Li Wei Zhen 1,2,3Jiang Yang 1,2,3, Yin beautiful1,2,3
(1. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640; 2. Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640; 3. Guangdong Key Laboratory of New Energy and Renewable Energy Research and Application, Guangzhou 510640)
Abstract: Biomass briquette has developed rapidly in recent years due to its characteristics of being clean and renewable, and the research on its forming mechanism is also becoming more and more profound.This paper mainly introduces the bonding modes of solid billet and the thermal transition characteristics of lignin And the bonding, raw material composition, forming parameters and other aspects of the research status, the proposed mixing and molding technology, the thermal transition characteristics of raw materials, the impact of lignin structure and species, 'solid bridge' structure of the construction conditions and the microscopic morphology, functional groups and The changing rules of chemical bond should be the main direction of the research on forming mechanism and provide a reference for further research on the forming mechanism of biomass.
0 Foreword
Biomass forming technology is one of the effective use of biomass energy technology, refers to the role of a certain temperature and pressure, all kinds of scattered, non-uniform shape of agricultural and forestry residues processed into a certain shape, density The technology of various fuel products [1]During the molding process, particles undergo rearrangement, mechanical deformation, plastic rheology, and density increase. The fuel quality is also affected by the chemical composition of the raw material and the external forming parameters [2], Force and particle binding mechanism shown in Figure 1. This article describes the particle forming process, lignin bonding, raw material composition and forming parameters of the research status, put forward future research directions for the development of biomass forming mechanism to provide reference.
1 molding process
Molding process, according to the deformation of raw materials, can be divided into four stages [3-5], Shown in Figure 2. a for the loose stage, to overcome the gap between the raw material-based, raw materials to a certain extent, the air is excluded, the pressure and deformation was linear, smaller pressure increase can be greater deformation increment. b is the transitional phase, the pressure increases, large particles break into small particles, the elastic deformation and dominates, the particle gap is filled, the pressure and deformation are exponential relationship.C is the compression stage, the main plastic deformation occurs, the particles In the deformation of the fracture or slip occurs: the direction of the vertical principal stress, particles fully extended, closely integrated by the meshing mode; parallel to the main stress direction, the particles thinner, closely bonded by way of bonding. Fundamental formation of the fuel, the pressure and plastic deformation of raw materials . d is the stage of transition, the plastic material and elastic visco-elastic deformation occurs, the main elastic deformation. Raw material stress relaxation and creep phenomena, the pressure will be significantly reduced.
2 particle combination
KALIYAN and so on [6]In this paper, two kinds of theories are put forward: (1) The distance between the particles is close enough and combined by the attractive force, the electrostatic attractive force due to the friction between the particles or the inner part can make the particles combine with each other. When the distance between the particles is less than 0.1μm, van der Waals forces become the main attraction of the interparticle bonding. ② The particles are bound by the "solid bridge" structure.
Some materials or additives in the raw materials, due to chemical reaction, crystallization or solidification, interdiffuse diffuse and form interdisciplinary bonds to form a "solid bridge" structure and become the main mode of particle-particle combination. KALIYAN et al. The lignin, carbohydrates, starch, protein and fat in the straw and switchgrass themselves soften or deform, forming a 'solid bridge' structure. KONG et al. [7]The study confirmed that the addition of waste wrapper fibers during sawdust formation creates a 'solid bridge' structure with better mechanical durability.
KONG and so on [8]The effect of adding four kinds of fiber, such as rice straw, wheat straw, rubber tree and nylon, on sawdust formation was further studied. As shown in Figure 3, it was found that rice straw and rubber tree can improve the physical properties of the grain. Rice straw, Sawdust is the same hydrophilic raw materials, the particles can be effectively entangled with each other to form a 'solid bridge' structure wheat straw, nylon hydrophobic raw materials, to improve the quality of particles play a negative role.
3 lignin bonding effect
3.1 thermal transition characteristics
Biomass is a kind of natural macromolecule polymer, its thermal transition characteristic refers to the glass transition temperature (Tg) and the melting temperature.The glass transition temperature refers to the temperature at which the polymer softens and transitions from the glass state to the plastic state. Molecular weight and chain length of the different structural monomer composition, the glass transition occurs in a temperature range, is an important property of the polymer. Melting temperature refers to the polymer from solid to liquid transition temperature. Lignin thermal transition characteristics In the molding process plays a key role in the glass transition temperature below the covalent force formed by the valence and secondary bonds, showing higher mechanical strength, greater elastic modulus; glass transition temperature above the glass transition temperature, Part of lignin molecular rotation or displacement gradually become the molecular thermal expansion movement, increased mobility, greater viscosity [9]The glass transition temperature of lignin depends on its source and is related to the type, moisture content and extraction process [10-11]STELTE et al [12]Research shows that hardwood lignin has more acetyl, methoxy structure and a small amount of phenolic hydroxyl structure, the glass transition temperature is lower than the softwood [12]STELTE et al [13]The effect of glass transition temperature on the formed pellets was studied.The glass transition temperature of wheat straw and wheat straw extracted with n-hexane was 53 ℃ and 63 ℃, respectively, when the moisture content was 8%. At the glass transition temperature Below (30 ° C) is lower than the above (100 ° C), the particle density and the strength are lower, and the axial extension is larger.
KALIYAN and so on [14]The glass transition temperature of maize stalk and switchgrass under different water contents was determined by differential scanning calorimetry (DSC). The results are shown in Table 1. It is found that the increase of moisture will decrease the glass transition temperature because water can act as plasticizer, lignin - The lignin-hydrogen bonds between lignin molecules are replaced by lignin-water linkages [15]The glass transition temperatures of the three industrial lignins were measured by DSC, differential scanning calorimetry (TMDSC), thermomechanical (TMA) and rheology methods, and were found to be directly The sample is warmed up and the DSC curve shows a wide endothermic peak due to the evaporation of moisture resulting in a larger error as shown in Figure 4. Therefore, the sample must be preheated to evaporate the water while eliminating the preheating history , While the lignin component is more sensitive to temperature, pre-heat treatment temperature reached 120 ℃ will bring about thermal decomposition, resulting in structural changes [16]. Therefore, the termination temperature of the preheating treatment was set to 90 ° C and the results are shown in Table 2. Methanol-soluble lignin (MS) was found to contain less condensation than the original lignin and methanol-insoluble components (MI) Structure, with a lower glass transition temperature.
3.2 lignin bonding effect
Molding process, lignin from the thermal transition from the role of bonding and solidification and filling, is the main component of the biomass itself from the role of bonding [17-18]At 70 ~ 110 ℃, lignin began to soften, with a certain viscosity, at 200 ~ 300 ℃, showing a molten state, the viscosity becomes larger at this time, under a certain pressure, with the raw materials of cellulose, hemicellulose Through the molecules attract each other and winding bonding molding[19-20].
3.3 lignin structure
STELTE and so on [9]It is believed that the hydroxyl groups, especially the phenolic hydroxyl groups, are easy to form abundant hydrogen-bonding structure, which will promote the bonding formation and increase the mechanical strength of the particles.The eucalyptus and hardwood lignin contents are similar (according to the American Renewable Energy Laboratory Database, The wood chips contained 26.91% ~ 28.16% of lignin and the hardwood chips contained 23.87% ~ 28.55% of lignin), but the actual energy consumption of eucalyptus chips was high and the density and intensity were low after forming, probably because eucalyptus lignin Based structural unit, hardwood lignin to guaiac wood based structural unit [21], The lignin-based structural unit lignin bonding index is lower than guaiac wood based structural unit, the forming effect is quite different[13].
4 raw chemical composition
Biomass has a complex structure, including cellulose, hemicellulose and lignin, as well as extractives and ash [22-23]. The different components in the molding process has a different role, shown in Figure 5. Different types of biomass composition and structure of different molding difficulty and the effect is quite different[24].
4.1 Cellulose
Cellulose is a highly ordered linear macromolecule polymer formed by β-1,4 glycosidic bonds linking hundreds to thousands of D-glucose. The smallest repeat unit is cellobiose (C 6H10O5)nIn plant cells, cellulose forms crystalline microfibrils that are surrounded by amorphous fibers that have both crystalline and noncrystalline regions [25]Cellulose crystal structure and rich hydrogen bond make it can not be used as a binder in forming, but it will become softer after heating.The filaments connected by hydrogen bond play a similar role of 'rebar' in the fuel, Become a 'skeleton' of the fuel. "Jiang et al. [26] studied the properties of cellulose powder alone and found that increasing the water content, pressure and temperature within a certain range increased the particle quality with a water content of 14% -29% and a pressure of 3 ~ 4kN, the best molding temperature is 100 ℃.
4.2 Hemicellulose
Hemicellulose is a polysaccharide polymerized from different kinds of monosaccharides, the polymer chain is amorphous and short chain exists.In the hardwood, the hemicellulose main chain from the xylose unit by β -1, 4 glycoside Bond, the chain consists of β-1,2 glycosidic bond and 4-O-methyl glucuronic acid linkages composition of cork hemicellulose backbone contains less acetyl, but there are connected to the main Chain arabinofuranose side chain molding process, under the combined effect of pressure and hydrolysis of hemicellulose can be degraded to lignin, play a binder role.
4.3 Lignin
Lignin is a kind of aromatic compound with three-dimensional polyphenol network structure formed by polymerizing phenylpropane monomer of guaiacyl, lilac and p-hydroxybenzene, which has different content of the three monomers.
Cork in more guaiac wood-based structural units, hardwood to lilac-based structural units .Lignin content on the molding has been studied [27]VANDAM et al [28]Found that the temperature above 140 ℃ can increase the lignin bond strength; CASTELLANO and so on [29]Found that raw material components are the key factors affecting particle quality, high lignin content, low extract content of raw materials molded particles with better physical quality; LEHTIKANGAS'30 'found for fresh and stored bark, Sawdust, raw materials for harvest residues, particles with high lignin content have good durability; HOLM, etc. [31]The higher the content of lignin is, the better the internal bonding of the particles is. The higher the temperature is, the higher the glass transition temperature is, the higher the mechanical strength of the particles is. However, BRADFIELD et al. [32]Lignin is considered as poor internal strength of the viscous material, within a certain range of crystal structure of wood polymers play a role in bonding, but its content exceeds the critical value, excessive accumulation of adhesive material between the crystals, reducing Strength and Durability of Granules; WILSON [33]It was found that the relationship between lignin content and grain durability was not obvious for hardwood and softwood.
4.4 Starch
Starch is a D-glucose polymer divided into branched amylopectin and amylopectin without branch, insoluble in water at room temperature, the formation of starch paste under the action of certain temperature, moisture, pressure and compression time (Irreversible), play the role of binder and lubricant '34', which will help the fuel discharged from the mold.There are two starch gelatinization mechanism: ① under the action of moisture and temperature, the crystal structure is damaged; ② in the subject In the process of pressing, the starch grains are broken by shearing and extrusion, the higher the degree of starch gelatinization, the more obvious the bonding effect, and the greater the mechanical strength of the fuel[35].
4.5 protein
Under certain temperature and moisture conditions, the protein in the raw material will be denatured, and proteins, fat, starch and the like will be transformed into new substances, which will help to improve protein cohesion. According to BRIGGS et al [36]Research, increase the protein content of raw materials, can improve the mechanical durability of products, and the undenatured protein can improve the product physical quality more than the denatured protein. [37]It is considered that the raw material contains enough natural protein to enhance its function as a binder.30 SOKHANSANJ et al found that the higher starch and protein content of the raw material than the cellulose-only raw material prepared mechanical durability is better, The optimum moisture content for raw materials containing only cellulose is 8% to 12%, while for starch and protein-rich raw materials, the optimal moisture content is up to 20%. The protein extracted from soybeans, wheat, rye and barley Forming a role in promoting the extraction of protein from corn on the contrary [39]The promotion effect of protein and starch on the forming was studied. It is considered that the crude protein is better than the denatured protein to promote the forming. Compared with the added starch, crude protein can promote the formation of crude protein better than the native starch or gelatinized starch.
4.6 fat
The fat in the raw material acts mainly as a lubricant in the forming process and a small amount of fat promotes the formation because the natural fat in the cell wall is squeezed out during the compression process to act as a "solid bridge" to improve the durability. Fat blocks the binding between the particles because the fat is distributed between the particles and its hydrophobicity can inhibit the binding of other water-soluble components (such as lignin, starch, proteins, etc.) and reduce the binding strength between the particles [30]CAVALCANTI'40 'was used to study the bonding properties of starch, protein and fat in 13 raw materials. The results showed that the fat content was more than 6.5%, the durability of CAVALCANTI'40' was poor, which was unfavorable to improve the bonding effect of starch and protein.
5 forming parameters
5.1 pressure
Pressure is a necessary condition for forming, under certain pressure, the raw material can be compressed and formed. Within a certain initial pressure range, the pressure and product density are basically linear relationship. Exceeding the pressure range, the pressure and product density are exponential, Pressure reaches a certain value, the product density increases with pressure is not obvious [41]Wu Kai and so on [42]It has been found that the toroidal mode torque has an exponential relationship with the Poisson's ratio of the material in the ring mold forming equipment and the exaggerated mode compression ratio also has an exponential relationship with the torque. [43]The study found that the pressure exponential relationship with the particle length, the temperature increase can reduce the required pressure.
5.2 moisture
Moisture is an important parameter to be controlled in the forming process [17]The results show that water can reduce the glass transition temperature and promote the formation of 'solid bridge' structure to increase the contact area between particles.Water is a natural binder and lubricant, a certain amount of water can form a thin film between the particles, increase Large contact area between particles and the interaction force (Van der Waals force), the film can also reduce the friction between raw materials and mold and raw material particles, reducing energy consumption[44-45].
However, too much water will reduce the product quality, because too much water can not be absorbed by the particles and attached to the surface, so that the particles are not easy to compress different raw materials required for the best moisture content is not the same '46', greater than or less than The best value, product quality will be reduced. LI and so on [47]Bark, sawdust, alfalfa briquetting and found that the best moisture content of about 8%.
MANI [44]It is pointed out that the optimal moisture content of fiber raw materials is 8% ~ 12%. OBEMBERGER et al [48]The results showed that when the moisture content of rice straw was 8% ~ 12%, the product quality was the best.Gang Yang et al '24 'studied the relationship between the moisture content and the density of corn straw, soybean straw, reed and so on, and considered the moisture content as 12% ~ 18% is more appropriate.
5.3 particle size
Particle size is also one of the factors that affect the molding, the smaller the particle size, the more easily compressed, the better the quality of the product [49]Inhomogeneous particle size, large differences in morphology, or larger particle sizes all lead to reduced product density, reduced strength and cracks on the surface and interior HARUNA et al. [50]According to the forming experiment of agriculture and wood raw materials, it was found that the smaller the particle size, the larger the density of the formed particles.The MANI study found that when the size of the crushed mesh is gradually reduced from 3.2mm to 0.8mm, the product density becomes larger.In fact, The quality of the product is better because the raw materials of different sizes are mixed and shaped, because the fibers or twisted sheets in the particles have bending and winding properties, and when they are gathered, they are entangled with each other to form a 'solid bridge' structure 6 so as to improve product quality[44].
5.4 Temperature
Molding process, raising the temperature can soften the lignin, play a role in bonding, but also make the material itself become soft, easy to compress, but the temperature should not be too high, or serious carbonization of raw materials, molding failure. The temperature is generally 80 ~ 150 ℃[51-53]Wang Gongliang and other '54' used response surface methodology to study the forming characteristics of corn stalks and found that there was an interaction between moisture and temperature. When the temperature was lower than 100 ℃, the moisture content decreased. When the temperature was higher than 100 ℃, the moisture content increased, The same, there is the lowest energy consumption at 100 ℃.
6 Discussion and Suggestions
(1) Compounding and forming technology utilizes the difference of physical and chemical characteristics of different raw materials, and adjusts and shapes them according to a certain proportion to realize the complementarity of raw material components, enhance the mechanical interlocking performance between particles and improve the molding effect, and is effective for solving the single raw material that can not meet the standard requirements Technical means should be one of the main directions of future research.
(2) different scholars adopt different research methods, different from the original
Therefore, it is necessary to study the thermal transition characteristics of raw materials in depth, which can provide a theoretical basis for maintaining a reasonable temperature range for the production of molding fuels and reducing the energy consumption of production.
(3) Lignin plays the main role in the forming process, but it has the softening bonding effect only after the thermal transformation.Lignin plays a different role in the forming process of different raw materials, the effect of lignin content on the forming There is not a unified understanding of the impact of lignin structure on molding is still very small, therefore, it is necessary to study its thermal transition characteristics, based on its structural characteristics and content of different raw materials forming classification research, which can be Production of raw materials deployment, forming a reasonable set of parameters to provide theoretical guidance.
(4) The construction of 'solid bridge' structure can obviously improve the quality of the product, but the study on its construction conditions and microstructural features in the process of building is still not deep enough. The research in this aspect can clarify the way of the inter-particle binding and improve Fuel quality indicates the direction.
(5) The fundamental reason for the improvement of molding fuel quality is the change of functional groups and chemical bonds in the molding process.At present, the research in this area is not enough, and we can get a better understanding of the combination of functional groups and chemical bonds and the way of activation and fracture in the molding process, Micro-perspective reveals the lignin bonding, is the basis for the study of forming mechanism.
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