Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 113

energy-lean gases. So, the energy density of the solid mass will increase from
[100/100] or 1.0 to [90/(100

30)] or 1.28 in that unit. The gaseous mass or the
volatile part of the biomass (30 unit) carries the residual energy (100 90)/30 or

0.33 energy density. Thus, we see from Figure 4.1 how the torrefaction process
increases the energy density of the biomass through this pretreatment.

4.5.3 Energy Yield
Energy yield gives the fraction of the original energy in the biomass retained
after torrefaction. After torrefaction, energy-rich components remain in the
biomass, but some energy-lean components are lost. This leads to some loss
in the overall energy content of the biomass, though there is an increase in
the energy density as illustrated in Figure 4.1 . Energy yield defines this
retention, and as such it is of great practical importance especially where the
biomass is used for energy conversion.
Torrefaction makes biomass use convenient especially in energy systems
like a boiler but at the expense of some energy loss. Energy yield gives quan-
titative value of this loss and is defined as:

energy in torrefied product
energy in raw biomass

Energy yield ð EY Þ 5 (4.14)

Energy yield may be written in terms of heating values of the biomass
before and after torrefaction:

heating value of product
mass of biomass feed

mass of product

3

EY

5

heating value of feed
By expressing the heating value on dry ash free basis (HHV daf ), one can
relate it to the mass yield, MY daf , as:

3

daf 3 daf 5 daf

product mass
feed mass

HHV product
HHV feed

HHV product
HHV feed

EY daf 5 MY daf 3 (4.15)

Unlike mass yield, energy yield does not depend on how the product or
feed is expressed as:

Example 4.2
Using data from Example 4.1, calculate the following:
a. Energy yield on “dry” and “dry ash free” basis.
b. HHV of torrefied biomass on “dry” and on “dry ash free” basis.

Solution
a. Energy yield:
Example 4.1 states that volatiles carried 5% of the total energy. So, the
amount of energy that remains in the solid is
Energy yield ð EY Þ 5 1 energy lost 100

3 ð

1 0 : 05 Þ 5 95%

2 5