5.2.3.1 Torrefaction

Torrefaction is used as a pretreatment prior to pelletization for upgrading the woody biomass primarily for energy production [36]. Torrefaction is a thermochemical treatment that subjects the biomass under heat at the reaction temperature between 200 and 300 °C in an inert medium (e. g., nitrogen) for a certain period of time (ranging between several seconds and an hour) depending on the particle size.

Reaction Chemistry

A detailed review of torrefaction chemistry and its process conditions is summa­rized in the literature [37, 38]. The reactions that take place for torrefaction are mainly decomposition and which can further divide into (1) drying, (2) depoly­merisation and recondensation, (3) limited devolatilization and carbonisation, (4) extensive devolatilization and carbonisation throughout the reaction temperature regime (Fig. 5.3). Hemicelluloses soften at temperatures between 150 and 200 °C and undergo dehydration, deacetylization, and depolymerization reactions at processing temperatures in the 200-300 °C [ 39,40]. Xylan is the predominant form of hemicel — lulose for hardwood while glucomannan is the predominant form of hemicellulose of softwood. Xylan tends to break down more quickly than glucomannan at lower tem­peratures. Therefore, hardwood has a higher breakdown of hemicellulose (or higher mass loss) than that of softwood when treated at the same temperature. This suggests

that different species of wood have different torrefaction kinetics which is worth for further in-depth investigation. A simple one-step (single stage) kinetic model with the first-order reaction of chemical reactions of cellulose, hemicellulose, and lignin during torrefaction of British Columbia (BC) softwoods was reported for slow resi­dence time reaction while a two-component and one-step first-order reaction yielded a better prediction of chemical components for the torrefaction with short residence time [41].

A small degree of degradation of cellulose and/or lignin also occurs during tor — refaction [43]. When the reaction temperature is high (>270 °C), a greater proportion of cellulose degradation was reported. In contrast, lignin is relatively stable and does not undergo significant chemical changes during torrefaction even at high temperature.

Particle Size and Residence Time

Particles with small size distribution would be recommended for some reactor types to achieve an optimized torrefaction efficiency and product quality. Note that different biomass species have different physical properties (porosity, specific heat capacity, thermal conductivity, particle size distribution of grinds under the same size reduction process, etc.) that further introduces a non-homogenous reaction due to heat and mass transfer limitation and subsequently results in a non-homogenous torrefied product.

In general, large particles within a wider range of particle size distributions may not be completely torrefied, and small particles may be over-carbonized under the same torrefaction condition. Therefore, a narrow or mono-disperse particle size dis­tribution of a certain specific wood particles may be ideal for a certain type of torrefaction reactor operating at a specific temperature and time. If the particle size distribution of raw materials is a bi-modal distribution, sieving and separation of mixtures would be recommended instead of torrefying the whole mixture of par­ticles. Large particles may require more than one pass milling in order to achieve smaller particle size suitable for homogenous torrefaction. The particle size distri­bution of ground particles depends on the biomass species, the types of mill, the MC of the biomass, and other factors, etc. Pretreatments (e. g., drying, size reduction) prior to torrefaction are necessary and critical to control the feedstock properties for producing a homogenous product quality.

Residence time also affects the mechanism of torrefaction decomposition. Resi­dence time is related to the reactor design (i. e., size) and operation condition (e. g., feeding velocity). Long residence time allows a greater degree of devolatilization from the biomass. Particle size and MC of wood particle also affect the heat and mass transfer. Small particles require less residence time to be treated in order to achieve the same degree of chemical reaction as the longer residence time is required for the large particles to allow nitrogen diffusion to initiate the decomposition reac­tion. A slow heating rate is critical for product homogeneity. Slow heating facilitates a uniform temperature gradient across the particle during torrefaction. Less volatile with low heating value is vaporized and thus results in a higher yield of solid residue with high heating value for pellet production.