Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 96

torrefaction, the piece retains only 43% of the original mass but loses only
13% of its energy content.
The wood sample is then heated to a further high temperature of 310 C
for 1 h when the sample wood becomes much darker and brittle. This is a
severe form of torrefaction when it loses another 11% of the original mass
due to loss of moisture, volatiles, and gases. The energy density increases
but much of the energy is lost in the process. The mass-based energy density
increases at the expense of decreased energy yield.

4.4.1 Heating Stages
Different stages of heating as illustrated in Figure 4.2 are examined in more
detail in the following sections. Figure 4.3 shows schematically the historical
changes (in ideal condition) in mass, temperature, and the energy consump-
tion of a biomass piece during torrefaction. Temperature of the heating
medium (furnace) was kept at the design torrefaction temperature, while bio-
mass was heated and its temperature change was recorded. From this dia-
gram, one notes the following stages of thermal treatment.

4.4.1.1 Predrying
This is the first step in the process. When the biomass is heated from room
to the drying temperature ( 100 C), its temperature rises steadily receiving

sensible heat from the reactor or the heating medium. For predrying heating,
the energy required, Q ph ,isM f C pw (100

B

T 0 ). Accounting for heat loss, one

2

can write the heat required, Q pd , as:

M f C pw ð 100 T 0 Þ

2

Q pd 5 (4.1)

h upd

where, C pw is the specific heat of wet or as-received biomass, M f is mass of
raw biomass, and T 0 is feed temperature. A heat utilization efficiency factor,
h u,pd of the system is used here to account for some heat that may be lost
from the drier. Energy required, Q pd , is generally a relatively small fraction
of the total heat requirement, Q total .

4.4.1.2 Drying
Drying on the other hand is the most energy-intensive step of torrefaction.
It is especially so for high-moisture biomass because the moisture in biomass
is evaporated during this stage. There is very little change in biomass tem-
perature as evaporation takes place at constant temperature till all the surface
moisture or free water is driven off. The temperature starts to climb after the
critical moisture is reached when the rate of evaporation starts to decrease.
This stage makes the biomass bone dry. In most cases, one notes a sharp
increase in the total energy demand in this stage ( Figure 4.3 ).