Pyrolysis kinetics of live and dead wildland vegetation from the Southern United States

Abstract

The fundamental combustion behavior of live wildland vegetation is not fully understood. Since the combustion process during wildland fire starts with pyrolysis, there is a need for better understanding of pyrolysis to develop improved wildland fire models. The kinetics of pyrolysis of live and dead wildland vegetation has not been explored in detail. In this study, the pyrolysis kinetics were determined for 14 different plant species (live and dead) which are all native to the forests in the southern United States. Pyrolysis experiments were carried out in a Thermogravimetric analyzer (TGA) under inert conditions at 5 different heating rates ranged from 10 to 30 °C min−1. The iso-conversional and model-free method of KAS was used to calculate the activation energies and pre-exponential factors. The rates of pyrolysis as a function of conversion were calculated using the KAS method for all plant species (live and dead), and the results showed that during the sample decomposition, the highest rates were observed at the lower conversions. The dependence of activation energy on conversion fraction, type of plant species, and plant condition (live and dead) was investigated. For some plant species, an increase in activation energy with conversion was observed in the major pyrolysis zone for live samples. The effect of aging on apparent activation energy was studied by comparing the pyrolysis rates of live and dead longleaf pine foliage with longleaf pine litter (pine straw). The results showed that length of time since living increased the activation energy in the major pyrolysis zone. The average activation energy based on the KAS method for pyrolysis of abscised pine foliage (pine straw) was 183.4 kJ mol−1, while the activation energies were lower for live and dead longleaf pine foliage (167.3 and 167.4 kJ mol−1, respectively). Finally, the activation energies obtained in this study were compared with literature and the results showed that the average activation energies obtained in this study for palmetto, grass, broadleaf, and needle types are very close to the average activation energies obtained for different kinds of straw reported in the literature.