Effect of Torrefaction on Properties of Pellets Produced from Woody Biomass

Abstract

Torrefaction has been recognized as a promising strategy to improve handling and storage properties of wood-based pellets, thus producing a uniform-quality commodity with high energy density and hydrophobicity. In this work, pellets produced from spruce stem wood, bark, and forest residues were torrefied in a bench-scale tubular reactor at 225 and 275 °C with two residence times (30 and 60 min). The effects of torrefaction on general properties, grindability, mechanical properties, hydrophobicity, and microstructure of the studied pellets were investigated. The increase of torrefaction severity reduced mass yields, but the heating values and the fixed carbon content of the torrefied pellets considerably increased. The grindability of raw pellets was substantially improved after torrefaction treatment. The energy required for grinding torrefied pellets is less than 50% of the energy needed for grinding the untreated pellets. In comparison to untreated pellets, the particles from ground torrefied pellets have clearly smaller sizes in a narrower size range. The increase of torrefaction severity improved hydrophobicity of the pellets, which have high resistance to water uptake and maintain their integrity after immersion testing. Upon torrefaction treatment, the durability and tensile strength of the pellets slightly decreased. Scanning electron microscopy analysis results show that particles from wood pellets torrefied at 275 °C lost their fibrous structure with an evident decrease of length/diameter ratios compared to untreated wood pellets. The particles from ground torrefied pellets are more uniform in terms of shape and size. Torrefaction can considerably improve grindability and uniformity of wood-based pellets and make them more acceptable in pulverized fuel applications.

Effect of Torrefaction Conditions on Physical and Thermal Properties of High- and Low- Quality Palm Pruning Residue Pellets

“…With increasing torrefaction temperature and residence time, mass yield, and energy yield decreased, weight loss and enhancement factor increased in HQ and LQ pellets. As stated in Wang's study [49], it has been revealed that the temperature is more effective on torrefaction efficiency than the residence time. The increase in temperature and residence time caused the loss of mass by removing the moisture content and volatiles.…”

“…This shows that high torrefaction temperature and low residence time can replace low torrefaction temperature and high residence time. Increasing temperature and residence time decreased the O:C and H:C ratios of the pellets, causing an increase in the HHV and having better combustion characteristics [49]. As a result of the torrefaction process, the atomic ratios of high-quality pellets have progressed in a relatively linear direction (R 2 :0.93) due to their smooth shape and homogeneous physical properties.…”

Medical Peat Waste Upcycling to Carbonized Solid Fuel in the Torrefaction Process

“…Such positive effects are mainly due to hemicellulose decomposition. The hemicellulose is rich in hydroxyl groups (including carboxylic and phenolic groups), which can form a hydrogen bond with water increasing bonded water content [30,32]. When hemicellulose is decomposed because of depolymerization, demethoxylation, and bond cleavage reactions, these groups are removed as well.…”

“…When hemicellulose is decomposed because of depolymerization, demethoxylation, and bond cleavage reactions, these groups are removed as well. As a result, fuel becomes hydrophobic and more fungal-resistant [32]. In addition, changes in chemical composition (decrease in available nutrients and production of toxic chemicals) the resistance to biodegradation increases [30].…”

Energy efficiency challenges in pulp and paper manufacturing: A tutorial review

“…Another option is to use low-grade steam for a pulp and paper mill system to achieve partial drying of the biomass (Erixon and Bjorklund 2017). A related option to consider is torrefaction (Nosek et al 2017;Mostafa et al 2019;Wang et al 2020), which entails heating up of the material to a range of about 200 to 300 C (Picchio et al 2020). Torrefaction, which can be carried out either before or after compressing the material into pellets, results not only in water removal, but also in partial decomposition of hemicellulose, yielding higher density and lower tendency of the material to absorb moisture from the air during storage.…”