Environmental Engineering Reference
In-Depth Information
6.2.1
F
D
F
ORMATION
OF
IOXINS
AND
URANS
The dioxins and furans are polychlorinated dibenzo-para-dioxins (PCDDs) and poly
-
chlorinated dibenzofurans (PCDFs). Dioxins and furans are chlorinated tricyclic
aromatic compounds with similar chemical properties. Of some 210 dioxin com-
pounds, only 17 compounds are of significant interest because of their toxicity. The
most toxic compound is 2,3,7,8-tetrachlorodibenzo-para-dioxin (2,3,7,8-TCDD),
and a few other dioxin compounds cause carcinogenic or co-carcinogenic effects on
humans. Further, chlorine and fluorine in wastes is directly responsible for the
formation and emission of dioxin and furan from incinerators. Chlorine in municipal
solid waste can characteristically vary in the range of 0.3 to 0.7% by weight and
the fluorine content can be 0.5 to 2% of the chlorine content. Factors influencing
the formation of dioxin/furan are given below:
1.
Oxygen: Thermal degradation experiments using polychlorinated biphe-
nyls (PCB) have shown that the yield of dioxins increases with the increase
in oxygen concentration. However, it is also reported that increased oxy-
gen concentration decreases the yield of dioxins when dioxins are pro-
duced only from their precursors.
2.
Temperature: The temperature band of 250 to 400˚C is most critical for
the dioxin and furan formation. Maximum levels of dioxins and furans
are formed at a temperature of about 300˚C . The levels of both dioxins
and furans are about ten times higher at 300˚C as compared with 100˚C.
3.
Water vapor: Dioxins and furans increase significantly (by some 76%) in
wet samples as compared with dry samples.
4.
Carbon: High carbon content in the fly ash results in higher formation of
dioxins. Carbon acts as a sorbent for the precursor compounds for dioxin
and furan formation.
5.
Chlorine: High chlorine content in the fly ash favors the formation of
PCDD/PCDF.
6.
Hydrogen chloride/hydrochloric acid (HCl): Presence of HCl is not
directly responsible for dioxin and furan formation, but it promotes the
formation of chlorobenzenes and precursors of dioxins and furans. Excess
oxygen leads to the formation of chlorine gas by the following chemical
reaction:
1
2
OHO
2HCl
+
+
l
2
2
2
High levels of chlorine gas present simultaneously with fly ash favor the forma-
tion of PCDD/PCDF.
6.2.1.1
Refuse (or Waste) Derived Fuel
RDF or
WDF (waste derived fuel) is a fuel produced from the combustible (organic)
portion of the waste. The composition of RDF is better defined as compared with
Search WWH ::
Custom Search