Effects of Preliminary Thermal Extraction on De Novo Synthesis †

Abstract

Abstract

This study systematically compared the evolution of a wide selection of dioxin-relevant compounds during thermal extraction (TE) of filter dust in helium and during de novo (DN) tests in a flow of synthetic air. TE yielded a wide range of dioxin-relevant compounds; however, chlorinated aromatics appeared only in minor amounts. Laboratory DN tests were conducted on this same dust, and also after a mild, medium, or severe TE-procedure, involving final TE-temperatures of 300°C, 420°C, and 540°C, respectively. The same compounds were again analyzed. Chlorinated compounds (Cl-compounds) became predominant DN-products. For each DN-test and for each product, it was verified as to how far preliminary TE and consecutive DN each contribute to the sum of (TE + DN) products. Therefore, for each test, an individual TE- and DN- contribution level could be assigned to each compound monitored. Most oxygenated-compounds were TE-derived, suggesting that they formed from pre-existing oxygenated carbon structures, rather than from oxidation of carbonaceous matter. Conversely, Cl-compounds were typical DN-products and selectively gained importance as TE became more severe. The tests generated much more chlorobenzenes than chlorophenols, in line with the high polychlorinated dibenzofuran to polychlorinated dibenzo-p-dioxin ratio found by conventional analysis of the dust tested. The tests were designed to compare DN- and TE-products and, thus, distinguish between dioxin formation by the precursor route (i.e., formation from structurally related compounds) and the DN route (i.e., low- temperature, catalytic oxidation of the carbonaceous matrix). There was substantial difference between TE- and DN-products, the former being lean in Cl-compounds and the latter being rich. The sample tested was particularly rich in volatile matter and, hence, atypical. The procedure followed stepwise eliminates this volatile matter, so that DN-products from the original sample could be compared with those in which fixed carbon is more important. Preliminary TE strongly enhanced the relative importance of Cl-compounds and reduced that of oxygenated and hydrocarbon compounds during the consecutive DN tests.