Page 141 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

118 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 4.7 Rise in Moisture Content After Submerging in Water for 2 h
Moisture % on Dry Ash Free Basis

Condition of the Feed Sawdust Water Hyacinth
25 C, Raw biomass before 150.3 197.5

torrefaction
After torrefaction at 250 C 7.8 17.7

After torrefaction at 270 C 3.3 14.9

After torrefaction at 300 C 2.1 8.8

Source: From Pimchua et al. (2010).

raw biomass. It further shows that the higher the torrefaction temperature,
the lower is its water absorption ability. Additionally, it also depends on the
type of biomass.
A more severe torrefaction (higher temperature and or longer residence
time) could make torrefied products more hydrophobic (Verhoeff et al.,
2011), but some researchers (Medic et al., 2012) noted that improvement in
hydrophobicity above 250 C is not significant (Yan et al., 2009).

Wet torrefaction, though done in water, interestingly makes the product
more hydrophobic (Yan et al., 2009) than dry torrefaction.

4.6.4 Explosion Potential of Torrefied Dust

Dust explosion is a major problem in handling and conveying fine dusts
especially of easily ignitable materials. Torrefaction makes biomass brittle
and could result in more dust during handling. Additionally, due to its high
reactivity and low moisture content, torrefied biomass could more easily
ignite than coal, which in turn increases the explosion potential of the torre-
fied biomass within mills or conveying pipes. Chapter 10 discusses this
aspect further.
In addition to the explosion potential, torrefied biomass also carries a risk
of fire because of its low ignition temperature. Some biomass plants have
experienced this.

4.6.5 Densification or Pelletization
Biomass is an energy-lean fuel. This makes its transportation more expen-
sive in terms of megajoule energy transported. So, to improve its energy
density, biomass is often compressed into denser pellets or briquettes.