Page 136 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 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 yield ðEYÞ 5 (4.14)
energy in raw biomass
Energy yield may be written in terms of heating values of the biomass
before and after torrefaction:
mass of product 3 heating value of product
EY 5
mass of biomass feed 3 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:

EY daf 5 product mass 3 HHV product 5 MY daf 3 HHV product (4.15)
feed mass HHV feed HHV feed
daf daf daf
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 ðEYÞ 5 1 2 energy lost 5 100 3 ð1 0:05Þ 5 95%