Introduction - Biomass Gasification, Pyrolysis and Torrefaction - page 25

1.6 BRIEF DESCRIPTION OF SOME BIOMASS CONVERSION
PROCESSES
The following section presents a brief description of reactions that take place
in different thermal conversion processes of biomass.

1.6.1 Torrefaction
It is a process of production of carbon-rich solid fuels from biomass. So, gas
and liquid parts of the conversion do not form a part of the product.
Torrefaction has some similarity with the process of carbonization, but there
are some important differences as explained in Section 4.1.1:

C n H m O p 1 heat - char CO CO 2 1 H 2 O condensable vapors (1.3)

1 1 1

1.6.2 Pyrolysis
In pyrolysis, heavier hydrocarbon molecules of biomass are broken down
into smaller hydrocarbon molecules, noncondensable gases like CO, CO 2 ,
and solid carbon as char:

X X X

C n H m O p 1 heat - C a H b O c 1 C x H y O z 1 C (1.4)

Liquid Gas Solid

1.6.3 Combustion of Carbon
In subsequent discussion, we take simple carbon as the feedstock and write
the chemical reaction of the process to illustrate the conversion process. The
positive sign on the right side ( Q kJ/kmol) of the reaction equations

implies that heat is absorbed in the reaction. A negative sign (

1

2 Q kJ/kmol)

means that heat is released in the reaction.
When 1 kmol of carbon is burnt completely in adequate air or oxygen, it
produces 394 MJ heat and carbon dioxide. This is a combustion reaction:

C 1 O 2 - CO 2 393 ; 770 kJ = kmol (1.5)

1.6.4 Gasification of Carbon
If the oxygen supply is restricted, one can gasify the carbon into carbon
monoxide. The carbon then produces 72% less heat than it would have in
complete combustion, but the partial gasification reaction as shown below
produces a combustible gas, CO:

C 1 = 2O 2 - CO 110 ; 530 kJ = kmol (1.6)

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