Pyrolysis - Biomass Gasification, Pyrolysis and Torrefaction - page 165

5.4.3 Kinetic Models of Pyrolysis
To optimize the process parameters and maximize desired yields, knowledge
of the kinetics of pyrolysis is important. However, it is very difficult to obtain
reliable data of kinetic rate constants that can be used for a wide range of
biomass and for different heating rates. This is even more difficult for fast
pyrolysis as it is a nonequilibrium and non-steady-state process. For engineer-
ing design purposes, a “black-box” approach can be useful, at least for the first
approximation. The following discussion presents a qualitative understanding
of the process based on data from relatively slow heating rates.
Kinetic models of the pyrolysis of lignocellulosic fuels like biomass may
be broadly classified into three types (Blasi, 1993):
1. One-stage global single reactions. The pyrolysis is modeled by a one-step
reaction using experimentally measured weight-loss rates.
2. One-stage, multiple reactions. Several parallel reactions are used to describe
the degradation of biomass into char and several gases. A one-stage sim-
plified kinetic model is used for theseparallelreactions.Itisusefulfor
determination of product distribution.
3. Two-stage semiglobal reactions. This model includes both primary and
secondary reactions, occurring in series.

5.4.3.1 One-Stage Global Single-Reaction Model
This reaction model is based on a single overall reaction:

Biomass - volatile 1 char

It neglects presence of ash and assumes moisture remains in volatile. The
rate of pyrolysis depends on the unpyrolyzed mass of the biomass. Thus,
the decomposition rate of mass, m b , in the primary pyrolysis process may be
written as:

dm b
dt 52

k ð m b 2 m c Þ (5.2)

Here, m c is the mass of char remaining after complete conversion (kg), k is
the first-order reaction rate constant ( σ 2 1 ), and t is the time (s).

The fractional change, X, in the mass of the biomass may be written in
nondimensional form as:

5 ð

m 0 2 m b Þ

X (5.3)

ð m 0 2 m c Þ

where m 0 is the initial mass of the biomass (kg).
Substituting fractional conversion for the mass of biomass in Eq. (5.2) ,
dX
dt 52

ð Þ

k 1 2 X (5.4)