Burner for heating systems

A packing as a porous body for burners, in particular for heating systems, is provided with a housing having an inlet for a gas/air mixture as a combustable gas mixture, a combustion chamber, an ignition device in the combustion chamber and an exhaust gas outlet. The combustion chamber is at least partly filled with a three-dimensional ordered packing of heat resistant ceramic material, foil material, or sheet metal material having continuous hollow cavities for the formation of a defined flame zone.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a burner, and more particularly, to a 
burner for heating systems. 
2. Description of Prior Art 
Various concepts are known from the prior art for the reduction of the 
noxious substances, such as NO.sub.x or CO, which arise during combustion. 
Since the NO.sub.x production is large at high combustion temperatures, 
one attempts, for example, to keep the flame temperature low. For this 
purpose a heating boiler has been proposed, for example in EP 0 256 322 
B1, in which a fuel gas is burned at a temperature of less than 
700.degree. C. through the use of a catalyst of the platinum group, 
whereby the creation of nitrogen oxides is prevented. However, such 
catalysts have only a relatively low working life and are, moreover, very 
costly. The essential disadvantage of catalytic combustion, however, lies 
in the fact that its flame temperature is too low, which does not permit 
any effective exploitation of the heat and thereby only allows the 
construction of a burner with a low power density. 
In addition to this, there are burners which operate in accordance with the 
process of exhaust gas recirculation. Here a part of the exhaust gas is 
returned into the flame, whereby an optimized, pollution reduced 
combustion is achieved. A stable flame arises with the burner model 
"RotriX" of the Viessmann company through an intentional decay of the 
turbulent fuel/air mixture, which has been set into rotation. The exhaust 
gas recirculation rate can be further increased by a flameless oxidation 
at a free surface. According to the specialist paper by J. A. Wunning and 
J. G. Wunning: "Brenner fur die flammlose Oxidation mit geringer 
NO-Bildung auch bei hochster Luftvorwarmung" (Burner for the flameless 
oxidation with low NO-formation even with the highest air pre-heating), in 
GASWARME International, Vol. 41 (1992), No. 10, pages 438 to 444, the 
flameless oxidation is usable in burners with process temperatures over 
850.degree. C. This process, however, involves high constructional cost 
and complexity because auxiliary burners are required, for example, for 
the heating up of the fuel/air mixture to ignition temperature. 
A further concept is present in the form of the "Thermomax-Burner" of the 
company Ruhrgas AG, which is treated in the specialist paper by H. Berg 
and T. Jannemann "Entwicklung eines schadstoffarmen Vormischbrenners fur 
den Einsatz in Haushalts-Gaskesseln mit zylindrischer Brennkammer" 
(Development of a low-pollution pre-mixing burner for use in domestic gas 
boilers with a cylindrical combustion chamber), in GASWARME International, 
Vol. 38 (1989), No. 1, pages 28 to 34. The combustion takes place there in 
a flameless manner at the surface of a metallic, apertured sheet, which 
transmits the heat energy produced out of the reaction zone principally by 
radiation. The combustion temperature is kept to approximately 800.degree. 
C. through this giving off of heat, which in turn has the consequence of a 
reduction of the emission of pollution. Burners of this type of 
construction typically have a thermal surface loading of 300 kW/m.sup.2. 
An increase of the thermal loading to approximately 3000 kW/m.sup.2 is 
achieved by a burner which is known from DE 43 22 109 A1. There, a part of 
the combustion chamber, in which a flame propagates, is completely filled 
with a porous material whose porosity changes along the flow direction of 
the fuel gas/air mixture in such a way that a critical Peclet number 
results at a boundary surface, or in a specific zone of the porous 
material, from which point on a flame can arise. With regard to the Peclet 
number, the following should be explained: 
With a specific pore size of the porous material, the production of heat by 
chemical reactions in the flame and the dissipation of heat by the porous 
medium are equal so that beneath this pore size no flame can arise but 
above it a free ignition occurs. 
This condition is described with the aid of the Peclet number, which 
recites the ratio of heat production to heat dissipation. In this way a 
critical Peclet number results for the flame propagation. A 
self-stabilizing flame within the supercritical zone results through the 
provision of a subcritical zone and a supercritical zone with respect to 
the Peclet number. 
Through the arrangement set forth in DE 43 22 109 A1, the problem of the 
stability of a flame burning in a porous medium is solved under the side 
conditions of a low temperature and thus a low emission of pollution. 
Ceramic foams or bulk fillings of balls are proposed as porous material. 
These materials have, however, a relatively low porosity, whereby 
combustion space is wasted and the gas/air mixture is exposed to a higher 
flow resistance. Moreover, these materials restrict, as a result of their 
low optical permeability, the energy transport on the basis of the thermal 
transport mechanism of thermal radiation which dominates in the present 
temperature range. This leads to a situation--from a specific 
constructional size of a burner of this kind onwards--in which the heat 
produced cannot be dissipated sufficiently well outwardly from the inner 
region of the combustion space. The local overheating in the porous 
material brought about in this way leads to material damage by thermal 
strains and an increased output of pollutants. 
SUMMARY OF THE INVENTION 
The present invention is thus based on the object of providing a porous 
medium for a burner which has a high porosity and thus a high optical 
permeability and which is also insensitive with respect to thermal 
strains. Moreover, it should be possible to manufacture the porous medium 
in a simple manner from the technical manufacturing viewpoint, at a 
favorable cost and with constant precision. 
In accordance with one aspect of the present invention the combustion space 
of the burner is at least partly filled by a three-dimensional ordered 
packing having connected cavities and consisting of ceramic material, foil 
material or sheet metal material for the formation of a defined flame 
zone. 
Such ordered packings can basically be manufactured with the required high 
porosity of up to approximately 99% and thus offer a larger combustion 
space than, for example, ceramic foams or bulk fillings of ceramic bodies. 
As a result of the high optical permeability of such packings, the thermal 
transport by thermal radiation is not blocked so that a rapid and 
effective heat dissipation to the thermal transfer medium is ensured. 
Furthermore, these packings have a low flow resistance as a result of the 
open structure. 
Thus, the pressure drop of the gas flow when flowing through the combustion 
space can be reduced, which lowers the required energy input. The known 
manufacturing methods for such packings furthermore enable their 
production in a simple manner from a technical manufacturing viewpoint and 
at favorable cost, with invariable precision with respect to the 
dimensioning of the hollow cavities. The latter can be varied in their 
size without great complexity. The packings have, as a result of their 
three-dimensional structure, the further advantage that they react 
resiliently to thermal or mechanical loading, whereby the danger of points 
of fracture, such as exists, for example, with the foam-like ceramic parts 
used in the prior art, is overcome. 
Since the packings of the invention can be manufactured with much higher 
degrees of porosity when compared with the prior art, the proportion of 
material related to the total volume is very low. This leads to a 
considerable shortening of the response times of the burner in comparison 
to the previously known porous media. Moreover, such packings can be made 
variable with respect to their diameter, length, hydraulic diameter etc., 
whereby an ideal fluid dynamic design can be achieved. 
Ordered packings which are used as static mixers have, in addition to a low 
pressure drop and optical permeability, also other characteristics which 
have a positive benefit. The pronounced transverse mixing leads to 
homogeneous concentration profiles and temperature profiles of the 
combustion gases, which favourably influences the combustion process and 
further reduces the production of pollution because no cold points and no 
so-called hot spots occur. Stagnating zones and also break-throughs of the 
flow media are prevented because of the low back mixing, and the 
combustion zone is additionally stabilized in the flow direction. 
Furthermore, it can be of advantage to use two or more packing elements, 
which are arranged rotated relative to one another. In this way a 
homogeneous distribution of concentration, temperature and flow speed is 
ensured over the entire flow cross section. 
The above advantages are in particular achieved by a packing which consists 
of a material which is resistant to temperatures in the range between 
1200.degree. C. and 2000.degree. C. 
Ordered packings such as, for example, static mixers, which are built up of 
layers of corrugated lamella, or lamella folded in zig-zag-like manner, 
which form channels have proved to be particularly suitable, with the 
channels of neighboring layers crossing one another and with the lamella 
consisting of metallic and/or ceramic materials, whereby the packing can 
also be of monolithic construction. 
In accordance with another aspect of the present invention the lamella can 
be foils or metal sheets which are arranged loosely alongside another and 
which have a plurality of perforations. 
Another type of ordered packing is made of webs which intersect cross-wise 
and have the same features as packings which are formed of corrugated 
lamella. 
In accordance with another aspect of the present invention the packing can 
be built up of layers of webs which cross each other and consist of 
metallic and/or ceramic materials. 
In accordance with another aspect of the present invention the packing can 
consist of ceramic materials, with the principal components being Al.sub.2 
O.sub.3, ZrO.sub.2 or SiC. These materials have advantages with respect to 
temperature resistance and corrosion resistance. 
The advantages listed are in particular achieved by a packing which has a 
high hollow space component or proportion, i.e. a high porosity of at 
least 70% and a wave height of the layers, or a web width, of between 3 mm 
and 15 mm (claim 7). With these geometrical data, low pressure drop and 
low emissions of pollutants can be realized. 
In accordance with another aspect of the present invention the packing in 
the combustion chamber can be catalytically coated or can be manufactured 
of a catalytically active material, i.e. it is itself catalytically 
active. In this way very low pollution emission values are achieved. 
In accordance with a further aspect of the present invention a porous body 
is placed in front of the inlet zone of the ordered packing. It functions 
as a flame holder or flame barrier in that it ensures that the Peclet 
number present there is subcritical, preferably smaller than 65. This 
porous body can be formed as an ordered packing. 
The present invention also provides measures for the defined restriction of 
the flame zone of the burner with the operation taking place with a flame 
holder of conventional construction known from the prior art; at the same 
time the mixing between the gaseous or vaporous fuel in the air is made 
more intense by the finely pored body. In this way conventional burners 
with free flame formation, which normally have such flame holders, can be 
retrospectively equipped with packings in accordance with the invention. 
In this way a cost favorable possibility is provided for the reduction of 
pollution of burners which are already in use. 
In an alternative embodiment finely porous material is arranged in the 
throughflow direction of the gas/air mixture upstream of the flame zone 
defined by the packing. No flame can form in this finely porous material 
because of its subcritical Peclet number. Thus, the concept known from DE 
43 22 109 A1 for flame stabilization can be combined with the present 
invention. 
The finely pored material, which can be produced without problems as an 
ordered packing with a porosity having a Peclet number which is in 
particular smaller than 65, can be manufactured in analogous manner from 
temperature resistant ceramic material, foil material or sheet metal 
material, in the same way as the actual ordered packing in the combustion 
chamber. 
In this arrangement this finely pored packing not only serves for the flame 
stabilization but rather the combustion gases such as, for example, 
natural gas, methane or heating oil vapor are homogeneously mixed with air 
before the actual combustion chamber as a result of the transverse mixing 
characteristics. This additionally favorably influences the combustion 
process, in particular with respect to the emission of pollutants.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS 
The ordered packing shown in FIG. 1 is put together from a plurality of 
corrugated ceramic plates. These ceramic plates are so arranged that the 
corrugations of two neighboring corrugated plates form an angle of 
60.degree.. In this way open channels result which cross each other. 
The burner shown in FIG. 2 has a housing 1 comprising a cylindrical main 
portion 2 and a truncated, cone-like upper end part 3. The latter has at 
its upper side an inlet 4 for a gas/air mixture as the combustible gas 
mixture. In the throughflow direction D of the gas/air mixture the 
prechamber 5 formed by the end part 3 is followed by a conventional flame 
holder or a perforated plate 6, through which the gas/air mixture enters 
into the subsequent combustion chamber 7. This combustion chamber is 
filled out with an ordered packing 8, which has, for example, the 
following specifications: 
diameter: 70 mm 
height: 90 mm 
porosity: ca. 95% 
corrugation height: 8 mm 
material: heat resistant ceramic 
The gas/air mixture entering into the ordered packing 8 is ignited by an 
ignition device 9 sitting at the side in the housing 1 at the level of the 
combustion chamber 7 and burns while forming a defined flame zone within 
the ordered packing 8 while producing thermal energy. The latter arises to 
a large part as thermal radiation which heats the main part 2 of the 
housing. The main part 2 is surrounded by a heat transfer jacket 10 in 
which helically extending channels 11 are provided. A heat exchanger 
medium, such as for example water, which circulates through a heating 
system, flows through these channels. 
After the combustion space in the passage direction D, there is further 
provided an exhaust gas space 12, in which temperatures between 
700.degree. C. and 1300.degree. C. prevail at the inlet of this zone and 
between 35.degree. C. and 1500.degree. C. at the outlet of this region. 
The exhaust gas space 12 serves as a cooling zone, with the cooling coil 
of stainless steel 14 extracting heat from the exhaust gas, which can be 
used as useful heat. The cooling coil 14 is kept at temperatures below 
200.degree. C. by the heat exchanger medium flowing through it so that 
other materials, in particular aluminium, brass or copper are also 
possible. The exhaust gas space 12 opens into the exhaust gas outlet 13 of 
the burner. 
The burner shown in FIG. 3 is distinguished from the burner in FIG. 2 only 
in two details. To this extent components are provided which otherwise 
correspond with the same reference numerals as in FIG. 2 and do not 
require repeated explanation. 
In distinction to FIG. 2, the burner of FIG. 3 has no conventional flame 
holder. On the contrary, a finely pored packing 15 is arranged in front of 
the ordered packing 8 when viewed in the throughflow direction D of the 
gas/air mixture, and is likewise formed from an ordered packing. The 
latter has a smaller pore size and porosity than the ordered packing 8, so 
that its Peclet number is smaller than 65 and is thus subcritical. This 
signifies that no flame can form in the ordered packing 15. The ordered 
packing 8 is so specified that the Peclet number is supercritical, so that 
a flame can form there in defined manner. 
Moreover, a static mixer 16 is inserted in front of the burner. It brings 
about a very homogeneous gas/air mixture. 
The temperature profile shown in FIG. 4 for a 6 kW natural gas burner with 
a power of 3 kW and an air number of 1.2 shows that the maximum 
temperatures arise shortly after the transition between the finely pored 
region A and the coarsely pored region C and can lie in the range of 
approximately 1400.degree. C. to 1500.degree. C. In the region D which 
follows it, the temperatures lie at around 1100.degree. C. at the inlet 
and sink towards the outlet to temperatures which are of the same order of 
magnitude as those of the heat exchanger medium. 
The gaseous fuel can, for example, also be vaporized heating oil or diesel 
oil. 
Moreover, it should be pointed out that the flame which forms through the 
ignition of the gas/air mixture in the flame zone defined by the ordered 
packing 8 propagates in dependence on the ratio of gas to air and also of 
the quantities thereof. To this extent the power of the burner can be 
regulated via the quantity of the gas and also of the gas/air mixture.