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  Porous burner having a flame barrierUSPTO Application #: 20060286498Title: 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. (end of abstract) Agent: Davidson, Davidson & Kappel, LLC - New York, NY, US Inventors: Dimosthenis Trimis, Stefan Diezinger USPTO Applicaton #: 20060286498 - Class: 431354000 (USPTO) Related Patent Categories: Combustion, Mixer And Flame Holder The Patent Description & Claims data below is from USPTO Patent Application 20060286498. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] 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 [0002] 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. [0003] 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. [0004] 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 .mu.m, while their length is approximately 2-3 mm, and the fibers comprise 95-97% Al.sub.2O.sub.3. 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. [0005] 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 (.rho..sub.F-.rho..sub.T/.rho..sub.F, where .rho..sub.F stands for the density of the nonporous solid, .rho..sub.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. [0006] Examples of structures and applications for porous burners of this type can be found, for example, in [0007] Durst, F., Kesting, A., Mo.beta.bauer, S., Pickenacker, K., Pickenacker O., Trimis, D., (1997), Der Porenbrenner--Konzept, Technik und Anwendungsgebiete [The Porous Burner--Concept, technique, and fiels of application], Gaswarme International, Vol. 46, No. 6, pp. 300-307; [0008] or in [0009] Diezinger, S., Talukdar, P., von Issendorff F., Trimis, D., (2005), Verbrennung von niederkalorischen Gasen in Porenbrennern [Combustion of low calorific gases porous burners], Gaswarme International, Vol. 54, No. 3, pp. 187-192. SUMMARY OF THE INVENTION [0010] 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. [0011] 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/m.sup.3. 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. [0012] 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/m.sup.2 and with a considerably lower air/fuel ratio of .lamda.=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. [0013] 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 [0014] 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: [0015] the sole appended FIGURE shows a diagrammatic cross section through a porous burner. DETAILED DESCRIPTION [0016] 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. [0017] 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. [0018] 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. [0019] 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. [0020] 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. [0021] 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. [0022] The flame barrier 2 should have a density of at least 1300 kg/m.sup.3 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/m.sup.3, while the typical manufacturing-related lower limit for the material porosity can be specified as 40%. [0023] In tests which have been carried out, values of the order of magnitude of .rho.=1750 kg/m.sup.3 as the density and approx. 50% as the material porosity have proven ideal. Mixtures of Al.sub.2O.sub.3 and SiO.sub.2, 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.degree. C. which occur in the region of the flame barrier 2, and here in particular in its region facing the combustion space 4. Continue reading... Full patent description for Porous burner having a flame barrier Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Porous burner having a flame barrier patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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