FR2710967A1 - Method and unit for treating dangerous waste by calcining - Google Patents

Abstract

The subject matter of the invention relates to a unit for treating dangerous waste by calcining. The unit includes: . a confinement enclosure (2) defining, above the upper level (31) of the fluidised bed (3), a post-combustion chamber (18) for the combuston fumes leaving the said bed, . means (20), separate from the heating means (16) of the fluidised bed, for heating the post-combustion chamber in order to make it possible to heat the post-combustion chamber to a temperature which is different from the combustion temperature and is designed to ensure combustion of the fumes, . and means (27) for supplying the post-combustion chamber with oxidant.

Description

CALCINATION TREATMENT METHOD AND UNIT
DANGEROUS WASTE
The present invention relates to the technical field of treatment by calcination of waste having a hazardous character.

 The invention finds a particularly advantageous application for the incineration of waste relating, for example, to the industrial or household sector having a hazardous character and to the nuclear or hospital fields.

 The incineration of hazardous waste must be carried out in accordance with various regulations intended to preserve the environment. In order to satisfy the imposed requirements, the methods of incineration of such waste require the implementation of increasingly complex installations.

 Conventionally, an incineration plant for hazardous waste includes a waste preparation system, intended to supply a calcination furnace. The fumes leaving the oven are sent to an afterburner system, the heat of which is generally provided by the combustion of a fossil fuel. The incineration plant also includes a smoke treatment system to recover, in particular, sulfur and chlorine compounds, a dedusting system and an ash cooling system.

 The state of the art has described, in particular by US Pat. No. 4,755,138, in particular a furnace for calcining toxic or dangerous products comprising a confinement enclosure for a fluidized bed formed from a mixture of waste and heat transfer grains. The calcination furnace is equipped with a circuit for supplying a pressurized fluid opening into the bottom of the enclosure, so that the bed is fluidized. Such a bed comprises, at its lower part, means for supplying waste and, at its upper part, extraction means known per se. The fluidized bed is maintained at a given combustion temperature by means of heating means.

 A calcination installation has a relatively high cost because of the price involved in producing, in particular, the oven, the afterburner and the smoke treatment system. It turns out, in particular, that the afterburner system requires the use of a large volume unit, due, on the one hand, to the difficulty of controlling the quantities of smoke to be calcined and, d on the other hand, the creation of additional smoke from fossil fuel.

 The calcination plants known to date do not allow the realization of incineration units having a relatively low treatment capacity and satisfactory profitability. However, it is clear that the need appears to have such facilities, so as to be able to treat waste on a site close to their place of production, in order to avoid the risks taken during their transport.

 The present invention therefore aims to satisfy this need by proposing a method of treatment by calcination of hazardous waste making it possible to offer the possibility of constituting an incineration unit having a relatively low treatment capacity, while retaining satisfactory profitability.

 The present invention therefore aims to propose a method for designing, for example, cost-effective treatment installations offering a capacity, for example of the order of 200 to 500 kg / hour and respecting the various regulatory provisions relating to the incineration of dangerous waste.

To achieve these objectives, the method of treatment by calcination of hazardous waste consists of - creating in the containment of a furnace, a fluidized bed formed of a
mixture of waste and inert materials, at the base of which is injected a fluid
under pressure, - heating the fluidized bed to a combustion temperature to ensure the
calcination of the chemicals of the fluidized mixture, - to feed the fluidized bed, at its lower part, using waste, - and to extract, from the upper level of the fluidized bed, part of the fluidized bed.

According to the invention, the method consists - of constituting in the confinement enclosure of the oven, above the upper level
fluidized bed, a post-combustion chamber for combustion fumes
from the fluidized bed, - heating the post-combustion chamber to a post-combustion temperature
different from that of combustion, to ensure combustion of the fumes, - and to supply the afterburning chamber with an oxidizer for the
fumes.

The object of the invention is also to propose a unit for the treatment by calcination of hazardous waste of the type comprising - a calcination oven delimiting a confinement enclosure for a fluidized bed
formed of a mixture of waste and inert materials, - a circuit for supplying a pressurized fluid opening into the bottom of
the enclosure to ensure at least the fluidization of the bed, - means for heating the fluidized bed to a given combustion temperature, - means for supplying the waste, opening out substantially at the bottom of
the enclosure, - and means for extracting part of the fluidized bed, from the upper level
of said bed.

According to the invention, the processing unit comprises - a containment enclosure delimiting above the upper level of the bed
fluidized, an afterburner for combustion fumes from
said bed, - means for heating the afterburner, separate from the means
for heating the fluidized bed, to allow the heating of the
post-combustion at a temperature different from that of combustion and suitable
to ensure the combustion of smoke, - and means for supplying an oxidizer to the afterburner.

 Various other characteristics will emerge from the description given below with reference to the appended drawings which show, by way of nonlimiting examples, embodiments and implementation of the subject of the invention.

 Fig. 1 is a schematic general view in elevation of an incineration unit according to the invention.

 Fig. 2 is a schematic view showing a preferred variant embodiment of the processing unit according to the invention.

 Fig. 1 shows a calcination treatment unit for hazardous waste comprising a calcination oven 1 delimiting an enclosure 2 for confining a mixture 3 including, in particular, waste to be calcined.

 In a conventional manner, the furnace 1 comprises an insulating envelope 4 suitably constituted to form a barrier limiting the propagation of calories from the interior of the furnace to the surrounding medium. Inside the insulating envelope 4 is mounted a tank 5 comprising a bottom 6 and a double bottom 7 defining between them an intake manifold and pressure distribution.

The double bottom 7 is provided with fluidization nozzles 8 opening inside the tank 5 and communicating with a circuit 9 adapted to supply a gaseous fluid under pressure which allows a fluidization of the mixture 3. Advantageously, the gaseous fluid is chosen for its oxidizing nature for waste and, for example, such a fluid may simply be compressed air. The circuit 9 is provided with a system 10 for heating the fluidizing air, placed downstream of a pressure relief device 11.

 The tank 5 is provided with means 12 for supplying or supplying waste to be calcined. Such means 12 may consist of a sheath containing an endless screw opening at the bottom of the tank 5, substantially at the level of the double bottom 7. The supply means 12 are supplied with waste by an installation 13 for preparing waste to be calcined.

The purpose of the installation is to carry out grinding, sieving and / or mixing of waste with inert materials, such as preferably sand, in order to give the mixture the properties of a fluidizable powder. Such a powder is intended to be incorporated into inert materials of fluidizable nature, previously placed inside the tank, together forming the mixture 3 to be fluidized. These inert materials constitute heat transfer elements and also make it possible to ensure filtration for non-combustible solid waste.

 The tank S is also equipped with means 14 ensuring extraction of the mixture 3 including the non-combustible solid waste. The extraction means 14 are placed at a given level of the tank to maintain a constant height of the mixture 3. The extraction means 14 are constituted by an endless extraction screw or involve, as illustrated, a simple flow gravity from a spillway of predetermined level communicating with a device 15 for cooling and storage of non-combustible solid waste.

Advantageously, the solid waste recovered can be used in whole or in part by the installation 13 for preparing the waste.

 The calcination unit further comprises heating means 16 which are constituted by static means capable of heating the mixture 3. Such means are preferably constituted by electrical resistors arranged between the tank 5 and the insulating jacket 4, being distributed over the entire periphery of the tank to cover the height corresponding to that of the charge of the mixture 3. The heating means 16 are controlled by a circuit 17 for servo-control, regulation or piloting , controlling the supply of resistors as a function of the internal temperature of the mixture 3 detected by any appropriate means.

 According to the invention, the tank 5 delimits, above the upper level 3, of the mixture 3, a post-combustion chamber 18 for the combustion fumes from the mixture 3. The post-combustion chamber 18 is arranged so as to provide the fumes with a residence time determined in said chamber to obtain their combustion.

 According to the invention, the calcination unit comprises means 20 for heating the afterburning chamber 18, distinct from the heating means 16, so as to maintain the chamber 18 at a temperature different from that of combustion of the mixture 3. The heating means 20 are constituted by static means, such as electrical resistors, arranged between the insulating envelope 4 and the tank 5, being distributed over the entire device of the tank to cover the height of the afterburner 18 s extending substantially above level 3, of the mixture 3 to a cover 21 for closing the tank 5. The heating means 20 could also consist of rods or coils mounted inside the tank 5 to provide calories supplied by electrical resistances or heat transfer fluids. The heating means 20 are connected to a servo, regulation or piloting circuit 23, controlling the supply of the heating means 20 as a function of the internal temperature of the afterburner 18, detected by any suitable means, such as as temperature sensors 24 mounted on the internal wall of the tank 5.

 According to the invention, the calcination unit also comprises means 27 for supplying an oxidizer to the afterburner chamber 18. In the example illustrated in FIG. 1, the means 27 for supplying an oxidizer are constituted by a supply rod 28, mounted inside the tank 5 and supplied by a circuit 29 for supplying the oxidant which is previously heated by a system 30.

Advantageously, the oxidizer is simply compressed air, so that the supply circuit 29 and the fluidization circuit 8 constitute a single supply circuit for the air mounted under pressure by the booster 11.

 In the example illustrated, the supply rod 28 passes through the cover 21 of the tank and extends vertically inside the tank and comprises, at its end, an injection nozzle 30. The nozzle 30 is arranged to allow the addition of the oxidant to the lower part of the afterburner 18 and / or to the upper part of the mixture 3, as is clear from FIG. 1.

 From the cover 21, rises a chimney 31 connected to a smoke evacuation circuit comprising a system 32 for cooling the fumes and a device 33 for treating fumes of all known types ensuring, for example, filtration, washing and desulfurization.

 The processing unit described above comprises a furnace 1 with a general vertical axis allowing the maintenance of a large load height of mixture 3 containing waste brought continuously or not to the base of the tank 5. The means extraction 14 are provided at the upper level of the mixture 3, so as to have a large and known load height and to impose, on each waste, a very specific residence time by forcing it to progress, so upward, inside the mixture 3 constituted in the form of a fluidized bed by the injection of fluid through the nozzles 8.

 Thus, the mixture 3 is constituted in the form of a fluidized bed according to particularly precise conditions, since they depend solely on the parameters for supplying the oxidizing fluid coming from the circuit 9. It is thus possible to obtain a fluidized bed according to constant conditions which guarantee a well-known upward path and a constant residence time of all the waste from its low injection level, until reaching the upper recovery level.

Such certain path conditions, combined with precise temperature maintenance by the means 16, makes it possible to involve calcination of the waste. For example, the waste is calcined at a constant temperature between 600 "C and 950" C, inside the fluidized bed 3, with a residence time greater than one hour. By way of example, the enclosure 2 has a diameter of between 0.5 and 2 meters and a height suitable for obtaining the desired residence time. It should be noted that the residence time of the waste in the fluidized bed can be increased by increasing the height of the bed 3 by closing the outlet outlet with a register 141.

 The fumes generated during combustion remain inside the fluidized mass 3, at the combustion temperature for a determined time which can be, for example, of the order of 10 seconds. For example, the fluidized height is between 0.5 and 2.5 meters. The fumes then arrive in the post-combustion chamber 18. The fumes are thus mixed with the oxidant delivered by the circuit 27. After their mixing with the oxidant, the fumes are subjected, for a determined time, for example of the order of 2 seconds, at the post-combustion temperature imposed by the heating means 20. For example, the post-combustion temperature is between 850 and 1,200 "C. The fumes are then extracted from the tank 5 to cool in the heat exchanger-recuperator 32 before passing through the smoke treatment device 33 which releases the gaseous fluid into the atmosphere.

 The object of the invention makes it possible to reduce the cost of an incineration unit due to the reduction in the flow rate of the fumes discharged into the atmosphere. Such a reduction is due to the use, for the oven and the afterburner, of static heating means, preferably electric, making it possible to contain the flow of fumes to those corresponding to the combustion of waste.

 The object of the invention also aims to overcome the use of a calcination oven and a separate system for afterburning the fumes. Indeed, the calcination unit comprises a single treatment enclosure where the waste incineration operations are carried out in the lower part and the afterburners of the fumes in the upper part. The object of the invention is to operate the two parts of the enclosure at two distinct temperatures, using two heating means 16 and 20 regulated separately, respectively as a function of the temperature of the mixture and as a function of the temperature of the upper wall of the enclosure forming the afterburner 18.

 Thus, the treatment unit according to the invention associated with a gas treatment system which sees a flow rate ten times lower than the flow rate of a traditional incineration plant, must make it possible to design profitable treatment plants of a capacity from 200 to 500 kg / hour and perfectly meeting the most restrictive regulatory provisions on the incineration of hazardous waste.

 Fig. 2 illustrates another alternative embodiment of the invention in which the supply rod 28 crosses the bottom 6 and the double bottom 7 of the tank 5 to extend vertically inside the fluidized bed 3. The cane supply 28, which crosses the fluidized bed 3, makes it possible to obtain the heating of the oxidant circulating inside the cane, by means of the heat available inside the fluidized bed 3. According to this variant, the cane 28 ensures the supply of the oxidant in the lower part of the combustion chamber 18 and / or in the upper part of the fluidized bed 3. It should be noted that the afterburner chamber 18 can be supplied, only or in combination with the cane inlet 28, using an oxidizer corresponding to a surplus of the fluid which ensures the fluidization of the bed 3.

 In the examples illustrated, the containment consists of a circular shell with a single diameter over its entire height. However, when the quantity of gas to be oxidized is particularly large, the upper part of the enclosure can advantageously consist of a ferrule with a diameter greater than that of the lower part, in order to reduce the total height of the incinerator, and avoid large shipments of fluidized solids at the bottom. In this case, the two parts are connected, for example, by a frustoconical part.

Claims (14)

CLAIMS:  1 - Method for the treatment by calcination of hazardous waste, of the type consisting in - creating in the enclosure (2) confinement of an oven (1), a fluidized bed (3) formed  of a mixture of waste and inert materials, at the base of which is injected a  pressurized fluid, - heating the fluidized bed to a combustion temperature to ensure  calcination of the chemicals of the fluidized mixture, - to feed the fluidized bed, at its lower part, using waste, - and to extract, from the upper level of the fluidized bed, part of the fluidized bed,  characterized in that it consists - of constituting in the enclosure (2) for confining the oven, above the level  upper (3,) of the fluidized bed (3), a post-combustion chamber for the fumes  combustion from the fluidized bed (3), - heating the post-combustion chamber (18) to a temperature of  post-combustion different from that of combustion to ensure the combustion of  fumes, - and to supply the afterburner with an oxidizer for  fumes.  2 - Method according to claim 1, characterized in that it consists in supplying the afterburner chamber (18) using an oxidizer corresponding to a surplus of the fluid which ensures the fluidization of the bed.  3 - Method according to claim 1, characterized in that it consists in supplying by combustion of the afterburner chamber (18) at its lower part.  4 - Method according to claim 1 or 3, characterized in that it consists in supplying with combustion the afterburner chamber (18), by injecting it into the upper part of the fluidized bed (3).  5 - Method according to claim 3 or 4, characterized in that it consists in supplying by combustion of the afterburner chamber (18) using a supply rod (28) passing through the fluidized bed (3).  6 - Method according to claim 1, characterized in that it consists in producing in the confinement enclosure (2), the afterburner chamber (18), so as to provide the fumes with a determined residence time in said chamber for get them burned.  7 - Unit for the treatment by calcination of hazardous waste of the type comprising - a calcination oven (1) delimiting a confinement enclosure (2) for a bed  fluidized (3) formed of a mixture of waste and inert materials, - a circuit (9) for supplying a pressurized fluid opening into the bottom of  the enclosure (2) to ensure at least the fluidization of the bed, - means (16) for heating the fluidized bed to a given temperature of  combustion, - means (12) for supplying the waste, opening out substantially at the bottom  of the enclosure, - and means (14) of extracting part of the fluidized bed, from the level  upper of said bed.  to ensure combustion of the fumes, - and means (27) for supplying an oxidizer to the afterburner chamber.  afterburning at a temperature different from that of combustion and suitable  heating means (16) of the fluidized bed, to allow the chamber to be heated  combustion from said bed, - means (20) for heating the afterburner, separate from  fluidized bed (3), a post-combustion chamber (18) for the smoke from  characterized in that it comprises - a confinement enclosure (2) delimiting above the upper level (3,) of the  8 - Processing unit according to claim 7, characterized in that the means for supplying an oxidant (27) consist of a supply rod (28) mounted inside the confinement enclosure (2) and supplied by a circuit (29) for supplying an oxidizer.  9 - Treatment unit according to claim 8, characterized in that the supply rod (28) of the oxidant is mounted to extend from the bottom (6, 7) of the enclosure and pass through the fluidized bed (3 ), in order to bring the oxidant to the lower part of the afterburner and / or to the level of the upper part of the fluidized bed (3).  10 - Processing unit according to claim 7 or 8, characterized in that the supply circuit of an oxidant (29) and the supply circuit of a pressurized fluid (9) form a single supply circuit.  11 - Processing unit according to claim 7, characterized in that the means (20) for heating the afterburner chamber are controlled by a regulation circuit (23) ensuring the maintenance of the wall of the afterburner chamber at temperature desired afterburner.  12 - Processing unit according to claim 7, characterized in that the containment (2) has, for the part defining the afterburner (18), a diameter greater than the part defining the fluidized bed (3).  13 - Processing unit according to claim 7, characterized in that the means (12) for supplying the waste are supplied by a system (13) for processing the waste.  14 - Processing unit according to claim 7, characterized in that the means (20) for heating the afterburner chamber are of the static type, in particular electrical.