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How Sawdust Pellet Machine Makes Quality Sawdust Pellets?

By Jack Huang    May 15, 2014
sawdust, the raw material for making wood pellets

What is sawdust and sawdust potential world widely?

Sawdust or wood dust is a byproduct of cutting, grinding, drilling, sanding, or otherwise pulverizing wood with a saw or other equipment. Sawdust as one of the best raw materials for fuel pellet production is abundant world widely. The global potential of sawdust is around 75Mm³, and 2/3 of which are potentially located in Brazil, Russia, Canada, and China, according to the diagram of Dr.M.Junginger, presented in Global sawdust potentials and pellet supply chains. The availability of large amount of sawdust at a fair price from the sawmills and planning mills enables the large scale sawdust pellet production possible.

sawdust potential for pellet production by sawdust pellet machines

 

The theoretical excess sawdust potential after meeting raw materials demand of the forest industry.

 
Which is the better raw materials for pellet making, the softwood sawdust or hardwood sawdust?

The heat value of sawdust depends on the density, lignin and extractives contents of its wood species. Since softwoods contains more lignin than hard woods, the calorific values of the softwood sawdust are higher than that of the hardwood. And also lignin is a natural binding materials for pellet production, so the softwood sawdust is better than the hardwood sawdust in terms of sawdust pellet production. Therefore, the sawdust of hardwood are usually mixed with softwood sawdust for pellet production.
 

sawdust for pellet production by sawdust pellet machine

Piles of sawdust for pellet production
 

How to make quality sawdust pellets with sawdust pellet machine?

Sawdust preparation

As the sawdust arrived in the sawdust pellet machine plant, the moisture content should be tested. The moisture content gives the proportion of evaporable water in the total weight of material. The sawdust with high moisture content should be stored separately from the low moisture content sawdust. The production of 1 ton pellets needs the about 7 bulk m³ sawdust with the moisture content around 50-55%. The optimum moisture content of the sawdust is 10-15%. The sawdust with higher moisture content should be dried before entering the sawdust pellet machine while the lower one can skip the procedure. If you want to know more about raw material moisture, read this article.

Drying

The wet sawdust should be dried before size reduction because the wet sawdust consume more power to be hammered than the dried one does. And also the wet sawdust may risk the screening by clogging or smearing the screen. Drying can be done by the dryers.

Screening

The screening of the sawdust is to remove the stones, plastics, metals, or other hard materials that may damage the equipment. The stones and stone like hard materials are removed by a stone trap when the sawdust passes over the screen while the metals are picked up by a magnet .

Hammering

The hammering is meant to homogenize the sawdust to an even-size. In the piles and piles of sawdust there are might be wood lumps, dead knots, etc. which needed to be hammered so that they can pass the die holes of the sawdust pellet machines.

Pelletizing

When the sawdust goes into the pelletizing chamber of the sawdust pellet machine, the lignin of the sawdust will be heated up to 120-130 ℃ by the heat produced by the running sawdust pellet machine and by which the lignin will be plasticized and binds the particles together to help to mold the pellets. The pellets are molded by being pressed through the pellet dies of the sawdust pellet machines and then are cut off by the cutter with the desired length. During the pelletizing, the binder may be needed to improve the pellet strength and durability. Watch this video to see how a sawdust pellet mill works.


2T sawdust pellet machine plant running in Serbia
 

Cooling

The moment when the pellets leave the sawdust pellet machines, they are hot and plastic. Then they need to be cooled down to become hard and rigid and to dry up so that the final moisture content after the cooler can be as low as 6% which may be increased up to 8 to 10 % by absorbing moisture from the surround air.

Packaging

Before packaging, the pellets will be screened again for recycling the fines. And then the pellets will be packaged. For domestic users, the pellets are preferred to be packed into bags with the sizes of 12, 15, or 20 kg. while for exporting, the pellets are usually shipped in one tone bags or in bulk which are usually wrapped in plastic to keep the moisture out. Compare with the large bulks or bags, the small bags are better for the pellets because the pellet are subject to less abrasion during delivery.

Sawdust pellets content and quality analysis

Since the determination of the content of sawdust pellets is connected with the application of relatively complicated apparatus and is also geographically dependent, it is important to know the content of the wood.

The heat value of fuel wood depends primarily on its moisture content, which is governed by its element composition: carbon, hydrogen, oxygen, nitrogen and a some extent sulphur, non-compounds-ashes, as well as moisture content.

Generally, the wood comprises 49 -50% of carbon, 43-44% of oxygen, about 6% of hydrogen, 0.1-0.3% of nitrogen and 0.1-1.0 %of ash, The ash contains calcium, potassium, sodium, magnesium, small amounts of phosphorus, sulphur and other elements(UGOLEV, 2001).

The content of the sawdust pellets determines the quality of the pellets. And the processing variables related the sawdust pellet machines also influence the pellet quality significantly. While according to international standards, the content of sawdust pellets should be as follow:

information on sawdust pellets made by sawdust pellet machine
Sawdust pellet information table
 

High quality sawdust pellets are with low moisture content and high heating energy and efficiency. They are easy to be handed, burned, transported and stored. The sawdust pellets are usually used for heating by larger scale power plants, biomass boilers, and centralize steaming boilers and also popular with the household for heating and cooking.