Table of Contents
In the context of today’s quest for sustainable development and clean energy, wood pellets are gradually coming to the fore as an environmentally friendly and efficient energy product. So how exactly are these seemingly ordinary wood pellet making?
Collection and selection of raw materials
The production of wood pellets begins with the collection of suitable raw materials. The main raw materials come from a wide range of sources, including wood processing residues, waste wood from the furniture industry, waste wood from the construction industry and forest residues.
Wood processors generate large quantities of sawdust, chips and flakes during the production process. These residues are usually rich in cellulose and lignin, which are high quality raw materials for the production of wood pellets. The furniture industry produces offcuts such as strips and blocks during the production of furniture, which can also be recycled. Waste wood from the construction industry, such as waste squares and waste stencils, can also be used as raw material for wood pellets after appropriate treatment. Forest residues such as branches and tops, although more irregular in size and shape, can also contribute to the production of wood pellets after collection and processing.
When raw materials are collected, they must be rigorously screened. Firstly, it is necessary to remove the impurities in the raw materials, such as sand, gravel, metal, etc. These impurities will not only damage the production equipment, but also affect the quality and combustion performance of the wood pellets. Secondly, the moisture content of the raw material should be controlled, generally speaking the moisture content should be between 10% – 15%, too high or too low moisture content will have a negative impact on the subsequent production p
Pre-treatment of raw materials
Crushing and sizing
The collected raw material is usually of different sizes and needs to be crushed and sized for subsequent processing. Larger pieces of wood need to be processed into chips using a large wood chipper, and larger chips need to be processed into smaller particles using a hammer mill. Generally, the size of the crushed particles is required to be between 3 – 5 mm, which not only increases the surface area of the raw material and improves the efficiency of the subsequent processing, but also ensures uniformity of the particles.
Dry Processing
The moisture content of the raw material is critical to the quality of wood pellets. If the moisture content is too high, the pellets will be difficult to form during the pelleting process and will be susceptible to mould; if the moisture content is too low, it will increase the energy consumption during the production process. It is therefore necessary to dry the raw material to control its moisture content within the appropriate range. Commonly used drying equipment includes drum dryers, air-flow dryers, etc. In the drying process, the drying temperature and time should be strictly controlled to avoid over-drying or under-drying of raw materials. Generally, the drying temperature is between 100 – 150 ℃, and the drying time depends on the initial moisture content of the raw material and the performance of the drying equipment, usually ranging from several minutes to several hours.
Press pellet
Compression pelletisation is the core aspect of wood pellet production. The pre-treated raw material is fed into the pellet press where it is compressed into pellets by the action of the pellet press. The operating principle of the pelletiser machine is to force the raw materials through the small holes in the dies to form cylindrical pellets by extrusion and friction.
During the granulation process, parameters such as pressure, temperature and speed of the granulator must be strictly controlled. Too much pressure will cause the particles to be too dense, affecting combustion performance; too little pressure will cause the particles to be loose and not strong enough. Too high a temperature will cause the cellulose and lignin in the feedstock to break down, affecting the quality of the pellets; too low a temperature will make the pellets difficult to form. The choice of rotation speed is also very important, too fast will make the surface of the particles rough, affecting the appearance of the quality; too slow will reduce the production efficiency. In general, the pellets should have a density of 1 – 1.2 g/cm3, a strength of more than 95 per cent, a cylindrical shape and a length of 10 – 30 mm.
Cooling and screening
Freshly produced wood pellets have a high temperature and need to be cooled. Cooling can be either natural or forced. Natural cooling means that the pellets are placed in a well-ventilated place to cool down naturally; forced cooling means that cooling equipment such as cooling towers, cooling fans, etc. is used to accelerate the cooling rate of the pellets.
The cooled pellets must be screened to remove broken particles and impurities. Screening can use vibrating screen and other equipment, according to the size and shape of the particles to be screened, to ensure that the quality of wood pellets is uniform and stable.
Packaging and storage
Screened quality wood pellets are packaged for storage and transport. Packing can be done in plastic bags, woven bags and other materials and can be rationed according to the customer’s needs. Packaged wood pellets should be stored in dry, well-ventilated warehouses to avoid moisture, mould and pests.
In a word, the production of wood pellets is a complex and delicate process, which involves the collection and screening of raw materials, pre-treatment, pellet making, cooling and sieving, packaging and storage and so on. By strictly controlling the quality of each link, we can help pellet plants produce high quality wood pellets and contribute to environmental protection and sustainable development of energy. We believe that wood pellets will be widely used in more fields in the future and become an important force to promote economic and social development.
