PCBs AND ASSOCIATED AROMATICS 885
indicated that it is unlikely that PCNs would add signifi-
cantly to the toxicity of PCBs at the concentrations which
are typically observed.
Polychlorinated biphenylenes (PCPs), because of their
structure, are expected to be as toxic as the correspond-
ingly substituted PCDDs. Accordingly to Poland and Glover
(1977) 2,3,6,7-tetrachloro-biphenylene is approximately as
potent as 2,3,7,8-TCDD. PCPs have been found to result
from the reductive conditions which occur in the early stages
of askarel transformer fires and are therefore compounds
of concern [see Smith et al. (1982); Rappe et al. (1982);
Rappe et al. (1983)].
Polychlorinated pyrenes (PCPYs) and polychlorinated
chrysenes (PCCYs) have been identified as components of the
soot from an askarel transformer fire (Rappe et al., 1983).
Rubey^64 was studied the thermal decomposition of 2,3′,5,6′-
pentaCB in different atmospheres and temperatures with a
residence time of 2s. The decomposition temperatures range
from about 700°–950°C. Similarly, a plot of PCB degradation
in which the formation and decomposition of PCDFs is super-
imposed on the destruction of 2,3′,4,4′,5- pentaCB shows that
the formation of PCDFs occurs at 700°–800°C and their yield
increases up to about 900°C. Above 900°C there is rapid
decomposition of both the PCB and the PCDFs.
Several physico-chemical factors influence incineration
and are listed here to indicate the complexity of the process.- exposure temperature
- the oxygen composition of flame and non-flame
atmospheres- gas phase residence times in different heated zones
- oxygen concentrations and associated gradients
- pressure
- flame contact time
- spatial and temporal variations in temperature.
- thermodynamic and kinetic properties of the com-
pounds involved.
An extensive treatment of the quantitative prediction of the
formation of PCDFs from PCBs under pyrolysis conditions
is beyond the scope of this paper but a review of the litera-
ture does allow a semi-quantitative assessment of the likeli-
hood of PCDF formation and decomposition in flames under
a variety of conditions. In addition, thermodynamic factors
will influence the enthalpies of activation and the frequency
factors for decomposition of PCDFs depending upon their
degree of chlorination (Tables 20, 21).
The chemical reactions and mechanisms which take
place in combustion systems can be extremely complex and
different systems of reactions, such as free radical decompo-
sitions, can occur simultaneously. The intractability of the
mathematics used to model such systems depends to some
degree upon the number and type of simplifying assump-
tions which are applied. For example, a steady-state approxi-
mation, in which the rate of formation and decomposition
of a species is constant, can be applied in certain situations.
Then, the equation which relates the rate of change of a spe-
cies in its steady-state with the concentrations of the other
species, can be solved in terms of the concentrations of
the other species by setting the differential equal to zero.FIGURE 22 Compounds of concern.C016_003_r03.indd 885C016_003_r03.indd 885 11/18/2005 1:12:33 PM11/18/2005 1:12:33 PM