5.3.1 Drying

Drying is a crucial step in the densification process of moist material. The optimum MC for pelletization was reported between 9 % and 15 % depending on feedstock species. However, the final quality of the feedstock can be affected by different factors during drying, and these include the type of dryer, drying conditions, drying medium, and the biomass characteristics. The degree to which the material is agitated and broken up in the drying process, the residence time and temperature of the material in the dryer influence the downstream densification process. In rotary dryers, the biomass is agitated to some extent and thus a significant amount of fines is usually produced in these dryers. The prolong drying of the fine particles caused by lag time of rotary dryer may degrade the biomass that affects the final quality of the resulted pellet. In contrast, PMB dryers are designed to circulate the drying medium through biomass layers, and the biomass is relatively stable on the bed, which can act as a filtration bed to trap any fines from the exhaust, resulting in low particulate emission.

Different components and products may be emitted in gas phase during drying. These include entrained fine particulates, volatile organic components (VOCs), and products of thermal degradation of the biomass [13]. The vaporized components can be further categorized into those that remain volatile at ambient conditions and those that condense after drying the stack. The most volatile components consist of mono-terpenes, which are naturally emitted from wood at ambient temperatures and the emission rate increases with temperature, particularly above 100 °C.

The condensable category consists of extractable components such as fatty acids, resin acids, di-terpenes, and tri-terpenes. Although these have high boiling points, they have sufficient vapor pressure at high drying temperatures (180-220 °C) to be released from wood. They are responsible for the formation of blue haze, a blue-gray discoloration of the exhaust gas from a wood dryer [16].

Thermal degradation products, such as formic and acetic acids, alcohols, aldehy­des, furfurals, and carbon dioxide, are released at higher drying temperatures (200 °C or higher) when pre-pyrolysis occurs. Increasing the wood temperature rapidly in­creases the amount of thermal degradation products that have a strong smell. The formation of these degradation products (pseudo-lignin) can influence the quality of produced wood pellets and improve the binding characteristics of woody pellets [63].

The wood color change (darkening) during drying is another issue to produce high-value pellets. Chromophoric groups (carboxylates and phenol) may be produced within the lignin or extractive molecules at high temperatures and humidity. During drying furfural and some polysaccharides with low molecular weights are created from hemicelluloses degradation. As these components are dark in color (blue green), they lead to the darker color in wood appearance [64].