Method of operating a premixing burner

A premixing burner (X) which consists essentially of at least two hollow partial bodies (1, 2), which are positioned one above the other and whose center lines (1b, 2b) extend offset relative to one another in the longitudinal direction of the partial bodies (1, 2), is employed for hot gas generation, for example in a firing plant. Due to this offset, tangential inlet slots (21, 22) respectively occur through which a combustion airflow (15) flows into the internal space (14) of the premixing burner (X). Venturi mixers (32) with fuel nozzles (36), through which a fuel (31) is introduced into the combustion air (15) flowing past at this point, are arranged in the region of these tangential inlet slots (21, 22).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a premixing burner and a method for 
operating the premixing burner of the invention. 
2. Discussion of Background 
In view of the extremely low NO.sub.x, CO and UHC emissions specified for 
the operation of a heat generator, it has become practice to replace 
diffusion combustion by a premixing distances and, correspondingly, by 
premixed combustion. A premixing burner of this type is disclosed in EP-0 
321 809. The disclosure of this publication involves replacing the 
conventional premixing distances by a premixing burner which essentially 
consists, in the flow direction, of at least two hollow partial conical 
bodies positioned one upon the other, the center lines of these partial 
bodies extending offset relative to one another. By this means, tangential 
entry slots are formed along the premixing burner formed in this way and 
these entry slots are opposite to one another with respect to the flow. 
There is an airflow through them into the internal space of the premixing 
burner. The formation of the mixture--from fresh air, possibly enriched by 
a quantity of recycled exhaust gas, and fuel--for the formation of a 
combustion airflow takes place in such a way that the premixing burner can 
have different fuel inlet nozzle arrangements. A first possibility 
consists in at least one fuel nozzle being provided at the beginning of 
the premixing burner, i.e. in the region of its smallest cross-section. 
This fuel nozzle is placed centrally relative to the center lines of the 
partial bodies extending offset relative to one another. A further fuel 
inlet nozzle arrangement, which can either be operated individually or is 
in effective connection with the fuel nozzle previously mentioned, is made 
available by providing a series of fuel nozzles along the tangential inlet 
slots at the transition to the internal space. As an example, the 
injection of a liquid fuel through the centrally placed nozzle takes place 
in such a way that a conical spray-type fuel column, which does not, 
however, wet the inner walls of the hollow conical space, forms in the 
flow direction of the premixing burner. This fuel column is surrounded by 
the airflow flowing into the internal space--and, if necessary, by a 
further axially introduced airflow--in such a way that mixture formation 
takes place within the premixing burner. This mixture is ignited at the 
outlet from the premixing burner and stabilization of the flame front is 
induced in the region of this burner mouth by a reverse flow zone which 
forms there. 
If, however, fuels with a high hydrogen content are burned in such a 
premixing burner, problems occur with the flame stabilization explained 
above. Due to the higher flame speed of the hydrogen, transition occurs in 
the burner from premixing operation to diffusion operation. This causes 
the following problems: 
the burner overheats, 
the NO.sub.x emissions increase greatly, 
pulsations occur in the transition range between 
diffusion and premixing operation. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention, as claimed in the claims, is to 
provide a remedy and to provide novel means--in a method and in a burner, 
for carrying out the method, of the type described at the beginning--which 
ensure stable premixing combustion with the lowest possible level of 
turbulence and minimized NO.sub.x emissions. 
In the present invention, the remedy is achieved by the introduction at a 
suitable position of a venturi mixer which is fitted in the combustion 
airflow upstream of the internal space of the burner. The venturi mixer 
can then be extended by means of a cooling airflow. This type of layout 
offers large advantages, particularly where the fuel has high hydrogen 
proportions. The fuel is then introduced at a position where the maximum 
combustion air velocity is present. 
Further advantages of the invention may further be seen in that, in 
particular, the fuel injection location is in the region of the venturi 
section; there is a relatively high combustion air velocity at this point 
so that rapid and comprehensive mixing of the fuel added takes place with 
the other medium. 
Furthermore, the lower flame velocities at the outlet from the burner can 
induce better flame stability, i.e. initiate smaller pulsations. 
Good mixture formation between the fuel and the air can be achieved with 
low pressure loss by means of the venturi mixer. 
The invention has, furthermore, another essential advantage which consists 
in the fact that it is not necessary to provide a premixing distance above 
the tangential entry slots for a fuel introduced there so that the 
original compactness of the burner is not lost due to the extension to 
fuels with a high hydrogen content. 
The invention also dispenses with the need to increase the fuel 
pressure--for the purpose of aiding better mixture formation--before the 
fuel is introduced. This increase in pressure is always needed for 
conventional premixing distances. 
Further advantages with respect to minimizing the turbulence in the region 
of the inlet slots leading to the internal space of the burner occur if 
the venturi mixer is placed precisely in the region of these inlet slots 
or can develop its effect there. 
Advantageous and expedient further developments of the solution to the 
object of the invention are described in the further dependent claims.

DETAILED DESCRIPTION 
Referring now to the drawings, all elements not necessary for immediate 
understanding of the drawing are omitted, and the flow direction of the 
media is indicated by arrows and wherein like reference numerals designate 
identical or corresponding parts throughout the several views. It is 
advantageous to refer simultaneously to FIG. 1, FIG. 2 and, if need be, 
also FIG. 3--which show a radial section through the premixing burner 
X--in order to gain better understanding of the construction of the 
premixing burner X. In order not to make FIG. 1 unnecessarily difficult to 
understand, furthermore, the combustion air supply and the venturi mixers 
in the region of or above the tangential air inlet slots, which are 
illustrated in FIG. 2 and 3, are not shown in FIG. 1. In the description 
of FIG. 1, reference is also made below to the other figures where 
required for clarity. 
The premixing burner X of FIG. 1 consists of two semi-conical partial 
bodies 1, 2 which are located one upon the other and offset relative to 
one another. It is understood that the number of partial conical bodies 
necessary for forming the premixing burner X is not limited to two. The 
conical shape of the partial bodies 1, 2 shown has a certain fixed angle 
in the flow direction. The partial bodies 1, 2 can have a different 
opening configuration in the flow direction, for example a regularly or 
irregularly increasing conical inclination which leads, pictorially, to an 
approximately trumpet shape, or a regularly or irregularly decreasing 
conical inclination which leads, pictorially, to an approximately tulip 
shape. The two shapes last mentioned are not included in the drawing 
because they can be readily visualized. The form which is finally selected 
depends on the various parameters of the particular combustion process. 
The offset of the respective center lines 1b, 2b of the partial conical 
bodies 1, 2 relative to one another creates respective tangential air 
inlet slots 21, 22 (FIG. 2 and 3) on both sides in an axisymmetrical 
arrangement and frees an axial inlet flow cross-section 18 through which 
the combustion air, 15, 16, consisting of fresh air or a mixture of fresh 
air and combustion gas, flows into the internal space 14 of the premixing 
burner X. The two partial conical bodies 1, 2 have respective cylindrical 
initial parts 1a, 2a which likewise extend offset in a manner analogous to 
the partial bodies 1, 2 so that the tangential air inlet slots 21, 22 are 
present over the complete length of the premixing burner X. 
The premixing burner X can be configured to be purely conical, i.e. without 
cylindrical initial parts 1a, 2a. At least one fuel nozzle 3 is 
accommodated within this cylindrical initial part 1a, 2a and this is, for 
example, particularly suitable as the seating for anchoring the complete 
premixing burner X. 
If required, both partial bodies 1, 2 have a fuel conduit 8, 9, which 
extends in the axial direction and which is provided with a number of 
nozzles 17. A preferably gaseous fuel 13 is guided through these conduits 
and this gaseous fuel 13 is added through the nozzles 17 mentioned in the 
region of the tangential air inlet slots 21, 22 (see FIG. 2) to the 
combustion air 15 which flows through these slots. It is therefore also 
possible to operate the premixing burner X by means of the fuel supplied 
via the nozzle 3 alone or via the nozzles 17. Mixed operation by means of 
both nozzles 3, 17 is possible, in particular where different fuels are to 
be supplied by means of the individual nozzles. 
At the combustion space end 11, the premixing burner X has a collar-shaped 
plate 10 which has a number of holes 10a through which dilution or cooling 
air is supplied to the front part of the premixing burner X. 
If a liquid fuel is supplied via the nozzle 3, this fuel is injected with 
an acute spray angle into the internal space 14 of the premixing burner X 
in such a way that a spray pattern 5 which is, as far as possible, 
homogeneously conical occurs as far as the burner outlet plane. The fuel 
inlet nozzle arrangement can be an air-supported nozzle or a nozzle which 
operates in accordance with a pressure atomization principle. The conical 
spray pattern 5 is surrounded by tangentially entering combustion airflows 
15, corresponding to the number of air inlet slots 21, 22, and by the 
axially introduced further combustion air 16. 
The concentration of the fuel 12, which has been introduced, is 
continuously reduced in the flow direction of the premixing burner X by 
the combustion airflows 15, 16 already mentioned. If a gaseous fuel 13 is 
introduced in the region of the tangential inlet slots 21, 22, the 
formation of the mixture with the combustion air 15 has, generally 
speaking, already commenced in this region. When a liquid fuel 12 is used, 
the optimum, homogeneous fuel concentration over the cross-section is 
achieved in the region where the vortex collapses, i.e, in the region of 
the return flow zone 6 at the end of the premixing burner X. 
The ignition of the fuel/combustion air mixture begins at the apex of the 
reverse flow zone 6. It is only at this point that a stable flame front 7 
can occur. A flash-back of the flame into the premixing burner X--as is 
always to be feared in the case of the previously revealed premixing 
distances and against which a remedy is sought in such cases by means of 
complicated flameholders--need not be feared in the present case. 
If the combustion air 15, 16 is preheated, however, accelerated overall 
evaporation of the liquid fuel 12 takes place before the point at the 
outlet from the premixing burner X is reached at which ignition of the 
mixture takes place. The degree of evaporation depends on the size of the 
premixing burner X, on the droplet size of the injected fuel 12 and on the 
temperature of the combustion airflows 15, 16 and their intensity. 
Minimization of the pollutant emissions also depends to an important 
extent on the exhaust gas recirculation, which contributes to the 
possibility of complete evaporation of the fuel taking place before entry 
to the combustion zone. 
When designing the partial conical bodies 1, 2 in terms of conical 
inclination and width of the tangential air inlet slots 21, 22, it is 
advantageous to maintain close limits at this point so that the desired 
flow field of the combustion air with its reverse flow zone 6 in the 
region of the mouth of the premixing burner X for flame stabilization is 
established. In general, it may be stated that decreasing the size of the 
air inlet slots 21, 22 displaces the reverse flow zone 6 further upstream 
although in that case the mixture ignites earlier. However, it should be 
stated that the reverse flow zone 6 is intrinsically stable positionally 
once its location has been fixed because the swirl number increases in the 
flow direction in the conical shape region of the premixing burner X. 
Furthermore, the axial velocity of the mixture can be influenced by the 
axial supply, already mentioned, of combustion air 16. The construction of 
the premixing burner X is eminently suitable for modifying the gap width 
of the tangential air inlet slots 21, 22 in the case of a specified design 
length of the burner which is not to be exceeded. This is because the 
partial conical bodies 1, 2 can be displaced towards or away from one 
another so that, as a result, the distance between the two center lines 
1b, 2b is respectively reduced or increased, as can be easily derived from 
FIG. 2. It is also easily possible to displace the partial conical bodies 
1, 2 into one another by a rotary motion. Given appropriate arrangements, 
it is therefore possible to vary the shape and the size of the tangential 
air inlet slots 21, 22 during operation so that the same premixing burner 
X can cover a wide functionality without the installation length being 
changed. 
As already stated above, problems with respect to flame stabilization occur 
during the operation of the premixing burner X with a fuel having a high 
hydrogen content. For this reason, the premixing burner X is to be 
extended in the region of the tangential air inlet slots 21, 22. These 
extensions are represented in the subsequent FIGS. 2 and 3, the original 
introduction of fuel via the nozzles 17 (see FIG. 1) being dispensed with 
in these embodiments. It is understood that it still remains possible to 
put the fuel nozzle 3 into operation. 
As shown in FIG. 2, the fuel 31 is introduced at the location of maximum 
velocity of the combustion air-flow 15, i.e. again in the region of the 
tangential air inlet slots 21, 22. In this region, a venturi mixer 32 is 
provided over the complete and length of the premixing burner. This 
venturi mixer 32 consists of a double passage, i.e. of tangential flow 
paths 33, 34. The central venturi body 35 is simultaneously used as the 
fuel supply conduit. In addition, it is provided with nozzles 36 on both 
sides in the direction towards the flow paths 33, 34 mentioned. The flow 
paths 33, 34 develop a venturi effect because the two other adjacent 
bodies 37, 38 are likewise configured in venturi fashion. This simple 
division of the venturi mixer 32 has itself the effect that the design 
length of the mixture-forming region can be effectively minimized without 
having to omit the advantages of individual venturi sections. This 
division can be a multiple and can also be embodied as a venturi matrix 
(not shown). In the case of the possible embodiment last mentioned, this 
involves occupying the flow path into the internal space 14 of the 
premixing burner X with a large number of small tubular venturis. 
FIG. 3 differs fundamentally from FIG. 2 in that the partial conical bodies 
1, 2 are extended by additional guide plates 41, 42 which permit an 
additional cooling airflow 43 which inter alia cools the partial bodies 1, 
2. In the case of the venturi mixer 44 shown here, a simple venturi-type 
flow path 45 is formed opposite the combustion air 15 so that, again, the 
fuel nozzles 46 only act on the flow path 45. A different venturi effect 
occurs with respect to the cooling airflow 43. The venturi body 47 is, 
here again, configured as a fuel supply pipe. 
In general, it is postulated that, independent of their embodiment, the 
venturi mixers 32, 44 are fitted upstream of the internal space 14 of the 
premixing burner X. 
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.