Page 114 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

92 Biomass Gasification, Pyrolysis and Torrefaction

Both carbonization and torrefaction require relatively slow rates of heating,
while pyrolysis relies on fast pace of heating to maximize the liquid yield.
Carbonization takes place at higher temperatures with a certain level of oxy-
gen that allows sufficient combustion to supply the heat for the process. The tor-
refaction process on the other hand tries to avoid oxygen as well as combustion.
Torrefaction is a thermal decomposition that takes place at low temperature
and within a narrow temperature range of 200 300 C, while carbonization is

a high temperature (.300 600 C) destructive distillation process.
Carbonization produces more energy dense fuel than torrefaction, but it has a
much lower energy yield.
The following sections provide further details of these processes. Since
Chapter 5 presents a comprehensive discussion on pyrolysis, that process is
not described here. Torrefaction is discussed in detail in this chapter with a
brief introduction to carbonization.

4.3 CARBONIZATION
Solid products of carbonization of biomass include fuel charcoal, activated
charcoal, biocoke, and biochar. All of these are produced in processes similar
to that of torrefaction, that is, slow heating in absence or low oxygen. The
major difference lies in the process temperature. While torrefaction is carried
out at a low and narrow temperature range of 200 300 C, other processes

are carried out at much higher temperature. Such high temperature product
of wood has the generic name charcoal. Within the charcoal class there are
several divisions like fuel charcoal, activated charcoal, biocoke, and biochar.
These have some fine differences primarily for application considerations.
Because of its stable pore structure with high surface area, charcoal is a
good reducing and adsorbent agent. It finds use in the following industries:

1. Fuel charcoal for energy, barbeque, and so on
2. Manufacture of carbon disulfide, sodium cyanide, and carbides
3. Smelting and sintering of iron ores, case hardening of steel, and purifica-
tion in smelting of nonferrous metals
4. Water purification, gas purification, solvent recover, and waste water
treatment
5. Carbon sequestration and soil remediation.

The carbonization product may be divided into several types from its
application consideration. A brief description of these products is given in
the following sections.

4.3.1 Charcoal Fuel

Charcoal is one of the earliest fuels used by human race. Presently it is used
as a smokeless fuel in many countries and a feedstock for barbeque fuel.