Wang Meimei 1, Wang Yumei 2, Han Wenqing2
(1. School of Mechanical Engineering, Anyang Institute of Technology; 2. Anyang Jimke Energy Machinery Co., Ltd., Anyang, Henan 455000)
Abstract: As an important form of biomass energy, biomass particles have important significance in studying the influencing factors in the molding process. This paper mainly analyzes the factors affecting the solid-state forming of biomass particles from the three aspects of raw materials, process and equipment. The effect of various factors on the influence of indicators such as particle quality factor (PQF) and particle durability index (PDI) is related to each other. Among them, the equipment hole type parameter is a key factor in the analysis and research, and it is the research and development of particle molding equipment. Preliminary basic research.
With the rapid development of the world today, production and life are increasingly dependent on energy, and energy has become a basic material that restricts the country’s economic development. As a primary energy source, petrochemical energy cannot be regenerated due to limited reserves, causing global It is highly valued. Biomass energy, as a renewable resource, occupies an important position in petrochemical alternative energy sources. [1]The development of biomass energy can not only supplement the shortage of petrochemical energy, but also produce significant environmental benefits. The conversion of loose biomass feedstocks into solid fuels through particle formation is a highly efficient, simple, and practical form of biomass energy use. It has abundant biomass resources and provides a wide range of raw material guarantees for the utilization of physical energy generation technology. Although China's research and development in this technology field started late, but with the increasing demand of the market and the environment, the forming factor of graduate students' material pellet fuels. , To promote the development of China's biomass solidification molding technology and its equipment to play a large role in promoting the scale.
The process of forming biomass pellets is actually a process in which some of the materials in the biomass are interdigitated with each other and accompanied by a physical process of softening and gluing of organic materials. This process is mainly based on physical changes, chemical changes are assisted, and raw materials are used. Raw material particle size, raw material humidity, molding temperature, material layer thickness, extrusion speed, extrusion pressure and molding hole parameters and many other aspects[2-3].
1 raw material factors
1.1 raw material composition
Since the raw materials are generally composed of several different components, the influence of different components on the solidification of the particles is different. Some components, such as proteins in the biomass, lignin, etc., soften at a certain pressure or temperature, and change to form The organic substance having a cementing property, some components such as cellulose in wood chips, inorganic particles in biological waste, etc., are not easily decomposed or cracked, and hinder the molding of particles in the process of particle formation like a dispersant, while the biomass raw material is large Some of the substances are not easily decomposed or softened. Raw material components are the main factors affecting the particle durability index (hereinafter referred to as PDI). The influence of raw material components on PDI is about 40% of the various factors. [4]The contribution rate of various components in raw materials to the entire PDI is different. According to the different contribution rates of different substances to PDI, some commonly used raw materials give different particle quality coefficients (hereinafter referred to as PQF).[5].
The larger the PQF raw material, the tighter the pellets produced, the higher the PDI, and vice versa. The bentonite, lignin, etc., with higher PQF, can generally be used as binders, such as acid oil, etc., its PQF The value is -40, which is due to a large number of fats and oils as raw material components, resulting in mutual generation of oil film between the particles. The presence of the oil film leads to the dispersion and isolation of the particles, so that the produced particles can not be effectively combined, the looser the particles The lower the PDI, the better the particle quality when the particle mass index of the ingredients in the raw material is weighted and 4.7, see (1):
1.2 raw material granularity
The finer the particle size, the larger the surface area of the particles, the easier the particles to bind, and the easier to absorb heat and moisture during mixing and tempering. The better the softening of the biomass, the more dense and smooth the particles are. Cracking and fines are less likely to occur. However, the finer the particle size, the higher the energy consumption of the crushing process, and therefore it is not too much to pursue a smaller particle size. [6]The homogeneity of the particle size of the raw materials is also very important. The raw materials after the grinding cannot have too large particles. The excessively large particles are not easy to combine with other raw materials and affect the uniformity of mixing and tempering. After the granulation, the surface of the particles is prone to irregularities. Phenomenon, and easy to produce radiation-type cracks around large particles, causing particles easily damaged. From the granulation point of view, the finer the raw material particle size, the granulation intensity is high, but should not adjust the humidity, easy to stick together, and the material is crushed too fine , Crushing energy consumption is too high; Particle size is too thick, the hole is difficult to enter, increase the wear of the die hole and the pressure roller, granulation forming is difficult, especially the small-aperture mold is more difficult to form, and the material softening and gelatinization effect is poor, resulting in High material consumption, low yield, and high powder content of the produced particles. Therefore, the particle size of the raw materials is generally 1/2~2/3 of the particle diameter or the sieve plate used for the half particle diameter in the crushing control. The maximum size should not be > 10mm.
In this way, excessive energy consumption of crushing can be avoided, and the required particle size for humidification, mixing and conditioning is also ensured, which is advantageous for reducing the powder content of the particles. In addition, the mixing uniformity before granulation should be emphasized so that the coefficient of variation of the mixing uniformity can reach 5%. About, will lay a good foundation for the back granulation process.
1.3 raw material humidity
In the biomass raw materials, the combined water and the free water act as a lubricant to reduce the internal friction between the particles, enhance the fluidity, thereby promoting the binding between the particles, and enhance the plasticity due to the hydrolysis and the water-absorption and softening of the biomass. Change adhesion, the appropriate humidity can promote the formation of particles, generally different raw materials applicable humidity is not the same, should be affected by temperature components, most of the humidity of the material is 8% to 15%, but there are individual materials, humidity The requirements are relatively high, such as teak wood chips even up to 50%, etc. Unusual raw materials are best tested to ensure the effect of particles. For raw materials with excessive humidity, they must be processed to reduce moisture before pelleting. Usually, the drying process is used. deal with[7-8].
2 process factors
2.1 molding temperature
The process of biomass forming is mainly the process of softening, plasticizing, and bonding of some organic components under certain temperature and pressure, so the forming temperature is an important influence factor for the formation of biomass particles '9'. In general, the biomass The temperature at which the organic component starts to soften is 70°C to 80°C, and the plasticity changes at about 160°C, but some organic components start to liquefy at 240°C. Therefore, when the molding temperature reaches 60°C, the raw materials begin to soften and fluidity. With the increase of temperature, the material movement resistance becomes smaller and smaller, and the forming resistance also decreases. According to the determination of the pressure difference between normal temperature molding and hot molding, the difference is nearly 2 times. However, it is not that the higher the temperature, the better. High will cause carbonization of biomass, which is not conducive to molding, and due to the uneven and uncontrollable carbonization, it will affect the particle quality and calorific value.
2.2 Material thickness
The material enters the squeezing area, so that a gap is formed between the press roller and the mold. This gap is the thickness of the material layer. The material layer thickness is a pre-treatment stage before the material enters the squeezing area. At this time, the material in the material layer is pressed by the press roller. There is a pre-compression force. For molding die with different die hole diameters, generally smaller gaps are used to select smaller gaps, and larger gaps are used to select larger gaps.
The thickness of the material layer is not only related to the particle diameter of the finished product, but also affects the power consumption of the granulation process. Under the same conditions, the thickness of different material layers will cause the change of the extrusion force. When the material layer thickness is too small, the extrusion force is small and enters the die hole. The material is also low, the productivity is low, and even the material entering the die hole is too small and cannot be squeezed. The particles cannot be produced. When the material layer thickness is too large, the extrusion force increases rapidly, and the material enters the die hole too much, causing particles Difficult to discharge too dense, power increases, or even cause overload failure equipment. Figure 1 shows the effect of the material layer thickness on the extrusion force, where B is a reasonable material layer thickness, A is the thickness of the material layer in an excessive state, C is too thin thickness of the material layer. The extrusion force of the material is the vertical force component of the contact between the press roller and the material, that is, formula (2):
It can be clearly seen from Fig. 1 that the extrusion force F at the layer thickness A is due to the long squeeze zone, which results in a significant increase in the extrusion force, while the compression force in the C zone is too small. Therefore, a reasonable material Thickness is the guarantee of efficient production.
3 equipment factors
3.1 extrusion speed
The extrusion speed directly affects the residence time of the material in the die hole. The residence time of the material in the die hole has a great influence on the forming quality. When the material is squeezed in the die hole for a short time, the power consumption per unit of output is consumed. It is less, but it tends to cause insufficient compaction of the produced particles; the longer the material stays in the die hole, the longer the time it takes to squeeze, the denser the structure of the particles, and the particle quality will be improved, but the relative The unit energy consumption also rises; When the material stays in the die hole for too long, the compression is too dense, resulting in increased friction and movement difficulties, and the resulting charring of the particles due to overheating may even result in compaction of the die holes and shutdown of the die holes. Pressure roller speed, pressure roller diameter, etc. Reasonable extrusion speed is the guarantee for continuous operation and lower energy consumption.
3.2 Extrusion strength
Extrusion strength is the physical compression force of the material, which is the main factor of particle compactness '10'. Only with higher crushing strength can the partial biomass in the raw material be softened and higher The frictional heat generated under the crushing strength also promotes the plasticization and adhesion of some of the components in the raw material, thus making the particle gel synthetic type. When the compressive strength is insufficient, the raw material is not effectively squeezed and the particles cannot be molded; Excessive compression strength will intensify the wear of the mold, which is not conducive to cost control.
3.3 molding hole parameters
Forming hole parameter is a comprehensive factor, which is constrained by many factors such as raw material composition, raw material granularity, extrusion speed, extrusion strength, etc., which in turn affect these factors and comprehensively affect the process of particle formation. Compression ratio, greater compression ratio increases extrusion strength, reduces extrusion speed, and has a positive effect on materials with lower quality factors of raw material pellets; inlet type in the form of hole affects the amount of material entering the hole And speed, and affect the thickness of the material layer changes.
After previous experimental studies, a certain amount of data has been accumulated on the pore type data of the biomass particle forming molds on the basis of the tests, which laid the foundation for the improvement of the molding theory. Currently, the mold disks for manufacturing particles are based on different raw materials. Ratio, generally according to the required forming aperture φ, choose the appropriate coefficient to determine the disc thickness H, cone D, cone depth h.
For different raw materials, the forming parameters of the forming holes of the granulation equipment are adjusted to obtain particles with excellent forming granulation effect and low unit production energy consumption, which is conducive to the expansion of production profits.
4 Conclusion
This article analyzes the main factors influencing the particle molding. The influence of these factors on the particle molding is manifold. The raw material composition and moisture content directly determine the effect of granulation molding. For high humidity raw materials, the treatment to reduce the moisture content must be performed. Can not form granulate; granulation process is a relatively strict control process, temperature, layer thickness can not exceed the required range or it can not be normal grain; and equipment factors is a complex comprehensive factor, in which the choice of shape hole parameters can be adjusted Improve the effect of adjusting other factors. Good hole forming parameters have a wide range of applications for raw materials, and the unit energy consumption of the production process is also low, and the particle molding quality is good. Through the optimization and improvement of molding equipment and hole parameters, it can effectively Improve particle quality and yield, and promote the application and promotion of biomass particles.
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