Wood pellet is a solid fuel that is produced by crushing and densifying waste timber like sawdust, forestry residues, industrial byproducts like old paper and forestry wastes. The length is 1-2 cm and the diameter is generally 6, 8, 10, or 12mm. Wood pellet production doesn't need to add binder as lignin, an element of wood, plays a role of binder and helps to pelletize when it is melted by heat.
Table. Characteristic of wood pellet fuel
|Amount of heating generation (lower limit)
||4.7 kWh／kg ＝ about4,000 kcal／kg
|Kerosene conversion value
||pellet：kerosene ＝ about2.1tons：1m3
||wood except bark: under 0.5%, bark: over 2.5%
Characteristic of wood pellet is very easy to manage compared to other biomass fuel. As the shape and hydrous rate are stable, wood pellet is suitable to use in automatic combustion installation and it can remarkably save time and labor for both of electric power generation boiler and pellet stove for house. It is possible to transport long distance because wood pellet has high energy density and much energy can be transported at once. Also wood pellet which is heat-treated, can be stored in long term with less risk for molding.
How to Make Wood Pellet
Flow chart of wood pellet plant production line
(1) Initial size reduction
For large pellet plant, when the raw materials are ready, the first thing to do is raw material size reduction. The raw materials to produce wood pellet must be under the diameter of 5mm, so the logs and branches must be processed into small pieces. In this stage, the large pellet plant usually uses the wood chipper, which is a common size reduction equipment that can cut the wood logs and branches into small wood chips with a diameter no mor than 40mm in order to be further processed. On the contrary, this is not a necessary step for the small pellet plant. If the pellet plant use wood shavings as a raw material, that means the raw materials has already been processed into small pieces in the saw mills, so there is no need to use the wood chippers to cut the raw materials agairn.
For large wood pellet plants, after the initial size reduction step, the raw materials must be dried. In order to produce high quality wood pellets, the raw material must maintains a moisture content of 8% – 12%, so the raw material must be put into a dryer to reduce its moisture content. The most common dryer for large pellet plant is the rotary drum dryer, which is the most cost effective choice for large pellet plant. The drying process is also a necessary step for the small scale pellet plant to produce commercial wood pellet.
(3) Initial Sieving
For large wood pellet plants, after the drying process, the raw material will be sieved to separate the contaminants such as stone and metal particles away from the wood raw materials. These contaminants may cause the mechanical failure of the pellet mills, so if the raw materials of your pellet plant has the potential to be contaminated by these solid particles, it is necessary for you to sieve the raw materials before fed them into the pellet mill. The machines for the initial sieving process is normally a de-stoner or magnetic separator. Fo small scale pellet plant, due to its limited capital, this is not a necessary step.
Grinding is also a size reduction step to make wood pellet, it is a necessary step for both large and small pellet plants. During the grinding process, the raw materials will be fed into a hammer mill which will grind the raw materials into smaller pieces with a diameter under 5mm. But the hammer mill can only process the wood raw materials with a diameter no mor than 50mm, therefore, the wood logs and branches can not be fed directly into the hammer mill, they must be processed by the wood chipper first.
After the grinding process, the wood raw materials are reached the size and moisture requirements for wood pellet manufacture. Therefore, the following step is to pelletize the raw materials into wood pellet. The machine used to pelletize the raw materials into wood pellet is called pellet mill. There are two kinds of pellet mills, ring die pellet mill and flat die pellet mill. Ring die pellet mill has a capacity over 1ton per hour, it is used for large pellet plant. Flat die pellet mill has a capacity about 500kg per hour, which used for small pellet plant.
After the pelletizing process, the wood pellet is very vulnerable. It has been press over and over in the pellet mill, which makes it hot and easy to be deformed. To make the wood pellet regain its proper rigidity and temperature, the wood pellet will be fed into a pellet cooler right after the pelletizing process. The types of pellet cooler can be divided into three types, the horizontal cooler, the vertical cooler and the counter flow cooler. All of them can be used to cool the wood pellet, but the counter flow cooler is my recommendation, as it is the most advanced pellet cooler in the market.
(7) Final sieving
At this stage, the wood pellet sieving purpose is different from the initial sieving. This step only exist in the commercial wood pellet plant. As the commercial wood pellet has a strict limit on fine content, therefore, after the pellet cooling process, the wood pellet products will be sieved in order to separate the fine particles from the well made wood pellet. For the final sieving process, the vibrating screen is a common equipment for wood pellet plant.
Advantages of Wood Pellets
Table. Advantages and disadvantages of wood pellet fuel
||High quality (shape and hydrous rate are stable)
|dry, easy to store
|burning under environmental compliance is possible
|suitable for automatic combustion operation
|made of regional renewable resources
|energy density is higher than wood chips, suitable for transportation and storage
|transportable to areas with high energy demand density because of its high energy density
|ideal for closed renewable energy system
|possibility to be an alternative for energy system from smaller to larger scale
||labor-intensive more than heating with gas, oil and electric power in house
|credibility in fuel supply, transportation and combustion is inferior to gas, oil and electric power
|need storage capacity three times as large as oil's
|weak to moisture
Dried timber 1ton can generate energy equivalent to electric power 1,000 kWh or heat 4,500kWh. Energy ratio for pelletizing in comparison with wood pellet product's is as follows:
To pelletize dried materials needs energy equivalent to 8-13% of energy contained in wood pellet product
To pelletize materials (sawdust) after drying needs energy equivalent to 10-25% of energy contained in wood pellet product
Whole process of pelletization of wood chip (including drying and crush) needs energy equivalent to 18-35% of energy contained in wood pellet product
We provide customers with cost-effective solutions that are reliable, professional, affordable and add value to their businesses.