DE3433913A1 - Process and apparatus for ashing sewage sludge - Google Patents

Abstract

A process is described in which sewage sludge, which in one procedure by means of spray-drying which is arranged immediately above a fluidised-bed furnace, is first dewatered and then, after conversion into solid particles, is incinerated in the fluidised-bed furnace situated below, is thus ashed in an environmentally friendly manner. The remainder of the solid particles carried over with the combustion air are separated off in a downstream hot de-dusting plant and then fed to the fluidised-bed furnace. Under suitable process conditions, the plant can also be utilised for energy production and the sewage sludge ashing can be carried out intermittently as a subsidiary function.

Description

Method and device for incinerating sewage sludge

The invention relates to a method and a device for environmentally friendly Incineration of sewage sludge.

In addition to a great deal of water, urban wastewater contains a wide variety of materials Solids collected in sewage treatment plants and fed to digestion towers after enrichment where they are subjected to fermentation under anaerobic conditions, whereby Sewage gas with a high methane content is produced. This process leaves a very watery one Sewage sludge, its further use or disposal increasing difficulties prepares. On the one hand, the drainage takes place before any further treatment must, big problems, on the other hand, limits the content of harmful polluting Substances have a very strong effect on further use, for example for fertilization purposes. the direct combustion is almost always ruled out due to the high water content. Consistently Combustion or incineration processes are therefore expensive drying processes upstream. After it became known that high combustion temperatures of 7000 up to 9000 C (973 - 1173 K) toxic heavy metals in oxide form in Released into the atmosphere, the possibilities for incinerating sewage sludge became further narrowed.

First transferring sewage sludge to air-dry condition and then converting it Degassing in externally heated retort furnaces is - according to the application CH-A 478 216 known.

Without pyrolysis only with combustion of the in direct current with the Combustion gases of dried sewage sludge works according to DE-A-21 42 439.

According to AT-B-266 723 it is known to remove sewage sludge after it has been sucked up a large part of the water by carrier material, e.g.

Sawdust or peat to be fed to a drum dryer and the resulting Use product as fuel.

From CH-A-499 750 a method of incinerating sewage sludge is known, in which the same is fed to a rotary kiln after being centrifuged several times.

A pyrolysis together with lumpy garbage in an indirectly heated rotating drum is described in DE-B-24 32 504. For the incineration of pure sewage sludge this method is not suitable. This also applies to a process for the conversion of high polymers Waste materials such as polyethylene, polypropylene and the like by thermal decomposition associated with the production of gaseous or liquid fuels according to DE-B-22 22 267.

From a publication in the VDI-Nachrichten of October 26, 1979 a method known in which the sewage sludge in a drum furnace dried and then offered for fertilization purposes.

DE-PS-27 53 537 describes a pressure incineration of wet waste, especially of sewage sludge, which makes a fairly good use of the incineration released energy in a, however, quite complex apparatus under one Pressure of more than 11 bar should take place.

It is common to all processes that sewage sludge is usually processed takes place in several operations, ultimately leads to products that because of their Contaminants or their heavy metal constituents can only be used to a very limited extent convert to oxide form at combustion temperature and then pollute the environment when they enter the atmosphere.

The invention was based on the object of something vo in the air to dewater dried, but still pumpable sewage sludge in one operation and to be incinerated at the lowest possible temperature in an environmentally friendly manner.

According to the invention, this is achieved in that the sewage sludge is finely distributed, e.g. atomizing nozzles is fed that the atomized sewage sludge in the upper Area of a drying shaft is entered that the atomized sewage sludge is passed in free fall to the bottom area of the drying shaft and that in A fluidized bed furnace is operated in the bottom area of the drying shaft, the Exhaust gases are led through the drying shaft in countercurrent to the sewage sludge.

The incineration necessary for incineration is carried out in a fluidized bed furnace, known for its relatively low combustion temperatures, with the resulting combustion or smoke gases located immediately above it Spray drying shaft are fed.

So that in countercurrent from the atomized sewage sludge by evaporation of the water contained in the resulting solid particles to the greatest possible extent due to the effect of gravity in the fluidized bed of the furnace below can fall, the cross-section of the drying shaft is much larger than the same of the fluidized bed shaft. The flow rate of the ascending Flue gas is reduced in this way. The solid particles that were nevertheless carried away are separated in an insulated hot dedusting system and via the conveying air line fed to the fluidized bed. It can be assumed that the spray drying as well as the fluidized bed combustion, as well as the necessary heat recovery systems, superheater, etc., can be considered state of the art and no further description need.

The drawing from which further inventive features emerge, shows the system required for the process in the scheme: In the combustion shaft A layer of quartz sand 8 is introduced into a fluidized bed furnace 7 for start-up. This rests on a perforated grate 11 Start up was heated by external heating, for example also a gas flame, this is Sand set in a whirling motion. Once the combustion temperature has been reached, finely ground coal from the bunker 14 is temporarily added to the conveying air 12. With the help of the preheated combustion air 10, this coal is in the fluidized bed 8 burned. The combustion gases heat the drying shaft above 5 on.

After the entire system has been sufficiently heated, it can still be pumped Sewage sludge is supplied to one or more atomizer nozzles 3 via the supply line 1. The finely divided sewage sludge first sinks down in the form of drops (4). Here it releases its water content to the hot combustion gases.

Part of the resulting solid particles continue to sink down and are captured and burned at the surface 6 of the fluidized bed. Another part is caused by the combustion gases rising upwards in countercurrent promoted. In an isolated 16 hot dedusting 17, the last solid particles are deposited and fed through line 13 of the conveying air 12, which they the Incineration.

The flue gases reach 15 for heat recovery, which in itself can serve as a known way of preheating the combustion air and the conveying air, and then in the fireplace.

It depends on the quality of the sewage sludge whether and in which one Extend the supply of additional fuel via the Bunker 14 can be adjusted or throttled. The modest proportion of combustible parts in sewage sludge can usually not fully cover the energy demand, which is why mostly a coal addition will remain necessary.

The ash extractor 9 is indicated schematically and belongs to fluidized bed combustion technology.

In the same way as adding coal, you can add aggregates are added to the incineration, whereby, for example, the emergence of environmentally harmful Fractions in the combustion gases (e.g. sulfur dioxide) completely or partially prevented can be. If the system is designed for a permanent and larger addition of coal, an additional energy generation is possible during the incineration of sewage sludge is done intermittently after a seizure.

Claims (8)

Claims: 1. Process for the environmentally friendly incineration of Sewage sludge, characterized in that the sewage sludge is finely distributed, e.g. atomizing nozzles (3) is fed that the atomized sewage sludge in the upper area of a drying shaft (5) it is entered that the atomized sewage sludge in free fall to the bottom area of the drying shaft and that in the bottom area of the drying shaft a fluidized bed furnace (7) is operated, its exhaust gases in countercurrent to the sewage sludge be passed through the dryer shaft. 2. The method according to claim 1, characterized in that the fluidized bed furnace (7) Aggregates are fed. 3. The method according to any one of claims 1 and 2, characterized in that that the exhaust gases emerging from the drying shaft (5) in a dedusting device (17) are passed and that the dust accumulating in the dedusting device the fluidized bed furnace (7) is fed again. 4. Device for performing the method according to the claims 1 to 3, characterized by an approximately perpendicular drying shaft (5), the upper area of which has fine distributors, e.g. atomizing nozzles (3) for the Dry has nungsschacht pumpable sewage sludge and in which A fluidized bed furnace (7) known per se is arranged in the bottom area. 5. Apparatus according to claim 4, characterized in that the drying shaft (5) A dedusting device (17) is connected downstream in the direction in which the exiting exhaust gases are drawn off is. 6. Device according to one of claims 4 and 5, characterized in that that a conveyor line (12) for aggregates, such as coal dust, chemicals and. Like., Is provided, which opens into the operating range of the fluidized bed furnace (7). 7. Device according to one of claims 4 to 6, characterized in that that one of the dedusting device (17) outgoing conveyor element (13) for incurred Dust flows into the operating area of the fluidized bed furnace (7). 8. Device according to one of claims 4 to 7, characterized in that that the cross section of the drying shaft (5) is larger than the cross section of the operating area the fluidized bed furnace (7).