PCDD/F formation in milled fly ash: Metal chloride catalysis

Abstract

Mechanochemical treatment reduces the particle size of fly ash and disperses catalytic metals, raising the potential reactivity of fly ash to form and destroy ‘dioxins’, i.e., polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD + PCDF or PCDD/F). To test this issue, model fly ash (MFA) samples were artificially composed by mixing silica, sodium chloride, and activated carbon, and doping this matrix with five selected catalytic metal chlorides administered as CuCl2•2H2O, CrCl3•6H2O, FeCl3 (anhydrous), ZnCl2 and anhydrous CaCl2. Without additives, these samples were first finely milled for 0 (blank), 1 and 8 h, and the effect on the formation of PCDD/F was investigated during de novo tests. These simulate the conditions prevailing in deposits in the heat-recovery zone of an incinerator, where dioxins are formed and destroyed, and the tests were conducted at the same temperature, reaction time and air flow rate. Metal chlorides produce specific and distinct homologue and isomer patterns. The isomer signatures of 2,3,7,8-PCDD/F, precursor route, and other congeners were recognised and are studied in some detail. Principal component analysis (PCA) was applied to the 2,3,7,8-PCDD/F-congeners, indicating CuCl2 as major contributor of WHO2005-TEQ values. Surprisingly, for CuCl2•2H2O the total yield of dioxins reduced drastically with milling. Since this study revealed various unexpected results, as well as experimental limitations, some suggestions for further ongoing work are formulated.