Table of Contents
Biomass pellets are playing an increasingly important role in the energy sector as a renewable energy source. Understanding what biomass pellets are made of is important to gain a deeper understanding of their properties, applications and sustainable development.
Wood processing residues
Sawdust
Wood processing, such as sawmills turning logs into panels, produces large quantities of wood chips. These chips come mainly from sawing, planing and other processes. They come in different sizes and shapes, but they are all high quality raw materials for biomass pellets. Wood chips are rich in cellulose, hemicellulose and lignin, and their chemical composition allows them to retain good structural stability after compression moulding.
The properties of chips from different types of wood vary. For example, coniferous wood chips (e.g. pine) may have a higher turpentine content, which to some extent affects the combustion characteristics of biomass pellets, which may produce more volatile organic compounds (VOCs), but at the same time provide a higher level of heat.
Wood chips
Wood chips are usually made from larger pieces of wood, e.g. at the feedstock pre-treatment stage in the paper industry. Wood chips are relatively large in size and usually need to be further reduced in size by a Hammer Mill before they can be used for biomass pellet production. The advantage of wood chips is that they are produced from a wide range of sources, including branches from forestry residues.
Wood chips have a high lignin content and lignin acts as a binder in the biomass pellet moulding process. When the wood chips are compressed, the lignin softens under high temperature and pressure and combines components such as cellulose and hemicellulose to form dense biomass pellets.
Forestry residues
Twigs and branches
Forestry operations leave a large amount of branches and twigs in the forest. If left unused, these branches and twigs are not only a waste of resources, but can also cause safety hazards such as forest fires. They are a natural raw material for biomass pellets and can be processed into biomass pellets after collection, drying and grinding.
Tree branches and twigs are made up of relatively hard materials, with different proportions of lignin and cellulose depending on the tree species. In general, hardwood branches and twigs contain relatively high levels of lignin, which contributes to the strength of biomass pellets.
Tree bark
Bark is another type of forest residue. Bark is usually removed during wood processing. Bark is rich in cellulose, hemicellulose and lignin, which can still be used as a raw material for biomass pellets, although the ratio of its composition differs from that of wood. However, bark can contain relatively high levels of contaminants such as soil and micro-organisms, which must be effectively cleaned and treated before processing.
Agricultural straw
Wheat straw
After the wheat harvest, a large amount of straw is left in the field. The main constituents of wheat straw are cellulose, hemicellulose and lignin, but its lignin content is relatively low, making it suitable for processing into biomass pellets with the addition of certain binders or blending with other feedstocks that have a higher lignin content.
Wheat straw has a looser structure and requires higher pressure during pressing to ensure pellet density and strength. It has the advantage of being a large producer and there is an abundance of resources available in wheat producing areas.
Maize straw
Corn stover is another common crop residue feedstock. It has a high cellulose content and the availability of maize stover during the harvest season is considerable. Corn stover is relatively hard on the outside and loose on the inside, and attention must be paid to the degree of comminution during processing to ensure particle uniformity.
Similar to wheat straw, maize stover must also be dried as its high water content can affect the quality of the biomass pellets, for example by causing mould and deterioration during storage.
Agro-processing residues
Rce husk
Processing paddy into rice produces a large amount of rice husk. Rice husk is rich in cellulose and has a unique structure with high hardness. As a feedstock for biomass pellets, rice husk requires special handling during processing, such as pre-crushing to reduce its size, and can cause some wear and tear on processing equipment due to its high silica content.
Despite these challenges, rice husk is a huge resource, especially in rice-producing areas. Converting rice husk into biomass pellets can be an effective solution to its processing challenges while achieving efficient resource utilisation.
Fruit shell
Fruit shells such as walnut shells and almond shells are also potential biomass pellet feedstocks. These shells typically have high hardness and lignin content, which can provide better structural support during the biomass pellet forming process. For example, walnut shell biomass pellets can generate high heat during combustion and burn for a relatively long time.
The collection of fruit shells is relatively decentralised and needs to be collected from numerous fruit processing companies or fruit farmers, which increases the difficulty of raw material collection, but with the increased emphasis on biomass energy, the prospects for the use of fruit shells are still wide.
Herbaceous plant
Reed
Reed is a herbaceous plant that grows in wetlands and has a high cellulose content. Reeds are able to absorb large amounts of carbon dioxide during the growth process and convert it into their own biomass.
When processed into biomass pellets, reed must be dried and ground. As the stem of the reed is hollow, care must be taken during the crushing process to ensure that the density of the pellets is not affected. Reed biomass pellets have a better combustion performance and reed resources are more abundant in some wetland areas, providing a potential raw material for biomass pellets.
Sugaw
Sugaw is a common wild herb with a high biomass. The composition of sugargrass consists mainly of cellulose and hemicellulose, and its lignin content is relatively low.
When producing biomass pellets, sugargrass must be mixed with other raw materials with a higher lignin content to improve the pellet forming effect. Sugargrass is highly adaptable and can grow on some marginal land, providing a resource base for its use as a biomass pellet feedstock.
Animal manure
cattle manure
Cattle manure is a common waste product of livestock production. Cow manure contains organic materials such as cellulose and protein that have not been fully digested. Before cow dung can be processed into biomass pellets, it must be dried to remove the moisture in it to prevent problems such as mould during pellet production.
Due to the complexity of the composition of cow dung, using cow dung alone to make biomass pellets can cause some problems, such as odour during combustion. Therefore, cow dung is usually mixed with other biomass feedstocks (e.g. crop residues), which can improve both the pellet shape and combustion performance.
chicken manure
Chicken manure contains high levels of nutrients such as nitrogen, phosphorus and potassium, as well as some organic matter. Chicken litter also requires drying treatment when it is processed into biomass pellets. The advantage of chicken manure biomass pellets is that the ash produced after combustion can be used as fertiliser to return to the field, which has a certain fertiliser value.
However, the uric acid and other components in poultry manure can produce harmful gases when burned, so it needs to be effectively treated during processing, such as high-temperature fermentation, to reduce the content of harmful components, and then mixed with other suitable biomass raw materials to make biomass pellets.
Biomass pellets are produced from a wide range of feedstocks, including woody feedstocks (e.g. wood processing residues and forestry residues), agricultural feedstocks (e.g. crop residues and agricultural processing residues), herbaceous feedstocks and animal manure feedstocks. Different feedstocks have different chemical compositions and physical properties and need to be processed into biomass pellets accordingly. With the increasing demand for renewable energy and the emphasis on resource recycling, the development and use of biomass pellet feedstocks will continue to expand and improve, making a greater contribution to sustainable energy development.
