Pyrolysis - Biomass Gasification, Pyrolysis and Torrefaction - page 154

TABLE 5.2 Characteristics of Some Thermal Decomposition Processes

Pyrolysis
Process

Residence
Time

Heating
Rate

Final
Temperature ( C) Products

Torrefaction 10 60 min Very small 280 Torrefied

biomass

Carbonization Days Very low 400 Charcoal

.

Fast 2 s Very high 500 Bio-oil

, B

Flash , 1 s High , 650 Bio-oil,

chemicals, gas

Ultrarapid , 0.5 s Very high B 1000 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 (

400 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.

B