Production process for preparing biomass particles by drying and extruding furfural residues at low temperature

Abstract

The invention discloses a production process for preparing biomass particles by drying and extruding furfural residues at low temperature, and belongs to the technical field of biomass briquette fuel preparation. The process comprises the following steps: providing furfural residue with the initial water content of 50-60% and a coking polymer generated in furfural production; an indirect heat exchange type roller dryer is adopted for low-temperature steam drying, the heat exchange surface temperature is made to be 90-100 DEG C, air inducing and dehumidification are matched, the moisture content of discharged furfural residues is controlled to be 15-20%, and condensate water is recycled; crushing a coking polymer to be not greater than 50 meshes, and mixing the coking polymer with the dried furfural residues according to the ratio of (2-5): (95-98); carrying out winnowing and cyclone separation grading ash removal, outputting a main material of more than 50 meshes, and removing fine powder of less than 50 meshes and entrained ash; 0.5-5wt% of a mineral functional agent is doped into the main material and is uniformly mixed. According to the process, stable water control, graded ash removal, low-temperature forming interlocking stability, mineral internal and external synergistic slagging resistance and hydrophobic moisture-proof pulverization resistance are achieved, and the quality and storage and transportation stability of the granular fuel are improved.

Description

Technical Field

[0001] This invention relates to the field of biomass briquette fuel preparation technology, specifically a production process for producing biomass pellets by low-temperature drying and extrusion of furfural residue. Background Technology

[0002] Biomass pellet fuel is typically produced from agricultural and forestry residues, industrial byproducts, or organic solid waste through processes such as drying, crushing / mixing, extrusion molding, cooling, and screening. It can be used for residential heating and industrial heating. Existing production lines mostly employ a process route that combines drum dryers with fan dehumidification, pellet mill extrusion molding, and screening of recycled materials to achieve a certain molding strength and combustibility, and to realize large-scale continuous production.

[0003] However, materials such as furfural residue, xylose residue, distiller's grains, and cow dung generally have high moisture content, fine particles, high ash content, and large fluctuations in physical properties, resulting in insufficient stability of traditional processes in engineering operation. Taking furfural residue as an example, its initial moisture content can typically reach 50-60%, and it is acidic and contains fine ash and microparticles. During the operation of furfural units, coking polymer byproducts are also generated, whose morphology, ash content, and combustion characteristics differ significantly from furfural residue. The combination of these factors makes the impact of raw material fluctuations on drying, grading, and molding processes more sensitive.

[0004] In the prior art, there are already disclosed methods for preparing pellet fuel using furfural residue. For example, patent document CN105018178A discloses a process for preparing plant pellets using furfural residue. Such methods have achieved resource utilization of furfural residue to a certain extent. However, some methods introduce substances such as potassium permanganate, waste cooking oil, and magnesium powder, or use complex proportions of multiple materials to improve molding or calorific value performance. This may bring safety risks, raw material compliance issues, and increased costs. At the same time, the proportion and fluctuation of multiple raw materials can also easily cause instability in pellet calorific value, ash content, and appearance quality. Patent document CN105482867B includes steps such as sieving / crushing, spraying water to adjust humidity (e.g., controlling the moisture content to 9-13%), composting and fermentation, molding, and natural drying to reduce moisture. This type of solution can realize the fuel utilization of furfural residue, but it relies on multiple material ratios and fermentation, natural drying and other processes, and is easily affected by raw material fluctuations and environmental conditions. The continuous and stable quality control is relatively limited, and the introduction of external components such as lime powder may also bring constraints on batching management and product composition consistency. Similarly, patent document number CN120521219A focuses on making furfural residue meet the pre-treatment requirements for co-firing, and has certain process control and energy recovery ideas, but has certain defects in the continuous and stable preparation of shaped fuel pellets and the improvement of storage and transportation performance.

[0005] There are also existing "multi-layer / coated structure" solutions for biomass pellet fuel: for example, patent document CN111019730A / B adopts a layered structure of inner core / control layer / shell layer, and introduces inorganic components such as bentonite, bone meal, fly ash, and lime powder into different layers to regulate the combustion process and exothermic stability, focusing on improving combustion stability and combustion rhythm through functional layer formulation. In addition, there are also solutions in the utility model field that reduce problems such as coking and sticking to the furnace through structural layers. For example, the composite biomass pellet disclosed in CN207862279U adopts an inner and outer layer composite structure to improve the coking / adhesion problem during use. Meanwhile, there are also structural improvement schemes for coated biomass pellets (patent documents CN207862927U, CN207862281U, etc. belong to this type of idea); although the above technologies belong to the "multi-layer / coated" direction as in this application, they are mostly biased towards the design of combustion control layer or structural composite layer, and may not cover the technical path of solving the problem of stacking adhesion and transportation wear and powdering by taking the hydrophobic shell layer as the core of water resistance and wear resistance and further introducing an outer anti-sticking wear-resistant pulverization inhibition layer.

[0006] The technical solution disclosed in patent document CN111530429A involves pre-treatment by screening, low-temperature drying and pulverization, adding conditioning agents or binders (such as chitosan or hydroxymethyl cellulose) to compress into granules, and then performing surface micro-oxygen carbonization at 250-300℃, followed by cooling / activation with steam to obtain carbonized materials. However, this solution aims to obtain biomass carbonized materials / adsorbents, requiring the installation of a carbonization device and micro-oxygen carbonization and steam activation, resulting in a long process chain, high energy consumption, and high equipment investment. Furthermore, it relies on external binders and involves pyrolysis flue gas control, which does not fully match the requirements of continuous fuel pellet production, such as stable low-temperature drying and water control, graded ash removal and reduction, stable extrusion granulation load and temperature interlocking, and the hydrophobic shell for moisture prevention and anti-pulverization, as well as the synergistic anti-caking of minerals. Therefore, it is difficult to directly solve the problem of stable preparation and improved storage and transportation performance of furfural residue fuel pellets. Similarly, patent document CN106318418A... The disclosed technical solution proposes to use the combustible gas generated during the carbonization process as a heat source after dust removal, condensation, and purification to improve heat utilization efficiency. It focuses on the biomass charcoal fuel preparation route, which is significantly different from the issues of drying and water control, graded ash removal, extrusion granulation, and improvement of pellet storage and transportation performance in the production of fuel pellets made from furfural residue.

[0007] The applicant's prior patent application, CN119701874A, disclosed a method for preparing biochar for adsorption and other applications by drying and granulating furfural residue, followed by low-temperature carbonization, and then combining this with acid / alkali activation and high-temperature heat treatment. Its disadvantages include a long process chain, high equipment and energy consumption, the introduction of acid and alkali agents, and the generation of washing wastewater, posing safety and environmental disposal challenges. Furthermore, it did not provide corresponding solutions to the issues of low-temperature drying for stable water control, graded ash removal and reduction, mineral agent inhibition of slagging, and hydrophobic shell for moisture prevention and anti-pulverization in the continuous production of fuel pellets, which are of concern in this application. Patent publication CN114276199A discloses a method that... The process involves grinding and screening furfural residue, washing and fermenting it, adding aluminum silicate, potassium chloride, calcium sulfate, wood ash and fermentation agents, drying and granulating it, and then carbonizing it once at 300-450℃ and twice at 400-550℃ to obtain biochar products. This method overlaps with the present application in that both involve furfural residue as a raw material source and include drying and granulation processes. However, its technical goal and product positioning are biochar / soil improvement, which requires high-temperature carbonization and the introduction of various inorganic salts and fermentation systems. The process route is completely different from the route of the present application, which focuses on the incorporation of mineral functional agents into fuel biomass pellets and the core-shell coating on the surface to prevent moisture, pulverization and slagging.

[0008] In summary, existing processes still have shortcomings in key process control and quality stability: Regarding the problem of "ash fluctuation and fine powder entrainment", many processes have failed to establish a stable and effective graded ash removal mechanism before granulation. After fine particles and entrained ash enter the granulation section, they will cause abnormal molding friction, increased die wear or blockage risk, and lead to increased ash content and combustion slagging tendency in the product, which in turn affects the terminal grate operation and emission stability. In the storage, transportation and use of pellets, traditional biomass pellets often have problems of moisture absorption and surface wear and pulverization, especially when the proportion of fine powder in the raw materials is high or the surface structure of the pellets is loose. In addition, although there are ideas in existing technologies to use mineral additives to improve ash melting characteristics in order to address the combustion slagging problem, most of them only stay at "internal addition" or "single treatment", lacking the coordinated design of mineral agent particle size, application method and surface structure, making it difficult to simultaneously meet the multiple objectives of molding strength, anti-slagging and moisture-proof and anti-pulverization.

[0009] In the prior art, there are solutions to improve the moisture resistance and durability of biomass pellet fuel by introducing a wax system during the pelleting and batching stage. For example, US Patent Document No. 20190031972A1 discloses that a wax composition is incorporated into a wood biomass mixture to prepare pellets, thereby improving pellet durability, reducing the tendency to absorb moisture during storage, and potentially reducing pelleting energy consumption. This type of technical route focuses on "inserting wax into the mixture / optimizing the wax formulation", which differs from the structured coating approach of "spraying a hydrophobic shell layer on the outer surface after molding and working in conjunction with a mineral protective layer" that is emphasized in this application. Patent document EP3476921A1 discloses a method in which dried waste particles are first coated with waste oil and then mixed with sawdust, coal and catalyst. A coating composition containing asphalt emulsion, coal powder and red clay is then applied to the surface of the particles, which are then extruded and cut to obtain particles. This type of scheme reflects a similar technical idea of ​​"coating / secondary coating", but its coating system and target use are more inclined to construct the extruded raw material with oil / asphalt coating formulation, which is different from the moisture-proof and wear-resistant structured coating design of biomass particles in this application.

[0010] Therefore, there is still an urgent need for a biomass pellet production process for fine-grained materials with high moisture and high ash content, such as furfural residue, to achieve stable and controllable drying and molding, effective grading and ash removal, improved moisture resistance, anti-pulverization and anti-slagging performance, and to take into account waste heat utilization and environmental compliance. Summary of the Invention

[0011] The technical problem this invention aims to solve is to overcome existing defects and provide a production process for low-temperature drying and extrusion of furfural residue into biomass pellets. Addressing issues such as high and fluctuating initial moisture content of furfural residue, unstable molding due to entrained fine powder and ash, easy mold blockage and large temperature fluctuations during pelleting, easy moisture absorption and pulverization of finished pellets during transportation, easy slagging at the combustion end, and high energy consumption and environmental pressure, this invention achieves continuous and stable production by employing low-temperature indirect steam drying with online closed-loop moisture control, pre-granulation grading and ash removal, torque / temperature interlocking for stable control during pelleting, internal incorporation of mineral functional agents for synergistic surface layer formation, and surface functionalization coating and curing to form a core-mineral layer-hydrophobic layer, combined with wind-coupled waste heat utilization. This results in moisture-proof, pulverization-resistant, slagging-resistant, and high-quality biomass pellet fuel, effectively solving the problems in the background technology.

[0012] To achieve the above objectives, the present invention provides the following technical solution: a production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion, comprising the following steps:

[0013] S1) Raw material preparation: Provide furfural residue with an initial moisture content of 50-60%; provide coking polymers generated during the furfural production process;

[0014] S2) Low-temperature indirect steam drying: Saturated steam is introduced into an indirect heat exchange rotary drum dryer to preheat its heat exchange components, raising the temperature of the heat exchange surface in contact with the material to 90–100°C. Then, furfural residue is fed into the inner drum of the rotary drum dryer. The rotary drum dryer is an indirect heat exchange structure with an inner and outer drum. The outer drum is made of carbon steel, and the inner drum is made of acid-resistant steel. The furfural residue does not directly contact the steam. The inner drum diameter is φ1200mm, the outer drum diameter is φ1800mm, and the drum length is 18–20m. Moisture evaporation is achieved through heat exchange between steam and the heat exchange components, and between the heat exchange components and the furfural residue. Simultaneously, a blower extracts humid gas from the drum to enhance convective heat exchange, controlling the moisture content of the discharged furfural residue to 15–20%.

[0015] In step S2, the condensate after steam heat exchange is recovered and reused in the production system;

[0016] S3) Coking polymer pretreatment: The coking polymer is crushed to obtain a pulverized material with a particle size of no more than 50 mesh;

[0017] S4) Mixing: The dried furfural residue obtained in step S2 and the pulverized material obtained in step S3 are mixed evenly at a mass ratio of coking polymer: furfural residue of 2-5: 95-98.

[0018] S5) The mixture is then air-classified and sent to a cyclone separator for classification. The particle size of the main material after classification is controlled to be above 50 mesh, and fine particles smaller than 50 mesh and entrained ash are separated and removed to obtain the main material for granulation.

[0019] S6) Adding mineral functional agents: Add mineral functional agents to the main material obtained in step S5 and mix evenly. The mineral functional agents are at least one of silica-alumina minerals, carbonate minerals and zeolites. The amount of mineral functional agents added is 0.5 to 5 wt% based on the mass of the main material. Furthermore, the added mineral functional agents have a first particle size grade of ≤100 mesh.

[0020] The particle size of the mineral functional agent added in step S6 is no greater than 100 mesh. The mineral agent added in step S6 participates in the internal skeleton of the particles and the regulation of the combustion ash phase, reducing the tendency of ash melting. At the same time, it also affects the forming friction and particle hardness.

[0021] S7) Extrusion granulation: The material obtained in step S6 is fed into an extrusion granulator and extruded through a die to obtain biomass pellets. The temperature of the extruded pellets is controlled to be no higher than 80°C.

[0022] The motor current and / or torque of the extrusion granulator are monitored online and the monitored values ​​are maintained within a preset operating window. At the same time, the temperature of the extruded granules is monitored. When the motor current and / or torque deviate from the operating window and / or the temperature of the extruded granules approaches or reaches 80°C, the motor current and / or torque are adjusted back to the operating window and the temperature of the extruded granules is kept below 80°C.

[0023] In step S7, the extrusion granulator is preheated and residual material in the mold is cleaned before feeding; the mold hole diameter is 5mm and the cutting length is 15-20mm.

[0024] S8) Surface functional coating and curing drying: After step S7 and before screening, the biomass particles are coated with a double-layer surface functional layer and cured and dried under the condition that the surface temperature of the extruded biomass particles is 40-80℃. The mineral protective layer is coated first and then the hydrophobic shell layer is coated to form a core-mineral layer-hydrophobic layer structure particle, wherein the mineral layer is located inside the hydrophobic layer.

[0025] Mineral protective coating: When the surface temperature of the particles is 40-70℃, the mineral functional agent is applied to the surface of the biomass particles in the form of a slurry or powder, and the amount of the mineral functional agent applied is 0.2-2wt% (based on solids) based on the mass of the biomass particles. The particles are then dried and fixed under the action of airflow in the drying section to form a mineral protective layer.

[0026] Hydrophobic shell coating: When the surface temperature of the particles is 50-80℃, the hydrophobic coating material is applied to the surface of the biomass particles by melt atomization spraying. The hydrophobic coating material is at least one of natural wax, plant wax, and bio-based hydrophobic agent, and the amount of hydrophobic coating material applied is 0.2-2.0 wt% based on the mass of the biomass particles. After spraying, it is cured into a film under the action of airflow in the drying section to form a hydrophobic shell.

[0027] In step S8, the mineral functional agent used to form the mineral protective layer is the same mineral functional agent as the mineral functional agent added in step S6, and the surface mineral functional agent in step S8 is of the second particle size grade, which is finer than the first particle size grade; and the second particle size grade is 200-500 mesh micro powder.

[0028] Furthermore, the hydrophobic coating material is a compound system, and when the hydrophobic coating material contains tal oil, the mass ratio of the wax component to the tal oil is 9:1 to 6:4.

[0029] Single wax films are prone to "brittleness," which is more pronounced under transportation impact / low temperature conditions, leading to powdering and localized failure; wax plus tal oil or rosin derivatives can improve film toughness and adhesion; a small amount of lignin modifiers can enhance polar interface interactions and improve adhesion to the mineral layer.

[0030] Meanwhile, the air supplied for the induced draft in step S2 is first exchanged with the particles through the pallet / tray before entering the drum dryer, so as to achieve particle cooling and induced draft preheating.

[0031] In addition, the particle size of the mineral functional agent used to form the mineral protective layer in step S8 is 200-500 mesh, and the hydrophobic coating material in step S8 is a compound system, which includes wax components, lignin-based hydrophobic modifiers and / or rosin derivatives, wherein the total amount of lignin-based hydrophobic modifiers and / or rosin derivatives added is 0.5-10 wt% based on the total mass of the hydrophobic coating material.

[0032] The double-layer core-shell sequential temperature window; 40-70℃ mineral layer; 50-80℃ hydrophobic layer; enables the mineral layer (micro powder / thin paste) to more easily form a continuous, rough, and anchorable base layer on the particle surface at 40-70℃; subsequently, when the hydrophobic material is melt-atomized and sprayed at 50-80℃, the hydrophobic material can form a stronger mechanical interlocking effect and a more stable film adhesion on the rough mineral surface.

[0033] The mineral layer should be kept at 40–70°C to avoid poor adhesion of the thin paste due to excessively low temperatures, or instantaneous vaporization of moisture due to excessively high temperatures, resulting in "holes / detachment".

[0034] The hydrophobic layer is kept at 50–80°C to ensure that the wax / bio-based hydrophobic agent is in a low-viscosity flow zone, which is conducive to atomization and film formation. At the same time, it does not exceed the preset safe upper limit temperature to avoid excessive softening and adhesion.

[0035] Through the double-core process design, the mineral layer provides the function of "isolation / dilution / increasing ash melting temperature" in the early stage of combustion, inhibiting ash adhesion and coking; the hydrophobic layer reduces moisture absorption and moisture regain, and reduces powder shedding during transportation friction; moreover, the interlayer bonding force is stronger and the shell layer is more uniform: reducing "local delamination / cracking", improving storage and transportation stability and appearance consistency.

[0036] Internal and external synergy: It improves the combustion ash phase (internal) and inhibits slagging (surface) without significantly sacrificing molding strength.

[0037] After the hydrophobic shell layer is melt-atomized and sprayed, while the hydrophobic shell layer is in a slightly viscous state, an anti-adhesion and wear-resistant powder is applied to the surface of the extruded biomass pellets to form an outer anti-adhesion and wear-resistant layer. The anti-adhesion and wear-resistant powder is selected from one or more of talc, ultrafine calcium carbonate, silica-alumina mineral powder, and finer-grained mineral functional agents. The particle size of the anti-adhesion and wear-resistant powder is 500-1200 mesh, and the application amount is 0.05-0.8 wt% based on the biomass pellet mass. After applying the anti-adhesion and wear-resistant powder, the pellets undergo a secondary reflow and embedding treatment, causing slight softening and reflow on the surface of the hydrophobic shell layer and semi-embedding and anchoring the anti-adhesion and wear-resistant powder. Subsequently, the pellets are cooled to below 40°C for shaping, thereby forming a multi-layer structure of core-mineral protective layer-hydrophobic shell layer-anti-adhesion and wear-resistant layer in the resulting biomass pellet fuel.

[0038] S9) Screening and recycling: Screen the biomass pellets dried in step S8. The whole pellets on the screen are used as finished products, and the material under the screen is returned to step S7 for re-granulation.

[0039] In step S9, the sieve is 3mm × 3mm.

[0040] S10) Waste gas treatment: The humid waste gas extracted in step S2 is discharged after spraying, alkali neutralization and gas-water separation, and the dust from granulation, coating, screening and packaging is collected under negative pressure.

[0041] Furthermore, in step S2, the discharge moisture content of the drum dryer is detected online, and the discharge moisture content is set to 15-20%. Based on the deviation between the detected discharge moisture content and the target moisture content, at least two of the following are adjusted in a closed loop: steam supply, drum speed, induced draft, and feed rate, so that the discharge moisture content is controlled at 15-20%.

[0042] Because furfural residue has a high initial moisture content (50-60%) and fluctuates significantly with incoming material, it is difficult to maintain a stable output moisture content of 15-20% over a long period if only a fixed steam supply or fixed induced draft volume is used for drying control. Deviations in output moisture content from the target range directly affect the stability of subsequent processes: over-drying increases system energy consumption, increases material brittleness and easily produces fine powder, leading to poor grading and ash removal effects and granulation stability; under-drying easily causes die blockage and particle strength fluctuations during granulation, making subsequent surface coating and film-forming processes unstable. Therefore, this invention introduces online closed-loop control with output moisture content as the target amount in the low-temperature indirect steam drying step, transforming the drying section from an experience-based operation into a quantifiable, repeatable, and controllable process.

[0043] The closed-loop control of this invention uses the moisture content of the drum dryer discharge as the controlled variable (output) and a target control range of 15-20%. To achieve stable control of the discharge moisture content, an online moisture detection device is installed to detect the discharge moisture content in real time. After the detection signal is input into the controller, it is compared with the set value, and at least one or more controllable quantities are adjusted according to the deviation. The controllable quantities include at least: steam valve opening / steam flow rate, drum rotation speed (used to adjust the material residence time), induced draft fan air volume (used to enhance convective heat transfer and remove moisture in time), and feed rate (used to adjust the drying load). Through the above closed-loop control, the discharge moisture content can be stably fluctuated within the target range, and a stable material state basis is provided for subsequent grading, feeding, granulation and coating processes.

[0044] By adopting online closed-loop moisture control, the moisture content of the drum dryer output is stable, making the separation state of the grading and ash removal process more stable, which in turn makes the extrusion temperature and load (current / torque) of the granulation process fluctuate less. At the same time, it can significantly reduce the frequency of mold blockage and shutdown for mold cleaning caused by under-drying, reduce pulverization and ash enrichment caused by over-drying, improve the consistency and stability of finished granules, and reduce the overall energy consumption of the system.

[0045] Furthermore, the saturated steam temperature in step S2 is 150°C, and the preheating time is 20–30 min.

[0046] Furthermore, in step S7, the material level in the pellet mill hopper is controlled to be 1 / 2 to 2 / 3 of the effective volume of the hopper, and the operating load of the pellet mill is controlled to be 60 to 70% of the nameplate capacity.

[0047] Furthermore, after step S8 curing and air drying, the particle temperature drops below 40°C before packaging.

[0048] Furthermore, the silica-alumina minerals include kaolin and / or bentonite, and the carbonate minerals include dolomite and / or limestone powder.

[0049] Furthermore, in step S6, the amount of mineral functional agent added is 1-3 wt% based on the mass of the main material, and the mineral functional agent includes zeolite.

[0050] Furthermore, in step S8, a mineral protective layer is first applied, followed by a hydrophobic shell layer, thereby forming a core-mineral layer-hydrophobic layer granular structure.

[0051] Furthermore, a production system for low-temperature drying and extrusion of furfural residue to produce biomass pellets includes: a drum drying unit, a coking polymer pulverizing unit, a mixing unit, a grading and ash removal unit, a mineral functional agent feeding unit and a mixing and homogenization unit, an extrusion granulation unit, and a surface coating and curing air-drying unit; the drum drying unit is an indirect heat exchange drum dryer, equipped with steam preheating pipelines and condensate recovery pipelines; the drum drying unit is connected to an induced draft unit for extracting humid gas within the drum drying unit; the coking polymer pulverizing unit is used to pulverize the coking polymer generated during the furfural production process; the mixing unit is connected to the discharge end of the drum drying unit and the coking polymer pulverizing unit, and is used to mix the dried furfural residue with the pulverized coking polymer; the grading and ash removal unit includes an air classifier and a cyclone separator, and is connected to the mixing unit. The unit discharge end is connected to the material supply. The grading and ash removal unit is set to output granulation material with a particle size of 50 mesh or larger, and to separate and remove fine particles smaller than 50 mesh and entrained ash. The mineral functional agent feeding unit and the mixing and homogenizing unit are connected to the granulation material output from the grading and ash removal unit, and are used to add mineral functional agents to the granulation material and mix them evenly. The extrusion molding granulation unit is connected to the material supply at the discharge end of the mixing and homogenizing unit, and is used to extrude and mold biomass pellets. The surface coating and curing drying unit includes a pallet / tray conveying structure and a spraying device. The spraying device is used to apply a mineral protective layer coating and / or a hydrophobic shell coating to the biomass pellets, and to cure them under the action of the drying airflow. The screening unit is used to screen the biomass pellets, and the return material conveying unit is used to return the undersize material to the extrusion molding granulation unit.

[0052] Furthermore, it also includes an exhaust gas treatment unit, which is connected to the exhaust gas outlet of the drum drying unit. The exhaust gas treatment unit includes a spray section, an alkali neutralization section, and a gas-water separation section; a dust negative pressure collection unit, which covers at least the granulation, coating, screening, and packaging stations; wherein, the air intake path of the induced draft unit is arranged such that it first passes through the surface coating and curing drying unit to exchange heat with the particles before entering the drum drying unit, so as to realize the utilization of waste heat through air path coupling.

[0053] Furthermore, a biomass pellet fuel is characterized in that the biomass pellet fuel comprises a biomass matrix, an internally incorporated mineral functional agent, and a surface functional layer; wherein, the biomass matrix is ​​obtained from furfural residue and coking polymer produced during the furfural production process, and the mass ratio of coking polymer to furfural residue is 2-5:95-98; the internally incorporated mineral functional agent is at least one selected from silica-alumina minerals, carbonate minerals, and zeolites, and the mass fraction of the internally incorporated mineral functional agent in the biomass pellet fuel is 0.5-5 wt%; the surface functional layer comprises... The surface functional layer comprises a mineral protective layer and / or a hydrophobic shell, wherein the content of the mineral protective layer is 0.2–2 wt% (based on solids) and / or the content of the hydrophobic shell is 0.2–2.0 wt% (based on particle mass); when the surface functional layer includes both a mineral protective layer and a hydrophobic shell, the surface functional layer has a core-mineral layer-hydrophobic layer structure with the mineral layer located inside the hydrophobic layer; the biomass pellet fuel has a particle diameter of 5 mm, a particle length of 15–20 mm, an ash content of less than 8%, and a lower heating value of greater than 3000 kcal / kg.

[0054] Compared with the prior art, the beneficial effects of the present invention are:

[0055] 1. This invention uses an indirect heat exchange drum dryer for low-temperature steam drying, so that furfural residue does not come into direct contact with steam and the temperature of the heat exchange surface is controlled at 90-100℃. Combined with induced draft to enhance convective heat exchange, the moisture content of the furfural residue is stably controlled at 15-20%, thereby avoiding embrittlement and pulverization and increased energy consumption caused by over-drying, and also avoiding subsequent granulation blockage and strength fluctuations caused by under-drying, significantly improving the stability and controllability of high moisture content furfural residue treatment.

[0056] 2. This invention sets up an air classifier and cyclone separator for grading and ash removal before granulation, outputting granulation material of 50 mesh or higher and separating and removing fine particles smaller than 50 mesh and entrained ash. This can effectively reduce the impact of fine powder and ash fluctuations on granulation friction, die wear and blockage risk, reduce the tendency of ash entrainment and slagging at the combustion end of the finished product, thereby improving the appearance, quality stability and combustion adaptability of the granules.

[0057] 3. This invention achieves a composite function of anti-caking and anti-dust powdering through the synergistic combination of "internal admixture + surface layer" of mineral functional agents and a double-layer core-shell structure design: 0.5-5 wt% of silica-alumina minerals / carbonate minerals / zeolite are added to the main material to improve the combustion ash phase characteristics. At the same time, a 0.2-2 wt% mineral protective layer is first formed on the particle surface at a temperature range of 40-70℃. Then, the hydrophobic material is melt-atomized and sprayed at 50-80℃ and solidified into a film to form a hydrophobic shell layer, resulting in a core-mineral layer-hydrophobic layer structure. The surface mineral uses a finer particle size (e.g., 200-500 mesh) to form a continuous protective layer. The hydrophobic layer is made of waxes and talc / rosin derivatives and can be modified with lignin-based hydrophobic modifiers to improve film toughness and adhesion, thereby reducing moisture regain and clumping and transport powdering, reducing delamination cracking, and improving storage and transportation stability.

[0058] 4. Furthermore, this invention couples the particle air-drying and cooling section with the drum drying induced draft air, allowing the supplied air to exchange heat with the hot particles before entering the dryer. This achieves synergy between rapid and uniform particle cooling and air drying and induced draft preheating, improving waste heat utilization efficiency and reducing energy consumption. Combined with the closed-loop screening and recycling process, it improves the yield and particle size consistency. Moreover, through drying exhaust gas spraying—alkali neutralization—gas-water separation and dust negative pressure collection, it achieves environmentally compliant operation, thus possessing significant resource utilization, economic benefits, and environmental advantages. Attached Figure Description

[0059] Figure 1 This is a process flow diagram of the present invention;

[0060] Figure 2 This is a system block diagram of the present invention. Detailed Implementation

[0061] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0062] Example 1

[0063] Please see Figure 1-2 The present invention provides a technical solution: This embodiment provides a production process for producing biomass pellets by low-temperature drying and extrusion of furfural residue, which specifically includes the following steps;

[0064] S1 Raw material preparation, providing furfural residue, the initial moisture content of furfural residue is 50-60%; furfural residue can come from the solid-liquid separation slag discharge section of conventional furfural production unit, the material is usually acidic and contains certain fine particles and ash.

[0065] It also provides coking polymers generated during the furfural production process, which can be derived from coking by-products cleaned from reaction, evaporation, or heat exchange sites.

[0066] To ensure production stability, sampling tests can be conducted when raw materials enter the factory or warehouse, including at least moisture content, particle size distribution, and ash content, to guide the parameter settings of the drying and grading sections.

[0067] S2 Low-Temperature Indirect Steam Drying (Indirect Heat Exchange Rotary Drying) involves introducing saturated steam into the indirect heat exchange rotary dryer to preheat the heat exchange components, raising the temperature of the heat exchange surfaces in contact with the material to 90–100°C. The furfural residue is then fed into the inner cylinder of the rotary dryer for drying. The rotary dryer employs an indirect heat exchange structure with an inner and outer cylinder. The outer cylinder is made of carbon steel, and the inner cylinder is made of acid-resistant steel. This prevents the furfural residue from directly contacting the steam, reducing the risk of corrosion from acidic materials on the steam side and the outer shell, and avoiding secondary wetting caused by direct steam contact.

[0068] In this embodiment, the inner cylinder diameter of the drum dryer is φ1200mm, the outer cylinder diameter is φ1800mm, and the drum length is 18-20m. Moisture evaporation is achieved through heat exchange between steam and heat exchange components, and heat exchange between the heat exchange components and furfural residue. At the same time, a blower extracts the humid gas inside the drum to enhance convective heat exchange and remove moisture in a timely manner, so that the moisture content of the furfural residue output is controlled at 15-20%.

[0069] The condensate after steam heat exchange is recovered and reused in the production system to reduce steam water consumption and emissions.

[0070] Furthermore, in step S2, an online detection device for the discharge moisture content can be set up, with a discharge moisture content of 15-20% as the control target. Based on the deviation between the detected value and the target range, at least two of the following should be adjusted in a closed loop: steam supply, drum speed, induced draft volume, and feed volume, so that the discharge moisture content is stabilized at 15-20%. This closed-loop control is used to deal with the fluctuation of furfural residue moisture content in the range of 50-60%, to avoid under-drying leading to subsequent mold blockage and unstable coating film formation, and also to avoid over-drying leading to embrittlement, pulverization, and increased energy consumption.

[0071] In a preferred embodiment, the saturated steam temperature in step S2 is 150°C and the preheating time is 20-30 minutes, so that the heat exchange surface enters a stable hot state before feeding, thereby reducing the fluctuation of moisture content in the initial stage of feeding.

[0072] As one implementation method, the furfural residue is acid-controlled before the drying step: an alkaline control solution is sprayed or atomized onto the furfural residue and mixed evenly to adjust the pH of the furfural residue to 5.0-7.5; the alkaline control solution is at least one of sodium carbonate solution, calcium hydroxide slurry, ammonia water, and circulating liquid from the alkaline washing / alkaline mist neutralization section of the waste gas treatment unit, and the amount of alkaline control solution added is 0.2-2.0 wt% based on the mass of wet furfural residue (based on the effective alkaline component); preferably, the amount of alkaline control solution added is adjusted by online or sampling pH detection, and solid-liquid separation is performed on the circulating liquid before reuse to remove suspended solids.

[0073] Since furfural residue (or similar biomass residue containing wood fibers) is usually acidic and its acidity fluctuates greatly, the acidic components may cause equipment corrosion, release of irritating odors, and risk of extrusion friction heating and coking during drying and extrusion. At the same time, excessive acidity will weaken the interfacial interaction between mineral functional agents and the matrix surface, which is not conducive to the wetting, fixation and wear resistance of the subsequent mineral protective layer. Therefore, before entering the drying process, the raw materials are acidified to reduce the impact of acidity fluctuations on subsequent processes and product performance.

[0074] Specifically, the alkaline conditioning solution is sprayed into a mixing and homogenizing device (such as a ribbon mixer, a twin-shaft paddle mixer, or a silo with agitation) through an atomizing nozzle, and simultaneously stirred to ensure uniform distribution of the alkaline conditioning solution on the surface of the furfural residue particles. The preferred amount of alkaline conditioning solution added is 0.2–2.0 wt% based on the mass of the wet furfural residue (based on the effective alkaline component), more preferably 0.3–1.5 wt%. The preferred mixing time is 1–10 min, more preferably 2–6 min. The standard is to ensure that the sampled pH is stable and there is no obvious local over-alkalinity / over-acidity agglomeration; the treatment temperature can be in the range of room temperature to 60℃, without limitation; furthermore, online pH detection or sampling detection can be set as feedback, with the pH of the mixed and homogenized raw material as the control variable, and the target pH is set to 5.0 to 7.5 (preferably 5.5 to 7.0); when the detected pH is lower than the target lower limit, the amount of alkaline control solution injected is increased and / or its effective alkali concentration is increased; when the detected pH is higher than the target upper limit, the injection amount is reduced or switched to water mist / low alkalinity circulating liquid to correct, thereby forming a closed-loop or semi-closed-loop dosing control.

[0075] Acidic regulation treatment can reduce the acid load and irritating odor release in the drying section exhaust gas and mitigate corrosion risks; it can suppress abnormal frictional heating and coking tendency in the extrusion section caused by acidity, improving continuous operation stability; at the same time, it can stabilize the interfacial interaction between mineral functional agents and the matrix surface, thereby improving the wettability and adhesion continuity of the subsequent mineral protective layer, and thus improving the wear resistance, anti-pulverization performance and stockpiling stability of the particles; when using alkaline washing / alkaline mist neutralization liquid reuse for waste gas treatment, it can also realize the recycling of reagents and water resources, reduce operating costs and reduce wastewater discharge.

[0076] S3 coking polymer pretreatment involves crushing the coking polymer to obtain a pulverized material with a particle size of no more than 50 mesh. The purpose of crushing is to reduce the particle size dispersion of the coking polymer so that it can be fully and evenly mixed with the dried furfural residue, avoiding stress concentration or die blockage caused by large particles in the extrusion section.

[0077] The crushing method can be hammer mill, shear mill, or other equivalent crushing equipment, as long as the particle size does not exceed 50 mesh.

[0078] S4 Mixing: The dried furfural residue obtained in step S2 and the pulverized material obtained in step S3 are mixed evenly at a mass ratio of coking polymer: furfural residue of 2-5:95-98 to obtain a mixture to be graded. This ratio is used to balance combustibility, molding stability and product composition consistency, and to avoid introducing too many external components that may cause fluctuations in calorific value and ash content.

[0079] Mixing can be achieved using a continuous mixer or a batch mixer, as long as the mixing uniformity meets the requirements for subsequent grading and granulation stability.

[0080] S5 Air classification + cyclone separation and grading (grading before granulation): The mixture obtained in step S4 is air classified and sent to a cyclone separator for grading. The particle size of the main material after grading is controlled to be above 50 mesh, and fine particles smaller than 50 mesh and entrained ash are separated and removed to obtain the main material for granulation.

[0081] In the above scheme, graded ash removal is used to reduce the proportion of fine powder and the fluctuation of entrained ash before granulation, reduce the risk of high friction, abnormal temperature rise, die wear or blockage caused by fine powder entering the extrusion section, and reduce the ash content of finished product and the tendency of slagging at the combustion end.

[0082] In a preferred control method, the material level in the pellet mill hopper is controlled to be 1 / 2 to 2 / 3 of the effective volume of the hopper, and the operating load of the pellet mill is controlled to be 60 to 70% of the nameplate capacity, so as to reserve operating margin for raw material fluctuations and improve the stability of the extrusion section.

[0083] In another preferred control method, the air classification and cyclone separation grading and ash removal described in step S5 can be achieved using a combination of negative pressure pneumatic conveying and cyclone separation. The key adjustable parameters of the grading and ash removal unit include at least: induced draft fan airflow / negative pressure (or frequency converter), feed rate (or airlock feeder speed), material gas-solid ratio, and the opening of the cyclone separator inlet valve and the ash return (fine powder) discharge valve. Through the coupled adjustment of the above parameters, the grading and cutting particle size can be stably controlled at around 50 mesh, ensuring that the output main material meets the particle size requirement of "above 50 mesh," and stably separating and removing fine particles smaller than 50 mesh and entrained ash.

[0084] The online / offline screening results (pass rate above 50 mesh) of the graded main material and the collection amount of fine powder (cyclone discharge / ash hopper) are used as feedback parameters: when an increase in the proportion of fine powder smaller than 50 mesh in the main material or an upward trend in the ash content of the finished product is detected, the induced draft volume or system negative pressure is increased, and / or the feed rate is reduced to improve the separation intensity; when a large entrainment loss of the main material is detected (i.e., main material above 50 mesh is carried away with the fine powder) or an abnormal increase in the collection amount of fine powder is detected, the induced draft volume or system negative pressure is reduced, and / or the feed rate is increased to reduce the separation intensity. Through the above closed-loop or semi-closed-loop adjustment, the grading and ash removal process becomes adaptable to fluctuations in the moisture content of the incoming material, the proportion of fine powder, and the ash content, thereby providing a more stable raw material state basis for the load / temperature rise interlock stability of the subsequent granulation section.

[0085] S6 Mineral functional agent addition and mixing homogenization (internal doping): Mineral functional agent is added to the main material obtained in step S5 and mixed evenly; the mineral functional agent is at least one of silica-alumina minerals, carbonate minerals and zeolite, and the amount added is 0.5 to 5 wt% based on the mass of the main material, and the added mineral functional agent has a first particle size grade ≤100 mesh.

[0086] The particle size of the internally added mineral functional agent is no larger than 100 mesh. After entering the interior of the particles, the internally added mineral functional agent participates in the regulation of the internal skeleton and friction state of the particles, affecting the forming friction, particle hardness and density. On the other hand, it participates in ash phase regulation during combustion, reducing the tendency of ash melting and reducing the risk of slagging.

[0087] In a preferred embodiment, the amount of mineral functional agent added in step S6 is 1 to 3 wt% based on the mass of the main material, and the mineral functional agent includes zeolite, in order to achieve a better balance between ash phase control and calorific value dilution.

[0088] In one alternative mineral system, the silica-alumina minerals include kaolin and / or bentonite, and the carbonate minerals include dolomite and / or limestone powder.

[0089] S7 Extrusion Granulation (Load / Temperature Interlock Stable Control): The material obtained in step S6 is fed into the extrusion granulator and extruded through the die to obtain biomass pellets. The temperature of the extruded pellets is controlled not to exceed 80°C. The extrusion granulator is preheated and the residual material in the die is cleaned before feeding to avoid carbonization or agglomeration of the residual material, which may affect the smooth flow of the die.

[0090] In this embodiment, the die hole diameter is 5mm and the cutting length is 15-20mm to obtain a particle shape with consistent particle size and length.

[0091] The motor current and / or torque of the extrusion granulator are monitored online and maintained within a preset operating window. The temperature of the extruded granules is also monitored. When the motor current and / or torque deviate from the operating window and / or the temperature of the extruded granules approaches or reaches 80°C, the motor current and / or torque are adjusted back to the operating window by at least adjusting the feeding rate and / or the amount of undersize material returned, and the temperature of the extruded granules is kept below 80°C. The essence of this interlocking adjustment is to dynamically constrain the material load and fine powder ratio entering the die per unit time to suppress the accumulation of frictional heat and the risk of die blockage.

[0092] S8 Surface Functional Coating and Curing (Double Core-Shell Structure, Surface Synergy): After step S7 and before screening, the biomass particles are coated with a double surface functional layer and cured and dried under the condition that the surface temperature of the extruded biomass particles is 40-80℃. First, a mineral protective layer is coated, and then a hydrophobic shell layer is coated to form a core-mineral layer-hydrophobic layer structure particle, wherein the mineral layer is located inside the hydrophobic layer.

[0093] 1) Mineral protective coating: When the surface temperature of the particles is 40-70℃, the mineral functional agent is applied to the surface of the biomass particles in the form of a slurry or powder, and the amount of mineral functional agent applied is 0.2-2wt% (based on solids) based on the mass of the biomass particles. The particles are then dried and fixed under the action of airflow in the drying section to form a mineral protective layer.

[0094] 2) Hydrophobic shell coating: When the surface temperature of the particles is 50-80℃, the hydrophobic coating material is applied to the surface of the biomass particles by melt atomization spraying. The hydrophobic coating material is at least one of natural wax, plant wax, and bio-based hydrophobic agent. The amount applied is 0.2-2.0 wt% based on the mass of the biomass particles. After spraying, it is cured into a film under the action of airflow in the drying section to form a hydrophobic shell.

[0095] In the above scheme, the mineral functional agent used to form the mineral protective layer is the same mineral functional agent as the mineral functional agent added in step S6, so as to ensure the consistency between the gray phase regulation inside the particles and the material of the protective layer on the particle surface; the surface mineral functional agent in step S8 is of the second particle size grade, and the second particle size grade is finer than the first particle size grade. The second particle size grade is 200-500 mesh micro powder, so as to facilitate the formation of a more continuous and uniform surface mineral layer.

[0096] In a preferred hydrophobic system, the hydrophobic coating material is a compound system. When the hydrophobic coating material contains talc oil, the mass ratio of the wax component to the talc oil is 9:1 to 6:4. In addition to the wax component, the hydrophobic coating material also contains lignin-based hydrophobic modifiers and / or rosin derivatives. The total amount of lignin-based hydrophobic modifiers and / or rosin derivatives added is 0.5 to 10 wt% based on the total mass of the hydrophobic coating material. This is used to improve the toughness and interfacial adhesion of the hydrophobic film and reduce the risk of powdering and local failure caused by film brittleness due to transportation impact and low temperature environment.

[0097] In the above embodiments, the double-layer coating is carried out in the order of temperature zone windows: a mineral base layer is formed in the 40-70℃ stage to avoid poor adhesion of the thin paste due to excessively low temperature or instantaneous vaporization of water due to excessively high temperature, resulting in pores; melt atomization spraying is carried out in the 50-80℃ stage to ensure that the hydrophobic material is in the low viscosity spreadable zone and does not exceed the safe upper limit of 80℃, so as to avoid the particles softening and sticking together.

[0098] To reduce the risk of surface adhesion and clumping of the hydrophobic shell during the initial stage of curing, and to improve the wear resistance and anti-powdering properties of the outer surface of the particles, after the hydrophobic shell is melt-atomized and sprayed in step S8, a small amount of anti-adhesion and wear-resistant powder can be applied to the particle surface to form an outer anti-adhesion and wear-resistant layer, thereby forming a multi-layer structure of "core - mineral protective layer - hydrophobic shell - anti-adhesion and wear-resistant layer" in the obtained biomass pellet fuel; the anti-adhesion and wear-resistant powder can be selected from one or more of talc, ultrafine calcium carbonate, silica-alumina mineral micro powder, or finer-grained micro powder of the same mineral functional agent as the mineral protective layer in step S8; the particle size of the anti-adhesion and wear-resistant powder is preferably 500-1200 mesh, and the application amount is preferably 0.05-0.8 wt% based on the particle mass.

[0099] In a preferred embodiment, the anti-adhesion and wear-resistant powder is applied while the hydrophobic shell is still in the "surface micro-adhesion" window, causing the powder to mechanically interlock and anchor at points on the surface of the hydrophobic shell. Subsequently, air drying / cooling curing is performed, causing the outer powder layer to be locally embedded or anchored within the surface's micro-uneven structure by the hydrophobic shell. Through this outer anti-adhesion and wear-resistant layer: firstly, it significantly reduces the adhesion tendency of the hydrophobic shell surface, reducing agglomeration during storage and transportation; secondly, the powder acts as a micro-bearing / wear-resistant filler under transportation friction, reducing direct frictional wear of the hydrophobic shell and minimizing powder shedding; and thirdly, it can mitigate stress concentration in the hydrophobic shell at low temperatures, reducing localized brittle fracture propagation, thereby improving particle storage stability and long-distance transportation stability.

[0100] The air supplied for step S2 is first heat-exchanged with the particles via a pallet / tray before entering the drum dryer, so that the hot particles are cooled in the drying section and the air is preheated, thus realizing the utilization of waste heat through air path coupling.

[0101] In a preferred sequence, step S8 involves first coating with a mineral protective layer, followed by coating with a hydrophobic shell, thereby forming a core-mineral layer-hydrophobic layer structured particle.

[0102] The "thick slurry" mentioned in step S8 can be prepared by dispersing mineral functional agents (200-500 mesh micro powder) of the second particle size grade using water as a carrier; the solid content of the thin slurry is preferably 10-40 wt%, more preferably 15-30 wt%, to ensure both atomization spraying and sufficient solid deposition efficiency. The thin slurry can be prepared by stirring and dispersing, and if necessary, a circulating stirring or circulating pump can be set to suppress sedimentation; the amount of thin slurry applied remains constant at 0.2-2 wt% (based on solids) of mineral solids, calculated based on the mass of biomass particles.

[0103] In a preferred embodiment, the mineral slurry is sprayed onto the surface of particles at 40–70°C using a spray gun / atomizing nozzle, while simultaneously activating the airflow in the drying section to achieve a continuous "spraying-drying-fixing" process. To avoid nozzle clogging, an atomizing nozzle with an orifice diameter larger than the maximum particle size of the mineral powder can be used, and the slurry viscosity is controlled within a range suitable for stable atomization (e.g., to prevent significant stringing, continuous flow, and nozzle clogging). When dry application is more suitable on-site, the mineral functional agent can also be applied in powder form via a metering screw or pneumatic spraying method, while still meeting the requirement of a surface mineral application amount of 0.2–2 wt% (based on solids). By providing the slurry carrier, solids content range, and key points for spraying / anti-clogging, those skilled in the art can achieve equivalent mineral protective layer deposition and fixation under different equipment conditions, thereby improving the repeatability and sufficiency of the process.

[0104] S9 Screening and Recycling: The biomass pellets dried in step S8 are screened. The intact pellets on the screen are used as the finished product, while the undersized material is returned to step S7 for re-granulation, thus achieving a closed-loop particle size distribution and improving the yield. In this embodiment, the screening screen is 3mm×3mm to achieve effective separation and recycling of pulverized fine materials and short materials.

[0105] In a preferred packaging condition, after the particles are cured and air-dried in step S8, the temperature drops to below 40°C before packaging, in order to reduce the risk of film adhesion, clumping, and condensation and moisture regain inside the bag.

[0106] S10 Waste Gas Treatment and Dust Collection: The humid waste gas extracted in step S2 is discharged after spraying, alkali neutralization, and gas-water separation; negative pressure collection is adopted for dust-generating stations such as granulation, coating, screening, and packaging to reduce dust escape and improve the operating environment.

[0107] Beneficial effects:

[0108] In the above embodiments, low-temperature indirect steam drying controls the heat exchange surface temperature at 90-100℃ and, combined with induced draft dehumidification to enhance convective heat transfer, allows high-moisture furfural residue to achieve stable dehydration to 15-20% in a relatively mild temperature range. This reduces problems such as embrittlement and pulverization caused by over-drying and mold blockage and strength fluctuations caused by under-drying. Simultaneously, steam condensate is recovered and reused, improving the utilization of steam-side energy and water resources. Air classification and cyclone separators are used to grade and remove ash, outputting main material of 50 mesh or higher and removing fine particles smaller than 50 mesh and entrained ash. This reduces the impact of ash and fine powder fluctuations on molding friction, die wear, and blockage risks, and helps reduce the tendency for ash entrainment and slagging at the combustion end of the finished product. Mineral functional agents are internally added (0.5-5 wt%, ≤100 mesh) and... The surface layer (0.2–2 wt%, 200–500 mesh) uses a synergistic mineral-core structure with a core-mineral-hydrophobic layer and a temperature window (40–70℃ mineral layer, 50–80℃ hydrophobic layer). This allows for both internal ash phase control and surface anti-slagging / anti-pulverization requirements without significantly sacrificing molding strength. The composite system of the hydrophobic shell reduces granule moisture absorption and improves storage and transportation stability after curing. The granulation section uses online monitoring and interlocking adjustment of motor current and / or torque with extrusion temperature to control load and temperature rise, thereby improving continuous production stability and finished product consistency. Waste gas spraying—alkali neutralization—gas-water separation and dust negative pressure collection reduce the risk of odor, acidic components and dust escape, facilitating compliant emissions and clean on-site management.

[0109] As one embodiment, to further reduce the risk of surface adhesion and clumping during the initial stage of hydrophobic shell curing and to improve the wear resistance and anti-powdering performance of the outer surface, after the hydrophobic shell is melt-atomized and sprayed in step S8, an anti-adhesion and wear-resistant powder can be applied to the particle surface to form an outer anti-adhesion and wear-resistant layer. Unlike simply applying conventional powder, the anti-adhesion and wear-resistant powder is preferably first subjected to hydrophobic modification treatment to give the powder surface and the hydrophobic shell a higher interfacial compatibility and anchoring stability. The hydrophobic modification can be achieved by coating the powder surface with a thin layer of fatty acids and / or rosin derivatives, and / or waxes / bio-based hydrophobic agents. Furthermore, the anti-adhesion and wear-resistant powder preferably adopts a bimodal particle size system, including coarse powder with a particle size of 500-1200 mesh and fine powder with a particle size of 2000-5000 mesh. The mass ratio of coarse powder to fine powder is preferably 7:3-9:1, and the total amount of anti-adhesion and wear-resistant powder applied is preferably 0.05-0.8 wt% based on the particle mass.

[0110] The anti-adhesion and wear-resistant powder is applied when the hydrophobic shell is in the "surface micro-adhesion" window, followed by a secondary reflow and embedding treatment of the particles: the particles are briefly treated with hot air at 45-65℃ for 0.5-3 minutes to cause slight softening and reflow of the hydrophobic shell surface and to semi-embed and anchor the powder; then cooling or forced cooling with cold air is performed to reduce the particle temperature to below 40℃ for rapid shaping; through hydrophobic modification of the powder, bimodal particle size gradient filling, and secondary reflow and embedding, the adhesion tendency caused by the free wax phase on the surface can be reduced, the surface layer density and wear resistance can be improved, and the film layer brittle fracture propagation and powder loss caused by transportation impact under low temperature environment can be suppressed, thereby further improving the storage stability and long-distance transportation stability of the particles.

[0111] Example 2: Production system for implementing the above process

[0112] Reference Figure 2 The system block diagram shown in this embodiment provides a production system for implementing the above-mentioned production process, including: a drum drying unit, a coking polymer crushing unit, a mixing unit, a grading and ash removal unit, a mineral functional agent feeding unit and a mixing and homogenization unit, an extrusion granulation unit, a surface coating and curing drying unit, a screening unit and a return material conveying unit.

[0113] 1) The drum drying unit is an indirect heat exchange drum dryer, equipped with steam preheating pipeline and condensate recovery pipeline; the drum drying unit is connected to the induced draft unit, which is used to extract the humid gas in the drum drying unit and enhance convective heat transfer and humidification.

[0114] 2) The coking polymer pulverizing unit is used to pulverize the coking polymer produced in the furfural production process to a size not greater than 50 mesh;

[0115] 3) The material outlet of the mixing unit is connected to the drum drying unit and the coking polymer pulverizing unit, and is used to mix the dried furfural residue and the pulverized coking polymer evenly according to the set ratio.

[0116] 4) The grading and ash removal unit includes an air classifier and a cyclone separator, which are connected to the material at the discharge end of the mixing unit. The grading and ash removal unit is set to output granulation material with a particle size of 50 mesh or more, and to separate and remove fine particles smaller than 50 mesh and entrained ash.

[0117] 5) The mineral functional agent feeding unit and the mixing and homogenizing unit are connected to the granulation main material output from the grading and ash removal unit, and are used to add mineral functional agents to the granulation main material and mix them evenly.

[0118] 6) The extrusion molding granulation unit is connected to the discharge end of the mixing and homogenization unit for extrusion molding to obtain biomass pellets; it can be configured with online monitoring of motor current and / or torque and extruded pellet temperature, and form an interlock control with feeding and return feeding.

[0119] 7) The surface coating and curing drying unit includes a pallet / tray conveying structure and a spraying device. The spraying device is used to apply a mineral protective layer coating and / or a hydrophobic shell coating to the biomass pellets and cure them under the action of the drying airflow.

[0120] 8) The screening unit is used to screen biomass pellets, and the return conveying unit is used to return the undersize material to the extrusion molding and pelletizing unit.

[0121] Furthermore, the system also includes an exhaust gas treatment unit and a dust negative pressure collection unit: the exhaust gas treatment unit is connected to the exhaust gas outlet of the drum drying unit, and the exhaust gas treatment unit includes a spray section, an alkali neutralization section and a gas-water separation section; the dust negative pressure collection unit covers at least the granulation, coating, screening and packaging stations; and the air intake path of the induced draft unit is arranged as follows: it first passes through the surface coating and curing air drying unit and exchanges heat with the particles before entering the drum drying unit, so as to realize the utilization of waste heat through air path coupling.

[0122] Example 3: Biomass Pellet Fuel Products

[0123] This embodiment provides a biomass pellet fuel, which includes a biomass matrix, a mineral functional agent, and a surface functional layer;

[0124] The biomass matrix is ​​made from furfural residue and coking polymer produced during the furfural production process, and the mass ratio of coking polymer to furfural residue is 2-5:95-98; the mineral functional agent is at least one of silica-alumina minerals, carbonate minerals and zeolite, and the mass fraction of the mineral functional agent in the biomass pellet fuel is 0.5-5 wt%.

[0125] The surface functional layer includes a mineral protective layer and / or a hydrophobic shell, wherein the mineral protective layer contains 0.2 to 2 wt% (based on solids) of particle mass and / or the hydrophobic shell contains 0.2 to 2.0 wt% of particle mass; when a double-layer coating is used, the surface functional layer has a core-mineral layer-hydrophobic layer structure, and the mineral layer is located inside the hydrophobic layer.

[0126] The biomass pellet fuel in this embodiment has a pellet diameter of 5 mm, a pellet length of 15-20 mm, an ash content of less than 8%, and a lower heating value of more than 3000 kcal / kg, in order to meet the needs of conventional conveying, feeding and combustion applications.

[0127] Replacement or Modification Instructions

[0128] Based on the above embodiments, the following equivalent substitutions or modifications may be adopted without departing from the scope of the claims:

[0129] 1) The drying equipment can adopt an equivalent indirect heat exchange drum structure and an equivalent corrosion-resistant inner cylinder material, as long as the furfural residue and steam do not come into direct contact, the heat exchange surface temperature is 90-100℃, the output moisture content is 15-20%, and the condensate is recycled and reused.

[0130] 2) The specifications, air volume and separation efficiency of the air classifier and cyclone separator can be adjusted according to the production capacity and the particle size distribution of the incoming material, as long as the final output is a main material of 50 mesh or above and the fine powder of less than 50 mesh and the entrained ash are removed.

[0131] 3) Mineral functional agents can be selected individually or in combination from silica-alumina minerals, carbonate minerals, and zeolites; the relationship that the internal particle size is no greater than 100 mesh and the surface minerals are 200-500 mesh and finer than the internal particle size remains unchanged;

[0132] 4) The wax component of the hydrophobic coating material can be natural wax, plant wax or bio-based hydrophobic agent. In the compound, tal oil and rosin derivatives can be used alone or in combination. In addition, lignin-based hydrophobic modifiers and / or rosin derivatives can be added in the range of 0.5 to 10 wt% to improve the toughness and adhesion of the film.

[0133] 5) The extrusion section can use current monitoring, torque monitoring, or both; interlocking can be achieved by feeding rate, under-screen return material amount, or a combination thereof, but it is necessary to ensure that the temperature of the extruded particles does not exceed 80℃ and that the load returns to the operating window;

[0134] 6) The structure of the screening unit and the return conveying unit can be equivalently replaced, as long as the finished product is on the screen and the material under the screen is returned to the extrusion granulation section to form a closed loop; the scheme with a screen size of 3mm×3mm is preferred.

[0135] The foregoing has shown and described the basic principles, main features and advantages of the present invention. Various changes and modifications can be made to the present invention without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion, characterized in that, Includes the following steps: S1) Raw material preparation: Provide furfural residue with an initial moisture content of 50-60%; provide coking polymers generated during the furfural production process; S2) Low-temperature indirect steam drying: Saturated steam is introduced into the indirect heat exchange drum dryer to preheat its heat exchange components, so that the temperature of the heat exchange surface in contact with the material reaches 90-100℃; then the furfural residue is fed into the inner cylinder of the drum dryer, so that the moisture content of the furfural residue is controlled at 15-20%; S3) Coking polymer pretreatment: The coking polymer is crushed to obtain a pulverized material with a particle size of no more than 50 mesh; S4) Mixing: The dried furfural residue obtained in step S2 and the pulverized material obtained in step S3 are mixed evenly at a mass ratio of coking polymer: furfural residue of 2-5: 95-98. S5) The mixture is then air-classified and sent to a cyclone separator for classification. The particle size of the main material after classification is controlled to be above 50 mesh, and fine particles smaller than 50 mesh and entrained ash are separated and removed to obtain the main material for granulation. S6) Adding mineral functional agents: Add mineral functional agents to the main material obtained in step S5 and mix evenly. The mineral functional agents are at least one of silica-alumina minerals, carbonate minerals and zeolites. The amount of mineral functional agents added is 0.5 to 5 wt% based on the mass of the main material. Furthermore, the added mineral functional agents have a first particle size grade of ≤100 mesh. S7) Extrusion granulation: The material obtained in step S6 is fed into an extrusion granulator and extruded through a die to obtain biomass pellets. The temperature of the extruded pellets is controlled to be no higher than 80°C. S8) Surface functional coating and curing drying: After step S7 and before screening, the biomass particles are coated with a double-layer surface functional layer and cured and dried under the condition that the surface temperature of the extruded biomass particles is 40-80℃. The mineral protective layer is coated first and then the hydrophobic shell layer is coated to form a core-mineral layer-hydrophobic layer structure particle, wherein the mineral layer is located inside the hydrophobic layer. Mineral protective coating: When the surface temperature of the particles is 40-70℃, the mineral functional agent is applied to the surface of the biomass particles in the form of a slurry or powder, and the amount of the mineral functional agent applied is 0.2-2wt% (based on solids) based on the mass of the biomass particles. The particles are then dried and fixed under the action of airflow in the drying section to form a mineral protective layer. Hydrophobic shell coating: When the surface temperature of the particles is 50-80℃, the hydrophobic coating material is applied to the surface of the biomass particles by melt atomization spraying. The hydrophobic coating material is at least one of natural wax, plant wax, and bio-based hydrophobic agent, and the amount of hydrophobic coating material applied is 0.2-2.0 wt% based on the mass of the biomass particles. After spraying, it is cured into a film under the action of airflow in the drying section to form a hydrophobic shell. In step S8, the mineral functional agent used to form the mineral protective layer is the same mineral functional agent as the mineral functional agent added in step S6, and the surface mineral functional agent in step S8 is of the second particle size grade, which is finer than the first particle size grade; and the second particle size grade is 200-500 mesh micro powder. Furthermore, the hydrophobic coating material is a compound system, and when the hydrophobic coating material contains tal oil, the mass ratio of the wax component to the tal oil is 9:1 to 6:

4. Meanwhile, the air supplied for the induced draft in step S2 is first exchanged with the particles via a pallet / tray before entering the drum dryer, so as to achieve particle cooling and induced draft preheating. S9) Screening and recycling: Screen the biomass pellets dried in step S8. The whole pellets on the screen are used as finished products, and the material under the screen is returned to step S7 for re-granulation. S10) Waste gas treatment: The humid waste gas extracted in step S2 is discharged after spraying, alkali neutralization and gas-water separation, and the dust from granulation, coating, screening and packaging is collected under negative pressure.

2. The production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 1, characterized in that: The saturated steam temperature in step S2 is 150°C, and the preheating time is 20-30 minutes.

3. The production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 1, characterized in that: In step S7, the material level in the pellet mill hopper is controlled to be 1 / 2 to 2 / 3 of the effective volume of the hopper, and the operating load of the pellet mill is controlled to be 60 to 70% of the nameplate capacity.

4. The production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 1, characterized in that: After curing and air drying in step S8, the particle temperature should be reduced to below 40℃ before packaging.

5. The production process for low-temperature drying and extrusion of furfural residue to produce biomass pellets according to claim 1, characterized in that: The silicate minerals include kaolin and / or bentonite, and the carbonate minerals include dolomite and / or limestone powder.

6. The production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 1, characterized in that: In step S6, the amount of mineral functional agent added is 1-3 wt% based on the mass of the main material, and the mineral functional agent includes zeolite.

7. The production process for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 1, characterized in that: In step S8, a mineral protective layer is first applied, followed by a hydrophobic shell, thus forming a granular structure of core-mineral layer-hydrophobic layer.

8. A production system for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to any one of claims 1 to 7, characterized in that, include: The system comprises a drum drying unit, an induced draft unit, a coking polymer pulverizing unit, a mixing unit, a grading and ash removal unit, a mineral functional agent feeding unit and a mixing and homogenization unit, an extrusion molding and granulation unit, a surface coating and curing drying unit, a screening unit, and a return material conveying unit. The drum drying unit is an indirect heat exchange drum dryer, equipped with steam preheating pipelines and condensate recovery pipelines. The drum drying unit is connected to the induced draft unit. The coking polymer pulverizing unit is used to pulverize the coking polymer produced during furfural production. The mixing unit is connected to the discharge end of both the drum drying unit and the coking polymer pulverizing unit for... The dried furfural residue is mixed with the pulverized coking polymer; the grading and ash removal unit, including an air classifier and a cyclone separator, is connected to the material outlet of the mixing unit. The grading and ash removal unit is set to output granulation material with a particle size of 50 mesh or larger, and to separate and remove fine particles smaller than 50 mesh and entrained ash; the mineral functional agent feeding unit and the mixing and homogenizing unit are connected to the granulation material output from the grading and ash removal unit, and are used to add mineral functional agents to the granulation material and mix them evenly; the extrusion molding granulation unit is connected to the material outlet of the mixing and homogenizing unit, and is used to extrude and mold biomass pellets. The surface coating and curing drying unit includes a pallet / tray conveying structure and a spraying device. The spraying device is used to apply a mineral protective layer coating and / or a hydrophobic shell coating to the biomass pellets and cure them under the action of air drying airflow. The screening unit is used to screen the biomass pellets, and the return material conveying unit is used to return the undersize material to the extrusion molding granulation unit.

9. The production system for producing biomass pellets from furfural residue by low-temperature drying and extrusion according to claim 8, characterized in that, It also includes a waste gas treatment unit, which is connected to the waste gas outlet of the drum drying unit. The waste gas treatment unit includes a spray section, an alkali neutralization section, and a gas-water separation section; a dust negative pressure collection unit, which covers at least the granulation, coating, screening, and packaging stations; wherein, the air intake path of the induced draft unit is arranged as follows: it first passes through the surface coating and curing drying unit to exchange heat with the particles before entering the drum drying unit, so as to realize the utilization of waste heat through air path coupling.

10. The biomass pellet fuel produced by the production process according to any one of claims 1 to 7, characterized in that, The biomass pellet fuel comprises a biomass matrix, an internally blended mineral functional agent, and a surface functional layer; wherein, the biomass matrix is ​​obtained from furfural residue and coking polymer produced during the furfural production process, and the mass ratio of coking polymer to furfural residue is 2-5:95-98; the internally blended mineral functional agent is at least one of silicate minerals, carbonate minerals, and zeolites, and the mass fraction of the internally blended mineral functional agent in the biomass pellet fuel is 0.5-5 wt%; the surface functional layer comprises a mineral protective layer and / or a hydrophobic shell. The surface functional layer comprises a mineral protective layer containing 0.2–2 wt% (based on solids) of particle mass and / or a hydrophobic shell containing 0.2–2.0 wt% of particle mass. When the surface functional layer includes both a mineral protective layer and a hydrophobic shell, the surface functional layer has a core-mineral layer-hydrophobic layer structure with the mineral layer located inside the hydrophobic layer. The biomass pellet fuel has a particle diameter of 5 mm, a particle length of 15–20 mm, an ash content of less than 8%, and a lower heating value of greater than 3000 kcal / kg.

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