Wildland fire is an important component of many North American ecosystems and has been used by humans to accomplish various objectives for several thousand years. Prescribed burning in the southern United States is an important tool used by the Department of Defense and other land managers to accomplish several objectives including hazardous fuel reduction, wildlife habitat management, critical training area maintenance, ecological forestry and infrastructure protection. The vegetation on Department of Defense installations is heterogeneous, unlike the homogeneous fuel beds assumed by current operational fire spread models. These models do not contain fundamental descriptions of chemical reactions and heat transfer processes necessary to predict fire spread and energy release needed for process-based fire effects models. To improve prescribed fire application to accomplish desired fire effects and limit potential escapes, an improved understanding is needed of the fundamental processes related to pyrolysis and ignition in heterogeneous fuel beds of live and dead fuels. The objective of this project is to address several fundamental questions to improve our understanding and modeling capability of fire propagation in natural fuel beds including 1) detailed description of pyrolysis and the evolution of its products for a greater variety of southern fuels than is currently known, 2) how convective and radiative heat transfer from flames to live fuel particles influences pyrolysis and ignition at laboratory and field scales, and 3) more detailed insight into pyrolysis, combustion and heat transfer processes in wildland fire spread through the use of high-fidelity physics-based models.
During this research, fast pyrolysis of 14 live and dead plant species which are native to the southern United States was studied using a flat-flame burner (FFB) system. The FFB system enables experiments at a high heating rate (~100°C s-1) and moderate temperature (~765°C) to imitate pyrolysis during typical fire spread conditions. Pyrolysis products were analyzed in detail using a gas chromatograph equipped with a mass spectrometer (GC-MS) for analysis of tars, and a gas chromatograph equipped with a thermal conductivity detector (GC-TCD) for analysis of permanent (non-condensable) gases. Permanent gases were also monitored with an FTIR system. Differences between yields of permanent gas species were small between plant species. Composition of tars included aromatic compounds with 1 to 5 rings with very few attachments. The pyrolysis products observed at this temperature and heating rate appear to have experienced secondary pyrolysis. The tar composition showed some large changes with plant species. Comparison of products from pyrolysis of live vegetation and dead vegetation of the same plant species showed differences in tar, gas, and char yields, but no major changes in the types of chemical compounds observed.