PCDD/F Formation from Heterogeneous Oxidation of Wood Pyrolysates

This paper examines the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) during oxidative pyrolysis of wood. Model lignocellulosic compounds and fractions isolated from wood serve to generate volatiles which then interact with copper-loaded surrogate ash to produce PCDD/F. The measurements indicate a preference for the formation of PCDF over PCDD in all experiments. Lignin yields considerably more PCDD/F than the carbohydrates, in agreement with the previous measurement in experiments performed under inert conditions [Tame, Dlugogorski, Kennedy, Conversion of Wood Pyrolysates to PCDD/F, Proc. Combust. Inst. 32, submitted]. The ratio of PCDD to PCDF varies according to the chemical structure of the feed; with carbohydrates demonstrating greater relative propensity for PCDF than lignin. The lignin monomers with methyl ether subsitituents (i.e., vanillin and syringol) produce less PCDD/F than those without (i.e., catechol and p-hydroxybenzaldehyde). Ensuing experiments, to examine yield of PCDD/F as a function of temperature and mass loading of vanillin, reveal the mechanism of heterogeneous condensation of chlorophenols with subsequent chlorination/dechlorination.