Page 111 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 89

4.2.1 Pyrolysis, Carbonization, and Torrefaction
The torrefaction process is sometimes confused with related processes like
carbonization, mild pyrolysis, roasting, and wood cooking, but the motiva-
tion and process conditions of these processes are not necessarily the same.
A major objective of torrefaction is to increase the energy density of the bio-
mass by increasing its carbon content while decreasing its oxygen and hydro-
gen content. This objective is similar to that of carbonization that produces
charcoal but with an important difference that the latter does not retain maxi-
mum amount of energy of the biomass, and thereby gives low energy yield.
Difference between torrefaction, pyrolysis, and carbonization is delineated
further in this section and in Table 4.2.
Pyrolysis, carbonization, and torrefaction are all parts of the thermal
decomposition process of biomass. Table 4.2 illustrates the changes that occur
when a piece of wood or any biomass is heated in an inert atmosphere.
Though the thermal degradation processes are listed as separate sequential pro-
cesses they could overlap each other to some extent during the heating process.
Each process has its own motivation. For example, the major motivation of
pyrolysis is the production of liquid stuff, while that of carbonization is solid.
The term “pyrolysis” means thermal decomposition or chemical change
brought about by heat (see Chapter 5). This dictionary definition could cover
torrefaction, carbonization, and pyrolysis processes, but generally we use the
term “pyrolysis” in a more restrictive sense for the thermal process for pro-
duction of liquid extracts from biomass.
Carbonization is perhaps the oldest biomass conversion process that came
to the service of humankind. It requires relatively high temperature and is a
slow and long process (Table 4.2). For centuries, people have been using car-
bonization to produce charcoal from biomass. Charcoal has been used for
thousands of years for many applications including heating, production of
gunpowder, and metal extraction. Even now, charcoal has important com-
mercial use in a number of applications including the following:
a. Fuel in domestic oven or barbeque. Fuel for steam generation or cement
production.
b. Reducing agent in metallurgical industries.
c. Filter medium for water filter.
d. Pollutant capture and reaction site in chemical industries.

Traditional carbonization process uses beehive retort (Figure 5.2) where
wood is piled inside a mud covered pit to restrict air entry, and it is ignited
at the base. A part of the combustion heat provides the energy needed for
carbonization. Such plants suffer from a high level of smoke production.
Modern plants are relatively smoke free and typically operate at about
900 C. The amount of charcoal produced per unit weight of raw wood is

low, and it depends on the peak temperature of carbonization.