Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 88

TABLE 4.1 Torrefaction Temperature Range as Suggested by Different

Researchers

Temperature Range ( C) Researchers

Arias et al. (2008) 220 300

Chen and Kuo (2010), Prins (2005), Zwart et al. (2006) 225 300

Pimchuai et al. (2010), Prins et al. (2006) 230 300

Bergman et al. (2005), Tumuluru et al. (2011a), Rouset
et al. (2011), Sadaka and Negi (2009)

200 300

temperature would result in extensive devolatilization and carbonization of the
polymers both of which are undesirable for torrefaction (Bergman et al., 2005).
Also, the loss of lignin in biomass is very high above 300 C. This loss could
make it difficult to form pellets from torrefied products. Furthermore, fast ther-
mal cracking of cellulose causing tar

formation starts at temperature

320 C (Prins et al., 2006). These reasons fix the upper limit of torrefaction
temperature as 300 C.
Another important aspect of torrefaction's definition is oxygen concentra-
tion in the reactor. Studies (Basu et al., 2013; Uemura et al., 2011) on the effect
of oxygen concentration on torrefaction suggest that it is not essential to have
oxygen-free environment for torrefaction. Presence of a modest amount of oxy-
gen can be tolerated and may even have a beneficial effect on the torrefaction.
A major motivation of torrefaction is to make the biomass lose its fibrous
nature such that it is easily grindable, while it is still possible to form into
pellets without binders. Such requirements limit the torrefaction temperature
range to 200

300

300 C.
Slow heating rate is an important characteristic of torrefaction. Unlike in
pyrolysis, the heating rate in torrefaction must be sufficiently slow to allow
maximization of solid yield of the process. Typically the heating rate of tor-
refaction is less than 50 C/min (Bergman et al., 2005). A higher heating rate
would increase liquid yield at the expense of solid products as is done for
pyrolysis.
The thermal decomposition of biomass occurs via a series of chemical
reactions coupled with heat and mass transfer. Within the temperature range
of 100

260 C, hemicellulose is chemically most active, but its major degra-
dation starts above 200 C. Cellulose degrades at still higher temperature
(

275 C), but

its major degradation occurs within a narrow temperature

.

350 C (Chen et al., 2011). Lignin degrades gradually over the
temperature range of 250

band of 270

500 C, though it starts softening in the tempera-

90 C (Cielkosz, 2011). ture range of 80