EP0685551A1 - Internally heated rotary kiln for the pyrolysis of waste - Google Patents

Abstract

In a rotary solids heat treatment furnace (1) which contains a longitudinally extending heating arrangement (9) and which has a longitudinal solids travel distance of about twice the length of the furnace interior, the novelty is that the heating arrangement (9) is fixed and provides piping of and preheating and/or heating of the solids. <IMAGE>

Description

The present invention relates to the field of heat treatment furnaces for solid materials, and more particularly pyrolysis (or thermolysis) furnaces intended to treat solids such as waste of any kind.

The ovens concerned are generally cylindrical and rotate around their axis of symmetry. The heat input necessary for pyrolysis can consist of solids or gases in contact or not with the solids to be treated.

Thus patent application FR 2 668 774 shows a pyrolysis oven in which the heating can be carried out outside the oven by burners supplied with pyrolysis gases; according to another embodiment, heat transfer solids are brought into contact with the waste to be pyrolyzed in the furnace itself. An additional reactor is then necessary to heat the heat transfer solids.

This type of installation is a complex implementation, and proves to be a heavy consumer of energy.

In addition, the numerous connections make the installation more expensive, less reliable and above all lower its thermal efficiency.

One way of partially solving these problems is, for example, to make the oven more compact. Document DE 29 03 280 discloses a compact rotary kiln in which the waste to be treated successively passes through a first rotating cylinder then a rotating ring coaxial with the cylinder. The waste is heated by heat transfer gases circulating around the cylinders. An external rotating jacket delimits the heating zone.

If it allows, notably by its compactness, to obtain a better thermal efficiency, this type of installation remains complex in particular in terms of the shape of the cylinders; moreover the heating and smoke evacuation means not being fixed, problems may arise at the connections. This type of installation, like most ovens known rotary kilns, must be considered rotary kilns with external indirect heating.

The present invention represents an improvement of known rotary pyrolysis ovens;

It has construction advantages since the rotating cylinder is simple.

In addition, only one element is rotatable, which simplifies connections.

Furthermore, in terms of operation, the present invention avoids the problems of obstruction by large solids, inert or not yet thermally degraded; the risks of snagging are almost nonexistent, very much reduced compared to known systems.

According to the present invention, access and disassembly are particularly facilitated; which is very important and much appreciated by operators.

Finally, the thermal efficiency is clearly improved by the extreme compactness of the device according to the invention.

The thermal efficiency is further improved by the fact that the highest temperatures are clearly located in the center of the system.

In document DE 2 903 280 for example, the hottest zone is located on the periphery which is detrimental in terms of heat losses.

The objectives, advantages and improvements which have just been stated are therefore achieved according to the present invention thanks to an oven for heat treatment of solid materials comprising a rotating element in which said solid materials circulate longitudinally, and a heating means for said materials, which extends longitudinally in the furnace, the solid materials progressing along approximately two lengths inside the rotating element.

According to the invention, the heating means is fixed, intended to channel the solid materials and to ensure their preheating and / or their heating.

More specifically, the heating means is fixed, arranged coaxially and inside the rotating element so that in the axial zone delimited by the heating means, the solid materials have the highest temperatures.

Advantageously, the heating means comprises at least one tubular element whose branch or branches are parallel to the longitudinal axis of the furnace, the branch or branches are connected by membranes, the assembly having a generally cylindrical shape.

This arrangement allows a very interesting thermal yield.

Without departing from the scope of the invention, a heat transfer fluid circulates in the co-current and / or counter-current heating means for solid materials.

Preferably, the oven according to the invention can comprise tubular elements which are supplied independently of each other.

By using a fixed heating system, the oven according to the invention allows reuse of hot fluids easier than in the case of a rotary heating system; this is another way of increasing the energy performance of the installation, since the sensible heat of the fumes produced can be efficiently recovered.

Without departing from the scope of the invention, the heating means can constitute an element of generally cylindrical shape around or inside which are arranged resistive elements.

• La figure 1 représente une coupe longitudinale simplifiée de l'installation selon l'invention; et
• La figure 2 montre l'installation selon la coupe AA de la figure 1.

Other details, characteristics and advantages of the 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 drawings according to which:

• Figure 1 shows a simplified longitudinal section of the installation according to the invention; and
• Figure 2 shows the installation according to section AA of Figure 1.

The type of oven illustrated in FIG. 1 comprises a rotating cylinder 1 with a horizontal axis or slightly inclined according to the needs for inventory and flow of solids. This cylinder turning is mounted for example on rollers 2, themselves fixed on a base 3 which supports most of the equipment of the oven. Said rotating cylinder is equipped with an assembly 4 which ensures its rotation at a speed preferably between 0.1 and 20 rpm.

The cylinder is connected to a fixed part 5, by means of a link 6 which provides a complete seal between the atmosphere in the oven and the outside. This connection 6 can be what a person skilled in the art commonly calls a rotary joint. Said fixed part 5 has an outlet 7 for the gases produced in the rotating cylinder and an outlet 8 for the solids resulting from the treatment.

This same fixed part supports a fixed element 9, of substantially cylindrical shape having one end or "outlet" 9a. The element 9 preferably extends over almost the entire length of the cylinder 1. The device 9 is used to heat the interior of the furnace and to channel the waste to the end of the rotating cylinder 1 opposite to that where said waste to be treated is introduced. The waste comes from a hopper or any storage device 10. They are introduced into the oven at a controlled rate using a pusher device 11 as shown in Figure 1, or using any other known device skilled in the art (worm gear for example).

The transfer from the hopper to the oven takes place in a line 12, in which the waste is more or less compacted if it is present in an abundant form in the raw state. The waste progresses in the device 9 according to a more or less compact piston flow, while being preheated. At the end 9a of the heating element 9, the waste falls by gravity into the rotating cylinder, and gradually flows longitudinally towards the outlet 8. The waste therefore goes back and forth in the oven.

As shown in FIG. 2, the heating device 9 can consist of a set of tubular elements 20 connected to one another by membranes 21, ensuring continuity over the whole of the device which has a generally cylindrical shape.

The tubular elements are preferably grouped by 2, 4, 6, etc. in the form of pins in which a hot fluid circulates or in which a combustion operation is carried out. These pins are connected to a collector 13 fixed on the fixed part 5 or placed near this fixed part 5.

The manifold 13 comprises a zone 13a for the supply of hot fluid or for the supply of air and fuel, and a zone 13b for the discharge of the cooled fluid or the fumes. In any case, there is contact between the fluid or the fumes used for heating the tubes 20 on the one hand, and the gases contained in the cylinder 1 and the fixed part 5 on the other hand.

In the case of FIG. 2, the device 9 consists of simple pins having two types of tubes 20: tubes of type 22 in which the fluid circulates cocurrently with the waste inside the device 9 and the tubes of type 23 interposed between the tubes 22 in which the fluid circulates against the current of the waste.

The tubes can be heated for example by combustion of a gaseous fuel.

The gaseous fuel can be natural gas or even gases resulting from the pyrolytic treatment of waste, said gases having been previously and preferably treated to remove tar and particles which could lead to unwanted fouling phenomena.

The description which has just been made is of course not limiting, and the heating device 9 could just as easily consist of a substantially cylindrical tube, equipped with electric heating means such as resistors, wound around or at the inside said tube, helically, with connections carried outside to the level of a power supply unit.

The oven according to the invention is preferably intended for the treatment of waste at temperatures between 50 and 900 ° C., with final temperatures of the pyrolysis products leaving the oven preferably between 400 and 600 ° C. The temperatures at the heater 9 are between 100 and 2000 ° C and preferably between 600 and 1000 ° C. The tubes 20, 21, 22 provide a first heating (or preheating) of the waste by the internal face of the device 9, during the progression of the latter inside the device 9 itself. The calorific contribution to the waste continues when these have left the device 9, via the external face of said device 9. The heating of the waste is then carried out by the external face of the device 9 which emits energy by radiation, either directly on the waste , or on the internal wall of cylinder 1 which then returns it to the waste.

A part of the internal face or of the external face can be hidden by an insulating mask 14, as indicated in FIG. 1, to control and limit the supply of heat at any point of the system.

The oven according to the present invention presupposes, a priori, the use for heating, of hot fluids or of clean fuels such as natural gas or pyrolysis gases rid of their tars and other particles.

Without departing from the scope of the present invention, the hot fluid can be air preheated, for example to a temperature between 500 and 1000 ° C., in a boiler which burns raw pyrolysis gases.

From the above it appears that the present invention has a number of advantages in particular with respect to a conventional installation comprising a rotating cylinder heated externally by burners or a heat transfer fluid which surrounds the rotating cylinder.

• L'absence de moyens de chauffage externes réduit les coûts d'investissement, et d'une manière certaine, diminue les pertes thermiques pour au moins deux raisons : les points les plus chauds en température sont au centre du dispositif et non à la périphérie et la surface externe des parties chaudes est sensiblement réduite avec l'absence d'enveloppe entourant le cylindre tournant.
• La présente invention permet un accroissement très sensible de l'efficacité énergétique de l'installation, parce que les pertes thermiques sont considérablement réduites d'une part, et parce que les fumées ou le fluide chaud utilisés pour le chauffage, sont parfaitement canalisés et peuvent être réemployés sans difficulté en un autre point du procédé, alors qu'avec un système classique, la dilution des fumées par des entrées d'air parasites inévitables, obhèrent quelque peu les possibilités d'une utilisation optimale du contenu énergétique des déchets.
• En outre la présente invention crée de plus grandes facilités pour contrôler et moduler le chauffage du cylindre tournant, grâce notamment à la possibilité d'alimenter de manière indépendante les différents tubes constitutifs du système de chauffage.
• De surcroît, le four selon l'invention présente une moindre inertie thermique de l'ensemble, ce qui autorise des démarrages beaucoup plus rapides, et accroît les conditions de sécurité dans les cas d'arrêts impromptus. Par exemple, lorsque un four classique doit être stoppé rapidement à la suite d'un incident sur l'incinérateur de gaz de pyrolyse, toute l'énergie accumulée dans le réfractaire continuant à chauffer le cylindre tournant, la pyrolyse se poursuit et génère des gaz qui ne peuvent plus être brûlés, pouvant ainsi créer une situation explosive. Selon l'invention, les moyens de chauffage ont une inertie beaucoup plus faible, et l'arrêt de l'alimentation en fluide chaud ou en combustible étant instantané, on peut stopper le processus de pyrolyse en des temps beaucoup plus brefs.
• Par ailleurs des simplifications au plan de la conception et de la construction du four sont réalisées selon l'invention, parce que, pour une puissance donnée, la taille du cylindre tournant est plus petite qu'avec un four traditionnel. D'autre part, une seule étanchéité est à assurer au lieu de deux et enfin les problèmes de dilatation thermique sont simplifiés, le cylindre tournant étant relié à un seul point fixe, alors qu'avec les montages connus le cylindre tournant est généralement pris entre deux points fixes.

More precisely :

• The absence of external heating means reduces investment costs, and certainly reduces thermal losses for at least two reasons: the hottest points in temperature are at the center of the device and not at the periphery and the external surface of the hot parts is significantly reduced with the absence of an envelope surrounding the rotating cylinder.
• The present invention allows a very significant increase in the energy efficiency of the installation, because the heat losses are considerably reduced on the one hand, and because the fumes or the hot fluid used for heating, are perfectly channeled and can to be reused without difficulty in another point of the process, whereas with a conventional system, the dilution of the fumes by inevitable parasitic air inlets, somewhat obstruct the possibilities of an optimal use of the energy content of the waste.
• In addition, the present invention creates greater facilities for controlling and modulating the heating of the rotating cylinder, in particular thanks to the possibility of supplying independently the various tubes constituting the heating system.
• In addition, the oven according to the invention has a lower thermal inertia of the assembly, which allows much faster starts, and increases the safety conditions in the event of impromptu stops. For example, when a conventional oven must be stopped quickly following an incident on the pyrolysis gas incinerator, all the energy accumulated in the refractory continuing to heat the rotating cylinder, the pyrolysis continues and generates gases which can no longer be burned, which could create an explosive situation. According to the invention, the heating means have a much lower inertia, and the stopping of the supply of hot fluid or of fuel being instantaneous, the pyrolysis process can be stopped in much shorter times.
• Furthermore, simplifications in terms of the design and construction of the oven are made according to the invention, because, for a given power, the size of the rotating cylinder is smaller than with a traditional oven. On the other hand, a single seal is to be provided instead of two and finally the thermal expansion problems are simplified, the rotating cylinder being connected to a single fixed point, whereas with known arrangements the rotating cylinder is generally taken between two fixed points.

Claims (9)

1) Oven for the heat treatment of solid materials comprising a rotating element (1) in which said solid materials circulate longitudinally, and a heating means (9) for said solid materials, which extends longitudinally in the oven, the solid materials progressing along approximately two lengths inside the rotating element (1), characterized in that the heating means (9) is fixed, intended to channel the solid materials and to ensure their preheating and / or heating. 2) Oven according to claim 1, characterized in that the heating means (9) is arranged coaxially and inside the rotating element (1) so that in the axial zone delimited by the heating means, solid materials have the highest temperatures. 3) Treatment oven according to one of claims 1 or 2, characterized in that said heating means comprises at least one tubular element (22, 23) whose branch or branches are parallel to the longitudinal axis of the oven, that the branch or branches are connected by membranes (21), the assembly having a generally cylindrical shape. 4) Oven according to claim 3, characterized in that a heat transfer fluid circulates in the heating means (22, 23) co-current and / or against the current of solid materials. 5) Oven according to any one of claims 3 or 4, characterized in that the tubular elements (22, 23) are fed independently of each other. 6) Oven according to any one of the preceding claims, characterized in that it further comprises a first means (7), fixed, intended to evacuate the gases produced in the rotating element (1) and a second means (8 ), fixed, to evacuate the solids resulting from the heat treatment. 7) Oven according to one of claims 1 or 2, characterized in that the heating means (9) constitutes a form element general cylindrical around which or inside which are arranged resistive elements. 8) Oven according to any one of the preceding claims, characterized in that it further comprises an insulating element (14) surrounding said heating means (9). 9) Application of the oven according to any one of the preceding claims to the pyrolysis of waste.

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