Patent Publication Number: US-9835327-B2

Title: Gas burner for cooking appliances

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to gas-fired cooking appliances, in particular of household type, and regards specifically the burners for such appliances. 
     (2) Description of Related Art 
     As is well known, the combustion process that takes place in these appliances generates various noxious substances, such as nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon oxides (CO and CO2). 
     The problem of eliminating or reducing these substances to improve the working conditions in cooking environments has been tackled for a long time with various technical solutions. 
     One of the known solutions provides for the use of so-called “catalytic” burners, i.e., burners in which a gas-air mixture is passed through a structure constructed or coated with a material that produces a flame-less combustion of the mixture. These burners act substantially as filters designed to absorb the combustion gases or produce an exothermic oxidation of the same, so as to eliminate the noxious substances resulting from combustion. 
     GB 2,347,362 discloses a burner of this type, with a structure made of ceramic material, such as cordierite, and the catalyst includes at least one metal selected from among platinum, rhodium, palladium and iridium, with the preferred metal being platinum. Cordierite is chosen because it displays a surface porosity necessary to achieve the deposition of the catalyst, thus increasing the active surface in the elimination of noxious gases. However, the construction of catalytic burners with a structure of ceramic material has not proved to be advantageous in household applications for various reasons, such as, for example, the fragility of the material, which is scarcely suitable for an object, such as a burner, consisting of a plurality of pieces which need to be frequently disassembled for cleaning and maintenance. In addition, the catalytic material is applied to only one part of the surface of the burner, particularly on the outlet surface of the structure, as it is believed it should act on the gaseous products of combustion, that is, after the combustion has occurred. 
     A similar solution is disclosed in JP 07091622, where the surfaces that come into contact with gas emissions are coated with catalyzing material to produce an oxidation-reduction of the same emissions. 
     The known catalytic burners act by eliminating the noxious substances produced by combustion because, as already mentioned, the catalyst is made to act downstream of combustion. Thus, the main advantage obtainable with the use of catalytic burners has been to facilitate the maintenance of the cleanness of the surfaces in contact with the flame, with the so-called self-cleaning burners. Examples of catalytic burners of this type are described in U.S. Pat. No. 3,817,689 and U.S. Pat. No. 3,921,913. 
     BRIEF SUMMARY OF THE INVENTION 
     The main objective of this invention is to provide a burner for cooking appliances, particularly of household type, that effectively resolves the problem of eliminating the noxious products of combustion, by bringing the air-gas mixture in contact with a catalytic surface before combustion takes place. 
     Another objective of the invention is to provide a burner of catalytic type that offers a greater thermal efficiency and reduces the energy required for combustion. 
     A further objective of the invention is to provide a burner of catalytic type whose structure is realized with metal materials suitable for use in household cooking appliances, particularly aluminium alloys, which ensure the required mechanical sturdiness. 
     These and other objectives of the invention will be achieved with a burner as described hereunder and with specific reference to the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of the present invention will become clear from the following description, given by way of example and not by way of limitation, with reference to the accompanying drawings, wherein: 
         FIG. 1  is an exploded perspective view of a burner structure according to the invention; 
         FIG. 2  is a schematic cross section of the burner structure of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating the energy required to activate the catalytic reaction in the combustion process; 
         FIG. 4  is a diagram showing the quantities of catalyst that are activated to generate combustion as a function of the energy supplied. 
         FIGS. 5A and 5B  each show a cross sectional view of a surface of the burner. 
     
    
    
     A burner according to the invention has a structure ( FIG. 1 ) that substantially consists of: a body  10  defining a chamber  12 , wherein an injector  14  inputs the gas that upon mixing with the air forms the combustible air-gas mixture; a ring-shaped element  16  on the upper side of the body, having a periphery provided with the combustion mixture outlet ports  18 , and a burner-covering circular plate  20 . 
     According to the invention, at least the ring-shaped element  16  is made of a metal or metal alloy, preferably an aluminium alloy such as Pyral (96% Al, 2% Mg, 2% Si), a material widely used in the production of gas-fired burners. Naturally, the body  10  and the circular plate can also be made from metal material or a metal alloy. 
     As is well known, the combustible mixture issues from the outlet ports  18  and is ignited by an ignition device (non shown), forming a crown of flames around the periphery of the burner. The heat generated by combustion is transmitted to the whole structure of the burner, which reaches a high steady-state temperature (in the order of several hundred degrees Celsius). 
     According to the invention, at least the ring  16  ( FIG. 2 ) is coated with a thin layer of material having a catalytic activity, for the purpose of reacting with the gas-air mixture that flows out along the surface of the ring. 
     As shown in  FIG. 5A , the thin layer of material  110  having catalytic activity is formed on a surface  120  of the burner. This surface  120  may be at least an underside surface of the burner-covering plate. As mentioned above, this surface  120  may also be on the ring  16 . This surface may also be at least an internal surface of the burner body  10 . As shown in  FIG. 5B , a support layer  130  may be formed on the surface  120 . The layer  130  may also be a buffer layer or substrate. 
     The coating material having catalytic activity is made up of metal oxides, either simple or mixed, in particular oxides of alkaline or alkaline-earth metals, that are coated on the burner surfaces by means of known procedures, for example by immersion in a catalyst bath. 
     To obtain a suitable coating, the surfaces can be, if necessary, prepared by forming on them the support layer  130  that serves as suitable precursor of the catalyst. When the burner is made of Pyral, which has a compact surface with low porosity, the surfaces can be prepared by coating them with an alumina layer AI2O3, for example by electrochemical oxidation, so as to form a buffer layer or substrate. 
     The catalysts used, which are active at the typical temperatures of household gas burners (200-4000 C), enable the gas-air combustible mixture to burn with a better combustion, reducing the production of noxious gases, while lowering the quantity of energy required for combustion, with the result of improving its efficiency and consequently reducing the output of noxious gases. In fact, the contact of the combustible mixture with the catalyst-coated and activated burner surfaces has the effect of preoxidizing the air-gas mixture within the burner body. 
     The combustion reaction requires considerable quantity of activation energy. This activation energy is considerably reduced in a burner coated with catalyzing material according to the invention. 
     As shown in the diagram of  FIG. 3 , the use of the catalyst makes it possible to lower the priming energy necessary to activate the combustion process. 
     The reduction of the combustion activation energy is due to the fact that the catalytic reaction brings about an increase in the quantity of fuel particles that acquire the energy necessary for combustion. Normally, the quantity of particles provided with such energy is represented by area A in the diagram of  FIG. 4 , while area B represents the additional quantity of particles that are activated by the catalytic reaction to generate combustion. Finally, area C represents the quantity of particles that do not have sufficient energy to take part in the reaction.