CH656114A5 - Process and apparatus for drying and incineration of sludges - Google Patents

Description

The invention relates to a method and a device for drying and burning sludge using a fluidized bed provided with a loosening / heat transfer bulk material and a solid fuel burner downstream of the fluidized bed with a subsequent solid separator, the sludge being fed into the lower region of the fluidized bed.

The drying and combustion of sludges, e.g. B. from sewage treatment plants, is becoming increasingly important as a solution to an environmental problem. To provide the energy required for the drying process, it is possible to use the heat chemically bound in the sludge by burning the sludge.

It is known to remove the wet sludge of a fluidized bed from a loosening / heat transfer bulk, e.g. Sand to give up and dry and burn the wet mud in this fluidized bed. The fluidizing gas that escapes from the fluidized bed is with the hot Ver5

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combustion gases mixed and is supplied to the extent of the fluidized bed as the fluidizing gas which operates it, which is necessary to maintain the desired fluidizing conditions; some of the sensible heat of the combustion exhaust gas is used for the drying process in the fluidized bed. This method has the disadvantage that the entire water vapor of the moist sludge that is released in the fluidized bed must be brought to the high temperature level during the sludge combustion. In order to provide the energy required for this, an additional supply of heating gases into the fluidized bed is generally necessary. In addition, the fluidized bed is inevitably operated at a higher temperature than is necessary for drying, and the resulting gases, which are not circulated as fluidizing gas, are dissipated at a correspondingly high temperature level without their sensible heat being used in a sufficiently meaningful manner can be.

In addition, from DE-OS 2 851 609 a method and a device of the generic type are known, in which the solid fuel burner is also designed as a fluidized bed, from which on the one hand the loosening / heat transfer bulk material into the drying fluidized bed and on the other hand dried Sludge together with loosening / heat transfer bulk material must be transferred to the solid fuel burner. This double solids transport, which is obviously carried out with regard to the loosening / heat transfer bulk material for thermal reasons, represents a process disadvantage when operating the plant and an investment disadvantage when it is erected. In addition, a heat exchanger must be provided in which the hot combustion gases are indirect heat the heat gases used for the fluidized beds by heat exchange, so that a not inconsiderable part of the sensible heat of the combustion gases is lost for the drying process.

It is therefore the object of the invention to implement an improved method and a device of the generic type, the aforementioned disadvantages being avoided by providing the thermal energy required for drying the sludge in a simpler manner exclusively by burning the sludge.

The object is achieved according to the invention in relation to a method of the generic type by the characterizing features of claim 1. This enables drying treatment of all sludges containing combustible substances.

Sludges e.g. Sewage sludge, such as fresh digested sludge, digested sludge from belt presses or from dry beds in question. The dry matter content after pre-dewatering is usually between 10 and 40%. The ash content of such dry substances is usually between 50 and 85%. The calorific value of sludges can e.g. B. depending on the ash content, be quite different; e.g. the upper calorific value of the digested sludge examined here was around 23,000 kJ / kg ash-free dry matter.

Sand can be used as a loosening / heat transfer bulk material. Such a bulk material has a more or less uniform particle size distribution, the choice of particle sizes being based on the criteria known for fluidized beds. With regard to the average grain diameter and the bulk density, care must be taken that the fluidizing gas velocity corresponding to these basic parameters is so high that the fluidizing gas is also able to guide the sludge upwards through the fluidized bed during drying and, if possible, also pneumatically discharge from the fluidized bed furnace. Quartz sand with a grain size of 1-2 mm is preferred; the narrower the grain spectrum, the better the results are obtained, and it is advisable to sieve them accordingly before use. In addition, chemical neutrality of the bulk material in relation to the sludge to be dried and the fluidizing gas atmosphere is desirable.

The bulk material promotes a disperse distribution of the sludge and thus prevents the occurrence of agglomerations from the outset. According to the invention, the sludge is to be fed in in the lower region of the fluidized bed, the feed points preferably being somewhat higher than the introduction point for the fluidizing gas (inflow floor), since it has been found that this measure achieves a more uniform sludge distribution over the cross-sectional area of the fluidized bed . The conveying of the sludge with simultaneous drying through the swirling loosening / heat transfer bulk material should not take place too quickly, so that a thorough heat transfer between the fluidizing gas and the sludge or the bulk material and the sludge is possible, so that the desired degree of drying is actually achieved is achieved.

The removal of the dried sludge together with the fluidizing gas from above the fluidized bed can take place in a wide variety of ways, it being preferred that the flowing fluidizing gas entrains the dried sludge particles. This gas / solid mass flow is separated into gas and solid in a downstream solid separator; a cyclone known per se is usually used for this.

The dry substance is subsequently e.g. abandoned via a rotary valve, preferably an intermediate bunker, from which z. B. with the aid of an ejector or also by means of a rotary valve, a solid burner known per se; In principle, all solid fuel burners available on the market are suitable for the latter. The dried sludge particles, e.g. B. with the supply of air, which can also be preheated, burned at temperatures of usually between 1200 and 1800 ° C. The residue ash is discharged together with the hot combustion exhaust gas from the burner and discharged in a downstream separator. As a result, an essentially ash-free, hot combustion exhaust gas reaches the inflow floor of the fluidized bed, which is supplied with the heat necessary for drying the sludge.

A typical value for the residual moisture is <3% by weight. If the water content of the sludge is different, compliance with such a value is preferably achieved by varying the fluidizing gas temperature below the inflow floor. The height of such a fluidized bed, regardless of the size of the inflow surface, depends on the maximum desired residual moisture that the dried sludge may still have. The fluidized bed heights when using sand as bulk material are e.g. 500-800 mm in question.

The sludge is fed in by a suitable conveyor, preferably from a storage bunker.

According to a development of the invention, the fluidizing gas obtained in the solid separator downstream of the fluidized bed is at least partially mixed with the hot combustion exhaust gases. This makes it possible to feed the required amount of gas to the fluidized bed and to set the gas temperature to the best level for drying. These temperatures should not

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are too high to limit the heat losses of the fluidizing gas leaving the fluidized bed; Temperatures for the fluidizing gas entering the fluidized bed of 500-800 ° C have proven to be particularly advantageous.

The gas to be admixed can alternatively or cumulatively be admixed to the hot combustion exhaust gases at various points; so e.g. in the area of the solid fuel burner in order to achieve any necessary exhaust gas cooling to protect the material. However, it is also advisable to mix in before the ash separator, provided that it has a maximum permissible operating temperature, the design then of course having to take the increased gas flow into account. And finally, direct admixing can also take place in the region of the fluidized bed furnace, preferably in a mixing chamber which has a homogenizing effect with respect to the temperature.

According to a development of the invention, the fluidizing gas produced in the solid separator downstream of the fluidized bed can be at least partially freed of water vapor in a condensation stage. This prevents excess, that is to say not circulated, fluidizing gas from emitting too much steam into the atmosphere; in addition or alternatively, the absorption capacity of the circulating fluidizing gas for the moisture of the sludge is thereby improved.

Because of such a condensation process, it is of course also recommended that the fluidizing gas temperature is not too high when it emerges from the fluidized bed, since otherwise intensive cooling would be unnecessarily necessary.

It is particularly advisable if, according to the invention, the combustion air supplied to the solid fuel burner is preheated, preferably with cooling of the condensation stage. This minimizes energy losses. Preheating has proven to be particularly important for sludges with a low calorific value.

To start up or keep the sludge drying and incineration plant warm, it is recommended that, according to the invention, the solid fuel burner is operated with a separately supplied fuel. This can also be gas or oil or dried sludge from a storage container. However, such external energy is only required for the purposes mentioned and is not necessary in the normal drying / combustion process, although it cannot be ruled out, of course, that if larger amounts of heat are required for a short time, e.g. due to an inadequate preliminary dewatering of the sludge by additional combustion of an external fuel supplied to the solid fuel burner, the necessary amount of heat is supplemented.

It has proven to be particularly advantageous if, according to the invention, components which are undesirable for the eddy gas which is not recirculated are released for the release into the atmosphere or are recycled into bio-tanks such as the aeration tanks of sewage treatment plants. This eliminates the need for additional aftertreatment and avoids releasing such gases into the atmosphere or continues to use combustible materials extracted from the dried sludge. Pollutants present in the exhaust gas are biodegraded in the aeration tank.

If sludge has a relatively low calorific value, it is advisable, according to the invention, to carry out a correspondingly increased mechanical preliminary dewatering of the sludge; this happens e.g. on a preferably continuously operating belt press. "Relatively low calorific value" means e.g. up to 5000 kJ per kg ash-containing dry matter, in which case the sludge should be dewatered to a dry matter content of 30 to 35%.

The stated object is achieved in relation to a device for performing the method by the features of patent claim 8. It has proven to be particularly advantageous if the length-to-width ratio of the fluidized bed furnace is greater than / equal to 4. A particularly good utilization of the inflow surface with regard to the thermal energy and in relation to the sludge distribution is thereby achieved; It was surprising that no negative mutual influence was found by using several different posting points.

According to a further development of the invention, sludge feed lances introduced laterally into the fluidized bed furnace have proven to be particularly advantageous. These are preferably such that they avoid selective sludge application, which z. B. can be realized by a tube which has a larger-tapered exit surface at its end in the flow direction of the fluidizing gas by a strong bevel. As a result, the rate of entry of the sludge into the fluidized bed furnace is reduced and an area distribution is achieved. Interchangeable lances are particularly preferred.

According to a development of the invention, the height of the sludge feed points above the inflow floor is approximately from V20 to Vs of the fluidized bed height. In the case already described above, a height of 100 mm would be particularly favorable. Higher feed points reduce the dwell time and the degree of drying of the sludge in the fluidized bed furnace, while lower heights can lead to the risk of caking and incrustation of the inflow floor; in the latter case the dispersing effect of the loosening / heat transfer bulk material would be reduced.

It has been found that, in contrast to what is customary according to the prior art, an inflow base designed as a grate is particularly recommended according to the invention. The systems used for gas deflection, e.g. by bells above the gas outlet points are disadvantageous insofar as they prevent the passage of solid particles still present in the fluidizing gas; this is avoided by a grate, preferably a grate bar floor. As a result, the separation from the hot combustion gases does not have to be one hundred percent, although it is undesirable to circulate excessive ash components through the system. On the other hand, surprisingly, no diarrhea of the fluidized material through the inflow floor - especially if it was a grate bar floor - was observed.

Finally, it has proven to be particularly favorable in terms of energy and apparatus if, according to the invention, a gas mixing chamber is present below the inflow floor in the fluidized bed furnace, which leads to temperature-related homogenization of the fluidized gas, which avoids an additional apparatus part, namely a gas mixing chamber separately upstream of the fluidized bed furnace .

Further objectives, features, advantages and possible uses of the present invention result from the following description of an exemplary embodiment with reference to the accompanying drawings. All of the described and / or illustrated features, alone or in any meaningful combination, form the subject matter of the present invention, regardless of their summary in the claims or their relationship.

Show it:

Figure 1 is a schematic representation of a system for performing the inventive method.

Fig. 2 shows a sludge feed lance according to the invention, partly in section.

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In FIG. 1, 1 denotes a fluidized bed furnace, in which the sludge to be treated is fed from a storage bunker 2 via a conveying element 3 and one or more sludge feed points 4. The conveying element 3 can be a metering device, e.g. a swash plate pump. The fluidized bed furnace has e.g. a rectangular cross-section with a length-to-width ratio of preferably 4: 1 or larger.

Above an inflow floor 5, a loosening / heat transfer bulk layer swirls, which is preferably made of quartz sand with a uniform grain size of e.g. 1-2 mm and has a height of 500-800 / mm.

Hot fluidizing gas flows through the inflow base 5 into the fluidized bed and preferably has a very low, but possibly no, oxygen content. Usual fluidizing gas speeds are between 1 and 2 m / s, while the temperatures are usually between 500 and 800 ° C.

The sludge to be dried, which is already in the fluidized bed furnace, z. B. has been homogenized in the conveyor and possibly crushed, after drying and subsequent gas separation e.g. in a cyclone 6 an average particle diameter of approx. 500 (xm and a density of approx. 2000 kg / m3.

Via line 7, the dried sludge is fed into a solid fuel burner 8 in a suitable manner known per se, with the interposition of a storage bunker, not shown, with an upstream rotary valve in a suitable manner. The latter is preferably supplied with preheated air via line 9, which e.g. after passing a pre-filter 10 and blower 11 has cooled a heat exchanger 12.

The solid fuel burner 8 can be equipped with an additional feed line 13 for another fuel, e.g. Gas or oil, be provided to allow starting or keeping the entire system warm.

The hot combustion exhaust gases, together with the ash components of the burned sludge, leave the solid fuel burner 8 via line 14 and reach a gas separator 15. There, the ash is largely discharged via line 16, while the hot combustion exhaust gases via line 17 into the fluidized bed furnace 1 below the inflow floor 5 and get into the fluidized bed from there.

The vortex gases freed from solid matter in the cyclone 6 are fed via line 18, which can be provided with a control flap 19, to the heat exchanger 12, in which extensive condensation of the water vapor contained in the vortex gas takes place, before the blowers 20 treat the vortex gases treated in this way Lines 21, 22 take place with the interposition of a control flap 23 and, alternatively or cumulatively, via lines 21, 24 with the interposition of a control flap 25 in line 14. In addition, it is of course also conceivable to introduce these gases into the solid fuel burner 8 (not shown).

The various gas streams can be homogenized in a gas mixing chamber 26 arranged inside the fluidized bed furnace 1 below the inflow base 5.

Eddy gas which is not recirculated and which corresponds approximately to the volume of the air supplied to the combustion process - increased by the water content of the sludge released by the drying process - is discharged from the process via line 27 and control flap 28. B. supplied to an afterburning or z. B. returned to a sewage plant in Biobek-ken

The energy balance of the entire device is generally positive, since the chemical heat generally bound in the sludge is used particularly well by this process. In the case of sludges with a particularly high calorific value, the dried sludge can be recycled to a certain extent in a manner other than sensible by combustion, or the excess thermal energy can be converted into another form of energy in a manner known per se, or the sludge is pre-dewatered to a lesser extent, which can also be useful in this area. - For sludges with a relatively low calorific value, the mechanical pre-dewatering, as already mentioned, is driven so far that the amount of heat required for the sludge drying can be provided solely from the sludge combustion.

2, the side walls of a fluidized bed furnace 1 are shown in detail by 29 and 30, a sludge feed lance 31 in the direction of the arrow 32 being flowed through by the sludge to be dried and discharged into the fluidized bed 34 through an obliquely upward elongated mouth opening 33, which flows through fluidizing gas in the direction of arrow 35. Such sludge feed lances are preferably arranged staggered transversely to the image plane, the view side shown in FIG. 2 representing the narrow side of the fluidized bed furnace 1.

Embodiment

A test facility corresponding to the principle of FIG. 1 was available. A fluidized bed furnace (WSO) was used with a rectangular inflow surface (inflow floor) with a length of 0.75 m and a width of 0.2 m in the form of a gas-permeable grate with simple openings distributed over the surface. Four sludge feed points were evenly distributed over the long side at a height of 100 mm above the inflow floor; 2 with a mouth opening inclined upwards by 30 ° with respect to the horizontal, approximately in the middle above the inflow floor.

The WSO was filled with quartz sand with a grain size of 1-2 mm with a layer height of 0.6 m at rest.

Eddy gas passed through the inflow floor at a temperature of 590 ° C and had a speed of 1.2 m / s based on the free cross-section of the WSO - based on a temperature of 250 ° C and atmospheric pressure. The fluidizing gas consisted of 25% of the combustion gases obtained by the combustion of dried sludge at a temperature of approx. 1700 ° C and the rest of the fluidizing gas at the temperature behind the fluid separator downstream of the fluidized bed at a temperature of approx. 180 "C.

During the sludge drying operation in the WSO, the height of the quartz sand fluidized bed was between 0.7 and 0.8 m and the fluidizing gas leaving the WSO together with the dried sludge had a temperature of approximately 250 ° C.

The sludge to be treated was a digested sludge from a municipal wastewater treatment plant, which was mechanically dewatered to 29% dry matter content and had a calorific value of 10,130 kJ / kg dry matter with an ash content of 48.7% dry matter. - Over the test period of 5.3 hours, an average of 1181 sludges per hour were fed into the WSO.

The dry matter was separated from the fluidizing gas leaving the WSO in an aero cyclone with tangential gas entry. The degree of separation was approx. 83%, based on the dry matter leaving the WSO. The residual moisture in the dry matter was about 1.8%.

The dry matter was burned in one

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simple solid fuel burner with an inner diameter of approx. 300 mm, a tangential dry substance inlet for pneumatic solid feed and an air connection at the front with an upstream fan. In the exit region of the combustion gases from the combustion chamber, the recirculated gas stream mentioned was mixed in. Around 80% of the combustion residues were separated in a downstream cyclone of the same type as the aforementioned. The latter consisted of 80% ash - the combustion of the dry matter was therefore not complete. The gas mixture leaving the latter cyclone was fed to the WSO together with the non-separated solid particles as fluidizing gas.

About 60% of the fluidizing gas produced in the solid separator downstream of the fluidized bed was returned uncooled to the solid fuel burner for producing the mixed gas. The rest served in a heat exchanger to preheat the combustion air, which reached about 70 Nm3 / h and a temperature of 115 C, whereby the residual gas also cooled down to 130 C. The latter was subsequently fed to a generally known thermal afterburning.

No external thermal energy was required in this process. Rather, there was even a residue with a residual calorific value in the cyclone downstream of the solid fuel burner, and the residual fluidizing gas to be burned still had a certain, but not determined, calorific value, although only 83% of the dry substance accumulating in the cyclone upstream of the solid fuel burner was burned in the solid fuel burner , while the remaining 17% dry matter could be used for other purposes.

The entire plant was started up by burning natural gas and operated with it until sufficient dry matter for the feed-in and combustion was obtained. At this point the natural gas supply was switched off and the burner switched to solid fuel combustion.

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2 sheets of drawings

Claims (12)

656114 1.4. subsequently burned in the solid fuel burner and 1.3. separated from the fluidizing gas in a downstream solid separator and 1.2. discharged together with the fluidizing gas from above the fluidized bed, 1.1. conveyed upwards while drying through the swirling loosening / heat transfer bulk material, 1. all of the mud 1.4. subsequently burned in the solid fuel burner and 1.3. separated from the fluidizing gas in a downstream solid separator and 1.2. discharged together with the fluidizing gas from above the fluidized bed, 1.1. conveyed upwards while drying through the swirling loosening / heat transfer bulk material, 1. all of the mud 1.4. subsequently burned in the solid fuel burner and 1.3. separated from the fluidizing gas in a downstream solid separator and 1.2. discharged together with the fluidizing gas from above the fluidized bed, 1.1. conveyed upwards while drying through the swirling loosening / heat transfer medium, 1. all of the mud 1. A process for drying and burning sludge using a fluidized bed provided with a loosening / heat transfer bulk material and a solid fuel burner downstream of the fluidized bed with subsequent solid separator, the sludge being fed into the lower region of the fluidized bed, characterized in that 2. The hot combustion gases after the extensive separation of the combustion residues of the fluidized bed are fed as the heat-supplying fluidizing gas for the sludge drying, a correspondingly increased mechanical pre-dewatering taking place for sludges with a relatively low calorific value. 2. the hot combustion gases after the extensive separation of the combustion residues of the fluidized bed are fed as the heat-producing fluidizing gas for the sludge drying, whereby undesired components of the non-circulating fluidizing gas are subsequently burned or returned to bio-tanks for release to the atmosphere. 2. The method according to claim 1, characterized in that the resulting in the fluid separator downstream of the fluidized bed fluidizing gas is at least partially mixed with the hot combustion gases. 2. The hot combustion gases after the extensive separation of the combustion residues of the fluidized bed are fed as the heat-producing fluidizing gas for the sludge drying. 2nd PATENT CLAIMS 3. The method according to claim 1 or 2, characterized in that the resulting in the fluid separator downstream of the fluidized bed fluidizing gas is at least partially freed from steam in a condensation stage. 4. The method according to claim 3, characterized in that the combustion air supplied to the solid fuel burner is preheated. 5. The method according to claim 4, characterized in that the solid fuel burner for starting or keeping the sludge drying and incineration plant operated with a separately supplied fuel. 6. A process for drying and burning sludge using a fluidized bed provided with a loosening / heat transfer bulk material and a solid fuel burner downstream of the fluidized bed with a subsequent solid separator, the sludge being fed into the lower region of the fluidized bed, characterized in that 7. Process for drying and burning sludge using a fluidized bed provided with a loosening / heat transfer bulk material and a solid fuel burner downstream of the fluidized bed with subsequent solid separator, the Sludge is fed into the lower region of the fluidized bed, characterized in that 8. Apparatus for performing the method according to one of claims 1 to 7 using a fluidized bed provided with a loosening / heat transfer bulk material and a fluid burner connected downstream of the fluidized bed with subsequent solid separator, characterized by an elongated fluidized bed furnace (1) from which flow flows from below its longitudinal extent was distributed by several sludge feed points (4), a solids separator (6) downstream of its gas space for separating all of the dried sludge from the fluidizing gas and an inflow floor (5) of the fluidized bed furnace (1) charged with the exhaust gases from the solid fuel burner (8). 9. The device according to claim 8, characterized by laterally in the fluidized bed furnace (1) introduced sludge feed lances (31) at the sludge feed points (4). 10. The device according to claim 8 or 9, characterized by a height of the sludge feed point (4) above the inflow floor (5) from V20 to Vs of the fluidized bed height. 11. The device according to any one of claims 8 to 10, characterized by an inflow base designed as a grate (5). 12. The device according to one of claims 8 to 11, characterized by a gas mixing chamber (26) below the inflow floor (5).