PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 253

M ar , 1
100

W cv = φ m , 1 , moist h 1 , moist

φ m , 1 , b , d h 1 , b , d +

− M ar , 1

M ar , 2
100

φ m , 1 , b , d h 2 , moist

− φ m , 1 , b , d h 2 , b , d −

− M ar , 2

100 M ar , 1 − M ar , 2

100 φ m , 1 , b , d h 2 , moist

−

100 − M ar , 1 − M ar , 2

So,

W cv
φ m , 1 , b , d

+

M ar , 1
100

= h 1 , b , d − h 2 , b , d h 1 , moist − h 2 , moist

− M ar , 1

( )

M ar , 1
100

Δ

= − Δ h b , d + h moist

− M ar , 1

( )

W cv
φ m , 1 , b , d

M ar , 1
100

c p , moist Δ

= − c p , b , d Δ T+ T

− M ar , 1

kg −1 = kg −1

= − f 1200 × 1 + 3 × 4180 × 1 g J − 13 : 7kJ

The minus sign indicates that work is done on the biomass.

Question : What would be the (minimum amount of) energy needed to evaporate
the same amount of water, now removed mechanically?

8.8.3 Thermal Drying

Drying using (waste) heat is a more energy-intensive process than the other pretreat-
ment techniques for biomass. This form of dewatering has been investigated widely
and is commonly used in industrial practice, e.g., in the food processing industry
where drying is needed to prevent degradation but also for easy handling/packaging
and for giving the product the right flow characteristics (not sticking). For most
bioenergy purposes, the requirements of thermal drying are less strict. Important
aspects are energy consumption, emission of volatile organic compounds (VOCs)
and dust, and, most importantly, safety (fire and dust-explosion hazards when drying
with oxygen-containing gas).
There are different ways to classify existing drying technologies (see for a thorough
treatise, e.g., Mujumdar, 2006). An important distinction can be made between direct
and indirect drying . Direct drying takes place with a flow of hot air, steam, or flue gas
in direct contact with the wet biomass. Indirect drying prevents this contact, but heat is
transferred through a casing by conduction.