Reduction of Dioxins and Furans

Although dioxins and furans are destroyed at above 850 °C, they re-form in the temperature range 200 to 450 °C [29.3]. It is understood that, without abatement techniques, municipal incinerators may emit at levels in the range 1 to 10 ng/Nm. Emission limits range from 0.1 to 1 ng/Nm.  

Emission levels can be reduced by two techniques — rapid cooling between 450 and 200 °C, to minimise their re-formation, and adsorption on activated carbon. 

The semi-dry scrubbing technique can be used to achieve the required rapid cooling from 450 °C, but at a cost, as this reduces the amount of recoverable heat (boiler flue gases are normally cooled to about 200 °C in a waste heat boiler). 

Alternatively, activated carbon (or a less expensive equivalent such as lignite coke) can be injected into the exhaust gases and collected in a bag filter. An addition of 1 kg of activated carbon per tonne of organic matter is generally sufficient to reduce the dioxin and furan level to below 0.1 ng/Nm [29.10]. A patent application describes the use of a blend of hydrated lime and activated carbon/lignite coke [29.11] uses the hydrated lime to effectively disperse the carbon and to eliminate the fire/explosion hazard associated with finely divided activated carbon. 

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