Page 127 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 105

any other solid components of the biomass (Basu et al., 2013a c, 2012; Prins,
2005). For this reason, the mass loss is greatly influenced by the xylan content in
the biomass rather than the hemicellulose content alone. Table 4.4 shows values
of xylan in some hardwood and softwood.
During torrefaction, hardwood releases mainly acetic acid and water dur-
ing torrefaction, while softwood releases mostly formic acid. Since hardwood
experience higher mass loss on torrefaction without much effect on the
energy loss, it would have a higher gain in energy density compared to that
in softwood (Prins, 2005; Prins et al., 2006).

4.4.3.4 Feed Size
The size of biomass particles or pieces is another parameter that could affect
the torrefaction yield. This effect may not be prominent for fine size of parti-
cles but could be measurable for large sizes.
The mass yield showed a modest increase with increase in volume mean
diameter (Basu et al., 2013a) or the length of a piece of constant diameter.
An opposite result (mass yield reduced) was found when the diameter was
increased keeping the length constant (Basu et al., 2013a). These are direct
result of heat transfer to biomass interior and the temperature dependent
reaction within it. It is discussed further below.
Torrefaction involves convective heat transfer from the reactor to the bio-
mass surface, conduction of the heat into the biomass interior, and finally the
reaction within it. Relative magnitude of these three rates decides which
parameter might influence the overall torrefaction process. Biot number,
which is the ratio of heat convection to the outer surface and conduction of
the same into the interior of the particle, is given as:
hV hr p
Bi 5 B (4.6)
λS λ
where V is the particle volume, S the external surface area, h the convective heat
transfer coefficient on biomass particle, r p is characteristic particle size taken
here as the radius of particle, and λ is thermal conductivity of biomass particle.
Pyrolysis number, Py, is another parameter that could influence the
torrefaction process. It relates the external surface heat transfer rate to the
torrefaction reaction rate (Pyle and Zaror, 1984).
h
Py 5 (4.7)
KρC p r p
21
where, K is reaction rate of torrefaction, s , ρ is density of particle, and C p
is specific heat of particle.
If the Biot number is sufficiently small, as is the case for fine particles,
the internal thermal resistance is negligible, and if the pyrolysis number is
very high, the reaction will be rate controlled. Larger particles on the other
hand would have higher Bi and lower Py. So, the torrefaction would be