Heating rate and temperature effects on pyrolysis products from live wildland fuels

Wildland fire, which includes both planned (prescribed fire) and unplanned (wildfire) fires, is an important component of many ecosystems. During wildland fires, low heating rate pyrolysis (slow pyrolysis) occurs during preheating and/or smoldering of plant material. High heating rate pyrolysis (fast pyrolysis) exists in the flame region. Pyrolysis temperature and heating rate play important roles on the yields and the compositions of pyrolysis products. In this work, the effects of pyrolysis temperature and heating rate on the yields and the compositions of pyrolysis products from 14 plant species native to the forests of the southern United States are shown. The slow pyrolysis experiments were performed at a low heating rate of 0.5 °C s−1 and an operating temperature of 500 °C. However, the fast pyrolysis experiments were operated at a high heating rate of 180 °C s−1 and a temperature of 765 °C. The yields and compositions of the pyrolysis products during the slow and fast pyrolysis experiments were analyzed in detail. The results showed that the average tar yield for all plant species (live and dead) was 58 wt% on a dry-ash free (daf) basis for the fast pyrolysis experiments compared to 49 wt% (daf) for the slow pyrolysis experiments, an increase of 9 wt%. The average gas yields for the slow and fast pyrolysis of the plants were 20 and 22 wt% (daf), respectively. The average volatile yield increased from 69 wt% (daf) at the low heating rate experiments to 80 wt% (daf) for the high heating rate experiments. The major light gas species for both the slow and fast pyrolysis experiments (wt% basis) were CO, CO2, CH4, and H2, with higher yields of CO observed in the high heating rate experiments and higher yields of CO2 in the slow pyrolysis experiments. The slow pyrolysis experiments led to formation of aliphatic and 1-ring aromatic compounds with large number of attachments on their rings, such as phenol, 1,2-benzenediol, 2-methoxy phenol, etc. In the fast pyrolysis experiments, phenol was still one of the major products. However, in contrast with the slow pyrolysis experiments, 1- to 5-ring aromatic compounds with very few attachments, such as fluorene, anthracene, phenanthrene, fluoranthene, pyrene, etc. were major tar compounds during the fast pyrolysis experiments.