Page 159 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

136 Biomass Gasification, Pyrolysis and Torrefaction

Thereafter, the hot gas would enter the drier in a directly heating system. So,
the temperature of the gas leaving the drier (T g0 ) should preferably be above

100 C to avoid condensation.
The flue gas, W , leaves the drier carrying with it moisture and product
0
g
gases from the torrefaction. So, the total flow-rate of product gas:
0
W 5 W f 1 W g 2 W t (4.25)
g
The burner burns an amount of oil, W oil ,in combustion air, W air ,toproduce
0
a product gas that is mixed with recycled flue gas (xW ) to produce the hot gas,
g
W g , to be used as the heating medium. The combustion air could be preheated
in the cooling section of the torrefier to a preheat temperature of T : The mass
0
0
balance around the burner system (Figure 4A.2)may be written as:
0
W g 5 xW 1 W oil 1 W air (4.26)
g
An energy balance of the torrefier can give the amount of diluted flue
gas W g as:
A
W g 5 kg=s (4.27)
C g ðT gi 2 T g0 Þ
where, A 5 (W v C v 1 W vl C g )T g0 1 W t C d T t 2 W f C b T 0 1 W v L + Losses
C v is the specific heat of steam and C g is the average specific heat of flue
gas between 300 C and T g0 .

An energy balance of the burner could give the amount of oil that must
be consumed to provide necessary energy for torrefaction (see Appendix).
" #
1 W g C g T gi
W oil 5 3 2 1 (4.28)
K 2 P W 0 0
g C g T g0 1 VL LHV v1
fr
where,
0
ðαðA=FÞC a T 1 LHVη 1 C oil Tv 0 Þ ðαðA=FÞ 1 1Þ W vl
0
K 5 0 5 and VL 5
W C g T g0 1 W vl LHV vl η W 0 fr W 0
0
g g g
0
where, η is the efficiency of the burner, T is the temperature of the pre-
0
heated air entering the burner, and VL fr is the fraction of volatiles in the
product gas of torrefaction. (The Appendix at the end of Chapter 4 gives the
derivations of Eqs. (4.27) and (4.28).)
Unit Sizing
The above calculations give the heat and mass balance of the system. Now,
we find the sizes of different functional sections of the torrefier that would
permit the required heat and mass transfer for the system to take place.
We take a simple case of a moving bed torrefier characterized by a uni-
form downward flow of solids and upward flow of gas through a vertical