(291c) Effect of Particle Size and Temperature On Woody Biomass Slow Pyrolysis Process

            Slow
pyrolysis of dried pine spheres was carried out in a flow reactor to study the
influence of temperature (378-465 °C) and particle size (2.54-3.81 cm) on the
product distribution and temperature profiles. Pyrolysis was performed by
inserting spheres into a heated nitrogen flow for a residence time of 40 min. The
particle size of the samples used in the experiment is limited by the reactor
diameter.  The temperature histories within the wood spheres
were measured at three radial locations: center (r=0), and two ?middle?
locations (r= R/2), using three sheathed 0.5 mm OD K-type thermocouple inserted
through holes drilled in the particle. Figure 1 shows the installation of
thermocouples inside the sphere. The temporal evolution of the species (CO,
CO₂, CH₄, HCHO, CH₃OH, HCOOH, and CH₃COOH) released during the sample pyrolysis was measured using Fourier
Transform Infrared (FTIR) analyzer. TCD and MS detectors on a gas chromatograph
were used to analyze the time-resolved concentration of hydrogen, and the tars
collected over the course of the entire experiment, respectively.

Figure
1: Thermocouples locations within the woody sphere.  The arrow indicates the
nitrogen flow direction.

            Figure 2
illustrates the temperature history of the two particle sizes (2.54 cm and 3.81
cm) heated with a nitrogen flow at 466 °C. Both profiles show two thermal
events; an endothermic reaction, followed by an exothermic reaction. The
temperature peak appears early for the 2.54 cm sphere at 5.33 min however for
3.81 cm, it appears at 9.77 min. This trend is
reasonable because for the large particle the heat transfer is slower.

Figure
2: Particle temperature history comparison at a nitrogen temperature of 466 °C.

            Figure 3
below illustrates the species time histories at a pyrolysis temperature of 466⁰C
and for sphere 2.54 cm diameter, displaying a high degree of dilution caused by
the large nitrogen flow (5.8E-03 Kg/s). Different classes of species were
identified, including hydrocarbons, aldehydes, alcohols, and carboxylic acids,
in addition to CO, CO₂ and H₂. Good repeatability was
obtained over at least two runs. As observed in this figure, the
formation of all detected species, except for methane, occurs very early
(starting earlier than 1 min after sphere insertion). The peak concentrations
of CO₂, HCHO, CHOOH and CH₃COOH appear earlier than the
other measured species. Furthermore, the results indicated the presence
of the same main species for the both particle sizes and the three temperature
conditions.

            Figure 4 shows
the char, tar and gases yields measured for the two particle sizes and the
three pyrolysis temperatures. Gases are defined as CO, CO2,
and CH4
only,
and tar yields in the experiment are obtained by difference.

Figure
3: Mole fractions of species released from 2.54 cm pine sphere at a nitrogen
temperature of 466 °C.

Figure
4: Experimental products yields (mass % as a percent of initial wood mass)
obtained for the two particle sizes at three temperatures.

            The
experimental results were then compared to those calculated with a 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 multistep kinetic
mechanism describing direction production of gas and tar, and indirect
production of char. Each reaction is assumed to be first order with Arrhenius
type rate expression.

            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 an adaptation of some
wood and char properties, such as thermal diffusivity, and an experimental
determination of thermal boundary conditions. The modified pyrolysis model was
implemented in COMSOL, a commercial Multiphysics software. ______________________________

[1] W.C. Park, A.
Atreya and H.R. Baum, Combustion and Flame 157 (2010) 481-494