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Incineration of MSW 257

TABLE 9.2
Simplified Reaction Scheme for Photochemical Smog

NO light → NO O
2
O O 2 → O 3
O NO → NO O 2
3
2
O (HC) x → HCO o
HCO O 2 → HCO 3 o
o
HCO 3 o HC → Aldehydes, and ketones .
HCO 3 o NO → HCO NO 2
o
2
HCO 3 o O 2 → O HCO 2 o
3
o
HCO NO 2 → Peroxyacetyl nitrates
x
Adapted from Vesilind, P.A. et al., Solid Waste Engineering, 1st Ed., Brooks/Cole, Pacific
Grook, CA, 2002. Reproduced with kind permission of Brooks/Cole, a division of Thomson
Learning: www.thomsonrights.com.
water to form the corresponding hydrochloric acid. This corrosive liquid affects eyes, skin, and
mucosa, and is linked with acid rain.
HCl (g) H O → HCl (aq) (9.9)
2
9.4.6 TRACE GASES
In this category are gases that may occur at levels of a few parts per million (ppm), yet they may still
exert a hazardous effect on living systems. Polychlorinated dibenzodioxins (PCDDs) and polychlori-
nated dibenzofurans (PCDFs) (Figure 9.5), some of which are highly toxic, are now known to form
during the combustion of chlorine-containing wastes. There are 75 possible isomers of PCDD and 135
of PCDF (Lisk, 1988). The 2,3,7,8-tetrachlorodibenzodioxin isomer (2,3,7,8-TCDD) (Figure 9.5) is
an animal teratogen and by far the most toxic, but its toxicity varies over 5000-fold among species.
A draft report released for public comment in September 1994 by the U.S. EPA described PCDDs
as a serious public health threat. The EPA report confirmed that PCDDs are a cancer hazard to
humans; that exposure to PCDDs, even at extremely low levels, can cause severe reproductive and
developmental problems; and that PCDDs can cause immune system damage and interfere with reg-
ulatory hormones. The International Agency for Research on Cancer of the World Health Organization
declared in 1997 that 2,3,7,8-TCDD is a Class 1 carcinogen, i.e., it is a known human carcinogen.
Various isomers of PCDD and PCDF have been detected at parts per billion (ppb) levels in fly
3
ash and ng/m concentrations in emissions from incinerators in many countries. The concentrations
of several isomers of PCDD and PCDF in fly ash samples from MSW incinerators are shown in
Table 9.3. Formation of PCDDs and PCDFs in the combustion chamber itself is unlikely due to the
high temperatures present; however, these compounds form in the cooling gases as they exit the
flue. Three possibilities have been proposed to account for the presence of PCDDs and PCDFs in
MSW incinerator emissions (Hutzinger et al., 1985; Lisk, 1988):

● They are already present in the refuse to be burned and are not completely destroyed dur-
ing incineration.
● They are produced from chlorinated precursors such as PCBs, chlorophenols, and
chlorobenzenes contained in the refuse.
● They result from the cracking of complex organic substances (such as lignin to produce
phenol) and are subsequently synthesized in the presence of chlorine at high tempera-
tures, perhaps catalyzed by metal ions. Formation of chemically unrelated chlorinated
organics such as PVC after pyrolysis is also possible.