EP0686686A1 - 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 to the pyrolysis of solids, in particular industrial and / or household waste.

The thermal treatment of waste and in particular its pyrolysis is an increasingly used technology because it offers a certain number of advantages over other means such as landfilling, compaction or even so-called treatments. "wet" that is to say comprising at least one step of washing the 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 maîtrisés 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.

Various technologies have already been proposed aiming to more specifically solve one or the other of these problems.

Thus patent application FR 2 668 774, filed in the name of the applicant, describes an installation comprising a rotary pyrolysis oven heated externally and having in particular a specific means for trapping the pollutants produced by the pyrolysis.

The French patent application EN. 94/06660 filed in the name of the applicant, discloses a rotary pyrolysis oven comprising heating means placed inside the pyrolysis oven in order to reduce energy consumption 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 N0x 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 any particular treatment, that is to say with their tars and their particles if these remain in limited quantities, to heat the rotating cylinder 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 by a dry treatment, the dechlorination of the pyrolysis gases already started in the rotating cylinder.

The present invention makes it possible in particular to solve the problems of fouling in the discharge lines of the pyrolysis gases.

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

The objectives set out above are achieved according to the invention which relates to an oven intended for the thermal 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 oxidant into said chamber.

According to the invention, the means for introducing fuel and oxidizer are oriented tangentially to the wall of said combustion chamber, so that the flame or flames which 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 in order to carry out staged combustion in said combustion chamber.

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

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

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

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

The invention further relates to a process for the thermal treatment of waste comprising pyrolysis of the waste in an essentially cylindrical cavity which rotates around its longitudinal axis, heating of the cavity by means placed in a combustion chamber surrounding said cavity 1, the injection of fuel and oxidant into the combustion chamber being carried out tangentially to the internal wall of said combustion chamber.

Preferably, a stepwise combustion is carried out in said combustion chamber.

Advantageously, the method can also comprise a step of dechlorination of the pyrolysis gases coming 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 FIG. 1. The oven comprises an elongated cylindrical rotating part 1 connected at each end to fixed parts 2 and 3 by means of tight seals 4 and 5. The rotating part 1 is always either horizontal or slightly inclined according to the requirements for flow rate and residence time of the solids in the system.

The waste is brought by a line 6 fixed to the fixed element 3 at the top of the furnace. At the outlet of line 6, the waste falls into the rotating part 1, forms a bed 7 there and is heated then pyrolyzed as it progresses through the oven towards the fixed part 2.

The fixed part 2 includes an area 2a for withdrawing the solid phase, and a line 8 maintained over its entire length at a temperature close to that of the waste at the outlet of the rotating part 1 and intended for the evacuation of the pyrolysis gases. .

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

The means 9 are essentially means for injecting and controlling the air and fuel flow rates, the air being supplied by a line 10 and the fuel being supplied by the line 8, which make it possible to generate one or more enveloping flames. the rotating part 1.

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

The positioning of the means 9 will be better understood with the aid of FIG. 2, which shows that they are installed so as to ensure a substantially tangential introduction of the fuel and of the oxidant, 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 in a helical movement which completely envelops the rotating part 1. They leave the chamber 12 by a line 11.

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

According to a particular mode of operation of the invention, it is possible to achieve staging of the combustion by supplying only a part of the air necessary for combustion by the line 10 and by providing the complement by 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 is formed. This characteristic in particular avoids the self-ignition problems which arise when a mixture of air and combustion is created upstream of the burners.

The staging of the combustion also makes it possible to modify the heat fluxes along the rotating cylinder. Staging can concern both fuel and air, although fuel staging is a priori less desirable, since it presupposes the installation of flow control devices, all of which increase the risk of fouling. and plugging the pipes carrying the pyrolysis gases.

Preferably, the oven according to the invention can allow waste treatments 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 entrain hydrocarbon particles or not, which generate ashes in the chamber 12. To this end, the lower part of the chamber 12 can include conventional devices for collecting and evacuating these 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 operation with dusty gases in order to have sufficiently high initial fluid velocities, so that the flow of the smoke in the chamber 12 retains its helical character until the outlet 11. Typically the initial speeds of the air and of the pyrolysis gases are between 10 and 250 m / s, preferably between 50 and 100 m / s. The velocities of air and pyrolysis gases are not necessarily the same. The restrictions on the means 9a, 9b and 15 also make it possible to regulate the flow of pyrolysis gas coming from the rotating cylinder, and thus to ensure more stable combustion.

When the heating requirements of the cylinder are significantly lower than the energy contained in the pyrolysis gases, part of said gases taken from the circuit can then be evacuated to the outside by a line 16. In the opposite case, it is that is to say when the pyrolysis gases are not sufficiently energetic to provide for the needs of the furnace, it is possible, for example, to boost said pyrolysis gases, with an auxiliary fuel.

To function optimally from an energy point of view, it is in particular necessary to avoid the entry of air into the combustion chamber 12 through the connections 13, connections which for mechanical reasons are not always perfectly sealed. It is therefore important that the pressure in said chamber 12 is maintained at all times at atmospheric pressure; which can be done as indicated in FIG. 1, by means of a controlled extraction of the fumes carried out for example using an extractor 17 associated with a recycling loop whose flow rate is controlled by a valve 19 controlled 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 FIG. 3, where a step of dechlorination of the pyrolysis gases has been added after they have left the oven. Line 8 of the furnace shown in Figure 1 is replaced by a line, devoid as much as possible, of singularities to avoid blockages by deposits of solid materials 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 mixing of the absorbent with the pyrolysis gases. The dechlorination process takes place throughout the course of the gases in a line 33. The mixture then preferably enters a gas-solids separator 34 which can for example be a cyclone or possibly a set of cyclones arranged in series or in parallel. The pyrolysis gases, freed from most of their particles, are then sent to the combustion means 9 by a line 35, while the collected solids are evacuated by another line 36. The absorbent, partially used, can advantageously be returned to the rotating enclosure 1, where it can again participate in the dechlorination process of the pyrolysis gases.

Of course, minor additions and / or modifications may be made by those skilled in the art to the oven and to the process just described without going beyond the ambit of the present invention.

Claims (11)

1) Oven intended for heat treatment and in particular for pyrolysis of waste, comprising a cavity (1) essentially cylindrical for the waste to be treated, rotating around its longitudinal axis; a combustion chamber (12) located around said cavity and means (9a, 9b, 15) intended to introduce fuel and oxidant into said chamber (12), characterized in that the means (9a, 9b, 15) fuel introduction and oxidizer are oriented tangentially to the wall of said combustion chamber (12), so that the flame or flames they create develops helically around said cavity (1) containing the waste and in that the means (9a, 9b, 15) for introducing fuel or oxidizer are different and spaced longitudinally along the cavity (1) in order to carry out staged combustion in said combustion chamber (12). 2) Oven according to claim 1, characterized in that it further comprises an outlet for the pyrolysis gases associated with a line (8; 33, 37) intended to bring part of said pyrolysis effluents to the means (9b) for introducing fuel into said combustion chamber (12). 3) Oven according to claim 2, characterized in that the line (37) for the evacuation of the pyrolysis gases comprises a means (31) for absorbing the absorbent and a means (34) for separation between the gases and the solids circulating in said line. 4) Oven according to claim 3, characterized in that the means (31) of absorbent injection cooperate with means (32) such as venturi which have non-constant sections in order to increase the speed of flow of fluids passing through them and the intensity of the particular gas mixture. 5) rotary kiln according to any one of the preceding claims, characterized in that it further comprises means (17, 18, 19, 30) intended to maintain inside the combustion chamber (12) a pressure substantially equal to atmospheric pressure. 6) Oven according to claim 5, characterized in that said means for maintaining pressure in particular comprise means (17) for extracting combustion fumes, a valve (19) for controlling the flow rate controlled by a pressure sensor (30 ) placed inside said combustion chamber (12). 7) Oven according to any one of the preceding claims, characterized in that the rotary cavity (1) cooperates with a first fixed means (3) through which the waste is introduced and with a second fixed means (2) in which the waste is discharged once treated. 8) Oven according to any one of claims 2 to 7, characterized in that said line (8; 37) for discharging the pyrolysis gases outside the rotary oven (1) opens into a fixed end (2) of said oven ( 1). 9) A method of heat treatment of waste comprising a pyrolysis of the waste in an essentially cylindrical cavity (1) which rotates around its longitudinal axis, heating of the cavity by means (9a, 9b, 15) in a combustion chamber ( 12) surrounding said cavity (1), characterized in that the injection of fuel and oxidant into the combustion chamber (12) is carried out tangentially to the internal wall of said combustion chamber (12). 10) Method according to claim 9, characterized in that one carries out a staged combustion in said combustion chamber (12). 11) A method according to any one of claims 9 or 10, characterized in that it further comprises a step of dechlorination of the pyrolysis gases from the cavity (1).

Images