Wood pellet manufacturing process

Abstract

The invention relates to a process for manufacturing wood pellets from woody green waste, comprising the following steps: - collection of woody green waste (1), - coarse shredding of the woody green waste (2), - fine shredding of the first shredded material and a second shredded softwood material (3), by inserting the first and second shredded materials into at least one shredder, - drying of the first and second shredded materials (4), in at least one dryer, - mixing of the first and second shredded materials (5), in at least one mixer, - granulation of the resulting mixture (6), in at least one pellet press. The invention further relates to an installation for implementing a process according to the invention. Figure for the abstract: Fig 1

Description

Title of the invention: Process for manufacturing wood pellets technical field

[0001] The present invention relates to a method for manufacturing wood pellets. More particularly, it relates to a method for manufacturing wood pellets that allows for the valorization of woody green waste. Previous technique

[0002] Wood pellets are a fuel usually made from sawmill residues. The advantage of this fuel is that, in most regions of the world, it can be made from local raw materials. It is therefore a particularly attractive alternative to traditional heating methods based on hydrocarbon combustion or electricity. Compared to firewood, pellets allow for more easily regulated heating, particularly with a thermostat, making it possible to optimize energy consumption in relation to needs, and thus reduce costs and CO2 emissions.

[0003] However, since the primary raw material used for manufacturing wood pellets comes from sawmills, the wood used is softwood from forests. Yet, in many regions of the world, forests are being disrupted by climate change, and pellets can no longer be produced locally. Furthermore, it is now important to limit forestry operations, particularly tree felling, in order to preserve biodiversity and the role of trees as CO2 sinks. Description of the invention

[0004] The present invention aims to overcome these drawbacks by proposing a process for manufacturing wood pellets from woody green waste, characterized in that it comprises the following steps in order: - collection of woody green waste, - coarse grinding of woody green waste, - fine grinding of the first and second batches of softwood chips, by inserting the first and second batches into at least one defibrating shredder, - drying of the first and second defibrated shreds, in at least one dryer, - mixing of the first and second defibrated shreds, in at least one mixer, - granulation of the resulting mixture, in at least one pellet press.

[0005] Thanks to these provisions, the invention makes it possible to manufacture pellets using waste that is not usually or only minimally recovered, and in particular wood from hardwood trees. These pellets are efficient in terms of lower heating value (LHV) and ash content.

[0006] The wood pellet manufacturing process may include, prior to the fine grinding stage, a fine screening stage of the ground woody green waste, with selection of the coarse grinding products whose particle size exceeds a fine screening threshold, in order to obtain a first batch of ground material having a woody content exceeding a target lignin threshold. This makes it possible to eliminate a certain number of impurities, i.e., non-woody material, from the process.

[0007] Said manufacturing process may further comprise the following steps, prior to the coarse grinding step: - Preparatory shredding of woody green waste, - coarse screening of woody green waste, with selection of the products of the preparatory shredding whose particle size is greater than a coarse screening threshold, in order to obtain a preparatory shredd material.

[0008] This makes it possible to eliminate a number of non-woody impurities from the process and thus to obtain higher-performing granules.

[0009] Said manufacturing process may include, before the coarse grinding stage, a preparatory drying stage of the woody green waste by open-air storage, which allows the grass and leaves, in particular, to wither, and during the coarse grinding stage these elements form fine particles which can be eliminated during fine screening.

[0010] The preparatory drying step can last at least 72 hours, which allows for a sufficient effect under certain weather conditions. If conditions are unfavorable, the preparatory drying step can last up to one or more weeks.

[0011] The first ground material and the second ground material can be ground together in at least one defibrating mill, which ensures a more homogeneous grinding of the two ground materials and thus a more efficient mixing of these ground materials in the granules.

[0012] The first shredded material can be sent to the shredder via a first conveyor, and the second shredded material can be sent to the shredder via a second conveyor separate from the first conveyor, which allows control of the quantity of each of the shredded materials entering the shredder, and thus the content that each of these shredded materials will have in the granule.

[0013] The moisture content of the shredded material entering the shredder can be between 25 and 50%, which helps to reduce the risk of fire.

[0014] At the mixer level, the weight content of first ground material can be greater than or equal to 30%, which allows a large quantity of woody green waste to be recovered for a given quantity of granules produced.

[0015] The shredding products not selected during at least one screening stage can be sent to a composting unit, which allows for the best possible recovery of all green waste, woody and non-woody.

[0016] The present invention also relates to an installation for implementing a process for manufacturing wood pellets, comprising: - at least one coarse crusher, configured to perform the coarse crushing step, - at least one fine screener, configured to perform the fine screening step, - at least one means of transferring material from the coarse crusher to the fine screener, - at least one fine crusher, configured to perform the fine crushing step, - at least one means of transferring material from the fine screener to the fine crusher, - at least one dryer, configured to perform the drying step, - at least one means of transferring material from the fine grinder to the dryer, - at least one mixer, configured to perform the mixing step, - at least one means of transferring material from the dryer to the mixer, - at least one pellet press, configured to perform the pelletizing step, - at least one means of transferring material from the mixer to the pellet press.

[0017] Thanks to these provisions, the invention allows the manufacture of pellets using waste that is not usually or only minimally recovered. These pellets are efficient in terms of lower heating value (LHV) and ash content. Brief description of the drawings

[0018] The present invention and its advantages will become more apparent from the following description of several embodiments given by way of non-limiting examples, with reference to the accompanying drawings, in which:

[0019] [Fig.1] [Fig.1] is a schematic view of the process according to the invention. Description of the implementation methods

[0020] In the illustrated embodiment examples, identical elements or parts bear the same reference numbers.

[0021] With reference to [Fig.1], the process according to the invention makes it possible to manufacture wood pellets from woody green waste.

[0022] In the present invention, the term "woody green waste" refers to all materials derived from woody plants and resulting from the pruning, cutting, or maintenance of these plants, for example, during the maintenance of gardens, green spaces, forests, hedges, or trees. This can include both deciduous and coniferous plants.

[0023] The process according to the invention begins with a first step 1, of collecting woody green waste. Given the conditions under which this waste is produced and collected, it is usually accompanied by a number of impurities, such as than soil, pebbles, etc. They are also most often accompanied by non-woody green waste such as grass clippings, collected leaves, etc.

[0024] After collection, woody green waste may undergo pre-treatment comprising a preliminary shredding stage and a coarse screening stage. Pre-treatment is optional, as it may be omitted depending on the type of woody green waste collected.

[0025] Preparatory shredding allows the largest dimension of green waste to be reduced to a dimension less than 1 m, for example between 100 and 600 cm, preferably between 30 and 50 cm.

[0026] The preparatory grinding is preferably a slow grinding, that is to say carried out by means of a slow grinder, also called a pre-grinder, whose operating speed is for example between 30 and 50 revolutions / minute.

[0027] Coarse screening removes the finest particles from the process, which have a low woody content and are therefore of no use as fuel. For the remainder of the process, the particles with a size greater than a certain threshold, for example between 60 and 100 mm, preferably between 70 and 90 mm, are retained after coarse screening.

[0028] Pre-treatment makes it possible to get rid of some of the impurities collected with the green waste, including some of the twigs, grass, soil, or even pebbles.

[0029] The elements removed from the process during the coarse screening can be sent to a composting unit.

[0030] After the first step 1, and optionally after pre-treatment, the process may include a preparatory drying step. During this step, the collected woody green waste, which may have undergone preparatory shredding and coarse screening, is stored outdoors, preferably sheltered from the weather. Outdoor storage may last several days, for example, at least 72 hours. The duration may vary depending on the season and weather conditions, the objective being, for example, for the leaves to be sufficiently dry to crumble easily. The duration of outdoor storage can therefore, in practice, reach at least one week, and is preferably between two and six weeks. Preparatory drying is optional, as it may be omitted depending on the type of woody green waste collected.

[0031] The process then comprises a second step 2 of coarse shredding the green waste. Coarse shredding reduces the largest dimension of the green waste to a dimension below a certain threshold. This threshold is preferably less than or equal to 150 mm, and is, for example, between 70 and 130 mm, preferably between 80 and 120 mm.

[0032] Coarse grinding is preferably rapid grinding, i.e. carried out using a rapid grinder, the operating speed of which is for example between 800 and 1200 revolutions per minute.

[0033] After coarse grinding, the process according to the invention may include a fine screening step. Fine screening removes the finest particles from the process, which have a reduced woody content and are therefore unsuitable for fuel. For subsequent stages of the process, particles with a particle size exceeding a certain threshold, for example 10 mm or greater, or for example between 20 and 30 mm, can thus be retained after fine screening.

[0034] The elements removed from the process during fine screening can be sent to a composting unit.

[0035] During coarse shredding, if the green waste has undergone the preparatory drying stage and contains leaves, grass, or twigs, these elements, weakened by drying, will form extremely fine particles. These fine particles, containing little or no woody material, can be removed from the process during fine screening and possibly sent to a composting facility.

[0036] Following coarse grinding, and possibly fine screening, a first ground material is obtained preferably having a woody content exceeding a target lignin threshold. This threshold is, for example, between 90 and 99%, for example, it could be 95%.

[0037] After the coarse grinding stage, and optionally the fine screening stage, the process includes a third fine grinding stage, in at least one shredder. Two ground materials undergo this fine grinding: the first ground material, resulting from the coarse grinding, and optionally the fine screening, and a second ground material, which is softwood mulch. The second ground material may, in particular, be the mulch traditionally used to make wood pellets.

[0038] The use of this second mulch ensures a certain stability in the properties of the wood pellets obtained by the process according to the invention. Woody green waste is by nature very heterogeneous and can vary greatly in terms of the type and quantity of impurities it contains, the wood species it contains, the type and quantity of non-woody green waste present, etc. It is therefore very difficult to obtain a pellet with stable performance that is made entirely from green waste. The use of a second, more stable mulch, in addition to the first mulch, thus makes it possible to use woody green waste from various sources without the need for extensive sorting operations, while obtaining pellets with a high level of performance, in particular in terms of calorific value and ash content, equivalent to existing pellets.

[0039] The ground materials can, for example, be used in a ratio of 50% primary ground material and 50% secondary ground material. Depending on the materials used, the primary ground material content can be greater than 30%, preferably greater than 40% or even 50%, and can reach 60% or even 70%. These percentages can be observed in the mixer described below.

[0040] The first and second ground materials can undergo fine grinding together, either in a common shredder or in separate shredders. Preferably, they are fed simultaneously into at least one common shredder, ensuring uniform grinding of both materials. In this case, the first and second ground materials can be conveyed to the shredder on separate conveyors, allowing control of the proportion of each material in the pellet. For this control, the speed of each conveyor can, for example, be controlled by a single controller configured to adjust the conveyor speeds to maintain a target proportion by weight or volume of each material. To simplify this control, the mixer can include a feeding means, for example, an auger.This supply method can be configured with conveyors to be continuously fed at the maximum volumetric flow rate it can support. This allows for volume control, which is more relevant than weight because the moisture content of the ground material can vary.

[0041] Fine grinding makes it possible to obtain a particle size below a certain threshold, for example between 6 and 9 mm, preferably between 7 and 8 mm.

[0042] The shredder(s) used are, for example, of the type comprising rollers driven in rotation by a rotor, the rollers rotating about axes perpendicular to the axis of rotation of the rotor. The material is crushed between the outer surface of the rollers and a sieve arranged perpendicular to the axis of rotation of the rotor. The material is crushed until it reaches a size smaller than the particle size of the sieve. This type of shredder allows operation with relatively moist material, in particular with shreds having a moisture content between 25 and 50%, for example, between 25 and 40% for the first shred and between 35 and 45% for the second. This "wet" shredding allows for safer operation, as the risk of fire due to heating of the material during shredding is reduced.

[0043] After fine grinding, the process according to the invention includes a fourth step 4 of drying in a dryer.

[0044] The dryer is preferably of the low-temperature type, that is to say, it operates, for example, at a temperature between 120 and 150°C, preferably between 130 and 140°C. The dryer includes, for example, a tunnel traversed by one or more perforated moving belts, and the product to be dried travels through the tunnel on the moving belt. Hot air is blown into the tunnel from below. Two superimposed belts can be used, so that the material makes an initial pre-drying pass on the upper belt, then a second drying pass on the lower one. The moving belts can travel at a speed of between 2 and 4 m / s, for example 3 m / s, and the drying time lasts between 40 and 80 minutes, preferably between 50 and 70 minutes.

[0045] After drying, the process according to the invention includes a fifth step 5 of mixing in a mixer, in order to obtain a certain homogeneity among the granules with regard to their moisture content, their content of first and second ground material, but also their content of different essences composing the first ground material.

[0046] During the mixing stage, water can be added to the material to reach a target moisture content. A certain moisture content is necessary for the subsequent granulation stage. This threshold is, for example, between 12 and 15%.

[0047] The mixer can operate continuously, or in batch.

[0048] If it operates by batch, the mixer may include a tank through which a horizontal rotor passes, on which are fixed helical elements extending around the axis of the rotor.

[0049] Following the mixing step, the process includes a sixth step 6 of granulation in a pellet press. In such a press, the material is compressed by rollers to pass through a die, the holes of which correspond to the diameter of the pellets. The process takes place at high pressure and at a temperature, for example, between 70 and 100°C.

[0050] After the granulation step, the process may include a cooling step. This step is optional. For example, it may involve cooling by ventilation, which isolates the fine particles lifted by the ventilation and returns them to the mixer. Ventilation also reduces the surface moisture content of the granules.

[0051] The granules obtained by the process according to the invention have been tested and exhibit performance similar to existing granules: - lower heating value (LHV) greater than or equal to 4.6 kWh / kg, - ash content less than 2.5%.

[0052] The present invention also relates to an installation for implementing a manufacturing process according to the invention, comprising: - at least one coarse crusher, configured to perform the coarse crushing step, - at least one means for transferring material from the coarse crusher to the fine screener, - at least one fine crusher, configured to perform the fine crushing step, - at least one means for transferring material from the fine screener to the fine crusher, - at least one dryer, configured to perform the drying step, - at least one means of transferring material from the fine mill to the dryer, - at least one mixer, configured to perform the mixing step, - at least one means of transferring material from the dryer to the mixer, - at least one pellet press, configured to perform the pelletizing step, - at least one means of transferring material from the mixer to the pellet press.

[0053] The present invention is not limited to the embodiments described but extends to any modification and variant obvious to a person skilled in the art, within the limits of the appended claims. Furthermore, the technical features of the various embodiments and variants mentioned above may be combined, in whole or in part.

Claims

Demands

1. A process for manufacturing wood pellets from woody green waste, characterized in that it comprises the following steps in order: - collection of woody green waste (1), - coarse grinding of the woody green waste (2), - fine grinding of the first and second softwood grinding (3), by inserting the first and second grinding into at least one defibrating grinder, - drying of the first and second defibrated grinding (4), in at least one dryer, - mixing of the first and second defibrated grinding (5), in at least one mixer, - granulation of the resulting mixture (6), in at least one pellet press.

2. A method for manufacturing wood pellets according to claim 1, characterized in that it comprises, before the fine grinding stage, a fine screening stage of the ground woody green waste, with selection of the products of the coarse grinding whose particle size is greater than a fine screening threshold, in order to obtain a first ground material having a woody matter content greater than a target lignin threshold.

3. A method for manufacturing wood pellets according to any one of claims 1 or 2, characterized in that it further comprises the following steps, before the coarse grinding step (2): - preparatory grinding of woody green waste, - coarse screening of woody green waste, with selection of the products of the preparatory grinding whose particle size is greater than a coarse screening threshold, in order to obtain a preparatory mulch.

4. A method for manufacturing wood pellets according to any one of claims 2 to 3, characterized in that the grinding products not selected during at least one screening step (3) are sent to a composting unit.

5. A method for manufacturing wood pellets according to any one of claims 1 to 4, characterized in that it comprises, before the coarse grinding step (2), a preparatory drying step of woody green waste by open-air storage.

6. A method for manufacturing wood pellets according to claim 5, characterized in that the preparatory drying step lasts at least 72 hours.

7. A method for manufacturing wood pellets according to any one of claims 1 to 6, characterized in that the first ground material and the second ground material are ground together in at least one defibrating mill.

8. A method for manufacturing wood pellets according to claim 7, characterized in that the first shredded material is sent to the defibrating crusher via a first conveyor, and the second shredded material is sent to the defibrating crusher via a second conveyor separate from the first conveyor.

9. A method for manufacturing wood pellets according to any one of claims 1 to 8, characterized in that at the mixer, the weight content of first ground material is greater than or equal to 30%.

10. Installation for carrying out a process for manufacturing wood pellets according to any one of claims 1 to 9, comprising: - at least one coarse grinder, configured to carry out the coarse grinding step (2), - at least one means for transferring material from the coarse grinder to the fine screener, - at least one fine grinder, configured to carry out the fine grinding step (3), - at least one means for transferring material from the fine screener to the fine grinder, - at least one dryer, configured to carry out the drying step (4), - at least one means for transferring material from the fine grinder to the dryer, - at least one mixer, configured to carry out the mixing step (5), - at least one means for transferring material from the dryer to the mixer, - at least one pellet press, configured to carry out the pelletizing step (6), - at least one means for transferring material from the mixer to the pellet press.

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