How the Quality of Pellets Machine Made Pellets Are Attributed?
Pellet quality is always the concern of the end pellet users. As a pellet manufacture, do you know what factors attribute your pellet quality? Generally speaking, the composition of your raw materials, the binding agent used in the pelletizing process, the conditions of the feed stocks and the pellet machine conditions will significantly influence your pellet quality. However, how to evaluate your pellet quality? What factors attribute your pellet quality? Let us talk these factors one by one.
Moisture content is always important in the whole process of pellet production. Higher moisture content of the raw materials may result in higher moisture content in the final pellet moisture content. When the moisture content of the pellets extruded out of the pellets machines is higher than 15%, it will influence the pellet quality badly. Because the high moisture may rotten the pellets because of the decomposition of the batteries and fungus. When the final pellets moisture content is lower than 5 %, the pellet may easy be broken during fraction caused in transportation and storage.
The density refers to the unit density and bulk density. The density of the pellets are large decided by the moisture content of the raw materials and the size of the particles, the processing pressure of the pellets machines, and the die temperature of the pellets machines. Generally, higher moisture content and larger particles may reduce the unit density and bulk density of the pellets while higher pressure and temperature casted by the pellets machines will increase the unit and bulk density of the pellets. The unit density and the bulk density of the pellets are largely depend on the factors, such as moisture content, particles size, die temperature and processing pressure of the pellets machines. Higher die temperature, lower moisture content, smaller particle size produce high density pellets with proper pressure.
The durability of the pellets refers to the ability or the strength and resistance of the pellets to remain intact from being broken during transporting and storing. The durability of the pellets can be measured by the amount of the fines or dusts produced by spinning a quantity of pellets in a metal box, based on the study of Kaliyan and Morey in 2009.
Usually, the moisture together with the water soluble compounds such as starch and lignin functions as a binding agent as well as a lubricant to increase the pellet durability by increasing the contact area between the particles. However, the durability value will be decreased when the lignin and other extractives is more than 35%. Also, the fat content in the feed stocks will decrease the pellet durability for which will act as lubricant during the processing of the pellets machines.
What’s more, the pellet durability also is also subjected to the particle size. The finer of the particles, the higher durability of the pellets is . The finer particles undergo a higher degree of preconditioning, such as high degree of preheating or steaming which increase the activity of the natural binders and eventually increase the durability of the pellets.
Last but not least, the pellet density is also affected by the pellets machine variables, such as the dimensions and speeds of the dies of the pellet machines. Smaller pellets machine die dimension produce higher pellet durability. Die speeds of about 126 – 268 rpm are suitable for small pellet production while the die speeds of about 6- 7 m/s are always the choices for large sized feed pellet production, according to the study of the Stevens in 1987.
The fines are produced by the broken pellets caused by frictions during transporting and storing. It is simple to understand that the less of the fines or dusts are, the better of the pellet quality is . The bad quality pellets intend to produce more fines which are endanger the burning equipment, by increasing the chance of dust explosion and other combustion problems. According to Shanke et al’s study, the storage temperature will affect the quality of the pellets and the higher storage temperature at around 30 to 50℃ will increase the fines percentage during two months storage time by more than 1 %.
The heating value of the pellets are usually affected by processing variables, such as the die temperature of the pellets machines, particle size of the feed stocks, and the pretreatment of the feed stocks. Usually, the higher heating values come with higher pellet density. The general heating value of wood pellets are around 17 to 18 MJ/kg. However, the proper pretreatment can increase the heating value of the pellets to 20-22 MK/kg when the pellets are torrefied under the temperature of 200 to 300 ℃。