EP0686686B1 - Furnace for the thermal treatment of waste and process therefor - Google Patents

Abstract

Waste travels along a rotating cylinder inside a combustion chamber. Combustible pyrolysis gases from the waste are piped to a nozzle (9b) in the combustion chamber. Air is admitted through two nozzles (9a,15). All three nozzles are oriented tangentially to the cylinder so that the flames travel helically along the cylinder. <IMAGE>

Description

The present invention relates to heat treatment and more specifically the pyrolysis of solids, in particular waste industrial and / or household.

The heat treatment of waste and in particular its pyrolysis is an increasingly used technology because it offers a number of advantages over other means such as that landfill, compaction or treatments said to be "wet", that is to say comprising at least one step of washing solids.

• Il nécessite un apport énergétique important car les solides doivent être portés à des températures pouvant dépasser 1000°C ; d'où des coûts de fonctionnement très élevés,
• La nature parfois très hétérogène des déchets nécessite une souplesse de fonctionnement - souvent incompatible avec l'inertie thermique des installations, et les moyens de contrôle de débits/de chlores,
• Les rejets de matières polluantes tels que les oxydes de soufre créées lors de la pyrolyse doivent être maítrisés puis inhibés car ces rejets sont bien entendu nocifs pour l'environnement. Des normes de plus en plus sévères rendent le problème de rejet de matières polluantes très sensible.

However, heat treatment sometimes has drawbacks:

• It requires a significant energy contribution because the solids must be brought to temperatures which can exceed 1000 ° C; hence very high operating costs,
• The sometimes very heterogeneous nature of the waste requires flexibility of operation - often incompatible with the thermal inertia of the installations, and the means of controlling flows / chlorines,
• Releases of pollutants such as sulfur oxides created during pyrolysis must be controlled and then inhibited because these releases are of course harmful to the environment. Increasingly strict standards make the problem of rejection of pollutants very sensitive.

Different technologies have already been proposed aimed at more specifically solve one or the other of these problems.

Patent application FR-A- 2 668 774, filed at name of the applicant, describes an installation comprising a externally heated pyrolysis oven having in particular a specific means of trapping pollutants produced by pyrolysis.

The French patent application EN. 94/06660 (FR-A- 2 720 487 or EP-A- 0 685 551) deposited at applicant's name discloses a rotary pyrolysis oven comprising heating means placed inside the same pyrolysis oven to reduce consumption energy and thermal inertia.

• La possibilité d'utiliser les gaz de pyrolyse sans traitement particulier, c'est-à-dire avec leurs goudrons et leurs particules si celles-ci restent en quantités limitées, pour chauffer le cylindre tournant de manière indirecte;
• Un dispositif de combustion simple qui supprime les problèmes d'encrassement;
• Un dispositif de combustion qui peut réduire les émissions de NOx par étagement de l'injection du combustible et/ou du comburant;
• La possibilité de compléter par un traitement à sec, la déchloration des gaz de pyrolyse déjà commencée dans le cylindre tournant.

The present invention represents an improvement of this type of installation since it provides in particular:

• The possibility of using the pyrolysis gases without special treatment, that is to say with their tars and their particles if these remain in limited quantities, to heat the cylinder rotating indirectly;
• A simple combustion device which eliminates fouling problems;
• A combustion device that can reduce NOx emissions by staging the injection of fuel and / or oxidizer;
• The possibility of supplementing with a dry treatment, the dechlorination of the pyrolysis gases already started in the rotating cylinder.

The present invention makes it possible in particular to resolve the fouling problems in the exhaust gas lines of pyrolysis.

Control and control of pressures in the installation is also advantageously taken into account according to the invention.

The above objectives are achieved according to the invention which relates to an oven intended for the heat treatment of waste, comprising an essentially cylindrical cavity for the waste to be treated rotating around its longitudinal axis, a combustion chamber located around said cavity and means for introducing fuel and oxidizer in said room.

According to the invention, the means for introducing fuel and oxidant are oriented tangentially to the wall of said combustion chamber, so that the one or more flames they create develops helically around said cavity containing the waste.

Advantageously, the means for introducing fuel and / or oxidizer are spaced longitudinally along the cavity to achieve staged combustion in said combustion chamber.

In particular, the oven further includes an outlet for pyrolysis gases associated with a line intended to bring said pyrolysis gases to the means for introducing fuel in said combustion chamber.

In accordance with the invention, the gas recycling line of pyrolysis comprises an absorbent injection means and a means of separation between the gases and the solids circulating in said line.

In addition, the rotary kiln may further include means intended to maintain inside the chamber combustion a pressure substantially equal to the pressure atmospheric.

Without departing from the scope of the invention, the holding means in pressure include in particular a means of extracting combustion fumes, a flow control valve controlled by a pressure sensor placed inside said combustion chamber.

The invention further relates to a method of treatment waste thermal including pyrolysis of waste in an essentially cylindrical cavity which revolves around its longitudinal axis, heating of the cavity by means placed in a combustion chamber surrounding said cavity 1, injecting fuel and oxidizer into the combustion being carried out tangentially to the internal wall of said combustion chamber, so that the flame or flames they create develop (s) helically around the cavity (1) containing the waste.

We perform a staged combustion in said combustion chamber thanks to different introductions and longitudinally spaced fuel and fuel.

Advantageously, the method can further comprise a step of dechlorination of the pyrolysis gases from the cavity.

• La figure 1 montre de façon schématique en coupe longitudinale un four tournant selon un mode de réalisation de l'invention;
• La figure 2 est une coupe transversale simplifiée d'un four selon l'invention; et
• La figure 3 montre de façon schématique, en coupe longitudinale, un four tournant selon un autre mode de réalisation de l'invention.

Other details, characteristics and advantages linked to the present invention will appear better on reading the description which follows, given by way of illustration and in no way limiting, with reference to the appended figures according to which:

• Figure 1 shows schematically in longitudinal section a rotary oven according to an embodiment of the invention;
• Figure 2 is a simplified cross section of an oven according to the invention; and
• Figure 3 shows schematically, in longitudinal section, a rotary oven according to another embodiment of the invention.

An illustration of the device according to the invention is therefore given in Figure 1. The oven includes a rotating part elongated cylindrical 1 connected at each end to parts fixed 2 and 3 by means of tight seals 4 and 5. The part 1 is always either horizontal or slightly inclined according to solid flow and residence time requirements in the system.

The waste is brought by a line 6 fixed to the element fixed 3 at the top of the oven. At the end of line 6, the waste falls in the rotating part 1, there constitute a bed 7 and are heated then pyrolyzed as they progress in the oven to the fixed part 2.

The fixed part 2 comprises a zone 2a for the withdrawal of the solid phase, and a line 37 maintained over its entire length at a temperature close to that of the waste leaving the rotating part 1 and intended for the evacuation of pyrolysis gases.

The rotating part 1 is heated by the combustion of pyrolysis gases, using means 9 fixed to a fixed chamber 12 of substantially cylindrical shape and completely enveloping most of the cylinder turning 1.

The means 9 are essentially injection means and control of air and fuel flows, the air being brought by a line 10 and the fuel being brought by the line 8, which generate one or more flames wrapping the rotating part 1.

The injection means are preferably placed opposite of the first part of the rotating cylinder, where the waste is still cold and humid and where energy needs are the biggest.

The positioning of the means 9 will be better understood at using Figure 2, which shows that these are installed from so as to ensure a substantially tangential introduction of the fuel and oxidizer, which generates a flame which develops near the wall 20 of the chamber 12, which is preferably made of refractory and insulating materials.

The fumes produced by the combustion means 9 progress from the end 12a towards the end 12b according to a helical movement which completely envelops the part rotating 1. They leave chamber 12 by a line 11.

The tightness of the chamber 12 with respect to the outside is ensured by rotating joints or equivalent systems 13.

According to a particular mode of operation of the invention, one can achieve a staging of the combustion by not bringing that part of the air necessary for combustion via line 10 and by adding the complement with an additional line 14. This staging strategy reduces NOx emissions when the fuel contains nitrogen compounds.

The staging of combustion therefore allows the flame to develop as the mixture forms. This characteristic in particular avoids the problems of auto-ignition which arise when a mixture of air and combustion is created upstream of burners.

The staging of the combustion also makes it possible to modify heat flows along the rotating cylinder. The tiering can affect both fuel and air, although fuel staging is a priori less desirable, because it supposes the installation of flow control devices, all devices that increase the risk of fouling and plugging of pipes carrying pyrolysis gases.

Preferably, the oven according to the invention can allow waste treatment at temperatures between 150 and 900 ° C, with a preference for the range 400-600 ° C.

Advantageously, the pyrolysis gases are burned without treatment. They can therefore cause particles hydrocarbon or not, which generate ash in the chamber 12. To this end, the lower part of chamber 12 can understand conventional collection and disposal systems of ashes such as scrapers (which are not shown in Figure 1).

The means for introducing air and fuel 9a, 9b, and 15 may include restrictions (Venturi) adapted to the operation with dusty gases in order to have speeds fluid initials large enough that the flow of smoke in chamber 12 retains its helical character up to exit 11. Typically the speeds initials of air and pyrolysis gases are between 10 and 250 m / s, preferably between 50 and 100 m / s. Air velocities and pyrolysis gases are not necessarily the same. The restrictions on means 9a, 9b and 15 also allow regulate the flow of pyrolysis gas from the cylinder turning, thus ensuring more stable combustion.

When the heating needs of the cylinder are significantly lower than the energy contained in the gases of pyrolysis, part of said gases taken from the circuit can then be evacuated to the outside by a line 16. In the case reverse, i.e. when the pyrolysis gases are not enough energy to meet the needs of the oven, can for example dop said pyrolysis gases, with a auxiliary fuel.

To function optimally from the point of view energy, in particular avoid the entry of air into the combustion chamber 12 by connections 13, connections which for mechanical reasons are not always perfectly waterproof. It is therefore important that the pressure in said chamber 12 is maintained at all times under pressure atmospheric; which can be done as shown in the figure 1, thanks to a controlled smoke extraction carried out by example using an extractor 17 associated with a loop recycling, the flow rate of which is controlled by a valve 19 controlled at from the pressure information obtained by a sensor 30 placed inside the chamber 12.

Another embodiment of the oven according to the invention is given in Figure 3, where a dechlorination step of the pyrolysis gas after removal from the oven. Line 8 of the oven shown in Figure 1 is replaced by a line, devoid as much as possible, singularities to avoid blockages by solid deposits or tars. Said line is preferably maintained at a temperature close to that of the gases leaving the fixed part 2. It comprises an absorbent injection device 31, preferably associated with a means 32 such as for example a venturi, which facilitates the mixing of the absorbent with the pyrolysis gases. The process of dechlorination develops along the gas path in a line 33. The mixture then preferably enters a gas-solid separator 34 which can for example be a cyclone or possibly a set of cyclones arranged in series or in parallel. Pyrolysis gases, free of most of their particles are then sent to the means of combustion 9 by a line 35, while the solids collected are discharged through another line 36. The absorbent, partially used, can advantageously be returned to the enclosure revolving 1, where he can again participate in the process of dechlorination of pyrolysis gases.

Of course minor additions and / or modifications may be brought by the skilled person to the oven and process have just been described without departing from the scope of the present invention.

Claims (10)

1. A furnace designed for the thermal treatment and in particular pyrolysis of waste, comprising a substantially cylindrical cavity (1) for the waste to be treated, rotating about its longitudinal axis; a combustion chamber (12) arranged around said cavity and means (9a, 9b, 15) for introducing combustible and fuel, characterised in that the means (9a, 9b, 15) for introducing combustible and fuel are arranged at a tangent to the wall of said combustion chamber (12) so that the flame or flames which they create build up in a helical pattern around said cavity (1) containing the waste and the means (9a, 9b, 15) for introducing combustible or fuel are separate and spaced longitudinally along the cavity (1) in order to produce a staggered combustion inside the combustion chamber (12).
2. A furnace as claimed in claim 1, characterised in that it also has an outlet for the pyrolysis gases connected to a line (8, 33, 37) provided as a means of feeding some of said pyrolysis effluents to the means (9b) for introducing combustible into said combustion chamber (12).
3. A furnace as claimed in claim 2, characterised in that the line (37) for discharging the pyrolysis gases has a means (31) for injecting absorbent and a means (34) for separating the gases and solids circulating in said line.
4. A furnace as claimed in claim 3, characterised in that the means (31) for injecting absorbent cooperate with means (32) such as venturis having non-constant sections in order to increase the flow speed of the fluids passing through them and the intensity with which the specific gas is mixed.
5. A rotating furnace as claimed in any one of the preceding claims, characterised in that it also has means (17, 18, 19, 30) designed to maintain a pressure substantially equal to atmospheric pressure inside the combustion chamber (12).
6. A furnace as claimed in claim 5, characterised in that said pressure-maintaining means have in particular a means (17) for extracting the combustion fumes and a valve (19) for regulating the flow rate controlled by a pressure sensor (30) positioned inside said combustion chamber (12).
7. A furnace as claimed in any one of the preceding claims, characterised in that the rotating cavity (1) co-operates with a first fixed means (3) through which the waste is introduced and with a second fixed means (2) into which the waste is dropped once treated.
8. A furnace as claimed in any one of claims 2 to 7, characterised in that said line (8, 37) for discharging the pyrolysis gases outside the rotating furnace (1) opens into a fixed end (2) of said furnace (1).
9. A method of thermally treating waste including pyrolysis of the waste in a substantially cylindrical cavity (1) which rotates about its longitudinal axis, the cavity being heated by means (9a, 9b, 15) inside a combustion chamber (12) surrounding said cavity (1), characterised in that the combustible is injected and the fuel is injected into the combustion chamber (12) respectively at a tangent to the internal wall of said combustion chamber (12) so that the flame or flames which they create build up in a helical pattern around the cavity (1) containing the waste and in that combustion inside said combustion chamber (12) is staggered due to the fact that the combustible and fuel are introduced separately and in a longitudinally spaced arrangement.
10. A method as claimed in claim 9, characterised in that it also includes a step during which the pyrolysis gases leaving the cavity (1) are dechlorinated.

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