Page 140 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 117

terms of the energy spent in evaporating the moisture during combustion or
gasification. Coal on the other hand is hydrophobic, that is, it does not
absorb moisture or is less hygroscopic. So, this penalty for extended storage
is absent for coal. The extent of water repellant property is described by its
hydrophobicity.
Presence of moisture in fuel is undesirable for several reasons:

1. Moisture not only reduces the heating value of the fuel but also it
greatly increases the stack loss in a combustion system. For example,

1 kg evaporated moisture at 150 C carries away 2698 kJ of moisture
while the same mass of dry flue gas will carry only 160 kJ at that
temperature.
2. Moisture increases the potential for fungus development in biomass when
stored.
3. Moisture increases the cost of transportation and handling and feed prepa-
ration without making any useful contribution to the fuel’s use.
Thus, the lower the moisture in the fuel, the better is its end use.
Torrefaction can address these problems in the following ways:

1. Drying in pretorrefaction stage reduces the moisture of raw biomass from
10 50% to about 1 5%.
2. After torrefaction, biomass becomes largely hydrophobic, or resistant to
water, and thus it absorbs very little moisture.
3. The hydrophobic character of torrefied biomass allows its extended
storage without biological degradation (Tumuluru et al., 2011).

4.6.3.1 Why Biomass Becomes Hydrophobic after Torrefaction?
In biomass, the moisture absorption capacity of its hemicellulose constituent
is highest. The capacity of cellulose and lignin follows that (Li et al., 2012).
Since torrefaction involves near-complete breakdown of hemicellulose, the
process makes biomass hydrophobic. Raw biomass readily absorbs moisture
due to the presence of its hydroxyl (aOH) groups that form hydrogen bonds
to retain additional water. The torrefaction process destroys the OH groups
and thereby reduces its capacity to absorb water (Pastorova et al., 1993).
Additionally, due to the chemical rearrangement during torrefaction, nonpolar
unsaturated structures are formed in biomass after torrefaction. The nonpolar
character of condensed tar on the solid also prevents condensation of water
vapor inside the pores. Felfli et al. (2005) attributed the hydrophobicity of
torrefied biomass to tar condensation inside the pores that obstruct the passage
of moist air through the solid, which then avoids the condensation of
water vapor.
Table 4.7 compares the hygroscopic character of raw biomass with that
of torrefied biomass. After immersing in water for 2 h, the water uptake of
the torrefied biomass is nearly two orders of magnitude lower than that of