THERMOCHEMICAL CONVERSION: TORREFACTION - Biomass as a Sustainable Energy Source for the Future - page 389

improving the physicochemical properties of biomass as a fuel or a feedstock. The term
torrefaction is derived from the French verb torréfier , meaning roasting (typically that
of coffee beans).
Ever since man could control fire, biomass has been harvested, collected, stored,
traded and used as a fuel. However the technologies applied in combustion have
become increasingly sophisticated in time, with a view to achieving higher fuel effi-
ciency and reducing environmental impacts. To meet the increasingly stringent quality
requirements of combustion processes, fuel pretreatment technologies have been
developed. The severity of treatment (in terms of applied temperatures) depends on
the intended application of the product. As an example, the most basic and oldest
of these thermal treatment processes involves low-temperature sun drying of wood
or dung cakes for combustion in order to meet domestic energy requirements. How-
ever, developments in metallurgy necessitated higher-temperature furnaces and redu-
cing agents, resulting in the development of slow pyrolysis for producing charcoal
fromwood. Charcoal production was widespread until the nineteenth century, but later,
charcoal was slowly replaced by coke (in metallurgy) or coal (as an industrial fuel) due
to cost considerations. In general, the use of thermal pretreatment technologies imply
energetic and economic costs, which become higher with greater improvements in fuel
properties. For drying, the energy cost is the heat required in evaporating the moisture
from the biomass. For charcoal production, in addition to the drying cost, there is a
significant (up to 70 80%) dry mass loss with an associated energy content, leading

to a much lower energetic efficiency. However, the resulting charcoal is undoubtedly
a better fuel than dry wood. Torrefaction, conceptually, can be placed in between
drying and charcoal production with respect to pretreatment severity. Unlike charcoal
production, torrefaction does not aim at removing all the volatiles from biomass.
Instead, it aims at maximizing the energy yield of the fuel while bringing about the
necessary improvements in the fuel properties suiting logistics and end use.
Charcoal production at relatively low temperatures has been mentioned already in
the 1800s (Percy, 1861), but the first reference to a torrefaction process is of more
recent date; in the 1930s in France, research on gasification of fuels was performed.
In the 1980s, a pilot plant was set up by Pechiney in Laval-de-Cere, France, for the
production of torrefied material as a reducing agent in metallurgical applications
(Bergman et al., 2005b). With increasing focus on biofuels in the twenty-first century,
several start-ups in Europe and North America have initiated work on torrefaction, and
several pilot/demo plants have been built.

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12.2 FUNDAMENTALS OF TORREFACTION

The optimum choice of conditions (temperature and time) used in torrefaction
depends on the nature of the input as well as on the application of the end product.
In case of woody biomass, it may be assumed that the optimum conditions are reached
with a mass loss of about 30 wt% of the dry solid matter. At this point, the torrefied
wood typically contains 90% of the energy content (LHV basis) of the input (Bergman
et al., 2005b).