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Kinetic Parameters for Biomass under Self-Ignition Conditions: Low-Temperature Oxidation and Pyrolysis
Version 3 2019-09-19, 13:43
Version 2 2019-09-17, 20:03
Version 1 2019-08-07, 18:04
journal contribution
posted on 2019-09-19, 13:43 authored by Lars Schwarzer, Zsuzsa Sárossy, Peter Arendt Jensen, Peter Glarborg, Oskar Karlström, Jens Kai Holm, Kim Dam-JohansenPulverized biomass
may self-heat and spontaneously ignite when
stored or processed at intermediate or even low temperatures. In this
work, reaction kinetic parameters for biomass oxidation and pyrolysis
were determined for the temperature range 423–523 K. Thermogravimetric
analysis was used to determine mass loss kinetics in a stepwise-isothermal
heating program. Two wood species (pine and beech), two agricultural
residues (wheat straw and sunflower husks), and two commercial wood
pellet samples were investigated. Atmospheres with 0, 20, and 80%
oxygen were used in the experiments. A pyrolysis model of four parallel
reactions for extractives, hemicellulose, cellulose, and lignin fit
the experimental data for 0% O2 well. Oxidation kinetics
could be modeled by additional reactions in parallel to the pyrolysis
mechanism. Two mechanisms were tested: (1) considering oxidation of
a lumped “volatilizable” component plus oxidation of
char; and (2) separate oxidation reactions for volatilizable extractives,
hemicellulose, cellulose, and lignin, plus char. The more complex
mechanism did not give a clear advantage over the simpler mechanism.
It was further found that pyrolysis and oxidation reactions for the
components could be modeled with the same activation energy, regardless
of which biomass they appear in. For the lumped component oxidation
model, an apparent activation energy of 130 kJ/mol was found. The
observed reaction order in oxygen was in the range 0.4–0.5.
The models also compared favorably to additional experimental data
between 373 and 773 K for a heating rate of 5 K/min. The kinetic models
presented here are intended mainly to describe low-temperature reactions,
such as self-heating of biomass and the onset of smoldering combustion.