Effects of Preliminary Thermal Extraction on De Novo Synthesis

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 degrees C, 420 degrees C, and 540 degrees 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

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.