EP0011037A1 - Process for gasification and device for such process - Google Patents

Abstract

A method for the gasification of large-sized vegetable materials using a fixed bed gasogene. This gasification method includes the steps of moving the materials in a fixed treatment chamber extending substantially horizontally and having opposite rear and forward ends, the materials being moved from the rear end towards the forward end through substantially the whole transverse section of the chamber and being successfully subjected to drying, pyrolysis and gasification in successive areas of the chamber. Hot gases are generated by combustion in a zone free of materials to be treated and located in the chamber in front of the natural talus created by the front of the whole bulk of materials which occupy substantially the whole transverse section of the chamber. Gases are extracted from the chamber and recycled into the zone in front of the natural talus, whereby tar produced from the pyrolysis and carried away by the extracted gases is eliminated by heating the recycled gases in the zone. The temperature of the extracted gases is at least 500 DEG C. after having passed through the materials contained in the chamber. The gas having passed through the material in the gasification area is taken out from the chamber at a location in front of the pyrolysis area product, whereby the product gas is freed from tars.

Description

The present invention relates to a fixed bed gasifier for the treatment of large vegetable matter.

In a fixed bed gasifier, contact between the treated plant materials and the hot treatment gases is achieved by circulation of these gases through a bed of materials. These must be in the form of particles of sufficient weight not to be entrained by the hot gases. The field of application of the gasifier according to the invention is therefore limited to large vegetable matter. To illustrate what is meant here by these terms, mention may be made, by way of nonlimiting examples, of wood logs, nut or coconut shells, palm kernel hulls, walnut husks coconut, corn cobs, peat, etc. Other finer vegetable materials, for example straw, can also be used if they have been previously agglomerated.

Fixed bed gasifiers have been known for a long time, for example De Lacotte gasifiers. These consist of a tower loaded from above with the materials to be treated and at the base of which the ash and the gases produced are recovered, the materials successively undergoing drying, pyrolysis and gasification.

Gases are recovered at the top of the gasifier, mixed with combustion air, and the very hot gases produced are reinjected into the gasifier substantially at the limit of the pyrolysis and gasification zones. A part of the gases reinjected back into the gasifier, ensuring pyrolysis and drying and taking care of tars produced by pyrolysis, these tars being removed by cracking after mixing with combustion air. The other part of the gas descends into the gasifier and ensures gasification.

These known gasifiers have various drawbacks.

First, the treatment is very slow and this leads to large gasifiers. Thus, De Lacotte wood gasifiers up to 20 m high were produced with a diameter of 3 m and a flow rate of 1 T / h, 24 hours which can elapse between the time when a log is introduced at the top gasifier and the one where the ashes of this log are collected at the bottom of the gasifier. The size and construction cost of such gasifiers are prohibitive and their operation is difficult.

Secondly, it is extremely difficult to inject the hot gases uniformly throughout the cross section of the product contained in the gasifier, at the level where this injection takes place. Even if several injection orifices are provided all around the gasifier, there is a noticeable cooling of the gases before they come into contact with the product located in the center. To remedy this, it is known to form a narrowing of the cross section of the gasifier at the injection of hot gases. In order to carry out this injection as close as possible to the axis of the gasifier. However, it turns out that the shrinkage disrupts the flow of the treated materials due in particular to solidification of the product and arching caused by the presence of tar produced during pyrolysis.

Yet another drawback of vertical fixed bed gasifiers of the De Lacotte type is that they offer very limited possibilities for adjusting the flow rate of gases produced. This adjustment can in fact only be done by modifying the quantity of combustion air used to produce the hot gases, hence a limited adjustment power. In addition, the response time of the gasifier is very long, that is to say that there is a long time interval between the moment when the adjustment is made and the moment when it produces its effects.

It was proposed in patent GB 352,271 to produce a gasifier comprising a vertical feed and pyrolysis zone and a horizontal gasification zone, the lower wall of which is formed by the conveyor belt. The recycled gases are mixed with the combustion air under the horizontal chamber and injected from below into the layer of materials to be treated. Recycling is carried out in much the same way as in the De Lacotte gasifier and does nothing more. It will also be noted that the gasifier of patent GB 352,271 cannot be used for the gasification of materials whose ashes have a low melting temperature, in particular vegetable materials, at the risk of blocking the passages provided through the conveyor.

• - à production égale, une durée de traitement, un encombrement et un coût d'installation et d'exploitation très nettement réduits par rapport aux gazogènes à lit fixe connus évoqués plus haut,
• - traitement par les gaz chauds du contenu du gazogène de façon uniforme dans toute sa section transversale,
• - gazéification totale avec production de gaz exempts de goudrons,
• - possibilités de réglage plus étendues que dans les gazogènes connus et avec un temps de réponse plus court.

The object of the present invention is to provide a gasifier of the fixed bed type which does not have any of these drawbacks. More specifically, the purpose of the present invention is to provide a gasifier which is particularly suitable for the treatment of large plant materials having in particular the following properties:

• - for equal production, a treatment time, a space requirement and a cost of installation and operation very significantly reduced compared to the known fixed bed gasifiers mentioned above,
• - treatment by hot gases of the content of the gasifier uniformly throughout its cross section,
• - total gasification with production of tar-free gases,
• - more extensive setting possibilities than in known gasifiers and with a longer response time short.

This object is achieved by a gasifier which comprises: a horizontal treatment chamber, a device for supplying the chamber with materials to be treated situated at a first end, or rear end, of the chamber, a drive device for advancing the materials to be treated between the rear end and the other end, or front end of the chamber, a discharge opening formed in the lower part of the chamber at the front thereof, an outlet for the gases produced by gasification, a hot gas generator connected to oxidizing gas supply means and to a recycle pipe for gases taken off after passing through the materials to be treated, a gasifier in which, according to the invention, the recycle pipe is connected to a second outlet formed in the wall of the chamber at the rear thereof and the combustion gases are produced by the hot gas generator in a space located inside the chamber and at the front the natural slope of the materials contained in the chamber to attack these materials by the combustion gases produced over the entire section of the chamber; and a suction device is arranged in the recycling duct to establish a forced circulation of gases through the bed of materials, and to recycle gases of temperature preferably greater than about 500 ° C.

Recycling with suction establishes a forced circulation of hot gases in the gasifier, which makes it possible to bring the materials to be treated introduced into the treatment chamber directly into contact with gases at relatively high temperature. Drying and pyrolysis are therefore carried out very quickly. The pyrolysis gases are brought into the free space located in front of the natural slope, the space where their combustion. By natural slope is meant here the shape which freely takes under the action of gravity the front end of the bed of materials contained in the treatment chamber. As this natural slope does not need any support piece, such as grid or wall, fragile under the thermal conditions required by gasification and limiting the working temperature, this slope is attacked directly from the front and over the whole section of the gasifier by the combustion gases. This, combined with the forced circulation of gases through the bed of materials is an additional factor in the speed and completion of chemical reactions. The use of hot combustion gases is therefore optimized to obtain rapid gasification. This high processing speed results in a gasifier having relatively small dimensions and a reasonable cost.

The horizontal arrangement of the treatment chamber makes it possible to solve the problem of uniform injection of hot gases throughout the cross section of the content of the gasifier since this injection takes place over the entire surface of the natural slope. It will be noted, in this regard, that the admission of hot gases through orifices formed at the lower end of a vertical gasifier known for carrying out an injection over the entire cross section, is impracticable because these orifices would be subjected to forces important mechanical conditions under severe thermal conditions and very quickly blocked by accumulated and agglomerated ash.

It should also be noted that the possibility of modifying the flow rate of produced gases is offered by adjusting the flow rate of recycled gases. In addition, because the reaction rate is high, and depends essentially on recycling, the response time of the gasifier to such a setting is very short.

The oxidizing gas used can be air or oxygen. Now, oxygenation gasification with a view in particular to producing synthesis gas causes temperatures such that the ashes of plant materials, including peat, inevitably melt in the gasification zone, their melting temperature being approximately 800 ° C. An advantage of the gasifier according to the invention is that in the case of combustion with air or with oxygen, the combustion gases are not injected directly into the mass of materials, but into a free space located at the front of the slope of materials and authorizing their dilution before injection.

According to another feature of the gasifier according to the invention, said second outlet and the combustion air supply line are connected to a heat exchanger so as to heat the combustion air by the calories conveyed by the gases produced by the gasifier.

• - la figure 1 est une vue très schématique en élévation latérale d'un gazogène conforme à l'invention,
• - la figure 2 est une vue très schématique du gazogène illustré par la figure 1, vue en élévation et en coupe longitudinale médiane suivant la ligne II-II de la figure 4,
• - la figure 3 est une vue en coupe transversale suivant la ligne III-III de la figure 2, et
• - la figure 4 est une vue de dessus en coupe suivant la ligne IV-IV de la figure 3.

Other particularities and advantages of the gasifier in accordance with the invention will emerge on reading the description of a particular embodiment of this gasifier, description given below, by way of indication but not limitation, with reference to the drawings attached to which :

• FIG. 1 is a very schematic view in side elevation of a gasifier according to the invention,
• FIG. 2 is a very diagrammatic view of the gasifier illustrated by FIG. 1, seen in elevation and in median longitudinal section along line II-II of FIG. 4,
• FIG. 3 is a cross-sectional view along line III-III in FIG. 2, and
• - Figure 4 is a top view in section along the line IV-IV of Figure 3.

The gasifier represented in FIGS. 1 to 4 essentially comprises a horizontal treatment chamber 10, a supply device 20 for supplying the chamber 10, at its rear end 10a, with materials to be treated, a feed device 21 for advancing the materials to be treated through the chamber 10, a generator 30 for supplying hot gases to the chamber 10 at its front end 10b, and a recycling device 40.

The chamber 10 has the form of a horizontal tunnel whose side wall 11 is made of refractory material and which is extended, at the rear, by a supply chamber 22 and closed, at the front, by a front partition 10c . In the floor 11a of the chamber 10 are formed, from the rear to the front, a first outlet 12 provided with a grid 12a, a second outlet 13 provided with a grid 13a and a discharge opening 14.

The outlet 12 is formed at the rear end of the floor 11a, or in the immediate vicinity of this rear end.

The outlet 13 is the outlet for gases produced by the gasifier; it is formed in the middle part of the tunnel or slightly on the front side of it. An outlet pipe 18 is connected to the outlet 13 and opens, at 18a, towards the rear of the gasifier, in a compartment 13b located under the grid 13a.

The opening 14 is formed over the entire width of the floor 11a at its front end. It communicates, via a vertical duct 15, with a tank 16 which is located below the chamber 11 and where the ashes of the materials treated in the gasifier are collected. A water seal shown diagrammatically at 17 is provided to compensate for the difference in pressures prevailing outside and inside the gasifier when the latter is in operation.

The supply chamber 22 is surmounted by an airlock 23 for admitting the materials to be treated. A pusher or piston 21 can slide horizontally in the chamber 22 over the entire length of the latter. The piston 21 is provided with a cap 21a fixed on its front face, overhanging on it.

When the piston is in its rear extreme position (in broken lines in FIG. 2), the content of the airlock 23 can be introduced into the chamber 22. The advance of the piston 21 is then controlled at a constant and slow speed equal to that to which the heat treatment of the materials is carried out in the chamber 10. Under the thrust of the piston 21, the materials to be treated when freshly introduced tend to rise upwards. However, this rise is limited by the cap 21a and there can be no jamming of materials to be treated between the front face of the piston and the lower front edge 23a of the airlock 23, jamming which would block the advance of the piston. When the piston is in its extreme front position (shown in solid lines in FIG. 2), it is quickly brought back to the rear position to allow a new loading of the chamber 22.

At the front of the chamber 10 opens at least one injector 31 of oxidizing gas, for example oxygen or combustion air. Preferably, several injectors 31 form a horizontal row, at a level located immediately above that of the floor 11a. The, or each, injector 31 is supplied by a pipe 36 coming from a heat exchanger 33. The latter receives the pipe 18 and a pipe 34 for supplying cold oxidant gas, for example cold air. This cold air is heated in the exchanger 33 by the calories conveyed by the gases produced by the gasifier which, cooled, are evacuated via a pipe 19.

At each injector 31, there opens an opening 32 which is supplied with gas by a recycling conduit 41 connected to the outlet 12. The assembly 31-32 is arranged to ensure a good mixture between oxidizing gas and combustible recycled gases, such as in a conventional gas burner.

A suction device 42 constituted by a refractory fan supporting high temperatures is disposed between a conduit 43 connected to the outlet 12 and the conduit 41. The conduit 43 opens, at 43a, towards the rear of the gasifier, in a compartment 12b located under the grid 12a.

At least one burner 35, capable of being supplied with fuel, is disposed at the rear of the chamber 10.

• La chambre de traitement 10 étant chargée en matières à gazéifier, le ou chaque brûleur 35 est mis en marche, ainsi que l'aspirateur 42.

The operation of the gasifier described above is as follows:

• The treatment chamber 10 being loaded with materials to be gasified, the or each burner 35 is started, as well as the vacuum cleaner 42.

The chamber 22 being loaded and the piston 21 being in the rear position, the advance movement of the latter is controlled as soon as the drying, pyrolysis and gasification operations have started.

The start-up phase of the operation of the gas generator having ended, which can be seen by observing the flow of gas produced, the burner 35 is switched off. The hot gases are then produced by the reaction of the gases recycled through the pipe 41 with the air or the combustion oxygen injected at the front of the chamber 10.

The hot combustion gases are produced at the front of the chamber 10 in a free space contained in this chamber and located in front of the natural slope formed at the front of the materials contained in the chamber 10. The particles of material closest to the opening 14 are attacked by the hottest gases (around 1200 ° C). The carbon possibly remaining at the heart of these particles is thus reached and the rejected ashes have been completely gasified.

The hot gases progress from front to rear through the materials to be treated under the effect of suction by the fan 42. The arrangement of the outlets 12 and 13 in the floor of the chamber 10 requires combustion gases to cross the bed of materials to be treated over its entire thickness and practically throughout the room.

In the gasification zone G, the temperature of the gases gradually decreases, and reaches approximately 800 ° C. at the level of the outlet 13. The gases not captured by this outlet pass, towards the rear, the pyrolysis P and drying S zones. before being taken up in the recycling conduit by outlet 12. The temperature of the recycled gases is at least 500 ° C, approximately 600 to 700 ° C.

In the drying and pyrolysis zones, the temperature of the gases is relatively high. The time required for these operations is therefore short. The length of the drying and pyrolysis zones is therefore quite short.

When crossing the pyrolysis zone, the gases take up the tar produced by the pyrolysis. These tars _' are transported by recycled gases. As these are injected into the flame created at the front of the chamber 10, the temperature in this zone ensures their elimination by cracking or by combustion.

As indicated above, the outlet 13 is the outlet from which the gas produced by the gasifier is taken. To avoid the presence of tar in this gas, it is desirable that the outlet 13 is located in front of the pyrolysis zone or at the limit between it and the gasification zone. In practice, this means that the outlet 13 is located at a distance from the opening 14, at the front of the chamber 10, between approximately L / 4 and L / 2, L denoting the distance between the rear end of the chamber 10 and opening 14.

The outlets 12 and 13 are formed in the floor 11a of the chamber 10. The hot gases passing through the materials contained in the chamber thus necessarily come into contact with the materials located in the bottom of the chamber 10, which could not be the case. if the openings 12 and 13 were located at a certain height above the floor lla. However, it is possible to envisage, in particular for the outlet 12, to form several openings formed in the floor and the sides of the chamber 10 and connected to the recycling conduit.

The advance of the materials into the chamber 10 is produced by the piston 21. It will be noted that the materials entering the chamber are brought straight away to a high temperature (several hundred ° C.). The carbonization of materials is very fast. The materials to be treated therefore very quickly become brittle, in particular in the vicinity of the wall 11 which prevents any jamming of these materials which may oppose the force exerted by the piston 21. The advance of the piston 21 is interrupted when 'It reaches the front of the chamber 22, this interruption only during the time necessary to return the piston 21 to its rear position and fill the chamber 22. The advance movement of the piston 21 can then immediately resume.

The advance speed of the piston 21 is determined as a function of the treatment speed in the chamber.

This processing speed is relatively high, in the present case, because the materials to be treat are subjected to high temperatures upon entering chamber 10.

The processing speed is not only a function of the temperature of the gases passing through the bed of materials to be treated, but also, and above all, a function of the flow rate of the recycled gases and, thereby, the flow rate of the gases passing through the chamber 10. Thus , a strong recycling results in a strong gas flow in the gasification zone and, consequently, in a high reaction rate.

In addition to adjusting the amount of air or combustion oxygen injected, there is therefore a possibility of adjusting the flow rate of recycled gases to adjust the processing speed and the amount of gas produced to desired values.

The adjustment of the flow rate of recycled gases can be carried out by adjusting the position of an adjustable flap inserted in the duct 43 or by modifying the speed of the suction fan 42.

As the processing speed is quite high and depends essentially on recycling, the response time of the gasifier to a change in the flow rate of recycled gases is relatively short.

Of course, various modifications or additions may be made to the embodiment described above of a gasifier in accordance with the invention, without thereby departing from the protective framework defined by the appended claims.

Note in particular that it is possible to increase the speed of the piston to bring it to a value such that gasification cannot take place in order to produce charcoal.

Claims (6)

1. Gasifier comprising a horizontal treatment chamber, a device for supplying the chamber with materials to be treated situated at a first end, or rear end, of the chamber, a drive device for advancing the materials to be treated between the rear end and the other end, or front end of the chamber, a discharge opening formed in the lower part of the chamber at the front thereof, an outlet for the gases produced by the gasification, a generator hot gas connected to means for supplying oxidizing gas and to a pipe for recycling gases withdrawn after passing through the materials to be treated,
characterized in that the recycling duct is connected to a second outlet formed in the wall of the chamber at the rear thereof and the combustion gases are produced by the hot gas generator in a space located inside from the chamber and at the front of the natural slope of the materials contained in the chamber to attack these materials because the combustion gases produced over the entire section of the chamber; and a suction device is arranged in the recycling duct to establish a forced circulation of gases through the bed of materials, and to recycle gases of temperature preferably above about 500 ° C. 2. Gasifier according to claim 1, characterized in that the first and second outlets are formed in the lower wall of the chamber. 3. Gasifier according to any one of claims 1 and 2, characterized in that the outlet of the gases produced by the gasifier and the means for supplying oxidant gas are connected to a heat exchanger to heat the oxidant gas by the calories conveyed by the gases produced by the gasifier. 4. Gasifier according to any one of claims 1 to.3, characterized in that it comprises means for adjusting the flow rate of the recycled gases. 5. Gas generator according to any one of claims 1 to 4, characterized in that the drive device is a piston movable horizontally in a supply chamber located in the rear extension of the treatment chamber. 6. Gasifier according to claim 5, characterized in that the piston is provided with an overhanging cap on its front face.

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