Ruud Addink

Mechanisms of Formation and Destruction of Polychlorinated Dibenzo=p=dioxins and Dibenzofurans in Heterogeneous Systems

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are toxic compounds formed during natural processes and human activities. The basic questions about PCDD/F formation such as (1) what is the influence of process parameters on the formation process, (2) what reaction mechanisms are involved in formation, and (3) what kinetics describes PCDD/F formation are discussed, and recommendations are given.

Metals as Catalysts during the Formation and Decomposition of Chlorinated Dioxins and Furans in Incineration Processes

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are found on fly ash and in flue gas of municipal waste incinerators. Municipal waste incinerators seem to be the most important source for PCDDs and PCDFs emitted into the air during the last decades. PCDD/F formation takes place in the cooler zones (250 °C-450 °C) of a municipal waste incinerator, at the electrostatic precipitator. Copper is an important component of the fly ash that can catalyse the formation of PCDDs and PCDFs. Inhibition of the formation can be done by complexation of the metals (Cu) with EDTA or NTA.

Role of oxygen in formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from carbon and fly ash.

Under an oxygen-deficient flow, no PCDD/F is formed from a carbon/fly ash mixture, but with a mixture of 1% O 2 in N 2 de novo synthesis of PCDD/F sets in. The O 2 concentration was varied 0-10%, formation was highest at 5-10% O 2 . The [PCDD] :[PCDF] ratio rises with increasing [O 2 ], but the average degree of chlorination of PCDD and PCDF does not depend on the O 2 concentration. Especially 3,4,6,7-substituted PCDD/F congeners are formed. Without a gas flow, carbon/fly ash mixtures can give PCDD/F formation too : the O 2 now enters the mixture by diffusion. This formation process can continue for several hours. Isomer distributions do not change with increased reaction times. Formation rates under conditions without a gas flow are higher than with a gas flow present, suggesting that O 2 diffusion into the fly ash bed is more efficient than the supply of O 2 through a gas flow.

Influence of HCl and Cl2 on the Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans in a Carbon/Fly Ash Mixture.

Formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) from carbon on incinerator fly ash was investigated using HCI and Cl 2 as a chlorinating agent. The formation proceeds roughly at an equal rate with both gases. These results do not support the theory that chlorination reactions on the fly ash surface take place through free Cl 2 generated by the Deacon reaction. Once formed, PCDD/F remain adsorbed on the fly ash surface until ca. 350°C ; at ca. 400 °C, the major part of these compounds has evaporated from the fly ash to the gas phase.

Formation of polychlorinated dibenzo-p-dioxins/dibenzofurans in the carbon/fly ash system

Abstract The effect of the reaction time and temperature on PCDD/F formation on a fly ash surface was studied. Carbon was used as a source for de novo synthesis. There is a great rise in PCDD/F formation between 2 and 4 hours annealing time. This is probably due to changes in the fly ash surface. The optimum temperature for formation was found to be around 350 °C, which is higher than the 300 °C found in other publications. Whereas the congener distribution changes as function of both reaction time and temperature, the isomer distribution shows little reaction time or temperature dependence.

Surface catalyzed formation of polychlorinated dibenzo-p-dioxins/dibenzofurans during municipal waste incineration

A series of reactions was carried out on different surfaces using carbon and CuCl2, and KCl or HCl as a Cl-source. The PCDD/PCDF congener distribution was seen to vary to a great extent but the isomer distribution remained fairly constant. It is concluded that thermodynamic properties are largely responsible for the PCDD/PCDF isomer distribution. With pentachlorobenzene Cl2 elimination, assisted by oxygen, appears to be the major pathway for PCDD/PCDF formation.

Desorption behaviour of polychlorinated dibenzo-p-dioxins/dibenzofurans on a packed fly ash bed.

Abstract When incinerator fly ash is heated at 398 °C under a stream of N 2 O 2 polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) concentrations are lowered. Dechlorination/dtx;omposition and desorption are competing processes. In this paper the desorption was studied: within PCDD/F homologues, all isomers desorb at an equal rate. Obviously, values of the activation energy of desorption from the fly ash surface (ΔG#398,des do not differ significantly between these isomers.

Kinetics of formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from carbon on fly ash

Abstract Experiments were carried out to study the rate of PCDD/F formation from carbon on fly ash. The formation follows first order behaviour during the first part of the experiment. With longer reaction times, degradation reactions become more important. Rate constants found were in between 0.01–0.04 min −1 .