EP0716264A2 - Method and installation for sludge combustion - Google Patents

Abstract

In a process for the combustion of sewage sludge, in which the sewage sludge is dried by means of a dryer (3) heated by a heat transfer medium and then burned in a furnace, in particular cycloidal combustion (6), in which the heat transfer medium is heated by the combustion and in which the flue gases of the furnace are subjected to dedusting and flue gas cleaning, for the unloaded removal of at least some of the solids attributable to the combustion and gas purification, that a basic additive (9, 10, 12) is added to the furnace and then heat to the flue gas (13) for the drying of the sewage sludge with cooling of the flue gases to a first temperature above the condensation temperature of volatile heavy metals is withdrawn, then a first dedusting (17) takes place, then the flue gas again heat (19) while cooling the flue gas to a lower level, however above the cooling limit temperature de s the temperature of the flue gas is withdrawn, then further basic additive (22) is added to the flue gas and the flue gas is then fed to a filtering dedusting device (23).

Description

The invention relates to a process for the combustion of sewage sludge, in which the sewage sludge is dried by means of a dryer heated by a heat transfer medium and then burned in a furnace, in particular cycloidal combustion, in which the heat transfer medium is heated by the combustion and in which the flue gases from the furnace dedusting and flue gas cleaning.

Such a method is known from DE-Z.
BWK / TÜ / UMWELT / SPECIAL, October 1993, p. E 32-E43, especially p. E40 known. In the known method, the dried sewage sludge granulate is fed into a cycloid furnace in doses and burned in its swirl flow. In addition, the non-condensable vapor components are passed into the combustion chamber for thermal deodorization. The flue gases emerging from the combustion chamber flow into a waste heat boiler, where they are cooled to approx. 200 ° C. The steam generated in the waste heat boiler is fed to the dryer as heating steam. A fabric filter for separating the sewage sludge ash is arranged behind the waste heat boiler and the ash freed from it Flue gases are then subjected to a flue gas wash and leave the plant cleaned.

Cycloidal firing in the description and in the claims is understood to mean a firing in which fuel is burned in a swirl flow. Vortex air is preferably fed into the combustion, as shown in Figure 3 on page E36.

In the known method, cooling takes place in the waste heat boiler to 200 ° C., so that the filter ash drawn off in the fabric filter is contaminated with pollutants, in particular condensable heavy metals and their compounds, as well as products of incomplete combustion.

It is the object of the present invention to provide a generic method in which at least some of the solids due to the combustion and gas cleaning can be removed from the method essentially without contamination.

This object is achieved in that a basic additive is added to the furnace, then heat is removed from the flue gas for drying the sewage sludge while the flue gases are cooled to a first temperature above the condensation temperature of volatile heavy metals, followed by a first dedusting, then again from the flue gas Heat is extracted while cooling the flue gas to a lower temperature, but above the cooling limit temperature of the flue gas, then further basic additive is added to the flue gas and the flue gas is then fed to a filtering dedusting device.

The flue gases are preferably cooled from the combustion temperature to a temperature of 400-500 ° C, more preferably 420-470 ° C, even more preferably 450 ° C, and the cooling before the further addition of basic Additive to a temperature of 100-150 ° C, it being assumed that the furnace has a temperature of 1000-1100 ° C, preferably 1050 ° C.

After the first cooling, a mixture of flying dust and the at least partially reacted basic additive is separated in the first dedusting. This dust has essentially no condensed heavy metals and their compounds. There are essentially no products of incomplete combustion, since the heavy metals only condense at lower temperatures or the products of incomplete combustion only regress in the course of a de novo synthesis. Consequently, this product can e.g. B. can be used as a soil conditioner or as an additive in the construction or building materials industry. In this way, the sewage sludge is at least partially recovered. Only the dust drawn off during the second dedusting, which contains practically all condensed heavy metals, but hardly any organics, has to be disposed of.

If there is still a residual dust, acid pollutant gas component, uncondensed mercury and organics after the addition of further additive after the second dedusting, it is advisable to add a fine cleaning filter.

Subclaims 2-9 relate to further refinements of the method according to the invention.

The invention is also directed to a plant for the combustion of sewage sludge with a dryer for drying the sewage sludge, a combustion device downstream of the dryer, in particular cycloid firing, a heat sink downstream of the firing for the provision of heat for sewage sludge drying, dust separation and flue gas cleaning.

According to the invention, the system is characterized in that a device is provided for the supply of a basic additive for firing, that the dust separation is followed by a further heat sink and this is another device for the supply of basic additive, and that the further device is followed by a filtering dust separation .

FIG. 1

eine Verfahrensführung mit indirekter Klärschlammtrocknung und

FIG. 2

eine Verfahrensführung mit direkter Klärschlammtrocknung.

The invention will now be explained in more detail with reference to the accompanying figures. Show it:

FIG. 1

a procedure with indirect sewage sludge drying and

FIG. 2nd

a procedure with direct sewage sludge drying.

Mechanically pre-dewatered sludge with a dry matter content above 20% TS, but preferably above 25% TS (mass percent), is placed in a bunker 1. From there, the metered conveyance of the sludge takes place with the aid of a sludge pump 2, which, for. B. as a piston pump, eccentric screw pump, in an indirectly heated dryer 3. If a thin sludge with 3 - 5% TS is to be used, the dryer is a mechanical pre-dewatering, eg. B. upstream a decanter.

A ploughshare mixer is shown as the dryer 3, the double jacket 3a of which is heated with steam. During the drying process, the sludge is constantly whirled up and divided. The residence time of the sludge is set so that the sludge is brought to over 80% DM, preferably 85-92% DM. This dryer also works optimally when the sewage sludge passes through the glue phase during drying. As a result, it is not necessary to mix the sewage sludge with sludge that has already dried before entering the dryer. The glue phase is determined by a dry matter content of 50 - 60% dry matter, within which the sewage sludge tends to stick together to such an extent that most dryer types including a disc dryer cannot process such a sludge. Such systems therefore require the dry sludge recycle also shown in DE-ZS E40, Figure 6, which can be omitted here. In addition to steam, an indirectly heated dryer can also be operated with heated thermal oil or with flue gases. It is only important that the drying takes place indirectly.

After exiting the dryer 3, the dry sludge is thrown into a buffer tank 4. From there, the task takes place with the aid of a metering device 5, which is shown in FIG. 1 is shown as a motor-driven discharge screw into a cycloid combustion chamber 6. The cycloid combustion chamber is supplied with primary air PL via a blower 7 and a line 7a such that the primary air swirls into the combustion chamber via the lower combustion chamber cone 6a. A start-up and support burner 8 and a secondary air supply 9 including a blower 10 are assigned to the combustion chamber. The combustion takes place automatically, i. H. the burner 8 is usually out of operation.

A basic additive, preferably hydrated lime, from a silo 12 is added to the secondary air SL via an injector 11a. In this way, a first dry flue gas cleaning stage is implemented, since the combustion chamber 6 has a very balanced temperature distribution, preferably in the range of 900-1050 ° C. With this temperature distribution, very good integration of acidic noxious gas components, in particular SO _x , can be achieved with the formation of the corresponding calcium compounds.

The flue gases then flow through a waste heat boiler 13 together with the fly ash formed during the combustion and the at least partially reacted additive in a water vapor circuit 14 with flow 14a and return 14b including a circuit pump 15 and a drum 16 in the from FIG. 1 apparent way is switched on by means of a feed line 13a from drum 16 and a discharge line 13b to drum 16. It is preferred to design the waste heat boiler as a saturated steam smoke tube boiler, as shown in FIG. 1 is shown. Steam is applied to the double jacket 3a of the dryer and the condensate that forms is returned to the drum via the pump 15.

After the flue gases in the waste heat boiler 13 have cooled to a first temperature, preferably 450 ° C., the flue gases flow through a dust separator, which is preferably designed as a high-temperature cyclone 17. In the high-temperature cyclone 17, 90%, but preferably even more, of the entrained mixture of fly dust and reacted additive is separated out as the first product 18. This product contains practically no condensed heavy metals and their compounds, as well as no products of incomplete combustion.

The high-temperature dedusting in the cyclone 17 is followed by a heat exchanger 19 in which heat is transferred to water, steam, air or the like. For example, water from the steam circuit associated with the waste heat boiler 13 can be preheated.

The flue gas, cooled, for example, in the heat exchanger 19 to 200 ° C., is then fed to a quench 20, with the aid of which fresh water 21 is added to cool the flue gas to the operating temperature of a downstream, second, dry flue gas cleaning stage. The operating temperature of this flue gas cleaning stage is above the cooling limit temperature (adiabatic saturation temperature) of the flue gases, preferably in the range of 100-150 ° C. To form the second dry flue gas cleaning stage, over an injector 11b, likewise acted upon by the blower 10, is blown in upstream of a fabric filter 23 via line 22, a basic additive, preferably calcium hydrate. The second flue gas cleaning stage represents fine cleaning with regard to the acidic harmful gas components and the substance. A filter auxiliary layer is formed in the fabric filter 23 from the separated solids. The water accumulation in the flue gas due to the quenching is conducive to the separation performance of the fabric filter 23.

The solids separated in the fabric filter can be recirculated back into the flue gas stream upstream of the fabric filter 23 via a rotary valve 24 and a return line 25. A partial stream 26 is discharged. This contains practically all condensed heavy metals, but hardly any organics. The second dry flue gas cleaning stage is followed by an activated coke filter 27, which fulfills a police filter function with regard to dust and acidic harmful gas components. With regard to mercury and organics, the fine cleaning function is fulfilled. Instead of an activated coke filter, a zeolite filter can also be used. Because of the low load, the filter material of the filter 27 can be exchanged at more or less long intervals. It is regenerated or, in the case of the activated coke, co-combustion in the furnace 6 can also be considered.

A suction fan 28 connects to the filter 27 and supplies the flue gases to a chimney (not shown).

The vapors formed during drying can be according to Figure 6 of page E40 of the aforementioned DE-Z. treated. However, in order to avoid a condensate to be treated separately, it is more expedient to feed the vapors directly to the furnace 6 via a line 29.

In the embodiment shown, the additive for the second cleaning stage between quench 20 and fabric filter 23 is added. However, it can also make sense to add the second additive between the heat exchanger 19 and the quench 20.

In the embodiment shown, the secondary air is supplied in stages via lines 9a and 9b. Of course, it is also possible to remove coarse ash 30 from the cycloid combustion chamber itself in addition to products 18 and 26 if the combustion process is carried out in such a way that coarse ash is formed.

At least a partial flow of the flue gases leaving the furnace, steam generated by heat exchange by means of the flue gases or air heated by heat exchange by means of the flue gases can be used as the heat carrier for indirect sewage sludge drying.

It is also possible that a cycle gas is used as the heat transfer medium for direct sewage sludge drying, preferably air and / or vapors formed during the drying, which is heated by heat exchange with the flue gases.

An embodiment for the process control with direct sewage sludge drying will be based on FIG. 2 are described. In FIG. 2 the same reference numerals have been adopted as far as possible for the same technical objects.

In the procedure according to FIG. 2, the sewage sludge is fed to a screw 29 by means of the sludge pump 2 and from there enters a fluid bed dryer 30. In the fluidized bed 30a of the fluidized bed dryer 30, the sludge layer is directly flowed through and thus dried by a heat transfer medium guided in a circuit 31, which essentially consists of those during drying vapors formed. The vapors are fed via a vapor extraction line 31a to a fabric filter 32, in which they are dedusted. From there they are returned to the fluid bed dryer 30 via a circuit line 31b containing a vapor compressor 33 and a heat exchanger 34. The heat exchanger 34 is switched on in the water-steam circuit 14 instead of the indirect heat dryer 3. With the help of the steam of the water-steam circuit 14, the vapors compressed in the vapor compressor 33 are brought to the required drying temperature.

As shown in FIG. 2, fresh air may possibly be fed into the vapor circuit 31 upstream of the heat exchanger 34 via a fan 35.

A part of the vapors (evaporated water and possibly supplied air) is fed via a line 36 to the cycloid combustion chamber 6 with the interposition of a heat exchanger 37. The heat exchanger 37 is comparable to the heat exchanger 19. In the heat exchanger 37, the heat to be removed from the flue gases after the dust separation 17 is transferred to the vapors to be fed to the cycloid combustion chamber 6.

The solids deposited on the fabric filter can also be fed to the screw 29 via a line 38 with the interposition of a buffer tank 39 and a screw conveyor 40 in order to adjust the consistency of the sewage sludge to be fed in the dryer (glue phase).

For this purpose it would also be conceivable to return part of the dry material leaving dryer 30 via line 41. (Such a return of dry material before the entrance of the dryer would also be conceivable in the process according to FIG. 1 (before dryer 3)).

Instead of a fluid bed dryer, other direct drying dryers can also be used.

Claims (11)

Process for the combustion of sewage sludge, in which the sewage sludge is dried by means of a dryer heated by a heat transfer medium and is then burned in a furnace, in particular cycloidal combustion, in which the heat transfer medium is heated by the combustion and in which the flue gases from the furnace are dedusted and removed Be subjected to flue gas cleaning,
characterized in that a basic additive is added to the furnace, then heat is removed from the flue gas for drying the sewage sludge while the flue gases are cooled to a first temperature above the condensation temperature of volatile heavy metals, then there is a first dedusting, then heat is again cooled from the flue gas of the flue gas is withdrawn to a lower temperature, but above the cooling limit temperature of the flue gas, then further basic additive is added to the flue gas and the flue gas is then fed to a filtering dedusting device. Method according to claim 1,
characterized in that the first temperature is in the range of 400-500 ° C, more preferably 420-470 ° C and the second temperature is in the range of 100-150 ° C. The method of claim 1 or 2,
characterized in that at least a partial flow of the flue gases leaving the furnace, by means of the flue gases generated by heat exchange or by means of the flue gases heated by heat exchange is used as the heat transfer medium for indirect sewage sludge drying. The method of claim 1 or 2,
characterized in that a cycle gas is used as the heat transfer medium for direct sewage sludge drying, preferably air and / or vapors formed during drying, which is heated by heat exchange with the flue gas. Method according to at least one of claims 1-4,
characterized in that at least some of the vapors produced during drying are fed to the furnace. Method according to at least one of claims 1-5,
characterized in that a centrifugal separator is used for the first dedusting. Method according to at least one of claims 1-6,
characterized in that a fabric filter is used to form a filter auxiliary layer for the second dust separation. Method according to at least one of claims 1-7,
characterized in that after the second dust separation, the flue gases are passed through a police filter. Method according to at least one of claims 1-8,
characterized in that the cooling to the second temperature takes place by a heat exchange with a gaseous or liquid heat transfer medium and then the flue gases are quenched with a quench medium, in particular water. Plant for the combustion of sewage sludge with a dryer for drying the sewage sludge, a combustion device downstream of the dryer, in particular cycloid firing, a heat sink downstream of the firing for the provision of heat for sewage sludge drying, a dust separation and a flue gas cleaning, in particular for carrying out the method according to one of the Claims 1-9,
characterized in that a device (9) for supplying a basic additive for firing (6) is provided, that the dust separator (17) has a further heat sink (19; 37) and this another device (22) for the supply of basic Additive is connected downstream and that the further device is followed by a filtering dust separation (23). System according to claim 10,
characterized in that the second heat sink (19) is followed by a quench (20) and the further basic additive is supplied upstream or downstream of the quench.

Images