Material input into the pelleting chamber
Regulated material input into the pellet mill is important for maintaining consistent pellet quality. During the pellet production process moisture is released as steam. If the rate of material feed is greater than the productivity rate of the pellet mill, problems may occur. If the steam generated from pelletization cannot escape the pelleting chamber, the material above the roller will absorb the steam. This can cause bridging of the input material, which will stop material feed or adversely affect pellet quality. If the material above the roller absorbs the steam this will increase that material’s moisture content, and the characteristics of the material in the pellet mill, as well as pellet quality, will change.
Therefore the material feed rate should be regulated as a factor of pellet mill productivity. At the beginning of production, the pellet mill should be fed at a lower rate and increased as the pellet mill temperature increases. Therefore the feed rate should mirror the performance of the pellet mill and slowly increase up to maximum productivity. As the productivity of the pellet mill is variable based on the characteristics of the raw material, the feed rate must also be adjusted. The target feed rate should be that which covers the rollers but no more.
Pelleting chamber temperature
As heat and pressure are key factors for pellet production, the die’s temperature is essential to pellet quality and mill productivity. When starting pellet production, the material should be slowly fed into the pellet mill to increase the die’s temperature. If too much material is fed into the pellet mill at start-up there is a high chance of blocking the pellet mill die. This is where a high chrome die is an advantage as the chances of blocking the die at start-up are reduced. To get the die to optimum temperature could take from several minutes to half an hour. Once the die is up to temperature, maximum productivity will be obtained.
Pelleting conditions with dry feedstock
A: As the dry material enters the pellet mill, due to the action of the rotating roller and die, material will engulf the pelleting chamber.
B: Due to the material’s low density and lack of moisture the roller will not be able to compress the material through the die.
C: Dry material will not form a carpet, as there is not enough friction to generate pressure as the material passes under the roller. The lack of pressure therefore means a lack of heat and that the lignin cannot melt to bind the material to form a carpet.
D: As the dry material does not generate enough friction and pressure the material will run freely through the die without compression
E: The end result is loose dry materials freely passing through the pellet mill, and no pellets produced.
To make the material form pellets, water must be added to add body to the material. More water adds more pressure—to a certain extent. Slowly adding water to the material and testing the results will eventually generate the necessary friction to produce pressure and heat. The production of dense, shiny pellets gives and indication to the necessary amount of water to add. For very dense and low lignin materials, additional vegetable oil maybe required to lubricate the process and aid binding.
Pelleting conditions with overly moist feedstock
A: As the wet material enters the pellet mill the high moisture content will produce very high pressures as the roller tries to compress the material through the die. The high pressure will result in high temperatures and large amounts of steam will emerge from the pellet mill.
B: Due to the high pressures generated as the material is compressed through the die, a high load will be placed on the pellet mill motor. If the high pressure continues this could stall the pellet mill motor, and could also affect the roller bearings.
C: Wet materials will form a carpet; again this will be release large amounts of steam.
D: The wet material will create high pressures as it passes through the die; this may result in the material passing through very slowly. However, even though the wet material generates high pressures, the temperature needed to release the material’s natural lignin is not reached. If the material’s lignin does not melt, the material will have no binding qualities.
E: The pellets that exit the pellet mill will be soft and loose, perhaps in small moist lumps. As there is still too much moisture in the pellet, it will expand and release moisture. Therefore the pellet cannot form a smooth, compact cylinder, as it should.
Processing wet materials can in some cases block the die due to the pressure it generates. In this case the material generates a greater, more resistant force when it passes through the die, than the force generated by the roller. A blocked die can also occur due to corrosion within the die holes. If a die has become blocked, drilling the material out may be required. This is a lot easier if the die is still warm, as the material has not dried and cooled and the lignin has not set.
A: The material first enters the pellet mill and comes into contact with the roller.
B: Some of the material will be compressed under the roller through the die holes. A material, which lacks a binder, may struggle to do even this. It may display properties more like a dry material, as the materials cannot bind together under compression.
C: Due to a lack of a binder the material may be unable to form a carpet, and again it may display similar properties to a dry material.
D: Even with sufficient moisture to give the required pressure and heat, the material may not compress at all. Generally though some compression and binding will take place, but not to the levels required.
E: If the lack of a binder is extreme the material may not form a pellet at all. In many cases small layers of compressed material will emerge, but not a complete pellet. A short pellet is not detrimental to its purpose, however it should be checked that once the pellets cool they do not easily crumble into dust. If this occurs the lack of binder needs to be addressed.
The above issue could be due to a lack of binder or an inability to release the naturally occurring binder in the material. For example, the material may possess a sufficient amount of lignin naturally, yet if there is insufficient moisture to generate the required pressure to melt the lignin, the material will not bind. Some materials however contain insufficient amounts of lignin and must be supplemented with a binder. As additional binders increase production costs, it is worth experimenting with increasing the moisture content to aid material binding.