Process for manufacturing wood pellets from fines

Abstract

Process for manufacturing wood pellets from fines. The manufacturing process includes passing wood fragments through a die to compress them into pellets. At least 25% by mass of the wood fragments are fines with a particle size of 2 mm or less. Prior to passing through the die, the process includes a step of moistening the wood fragments. See Figure 1 for the abbreviated version.

Description

Title of the invention: Process for manufacturing wood pellets from fines

[0001] The present invention relates to a method for manufacturing wood pellets. Such wood pellets are also called "pellets".

[0002] A method for manufacturing wood pellets is already known in the prior art, comprising passing wood fragments through a die to compress the wood fragments into pellets. In the prior art, the wood fragments from which the pellets are obtained are generally chips or sawdust, the particle size of which is typically between 3 and 15 mm, with a variability that can be significant over time. Furthermore, since these wood chips or sawdust are wet, typically with a moisture content of around 30% (which can go up to 55%), the methods of the prior art usually comprise drying the wood fragments prior to their use in the pelletizing die.

[0003] The handling of wood pellets (unloading, storage, bagging) usually generates particles, called fines, by abrasion. These fines can be recovered at the screen outlet as well as at the bag filter outlet.

[0004] When fines are generated at pellet producers, these fines are reincorporated into the raw materials, therefore mixed in minor quantities with sawdust and wood chippings (chips, cuttings, etc.) in order to manufacture pellets. The quantity of fines thus reincorporated into the main load of sawdust and wood chippings does not exceed 20% by mass, and is usually less than 5% by mass.

[0005] When generated at storage sites and / or pellet import sites, these fines cannot be reincorporated into any production and are disposed of as a by-product and therefore receive very little value. They are sold to producers of agro-pellets or densified logs who incorporate them (typically at a rate of less than 10%) into other raw materials. In all cases, the fines receive very little value.

[0006] In particular, these fines are in no way proposed for obtaining class Al pellets, i.e. pellets of maximum quality as defined by the DIN ISO 17225-2 standard.

[0007] The NF EN ISO 17225-2 standard defines in particular the following characteristics for class Al granules sold in bags:

[0008] - A generation of fines with a particle size of less than 3.15 mm, measured according to the DIN EN 15149-2 standard, less than 0.5% by mass;

[0009] - Humidity, measured according to DIN EN 14774-2, ISO 18134-1 and ISO 18134-2, less than 10% by mass;

[0010] - Durability, measured according to DIN EN 15210-1 and ISO17831-1, su above 98%;

[0011] - The presence of additive less than 2% by mass.

[0012] In particular, since all the wood fibers are broken into fines, they do not allow, in the state of the art, the manufacture of pellets having satisfactory mechanical strength meeting the specifications of class Al. Thus, pellets, prepared from wood fragments incorporating a minor quantity of fines, usually generate a level of fines higher than that specified by the DIN plus and EN plus standards in connection with the requirements of the ISO 17225-2 standard for class Al pellets, due to the low durability of such pellets.

[0013] The invention aims in particular to remedy this problem by proposing a process for manufacturing wood pellets which makes it possible to recover the fines while manufacturing good quality pellets.

[0014] To this end, the invention relates to a method for manufacturing wood pellets, comprising passing wood fragments through a die to compress the wood fragments into pellets, characterized in that:

[0015] - at least 25% by mass of the wood fragments are fines of granulometry in less than or equal to 2 mm, and

[0016] - the process comprises, prior to passing through the die, a step humidification of wood fragments.

[0017] Thus, unlike the processes of the state of the art where the wood fragments are dried before granulation, the process for manufacturing wood pellets according to the invention involves humidifying the wood fragments based on fines, in order to compress them into pellets of satisfactory strength.

[0018] Advantageously, the manufacturing method according to the invention makes it possible to obtain, from a load of wood fragments comprising a significant proportion of fines, granules (also referred to interchangeably as "pellets") of superior quality of the so-called Al class according to the DIN plus and EN plus standards in connection with the requirements of the ISO 17225-2 standard, as recalled previously.

[0019] By "fines", as mentioned above, we mean the particles generated by abrasion from the wood pellets (during unloading, storage, bagging operations), which can be recovered for example at the screen outlet as well as at the bag filter outlet. The fines have an average size, measured for example by a sieve oscillating in the horizontal plane, strictly less than 3.15 mm.

[0020] In a particularly advantageous embodiment, as detailed in the remainder of the text, the method of the invention involves the manufacture of wood pellets from Al quality according to DIN plus and EN plus standards in connection with the requirements of ISO 17225-2 from a load consisting mainly of fines. In particular, the load of wood fragments from which pellets according to the invention are manufactured may come from a stock of fines directly generated during the pellet manufacturing and / or handling operations, without the incorporation of other particles, and consisting almost exclusively of fines, with the exception of a few residual larger particles within the stock of fines.

[0021] The strength of the granules obtained according to the invention can be further optimized using optional characteristics which will be defined below. Thus, according to other advantageous aspects of the invention, the manufacturing method comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0022] - At least 50% by mass of the wood fragments are fines of granulometry, assessed according to standard NF EN ISO 17827-1 / 2, less than or equal to 2 mm.

[0023] - At least 75% by mass, in particular at least 80% by mass, and more particu Generally at least 90% by mass, wood fragments are fines with a grain size, evaluated according to standard NF EN ISO 17827-1 / 2, less than or equal to 3.15 mm.

[0024] - Said wood fragments consist of a stock of fines directly generated at the level of production and / or handling operations of wood pellets, and formed almost exclusively of fine particles with a particle size, evaluated according to standard NF EN ISO 17827-1 / 2, less than or equal to 3.15 mm.

[0025] - The die is sized to compress the wood fragments with a rate of compression greater than or equal to 6, preferably greater than 6.5.

[0026] - The die is cylindrical, and preferably has a compression ratio of 7, for example a diameter of 6 mm and a length of 42 mm.

[0027] - The humidification of the wood fragments is carried out to obtain a humidity level between 10 and 15%, preferably 12%.

[0028] - The humidification step is followed, possibly after a setting step resting of the moistened fragments, a homogenization step, prior to the compression step, preferably carried out for a period ranging from 5 minutes to 60 minutes, in particular from 15 minutes to 30 minutes, and, preferably, at a temperature between room temperature (Tamb) and room temperature + 30°C (Tamb+30°C).

[0029] By "ambient temperature" is meant the temperature of the environment surrounding the wood fragments, generally between -10°C and +40°C.

[0030] - The manufacturing process involves the addition of binder, in particular starch, mixed with the wood fragments prior to compression.

[0031] - The manufacturing process does not require the addition of a binder such as starch.

[0032] - The manufacturing process is used for the manufacture of wood pellets from Al quality defined according to DIN plus and EN plus standards in connection with the requirements of ISO 17225-2 standard.

[0033] Various aspects and advantages of the invention will be highlighted in the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0034] [Fig.l] [Fig.l] is a schematic flowchart representing steps of a manufacturing method according to an exemplary embodiment of the invention.

[0035] The method comprises a step 10 of providing wood fragments. The wood fragments from which the pellets are prepared according to the invention comprise fines with a particle size less than or equal to 2 mm. More particularly, at least 25% by mass, preferably at least 50% by mass, of the wood fragments are fines with a particle size less than or equal to 2 mm.

[0036] The wood fragments may also contain fines with a grain size greater than 2 mm and less than or equal to 3.15 mm.

[0037] In a particular embodiment, at least 75% by mass of the wood fragments from which the pellets are prepared according to the method of the invention are fines with a particle size of less than 3.15 mm (including the aforementioned fines with a particle size of less than 2 mm). It will be noted that this percentage can be even higher, in particular 80% by mass of fines, 90% by mass of fines, or even brought to 100% by mass of fines.

[0038] The remainder of the wood fragments are unwanted residues but present in minor proportions mixed with the fines. These may be larger particles, wood chips and / or sawdust, as long as the percentages of fines correspond to the values ​​mentioned above. Indeed, the aim of the invention is to recover the fines to make wood pellets, so that a process using fewer fines would be outside the scope of the invention.

[0039] Thus, as mentioned above, in a particularly advantageous embodiment, the method is implemented with a stock of fines directly generated at the level of the pellet manufacturing operations, without incorporation of other particles. The stock of fines is thus almost exclusively made up of fines with the exception of the presence in minor quantity of a few larger particles, by-products generated concomitantly with the fines.

[0040] In a particular embodiment, the method for manufacturing the granules according to the invention may comprise a step 20 of adding a binder, for example natural or modified starch, mixed with the wood fragments. For example, the mixture of starch and wood fragments comprises between 1 and 2% by mass of binder, for example starch.

[0041] The addition of binder is entirely optional. In fact, the method according to the invention makes it possible to produce granules of satisfactory quality without requiring a binder. Thus, in an alternative embodiment, the method for manufacturing granules according to the invention does not involve any step of adding binder.

[0042] The method for manufacturing the pellets according to the invention then comprises a step 30 of humidifying the wood fragments, preferably by adding water.

[0043] Advantageously, the humidification of the wood fragments is carried out to obtain a final humidity level of between 10 and 15% (i.e. the ratio between the final mass of water at the level of the moistened fragments and the total mass of the moistened fragments). Preferably, the final humidity level is 12%. This humidity level is optimal for obtaining the best quality of pellets. In particular, the durability of the pellets is lower with a lower humidity level, and a higher level can lead to pellet humidity exceeding the standards, and / or difficulties in handling the wood fragments during the manufacturing process.

[0044] Optionally, the method for manufacturing the pellets according to the invention comprises, after humidification of the wood fragments, and before homogenization, a step 40 of resting the moistened wood fragments. This step is carried out without the addition of heat, at a temperature not conducive to the evaporation of water, for example at a temperature between room temperature (Tamb) and room temperature plus 30°C (Tainb+30oC).

[0045] This resting step 40 can be carried out for a duration ranging from 0 to 15 minutes.

[0046] The humidification step 30, possibly after the resting step 40, is advantageously followed by a step 45 of homogenization of the moistened wood fragments, prior to a compression step 50.

[0047] This homogenization step 45 is such that it ensures the imbibition of the wood fragments, and good homogenization of the water within the wood fragments, in particular the fines, moistened. It can be carried out by any physical mixing technique.

[0048] Preferably, this homogenization step 45 is carried out for a duration ranging from 5 minutes to 60 minutes, in particular from 15 minutes to 30 minutes.

[0049] Preferably, this homogenization step is carried out at a temperature between room temperature (Tamb) and room temperature + 30°C (Tamb+30°C).

[0050] By "ambient temperature" is meant the temperature of the environment surrounding the wood fragments, generally between -10°C and +40°C.

[0051] The method for manufacturing pellets according to the invention then comprises a step 50 of pressing the wood fragments in a die, in order to recover the pellets at the outlet of the die.

[0052] In order to produce granules of optimal quality, the die is sized to apply a compression ratio (length of the compression channel over diameter of the granules) greater than or equal to 6, preferably greater than 6.5. In a preferred example, a compression ratio of 7 is used.

[0053] For this purpose, the die preferably has a compression channel of generally cylindrical shape, with a length to diameter ratio of 7, for example a diameter of 6 mm for a length of 42 mm.

[0054] It will be noted that the process for preparing the granules according to the invention is carried out under conditions such that the temperature of the die does not exceed 90°C. Typically, the temperature is between 75 and 90°C and ideally between 85°C and 87°C. Such temperatures advantageously make it possible to avoid any denaturation of the granule and to avoid the need to cool the die. In fact, a temperature above 100°C would have the effect of denaturing the granules and increasing the thermal stress on the die, which would then need to be cooled, either by stopping or reducing production, or by adding water, which could result in granules not conforming to the specification required for Al quality granules in terms of moisture.This maintenance of the temperature below 90°C is partly due to the presence of water uniformly distributed at the level of the charge based on fines implemented according to the invention, resulting from the humidification step.

[0055] It will be noted that the invention allows the granulation of fines resulting from the handling of granules. In addition to bagging sites, the fines granulation unit will be able to process waste from intermediate storage and bulk loading sites, the number of which is increasing to ensure the network of bulk deliveries currently being developed.

[0056] As indicated previously, the invention advantageously allows the production of granules of class Al defined by standard DIN ISO 17225-2.

[0057] The following example is given as an illustration of the invention, and should not be interpreted in such a way as to limit its scope. EXAMPLE

[0058] The fines load used in this test consists of a mixture of fines recovered at the screen outlet and fines recovered at the bag filter outlet of a pellet unloading / storage / bagging site in mass proportions of 90 / 10.

[0059] Table 1 below brings together the specificities of the fines, as well as the granules from which they are derived.

[0060] The particle size (proportions of fines of different diameters (Pfines)) is determined according to standard NF EN ISO 17827-1 / 2.

[0061] The humidity level is determined by the standards DIN EN 14774, ISO 18134-1 and NF EN ISO 18134-1,2.

[0062] The apparent density is determined according to standard NF EN ISO 17828.

[0063] The average diameter of the pellets is determined according to the NF EN ISO 17829 standard.

[0064] The durability of pellets is determined according to DIN EN 15210-1 and NF EN ISO17831-1.

[0065] [Table 1] Characteristics of fines Characteristics Unit Original pellets Screen fines Bag filter fines Bulk density kg / m3 660 459.07 474.87 Particle size distribution of fines Pfines <3.15 mm % 0.8 76.71 69.39 0 < Pfmes < 0.2 mm % 0.26 0.23 0.2 < Pfines < 0.5 mm % 9.87 38.67 0.5 < Pfines < 1 mm % 27.14 16.19 1 < Pfmes < 2 mm % 33.37 11.76 2 < Pfmes < 3.15 mm % 6.07 2.55 3.15 < Pf^es < 8 mm % 23.2 28.58 8 < Pfines % 0.09 2.03 Granules: proportion between 3.15 and 40 mm % 99.2 0 0 Average diameter mm 6.1 SO SO Humidity % 6.1 6.55 7.42 Durability % 99 SO SO

[0066] The fines are pre-humidified by adding water, with a humidification rate (ratio between the mass of water added and the total mass of the humidified fragments) of 12%, then left to stand at a temperature between room temperature and room temperature plus 30°C. for a period of time ranging from 0 to 15 minutes. This is followed by a homogenization step for a period of time ranging from 5 minutes to 60 minutes (ideally 15 to 30 minutes) at a temperature between room temperature and room temperature plus 30°C.

[0067] The charge of moistened fines is then conducted into a granulation die. The granulation conditions are as follows: - Die 6 / 42 mm (diameter / length), - Compression channel type: cylindrical, - Compression ratio: 7, - Additive rate (binder): 0, - Production rate: 45 kg / h; example of the feasibility study, the production rate could reach 3 t / h - Average temperature of the sector: 79°C.

[0068] The granules obtained are analyzed according to the criteria defined according to standard EN 17225-2.

[0069] The results of the analysis of the granules are collected in the following table 2.

[0070] [Tables2] Pellet characteristics Unit DIN plus and EN plus specifications Pellets Al ISO 17225-2 Pellet analysis results Diameter mm 6+1 6.02 Length mm 3.15 < <40 19.93 Pfines <3.15 mm % by mass <0.5 0.44 Humidity % by mass < 10 8.07 Durability % by mass >98 99.24 Density kg / m3 600< <750 663.5 Additive added % by mass <2 0

[0071] The results of analysis of the pellets, produced from fines according to a granulation process in accordance with the invention, show that the pellets thus obtained satisfy the specificities of pellets of Al quality defined according to the DIN plus and EN plus standards in connection with the requirements of the ISO 17225-2 standard.

Claims

Claims

1. A method of manufacturing wood pellets, comprising passing wood fragments through a die to compress the wood fragments into pellets, characterized in that: - at least 25% by mass of the wood fragments are fines with a particle size less than or equal to 2 mm, - the method comprises, prior to passing through the die, a step of humidifying the wood fragments.

2. Manufacturing method according to claim 1, in which at least 50% by mass of the wood fragments are fines with a particle size less than or equal to 2 mm.

3. Manufacturing method according to claim 1 or 2, in which at least 75% by mass, in particular at least 80% by mass, and more particularly at least 90% by mass, of the wood fragments are fines with a particle size of less than or equal to 3.15 mm.

4. Manufacturing method according to any one of the preceding claims, in which said wood fragments consist of a stock of fines directly generated during the production and / or handling operations of wood pellets, and formed almost exclusively of fines with a particle size of less than or equal to 3.15 mm.

5. Manufacturing method according to any one of the preceding claims, in which the die is sized to compress the wood fragments with a compression ratio greater than or equal to 6, preferably greater than 6.

5.

6. A manufacturing method according to any preceding claim, wherein the die is cylindrical, and preferably has a compression ratio of 7, for example a diameter of 6 mm and a length of 42 mm.

7. Manufacturing method according to any one of the preceding claims, in which the humidification of the wood fragments is carried out to obtain a humidity level of between 10 and 15%, preferably 12%.

8. Manufacturing method according to any one of the preceding claims, in which the humidification step is followed, optionally after a resting step, by a homogenization step, preferably carried out for a period ranging from 5 minutes to 60 minutes, in particular from 15 minutes to 30 minutes, and at a temperature between between room temperature and room temperature plus 30°C prior to the compression stage.

9. Manufacturing method according to any one of claims 1 to 8, comprising the addition of binder, in particular starch, mixed with the wood fragments prior to compression.

10. A manufacturing method according to any one of claims 1 to 8, wherein said method is devoid of the addition of a binder such as starch.

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