(318b) Experimental and Modeling Study of Woody Biomass Pyrolysis Under Low Temperature Conditions

Experimental
and Modeling Study of Woody Biomass Pyrolysis at Low Temperatures

Pyrolysis is one of the
most common thermo-chemical conversion processes of biomass. Little modeling
and experimental data exist in the literature for low temperature pyrolysis of
wood. To better understand the chemical and physical processes involved, dried poplar
wood cylinders of 1.9 cm diameter and 4 cm length were pyrolyzed using a flow
of nitrogen heated to a temperature range of 390-440⁰C.
Experiments were run with a nitrogen velocity of 2.3 m.s^-1 at
atmospheric pressure for 30 minutes. Temperature profiles at the center of the
poplar cylinder were measured using a thin sheathed 0.25 mm OD K-type
thermocouple. Figure 1 shows the centerline
wood temperature history at a gas temperature of 420⁰C.
The temperature history is characterized by two distinct events: the first an
endothermic reaction, followed by an exothermic reaction.

Figure 1.
Center Temperature Profile for Pyrolysis at 420⁰C

Concentration histories
of gaseous species produced through the pyrolysis process were measured by
Fourier Transform Infrared (FTIR) analysis; a unique data set because, to our
knowledge, no previous studies have been reported in the literature with time
resolved species data from pyrolysis. Tars were trapped over the course of the
entire experiment and analyzed using gas chromatography/mass spectrometry
(GC/MS) at the conclusion of the experiment. Char yields were also determined
at the end of the experiment, and under the given experimental conditions,
ranged from 24-28 wt %. Figure 2 below shows the
species time histories at a pyrolysis temperature of 420⁰C,
displaying a high degree of dilution caused by the large nitrogen flow. Different
classes of species were identified, including hydrocarbons, aldehydes, alcohols,
and carboxylic acids, in addition to CO and CO₂. Unfortunately,
water (H₂O) and hydrogen (H₂), which are expected to be
among the major species, were not analyzed in this study. As observed in Figure
2, the formation of all detected
species, except for methane, occurs very early (starting earlier than 1 min
after insertion). Furthermore, the peak concentrations of CO₂, HCHO,
CHOOH and CH₃COOH appear earlier than the other measured species.

Figure
2.
Species Profiles for Pyrolysis at 420⁰C

Experimental temperature
profiles were compared to predictions of the pyrolysis model developed by Park
et al. 2010[1].
This model was developed to determine the kinetics involved in the slow
pyrolysis of large wood particles under conditions of low temperature and long
residence times. It includes transient mass, species, and energy conservation
equations, in addition to the Darcy momentum equation. Coupled to these equations
is the kinetic mechanism, shown in Figure
3.
Each reaction rate is assumed to follow a first order Arrhenius type reaction.

Figure 3.
Kinetic Mechanism

Because the pyrolysis
model was developed for a specific experimental configuration, changes were made to better represent the results obtained from the
experimental setup presented here. Modifications included a change to a
cylindrical coordinate system, adaption of wood and char properties, such
thermal diffusivity, for poplar wood, and experimental determination of thermal
boundary conditions. The modified pyrolysis model was implemented in COMSOL, a commercial
Multiphysics software.