Page 122 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

100 Biomass Gasification, Pyrolysis and Torrefaction

The basic polymeric constituents of biomass, namely hemicellulose, cel-
lulose, and lignin, are believed to react independently, and as such they do
not show the synergetic effect (Chen and Kuo, 2011). Thus, mass loss of
individual components can be simply added to get the total mass loss during
torrefaction as shown in Figure 4.5B.
Major attractions of torrefaction pretreatment stem from the degradation
of the hemicellulose content of the biomass. So, torrefaction is characterized
primarily by the degradation of hemicellulose. Dehydration and decarboxyl-
ation are the main reactions in this degradation that produce both condens-
able and noncondensable products. The torrefaction process produces solid,
liquid, and gaseous products as shown in Figure 4.4. The solid component is
made primarily of char along with items like some sugar and polymeric
structures and ash (Bergman et al., 2005a). The noncondensable gases com-
prise CO, CO 2 , and small amounts of CH 4 . Condensed liquid contains water
from thermal decomposition, lipids such as terpenes and waxes, and organics
such as alcohols and furans.
Torrefaction products comprise carbon water, carbon dioxide, carbon
monoxide, acetic acid, methanol, and formic acid. The formation of CO 2 is
due to decarboxylation. The acetic acid comes from the decomposition of
acetyl pendant group in cellulose. Carbon monoxide comes mainly from the
reaction between CO 2 and steam with porous char surface of the biomass
(White and Dietenberger, 2001).
Though torrefaction is characterized mainly by the degradation of hemi-
cellulose, other polymers, cellulose, and lignin also degrade to some extent
that depends on the temperature (Figure 4.5A). The mass loss due to torre-
faction at a given temperature is the sum of degradation of each of the three
polymers (Chen et al., 2011) and moisture if any. Figure 4.5B shows a sim-
ple qualitative diagram of decomposition of the polymers. Three figures here
give mass losses of hemicellulose, cellulose, and lignin when subjected to
torrefaction at different temperatures. It plots the mass as percentage of the
original mass of the biomass (dry ash free (daf)). By drawing a horizontal
line at the given temperature and by adding the intercept, one can get the
projected mass loss at that temperature. This is plotted on the extreme right
graph in Figure 4.5B as the same function of temperature.

Torrefaction products

Liquid Solid Gas
Water Original and modified H , CO , CO, CH 4
2
2
Organics sugar structures C x H y , tolune, benzene
Lipids New polymeric
structures
Ash
Char
FIGURE 4.4 Products of torrefaction of biomass.