Kevin R. Bruce

The effect of metal catalysts on the formation of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran precursors

The catalytic effects of copper and iron compounds were examined for their behavior in promoting formation of chlorine (Cl2), the major chlorinating agent of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), in an environment simulating that of municipal waste fly ash. Formation of Cl2 occurred as a result of a metal-catalyzed reaction of HCl with O2. Catalytic activity was greatest at a temperature of approximately 400 °C, supporting a theory of de novo synthesis of PCDDs and PCDFs on fly ash particles downstream of waste combustion.

Mechanistic steps in the production of PCDD and PCDF during waste combustion

Abstract Research has shown that synthesis of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) during municipal waste combustion can proceed through a three step mechanism including 1) production of Cl2 from a metal-catalyzed reaction of HCl and O2, 2) Cl2 chlorination of aromatic rings through substitution reactions, and 3) formation of dual ring structures by a second metal-catalyzed reaction. Formation of the dual ring PCDD structure, likely through condensation reactions of chlorophenols, is enhanced up to three orders of magnitude in the presence of metal catalysts, such as Cu (II), reaching a maximum around 400°C.

Formation of Chlorinated Organics During Solid Waste Combustion

The formation mechanisms of the precursors of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) were examined in a laboratory reactor. Both homogeneous and heterogeneous reactions were studied between 200 and 800 °C with HCl, Cl2, and phenol as reactants in a simulated flue gas containing oxygen. Analysis of the reactor effluent showed that homogeneous phase production of chlorophenols and non-chlorinated dioxin and dibenzopdioxin and dibenzofuran, benzofuran potential precursors to PCDD and PCDF, was related to HCl concentration, reaching a maximum formation level around 650 °C. However, Cl2 produced a greater variety of chlorinated aromatics at levels over three orders of magnitude greater than with HCl, with product concentrations reaching maximum formation levels around 350 °C. Heterogeneous tests at 450 °C using a CuCl catalyst increased formation of chlorinated organics and PCDDs and identified the major chlorinating reactant to be Cl2.

The role of gas-phase Cl2 in the formation of PCDD/PCDF during waste combustion

Results of previous experiments investigating formation of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/PCDF) through low-temperature (300°C), fly-ash-catalyzed reactions are demonstrated to have occurred through intermediate formation of gas-phase Cl2 by decomposition of the added catalyst, CuCl2. The dependence of PCDD/PCDF formation rates on Cl2 concentration is shown and the implications of the Deacon process on these rates discussed. A scheme for controlling the formation of PCDD/PCDF during incineration using sorbent materials to remove the source of Cl at high temperatures is proposed.

Apparatus for short time measurements in a fixed‐bed gas/solid reactor

An apparatus for exposure of a solid to reactive process gas is described which makes possible short time (≥0.3 to 15 s) exposures in a fixed‐bed reactor. Operating conditions for differential reaction with respect to the gas concentration and rapid quench for arresting high‐temperature reactions are made possible by high gas flowrates through the sample bed. The residence time of solid exposure to the reactive gases is timed by a photosensor emitter/detector and oscilloscope. For times under 2 s, the standard deviation of the residence times is less than 10 ms. The ease of apparatus construction and operation, the operable temperature range (ambient to 850 °C), and the reproducibility of residence times down to 0.3 s makes the apparatus ideal for kinetic studies of rapid gas/solid reactions.