A methane catalytic combustion boiler for obtaining hot water for household and industrial uses

Abstract: A methane catalytic combustion boiler for obtaining hot water for household and industrial uses, such as room heating, sanitary services, heat transfer in industrial processes is described. The boiler includes a container for the combustion catalyst, which consists of pure or supported metal oxides, a heat exchanger between the combustion gases and the water to be heated, a system for starting the methane combustion and a system for controlling the combustion.

Description: The present invention relates to a boiler for obtaining hot water for household and industrial uses by combustion of methane on a catalytic bed. The availability and cheapness of methane have brought about a demand by the consumers of plants employing methane both for room heating and for sanitary and industrial uses. The boilers now available on the market present many shortcomings. These originate particularly from the fact that the temperature of free flame methane fuel gases may reach up to 1300-1950°C. This causes serious problems in connection with the quality of the construction materials, the thermal exchange efficency, the fuel consumption, the production of obnoxious gases and the safety. EP-A-0 256 322 describes a methane catalytic boiler comprising a container almost completely full of a catalyst mass consisting of metals of the platinus group, wherein the methane gas and the air are fed together; heat transfer means between the gaseous combustion products and the water to be heated, said heat transfer means preferably consisting of finned tubes included in said catalyst mass; and a control system for said combustion. The above mentioned problems are solved with full satisfaction by the catalytic boiler for methane combustion of the present invention. We have found in fact that it is possible to obtain the complete combustion of methane at decidedly lower temperature than in the case of the free flame combustion, if the methane-air mixture is contacted with a catalyst consisting of pure or supported metal oxides. The oxidation state of said oxides varies depending on the temperature and on the excess or defect of oxygen, and, because of this, an efficient combustion results in different feeding conditions of the mixture. The boiler comprises a container for the catalyst, a catalyst, means for heat exchange between the fuel gases and the water to be heated, means for starting the methane combustion and a system for controlling the combustion. Said boiler is useful for the production of hot water for household and commercial uses. The characteristics and advantages of the methane catalytic combustion boiler for obtaining hot water for household and industrial purposes according to the invention will be put in better evidence by the following detailed description and by the enclosed figures 1 and 2, which are reported for illustrative, but not limitative, purposes. The catalysts employed in the boiler according to the present invention consist of metal oxides, pure or supported, single, mixed or admixed, of metals selected from the group consisting of Cr, Mn, Fe, Ca, Ni, Cu, Zn, Sn. An example of a catalyst particularly suited to the low temperature combustion of methane (ignition at 270-300°C) is a mixture of Cu and Cr oxides in various oxidation states. The catalyst may be in pellets, tablets, spheres of 1 to 20 mm diameter. These dimensions allow the gases to pass through the catalytic bed with only a moderate pressure drop, this avoiding the need of pumps or other devices to facilitate the gas flux. Said catalysts have a specific surface area comprised between 1 and 200 m²/g. These catalysts are very active, allowing very high flow capacities, tipically comprised between 2.000 and 100.000 volumes of gas for catalyst volume per hour. The catalyst is placed in a layer of a thickness variable according to the power of the boiler. The heat exchange between the catalytic bed, the combustion gases and the water to be heated is obtained by means of a metal heat-sink supported on the outer surface of the catalyst container. In the catalyst container, heat-sink system, a direct contact between heat-sink and catalyst, which would impair the reaction, is avoided. The system is make out of a metal with good heat transfer properties, such as copper, in order to obtain an efficient heat exchange and a good uniformity of the thermal profile in the catalytic bed. We have found that, if the heat exchange takes place with the exchanger directly included in the catalytic mass, the temperature falls at the contact surface of the catalytic grains with the exchanger is such that the temperature of the system falls below the ignition temperature, this leading progressively the combustion to stop. With the catalysts according to the present invention, the ignition temperatures of the methane-air mixture are comprised between 200 and 400°C, while the catalytic bed temperature during the normal working of the boiler is comprised between 350 and 750°C. Such temperature levels allow the use of common construction materials. At a reaction temperature lower than 750°C furthermore the formation of carbon monoxide and nitrogen oxides is avoided, while they are always present when burning methane in a free flame at temperature higher than 1.000°C. The catalyst's property of varying its oxidation state depending on the thermal profile leads also to the reduction to elemental nitrogen of nitrogen oxides which may be present. A further characteristic of the boilers according to the present invention is to comprise a reaction control system simply consisting of one or more thermocouples sunk in the catalyst bed, which signalize, to a system blocking the methane feed, falls of temperature below the ingnition limits which may take place. This control system is an additional advantage of the present invention, in that it avoids the inconvenience of fooling of the photocells used for the control of the conventional burners. The system blocking the methane feed also controls the electrical start of the gas ignition each time the boiler is started. It is possible, without departing from the essence of the invention, to recycle combustion gases, for pre-heating gas and water feeds, thus recovering also such heat dispersions. The methane-air admixture is made according to a particular embodiment of the invention prior to admitting the gases in the catalyst container; it is however possible to introduce the two gases separately in a chamber situated below the catalyst container and comprising the ignition system. According to a typical embodiment of the present invention, the combustion gases, after pre-heating the feeds, are particularly recycled to the combustion together with the air and methane feed. Thus a futher control of the desired temperature level is provided. The amount of air fed is adjusted at any rate so as to have an at least stoichiometric ratio between oxygen and methane. Figures 1 and 2 schematically show an embodiment of the boiler according to the present invention. Referring to the reference numerals in the figures, the boiler consists essentially of three superimposed cylindrical chambers, 1, 2 and 3, respectively. Chamber 1 is connected with chamber 2 through a porous wall, and so does chamber 2 with chamber 3 through porous wall 4. Chamber 2 contains the electrical ignition system 5, which starts the methane combustion. Chamber 3 contains the catalyst 6. Chamber 1 has the purpose of pre-mixing the gases fed; in chamber 2 the combustion starts whereas chamber 3 has the function of completing the combustion and the heat exchange. Methane is fed through pipe 7, through valve 8, while air is fed through pipe 9. Pre-mixing chamber 1 should be so constructed as to facilitate a homogeneous mixing of the gases. Once the combustion is started, the temperature in the catalytic bed remains higher than the ignition temperature, due to the reaction heat, and the electrical ignition system is automatically disconnected, to be re-inserted at each new start of the boiler. The combustion is controlled by means of thermocouples 16 immersed in the catalyst and which signalize to a system 10, blocking the methane feed, possible temperature falls below the reaction ignition value. The blocking system operates through valve 8. The heat exchange for obtaining hot water is performed by means of a metallic dissipator (heat sink) supported on the outer surface of the catalyst container. Said dissipator may for instance be in the form of a coil 11, or of a jacket 15. The dissipator is fed with water from a main through pipe 12, while the hot water proceeds to the use via pipe 13. The combustion fumes exit through chimney 14. Experiments on methane combustion in a boiler of the type described were carried out for a long period (7 months), using as catalyst 350 ml of copper chromite of the Harshaw Co. and as heat exchanger a coil consisting of a copper pipe of 4x6 mm diameter wound in five spirals of 50 mm diameter. Methane was fed at a rate of 60-80 Nl/h and air at 690 to 1800 Nl/h. Water passed through the coil at the rate of 8 l/h, entering at 20°C and being collected at the exit at 47-50°C. The temperature at the center of the catalyst bed was comprised between 514 and 740°C. Futher experiments were carried out in a similar way using in the combustion of methane various catalyst types. Table 1 summarises the experiments. In all cases the methane combustion was complete. No carbon monoxide or nitrogen oxides were detected at the stack in most of the experiments, while traces were present in some. By comparison, the analysis of the combustion gases of a conventional methane boiler gave a C0 content of 60-75 ppm and a nitrogen oxide content of 60-66 ppm.

Claims: Methane catalytic combustion boiler for obtaining hot water for household and industrial uses comprising three superimposed cylindrical chambers where a first chamber (1) has the purpose of pre-mixing the gases fed, a second chamber (2) contains the electrical ignition system (5) and a third chamber (3) contains a catalyst (6), means for heat exchange between the combustion gases and the water to be heated and a system for controlling the methane combustion into said chamber (3) consisting of at leat one thermocouple (16) inserted in said third chamber (3) and connected to a system (10) for blocking the methane feed through valve (8), characterized in that said catalyst (6) is a metal oxide or a mixture of metal oxides and in that said heat exchange means consist of a metallic heat-sink supported on the outer surface of the third chamber (3) container. Boiler according to claim 1, characterized in that said metal oxides are selected from the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Sn and Zn oxides. Boiler according to claim 1, characterized in that said catalyst consists of copper chromite. Boiler according to claim 1, characterized in that said catalyst is in the form of pellets, tablets or spheres of 1-20 mm diameter. Boiler according to claim 1, characterized in that said catalyst has a specific surface comprised between 1 and 200 sq.m/g.

Inventors: ITALIANO PIETRO, ITALIANO, PIETRO
Applicants: GRIV SRL, GRIV S.R.L.
Application year: 1989
Main classification: F24H1
IPC codes: F24H1