Page 132 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 109

basis (db) is more scientific, and it distinguishes torrefaction from drying or
deashing processes of a biomass.
So, logically mass yield, MY, should be defined as the fraction of the
original organic component of biomass that is converted into solid char.
Thus, mass yield should be defined on a “dry ash free” basis.
Mass yield of torrefied biomass on dry ash free basis, MY daf ,is
mass of torrefied biomass on daf basis
MY daf 5 (4.8)
mass of original biomass on daf basis
Although, the above definition is most accurate, its effectiveness in
design calculations is somewhat restricted. It does not give the actual solid
mass in the torrefied product; it gives only the residue of the organic compo-
nent. When one calculates the sensible heat or the mass of solid handled, the
total solid amount that includes the ash is needed. This difference could be
substantial for organic feedstock such as rice husk and sewage sludge where
inorganic content is high. For this reason, an alternative definition of mass
yield on “db” or dry basis alone can be used.
Mass yield on dry basis, MY db :
mass of torrefied biomass on dry basis
MY db 5 (4.9)
mass of original biomass on dry basis
Mass yield is not generally expressed on “as-received” (ar) basis.
However, for quick but rough design calculations, one may use it to deter-
mine the overall material flow in and out of the system. For such design pur-
pose, one may use a more practical definition on as-received basis as below:
Mass yield on “as-received” basis, MY ar :
total mass of torrefied biomass
MY ar 5 (4.10)
mass of wet biomass as-received
Relationship between these three definitions of mass yield may be
derived as below:
MY ar 5 ð1 MÞ MY db (4.11)
MY db 2 ASH db
MY daf 5 (4.12)
1 2 ASH db
or
MY db 5 MY daf ð1 2 ASH db Þ 1 ASH db
where M and ASH are fractions of moisture and ash, respectively on
as-received basis.
The ash fraction on dry basis can be related to that on as-received basis as:
ASH
ASH db 5
1 M