A method and device for combustion at high temperatures

Abstract: The invention concerns the combustion at high temperatures (900 - 1,800 degrees C) specially of substances containing toxic compounds. The resulting ashes melt and exert a corrosive action on refractories which sums up with that of the combustion products. In order to avoid such inconvenience, the combustion chamber consists of a rotary crucible, on the walls of which a layer of melted ashes acting as refractory is formed and maintained.

Description: The invention concerns a method and a device to perform the combustion and ashing of substances at high temperatures, mainly substances containing toxic products. As known the elimination of toxic compounds, particularly organic ones, is often carried out by demolition and oxidation of the molecules of said toxic compounds in ashing furnaces at high temperatures. For a complete elimination of some particularly stable compounds, such as chloro-substituted hydrocarbons and chlorofluorohydrocarbons, in order to avoid too long stay times, it is preferred to operate at very high temperatures (900-1800 degrees C). Patent specification US-A-3357383 discloses a furnace for burning a pulverized fuel wherein a film of liquid slag formed on the walls is likely to retain the coarse particles of fuel adhering to said film. The internal walls of the furnace are lined with a permanent layer of refractory coating. At very high temperatures, however, many problems arise, among which the major ones are: corrosive action of the combustion products on the refractories of the walls, in particular in the flame impact area; melting of the ashes resulting from the load of inorganic substances present in the compounds to be eliminated; corrosive and erosive action of melted ashes on the refractories of the walls and the bottom they are in contact with. DE-B-1024191 shows a fixed furnace wherein a layer of solid ash is obtained by cooling the combustion products. A finely pulverized fuel is supplied through a tangential admittance to form a centrifugal circulation of the burnt gases in the furnace, on the internal walls of which a cooling pipe system is disposed. In furnaces of said type it is very difficult to form and maintain an effective layer of solid ash on the very large surface of the cooling pipes because it is not possible to control all the factors which affect the formation of the refractory layer even when the furnace operates under normal conditions. This invention concerns a method and device, of combustion specially an ashing furnace at very high temperatures, wherein the abovesaid problems are reduced or solved. This is essentially achieved by a method to perform combustion at high temperatures (> 900 degrees C) within a combustion chamber using ash obtained from combustion, at the melted, semi-melted or dust-like state as a protective refractory layer on the internal walls of the combustion chamber, characterized in that said combustion chamber rotates at speed such that the centrifugal force forms and maintains a continuous layer of said ash on the internal walls of the combustion chamber. The use of the melted ash itself as refractory on the walls of the combustion chamber eliminates any problem of corrosion simply because the corrodible substance is substituted by the substance which is the source of corrosion and has proved to have sufficient refractory properties. To perform the abovementioned method, the invention provides a device for the combustion of products, specially harmful toxic ones, at temperatures exceeding 900 degrees C, comprising a combustion chamber defined by a rotary container or crucible; means to rotate the container about its own axis at a given speed; a crown or shutter, fixed in correspondence to one of the axial openings of the container and bearing at least the systems for the introduction of fuel and air and/or oxygen for combustion; and means for discharging the products of combustion and the ashes, characterized in that the shape, the radial dimension, the position of the axis and the rotation speed of the container are such as to create a centrifugal force whose value is at least sufficient to maintain on the whole internal wall a continuous layer of ash at its melted, semi-melted or dust-like state. Said device, which may be a furnace with horizontal or vertical axis or even the combustion chamber for a gas turbine, is started substantially according to the following steps: controlling the rotation of the crucible at a preset speed; introducing into the crucible a filling of glass powder, silica or ash from previous processes; waiting for a uniform distribution of dust on the internal walls of the crucible due to the centrifugal force; heating and simultaneously actuating cooling on the outside; and recording the speed of rotation possibly adding other powdered material until a continuous layer on the internal walls of the crucible is formed and maintained. What stated above will be better understood from the following description of two preferred embodiments, illustrated in the accompanying drawings, where: Figure 1 is a schematic section of an ashing furnace with rotary crucible, according to the invention, of the type with vertical axis. Figure 2 is a schematic section of an ashing furnace with rotary crucible, according to the invention, of the type with horizontal axis. With reference to figure 1, the furnace consists of a container 1 for example in the form of a revolution solid, made of a material having good mechanical resistance (i.e.: welded carbon steel plate possibly coated with thin-layer refractory) and is cooled from the outside by suitable means such as water jets coming from one or more tubes 2 provided with several nozzles 3. Said container will be hereinafter indicated by the term "rotary crucible". The rotary crucible is mounted on suitable wheels or rolling bearings to be put and maintained in rotation at the desired speed about a vertical axis a. In the figure, for exemplifying purposes, the crucible is supported by three motor wheels 4 holding a circular rail 18 integral to the rotary crucible, the crucible is provided with a central discharge duct 5, preferably made of refractory with high thermal mechanical and chemical resistance. On the crucible top, there is provided a fixed crown 6 having two or more holes for feeding systems of the wastes to be ashed, of the supporter of combustion, of possible additions of inoculants, as well as for discharching combustion products, for inspection, for sampling operations, etc., and for an auxiliary and/or starting burner. In the drawing only the main burner 19 and the discharge opening 16 for burnt gases are illustrated. The fixed crown can be made of double-wall metal with water cooling, according to the technique used in electric furnaces for metallurgy, or of refractory provided with adequate thermal and mechanical resistance. On the outside of the rotary crucible and in a way as to support the crown 6, there is provided an external shroud 7, for example made of galvanized sheet iron, possibly thermally insulated by means of suitable material. The external shroud 7 serves to separate from the ambient the volume 8 outside of the crucible 1 wherein the nozzles 3 operate, in a way that water vapor generated by the cooling action of the water jets on the rotary crucible is not dispersed in the atmosphere and in a way that pressure around the rotary crucible can be kept at the desired level. Steam may be partly exhausted through a duct 9 or can flow, depending on the pressure levels, into the rotary crucible through an annular space 20 existing between the fixed portion and the rotary portion 10, as well as possibly through the discharge duct 5. The internal volume of the shroud can be divided by a septum 11 serving to collect the exceeding water coming from the jets 3 (the septum shape and position are given for exemplifying purposes only). The collected water, after proper restoration and treatment (for example, filtration) to avoid the formation of deposits, is sent again to the pumps 12. The shroud is equipped with sealing doors for the removal of the melted ash collecting container 13 and is designed in a way as to allow the rotary crucible removal for maintenance purposes. The shroud must be provided with proper easily yielding sections as to allow large steam volumes to be exhausted, for example in the catastrophic event of the rotary crucible collapse and subsequent interaction between cooling water and melted ash. Below the discharge opening 5 there is provided a system to collect melted ash, consisting of a container 13 (of the disposable type or in the form of re-usable ingot mold or other), possibly mounted on a trailer 14 for its easier removal from the furnace. Below the container a dripping-pan 15 may be positioned sized as to be able to receive the largest amount of melted ash which may possibly escape from the hole 5 in case of the equipment malfunctioning. Preferably a few auxiliary jets serve to cool the container 13, in order to avoid dangers during the container removal and in particular to increase the steam flowrate in case of use for energetic purposes. In case of liquid products or mainly liquid products to be ashed, the feeding takes place through a burner 19, for example of the multi-tubular type, allowing a regular and well directed flame beam 21 to form, which is oriented in a way as to optimise the burnt quantity of combustion supporter or waste. Air optionally enriched with oxygen or even pure oxygen can be used as combustion supporter. Preferably the fluid coming from the burner and therefore the combustion products will be provided with a speed having a wide enough tangential component with respect to the axis of the rotary crucible as to facilitate the separation of the particulate present in the flame and its deposition on the melted ash bath. Possible additions (such as water or steam intended to lower the operating temperatures, inoculants to modify the ash composition, further combustion supporter to maintain a proper O² percentage in the chamber) can take place through the burner itself or through other openings (not shown) in the crown. Furthermore, should the adopted burner require it, an auxiliary burner (a fossil fuel one) may be installed in the crown for starting the equipment and maintaining the set temperature. The burnt gas coming out through the exhaust port 16 will be preferably conveyed to a post-combustion chamber for completion of oxidation and subsequently to a possible energetic use and treatment before being discharged to the atmosphere. At first starting the crucible 1 is empty and it is therefore necessary to make it rotate by means of motors 4 and then to introduce through the means provided in the fixed crown 6 (not shown) a suitable filling, such as glass powder, silica or ash resulting from previous processes. When the filling is correctly distributed on the surface, heat is introduced through the auxiliary burner or directly through the main burner until the crucible filling is brought to melt, while the water jets are operated to keep the container 1 cold. At this point the melted ash takes approximately the shape of a rotation paraboloid and care is taken, by possible additions and variations in the rotation speed, that portions of the container do not remain uncovered. Once steady conditions achieved normal operation can start, with the introduction of the waste to be ashed in replacement of all or part of the fossil fuel. Ashes resulting from combustion are added to the crucible filling and, after some time, they are discharged, through the port 5, into the container 13, by natural overfall or by way of a slight decrease in the set speed of rotation. During operation, the external wall of the container 1 is kept at moderate temperature (for example less than 100 degrees C) by the water jets, so the internal face of the container is covered by a layer of solid ash protecting the container from the thermal, chemical and erosive action of the melted mass. The flame beam may therefore attain very high temperatures, in particular by combustion in pure oxygen, without the crucible wall being damaged. The tolerable average temperatures for the melted mass and combustion gases mainly depend on the crown's characteristics. The steam in the volume 8 must be kept at slightly higher pressure than the one existing within the crucible, in a way that steam flow towards the crucible takes place and burnt gases do not escape. This situation naturally occurs when the shroud is pneumatically sealed and no steam is drawn from the duct 9 or it is drawn without the use of pumping means. The steam flow entering through the slot 20 laps the crown and can therefore exert a protective action thereon against the most aggressive products of combustion and those at high temperatures. The addition of a certain amount of steam to burnt gases can be advantageous from the viewpoint of subsequent energetic use (see previous european patent application N. 90103109.6 by the same Applicant). A furnace of this type, in the embodiment with crown provided with water jacket, has no pieces which may be damaged by rapid temperature variations, therefore quick startings and quick operative variations are possible. Figure 2 shows an embodiment with horizontal axis, wherein equal or corresponding pieces are indicated by the same references as those in the embodiment of figure 1. In the illustrated case, the container 1 has a cylindrical shape with horizontal axis b and is supported by four motor wheels 4 which hold two circular rails 18. At one end of the rotary crucible there is provided a fixed shutter 6 bearing the systems for the introduction of wastes, combustion supporter, possible additions, for inspection and sampling operations, as well as optionally the auxiliary and/or starting burner. At the opposite end, an opening 16-5 is provided for burnt gases exhaust and for the elimination of exceeding melted ash. The external shroud 7 has here a different shape, but it has essentially the same function as the shroud 7 examined in figure 1. The septum 11 is vertical and defining a space 22 wherein exceeding ashes can fall, which are cooled by means of water jets 23 before being conveyed towards the outside, for example by a scroll 24. Alternatively, instead of using a continuous system of ashes discharge, the furnace may be periodically stopped and cooled, the crucible opened at one end (for example at the opposite end to the shutter 6, which then will only have an exhaust opening for burnt gases) and all ashes discharged. For the rest, the starting and operation of this furnace are the same as described for the furnace of figure 1, therefore a detailed description is not repeated. It must be noted that, though the exemplified embodiments referred to ashing furnaces, the principles of this invention can be applied also to different combustion devices, for example to combustion chambers for gas turbines, in particular for combustion of dust-like solid substances, such as for instance saw dust.

Claims: A method to perform combustion at high temperatures (900 degrees C) within a combustion chamber using ash obtained from combustion, at the melted, semi-melted or dust-like state as a protective refractory layer on the internal walls of the combustion chamber, characterized in that said combustion chamber rotates at speed such that the centrifugal force forms and maintains a continuous layer of said ash on the internal walls of the combustion chamber. A method according to claim 1, characterized in that the external walls of said combustion chamber are cooled. A method according to claim 1, characterized in that the rotation speed of said combustion chamber is controlled. A device for the combustion of products, specially harmful toxic ones, at temperatures exceeding 900 degrees C, comprising a combustion chamber defined by a rotary container or crucible; means to rotate the container about its own axis at a given speed; a crown or shutter, fixed in correspondence to one of the axial openings of the container and bearing at least the systems for the introduction of fuel and air and/or oxygen for combustion; and means for discharging the products of combustion and the ashes, characterized in that the shape, the radial dimension, the position of the axis and the rotation speed of the container are such as to create a centrifugal force whose value is at least sufficient to maintain on the whole internal wall a continuous layer of ash at its melted, semi-melted or dust-like state. A device according to claim 4, characterized in that it comprises means to cool said rotary container on its external surface. A device according to claim 5, characterized in that said means to cool the rotary container comprise water jets directed towards the external surface of the crucible wall. A device according to claim 4, characterized in that it further comprises a shroud essentially closed towards the outside and defining a closed space in which steam generated by the cooling action of the water jets develops. A device according to claim 7, characterized in that said shroud has at least one passage for discharging at least part of the steam under pressure within said shroud toward said crucible. A device according to one of the claims 4 to 8, characterized in that, at its end opposite to the crown or shutter, a discharge opening for exceeding melted, semi-melted or dust-like ash is provided. A device according to claim 9, characterized in that a unit for the elimination of excess ash is placed at the outlet of said opening. A device according to claim 9 or 10, characterized in that means to control the crucible rotation speed are provided to cause the excess ash to be discharged. A device according to one of the claims 4 to 10, characterized in that its axis of rotation is vertical; the crucible has a cup shape, upperly closed by a crown having at least one opening for the exhaust of burnt gases; and said discharge opening for ashes is placed on the bottom and protected by an overflow projection defining, with the other parameters, the thickness of melted, semi-melted or dust-like ash layer. A device according to one of the claims 4 to 10, characterized in that its axis of rotation is horizontal and in that it has a tubular shape closed at one end by a shutter and open at the opposite end for the discharge of fumes and possibly of ashes. A device according to one of the claims 4 to 13, characterized in that it consists of a furnace for ashing wastes, specially containing toxic compounds. A device according to one of the claims 4 to 7 and 9 to 13, characterized in that it consists of a combustion chamber for a gas turbine, in particular for combustion of solid dust-like substances. A process for starting and operating a device according to one of claims 4 to 15, characterized in that it comprises the steps of: controlling the rotation of the crucible at a preset speed; introducing into the crucible a filling of glass powder, silica or ash from previous processes; waiting for a uniform distribution of dust on the internal walls of the crucible due to the centrifugal force; heating and simultaneously actuating cooling on the outside; and recording the speed of rotation possibly adding other powdered material until a continuous layer on the internal walls of the crucible is formed and maintained.

Inventors: GAIA MARIO, GAIA, MARIO
Applicants: GAIA MARIO, GAIA, MARIO
Application year: 1990
Main classification: F23G5
IPC codes: F23G5, F23M5