Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 135

Dioxins are highly toxic compounds that are created unintentionally during
combustion processes. The most toxic form is the 2,3,7,8-tetrachlorodibenzo- p -
dioxin (TCDD) isomer. Other isomers with the “tetra” configuration are also
considered to have higher toxicity than the dioxins and furans with different
chlorine atom arrangements.
Knowing how these compounds are formed is the first step in reducing or
eliminating them. The chemical and physical mechanisms that lead to the pro-
duction of dioxin involve the halogen chlorine. Incinerators of chlorinated wastes
are the most common environmental sources of dioxins, accounting for about
95% of the volume produced in the United States.
The emission of dioxins and furans from combustion processes may follow
three general physicochemical pathways. The first pathway occurs when the feed
material going to the incinerator contains dioxins and/or furans and a fraction of
these compounds survives thermal breakdown mechanisms and pass through to be
emitted from vents or stacks. This is not considered to account for a large volume
of dioxin released to the environment, but it may account for the production of
dioxinlike, coplanar PCBs.
The second process is the formation of dioxins and furan from the ther-
mal breakdown and molecular rearrangement of precursor compounds, such as
the chlorinated benzenes, chlorinated phenols (such as pentachlorophenol), and
PCBs, which are chlorinated aromatic compounds with structural resemblances
to the chlorinated dioxin and furan molecules. Dioxins appear to form after the
precursor has condensed and adsorbed onto the surface of particles, such as fly ash.
This is a heterogeneous process, ∗ where the active sorption sites on the particles
allow for the chemical reactions, which are catalyzed by the presence of inorganic
chloride compounds and ions sorbed to the particle surface. The process occurs
within the temperature range 250 to 450 ◦ C, so most of the dioxin formation un-
der the precursor mechanism occurs away from the high-temperature zone in the
incinerator, where the gases and smoke derived from combustion of the organic
materials have cooled during conduction through flue ducts, heat exchanger and
boiler tubes, air pollution control equipment, or the vents and the stack.
The third means of synthesizing dioxins is de novo within the “cool zone” of
the incinerator, wherein dioxins are formed from moieties different from those
of the molecular structure of dioxins, furans, or precursor compounds. Generally,
these can include a wide range of both halogenated compounds such as polyvinyl
chloride, and nonhalogenated organic compounds such as petroleum products,
nonchlorinated plastics (polystyrene), cellulose, lignin, coke, coal, and inorganic
compounds such as particulate carbon and hydrogen chloride gas. No matter
which de novo compounds are involved, however, the process needs a chlorine
donor (a molecule that “donates” a chlorine atom to the precursor molecule).
∗ A heterogeneous reaction occurs in more than one physical phase (solid, liquid or gas).