Multiple hearth furnace for placing a solid and a gas in contact

Abstract

The invention relates to a multiple hearth furnace for placing a solid and a gas in contact, comprising: a chamber (30); a plurality of trays (41, 42) which are distributed along the longitudinal axis (AA) of the chamber (30); means for circulating the solid in the chamber (45, 46); and means for circulating the gas (31, 32, 33, 45, 46, 43, 50, 51, 52) in the chamber (30) which comprise a first opening (32) for discharging the gas from the chamber (30) and first gas distribution means (31, 43, 50, 51, 52) which are associated with at least one of the trays (42), the first gas distribution means (31, 43, 50, 51, 52) comprising a plurality of openings (43) which are arranged in the tray (42) in question, an air box (50) which is arranged under the tray (42), and means for supplying gas to the air box (31, 51, 52).

Description

[0001] MULTIPLE-DECK OVEN FOR BRINGING A SOLID AND A GAS INTO CONTACT

[0002] Technical field

[0003] The invention relates to the field of multiple hearth furnaces for bringing a solid into contact with a gas.

[0004] Generally speaking, multi-hearth furnaces can be used in different types of processes, including but not limited to waste incineration, calcination for cement manufacturing for example, regeneration of activated carbon, or even pretreatment of biomass.

[0005] More specifically in the field of biomass transformation processes, the objective is to produce advanced biofuels (biodiesel and biokerosene) by thermochemical conversion of lignocellulosic biomass. The different stages of this process include (i) pretreatment, (ii) gasification, (iii) syngas conditioning and (iv) Fisher-Tropsch synthesis.

[0006] The first step of biomass pretreatment is carried out in order to prepare the biomass for its future injection into the gasifier. This step consists of drying and gentle thermal degradation of the wood called torrefaction. This pretreatment requires a significant heat input to, firstly, raise the temperature of the biomass to a level suitable for the drying phase, then eliminate the water present in the biomass, and finally raise and maintain the temperature of the dry biomass at a level suitable for the torrefaction operation. This heat input is generally carried out by contact of the biomass with a flow of hot gas. This pretreatment step can in particular be carried out using a multiple deck furnace. Multiple deck furnaces can then serve as dryers and / or torrefactors.

[0007] Prior art

[0008] Figure 1 shows an example of a multiple-hearth furnace for bringing a solid material into contact with a gas according to the prior art, described as such in patent application US2016356548A. According to this embodiment, the furnace (10) consists of a closed cylinder (11), (12) and (13) comprising 8 trays (1) to (8). A central shaft (15) arranged in the cylinder is capable of being rotated by a mechanism (17) around the vertical axis (16). The shaft makes it possible to rotate arms (18), integral with the shaft, comprising teeth (19), the arms being arranged above the trays. When the shaft rotates, the teeth are set in motion and, depending on their orientation, move the solid either towards the inside or towards the outside of the tray.The solid is introduced at the top of the furnace through a conduit (20) and circulates downwards, from tray to tray, either through the outer holes (222), (224), (226) and (228) or through the central holes (211), (213), (215) or (217), before leaving the reactor through an outlet conduit (24). The gas, bringing its heat to the solid, is introduced tangentially either at the bottom of the furnace (countercurrent movement of the solid via the orifices 27), or at the top of the furnace (co-current movement) and circulates through the same central or peripheral holes on each tray. During its travel, the gas then gives up its heat to the solid to carry out the heat treatment. However, the contact between gas and solid is not optimal, because the gas is injected tangentially and licks the surface of the solid. The heat brought by the gas to the solid is therefore transferred by convection to the surface then by conduction in the layer of solid.To promote heat and material transfer, patent application US2016356548 proposes placing vertical baffles above each tray to increase the gas residence time and thus the contact time between the gas and the solid. This phenomenon tends to improve the furnace's performance, but insufficiently.

[0009] Document US4050389A is also known, which relates to a method and device for incinerating waste in a multiple-hearth furnace. More specifically, this document describes means for introducing air in a conventional manner, i.e. at the base of the furnace, but also at the level of the different trays, with the objective of adding sufficient air at the level of each tray to maintain the temperature below a predetermined maximum limit. The means for introducing air at the level of the trays are in the form of nozzles arranged on a side face of the furnace, above each tray. Such a configuration allows injection of the gas at different levels of the furnace but has the disadvantage of having limited contact between the solid and the gas.Indeed, in this design, which aims for optimal control of incineration, the gas does not penetrate through the solid bed, which limits the transfer of heat and matter. Also known is document US2376177A which relates to a multiple hearth furnace comprising a rotating central shaft carrying arms at each tray on which the solid material is located, said arms being hollow and formed of openings for discharging the gas onto the material. Such a configuration allows injection of the gas at different levels of the furnace but has the disadvantage of injecting the gas above the solid, and therefore of having limited contact between the solid and the gas because the gas does not penetrate into the solid bed.

[0010] Document US2014298716 is also known, which relates to a biomass processing installation in the form of a multi-hearth furnace, comprising a gas injection system at each tray but with openings at the side walls of the furnace. Optionally, in order to better control the temperature and homogenize it, the injection system can be coupled to a system for optimizing heat exchanges, for example in the form of a heat pipe connecting the side wall of the furnace and each tray. However, even in this embodiment, the gas is injected at the side wall of the enclosure, which does not allow for homogeneous distribution. This document also describes means for promoting the rotational movement in the furnace, by means of a rod, a metal plate forming a deflector or a particular opening.In all cases, these embodiments aim to diffuse heat towards the center of the furnace chamber, but do not promote contact between the solid and the gas in a homogeneous manner on the plate.

[0011] Also known is GB941635A, which relates to a multi-hearth furnace for regenerating activated carbon, comprising means for introducing additional air into the furnace at a plurality of points. However, the disclosed technical solutions have air tubes arranged at least above the furnace trays. Such a configuration has the disadvantage of injecting the gas above the solid, and therefore of having limited contact between the solid and the gas. In the case where the air tubes were arranged below the furnace trays, this would not allow contact between the solid and the homogeneous gas on the tray.

[0012] The present invention overcomes these drawbacks. In particular, the present invention aims to improve the contact between the gas and the solid when injecting the gas into the furnace, via a homogeneous distribution of the gas under the layer of solid for at least one of the furnace trays. This contributes to greater efficiency in industrial processes using multi-hearth furnaces to bring a solid and a gas into contact.

[0013] Summary of the invention

[0014] The present invention relates to a multiple hearth furnace for bringing a solid and a gas into contact, said furnace comprising an enclosure with a vertical longitudinal axis in the operating position, a plurality of trays arranged in said enclosure and distributed along said longitudinal axis, means for circulating the solid in said enclosure, and means for circulating the gas in said enclosure comprising at least a first opening arranged in a wall of said enclosure to evacuate the gas from said enclosure.

[0015] According to the invention, said means for circulating the gas in said enclosure further comprise first means for distributing gas in said furnace associated with at least one of said trays, said first means for distributing gas associated with at least one of said trays comprising a plurality of orifices arranged in said at least one tray, a wind box arranged under said at least one tray and in fluid communication with said tray, and means for supplying gas to said wind box.

[0016] According to one implementation of the invention, said first gas distribution means in said oven can be associated with each of said trays of said oven.

[0017] According to one implementation of the invention, the number of orifices of said at least one plate associated with said first gas distribution means may be between 5 and 60 per m 2 , preferably between 8 and 40 per m 2, and very preferably between 10 and 30 per m 2 .

[0018] According to one implementation of the invention, a bed of particles may be arranged on a surface of said at least one tray associated with said first gas distribution means.

[0019] According to one implementation of the invention, said particles of said particle bed may be sand particles, crushed glass beads or raw glass beads, particles formed from natural or synthetic zeolite, from alumina, or from resins or plastic.

[0020] According to one implementation of the invention, when said solid is in the form of particles, said orifices of said at least one plate associated with said first gas distribution means can be sized so as to respect a predefined criterion and expressed according to a formula of the type: àP P = a. to P s in which AP Pis the pressure drop of the gas at the outlet of said orifices arranged in said plate, AP S is the pressure drop of the gas leaving said solid in the form of particles arranged on said plate, and a is a coefficient of proportionality between 0.01 and 0.7, preferably between 0.05 and 0.5 and more preferably between 0.1 and 0.4.

[0021] According to one implementation of the invention, said orifices of said at least one plate associated with said first gas distribution means can be sized to allow a gas speed in said orifices of between 0.1 and 20 m / s, preferably between 0.5 and 10 m / s, and very preferably between 0.8 and 5 m / s.

[0022] According to one implementation of the invention, said oven may further comprise second means for distributing gas in said oven, comprising at least one opening arranged in a wall of said enclosure and configured for a gas supply directly into said enclosure.

[0023] According to one implementation of the invention, said first gas distribution means associated with at least one of said trays may comprise a second opening arranged in a wall of said enclosure, a conduit optionally provided with a valve or a non-return valve, said conduit connecting said second opening to an external gas supply circuit.

[0024] According to one implementation of the invention, said means for circulating the solid in said enclosure may comprise a shaft passing through said longitudinal axis of said enclosure to rotate means for moving the solid on each of said plates, said means for moving the solid on each of said plates being able to comprise a scraping arm.

[0025] According to one implementation of the invention, said means for circulating the solid in said enclosure may comprise openings arranged on said trays, said openings arranged on said trays being able to be configured so that said trays alternately have either a central opening or a peripheral opening to allow the solid to move from one tray to another when said furnace is in operation.

[0026] The invention further relates to a method for drying and / or roasting a solid of the biomass feed type, said method being carried out using the multi-hearth furnace according to any one of the embodiments described above.

[0027] Other characteristics and advantages of the multiple-hearth furnace and of the method according to the invention will appear on reading the following description of non-limiting examples of embodiment, with reference to the figures appended and described below.

[0028] List of figures

[0029] Figure 1 shows an example of a multiple-hearth furnace for contacting a solid material with a gas according to the prior art.

[0030] Figure 2 illustrates in a schematic and non-limiting manner a first embodiment of the multiple-hearth furnace according to the invention, in countercurrent operation.

[0031] Figure 3 illustrates, in a schematic and non-limiting manner, a first variant of the embodiment of Figure 2.

[0032] Figure 4 illustrates, in a schematic and non-limiting manner, a second variant of the embodiment of Figure 2.

[0033] Figure 5 illustrates, in a schematic and non-limiting manner, an evolution of the variant of Figure 4.

[0034] Figure 6 illustrates, in a schematic and non-limiting manner, a fourth variant of the embodiment of Figure 2. Figure 7 illustrates, in a schematic and non-limiting manner, a fifth variant of the embodiment of Figure 2.

[0035] Figure 8 illustrates, in a schematic and non-limiting manner, a second embodiment of the multiple hearth furnace according to the invention, in co-current operation. Figure 9A (respectively Figure 9B) shows the evolution over time of the average temperature (respectively of the heating rate) of the biomass particles in the case of the multiple hearth furnace according to the prior art and the multiple hearth furnace according to the invention.

[0036] Description of the embodiments

[0037] The present invention relates to a multiple hearth furnace for contacting a solid in a flow of a gas.

[0038] The solid according to the invention can be any type of solid, such as for example a powder of a mixture of limestone, clay and sand, ore, plastic or a biomass filler.

[0039] According to a preferred implementation of the invention, in which the system is implemented as part of a process for drying and / or roasting biomass, the solid may correspond to a biomass feedstock. The biomass feedstock may advantageously vary depending on its origin. It may be wood or wood by-products, such as waste produced by logging (forest residues), sawmills, wood processing industries. It may also come from industrial by-products such as sludge or agri-food waste. The biomass may also come from traditional agriculture and consist of residues such as straw, coppice, bagasse, as well as crops dedicated to energy production (miscanthus, short-rotation coppice, etc.).Finally, it may correspond to organic waste, such as urban waste including sewage sludge, household waste may also constitute the feedstock. Preferably, the biomass feedstock may be lignocellulosic biomass or cellulose, and more preferably lignocellulosic biomass. The biomass feedstock may also come from algae. The feedstock may be composed of solid particles. In particular when the solid according to the invention corresponds to a biomass feedstock, it may contain a certain water content.

[0040] The gas according to the invention may be, depending on the intended application, air but also recycled combustion gases, carbon dioxide, water vapor, or an inert gas such as helium or nitrogen.

[0041] According to a preferred implementation of the invention in which the system is implemented as part of a process for drying and / or torrefying biomass, the gas may be an inert gas such as helium, nitrogen, or a mixture of these gases with recycled gases from a previous drying and / or torrefying step. According to an implementation of the invention in which the system is implemented as part of a process for torrefying biomass, the gas may also be water vapor.

[0042] According to the invention, the multiple-hearth furnace comprises an enclosure whose longitudinal axis, in the operating position (i.e. when the device is in use), is oriented vertically. According to the invention, a plurality of trays (also called hearths) are arranged (radially) along this longitudinal axis. The trays (or at least their median plane) are therefore, in the operating position of the multiple-hearth furnace, substantially horizontal.

[0043] The multiple hearth furnace according to the invention comprises means for circulating the solid in the enclosure. It is clear that the solid circulates, in the operating position, from the top of the enclosure to the bottom of the enclosure, by gravity.

[0044] According to one implementation of the invention, the means for circulating the solid may comprise a first opening arranged in the enclosure above the upper tray of the multi-hearth furnace (the term "upper" is taken with reference to the operating position), and a second opening arranged in the enclosure below the lower tray of the multi-hearth furnace (the term "lower" is taken with reference to the operating position). According to one implementation of the invention, the means for circulating the solid may comprise openings arranged on the trays, to allow the solid to pass from one tray to another. According to one implementation of the invention, the openings of the trays are configured so that the trays alternately have either a central opening or a peripheral opening to allow the solid to pass from one tray to another when the furnace is in operation.Advantageously, the means for circulating the solid may comprise a shaft passing through the longitudinal axis of the enclosure to rotate means for moving the solid on each of the plates. According to one implementation of the invention, the means for moving the solid on each of the plates may comprise a scraping arm, for example an arm provided with teeth.

[0045] The multiple-hearth furnace according to the invention comprises means for circulating the gas in the enclosure, comprising primary gas distribution means associated (or dedicated, or specific, or relative) to at least one of the trays of the multiple-hearth furnace. In other words, at least one of the trays of the multiple-hearth furnace comprises (or is associated with) primary gas distribution means (or primary gas distribution means). The primary gas distribution means associated with at least one tray comprise a plurality of orifices arranged in this tray, at least one windbox arranged under this tray and in fluid communication with said tray (i.e. the windbox and the tray are configured to allow the passage of the primary gas between the windbox and the tray), and means for supplying gas to the windbox.Thus, during its injection, the gas first fills the air box before penetrating and uniformly crossing the thickness of the solid located on the upper part of the tray having a plurality of orifices. This configuration makes it possible to force the gas to pass through the solid, which promotes gas-solid contact and allows better transfer of heat and material between gas and solid at the level of the tray(s) having primary gas distribution means. Subsequently, we speak of a perforated tray to designate a tray of the multiple hearth furnace having a plurality of orifices.

[0046] By air box is meant a chamber intended to be connected to gas supply means and comprising a plurality of openings intended for the distribution of gas. Thus, a air box allows homogenization of the gas introduced into the chamber and stabilization of the gas pressure in the chamber, before its distribution. It is clear that the air box according to the invention associated with a tray is supplied with gas by the means for supplying gas to primary gas distribution means according to the invention.

[0047] Very preferably, the roof (in other words the upper face) of the wind box according to the invention may coincide with the perforated plate with which it is associated. Thus, the orifices of the perforated plate allow the distribution, above the perforated plate, of the gas contained in the wind box.

[0048] According to one embodiment of the invention, the base (i.e. the lower face) of the wind box associated with a perforated tray may extend to the walls of the enclosure of the multi-hearth furnace. The chamber of the wind box according to this embodiment, when combined with the highly preferred variant according to which the roof of the wind box coincides with the perforated tray with which it is associated, is then formed by the perforated tray, the base, and the walls of the enclosure of the multi-hearth furnace. The wind box according to this design may be supplied with gas through an opening provided in a wall of the enclosure between the base of the wind box and the perforated tray. The base of a wind box may have any shape, such as for example a flat or curved plate.

[0049] Preferably, a plurality of trays of the multi-hearth furnace may comprise primary gas distribution means, and very preferably, all of the trays of the multi-hearth furnace comprise primary gas distribution means. In this way, the transfer of material and heat between gas and solid is optimized over the entire height of the multi-hearth furnace.

[0050] According to the invention, the means for circulating the gas further comprise at least one opening arranged in the enclosure to allow the gas to escape. Advantageously, this opening is either arranged above the upper tray of the multiple-hearth furnace in the case of countercurrent gas circulation (the term "upper" is taken with reference to the operating position), or arranged below the lower tray of the multiple-hearth furnace (the term "lower" is taken with reference to the operating position) in the case of cocurrent circulation.

[0051] According to one implementation of the invention, the means for supplying gas to the air box of at least one of the trays associated with primary gas distribution means may comprise at least one opening provided in the furnace enclosure. According to one implementation of the invention, the opening provided in the enclosure for supplying the air box of at least one of the trays associated with primary gas distribution means may be connected to a conduit which may itself be connected to a gas supply circuit (external to the multiple-hearth furnace according to the invention), optionally by means of a 2-way valve, a non-return valve, or gas flow regulation means. A 2-way valve, a non-return valve, or gas flow regulation means, specific to each tray associated with primary gas distribution means, make it possible to control the injection of gas from one tray relative to another.This allows for better adaptation to the specificities of the variety of industrial processes using multi-hearth furnaces as described above. In particular, the gas flow control means (such as a fan, a butterfly valve or a needle valve) allow a separate quantity of gas to be injected from one tray to another, for greater flexibility.

[0052] According to one implementation of the invention, the means for circulating the gas in the furnace may further comprise secondary gas distribution means, in addition to the primary gas distribution means according to the invention. These secondary (or additional) gas injection means may take any form of the gas distribution means according to the prior art. According to one implementation of the invention, the secondary gas distribution means may comprise an opening arranged in a wall of the enclosure and configured for a gas supply directly into the enclosure (unlike the primary gas distribution means which first supplies a wind box).According to one implementation of the invention, the secondary gas distribution means may comprise an opening arranged in the enclosure below the lower plate of the multi-hearth furnace in the case of countercurrent gas circulation (the term "lower" is taken as reference to the operating position), or arranged above the upper plate of the multi-hearth furnace (the term "upper" is taken as reference to the operating position) in the case of cocurrent circulation. These secondary gas distribution means make it possible to supplement the primary gas distribution means according to the invention, for greater flexibility of the multi-hearth furnace, in particular better adaptation to the specificities of the variety of industrial processes using multi-hearth furnaces as described above.

[0053] According to one implementation of the invention, the orifices of at least one of the trays associated with the primary gas distribution means are distributed regularly on the tray, i.e. with a constant number of orifices per unit area. Advantageously, the number of orifices of at least one of the perforated trays may be between 5 and 60 per m 2 , preferably between 8 and 40 per m 2 , and very preferably between 10 and 30 per m 2 Such orifice distributions contribute to a homogeneous distribution of gas over the upper part of the perforated plate(s).

[0054] Advantageously, when the solid is in the form of particles, the orifices of at least one of the plates associated with the primary gas distribution means have dimensions smaller than the dimensions of the particles of the solid, in order to avoid the phenomenon of weeping (particles which fall into the injection holes).

[0055] According to one implementation of the invention, the orifices of at least one of the plates associated with the primary gas distribution means are sized to allow a gas speed in each of the orifices of between 0.1 and 20 m / s, preferably between 0.5 and 10 m / s, and very preferably between 0.8 and 5 m / s.

[0056] According to one implementation of the invention, the orifices of at least one of the trays associated with the primary gas distribution means can be sized so as to comply with a predefined criterion relating to the pressure drop of the gas passing through the distribution orifices of the tray in question, and to the pressure drop of the gas passing through the solid arranged on the tray in question. According to one implementation of the invention, when the solid is in the form of solid particles and when the number of orifices of at least one of the perforated trays is between 5 and 60 per m 2, preferably between 8 and 40 per m 2 , and very preferably between 10 and 30 per m 2 , the orifices of at least one of the trays associated with the primary gas distribution means are sized so as to respect a predefined criterion expressed according to a formula of the type: Pp = a. P S in which P P is the pressure drop of the gas passing through the distribution orifices of the plate considered, AP Sis the pressure drop of the gas passing through the solid, in particular when it is in the form of particles, placed on the plate considered, and a is a coefficient of proportionality between 0.01 and 0.7, preferably between 0.05 and 0.5 and more preferably between 0.1 and 0.4. According to a particular implementation, the coefficient of proportionality a may be equal to 0.3. When the solid is in the form of particles, the solid may then take the form of a bed of particles, and the pressure drop through the solid may then be a function of the height and the porosity of the bed of solid particles. It is clear to a person skilled in the art that such ranges for the coefficient of proportionality a, combined with a number of orifices per m 2defined according to the aforementioned ranges, are sufficient to allow a person skilled in the art to size the orifices of a gas distributor plate on which solid particles would be placed.

[0057] According to one implementation of the invention, the enclosure of the multiple hearth furnace may be cylindrical or parallelepipedal in shape. According to this implementation, the openings for the entry and exit of the solid from the enclosure may advantageously be arranged respectively in the upper wall and in the lower wall of the enclosure. Advantageously, the section of the enclosure may be constant along the longitudinal axis. According to one implementation of the invention in which the enclosure is cylindrical in shape, the diameter of the enclosure may vary between 80 cm and 120 cm in internal diameter, preferably between 90 and 110 cm in internal diameter, and may very preferably be 1 m, and the height of the enclosure may vary between 80 cm and 120 cm, preferably between 90 and 110 cm, and may very preferably be 1 m.Such dimensions are typical for multiple hearth furnaces used in industrial processes such as those described above.

[0058] According to one implementation of the invention, the material of the various elements (enclosure, trays, wind box, etc.) of the multi-hearth furnace may be ceramic, a metal preferably resistant to corrosion, in particular any type of stainless steel. These materials are in fact resistant to the very high temperatures and acid compounds involved during the implementation of processes using this type of furnace, and limit the extractable compounds that can migrate into the solid and / or into the gas.

[0059] According to a preferred implementation of the invention, the multi-hearth furnace comprises a bed of particles arranged on the surface of at least one of the perforated trays, preferably on the surface of each of the perforated trays of the multi-hearth furnace. The bed of particles will increase the pressure drop above the perforated tray and thus improve the gas distribution over the entire surface of the perforated tray(s). Advantageously, the bed of particles is surmounted by a grid, so that they are not carried away by the gas flow coming from the perforated trays. In addition, such a grid can also allow a more homogeneous distribution of the gas, thus improving the gas-solid contact. It is clear that the size of the openings of the grid must be both smaller than the size of the particles of the bed of particles and smaller than the size of the particles of the solid charge.

[0060] According to one implementation of the invention, the particles of a bed can be any type of particles that are non-reactive (or inert) to gas and / or have a high thermal conductivity (to promote heat transfer in the solid), such as, for example, sand particles, or crushed glass beads or raw glass beads, particles formed from natural or synthetic zeolite, from alumina, or from resins or plastic. The materials mentioned above have the advantages of being easily accessible commercially and of having advantageous thermal and mechanical properties (high thermal conductivity and good resistance to attrition).According to one implementation of the invention, the height of the particle bed may be greater than 50% of the spacing between the base of the plate and the lower part of the means for moving the solid on the plate in question (for example, in the case of an arm provided with teeth, the lower part of the means for moving the solid on the plate in question corresponds to the base of the teeth), preferably greater than 60% and more preferably greater than 70%. Advantageously, the particle size of a bed may be greater than the dimensions of the orifices of the plate in question.

[0061] Figure 2 illustrates, in a schematic and non-limiting manner, a first embodiment of the multiple-hearth furnace according to the invention, in countercurrent operation. More precisely, this figure shows a multiple-hearth furnace comprising an enclosure 30 traversed by a central shaft 36 driven in rotation R relative to the longitudinal axis AA of the enclosure and which is arranged along the vertical axis when the furnace is in operation. The multiple-hearth furnace comprises four trays 41, 42 arranged along the longitudinal axis. According to this embodiment, the trays 41, 42 are equipped with either a central hole 45 or a peripheral hole 46, and this alternately from one tray to another along the longitudinal axis, to allow the passage of the solid charge from one tray to another (openings in the enclosure for the passage of the solid charge not shown).The means for circulating the gas in the enclosure 30 according to this design comprise primary gas distribution means associated with a single tray, in this case the tray 42 located lowest in the enclosure 30. The primary gas distribution means associated with the tray 42 comprise orifices 43 distributed homogeneously on the tray 42, a wind box 50 arranged under the tray 42, the wind box 50 being connected to a gas supply circuit 60, 61 external to the furnace by means of a conduit 52 connecting the opening 51 provided in the wind box and the opening 31 provided in the side wall of the enclosure. The means for circulating the gas in the enclosure 30 according to this design further comprise an opening 32 provided in the upper wall of the enclosure to allow the evacuation of the gas in countercurrent operation.Thus, during its injection, the gas first fills the wind box 50 of the perforated plate 42 before penetrating and uniformly passing through the thickness of the solid (not shown) located on the upper part of the perforated plate 42. This configuration makes it possible to force the gas to pass through the solid, which promotes gas-solid contact and allows better transfer of heat and material between gas and solid at the level of the perforated plate.

[0062] Figure 3 illustrates, in a schematic and non-limiting manner, a first variant of the embodiment of Figure 2, in every respect identical to this first embodiment (thus, the common elements will not be described again), with the exception of the presence of a valve 53 along the conduit 61 of the gas supply circuit 60, 61 external to the furnace, so as to be able to control the injection of gas into the enclosure 30.

[0063] Figure 4 illustrates, in a schematic and non-limiting manner, a second variant of the embodiment of Figure 2, in every respect identical to this first embodiment (thus, the common elements will not be described again), with the exception of the fact that all the trays 42 of the enclosure are trays comprising primary gas distribution means, in this case identical to the primary gas distribution means of the tray 42 of Figure 2. This configuration makes it possible to promote gas-solid contact for each of the trays of the multiple-hearth furnace, and thus to obtain better efficiency of the multiple-hearth furnace with regard to the heat transfer between the gas and the solid.

[0064] Figure 5 illustrates, in a schematic and non-limiting manner, an evolution of the variant of Figure 4, in all respects identical to this variant (thus, the common elements will not be described again), with the exception of the presence of a valve 53 along each conduit 61 of the gas supply circuit 60, 61 external to the furnace. This configuration makes it possible to control the injection of the gas into the enclosure 30 independently from one tray to another, depending for example on the results of control measurements of the process implemented, such as measurements of gas flow rate, temperature, pressure, gas composition, etc.

[0065] Figure 6 illustrates, in a schematic and non-limiting manner, a fourth variant of the embodiment of Figure 2, in all respects identical to this first embodiment (thus, the common elements will not be described again), with the exception of the fact that a bed of particles 54 is arranged on the surface of the perforated plate 42 and is surmounted by a grid 55 on which the solid to be heat-treated is distributed. The bed of particles makes it possible to increase the pressure drop above the perforated plate and thus to improve the gas distribution over the entire surface of the plate. The grid makes it possible to prevent the particles of the bed of particles from being carried away by the gas flow and from mixing with the particles of solid circulating on the plates. In addition, the grid makes it possible to contribute to a more homogeneous distribution of the gas.

[0066] Figure 7 illustrates, in a schematic and non-limiting manner, a fifth variant of the embodiment of Figure 2, in all respects identical to this first embodiment (thus, the common elements will not be described again), with the exception of the fact that the means for circulating the gas comprise secondary gas distribution means, in the form of an opening 33 arranged above the lower tray (in this case the perforated tray 42) of the furnace, allowing a supplementary gas supply via the conduit 55 connected to the gas supply circuit 60, 61 external to the furnace. These secondary gas distribution means make it possible to supplement the primary gas distribution means associated with the tray 42, for better adaptation to the specificities of the variety of industrial processes. It is also possible to connect the conduit 55 to a dedicated secondary gas circulation line, different from the line of the primary gas supply circuit 60.

[0067] It is clear that the variants of figures 3 to 7 can be combined with each other.

[0068] Figure 8 illustrates, in a schematic and non-limiting manner, a second embodiment of the multiple-hearth furnace according to the invention, in co-current operation. This second embodiment is in all respects identical to the first embodiment (thus, the common elements will not be described again), except that in this design, the means for circulating the gas in the enclosure 30 comprise an opening 32 arranged in a part of a side wall of the enclosure 30 located under the lower plate (in this case the perforated plate 42) of the furnace, to allow the evacuation of the gas in co-current operation of the furnace. It is clear that the variants of Figures 3 to 7 can be combined with the embodiment of Figure 8.

[0069] The invention further relates to a method for drying and / or roasting a biomass feedstock implemented using the multi-hearth furnace as described above according to any of its variants.

[0070] Examples

[0071] The characteristics and advantages of the system according to the invention will appear more clearly on reading the application example below.

[0072] This application example aims to compare the heating rate of a biomass-type material placed on a tray of a multiple-hearth furnace according to the prior art (i.e. with tangential gas injection) described in figure 1, and the heating rate of a biomass-type material placed on a tray associated with primary gas distribution means of a multiple-hearth furnace according to the invention.

[0073] Heating rates are estimated using numerical simulations performed with CPFD Barracuda™ software. The simulation geometric data and operating / boundary conditions are shown below:

[0074] Characteristics of the enclosure: cylinder 1 m in diameter and 1 m in height, comprising an opening for the gas outlet in the form of a cylindrical tube 0.17 m in diameter and 0.1 m in height fitted in the upper wall of the cylindrical enclosure, this opening being placed under atmospheric conditions;

[0075] The oven is maintained at a constant temperature of 150°C;

[0076] The enclosure of the multiple-hearth furnace according to the prior art comprises 3 tangential gas injection cylinders of 0.1 m diameter located 0.3 m above the tray in question. The perforated tray of the multiple-hearth furnace according to the invention comprises 21 orifices distributed uniformly on the tray, corresponding to a number of orifices per m 2 of plateau of approximately 27.

[0077] The gas used is nitrogen at 280°C. The total inlet gas flow rate is 0.13 kg / s, representing a superficial gas velocity in the furnace of 0.3 m / s.

[0078] A layer of biomass with a height of 0.2 m is arranged above the tray of the multiple hearth furnace according to the prior art and the perforated tray of the multiple hearth furnace according to the invention. The biomass particles have an average diameter of 2 cm and an initial temperature of 20°C.

[0079] A numerical simulation lasting 100 s is carried out for the multiple hearth furnace according to the prior art and for the multiple hearth furnace according to the invention. The average temperature of the biomass particles is recorded over time, from which the heating rate of the biomass particles can be deduced. Figure 9A (respectively Figure 9B) shows the evolution over time t of the average temperature (respectively the heating rate) of the biomass particles in the case of the multiple hearth furnace according to the prior art (curve and scale TAA; respectively curve and scale VAA) and the multiple hearth furnace according to the invention (curve and scale Tl NV; respectively curve and scale VINV). A much faster temperature rise can be observed in the case of the multiple hearth furnace according to the invention, which makes it possible to increase the heating rate by a factor of 600 compared to the multiple hearth furnace according to the prior art.Such an increase in heating speed not only reduces production time, but also the energy consumption of the furnace.

[0080] Thus, the multiple-hearth furnace according to the invention allows a more homogeneous distribution of the gas on the surface of a tray comprising primary gas distribution means, which allows better contact between the gas and the solid placed on this tray, and thus greater efficiency of the industrial processes using such a furnace.

Claims

Claims 1. Multiple hearth furnace for bringing a solid and a gas into contact, said furnace comprising an enclosure (30) with a vertical longitudinal axis (AA) in the operating position, a plurality of trays (41, 42) arranged in said enclosure and distributed along said longitudinal axis (AA), means for circulating the solid in said enclosure (45, 46), and means for circulating the gas (31, 32, 33, 45, 46, 43, 50, 51, 52) in said enclosure (30) comprising at least one first opening (32) arranged in a wall of said enclosure for evacuating the gas from said enclosure (30), characterized in that said means for circulating the gas (31, 32, 33, 45, 46, 43, 50, 51, 52) in said enclosure (30) further comprise first gas distribution means (31 , 43, 50, 51, 52) in said oven associated with at least one of said trays (42), said first gas distribution means (31, 43, 50, 51,52) associated with at least one of said trays (42) comprising a plurality of orifices (43) arranged in said at least one tray (42), a wind box (50) arranged under said at least one tray (42) and in fluid communication with said tray, and means for supplying gas to said wind box (31, 51, 52)., 2. Multi-hearth furnace according to the preceding claim, wherein said first gas distribution means (31, 43, 50, 51, 52) in said furnace are associated with each of said trays (41, 42) of said furnace.

3. Multiple hearth furnace according to one of the preceding claims, in which the number of orifices (43) of said at least one tray (42) associated with said first gas distribution means is between 5 and 60 per m 2 , preferably between 8 and 40 per m 2 , and very preferably between 10 and 30 per m 2 .

4. Multi-hearth furnace according to one of the preceding claims, in which a bed of particles (54) is arranged on a surface of said at least one tray associated with said first gas distribution means.

5. A multiple hearth furnace according to claim 4, wherein said particles of said particle bed (54) are sand particles, crushed glass beads or raw glass beads, particles formed from natural or synthetic zeolite, alumina, or resin or plastic.

6. Multiple hearth furnace according to claim 3, wherein, when said solid is in the form of particles, said orifices (43) of said at least one tray (42) associated with said first gas distribution means (31, 43, 50, 51, 52) are sized so as to comply with a predefined criterion and expressed according to a formula of the type: P P = a. P S in which AP Pis the pressure drop of the gas at the outlet of said orifices (43) arranged in said plate (42), AP S is the pressure drop of the gas leaving said solid in the form of particles arranged on said plate (42), and a is a coefficient of proportionality between 0.01 and 0.7, preferably between 0.05 and 0.5 and more preferably between 0.1 and 0.

4.

7. Multiple hearth furnace according to one of the preceding claims, wherein said furnace further comprises second gas distribution means (33) in said furnace, comprising at least one opening (33) arranged in a wall of said enclosure and configured for a gas supply directly into said enclosure.

8. Multiple hearth furnace according to one of the preceding claims, in which said first gas distribution means (31, 43, 50, 51, 52) associated with at least one of said trays (42) comprise a second opening (31) arranged in a wall of said enclosure, a conduit (61) optionally provided with a valve (53) or a non-return valve, said conduit (61) connecting said second opening (31) to an external gas supply circuit (60).

9. A multiple hearth furnace according to one of the preceding claims, wherein said means for circulating the solid (45, 46) in said enclosure (30) comprise a shaft (36) passing through said longitudinal axis (AA) of said enclosure (30) to rotate means for moving the solid (45, 46) on each of said trays (41, 42), said means for moving the solid (45, 46) on each of said trays (41, 42) comprising a scraping arm.

10. Multiple hearth furnace according to one of the preceding claims, wherein said means for circulating the solid (45, 46) in said enclosure (30) comprise openings (45, 46) arranged on said trays (41, 42), said openings (45, 46) arranged on said trays (41, 42) being configured so that said trays (41, 42) alternately have either a central opening (45) or a peripheral opening (46) to allow the solid to move from one tray (41, 42) to another when said furnace is in operation.

11. Method for drying and / or roasting a solid of the biomass feed type, said method being carried out by means of the multiple hearth furnace according to any one of claims 1 to 10.

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