METHOD OF INCINERATING WASTE MATERIAL, AND INCINERATOR FOR CARRYING OUT THE METHOD

Abstract: In a method of incinerating waste material in a first combustion zone (2, 3), in which uncombusted or partly combusted material from said first zone (2, 3) is subjected to additional incineration in a second combustion zone (6), the main novel feature is the use of a fluidized-bed furnace (6), preferably with circulating sand and a separator (7) to remove ashes, slag and/or other incombustible matter. With this arrangement, increased adaptability of the second combustion zone to variations in the amount per unit time of material received from the first combustion zone is achieved.

Description: 1 METHOD OF INCINERATING WASTE MATERIAL. AND INCINERATOR FOR CARRYING OUT THE METHOD TECHNICAL FIELD The present invention relates to a method of incinerating waste material, said method being of the kind set forth in the preamble of claim 1. BACKGROUND ART Methods of the kind referred to above are disclosed in the European patent specifications Nos. 536,268 and 573,756. According to both of these specifications, a grate arrangement is used for the first combustion zone. According to the former specification, a rotary kiln is used for the second combustion zone, whilst according to the latter specification, an ash-burning vibratory conveyor constitutes the main component of the second combustion zone. If, with arrangements of the types referred to above, the first combustion zone is used to carry out an amount of incineration work per unit time below and up to a maximum determined by its capacity, and the second zone is used to deal with peak loads beyond the capacity of the first zone, then it is important that the second zone be as flexible and capable of adjustment as possible without requiring costly and/or space-consuming equipment. As is well-known, rotary kilns are rather costly and require considerable space as well as means to support them for rotation, and cannot quickly be adjusted to adapt to changes in the load, i.e. the amount per unit time and nature of the material to be treated after the initial processing in the first combustion zone. 2 An ash-burning vibratory conveyor is probably considerably less costly and more adaptable than a rotary kiln, but - like the latter - it suffers from the disadvantage of having moving mechanical parts subjected to high temperatures. DISCLOSURE OF THE INVENTION It is the object of the invention to provide a method of the kind referred to above, with which it is possible to effect a highly efficient two-stage incineration well adapted to the load being presented at any moment in time whilst using relatively low-cost and reliable equipment, and this object is achieved by, according to the invention, proceeding in the manner set forth in the characterizing clause of claim 1. By using a fluidized-bed furnace in the second, final combustion zone, it is possible to achieve a very high degree of flexibility and adaptability. Thus, the fluidizing gas may be used as a vehicle for effecting adjustments of the combustion process taking place in the second combustion zone. It is e.g. possible to vary the intensity of combustion by varying the volume per unit time of the fluidizing gas and /or its composition by varying the percentage of e.g. oxygen, carbon dioxide or nitrogen. Another advantage of a fluidized-bed furnace is that - at least in principle - it comprises no moving parts with the exception of the particles of the fluidized bed itself, and it can also be constructed at a relatively low cost as compared to a vibratory ash-burning conveyor and - not least - a rotary kiln. The present invention also relates to an incinerator for carrying out the method of the invention. This incinerator is of the kind set forth in the preamble of claim 8, and according to the invention it also comprises the features set forth in the characterizing clause of this claim 8. Further advantageous embodiments of the method and the incinerator, the effects of which - beyond what is obvious - are explained in the following detailed part of the present description, are set forth in claims 2-7 and 9-15, respectively. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed part of the present description, the invention will be explained in more detail with reference to the exemplary embodiments of an incinerator according to the invention shown in a highly diagrammatic manner in the drawings, in which Figure 1 is an overall view of the incinerator, taken mostly as a longitudinal vertical section, Figure 2 at an enlarged scale shows an important furnace component of the incinerator shown in Figure 1 , and Figure 3 is an overall view, taken in the same manner as Figure 1 , but showing an altered embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS The incinerator shown in Figures 1 and 2 comprises the following well-known components: - an inlet chute 1, through which waste material to be incinerated (not shown) is introduced, and the lower end of which, possibily through infeed means (not shown), opens into a furnace chamber 2, the bottom of which is constituted by a grate 3 for supporting the waste material being incinerated during the combustion process, said furnace chamber 2 opening into a boiler 4 for extracting heat from the flue gas received from the furnace chamber 2, and finally a flue-gas cleaner 5 for removing dust, noxious gases and other components from the flue gas received from the boiler 4. The boiler 4 and the flue-gas cleaner 5 are shown in "block-diagram" form, as they can be constructed in any suitable manner, known or unknown, to fulfil their functions. 4 The functioning of an incinerator with the components mentioned above is well known and needs no further explanation. It should, however, be mentioned that in previously known incinerators of this kind, some kind of collector for collecting ashes and/or slag from the lower end of the grate 3 is provided. During normal operation of such previously known incinerators, it may happen every now and then that some of the waste material is not completely burned-out when leaving the grate 3, and to prevent such partly burned material from contaminating the environment it is necessary to return it to the inlet end of the grate for another pass. It is, however, difficult to separate partly-burned material from e.g. lumps of slag or large pieces of incombustible material, and to ensure that only the material requiring another pass is returned to the inlet end of the grate. It is, of course, possible to ensure that no material leaves the grate without being completely burned-out, but this would require a very long grate or adjusting the incineration process to ensure that the material to be incinerated rests upon the grate for a considerable period of time - in either case a costly arrangement. The problem referred to above is solved by, in addition to the first combustion zone represented by the space above the grate 3, providing a second combustion zone. As mentioned initially, previous incinerators have comprised rotary kilns or ash- burning vibratory conveyors as active parts of the second combustion zone. According to the present invention, however, the active component of this second zone is constituted by a fluidized-bed furnace 6, preferably of the type comprising a body of sand or similar refractory particulate material constantly agitated by suitable means (not shown), such as jets of gas that may be heated or combustible to produce heat within the furnace 6. The sand in the fluidized-bed furnace 6 is not only being constantly agitated by fluidizing gas, that may be air, through a plenum 17 and the perforated bottom of the furnace 6, but is also preferably kept in constant circulation by e.g. a lifting conveyor 18, partly through the fluidized bed itself, partly through a separator 7 having an outlet 8 for ashes and/or slag, the sand being returned to the bed. As will appear from the arrows shown in Figure 2, the sand in the fluidized bed itself moves in an upstream direction, i.e. towards the 5 place, where material from the grate 3 falls onto the bed. This makes it possible, should need arise, to remove lumps of agglomerated slag or other large objects immediately after having fallen from the grate 3, and smaller lumps will reach the outlet leading to the separator 7 without travelling along the full extent of the fluidized bed. This does not, however, prevent combustible material from being distributed in the fluidized bed. When operating an incinerator of this type comprising a grate like the grate 3 shown, some of the material resting and/or moving downwardly on the grate will unavoidably pass through the openings in the grate into the space 9 below the grate. These "grate droppings" will, likewise unavoidably, comprise some incompletely burned material, and in earlier practice this material was returned to the inlet end of the grate 3 or even to the inlet end of the chute 1 , or else directly into the clinker outlet. According to a preferred feature of the present invention, the grate droppings are conveyed in a downstream direction to the fluidized-bed furnace 6 by means of a conveyor, in the exemplary embodiment shown as a worm conveyor 10, but other types of conveyors could, of course, be used, such as grate-belt conveyors, scraper conveyors, of the dry or wet type, or simply an inclined chute or skid. In addition to the inlet chute 1 , the incinerator shown in Figure 1 comprises a number of auxiliary inlets, i.e. a first auxiliary inlet 11 leading into the side of the inlet chute 1 at some distance below its top, - a second auxiliary inlet 12 leading into the furnace chamber 2 in the roof or in the side wall above the grate 3, and a third auxiliary inlet 13 leading into the space above and placed vertically above the fluidized-bed furnace 6. The first, second and third auxiliary inlets 11 , 12 and 13, respectively, are mainly intended for the introduction of sludge, such as sewage sludge, into the incinerator. Which of these three auxiliary inlets to choose for this purpose will in each case depend on the operating conditions, such as the quality and quantity of the "normal" waste material being introduced to the inlet chute, as well as on the 6 quality, such as combustibility and water content, and quantity of the sludge to be incinerated. If, for instance, the temperature on the grate 3 is very high, the heat present here can be exploited to evaporate water from sludge being introduced through the second auxiliary inlet 12, at the same time lowering the temperature to an acceptable operating level. The same applies to the third auxiliary inlet 13 in relation to the fluidized-bed furnace 6. The second auxiliary inlet 12 may be used for introducing waste material that for some reason or other is considered unsuitable for feeding through the chute 1. Such waste material could comprise waste material from hospitals etc., as well as sludge of various types. The exemplary embodiment shown in Figure 3 uses a rotary kiln 14 as the active part of the first combustion zone. Naturally, the use of a rotary kiln instead of the furnace chamber 2 of Figure 1 with the grate 3 and the space 9 with the conveyor 10 makes these latter components superfluous, and an auxiliary inlet corresponding to the second auxiliary inlet 12 is simply not possible. Otherwise, the exemplary embodiment of Figure 3 corresponds to that of Figure 1 , and its way of functioning should need no further explanation. Unless precautions are taken, the use of the fluidized-bed furnace 6, when supplied from outside with fluidizing gas, could cause the pressure within the incinerator, including the boiler 4 and the flue-gas cleaner 5, to rise above that of the surrounding atmosphere. According to a preferred embodiment of the present invention, this problem can be avoided by providing the flue-gas cleaner 5 with an exhaust fan 15 adapted to keep the pressure in the spaces upstream of it below atmospheric. The cleaner 5 also comprises an outlet 16 for dust separated from the flue gas. By carrying out the invention according to the principles described above, the first and second combustion systems in combination can be fed with a full variety of municipal and industrial waste types, e.g. completely filled waste bags, solid waste of similar bulk size, shredded waste, sludges, hospital waste, waste oil and bio-fuels, and at the same time it can be ensured that the slag and/or ashes 7 discharged from the incinerator after combustion will have a content of unburnt matter typically below 0.5%. LIST OF PARTS 1 inlet chute 2 furnace chamber 3 grate 4 boiler 5 flue-gas cleaner 6 fluidized-bed furnace 7 separator 8 outlet (for ashes/slag) 9 space 10 worm conveyor 11 first auxiliary inlet 12 second auxiliary inlet 13 third auxiliary inlet 14 rotary kiln 15 exhaust fan 16 dust outlet 17 plenum 18 lifting conveyor

Claims: 9 CLAIMS 1. Method of incinerating waste . material, said method being of the kind comprising the following steps a to e: a) introducing the waste material into and subjecting it to combustion in a first combustion zone, b) from said first combustion zone conducting gaseous combustion products through heat-exchange means, and removing remaining solids, c) from said first combustion zone transferring said remaining solids to a second combustion zone, d) in said second combustion zone subjecting said remaining solids received from said first combustion zone to further combustion, and e) from said second combustion zone e1) conducting gaseous combustion products through heat-exchange means, and e2) removing solids remaining after said further combustion, characterized by f) for said second combustion zone using a fluidized-bed furnace. 2. Method according to claim 1 , characterized by a) for said second combustion zone using a fluidized-bed furnace of the kind containing particles of refractory material, e.g. sand, and b) continuously moving said particles together with any ashes and/or slag out from said fluidized-bed furnace through means for separating said ashes and/or slag from said particles and returning the latter to said fluidized-bed furnace. 3. Method according to claim 1 or 2 , characterized by a) for said first combustion zone using a furnace with a grate, and b) using means below said grate for receiving material falling through the openings in said grate and transferring it to said second combustion zone. 4. Method according to claim 1 or 2, characterized by for said first combustion zone using a rotary kiln. 10 5. Method according to any one or any of the claims 1-4, characterized by introducing material not suitable for combustion in said first combustion zone directly into said second combustion zone. 6. Method according to any one or any of the claims 1-5 and comprising the incineration of flowable water-containing material, such as sludge from sewage or industrial processes, characterized by introducing said flowable material in a manner a) facilitating the inflow of solids to said first combustion zone, and/or b) reducing the temperature on said grate, and/or c) reducing the temperature in said fluidized-bed furnace. 7. Method according to any one or any of the claims 1-6, characterized in that the pressure within the first and second combustion zones and spaces communicating directly therewith is kept below the pressure of the surrounding atmosphere. 8. Incinerator for carrying out the method according to any one or any of the claims 1-7 and being of the kind comprising a) an inlet chute (1) or equivalent means for introducing waste material to be incinerated, said inlet chute (1) feeding b) a first combustion zone (2,3; 14) with means for supporting the material being incinerated and means (4), such as a boiler, for extracting the heat of combustion in the gases produced by the combustion, and c) a second combustion zone (6) adapted to receive solid residue from said first combustion zone (2,3; 14) and to incinerate same, characterized in d) that said second combustion zone comprises a fluidized-bed furnace (6). 9. Incinerator according to claim 8, characterized in that said fluidized-bed furnace (6) is of the kind containing a filling of refractory particulate material, such as sand, and is provided with means for agitating said particulate material and circulating it through a circuit comprising the furnace (6) itself and a separator (7) 11 for separating other material, such as ashes and/or slag and/or other incombustible matter, from said particulate material, and removing (8) said other material from said circuit (6,7). 10. Incinerator according to claim 9, characterized by such an arrangement of said fluidized-bed furnace (6), that said particulate material in the furnace itself moves in a direction towards said first combustion zone. 11. Incinerator according to any one or any of the claims 8-10, characterized in that said first combustion zone comprises a rotary kiln (14) adapted to allow or cause the material being incinerated to move towards said fluidized-bed furnace (6). 12. Incinerator according to any one or any of the claims 8-10, characterized in that said first combustion zone comprises a furnace chamber (2) with a grate (3) that is inclined downwards in the downstream direction or otherwise adapted to cause or allow the material being incinerated to move towards said fluidized-bed furnace (6). 13. Incinerator according to claim 12, characterized by means (10) in the space (9) below said grate (3) to convey or allow movement of material having fallen through openings in said grate towards said fluidized-bed furnace (6). 14. Incinerator according to any one or any of the claims 8-13, characterized by at least one auxiliary inlet (11,12,13) adapted to introduce flowable material, such as sludge, either (11) into said chute (1) and/or (12) above said first combustion zone (2,3) and/or (13) above said fluidized-bed furnace (6). 15. Incinerator according to any one or any of the claims 8-14, characterized by means (15) to keep the pressure within said first and second combustion zones and in spaces communicating directly therewith below the pressure in the surrounding atmosphere.

Inventors: NIELSEN JOERGEN VITUS, NIELSEN, JOERGEN, VITUS
Applicants: ANSALDO VOLUND AS, NIELSEN JOERGEN VITUS, ANSALDO VOELUND A/S, NIELSEN, JOERGEN, VITUS
Application year: 1998
Main classification: F23G5
IPC codes: F23G5, F23G7