Page 184 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 161

Larger particles, on the other hand, facilitate secondary cracking due to the
higher resistance they offer to the escape of the primary pyrolysis product.
For this reason, older methods of charcoal production used stacks of large-
size wood pieces in a sealed chamber (Figure 5.2).

5.4 PYROLYSIS KINETICS

A study of pyrolysis kinetics provides important information for the engi-
neering design of a pyrolyzer or a gasifier. It also helps explain how different
processes in a pyrolyzer affect product yields and composition. Three major
processes that influence the pyrolysis rate are chemical kinetics, heat trans-
fer, and mass transfer. This section describes the physical and chemical
aspects that govern the process.

5.4.1 Physical Aspects
From a thermal standpoint, we may divide the pyrolysis process into four
stages. Although divided by temperature, the boundaries between them are
not sharp; there is always some overlap:

1. Drying (B100 C). During the initial phase of biomass heating at low
temperature, the free moisture and some loosely bound water is released.
The free moisture evaporates, and the heat is then conducted into the
biomass interior (Figure 5.4). If the humidity is high, the bound water aids
the melting of the lignitic fraction, which solidifies on subsequent cooling.
This phenomenon is used in steam bending of wood, which is a popular
practice for shaping it for furniture (Diebold and Bridgwater, 1997).
2. Initial stage (100 300 C). In this stage, exothermic dehydration of the

biomass takes place with the release of water and low-molecular-weight
gases like CO and CO 2 . Torrefaction takes place in this stage.
3. Intermediate stage (.200 C). This is primary pyrolysis, and it takes place

in the temperature range of 200 600 C. Most of the vapor or precursor
to bio-oil is produced at this stage. Large molecules of biomass particles
decompose into char (primary char), condensable gases (vapors and
precursors of the liquid yield), and noncondensable gases.

4. Final stage (B300 900 C). The final stage of pyrolysis involves secondary
cracking of volatiles into char and noncondensable gases. If they reside in
the biomass long enough, relatively large-molecular-weight condensable
gases can crack, yielding additional char (called secondary char) and gases.

This stage typically occurs above 300 C (Reed, 2002, p. III-6). The con-
densable gases, if removed quickly from the reaction site, condense outside
in the downstream reactor as tar or bio-oil. It is apparent from Figure 5.6
that a higher pyrolysis temperature favors production of hydrogen, which
increases quickly above 600 C. An additional contribution of the shift

reaction (Eq. (7.16)) further increases the hydrogen yield above 900 C.