CERAMIC HOT PLATE

Abstract: The present invention relates to a gas-heated ceramic hot plate comprising a ceramic top plate and, arranged directly under this plate, a combustion chamber for gas, in which the combustion of the gas occurs at least partly catalytically.

Description: Title: Ceramic hot plate This invention relates to a new development in the field of ceramic hot plates. Current ceramic hot plates consist of a glass-ceramic top plate having arranged thereunder an electric radiant burner by which the top plate and, if present, anything placed thereon is heated. Although such hot plates constituted a clear improvement over the conventional electric hot plates, having a metal top plate, there still exists a need for a system that has a faster control in respect of the heat emission and has a higher power available without a special power current connection being necessary. The use of the combustion of gas as a heat source for the ceramic hot plate is an excellent solution for achieving these objectives, as appears from the fact that hot plates with radiant heaters on natural gas under a plate of glass ceramics are already on the market. However, there remains a need for ceramic hot plates on gas which are less environmentally objectionable in the area of N0 X emissions and/or whose controllability is better. It has been found that the use of catalytic combustion of gas as a heat source for the ceramic hot plate is an excellent solution for achieving these objectives. The NO x emission is much lower than with a conventional burner and than the emission accompanying the generation of electricity for an electric hot plate. Moreover, the heat emission of the hot plate is easy to control. US-A-5,251, 609 and FR-A-2 , 708 , 337 disclose ceramic hot plates where a combustible gas is catalytically combusted to emit heat to a ceramic plate. These systems are based on a ceramic plate under which a gas distribution space is located, which is bounded at the underside by a monolithic structure provided with a combustion catalyst. These systems, however, still have inherent disadvantages. For instance, the combustion of the gas is suboptimal, and notably in the start-up phase production of N0 X can occur. Accordingly, it is a first object of the invention to provide a hot plate which does not have the disadvantages of the prior art, and which furthermore has still other advantages, as will appear from the following description. The invention concerns a gas-heated ceramic hot plate comprising a ceramic top plate and, arranged directly under this plate, a combustion chamber for gas, in which the combustion of the gas occurs at least partly catalytically. An important aspect of the invention is that the combustion space or catalytic reactor is arranged directly under the ceramic plate. The combustible gas, that is, the gas-air mixture, is supplied either from below or from the center. According to a preferred embodiment, there is arranged under the combustion chamber a catalyst body with a distributing space for gas under it. In the hot plate according to the invention, preferably means are present for allowing the combustion to occur in two steps, and in the first step a part of the combustible gas is catalytically combusted, with the heat produced being emitted for the most part directly via the ceramic plate. The second step can occur, for instance, in a catalytic reactor, with a smaller part of the total heat produced being removed. In that case, in the first step mentioned, more than 80%, preferably more than 90%, and still more preferably more than 99% of the combustible gas is combusted, with emission of the heat produced via the overlying top plate. If desired, the second step can occur in a flame, it being then preferred that in the first step between 10 and 80% of the combustible gas is combusted, preferably between 20 and 70%, still more preferably between 30 and 60%. The first catalyst compartment is located directly under the hot plate. The heat produced upon combustion of the gas is then directly transmitted, partly by radiation and partly by convection and conduction, to the pan on a preferably IR- transparent plate. Any unburned part of the gas can be combusted in a second, more adiabatic, catalytic reactor, or in a flame. The hot flue gas can optionally preheat the combustion air. In the case where a catalyst is used, this can be a simple combustion catalyst or a three-way catalyst. With the latter catalyst, it can be ensured that any NO x formed is converted to less harmful components. Under the ceramic top plate, one or more bodies can " be arranged, which consist of a heat-resistant material, preferably resistant to temperatures of up to more than 600°C, more preferably of up to more than 800°C, and still more preferably of up to more than 1,000°C, of which the surface itself, or the surface of the layer provided thereon, is catalytically active for the combustion of hydrocarbon- containing gas, more particularly methane, propane and/or butane. According to a preferred embodiment, specifically the surface facing the ceramic plate is catalytically active. For instance, when a monolith is used, it is preferred that the catalytically active material be provided only in the upper part of the monolith, being the part proximal to the ceramic plate. This prevents a possibly too high temperature in the monolith. The material of which the body consists generally has a coefficient of heat conduction of more than 5 W/mK, preferably more than 10 W/mK, still more preferably of more than 15 W/mK. This body can be a gauze (metal or a different material), a metal foam, sintered material, a metal or ceramic monolith, a fiber metal body, one or more annular concentric bodies with radial channels or a preformed metal plate. The hot plate according to the invention can be provided with a central supply of gas and air and an annular combustion space arranged around the supply. In another embodiment the situation is the obverse: the gas is homogeneously supplied to the combustion space from below, while the flue gases are discharged centrally. In general, it will be provided that means are present for heating up the gas to be combusted and/or the combustion air by means of the flue gases of the combustion, in order to obtain the best possible efficiency of the combustion. For the catalytic combustion, a conventional combustion catalyst is used, which is resistant to the temperatures to be generated. Such catalysts are generally provided on the surface of a support material, although it is also possible to use materials which contain no support but whose surface is catalytically active. Known catalytically active materials for the combustion of hydrocarbons such as methane, ethane, butane, propane and pentane (more particularly methane, propane and butane) are noble metal catalysts and transition metal oxides, such as platinum, palladium, and oxides, such as of copper, manganese, iron, palladium, nickel and/or cobalt. Preferably used is a combustion catalyst selected from the group consisting of noble metal such as platinum, on stabilized ceramic supports, palladium oxide, rhodium oxide, transition metal oxides, such as of copper, manganese, iron, nickel and/or cobalt, on stabilized ceramic supports, or noble metal combined with transition metal oxides on support. For optimum operation of the catalyst, it should be heated up when the hot plate is switched on. The heat for this warming of the catalyst is preferably generated by combustion of combustible gas in a flame or by electric heating of the catalyst or the combustion air, the combustible gas-air mixture and/or the combustible gas. The structure of the hot plate according to the invention is further elucidated in Fig. 1, which represents a possible embodiment. The combustion air flows, optionally via the heat exchanger (1) in which the air is pre-heated, through the first catalyst in the first catalyst space (2) right under the hot plate (3) . The combustible gas is added to the combustion air before it enters the first catalyst space, in this case preferably after the heat exchanger (1) , if present. As soon as the catalyst is at a certain temperature, a large part of the combustible gas is catalytically burned. The heat thus produced is emitted for a large part to the hot plate (3) . If the hot plate can transmit infrared radiation, heat is also emitted through the hot plate by infrared radiation. Preferably, during the increase of the conversion in this first catalyst space the gas moves, on average, parallel to the plane of the hot plate. This yields a temperature profile over the surface of the hot plate. As a result, it is possible for the partly cooled gas too, still to impart heat to the hot plate, which is of benefit to the efficiency. The gas leaving the first catalyst space contains a highly reduced concentration of combustible gas. As a consequence, it can be combusted in a catalyst space where less heat can be emitted (4) . The flue gas is then cooled by the combustion air in the heat exchanger (1) mentioned earlier. At the start, the catalyst, the combustion air, the combustible gas, or combinations thereof, can be heated electrically or by means of a flame. The following example, which is not intended as a limitation of the invention and which is shown in Fig. 2, gives an elucidation of the invention. In Fig. 2 the gas first flows through a heat exchanger (1) by which the combustion air is pre-heated. The combustion air also absorbs the part of the heat escaping from the second reactor space (4) via the insulating wall at the underside thereof. In the center, upon leaving the heat exchanger (1) , the combustible gas is added to the combustion air. The gas-air mixture then flows in radial direction, parallel to the glass-ceramic plate (3) , through the first catalyst space (2) which is filled up by two stacked annular gauzes, made of a high temperature-resistant alloy, which are covered with a catalytically active material. The metal filaments from which the gauzes are manufactured are 0.5 millimeters in thickness. This first catalyst space (2) is located directly under the glass-ceramic plate (3) . The first catalyst space is separated from the second catalyst space (4) by a heat-resistant metal foil. The gas mixture, for the most part catalytically combusted in the first catalyst space, then flows radially towards the center through the second catalyst space. This space is filled up with five stacked gauzes which are identical to the gauzes disposed in the first catalyst space and hence are also covered with a combustion catalyst. In this catalyst space the unburned part of the combustible gas is catalytically burned in the second instance. Thereafter heat is recovered from the flue gas in the heat exchanger (1) . In Fig. 3 a second embodiment of the invention is shown. In this embodiment, natural gas and air, after being preheated with the flue gases, are homogeneously supplied to the underside of the catalytic combustion space. After combustion and emission of heat to the ceramic top plate located directly above the combustion space, the flue gases formed are carried off via a central discharge, with catalytic combustion and heat exchange, as indicated hereinabove .

Claims: ri aims 1. A gas-heated ceramic hot plate comprising a ceramic top plate and, arranged directly under this plate, a combustion chamber for gas, in which the combustion of the gas occurs at least partly catalytically. 2. A hot plate according to claim 1, wherein under the combustion chamber provided with a catalyst body a distributing space for gas is arranged. 3. A hot plate according to claim 1 or 2 , wherein means are present for allowing the combustion to occur in two steps, and in the first step a part of the combustible gas is catalytically combusted, with a part of the heat produced being removed. 4. A hot plate according to claim 3 , wherein the second step occurs in a catalytic reactor, with a smaller part of the total heat produced being removed. 5. A hot plate according to claim 4, wherein in the first step more than 80%, preferably more than 90%, and still more preferably more than 99% of the combustible gas is combusted. 6. A hot plate according to claim 4 or 5, wherein the catalyst is a combustion catalyst or a three-way catalyst. 7. A hot plate according to claim 3, wherein the second step occurs in the presence of a flame. 8. A hot plate according to claim 7, wherein in the first step, between 10 and 80%, preferably between 20 and 70%, more preferably between 30 and 60% of the combustible gas is combusted. 9. A hot plate according to claims 1-8, wherein under the ceramic top plate one or more bodies are arranged which consist of a heat-resistant material, preferably resistant to temperatures of up to more than 600°C, more preferably of up to more than 800°C, still more preferably of up to more than 1,000°C, of which the surface itself, or the surface of the layer provided thereon, is catalytically active for the combustion of hydrocarbon-containing gas, more particularly methane, propane and/or butane. 10. A hot plate according to claim 9, wherein the catalytically active material is provided on the side of said bodies that faces the top plate. 11. A hot plate according to claim 9 or 10, wherein the material of which the body consists has a coefficient of heat conduction of more than 5 W/mK, preferably of more than 10 W/mK, still more preferably of more than 15 W/mK. 12. A hot plate according to claims 9-11, wherein the body can be a gauze, a metal foam, a sintered material, a metal fiber mat, a ceramic monolith, a metal monolith, one or more annular concentric bodies with radial channels, or a preformed metal plate. 13. A hot plate according to claims 1-12, comprising a central supply of gas and air and a combustion space arranged in the form of a ring around the supply. 14. A hot plate according to claims 1-12, wherein the combustion chamber is provided at the underside with a supply for the homogeneous distribution of gas over the surface of the combustion space, and with a central discharge of combustion gases. 15. A hot plate according to claim 14, wherein the central discharge is provided with means for said second combustion step. 16. A hot plate according to claims 1-15, wherein means are present for heating up the gas to be combusted and/or the combustion air by means of the flue gases of the combustion. 17. A hot plate according to claims 1-16, wherein a combustion catalyst is used, selected from the group consisting of platinum on stabilized ceramic supports, palladium oxide, rhodium oxide, and/or transition metal oxides, such as of copper, manganese, iron, nickel and/or cobalt, on stabilized ceramic supports. 18. A hot plate according to claims 1-17, wherein the gas is selected from the group consisting of methane, ethane, propane, butane, pentane. 19. A hot plate according to claims 1-18, wherein the heat for the heat-up of the catalyst is generated by combustion of combustible gas in a flame. 20. A hot plate according to claims 1-19, wherein the heat . for the heat -up of the catalyst is generated by electric heating of the catalyst or the combustion air, the combustible gas-air mixture and/or the combustible gas. 21. A hot plate according to claims 1-20, wherein the ceramic material of the top plate is at least partly transparent to infrared radiation.

Inventors: SCHOLTEN ANTON, SCHOLTEN, ANTON
Applicants: GASTEC NV, SCHOLTEN ANTON, GASTEC N.V., SCHOLTEN, ANTON
Application year: 1997
Main classification: F24C3
IPC codes: F23D14, F24C3