EP0747462A1 - Fluidized bed reactor for the thermal treatment of waste materials - Google Patents

Abstract

Fluidised bed reactor for the heat treatment of waste and heat exchange between the circulating solids and a heat exchange device such as an evaporator and/or superheater. The reactor contains an axially circulating fluidised bed (18-18') and at least a first (28) and a second (48) dense lateral fluidised bed established along the length of a first (11) and second (12) wall of the reactor. The waste is fed in at at least one point in the first wall, above the first bed. The reactor also contains at least one extraction pipe (39) for the non-fluidisable heavy elements, located at the base of the first bed.

Description

The present invention relates to the thermal treatment of waste. By heat treatment, we mean not only the destruction of waste, but also its recovery by recovering most of their heat energy.

One way of treating waste consists in subjecting them to a heat treatment, after which they transform into an inert material of small volume compared to their initial volume and during which they transfer a large part of the calorific energy which they contain thermal energy exchanged for example with an energy recovery unit.

A problem often encountered in the treatment of waste is the need to sort it beforehand and to carry out a separation between combustible materials and the others, and to fragment this waste into very small pieces to facilitate their treatment. Furthermore, the known grate devices do not always make it possible to treat any type of waste, in particular they can only treat the settling sludge from wastewater treatment plants to a limited extent.

An object of the present invention is to define a reactor capable of receiving all types of waste, including decantation sludge, without requiring prior sorting and by being satisfied with a summary fragmentation.

An important problem encountered in the treatment of waste is the presence in this waste of compounds whose thermal decomposition produces a release of chlorine which very quickly deteriorates by corrosion the exchange tubes of the superheaters of the energy recovery unit.

Another object of the present invention is to define a reactor in which the heat exchange zone, by example with the tubes of the superheaters of an energy recovery unit, is free of chlorine.

All these aims are achieved by means of a fluidized bed reactor derived from that described in French patents Nos. 90 05 060 and 92 05 165 filed in the name of STEIN INDUSTRIE which are incorporated by reference into the present application.

The subject of the invention is a fluidized bed reactor for the thermal treatment of waste and the heat exchange between the solids in circulation and a heat exchange member, such as an evaporator and / or a superheater, the reactor being type comprising an axial circulating fluidized bed and at least first and second dense lateral fluidized beds established respectively along a first and a second wall of the reactor shell, characterized in that the supply of waste is carried out at at least one point on said first wall, above said first dense lateral fluidized bed, said reactor further comprising at least one non-fluidizable heavy element extraction duct, located at the base of said first fluidization bed.

The heat exchange is preferably carried out in said second dense lateral fluidized bed.

The heavy elements extraction duct comprises an inclined part arranged in the extension of an inclined sole of said first dense lateral fluidized bed, air blowing nozzles, directing an orientable flow, flush with the surface of said sole.

Said extraction duct is provided with adjustable air injection means for regulating the flow of solids passing through said duct.

The base of the second dense fluidized bed is connected by at least one extraction duct to the base of said circulating fluidized bed.

Said extraction duct is provided with adjustable air injection means for regulating the flow of solids passing through said duct.

The circulating fluidized bed and the first dense lateral fluidized bed are fluidized by a mixture of primary air and recycled fumes.

The second dense lateral fluidized bed is fluidized by a mixture of air and dechlorinated recycled fumes.

Air, known as tertiary air, is sent by means of adjustable flow injectors, at an altitude higher than that of the waste introduction points, through each of the walls of the reactor.

The tertiary air is mixed with recycled and dechlorinated fumes at least for the tertiary air injected into the wall where the dense fluidized heat exchange bed is located.

The base of the reactor includes means for extracting inert materials.

Said duct for extracting non-fluidizable heavy elements is associated with a device for sorting and extracting said inert elements.

The reactor comprises a third lateral fluidized bed, the heat exchange being carried out with at least one of said second and third lateral fluidized beds.

The reactor comprises a third and a fourth lateral fluidized beds, the heat exchange being carried out with at least one of said second, third and fourth lateral fluidized beds.

The reactor comprises means for supplying the fluidized waste supply bed with part of the materials from at least one heat exchange bed.

The reactor is connected to a hot cyclone, the wall where the connection of the reactor to the cyclone is carried out opposite the wall where the waste is fed.

The reactor walls have heat exchange tubes.

The reactor comprises an annexed fluidized bed, containing a heat exchange member, supplied with a portion of the solids from the second dense fluidized bed, provided with fluidization means, with at least one line for returning the solids to the bottom of the fluidized bed. circulating, and a vent directing the gases up the circulating fluidized bed.

The reactor is supplied by waste consisting of a mixture of urban residues and / or waste chosen in particular from settling sludge, biomass residue, industrial waste and the refusal of grinding, and / or selected fossil fuels especially among coal and petroleum residues.

• la figure 1 est une vue schématique d'une partie des installations thermiques d'une unité de traitement de déchets,
• la figure 2 est une vue schématique en élévation d'un réacteur faisant partie de ladite installation,
• la figure 3 est une vue en coupe selon la ligne III-III de la figure 2
• la figure 4 est une vue schématique partielle du réacteur selon une variante.

The invention is explained by the following description of an exemplary embodiment, with reference to the appended drawing in which:

• FIG. 1 is a schematic view of part of the thermal installations of a waste treatment unit,
• FIG. 2 is a schematic elevation view of a reactor forming part of said installation,
• Figure 3 is a sectional view along line III-III of Figure 2
• Figure 4 is a partial schematic view of the reactor according to a variant.

Fig. 1 shows schematically and in a simplified manner a part of the thermal installations of a waste treatment unit according to the invention. The steam turbine and the electric generator are not shown.

In FIG. 1, a fluidized bed reactor 1, the subject of this patent application, supplied with products to be treated. This reactor is connected by its upper part to the upper part of a hot cyclone 2 in which the separation takes place between the gases and most of the solid materials with which they are charged, the latter returning to the reactor.

• un échangeur de surchauffe à basse température SBT recevant de la vapeur d'un ballon B de chaudière et envoyant de la vapeur surchauffée à l'échangeur de surchauffe haute température SHT placé dans le réacteur,
• un vaporisateur V envoyant la vapeur produite au ballon de chaudière B,
• un économiseur recevant de l'eau alimentaire et l'envoyant au ballon de chaudière B.

The hot gases from the hot cyclone 2 are sent to a heat exchange unit 3, comprising, for example:

• a low temperature superheat exchanger SBT receiving steam from a boiler cylinder B and sending superheated steam to the high temperature superheat exchanger SHT placed in the reactor,
• a vaporizer V sending the vapor produced to the boiler cylinder B,
• an economizer receiving drinking water and sending it to the boiler tank B.

The gases leaving the heat exchange unit 3 are sent to a gas-solid separator 4 where the gases are separated from the coarse fraction C of the fly ash which is collected at the base of the cyclone 4.

The gases, thus separated from said fraction, are sent to a smoke treatment device TF from where they are extracted by a draft fan V and sent to the base of a chimney CH.

The reactor 1, which is the subject of the present invention, is shown in more detail in FIGS. 2 and 3.

The example described, without any limitation being implied, is that of a reactor with two dense lateral fluidized beds.

The reactor comprises a casing 10, which can have a rectangular section and thus have four walls 11, 12, 13 and 14.

The lower part 16 of the reactor is in the form of an inverted pyramid trunk, or in the form of an inverted pseudo-pyramid with two parallel faces, or in the form of a truncated cone; it is in this part that an axial circulating fluidization bed is installed comprising a lower part 18 surmounted by an upper part 18 ′, as was explained in the second of the aforementioned documents and to which the reader is referred. The lower part 18 of the circulating fluidized bed is supplied with pyrolyzed fuel originating in particular by the overflow of the dense lateral bed 28, as will be seen below. A grid 19, at the base of the part 18 of the bed, is equipped with nozzles 20 for injecting air, called primary air, identified in the drawing by the arrow AP, which can optionally be mixed with fumes taken in El input of the TF smoke treatment.

Above the grid 19 are arranged secondary air inlets AS which can also be optionally mixed with fumes taken from the inlet E1 of the smoke treatment TF.

Reactor preheating means, not shown, located above the grate 19, make it possible to heat the entire reactor from a cold or lukewarm state, and bring it to the temperature necessary for ensuring combustion garbage.

Under the grid 19 is disposed a device 22 for extracting residues, such as a cooled screw extractor or a dry extractor.

In the example shown in Figs. 2 and 3, the reactor comprises two dense lateral fluidized beds 28 and 48.

The lateral fluidized bed 28, installed against the wall 11, comprises a floor 29, preferably inclined and in which are placed directional blowing nozzles 30, that is to say those in which the direction of the blowing flow can be oriented between a direction perpendicular to the plane of the sole and a direction parallel to this plane. The nozzles are supplied with air (reference AF), possibly mixed with recycled fumes from point E1. The dense fluidized bed 28 is partially contained by an overflow wall 31 raised at the end of the floor 29 parallel to the wall 11.

The dense fluidized bed 28 is supplied with the waste to be treated which, according to a fundamental characteristic of the invention, is injected into the dense fluidized bed in several points of the wall 11, preferably at a height greater than that of the upper part of the overflow wall 31.

The feeding is carried out from shears 33 or 34 which carry out a summary fragmentation of the waste at a maximum size of between 200 and 400 mm.

The fragmented waste is introduced either by conduits 35 provided with injection screws such as the screw 36, or by simple gravity in the conduits 37. Conventional means not shown make it possible to adjust the flow rate of the introduced waste.

In the extension of the sole 29 is disposed a conduit 39 (or more conduits if necessary) collecting the heavy non-fludisable elements such as scrap, bottles, pieces of glass, etc. or the elements which have not been pyrolyzed. These elements are sent to the base of the part 18 of the circulating fluidized bed, with, possibly, prior passage through a sorting device 40 from which are extracted the elements which do not disturb the fluidization at the base of the circulating fluidized bed 18.

It is noted that recycled fumes, for example taken from E1 upstream of the treatment of TF fumes, can be injected into the conduits 39.

The conduits 39 are provided, in the vicinity of the grid 29, with means for adjusting the flow rate of the solids passing through these conduits. These means can be adjustable air inlets 39 ′.

The wall 11 where the waste is introduced is protected by a coating of silicon carbide or any other material resistant to the reducing medium; the usefulness of this coating will be shown later.

According to the invention, a second dense lateral fluidized bed 48 is installed along the wall 12 which, in the example described, is the wall opposite the wall 11. It comprises an overflow wall 51, a grid 52 and blowing nozzles 53 supplied with air (referenced AF), possibly supplemented with dechlorinated recycled fumes, for example coming from an outlet E2 downstream of the treatment of fumes TF.

The base of the dense bed 48 is connected by conduits 59 to the lower part 18 of the circulating fluidized bed.

Means are provided for adjusting the flow rate of solids circulating in these conduits 59; these means can be constituted by air inlets 59 'with adjustable flow.

In this bed 48 is arranged the heat exchange element SHT which can include evaporators and / or superheaters of high temperature steam.

The reactor is completed by air injectors 54, said tertiary air, which are arranged on the four faces of the reactor at a height greater than that of the dense lateral fluidized beds. Again the tertiary air can be mixed with recycled fumes from the outlet E1 for the injectors placed on the dense feed bed 28 and dechlorinated from the outlet E2 for the dense exchange bed 48 .

As explained in the aforementioned documents, the temperature prevailing at the center of the reactor is greater than 850 ° C. and generally between 850 and 950 ° C. in order to comply with the regulations concerning the combustion of waste.

The ratio of the section S2 measured above the overflow walls 31 and 51 to the section S1 measured between these overflow walls is between 1.05 and 2.

The speed of the fluidizing gases in the lower axial part 18 (rising arrows F1) is, in empty barrel, between 3 and 12 m / s.

The surface velocity of the fluidizing gases, in empty barrels, in the dense lateral beds is between 0.3 and 2.5 m / s.

• le débit des déchets introduits,
• les débits d'air primaire, secondaire et tertiaire,
• la granulométrie du matériau du lit en circulation,
• le taux de remplissage en solides du réacteur.

Once the S2 / S1 ratio has been set, the above values of speeds and temperatures can be adjusted using the parameters consisting of:

• the flow of waste introduced,
• primary, secondary and tertiary air flows,
• the grain size of the bed material in circulation,
• the filling rate of solids in the reactor.

The operation of the reactor is as follows.

The roughly fragmented waste is introduced into the reactor. On abrupt contact with the descending layer of solid wall material which is at a temperature above 850 ° C (generally between 870 and 900 ° C), the chlorine contained in the waste is immediately released, by a pyrolysis-flash effect ( or instant pyrolysis), and almost all of this gas is entrained towards the top of the reactor (arrows F2) and passes into the hot cyclone 2. The coating of silicon carbide in the wall 11 where the waste is supplied protects this against the corrosive effect of hot chlorine combined with reducing gases (mainly CO).

This pyrolysis effect is obtained by mixing the waste introduced and the solids falling on the wall in the form of a dense layer resulting from the operation of the upper part 18 ′ in a circulating fluidized bed.

It is observed that the dense fluidized bed 48 is fed by the dense layer of circulating solids resulting from the operation of the circulating fluidized bed 18-18 '. Furthermore, the dense bed 48 is fluidized by a mixture of air and dechlorinated fumes, as was mentioned above. This bed 48 is therefore free of chlorinated products. It is therefore possible to house the heat exchange member SHT there; this organ will be placed in a medium whose temperature is approximately 870 ° C. so that superheated steam will be obtained at 450 or 500 ° C. (instead of 360 ° C. in the reactors of the prior art since cannot place the exchanger in a medium with a temperature higher than 600 ° C without very rapid corrosion). This significant increase in the temperature of the superheated steam, which is used in a turbine not shown, makes it possible to increase the efficiency of the installation cycle (Carnot law), and therefore the energy recovery of the waste, and to SHT exchangers have a longer service life, which increases the availability of the installation.

Furthermore, it is possible to use heat exchange members having external exchange coefficients of 450 W / m ² ° K (instead of 35 W / m ² ° K in the prior art) and a DTLM (much larger mean log temperature difference) (450 ° C instead of 250 ° C.

Finally, there is no risk of fouling of the heat exchange member, this risk leading, in conventional technique, to almost double the heat exchange surface and to install expensive chimney sweeps.

The use of this type of heat exchange member allows a significant reduction in size and therefore in cost.

The fluidized reactor can operate with an excess of overall air, compared to stoichiometric conditions, limited to 1.4, which has the double advantage of being in accordance with the regulations and more economical than certain installations which require for their operation a 1.8 or 1.9 excess air.

Circulating fluidized beds inherently generate low nitrogen oxides by the low temperature, the air staging and the low air flow; if necessary, an injection of ammonia upstream of the hot cyclone 2 can be envisaged. In this way, the limitation to 200 mg / m ³ can be easily respected.

The adjustable air injection, said tertiary air, provided on the four faces 11 to 14, and materialized in the drawing by the arrows 54, serves as auxiliary combustion air for the volatile combustible materials released by pyrolysis-flash; this air allows rapid mixing of the gases from the upper part of the reactor which promotes the evacuation of chlorine. A terminal air injection may be provided optionally upstream of cyclone 2.

It will be noted that, preferably, the wall in which the connection is made between the reactor 1 and the cyclone 2, here the wall 12, is chosen opposite to the wall 11 where the reactor is supplied with power.

The invention also applies to the simultaneous treatment of waste increased by a certain proportion of sludge from treatment plants, biomass residues, as well as ordinary industrial waste and to the refusal of automobile grinding.

The invention also applies to the simultaneous treatment of waste with fossil fuels of the coal or petroleum residue type. The latter are introduced at the bottom of the lower zone 18 of the circulating fluidized bed 18-18 ′, for example through the return pipe of cyclone 2.

The reactor may include means for injecting agents for fixing the sulfur contained in the fumes, such as limestone.

If for any reason the temperature of the dense waste introduction bed is desired lower than 870 to 900 ° C., it is possible to supply the dense waste introduction bed 28 with a portion of the products from bed 48 heat exchange.

The walls of the reactor can be cased in whole or in part. The tubes, some of which are shown and referenced 60, are traversed by a mixture of water and steam coming from and returning to the boiler flask. The invention is not limited to the embodiment described and shown.

In particular, provision can be made for the reactor to comprise a third dense lateral fluidized bed, the heat exchange being carried out with at least one of the second and third dense lateral fluidized beds.

In another variant, the reactor according to the invention will comprise a third and a fourth lateral fluidized beds, the heat exchange being carried out with at least one of the second, third and fourth lateral fluidized beds.

In these two variants, it is possible to feed the dense feed bed with waste by one or more of the other dense beds.

Fig.4 illustrates a variant in which solids collected by the dense bed 48 are transferred to an annexed fluidized bed exchanger 70 before being reinjected into the zone 18 by means of conduits 71 provided with air means 71 'of flow regulation. It is in this bed 70 that the SHT heat exchange member is placed.

The fluidization gases from bed 70 are reinjected through a conduit 72 into the upper part 18 'of the bed 18-18'.

This variant makes it possible to decouple the design constraints due on the one hand to the requirements of l (hydrodynamics of the gas-solid flow and on the other hand to the requirements of compliance with the thermal balance of the installation which may require dimensions of exchangers important.

Claims (19)

Fluidized bed reactor for the thermal treatment of waste and the heat exchange between the circulating solids and a heat exchange member, such as an evaporator and / or a superheater, the reactor being of the type comprising an axial circulating fluidized bed (18-18 ') and at least a first (28) and a second (48) dense lateral fluidized beds established respectively along a first (11) and a second (12) walls of the reactor shell , characterized in that the supply of waste is carried out at at least one point of said first wall (11), above said first dense lateral fluidized bed (28), said reactor further comprising at least one extraction duct (39) non-fluidizable heavy elements, located at the base of said first fluidization bed. Reactor according to claim 1, characterized in that the heat exchange is carried out in said second dense lateral fluidized bed (48). Reactor according to claim 2, characterized in that the extraction duct (39) for the heavy elements comprises an inclined part arranged in the extension of an inclined bottom (29) of said first dense lateral fluidized bed (28), nozzles ( 30) of air blowing, directing a directional flow, flush with the surface of said sole. Reactor according to one of the preceding claims, characterized in that said extraction duct (39) is provided with adjustable air injection means (39 ') for regulating the flow of solids passing through said duct. Reactor according to one of the preceding claims, characterized in that the base of the second dense fluidized bed (48) is connected by at least one extraction duct (59) to the base (18) of said circulating fluidized bed. Reactor according to claim 5, characterized in that said extraction duct (59) is provided with adjustable air injection means (59 ') for regulating the flow of solids passing through said duct. Reactor according to one of the preceding claims, characterized in that the circulating fluidized bed (18-18 ') and the first dense lateral fluidized bed (28) into which the waste is introduced are fluidized by a mixture of primary air and recycled fumes. Reactor according to one of the preceding claims, characterized in that the second dense lateral fluidized bed (48) is fluidized by a mixture of air and dechlorinated recycled fumes. Reactor according to one of the preceding claims, characterized in that air, called tertiary air, is sent by means of adjustable flow injectors (54), at an altitude higher than that of the waste introduction points, through each of the walls (11, 12, 13, 14) of the reactor. Reactor according to claim 9, characterized in that the tertiary air (54) is mixed with recycled and dechlorinated fumes at least for the tertiary air injected into the wall where the dense fluidized heat exchange bed is located. Reactor according to one of the preceding claims, characterized in that the base of the reactor comprises means (22) for extracting inert materials. Reactor according to one of the preceding claims, characterized in that said extraction duct (39) for non-fluidizable heavy elements is associated with a sorting device (40) and for extraction of said inert elements. Reactor according to one of the preceding claims, characterized in that the reactor comprises a third lateral fluidized bed, the heat exchange being carried out with at least one of said second and third lateral fluidized beds. Reactor according to one of claims 1 to 12, the reactor comprises a third and a fourth lateral fluidized beds, the heat exchange being carried out with at least one of said second, third and fourth lateral fluidized beds. Reactor according to one of the preceding claims, characterized in that it comprises means for supplying the fluidized waste supply bed with part of the materials from at least one heat exchange bed. Reactor according to one of the preceding claims, characterized in that it is connected to a hot cyclone (2), the wall (12) where the connection of the reactor to the cyclone is effected opposite the wall (11) where is made waste supply. Reactor according to one of the preceding claims, characterized in that the walls of the reactor comprise heat exchange tubes (60). Reactor according to one of the preceding claims, characterized in that it comprises an annexed fluidized bed (70), containing a heat exchange member (SHT), supplied with a part of the solids of the second dense fluidized bed (48), provided with means (AF) for fluidization, at least one line (71) for returning the solids to the bottom (18) of the circulating fluidized bed (18-18 '), and a vent (72) directing the gases upwards (18 ') of the circulating fluidized bed (18-18'). Reactor according to one of the preceding claims, characterized in that the waste consists of a mixture of urban residues and / or waste chosen in particular from settling sludge, biomass residues, industrial waste and the refusal of grinding, and / or fossil fuels chosen in particular from coal and petroleum residues.

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