A METHOD AND A SYSTEM FOR DRYING SLUDGE

Abstract: A method and a system serve to completely or partially dehydrate sludge by evaporation which originates from e.g. sewage. The sludge is dried with addition of heat in a drying chamber (2), the sludge being transported on an endless travelling grate (3) from an inlet opening (8) in the drying chamber (2) to a discharge opening (9) in it. The sludge is deposited on the grate in a plurality of juxtaposed sludge strings (18) by means of an extruder (6), in which the sludge is preheated to almost 100 DEG C prior to extrusion. The extruded strings (18) are then heated with such an intensity that the water in the sludge immediately begins to boil, and that a zone of preferably saturated water steam is formed at any rate in the immediate vicinity of the sludge. This provides a light and porous, dried sludge product which is in no way either burnt or scorched, and which constitutes excellent fuel as well as excellent compost. The system has a simple and compact construction and is inexpensive to produce, just as it works at moderate operation costs.

Description: A method and a system for drying sludge The invention concerns a method of wholly or partly de- hydrating sludge by evaporation which originates from e.g. sewage, said sludge being transported on a travelling grate from an inlet to a discharge opening in a drying chamber with addition of heat. To protect nature and to prevent environmental damage very strict requirements are made today with respect to the purification of the sewage which is discharged to a reci¬ pient from municipal or industrial purification systems. An essential part of the required purification consists in passing the sewage through relatively large settling basins or tanks, in which the sewage then stays so long and under so quiet conditions that its content of sewage settles, or as is more common today, is subjected to me¬ chanical treatment during which the sludge is dehydrated to a suitable extent by means of e.g. centrifugation. In some cases, the sludge, which generally mainly consists of organic materials, may be spread directly over agricul¬ tural areas and be of use here as a fertilizer for the grown plants, but this will frequently not be possible since the sludge may have a content of substances which are harmful to animals and humans, and it is consequently not acceptable that these are included in the food chain via the crops in question. Accordingly, methods have been developed for drying the sludge and making it useful as fuel or compost. For the latter purpose, additional heat treatment frequently takes place in connection with the drying to free the sludge of its content of harmful substances to the necessary extent. It is preferred in many cases to initiate drying of the sludge directly in the wet and tacky state in which the sludge is conveyed to the drying system. In such a known method the sludge is thus thrown from e.g. a rotating wheel in countercurrent to a hot stream of air in a more or less finely divided form, so that the sludge rapidly dries to a powder and/or granulate. This method is ex¬ tremely efficient, but on the other hand uneconomic in operation since it necessarily requires large amounts of air which must first be heated and is then discharged to the atmosphere, where the air per se, without a preceding expensive and cumbersome separation and filtration pro¬ cess, may constitute a source of environmental pollution since it very easily entrains pulverized sludge from the drying process. WO 88/02284 discloses a method of drying sludge in a ro¬ tary kiln. In this case, an extruder is inserted in front of the rotary kiln, said extruder kneading the sludge with a separate binder and producing pellets from the kneaded mixture which are supplied to the rotary kiln and burnt to a solid, almost ceramic-like form which is suitable for disposal. However, the method cannot be used for producing pellets which can be burnt or composted. Further, DE Offenlegungsschrift 3 531 748 discloses a method where the sludge is dried on a conveyor belt in a traverse kiln. In order that the sludge can be treated in this manner, the wet sludge is mixed in a suitable propor- tion with already dried sludge to form a sufficiently loose and friable material which can be distributed on the grate without sticking to it. During drying the sludge shrinks greatly already from the beginning and will hereby form a granulate with a dense and compact structure, which impedes the biological processes which are subsequently to take place during composting. The granulate cannot be used as a fuel either, since the end product has a much to high water content for this purpose. To this should be added that the system is relatively uneconomic in operation, since a considerably portion of the already dried sludge must constantly be recirculated and mixed with new and wet sludge. Moreover, WO 90/09967 discloses a device for dehydration of sewage sludge, which is subjected to relatively high thermal impacts during transport through a plurality of augers. During this transport, the sludge is constantly rotated about, while relative movement takes place between the sludge and the augers, whereby the material, which is initially in a viscous state, is subjected to compressive forces counteracting the formation of gas pores and channels. This device cannot therefore dehydrate sludge in the form of stable, relatively thin-walled strings, which are immediately heated through the entire cross-section to such an extent that the water content of the sludge begins to boil, thereby imparting to the dehydrated sludge a light and porous structure, while the rapid steam dis¬ charge through the resulting gas channels serves to pre¬ vent the sludge from baking on to the transport means, e.g. a travelling grate, on which the sludge rests immov- ably during transport. The object of the invention is to provide a method of the type stated in the opening paragraph, which, with less current and investment expenditure, more efficiently and with less environmental impact than known before, is cap¬ able of drying wet and sticky sludge originating from e.g. sewage, and thereby converting this sludge tb a dry and uniform porous product which constitutes a valuable fuel or valuable compost. This is achieved in that the method of the invention is characterized in that the heat is added with such an in¬ tensity that the water in the sludge immediately begins to boil, and that a zone of preferably saturated water steam is formed at any rate in the immediate vicinity of the sludge. This entails that the water content of the sludge is rapidly expelled to the surface as either water or steam, whereby a network of pores and channels is left in the dried sludge which hereby maintains its original vol- ume or even increases it. The sludge hereby obtains a structure which is extremely suitable both when the sludge is to be used as fuel and when it is to be used as com¬ post. During drying, there is no risk of the sludge get¬ ting burnt or scorched, since the surrounding steam zone keeps all oxygen spaced from the sludge, while the rapid discharge of water and steam from the sludge prevents the sludge from adhering to the transport means in the drying chamber. Because of the conditions described, drying may take place at an unprecedented rate and by means of a structure which is very compact and inexpensive with re¬ spect to the known systems. According to the invention, the sludge may advantageously be conveyed to the drying chamber by means of an extruder, which may moreover be heated such that the sludge has a temperature of up to 100 °C already when it is deposited on the conveyor belt. The water content of the sludge hereby immediately begins to boil after the sludge has entered the drying chamber, so that the above-mentioned advantages are obtained already from the commencement of the drying process. This is of particularly great impor¬ tance to prevent the sludge from initially adhering to or baking on to the transport means. According to the invention, in cross-section the sludge strings deposited by the extruder on the transport means have a relatively large peripheral length of preferably at least 3 times the smallest transverse dimension of the cross-section to make the water content boil rapidly to promote evaporation. It is particularly advantageous when each string is provided with longitudinal ribs. When the sludge is heated by means of the generated heat from a combustion process, an economic advantage will be obtained according to the invention when the combustion air for this process is first passed through the drying chamber, whereby the temperature and flow of the air are additionally controlled such that a zone of preferably saturated water steam is constantly left around the sludge. Advantageously, by connecting the drying chamber with a condenser and cooling its contents to a temperature below 100 °C, it is possible according to the invention succes¬ sively to remove the generated steam in the drying chamber and to use the heat content of this steam in e.g. a dis¬ trict heating system. The invention also concerns a system which serves to per¬ form the above-mentioned method, and which comprises a drying chamber with an inlet opening and a discharge opening arranged at their respective ends of said chamber, as well as an endless travelling grate arranged in the drying chamber for transporting the sludge to be dried from one opening to the other in the drying chamber with simultaneous addition of heat from a heat releasing means. This system is characterized according to the invention in that it moreover comprises an extruder which has a mouth¬ piece arranged relatively closely spaced upwardly from the feed end of the travelling grate, as well as one or more heat releasing means which are arranged around the ex- - 6 - truder and/or in its transport means, and which are so adapted that the sludge leaves the mouthpiece of the ex¬ truder with a temperature which is preferably between 80 and 100 °C, in particular between 95 and 100 °C. The invention will be explained more fully by the follow¬ ing description of embodiments which are just given by way of example, with reference to the drawing, in which fig. 1 schematically shows a first embodiment of a system according to the invention, fig. 2 is an enlarged section along the line II-II in fig. I, and fig. 3 schematically shows a second embodiment according to the invention. Fig. 1 schematically shows a drying system which is gene- rally designated 1. The drying system substantially con¬ sists of a drying chamber 2 with an endless travelling grate 3 running in the direction of the arrow around two rollers 4 and 5, of which the first roller 4 is driven by means of a motor (not shown), and the other roller 5 is idling. The drying system moreover comprises an extruder 6 with a mouthpiece 7 which protrudes through an inlet open¬ ing 8 in the drying chamber 2 directly above the feed end of the travelling grate 3. The opposite end of the drying chamber is formed with a discharge opening 9 which accom- modates a sluice 10, whose importance will be described below. The extruder 6, which comprises at least one conveyor worm II, receives the sludge in a funnel 12, to which the sludge is conveyed from the sludge basin or tank by means of a pump and/or transport system (not shown), during which the sludge is optionally dehydrated to a solid con¬ tent of between 15 and 40% in a manner known per se, which will therefore not be mentioned in greater detail here. The conveyor worm 11 is driven by a motor 13 via a trans- mission 14 and, in the shown embodiment, is provided with an axially extending cavity 15 which accommodates an elec¬ tric heating member 16 which is connected to a power supply 17. When the system is in operation, the wet sludge is con¬ veyed from the funnel 12 forwardly through the extruder to the mouthpiece 7 by the rotating conveyor worm 11, the sludge being heated during this process to almost 100 "C by the heating member 16. This heating in connection with the simultaneous mechanical stirring imparted to the sludge by the conveyor worm, entails that the sludge will be pliable and easy to transport and extrude, and that the extruded string 18 has exactly the same temperature over its entire cross-section when it leaves the mouthpiece 7. Then the string 18 slides via an optional chute 19 down to the travelling grate 3, which runs with a speed adapted to the extrusion rate, such that the string is continuously moved towards the opposite end of the conveyor belt. The extruded string 18 is heated during this by means of a plurality of electric heating members 20 which, like the heating member 16, are connected to the power supply 17. As shown, heating members 20 are provided both above and below the extruded string 18, and the heat released by the heating members is most intensive initially where the need for heat to evaporate the water content of the sludge is greatest. To avoid loss of heat to the surroundings, the drying chamber is exteriorly provided with a heat insulat¬ ing jacket 21. - 8 - Electric heating members are used in this case for the heating of the sludge in both the extruder and the drying furnace, but this is just an example of course, since it is possible to use any suitable heating source instead, such as the flue gases from a combustion furnace or gas or oil burners. However, in all cases the thermal capacities must be adjusted such that the temperature of the sludge in the extruder is brought almost to the boil before the sludge leaves the mouthpiece, following which the water content in the extruded string immediately begins to boil violently over the entire cross-section of the string, and this state is maintained until the string has reached the other end of the drying furnace. Several remarkable advantages are obtained by means of the above-mentioned special combination of heating the sludge from the moment this is supplied to the extruder and until it leaves the travelling grate in a dried state. Thus, the sludge does not adhere to and/or bake on to the travelling grate, as might otherwise be expected, because the water and steam, which are rapidly expelled to the surface of the sludge because of the boiling, settle as a protective layer between the sludge surface and the grate. Another considerable advantage is that the sludge will not be burnt or scorched, since the strong boiling and evapora¬ tion forms a zone of saturated water steam which settles around the string of sludge and keeps all oxygen spaced from it. The strong boiling along the entire drying zone in the drying chamber also entails that the water and the steam, which are expelled to the surface of the string by the steam pressure in the sludge, form a finely branched network of pores and channels which remain in the finished product, which will therefore be light and porous with ex¬ cellent properties both as fuel and compost. Having passed the drying zone, the now dry string leaves the travelling grate, and during this movement the string impinges on an inclined abutment plate 22 which breakes the relatively fragile string into suitable pieces 23. However, in some cases the string may already have been broken to a certain extent because of i.a. thermal stresses. The broken pieces 23 fall directly down into the sluice 10, which has a mill 24 that rotates in the direc¬ tion of the arrow in operation and thereby chrushes the pieces 23 additionally to bits 25 which fall through a lower opening 26 in the sluice 10. These bits 25 of dried sludge can now be used as fuel in e.g. a combustion system or as compost, if the sludge is free of harmful compo¬ nents, such as heavy metals and poisonous substances. Considerable amounts of steam are generated during drying, which are successively removed via a vent pipe 27 with a damper 28 to regulate the steam pressure in the actual drying chamber. The steam contains a considerable amount of energy which can be turned to account via a heat ex¬ changer in e.g. a district heating system. The embodiment shown in fig. 1 is of a very simple and inexpensive structure, which, however, is nevertheless extremely efficient. The streams of energy generated be¬ cause of the drying are not recycled in a closed circuit in the system shown, but are discharged from it and uti¬ lized in a greater system, which may e.g. be a district heating system which is supplied with heat from a munici- pal combustion system. The flue gases from this may be used as a heat energy source for the drying process, the flue gases still liberating heat and the generated steam from this for heating the water of the district heating system, and the dried sludge as fuel in the combustion system. An excellent operation economy at relatively mo¬ dest initial costs is obtained by means of such a combi- - 10 - nation between the shown drying system and an already existing larger energy system. Fig. 2 is a cross-sectional view of a fragment of the travelling grate 3 with a plurality of extruded strings 18 which are disposed closely to each other. Each of these strings is tubular and has a base 29 by which the string rests on the grate, and an upper portion 30 which arches above the base in e.g. a cycloide shape, which ensures that the weight of the string can only apply compressive stresses to the at any rate initially very weak wall of the upper portion. As shown, the upper portion 30 is more¬ over provided with a plurality of longitudinal ribs 30a, which serve to considerably increase the surface of the extruded string to promote evaporation. The arched shape of the upper portion moreover entails that the radiant heat from the heat sources in the drying chamber can reach substantially all portions of the outer surface of the string. In this case, the water can also advantageously evaporate from the inner surface of the string, since the generated steam can escape through a longitudinal gap 29a in the base 29. Of course, the shape of the string cross-section shown in fig. 2 is just an example, since the cross-section may have any other expedient shape within the scope of the in¬ vention. Thus, the cross-section may be four-sided, oval, round or star-shaped, just as it may optionally be solid or hollow and does not have to be provided with longitu- dinal ribs. However, under any circumstances, a large surface with respect to the area of the cross-section is desired. Fig. 3 schematically shows a second embodiment of the dry- ing system, where parts corresponding to those shown in fig. 1 are provided with the same reference numerals. The actual drying system is substantially constructed in the same manner as described in connection with fig. 1 and operates in the same manner, but in this case the drying system is additionally provided with a heat recovery sys- tern, and the primary heat for the drying process is added by means of one or more gas burners 31 which burn with a flame 32 in a separate combustion chamber 33 spaced from the drying chamber 2 by means of a partition 34, from which the heat from the gas flame 32 is transferred in- directly as radiant heat to the extruded string 18. The flue gases from the combustion of the gas in the com¬ bustion chamber 33 is drawn out of this and discharged to the atmosphere by means of a ventilator 36 via a vent pipe 35 in which a cross heat exchanger 37 is inserted. The necessary amount of air for the combustion process is likewise added through this cross heat exchanger 37, but in counterflow to the hot flue gases so that the heat energy of these is utilized for heating the combustion air. As shown, this is introduced by means of a blower 39 and conveyed, as mentioned, through the cross heat ex¬ changer 37 and further on from this via an air channel 40 into the drying chamber 2. This chamber accommodates a plurality of baffle plates 41, which are so arranged that the hot combustion air is caused to pass up and down be¬ tween the extruded sludge strings 18 before the air leaves the drying chamber 2 via the vent pipe 27. The air absorbs and entrains the steam during this passage which is gene¬ rated by the evaporation of the water content of the sludge, it being observed in this connection that because of the extremely rapid evaporation process there will still be a cushion of steam close to the surface of the sludge strings to keep the oxygen of the air spaced from the strings, so that these are not burnt or scorched. A contributory reason of this advantageous circumstance is that a network of pores and channels is formed already from the beginning of the evaporation process which ra¬ pidly convey water and steam to the surface also at the end of the evaporation process. As appears, the heat con¬ tent in the preheated combustion air effectively contri- butes to heating the sludge strings and promoting the evaporation of the water content of these, while the air convection equalizes differences in temperature between surface portions which receive different amounts of ra¬ diant heat from the partition 34 because of their loca- tion. Having passed the drying chamber 2, the air may contain other gases in addition to water steam which are released by heating of the sludge strings. The air thus mixed is then conveyed via the vent pipe 27 to a condenser 42 in which the air is cooled down to a temperature of slightly below 100 °C, so that the water steam is condensed to generate a slight negative pressure in the drying chamber 2 to ensure that poisonous or malodorous gases from the drying process do not escape through possible leaks in the drying chamber to the surroundings. In the shown case, cooling of the air takes place by means of a water mist 43 which is ejected by an atomizer nozzle 44, which is supplied with water under pressure via a water conduit 47 by a pump 46, which sucks up the neces¬ sary water from a sewage basin 45 by means of a suction line 48. The cooling water is then conveyed together with the condensed water steam by means of a pump back to the sewage basin 45 again via a water conduit 51, which accom¬ modates a cross heat exchanger 50. An air flow from an air inlet 53 is moreover passed through the cross heat ex¬ changer 50 by means of a blower 52 in co-current with the warm cooling and condensed water which thereby releases its heat content to the air, which, in heated state, is conveyed further on via an air channel 54 to a storage and drying silo 55, which receives the resulting dried sludge in bits 25 from the sluice 10 by means of a conveyor 56. The heat from the condensed water is hereby utilized for after-drying the sludge so that the end product will be very dry and suitable for use in particular as a fuel. After the steam content of the air has condensed in the condenser 42, the air is conveyed further on via an air channel 58 to a cyclone drop separater 57 by means of a blower 59, which is inserted in an air channel 60 con¬ nected with the upper part of the cylone drop separater 57. Water drops which might have been entrained by the air flow from the condenser 42, are now separated in the cy¬ clone drop separater 57 and are conveyed via the water conduit 51 and the cross heat exchanger 50 together with the cooling and condensed water to the sewage basin 45. In addition to the amount of sewage which is thus recircu¬ lated in the above-mentioned circuit, fresh sewage from the drying process will of course continuously be supplied to the sewage basin 45. However, it has been found that this sewage is so clean that it can readily be used for several different industrial purposes or for watering of fields. As appears, the steam content of the air is thus removed in the condenser 42 and water drops, if any, in the cy¬ clone drop separater 57. However, the air may still have a content of hydrocarbons and other non-condensable gases. Since the air is finally fed to the gas burner 31 by means of the blower 59 as combustion air, these gases are inci¬ nerated because of their high temperatures which occur during the combustion. The air is then completely cleaned of hydrocarbons and malodorous gases and can readily be discharged to the atmosphere. If desired, a catalyst and optionally a soot separater may additionally be inserted in the vent pipe 35 to ensure that even the strictest threshold requirements are observed. It is mentioned above how the sludge is preheated in the extruder, and the advantages involved hereby for the over- all drying process have been explained. However, the essential feature is that the sludge is fed to the drying chamber with a relatively high temperature, and the sludge may therefore just as well be preheated before reaching the extruder, e.g. during transport to it. In this case the extruder is expediently insulated to keep the tempera¬ ture of the preheated sludge at a desired level. For the preheating, it is then possible to use e.g. the flue gases which are discharged from the cross heat exchanger 37, and which are preferably discharged with a temperature of above 100 °C to avoid deposits of corrosive conden¬ sates. In a particularly simple and advantageous embodi¬ ment of the invention in this connection, the hot flue gases are caused to bubble directly up through the sludge which will thereby advantageously serve as a regular after-filter for the flue gases. EXAMPLE A tubular drying chamber with a length of 3 m and a dia- meter of 0.6 m was used in a test. The upper half of the tubular wall formed a partition to a semicircular combus¬ tion chamber which has heated by means of a gas burner. A conveyor belt of almost the same length as the drying chamber and with a width of 0.5 m was placed in the centre of the drying chamber. A round sludge strings were evenly distributed over the width of the belt by means of an ex¬ truder which had a mouthpiece with 57 nozzles with a dia¬ meter of 6.5 mm. The flue gases from the combustion were discharged directly to the atmosphere from the combustion chamber end nearest to the extruder, while the generated water steam from the drying process was sucked out of the drying chamber to the atmosphere from the opposite end. The extruded amount of sludge was 0.8 kg per min. , and the speed of the belt was adapted to the rate of 0.6 m per min. at which the sludge left the mouthpiece of the ex¬ truder, so that the complete drying time was about 5 min. The temperature in the drying chamber was 435 °C, and the discharge temperature of the flue gases from the combus¬ tion chamber was 550 °C. After drying, the sludge was re- moved from the drying chamber via a rotating gate in which the strings of sludge were broken down into small pill- shaped pieces which were found to have a dry matter con¬ tent of 90%. This is sufficient if the dried sludge is to be used e.g. for composting, and if it is to be used as fuel, additional water can easily be removed to the neces¬ sary extent in a storage and drying silo. As appears, the test system have a capacity of 48 kg of wet sludge per hour with an evaporation capacity of 25.6 kg of water per cm 2 conveyor belt. During drying no form of volume shrinkage was observed, and the resulting dried product was found to have a light and porous structure and was in no way either charred or scorched in spite of the high temperature impact in the drying chamber. Because of the porous structure the dried sludge product could easily be converted biologically to compost, just as the product was found to be an excellent fuel. Although the method and the system of the invention have been described in the foregoing on the assumption that it was sewage sludge that was to be dried, the method and the system can be used equally well and with the same advan- tages for drying many other types of sludge which might occur in connection with e.g. industrial processes, with- out departing from the scope of the invention.

Claims: P a t e n t C l a i m s : 1. A method of completely or partially dehydrating sludge which originates from e.g. sewage, said sludge being transported on a travelling grate from an inlet to a dis¬ charge opening in a drying chamber with addition of heat, c h a r a c t e r i z e d in that the heat is added with such an intensity that the water in the sludge immediately begins to boil, and that a zone of preferably saturated water steam is formed at any rate in the immediate vicini¬ ty of the sludge. 2. A method according to claim 1, c h a r a c t e r - i z e d in that the contents in a condenser connected with a drying chamber are cooled to a temperature of below 100 °C. 3. A method according to claim 1 or 2, wherein heating takes place by means of the heat generated from a combus¬ tion process, c h a r a c t e r i z e d in that the com¬ bustion air for this process is first conveyed through the drying chamber with such a temperature that a zone of pre¬ ferably saturated water steam is constantly left around the sludge. 4. A method according to claim 1, 2 or 3, c h a r a c ¬ t e r i z e d in that the sludge is fed to the drying chamber in one or more strings by means of an extruder. 5. A method according to claim 4, c h a r a c t e r ¬ i z e d in that in cross-section each string has a peri¬ pheral length which is at least three times as great as its smallest transverse dimension. - 18 - 6. A method according to claim 4 or 5, c h a r a c ¬ t e r i z e d in that each string is provided with longi¬ tudinal ribs. 7. A method according to claim 4, c h a r a c t e r ¬ i z e d in that the transport means of the extruder are kept heated, or that the sludge is fed to the extruder in a preheated state. 8. A system for performing the method stated in claims 1- 7 and comprising a drying chamber with an inlet and a dis¬ charge opening arranged at their respective ends of said chamber, as well as an endless travelling grate arranged in the drying chamber for transporting the sludge to be dried from one opening to the other in the drying chamber with simultaneous addition of heat from one or more heat releasing means, c h a r a c t e r i z e d in that the system moreover comprises an extruder which has a mouth¬ piece relatively closely spaced above the feed end of the travelling grate, as well as one or more heat releasing means arranged around the extruder and/or in its transport means and adapted such that the sludge leaves the mouth¬ piece of the extruder with a temperature which is prefer¬ ably between 80 and 100 °C, in particular between 95 and 100 °C. 9. A system according to claim 8, c h a r a c t e r ¬ i z e d in that it comprises a steam condenser which is connected with the drying chamber and contains a cooling means for keeping the temperature in the condenser below 100 °C. 10. A system according to claim 8 or 9, c h a r a c ¬ t e r i z e d in that it comprises a gate with a rorat- able mill arranged in the outlet opening of the drying chamber to divide the dried sludge strings into suitable smaller pieces and remove these from the drying chamber.

Inventors: BERNES STEEN, JENSEN JOHN ROSING, BERNES, STEEN, JENSEN, JOHN, ROSING
Applicants: BLUE TEC AS, BLUE-TEC A/S
Application year: 1991
Main classification: F26B17
IPC codes: C02F11, F26B17, F26B25