Page 156 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 133

Input Parameter:
Capacity of the torrefier 5 W t kg/s of torrefied wood including ash and
moisture
Desired mass yield 5 MY daf
Moisture fraction of raw biomass 5 M
Ash fraction of raw biomass 5 ASH
Corresponding values of the torrefied product are M and ASH .
0
0
Mass and Energy Balance
For the given capacity, W t , of the unit, we calculate the required feed rate of
raw biomass, W f , entering the drier, and the flow-rate of dried biomass W d
entering the torrefier section.
From Eq. (4.1), the flow-rate of the torrefied product on dry ash free
basis is
0
0
W t;daf 5 W t ð1 2 M ASH Þ 5 MY daf 3 W daf kg=s
where W daf 5 W f (1 2 M 2 ASH)
Combining the above two, we get the feed rate of raw biomass as:

0 0
W t ð1 2 M 2 ASH Þ
W f 5 kg=s (4.17)
MY daf ð1 2 M 2 ASHÞ
Moisture in the biomass is reduced through torrefaction, but that is not
the case for ash. Its absolute amount remains unchanged after torrefaction.
Neglecting any loss of ash between the units, we get:
W t ASH 5 W f ASH (4.18)
0
For further analysis, we consider a single integrated torrefaction system
as shown in Figure 4.11 where each zone requires specific amounts of heat.

Example 4.4
Design a moving bed torrefier to produce 1 ton/h (daf) of torrefied biomass from
raw biomass containing 30% moisture but negligible amount of ash.
Torrefaction at 280 C yields 70% mass (daf). Biomass and air enter the unit at

ambient temperature of 20 C. Hot gas leaves torrefier at 105 C.

Solution
Desired output on daf basis:
W t 5 1TPH 5 1000=3600 5 0:277 kg=s
Moisture in raw feed 5 30%
Using Eq. (4.17), we calculate the feed rate of raw biomass (neglecting
any loss):
W f 5 ððW t =MY daf ð1 2 M 2 AÞÞÞ 5 ½0:277=ð0:7 3 ð1 2 0:3Þފ 5 0:566 kg=s
Moisture in biomass: W v 5 0.566 3 0.3 5 0.17 kg/s