Page 526 - Materials Chemistry, Second Edition

CAT3525_C15.qxd 1/27/2005 12:40 PM Page 497
Incineration of Hazardous Wastes 497
Disadvantage includes:

● A large amount of particulates are produced, requiring extensive pollution control.

15.7 AIR POLLUTION AND ITS CONTROL

A major regulatory concern associated with hazardous waste incineration (and a major justification
for the NIMBY [Not in My Backyard] syndrome) is the emission of air pollutants. For a simple
hydrocarbon compound (e.g., methane, propane), the primary end products from combustion, given
adequate amounts of air, are carbon dioxide, water vapor, and heat. When hazardous wastes are
incinerated, however, gaseous wastes posing a threat to public health or the environment often
result. The types and amounts of emissions from hazardous waste incineration depend upon a num-
ber of variables including the chemical composition of the waste, waste incineration rate, incinera-
tor type, incinerator operating parameters, and air pollution control equipment.

15.7.1 ATMOSPHERIC PRODUCTS FROM COMBUSTION
The greatest mass of air contaminants consists primarily of particulate matter and oxides of sulfur
and nitrogen. Acid gases such as HCl, HBr, and HF may be produced in significant quantities,
depending on feedstock. Trace levels of various other oxides, hydrocarbons (including chlorinated
hydrocarbons), and heavy metals are also generated. Particulate matter consists of metal salts from
the waste, metal oxides formed by combustion, fragments of incompletely burned material (prima-
rily carbonaceous), and condensed gaseous contaminants (i.e., droplets). The metals and toxic by-
products condense as or on fine particles as the exhaust gas stream cools.
Sulfur oxides, mostly as sulfur dioxide (SO ), but also including small amounts of sulfur triox-
2
ide (SO ), originate from sulfur or sulfur compounds present in the waste or fuel mixture. Nitrogen
3
oxides (NO ) originate from nitrogen in the combustion air or from organic nitrogen compounds
x
present in the waste. HCl and chlorine (Cl ) are derived from the incineration of chlorinated hydro-
2
carbons such as polyvinyl chloride. Phosphorus pentoxide and phosphoric acid are formed from the
incineration of organophosphorus compounds such as malathion or parathion.
15.7.2 PRODUCTS OF INCOMPLETE COMBUSTION (PICS)
Even in a well-designed incinerator, the firebox may contain areas of incomplete oxygen incorpora-
tion or other, similar, quench zones. At temperatures common to hazardous waste incinerators,
hydrocarbons will not oxidize in these zones but will decompose pyrolytically, forming PICs. The
primary PICs include carbon monoxide, carbon soot, hydrocarbons, organic acids, polycyclic
organic matter, and any other waste constituents that escape complete thermal destruction in the
incinerator. In well-designed and well-operated incinerators, these incomplete combustion products
are emitted only in trace amounts.
In a study by the U.S. EPA (1986), combustion by-products were examined from 23 emissions
tests at thermal destruction facilities, including eight incinerators, nine industrial boilers, and six
industrial kilns. The organic emissions were compared with emissions from facilities burning coal
only, and with municipal solid wastes (MSW) incinerators. A total of 28 volatile and 27 semivolatile
compounds were detected in stack emissions. The compounds were emitted at rates that spanned
five orders of magnitude, i.e., 0.09 to 13,000 ng/kJ of combustor heat input. Emission rates for 12
compounds emitted from the three sources are shown in Table 15.2.
Metals occurring in hazardous wastes being combusted are usually either collected as bottom
ash or are emitted as particulate matter. In some cases, however, some of the more volatile elements
(e.g., Hg, Cd, and Se) are emitted as vapors. Emission rates for a number of metals from hazardous
waste combustion appear in Table 15.3.