THERMOCHEMICAL CONVERSION: TORREFACTION - Biomass as a Sustainable Energy Source for the Future - page 392

12.3 ADVANTAGES OF TORREFACTION
1. Improvement in Lower Heating Value . Torrefaction results in the loss of
approximately 30% of the mass of the fuel (for woody biomass), primarily
chemically bound water, and other light volatiles that do not contribute signif-
icantly (around 10%) to the energy content. Consequently, the specific energy
content of the solid matter left behind after torrefaction is higher (Bridgeman
et al., 2008; Patel et al., 2011; Pimchuai et al., 2010; Prins et al., 2006b; Rousset
et al., 2011). A significant increase in the LHV is also due to drying of the
biomass during the torrefaction process; the resulting increased hydrophobicity
largely prevents the reabsorption of moisture.
2. Improved Grindability . For application in large pulverized fuel boilers and
(dry feed) entrained flow gasifiers, biomass must be ground to (sub)millimeter
sizes. Woody biomass typically consists of long tenacious fibers, the pulveriz-
ing of which requires a significant amount of energy. Torrefaction (in addition
to dehydration) leads to shorter fibers due to cellulose depolymerization with
less interconnections owing to the hemicellulose decomposition. This reduces
the energy consumption in grinding and results in a more even particle size dis-
tribution and particles with increased sphericity (Arias et al., 2008; Bergman
et al., 2005a; Phanphanich and Mani, 2011).
3. Increased Hydrophobicity and Decreased Biological Activity . Hemicellulose is
the most hydrophilic compound in the biomass structure and is preferentially
decomposed in the process of torrefaction. This is associated with the loss of
free hydroxyl groups that can act as sites for hydrogen bonding, the absence
of which lead to increased hydrophobicity (Chew and Doshi, 2011; van der Stelt
et al., 2011).
Due to the sustained high temperatures, torrefaction leads to sterilization of the
feedstock. Furthermore, the increased hydrophobicity and the removal of monosac-
charides and hemicelluloses serve to prevent the recurrence of fungi/mold in storage.
The elimination of biological activity in the feedstock prevents decomposition and
loss of solid matter and also leads to limitation of biochemically induced self-heating
of the biomass in stockpiles. The improved storage properties of torrefied biomass
may enable outdoor storage, which can lead to significant savings.
Typical properties of torrefied wood pellets in comparison with other solid fuels are
shown in Table 12.1.
12.4 TORREFACTION TECHNOLOGY
Many research groups are involved in the research and development of torrefaction
technologies. Rather than developing entirely new reactor designs, most developers
rely on reengineering of proven technologies developed for other applications, such
as drying, pyrolysis, combustion, and ore roasting.