PNNL

Abstract

Background. Fire models typically use pyrolysis data from non-oxidizing environments. Pyrolysis and combustion in wildland fires occur in an oxidizing environment. Data from experiments measuring pyrolysis gases in both environments are available for comparison. Aims. To determine if the composition of pyrolysis gases measured at bench-scale in a non-oxidizing environment is like the composition measured in ambient atmospheric conditions (oxidizing environment) in wind tunnel and field settings using compositional data analysis. Methods. Permanent gases and tars were measured in a fuel-rich (oxygen-free) environment in a flat flame burner using GC/MS and GC/TCD. Permanent and light hydrocarbon gases were measured for the same fuels heated by flames in ambient atmospheric conditions (wind tunnel, prescribed burns) using GC/FID and FTIR. Log-ratio balances of CO, CO2, CH4, H2, C6H6O, and other gases in the non-oxidizing and oxidizing environments were examined by PCA, CDA and PERMANOVA. Key results. Mean compositions changed notably between the non-oxidative pyrolysis (bench) samples and the oxidative (wind tunnel, field) samples. PCA showed that bench samples were tightly clustered and distinct from the wind tunnel and field samples. CDA found that the difference between environments was defined by the CO-CO2 log-ratio balance. PERMANOVA and pairwise comparisons found bench samples differed from the wind tunnel and field samples which did not differ from each other. Conclusion. The relative composition of these pyrolysis gases differed between the oxidizing and non-oxidizing environments. Implications. These results indicate the need for more fundamental research on the early time-dependent pyrolysis of vegetation in the presence of oxygen rather than using a bulk composition for the entire pyrolysis process taken from non-oxidizing experiments.