High-density straw biomass pellet fuel and preparation method thereof

Abstract

The application discloses a kind of high-density straw biomass granular fuel and its preparation method, first straw material retains straw skin and is involved in pressing with larger size, so that the natural wax in the skin of straw is migrated in the process of heat conditioning and pressing with material heated and pressed, the second straw material is formed by crushing the core of straw after removing the skin, which can be embedded in the gap between the first straw material with smaller particle size, providing continuous filling support for the first straw material and reducing the internal void of the pressed granule;At the same time, the pressure holding cooling makes the pressed granule complete cooling in a limited state, so that the material form after heat conditioning and pressing is maintained, and the migrated natural wax is stably distributed at the periphery of the granule and the contact interface, the skin wax migration of the first straw material, the interstitial compaction of the second straw material and the limited shape of the pressure holding cooling make the straw biomass granular fuel have higher compactness, better forming stability and lower surface pulverization tendency.

Description

Technical Field

[0001] This invention relates to the field of biomass solid fuel preparation technology, and in particular to a high-density straw biomass pellet fuel and its preparation method. Background Technology

[0002] Straw is a major renewable biomass resource in agricultural production, mainly including corn straw, wheat straw, rice straw, rapeseed straw, cotton straw, and sorghum straw. Preparing straw into pellet fuel is an important way to improve the resource utilization rate of straw, reduce bulk transportation costs, and improve combustion stability. Existing straw biomass pellet fuels are typically prepared through steps such as drying, crushing, moisture conditioning, mixing, pressing, and cooling, which can improve the bulk density and molding stability of the straw raw material to a certain extent.

[0003] However, compared with woody biomass, straw raw materials have characteristics such as loose fiber structure, uneven distribution of epidermal wax, higher ash and silica content, and relatively lower lignin content. In the conventional pelleting process, if the straw is crushed into a uniform powder, although the feeding stability can be improved, it is easy to damage the original epidermal structure of the straw. This causes the natural wax of the straw to disperse inside the pellet during crushing and mixing, making it difficult to form a stable surface phase on the outer periphery of the pellet. At the same time, the uniform powder lacks stable internal support units, making it prone to elastic rebound and surface powder shedding after pressing.

[0004] To improve the density and strength of straw pellet fuel, existing technologies typically employ methods such as increasing compression pressure, raising die temperature, increasing compression ratio, adding binders like starch, molasses, lignin, and bentonite, or compounding with high-lignin raw materials like sawdust, bamboo powder, and nutshell powder. While these methods can improve pellet formation, increasing pressure and temperature increases equipment load and energy consumption, adding binders increases raw material costs and may alter ash composition, and compounding with woody raw materials reduces the utilization rate of straw. Furthermore, these methods are mostly based on uniform crushing and mixing, and their core function remains improving pellet formation through external pressure or added binders, failing to fully utilize the straw's own surface structure and natural wax components.

[0005] Therefore, it is necessary to provide a high-density straw biomass pellet fuel and its preparation method. By utilizing different parts of the straw raw material in a graded manner, the straw material with the outer skin retained and the straw core crushed material after the outer skin has been removed play different roles in the pressing process. Combined with heat conditioning, pressing and pressure holding cooling processes, high-density pellet fuel can be prepared from straw raw materials without relying on microcapsules, grafting or adding groups. Summary of the Invention

[0006] The purpose of this invention is to provide a high-density straw biomass pellet fuel and its preparation method, so as to solve the problems of the straw surface wax being difficult to effectively utilize, insufficient internal support of uniformly crushed material, easy rebound of pellets after pressing, and surface pulverization in the existing straw pellet fuel preparation process.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing high-density straw biomass pellet fuel includes the following steps:

[0009] S1. The straw raw material is cleaned, dried and graded to obtain the first straw material with the outer skin retained and the second straw material obtained by crushing the straw core after the outer skin is removed. The length of the first straw material is 15-45mm and the width is 2-8mm. The particle size of the second straw material is 0.15-1.20mm.

[0010] S2. Mix the first straw material with the second straw material at a mass ratio of 20-40:60-80, and adjust the moisture content of the mixed material to 10%-18%.

[0011] S3. Heat-condition the moisture-treated material at 65-105℃ for 3-20 minutes to obtain the mixture to be pressed.

[0012] S4. Press the mixture to be pressed at 80-135℃ and 90-260MPa for 5-60s to obtain pressed granules.

[0013] S5. Press the pellets and cool them under pressure at 10-35℃ for 30-300s to obtain high-density straw biomass pellet fuel.

[0014] In some embodiments, in step S1, the straw raw material includes corn straw, wheat straw, rice straw, rapeseed straw, cotton straw, sorghum straw, or any combination thereof; the straw raw material has a moisture content of 6%-14% and an ash content of 3%-12% by mass before grading, and an impurity content of ≤2% by mass.

[0015] In some embodiments, in step S1, the first straw material is obtained by cutting along the straw axis, and the mass fraction of the outer skin retained in the first straw material is 60%-95%; the second straw material is obtained by crushing and sieving the straw core after peeling off the straw outer skin, and the mass fraction of the second straw material passing through a 0.15mm sieve is ≤10%, and the mass fraction passing through a 1.20mm sieve is ≥90%.

[0016] In some embodiments, in step S2, the first straw material and the second straw material are mixed in two stages. In the first stage of mixing, 40%-70% of the total mass of the second straw material is added, and the mixing time is 2-8 minutes. In the second stage of mixing, the remaining second straw material is added, and the mixing time is 3-12 minutes.

[0017] In some embodiments, in step S2, the humidity conditioning is performed using atomized water, saturated steam, or a combination of atomized water and saturated steam. The material temperature during the humidity conditioning process is 25-60°C, the settling time after humidity conditioning is 5-30 minutes, and the relative humidity of the settling environment is 45%-75%.

[0018] In some embodiments, in step S3, heat conditioning is carried out in a closed conveying state. During the heat conditioning process, the material layer thickness is 20-80mm, the material turning frequency is 2-15 times / min, and the material discharge temperature is 65-105℃ after heat conditioning.

[0019] In some embodiments, in step S4, the pressing is performed using a pressing mold with a cylindrical compression channel, wherein the diameter of the compression channel is 6-12 mm, the length-to-diameter ratio of the compression channel is 4.5-9.0, and the compression ratio is 4:1-8:1.

[0020] In some embodiments, in step S4, the thickness of the material to be pressed before entering the pressing mold is 10-40mm, the feeding rate is 20-120kg / h, and the linear speed of the pressure roller during the pressing process is 0.8-3.5m / s.

[0021] In some embodiments, in step S5, the pressure holding and cooling is carried out by axial pressure holding, radial limiting, or a combination of axial pressure holding and radial limiting. The pressure holding and cooling pressure is 75-95MPa, the cooling medium temperature is 5-30℃, and the particle surface temperature is 10-35℃ when the pressure is released.

[0022] The present invention also provides a high-density straw biomass pellet fuel, which is prepared by the aforementioned method. The raw materials of the biomass pellet fuel include a first straw material and a second straw material. The first straw material is straw material with the outer skin retained, and the second straw material is straw core powder after the outer skin has been removed. The mass ratio of the first straw material to the second straw material is 20-40:60-80.

[0023] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0024] (1) The present invention provides a method for preparing high-density straw biomass pellet fuel. The first straw material retains the straw skin and participates in the pressing with a larger size, so that the natural wax in the straw skin migrates with the material under heat and pressure during the heat conditioning and pressing process. The second straw material is formed by crushing the straw core after removing the skin. Its smaller particle size can be embedded in the gaps between the first straw materials, providing continuous filling support for the first straw material and reducing the internal voids of the pressed pellets. At the same time, the pressure holding and cooling allows the pressed pellets to complete the cooling under a restricted state, so that the material morphology after heat conditioning and pressing is maintained, and the migrated natural wax is stably distributed on the outer periphery and contact interface of the pellets. The migration of the skin wax of the first straw material, the filling and compaction of the second straw material and the restricted shaping of the pressure holding and cooling make the straw biomass pellet fuel have a high degree of density, good molding stability and low surface pulverization tendency.

[0025] (2) In this invention, the mass ratio of the first straw material to the second straw material limits the relative proportion of the retained outer straw segments and the core crushed material in the mixture, so that the first straw material can provide a continuous source of outer wax and a larger size support unit; the second straw material can enter the gap between the first straw materials within this ratio range, and fill the gap between the first straw materials during the pressing process, promoting the compression arrangement of the first straw materials and the migration of wax. During the pressing process, the mixture avoids both excessive coarse material causing insufficient filling and excessive fine material causing insufficient support, thereby obtaining a more stable compacted structure.

[0026] (3) In this invention, the holding and cooling pressure keeps the pressed particles in a compressed or limited state during the cooling stage, inhibiting the size rebound of the pressed material due to elastic recovery; the wax migration state formed during the heat conditioning and pressing stages can maintain contact with the contact interface of the first straw material and the second straw material under the action of this pressure, and the second straw material can also maintain the filling state of the internal gap during the restricted cooling process. The holding and cooling pressure works in conjunction with the pressing process of the preceding graded material, so that the particles can complete the structural stability during the temperature reduction process, reducing the loosening, cracking and pulverization during the cooling stage. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a flowchart illustrating the method steps of a preferred embodiment of the present invention. Detailed Implementation

[0029] 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.

[0030] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0031] Application Overview:

[0032] Existing straw biomass pellet fuel typically uses uniformly pulverized straw powder as the main raw material, and improves the forming state by increasing pressing pressure, pressing temperature, adding binders, or adjusting moisture content. Although this method is simple, uniform pulverization destroys the natural structural differences between the straw outer layer and the core, causing the natural waxes in the straw outer layer to disperse throughout the material system, making them difficult to utilize effectively in subsequent pressing and cooling stages. Furthermore, uniform powder lacks large internal support units, making the pellets prone to springback after pressing, and the fine powder on the surface is easily detached during storage and transportation.

[0033] This invention separates straw raw materials into a first straw material with its outer skin retained and a second straw material formed by crushing the straw after removing its outer skin. The first straw material maintains a relatively large size during compression, while the second straw material fills the gaps between the first straw materials during mixing and compression. Based on this, the natural waxy coating on the surface of the first straw material softens during heat conditioning and compression, while the second straw material, through its particle size difference, helps to fill the gaps and achieve compaction. This method does not simply rely on increasing pressure or adding binders, but rather utilizes the differences in physical properties of different parts of the straw raw material itself for preparation.

[0034] Based on the above principles, this invention places the first straw material, the second straw material, and the natural waxy substance of the straw in the same continuous process path during the heat conditioning, pressing, and pressure holding cooling processes. The first straw material provides larger straw units with the outer skin retained, the second straw material provides interstitial components with smaller particle sizes, and the heat conditioning and pressure holding cooling provide control over the material temperature and pressure state, thereby obtaining straw biomass pellet fuel suitable for biomass combustion, molding, transportation, and storage.

[0035] This invention is applicable to the solid fuel utilization of agricultural straw biomass raw materials, especially to the preparation of pellet fuel from corn straw, wheat straw, rice straw, rapeseed straw, cotton straw, sorghum straw, and their mixtures. The biomass pellet fuel obtained by this invention can be used in biomass boilers, biomass hot air furnaces, biomass heating equipment, biomass gasification equipment, and other combustion or heat utilization scenarios using solid biomass pellets as fuel.

[0036] In the following embodiments, the purchased materials include: corn stalks, purchased from a local agricultural cooperative, naturally air-dried after harvest, with an initial moisture content of 10.5% and an impurity mass fraction of 1.1%; wheat stalks, purchased from a local agricultural cooperative, with an initial moisture content of 9.8% and an impurity mass fraction of 0.9%; and rice stalks, purchased from a local agricultural cooperative, with an initial moisture content of 11.2% and an impurity mass fraction of 1.4%. The water used is industrial-grade softened water with a conductivity ≤50μS / cm. The straw raw materials undergo dust removal, sand removal, and removal of metallic impurities before use.

[0037] The homemade materials include a first type of straw feed and a second type of straw feed. The first type of straw feed is prepared as follows: Cleaned straw raw material is taken and cut along the straw axis into sections 30mm in length and 5mm in width, retaining the straw outer layer. Broken powder is then sieved out to obtain the first type of straw feed. The second type of straw feed is prepared as follows: Cleaned straw raw material is taken, the outer layer is peeled off, retaining the straw core. The straw core is crushed and sieved, collecting the crushed material with a particle size of 0.30-0.85mm to obtain the second type of straw feed.

[0038] like Figure 1 As shown, the present invention provides a method for preparing high-density straw biomass pellet fuel, comprising the following steps:

[0039] S1. The straw raw material is cleaned, dried and graded to obtain the first straw material with the outer skin retained and the second straw material obtained by crushing the straw core after the outer skin is removed. The length of the first straw material is 15-45mm and the width is 2-8mm. The particle size of the second straw material is 0.15-1.20mm.

[0040] Step S1 involves the pretreatment and grading of the straw raw material. In this paper, the first straw material refers to larger straw segments retaining the outer skin, which can originate from the outer layer of straw or straw segments with the outer skin intact. The second straw material refers to the pulverized material obtained by crushing the straw core after removing the outer skin. It should be distinguished that the first straw material is not ordinary coarse powder, but rather straw segments retaining the outer skin; the second straw material is not arbitrary straw powder either, but rather pulverized material mainly obtained from the straw core after removing the outer skin. Through this grading method, the outer skin and core portions of the straw raw material exhibit different material forms in subsequent processes.

[0041] S2. Mix the first straw material with the second straw material at a mass ratio of 20-40:60-80, and adjust the moisture content of the mixed material to 10%-18%.

[0042] Step S2 involves mixing and conditioning the graded materials. Specifically, the first and second straw materials are added to the mixing equipment according to a set mass ratio, distributing the second straw material among the first straw materials. Conditioning can be performed using atomized water, saturated steam, or a combination of both. In this text, moisture content refers to the percentage of water mass in the material relative to the total mass of the wet material. After conditioning, the mixture can be allowed to stand, allowing the moisture to distribute between the first and second straw materials.

[0043] S3. Heat-condition the humidified material at 65-105℃ for 3-20 minutes to obtain the mixture to be pressed.

[0044] Step S3 is the heat conditioning step. Heat conditioning can be carried out in a closed conveyor system, allowing the conditioned material to undergo conditioning treatment at a controlled temperature through heating and turning. More specifically, the thickness of the material layer and the turning frequency can be controlled during heat conditioning to keep the first and second straw materials mixed. After heat conditioning, the material enters the pressing stage at a set temperature.

[0045] S4. Press the mixture to be pressed at 80-135℃ and 90-260MPa for 5-60s to obtain pressed granules.

[0046] Step S4 is the compression molding step. Specifically, the mixture to be compressed enters a compression mold with a cylindrical compression channel, and is compressed under a set temperature and pressure. The compression channel of the compression mold can be a cylindrical channel with a channel diameter of 6-12mm, a length-to-diameter ratio of 4.5-9.0, and a compression ratio of 4:1-8:1. Before the mixture enters the mold, the feed thickness can be controlled by spreading the material, and the continuity of the compression process can be controlled by the feed rate and the linear speed of the pressure rollers.

[0047] S5. Press the pellets and cool them under pressure at 10-35℃ for 30-300s to obtain high-density straw biomass pellet fuel.

[0048] Step S5 is the pressure holding and cooling step. Pressure holding and cooling can be performed using axial pressure holding, radial limiting, or a combination of axial pressure holding and radial limiting. Specifically, after the compressed pellets transition from the pressing stage to the cooling stage, they come into contact with the cooling medium while still under pressure. The cooling medium can be air, a cooling water jacket, or other indirect cooling media. After pressure holding and cooling is completed, the pressure is released and the pellet fuel is collected.

[0049] Further, in step S1, the straw raw material includes corn straw, wheat straw, rice straw, rapeseed straw, cotton straw, sorghum straw, or any combination thereof. The moisture content of the straw raw material before grading is 6%-14%, the ash content is 3%-12%, and the impurity content is ≤2%. Impurities include mud, sand, stones, metal fragments, and non-straw plant residues. Controlling the moisture content of the straw raw material before grading ensures stable cutting and crushing processes. The ash content and impurity content are used to define the basic state of the raw material and prevent abnormal raw materials from entering subsequent pressing processes.

[0050] Further, in step S1, the first straw material is obtained by cutting along the straw axial direction, with the mass fraction of the outer skin remaining in the first straw material being 60%-95%. Cutting along the straw axial direction means that the cutting direction is consistent with or substantially consistent with the natural extension direction of the straw, thereby obtaining strip-shaped straw material with the outer skin intact. When the length of the first straw material is 15-45mm and the width is 2-8mm, it can maintain a relatively intact strip shape during mixing and feeding. The second straw material is obtained by crushing and sieving the straw core after peeling off the outer skin. The mass fraction of the second straw material passing through a 0.15mm sieve is ≤10%, and the mass fraction passing through a 1.20mm sieve is ≥90%. This sieving is limited to remove excessively fine dust and excessively large particles, so that the second straw material has a particle size suitable for filling the gaps between the first straw materials.

[0051] Further, in step S2, the first and second straw materials are mixed in two stages. In the first stage, 40%-70% of the total mass of the second straw material is added, and the mixing time is 2-8 minutes. In the second stage, the remaining second straw material is added, and the mixing time is 3-12 minutes. In this text, "two-stage mixing" refers to the method of adding the second straw material to the first straw material in batches. The first stage of mixing allows some of the second straw material to enter the mixture first, while the second stage of mixing replenishes the remaining second straw material and adjusts the overall uniformity of the mixture. This mixing method differs from the method of adding all the pulverized material at once.

[0052] Further, in step S2, the humidification process employs atomized water, saturated steam, or a combination of both. Atomized water can be sprayed to form fine droplets that penetrate the mixture, while saturated steam can contact the material during mixing and provide moisture. The material temperature during humidification is 25-60℃, the settling time after humidification is 5-30 minutes, and the relative humidity of the settling environment is 45%-75%. Settling after humidification refers to placing the mixture in a designated environment after humidification to ensure a stable moisture content before entering the heat conditioning stage.

[0053] Furthermore, in step S3, heat conditioning is carried out under a closed conveying condition. A closed conveying condition means that the material is in a relatively enclosed conveying space during the heat conditioning process, reducing fluctuations in material temperature and moisture caused by large amounts of outside air entering. During heat conditioning, the material layer thickness is 20-80 mm, the material turning frequency is 2-15 times / min, and the material discharge temperature after heat conditioning is 65-105℃. Material layer thickness refers to the thickness formed by the accumulation or conveying of material within the heat conditioning space. Material turning frequency refers to the number of times the material is turned or disturbed per unit time.

[0054] Further, in step S4, the pressing is performed using a pressing mold with a cylindrical compression channel. The diameter of the compression channel is 6-12 mm, the length-to-diameter ratio is 4.5-9.0, and the compression ratio is 4:1-8:1. In this text, the diameter of the compression channel refers to the inner diameter of the compression particle forming channel; the length-to-diameter ratio refers to the ratio of the effective length of the compression channel to the diameter; and the compression ratio refers to the ratio between the volume of the material before entering the pressing area and the corresponding volume after pressing. The pressing mold can be a ring-shaped mold or a flat mold, as long as it has a cylindrical compression channel that can limit the outer diameter of the particles and the compression path.

[0055] Further, in step S4, the thickness of the material layer before the mixture to be pressed enters the pressing mold is 10-40 mm, the feeding rate is 20-120 kg / h, and the linear speed of the pressure roller during pressing is 0.8-3.5 m / s. The material layer thickness refers to the thickness of the material layer formed before the mixture enters the pressing area. The feeding rate refers to the mass of the mixture entering the pressing equipment per unit time. The linear speed of the pressure roller refers to the circumferential linear speed of the working surface of the pressure roller. By limiting the above parameters, the mixture to be pressed can continuously enter the pressing area.

[0056] Further, in step S5, the pressure-holding cooling is performed using axial pressure holding, radial limiting, or a combination of both. Axial pressure holding refers to maintaining pressure along the length of the particle or the pressing direction; radial limiting refers to restricting particle expansion in the radial direction. The pressure-holding cooling pressure is 75-95 MPa, the cooling medium temperature is 5-30℃, and the particle surface temperature is 10-35℃ when the pressure is released. Releasing the pressure refers to removing the external pressure or limiting condition applied to the particle after the pressure-holding cooling is completed.

[0057] Example 1:

[0058] A method for preparing high-density straw biomass pellet fuel includes the following steps:

[0059] S1. Take corn stalks as raw material, clean and remove mud, sand, metal impurities and non-straw debris, and dry the raw material to a moisture content of 10.5%, an ash content of 6.1% by mass, and an impurity content of 0.8% by mass. Cut a portion of the stalks into strips with a length of 30mm and a width of 5mm along the axial direction, retaining the outer skin, to obtain the first stalk material. Remove the outer skin from another portion of the stalks, take the core of the stalks, crush and sieve it to obtain the second stalk material with a particle size of 0.30-0.85mm.

[0060] S2. Mix the first straw material with the second straw material at a mass ratio of 20:80. Add 55% of the total mass of the second straw material in the first stage and mix for 5 minutes. Add the remaining second straw material in the second stage and mix for 8 minutes. Use a combination of atomized water and saturated steam to condition the moisture content of the mixed material to 14.0%. The material temperature during the conditioning process is 42℃. After conditioning, let it stand for 18 minutes. The relative humidity of the standing environment is 60%.

[0061] S3. Heat-condition the humidified material at 85℃ for 10 minutes. During the heat conditioning process, the material layer thickness is 50mm, the material turning frequency is 8 times / min, and the material discharge temperature is 85℃ after the heat conditioning is completed, thus obtaining the mixture to be pressed.

[0062] S4. The mixture to be pressed is fed into a pressing mold with a cylindrical compression channel. The compression channel has a diameter of 8mm, a length-to-diameter ratio of 6.5, and a compression ratio of 6:1. The thickness of the mixture before entering the pressing mold is 25mm, the feeding rate is 70kg / h, and the linear speed of the pressure roller is 2.0m / s. The mixture is pressed at 110℃ and 180MPa for 30s to obtain pressed granules.

[0063] S5. The compressed pellets are cooled by a combination of axial pressure holding and radial limiting. The pressure holding and cooling pressure is 75MPa, the cooling medium temperature is 18℃, the pressure holding and cooling time is 160s, and the surface temperature of the pellets is 25℃ when the pressure is released, thus obtaining high-density straw biomass pellet fuel.

[0064] Example 2:

[0065] A method for preparing high-density straw biomass pellet fuel is basically the same as that in Example 1, except that:

[0066] In step S2, the mass ratio of the first straw material to the second straw material is 30:70.

[0067] The remaining steps, material parameters, and process parameters are the same as in Example 1.

[0068] Example 3:

[0069] A method for preparing high-density straw biomass pellet fuel is basically the same as that in Example 1, except that:

[0070] In step S2, the mass ratio of the first straw material to the second straw material is 40:60.

[0071] The remaining steps, material parameters, and process parameters are the same as in Example 1.

[0072] Example 4:

[0073] A method for preparing high-density straw biomass pellet fuel is basically the same as that in Example 3, except that:

[0074] In step S5, the pressure holding and cooling pressure is 85 MPa.

[0075] The remaining steps, material parameters, and process parameters are the same as in Example 3.

[0076] Example 5:

[0077] A method for preparing high-density straw biomass pellet fuel is basically the same as that in Example 3, except that:

[0078] In step S5, the pressure holding and cooling pressure is 95 MPa.

[0079] The remaining steps, material parameters, and process parameters are the same as in Example 3.

[0080] Comparative Example 1:

[0081] A method for preparing straw biomass pellet fuel is basically the same as that in Example 1, except that:

[0082] In step S2, the mass ratio of the first straw material to the second straw material is 18:82.

[0083] The remaining steps, material parameters, and process parameters are the same as in Example 1.

[0084] Comparative Example 2:

[0085] A method for preparing straw biomass pellet fuel is basically the same as that in Example 1, except that:

[0086] In step S2, the mass ratio of the first straw material to the second straw material is 42:58.

[0087] The remaining steps, material parameters, and process parameters are the same as in Example 1.

[0088] Comparative Example 3:

[0089] A method for preparing straw biomass pellet fuel is basically the same as that in Example 2, except that:

[0090] In step S5, the pressure holding and cooling pressure is 74 MPa.

[0091] The remaining steps, material parameters, and process parameters are the same as in Example 2.

[0092] Comparative Example 4:

[0093] A method for preparing straw biomass pellet fuel is basically the same as that in Example 2, except that:

[0094] In step S5, the pressure holding and cooling pressure is 96 MPa.

[0095] The remaining steps, material parameters, and process parameters are the same as in Example 2.

[0096] Comparative Example 5:

[0097] A method for preparing straw biomass pellet fuel is basically the same as that in Example 4, except that:

[0098] In step S1, the first straw material with the outer skin is not prepared. The outer skin of all corn straw is removed, crushed and sieved into straw core crushed material with a particle size of 0.50 mm.

[0099] In step S2, the crushed straw core material is directly mixed and moistened to a moisture content of 14.0%.

[0100] The remaining steps, material parameters, and process parameters are the same as in Example 4.

[0101] Comparative Example 6:

[0102] A method for preparing straw biomass pellet fuel is basically the same as that in Example 4, except that:

[0103] In step S1, without distinguishing between the first and second straw materials, the corn stalks are crushed into homogeneous straw crushed material with a particle size of 0.30-0.85mm.

[0104] In step S2, corn starch is added to the homogeneous crushed straw material at a rate of 5% of the mass of the homogeneous crushed straw material, and the mixture is moistened to a moisture content of 14.0%.

[0105] In step S5, after the pressed particles leave the pressing mold, they are allowed to cool naturally in an environment of 25°C without pressure holding and cooling.

[0106] The remaining steps, material parameters, and process parameters are the same as in Example 4.

[0107] The following tests were performed on Examples 1-5 and Comparative Examples 1-6 respectively:

[0108] Particle density test: Select intact particle samples, measure particle length and diameter, calculate particle volume, weigh particle mass, and calculate particle density according to the mass-to-volume ratio. Test 10 particles for each sample group and take the average value.

[0109] Loose bulk density test: Pour the particle sample naturally into a container of known volume without external compaction. After leveling the surface of the container, weigh the sample and calculate the loose bulk density as the ratio of mass to container volume. Each group of samples is tested in parallel three times, and the average value is taken.

[0110] Compressive strength test: A single particle is placed radially in a compression testing device, and a load is applied at a constant loading rate. The maximum load at which the particle breaks is recorded and converted into compressive strength. Ten particles are tested in each sample group, and the average value is taken.

[0111] Abrasion resistance index test: A certain mass of particle sample is placed in a rotary abrasion device, processed at a specified speed and time, and then sieved. The mass of intact particles on the sieve is weighed, and the percentage of the mass of intact particles on the sieve to the mass of the sample before the test is taken as the abrasion resistance index.

[0112] Drop breakage rate test: Take a certain mass of particle sample and drop it freely from a height of 1.5m onto a hard surface. Repeat the drop 3 times, then sieve and weigh the mass of the broken fine powder. The percentage of the mass of the broken fine powder to the mass of the sample before the test is taken as the drop breakage rate.

[0113] 24h water absorption rate test: Weigh the initial mass of the dried granular sample, place it in an environment with a relative humidity of 90% for 24 hours, take it out and weigh it again. The percentage of the increase in mass relative to the initial mass is taken as the 24h water absorption rate.

[0114] Table 1. Test results for examples and comparative examples.

[0115] sample Particle density / kg / m3 Loose bulk density / kg / m3 Compressive strength / MPa Abrasion resistance index / % Drop breakage rate / % 24h water absorption rate / % Example 1 1286.4 706.2 4.72 96.8 3.2 7.4 Example 2 1378.5 752.6 5.83 98.6 1.6 5.0 Example 3 1336.7 731.8 5.31 97.9 2.1 5.8 Example 4 1392.8 761.4 5.96 98.9 1.3 4.6 Example 5 1368.2 748.5 5.67 98.4 1.7 5.2 Comparative Example 1 1198.3 659.5 3.88 94.7 5.4 9.3 Comparative Example 2 1164.9 641.7 3.51 93.2 6.5 10.4 Comparative Example 3 1216.8 667.4 4.02 95.0 5.1 9.0 Comparative Example 4 1187.6 654.3 3.76 94.1 5.8 9.8 Comparative Example 5 1096.5 606.8 2.94 89.6 9.4 13.7 Comparative Example 6 1128.2 622.5 3.18 91.0 8.1 12.5

[0116] As shown in Table 1, in Example 2, the mass ratio of the first straw material to the second straw material was 38:62. The particle density, loose bulk density, compressive strength, and abrasion resistance index were all higher than in Examples 1 and 3, while the drop breakage rate and 24-hour water absorption rate were lower than in Examples 1 and 3. In Comparative Example 1, the proportion of the first straw material was low, and the proportion of the second straw material was too high; in Comparative Example 2, the proportion of the first straw material was high, and the proportion of the second straw material was insufficient. The overall performance of both was lower than that of Examples 1-3.

[0117] In Example 4, the holding and cooling pressure was 85 MPa. Its particle density, loose bulk density, compressive strength, and abrasion resistance index were all higher than those of Examples 3 and 5, while its drop breakage rate and 24-hour water absorption rate were lower than those of Examples 3 and 5. In Comparative Example 3, the holding and cooling pressure was low, while in Comparative Example 4, the holding and cooling pressure was higher. The overall performance of the pellet fuel obtained in both examples was inferior to that of Examples 3-5.

[0118] Comparative Example 5 omitted the first straw material with the outer skin intact, using only the crushed straw core material after the outer skin was removed. Compared to Example 2, although Comparative Example 5 retained the steps of conditioning, heat conditioning, pressing, and pressure holding cooling, the raw material system no longer contained larger straw units with the outer skin intact. Therefore, it could not form a graded material system involving both the first and second straw materials. Consequently, its particle density, loose bulk density, compressive strength, and abrasion resistance index were all lower than all examples, while its drop breakage rate and 24-hour water absorption rate were higher than all examples.

[0119] Comparative Example 6 replaced the first and second straw material grading system of the present invention with a homogeneous pulverized material and added corn starch, and eliminated the pressure holding and cooling step. Compared with Example 2, although Comparative Example 6 added conventional binder materials, its raw material structure and cooling path are different from the scheme of the present invention, and the overall performance of the resulting pellet fuel is still lower than that of all examples. This result shows that simply using homogeneous pulverized material with conventional binders cannot replace the combination relationship between the first and second straw materials with the retained skin and the pressure holding and cooling in the present invention.

[0120] The results of the examples show that when the graded material system and the pressure-holding and cooling conditions are synergistically matched, the resulting pellet fuel exhibits good overall performance in terms of density, packing stability, mechanical stability, and moisture absorption control. The comparative results show that when the proportion of the first straw material is too low or too high, the pressure-holding and cooling pressure is below or above the appropriate range, the first straw material with its outer skin retained is omitted, or homogeneous pulverized material and an external binder are used to replace the scheme of this invention, the overall performance of the resulting pellet fuel decreases.

[0121] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing high-density straw biomass pellet fuel, characterized in that, Includes the following steps: S1. The straw raw material is cleaned, dried and graded to obtain the first straw material with the outer skin retained and the second straw material obtained by crushing the straw core after the outer skin is removed. The length of the first straw material is 15-45mm and the width is 2-8mm. The particle size of the second straw material is 0.15-1.20mm. S2. Mix the first straw material with the second straw material at a mass ratio of 20-40:60-80, and adjust the moisture content of the mixed material to 10%-18%. S3. Heat-condition the moisture-treated material at 65-105℃ for 3-20 minutes to obtain the mixture to be pressed. S4. Press the mixture to be pressed at 80-135℃ and 90-260MPa for 5-60s to obtain pressed granules. S5. Press the pellets and cool them under pressure at 10-35℃ for 30-300s to obtain high-density straw biomass pellet fuel.

2. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S1, the straw raw material includes corn straw, wheat straw, rice straw, rapeseed straw, cotton straw, sorghum straw or any combination thereof; the moisture content of the straw raw material before grading is 6%-14%, the ash content is 3%-12%, and the impurity content is ≤2%.

3. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S1, the first straw material is obtained by cutting along the straw axis, and the mass fraction of the outer skin retained in the first straw material is 60%-95%; the second straw material is obtained by crushing and sieving the straw core after peeling off the straw outer skin, and the mass fraction of the second straw material passing through a 0.15mm sieve is ≤10%, and the mass fraction passing through a 1.20mm sieve is ≥90%.

4. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S2, the first straw material and the second straw material are mixed in two stages. In the first stage of mixing, 40%-70% of the total mass of the second straw material is added, and the mixing time is 2-8 minutes. In the second stage of mixing, the remaining second straw material is added, and the mixing time is 3-12 minutes.

5. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S2, the humidity is adjusted using atomized water, saturated steam, or a combination of atomized water and saturated steam. The material temperature during the humidity adjustment process is 25-60℃, the settling time after humidity adjustment is 5-30 minutes, and the relative humidity of the settling environment is 45%-75%.

6. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S3, heat conditioning is carried out in a closed conveying state. During the heat conditioning process, the material layer thickness is 20-80mm, the material turning frequency is 2-15 times / min, and the material discharge temperature is 65-105℃ after heat conditioning.

7. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S4, the pressing is performed using a pressing mold with a cylindrical compression channel. The diameter of the compression channel is 6-12mm, the length-to-diameter ratio of the compression channel is 4.5-9.0, and the compression ratio is 4:1-8:

1.

8. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S4, the thickness of the material to be pressed before entering the pressing mold is 10-40mm, the feeding rate is 20-120kg / h, and the linear speed of the pressure roller during the pressing process is 0.8-3.5m / s.

9. The method for preparing high-density straw biomass pellet fuel according to claim 1, characterized in that: In step S5, the pressure holding and cooling is carried out by axial pressure holding, radial limiting, or a combination of axial pressure holding and radial limiting. The pressure holding and cooling pressure is 75-95MPa, the cooling medium temperature is 5-30℃, and the particle surface temperature is 10-35℃ when the pressure is released.

10. A high-density straw biomass pellet fuel, characterized in that: The biomass pellet fuel is prepared by any one of the preparation methods described in claims 1-9. The raw materials of the biomass pellet fuel include a first straw material and a second straw material. The first straw material is straw material with the outer skin retained, and the second straw material is straw core powder after the outer skin has been removed. The mass ratio of the first straw material to the second straw material is (20-55):(80-45).

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