Page 157 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

134 Biomass Gasification, Pyrolysis and Torrefaction

Flow-rate of dry biomass: W d 5 (1 0.3) 3 0.566 5 0.396 kg/s
W vl 5 W f 2 W t MW f
5 0:566 0:278 0:566 3 0:3 5 0:118 kg=s

Dryer The feed is heated to the torrefaction temperature, T t , in three stages:

i. Q ph is needed for preheating feed from room temperature, T 0 , to the dry-

ing temperature, taken as 100 C.
Q ph 5 W f C pw ð100 2 T 0 Þ
ii. Q dr is required for complete evaporation of moisture in biomass. For this
reason, there is hardly any change in the temperature in this zone. The heat
duty of this stage is generally highest especially for a high-moisture feed:
Q dr 5 W f ML
Here, L denotes the heat of vaporization at 100 C. So, we take this as

2260 kJ/kg.
iii. Q pd is needed for postdrying heating of the dry feed to the torrefaction
temperature, T t .

Q pd 5 W d C pd ðT t 2 100Þ

Total energy required or heat load, Q d , for drying biomass to the torrefac-
tion temperature, T t , is sum of above three.

Q d 5 Q ph 1 Q dr 1 Q pd
(4.19)
5 W f C pw ð100 2 T 0 Þ 1 W f ML 1 W d C pd ðT t 2 100Þ kW
where C pw and C pd are specific heats of raw biomass and dry biomass,
respectively.
Unlike in Section 4.5.1, we consider an overall heat loss fraction X d to
calculate the actual heat load Q of the dryer as below:
0
d
Q d
0
Q 5 (4.20)
d
ð1 2 X d Þ
Though torrefaction starts from its onset temperature, which is about

200 C, for convenience the entire heating, from 100 C to the torrefaction
temperature, T t , is included in the postdrying zone.
Torrefier One can see from Figure 4.3 that the heat load or the energy
required for torrefaction is very low. It is because the overall torrefaction
reaction is either mildly exothermic or endothermic depending upon the tor-
refaction temperature. So, neglecting this heat, we take heat required in the
torrefier Q as the loss from the reactor Q tL .
0
T