METHOD AND PLANT FOR PRODUCING HIGH STEAM TEMPERATURES WHEN BURNING PROBLEMATIC FUELS

Abstract: When burning problematic fuels, the hot flue gas produced is held physically separate from the superheater (4) of the plant, a part of the hot flue gas being made to flow towards a heat exchanger (3), in which an energy carrier is heated and transfers the heat to the superheater (4). In this manner, problems due to high-temperature corrosion are avoided.

Description: METHOD AND PLANT FOR PRODUCING HIGH STEAM TEMPERATURES WHEN BURNING PROBLEMATIC FUELS TECHNICAL FIELD The present invention relates to a method for producing high steam temperatures when burning problematic fuels. BACKGROUND ART When burning fuels like e.g. straw, wood and household refuse, the combustion process comprises the liberation of corrosive substances like sulphur, chlorine and alkali compounds. If the combustion occurs in connection with processes, in which the heat is utilized for producing steam, and in which the steam is subsequently to be superheated, e.g. for use in a steam turbine, the life span of the superheater depends in part on the content and concentration of corrosive substances in the flue gas, in part on the material, of which the superheater is manufactured, and the tempera¬ ture of the material. In a steam-producing plant, the superheater is normally an integral part of the total layout of the plant. The steam to be superheated flows under pressure in tubes made from metallic materials. The tubes are heated externally by the hot flue gases from the combustion process, and if the steam is to be used e.g. in a steam turbine for producing electric power, it is of decisive importance for the performance of the turbine that temperature and pressure are as high as possible, i.e. as high as permitted by the materials used. With the methods known at present for using flue gas from chlorine-containing fuels, it is not possible to achieve steam data as good as the data achieved with chlorine-free fuels, without the process causing high- temperatur corrosion, rapidly causing breakdown of the materials, of which the superheater is constructed. The literature in this field describes materials able to resist the aggressive attack from e.g. chlorine and alkali compounds, even at high temperatures. These materials are, however, extremely costly and can hardly be used at the high pressures occurring in a superheater. It is known to be possible, by using several fuels in combination, to achieve improved steam data compared to what is possible when using one single problematic fuel. This is done by using the problematic fuel to produce steam at low temperatures, while the superheating towards higher temperatures takes place in an external superheater, in which the high temperature is produced by burning a problem-free fuel, such as natural gas. Further, the literature describes the possibility of achieving superheating by means of pyrolysis gas produced from the problematic fuel at low temperatures, so that the corrosive substances, such as e.g. chlorine and alkali compounds, are retained in the pyrolysis equipment. This process step is, however, extremely complicated and expensive, and is hardly likely to find widespread use in commercial plants. DISCLOSURE OF THE INVENTION It is the object of the present invention to provide a method for superheating steam, in which the energy for the superheating is taken from the problematic fuel, without this fuel affecting the life span of the superheater. According to the present invention, this object is achieved in a method of the kind referred to initially, characterized in that the hot flue gases from the combustion of the problematic fuels are held physically separate from the superheater, intermediary equipment being used for transferring the heat from the corrosive flue gasses to the steam-filled superheater, said equipment in the following description being termed a high-temperature heat exchanger. The invention makes it possible to divide the problem complex associated with superheating of steam into two separate simple problems, each being possible to solve with known technology, but in combination making extremely high demands on the choice of materials. In this manner, it is possible to use a high- temperatur heat exchanger made from materials capable of withstanding high temperatures and aggressive flue- gas mixtures, without making high demands on the physical strength of the materials, as the heat exchanger is preferably used at very low pressures. BRIEF DESCRIPTION OF THE DRAWING In the following detailed portion of the present description, the invention will be explained in more detail with reference to the drawing, in a purely diagrammatic manner showing how the method according to the invention may be carried out. DESCRIPTION OF THE PREFERRED EMBODIMENTS The fuel is burned in a combustion space or fire box 1, that may be of the water-cooled or the brick- lined type. The hot flue gases are made to flow in part to a boiler 2, in part to a high-temperature heat exchanger 3. In the boiler 2, the hot flue gas from the fire box 1 is used for producing steam at a high pressure, the temperature being held at a low level in order to avoid high-temperature corrosion. In the high-temperature heat exchanger 3, which is made from corrosion-resistant materials, such as e.g. ceramics, the hot flue gases are used to heat a problem-free energy carrier, that may e.g. be air. The energy carrier being heated in this manner is made to flow towards a superheater 4 that receives the steam from the boiler 2. In the superheater 4, the steam is heated to high temperatures by means of the energy carrier, one such having being chosen that is not aggressive towards the materials used in the superheater 3. Typically, the superheater 3 is made from metallic materials able to withstand high pressures. The energy carrier may be made to circulate in a closed circuit to produce high temperatures and direct heat exchange as shown in Figure 1, or in an open system, possibly connected to a turbo charger or a gas turbine as indicated in Figure 2. The high-temperature heat exchanger 3 can constitute a separate unit as shown on the drawing, or it can be integrated with or in the fire box.

Claims: CLAIMS 1. Method for producing high steam temperatures when burning problematic fuels, characterized in that the hot flue gas from the combustion of the problematic fuels is held physically separate from the superheater of the steam plant, a part of the hot flue gas being made to flow towards a heat exchanger, in which an energy carrier is heated and transfers the heat to the super¬ heater of the steam plant. 2. Plant for carrying out the method according to claim 1 and of the kind comprising a) a combustion space or fire box (1) , b) a boiler (2) adapted to be heated by hot flue gas from the combustion space or fire box (1) and to deliver the steam so formed to c) a superheater (4) , characterized bv d) a heat exchanger (3) , the heat-receiving side of which is adapted to be heated by the hot flue gases from the combustion space or fire box (1) , and the heat-delivering side of which is adapted to supply heat to the superheater (4) through a separate heat-exchange medium. AMENDED CLAIMS [received by the International Bureau on 14 February 1994 ( 14.02.94 ) ; original claims 1 and 2 replaced by amended claims 1 and 2 ( 1 page ) ] 1. Method for producing high steam temperatures when burning problematic fuels, characterized in that the hot flue gas from the combustion of the problematic fuels is held physically separate from the superheater of the steam plant, a part of the hot flue gas being utilized by a heat exchanger, in which an energy carrier is heated and transfers the heat to the superheater of the steam plant. 2. Plant for carrying out the method according to claim 1 and of the kind comprising a) a combustion space or fire box (1), b) a boiler (2) adapted to be heated by hot flue gas from the combustion space or fire box (1) and to deliver the steam so formed to c) a superheater (4), characterized by d) a heat exchanger (1), the heat-receiving side of which is adapted to be heated by the hot flue gases from the combustion space or fire box (1), and the heat-delivering side of which is adapted to supply heat to the superheater (4) through a separate heat-exchange medium.

Inventors: KRISTENSEN OLE STIG, KRISTENSEN, OLE, STIG
Applicants: ANSALDO VOLUND AS, KRISTENSEN OLE S, ANSALDO VOELUND A/S
Application year: 1993
Main classification: F01K3
IPC codes: F01K3, F22G1