Burner

A burner with a conical shape opening in the flow direction is composed of two partial-conical bodies, which are positioned one upon the other and whose centerlines in the longitudinal direction extend offset relative to one another. Because of this offset, a tangential inlet slot to an internal space of the burner is formed in each case over the length of the burner. The fuel supply takes place centrally via a nozzle and tangentially in the region of the inlet slots via, in each case, a fuel line, which is provided with fuel openings which there undertake the injection of the fuel. A duct is formed above each inlet slot and this is equipped with an injector. A further fuel is introduced through this injector. The air/fuel mixture with fuel from the injector and/or fuel from the fuel line flows generally as an air/fuel mixture through the tangential inlet slots into the internal space of the burner. If needed, further mixing with the fuel from the nozzle takes place there.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a burner and as a method for operating 
such a burner. 
2. Discussion of Background 
A burner is known from EP-A1-0, 321,809 which consists of two half hollow 
partial-conical bodies which lie offset one upon the other. The conical 
shape of the partial-conical bodies shown in the figure of this patent 
extends in the flow direction at a certain fixed angle. The offset 
mentioned of the partial-conical bodies relative to one another creates a 
tangential inlet slot over the complete length of the burner on each of 
the two sides of the burner body, the width of the slot corresponding to 
the particular offset of the centerlines of the partial-conical bodies 
relative to one another and the combustion air flowing into the internal 
space of the burner through the slots. 
A fuel nozzle is located in the internal space at the beginning of the 
burner and its fuel injection preferably emerges centrally between the 
centerlines of the partial-conical bodies offset relative to one another. 
Further fuel nozzles are provided in the region of the tangential inlet 
slots. Liquid fuel is preferably introduced through the central fuel 
nozzle whereas the fuel nozzles in the region of the tangential inlet 
slots are preferably operated with a gaseous fuel. If such a burner is 
operated with a medium calorific value gas, which usually contains easily 
ignited hydrogen, there exists the real danger that this gas and the 
combustion air introduced will mix so strongly even in the inlet region, 
at the location where they meet, in such a way that premature ignition of 
the mixture can occur. This would in turn lead to diffusion-type 
combustion with greatly increased NO.sub.x emission. In addition, it may 
also be the case that shear layers can easily occur with such air/gas 
mixing and the result of this is instability in the mixing process due to 
strong eddying. If gas supply pressure pulsations occur because of the 
above-mentioned instability, this additionally leads to strong vibrations 
in the system. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention, is to provide, in a burner of 
the type mentioned, measures which make premature ignition of the mixture 
impossible when a medium calorific value gas is used as fuel. The measures 
should also permit stabilization of the mixing process. 
The essential advantage of the invention may be seen in the fact that the 
NO.sub.x emissions remain low because no premature ignition occurs. 
A further essential advantage of the invention may be seen in the fact that 
the injector, by which the objective is achieved, makes it possible to 
avoid substantial alteration to the flow field of the burner used despite 
the high mass flow proportion of the medium calorific value gas in the 
air/gas mixture. This is achieved by means of a suitable distribution of a 
number of injector holes of the same size or by means of an arrangement of 
holes whose diameter is varied in a suitable manner. The density of the 
gas inlet holes (.rho..sub.GB) is proportional to the radially averaged 
combustion air inlet velocity through the tangential air inlet slots of 
the burner. 
In addition, the injector in accordance with the invention does not permit 
the occurrence of shear layers during the mixing process. These shear 
layers, which always occur when the velocity of the gaseous fuel at the 
location of mixing is greater than the air velocity, cause strong eddies 
which initiate an instability of the system. Because the injector is 
designed in such a way that the two media meet at the mixing location with 
almost the same velocity, no turbulence occurs there; in addition, 
pressure pulsations which would have a negative effect on the mixing and 
combustion process do not occur at this location so that vibrations in the 
system are excluded. With respect to the flow velocity of the gaseous 
fuel, the mixing process is designed for full load and the gaseous fuel is 
"breathed" almost unpressurized into the airflow. Further advantages of 
the invention concern the avoidance of acoustic resonance in the injection 
of the fuel; because the gap width and the length of the injector are 
appropriately designed, the flow can recover to such an extent before 
leaving the injector that the acoustic resonance mentioned cannot occur. 
A further advantage of the invention may be seen in the fact that 
combustion is conceivable over suitable temperature and pressure ranges 
even in the case of gases with a low calorific value. 
Advantageous and desirable extensions of the way of achieving the 
objective, according to the invention, are claimed in the further claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts in the two views, the injectors shown in 
FIG. 2 are not included in FIG. 1 in order to make the latter more easily 
understood. FIG. 1 and FIG. 2 should be considered simultaneously in order 
to understand the structure of the burner better. 
FIG. 1 shows a burner 1, which consists of two half hollow partial-conical 
bodies which lie one upon the other and offset relative to one another. 
The conical shape of the partial-conical bodies 2, 3 shown has a certain 
fixed angle in the flow direction. The partial-conical bodies 2, 3 can, of 
course, have an increasing conical inclination in the flow direction 
(convex shape) or a decreasing conical inclination in the flow direction 
(concave shape). The two latter shapes are not included in the drawing 
because they can be envisaged without difficulty. The shape which is 
finally used depends on the various parameters of the combustion process. 
The shape shown on the drawing is preferably used. The offset of the 
respective centerlines 2a, 3a (see FIG. 2) of the partial-conical bodies 
2, 3 relative to one another creates a tangential inlet slot 2b, 3b in the 
flow direction on each of the two sides of the burner 1 with a certain free 
inlet slot width S (see FIG. 2) through which the combustion air 8 
(air/fuel mixture) flows into the internal space 17 of the burner 1. The 
tangential inlet slot width S is a dimension which results from the offset 
of the two centerlines 2a, 3a of the partial-conical bodies 2, 3. The two 
partial-conical bodies 2, 3 each have an initial cylindrical portion 2c, 
3c. These also extend offset relative to one another, in a manner 
analogous to the partial-conical bodies 2, 3, so that the tangential inlet 
slots 2b, 3b are present from the start. The burner 1 can, of course, 
describe a purely conical form, i.e. without an initial cylindrical body. 
A nozzle is located in this initial cylindrical body; this nozzle is 
preferably operated with a liquid fuel 5 and its fuel injection 15 is 
preferably located centrally between the two centerlines 2a, 3a. As a 
further fuel supply, both partial-conical bodies 2, 3 each have a fuel 
line 10, 11 which is provided in the flow direction with openings 21, 
which are distributed over the complete length of the fuel lines. A 
gaseous fuel 6 is preferably introduced through the fuel lines 10, 11, 
this fuel being injected in the region of the tangential inlet slots 2b, 
3b as can be seen particularly well from FIG. 2. The burner 1 also has a 
fuel supply, preferably a supply of a gaseous fuel 4, which takes place 
via injectors 12, 13 which also act in the region of the tangential inlet 
slots 2b, 3b via a number of gas holes 14, as can be comprehensively seen 
from FIG. 2. Reference should be made to FIG. 2 for the relevant 
description. The burner 1 can, fundamentally, be operated by individual 
fuel supplies or in a mixed operation with the available fuel 
possibilities. At the combustion space end 22, the burner 1 has a 
collar-shaped wall 20 through which, if need be, holes are provided which 
are not shown and through which dilution air or cooling air is supplied to 
the front part of the combustion space 22. The liquid fuel 5 preferably 
introduced through the nozzle 9 into the burner 1 is injected at an acute 
angle into the internal space 17 in such a way that a conical spray 
pattern which is as homogeneous as possible appears at the burner outlet 
plane. This fuel injection 15 can involve air-supported atomization or 
pressure atomization. The conical liquid fuel profile 16 is surrounded by 
a tangentially entering combustion airflow 8 and an axially introduced 
further airflow 7a. The composition of the tangentially entering air/fuel 
mixture 8 is dealt with in more detail in the description of FIG. 2. The 
concentration of the liquid fuel 5 injected is continuously reduced in the 
axial direction of the burner 1 by an airflow or by the air/fuel mixture 8. 
If gaseous fuel 6 is introduced via the two fuel lines 10, 11, mixture 
formation with the air supply (not shown) (see. FIG. 2, item 7), commences 
directly in the region of the tangential inlet slots 2b, 3b because fuel 
openings 21 are provided there. In the case of the injection of liquid 
fuel 5 via the nozzle 9, the optimum, homogeneous fuel concentration over 
the cross-section is attained in the region where the vortex bursts, i.e. 
in the region where a reverse flow zone 18 forms. The combustion process 
for each air/fuel mixture then begins at the apex of this reverse flow 
zone 18. It is only at this point that a stable flame front 19 can occur. 
Burnback of the flame into the interior of the burner 1 (which is always 
to be feared in the case of known premixed sections and for which a remedy 
is provided in known sections by means of complicated flame holders) does 
not have to be feared in the present case. If, in general, the air used 
(see FIG. 2, Item 7) is preheated if the need arises, accelerated overall 
evaporation of the liquid fuel 5 takes place before the point at the 
outlet of the burner 1 is reached where the combustion process of the 
mixture commences. The degree of evaporation depends on the size of the 
burner 1, the droplet size and the temperature of the airflows 7a, 7 or of 
the air/fuel mixture 8. Independent of whether, in addition to the 
homogeneous droplet mixing by a combustion airflow of low temperature, 
either additional partial or complete droplet evaporation is achieved by 
preheated combustion air, the nitrogen oxide and carbon monoxide emissions 
are low if the excess air is at least 60%, so that in this case an 
additional means of minimizing the NO.sub.x emissions is available. The 
pollutant emission values are lowest in the case of complete evaporation 
of the fuel used before inlet into the combustion zone. The same also 
applies for near-stoichiometric operation if the excess air is replaced by 
recirculated combustion gas. Narrow limits have to be maintained in the 
design of the partial-conical bodies 2, 3 with respect to their cone angle 
and the width of the tangential inlet slots 2b, 3b so that the desired flow 
field of the air (with its reverse flow zone 18) occurs, for flame 
stabilization purposes, in the region of the mouth of the burner. In 
general, it should be stated that a reduction in the tangential inlet 
slots 2b, 3b, i.e. a reduction in the inlet width S (see FIG. 2), 
displaces the reverse flow zone 18 further upstream so that then, however, 
the mixture would ignite earlier. It should be noted that the reverse flow 
zone 18, once fixed geometrically, is intrinsically stable with respect to 
position because the swirl increases in the flow direction in the region of 
the conical shape of the burner 1. In addition, the axial velocity can be 
affected by axial supply of the airflow 7a already mentioned. The design 
of the burner 1 is extremely suitable for adapting--for a given 
installation length of the burner 1--the size of the tangential inlet 
slots 2b, 3b to the requirement by moving the partial-conical bodies 2, 3 
towards or away from one another so that the distance between the two 
centerlines 2a, 3a is reduced or increased and the inlet slot width S also 
changes accordingly, as can be seen particularly well from FIG. 2. The 
partial-conical bodies 2, 3 can, of course, also be displaced relative to 
one another in a different plane. From this point of view, the burner 1 
can be individually adapted without changing its combustion length. 
FIG. 2 is a section approximately in the center of the burner 1, in 
accordance with the section plane II--II of FIG. 1. The 
axial-symmetrically arranged inlets 23, 24, which enter the internal space 
17 of the burner 1, each contain an injector 12, 13 which extends over the 
whole length of the burner 1. The injector 12, 13 is designed in such a 
way that the preferably used gaseous fuel 4 flows out from a gas supply 
pipe 12a, 13a (through which flow is possible) via a number of gas holes 
14 into a gas injector duct (blowing duct) 12b, 13b. The latter extends as 
far as the region of the tangential inlet slot 2b, 3b. The width of the 
injector 12, 13 is designed in such a way that the air introduced 7 flows 
along the flanks of the injector 12, 13 and starts to mix with the gaseous 
fuel 4 in the region of the tangential inlet slot 2b, 3b so that the 
air/fuel mixture 8 only appears then. The property of the injector 12, 13, 
that it hardly alters the flow field of the burner 1 despite the high 
mass-flow proportion of the medium calorific value gas used in the air/gas 
mixture, is of fundamental importance. This is achieved with the aid of a 
suitable distribution of the gas holes 14 of equal magnitude or with the 
aid of an arrangement of holes whose diameter varies in a suitable manner. 
The density of the gas holes, referred to as .rho..sub.GB, is then 
proportional to the radially averaged velocity of the air 7 in the inlet 
slots 2b, 3b of the burner 1, and is given by the following equation: 
##EQU1## 
where .alpha. is the included angle of the burner 1 (see FIG. 1), S 
indicates the inlet slot width and R is the average radius of the 
particular position considered in the inlet slot 2b, 3b (see FIG. 1). The 
directions of the gas holes 14 should preferably coincide with the 
prevalent flow direction in the inlet slot 2b, 3b. It is then important 
that the actual throttling of the gaseous fuel 4 takes place when entering 
the gas holes 14 from the gas supply duct 12a, 13a. Because medium 
calorific value gases generally contain easily ignitable hydrogen, the gas 
holes 14 are to be designed in such a way that they cannot blow freely into 
the internal space 17 of the burner 1. These gas holes 14 enter a gas 
injector duct 12b, 13b which extends as far as the inlet slot 2b, 3b. It 
is advantageous for this duct to be subdivided several times in the 
longitudinal direction by flow vanes (not visible) so that the gaseous 
fuel 4 is canalized in the direction of the combustion airflow under 
design conditions, for example full load. In addition, this helps permit 
the gaseous fuel 4 to be blown with the particular velocity of the air 7 
introduced in the region of the inlet slots 2b, 3b. This prevents the air 
7 and the medium value calorific gas 4 used from mixing strongly already 
in the inlet region of the internal space 17 of the burner 1 because this 
would necessarily lead to premature ignition which causes diffusion-type 
combustion with greatly increased NO.sub.x emissions. In order to achieve 
these desired objectives, the transition from the gas holes 14 to the 
subsequent gas injector duct 12b, 13b is preferably designed as a 
Borda-Carnot expansion. In terms of the minimum length of the gas injector 
duct, it is advantageous to employ the usual rule of 3-5 hydraulic 
diameters or 6-10 gap widths. Such a design ensures that the smoothed gas 
flow 4 can mix with the airflow 7 "as if breathed in" so that acoustic 
resonance is also avoided during the 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.