From Wikipedia, the definition of wood pellets is defined as the most common type of pellet fuel that are generally made from compacted sawdust and related industrial wastes from the milling of lumber, manufacture of wood products and furniture, and construction. Pellet plant is the facility that manufactured wood pellets and packed and sold them in bags for both commercial purpose and personal use. Typically, based on the scale of production, there are two kinds of pellet plant, the large pellet plant and the small pellet plant. The question of how to make wood pellet is literally the complete manufacture process of the pellet plant.
Wood Pellet Manufacture Process Step By Step
Raw material preparation
To prepare the raw materials is the first thing when we make wood pellet. For small wood pellet plant, the raw materials quantity is not much. If the wood pellet is for personal use, the raw materials are normally the crop waste in the farmland. If the small pellet plant produce wood pellet for commercial purpose, the raw materials are normally the wood shavings collected from saw mill nearby. But for large wood pellet plant, the preparation of raw material become a serious problem which impact the pellet plant capacity directly. As the demand of raw materials for a large pellet plant is massive, the wood shaving quantity of a saw mill is not enough for a large pellet plant. Therefore, a large pellet plant usually use wood logs and branches as raw materials, or to find some saw mills and sign a long term wood shavings supply contract with them.
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.
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.
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.
Above is the manufacture processes of wood pellet. Due to its different utilization, the wood pellet manufacture processes may be a little bit different in real pellet plants. For example, if the wood pellet is made for personal usage, there are several steps are not necessary. There may be no need to use the wood chipper as the materials size can be put directly into the hammer mill. Also the cooling and final sieving processes are not necessary, because the wood pellet will be piled in the barn. But if the wood pellet is made for commercial purpose, the commercial wood pellet also need a proper package, so the commercial wood pellet plant also need a package machine to pack the wood pellet into bulk bags or retail bags.