Porous burner having a flame barrier

Abstract

A porous burner has a mixing space for premixing reducing agent and oxidizing agent, and an adjoining combustion space filled with a porous material. A flame barrier is arranged between the mixing space and the combustion space. The flame barrier is made from a porous material with a material porosity of less than 60%. This material has a density of at least 1300 kg/m3. The preferred use of a porous burner configured in this way lies in the combustion of hydrogen or hydrogen-rich gas.

Description

This claims the benefit of German patent application DE 10 2005 027 698.9, filed Jun. 15, 2006 and which is hereby incorporated by reference herein.

BACKGROUND

The present invention relates to a porous burner having a mixing space for mixing reducing agent and oxidizing agent, and an adjoining combustion space, which is filled with a porous material, a flame barrier with holes being arranged between the mixing space and the combustion space. Furthermore, the present invention relates to the use of a porous burner of this type.

WO 95/01532 A1 has disclosed a burner which includes a combustion space filled with a porous material and a mixing space. This burner also has a flame barrier or flame trap between the mixing space and the burner space.

Premix combustion techniques, such as porous burner technology, require flame stabilization, which prevents a flashback of the flame front to the mixing space. Therefore, a flame barrier is arranged between the mixing space and the combustion space filled with a porous material. The conventional flame barriers used in porous burners are optimized for use with hydrocarbon-based fuels, such as natural gas or gasoline. They typically comprise generally ceramic fiber plates with parallel holes. The thickness of these fibers is generally approximately 3.0-3.5 μm, while their length is approximately 2-3 mm, and the fibers comprise 95-97% Al₂O₃. During production, they are arranged in such a manner that the plates which are formed have a high material porosity and, as a result, a low thermal conductivity and a high strength under the high spatial and temporal temperature gradients which occur. These perforated flame barriers, as described in the above-mentioned WO document, allow operation with a high power modulation (1:25) over a wide range of air/fuel ratios using hydrocarbon-containing fuels.

The typical material porosity of flame barriers of this type is approx. 90%. For the explanations given here, the material porosity is in each case determined from the quotient (ρ_F−ρ_T/ρ_F, where ρ_F stands for the density of the nonporous solid, ρ_T stands for the density of the porous material of the same composition under the same external conditions, e.g. standard conditions. In the context in which it is used here, therefore, the material porosity indicates the proportion of “empty” volume in the material.

Examples of structures and applications for porous burners of this type can be found, for example, in

• • Durst, F., Kesting, A., Möβbauer, S., Pickenäcker, K., Pickenäcker O., Trimis, D., (1997), Der Porenbrenner—Konzept, Technik und Anwendungsgebiete [The Porous Burner—Concept, technique, and fiels of application], Gaswärme International, Vol. 46, No. 6, pp. 300-307;
  • or in
  • Diezinger, S., Talukdar, P., von Issendorff F., Trimis, D., (2005), Verbrennung von niederkalorischen Gasen in Porenbrennern [Combustion of low calorific gases porous burners], Gaswärme International, Vol. 54, No. 3, pp. 187-192.

SUMMARY OF THE INVENTION

An object of the invention is to provide a porous burner as a premix burner which allows flashback-free use with hydrogen or hydrogen-rich gas.

The present invention provides a porous burner having a mixing space for mixing reducing agent and oxidizing agent, and an adjoining combustion space, which is filled with a porous material, a flame barrier with holes being arranged between the mixing space and the combustion space, characterized in that the flame barrier includes a porous material with a material porosity of less than 60%, this material having a density of greater than 1300 kg/m³. On account of the use of materials with a significantly lower material porosity and a correspondingly higher density than with conventional flame barriers, it has been possible to considerably extend the operating range and power modulation of porous burners during combustion, in particular of hydrogen and hydrogen-rich gas.

The flame barriers according to the invention then can permit reliable operation with hydrogen and air, for example at a surface loading of 1300 kW/m² and with a considerably lower air/fuel ratio of λ=1.8. This limit air/fuel ratio may be reduced accordingly for higher surface loading levels. Moreover, the novel design may allow a power modulation of 1:10.

The widening of the operating range using the flame barrier according to the present invention opens up the possibility of new application areas and increases the versatility for existing application areas.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous configurations of the invention are described in the exemplary embodiment which is explained in more detail below with reference to the drawing, in which:

the sole appended FIGURE shows a diagrammatic cross section through a porous burner.

DETAILED DESCRIPTION

The text which follows is intended to present a porous burner 1 having a flame barrier 2 according to the present invention, as well as the resulting options with regard to flexibility and widening the operating range, by way of example, on the basis of an afterburner or heating burner for a hydrogen-operated fuel cell system, for example based on a polymer membrane as electrolyte (PEM), for mobile applications. Naturally, the porous burner 1 is not restricted to an application of this nature.

In particular in mobile applications, the time taken to reach a steady operating state is a crucial factor. This time can be considerably reduced by heating. Either an electrical heating system or a burner which generates a hot exhaust-gas stream by burning, the system fuel hydrogen can be used for this purpose. The use of burners has the advantage that considerably shorter starting times can be achieved than with electrical systems, in particular since in many mobile applications insufficient electrical energy is available.

Since systems of this type impose very high demands on the discharge of emissions, the burners which are to be used, however, have to satisfy extremely high demands. In this context, widening of the air/fuel ratio range in the direction of low air/fuel ratios is crucial, since otherwise the exhaust-gas temperatures remain too low to significantly shorten the starting time. An additional range of applications for a starting burner of this type can be opened up by increasing the power modulation range.

The exhaust gases which are formed during what is known as purging, i.e. the blow-off of hydrogen-containing gases from the fuel cell or from an anode hydrogen circuit thereof, when shutting down or from time to time during operation, typically have a hydrogen content of between 5% and 10% by volume. One way of avoiding emission of these exhaust gases into the atmosphere is afterburning. However, the chemical energy contained in this exhaust gas is well below the thermal power required when starting. The use of a single burner both as a starting burner and for the afterburning is therefore only possible if this burner can be reliably operated with a sufficient load spread.

The only appended FIGURE shows a diagrammatic cross section through the porous burner 1 having the perforated flame barrier 2. The flame barrier 2 is located between a mixing space 3, in which air as oxidizing agent and hydrogen or hydrogen-rich gas as reducing agent/fuel are premixed, and a combustion space 4. As is customary in porous burners 1, the combustion space 4 is filled with a porous material, in which the actual combustion takes place. This porous material could, for example, be an SiC foam with 10 ppi (pores per inch). In addition, all other conceivable configurations of the combustion space 4 which have already been used in previous porous burners are also possible. The porous burner 1 illustrated here differs from known porous burners only by virtue of the flame barrier 2 or the material from which the latter is formed.

The material used for the flame barrier 2 is typically ceramic produced by a slip casting. To minimize the shrinkage during the production process, this material, in addition to the powder fraction, may contain less than 30%, preferably less than 15%, e.g. approx. 10%, by mass of fibrous constituents. These fibrous constituents have the additional advantage of increasing the stability of the material. To keep the material porosity at a sufficiently low level, however, it would also be possible to dispense with adding fibers altogether.

The flame barrier 2 should have a density of at least 1300 kg/m³ and a material porosity, as defined above, of less than 60%. In this context, a typical material-based upper limit for the density can be specified as 2000 kg/m³, while the typical manufacturing-related lower limit for the material porosity can be specified as 40%.

In tests which have been carried out, values of the order of magnitude of ρ=1750 kg/m³ as the density and approx. 50% as the material porosity have proven ideal. Mixtures of Al₂O₃ and SiO₂, preferably in a mass ratio of approx. 80% to approx. 20%, have been identified as a suitable material. This material is also able to withstand the temperatures of up to 1700° C. which occur in the region of the flame barrier 2, and here in particular in its region facing the combustion space 4.

With materials of this type for the perforated flame barrier 2, in a test carried out using hydrogen, it was possible to achieve safe operation under surface loading of 1300 kW/m² at an air/fuel ratio of λ=1.8. This limit air/fuel ratio is reduced further accordingly at higher surface loading levels. Moreover, the new type of flame barrier also allows power modulation of 1:10.

A perforated flame barrier 2 made from material of this type was used in tests. The holes in the flame barrier 2 were made with a diameter of d=0.5 mm. A likewise sufficient functionality is achieved with hole diameters of up to d=0.8 mm. For manufacturing reasons, hole diameters of less than approx. d=0.35 mm are scarcely feasible, even though a good functionality would be expected.

The sum of the cross sections of the holes in the flame barrier 2 should not exceed 5% of the total area of the flame barrier 2. Particularly good results can be achieved between 1% and 4%. The sum of the cross sections of the holes within the tests undertaken were, for example, approx. 1.4% of the total area of the flame barrier 2. This effectively avoided a flashback on the part of the flame front. However, since the pressure losses caused by the flame barrier 2 also increase with a decrease in cross-sectional area of the holes, in practice the porous burner 1 or the flame barrier 2 should be optimized on the basis of the pressure losses which are just still tolerable within the context given above. For example, if a high pressure is available, it is generally possible for the sum of the cross sections of the holes in the flame barrier 2 to be selected from 1% to 2% for ultrahigh security against flashbacks. If, on the other hand, the air and/or hydrogen is delivered at only a low excess pressure, for example by means of blowers, the sum of the cross sections of the holes in the flame barrier 2 should be selected to be from 3% to 4%, in order to still ensure highly safe operation while at the same time achieving minimal pressure losses in the flame barrier 2.

The widening of the operating range which is achieved with the perforated flame barrier 2 made from the material described above for the first time allows the use of the porous burner 1 for the conversion of hydrogen in industrial burners and in particular in fuel cell systems. In the fuel cell systems, it is possible on the one hand to shorten the cold-start time and on the other hand to reduce the system complexity, since there is no need for an additional burner.

The advantages of the invention are further clarified below.

Many scenarios foresee hydrogen as one of the energy carriers of the future, since it allows the generation of heat and power with very low pollutant emissions. NO_x is the only potential pollutant during the combustion of hydrogen, since carbon-based pollutants, such as carbon monoxide or particulates, cannot be formed. Possible application areas in which low-emission hydrogen burners are required even now include the chemical industry and use as afterburners and heating burners in fuel cell systems.

At present, it is primarily non-premix or catalytic burners which are used for the combustion of hydrogen, as opposed to premix combustion systems. In the case of non-premix burners, the fuel is only mixed with the air in the reaction area, so that a flashback fundamentally cannot occur. However, this concept involves relatively high NO_x emissions, since irrespective of the overall air excess there are always zones with a stoichiometric mixture composition and therefore high temperatures. By contrast, catalytic burners have the drawback that for many applications they are too susceptible to contamination and temperature fluctuations and have an insufficient power modulation. An additional problem is that they have to be operated with high excesses of air in order to avoid overheating of the catalyst. It is likely that with the state of the art as it stands, the desired low NO_x emissions can only be achieved using premix combustion systems.

In addition to the general benefits of premix burner designs, porous burner technology has further properties which make the use of a porous burner for the combustion of hydrogen an attractive option. With porous burners, the temperature in the combustion zone can be influenced in a targeted way. Moreover, on account of the high heat transport, the temperature field within the combustion zone is very homogeneous. Both these advantages allow very low NO_x emissions which cannot be achieved even by most other premix burners. Further advantages of porous burners over other premix burners are the high surface loading which can be achieved and the geometric flexibility in terms of the configuration of the combustion zone.

The present invention addresses the flashback problem found with porous premix burners. It has been found that when other porous premix burners are operated with hydrogen or hydrogen-rich gases, as are formed for example when hydrogen is obtained by reforming hydrocarbon-containing starting materials (methane, methanol, gasoline, etc.), a flashback of the flame into the mixing space occurs within a short time. Safe, i.e. flashback-free, steady-state operation is only possible if the air excess is greatly increased. The extent of the rise in the air/fuel ratio required depends mainly on the surface loading. Safe operation at a surface loading of 1300 kW/m², which is customary for porous burners, with the conventional flame barrier configuration is only possible beyond an air/fuel ratio of approximately λ=3.0. Furthermore, the power can only be modulated in the range of 1:2. It has been possible to demonstrate in tests that during the combustion of hydrogen the flame front penetrates into the porous material of the flame barrier, moves through the material in the direction of the mixing space and leads to a flashback. In practice the extremely high risk of flashbacks, which greatly restricts the operating range, constitutes a significant obstacle to the implementation of premix hydrogen burners. The present invention addresses this problem.

Claims

1. A porous burner comprising:

a mixing space for mixing reducing agent and oxidizing agent;

an adjoining combustion space filled with a first porous material; and

a flame barrier with holes being arranged between the mixing space and the combustion space, the flame barrier including a second porous material with a material porosity of less than 60%, the second porous material having a density of greater than 1300 kg/m3.

2. The porous burner as recited in claim 1 wherein the holes have a diameter of less than 0.8 mm and a sum of cross sections of the holes amounts to less than 5% of a surface area of the flame barrier.

3. The porous burner as recited in claim 2 wherein the holes have a diameter of 0.5 mm.

4. The porous burner as recited in claim 1 wherein the sum amounts to between 1% and 4% of the surface area.

5. The porous burner as recited in claim 1 wherein the second porous material has a density of between 1500 kg/m3 and 2000 kg/m3.

6. The porous burner as recited in claim 5 wherein the second porous material has a density of 1750 kg/m3.

7. The porous burner as recited in claim 1 wherein the second porous material has a proportion by mass of fibrous constituents of less than 30%.

8. The porous burner as recited in claim 7 wherein the second porous material has a proportion by mass of fibrous constituents of less than 15%.

9. The porous burner as recited in claim 1 wherein the second porous material includes a mixture of Al2O3 and SiO2.

10. The porous burner as recited in claim 9 wherein the mixture includes 80% Al2O3 and 20% SiO2 based on mass.

11. The porous burner as recited in claim 1 wherein the first and second porous materials are different.

12. A method for burning comprising:

burning a mixture of an oxidizing agent and a reducing agent using the porous burner as recited in claim 1, the reducing agent containing at least 75% by volume of hydrogen.

13. The method as recited in claim 12 wherein the oxidizing agent is air.

14. The method as recited in claim 1 wherein the reducing agent includes more than 90% by volume of hydrogen.

15. A method for burning comprising:

burning, in a fuel cell system, a mixture of air and hydrogen or hydrogen-rich gas using the porous burner as recited in claim 1 with a hydrogen content of more than 90% by volume in a fuel cell system.

16. The method as recited in claim 15 wherein the porous burner burns exhaust gases from a fuel cell in the fuel cell system.