Page 139 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

116 Biomass Gasification, Pyrolysis and Torrefaction

These along with the plastic behavior of biomass cause handling difficult espe-
cially its pneumatic transportation through pipes. The soft and plastic nature
of biomass makes it also difficult to grind and pulverize biomass to fine sizes.
Co-combustion of coal with biomass requires biomass to be ground to
sizes comparable to those of coal (B75 μm), and then conveyed pneumati-
cally through pipes (see Chapter 10). Because of its soft, nonbrittle charac-
teristics, considerably more energy is required to grind untreated biomass to
required fineness. For example, to grind a ton of coal to a fineness
(d 50 B500 μm) 7 36 kW h of grinding energy would be required, while
130 170 kW h of energy is needed to grind the same amount of raw poplar
wood to that fineness (Esteban and Carrasco, 2006). There is thus nearly an
order of magnitude increase in energy consumption when a coal pulverizer is
used for biomass grinding. Additionally, torrefaction also influences the final
particle size distribution.
Torrefaction results in complete breakdown of the cell structures of bio-
mass making its particle brittle, smooth, and less fibrous. By making biomass
particles more brittle, smoother, and less fibrous torrefaction addresses above
problems to a great extent. An absence of fibrous exterior, sharp ends of
the biomass particles after torrefaction (Phanphanich et al., 2011) reduces the
friction created by the interlocking of these fibers during handling a pneu-
matic transportation.

4.6.2.1 Effect of Torrefaction Parameters on Grinding
All torrefaction parameters like temperature, residence time, and original
particle size play a role in the reduction in the energy required for grinding
to a given fine fraction. Torrefaction parameters influence the grinding of
torrefied product in the following order (Joshi, 1979):
Temperature . residence time . original particle size (4.16)
Thus, torrefaction temperature is the most influential parameter for grind-
ing. The higher the torrefaction temperature, the lower the energy required
for grinding or for a given energy input a greater amount of finer particles
are obtained after grinding. After torrefaction, the particles are not only
smaller but their size distribution is also more uniform. The grinding energy
requirement for specified level of grinding decreases with torrefaction tem-
perature. For example, Phanphanich et al. (2011) noted that the specific
energy consumption reduced from about 237 KW h/t for raw biomass to
about 24 kW h/t for that torrefied at 280 C.

4.6.3 Hydrophobicity of Torrefied Biomass

Biomass is hygroscopic in nature. So, it absorbs moisture even when it is
stored after drying. Thus, extended storage of biomass is very expensive in