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Incineration of MSW 259
furnace designs to optimize conditions for more complete destruction of PCDDs and PCDFs. Some
newer incineration facilities employ auxiliary burners utilizing fossil fuels to maintain the temper-
ature in the combustion zone sufficiently high at critical times, e.g., when burning wet MSW or
when starting-up and shutting-down operations. The U.S. EPA recently issued new guidelines for
MSW incinerator emissions, and optimum operational parameters to meet these emission standards
have been published. Degradation of PCDDs and PCDFs requires sufficient oxygen, ample turbu-
lence in the combustion zone to avoid quench zones, and adequate residence time of the compounds
in the combustion zone. About 7 to 10% oxygen or 50 to 100% excess air and a residence time of
at least 1s are estimated to be required for adequate destruction.
Early theories of PCDD formation from MSW incineration centered on the content of PVC,
which typically accounts for 50% of the chlorine content of the original waste; however, later stud-
ies found that if the temperature, oxygen, turbulence, and residence time parameters are optimized
for the destruction of PCDDs and its precursors, the quantities of PCDDs emitted in the flue gas are
independent of the PVC content of the original MSW. PCDDs are known to form during wood
burning, so the chlorine content of wood is apparently sufficient to combine with precursors (e.g.,
phenols) released in the combustion process (Choudry et al., 1982; Olie et al., 1983). Removing
PVC from MSW before incineration in order to reduce PCDD emissions, therefore, may be of ques-
tionable benefit.
It has also been hypothesized that PCDDs and PCDFs may be formed in the pollution control
devices or in the stack well beyond the combustion zone. These compounds may be produced by
chlorination of precursors adsorbed to the surface of fly ash particles and subsequently desorbed for
release into the flue gases. Since PCDDs and PCDFs condense on to fly ash particles beyond the
combustion zone and are tightly adsorbed, these compounds may be removed in the adsorbed form
by conventional gas cleaning technologies. For example, electrostatic precipitators (see below) effi-
ciently trap large fly ash particulates but do not consistently remove fine particles (< 2 µm in diam-
eter) unless sophisticated multistage plates are incorporated (Lisk, 1988). Baghouses (fabric filters;
see below) are also highly efficient for particulate removal from the flue gas stream. The efficiency
of baghouses for fly ash removal can be improved by use of a dry scrubber upstream. It is suggested
that scrubbers, which remove acidic constituents by introduction of alkaline (e.g., lime) slurry into
the flue gas, will increase agglomeration of fly ash particles, thus further improving collection effi-
ciency by baghouses.
Measurement of PCDDs during waste combustion is difficult and expensive. The emissions of
PCDDs from a stack can be roughly estimated by measuring the emission of carbon monoxide.
According to Hasselriis (1987), the generation of PCDDs is proportional to the CO concentration as

PCDDs (CO / A) 2 (9.10)

where CO is the concentration of carbon monoxide in the flue gas as percent of total gas, and A is
a constant, a function of the operating system. PCDD concentrations in the off-gases are expressed
3
as ng/m .
The emission of PCDDs increases with increasing CO emissions, both of which are regulated
by the amount of excess air used and the combustion temperature. From empirical evidence, sev-
eral quantitative relationships have been developed that are good predictors of PCDD and PCDF
formation (Vesilind et al., 2002).
For modular incinerators,

PCDDs PCDFs 2670.2 – 1.37T 100.06CO (9.11)

For waterwall incinerators,

PCDDs PCDFs 4754.6 – 5.14T 103.41CO (9.12)