THERMOCHEMICAL CONVERSION: TORREFACTION - Biomass as a Sustainable Energy Source for the Future - page 390

Gas

0.3 0.1

Dry biomass

Torrefied biomass

Mass 1

Torrefaction
200-300 °C

0.7

Energy 1

0.9

FIGURE 12.1 Typical mass and energy balances in torrefaction of woody biomass.

Figure 12.1 shows a simplified schematic of the torrefaction reaction starting with
dry biomass as the input. The endo- or exothermicity of torrefaction is insignificant as
compared to the chemical energy potentials of the inlet and outlet flows. Conse-
quently, the heat of torrefaction is neglected in Figure 12.1. It can be seen that in com-
parison to the mass, a higher fraction of energy is retained in the torrefied biomass.
Consequently, there is an increase in the heating value (or specific energy content on a
mass basis) of the biomass. It can also be seen that the gas emanating from the torre-
faction process carries away part of the energy of the incoming fuel. Several process
schemes utilize the torrefaction gases for providing a substantial fraction of the heat
input required for either drying the biomass or heating up of the dried biomass prior to
torrefaction.
Biomass is primarily composed of organic matter, inorganic , and moisture. In

case of lignocellulosic biomass, such as wood or straw, the dominant polymeric struc-
tures in the organic matter are cellulose, hemicellulose, and lignin. Woody biomass
contains 20

“ ash ”

25 wt% lignin.
In reality, there are differences in composition, morphology, and decomposition behav-
ior for each class of biological macromolecules (celluloses, hemicelluloses, and lig-
nins) depending on their source. However, these (comparatively small) interclass
distinctions are not discussed here (see Chapter 2 for more background information).
The regimes of physicochemical changes on account of torrefaction are shown in
Figure 12.2. Upon heating biomass to temperatures of around 100 C, first, the phys-
ically bound water undergoes evaporation. As the temperature is increased further,
light volatiles and chemically bound water start to evaporate. The dehydration and
devolatilization reactions are endothermic, but condensation reactions may be initiated
as well, which are exothermic. Hemicelluloses are the first to decompose, with their
devolatilization starting at temperatures even below 200 C. Cellulose and lignin
are included in the decomposition process at higher temperatures of typically 240
−

− 40 wt% hemicellulose, 40 − 60 wt% cellulose, and 10 −

320 C. In this temperature range, also, extractives (e.g., resins, fats, and fatty acids),
which are present in lignocellulosic biomass in small quantities, may evaporate.
The gases emanating from the decomposition of the biocompounds are referred
to as torrefaction gases . The composition of torrefaction gases depends on the tor-
refaction conditions and extent. The gases are typically a combination of water
vapor and carbon dioxide along with condensable organic volatiles such as acetic