Page 137 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

114 Biomass Gasification, Pyrolysis and Torrefaction

As energy yield is independent of whether the mass is expressed on dry
or dry ash free basis. We can write
EY ar 5 EY daf 5 EY db 5 95%
b. HHV of torrefied biomass:
Equation (4.15) relates the HHV of torrefied product with that of the raw
feed as:

HHV product
EY daf 5 MY daf 3

HHV feed
daf
EY daf
HHV t;daf 5 HHV f;daf 3
MY daf
Example 4.1 gives HHV f,daf 5 20.52 MJ/kg and MY daf 5 79.35%.
0:95
HHV t;daf 5 20:52 3 5 25:36 MJ=kg
0:7935
The ash of the biomass is not lost during torrefaction, though the overall
mass of the biomass is reduced. The absolute amount of ash in the feed does
not change while the overall mass reduces. That is, the ash percentage
changes after torrefaction. Torrefaction does not change the absolute amount
of ash in the biomass though the overall mass reduces.
Example 4.1 gives mass yield and ash in feed on “as-received” basis as
52% and 2.03% respectively. So, the ash content of torrefied mass, ASH t is

ASH t 5 2:03=0:52 5 0:039 5 3:9%
From Eq. (3.31) we can get:
ð1 2 M 2 ASHÞ ð1 2 0 2 0:039Þ
HHV t;db 5 HHV t;daf 5 25:36 3 5 24:37 MJ=kg
ð1 2 MÞ ð1 2 0Þ

4.6 PHYSICAL PROPERTIES OF TORREFIED BIOMASS

Torrefaction brings about several important changes in the physical proper-
ties of biomass. Some of them are much relevant for cofiring of biomass
with coal (Chapter 10). This section discusses the modification of several
physical properties of biomass through torrefaction pretreatment.

4.6.1 Density and Volume
We have seen earlier that the mass of biomass reduces during torrefaction.
This change often brings about some reduction in volume as well. As a
result, the density change due to torrefaction could not be defined by the
mass yield alone. There is thus a distinct effect of torrefaction on the density
of biomass, which is a reduction with torrefaction (Table 4.6).