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222 Cha pte r Se v e n

Approximate Range of Validity
Model Bi Py Py
Non-controlling conditions All values
External heat transfer <1 >1 >1
Kinetics <1 >10 >10
Internal heat transfer >50 <10 –3 <<1
TABLE 7.1 Criteria for Identifying the Controlling Mechanism during
Biomass Devolatilization

Biot number values. The criteria to identify the controlling mechanism
for a single biomass particle devolatilization are listed in Table 7.1
(Pyle and Zaror 1984).
The poor thermal conductivity of biomass particles (0.1 W/m K
along the fibers and 0.05 W/m K cross-fiber) limits the maximum
heating rates that can be achieved. Large particles (diameter more
than 2 mm) can only achieve high heating rates if the char is continu-
ously removed mechanically (Bridgwater et al. 1999).

7.5 Pyrolysis Technologies
The reactors used for biomass pyrolysis can be classified into slow
and fast pyrolysis. Although slow pyrolysis reactors are mainly used
to produce charcoal, fast pyrolysis is the technology of choice to max-
imize bio-oil yields. Yields of oil as high as 80 mass% can be obtained
with this technology. Slow pyrolysis, on the other hand, will result
in much lower yields (30 to 50 mass%) of a liquid composed of
two phases (decanted oil or tars and pyroligneous acid) (Wood and
Baldwin 1985).

7.5.1 Slow Pyrolysis
Most of the charcoal producers in developing nations employ tempo-
rary earthen kilns in which the wood pile is buried under a layer of
soil. Charcoal makers are in charge of opening and closing vent holes
during the whole production period to control the amount of oxygen
supplied. Slow pyrolysis happens when the heating rate is less than
10°C/s, the pyrolysis temperatures are below 500°C, and gases and
solids stay inside the reactor for a long time (Graham et al. 1984).
A typical yield of products for these technologies is liquids, 30 mass%;
charcoal, 35 mass%; and gases, 35 mass%. Slow pyrolysis typically
achieves energy efficiencies between 20 and 30 percent.