A woody biomass fuel and its preparation method

Abstract

This invention discloses a wood biomass fuel and its preparation method, including the following steps: (1) crushing waste wood to a particle size of 5-20 mm, removing metal impurities by magnetic separation, and then separating by air separation according to density difference to obtain light components and heavy components; (2) sending the light components into the first stage baking reactor, and baking at low temperature under the conditions of 180℃-220℃, nitrogen and water vapor mixture as carrier gas, and 0.05-0.10 MPa pressure in the reactor to selectively remove acidic volatiles generated by hemicellulose pyrolysis, and controlling the mass loss rate to 8%-12%. The advantages of this invention are: differentiated baking pretreatment gives the material excellent plasticity and self-adhesion, providing a basis for high-pressure compaction molding; and directional molding and differentiated structure control feed back into the pretreatment process through waste heat recovery and structural optimization, ultimately realizing the efficient conversion of waste wood from inefficient waste to high-quality biomass fuel.

Description

Technical Field

[0001] This invention relates to the field of biomass energy technology, specifically to a woody biomass fuel and its preparation method. Background Technology

[0002] Wood-based biomass fuels, especially biomass pellet fuels, have become an important form of biomass energy utilization due to their advantages such as high energy density, convenient transportation and storage, and high combustion efficiency. However, traditional biomass pellet fuels mostly use agricultural and forestry waste (such as straw, rice husks, sawdust, etc.) as raw materials, and there is relatively little research on the efficient energy utilization of waste wood (such as construction templates, waste furniture, decoration waste, etc.).

[0003] Waste wood, as an important component of urban solid waste, is widely sourced and generated in large quantities. However, it has a complex composition, contains many impurities (such as nails, paint, and adhesives), and exhibits a wide range of moisture content fluctuations (typically between 15% and 45%), with an uneven lignin-to-cellulose ratio. These factors present numerous technical challenges to its direct use in the preparation of biomass fuel. Existing methods for preparing biomass fuel from waste wood mainly suffer from the following shortcomings: First, existing pretreatment technologies mostly employ single-temperature treatment or simple drying, failing to fully consider the differences in the internal components of waste wood. Lighter components (such as veneers and sawdust) in waste wood have a higher cellulose content, while heavier components (such as solid wood blocks and thick boards) have a higher lignin content. These two types of wood exhibit significant differences in pyrolysis characteristics, glass transition temperature, and volatile matter release patterns. Using uniform process parameters leads to poor pretreatment results, either resulting in overtreatment and energy waste, or undertreatment affecting subsequent molding quality. Second, existing molding technologies often employ the addition of exogenous binders (such as molten lignin, starch, and plastic coating) or single high-pressure molding, which presents problems such as increased costs from exogenous additives, high energy consumption from high-temperature treatment, and poor combustion performance of the molded particles. In particular, existing molded pellets are mostly uniformly dense or uniformly porous, failing to achieve the synergistic optimization of "surface densification to improve water resistance and mechanical strength" and "internal porosity to promote combustion and oxygen supply." This results in pellets being prone to moisture absorption and breakage during storage and transportation, and incomplete combustion during combustion. Thirdly, in existing processes, pretreatment and molding are mostly independent operations, resulting in low energy utilization efficiency. Volatile gases and pyrolysis products generated during pretreatment cannot be effectively recycled, causing resource waste and environmental pollution. Therefore, we propose a woody biomass fuel and its preparation method. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0005] The present invention solves the above-mentioned technical problems by adopting the following technical solution: providing a woody biomass fuel and a method for preparing the same, comprising the following steps: (1) The waste wood is crushed to a particle size of 5-20 mm, and metal impurities are removed by magnetic separation. Then, it is separated by air separation according to density difference to obtain light and heavy components. (2) The light components are fed into the first stage baking reactor and baked at a low temperature of 180℃-220℃, with nitrogen and water vapor as the carrier gas and a pressure of 0.05-0.10MPa inside the reactor. The acidic volatiles produced by the pyrolysis of hemicellulose are selectively removed, and the mass loss rate is controlled at 8%-12%. (3) The heavy component is fed into the second-stage baking reactor and baked at a temperature of 240℃-280℃, with a carrier gas of nitrogen and water vapor and a pressure of 0.10-0.15MPa to promote the glass transition and partial melting of lignin, and control the mass loss rate to 15%-20%; (4) The two sections of baked materials are homogenized in a closed mixing chamber at a mass ratio (light component: heavy component = 3:7 to 5:5) to obtain the baked material; (5) The baked material is fed into a ring die forming machine with a segmented heating mold sleeve, and passes through the feeding section, compression section and pressure holding and shaping section in sequence along the die hole axis to carry out gradient heating and high pressure forming; (6) The formed particles are immediately passed through a surface microporous treatment device and rapidly cooled in a controlled humidity environment to form a uniformly distributed microporous structure on the particle surface. (7) The pellets are treated in a climate drying chamber to reduce the surface moisture content of the pellets to 3%-5% and the internal moisture content to 6%-8% to obtain the finished wood biomass fuel.

[0006] Preferably, in step (2), the carrier gas for the first stage of baking is a mixture of nitrogen and water vapor in a volume ratio of 3:1 to 5:1, the carrier gas flow rate is 0.5-1.0 L / min·kg material, and the baking time is 20-40 min.

[0007] Preferably, in step (3), the carrier gas for the second stage of baking is a mixture of nitrogen and water vapor in a volume ratio of 2:1 to 4:1, the carrier gas flow rate is 0.8-1.5 L / min·kg material, and the baking time is 30-50 min.

[0008] Preferably, in step (5), the temperature of the feeding section is 80℃-100℃, the temperature of the compression section is 120℃-150℃, the pressure is 100-150MPa, and the temperature of the pressure holding and shaping section is 60℃-80℃.

[0009] Preferably, the inner wall of the mold in the compression section of step (5) is provided with a spiral guide groove with a spiral angle of 15°-25°, so that the material forms directional fiber arrangement and internal microchannels along the axial direction under high pressure.

[0010] Preferably, the diameter of the internal microchannel is 0.3-1.0 mm.

[0011] Preferably, the controllable humidity in step (6) is 40%-60% relative humidity, the rapid cooling time is 50-70s, the pore size of the formed surface micropores is 50-200μm, and the porosity is 8%-15%.

[0012] Preferably, the heat generated in the compression section in step (5) is recycled through a heat recovery system for preheating the baking reactor in steps (2) and (3).

[0013] Preferably, the condensable thermal decomposition gas generated during the baking process in steps (2) and (3) is condensed and recovered, and then reused as an auxiliary binder in the molding process in step (5).

[0014] Preferably, the woody biomass fuel has a differentiated structure of "dense on the outside and porous on the inside", with a dense surface layer and a porous structure that retains directional microchannels on the inside.

[0015] Compared with the prior art, the present invention provides a woody biomass fuel and its preparation method, which has the following beneficial effects: This invention relates to a wood-based biomass fuel and its preparation method. Addressing the complex and diverse composition of waste wood, this invention innovatively proposes a graded low-temperature baking-selective devolatilization pretreatment technology based on component differences. By separating waste wood into light components (high cellulose) and heavy components (high lignin) through air classification, and employing differentiated baking temperatures, carrier gas ratios, and pressure parameters for each component, the selective removal of hemicellulose acidic volatiles and precise control of lignin glass transition are achieved. This pretreatment technology not only eliminates the need for exogenous chemical additives and fully utilizes the natural binding properties of lignin in waste wood, forming a natural hydrophobic film on the surface of the baked material, but also effectively removes acidic substances that affect molding and combustion through selective devolatilization, significantly improving the energy density and molding adaptability of the material. Simultaneously, the condensable thermally decomposed gas generated during baking is recovered and reused in the molding process, and the waste heat from the compression section is recycled for preheating the baking reactor, achieving closed-loop utilization of energy and materials between processes. Overall energy consumption is reduced by 15%-20% compared to traditional processes, demonstrating significant energy-saving and consumption-reducing effects.

[0016] This invention relates to a woody biomass fuel and its preparation method. It proposes a technology for directional densification molding of roasted materials and synergistic control of surface micropores and internal channels. Gradient heating and high-pressure molding are achieved through a segmented heating mold. Under the action of the spiral guide groove on the inner wall of the mold, the material is densified while forming directional fiber arrangement and internal microchannel structure. Subsequently, surface micropores are formed by rapid cooling with controllable humidity. This differentiated structure, characterized by a dense outer layer and a porous inner layer, achieves synergistic optimization of multiple performance aspects: the surface densification layer effectively blocks moisture intrusion, significantly improving the water resistance of the particles (water absorption rate <8% and mass loss rate <5% after 24 hours of immersion in water), with mechanical durability exceeding 97%; the surface microporous structure acts as an ignition point in the early stages of combustion, reducing the ignition temperature by approximately 30-50°C and promoting volatile matter release and gasification; the internal directional microchannels create a chimney effect, increasing the effective oxygen diffusion coefficient by 60%-80%, fundamentally solving the problems of external charring and internal incomplete combustion in traditional dense particles, achieving a combustion efficiency of 92%-94%, an improvement of 8-12 percentage points compared to ordinary wood chips. Differentiated baking pretreatment endows the material with excellent plasticity and self-adhesion, providing a foundation for high-pressure densification molding; while directional molding and differentiated structural control, through waste heat recovery and structural optimization, further support the pretreatment process, ultimately achieving the efficient conversion of waste wood from inefficient waste into high-quality biomass fuel. Attached Figure Description

[0017] Figure 1 This is a flow chart of the waste wood low-temperature baking-selective devolatilization pretreatment process of the present invention; Figure 2 This is a schematic diagram of the structure of the directional densification molding and surface micropore-internal channel synergistic control device of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1-2 A woody biomass fuel and its preparation method, comprising the following steps: (1) The waste wood is crushed to a particle size of 5-20 mm, and metal impurities are removed by magnetic separation. Then, it is separated by air separation according to density difference to obtain light and heavy components. (2) The light components are fed into the first stage baking reactor and baked at a low temperature of 180℃-220℃, with nitrogen and water vapor as the carrier gas and a pressure of 0.05-0.10MPa inside the reactor. The acidic volatiles produced by the pyrolysis of hemicellulose are selectively removed, and the mass loss rate is controlled at 8%-12%. (3) The heavy component is fed into the second-stage baking reactor and baked at a temperature of 240℃-280℃, with a carrier gas of nitrogen and water vapor and a pressure of 0.10-0.15MPa to promote the glass transition and partial melting of lignin, and control the mass loss rate to 15%-20%; (4) The two sections of baked materials are homogenized in a closed mixing chamber at a mass ratio (light component: heavy component = 3:7 to 5:5) to obtain the baked material; (5) The baked material is fed into a ring die forming machine with a segmented heating mold sleeve, and passes through the feeding section, compression section and pressure holding and shaping section in sequence along the die hole axis to carry out gradient heating and high pressure forming; (6) The formed particles are immediately passed through a surface microporous treatment device and rapidly cooled in a controlled humidity environment to form a uniformly distributed microporous structure on the particle surface. (7) The pellets are treated in a climate drying chamber to reduce the surface moisture content of the pellets to 3%-5% and the internal moisture content to 6%-8% to obtain the finished wood biomass fuel.

[0020] In this embodiment, the carrier gas for the first stage of baking in step (2) is a mixture of nitrogen and water vapor in a volume ratio of 3:1 to 5:1, the carrier gas flow rate is 0.5-1.0 L / min·kg material, and the baking time is 20-40 min.

[0021] In step (3), the carrier gas for the second stage of baking is a mixture of nitrogen and water vapor in a volume ratio of 2:1 to 4:1, the carrier gas flow rate is 0.8-1.5 L / min·kg material, and the baking time is 30-50 min.

[0022] The temperature of the feeding section in step (5) is 80℃-100℃, the temperature of the compression section is 120℃-150℃ and the pressure is 100-150MPa, and the temperature of the pressure holding and shaping section is 60℃-80℃.

[0023] The inner wall of the mold in the compression section described in step (5) is provided with a spiral guide groove with a spiral helix angle of 15°-25°, so that the material forms directional fiber arrangement and internal microchannels along the axial direction under high pressure.

[0024] The diameter of the internal microchannel is 0.3-1.0 mm.

[0025] The controllable humidity in step (6) is 40%-60% relative humidity, the rapid cooling time is 50-70s, the surface micropore diameter is 50-200μm, and the porosity is 8%-15%.

[0026] The heat generated in the compression section in step (5) is recycled back to the baking reactor in steps (2) and (3) for preheating via a heat recovery system.

[0027] The condensable thermal decomposition gas generated during the baking process in steps (2) and (3) is recovered by condensation and reused as an auxiliary binder in the molding process in step (5).

[0028] The woody biomass fuel has a differentiated structure of "dense on the outside and porous on the inside", with a dense surface layer and a porous structure that retains directional microchannels on the inside.

[0029] Specifically, Example 1 The recycled construction waste templates (moisture content 32%) are crushed to a particle size of 10-15mm using a crusher, and then metal impurities such as iron nails are removed by a magnetic separator. Finally, the material is separated by density using an air separator to obtain the lightweight component (density <0.45g / cm³). 3 It contains approximately 55% cellulose and heavy components (density ≥ 0.45 g / cm³). 3 (The lignin content is approximately 32%).

[0030] The light components were fed into the first-stage baking reactor, with the temperature set at 200℃. The carrier gas was a mixture of nitrogen and water vapor in a volume ratio of 4:1, with a flow rate of 0.8 L / min·kg material. The reactor pressure was 0.08 MPa, and the baking time was 30 min. This stage selectively removed acidic volatiles such as acetic acid and formic acid produced by hemicellulose pyrolysis, with a material mass loss rate of 10.2%.

[0031] The heavy component was fed into the second-stage baking reactor, with the temperature set at 260℃. The carrier gas was a mixture of nitrogen and water vapor in a volume ratio of 3:1, with a carrier gas flow rate of 1.2 L / min·kg material. The reactor pressure was 0.12 MPa, and the baking time was 40 min. During this stage, lignin underwent a glass transition and partial melting, with a material mass loss rate of 17.5%.

[0032] The two roasted materials were homogenized in a closed mixing chamber at a mass ratio of 4:6 (light component: heavy component) for 20 minutes to obtain the roasted material. Testing showed that the roasted material had a moisture content of 4.8%, a higher heating value (HHV) of 5432 kcal / kg, and a natural hydrophobic film formed on its surface by the recrystallization of molten lignin.

[0033] The baked material is fed into a ring die forming machine. The die hole is divided into three temperature-controlled zones along the axial direction: the feeding zone has a temperature of 90℃, and the material is initially compressed to a density of 0.95g / cm³. 3 The compression section operates at a temperature of 135℃ and a pressure of 120MPa. The inner wall of the mold is equipped with spiral guide grooves (spiral angle 20°), ensuring the material is densely compacted to a density of 1.28g / cm³. 3 Under high pressure, lignin permeates and fills the gaps between fibers to form a natural bonding network. At the same time, the material forms oriented fiber arrangement and internal microchannels (channel diameter about 0.6 mm) along the axial direction. The temperature of the pressure holding and shaping section is 70℃ and the pressure holding time is 8s, so that the lignin bonding phase is fully cured and shaped.

[0034] After the shaped granules are demolded, they immediately enter the surface microporous treatment device and are rapidly cooled for 60 seconds in an environment with a relative humidity of 50% and a temperature of 25°C. Due to thermal expansion and contraction and the release of residual stress, the surface of the granules forms a uniformly distributed microporous structure (pore size 80-150μm, porosity 11.3%).

[0035] Finally, the pellets were sent to a climate drying chamber and treated for 4 hours at a temperature of 60℃ and a relative humidity of 25% to obtain the finished wood biomass fuel. The finished pellets had a surface moisture content of 4.2%, an internal moisture content of 7.1%, a diameter of 8mm, and a length of 25-35mm.

[0036] Specifically, Example 2 The recycled waste furniture wood (moisture content 28%) is crushed to a particle size of 8-12mm by a crusher, and then separated into light and heavy components by magnetic separation and air separation after impurity removal.

[0037] First stage baking: temperature 190℃, nitrogen / water vapor volume ratio 5:1, carrier gas flow rate 0.6L / min·kg material, pressure 0.06MPa, time 35min, mass loss rate 9.5%.

[0038] Second stage baking: temperature 250℃, nitrogen / water vapor volume ratio 2:1, carrier gas flow rate 1.0L / min·kg material, pressure 0.10MPa, time 45min, mass loss rate 16.8%.

[0039] The two materials were mixed at a mass ratio of 3:7. After baking, the moisture content of the material was 4.2%, and the higher calorific value was 5380 kcal / kg.

[0040] Molding process: Feeding section temperature 85℃, compression section temperature 130℃, pressure 110MPa, holding and shaping section temperature 65℃. The spiral guide channel has a spiral rise angle of 18° and an internal channel diameter of approximately 0.5mm. Waste heat from the compression section is recovered through a heat recovery system and used for preheating the baking reactor, which can reduce baking energy consumption by approximately 15%.

[0041] Surface microporous treatment: relative humidity 55%, rapid cooling for 50s, surface micropore diameter 60-180μm, porosity 10.5%.

[0042] Climate drying chamber treatment: temperature 55℃, relative humidity 22%, treatment time 5 hours. Finished product surface moisture content 3.8%, internal moisture content 6.5%, density 1.25 g / cm³. 3 .

[0043] Specifically, Example 3 Waste wood from renovation projects (moisture content 38%) is crushed to a particle size of 12-18mm using a crusher, and then separated by magnetic separation and air separation.

[0044] First stage baking: temperature 210℃, nitrogen / water vapor volume ratio 3:1, carrier gas flow rate 1.0L / min·kg material, pressure 0.10MPa, time 25min, mass loss rate 11.5%.

[0045] Second stage baking: temperature 270℃, nitrogen / water vapor volume ratio 4:1, carrier gas flow rate 1.5L / min·kg material, pressure 0.15MPa, time 35min, mass loss rate 18.2%.

[0046] The two materials are mixed at a mass ratio of 5:5. The condensable thermal decomposition gas generated during the baking process is condensed and recovered, and then reused as an auxiliary binder in the molding process, which can reduce the molding pressure requirement by about 10%.

[0047] Molding process: Feeding section temperature 95℃, compression section temperature 140℃, pressure 140MPa, holding and shaping section temperature 75℃. The spiral guide groove has a spiral helix angle of 22° and an internal channel diameter of approximately 0.8mm.

[0048] Surface microporous treatment: relative humidity 45%, rapid cooling for 70s, surface micropore diameter 100-200μm, porosity 13.8%.

[0049] Climate drying chamber treatment: temperature 65℃, relative humidity 28%, treatment time 3.5 hours. Finished product surface moisture content 4.5%, internal moisture content 7.5%, density 1.32 g / cm³. 3 .

[0050] The performance indicators of the woody biomass fuel product prepared by this invention are shown in Table 1: Table 1 Comparison of performance indicators of woody biomass fuel products from Examples 1-3

[0051] Note: In the moisture content column, "4.2 / 7.1" means that the surface moisture content is 4.2% and the internal moisture content is 7.1%, and so on.

[0052] In this invention, the higher heating value (HHV) of the baked material can be estimated using the following empirical formula: HHV = 0.3491×C + 1.1783×H + 0.1005×S - 0.1034×O - 0.0151×N -0.0211×A In the formula, C, H, S, O, N, and A represent the mass percentages (%) of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash in the material, respectively. After baking pretreatment, selective devolatilization removes some oxygen-containing acidic volatiles, resulting in a relatively enriched carbon content and a reduced oxygen content in the material. According to this formula, the higher calorific value is increased by 12%-18% compared to untreated waste wood.

[0053] During the molding process, the degree of densification of the material in the compression section can be characterized by its relative density ρr: ρr = ρp / ρt In the formula: ρp is the density of the molded particles (g / cm³) 3 ρt is the true density of the material (g / cm³). 3 In this embodiment of the invention, ρr reaches 0.85-0.92, which is significantly higher than that of ordinary particles (0.70-0.78), indicating that the densification effect of molding is significant.

[0054] During combustion, the oxygen diffusion rate within the particle's internal channels can be described by the effective diffusion coefficient, Deff: Deff = D0 × (ε / τ) Where: D0 is the free diffusion coefficient of oxygen in air (m 2 ε is the porosity inside the particle, and τ is the tortuosity factor. Due to the directional microchannel structure inside the particles of this invention, the tortuosity factor τ is significantly reduced (τ≈1.2-1.5, while that of ordinary particles is τ≈2.5-3.5), thus increasing the effective diffusion coefficient by about 60%-80%, which is the key mechanism for the significant improvement in combustion efficiency.

[0055] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A wood-based biomass fuel and its preparation method, characterized in that, Includes the following steps: (1) The waste wood is crushed to a particle size of 5-20 mm, and metal impurities are removed by magnetic separation. Then, it is separated by air separation according to density difference to obtain light and heavy components. (2) The light components are fed into the first stage baking reactor and baked at a low temperature of 180℃-220℃, with nitrogen and water vapor as the carrier gas and a pressure of 0.05-0.10MPa inside the reactor. The acidic volatiles produced by the pyrolysis of hemicellulose are selectively removed, and the mass loss rate is controlled at 8%-12%. (3) The heavy component is fed into the second-stage baking reactor and baked at a temperature of 240℃-280℃, with a carrier gas of nitrogen and water vapor and a pressure of 0.10-0.15MPa to promote the glass transition and partial melting of lignin, and control the mass loss rate to 15%-20%; (4) The two sections of baked materials are homogenized in a closed mixing chamber at a mass ratio (light component: heavy component = 3:7 to 5:5) to obtain the baked material; (5) The baked material is fed into a ring die forming machine with a segmented heating mold sleeve, and passes through the feeding section, compression section and pressure holding and shaping section in sequence along the die hole axis to carry out gradient heating and high pressure forming; (6) The formed particles are immediately passed through a surface microporous treatment device and rapidly cooled in a controlled humidity environment to form a uniformly distributed microporous structure on the particle surface. (7) The pellets are treated in a climate drying chamber to reduce the surface moisture content of the pellets to 3%-5% and maintain the internal moisture content at 6%-8% to obtain the finished wood biomass fuel.

2. The wood-based biomass fuel and its preparation method according to claim 1, characterized in that: In step (2), the carrier gas for the first stage of baking is a mixture of nitrogen and water vapor in a volume ratio of 3:1 to 5:1, the carrier gas flow rate is 0.5-1.0 L / min·kg material, and the baking time is 20-40 min.

3. The wood-based biomass fuel and its preparation method according to claim 1, characterized in that: In step (3), the carrier gas for the second stage of baking is a mixture of nitrogen and water vapor in a volume ratio of 2:1 to 4:1, the carrier gas flow rate is 0.8-1.5 L / min·kg material, and the baking time is 30-50 min.

4. The woody biomass fuel and its preparation method according to claim 1, characterized in that: The temperature of the feeding section in step (5) is 80℃-100℃, the temperature of the compression section is 120℃-150℃ and the pressure is 100-150MPa, and the temperature of the pressure holding and shaping section is 60℃-80℃.

5. The woody biomass fuel and its preparation method according to claim 1, characterized in that: The inner wall of the mold in the compression section described in step (5) is provided with a spiral guide groove with a spiral helix angle of 15°-25°, so that the material forms directional fiber arrangement and internal microchannels along the axial direction under high pressure.

6. The woody biomass fuel and its preparation method according to claim 5, characterized in that: The diameter of the internal microchannel is 0.3-1.0 mm.

7. The wood-based biomass fuel and its preparation method according to claim 1, characterized in that: The controllable humidity in step (6) is 40%-60% relative humidity, the rapid cooling time is 50-70s, the surface micropore diameter is 50-200μm, and the porosity is 8%-15%.

8. The woody biomass fuel and its preparation method according to claim 1, characterized in that: The heat generated in the compression section in step (5) is recycled back to the baking reactor in steps (2) and (3) for preheating via a heat recovery system.

9. The woody biomass fuel and its preparation method according to claim 1, characterized in that: The condensable thermal decomposition gas generated during the baking process in steps (2) and (3) is recovered by condensation and reused as an auxiliary binder in the molding process in step (5).

10. A woody biomass fuel, prepared by the preparation method according to any one of claims 1-9, characterized in that: The woody biomass fuel has a differentiated structure of "dense on the outside and porous on the inside", with a dense surface layer and a porous structure that retains directional microchannels on the inside.

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