Research On The Thermal Degradation Pathways Of UPOPs In Fly Ash

To implement the “Stockholm Convention on Persistent Organic Pollutants(POPs)”, the waste incineration is the priority controlled key source for its contributionto the unintentionally POPs (UPOPs) emissions. Under the current strict air pollutioncontrol technical levels, more than90%UPOPs was concentrated into the fly ash,which made the elimination of UPOPs in fly ash particularly crucial in the totalquanlity control of UPOPs in waste incineration source. Low temperature thermaldechlorination was proved to be effective in UPOPs elimination, however, thedechlorination pathway and the ultimate dechlorination products were not clear yet. Inthis study, the dechlorination mechanism of UPOPs in fly ash was confirmed.Hexachlorobenzene (HCB) was used as model UPOPs compound, thecatalyze effect of CaO on HCB thermal dechlorination was studied. The speciesof the dechlorination products were identified by a series of analytical methods,and the amorphous carbon containing the crystalline graphite plane structureproved existence of the dechlorination/polymerization pathway. Thedechlorination/polymerization products were complex mixtures of graphite andhigh-moleculer-carbon-rich compounds. Determination of elemental carbon and organiccarbon of the dechlorination/polymerization products was studied, and thedechlorination/polymerization was deduced as the major reaction pathway of the lowtemperature thermal dechlorination of HCB in CaO. Material balance calculation iscarried out based on hypothetical chemical equation, revealing over75%fate of theHCB thermal dechlorination products.The negative effect of nano-scale CaO power on HCB thermal dechlorinationcompared with industrial CaO, confirm the electron transfer mode hypothesis of thedechlorination/polymerization pathway. The dechlorination/polymerization initiatedfrom the HCB getting an electron from the solid matrix to form HCB anion. Thedissociation of a chloride ion happened forming a radical, and then the polymerizationof radicals led to the formation of high-molecular-weight compounds, such asamorphous carbon. The ablity of giving electron for nano-scale CaO is lower due to itsrare crystal defect. CaO in fly ash showed a notable HCB dechlorination potential, and the effect oflow content heavy metals, e.g Cu, was not notable in enhancing the dechlorinationreaction, indicated that the CaO was the major active component in fly ash. The majorreaction pathway of HCB in fly ash was the dechlorination/polymerization induced byCaO, the unbalance of material between dechlorination/hydrogenation products andinitial HCB in fly ash was explained.HCB was the appropriate indicater of dioxin concentrations and toxicity, theultimate fate of dioxin in fly ash thermal treatment de-chlorine/polymerization to formamorphous carbon and high-molercule compounds. The mechanism ofdechlorination/polymerization pathway inspired research into POPs pollutant except forfly ash.