As an important alternative of fossil fuels, wood pellets fuel is being widely used for home heating and industrial production at present. The newest market research shows that, biomass wood pellets are in great demand on a global scale. While, how to produce high quality wood pellets? It is a common worry for almost every manufacture. In this paper, I will give you a step-by-step guide on how to make wood pellets.
Raw materials for wood pellets
Before production, the first thing you should consider is where to build a biomass pellet plant, which depends on the availability of materials. So that it is a good choice to put your plant in agricultural or forest areas. Adequate supply of raw materials can reduce the production cost and transportation cost.
Raw materials for biomass wood pellets are various. In theory, almost any sort of vegetation can be compressed into pellets through a complete pellet plant. While in actual production, crop stalks and wood shavings are the two main widely used materials. For each sort of vegetation has its own characteristics, so you can gain the flexibility to choose different materials to meet the production needs.
Production steps of biomass wood pellets
Wood pellets production is generally divided into 8 steps, as you can see below
- Initial size reduction
In the first step, raw materials need to be crushed into small pieces. Whether the raw materials are logs, corn stalks, grasses or other types of vegetations, they must be reduced into a sufficient uniform size for a pellet mill. Because of the big size, putting them directly into a pellet mill would block the circle of die and rollers. There are many types of size reduction equipments, and you can choose disk chipper or wood waste shredder according to the different characteristics of materials.
During the process of pellet production, the moisture content of materials is an important factor that straightly affects the quality of wood pellets. Tested by application, the optimum moisture content of raw materials is from 10% to 15%. Too high or too low moisture content would result the failure of pellet forming. If the material’s moisture content is above 20%, it must be dried by a dryer or mixed with other dried materials. In the same way, it’s also important to keep down the moisture content of finished wood pellets. Wood pellets with higher moisture would cause more smoke when burning, which affects the combustion efficiency directly.
- Initial Sieving
The third step is using magnets and screens to separate the materials from foreign objects. Because the materials are gathered from forestry waste and crop stalks, they are inevitably mixed with contaminants like metal objects, stones and other foreign materials. On the process of initial sieving, bigger stones and other obvious objects can be picked away manually; ferrous metals can be separated by magnets covered on the surface of conveying belt. In some large-scale pellet plants magnetic separators are widely used to sieve the materials. The using of magnetic separators can save human resources and improve the production efficiency.
The next step is grinding the materials in a hammer mill. The hammer mill has a rotating shaft with free-swing hammers attached to. When working, the materials would be broken up until they are small enough to fall through a screen with openings corresponding to the desired particle size. Besides the grinding process, the mixing of the materials is also complete here. In the hammer mills, the materials could be mixed homogeneously, thereby enabling the high combustion efficiency of wood pellets. Facts proved that high efficiency of hammer mills depends on the coordination of every part, includes appropriate hammer patterns, well-sized screens, and well-designed air. So that keeping regular maintenance of every part is the key to make the process stable and smooth.
This step is the key of wood pellets production. After being grinded, the materials would be fed into a pellet mill. Under the compression of die and rollers, materials would be squeezed through the die holes and shaped into pellets. In this step, additives are often used for better binding, lubrication or to decrease combustion problems. There are typically two types of pellet mills, ring die pellet mill for large production scale and flat die pellet mill for small. After the wood pellets are produced, make a preliminary judgment of the quality by externals and constantly improve the ways of production.
After leaving the pellet mill, wood pellets are very hot, soft and releasing moisture. It’s necessary to cool and dry them at first before packaging. The simplest way of pellet cooling is spreading out the pellets and putting them in the open air.
- Final Sieving
As the materials compressed into pellets, some materials failed to be compressed into pellets for lacking of pressure in the pellet mill or lacking of binder, this dust is referred to as fines. If the forming pellets produced are not strong in density, dust exists when pellets collide with each other. The percentage of fines depends on the performance of the pellet mill and the quality of finished pellets. Pellets should be sieving before packaging. The fines can be recycled to the production line. Pellets with fewer fines would be popular with consumers for good looking means high quality in some sense.
- Pack and Store
In most cases, wood pellets may not be used for fuel immediately after produced. In order to keep the wood pellets dry and avoid the influence from water and dampness, the packing process is necessary. To improve work efficiency and reduce the cost, you should equip your plant with a pellet packing machine. As to storage, the packed pellets should be kept in the dry and cooling place with good ventilation and the product should avoid hot sunshine.