Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27L—REMOVING BARK OR VESTIGES OF BRANCHES; SPLITTING WOOD; MANUFACTURE OF VENEER, WOODEN STICKS, WOOD SHAVINGS, WOOD FIBRES OR WOOD POWDER
  • B27L11/00—Manufacture of wood shavings, chips, powder, or the like; Tools therefor
  • B27L11/005—Tools therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27M—WORKING OF WOOD NOT PROVIDED FOR IN SUBCLASSES B27B - B27L; MANUFACTURE OF SPECIFIC WOODEN ARTICLES
  • B27M1/00—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching
  • B27M1/02—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching by compressing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
  • B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
  • B27N3/08—Moulding or pressing
  • B27N3/28—Moulding or pressing characterised by using extrusion presses
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
  • B27N5/00—Manufacture of non-flat articles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/001—Combinations of extrusion moulding with other shaping operations
  • B29C48/0022—Combinations of extrusion moulding with other shaping operations combined with cutting
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
  • B29C48/04—Particle-shaped
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/285—Feeding the extrusion material to the extruder
  • B29C48/288—Feeding the extrusion material to the extruder in solid form, e.g. powder or granules
  • B29C48/2886—Feeding the extrusion material to the extruder in solid form, e.g. powder or granules of fillers or of fibrous materials, e.g. short-fibre reinforcements
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/30—Extrusion nozzles or dies
  • B29C48/345—Extrusion nozzles comprising two or more adjacently arranged ports, for simultaneously extruding multiple strands, e.g. for pelletising
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
  • B29C48/395—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using screws surrounded by a cooperating barrel, e.g. single screw extruders
  • B29C48/40—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using screws surrounded by a cooperating barrel, e.g. single screw extruders using two or more parallel screws or at least two parallel non-intermeshing screws, e.g. twin screw extruders
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
  • B29C48/395—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using screws surrounded by a cooperating barrel, e.g. single screw extruders
  • B29C48/40—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die using screws surrounded by a cooperating barrel, e.g. single screw extruders using two or more parallel screws or at least two parallel non-intermeshing screws, e.g. twin screw extruders
  • B29C48/405—Intermeshing co-rotating screws
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
  • B29C48/50—Details of extruders
  • B29C48/505—Screws
  • B29C48/57—Screws provided with kneading disc-like elements, e.g. with oval-shaped elements
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
  • C10L5/00—Solid fuels
  • C10L5/02—Solid fuels such as briquettes consisting mainly of carbonaceous materials of mineral or non-mineral origin
  • C10L5/34—Other details of the shaped fuels, e.g. briquettes
  • C10L5/36—Shape
  • C10L5/363—Pellets or granulates
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
  • C10L5/00—Solid fuels
  • C10L5/40—Solid fuels essentially based on materials of non-mineral origin
  • C10L5/44—Solid fuels essentially based on materials of non-mineral origin on vegetable substances
  • C10L5/442—Wood or forestry waste
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
  • B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
  • B27N3/02—Manufacture of substantially flat articles, e.g. boards, from particles or fibres from particles
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
  • C10L2290/00—Fuel preparation or upgrading, processes or apparatus therefore, comprising specific process steps or apparatus units
  • C10L2290/08—Drying or removing water
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
  • C10L2290/00—Fuel preparation or upgrading, processes or apparatus therefore, comprising specific process steps or apparatus units
  • C10L2290/50—Screws or pistons for moving along solids

Definitions

• Wood pellet extrusion process co-rotating twin-screw extruder for wood pellet extrusion, and related wood pellets
• the present invention relates to the field of the extrusion of wood pellets, in particular with a twin-screw extruder.
• Wood in the form of pellets is therefore recoverable as a source of energy by burning it, thanks in particular to a boiler that is a source of combustion performance and of lower environmental impact with gaseous and particulate emissions, harmful to the environment, minimized.
• the heat produced by combustion can either be used to produce heat to heat, for example individual or collective housing, or produce another type of energy such as electricity or mechanical work.
• the pellet manufacturing process available on the market consists of a circular press which forces by rollers, two in general, wood in the form of sawdust already strongly dried upstream (residual humidity rate, generally lower than 12% of the total mass), through multiple dies (holes) arranged radially in the circular metal casing of the press on which the rollers roll.
• the holes can be purely cylindrical or conical or mixed.
• the investment costs and the industrial infrastructure to achieve the pellet by drying, conveying, storing in waiting, the sawdust previously prepared for its final transformation, are very high.
• the infrastructures of a production site are very substantial in size. Associated with the large volumes processed in a production unit, the dimensions of the installations generate significant energy losses during the drying and conveying stages. Storage volumes of up to several hundred m 3 require proportionate land requirements. The size of a complete production unit requires investments exceeding ten million euros. Only the largest sawmills, producing sawdust and currently the only ones concerned by its transformation, are capable of investing and therefore of producing.
• Wood is in itself a heterogeneous material at all scales, molecular to macroscopic, and anisotropic. It consists of three main components: cellulose, hemicellulose and lignin with orientation effects of the cellulose fibers which constitute it and make it anisotropic. This anisotropy, due to the fact that the wood is fibrous, generates form factors and an anisometry of the wood particles obtained during a sawing or grinding step. Whether in the form of fine or coarse sawdust or in the form of chips, the wood comes with elongated geometries, in the direction of the fibers, or flattened.
• the anisotropy of the wood is therefore naturally present but, in addition, the current production processes of wood pellets undergo this anisotropy responsible for a form factor of the incoming material which has never been able to be removed to date by an action. swallows to the final transformation into pellet.
• a process for extruding wood pellets comprising at least the steps of:
• step (b) crushing and partial drying, in a twin-screw extruder comprising at least shear-generating elements, pieces of wood to obtain wood particles having a reduced water content compared to the pieces of wood, this is ie the pieces of wood from step (a) entering the extruder;
• a process for extruding wood pellets is proposed, the process being characterized in that it comprises at least the steps of:
• step (b) grinding and drying, in a twin-screw extruder comprising at least elements mixers and elements with reverse screw pitch, pieces of wood to obtain wood particles having a reduced water content compared to the pieces of wood, that is to say the pieces of wood of step ( a) entering the extruder;
• Steps (a), (b) and (c) are preferably successive one after the other.
• Step (a) of supplying pieces of wood only includes the supply of pieces of wood as is without the addition of an additive such as a fluid.
• the shearing elements include mixing elements, and the grinding of step (b) is carried out by applying a shearing force to the pieces of wood, thanks to the mixing elements of the extruder.
• the shear-generating elements comprise at least one element with a reverse screw pitch, and the pieces of wood are concomitantly moved in an axial direction, relative to the axis of the screws of the extruder, but retained by the element or elements ( s) with reverse thread.
• step (b) the partial drying is carried out using heat generated by grinding up to a temperature of at least 100°C.
• Step (b) comprises at least one phase of evacuation of water extracted from the wood fibers, outside the extruder.
• Step (c) of compression is carried out by applying a radial compression force, relative to the axis of the screws of the extruder, so as to push the wood particles in a substantially radial direction, relative to the axis of the screws of the extruder, in the at least one die to form at least one extruded rod and to obtain, after cutting, said wood pellets.
• the method comprises a step (d) of final drying of the wood pellets at the outlet of the die, in particular down to a humidity level of less than 12%, which makes it possible in particular to comply with the standards qualifying a wood pellet for energy.
• Step (d) is set to obtain wood pellets with a moisture content between 7% and 10%.
• Steps (b), (c) and (d) are set to obtain wood pellets with a moisture content of less than 12% and a density of at least 0.7.
• Steps (b), (c) and (d) are parameterized to obtain wood pellets having a moisture content of less than 12% and a density of at least 0.8.
• the pieces of wood provided in step (a) have a natural moisture content of more than 40%, preferably around 55%, and the wood particles obtained at the output of step (b) have a humidity level below 40%, preferably between 30% and 35%.
• the wood particles obtained at the output of step (b) have an ovoid, or even substantially spherical, shape.
• Steps (a) and (b) are repeated several times with the same pieces of wood and particles, before proceeding to step (c).
• step (a) pieces of wood from softwoods, or pieces of wood from hardwoods, or pieces of wood from a mixture of softwoods and hardwoods are provided.
• a twin-screw extruder suitable for implementing the process for extruding wood pellets according to the first aspect is proposed.
• twin-screw extruder preferably co-rotating, provided for the extrusion of wood pellets comprising: two profiles of identical screws, interpenetrating, and mounted in rotation (preferably in co-rotation) in a bore of a sheath , wherein each screw profile comprises a shaft on which are mounted shear generating elements for crushing and drying pieces of wood in order to obtain wood particles having a reduced water content compared to the pieces of wood, and at least one side die (40) forming a side outlet with respect to an axis of the screws of the extruder.
• Each screw profile comprises compression means mounted on the shaft, for radial compression, relative to an axis of the screws of the extruder, of the wood particles in the direction of the at least one side die.
• the compression means comprise direct-pitch screw elements and mixing elements placed at the end of the screw profile, the direct-pitch screw elements being arranged to push the wood particles onto the mixing elements.
• the sheath has no wood outlet in a direction parallel to the axes of the extruder screws.
• the shear generating elements include kneading elements (20).
• the shear generating elements include at least one reverse thread element (30).
• the at least one reverse thread element (30) is placed directly at the outlet of the mixing elements (20).
• Each screw profile comprises at least one direct screw pitch element directly placed at the outlet of the mixer elements (20) and just before the at least one reverse screw pitch element (30).
• a wood pellet which can be obtained according to the wood pellet extrusion process according to the first aspect.
• the granulate has a moisture content of less than 12% after final drying and a density of at least 0.7 and preferably greater than 0.8.
• the wood pellet has a moisture content of between 7% and 10%.
• the wood pellet is not disintegrable in water.
• the wood pellet is formed solely of wood and in particular does not include any additive.
• Figure 1 shows an example of a screw profile portion of a twin-screw extruder for implementing the proposed wood pellet extrusion process.
• Figure 2 shows an example of a mixer element of a twin-screw extruder for implementing the proposed wood pellet extrusion process.
• Figure 3 schematically represents the steps of an embodiment of the proposed wood pellet extrusion process.
• Figure 4 shows an example of an arrangement of lateral dies at the outlet of a twin-screw extruder for the implementation of the proposed wood pellet extrusion process.
• the invention provides a process for extruding wood pellets. Typically this process can be implemented with a co-rotating twin-screw extruder.
• a twin-screw extruder preferably co-rotating, suitable for implementing the proposed wood pellet extrusion process. It is a continuous process. It should be noted that it is possible in practice to use a succession of twin-screw extruders, so that the present method will not be limited to a single twin-screw extruder.
• a co-rotating twin-screw extruder is a transformation machine made up of two identical, interpenetrating screw profiles, rotating in the same direction in the bore of a barrel.
• the screws are driven by a motor block and both rotate at the same speed.
• One or more feeding zones can be set up. In general, a feed zone is chosen at the start of the screw profile if necessary with lateral or secondary feeders.
• At the outlet at least one die ensures the conformation of the material in the form of a rod whose section can take any shape.
• the die can be cylindrical or conical or a combination of cylindrical and conical sections in the case of a round section. Any other section is possible according to the same principle.
• screw profile we mean all the assembled elements, direct, reverse and kneading screw elements, on each shaft of the twin-screw extruder, as explained later.
• a screw profile of an extruder can be assembled to order by combining three main types of usual elements for this process which can have their own roles or be associated in synergy. They all have the same outer diameter but can be of varying lengths and assemble on screw shafts which can be splined or sections preventing their free rotation on the shafts, for example a hexagonal section, or another system preventing rotation such as a key system.
• all the elements chosen, whatever their type, whatever their number and length, must have an assembled length equal to the length of the screw shafts supporting them when operating in a conventional configuration.
• the assembly without play between the elements is usually maintained by tightening screws at the end of the shaft.
• These screw profiles can end in a point-shaped outer shape. The chosen assembly order of the screw elements will make it possible to generate zones with specific functionalities to carry out precise tasks in each specific zone elaborated.
• the sheath that surrounds the screw profiles provides overall sealing and resists pressure if it is completely closed, but also allows heat exchange by being heated or, conversely, cooled with an integrated regulated cooling system by circulation of a liquid or gaseous fluid. Heating or cooling is not constant across the entire profile. It can be adjusted in differentiated zones all along the barrel where each barrel section/element will have its own heating and cooling system
• the material leaving the extruder and its barrel / screw system will be shaped by means of a die.
• the die is fixed on the last element of the sleeve with an intermediate convergent, all in the axis of the profile of the screws.
• This channel can have a single outlet or combine several.
• the direct-pitch screw elements (10; 11) allow material to be conveyed in the direction of flow. These screw elements are characterized by their pitch (distance traveled per revolution) and the number of channels separated by threads, from 1 to 3 channels, generally 2. rotation and during the conveying of the material, the material circulates in continuous channels in the shape of a deformed 8 and passing from one screw to another through the zone of interpenetration between screws. The material there is also subjected to shear. Intense shear is also generated in the clearance between the screw threads and the barrel. With the only direct pitch elements, the extruder is not totally filled with the material and the material is therefore not under pressure before having reached the last elements before the die.
• FIG. 2 illustrates an example of the geometry of mixers 21, having a bilobe profile with a splined central axis.
• each screw is chosen to allow the implementation of the proposed wood pellet extrusion process. They are made up of the same sequences of elements so that they can interpenetrate, be combined and enter into rotation without mechanically interfering.
• the sheath is generally made up of a set of sheath elements (sections). These sheath elements being interchangeable in their positioning one after the other, as for the screw elements, their assembly can therefore be made to order. Able to be heated or cooled individually, each section can have its own set temperature.
• the sheath as a whole may have possible openings all along the sheath for: either feed the extruded system with a liquid, solid, or viscous system by means of a suitable feed system such as a volume or weight flow meter, a pump or even another extruder, or evacuate liquid (such as for example water) or vapours.
• the evacuation means comprise one or more openings in the sheath making it possible to evacuate water vapor resulting from the drying of the wood during the proposed extrusion process.
• Other additional openings can come in addition to evacuate liquid extractions, by simple gravity, based on water within the framework of the proposed method.
• the proposed twin-screw extruder has the particularity of having: two identical screw profiles, interpenetrating, and mounted in rotation (preferably in co-rotation) in a bore of a sleeve, in which each screw profile comprises a shaft on which are mounted shear generating elements for crushing and drying pieces of wood in order to obtain wood particles having a reduced water content compared to the pieces of wood, and at least one die side 40 forming a side outlet relative to an axis of the screws of the extruder.
• wood in its natural state wood is an anisotropic fibrous material and which remains so after transformation in the usual and current processes of transformation of wood as a structural material (beam, plank, etc.).
• a co-product of sawing wood it comes in the form of sawdust or chips.
• Due to the anisotropy of wood it is a material where the fibers are oriented which leads to co-products of sawing with elongated, flattened shapes and in any case with anisometric geometries which differ strongly from the ideal spherical shape. to hope for an isometry. These shapes are as much due to the natural structure of the wood as it is made, as to the usual tools that have sawn or crushed it.
• the method comprises in particular the following steps:
• the wood may in some cases have undergone prior drying or, on the contrary, have been re-moistened, insofar as it is simply a matter of restoring a natural water level in the wood before sawing (for example if the we want to use wood which has already dried over time, for example during storage, and which is no longer in its state as it is just after sawing a tree), and it will be understood that this possible modification of the moisture content will be made without or with mechanical treatment of the wood such as mixing.
• a rewetting of the wood can also be done in the same extruder by means of a feed pump as a peripheral connected to the sheath of the extruder or to one of its sections constituting it.
• the use of a natural wood as is will nevertheless remain preferred, as it allows for minimal costs and handling.
• the wood may have been rinsed with water or separated from its pollutants by simple flotation in water. water, ideally continuously and in this case integrated into the overall process, before extrusion.
• this wood may have ovoid shapes, and preferably substantially spherical, after grinding that it did not have at the extruder inlet. It should be noted that the drying of this stage is not a drying removing all the water present in the pieces of wood, we can therefore speak of partial drying;
• Step (a) Step (a) of supplying pieces of wood essentially consists of introducing pieces of wood into a twin-screw extruder.
• the wood is introduced continuously "as is", without adding a fluid or any additive.
• fluid it is understood the addition of water, an aqueous solution or any liquid that may have a lubricating function in the extrusion process and / or final binder extruded rod(s).
• One of the advantageous characteristics of the proposed extrusion process is that it is effective with any type of wood.
• it can be applied to pieces of wood from softwoods as well as from hardwoods.
• step (a) it is possible, for example, to provide pieces of wood from softwoods, or pieces of wood from hardwoods, or pieces of wood from a mixture of softwoods and hardwoods.
• step (b) comprises the grinding, in the twin-screw extruder, of the pieces of wood, whether in the form of sawdust from raw grinding, wood chips, wood shavings, or any other form of wood having been crushed beforehand or not, to obtain a wood powder having a particle size reduced in size compared to the incoming wood, completed with a form factor approaching the sphere.
• the wood resource can come as much from wood from recently felled trees or from wood to be recycled by the user.
• step (b) is carried out by generating shear on the extruded material.
• the two interpenetrating screws, of the co-rotating twin-screw extruder move the pieces of wood in a longitudinal direction thanks to direct-pitch screw elements, while exerting locally in one or more targeted zones in screw profile, shear, using specific elements of the screw profile.
• the twin-screw extruder may comprise in its screw profile at least a series of mixing elements 20 forming a block of this type of element (forming a “mixing” zone).
• the kneading elements 20 can be assembled directly at the inlet of the twin-screw extruder, or more ideally preceded by a section of direct-pitch screw elements 10, ideally short, to generate an axial thrust force in the block composed of several mixers 21 and thus effectively feed the mixing zone without clogging.
• elements with reverse thread 30 which are for example directly placed at the outlet of the mixer elements 20 constituting a block to retain the wood therein or separated by one or more elements to not straight.
• elements with reverse thread 30 are also provided.
• direct screw pitch elements 11 at the output of the reverse screw pitch elements 30 for conveying the particles which will be obtained.
• extruder kneading elements allow high shear to be exerted, especially in conjunction with the reverse thread elements which hold the wood in the kneader and increase the filling and therefore the shear.
• mixers make it possible to grind by breaking agglomerates or original particles into smaller particles. For this, they are assembled to several consecutively with angle offsets chosen because they are adjustable. An angle offset of approximately 90° is preferred here.
• screw profile we mean all the assembled elements, direct and reverse pitch screw elements and mixers, on each shaft of the twin-screw extruder.
• the mixer elements allow densification at the outlet by compression (strong pressure on the external thread of the mixer). As the wood can no longer pass through the axis at the end of the extruder, it is the pressure effect at the top of the threads of these mixers that dominates.
• Figure 1 illustrates an example of a screw profile portion of a twin-screw extruder for implementing the desired shear in the proposed wood pellet extrusion process.
• Step (b) is also a drying step, in the extruder, of the transformed wood to reduce its water content.
• Step (b) is also a drying step, in the extruder, of the transformed wood to reduce its water content.
• the extruder As specified above, in particular there is a partial drying of the wood since the water present is not entirely removed.
• the bis-extruder and in particular the combination of kneading elements and elements with reverse screw pitch, in fact makes it possible to simultaneously heat the wood while it is ground.
• the drying can in fact be carried out by using heat generated by work, on the material during grinding.
• the energy dissipated during the grinding of the transformed material by the mechanical work exerted on the sheared material, generates self-heating which makes it possible to dry the wood, advantageously at a temperature of at least 100° C approx.
• the elements with reverse thread can contribute to drying by wringing the wood to a certain extent (evacuation of water in the liquid state), and moreover thanks to the counter threads we manage to generate more self-heating and therefore a more efficient transition from water to vapor.
• this step does not exclude possible additional heating by external energy supply (for example heating collars), in particular beyond 120°C, more preferably in the interval 130°C-160°C.
• a proportion of between 10% and 80%, preferably between 20% and 60%, preferably between 30% and 40%, of the water naturally present in the wood, entering the extruder, can be extracted during step (b).
• the residual moisture content drops below 40%, preferably between 25% and 40%, preferably between 30% and 35%, in particular around 32%. This corresponds, starting from a humidity rate of 55%, to an extraction of approximately 35% of the water naturally present in the wood.
• step (b) can comprise a phase of evacuation of the water extracted from the wood, outside the extruder.
• this phase can be performed using a opening or several specific openings of the sheath to evacuate water in liquid and/or gaseous form.
• step (d) it is possible to repeat steps (a) and (b) several times before proceeding to step (d).
• the particles can be extracted from the extruder after step (b), then directly reintroduced into the extruder (or another extruder) to start the process again at step (a).
• Step (c) is a step of compressing the particles and shaping them in one or more dies to obtain one or more wooden rods which, cut after leaving the extruder, will give pellets.
• step (c) can be carried out by applying a radial compression force, relative to the axis of the screws of the extruder, on the wood, the wood being simultaneously pushed in a radial direction, relative to the axis of the screws of the extruder, in one or more dies to form granules.
• the die(s) are thus positioned laterally relative to the axis of the screws and to the sleeve, that is to say they are not positioned at the end forming the outlet of the extruder and along the axis of the extruder as is usually the case, but on the side. We are talking about lateral channels.
• each die is positioned laterally with respect to the axis of the screws of the extruder, forming a lateral exit, so as to allow lateral extrusion with respect to the axis of the screws of the extruder.
• radial die Even for the case where the axis of the die is not exactly radial with respect to the axis of the extruder.
• Each die can for example have an elongated shape extending laterally relative to the barrel of the extruder, and having a spinning orifice, preferably with a diameter of between 5 mm and 15 mm, for example of the order of 6 mm, 8mm, 10mm, or 12mm.
• FIG. 4 illustrates an example of the arrangement of several lateral dies 40 with respect to the sleeve 50 of a twin-screw extruder as proposed.
• the axial end 51 of the sheath has no outlet / orifice for the passage of wood.
• the assembly flange 51 is therefore full, with no outlet orifice allowing material to come out. If one or more axial holes exist, for example revealing the ends of the screw and their terminal nut tightening the profile of the screw, sealing against the wood is ensured in the axial direction between these ends of the screw and the radial compression zone where the radial extrusion dies.
• step (c) does not exclude material exits in an axial direction from the extruder, in particular gaseous fractions such as water vapor and/or liquid fractions such as water or water with a low content of submicronic wood particles giving a low viscous appearance. Wood can therefore possibly pass through the clearances between mechanical parts by being reduced to the submicron particulate state and to the pasty state because it is mixed with the liquid fraction.
• a sealing device can also prevent any liquid and/or gas passage in the axial direction as shown with the assembly flange 51.
• the method may comprise a step (d) of drying the granules at the exit of the die(s) used to take the wood out of the extruder, densified and shaped.
• the rods are cut into pellets before drying, even if the reverse is possible.
• step (b) allows the particles to be dried beforehand, which are then formed into granules.
• step (d) makes it possible to further reduce the moisture content of the granules, advantageously below 12%, preferentially between 7% and 10%, very preferentially up to about 8%.
• step (d) is a finishing drying step, which is optional insofar as the granules have already been dried beforehand.
• this step is necessarily shorter and above all less energy-intensive than the usual drying steps of the prior art, because the granules are already partially dried thanks to the mechanical work during step (b).
• wood pellets are proposed, the pellets being obtained according to the wood pellet extrusion process described above.
• these granules have densities of 1.1 to 1.2 when they are are wet (at the output of step (c), i.e. with a humidity rate of around 32%) and at least 0.7 or even around 0.8 once dried (at the output of step (d), i.e. with a moisture content of less than 12%) to be compared with the densities of 0.6 to 0.7 generally observed for the granules obtained by the known methods.
• the wood pellets obtained according to the proposed extrusion process have a moisture content of less than 12%, preferably a moisture content of less than 10%, and more preferably a moisture content of between 7 % and 10%, for example of the order of 8%.
• Such wood pellets also have a density of at least 0.7, preferably at least 0.8.
• the wood pellets obtained according to the proposed extrusion process have the particularity, and the advantage, of not disintegrating in water.
• the disintegration is defined by a return of the wood to the state of particulate powder close to its state before transformation, by a natural disintegration when a granule is placed in water at ambient temperature around 20° C. This this is observed in a few tens of seconds or minutes at most with the granules conventionally produced by the roller presses of the prior art described in the introduction.
• the granule produced by the proposed extrusion process does not disintegrate after 6 hours and even after several days of soaking.
• the wood pellets obtained according to the proposed extrusion process are advantageously wood pellets without additives. We can speak of wood pellets formed only from wood, that is to say consisting only of wood.

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• 2021
  • 2021-12-09 FR FR2113217A patent/FR3130183B1/fr active Active
• 2022
  • 2022-12-08 WO PCT/FR2022/052287 patent/WO2023105172A1/fr not_active Ceased
  • 2022-12-08 CN CN202280087871.4A patent/CN118871554A/zh active Pending
  • 2022-12-08 US US18/718,004 patent/US20250033239A1/en active Pending
  • 2022-12-08 CA CA3241921A patent/CA3241921A1/fr active Pending
  • 2022-12-08 EP EP22847536.4A patent/EP4444827A1/de active Pending

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