The pelletization of biomass fuel needs to meet the requirements on the species of raw materials, pellet sizes and water content. The plant cells contain cellulose, hemicelluloses and lignin. Lignin is a construction unit with aromatic characteristics and a kind of propane type high molecular compound with three-dimensional structure. The content of the lignin in the broad-leaved wood and coniferous wood is 27% to 32% (dry basis), grass weed 14% to 25%. Even lignin is contained in many plants, the composition and structure of lignin are various largely. Lignin is amorphous solid having no melting point but softening point. When the temperature gets up around 70 to 110 ℃， the adhesive force of lignin gets increased. When it gets the proper temperature (around 100 to 200℃), lignin will get softening and liquefaction. At the right moment, certain pressure can compact lignin, cellulose and other lignins together, then the desired pellets will be produced after cooling down. Therefore when heat-die-pressing technology is adopted in stalk pellet fuel production, no additives or adhesives are used, reducing the production cost and extrusion power by taking the advantage of the characteristics of lignin softening and liquefaction under high temperature.
It is obvious that when the temperature is lower than that of the requirement, the lignin can’t be pelletized and, on the contrary, when the temperature gets higher than that of the requirement, the raw materials will be decomposed at excessive speed, making the pelletization impossible. All in all, the temperatures needed for pelletization varies with the species of the materials. However, the proper temperature for the materials in general is around 100℃ to 200℃. As the requirement for productivity is getting higher, the increasing abrasion will produce plenty of heat, which will damage the equipment and plant badly. Therefore it is necessary to control and maintain the temperature at the proper range.
The water content of the raw materials is also a decisive factor for pelletization. When the water content is too high, the steam vapor produced during heating will not be expelled through the outlet in the middle of the materials in time, the outside will get cracked and in the serious situation, detonation is possible. However when the water content is too low, it is difficult to pelletizing the raw materials, for proper amount of water is needed to accelerate the speed of lignin softening and pelletization. As for sawdust and stalk-like materials, the proper water content is around 8% and 14% (naturally the air humid is also a factor that needs to be considered so that the water content needs to be controlled according to the situation). Even though the lignin contents of the materials are different, the water contents needed for pelletizing are basically the same.