Maintain of Screw Extrusion Pellet Mill

The screw extrusion pellet mill is to use the screw rod to compress the biomass material with heating from outside to keep a certain temperature for formation. Between the gaps between the screw rod and formation sleeve, the lignin content and fiber content is softened without any addictive or binder to compress the feedstock into pellets. The construction includes:

1. Screw rod
1) Construction of screw rod
During the formation process, the conveying and compressing of biomass material are completed by the screw rod and formation sleeve. The shape and size of screw rod has great effect to the formation of material. There is feeding section and compressing section on the screw rod, and the feeding section usually adopts rotation with cylinder-shaped constant pitch; the compressing section adopts the constant pitch with a certain taper or the rotation with varying pitch. The compressing section works under high temperature and high pressure, and the rod and the feedstock is in the state of constant drying friction, which is the main reason for the quick wearing of screw rod.



2) Method of decreasing the wearing of screw rod
At the end of the screw rod or the last ring of the top of the rod, the rotating blade undertakes the biggest pressure with heavy wearing. As long as this part of wearing has been deal with, the problem of wearing of screw rod is solved. The screw rod with varying pitch has high cost due to the complicated processing technology, it is not often been used for that reason. The top of screw rod with constant pitch which are mainly for compressing is wearing heavily during the formation process. In order to prolong the service life of it and solve the problem of heavy wearing of the top of the rod, the first method is to deal with the top of rod with heat which makes the surface hard. For example, the wearing part is treated with Tungsten cobalt alloy spray welding, bead weld 618, Tungsten carbide surfacing welding rod, tungsten carbide treatment or boriding, as well as vibration welding. But the service life of the screw rod has been improved with high cost, the clients are not accepted. The second method is to use the detachable movable rotating head installed at the top of rod which are heavy wearing. After wearing, the top can be replaced and the rod body can be used continuously. The third way is to contract the rotating blades at the last ring of rod toward the basis of screw rod, so that the back part of the blade undertakes part of the pressure. In addition to that, the screw grooves at the back part should be Wear - resistant alloy welding, to make them has a knuckle, increasing the strength and wearing-resistance of blades. The length of the rod is not too long or to short, if the diameter of screw rod is 76mm, then the length of the rod should be about 350mm. Length of optic axis at the top of the rod decided upon the spices of material and the requirement of the pellet fuel. Its function is to make the pellet stick is hollow. Generally speaking, the sawdust material for formation is short, the straw material or the formation fuel need to be carbonized should be long.

2. Formation sleeve
1) Structure of formation sleeve
The formation sleeve should have good cooperation of size with the rotating rod. During the process of compressing and pushing of biomass material, it mainly depends on the rotary rod to push the biomass to form layer by layer. It has comparative high speed movement between the biomass material during the pushing and extruding at the front side and top of the rotary rod, and it increase the energy consumption of product per unit. Due to the own features of biomass material and the comprehensive force of the rotary rod, part of the feedstock can adhere inside the rotary blades or the inside wall of formation sleeve, the movement of viscous material has big fiction force with the rotary rod and formation sleeve, it prevents the viscous material moving with the rotary rod inside the formation sleeve but not pushing the material. Therefore, the tangential friction force between the feedstock and inside wall of formation sleeve should be increased and the axial friction should be decreased. So, there are several portrait grooves inside the formation sleeve. The structure of grooves, amount and length of inside wall of formation sleeve should be on the basis of structure of rotary rod, types and physical texture of material for formation and the practical situation of rotating speed of rotary rod.

2) The measures of reducing the wearing of formation sleeve
The material of formation sleeve adopts 45# steel, nodular cast iron and varied alloy material. In order to lengthen the service life of formation sleeve, the followed measures can be adopted。 1）The inside wall of formation sleeve can be spayed with anti-wearing material to improve the wearing ability and lengthen the service time. 2) When connecting the formation sleeve and feeding tube, several A type and B type slim ring should be added inside the pressing ring. When the compression section of formation sleeve can not work any more, draw out A type ring and add a B type ring which pushes the compression section forward, keeping the gap with the screw rod which lengthen the service life of formation sleeve3) cone tank of formation sleeve which are the easiest wearing part should be processed through the way of splitting, the replaceable formation sleeve at the compression section can be replaced after wearing. Top of the screw rod and cone tank can be replaced without replacing the total rod and formation sleeve which can save part of the maintaining fee, to lengthen the service life of screw rod and formation sleeve, so it production cost for formation of biomass pellet fuel should be reduced.

3. Heating device
The most important function of heating device is to pre-heat the formation pellet mill at the time of starting the machine. There are many way of heating for screw biomass formation pellet mill, it can be classified into heat from inside and outside, heating with electricity and steam. And the heating device has many forms of structure, like electricity heating tube ring, cylinder heating ring and cast aluminum heating ring.

The general requirement for heating device is: a certain power for heating, high heating speed, adjustable heating temperature, good heat preserve ability, reliable and long service life. The middle and small pellet mill mostly adopts the heating way from outside, the large scale pellet mill adopts the inside heating way or steam for heating.

Another important function of heating device is for stable formation. When processing the biomass material by the screw pellet mill, there are a large amount of heat from friction among the screw rod, material and formation sleeve which make the formation part and material high in temperature. The temperature is difficult for formation. If without the assistant heating form outside, the machine is cold for starting up. Or the normal formation can not ensure the temperature for soften the lignin, and it increases the energy consumption of pellet mil and wearing of formation parts.

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Small Pellet Mill
Biomass small pellet mill is a kind of special equipment which takes sawdust, palm, peanut shell and other crops as raw materials and compressed them into pellets. The pellets produced by this kind of equipment have big density, high heat value, combustion resistance. Due to its small volume, simple operation, it can be used for making pellets at home.

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