Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 889

FIGURE 31

FIGURE 32

FIGURE 33

FIGURE 34

FIGURE 35

(13)

FIGURE 36

represent the same species as in reaction (3) or reactions (9),

(10), (11) or (13).

The unimolecular dissociation of dioxin takes place

when the molecule is heated sufficiently.

This reaction represents the disruption of the ring system

with an hydroxyl radical to yield non-dioxin products.

A major reaction pathway is expected to be the bimo-

lecular decomposition of phenoxy radicals with molecular

oxygen when oxygen is present in excess, as is usually the

case in incineration, as opposed to pyrolysis in a reductive

atmosphere.

The production of fuel radicals by hydrogen abstraction

from fuel is likely to be slower than the reaction between

hydroxyl radicals and the phenoxy radicals of reaction (8)

but this reaction will dominate especially in areas where

there are fuel-rich pockets in the incineration process.

The decomposition of fuel into products describes a

wide range of reactions.

Schaub and Tsang derived estimates of the rate constants

for each of the steps in the reaction sequence by comparing

literature values for similar chemistries and calculated four

situations governed by different mole fractions. The first

case, for example, assumed the condition that no O 2 or fuel

is available in the region in which the reaction mechanism

is taking place. This was modeled so that the formation of

dioxin could be exaggerated.

In the second case, concentrations were calculated on

the basis of there being oxygen available for reaction but at

an initial mole fraction which corresponded to about 60%

of the value needed for stoichiometric combustion. Under

normal conditions incinerators are run with excess oxygen

or at least a stoichiometric amount. This condition deem-

phasizes the rate of loss of chlorophenoxy radical in reaction

11 and consequently biases the calculation in favor of dioxin

formation. The third scenario models a fuel-rich pocket in

a post-combustion zone environment where no oxygen is

available for the reaction sequence.

Case four is similar to case three but includes the avail-

ability of oxygen (Figures 37, 38, 39).

The correlation between combustion temperature and

the concentration of dioxin in the example scenarios is illus-

trated in the above figures. The three curves were not plotted

together because the axes extend over significantly differ-

ent ranges. While the absolute concentrations predicted by

the model are relatively unimportant, it is significant that,

under post-combustion mixing, in an intermediate tempera-

ture zone, unburned chlorophenol is predicted to react with

hydroxyl radicals to produce PCDDs. Clearly, high tempera-

tures, and a sufficient quantity of air and fuel are able to

provide conditions which will combust chlorophenols into

products which are not PCDDs. Conditions can occur during

operation of the incinerator in which the amount of PCDD

produced is extremely small but, most importantly, some

conditions will allow the formation of PCDDs at concen-

trations which are 5 to 10-orders of magnitude higher than

those produced under ‘designed’ operating conditions.

The rate of destruction of organic material is deter-

mined from the kinetic differential equation which relates

concentration at a given moment of time to the rate constant

for the reaction:

d

d

C

t

−⋅kC

(1)

where

C  concentration at time, t

k  rate constant.

Upon integration,

t

k

C

C



(^10)
⋅
⎛
⎝⎜
⎞
⎠⎟
ln
(2)
where ,
C 0  initial concentration at time t  0.
C016_003_r03.indd 889C016_003_r03.indd 889 11/18/2005 1:12:35 PM11/18/2005 1:12:35 PM