Environmental Engineering Reference
In-Depth Information
Table 3.3
Comparison of biomass properties before and after torrefaction.
Original biomass
Torrefied biomass
Bulky
Densified
Wet
Dry and hydrophobic
Expensive to grind
Easily crushed
Low energy density
High energy density
Biodegradable
Non-biodegradable
Expensive to transport
Less expensive to transport
lower for agricultural residues) with energy densification ratios of 1.3. Torrefaction
also yields a material, which can be stored over prolonged periods of time and
feed more economic, year-round operations. Biomass is bulky and vulnerable to
degradation over time, making it difficult to store.
The main torrefaction product is the solid phase which, as for pyrolysis, is usually
referred to as the charred residue (or char). In the field of torrefaction the solid prod-
uct is also frequently called torrefied wood or torrefied biomass. By mass, important
reaction products other than char are carbon dioxide, carbon monoxide, water, acetic
acid, and methanol [67]. All these non-solid reaction products contain relatively
more oxygen compared to the untreated biomass. The O/C ratio of torrefied biomass
is therefore lower than untreated biomass, resulting in an increase of the calorific
value of the solid product. After the biomass has been torrefied it can be densified,
usually into briquettes or pellets using conventional densification equipment, to fur-
ther increase the density of the material and hence its energy content, as described
in Section 3.2.1.2. In addition, the biomass exchanges its hydrophilic properties to
hydrophobicity, allowing for effortless storage that goes hand-in-hand with a greater
resistance against biological degradation, self-ignition, and physical decomposition.
The results of torrefaction are different for different types of biomass and very much
dependent on the cell structure of the biomass. The different structural components
(cellulose, hemi-cellulose, and lignin) of biomass are affected by heat very differently.
Whereas the hemicellulose decomposes within the torrefaction temperature range
(about 290°C), the cellulose decomposition occurs at a much higher temperature
(about 360°C). In contrast to polysaccharides (cellulose and hemicellulose), phenolic-
based lignin is pyrolyzed at a broad range of temperatures between 250 and 400°C (see
Figure 3.11). It can be seen from Figure 3.12 that sample mass loss under torrefaction
conditions is lower than 20%, comprising: (1) 10% mass loss which corresponds to
water removal; and (2) 10% of actual organic matter decomposition. Hemicellulose is
the main component which is affected by torrefaction.
3.3.5.1
Microwave Torrefaction
Researchers have recently investigated the microwave-assisted torrefaction of
biomass and demonstrated microwave activation effects [41, 68-71]. Surprisingly,
microwave-mediated low-temperature torrefaction yields a variety of products
