A system for deep grinding of briquetted biomass pellets

Abstract

A kind of profiled biomass granular material grinding depth crushing system and method, including mill drive system, primary separator, mill feed pipe, mill outlet pipe and the secondary crushing separation device between primary separator and mill outlet pipe, secondary crushing separation device includes rotating shaft, N level airfoil propeller blade installed on rotating shaft and drive adjustment motor, rotating shaft is coaxially arranged with feed pipe, and inlet coarse screen and outlet fine screen are respectively arranged at the inlet and outlet of secondary crushing separation device.Profiled biomass granular material is ground after millstone and mill roller, in turn, primary separator is coarsely divided, inlet coarse screen is pre-screened, airfoil propeller blade is high-speed rotating shear impact centrifugal tearing depth crushing, and outlet fine screen is finally screened, and the high-quality biomass powder with adjustable fineness is obtained.The axial pumping aerodynamic force is generated simultaneously when the airfoil propeller blade of the application rotates, additional resistance is offset, and active auxiliary feeding is assisted, and the system resistance does not increase even slightly decreases.

Description

Technical Field

[0001] This invention belongs to the technical field of biomass solid fuel processing equipment, specifically relating to an integrated system that uses a medium-speed mill as the main body to simultaneously grind and deeply pulverize compressed biomass pellets, achieving high fineness and wide-range adjustable deep pulverization of compressed biomass raw materials, and is particularly suitable for the fine grinding of compressed biomass pellets. Background Technology

[0002] In biomass energy utilization, loose biomass raw materials are often compressed into cylindrical or block-shaped pellets with a diameter of 6–12 mm and a length of 10–40 mm for convenient storage and transportation. When these compressed pellets are further fed into a conventional pulverized coal furnace for suspension co-firing with pulverized coal, the powder fineness is usually required to reach R90 < 15%, or even finer, to ensure combustion efficiency. However, biomass materials are rich in cellulose, hemicellulose, and lignin, and the fibers are highly resilient. Conventional grinding methods are difficult to achieve the ideal fineness, and the powder is often in the form of fibrous clumps or flakes with poor flowability. Currently, the fineness of compressed biomass pellets is achieved through two methods: one is using a hammer mill in conjunction with a vibrating screen; the other is directly using a medium-speed coal mill commonly used in thermal power plants. However, hammer milling has problems such as difficulty in stabilizing the fineness and easy clogging of the screen, often resulting in a phenomenon of "unbreakable particles that clump together." The medium-speed coal mills equipped in conventional pulverized coal boilers mainly rely on the squeezing and grinding between the grinding disc and the grinding roller to crush materials. They are very effective for brittle coal. Since the crushing of biomass raw materials is mainly by breaking them apart, a large number of continuous fiber bundles and coarse particles that are not fully separated will be generated after the biomass raw materials enter the medium-speed coal mill for crushing. After a large number of coarse fiber bundles enter the separator of the coal mill, the coarse particles that are not fully separated will still be carried into the finished product by the airflow, making it difficult to meet the high fineness requirements.

[0003] If a separate dynamic classifier or high-speed impact mill is connected in series at the mill outlet, with the medium-speed mill performing coarse crushing first and then the high-speed pulverizer performing fine processing, the fineness can be improved. However, the series connection of these devices significantly increases the system's airflow resistance, and the external pipeline transportation increases the risk of finished product sedimentation and blockage. Achieving deep crushing and fineness adjustment of compressed biomass particles over a wide range using a medium-speed coal mill with minimal increase in system resistance has become an urgent problem to be solved. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention discloses a compressed biomass pellet. The technical solution for a granular grinding and deep crushing system is as follows: A deep grinding system for pressed biomass pellets, characterized in that it comprises: Coal mill drive system; The grinding disc of the coal mill is driven to rotate by the coal mill drive system; A grinding roller is disposed above the grinding disc of the coal mill, and a grinding zone is formed between the grinding disc and the grinding roller; A primary separator is disposed above the grinding zone; The coal mill feed pipe penetrates the central area of ​​the primary separator; Coal mill outlet pipe; A secondary crushing and separation device is installed between the primary separator and the coal mill outlet pipe. The secondary crushing and separation device includes a rotating shaft, N-stage airfoil propeller blades fixedly installed on the rotating shaft, and a drive and regulating motor, where N≥3. The rotating shaft is coaxially arranged with the coal mill feed pipe. The inlet of the secondary crushing and separation device is equipped with an inlet coarse screen, and the outlet of the secondary crushing and separation device is equipped with an outlet fine screen. The aperture of the inlet coarse screen is larger than the aperture of the outlet fine screen. The axial distance H between two adjacent airfoil propeller blades on the rotating shaft satisfies:

[0005] Where D is the outer diameter of the airfoil propeller blade, ω is the rotational angular velocity of the shaft, d is the characteristic particle size of the powder entering the secondary crushing and separation device after separation by the primary separator, ν is the kinematic viscosity of the airflow in the secondary crushing and separation device, and k is an empirical coefficient and 0.5≤k≤1.5; The installation angle of the airfoil propeller blades increases progressively along the upward flow direction of the gas-solid two-phase flow, and the rate of increase of the installation angle between adjacent blade stages satisfies the following:

[0006] Where βi is the installation angle of the i-th stage airfoil propeller blade, βi+1 is the installation angle of the (i+1)-th stage airfoil propeller blade, Re is the Reynolds number calculated based on the tip linear velocity of the airfoil propeller blade, and α is a correction coefficient with 3°≤α≤8°.

[0007] This invention also discloses a method for deep grinding of pressed biomass pellets, comprising the following steps: S1: The shaped biomass pellets enter the grinding zone of the coal mill body through the feed pipe of the coal mill. The shaped biomass pellets are ground by the grinding disc and grinding rollers of the coal mill. The grinding pressure Pm and the initial density ρ0 of the shaped biomass pellets satisfy the following relationship:

[0008] Where P0 is the reference grinding pressure, ρref is the reference density, and η is the pressure adjustment coefficient, and 0.5 MPa·m³ / kg≤η≤5 MPa·m³ / kg; S2: Primary air is sent in by the air ring around the grinding disc of the coal mill, carrying the ground material to form a gas-solid mixture that enters the primary separator from bottom to top. In the primary separator, coarse particles with a particle size greater than the first threshold and coarse fiber clumps that are not sufficiently ground are intercepted and returned to the grinding zone for re-grinding. Fine particles with a particle size less than or equal to the first threshold pass through the primary separator with the airflow. S3: The gas-solid mixture passing through the primary separator is pre-screened through the inlet coarse screen to retain particles with a particle size larger than the second threshold, wherein the second threshold is smaller than the first threshold; S4: The pre-screened gas-solid mixture enters the secondary crushing and separation device, where a drive regulating motor drives the N-stage airfoil propeller blades to rotate at high speed. This process shears, impacts, and centrifugally tears the suspended powder, causing the biomass fiber bundles to disintegrate and refine. The shear stress τ generated by the rotating airfoil propeller blades satisfies:

[0009] Where ρ is the density of the gas-solid two-phase flow in the secondary crushing and separation device, r is the radial position of the airfoil propeller blade, Cf is the friction coefficient, and 10 Pa ≤ τ ≤ 500 Pa; the negative pressure ΔP formed at the inlet of the secondary crushing and separation device when the airfoil propeller blade rotates satisfies:

[0010] in Let be the tip radius of the airfoil propeller blade. It is the pressure coefficient and 0.1≤Cp≤0.6; S5: The refined powder is finally screened by the outlet fine screen. The powder that passes through the outlet fine screen is discharged through the coal mill outlet pipe, while the powder that does not pass through the outlet fine screen is intercepted and continues to be processed in the secondary crushing and separation device.

[0011] Beneficial effects (i) Achieve efficient and deep pulverization of biomass fibers, significantly improving powder fineness. This invention integrates a two-stage crushing and separation device inside the medium-speed mill body. The N-stage airfoil propeller blades, rotating at high speed, apply forced shearing, impact, and centrifugal tearing forces to the suspended powder in the gas-solid two-phase flow, achieving thorough dissociation and refinement of the highly resilient biomass fiber bundles. Simultaneously, combined with pre-screening by the inlet coarse screen, coarse powder interception and reflux by the primary separator, and final screening by the outlet fine screen, a three-stage separation-crushing-re-separation cyclic refinement path is formed. After being ground by grinding discs and rollers, the shaped biomass pellets are sequentially intercepted and returned by a primary separator, pre-screened by an inlet coarse screen, deeply crushed by airfoil propeller blades, and finally screened by an outlet fine screen. The screening structures at each stage work together to intercept unqualified powder that fails to pass through the outlet fine screen and continue to be processed in the secondary crushing and separation unit. This ensures that the fineness of the output powder is stably controlled within the target range, the biomass fiber bundles are fully dissociated, and the uniformity of the powder is significantly improved. This effectively solves the industry problem of generating a large number of continuous fiber bundles and insufficiently dissociated coarse particles after conventional medium-speed coal mills crush biomass raw materials.

[0012] (ii) Achieve wide-range independent adjustment of powder fineness to adapt to different combustion conditions. The drive adjustment motor of this invention is a variable frequency speed control motor. By changing the rotation speed of the shaft, the shearing and impact intensity of the airfoil propeller blades on the powder can be adjusted independently. The fineness of the outlet powder can be flexibly adjusted within a wide range to adapt to the different requirements of different boiler furnaces for the fineness of biomass powder, and overcome the defect of traditional hammer mills where the fineness is not easy to be adjusted stably.

[0013] (iii) The system resistance does not increase or even decreases slightly, ensuring the stability of the coal mill operation. The airfoil propeller blades of this invention adopt an airfoil profile propeller blade shape, which generates axial pumping aerodynamic force from inside the coal mill to the discharge port when rotating at high speed. This aerodynamic force forms a significant negative pressure at the inlet of the secondary crushing and separation device, thereby offsetting the additional flow resistance brought about by the addition of the secondary crushing and separation device. This ensures that the total pressure drop of the system is not significantly increased or even slightly reduced compared to the reference system without the secondary crushing and separation device. This avoids the problem of a significant increase in system resistance caused by connecting an independent dynamic classifier or a high-speed impact mill in series at the mill outlet, and ensures the stable operation of the coal mill under primary air conveying conditions.

[0014] (iv) Actively assists feeding to effectively prevent blockage of the feed pipe. The axial pumping aerodynamic force generated by the high-speed rotation of the airfoil propeller blades creates a negative pressure at the inlet of the secondary crushing and separation device, which further enhances the air intake tendency of the upstream feeding system, forming an active suction effect on the shaped biomass pellets. This helps to actively suck in the shaped pellets, effectively prevents the accumulation and blockage of materials in the coal mill feed pipe, and improves the continuity and reliability of system operation.

[0015] (v) The system has a high degree of integration, avoiding the risk of external pipeline deposits and blockages. This invention integrates deep crushing functionality within a coal mill. Pressed biomass pellets undergo a complete process of grinding, primary separation, coarse screening, deep crushing, and final screening within the same mill body, eliminating the need for a separate external dynamic classifier or high-speed impact mill connected in series at the mill outlet. The secondary crushing and separation unit is directly located above the primary separator and before the mill outlet pipe, eliminating the need for external pipeline transportation and fundamentally avoiding the risk of finished powder deposition and blockage in external pipelines. The system has a compact structure and small footprint, making it particularly suitable for in-situ upgrades of existing medium-speed coal mills.

[0016] (vi) Significantly improve the combustion rate of biomass and pulverized coal suspension co-firing The high-fineness biomass powder with fully dissociated fibers obtained by this invention can meet strict requirements for fineness when it enters a conventional pulverized coal furnace for suspension co-firing with pulverized coal. The powder has good uniformity, significantly improved flowability, more uniform mixing with pulverized coal, and more complete combustion. This significantly improves the burnout rate of biomass fuel, effectively reduces the carbon content of fly ash and the loss of unburned carbon in flue gas, and has significant energy-saving and environmental protection benefits.

[0017] (vii) The equipment experiences minimal wear and tear and has a long service life. The inlet coarse screen of this invention pre-screens the gas-solid mixture between the primary separator and the airfoil propeller blades, effectively intercepting larger coarse particles and preventing excessive wear caused by direct impact of these particles on the airfoil propeller blades. Simultaneously, wear-resistant liners are embedded in the inner wall of the secondary crushing and separating device corresponding to the blade area, maintaining a radial gap between the blade tip and the wear-resistant liner. The blades themselves are made of wear-resistant alloy steel or have undergone surface hardening treatment. These multiple protective measures effectively mitigate the wear caused by small amounts of sediment that may be present in the biomass, significantly extending the equipment's service life.

[0018] (viii) Ensuring the certainty and controllability of the crushing process is conducive to large-scale industrial production. This invention achieves a high degree of certainty and controllability in the crushing process through the coordinated optimization of many structural parameters, such as the rotational speed, number of stages, incremental increase of installation angle, ratio of inlet and outlet screen apertures, and gap between blades and wear-resistant liners. This avoids the blindness of relying on experience-based operations and is conducive to the standardization and large-scale promotion of industrial production. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the present invention.

[0020] Among them, 1 is the coal mill drive system, 2 is the coal mill grinding disc, 3 is the grinding roller, 4 is the primary separator, 5 is the inlet coarse screen of the secondary crushing and separation device, 6 is the airfoil propeller blade of the secondary crushing and separation device, 7 is the outlet fine screen of the secondary crushing and separation device, 8 is the coal mill outlet pipe, 9 is the drive and adjustment motor of the secondary crushing and separation device, and 10 is the coal mill feed pipe. Detailed Implementation

[0021] Example 1 like Figure 1 As shown, the present invention provides a deep grinding system for pressed biomass pellets, including a coal mill drive system (1), a coal mill grinding disc (2), a grinding roller (3), a primary separator (4), a coarse screen at the inlet of a secondary crushing and separation device (5), an airfoil propeller blade of the secondary crushing and separation device (6), a fine screen at the outlet of the secondary crushing and separation device (7), a coal mill outlet pipe (8), a driving and regulating motor for the secondary crushing and separation device (9), and a coal mill feed pipe (10).

[0022] The coal mill body is a vertically arranged medium-speed coal mill structure. The coal mill drive system (1) is located at the bottom of the coal mill body, and its output shaft extends vertically upward and is fixedly connected to the center of the coal mill grinding disc (2). The coal mill grinding disc (2) is disc-shaped, and its outer periphery is provided with an air ring for primary air intake. Multiple tangential air inlets are evenly distributed along the circumference on the air ring. There are three grinding rollers (3). The three grinding rollers (3) are evenly pressed against the upper surface of the coal mill grinding disc (2) at an equal angle of 120° along the circumference of the coal mill grinding disc (2). An annular grinding zone is formed between the grinding rollers (3) and the upper surface of the coal mill grinding disc (2). Driven by the coal mill drive system (1), the coal mill disc (2) rotates around the vertical axis. The shaped biomass pellets fall into the central area of ​​the coal mill disc (2) through the coal mill feed pipe (10). Under the action of centrifugal force, they spread outward radially and enter the grinding zone between the grinding roller (3) and the coal mill disc (2), where they are crushed by the grinding action of the grinding roller (3). The coal mill feed pipe (10) is a vertically arranged cylindrical pipe. Its central axis coincides with the rotation axis of the coal mill disc (2) and the vertical central axis of the coal mill body. The lower end outlet of the coal mill feed pipe (10) is directly opposite the central area of ​​the upper surface of the coal mill disc (2).

[0023] Primary hot air is tangentially introduced into the coal mill body through an air ring located at the outer periphery of the coal mill disc (2). This primary hot air has two functions: firstly, it acts as a drying medium to heat and evaporate the moisture in the biomass material released during the grinding process, thus achieving a drying effect; secondly, it acts as a conveying medium to provide upward airflow, blowing the crushed material particles upward from the grinding zone to form a gas-solid two-phase flow carrying solid powder. The opening angle and number of tangential air inlets on the air ring are determined according to the required primary air velocity and volume. The tangential air inlets simultaneously supply air to both the radial and tangential directions of the coal mill disc (2), providing upward conveying power and assisting in the uniform spreading of the material on the coal mill disc (2).

[0024] The primary separator (4) is located above the grinding zone, in the upper middle space of the coal mill body. The primary separator (4) includes several adjustable baffles fixedly installed on the inner wall of the coal mill body. The baffles are evenly arranged along the circumference of the inner wall of the coal mill body. An annular airflow channel is maintained between the inner edge of the baffle and the outer wall of the coal mill feed pipe (10), allowing the gas-solid two-phase flow carrying the material to pass through from bottom to top. The inclination angle of each baffle can be adjusted independently or in conjunction with the required initial separation particle size threshold. When the gas-solid two-phase flow carrying the ground material enters the primary separator (4) area from bottom to top, the larger particles and the insufficiently ground coarse fiber clumps, due to their large mass and strong inertia, cannot quickly change direction with the airflow as they pass around the baffles. As a result, they collide with the baffle surface due to inertia. After the collision, these large particles and coarse fiber clumps lose their upward momentum and slide down the baffle surface under their own gravity. They return to the grinding area through the gap between the outer wall of the coal mill feed pipe (10) and the baffle, and are mixed with the new shaped biomass pellets before being ground again. The smaller fine particles, due to their small mass and good following ability, can pass around the baffles with the airflow and continue to move upward through the gap between the baffles. After passing through the primary separator (4), they enter the space above the primary separator (4). By adjusting the inclination angle of the baffles, the flow direction and velocity distribution of the airflow when passing through the baffle area can be changed, thereby adjusting the separation particle size threshold of the primary separator (4), that is, adjusting the maximum particle size of the powder that can pass through the primary separator (4).

[0025] The gas-solid two-phase flow after passing through the primary separator (4) continues to move upward and enters the inlet area of ​​the secondary crushing and separation device. The secondary crushing and separation device is located above the primary separator (4) and below the coal mill outlet pipe (8), that is, the secondary crushing and separation device is located between the primary separator (4) and the coal mill outlet pipe (8). The secondary crushing and separation device includes, from bottom to top, an inlet coarse screen (5), an airfoil propeller blade (6) area, and an outlet fine screen (7) along the airflow direction. The inlet coarse screen (5) is horizontally arranged below the airfoil propeller blade (6) area, and its screen holes are circular or rectangular. The screen hole diameter of the inlet coarse screen (5) is 3 to 10 times that of the screen hole diameter of the outlet fine screen (7). The inlet coarse screen (5) is arranged at an angle of 30°-60° relative to the horizontal direction. This inclined arrangement allows the coarse particles intercepted by the inlet coarse screen (5) to slide down the screen surface under gravity to the lower end of the inlet coarse screen (5). The lower end of the inlet coarse screen (5) is connected to a coarse particle return channel. This coarse particle return channel extends downward along the inner wall of the coal mill body or the outer wall of the coal mill feed pipe (10) and connects with the grinding zone, thereby returning the coarse particles intercepted by the inlet coarse screen (5) to the grinding zone for further grinding. The screen aperture of the inlet coarse screen (5) is larger than that of the outlet fine screen (7), and there is a clear multiple relationship between the two.

[0026] The gas-solid two-phase flow, after being screened by the inlet coarse screen (5), enters the area of ​​the airfoil propeller blades (6). The number of airfoil propeller blades (6) is four-stage, with each stage of blades arranged sequentially from bottom to top along the airflow direction on the rotating shaft. The rotating shaft is coaxially arranged with the coal mill feed pipe (10) and fixed to the outer wall of the coal mill feed pipe (10) by a sealed bearing. The rotating shaft is a hollow shaft, which is sleeved on the outside of the coal mill feed pipe (10). Rolling bearings or sliding bearings are provided between the rotating shaft and the coal mill feed pipe (10) to achieve relative rotation. A high-temperature resistant sealing structure is provided at the bearing to prevent powder from entering the bearing and causing wear or jamming. The upper end of the rotating shaft passes through the top of the coal mill body and is connected to the output shaft of the secondary crushing and separation device drive regulating motor (9) through a coupling. The secondary crushing and separation device drive regulating motor (9) is fixedly installed on the outer side of the top of the coal mill body, and its shell is sealed to the top of the coal mill body. Driven by the secondary crushing and separation device drive regulating motor (9), the rotating shaft rotates at high speed around its vertical axis, and the airfoil propeller blades (6) fixed on the rotating shaft rotate synchronously with the rotating shaft.

[0027] The airfoil propeller blades (6) are made of wear-resistant alloy steel or surface-hardened steel. Each stage of the airfoil propeller blades (6) consists of 3 to 6 blades evenly arranged along the circumference. The airfoil profile of each blade is that of a propeller blade, with a pressure surface, a suction surface, a leading edge, and a trailing edge. The leading edge radius is 5% to 15% of the maximum blade thickness, and the trailing edge thickness is 1% to 5% of the maximum blade thickness. The surface roughness of the blades is Ra≤1.6μm. The airfoil chord length of each stage of the airfoil propeller blades (6) decreases non-linearly from the hub to the tip along the radial direction of the blade, that is, the chord length is the largest at the hub and the smallest at the tip, and the rate of decrease of the chord length from the hub to the tip gradually increases. The installation angle of each stage of the airfoil propeller blades (6) increases progressively along the airflow direction, that is, the installation angle of the first stage blade is the smallest, the installation angle of the fourth stage blade is the largest, and the rate of increase of the installation angle between adjacent stages of blades satisfies a quantitative relationship. ,in Let be the installation angle of the i-th stage airfoil propeller blade (6). Let be the installation angle of the i+1th stage airfoil propeller blade (6), Re be the Reynolds number calculated based on the tip linear velocity of the airfoil propeller blade (6), and α be a correction factor with 3°≤α≤8°. The structural arrangement in which the installation angle increases step by step along the airflow direction causes each stage of the blade to generate axial pumping aerodynamic force that gradually increases from inside the coal mill toward the coal mill outlet pipe (8).

[0028] The axial distance H between two adjacent airfoil propeller blades (6) on the rotating shaft satisfies a quantitative relationship. Where D is the outer diameter of the airfoil propeller blade (6), ω is the rotational angular velocity of the shaft, and d is the characteristic particle size of the powder entering the secondary crushing and separation device after separation by the primary separator (4). The kinematic viscosity of the airflow within the secondary crushing and separation unit. It is an empirical coefficient and 0.5 ≤ ≤1.5. This quantitative relationship is determined based on fluid dynamics boundary layer theory and similarity criteria: when the ratio of the axial distance between two adjacent stages of blades to the blade outer diameter increases with the dimensionless number ω·d / When the size increases and decreases, it can ensure that the shearing and crushing effect applied by the upper-level blades to the powder will not be completely attenuated due to the re-agglomeration of the powder before the powder enters the area of ​​the next-level blade, and at the same time, it will not reduce the effectiveness of the independent action of each level of blades due to the small spacing.

[0029] When the gas-solid two-phase flow carrying powder enters the region of the airfoil propeller blade (6), the high-speed rotating blades exert multiple mechanical effects on the powder suspended in the airflow. First, there is a pressure difference between the pressure surface and the suction surface of the blade due to the airfoil flow. This pressure difference creates a strong velocity gradient and shear flow field near the blade surface. The powder particles suspended in this flow field are subjected to shear stress, which satisfies the following condition: , where ρ is the density of the gas-solid two-phase flow in the secondary crushing and separation device, and r is the radial position of the airfoil propeller blade (6). The coefficient of friction, Based on the thickness of the boundary layer on the blade surface, the range of shear stress τ is 10~500 Pa. Shear stress acts on the biomass fiber bundles in the powder, causing shear slip and separation at the lignin and hemicellulose interfaces connecting the individual fibers in the fiber bundle. Secondly, the leading edge of the blade collides with the suspended powder at a high linear velocity, applying an impact force to the powder, causing local stress and strain to be generated in the powder particles at the moment of collision. When the local stress exceeds the bonding strength of the internal interface of the fiber bundle, the fiber bundle breaks and disintegrates. Thirdly, when the powder flows along the blade surface with the airflow, it generates rotational motion under the guidance of the blade curvature. The coarse particles and fiber bundles in the powder are subjected to centrifugal force, which causes the components in the fiber bundle to separate radially, further promoting the disintegration and dispersion of the fiber bundle. The above three mechanical actions of shearing, impact and centrifugal tearing occur synergistically in the region of the airfoil propeller blade (6). Under the combined effect of the above multiple mechanical actions, the biomass fiber bundle gradually disintegrates into individual fibers or fine fiber clusters, and is further refined to the target particle size.

[0030] When the airfoil propeller blades (6) rotate at high speed, they simultaneously generate axial pumping aerodynamic force from inside the coal mill to the coal mill outlet pipe (8). The generation mechanism of this axial pumping aerodynamic force is as follows: The airfoil profiles of each stage of the airfoil propeller blades (6) have specific angles of attack and camber. When the blades rotate, they move relative to the surrounding gas. When the gas flows over the airfoil surface, a pressure difference is formed between the pressure surface and the suction surface. The integral of this pressure difference along the chord of the blade generates a component perpendicular to the direction of rotation, namely, axial thrust. This axial thrust acts on the gas and pushes the gas axially from the downstream direction of the blade to the upstream direction, thereby realizing the axial pumping function. Since the installation angle of each stage of the blades increases step by step along the airflow direction, the axial thrust generated by each stage of the blades increases step by step along the airflow direction, forming an increasing pumping capacity from the feed side to the discharge side. This axial pumping aerodynamic force forms a significant negative pressure zone in the inlet area of ​​the secondary crushing and separation device. satisfy ,in The tip radius of the airfoil propeller blade (6) is... The pressure coefficient is 0.1 ≤ ≤0.6, the absolute value of the negative pressure ΔP is 0.5~5 kPa. This negative pressure is transmitted upstream to the area of ​​the primary separator (4) and even the grinding area, and has a suction effect on the gas-solid two-phase flow in the grinding area and the feed pipe. This suction effect has a dual function: firstly, the direction of the aerodynamic force generated by the suction effect is consistent with the direction of the primary air conveying, which provides additional power for the movement of the gas-solid two-phase flow from the grinding area to the secondary crushing and separation device. This additional power compensates for the additional flow resistance loss introduced by adding the inlet coarse screen (5), the airfoil propeller blade (6) area and the outlet fine screen (7), so that the ratio of the total pressure drop ΔPtotal of the system to the total pressure drop ΔP0 of the reference system without the secondary crushing and separation device satisfies ΔPtotal / ΔP0=0.85~1.05, that is, the total pressure drop of the system is not significantly increased or even slightly decreased compared to the reference system. Secondly, the suction effect creates a downward suction effect at the inlet of the coal mill feed pipe (10), which actively tractions the shaped biomass pellets entering the coal mill feed pipe (10), helping the pellets overcome the frictional resistance and back pressure of the airflow in the feed pipe and fall smoothly to the coal mill grinding disc (2), thereby effectively preventing the coal mill feed pipe (10) from being blocked.

[0031] The inner wall of the secondary crushing and separation device is fitted with replaceable wear-resistant liners in the area corresponding to the airfoil propeller blades (6). The wear-resistant liners are made of high-chromium cast iron or ceramic-reinforced composite materials. There is a radial clearance δ between the wear-resistant liner and the tip of the airfoil propeller blade (6), where δ / D = 0.5% to 2.0%. This radial clearance provides a safe clearance for the rotation of the blade, preventing the blade from scraping and colliding with the casing during operation. At the same time, the size of this clearance determines the flow rate of the leakage flow between the blade tip and the wall surface, thereby affecting the pumping efficiency and shearing effect of the blade. The surface of the wear-resistant liner is provided with guide grooves extending along the airflow direction. The depth of the guide grooves is 0.5 to 3 mm, the width of the guide grooves is 2 to 10 mm, and the spacing between adjacent guide grooves is 5 to 20 mm. The function of the guide grooves is to form a micro-vortex boundary layer near the wall surface. On the one hand, this inhibits the deposition and adhesion of powder on the wall surface, and on the other hand, it guides the powder to flow orderly along the direction of the guide grooves, avoiding the generation of local vortices and back mixing of powder in the wall area.

[0032] The powder refined by the airfoil propeller blades (6) continues to move upward with the airflow and reaches the location of the outlet fine screen (7). The outlet fine screen (7) is located above the airfoil propeller blades (6) and below the coal mill outlet pipe (8). The outlet fine screen (7) is an arc-shaped screen, and the radius of curvature of its arc surface matches the radius of curvature of the rotation trajectory of the airfoil propeller blades (6). That is, the arc-shaped screen is arranged coaxially with the rotating shaft and surrounds the discharge side of the airfoil propeller blades (6) in the circumferential direction. The effective screening area of ​​the arc-shaped screen accounts for 60% to 90% of the total cross-sectional area at the outlet. The area without a screen is an airflow bypass channel, used to balance the airflow distribution and prevent excessive local air velocity from causing screen blockage or excessive wear. The aperture of the outlet fine screen (7) determines the maximum particle size of the final output powder. Qualified fine powder with a particle size smaller than the sieve aperture passes through the outlet fine screen (7) with the airflow, is discharged from the coal mill body through the coal mill outlet pipe (8), and is sent into the subsequent coal powder pipeline. Unqualified particles with a particle size larger than the sieve aperture are intercepted by the outlet fine screen (7). The intercepted particles return to the airfoil propeller blade (6) area under the action of gravity or the action of airflow back mixing, and are subjected to the airfoil propeller blade (6) for further shearing, impact and centrifugal tearing treatment until they are refined to the point that they can pass through the outlet fine screen (7).

[0033] The drive motor (9) of the secondary crushing and separation device is a variable frequency speed control motor, and its output speed can be continuously adjusted within a certain range. By changing the power supply frequency of the drive motor (9) of the secondary crushing and separation device, the rotational speed n of the shaft can be changed, thereby changing the rotational angular velocity ω and tip linear velocity vt of the airfoil propeller blade (6). The adjustment range of the shaft rotational speed n is 200~1500 rpm, and the corresponding tip linear velocity vt ranges from 30~120 m / s. The change of the shaft rotational speed n has multiple effects on the crushing and separation processes. When the rotational speed n increases, the shear stress τ applied by the airfoil propeller blade (6) to the powder increases with the square of the rotational speed, and the impact speed also increases. Therefore, the shear, impact and centrifugal tearing strength of the biomass fiber bundles are enhanced, the powder can be more fully refined, and the fineness R90 value of the output powder decreases. At the same time, the rotational speed As the speed increases, the axial pumping force generated by the blades also increases with the square of the rotational speed, the absolute value of the negative pressure ΔP increases, the system resistance further decreases, and the suction effect of the feed pipe is enhanced. Conversely, when the rotational speed decreases... When the speed is reduced, the crushing intensity weakens, the output powder fineness R90 value increases, and the axial pumping pneumatic power decreases. The quantitative relationship between the shaft rotation speed n and the target fineness R90 of the output powder at the outlet fine screen (7) satisfies ,in As the reference speed, Let λ be the baseline fineness, and 50 rpm ≤ λ ≤ 200 rpm. This quantitative relationship is determined based on regression analysis of a large amount of experimental data, where the logarithmic function reflects the nonlinear response characteristics of fineness changes to speed adjustment.

[0034] The aperture of the inlet coarse screen (5) is 3 to 10 times that of the outlet fine screen (7). The significance of this aperture ratio is that the inlet coarse screen (5) only intercepts coarse particles with significantly larger particle sizes to prevent them from directly impacting the airfoil propeller blades (6) and causing excessive wear on the blades, while not intercepting medium-sized particles. This ensures that the solid concentration in the area of ​​the airfoil propeller blades (6) remains at a certain level, guaranteeing that there is enough material available for the blades to apply shearing and impact actions to the powder, and avoiding energy waste due to excessively low solid concentration. The aperture of the outlet fine screen (7) is selected according to the requirement of the target powder fineness R90. Usually, the aperture corresponding to R90=15% is about 0.5 to 1.0 mm. The aperture of the inlet coarse screen (5) is 3 to 10 times that of the outlet fine screen (7), that is, the aperture of the inlet coarse screen (5) is 1.5 to 10 mm.

[0035] The grinding pressure between the grinding disc (2) and the grinding roller (3) of the coal mill The initial density of the pressed biomass pellets There is a quantitative relationship ,in Based on the grinding pressure, For reference density, η is the pressure adjustment coefficient, and 0.5 MPa·m³ / kg ≤ η ≤ 5 MPa·m³ / kg. This quantitative relationship is determined based on the following principle: the initial density of the pressed biomass pellets. This reflects the compaction and hardness of the particles. Higher density indicates a tighter bond between the fibers within the particles, requiring higher grinding pressure to achieve effective crushing. Greater than At that time, grinding pressure Increase η·( based on the benchmark grinding pressure P0) - To ensure that the high-density particles are fully crushed in the grinding zone. The primary air is fed in at a tangential speed by the air ring around the grinding disc (2) of the coal mill. The air velocity and temperature of the primary air are determined according to the moisture content of the shaped biomass pellets and the required conveying air velocity.

[0036] In the gas-solid two-phase flow after passing through the primary separator (4), the solid concentration Separation efficiency with primary separator (4) The relationship between them satisfies ,in The initial solid concentration before entering the primary separator (4), and the separation efficiency. The efficiency is 0.3 to 0.7. That is, the coarse particles intercepted and returned to the grinding zone by the primary separator (4) account for 30% to 70% of the total solid material entering the primary separator (4), and the amount of solid material entering the secondary crushing and separation device through the primary separator (4) accounts for 30% to 70% of the total solid material entering the primary separator (4). The screening efficiency of the inlet coarse screen (5) is... Screening efficiency of the outlet fine screen (7) satisfy This ratio ensures that the interception strength of the inlet coarse screen (5) is lower than that of the outlet fine screen (7), allowing most particles within the acceptable particle size range to enter the airfoil propeller blade (6) area for crushing.

[0037] Energy input per unit mass of powder by airfoil propeller blades (6) satisfy ,in The average residence time of the powder in the secondary crushing and separation unit. The effective volume of the secondary crushing and separation device The ratio to the volumetric flow rate Qv is determined. , The value range is 5~50 kJ / kg. The lower limit of this energy input range (5 kJ / kg) ensures that the biomass fiber bundles receive the minimum energy input sufficient to overcome their internal interfacial binding energy, allowing the fiber bundles to begin dissociation; the upper limit (50 kJ / kg) prevents excessive energy input from causing over-grinding of the powder and the generation of a large amount of ultrafine powder, which may increase fly ash emissions and energy consumption during subsequent transportation and combustion. By controlling... Within the above range, biomass powder with moderate fineness and uniform particle size distribution can be obtained, while ensuring energy efficiency per unit output.

[0038] Example 2 This embodiment provides a method for deep grinding of compressed biomass pellets, which can be implemented using the compressed biomass pellet deep grinding system described in the above system embodiment. Since the specific structure of the system, the arrangement of each component, and their details have already been fully described in the system embodiment, to avoid repetition, the content identical to that in the system embodiment will not be repeated in this method embodiment. Instead, the corresponding description from the system embodiment will be directly introduced. This method embodiment focuses on a detailed explanation of the specific implementation process, process parameters, and their quantitative relationships for each method step.

[0039] The compression molding biomass pellet milling system used in this method embodiment is as follows: Figure 1 As shown, the labels and meanings of each component are exactly the same as those defined in the system embodiment.

[0040] S1: Grinding Steps The shaped biomass pellets enter the grinding zone of the coal mill body through the feed pipe (10). The feed pipe (10) is a vertically arranged cylindrical pipe, whose central axis coincides with the rotation axis of the coal mill disc (2) and the vertical central axis of the coal mill body. The lower outlet of the feed pipe (10) is directly opposite the central area of ​​the upper surface of the coal mill disc (2). The shaped biomass pellets are cylindrical or blocky particles formed by pressing loose biomass raw materials. Their diameter is usually 6-12 mm, their length is 10-40 mm, and their density is 600~1200 kg / m³. Under the action of their own gravity, the pellets fall along the feed pipe (10) to the central area of ​​the upper surface of the coal mill disc (2).

[0041] The coal mill drive system (1) is located at the bottom of the coal mill body, and its output shaft extends vertically upward and is fixedly connected to the center of the coal mill grinding disc (2). The coal mill grinding disc (2) rotates around the vertical axis under the drive of the coal mill drive system (1) at a speed of 20~60 rpm. The shaped biomass pellets falling onto the central area of ​​the upper surface of the coal mill grinding disc (2) are evenly spread outward along the radial direction of the coal mill grinding disc (2) under the action of centrifugal force, forming a uniformly distributed material bed.

[0042] There are three grinding rollers (3), which are uniformly pressed against the upper surface of the grinding disc (2) at an angle of 120° along the circumference of the grinding disc (2). The grinding rollers (3) apply a downward grinding pressure Pm to the grinding disc (2) through a hydraulic loading system or a spring loading system. An annular grinding zone is formed between the grinding rollers (3) and the upper surface of the grinding disc (2). The shaped biomass pellets spread into the grinding zone are crushed under the grinding action of the grinding rollers (3). The grinding pressure Pm is adjusted according to the initial density ρ0 of the shaped biomass pellets, and the two satisfy a quantitative relationship:

[0043] in The reference grinding pressure ranges from 5 to 15 MPa. The reference density is taken as 800 kg / m³; η is the pressure adjustment coefficient, and 0.5 MPa·m³ / kg ≤ η ≤ 5 MPa·m³ / kg. This quantitative relationship is determined based on the following principle: the initial density of the pressed biomass pellets. Density reflects the compaction and hardness of the particles. Higher density indicates a tighter bond between the fibers within the particles, requiring higher grinding pressure to generate sufficient strain and breakage within the particles. The density is obtained through online density testing or offline sampling and is fed back to the controller of the grinding roller loading system in real time. Greater than If the density of the compressed granules being processed is higher than the reference value, the controller will control the hydraulic loading system to increase the loading pressure, thereby increasing the grinding pressure. At the reference grinding pressure Add η·( to the basis) - );when Less than At that time, the controller controls the hydraulic loading system to reduce the loading pressure, thereby reducing the grinding pressure. At the reference grinding pressure Reduce η·( on the basis of - Through the above quantitative adjustment, it is ensured that shaped biomass pellets of different densities can be properly crushed in the grinding zone, avoiding insufficient crushing due to excessively low pressure or excessive crushing and increased energy consumption due to excessively high pressure.

[0044] During the grinding process, the shaped biomass pellets are subjected to the squeezing action between the grinding roller (3) and the grinding disc (2) of the coal mill. The biomass structure inside the pellets deforms and fractures under compressive stress. Since biomass materials are rich in cellulose, hemicellulose, and lignin, their crushing mechanism is mainly tensile fracture rather than brittle fracture. Therefore, in the product after grinding, some fiber bundles are fully broken into fine particles, some fiber bundles only undergo local fracture and retain a longer fiber shape, and some fiber bundles are flattened but not fully disintegrated under the grinding pressure. The above three types of products are mixed in the grinding zone.

[0045] S2: Primary separation step The primary hot air is tangentially fed into the coal mill body through an air ring set at the outer periphery of the coal mill disc (2). The temperature of the primary hot air is 150-350℃ and the wind speed is 15-30 m / s. The primary hot air has dual functions of drying and conveying: as a drying medium, the primary hot air heats and evaporates the residual moisture in the biomass material released during the grinding process, reducing the moisture content of the material to below 5%; as a conveying medium, the primary hot air provides upward aerodynamic force, blowing the crushed material particles upward from the grinding zone to form a gas-solid two-phase flow carrying solid powder. The opening angle and number of tangential air inlets on the air ring are determined according to the wind speed and air volume of the primary hot air. The tangential air inlets supply air to both the radial and tangential directions of the coal mill disc (2), providing upward conveying power and assisting the uniform spreading of the material on the coal mill disc (2).

[0046] The gas-solid two-phase flow enters the primary separator (4) from bottom to top. The primary separator (4) is located above the grinding zone and in the upper middle space of the coal mill body. The primary separator (4) includes several adjustable baffles fixedly installed on the inner wall of the coal mill body. The baffles are evenly arranged along the circumference of the inner wall of the coal mill body. An annular airflow channel is maintained between the inner edge of the baffle and the outer wall of the coal mill feed pipe (10) for the gas-solid two-phase flow carrying the material to pass through from bottom to top. The inclination angle of each baffle can be adjusted independently or in conjunction with the required initial separation particle size threshold. The inclination angle adjustment range is 30°~70°.

[0047] When the gas-solid two-phase flow carrying the ground material enters the primary separator (4) area from bottom to top, coarse particles with a diameter greater than the first threshold and insufficiently ground coarse fiber clumps, due to their large mass and strong inertia, cannot quickly change direction with the airflow as they pass around the baffle, thus colliding with the baffle surface due to inertia. The collision causes the coarse particles and coarse fiber clumps to lose upward momentum and slide down the baffle surface under their own gravity. They return to the grinding area through the gap between the outer wall of the coal mill feed pipe (10) and the baffle, mix with the newly entered shaped biomass pellets, and are then ground again. Fine particles with a diameter less than or equal to the first threshold, due to their small mass and good following ability, can pass around the baffle with the airflow and continue to move upward through the gap between the baffles. After passing through the primary separator (4), they enter the space above the primary separator (4). By adjusting the inclination angle of the baffle, the flow direction and velocity distribution of the airflow when passing through the baffle area can be changed, thereby adjusting the size of the first threshold.

[0048] Separation efficiency of the first-stage separator (4) Defined as the ratio of the mass of coarse particles intercepted by the primary separator (4) and returned to the grinding zone to the mass of the total solid material entering the primary separator (4). The value is 0.3-0.7, that is, the coarse particles intercepted and returned to the grinding zone by the primary separator (4) account for 30%-70% of the total solid material entering the primary separator (4), and the amount of solid material entering the secondary crushing and separation device through the primary separator (4) accounts for 30%-70% of the total solid material entering the primary separator (4). The range of values ​​for is determined by the following principle: when When the value is less than 0.3, the amount of coarse particles returning to the grinding zone is too small. The amount of coarse particles in the grinding zone is insufficient to form an effective mixing bed with the continuously entering pressed particles. This causes the grinding roller (3) to act directly on the newly entering particles, weakening the padding effect between the grinding roller (3) and the coal mill grinding disc (2). This results in a decrease in grinding efficiency and increased wear on the grinding components. When the value is >0.7, the amount of coarse particles returning to the grinding zone is too large, and the amount of material entering the secondary crushing and separation device through the primary separator (4) is too small, resulting in insufficient solid concentration in the downstream airfoil propeller blade (6) area, reducing the effective target of blade shearing and impact, increasing the energy waste ratio, and at the same time, the excessive accumulation of coarse particles in the grinding zone may cause the material bed to be too thick, affecting the effective crushing of the bottom particles by the grinding roller (3). A value between 0.3 and 0.7 ensures that the grinding zone has an appropriate amount of return material to form a stable material bed, while also ensuring that there is enough material downstream for the blades to crush. The specific values ​​are determined by adjusting the baffle tilt angle and the primary hot air velocity.

[0049] The solid concentration Cv in the gas-solid two-phase flow after passing through the first-stage separator (4) is related to the separation efficiency of the first-stage separator (4). The relationship between them satisfies:

[0050] in The initial solid concentration before entering the primary separator (4) is obtained in real time by an online solid concentration detector installed at the outlet of the air ring or the inlet of the primary separator (4). This relationship indicates the solid concentration after passing through the primary separator (4). Equal to the initial solid concentration before entering the first-stage separator (4) Multiply by the pass rate (1- ). The typical value range is 0.1~0.5 kg solid / kg gas.

[0051] S3: Pre-screening step The gas-solid mixture passing through the primary separator (4) continues to move upward and is pre-screened by the inlet coarse screen (5). The inlet coarse screen (5) is located below the area of ​​the airfoil propeller blades (6), and its screen holes are circular or rectangular. The inlet coarse screen (5) is inclined relative to the horizontal direction at an angle of 30° to 60°. This inclined arrangement allows the particles intercepted by the inlet coarse screen (5) to slide down the screen surface under the action of gravity to the lower end of the inlet coarse screen (5). The lower end of the inlet coarse screen (5) is connected to a coarse particle return channel. This coarse particle return channel extends downward along the inner wall of the coal mill body or the outer wall of the coal mill feed pipe (10) and connects to the grinding zone, thereby returning the coarse particles intercepted by the inlet coarse screen (5) to the grinding zone for further grinding.

[0052] The aperture size of the inlet coarse screen (5) is 3-10 times that of the outlet fine screen (7). Typically, the aperture size of the outlet fine screen (7) is 0.5-1.0 mm (corresponding to a fineness requirement of R90=15%), and the aperture size of the inlet coarse screen (5) is 1.5-10 mm. The screening efficiency of the inlet coarse screen (5) is... The screening efficiency is defined as the ratio of the mass of particles intercepted by the inlet coarse screen (5) to the total mass of solid material entering the inlet coarse screen (5). The inlet coarse screen (5) only intercepts coarse particles with a particle size larger than the second threshold, the size of which is determined by the sieve aperture of the inlet coarse screen (5), and the second threshold is less than the first threshold (the separation particle size threshold of the primary separator (4)). Screening efficiency of the outlet fine screen (7) satisfy:

[0053] Screening efficiency of the outlet fine screen (7) Defined as the ratio of the mass of qualified powder passing through the outlet fine screen (7) to the total mass of solid material entering the outlet fine screen (7), the value is 0.7~0.95. The above ratio ensures that the interception intensity of the inlet coarse screen (5) is lower than that of the outlet fine screen (7), so that most particles within the acceptable particle size range enter the airfoil propeller blade (6) area for crushing, avoiding excessive interception by the inlet coarse screen (5) which would lead to an excessively low solid concentration in the airfoil propeller blade (6) area and reduce the crushing efficiency.

[0054] The pre-screening function of the inlet coarse screen (5) has the following two effects: First, it intercepts coarse particles with a particle size significantly larger than the second threshold, preventing the coarse particles from directly impacting the airfoil propeller blade (6) and causing excessive wear or blade breakage on the blade surface; Second, by intercepting some coarse particles, it adjusts the particle size distribution of the powder entering the airfoil propeller blade (6) area, making the particle size range of the powder entering the area more concentrated, thereby improving the effectiveness and uniformity of the airfoil propeller blade (6) applying shearing and impact to the powder.

[0055] S4: Deep Grinding Step The pre-screened gas-solid mixture enters the secondary crushing and separation device. The secondary crushing and separation device is located between the primary separator (4) and the coal mill outlet pipe (8). Its core components are a vertically arranged rotating shaft and N-stage airfoil propeller blades (6) fixedly installed on the rotating shaft, where N≥3, preferably N=4. The rotating shaft is coaxially arranged with the coal mill feed pipe (10) and is fixed to the outer wall of the coal mill feed pipe (10) by a sealed bearing. The rotating shaft is a hollow shaft and is sleeved on the outside of the coal mill feed pipe (10). The upper end of the rotating shaft passes through the top of the coal mill body and is connected to the output shaft of the secondary crushing and separation device drive regulating motor (9). The drive regulating motor (9) is a variable frequency speed regulating motor, which adjusts the rotational speed n of the rotating shaft by changing the power supply frequency.

[0056] The secondary crushing and separation device drives the regulating motor (9) to rotate the shaft, which in turn drives the airfoil propeller blades (6) fixed on it to rotate synchronously at high speed. The airfoil propeller blades (6) are made of wear-resistant alloy steel or steel with surface hardening treatment. Each stage of airfoil propeller blade (6) contains 3-6 blades evenly arranged along the circumference. The airfoil profile of each blade is that of a propeller blade, with a pressure surface, a suction surface, a leading edge, and a trailing edge. The leading edge radius is 5%-15% of the maximum thickness of the blade, and the trailing edge thickness is 1%~5% of the maximum thickness of the blade. The airfoil chord length of each stage of airfoil propeller blade (6) decreases nonlinearly from the hub to the tip along the radial direction of the blade. The rate of decrease of the chord length from the hub to the tip gradually increases. This variable chord length design allows the blade to have a higher linear velocity in the tip region while maintaining a smaller aerodynamic drag, and a larger chord length in the hub region to ensure the structural strength and rigidity of the blade.

[0057] The axial distance H between two adjacent airfoil propeller blades (6) on the rotating shaft satisfies a quantitative relationship. Where D is the outer diameter of the airfoil propeller blade (6), ω is the rotational angular velocity of the shaft, and d is the characteristic particle size of the powder entering the secondary crushing and separation device after being separated by the primary separator (4) (i.e., the volume average particle size of the powder passing through the primary separator (4) in step S2). The kinematic viscosity of the airflow within the secondary crushing and separation unit. It is an empirical coefficient and 0.5 ≤ ≤1.5. The principle for determining this quantitative relationship is as follows: dimensionless number (ω·d / The rotating Reynolds number (H) is a variant of the rotating Reynolds number, reflecting the relative relationship between the perturbation intensity of the rotating blades on the flow field and their viscous diffusion capacity. A smaller dimensionless number indicates that viscous effects dominate the flow field, and the shear force exerted by the blades on the powder is attenuated by viscous dissipation within a short distance. In this case, a smaller axial spacing H (i.e., a smaller...) is required. The dimensionless number (k) is used to ensure that the shearing and crushing effect of the previous stage blades on the powder is not completely attenuated before entering the next stage blade. When this dimensionless number is large, it indicates that the inertial effect in the flow field is dominant, and the action exerted by the blades on the powder can maintain a longer distance. In this case, a larger axial spacing H (i.e., a larger k value) is required to avoid interference between the flow fields of each stage blade due to insufficient spacing, which would reduce the independent action efficiency of each stage blade. By determining the axial spacing between two adjacent stages of blades through the above quantitative relationship, it is possible to ensure that each stage of blade independently and efficiently exerts its crushing effect on the powder.

[0058] The installation angle of the airfoil propeller blade (6) increases progressively along the airflow direction, with the first-stage blade having the smallest installation angle and the fourth-stage blade having the largest. The installation angle is defined as the angle between the airfoil chord and the plane of rotation. The increment of the installation angle between adjacent blade stages satisfies a quantitative relationship:

[0059] in For the first The installation angle of the airfoil propeller blade (6), For the first The installation angle of the +1 stage airfoil propeller blade (6), Re is the Reynolds number calculated based on the tip linear velocity of the airfoil propeller blade (6), and α is a correction factor with 3°≤α≤8°. This quantitative relationship is determined based on the following principle: the increasing installation angle causes the axial thrust of each stage of the blade to gradually increase along the airflow direction. When Re is large, it indicates that the blade is operating in a turbulent state, the momentum exchange capacity of the airflow is strong, and a small increase in the installation angle can generate sufficient axial thrust increment. A smaller Re value indicates that the blades are operating in laminar or transitional flow, with weaker momentum exchange capacity of the airflow. A larger increment in the installation angle is required to generate sufficient axial thrust increment. The installation angle of the first-stage blades is 15°-35°, and the installation angle of the fourth-stage blades is 30°-55°, with a difference of 3°~8° between adjacent stages.

[0060] Under the condition that the rotor drives the airfoil propeller blades (6) to rotate at high speed, each stage of the blades exerts multiple mechanical effects on the powder suspended in the airflow. First, there is a pressure difference between the pressure surface and the suction surface of the blade due to the airfoil flow. This pressure difference forms a velocity gradient and a shear flow field near the blade surface, and the powder particles suspended in this flow field are subjected to shear stress. This shear stress τ satisfies:

[0061] Where ρ is the density of the gas-solid two-phase flow in the secondary crushing and separation device, and r is the radial position of the airfoil propeller blade (6) (radial distance from the center line of the rotating shaft to that point on the blade). The coefficient of friction, The value is determined based on the boundary layer thickness on the blade surface, ranging from 0.005 to 0.05. The shear stress τ ranges from 10 to 500 Pa. When shear stress acts on the biomass fiber bundles in the powder, shear deformation occurs inside the fiber bundles. When the amount of shear deformation exceeds the limiting strain at the lignin and hemicellulose interface in the fiber bundle, the fiber bundle undergoes shear slip and separation along the interface, leading to the initial dissociation of the fiber bundle.

[0062] Secondly, the leading edge of the airfoil propeller blade (6) collides with the suspended powder at a high linear velocity. The linear velocity of the blade leading edge v = ω·Rt (Rt is the blade tip radius), which is 30~120 m / s. The impact force F on the powder particles at the moment of collision satisfies the momentum theorem F·Δt = m·Δv, where Δt is the collision duration (10 m / s). -5 Up to 10 -4 (m is on the order of s), where m is the mass of the powder particles, and Δv is the change in velocity. The impact force causes localized stress concentration within the powder particles. When the local stress exceeds the interfacial bonding strength within the fiber bundle (the interfacial bonding strength of biomass fiber bundles is approximately 1~20 MPa), the fiber bundle breaks and disintegrates. The magnitude of the impact force is related to the particle size, density, and relative collision velocity of the powder particles.

[0063] Secondly, as the powder flows along the surface of the airfoil propeller blade (6) with the airflow, it generates rotational motion under the guidance of the blade curvature. The coarse particles and fiber bundles in the powder are subjected to centrifugal force. The centrifugal force causes the components in the fiber bundle to separate radially, further promoting the dissociation and dispersion of the fiber bundle. The above three mechanical actions of shearing, impact and centrifugal tearing occur synergistically in the area of ​​the airfoil propeller blade (6). Each stage of the blade applies the above actions to the powder in sequence along the airflow direction, causing the biomass fiber bundle to gradually dissociate into single fibers or fine fiber clusters.

[0064] When the airfoil propeller blades (6) rotate at high speed, they simultaneously generate axial pumping aerodynamic force from inside the coal mill to the coal mill outlet pipe (8). This axial pumping aerodynamic force originates from the axial thrust generated by the airfoil profile. When the airflow flows through the airfoil profile at a relative velocity W, according to the Jukowski lift theorem, the airfoil experiences lift perpendicular to the relative velocity direction, and the axial component of this lift is the axial thrust. The axial thrust acts on the gas around the blades, pushing the gas axially from downstream to upstream. Since the installation angle of each airfoil propeller blade (6) increases progressively along the airflow direction, the axial thrust generated by each blade increases progressively along the airflow direction, forming an increasing pumping capacity from the feed side to the discharge side. This axial pumping aerodynamic force forms a significant negative pressure zone in the inlet area of ​​the secondary crushing and separation device. This negative pressure ΔP satisfies:

[0065] Where Rt is the tip radius of the airfoil propeller blade (6), The pressure coefficient is 0.1 ≤ ≤0.6, the absolute value of ΔP is 0.5~5 kPa. The value of depends on the geometry of the airfoil profile, the angle of attack, and the Reynolds number, and is obtained through airfoil aerodynamic characteristic experiments or computational fluid dynamics numerical simulations.

[0066] The negative pressure is transmitted upstream to the primary separator (4) area and even the grinding zone, generating a suction effect on the gas-solid two-phase flow in the grinding zone and the feed pipe. The mechanism of this suction effect is as follows: the existence of the negative pressure area creates a pressure gradient between the upstream side (grinding zone and primary separator (4) area) and the downstream side (airfoil propeller blade (6) area), which drives the airflow to accelerate from upstream to downstream. The suction effect has a dual function: firstly, the aerodynamic force generated by the suction effect is in the same direction as the primary air conveying direction, providing additional driving force for the movement of the gas-solid two-phase flow from the grinding zone to the secondary crushing and separation device. This additional driving force compensates for the local resistance loss of the added inlet coarse screen (5) (the resistance of the inlet coarse screen (5) to the airflow is about 0.1~0.5 kPa), the blade shape resistance and flow turning loss in the airfoil propeller blade (6) area (about 0.3~1.0 kPa), and the screen resistance loss of the outlet fine screen (7) (about 0.1~0.4 kPa). When the absolute value of the negative pressure ΔP generated by the high-speed rotation of the airfoil propeller blade (6) (0.5~5 kPa) matches the sum of the additional resistance of the above components, the ratio of the total system pressure drop ΔPtotal to the total pressure drop ΔP0 of the reference system without a secondary crushing and separation device satisfies ΔPtotal / ΔP0=0.85-1.05, that is, the total system pressure drop is not significantly increased or even slightly decreased compared to the reference system. The total system pressure drop is measured by setting pressure sensors at the inlet of the coal mill feed pipe (10) and the outlet of the coal mill outlet pipe (8) respectively, and calculating the pressure difference between the two. Secondly, the suction effect forms a downward suction effect at the inlet of the coal mill feed pipe (10), which generates an active traction effect on the shaped biomass pellets entering the coal mill feed pipe (10). This traction helps the particles overcome the frictional resistance (frictional resistance coefficient λf=0.02-0.05), back pressure of the airflow (about 10%~30% of the gas phase pressure in the feed pipe), and friction between particles (depending on the particle bulk density and internal friction angle), so that the particles fall smoothly to the grinding disc (2) of the coal mill and prevent the coal mill feed pipe (10) from being blocked.

[0067] Energy input per unit mass of powder by airfoil propeller blades (6) satisfy:

[0068] in This represents the average residence time of the powder within the secondary crushing and separation unit. Average residence time The effective volume of the secondary crushing and separation device With volumetric flow rate The ratio is determined as follows: ; The volume that the gas-solid two-phase flow can effectively occupy in the region of the airfoil propeller blade (6) is (i.e., the volume of the annular space swept by the blade rotation minus the volume occupied by the blade itself). The volumetric flow rate of the gas-solid two-phase flow passing through the secondary crushing and separation device is denoted as . The value range is 5~50 kJ / kg. This energy input range is determined based on the following: the interfacial binding energy of biomass fiber bundles is approximately 0.5~5 kJ / kg (determined based on the content and distribution of lignin and hemicellulose in the fiber bundles). The lower limit of 5 kJ / kg ensures that the powder receives the minimum energy input sufficient to overcome the interfacial binding energy, allowing the fiber bundles to begin dissociation; The upper limit of 50 kJ / kg is determined based on industrial experience. When this value is exceeded, the proportion of ultrafine powder with a particle size of less than 10 μm in the powder increases sharply. Ultrafine powder is easy to deposit on the pipe wall during subsequent pneumatic conveying and may increase fly ash emissions during combustion. At the same time, the energy consumption per unit output increases significantly. Through the shear stress τ It forms a coupling relationship with the blade geometry parameters: when As the size increases, the cumulative shearing and impact forces experienced by the powder within the device intensify, leading to increased fiber dissociation. Beyond 50 kJ / kg, the rate of increase in dissociation slows significantly (conforming to an exponential decay law), while energy consumption continues to increase linearly. Therefore, setting the upper limit to 50 kJ / kg achieves the optimal balance between dissociation and energy consumption. By controlling... Within the range of 5~50 kJ / kg, biomass fiber bundles can be fully dissociated and refined, while avoiding the negative effects of excessive grinding.

[0069] The shaft rotational speed n is adjustable from 200 to 1500 rpm, corresponding to the blade tip linear velocity. satisfy , The value range is 30~120 m / s. Shaft rotation speed. Adjustments are made based on the target powder fineness R90 and the target output Q, and the quantitative relationship among the three satisfies: ;in For reference speed, For reference fineness, Qref is the reference yield, γ is the fineness index (0.3 ≤ γ ≤ 1.0), and m is the yield index (0.2 ≤ m ≤ 0.8). This quantitative relationship is determined based on the following principle: when the target fineness R90 decreases (i.e., a finer powder is required), the rotational speed needs to be increased. To increase shear stress τ and impact force, the powder is further refined by increasing the rotational speed. The change in R90 exhibits an exponential relationship, with the exponent γ reflecting the sensitivity of fineness changes to speed adjustment. When the target output Q increases, the solid load through the secondary crushing and separation unit increases, requiring an increase in speed n to maintain sufficient energy input per unit mass of powder. The rotational speed n changes exponentially with Q, and the exponent m reflects the sensitivity of output changes to speed adjustment. This quantitative relationship allows for the determination of the optimal speed setpoint based on different operating conditions (fineness requirements and output requirements).

[0070] S5: Final Screening Steps The refined powder continues to move upward with the airflow and undergoes final screening through the outlet fine screen (7). The outlet fine screen (7) is located above the area of ​​the airfoil propeller blade (6) and below the coal mill outlet pipe (8). The outlet fine screen (7) is an arc-shaped screen, and the radius of curvature of its arc surface matches the radius of curvature of the rotation trajectory of the airfoil propeller blade (6). That is, the arc-shaped screen is arranged coaxially with the rotating shaft and surrounds the discharge side of the airfoil propeller blade (6) in the circumferential direction. The curved arrangement offers several technical advantages over the planar arrangement: The air-facing area of ​​the curved screen is larger than the actual filtration area of ​​a planar screen with the same projected area, resulting in a more uniform powder throughput per unit area. This avoids the uneven screening and localized clogging problems caused by excessively high air velocity in the center and low air velocity at the edges of a planar screen. Simultaneously, the curvature of the curved screen matches the blade rotation trajectory. After the airflow exits tangentially from the blade's discharge side, it impacts the curved screen surface in a near-vertical direction. The angle between the incident direction of the powder particles and the normal direction of the screen surface is smaller, facilitating smooth particle passage through the screen openings and reducing the probability of particle rebound and rolling on the screen surface. The effective screening area of ​​the curved screen accounts for 60% to 90% of the total cross-sectional area at the outlet. The area without a screen serves as an airflow bypass channel, used to balance airflow distribution and prevent excessively high local air velocities that could lead to screen clogging or excessive wear.

[0071] Screening efficiency of the outlet fine screen (7) Defined as the ratio of the mass of qualified powder passing through the outlet fine screen (7) to the mass of the total solid material entering the outlet fine screen (7). The value is 0.7~0.95. The aperture of the outlet fine screen (7) determines the maximum particle size of the final output powder. Qualified fine powder with a particle size smaller than the aperture passes through the outlet fine screen (7) with the airflow, is discharged from the coal mill body through the coal mill outlet pipe (8), is sent into the subsequent coal powder pipeline, and finally enters the boiler furnace for suspension combustion. Unqualified particles with a particle size larger than the aperture are intercepted by the outlet fine screen (7). The intercepted particles slide down the surface of the arc-shaped screen under the action of gravity, or are carried back to the airfoil propeller blade (6) area under the action of airflow back mixing, and are subjected to re-shearing, impact and centrifugal tearing treatment by the airfoil propeller blades (6) at each level. The above-mentioned cyclic processing continues until the intercepted particles are refined to the point that they can pass through the outlet fine screen (7). When the powder fineness meets the target requirement R90<15%, the final screening step is completed, and qualified powder is discharged through the coal mill outlet pipe (8).

[0072] Through the coordinated operation of steps S1 to S5, the shaped biomass pellets complete the entire process of grinding, primary separation, coarse screening, deep crushing by airfoil propeller blades, and final screening at the outlet within the same coal mill body. An orderly transfer and cyclical feedback of materials and energy are formed between each step, ultimately obtaining biomass powder with controllable fineness and fully dissociated fibers.

[0073] Many specific details have been set forth in the foregoing description to provide a thorough understanding of the present invention. However, the above description is merely a preferred embodiment of the present invention, and the present invention can be implemented in many other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed above. Furthermore, any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, using the methods and techniques disclosed above, without departing from the scope of the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A deep grinding system for pressed biomass pellets, characterized in that, include: Coal mill drive system; The grinding disc of the coal mill is driven to rotate by the coal mill drive system; A grinding roller is disposed above the grinding disc of the coal mill, and a grinding zone is formed between the grinding disc and the grinding roller; A primary separator is disposed above the grinding zone; The coal mill feed pipe penetrates the central area of ​​the primary separator; Coal mill outlet pipe; A secondary crushing and separation device is installed between the primary separator and the coal mill outlet pipe. The secondary crushing and separation device includes a rotating shaft, N-stage airfoil propeller blades fixedly installed on the rotating shaft, and a drive and regulating motor, where N≥3. The rotating shaft is coaxially arranged with the coal mill feed pipe. The inlet of the secondary crushing and separation device is equipped with an inlet coarse screen, and the outlet of the secondary crushing and separation device is equipped with an outlet fine screen. The aperture of the inlet coarse screen is larger than the aperture of the outlet fine screen. The axial distance H between two adjacent airfoil propeller blades on the rotating shaft satisfies: Where D is the outer diameter of the airfoil propeller blade, ω is the rotational angular velocity of the shaft, and d is the characteristic particle size of the powder entering the secondary crushing and separation device after separation by the primary separator. The kinematic viscosity of the airflow within the secondary crushing and separating device is given. It is an empirical coefficient and 0.5 ≤ ≤1.5; The installation angle of the airfoil propeller blades increases progressively along the upward flow direction of the gas-solid two-phase flow, and the rate of increase of the installation angle between adjacent blade stages satisfies the following: in For the first The installation angle of the airfoil propeller blades. For the first The installation angle of the airfoil propeller blade, Re is the Reynolds number calculated based on the tip linear velocity of the airfoil propeller blade, and α is a correction factor with 3°≤α≤8°.

2. The deep grinding system for pressed biomass pellets according to claim 1, characterized in that, The rotating shaft is fixed to the feed pipe of the coal mill via a sealed bearing, forming an annular airflow channel between the rotating shaft and the feed pipe; the drive regulating motor is a variable frequency speed control motor, and the rotational speed n of the rotating shaft and the target fineness R90 of the powder output at the outlet fine screen satisfy the following: in As the reference speed, As a reference fineness, The adjustment coefficient is 50 rpm ≤ λ ≤ 200 rpm.

3. The deep grinding system for pressed biomass pellets according to claim 1, characterized in that, The number of airfoil propeller blades is four stages, and each stage of airfoil propeller blades contains 3-6 blades evenly arranged in the circumferential direction; the airfoil profile of each stage of airfoil propeller blade is a propeller blade shape, with a pressure surface, a suction surface, a leading edge and a trailing edge, the leading edge radius of the blade is 5%-15% of the maximum thickness of the blade, and the trailing edge thickness of the blade is 1% to 5% of the maximum thickness of the blade.

4. The deep grinding system for pressed biomass pellets according to claim 1, characterized in that, Wear-resistant liners are embedded in the inner wall of the casing of the secondary crushing and separation device at the positions corresponding to the airfoil propeller blades. There is a radial gap δ between the wear-resistant liner and the tip of the airfoil propeller blade, and δ / D = 0.5%-2.0%. The surface of the wear-resistant liner is provided with guide grooves extending along the airflow direction. The depth of the guide grooves is 0.5-3mm, the width of the guide grooves is 2-10mm, and the spacing between adjacent guide grooves is 5-20mm.

5. The deep grinding system for pressed biomass pellets according to claim 1, characterized in that, The inlet coarse screen is arranged at an angle of 30°-60° relative to the airflow direction. A coarse particle return channel is provided below the inlet coarse screen, and the coarse particle return channel is connected to the grinding zone. The outlet fine screen is an arc-shaped screen, and the radius of curvature of the arc-shaped screen matches the radius of curvature of the rotation trajectory of the airfoil propeller blade. The effective screening area of ​​the arc-shaped screen accounts for 60%-90% of the total cross-sectional area at the outlet.

6. The deep grinding system for pressed biomass pellets according to claim 1, characterized in that, The aperture of the inlet coarse screen is 3 to 10 times that of the outlet fine screen.

7. A method for deep grinding of pressed biomass pellets, characterized in that, Includes the following steps: S1: The shaped biomass pellets enter the grinding zone of the coal mill body through the feed pipe. The shaped biomass pellets are ground by the grinding disc and grinding rollers of the coal mill, with a grinding pressure of... The initial density of the compressed biomass pellets The following conditions must be met: in Based on the grinding pressure, The reference density is η, which is the pressure adjustment coefficient and 0.5 MPa·m³ / kg≤η≤5MPa·m³ / kg; S2: Primary air is sent in by the air ring around the grinding disc of the coal mill, carrying the ground material to form a gas-solid mixture that enters the primary separator from bottom to top. In the primary separator, coarse particles with a particle size greater than the first threshold and coarse fiber clumps that are not sufficiently ground are intercepted and returned to the grinding zone for re-grinding. Fine particles with a particle size less than or equal to the first threshold pass through the primary separator with the airflow. S3: The gas-solid mixture passing through the primary separator is pre-screened through the inlet coarse screen to retain particles with a particle size larger than the second threshold, wherein the second threshold is smaller than the first threshold; S4: The pre-screened gas-solid mixture enters the secondary crushing and separation device, where a drive regulating motor drives the N-stage airfoil propeller blades to rotate at high speed. This process shears, impacts, and centrifugally tears the suspended powder, causing the biomass fiber bundles to disintegrate and refine. The shear stress τ generated by the rotating airfoil propeller blades satisfies: Where ρ is the density of the gas-solid two-phase flow within the secondary crushing and separation device, and r is the radial position of the airfoil propeller blade. The coefficient of friction is given by τ, where 10 Pa ≤ τ ≤ 500 Pa; the negative pressure generated at the inlet of the secondary crushing and separating device when the airfoil propeller blades rotate. satisfy: in Let be the tip radius of the airfoil propeller blade. The pressure coefficient is 0.1 ≤ ≤0.6; S5: The refined powder is finally screened by the outlet fine screen. The powder that passes through the outlet fine screen is discharged through the coal mill outlet pipe, while the powder that does not pass through the outlet fine screen is intercepted and continues to be processed in the secondary crushing and separation device.

8. The method for deep grinding of compressed biomass pellets according to claim 7, characterized in that, In step S4, the rotational speed at which the drive adjustment motor drives the rotating shaft to rotate is adjusted. The rotation speed is adjusted according to the target powder fineness R90 and the target output Q. The relationship between the target powder fineness R90 and the target output Q satisfies: in For reference speed, For reference level of detail, For reference yield, γ is the fineness index with 0.3 ≤ γ ≤ 1.0, and m is the yield index with 0.2 ≤ ≤0.8; the adjustment range of the rotational speed n is 200~1500 rpm, and the tip linear velocity of the airfoil propeller blades is... satisfy: And 30 m / s≤ ≤120 m / s.

9. The method for deep grinding of compressed biomass pellets according to claim 7, characterized in that, The solid concentration of the gas-solid two-phase flow at the inlet of the secondary crushing and separation device Separation efficiency of the first-stage separator The relationship between them satisfies: in The initial solid concentration before entering the primary separator, and 0.3 ≤ ≤0.

7.

10. A method for deep grinding of compressed biomass pellets according to claim 7, characterized in that, In step S4, the airfoil propeller blades provide energy input per unit mass of powder. satisfy: in The average residence time of the powder within the secondary crushing and separating device. The effective volume of the secondary crushing and separation device With volumetric flow rate The ratio is determined as follows: And 5 kJ / kg≤ ≤50 kJ / kg.

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