266605 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. 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.
 W.C. Park, A. Atreya and H.R. Baum, Combustion and Flame 157 (2010) 481-494
See more of this Group/Topical: 2012 International Congress on Energy (ICE)