Page 158 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 135

0
Q 5 Q tL (4.21)
T
The heat loss from the torrefier is a function of the size and level of insulation
of the torrefaction reactor. It is not necessarily a fraction of the input energy.
So, the total heat load of the torrefier and dryer, Q total , is sum of the
above two:
Q total 5 Q 1 Q 0 (4.22)
0
d T
Cooler The cooling section (Figure 4.10) cools the torrefied product from

the torrefaction temperature to a safe temperature T c (B , 50 C) that is
close to that of the atmospheric temperature. Heat extracted from the torre-
fied product Q c is therefore:

Q c 5 W t C d ðT t 2 T c Þ (4.23)
This heat Q c can be utilized through suitable arrangements to preheat the
burner air, and thereby reduce the oil consumption in the burner. Assuming a
heat loss fraction X c in the cooler, the preheat temperature T of the burner
0
0
air W air can be found from the following:
0
W air C air ðT 2 T 0 Þ 5 ð1 2 X c ÞQ c (4.24)
0
Burner Burner provides energy for the process. A major challenge in a
directly heated system is to avoid oxygen in the flue gas. Biomass ignition
temperature being exceptionally low, it could ignite even at temperatures as

low as 200 C. Thus, a low excess air burner is to be used for such systems.
Even then there could be air infiltration in a negative draft system raising the
oxygen in the heating medium. Thus, special care is needed in the design of
a burner system in a directly heated system like described here.
The total energy required Q total for the system is provided by the enthalpy
of flue gas from the burner. This energy may be supplemented by burning
along with the oil the volatiles released, W vl , in the torrefier (Figure 4.10).
The heating value of the volatile, LHV vl , is however relatively low.

The temperature of the oil flame generally exceeds 1000 C, which is
rather high, and could set fire to the torrefied biomass on first contact. So, it
is necessary to reduce the temperature of the heating medium, W g , to a lower
value, T gi , by diluting the burner flue gas with a part, x, of the relatively cold
product gas, W , leaving the torrefier. So, a mixture, W g comprising the
0
g
0
burner gas and the recycled torrefier gas, (XW ), enters the torrefier at tem-
g
perature T gi , which should be no more than the ignition temperature of the
torrefied product (B300 C) to avoid any risk of fire.

In a directly heated torrefier (Figure 4.11), the mixture flue gas would
typically first enter the torrefier that may absorb a small amount of heat.