Page 126 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

104 Biomass Gasification, Pyrolysis and Torrefaction

reason, two particles of different sizes could have different mass or energy
yields in otherwise identical torrefaction condition.

4.4.3.2 Residence Time
The residence time of biomass in the torrefier also influences the thermal deg-
radation of biomass. Slow heating rate is one of the distinguishing characteris-
tics of torrefaction that makes it different from the rapidly heated pyrolysis
process. It is typically less than 50 C/min (Bergman et al., 2005a). For this

reason, the residence time of biomass in a torrefaction reactor is much longer
in tens of minutes. Longer residence time gives lower mass yield and higher
energy density. Figure 4.8 illustrates the effect of residence time on the mass
yield as well as on the energy yield of torrefaction. Both yields reduce with
residence time. The influence of residence time on the torrefaction product is,
however, not as dominant as that of the torrefaction temperature. The effect of
residence time on mass loss diminishes after about 1 h (Stelt et al., 2011).
4.4.3.3 Biomass Type
The biomass type is another important parameter that could influence torre-
faction. As hemicellulose degrades most within the torrefaction temperature
range, one would expect a higher mass loss in a biomass with high hemicellu-
lose content. However, it is interesting that a hardwood and softwood with
similar hemicellulose content when torrefied under identical conditions could
show very different mass yields (Prins et al., 2006). Torrefaction of hardwood
gives lower mass yield than that of softwood because xylan, the active content
of hemicellulose of hardwood (deciduous), constitutes 80 90%, while in soft-
wood (coniferous), it constitutes only 15 30% (Sudo et al., 1989).
The xylan or 4-O-methylglucuronoxylan content of the hemicellulose is most
reactive within the torrefaction temperature range, and it degrades faster than

1
0.95
0.9
Yield in fraction (%) 0.75
0.85
0.8

0.7
0.65
0.6 Mass yield
Energy yield
0.55
0.5
0 20 40 60 80 100 120 140
Residence time in (min)
FIGURE 4.8 Effect of residence time on the mass and energy yield of torrefaction (poplar
wood: 25 mm diameter, 76 mm long, 250 C).