Page 143 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

120 Biomass Gasification, Pyrolysis and Torrefaction

utilized, but the temperature of the biomass must be increased to the range

of 50 150 C so that the lignin within biomass softens (Gilbert et al., 2009).
When the heated biomass particles are compressed, they form good physical
bonds between them. Thereafter, when cooled the lignin hardens holding the
compressed particles together providing it with a good mechanical strength
without an external binding agent.
Higher pressures produce pellets with higher bulk densities and improved
tensile strengths but slightly elevated temperatures (B70 C) could have a

greater beneficial effect on the quality of pellets due to the softening of the
lignin in biomass. So, the higher the lignin contents, the higher the pelletiza-
tion quality (Gilbert et al., 2009).

During torrefaction (200 300 C), the hemicellulose content of biomass
largely degrades while only a small part of its lignin breaks down. Thus,
after torrefaction the amount of lignin as a percentage of the total biomass
should ideally increase improving its binding property. Some experimental
data, however, suggest pelletization of torrefied wood to be harder due to its
brittle nature.
Additionally, torrefaction opens more lignin-active sites by breaking
down the hemicellulose matrix and forming fatty unsaturated structures,
which creates better binding. So, for torrefied wood, one could use lower
pressure and lower temperature for densification of biomass. Pelletization of
torrefied biomass, therefore, needs less energy than that by pelletization of
raw biomass pelletization (Tumuluru et al., 2011).
One drawback of torrefied pellets could be that due to the loss of hemi-
cellulose, pellets can be more brittle and less strong (Gilbert et al., 2009). To
avoid this shortcoming, one could carry out torrefaction and pelletization
simultaneously.

Example 4.3
The wood of Example 4.1 (M: 35%; HHV ar : 12.92 MJ/kg; apparent density:
3
300 kg/m ) is being considered for either pelletization or torrefaction followed
by pelletization. The pelletized raw wood is expected to have a moisture con-
3
tent of 7% and an apparent density of 650 kg/m .
The raw wood, when torrefied, suffers 39% reduction in density but is free
from moisture and its HHV increases to 24.59 MJ/kg. Neglect any change in its
energy content due to pelletization and assume a 20% reduction in volume due
to torrefaction.
Taking necessary values from Example 4.1:
a. Compare the volumetric energy density between raw wood and pelletized
wood.
b. Compute the increase in energy density if the wood is torrefied and the
pellets made from that.