Page 183 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

160 Biomass Gasification, Pyrolysis and Torrefaction

90 40
80 38
70 36
34 “Solid char”
60 32
Yield (%) 50 30 Heating value (Mj/kg) Gases (non
condensable)
40
30 28 “Heating value
of char”
26
20
24
10 22
0 20
200 400 600 800 1000
Temperature (°C)
FIGURE 5.8 Char yield from pyrolysis decreases with temperature while gas yield increases.
Heating value of the solid char produced increases with temperature. Source: Data for grape
bagasse (0.63 1.0 mm and HHV 17.2 MJ/kg dry) replotted from Encinar et al. (1996).

The heating rate alone, however, does not define the product. The resi-
dence time of the product in the reactor is also important. During slow heating,
a slow or gradual removal of volatiles from the reactor permits a secondary
reaction to occur between char particles and volatiles, leading to a secondary
char formation.
The operating parameters of a pyrolyzer are adjusted to meet the require-
ment of the final product of interest. Tentative design norms for heating in a
pyrolyzer include the following:
a. To maximize char production, use a slow heating rate (,0.01 2.0 C/s),

a low final temperature, and a long gas residence time.
b. To maximize liquid yield, use a high heating rate, a moderate final
temperature (450 600 C), and a short gas residence time.

c. To maximize gas production, use a moderate to slow heating rate, a high
final temperature (700 900 C), and a long gas residence time.

Production of charcoal through carbonization uses step (a). Fast pyrolysis
uses step (b) to maximize liquid yield. Step (c) is used when gas production
is to be maximized.

5.3.4 Effect of Particle Size
The composition, size, shape, and physical structure of the biomass could
exert some influence on the pyrolysis product through their effect on heating
rate. Finer biomass particles offer less resistance to the escape of condens-
able gases, which therefore escape relatively easily to the surroundings
before undergoing secondary cracking. This results in a higher liquid yield.