Page 135 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

112 Biomass Gasification, Pyrolysis and Torrefaction

From Eq. (4.12):
Mass yield on dry and ash free basis:
MY db 2 ASH d
MY daf 5
ð1 2 ASH d Þ
80 2 3:13
MY daf 5 5 79:35%
ð1 2 3:13Þ

4.5.2 Energy Density
Energy density is another important parameter of the product of torrefaction.
It gives the amount of energy released when unit mass of the torrefied prod-
uct is burnt and its product is cooled. Energy density is also associated with
terms like specific energy, calorific value, and heating value. Most applica-
tions use energy density on mass basis such as kJ/kg, Btu/lb, kCal/kg. Energy
density as defined here is equivalent to the HHV described in Section 3.6.5.
Energy density 5 amount of energy released when unit mass of the torrefied
biomass is fully combusted
5 higher heating value ðHHVÞ
(4.13)
Energy density may also be defined on volume basis, where it gives the
amount of useful thermal energy stored in unit volume of a substance. Here,
3
3
3
energy density is expressed as kJ/m , kCal/m , or BTU/ft . The volume-
based definition is used only in special cases like shipment of fuels. For
example, for ocean freight, the rate is generally based on volume basis sub-
jected to a maximum weight.
Biomass contains appreciable amount of moisture which evaporates during
combustion. So, energy density may also be expressed as lower heating value
(LHV), which is lower than the HHV as the former does not consider the heat
used in evaporating moisture. The heat of vaporization water being high
(B2260 kJ/kg), the difference between the HHV and net or LHV could be appre-
ciable for a wet, raw, or “as-received” biomass (see Section 3.6.5). Therefore,
one should pay particular attention to how the energy density or the heating
value is expressed as.
Energy density can be expressed on “as-received” basis, “dry basis”, or
on “dry ash free” basis. Equation (3.31) gives relations between these defini-
tions of energy density or heating value.
Figure 4.1 illustrates how the energy density of a sample biomass could
increase after torrefaction. Here, 100-unit mass of biomass with 100-unit energy
is torrefied losing certain amount (viz. 30 unit) of mass. The solid product of tor-
refaction generally retains a higher fraction of the energy (viz. 90 unit) of the
biomass because the lost mass comprises mostly of water, carbon dioxide, and