Dioxin prevention and abatement

Dioxins are relevant for thermal processes which have metals present. Dioxins or their precursors may be present in some raw materials and there is a possibihty of de-novo synthesis in furnaces or abatement systems. Dioxins are easily adsorbed onto solid matter and may be collected by all environmental media as dust, scrubber solids and filter dust. Field tests have shown that the formation of dioxins in cupola furnaces cannot be correlated to one (or a few single) operational parameter(s). A combination of measures is needed to minimise the risk of dioxin formation. 

In-process or primary measures to prevent dioxin emissions include  

Although dioxins are destroyed at high temperature (i.e. above 850 °C) in the presence of oxygen, the process of de-novo synthesis is still possible as the gases are cooled through the 

Sulphur has an inhibiting effect on the formation of dioxins, through depletion of molecular chlorine. The use of coal with a higher sulphur content in large combustion plants has been shown to provide lower PCDD/F concentrations. The reducing inhibiting effect is related to the S/Cl ratio, with a critical ratio of 0.64. A further increase does not result in less dioxins and furans. This effect has not been demonstrated in foundries, but may be studied. [231, UNEP, 2003] 

The great spreading and big variability in the dioxin emission levels (even for the same installation) show that primary measures alone may not allow a stable and low dioxin emission value. Therefore, besides primary measures, the following abatement measures may be considered  

---