Mechanism of Torrefaction

In biomass, hemicellulose is like the cement in reinforced concrete, and cel­lulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction.

During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction. As we can see from Figure 3.11, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Lignin, the binder component of biomass, starts softening above its glass-softening temperature (~130 °C), which helps densi — fication (pelletization) of torrefied biomass. Unlike hemicellulose, cellulose

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FIGURE 3.11 Weight loss in wood cellulose, hemicellulose, and lignin during torrefaction.

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shows limited devolatilzation and carbonization and that too does not start below 250 °C.

Thus, hemicellulose decomposition is the primary mechanism of torrefac­tion. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures (Bergman et al., 2005a). At higher temperatures (250-300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in tor — refaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.

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