Page 177 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

154 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 5.2 Characteristics of Some Thermal Decomposition Processes
Pyrolysis Residence Heating Final
Process Time Rate Temperature ( C) Products

Torrefaction 10 60 min Very small 280 Torrefied
biomass
Carbonization Days Very low .400 Charcoal
Fast ,2 s Very high B500 Bio-oil
Flash ,1 s High ,650 Bio-oil,
chemicals, gas
Ultrarapid ,0.5 s Very high B1000 Chemicals, gas
Vacuum 2 30 s Medium 400 Bio-oil
Hydropyrolysis ,10 s High ,500 Bio-oil
Methanopyrolysis ,10 s High .700 Chemicals

down into two types: carbonization and torrefaction. Torrefaction takes place

in a very low and narrow temperature (200 300 C), while carbonization
takes place at much higher and broad temperature.
In fast pyrolysis, the vapor residence time is on the order of seconds or
milliseconds. This type of pyrolysis, used primarily for the production of
bio-oil and gas, is of two main types: flash and ultrarapid.
Table 5.2 compares the characteristics of different thermal decomposition
processes and shows carbonization as the slowest and ultrarapid as the fast-
est. Carbonization produces mainly charcoal; fast pyrolysis processes target
production of liquid or gas.

5.2.2.1 Slow Pyrolysis
Carbonization is a slow pyrolysis process, in which the production of char-
coal or char is the primary goal. It is the oldest form of pyrolysis, which is in
use for thousands of years. The biomass is heated slowly in the absence of

oxygen to a relatively low temperature (B400 C) over an extended period of
time, which in ancient times ran for several days to maximize the char forma-
tion. Figure 5.2 is a sketch of a typical beehive oven in which large logs were
stacked and covered by a clay wall. It allows a certain amount of oxygen for
partial combustion of wood. A small fire at the bottom provided the required
heat for carbonization. The fire essentially stayed in the well-insulated closed
chamber. Carbonization allows adequate time for the condensable vapor to be
converted into char and noncondensable gases.