FR2624258A1 - PROCESS AND INSTALLATION FOR THE PYROLISE ELIMINATION OF ACTIVATED SLUDGE - Google Patents

Abstract

The invention relates to a method and an installation for the treatment of activated sludge from industrial wastewater treatment plants. The installation comprises a rotary furnace 2 with indirect heating, the latter being produced by the evacuation of the pyrolysis gases from the furnace 2, which is kept isolated and free of oxygen, combustion of these gases outside the furnace 2 with , if necessary, supply of an additional fuel, and circulation of the combustion gases on the outside of a heat exchange surface 7, with fins, of the furnace 2. Field of application: waste treatment industrial.

Description

The invention relates to a method for eliminating

  pollution-free pyrolysis of active sludge

  In particular, sludges from industrial wastewater treatment plants and an installation for the low-cost implementation of this process and / or simple drying of sludge. "Pyrolysis" is a known treatment method of severing chemical bonds using heat, which destroys pollutants by breaking down their molecules into simpler patterns to produce gases and solid carbon as residues. Applied to activated sludge,

  Pyrolysis poses a number of

  plexes due to the fact that existing ovens

  not an economical process, from the point of view of energy consumption, for indirectly heating the sludge to a temperature suitable for producing products.

  non-contaminating pyrolysis, which can be rejected.

  In addition, the direct heating, or oxypyrolysis, in which hot fumes are passed over the sludge, also has its disadvantages. All

  first, the difficulty of precisely adjusting the temperature

  erage causes local overheating requiring the use of refractory furnaces, and then

  the resulting combustible gases are strongly contaminated

  they must be removed in an afterburner chamber or directly inside the furnace, which in the first case increases the cost and causes considerable energy losses and, in the second case, causes a considerable increase in energy consumption. combustion of

  sludge whose solid residue is practically useless

  sand. In addition, the fact that rotary kilns are normally required for the treatment of sludge poses even more difficult problems, both economically and technically, because of the weight

  refractory lining required.

  The object of the invention is to provide a process for the pyrolysis elimination of activated sludge, in particular sludge from industrial wastewater treatment plants, which process offers a low energy consumption, produces a residual It can be used in industry and therefore has an added value that at least partially offsets the costs of sludge disposal, and causes virtually no air pollution due to the release of fumes polluting the air. Another goal

  of the invention is to propose an installation permitting

  both the implementation of this method and / or simple sludge drying, quickly and continuously, which installation is both easy to control and to

  regulate and inexpensive to manufacture.

  To achieve this goal, it is proposed,

  according to the invention, a method for eliminating

  by pyrolysis of activated sludge, in particular sludge from industrial wastewater treatment plants, characterized in that it comprises the steps of: - loading the sludge into a chamber

  isolated in a gas-tight manner from the atmosphere.

outer sphere;

  - to pyrolyze sludge in the absence of

  plete of oxygen gas to prevent any form of combustion, this pyrolysis being carried out by indirect heating of the sludge by means of an exchange

  of heat through the walls of the reaction chamber

  flowing a flow of combustion gas on the outside of the reaction chamber; - to evacuate the pyrolysis gases produced inside the reaction chamber by thermally breaking the sludge molecules

  - to produce the flue gas stream

  by burning the pyrolysis gases discharged from the reaction chamber, the combustion step being carried out exclusively outside the reaction chamber; and - to unload the pyrolyzed sludge from

the reaction chamber.

  According to the invention, it is also

  An installation for pyrolysis or drying, or both, of activated sludge has been proposed, this plant comprising an oven in which, at a first end, the sludge is continuously charged.

  and from which, at a second end, they are

  continuously, and means for indirect heating-

  sludge, the installation being characterized

  in that the indirect heating means com-

  take a skirt covering the outside of the furnace in order to define, with an outer heat exchange surface of the furnace, a cavity along which

  a stream of hot gas is circulated through

  through an entrance formed at a first

  of the skirt, to an exhaust formed at a second

  end of the skirt, the inside of the oven being

  gas-tight insulation of the external atmosphere

  and the installation also includes

  means to evacuate the gases produced within the

  heating the sludge, means for adding combustion support air to the gases and burning them to produce the hot combustion gas stream, and means for

  circulate the flue gas stream to the

of the cavity.

  The invention will be described in more detail

  with reference to the accompanying drawings as examples

  and in which: FIG. 1 is a diagram of the installation,

  as well as a functional diagram of the process according to

  vention; Figure 2 is a longitudinal section of the furnace constituting the main element of the installation according to the invention; FIGS. 3 and 4 are views on an enlarged scale of a detail of the oven of FIG. 2, FIG. 3 showing it in longitudinal section and FIG. 4 showing it in section along line IV-IV of FIG.

Figure 3; and -

  Figures 5 and 6 are graphs

  related to the process according to the invention.

  1 to 4, an apparatus for the pyrolytic elimination of activated sludge, in particular sludge from industrial wastewater treatment plants, and implementing the process according to the invention. invention. To allow a better

  understanding of the invention, the description of the process

  according to the invention will be preceded by a description

  of the installation 1 which represents only one advantageous embodiment of the invention, in that the method according to the invention can be

  implemented also and as well on

  completely different from those described here.

  The installation 1 comprises a reaction chamber defined by an oven 2 which, in the example shown, is a continuous feed rotary type furnace, a feed box 3 for untreated sludge,

  means 4 for loading the sludge into the

  2 of the chamber 2 and comprising, in accordance with the invention, a double screw device, as shown in detail in FIGS. 3 and 4, connected to the body

  in a known manner ensuring the

  fluidity with the inlet of the device 4 and, therefore, the reaction chamber or furnace 2 by means of sludge compressed inside the box 3 supply; a cavity or jacket 5 for indirect heating of the furnace 2, defined by a skirt 6 made of sheet metal (FIG. 2) covering the outside of the furnace 2, and by an external surface 7 of heat exchange on the furnace 2; an airtight collecting chamber 8 connected directly, by means of a known fixed hood 9, to an outlet 10 of the rotary kiln 2, so as to collect the sludge

  processed from oven 2 while maintaining simul-

  the interior of the furnace 2 is insulated in a gastight manner from the outside atmosphere and, consequently, free of gaseous oxygen; and means for

  circulate the hot combustion gases in the

  5. In accordance with the invention, the installation 1 comprises an ejector 12 supplied with ambient air, for example by means of a compressor 13, and connected by a duct 14 to the inside of the oven 2 in order to evacuate the gases inside

  of this oven; a combustion chamber, as shown in FIG.

  felt in dashed lines, intended to receive the air

  supplied to the ejector 12 and the gases evacuated by that

  deny the inside of the oven 2; and conduits 16 and 17 connected, respectively, in a known manner and not shown, to an inlet 18 of the cavity 5, formed through an end 19 of the skirt 6, and to an exhaust 20 of the cavity 5 formed through an end 21 of the skirt 6, opposite the end 19. The conduit 16 is connected directly to the combustion chamber, while the conduit 15 is connected to a chimney (not shown). The ducts 16 and 17 are also connected to each other by a connecting duct 22 equipped with a smoke recycling pump 23 intended to pass through the duct 16 a part of the exhaust gases flowing in the duct 17 and to recycle said gas inside the cavity 5 to adjust, in a known manner, the temperature of the gas stream flowing in this cavity. According to one possible variant, the combustion chamber 15 can be eliminated, in which case the ejector 12 is also designed to form a burner and is connected directly to the conduit 16. When

  is present, the combustion chamber 15 is

  lined with firebricks and, in addition

  a burner 15a of the air / gas mixture from the ejection

  12, it also comprises an additional burner supplied in known manner with air and fuel

normal.

  The rotary kiln 2 is defined by a cylindrical side wall 26 and rests so as to freely rotate on rollers 27 of support, of a type commonly used in the chemical industry, by means of rolling rings 28, so as to rotate around its longitudinal axis. With the aid of a toothed gear meshing with a ring gear 31 secured to the wall 26, a motor 29 slowly rotates the furnace 2 relative to the skirt 6 which, in the example shown, is fixed and rests on supports 32 which also support the rollers 27. The conduit 14 is fixedly supported on the hood 9 which is mounted in a gastight manner on the wall

  rotating device 26 by means of seals 33. In accordance

  According to the invention, the device 4 is placed sensitively

  on a relatively low generator of the cylindrical wall 26 so as to be so close

  as possible from the inner surface of it.

  The device 4, which is of a type already used in other technical fields, comprises a flattened ellipsoid envelope housing two coplanar worms 36 arranged longitudinally side by side and having the same pitch and a lower center distance.

  to the pitch diameter of each screw, so that

  helicoidal portions -37 located on the screws intersect in such a way that the scrolls

  of an element be placed between two adjacent scrolls

  of the other element. The two screws 36 are

  at the same speed by a common motor 38. Tests conducted by the present Applicant have shown that the structure of the device 4 is essential to ensure proper loading of the sludge in the furnace

  2. Moreover, the position of device 4 on the gen-

  The furnace 2 prevents the sludge from adhering to the wall 26. This wall 26 externally supports a set of radial fins which it is integral and which define, with the wall 26, said surface 7 of heat exchange, these fins being constituted,

  according to the invention, a piece of sheet metal

  continuous 40 welded to the outer surface of

  wall 26 and wrapped around it at a reduced step

  progressively clouding the end 41 of the oven 2,

  near the end 19 of the skirt 6, towards the end

  mité 42 of the furnace 2 near the end 21 of the skirt 6. The device 4 is attached to a panel 43 in sheet gastightly closing the open end 42 of the furnace 2, while the hood 9 is mounted on end 41. Finally, in accordance with the invention, the inlet 18 and the outlet 20 are formed so that each of them fits entirely between two adjacent volutes of the element 40. This, associated with the variable pitch of the element 40, in accordance with the invention, makes it possible to produce a strong vortex, as well as to reduce the pressure drop of the stream of combustion gas flowing through the cavity 5, which, together with the large surface area 'exchange

  heat of the element 40, allows, consequently, opti-

  bet the heat exchange between the gases flowing inside the cavity 5 and the sludge inside the furnace 2. In accordance with the invention, the sludge removal installation 1 operates in the manner now described. Untreated sludge, for example from a known type of biological scrubber (not shown) located in the vicinity of the plant 1, are introduced into the feed box 3 where they fall into the device 4. The latter, by means of the screws 36, forcibly displaces the sludge in a regular manner and in constant and dosable quantities (by adjusting the speed of the screws) towards the inside of the oven 2 o, thanks to the position of the device 4, the sludge immediately comes into contact with the wall 26. The slow rotation of the furnace 2 and the continuous loading of the sludge in the same furnace by the device 4 cause the charged sludge to move along the axis of rotation of the furnace 2, from the end 42 to the end 41, where the sludge, or rather their

  residues, are discharged into chamber 8 through

  9. By moving along the axis of the furnace 2, the sludge, according to the present invention, is pyrolyzed so that their molecules are broken and they gradually give a set of dry residues, gas and water vapor which, although combustible, consisting mainly of CO, methane and other hydrocarbons, fills the interior of the furnace 2 without combustion, since this furnace 2 is isolated from the outside atmosphere and is free of gaseous oxygen. The thermal energy for the pyrolysis process is supplied externally via the surface 7, by circulation of hot gases inside the cavity 5. Since the temperature required for the pyrolysis of the sludge is between 300 and 800 C, the hot gases must be produced by combustion at a temperature of at least 1000-1200 C. To reduce energy consumption and avoid the treatment of

  pyrolytic gases, according to the invention

  tion, said gases are continuously discharged from the

  of the reaction chamber 2 by means of the ejection

  12 during the pyrolysis and are introduced into the combustion chamber 15, with the air supplied to the ejector 12, the air supply of which is regulated so as to produce a fuel mixture in which the air supplied to the ejector 12 behaves like a

  combustion support and the pyrolytic gases

  bear as fuel. Inside the combustion chamber, the gas mixture is burnt, according to the invention, in a burner 15a to produce a flue gas stream at a temperature of 1000-1100 C. In accordance with the invention this stream is then introduced, by means of a conduit 16 and in the opposite direction to that of the sludge, into the cavity, where part of its heat is transferred to the sludge by heat exchange, through said exchange surface 7 of heat, between the gases inside the cavity 5 and the sludge lying

  inside the oven 2. The combustion gases introduced

  in the cavity 5 are then evacuated. at a temperature of about 100 C, through the evacuation 20 and they are partially recirculated downstream from the combustion chamber 15 to adjust the temperature inside the cavity 5. In the case the heat introduced is not sufficient to allow pyrolysis, the combustion gases produced by burning the gases discharged from the

  furnace 2 are supplemented with flue gases

  by burning a normal liquid or gaseous fuel within the burner 25, the resulting gases

  being mixed with the pyrolytic gases burned within the

  combustion chamber and introduced, with the latter, into the cavity 5, thus constantly ensuring a sufficient supply of heat. During the start-up of the installation, the combustion gases to start the pyrolysis process are obviously provided exclusively by the burner 25. However, once the process has begun, it produces gases having an extraordinary heating value such as

  to make the process autonomous and reduce practice

  zero fuel consumption. The pyrolysis is normally carried out at a temperature between 300 and 500 C, so that discharged into the chamber 8 of the ash and a high carbon residue. A surprising consequence of the process carried out as described above is that the residues obtained from activated sludge have a relatively high adsorption capacity, i.e. enough

  to be used without further treatment.

  as a filter medium. It is even more important

  note that they have physical and dietary characteristics

  enabling them to be activated in a known manner, in particular by being heated to approximately

  900 ° C. in carbon dioxide, to give a

  duit not differing in any respect from the best coal

  activated. This activation can be carried out

  in the collector chamber 8 itself, the heat

  necessary being supplied by the same fuel gases

  produced inside the chamber 15, which gases can be introduced into an external cavity (not shown) located on the chamber 8, similar to the cavity 5, before passing into the skirt 6. In this case also, the The process is surprisingly autonomous, or almost so, in that the sludge is finally converted into non-contaminating combustion gases (carbon dioxide and water) and into a product of some value, which can be marketed or used in the installation. purification plant producing the sludge,

  instead of the activated carbon normally used.

  The enormous economic and practical benefits

  invention will emerge from the description

  following, as well as the fact that the method described with reference to FIG. 1 can be implemented

  just as well on other types of installations.

  The invention will therefore still be described in the forms

  following implementation steps as a demonstration

tion.

  EXAMPLE 1 - Composition of pyrolytic gases

  Sludges having an average composition as indicated in the table were processed in a laboratory reactor. Instead of being burned, the resulting pyrolytic gases were collected and analyzed, as was the dry residue produced in the reactor. The composition of the organic phase condensate was calculated as the difference between the sludge and the sum of gases, solids and water, as shown in the table. The pyrolysis process was repeated at different temperatures and the fuel gas composition of the pyrolytic gases analyzed as indicated

on the graph of Figure 5.

BOARD

Sludge composition C H N 0 Cen-

  dried mud 36.8 4.9 6.9 34.2 17.1 solids 15.5 0.4 2.3 7.7 17.1

  Cond. Products 16.9 4.0 4.6 19.9 -

has 5pyrolyse

Egaz 4,4 0,5 - 6,7 -

1 2

  EXAMPLE 2 - Capacity of adsorption of the residue

  The dry residue of the sludge treated in Example 1 was activated in a furnace, in a carbon dioxide stream, at temperatures of 870 and 920 C for 15, 20, 30 and 40 minutes each. The methylene blue adsorption isotherms of the activated residue and activated carbon of the "Pittsburgh Filtrasob F440" type were then tested to give

  the results shown in Figure 6.

  It goes without saying that many changes

  may be made to the process and the installation

  described and represented without departing from

of the invention.

Claims (12)

  1. Process for the pyrolytic elimination of activated sludge, in particular sludge from   industrial wastewater treatment plants   trielles, characterized in that it comprises the steps of: loading said sludge into a reaction chamber (2) isolated in a gastight manner from the outside atmosphere; to pyrolyze said sludge in the absence   gaseous oxygen to prevent any forma-   combustion, the pyrolysis being carried out by indirect heating of the sludge by heat exchange through the walls (7) of the chamber (2) of reaction, by circulating a stream of combustion gas on the outside of the chamber reaction (2); - to evacuate the pyrolysis gases produced   inside the reaction chamber (2) by fractional   thermal generation of the molecules of said sludge;   to produce said gas stream   venting by burning the pyrolysis gases discharged from the reaction chamber (2), the combustion step being carried out exclusively outside the reaction chamber (2); and - to unload the pyrolyzed sludge from the reaction chamber (2).   2. The process according to claim 1, wherein   characterized by the fact that the sludge is pyrolyzed at a temperature of 300 to 500 ° C, the pyrolysis step   being continued long enough for us to obtain   pyrolyzed sludge consisting mainly of a carbon-based residue.   3. Process according to claim 2, characterized   characterized in that the pyrolyzed sludge discharged from the reaction chamber (2) is adsorbent and can be activated by the use of heat   in a stream of carbon dioxide.   4. Process according to any one of   preceding claims, characterized in that   the reaction chamber comprises a cylindrical rotary kiln (2) inside which the sludge is continuously charged and forced to move along the axis of rotation of the furnace, and from which the pyrolyzed sludge is discharged continuously for passing into an airtight collection chamber (8), the heating step by heat exchange with the flue gas stream being effected by circulating said flow of combustion gas in the opposite direction to that of the sludge inside a cavity (5) surrounding the rotary kiln (2) externally.   5. Process according to claim 4,   because the sludge is loaded into   the cylindrical oven (2) substantially according to a general   trice of it in order to immediately enter into   tact with the hot walls of the furnace (2), the sludge being loaded into the furnace (2) by means of a mechanism (4) with double screw whose inlet is closed in a fluid-tight manner by the sludge itself contained in a crate (3).   6. Process according to any one of   preceding claims, characterized in that   that the pyrolysis gases are discharged from the reaction chamber (2) by means of an ejector (12) supplied with a stream of air which also behaves as   combustion support for the combustion of gases.   7. Process according to any one of   preceding claims, characterized in that   that the flue gas stream is produced by the combustion of a fuel, in addition to said pyrolysis gases, and by mixing the resulting gases of the two combustion stages, which steps are advantageously carried out in a common refractory furnace (15). 8. Installation for pyrolysis or   or both, activated sludge, this plant   comprising an oven (2) at one end of which said sludge is continuously charged and at a second end of which it is discharged   continuously, and means. intended for indirect heating   the sludge, the installation being characterized   in that the indirect heating means com-   take a skirt (6) covering the outside of the furnace (2) to define, with an external heat exchange surface (7) of the furnace (2), a cavity (5) along which a stream of gas hot combustion gases is circulated through an inlet (18) formed at a first end of the skirt (6) to an outlet (20) formed at a second end of the skirt (6), the interior of which oven (2) being gastightly insulated from the outside atmosphere, and the installation also comprising means (12) for evacuating the gases produced inside the furnace (2) by heating the sludge, means (12,   ) intended to add combustion support air   to said gases and to burn them to produce said stream of hot combustion gases, and means (16) for introducing the gas stream   combustion inside the cavity (5).   9. Installation according to claim 8, characterized in that the surface (7) of heat exchange located on the oven (2) is defined by a cylindrical side wall (26) of the oven and by external radial fins ( 40) consisting of a continuous element wound around said wall   cylindrical side (26) which it is integral.   10. An installation according to claim 9, characterized in that the continuous element (40) is wound around the cylindrical wall (26) of the oven (2) at a step gradually decreasing from said second end towards said first end of the furnace (2), the skirt (6) being arranged so that its   first and second ends are close,   second and first ends of the furnace (2), the inlet (18) and the outlet (20) being each formed entirely in the space extending between   two adjacent volutes of said element (40).   11. Installation according to one of the   9 and 10, characterized in that the oven (2) comprises a device (4) for loading the sludge and which is located substantially on a generator   of said cylindrical side wall   the device (4) comprising an envelope (35) housing two side-by-side screws (36) which comprise   helicoidal elements (37) crossing one another.   12. Installation according to any one   Claims 8 to 11, characterized by the fact   the oven (2) is a rotary kiln resting on rollers (27) freely relative to the skirt (6) which is fixed.