THERMOCHEMICAL CONVERSION: TORREFACTION - Biomass as a Sustainable Energy Source for the Future - page 394

Low-grade heat
Fossil fuel
Biomass or product

Combustion

Biomass

Predrying Torrefaction Cooling Densiication Product

Heat
recovery

Flue gas/
moisture

FIGURE 12.3 Example of heat integration in a torrefaction process.

gases, supplementary fuel combustion (either fossil or biomass), and heat recovered
from cooling the solid product.
One of the primary choices to be made in designing the heat integration strategy
is the means of heating biomass and cooling the torrefied product. It is possible to
use the heat contained in the torrefaction gas or flue gas by bringing them into direct
contact with the biomass. Alternatively, heat exchangers can be used to transfer the
heat to an inert convective heating medium. Furthermore, the designer can choose
the reactor type (e.g., fixed bed, fluidized bed, moving bed) as well as the reactor
geometry (screw, conical, column). The mode of heat transfer can also be varied;
in addition to convection, conductive and radiative (infrared, microwave) heat
transfer modes have also been tested. While the design choices focus on achieving
high efficiencies and low costs, they also aim at avoiding operational and safety
complications emanating from, e.g., blockages, exothermicity, or condensation.
For example, directly contacting hot flue gas with biomass for reasons of more
efficient heat transfer or a strong overall exothermicity of the torrefaction reactions
may lead to uncontrolled temperature excursions. Using controlled quantities of an
inert convective medium instead of flue gas might be safer at the cost of a higher
energy penalty. Recirculation of torrefaction gases for effective heat management
might lead to condensation or polymerization of organic constituents leading to
fouling or blocking of pipes.
Current torrefaction technology initiatives are based on different reactor concepts,
some of which are illustrated in Figure 12.4.
The rotating drum reactor is widely used for drying and mixing of biomass
and waste streams and can therefore relatively easily be adapted to the torrefaction