Page 86 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

64 Biomass Gasification, Pyrolysis and Torrefaction

their residence time in the reaction zone. Thus, it increases the potential for
secondary cracking to produce char. The pores in wood are generally oriented
longitudinally. As a result, the thermal conductivity and diffusivity in the
longitudinal direction are different from those in the lateral direction. This
anisotropic behavior of wood can affect its thermochemical conversion. A
densification process such as torrefaction (Chapter 4) can reduce the anisotro-
pic behavior and therefore change the permeability of a biomass.

3.5.1.1 Densities
Density is an important design parameter for any biomass conversion system.
For a granular biomass, we can define four characteristic densities: true,
apparent, bulk, and biomass (growth).

True Density
True density is the weight per unit volume occupied by the solid constituent
of biomass. Total weight is divided by actual volume of the solid content to
give its true density.
total mass of biomass
ρ 5 (3.4)
true
solid volume in biomass
The cell walls constitute the major solid content of a biomass. For com-
3
mon wood, the density of the cell wall is typically 1530 kg/m , and it is con-
stant for most wood cells (Desch and Dinwoodie, 1981). The measurement
of true density of a biomass is as difficult as the measurement of true solid
volume. It can be measured with a pycnometer or it may be estimated using
ultimate analysis and the true density of its constituent elements.

Apparent Density
Apparent density is based on the apparent or external volume of the biomass.
This includes its pore volume (or that of its cell cavities). For a regularly
shaped biomass, mechanical means such as micrometers can be used to mea-
sure different sides of a particle to obtain its apparent volume. An alternative
is the use of volume displacement in water. The apparent density considers
the internal pores of a biomass particle but not the interstitial volume
between biomass particles packed together.

total mass of biomass
ρ apparent 5
apparent volume of biomass including solids and internal pores
(3.5)
The pore volume of a biomass expressed as a fraction of its total volume
is known as its porosity, A p . This is an important characteristic of the
biomass.