Emissions of dioxins and dioxin-like polychlorinated biphenyls from domestic sources Final Report May 2006
Enviros Consulting Ltd, UK

2.3 Mechanisms for release of dioxins and furans

Dioxins and dioxin-like PCBs are formed as unintentional by-products in certain processes and activities. As well as being formed as unintentional by-products of manufacturing or disposal processes, dioxins and dioxin-like PCBs can also be present as contaminants in raw materials. This means that dioxins and dioxin-like PCBs can be released into the environment even in situations where dioxins and dioxin-like PCBs are not being formed.

Dioxins and dioxin-like PCBs are formed in two main kinds of processes: thermal processes, and chemical processes. The domestic sources under consideration in this project are combustion (i.e. thermal) processes, and there are two mechanisms at work which may result in dioxin and furan emissions.

(a) Dioxins and dioxin-like PCBs can be formed from the chemical elements (chlorine, carbon and oxygen), or from the “building blocks” (for example, chlorinated aromatic chemicals, or vinyl chloride present in polyvinyl chloride, PVC). The precursors needed for dioxin and dioxin-like PCBs to be formed are carbon and chlorine, in the presence of a limited amount of oxygen. This process can be more efficient if the chemicals are already present in a form which is close to that found in dioxins and dioxin-like PCBs. For example, organic chlorine may be more readily converted to dioxins and dioxin-like PCBs than inorganic chlorine. Aromatic chemicals (that is, organic chemicals containing benzene rings) are more readily converted to dioxins and dioxin-like PCBs than aliphatic substances (that is, organic chemicals which do not contain benzene rings).

The following aspects of a combustion process are important factors in determining the potential for formation of dioxin and dioxin-like PCBs:

- The temperature of the process must be in the region of 200-450 °C for dioxin formation to occur;
- Organic carbon needs to be present for formation of dioxins and dioxin-like PCBs to occur;
- Chlorine needs to be present for formation of dioxins and dioxin-like PCBs to occur;
- The formation of dioxins and furans can be enhanced by the presence of particles which have a high surface area – for example, ash or soot;
- If metals such as copper or cadmium are present in the combustion process, this can catalyse the formation of dioxins and dioxin-like PCBs; DEPARTMENT FOR ENVIRONMENT, FOOD AND RURAL AFFAIRS 10 EMISSIONS OF DIOXINS AND DIOXIN-LIKE POLYCHLORINATED BIPHENYLS FROM DOMESTIC SOURCES
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