Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 93

Figure 5.2 shows a sketch of an oven typically used for the production of
charcoal. Wood is stacked on the ground and a clay covering is built over
this leaving a small opening at the bottom. This helps reduce oxygen supply
to the wood. The small opening provides just the amount of oxygen to burn
some wood to provide heat for carbonization. Since the oven is closed and
well insulated, whatever heat is generated is retained inside the oven and
that helps slow down the thermal degradation of the wood into charcoal. The
temperature inside the carbonizer could be as high as 800 C.
Modern industrial processes for charcoal making employ internal heating
(Missouri kiln), external heating (VMR retort), or heating by gas recircula-
tion (the Degussa process) (Antal and Gronli, 2003). A review of technolo-
gies for production of charcoal is given in FAO (2008). Fuel charcoal has
high fixed carbon content and a modest amount of volatile matter
( Table 4.3 ).
4.3.2 Activated Charcoal
Activated charcoal is a valuable product used in a host of chemical and envi-
ronmental industries. Its large pore surface area gives it an exceptionally
high adsorption capacity. As a result, this type of charcoal fetches a consid-
erably higher price from the market than by normal fuel charcoal.
Activated charcoal is produced by removing the tarry products from con-
ventional fuel charcoal. This makes the pores in charcoal more accessible for
adsorption. The activation process increases the pore surface area by orders
of magnitude.
There are several methods for making activated charcoal, but the basic
process is essentially the same. It involves heating ground charcoal to about
800 C in an atmosphere of superheated steam. The charcoal thus avoids con-
tact with oxygen while distilling away the tar that was blocking the fine
structures of the charcoal. Steam carries away the tarry residues. After this
the solid product is poured into a sealed container and allowed to cool.
4.3.3 Biocoke
This type of charcoal is produced specifically for metal extraction as a sub-
stitute for conventional coke that is produced from coking coal. When heated
with metallic ores with oxides or sulfides, carbon in biocoke combines with
oxygen, and sulfur allowing easy metal extraction. It is acknowledged to be
a better reductant than coke (FAO, 1983). Biocoke has been used for extrac-
tion of iron from iron ore during the very early days of metallurgical indus-
tries. Biocoke needs certain specific properties for its use in blast furnace. It
must have adequate compressive strength to withstand the pressure of heavy
burden of solids in the blast furnace. Additionally, it needs to have good
fracture resistance to maintain constant permeability of the furnace charge to