PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 254

Direct convection dryers can be divided into the following types:

￿ Belt conveyer

￿ Flash (pneumatic flow) dryer

￿ Fluidized bed

￿ Rotary drum

￿ Spray type

￿ Tray type

Indirect conduction dryers can be of the following types:

￿ Drum

￿ Steam jacket rotary drum

￿ Steam tube rotary drum

￿ Tray type

The selection of a dryer depends on the physical form of the biomass feedstock
(particle size distribution and morphology), the heat sensitivity of the material, the
required throughput capacity, the turndown ratio, and the pre- and postdrying
operations. Table 8.3 shows a dryer selection guide that has been adapted from
Mujumdar (2006).
Before going into details on drying characteristics, we need to deal with some
basics of moist air thermodynamics, i.e., the field of psychrometrics. Important defi-
nitions used in this field are given in the following.
Humidity, also called the humidity ratio, is defined as

= m w

m a = MW w p w V

ð = R u T Þ

= MW w p w 622 p w

p w

ω : : : :

MW a p a ≈

0

p a =0

622 ð Eq 8 11 Þ

ð MW a p a V = R u T Þ p − p w

sat . The (percentage)

When air is saturated, one speaks of the humidity of saturated air, ω

relative humidity is defined as

= p w
p sat
w

ϕ × 100 ð Eq : 8 : 12 Þ

This term is distinguished from the percentage humidity:

p sat
w

p −

ω
ω

f MW w p w = MW a p ð − p w Þ g

=

sat =

ϕ ð Eq : 8 : 13 Þ

ð Þ

MW w p sa w = MW a p − p sat

w

p − p w

When a gas flow is used for evaporating water from (the surface of ) a wet substance,
vaporization takes place provided that the air is not completely saturated with water
(at its dew point). The vaporization causes the temperature of the water to drop, and