Table of Contents
With the increasing global demand for clean energy, wood pellets, as a sustainable biomass energy product, have received widespread attention. Wood pellets are made from wood and other lignocellulosic biomass after processing, and have the advantages of high energy density, good combustion performance, easy storage and transportation, etc., and have important applications in the fields of home heating, industrial heating and power generation. This article will elaborate on the various aspects of making wood pellets.
Type and quality requirements of raw materials
Main wood material
Sawdust
Sawdust is one of the main raw materials for manufacturing wood pellets. Sawdust is widely available and can be collected from wood processing plants, furniture factories, etc. The sawdust produced in these places during the wood processing process is usually small in size, which is conducive to subsequent processing. For example, the particle size and shape of the sawdust produced by sawmills are already close to the requirements of wood pellets, thereby reducing the amount of pre-processing work.
Sawdust
Wood chips
Wood chips are also a common raw material. Waste wood, branches, etc., can be processed into wood chips using a special chipper. Reusing these waste wood resources not only reduces costs, but also has environmental benefits. When processing into wood chips, ensure that the quality of the wood chips meets the requirements. Generally speaking, thinner wood chips of uniform size are more conducive to crushing and pressing. When processing branches into wood chips,
Wood chips
Bark
Bark can also be used as part of the raw materials in the production of wood pellets. However, the proportion of bark should not be too high, because the composition and structure of bark are different from those of wood. Excessive bark may affect the quality and combustion performance of pellets. For example, the ratio of cellulose to lignin in bark is different from that in wood, and bark may contain more impurities and ash. However, an appropriate amount of bark can increase the diversity of raw materials and improve the physical properties of pellets to a certain extent.
Raw material quality requirements
Raw material moisture content
The moisture content of the raw material is one of the key factors affecting the quality of wood pellets. The ideal moisture content should be between 10% and 15%. If the moisture content is too high, the pellets are prone to deformation and cracking during the pressing process, and the finished pellets are prone to mould and deterioration, affecting storage life and use. Excessive moisture also requires additional energy to evaporate when the pellets are burned, reducing energy efficiency. Conversely, if the moisture content is too low, the raw materials are too dry and brittle, making it difficult to form them during pressing, while also increasing dust production, which is detrimental to both the equipment and the health of the operators.
Wood chips
Content of impurities in raw materials
Non-polluting wood and plant materials should be selected to avoid the use of raw materials containing harmful chemicals. For example, wood treated with chemicals such as paint and preservatives should not be used as raw materials, as these chemicals may release harmful substances during the combustion process, polluting the environment and endangering human health. At the same time, impurities such as soil and stones in the raw materials should be minimized as much as possible, as these impurities will affect the quality of the pellets and the normal operation of the pressing equipment.
Process of making wood pellets
Process of making wood pellets
Prepare ingredients
Collects fibrous biomass materials such as wood chips, sawdust and bark. During the collection process, the raw materials are initially screened.
Crush raw material
Milling is an important step to ensure that the material is uniform and easy to press. Commonly used milling equipment includes hammer mills and blade mills. The material is milled to particles smaller than 5 mm to facilitate better binding during the subsequent pressing process. This process helps to improve the molding rate and combustion efficiency of the particles.
Sieving and mixing of raw materials
The pulverized material may have a different particle size distribution, so it needs to be sieved to remove larger or substandard particles to ensure that all materials are uniform. In addition, raw materials from different sources can be mixed to achieve more uniform combustion and better molding quality. A small amount of natural binder (such as starch) is sometimes added to improve particle strength.
Raw material moisture control
Before the pressing step, the moisture content of the material should be checked again and adjusted as needed. This can be done by spraying on a small amount of moisture or by further processing in a drying device to ensure that the moisture content remains at the ideal 10%–15%. Too high or too low a moisture content will affect the pressing result: too high a moisture content will result in loose pellets, while too low a moisture content may make forming difficult.
Compression molding
Pressing is the most critical step in the production of wood pellets, and is usually carried out using a flat-die pellet press or a ring-die pellet press. The pellet press compresses the crushed and mixed raw materials into small pellets under high temperature and pressure. The high temperature during the pressing process softens the lignin in the wood, which acts as a natural binder, so that the pellets maintain a stable shape after cooling. Common pellet diameters range from 6 to 10 mm, suitable for both domestic and industrial fuel use.
Pellet cooling and drying
After pressing, the wood particles are still hot, often reaching temperatures above 90°C. The surface of the particles is relatively soft and therefore needs to be cooled and dried. Cooling towers can be used to cool the particles to room temperature. The cooling process further hardens the particles, making them less likely to crumble during transport and storage.
Screening and packaging
After cooling, the pellets are sieved again to remove any fine particles and impurities that may be present. This is to ensure consistent product quality, as too many fine particles may affect the flow and combustion of the pellets in the combustion equipment. The mesh aperture of the sieve used for secondary screening should be selected according to the product specifications.
After the second screening, qualified wood pellets can be packaged. Packaging materials can be plastic bags, woven bags, paper bags, etc. During the packaging process, care should be taken to ensure the airtightness of the packaging to prevent the pellets from becoming damp and impurities from mixing in. Warehouses storing wood pellets should be kept dry to prevent deterioration due to dampness and high temperatures. At the same time, they should be stacked reasonably to prevent the pellets from being crushed by excessive compression.
Screening and packaging
The production of wood pellets is an environmentally friendly measure and a significant way of using resources. By employing advanced production techniques and utilising waste wood resources in a strategic manner, we can convert this material into efficient biofuels for a range of applications, including home heating, industrial boilers, and power generation.
