EP0657011B1 - Burner - Google Patents

Abstract

PCT No. PCT/EP94/02156 Sec. 371 Date Mar. 1, 1995 Sec. 102(e) Date Mar. 1, 1995 PCT Filed Jul. 10, 1995 PCT Pub. No. WO95/01532 PCT Pub. Date Jan. 12, 1995.A burner with a housing enclosing a combustion chamber and with an inlet for a gas/air fuel mixture and an outlet for combustion gas is disclosed. The combustion chamber is filled with a porous material whose porosity varies along the combustion chamber in such a way that the pore size increases in the direction of flow of the gas/air mixture so that a critical Péclet number for the pore size and accordingly for flame development results at a boundary surface or in a determined zone of the porous material, above which number a flame can develop and below which number the flame development is suppressed.

Description

The invention relates to a burner with a housing that has a combustion chamber with an inlet for a gas / air mixture as fuel and an outlet for the exhaust gas having.

Burners of this type usually work with one that burns freely in the combustion chamber Flame that burns the gas / air mixture, using the hot exhaust gas as a heat source is used. In particular, the hot exhaust gas is used to exchange heat on water Pipes passed to produce hot water or steam in them.

Pollutants such as NO _x or CO are formed in such burners. These toxic and unhealthy gases are generated either at high flame temperatures or incomplete combustion in unstable flames or at low flame temperatures, which could be reduced, but then an unstable flame is produced. Furthermore, incomplete combustion of the gas / air mixture is also to be expected, that lowers efficiency.

To avoid these disadvantages, different types of burners have been developed. An overview is given in "Lean-Burn Premixed Combustion in Gas Turbine Combusters", A. Saul and D. Altemark, Vulkan-Verlag, Essen, Volume 40 (1991) Issue 7-8, pp. 336-342. A key feature in the developments described there for reducing pollutants is above all a low flame temperature, various measures being taken to burn the fuels as completely as possible. The most important measures to achieve a more efficient combustion are the superstoichiometry and the catalysis. For example, in the cited document, a fat-quensch-lean combustion chamber from General Electric "LM 2500" is specified, in which a fuel-rich mixture is burned in a first stage. In an intermediate area, air is supplied to the gas partially burned in the first stage and the resulting lean mixture is burned in a second stage. For this burner, the authors state a NO _x content of <190 mg / m ³ gas.

The above-mentioned document also describes the combustion with the aid of catalysts with which complete combustion can be achieved at a low temperature. The document specifies a NO _x content of <20 mg / m ³ for catalytic combustion. Catalytic combustion is under development at several research institutes but has not yet progressed beyond the research stage. According to the authors, this type of burner cannot be expected to be commercially available within the next 5 years.

Stability problems are not discussed in detail in the cited publication. she However, the lower the flame temperature is chosen, the more important.

One possibility for stable combustion at low temperatures is in "Neue Gasbrenner- und -gerätetechnik", a contribution of the gas industry to environmental protection, Otto Menzel, gwf Gas / Erdgas 130, 1989, Issue 7, pp. 355-364 and in "Development of a low-pollutant premix burner for use in domestic gas boilers with a cylindrical combustion chamber ", H. Berg and Th. Jannemann, Gas Wärme International, Volume 38 (1989), Issue 1, pp. 28-34, Vulkan-Verlag, Essen. The "Thermomax" burner described there has only a low NO _x emission. The flame stability in this burner is achieved by a heat-dissipating burner plate, which essentially consists of a perforated plate with circular bores through which the gas to be burned flows. Due to the heat dissipation via the perforated plate, the flame is practically held on the burner plate, which creates a stable flame.

The burner plate is also not sufficient to ensure flame stability in all operating parameters to ensure. So it is stated that at high air numbers a Mixture preheating of around 300 ° C should be provided as this will reduce the rate of combustion increases and thus reduces the tendency for the flames to withdraw becomes.

US-A-5 165 884 discloses flame stabilization by means of a control process in which a temperature detection takes place, as for example already from claim 1 feature b) of the document emerges. For this purpose, temperatures are measured at several points in the reactor and the flow rate and / or flow rate depending on the temperature changes in the matrix provided for combustion controlled.

From the cited prior art it is clear that a reduction is possible of pollutants to achieve through low flame temperature, while maintaining stability the flame continues to be an essential, unsolved problem.

The object of the invention is therefore to provide a burner in which the flame burns stably at low temperature and pollutant emissions.

Starting from US-A-5 165 884, the object is achieved by a burner according to claim 1. The housing contains contains a porous material with contiguous cavities, whose porosity changes along the combustion chamber so that the pore size in the direction of flow of the gas / air mixture increases from inlet to outlet, being in a zone or at a Interface of the porous material in the combustion chamber is critical for the pore size Péclet number for flame development results above which a flame can arise and below which the flame development is suppressed.

This proposal according to the invention is in contrast to the prior art the housing is filled with a porous material that has the property of flow of the gas / air mixture to resist, so that the combustion current gas volume is throttled. In addition, the Heat capacity of the porous material in the combustion chamber better the heat of combustion added and can therefore be cheaper than in the prior art for further use be transmitted. The porous material also creates one Cooling that reduces the flame temperature

For a given pore size, the chemical reaction of the flame and the thermal one Relaxation of the same size so that no flame is generated below this pore size can, but above that a free ignition takes place. This condition will suitably described using the Péclet number, which is the ratio of heat flow indicates due to transport to heat flow due to conduction. According to the porosity, in which a flame can set in, there is a supercritical Péclet number for flame development. Since the flame only in the area with the critical Péclet number can arise, a self-stabilizing flame front in the porous material generated.

The use of a porous material in the combustion chamber also requires a high heat capacity, whereby a high thermal energy stored locally in the porous material and high efficiency values can be achieved in an advantageous manner. Furthermore, this has high Heat capacity also has the advantage that a heat exchanger, for example, for heating of water, for the production of hot water or steam in the combustion chamber can, which achieves a much better heat transfer for heat exchange is considered the state of the art. The high power density is due to a higher combustion rate in the porous medium and a much larger flame front surface, which arises due to the porosity.

The porous material also has the advantage that in the flow of the gas / air mixture A high level of turbulence arises, which means that combustion speeds are up to 50 times higher than normal can be achieved. Above all, this means better degrees of combustion connected and higher power densities are achieved. On one below described embodiment, measurements were carried out which show that for the heat utilization efficiencies greater than 95% can be achieved.

Since the porous material itself cools the flame, the flame temperatures become correspondingly low achieved in connection with low emission values. It is therefore not one Cooling necessary, as in the prior art either by overstoichiometry or Exhaust gas recirculation is provided.

Since the porous material opposes the gas flow itself, the gas works Burner according to the invention essentially under a wide pressure range. Thereby operation under various pressures and even under high pressure is possible. For the burner according to the invention therefore has a wide range of applications.

According to a development of the invention, the critical Péclet number is 65 +/- 25 and in particular 65 for natural gas / air mixtures.
This number was determined based on tests for various gas / air mixtures. However, there is a large spread depending on the type of gas, but it was found that the natural Péclet number 65 is independent of the mixing ratio and the composition of the natural gas / air mixtures. This finding shows that the Péclet number is the suitable parameter for determining the porosity of the material to be selected in a burner according to the invention. The given teaching allows the person skilled in the art, without large preliminary tests, to fix a burner according to the invention by designing the porosity of the porous material to a critical Péclet number of 65 with regard to the operating mode.

A burner in accordance with the teaching according to the invention can have a continuous transition from a low porosity to a high porosity in the combustion chamber, the flame development then starting at a porosity with the critical Péclet number. As already discussed above, the critical Péclet number can also vary with different gas / air mixtures. If the porosity of the porous material in the jacket were to run continuously, this would have the disadvantage that the flame could shift under different conditions. In order to create a defined position for the flame development, in an advantageous development of the invention, two zones of different pore sizes lying one behind the other in the flow direction of the gas / air mixture are provided in the jacket, the first zone downstream of the inlet having a Péclet number for the flame development, which is smaller than the critical Péclet number, and the second zone further away from the inlet has a Péclet number which is greater than the critical Péclet number
As a result of these measures, the flame generation is fixed to the area or the area between the two zones, and essentially independent of operating parameters that could lead to a variation in the critical Péclet number. The measure mentioned for determining the location of the flame origin therefore further increases the stability and makes it possible to build a burner which can be used over a wide range of uses.

According to a preferred development, it is provided that the first zone has a pore size which gives a Péclet number ≤ 40 and the second zone a pore size has a Péclet number ≥ 90.

Because of this feature, the entire known range of variation is critical Péclet numbers, which, as mentioned above, can be 65 +/- 25. The specified values for the design of the zones for Péclet numbers <40 or> 90 are, as will become clear later in the exemplary embodiment, simple to implement, and allow a burner for a wide range of applications Gas / air mixtures.

According to a preferred development, the porous material is a heat-resistant one Foam plastic, a ceramic or metal or a metal alloy. Such as porous Materials can be manufactured is known from the prior art.

However, the heat resistance does not have to be particularly high for normal domestic burners, since the flame is cooled by the porous material itself. Experiments have shown that in burners according to the invention with a capacity of 9 kW, the temperatures remain below 1400 °. A preferred development of the invention therefore provides that the porous material is heat-resistant up to 1500 ° C.
On the basis of this feature, a large number of possible materials are available for a burner according to the invention, so that the material selection can be made not only from a technical point of view, but also a burner can be optimized with regard to an inexpensive construction and a low manufacturing outlay.

According to a preferred development of the invention, the porous material consists of packing material, for example in the form of bulk material, which can optionally be solidified, for example by sintering.
With the specified type of materials, a porosity can be generated in a simple manner. The porous material can consist of loosely layered grains, but it can also be solidified into a coherent porous mass.

Bulk material has the particular advantage that it can be easily filled into the housing and that it is manufactured can be handled very easily. But it's also burner maintenance, for cleaning, for example, easily possible, bulk goods out again to remove the housing.

According to a preferred development of the invention, the bulk material contains metal, a metal alloy or ceramic, in particular steatite, Stemalox or Al ₂ O _3. These materials correspond in every respect to the technical requirements for a burner according to the invention. The bulk material mentioned is readily available and is also reasonably priced. As a result of the further development, a burner according to the invention can be constructed in a cost-effective and technically simple manner.

According to a preferred development of the invention, the bulk material in the vicinity of the outlet consists of grains of spherical shape with average diameters of 5 mm and in the subsequent area with average diameters> 11 mm, if the diameter for achieving the critical Péclet number is between 5 and 11 mm and in particular Is 9mm.
If the grains of the bulk material are spherical, the uniformity of the bulk material can easily be checked during manufacture. In particular, this also applies to the attainable porosity, which is then only determined by the diameter of the spherical grains and their arrangement in the bed. With steel, steatite, Stemalox or Al ₂ O ₃ and with the use of natural gas / air mixtures, it has been found that the Péclet number of 65 for balls with a diameter of 9 mm and Péclet numbers of 40 and 90 for diameters of approximately 11 or 5mm can be reached. In the case of further training, the required porosity is therefore achieved with simple means, especially since bulk material of the type mentioned and the corresponding size is readily available. The required porosities for a burner according to the invention can thus be achieved without great effort.

As has already been mentioned in the prior art, the NO _x and CO emissions in particular can be reduced by using catalyst materials. Therefore, according to a preferred development, it is provided that the inner surfaces of the cavities of the porous material or the surfaces of the grains of the bulk material are coated with a catalyst material.

Because of the porosity, a burner according to the invention has a large surface area to interact with the gas. It is expected that a catalyst acts much more effectively than those known from the prior art Configurations. In addition, a burner according to the invention can be developed equip them with catalysts much easier, which means that they are very quickly ready for production, standard catalyst burner is made possible.

According to a preferred development of the invention, the housing has at least partially a cooling device.
In principle, the heat that flows into the housing could also be shielded from the outside world with insulating material, but cooling has the advantage that the heat can be absorbed by the coolant and then used again. As a result, the efficiency of a burner according to the invention can be increased further.

According to an advantageous development, the cooling device is designed as a cooling coil surrounding or forming the housing, through which a coolant, in particular water, flows. Furthermore, a monitoring device can be provided which prevents the supply of fuel to the combustion chamber in the event of a coolant failure.
Because of these features, the heat absorbed in the cooling can be reused, since the flowing coolant transports heat that can be removed at another location. In the case of coolant flows, however, it cannot be ruled out that the flow of the coolant is interrupted by a line break or blockage of the cooling coil, which could heat up the outer wall of the burner, which can lead to fire or burns. It is therefore expedient to provide a monitoring device which prevents the supply of fuel to the combustion chamber in the event of a coolant failure.

Due to the measures, a high efficiency of the burner can be achieved at the same time Generate cooling of the outer wall, whereby great security is guaranteed.

According to a preferred development of the invention is larger in one area Pore openings of the material provided a cooling device for heat exchange. With the help of this cooling device, which can be designed as a cooling coil, the Heat in the burner e.g. dissipated as hot water or steam and can be used in other processes continue to be used for heating or for operating turbines. In contrast to the state of the art, the heat transfer here is not only direct Interaction of the hot gas with the cooling device, but for the most part over the porous material, which provides better heat transfer than the state of the Technology is guaranteed. This feature also serves to increase efficiency.

According to a preferred development, cooling of the housing is provided, which is connected in series with the cooling device for heat exchange.
As a result of this measure, the energy which is absorbed by the cooling of the housing in the coolant is conducted into the same circuit in which the heat in the coolant is used for heat exchange. The coolant is preferably only used to cool the housing and then passed into the interior of the burner, where it interacts with the porous material at high temperature. In the development, the entire heat generated by the burner is absorbed in the coolant, which further increases the efficiency.

The more effective the transfer of the heat generated in the burner to the cooling device inside the burner, the more effectively the heat is transferred. Furthermore the cooling device in the burner forms a further flow resistance, which the design of the porous material in the area of the cooling device can. The cooling device then acts similarly to the porous material. The The amount of porous material can then be reduced, with a more effective one Heat transfer is achieved when the cooling device according to a further development itself is designed so that it acts at least partially as a porous material and / or porous material replaced.

When optimizing a burner, the distance of the cooling device from the Flame should be chosen as cheaply as possible. The highest temperature is reached in close to the flame, but it can also be suitable for lower temperatures Materials for forming the cooling device can be selected if they are outside of the flame area. In addition, the flame through the cooler not additionally cooled if it is outside the flame area, which is the Flame stability additionally increased. That is why a preferred further training sees the invention that the distance of the cooling device from the area with the critical Péclet number is at least so large that the cooling device with the flame does not is in contact. This has to do with the heat transfer from the flame to the cooling device due to the good heat conduction in the porous material only little influence.

In order not to affect the flame by cooling the outer casing, see a preferred development of the invention that by an additional device, e.g. a chip in the burner chamber with a dimension larger than 1mm between the inner wall of the housing and the insert in which the porous Material is created. This will reduce the CO emissions caused by incomplete or unstable burns occur, further suppressed.

Tests on exemplary embodiments have shown that the highest effectiveness is then achieved is when the porosity is generated with bulk material and the cooling device in one Distance of 2 to 4 grain sizes of the bed from the border area with the critical Péclet number 65 is arranged. According to further training, it can generally be expected that that the favorable conditions arise when the cooling device of the zone with the porosity required for the critical Péclet number is so far away that it does not dive into the flame area ..

According to another preferred development, an ignition device is arranged on the burner in such a way that the gas / air mixture is ignited in a region with a porosity that has the critical Péclet number.
In principle, the gas / air mixture could be ignited at all points on the burner where a combustible gas / air mixture is present, for example from the outlet. According to the further development, the ignition takes place in an area in which the porosity has the critical Péclet number. As a result, the flame is lit precisely in the area by burning even in the stable state. Because of this, a high stability is brought about already at the time of ignition, since the flame would have to be kicked back at other points, but this is not possible at high fuel flow rates. In this case, ignition could only take place if the fuel flow was reduced in the meantime. The feature of the development thus greatly reduces the expenditure on equipment for a burner according to the invention, since regulation of the ignition process can be omitted.

According to another advantageous development of the invention, a flame trap is arranged between the inlet and the porous material.
Because of the porous material, the flame is not expected to kick back, since the Peclet number in the inlet area does not permit the formation of a flame. Nevertheless, a flame trap is provided primarily for safety reasons, which can be important, for example, if the bulk material having the high porosity has been accidentally filled into the inlet area after cleaning work.

The flame trap should be as simple as possible since it is normally not required be. According to a preferred further development, the flame trap is a plate, which have a plurality of holes with a diameter smaller than that for each Fuels have critical "quenching" diameters. It has been shown that this Flame trap is effective with natural gas / air mixtures. Their main advantage lies in the simplicity of manufacture and the very inexpensive design. The effort for the flame trap is therefore kept low and remains reasonable, so that a additional flame trap can be used economically, although in Normal case for the burner according to the invention is not necessary.

Due to the high power density and the large amount of material to accommodate It is also easy to heat the burner according to the invention in the manner of a To operate condensing boilers because the exhaust gas temperature is greatly reduced. The resulting condensate must be removed. This is the case with the invention Burner easy to do, because it was in test models found that this in any position, even with flame development contrary to Gravity, can be operated. One with the outlet down Burner would be able to easily drain the condensate through this, so that no additional measures need to be taken. Therefore one sees preferred development of the invention before that inlet, outlet and porous material so are arranged so that condensate can flow through the outlet.

Further measures and advantages of the invention also result from the following embodiments described with reference to the drawings.

Fig. 1 eine erste Ausführungsform des Brenners mit drei Zonen;

Fig. 2 eine weitere Ausführungsform des Brenners mit zwei Zonen;

Fig. 3 ein Diagramm für Péclet-Zahlen in Abhängigkeit des Kugeldurchmessers bei einer Kugelschüttung,

Fig. 4 ein Diagramm für den Temperaturverlauf innerhalb des porösen Materials bei dem Ausführungsbeispiel gemäß Fig. 2,

Fig. 5 einen Schnitt durch einen als Wassererhitzer oder Dampferzeuger ausgelegten Brenner entsprechend der in Fig. 2 gezeigten, jedoch mit dem Auslaß nach unten angeordneten Ausführungsform und

Fig. 6 einen Schnitt durch einen mit einem Einsatz versehenen Brenner.

Show it:

Figure 1 shows a first embodiment of the burner with three zones.

2 shows a further embodiment of the burner with two zones;

3 shows a diagram for Péclet numbers as a function of the ball diameter in the case of a ball bed,

4 shows a diagram for the temperature profile within the porous material in the embodiment according to FIG. 2,

5 shows a section through a burner designed as a water heater or steam generator in accordance with the embodiment shown in FIG. 2, but with the outlet arranged downwards

Fig. 6 shows a section through a burner provided with an insert.

The flame development in porous material has already been done by several scientists has been examined and described, in particular by V.S. Babkin, A.A. Korzhavin and V.A. Bunev in "Propagation of Premixed Gaseous Explosion Flames in Porous Media, Combustion and Flame ", Volume 87, 1991, pp. 182 to 190. By these authors the following flame propagation mechanism has been described.

Turbulence is generated in the fuel flow in the porous material. A positive feedback between flame acceleration and the generation of turbulence by local suppression of chemical reactions due to intense heat exchange muffled in the turbulent flame zone. If the characteristic The time of the thermal compensation becomes smaller than the chemical conversion, the Prevents flame formation. Because there is also a wide variety of turbulent flows Speeds occur, the proportions of the flame are at maximum speeds suppressed, which creates a stable flame spread.

The authors' experiments resulted in a critical Péclet number of 65 +/- 25 for the Flame propagation in porous material, the variance being essentially extreme different gas compositions are given. With natural gas / air mixtures however, a Péclet number of 65 is essentially to be expected.

The Péclet number can be calculated using the following equation: P e = (P L d m c p ρ) / λ. where S _{L is} the laminar flame speed, d _{m is} the equivalent diameter for the central cavity of the porous material, c _{p is} the specific heat of the gas mixture, ρ is the density of the gas mixture λ and the thermal conductivity of the gas mixture. The equation shows that the conditions for flame development essentially depend on gas parameters, and the properties of the porous material are only included in the equation via d _m . The Péclet number is therefore essentially independent of the material properties and only dependent on the porosity. A wide variety of materials or geometric shapes can therefore be used as the porous material in the burners according to the invention.

In addition, all values in the equation can be measured, so that with the help of given equation is given a technical lesson that relates to the most diverse Gas mixtures can be used.

Fig. 1 shows a schematic representation of a burner with a housing 1, which has an inlet 2 for the gas / air mixture and an outlet 3 for the exhaust gases. In a distance from the inlet 2, a flame trap 4 is provided, which the interior of the housing 1 divided. The located between this flame trap 4 and the outlet 3 Part of the interior of the housing 1 is filled with a porous material 5. An ignition device 6 is also provided for igniting the gas mixture.

The gas / air mixture enters through inlet 2 and the exhaust gases leave the burner through the outlet 3. The porous material 5 has locally different porosities, in accordance with the different hatched zones A, B and C. In zone A the pores are so small that the resulting Péclet number is smaller than the critical one Péclet number (65 for natural gas / air mixtures) is. The critical Péclet number is that Limit above which a flame can develop or below which a flame is suppressed. In zone C, the Péclet number is significantly larger than the critical Péclet number, so that a flame can develop there. Zone B represents a transition area within which the porosity reaches the critical Péclet number.

According to the knowledge about the flame development in the porous material, the flame can only arise in zone B, and only at the points where the porosity reaches the critical Péclet number. The porous material cools the flame so that only little NO _{x is} generated. The inner surfaces of the cavities of the porous material, in particular that of zone B, can also be coated with a catalyst, as a result of which a further reduction in the NO _x and CO content in the exhaust gas is achieved.

Due to the physical laws for flame development described above in porous material the flame will stabilize in zone B, namely in places where the gas / air mixture is just reaching the critical Péclet number. This but also means that the flame approaches with strong changes in the physical Can move parameters within region B so that a local In principle there is no flame stability. On the other hand, the one given by Zone B. Transition layer the advantage that the flame front at the smallest possible Cavities stabilized, which ensures the best possible heat transfer from the Flame to the porous material is guaranteed.

However, if value is placed on a locally stable flame, a burner can be used after the in Fig. 2 shown embodiment can be used. This is compared to the described in Fig. 1, the zone B has been omitted so that only the two zones A and C are present. Here the flame stabilizes due to the one shown above Laws at the boundary layer between Zone A and Zone C. The flame is So determined by the interface and therefore stable in place. Due to the variance of +/- 25 of the specified Péclet number of 65, it is advantageous to provide a porosity in zone A, whose Péclet number is less than 40 and in zone C a porosity that of a Péclet number of greater than 90 corresponds. Then the boundary layer determines for a large area of gas / air mixtures the location of the flame development, which increases the stability for one large range of gas parameters is guaranteed.

Different materials, for example ceramic materials, can be used for the porous material. However, heat-resistant foam plastics are also possible. In the following considerations, bulk material is used as the porous material. For bulk material with round grains, the parameter d _m for the porosity, which is included in the equation for the Péclet number, can be calculated on the basis of geometric considerations as d _m = δ / 2.77, where δ is the diameter of the spherical grains of the bulk material.

According to the equation given above, Péclet numbers as a function of the diameter δ were calculated for natural gas / air mixture and are shown in FIG. 3. A stoichiometric laminar flame speed S _L of 0.4 mm per sec was assumed for the calculation. The Péclet number of 65 is achieved with a sphere radius of 9mm, while the above-mentioned Péclet numbers of 40 and 90 are given at 6mm and 12.5mm.

In a test set-up according to FIG. 2, grains with diameters of 5 mm in zone A and 11 mm in zone C were used. A wide variety of test materials were used, e.g. polished steel balls and ceramic grains of various compositions and sizes, such as steatite, Stemalox or Al ₂ O ₃ . It was found that the advantages according to the invention were achieved with all materials.

The temperature profile in the flow direction of the gas / air mixture in such a test burner is shown in Fig. 4 for different performances, the jacket of was cooled outside. It was shown that even with high outputs of 9 kW highest temperature was below 1500 ° C. Therefore all materials can be used which are temperature stable up to 1500 ° C.

In Fig. 4, a first vertical line is drawn, which is the interface between the Zone A and Zone C. It is clearly evident that the highest temperature at the interface or with respect to just behind the interface in zone C.

From Fig. 4 it can be seen that the temperatures to the outlet 3 (second vertical Line) drop sharply. So it can with the help of a burner according to the invention Exhaust temperature can be reached below the dew point, which gives the advantages of a Result in condensing boiler. However, the resulting condensate must be dissipated. It has been shown that the burner regardless of its location for Gravity field of the earth works stably, so that it is also horizontal or with the outlet 3 can be operated downwards. With this last arrangement, the condensate flow out of the burner.

The low gas temperature at the outlet also shows that the heat of the burned gas / air mixture is almost completely absorbed by the porous material, whereby the Construction of a heat exchanger with great efficiency is made possible. After with a burner the embodiment of Fig. 2, it is possible to use a water heater with an output from 5kW to an exhaust gas temperature of 60 ° C and an efficiency of 95% to build. The structural dimensions of the burner could be kept very small the length of the burner was only 15cm and the diameter was 8cm. The minor Dimensions are mainly due to the high power density that comes with Help of porous material can be achieved.

Figure 4 also shows that the highest temperatures are just behind the interface between Zone A and Zone C are created. It follows that for the production of hot steam the heat transfer from the flame to the water to be heated near it Interface should take place. One leading the water intended for steam generation Cooling device should therefore run in the area of the porous material that is about 3cm from the interface.

Regardless of this, it is generally advisable not to close the cooling device to be placed on the flame as the flame does not cool down itself to maintain its stability shall be. Therefore, it is advantageous to close the cooling device Lay the boundary layer, but not in the flame area. Should be due to material problems the high temperatures in the execution of the cooling device arise prefer larger distances.

Fig. 5 shows the schematic structure of one for heating water or for generating of steam suitable burner. This essentially includes the housing again 1, the inlet 2, the outlet 3, the flame trap 4, the ignition device 6 and the porous material 5. The burner is arranged with its outlet 3 downward, so that Condensate can drain off easily. The porous material 5 is only schematically by the same size Bullets indicated. This does not correspond to the real situation, because the porosity of the porous material changes along the direction of flow of the gas / air mixture, the balls in the inlet area have a smaller diameter than in Have outlet area.

The interface between zones A and C described above is broken by a Line 7 indicated. As already explained above, the flame is created this interface 7 and transfers its heat essentially in a range of a few cm in region C on the porous material.

In addition, there is an external cooling device which circumvents or even forms the housing 1 8 provided, which is designed as a cooling coil arranged around the housing 1 can be and prevents heat dissipation to the outside. The cooling coil is from Water flows through and is equipped with a water monitor, which in the event of failure of Coolant interrupts the inflow of the gas / air mixture into the inlet 2, so that the Housing 1 is always cooled when the burner is in operation. This ensures that the outer wall cannot heat up too much, which in turn prevents that you can burn yourself on the housing or cause a fire. The heat dissipated from the housing wall through the cooling coil can be reused efficiency increases when producing hot water or steam.

5 shows the arrangement of an internal cooling device 9 which extends from the outlet 3 extends until shortly before the interface 7 into the porous material of zone C.

The inner cooling device 9 is only indicated schematically, in practice it can e.g. have the shape of a spiral so that the best possible heat transfer from the porous Material 5 is guaranteed. But they are also more complicated embodiments conceivable for the cooling device 9. For example, this can even be the porous material form or contribute to porosity, resulting in an even better heat transfer becomes possible.

The outer cooling device 8 is connected in series with the inner cooling device 9, whereby the water already preheated by the housing 1 into the inner cooling device 9 is performed and for heating the water or for the production of steam is also used.

In order to avoid that the flame in the combustion chamber is not influenced by excessive cooling by the external cooling device 8, an insert 10, which consists of a suitable material, is provided in the flame region of the combustion chamber, as can be seen in FIG receives porous material 5 and shields the inner wall of the housing 1 against direct heat radiation. The insert 10 can also be designed such that it is arranged at a distance from the inner wall of the housing 1, so that a gap 11 between the inner wall and the insert 10 forms that is free of the combustible gas / air mixture.
This design of the combustion chamber in the flame area further suppresses the CO emissions caused by incomplete or unstable combustion.

The flame trap 4 is intended to prevent the flame from kicking back. Basically, it is not necessary in the burner according to the invention because the flame cannot penetrate to inlet 2 because of the low Péclet number in zone A, it is therefore only intended to increase safety. In the exemplary embodiment according to FIG. 5, the flame trap consists of a 4 mm thick steel sheet into which a large number of holes with a diameter of 1 mm have been drilled, the density of the holes being less than 20 / cm ² .

The ignition device 6 is in the vicinity of the interface 7, to a particular enable effective ignition. In the exemplary embodiment, the flame burns self-stabilizing at interface 7.

Experiments with ignition from outlet 3 have also been carried out. This kind the ignition was disadvantageous, however, because the speed of the flame front of the free Flame is comparatively low to the flame speed in the porous material. A flashback of the flame from the outlet 3 to the interface 7 was only possible if the average speed of the gas / air mixture at outlet 3 is kept low has been. Ignition from outlet 3 would therefore require additional regulation, where the flow velocity of the gas / air mixture is throttled first and then after ignition at the interface 7 is increased again. This gives the advantage an ignition near the interface 7, the complicated control solutions for the gas / air mixture does not require

The exemplary embodiments described above show the simple structure of the invention Burner at low temperature, good heat transfer and one stable flame. In the event of incomplete combustion, it is the case with the invention Brennem also possible to operate this stoichiometrically or by providing to perform better combustion of catalyst material in the porous material, the pollutant content in the exhaust gas is reduced even further.

Claims (24)

1. A burner, which is designed for a gas/air mixture as the fuel, which has a laminar flame velocity SL due to its physical properties and the design of the burner and is defined with reference to its specific heat c_p, a density ρ and a thermal conductivity λ, whereby in this burner a housing (1) is provided, which has a combustion space with an inlet (2) for the gas/air mixture and an outlet (3) for the exhaust gas, in which the housing (1) contains a porous material (5) with communicating voids, the porosity of which alters along the length of the combustion space, characterised in that the pore size increases in the direction of flow of the gas mixture from the inlet (2) to the outlet (3), whereby for the pore size a critical Peclet number (S_Ld_mc_pρ)/λ for the flame development of the gas/air mixture is produced by reason of the selected pore size in a zone (B) or at a boundary surface (7) of the porous material (5) in the combustion space, above which a flame can form and below which flame development is suppressed, d_m being the equivalent diameter of the mean void in the porous material in zone (B) or at the boundary surface (7), whereby the flame formation on the surface (7) or in the zone (B) is determined substantially independently of operating parameters which could result in a variation of the critical Peclet number.
2. A burner as claimed in Claim 1, characterised in that the critical Peclet number is 65 ± 25 and, particularly for natural gas/air mixtures, 65.
3. A burner as claimed in Claim 1 or 2, characterised in that two first and second zones (A, C) of different pore size are provided behind one another in the housing (1) in the flow direction of the gas/air mixture, between which the boundary surface (7) is situated, the first zone (A) arranged downstream of the inlet (2) having a Peclet number which is smaller than the critical Peclet number and the second zone (C) further away from the inlet (2) has a Peclet number which is larger than the critical Peclet number.
4. A burner as claimed in Claim 3, characterised in that the first zone (A) has a pore size which produces a Peclet number ≤ 40 and the second zone (C) has a pore size which produces a Peclet number ≥ 90.
5. A burner as claimed in one of Claims 1 to 4, characterised in that the porous material is heat resistant plastic sponge, ceramic material or metal or a metal alloy.
6. A burner as claimed in Claim 5, characterised in that the porous material is heat resistant up to a temperature of 1500°C.
7. A burner as claimed in Claims 1 to 4, characterised in that the porous material is filling bodies, e.g. in the form of granular material, which can optionally be stabilised, for instance by sintering.
8. A burner as claimed in Claim 7, characterised in that the granular material contains metal or ceramic material, particularly steatite, Stemalox or Al₂O₃.
9. A burner as claimed in Claim 7 or 8, characterised in that the granular material in the vicinity of the inlet (2) comprises grains of ball-like shape with a mean diameter of 5 mm and in the subsequent region with a mean diameter ≥ 11 mm if, at atmospheric pressure, the diameter to reach the critical Peclet number lies between 5 and 11 mm and is, in particular, 9 mm.
10. A burner as claimed in Claims 1 to 9, characterised in that the internal surfaces of the voids in the porous material or the surfaces of the grains of the granular material are coated with a catalyst material.
11. A burner as claimed in Claims 1 to 10, characterised in that the housing (1) has at least partially a cooling device (8).
12. A burner as claimed in Claim 11, characterised in that the cooling device (8) is constructed as a cooling coil, which surrounds the housing (1) or defines it and through which a coolant, particularly water, flows.
13. A burner as claimed in Claim 12, characterised in that a monitoring device is provided which shuts off the supply of fuel into the combustion space in the event of failure of the coolant.
14. A burner as claimed in Claims 1 to 13, characterised in that a cooling device (9) is arranged for heat exchange in a region of relatively large pore openings in the material.
15. A burner as claimed in Claim 14, characterised in that the cooling device (8) of the housing (1) is connected in series with the cooling device (9) for heat exchange.
16. A burner as claimed in Claim 14 or 15, characterised in that the cooling device (9) is itself so constructed that it acts at least partially as porous material and/or replaces porous material.
17. A burner as claimed in one of Claims 14 to 16, characterised in that the spacing of the cooling device (9) from zone (B) or the boundary surface (7) with the critical Peclet number is at least so large that the cooling device (9) does not come into contact with the flame.
18. A burner as claimed in Claims 14 to 16, characterised in that the internal wall of the housing (1) is shielded from direct heat radiation, at least in the flame region, by an additional device (10), for instance by means of an insert of suitable material.
19. A burner as claimed in Claim 18, characterised in that the device (10) is arranged at a spacing from the internal wall of the housing (1) which leaves free a gap (11) which is free of the gas/air mixture.
20. A burner as claimed in one of Claims 14 to 18, characterised in that the cooling device (9) is so far away from the zone with the porosity necessary for the critical Peclet number that it does not extend into the flame region.
21. A burner as claimed in Claims 1 to 20, characterised in that an ignition device (6) is so arranged that the ignition of the gas/air mixture occurs in a region with a porosity which has the critical Peclet number.
22. A burner as claimed in one of Claims 1 to 21, characterised in that a flame trap (4) is arranged between the inlet (2) and porous material (5).
23. A burner as claimed in Claim 22, characterised in that the flame trap (4) is a plate which has a plurality of holes with a diameter smaller than the critical "quenching" diameter for the fuel mixtures in question.
24. A burner as claimed in one of Claims 1 to 23, characterised in that the inlet (2), outlet (3) and porous material (5) are so arranged that condensate can drain away through the outlet (3).

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