AG3 battery.Research on Biomass Molded Fuel Technology

　　Facing the increasing depletion of oil, natural gas and coal fossil energy (see Table 1), increasingly serious environmental pollution, and global warming, as the Copenhagen International Climate Conference put forward the initiative to reduce carbon footprint, China has also formulated emission reduction measures. The goal is to strive to reduce carbon dioxide emissions per unit of GDP by 40% to 45% by 2020 compared with 2005. China Forestry Machinery Association non-fossil energy accounts for 15% of primary energy consumption. Biomass energy ranks fourth in total energy consumption in the world after oil, natural gas and coal. It occupies an important position in the entire energy system. Due to its advantages of large resource reserves, low carbon, environmental protection and renewable nature, biomass energy ranks fourth in total energy consumption in the world. Considered a hot area for energy development.

　　1The significance of biomass briquette fuel

　　Our country produces about 700 million tons of crop straw every year, and the total amount of forestry secondary residues is about 800 million tons to 1 billion tons, with abundant biomass raw material reserves. In addition, the biomass pellet fuel after densification processing has uniform particle size, increased unit density and strength, easy transportation and storage, and significantly improved combustion performance, which is of great significance for biomass raw materials to become commodities and truly enter the circulation field. my country's "Agricultural Biomass Energy Industry Development Plan (2007-2015)" and "Renewable Energy Mid- and Long-term Development Plan" clearly propose to focus on the development of biomass pellet fuels. By 2010, they will combine to solve the basic energy needs of rural areas and transform rural areas. In terms of energy use, about 500 straw densified fuel application demonstration sites will be built across the country. The annual utilization of straw densified fuel will reach about 1 million tons. By 2015, the annual utilization of straw densified fuel will reach about 20 million tons. By 2020 In 2016, the annual utilization of biomass solid pellet fuel reached 50 million tons. The National Development and Reform Commission issued a subsidy policy of 150 yuan/ton for the production of biomass briquette fuel, and the country formulated the standard GB/T21923-2008 "General Principles for the Validation of Solid Biomass Fuels", which was implemented on November 1, 2008. Currently, the Ministry of Agriculture is formulating a number of industry standards for biomass solid pellet fuel. However, according to statistics, the national output of biomass briquette fuel in my country in 2009 was still less than 200,000 tons, and the biomass briquette fuel industry still has a long way to go.

　　2 Introduction to the project results of "Introduction of High Pressure Densification Molding Technology for Biomass Molding Fuel"

　　In 2005, the "Introduction of High-pressure Densification Molding Technology for Biomass Molded Fuel" project chaired by Professor Yu Guosheng of Beijing Forestry University conducted research on the normal temperature and high-pressure densification molding method of biomass. The molding equipment is imported from the RB110 molding machine of the German RUF company. Research on the main factors in the molding process (pressure, raw material moisture content, raw material types, etc.) shows that the rate is well controlled during normal temperature and high pressure dense molding. Within the range of 5% to 15%, the borer height cannot exceed 22%; the pressure can be controlled between 15 and 35MPa to meet storage and transportation requirements; straw manure biomass is easy to shape, while shrubs are not easy to shape due to the hard fiber and good toughness of the raw material itself. forming. A stereomicroscope was used to observe the molded block. It was found that the bonding mechanism is mainly the mechanical inlay between particles. On the basis of studying the normal temperature molding mechanism of materials, Professor Yu Guosheng developed a biomass molding fuel normal temperature molding machine that is squeezed by a force ship (Figure 1). The machine has an installed power of 22kw and a processing capacity of 500~600kg/h. The actual molding energy consumption is not more than 40kw-h/t. The density of biomass molded blocks can be adjusted by adjusting the pressure of the hydraulic system of the molding equipment. It can not only meet the requirements for processing roughage required for livestock breeding, but also meet the requirements of biomass molded fuel processing. The maximum density can reach 1.2g/ cm3, has now been put into production practice.

　　3 Comparative study of heating molding technology and normal temperature molding technology

　　According to the heat generated during the molding process, it can be divided into normal temperature molding and heating molding. At present, there is a lot of research on heating molding technology in China.

　　3.1 Molding machines

　　Take the piston extrusion block fuel molding machine as an example. The molding of pellets is achieved by the reciprocating motion of the plug. The feeding, compression and discharging processes are all intermittent. There is a heating ring outside the molding cavity of the molding machine. It is a heating molding method, and some are molded at room temperature. Since the heating molding process is completed at a temperature above 200°C, the molded parts of the heating molding machine are easier to wear than those molded at room temperature. The maintenance cycle is about 200 hours, which is longer than that of molding at room temperature.

　　3.2 Molding process

　　The process flow of hot press molding technology is generally: raw material - pretreatment (crushing) - drying - heating forming - cooling packaging. The process of normal temperature molding technology is relatively simple. The process flow is generally: raw material - pretreatment (chipping or crushing) - Forming and packaging reduces energy consumption by 44% to 67% compared to heating forming technology, which requires three processes: raw material drying, heating and cooling during forming.

　　3.3 Factors affecting molding

　　The main factors that affect biomass dense molding are: raw material type, raw material moisture content, raw material particle size, molding pressure and mold size, and the heating molding method also includes heating temperature.

　　3.3.1 Molding pressure

　　There is no heating and softening process of raw materials in normal temperature molding, so the pressure required for molding is greater than that of heated molding. In the "Introduction of High-pressure Densification Molding Technology for Biomass Molded Fuel" project, room-temperature molding tests of various biomass raw materials were conducted. Here, only the test data of tetraploid black locust branch molding blocks are quoted.

　　After crushing, the particle size distribution of black locust branches is about 70% with a particle size distribution of 1 to 2 mm, 20% with a particle size distribution of 5 to 10 mm, and about 10% with a particle size larger than 10 mm. The moisture content is 7.65%, and the raw material density before molding is 0.195g/cm3. The main oil cylinder pressure is between 10~60MPa, every

　　An experiment was conducted at 2.5MPa, and the test results are shown in Figure 2. When the pressure is low (10~20PMa), the density of the briquettes increases to a large extent with the increase of the molding pressure. When the pressure is greater than 20MPa, the density of the briquettes tends to be stable with the increase of the molding pressure. Before and after compression, The volume ratio is distributed between 5.16 and 5.97. Tetraploid black locust branches have good toughness and high fiber content, and the formed blocks pressed under relatively small pressure are also very solid.

　　3.3.2 Moisture content of raw materials

　　The raw material requirements of biomass densified molding fuel technology include moisture content and particle size. The difference between heating and normal temperature molding technology is mainly reflected in the different requirements for raw material moisture content. Hot pressing molding technology has stricter requirements on raw material moisture content. Due to the absolutely dry production Materials have poor heat transfer properties, and moisture is the best heat transfer medium in biomass raw materials. Therefore, theoretically speaking, the higher the moisture content of biomass raw materials in hot pressing molding, the better the heat transfer, and the greater the softening degree of lignin and hemicellulose. The higher the moisture content, the easier it is to form; however, if the moisture content is too high, high-pressure steam will easily be generated during the compression process, and deflation or blasting will occur, interrupting the molding process. Hot press molding requires the moisture content of raw materials to be controlled at 8 to 12% for the best molding effect. Normal temperature molding technology requires that the maximum moisture content of the raw materials can reach about 22%, and no blasting will occur during the molding process of the materials.

　　3.3.3 Types of raw materials

　　Hot pressing molding technology has poor adaptability to the types of raw materials. Since different raw materials have different lignin and hemicellulose contents, the softening degree of the raw materials is different at the same heating temperature. When the molding pressure is constant, different raw materials need to be adjusted to different heating temperatures, which requires higher operating technology. Crop straw contains less lignin (about 15-25%) and is not suitable for softening and molding by heating. Densifying molding at room temperature and high pressure has advantages; forest biomass raw materials have long fibers and strong toughness, and are difficult to mold during molding. It is more difficult, but storage and transportation requirements can be met through dense molding at room temperature and high pressure. The molding effect is also very good, and the density value of the briquettes is relatively concentrated.

　　3.4 Forming mechanism

　　Cellulose is one of the main components of the cell wall of biomass raw materials, accounting for about 50% of the total cell wall material. The binding bonds of the cellulose macromolecule chain are mainly oxygen bonds, van der Waals forces and carbon-oxygen bonds. During biomass molding During the fuel production process, through mechanical compression of biomass raw materials, the distance between fibers can be shortened to facilitate the formation of hydrogen bonds and van der Waals forces, making the biomass raw materials easy to shape.

　　3.4.1 Heating forming mechanism

　　Hemicellulose and lignin in plants are amorphous thermoplastic polymers with glassy transition properties. When the temperature reaches 70 to 110°C, they begin to soften and the adhesive force begins to increase. The softening degree intensifies at 200 to 300°C. When it reaches melting, a certain amount of pressure is applied to make it closely bond with the cellulose and bond with adjacent particles. After cooling, it can be solidified and formed. Biomass heated molding fuel takes advantage of this characteristic of biomass. Compression molding equipment is used to pressurize and heat the dried and pulverized loose biomass raw materials to soften hemicellulose and lignin and then squeeze them into shape to obtain Molded fuel with certain shapes and specifications.

　　3.4.2 Normal temperature forming mechanism

　　Biomass raw materials are composed of fibers. The crushed biomass raw materials have a loose texture. After being subjected to a certain external pressure, the raw material particles undergo position rearrangement, particle mechanical deformation and plastic rheology. When the pressure is low at first, some particles enter the gaps between the particles, and the mutual positions of the particles continue to change. When all the larger gaps among the particles are occupied by particles that can enter, the only way to increase the pressure is through the deformation of the particles themselves. to fill the gaps around it. At this time, the particles are extended on a plane perpendicular to the maximum principal stress. When the particles are extended to contact two adjacent particles, and the pressure is increased, the particles will combine with each other. In this way, the originally dispersed particles are compressed and molded, while their volume is greatly reduced and their density is significantly increased. Due to the mutual entanglement and bite between inelastic or viscoelastic fiber molecules, they generally will not return to their original structural shape after the external pressure is relieved.

　　Master Hui Caijuan of Beijing Forestry University drew on the relevant theories of powder and granule molding proposed by Rumpf in Germany. She used a stereo microscope to observe the molded blocks and found that during the normal temperature dense molding process, the biomass raw material particles were embedded in each other due to the high pressure. Together, the binding force between particles mainly comes from the mechanical inlay between each other. The microstructure of the tetraploid black locust branch formed block is shown in Figure 3. From left to right are the microstructure diagrams of the formed block under 10MPa, 30Mpa and 60Mpa pressure.

　　3.5 Product quality

　　There are many indicators to measure the quality of biomass fuel blocks, including calorific value, molded fuel density and mechanical strength. Strength is related to density. Density is related to molding pressure, which will not be discussed here. Heating molding technology will cause carbonization and scorching of the product surface and loss of thermal energy of the biomass; normal temperature dense molding products do not destroy the molecular structure of the raw materials, and have no chemical reaction or thermal cracking and differentiation, so the molding fuel can maintain the calorific value of the raw materials. There is almost no heat loss. Formulating more complete biomass molding fuel industry policies and standards and developing more efficient, energy-saving and highly automated room temperature molding equipment are the efforts of experts and scholars in related fields.

　　4Conclusion and outlook

　　Biomass room-temperature densified molded fuel technology has the advantages of less wear on molded parts than heated molding, low pretreatment requirements for biomass raw materials, and the molded fuel products maintain the properties of the original material and have no thermal energy loss. Therefore, it has high value in research and promotion. By studying the influencing factors in the fuel block molding process and the microscopic images after molding, it was found that the normal temperature molding mechanism is the movement, deformation and mechanical inlay of biomass particles, which reveals the rules of dense molding of biomass. In addition, room temperature molding technology is more economical than heated molding technology, which is conducive to the commercial promotion of biomass molding technology.

　　At present, my country's biomass briquette fuel technology is in the research and demonstration pilot stage, with poor scale and marketization and slow promotion. In order to promote the development of my country's biomass briquette fuel industry, we call on the government to formulate more complete biomass briquette fuel industry policies and standards, and develop more efficient, energy-saving and highly automated room-temperature molding equipment. This is the direction of efforts of experts and scholars in related fields.

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