Page 289 - Materials Chemistry, Second Edition

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260 Waste Management Practices: Municipal, Hazardous, and Industrial
where T (ºC) is the temperature in the secondary chamber for modular combustors, and the furnace
temperature in waterwall incinerators, respectively.
PCDDs and PCDFs may be produced, albeit in extremely low quantities, in all incineration
processes, and not only those involving MSW. Their formation has been reported during the com-
bustion of paper, wood, vegetable wastes, chlorophenols, polychlorinated biphenyls (PCBs), and
from coal- and gasoline-powered engines (Lisk, 1988).
Berlincioni and di Domenico (1987) monitored vapor and smoke emissions from a MSW incin-
erator for PCDDs and PCDFs and found that the fraction of the compounds associated with fly ash
accounted for less than 10% of the total emitted. They also sampled soils up to 1 km in several
2
4
directions from the incinerator and found a maximum PCDD concentration of 7 x 10 ng/m of soil
surface. These compounds were not confined to the top 5 cm of soil and may have reached deeper
layers by leaching or plowing. The more highly chlorinated isomers accumulated to the greatest
degree. The authors postulated that the less chlorinated isomers were less persistent owing to their
high vapor pressure and reactivity with light (photolability).
Other chlorinated and organic compounds arising from MSW combustion include PCBs and
polycyclic aromatic hydrocarbons (PAHs). Compounds in the latter category include pyrene,
benzo[a]pyrene, and chrysene, among others (Figure 9.6). PAHs such as benzo[a]pyrene are car-
cinogenic. Similar to PCDDs, PAHs are produced as a result of incomplete combustion and have
been reported in gaseous emissions and fly ash from MSW incinerators. For example, the incom-
plete combustion of saturated hydrocarbons can form PAHs. At temperatures exceeding about
o
o
500 C (950 F), carbon–hydrogen and carbon–carbon bonds are broken, resulting in the formation
of free radicals. The radicals are dehydrogenated and combine to form aromatic rings that are resist-
ant to thermal degradation (Figure 9.7) (Manahan, 1994).
Concentrations of PAHs on fly ash from a MSW incinerator were found to vary markedly from
day to day and the variations were consistent with those of the total concentrations of organic com-
pounds on fly ash (Eiceman et al., 1981). Colmsjö et al. (1986) found that the concentration of PAHs
in stack gases from a MSW incinerator increased by more than 1000-fold during cold start-up of the
plant. Large PAH molecules were strongly adsorbed to fly ash particles. Pierce and Katz (1975)
found the highest concentrations of PAHs on the smallest particulates, those < 5 µm in diameter, and

Naphthalene Anthracene Phenanthrene
C H C H C H
14 10
14 10
10 10

Pyrene Chrysene
C H C H
18 12
16 10

Benz(a)anthracene Benzo(a)pyrene FIGURE 9.6 Structures of some common
C H C H polycyclic aromatic hydrocarbons.
18 12
20 12