Premix burner

Details Premix burner Premix burner

2000-07-24

2001-12-18

Yeung, James C. (Department: 3743)

Combustion

Flame holder having protective flame enclosing or flame...

C431S351000, C431S354000, C060S039300, C060S039300, C060S752000

Reexamination Certificate

active

06331109

ABSTRACT:

This application claims priority under 35 U.S.C. §§119 and/or 365 to Appln. No. 99114376.9 filed in Europe on Jul. 22, 1999; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The invention relates to premix burners according to the preamble of the independent patent claim.
BACKGROUND OF THE INVENTION
EP 0 321 809, EP 0 780 629 or WO 93/17279 in each case disclose premix burners for operating with gaseous and/or liquid fuels, these burners having essential features in common. Thus, in each case, a swirl generator having tangential air inflow orifices encloses a cavity, the cross-sectional area of which widens in the axial flow direction. In EP 0 321 809 and EP 0 780 629, this is implemented by the swirl generator being of conical design, whilst the fully equivalent solution proposed in WO 93/17279 is to make the swirl generator itself cylindrical and insert inside the cavity a conical displacement body narrowing in the axial throughflow direction. Fuel is supplied to the swirl flow within the swirl generator. It is known for means for supplying a liquid fuel to be arranged in the vicinity of the burner axis and for means for introducing gaseous fuels to be provided radially on the outside, preferably in the region of the tangential air inflow orifices. The introduction of the fuels into a highly swirled flow is aimed at good premixing of the fuel/air mixture, and, of course, the axial component of the flow velocities must be so high that the flame does not flash back into the cavity of the burner. For further intensifying the intermixing of fuel and air, EP 0 780 629 proposes that the swirl generator be followed by a mixing section and that the swirl flow be transferred into this mixing section, if possible, without any loss. At a downstream end, the burner types mentioned issue with a more or less sudden widening of the flow cross section, at a short axial distance, in a combustion space. The highly swirled flow bursts open at this sudden jumping cross section, and a backflow bubble is formed, which causes a flame to be stabilized, without mechanical flame holders which are at risk from latent heat.
Burners of the type known from EP 0 321 809 have proved appropriate for many years in practical applications in gas turbines and atmospheric firing installations. The burners known from EP 0 321 809 and from EP 0 780 629 have undergone constant further development, and improvement proposals are found in a multiplicity of published documents.
However, another result arising from the functioning of the burners is that they build up high thermal stresses during operation. Thus, a burner of this type has a front plate, on which the swirl generator and, if appropriate, a mixing tube are mounted. In this case, the front plate constitutes the closure of the burner to the combustion space and separates from the combustion space a space from which air flows through the tangential orifices into the interior of the burner. In this case, both a leakage of combustion air and an uncontrolled penetration of smoke gases into the fresh air are to be avoided under all circumstances. Moreover, the entire burner has to be anchored in some way to the combustion space wall. Consequently, in the burners known at the present time, the swirl generator or, if present, a mixing tube is connected fixedly to the front plate, for example by welding. The front plate is then subjected, during operation, to hot combustion gases, whilst the rest of the structure is surrounded by a medium having a markedly lower temperature. The swirl generator and the mixing tube impede the free thermal expansion of the front plate, and high mechanical stresses are induced, precisely at the connection point which for manufacturing reasons is often a weld seam.
In modern gas turbines, the temperatures of the combustion air reach an order of magnitude of around 500° C. However, with a rising pressure ratio of the working process or with pronounced external preheating of the combustion air, both of these being measures having a beneficial influence on the process efficiency, efficient cooling of the front plate is obviously more complicated to carry out.
It remains to be said, in conclusion, that, with a further rise in key process data, a limitation of the useful life of premix burners must be expected in the embodiments known at the present time, specifically, on the one hand, because of thermal stresses in the components due to impeded thermal expansion and, on the other hand, because the cooling of components exposed to hot gas will be always more complicated to achieve satisfactorily.
SUMMARY OF THE INVENTION
The invention is intended to remedy this. The object on which the invention, is based is to design a premix burner of the type initially mentioned, in such a way that relative displacements of the individual components of the burner due to thermal expansions can take place, unimpeded. Furthermore, the cooling of parts exposed to hot gas is to be ensured or parts not capable of being cooled effectively are to be protected against excessive thermal loads.
The essence of the invention, therefore, is for the front plate to be designed as a carrier structure, on which the swirl generator or, if appropriate, a mixing tube following the swirl generator is fastened. As already discussed with regard to the prior art, it is expedient, for various reasons, to connect these structural components firmly to the carrier structure. According to the invention, a heat shield is arranged downstream of the carrier structure, said heat shield being connected to the carrier structure in such a way that, within the limits of the thermal expansions to be expected, the free relative displaceability of the carrier structure and heat shield is impeded only slightly. This design affords a series of advantages. The front plate, which designed as a carrier structure is subjected to thermal load to a substantially lesser extent. As a result, substantially less cooling is necessary in order to maintain the carrier structure at a temperature which is compatible with that of the adjacent components of the burner. Under some circumstances, a special ceramic heat protection coating of the carrier structure may even be dispensed with, and therefore materials with compatible coefficients of thermal expansion are used at this point. By contrast, it would be conceivable to produce the heat shield, which naturally does not have to bear any mechanical loads, from a solid ceramic resistant to high temperature, and to dispense with cooling completely. Since largely unimpeded relative displaceability of the carrier structure and heat shield is ensured, in principle materials with completely different coefficients of thermal expansion may be used here.
The possible necessary cooling of the carrier structure and that of the heat shield may be combined in an expedient way. For this purpose, the carrier structure is provided with a multiplicity of orifices, through which a cooling medium, usually preferably air, flows out toward the combustion space. The carrier structure thus serves as a perforated plate for impact cooling of the heat shield, whilst the coolant flowing through simultaneously absorbs heat from the carrier structure. The interspace between the carrier structure and the heat shield is then designed as a cooling duct which is advantageously subdivided by means of a continuous web in order to avoid radial flows.
If the design conditions allow it, the carrier structure may itself be connected directly to the combustion chamber wall. Should the temperature distributions in the combustion space walls not allow it or allow it only in a disadvantageous manner, the carrier structure is held on the combustion space wall preferably by means of a number of tubes or rods oriented upstream of the carrier structure. This likewise ensures the absorption of thermal expansions. If these tubes are led through the heat shield in the axial flow direction, they may be utilized as a fuel gas supply for a so-called pilot mode of the burner. I

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