Method and Apparatus for Removal of Fly Ash From a Waste Incinerator With Liquid Slag Discharge

The invention relates to a method for the treatment of fly ash in a waste incinerating plant with liquid slag discharge, comprising a revolving cylindrical furnace provided with charging equipment for solid, pasty and liquid waste materials as well as sludge and which at its end has a melting slag bath, a secondary combustion chamber, and a flue gas dust separator, whereby the fly ash separated from the flue gases by means of the latter is discharged together with the slag by means of the incinerator slag discharge system.

The invention also relates to a plant for performing this method.

In order to avoid atmospheric pollution, particular importance is attached to removing dust from flue gases when burning waste materials which are rich in ballast. Whereas, when removing the dust from flue gases by means of electrostatic filters, cyclones or cloth and/or tube filters the separated dust is discharged in the dry state. When using a gas scrubber this leads to a wet dust discharge in the form of sludge. The present invention is in particular based on so-called dry flue gas dust separation.

Herein the dust separated by a flue gas dust separator is called "fly ash", whereas the dust still carried by the flue gases downstream of the gas dust separator and which finally pass through the chimney into the atmosphere is called "flue dust".

If the fly ash is discharged dry from a waste incinerator, it is in known manner stored under field conditions, either separately or together with the slag.

As is known, the composition of fly ash differs considerably from the composition of slag from municipal refuse incinerators and the composition of slag from industrial refuse incinerators which has been melted in a revolving cylindrical furnace. The following table of approximate compositions illustrates these differences:

The proportion of easily volatized heavy metals, such as e.g. zinc, manganese, or lead is many times higher in the fly ash than in the slag, while the melting point of the fly ash is generally lower than that of the slag. The water absorption capacity of molten slag is approx. 1 to 2% by weight.

In a known method of fly ash removal, the slag, together with the admixed fly ash, is dumped under field conditions. However, the fly ash, which contains in addition to its unburned organic constituents also a large proportion of water-soluble substances, as well as easily volatized heavy metals, such as e.g. tin, manganese, lead, etc. (cf. above table) can cause a contamination of the soil and the ground water. In addition, the transportation of the slag to a dump and the storage thereof in the latter can cause serious dust pollution of the surrounding atmosphere.

According to the present invention, the separated fly ash is initially conveyed into a storage tank and from the latter to a revolving cylindrical furnace or to an additional melting chamber which is also provided with a molten bath arranged in the secondary combustion chamber. The combustible constituent still contained in the fly ash is then burnt. The fly ash is melted in the molten bath and the molten fly ash is fed into the wet slag removal plant, from where it is removed with the slag via the slag discharge system.

The invention also relates to a plant for performing the method, wherein there is provided a fly ash conveying mechanism connecting the discharge side of the fly ash separator with the storage tank and at least one charging mechanism which also serves to feed the fly ash into the revolving, cylindrical furnace provided at the front wall of the furnace or a charging lance for the fly ash which extends into the revolving cylindrical furnace or the additional melting chamber and which is directed onto the molten bath located therein.

Five embodiments of the invention are explained hereinafter relative to the drawings. The drawings diagrammatically illustrate embodiments of a plant according to the present invention.

FIG. 1 shows a waste incinerating plant in accordance with a preferred embodiment of the present invention in a vertical longitudinal section.

FIG. 2 shows schematically a part of the plant of FIG. 1 for mixing the fly ash with liquid waste fuels or sludge prior to feeding it into the revolving cylindrical furnace of the plant of FIG. 1 in a flow diagram (second embodiment).

FIG. 3 shows a fragment section of the rear part of the plant of FIG. 1 with a water-cooled charging lance for introducing the fly ash into the slag molten bath of the revolving cylindrical furnace of the plant of FIG. 1 (fourth embodiment).

FIG. 4 shows a fragment section of the rear part of a plant, like that of FIG. 1, but with an additional melting chamber in its secondary combustion chamber (fifth embodiment).

FIG. 1 shows a waste incinerating plant fundamentally suitable for performing the method according to all five embodiments and designated by the reference numeral 1, whereby its secondary combustion chamber 2 is further developed in the manner described relative to FIG. 4 for performing the fifth embodiment.

The plant 1 according to FIG. 1 features a revolving cylindrical furnace 3 on whose front wall 3a are provided charging mechanisms for the furnace and on whose inwardly bent end 3b a molten slag bath 4 is provided within the furnace, together with the above-mentioned secondary combustion chamber 2, a boiler 5 connected thereto, a following apparatus 6 for dry flue gas dust separation, e.g. in the form of an electrostatic filter, optionally an apparatus for separating gaseous harmful substances from the flue gases from which the dust has been removed (not shown in FIG. 1) and a following chimney 37.

Pasty and solid waste materials in metal tanks are fed into the revolving cylindrical furnace 3 by means of a charging mechanism 39 equipped with a cap 40 and located on the furnace front wall 3a. Liquid waste material and sludge are also fed into the revolving cylindrical furnace 3 by means of a plurality of so-called fuel lances which are also arranged on the furnace front wall 3a. A supporting and pilot burner, e.g. operated with waste oil is also fitted on the furnace front wall 3a.

Due to the continuous revolving movement of furnace 3, the waste materials are transported to the rear end 3b thereof, in the course of which they ignite and gradually burn out. Due to the high temperature and the special construction of the revolving furnace shell 7 with an inwardly directed shell bend 8, the molten slag bath 4, which continuously overflows towards the rear edge 9 is formed at the rear end 3b of the revolving cylindrical furnace 3. The molten slag leaving furnace 3 drops through a slag shaft 33 into a wet slag removing apparatus 10, wherein it is quenched by water. The wet slag removing apparatus 10 also contains an endless scraper chain guided over rollers, by means of which the solidified slag can be discharged from the waste incinerating plant 1.

The necessary combustion air is fed into the revolving cylindrical furnace 3 as primary air by means of a blower provided on furnace front wall 3a. The dust-charged flue gases flow out of furnace 3 into secondary combustion chamber 2, in which they are completely burned with secondary air supplied by means of side wall nozzles 11. In the subsequently connected boiler 5, heat is removed from the flue gases. In the following dust separator 6, which can here be in the form of an electrostatic filter, the dust is separated in dry form from the flue gases and is discharged from the hopper 14 located beneath the same. If necessary, harmful gases contained in the flue gases can be separated in a wet scrubber, not shown, connected behind the dust separator 6.

In the plant according to FIG. 1 the fly ash is supplied from hopper 14 of dust separator 6 to a storage tank by means of a conveying mechanism which is not shown in FIG. 1. Depending on the size of the plant, the fly ash is fed either manually or automatically from the storage tank into metal containers. The filled containers are sealed with a sheet metal lid which, to avoid internal overpressures or explosions, is provided with at least one vent. The metal containers filled with fly ash in this way are introduced into the revolving cylindrical furnace 3 by means of the charging mechanism for solid waste arranged on furnace front wall 3a, together with said solid waste. The sealed metal containers pass through the furnace 3 up to the molten slag bath 4 arranged at its rear end 3b. Any still unburned combustible fly ash is thereby substantially completely burned. When they reach the molten slag bath 4 the metal containers melt. Since the melting temperature of the fly ash is lower than that of the metal casing container, when the latter melts, its content, i.e. the fly ash, is already in a pasty or even liquid state. As a result, no finely divided ash particles from the content are entrained by the flue gas flow passing over the molten slag bath 4 into secondary combustion chamber 2 and the following plant units (boiler 5, dust separator 6).

According to FIG. 2 in the plant of FIG. 1, the fly ash is fed by means of a conveying mechanism from the storage tank connected to hopper 14 of the flue gas dust separator by means of a feed mechanism into a container 15 for liquid waste or sludge with a fitted mixing device 16, the fly ash being simultaneously cooled. The fly ash supplied from above via a proportioning worm 17 is mixed with the waste fuels or sludge carried in the fuel circuit via container 15 by mixing device 16. To prevent fly ash from being deposited on the bottom 18 of the container 15, the content of the latter is kept continuously in motion by means of a stirring impeller 20 driven by a motor 19. A partial flow of the mixture formed from the waste fuel or industrial sludge and the fly ash which is circulated by means of a circulating pump 21 through a pipe 22 over mixing device 16 and container 15 is branched off from the circuit at a point 23 and is fed into the revolving cylindrical furnace 3 through a pipe 24, in a quantity controlled by a regulating valve 25, by means of the fuel lance for liquid waste or sludge located on the front wall 3a of furnace 3 (cf. FIG. 1) and therein burned. The resulting slag is melted again in the molten slag bath 4 provided at the rear end 3b of furnace 3 (cf. FIG. 1).

In a third embodiment of the present invention, the fly ash is supplied from a storage tank connected by means of a feed mechanism to the hopper 14 of the flue gas dust separator 6 (cf. FIG. 1) via a proportioning worm to a pelletizer and after the addition of water and binders is processed therein to form a coarse-grained material or pellets. These pellets are placed in open containers and are introduced into the revolving cylindrical furnace 3 by means of the charging mechanism located on the front wall 3a of furnace 3 for solid waste and barrels or tanks or by means of a special charging port provided especially for feeding in pellets. The pellets pass through the revolving cylindrical furnace 3 together with the solid and/or liquid waste, whereby they ignite and burn, their slag finally being melted in the molten slag bath 4 located at end 3b of the furnace 3.

In a fourth embodiment of the present invention, the fly ash is conveyed by means of a feed mechanism from the hopper 14 of the dust separator 6 to a storage tank. The fly ash removed from the latter is then supplied by means of a proportioning worm in controlled quantities to a water-cooled charging lance 27 via a line 38 (cf. FIG. 3). The fly ash is entrained in the lance 27 by the injector action of an air jet which, as shown in FIG. 3, is fed by means of a blower 28 via an air line 29 into the charging lance 27, which extends in a forward direction and nearly at right angles through the secondary combustion chamber 2. The lance discharges the fly ash at high speed into the molten slag bath 4 of the revolving cylindrical furnace 3. Charging lance 27 is so close to the molten slag bath 4, that most of the fly ash penetrates the bath 4, due to the kinetic energy of its dust particles where it melts. In the case of very fine dust, the dropwise addition of water to the delivery air flow via a line 30 connected to the air line 29 (cf. FIG. 3) can be used to increase the size of the dust particles, thereby increasing kinetic energy by raising the mass moment of inertia of the individual particles. As a result, the penetration depth of the dust particles into molten slag bath 4 is increased. The fly ash is melted in bath 4 and is bound into the molten slag. In the slag bath 4, the molten fly ash-slag mixture again finds an overflow and flows via the rear inner furnace edge 9 into the wet slag removal apparatus 10 (cf. FIG. 1), from where the mixture is discharged from plant 1 in quenched and solidified form.

The fifth embodiment of the present invention is shown in FIG. 4. Here, the fly ash is fed from a storage tank filled by means of a feed mechanism from the hopper 14 of the flue gas dust separator 6 (cf. FIG. 1) to a charging lance 27a by means of a proportioning worm via line 38, and is entrained by the injector action of an air jet, which, as for the 4th embodiment above, is introduced into charging lance 27a at the rear by means of a blower 28 via a line 29. The fly ash is discharged at high speed into an additional fly ash molten bath 31a, which is only for fly ash. For this purpose, the secondary combustion chamber 2 of plant 1 is provided with a separate melting chamber 31 for the fly ash. An opening 32 forms with its lower edge an overflow for the fly ash molten bath 31a. The molten fly ash falls through shaft 33, into which also falls the molten slag from the revolving cylindrical furnace 3, into the common wet slag removal apparatus 10, from which the quenched solidified slag and fly ash is discharged from the plant by its slag discharge system. The fly ash melting chamber 31 is provided with a burner 34 for an additional fuel, e.g. waste oil or liquid waste fuels, in order to produce the heat necessary for melting the fly ash. It comprises tubular boiler walls 35 which are connected to boiler 5 (cf. FIG. 1). Boiler tubes 35 are provided with a high temperature-resistant plastic refractory material. The combustion gases and delivery air, insofar as the latter does not participate in the combustion of the waste oil or liquid waste fuels, escape from the fly ash melting chamber 31, which is sealed at the top by an ash collecting grate 36, into the secondary combustion chamber 2. As in FIG. 3, a connection 30 for feeding in water in droplet form is provided on delivery air line 29.

The method and/or the plant for practicing the present invention as described hereinbefore relative to five embodiments and the drawings are based on dry flue gas cleaning, e.g. by means of an electrostatic filter. However, the second embodiment according to FIG. 2, in which the separated fly ash is mixed with liquid waste materials or sludge, can also be realized with wet flue gas cleaning, whereby the fly ash sludge obtained in the flue gas scrubber is fed into the storage tank and the following container.