
Burns Doane Swecker & Mathis L.L.P.
Cocks Josiah C.
Lazarus Ira S.
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