Fuel lance for liquid and/or gaseous fuels and method for operation thereof

A fuel lance for liquid and/or gaseous fuels for use in a combustion chamber includes a liquid fuel pipe extending along a lance center line and defining a liquid fuel passage, a gas pipe surrounding the liquid fuel pipe and forming therebetween a gas passage, and a lance outer shell surrounding the gas pipe and forming an air passage around the gas pipe for cooling air and atomizer air. At least one air/fuel nozzle is provided in a peripheral side of the lance outer shell at a downstream end of the fuel lance for air flow out of the air passage into the combustion chamber. At least one gas nozzle is provided in the gas pipe for gas flow out of the gas passage into the air passage, the gas nozzle is positioned relative to the air/fuel nozzle so that gas from the gas nozzle flows with air from the air passage through the at least one air/fuel nozzle into the combustion chamber. At least one liquid fuel nozzle is provided in the liquid fuel pipe for liquid fuel flow out of the liquid fuel passage, the liquid fuel nozzle being positioned relative to the gas nozzle and air/fuel nozzle so that liquid fuel from the liquid fuel nozzle flows through the air passage and, with the air, through the air/fuel nozzle into the combustion chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of combustion technology. It 
concerns a fuel lance for liquid and/or gaseous fuels for use in a 
combustion chamber, such as is used in gas turbines for example. 
2. Discussion of Background 
Fuel lances are used for the injection of liquid and/or gaseous fuels into 
the combustion chamber of a premixing burner and these fuel lances 
protrude into the combustion chamber and introduce the fuel or fuels in a 
suitable distribution into the combustion air which is flowing past. 
In the design of such fuel lances, various requirements have to be 
satisfied and these are provided partly by the environmental conditions 
and partly by the demands made on them: 
The combustion air flowing past the fuel lance has a temperature which is 
substantially independent of the flow of fuel in the lance. It can be 
necessary to protect the lance itself, and also the fuels carried in it, 
from an excessively high combustion air temperature. 
If the combustion chamber has to be operated with a high fuel quantity 
ratio between full load and part load, care must be taken to ensure that 
the fuel is present with a suitable distribution under every operating 
condition and can be introduced and mixed in the same manner into the flow 
of combustion air. Because the aerodynamics of the burner are practically 
independent of the fuel, the attainment of optimum combustion demands that 
the gaseous fuel and the liquid fuel should be injected in the same manner 
into the flow of the combustion air. 
To keep the efficiency of the burner as high as possible, as little carrier 
or auxiliary air as possible should be used in the lance. 
Furthermore, it is necessary to ensure that, as far as possible, no 
recirculation zones or wakes, which are filled with gas containing fuel 
and can lead to flash-back or thermo-acoustic vibrations, are formed in 
the region of the fuel lance. 
In the injection of liquid fuel, i.e. oil in particular, it is necessary to 
avoid the finely divided oil/air mixture igniting prematurely. 
In the case of liquid fuels, it is also necessary to avoid the formation of 
troublesome deposits within the lance owing to increased temperatures and 
evaporation of the fuel since this could impair the operation of the lance 
in the long term or make it quite impossible. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention is to provide a novel fuel lance 
and a method of operating it which satisfy the requirements mentioned 
above and ensure reliable injection of gaseous and/or liquid fuel with, at 
the same time, high efficiency and low pollutant emissions. 
The object is achieved in a fuel lance for liquid and/or gaseous fuels for 
use in the combustion chamber of a premixing burner, which fuel lance 
comprises 
(a) a liquid fuel pipe extending along a lance center line and surrounding 
a liquid fuel passage for carrying a liquid fuel; 
(b) a gas pipe surrounding the liquid fuel pipe and forming a gas passage 
between itself and the liquid fuel pipe for carrying a gaseous fuel; 
(c) a lance outer shell surrounding the gas pipe and forming an air passage 
between itself and the gas pipe for carrying cooling air and atomizer air; 
(d) at least one air/fuel nozzle which is provided in the side of the lance 
outer shell at the downstream end of the fuel lance and through which air 
can flow out of the air passage into the combustion chamber surrounding 
the fuel lance; whereby 
(e) arranged in the gas pipe, there is at least one gas nozzle through 
which gas can flow out of the gas passage through the air passage and, 
with the air, through the at least one air/fuel nozzle into the combustion 
chamber; and whereby 
(f) arranged in the liquid fuel pipe, there is at least one liquid fuel 
nozzle through which liquid fuel can flow out of the liquid fuel passage 
through the air passage and, with the air, through the at least one 
air/fuel nozzle into the combustion chamber. 
The core of the invention consists in equipping the lance with a suitable 
nozzle arrangement and a special cooling air supply which jackets the 
lance and makes it possible to employ the cooling air simultaneously for 
cooling the lance and the fuel, for atomizing liquid fuel, for preventing 
premature ignition and for generally supporting the mixing process. This 
provides optimum mixing and combustion which lead to a high efficiency 
with simultaneously low pollutant emissions. 
In a first preferred embodiment of the fuel lance according to the 
invention, the at least one air/fuel nozzle and the at least one gas 
nozzle are of circular configuration and are arranged one behind the other 
on a common nozzle center line, and the diameter of the gas nozzle is 
smaller than the diameter of the air/fuel nozzle. The gas flow emerging 
from the gas nozzle is, in this manner, jacketed by an airflow when 
passing through the air/fuel nozzle. On the one hand, this achieves the 
effect that practically the same injection path is provided for the 
gaseous fuel as for the liquid fuel. On the other hand, the airflow 
supports the gas injection substantially independently of the gas quantity 
so that even in the case of small gas flows, the aerodynamic relationships 
in the combustion chamber hardly change. 
Particularly simple and uniform flow relationships within the lance and at 
the nozzles are provided for the various fuels if, in accordance with a 
second preferred embodiment of the invention, the liquid fuel nozzle, 
together with the two other nozzles, is also arranged on the common nozzle 
center line, and the diameter of the liquid fuel nozzle is smaller than 
the diameter of the gas nozzle, and if the liquid fuel pipe and the gas 
pipe are firmly connected to the lance outer shell in the region of the 
nozzles. The fixed connection between the inner tubes and the lance outer 
shell then ensures that the position of the nozzles relative to one 
another can hardly be displaced even in the case of thermal expansions. 
A further preferred embodiment of the invention is one wherein the gas pipe 
and the liquid fuel pipe end, in the flow direction, before the at least 
one air/fuel nozzle, wherein the gas nozzle and the liquid fuel nozzle are 
arranged at the end of the respective pipe and are directed parallel to 
the lance center line and wherein a vane-shaped guide plate is provided 
for each air/fuel nozzle and the further nozzles, which guide plate 
deflects the gas and liquid flows emerging from the further nozzles by 
approximately 90.degree. and guides them into the respective air/fuel 
nozzle. By this means, an air-driven atomizer, which is known in the 
English language literature as a "prefilming atomizer" (on this point, see 
also A. H. Lefebvre, Airblast Atomization, Prog. Energy Combust. Sci., 
Vol. 6, pp. 233-261 (1980)), is brought into effect for the distribution 
and mixing of the liquid fuel. 
In a further preferred embodiment of the fuel lance according to the 
invention, the air passage is led around the downstream end of the fuel 
lance, and at least one auxiliary nozzle directed substantially parallel 
to the lance center line is provided in this end, and air can flow out of 
the air passage through this auxiliary nozzle into the combustion chamber. 
By means of the auxiliary nozzle, fuel-free air is injected into the space 
behind the tip of the lance in order to prevent the formation, at this 
critical location, of wakes and/or recirculation zones containing fuel. 
The method according to the invention of operating the fuel lance according 
to the invention is one wherein air with a temperature of up to several 
hundred degrees centigrade, but preferably less than 600.degree. C., is 
carried through the air passage to the air/fuel nozzle in order to cool 
the lance and distribute the fuel and is there blown into the combustion 
chamber as a flow jacketing the fuel flow. By this means, reliable cooling 
of the lance is achieved even where the combustion air or combustion gases 
flowing past the lance have relatively high temperatures. 
Further embodiments of the fuel lance according to the invention and 
embodiments of the method of operation according to the invention are 
given in the dependent claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, in FIG. 1 a 
possible arrangement of one example of a fuel lance in accordance with the 
invention is shown, in side view in a combustion chamber 2, of a gas 
turbine or the like, bounded by a casing 3 (only a partial excerpt of the 
chamber is shown). The lance center line 5 of the fuel lance 1 is arranged 
along the center line of the combustion chamber 2 in this example and (as 
is indicated by the three long arrows in FIG. 1) it has hot combustion air 
flowing around it. The fuel lance 1 is therefore matched to the 
aerodynamic relationships in the combustion chamber 2 and is streamlined. 
It is surrounded by an elongated lance outer shell 11 and is fastened to 
the casing 3 by means of a support arm 4 branching off to the side. The 
support arm 4 is likewise streamlined and can have an aerofoil-type 
support arm profile 14 in the cross-section drawn. 
A plurality of pipes extend through the support arm 4 and through the fuel 
lance 1 itself--as becomes clear from the opened-up part of the lance 
shown in FIG. 1. Gaseous and liquid fuel and cooling and atomization air 
are carried through these pipes to the downstream tip of the lance and are 
there injected into the combustion chamber 2 through appropriate air/fuel 
nozzles 12 and an auxiliary nozzle 13, in a manner to be described in more 
detail later. The pipes include an inner liquid fuel pipe 7 extending in 
the axial direction and a gas pipe 9 surrounding the liquid fuel pipe 7 
concentrically and at a distance. The gas pipe 9 is in turn surrounded 
concentrically and at a distance by the lance outer shell 11. Three 
passages, the inner liquid fuel passage 6, the gas passage 8 and the air 
passage 10, are formed by the concentric arrangement, at a distance from 
one another, of the pipes and outer shell. Depending on the type of 
operation of the fuel lance 1, the passages undertake different functions, 
which are explained in more detail below using three preferred embodiment 
examples represented in FIGS. 2 to 4. 
For the first embodiment example, FIG. 2 shows--in longitudinal 
section--the lance tip which, for the purpose of explaining different 
operating cases, is subdivided along the lance center line 5 into two 
separate halves. With the flows shown (marked by arrows), the upper half 
relates to the operating case with exclusively gaseous fuel and the lower 
half relates to the operating case with exclusively liquid fuel. A 
corresponding representation in two parts has also been selected, for the 
same reasons, in the case of the other FIGS. 3 and 4. 
Coming from the left, the inner liquid fuel pipe 7, the gas pipe 9 and the 
lance outer shell 11 end at the tip of the lance. At the end, the gas pipe 
9 merges into a hemispherical pipe end 17 which closes off the pipe. The 
liquid fuel pipe 7 is butt-welded (or brazed) to the inner surface of the 
pipe end 17 and is closed off towards the end by this means. The lance 
outer shell 11 surrounds the pipe end 17 at a distance in the form of a 
hemispherical shell so that the air passage 10 formed between the lance 
outer shell 11 and the gas pipe 9 extends into the immediate lance tip and 
surrounds the pipe end 17 on the outside. A plurality of connecting webs 
16 are welded in--or brazed--between the pipe end 17 and the front 
hemispherical shell of the lance outer shell 11. In this way, the two 
pipes 7 and 9 and the lance outer shell 11 form, in the region of the 
lance tip, a stable, firmly connected unit which prevents displacement of 
the pipes relative to one another due to thermal expansion. 
A plurality of (preferably four) sets of nozzles are provided in the region 
of the pipe ends and each of these nozzles is arranged along a nozzle 
center line 24 at right angles (or oblique) to the lance center line 5. 
The nozzle sets are distributed with respect to number and angular 
distance apart along the periphery of the fuel lance 1 in such a way as to 
ensure optimum mixing for a specified secondary pattern of the combustion 
chamber airflow, while avoiding wakes. Each set of nozzles includes a 
liquid fuel nozzle 18 let into the liquid fuel pipe 7, a gas nozzle 15 let 
into the gas pipe and an air/fuel nozzle 12 let into the lance outer shell 
11. Each of the nozzles 12, 15 and 18 is preferably circular. Their 
diameters are stepped, the inner liquid fuel nozzle 18 having the smallest 
diameter and the outer air/fuel nozzle having the largest diameter. The 
number and diameter of the liquid fuel nozzles 18 depends on the liquid 
fuel flow quantity which occurs in the normal case. Attention should be 
paid to ensuring that the nozzle diameters are not too small so that the 
nozzles do not become blocked if solid deposits are formed. Otherwise, the 
number of fuel jets injected into the combustion chamber through the 
nozzles must not be too large so as not to disturb the aerodynamics around 
the fuel lance 1 to such an extent that an increased number of wakes 
containing fuel are formed behind the lance. 
In the operating case with pure gas injection shown in the upper half of 
FIG. 2, the inner liquid fuel passage 6 is not used at all. The 
combustible gas flows through the gas passage 8 and the gas nozzle 15, 
where it forms a gas jet which is directed radially outwards and passes 
through the air/fuel nozzle 12 into the combustion chamber 2. At the same 
time, cooling air with a temperature up to several hundred degrees 
centigrade, but preferably less than 600.degree. C., is dispatched through 
the air passage 10 and likewise emerges radially into the combustion 
chamber from the air/fuel nozzle where it initially jackets the gas jet. 
The cooling air has a plurality of functions in this case. On the one 
hand, it cools the lance outer shell 11 and forms a thermal protective 
jacket for the fuel passages located further in. On the other hand, it 
generates a stable air jet at the air/fuel nozzle 12 and this jet remains 
the same irrespective of how much gas is fed in through the lance so that 
even in the case of small flow quantities of gaseous fuel, the 
configuration of the injection jets remains substantially unaltered. 
Finally, the jacket of relatively cool air permits and supports 
sufficiently lengthy mixing of the gaseous fuel with the combustion air in 
the combustion chamber 2. This is necessary for efficient combustion 
because premature self-ignition of the mixture is reliably avoided. 
In the case of the operating case with pure liquid fuel injection shown in 
the lower half of FIG. 2, a liquid fuel, usually an oil/water emulsion, is 
carried through the inner liquid fuel passage 6 to the liquid fuel nozzle 
18 and is there expelled radially outwards as a liquid jet. In this case, 
air is introduced through the gas passage 8 and emerges through the gas 
nozzle 15 where it interacts with the liquid jet likewise passing through 
the gas nozzle 15 to effect fine atomization of the liquid fuel into small 
droplets only ("plain-jet airblast atomization"). The atomization jet is 
then surrounded by a cooling air jacket (which also contributes to the 
atomization) at the air/fuel nozzle 12 in the same manner as described 
above and is finally injected into the combustion chamber 2. In addition 
to the cooling by the air flowing in the air passage 10, a further thermal 
screening stage is made available by the auxiliary air in the gas passage 
8. By this means, the liquid fuel in the liquid fuel passage 6 can be kept 
at temperatures at which solid deposits are reliably avoided. 
As may be seen from the above considerations, the cooling or auxiliary air 
in the lance according to the invention has several simultaneous 
functions: 
(i) It cools the lance and protects the fuel passages within it from 
excessive temperatures. 
(ii) It cools the fuel jets when they are injected and therefore delays 
their heating so that adequate mixing with the combustion air can take 
place before self-ignition. 
(iii) It effects, as auxiliary air, the necessary atomization of a liquid 
fuel. 
(iv) On emergence through the air/fuel nozzles 12, it supports--as a jacket 
flow--the mixing of the fuel jet in the combustion chamber. 
(v) Even in the case of small fuel flows, it maintains the jet system 
emerging from the nozzle sets. 
In all these processes, the special arrangement of the nozzles 12, 15 and 
18 achieves the effect that whether gaseous or liquid fuel is used, the 
same aerodynamic configuration always appears, i.e. the fuel jets are 
injected into the combustion chamber 2 in the same manner. Because of the 
strong connection between the pipes 7, 9 and to the lance outer shell 11, 
the single-axis arrangement of the nozzle sets, and therefore the 
aerodynamic configuration, is maintained even if thermal stresses are 
present in the lance due to different temperature distributions. 
The air from the air passage 10 can, advantageously, undertake a further 
function. For aerodynamic reasons, wakes--which fundamentally contain fuel 
and which lead to flash-back or thermo-acoustic vibrations 
(pulsation)--can form in the flow direction behind the lance tip. Such 
phenomena cannot be tolerated because they place loads on the combustion 
chamber and, more particularly, lead to increased pollutant emissions. In 
order to prevent them, an auxiliary nozzle 13 is preferably provided at 
the lance tip arranged centrally along the lance center line 5 and through 
it a fuel-free airflow is injected from the air passage 10 into the part 
of the combustion chamber located behind the tip. This measure has the 
simultaneous effect that the fuel lance 1 is cooled right up to the tip. 
A further preferred embodiment example of a fuel lance according to the 
invention is shown in FIG. 3. In this representation, FIG. 3A corresponds 
in the direction of the view to FIG. 2; FIG. 3B is a partial cross-section 
through the lance along the line A--A of FIG. 3A, the region with the 
liquid fuel nozzles 18 being shown rotated about the lance center line 5 
in FIG. 3A. The embodiment shown departs from that of FIG. 2 principally 
with respect to the arrangement of the liquid fuel nozzles 18. In this 
case, the nozzles 18 are no longer arranged, together with the other 
nozzles 12 and 15, on a common nozzle center line 24 but are displaced 
rearwards away from the lance tip and are simultaneously rotated about the 
lance center line 5 (FIG. 3B) so that a jet emerging from them no longer 
passes to the outside directly through the two other nozzles 15, 12. 
Because a rigid location of the liquid fuel nozzles 18 relative to the 
other air/fuel nozzles 12, 15 is no longer necessary in this case, the 
liquid fuel pipe 7 can end before the pipe end 17 and does not need to be 
fastened to the pipe end 17. FIG. 2 arises because a guide pipe 19 is 
fitted into each of the gas nozzles 15. This guide pipe 19 extends from 
the gas nozzle 15 through the air passage 10 and into the associated 
air/fuel nozzle 12. This supports the formation of the jacket flow already 
described above so that a gas flow through the guide pipe 19 reaches the 
combustion chamber 2 in a relatively protected manner when it emerges from 
the air/fuel nozzle 12. 
As in FIG. 2, the upper part of the illustration in FIG. 3A represents the 
operating case with gaseous fuel, the liquid fuel pipe 7 being empty and 
unused. In this case, the formation of the injection jet takes place in a 
manner completely analogous to that of FIG. 2. The lower part of the 
illustration shows the operating case with liquid fuel. The liquid fuel 
emerges as a jet from the liquid fuel nozzle 18, is carried--by the 
auxiliary air introduced in the gas passage 8--along the inner wall of the 
gas pipe 9 to the gas nozzle 15 and is there blown out, together with the 
auxiliary air, through the guide pipe 19, atomization taking place at the 
same time ("air assist atomizer"). Additional ring plates 20 on both sides 
of the liquid fuel nozzles 18 improve the flow relationships. 
A further preferred embodiment example of a fuel lance in accordance with 
the invention is shown in FIG. 4. FIG. 4A again corresponds in the 
direction of the view to FIG. 2 and FIG. 3A whereas the special shape of 
the guide plates used and their interaction with the nozzles are shown in 
FIG. 4B in a view in the flow direction. In the embodiment example of FIG. 
4, the air/fuel nozzles 12 are arranged at the same location as in the 
embodiment examples of FIGS. 2 and 3. The arrangement of the other 
nozzles, however, is clearly different. The gas pipe 9 and the liquid fuel 
pipe 7 end, in the flow direction, before the air/fuel nozzles 12. The gas 
nozzle 15 and the liquid fuel nozzle 18 associated with each air/fuel 
nozzle 12 are located at the end of the respective pipe (9 or 7) and are 
directed parallel to the lance center line 5. A vane-shaped guide plate 22 
is provided for each air/fuel nozzle 12 and the associated nozzles 15, 18; 
this guide plate 22 deflects the gas and liquid flows emerging from the 
associated nozzles 15, 18 by approximately 90.degree. and introduces them 
into the respective air/fuel nozzle 12. As may be recognized from FIG. 4B, 
the guide plates 22 are arranged like a clover leaf around the lance 
center line 5. 
Preferably in the region of the air/fuel nozzle 12, each guide plate 22 
ends in a closed sheet-metal ring 23 whose diameter is smaller than the 
diameter of the air/fuel nozzle 12. By this means, the deflected flows 
from the associated nozzles 15, 18 are again jacketed by an airflow on 
emergence from the air/fuel nozzle 12. A guide pipe 19 can be additionally 
fitted into each of the gas nozzles 15 in order to ensure reliable 
deflection of the gas flows by the guide plates 22. The guide plates 22 
are firmly connected to the lance outer shell 11 in the region of the 
nozzles (12, 15, 18) so that they cannot be displaced relative to the 
air/fuel nozzle 12. The connection takes place by means of a pipe end 21 
in the form of a hemispherical shell which takes up the position of the 
pipe end 17 from FIG. 2 and FIG. 3 and is anchored to the lance outer 
shell 11 by means of the connecting webs 16 already mentioned. 
The upper part of the illustration of FIG. 4 again represents pure gas 
operation in which the liquid fuel pipe 7 is not used. In this case, the 
gas flow emerges from the gas passage 8 through the guide pipe 19, is 
deflected by the guide plate 22, is concentrated by the sheet-metal ring 
23 and is expelled, jacketed by an airflow, through the air/fuel nozzle 12 
into the combustion chamber. In the case of liquid fuel operation in the 
lower part of the illustration, the gas passage 8 is not, in this case, 
used. The jet emerging from the liquid fuel nozzle 18 is guided without 
auxiliary air, as a liquid film on the inner wall of the guide plate 22, 
to the air/fuel nozzle 12 and is there atomized by very fine droplets 
being torn away at the outer edge of the sheet-metal ring ("prefilmer 
atomizer"). 
Another preferred embodiment example of a fuel lance in accordance with the 
invention is shown in FIG. 5. In this example, only the liquid fuel 
nozzles 18 and the corresponding air/fuel nozzles 12 are arranged along a 
nozzle center line 24. Independently of this, the gas nozzles 15 are 
placed in the flow direction before the other nozzles 12, 18. In the case 
of gas operation (upper half of the figure), the gas has already been 
intensively mixed with the cooling air in the air passage 10 before the 
air/fuel nozzle 12. The gas/air mixture then emerges through the air/fuel 
nozzle 12 into the combustion chamber. An air pipe 20, which starts before 
the gas nozzle 15 and leads past the gas nozzle, carries fuel-free cooling 
air into the end region of the lance where it is injected into the 
combustion chamber through the auxiliary nozzle 13 in order to prevent 
wakes. In the case of liquid fuel operation (illustration in the lower 
part of FIG. 5), the liquid fuel flows out of the liquid fuel nozzle 18 
accommodated in the pipe end 17, past the air pipe 20 and directly into 
the air/fuel nozzle 12 where it interacts with the cooling air from the 
air passage 10 in the manner already described. 
Overall, the invention provides a fuel lance which can inject gaseous and 
liquid fuels in the same aerodynamic configuration, operates reliably even 
at high combustion gas temperatures, permits optimum atomization of liquid 
fuels and makes very low pollutant emissions possible by means of a 
lengthened mixing process. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.