Page 299 - Materials Chemistry, Second Edition

CAT3525_C09.qxd 2/8/2005 10:11 AM Page 270
270 Waste Management Practices: Municipal, Hazardous, and Industrial
Advantages of gas washing include:

• Simultaneous removal of acid gases as well as particulates. It is recommended, however,
that particulates be removed first (e.g., using a baghouse). Particle removal allows for
greater efficiency of gas removal.
• The system, fed by a liquid mist, accommodates high-temperature flue gas streams.
• The scrubbing medium can be modified to increase removal efficiencies. For example, a
dilute NaOH solution can be used in place of water for treatment of especially acidic flue
gases.

Disadvantages:

• There is a high input of energy, primarily to pump liquids in and out of the system.
• A constant supply of water is necessary.
• Since aqueous acids and alkalis are formed, equipment corrosion is inevitable. Therefore,
maintenance costs may be substantial.
• There will be large quantities of wastewater produced. This liquid must be treated prior
to discharge into a receiving body of water.
• Acid gas removal is more efficient if particulates are removed first from the gas stream.

9.6 ASH QUALITY FROM MASS BURN

Incinerator residues consist of noncombustible materials such as metal, glass and stones, and also
incompletely burned combustibles. MSW incinerators produce two types of ash: (1) bottom ash, the
large, dense debris that falls through the grates by gravity and collects at the base of the combus-
tion chamber; and (2) fly ash, the fine particles transported out of the combustion chamber with the
air stream, which are removed by air pollution control devices. Most facilities combine the two
types of ash before disposal.
Incinerator fly ash and bottom ash are often a hazard to health and the environment. If prepro-
cessing of the MSW does not occur, there are a number of health and environmental concerns
regarding the generation, storage, and ultimate disposal of MSW ash. These are outlined in the fol-
lowing sections.
The composition of a typical MSW ash sample is shown in Table 4.17. The predominant health
and environmental concern with MSW incinerator ash is the presence of heavy metals. Table 9.6
lists a representative array of heavy metals found in combined fly ash and bottom ash from a MSW
waste-to-energy unit.
Based on chemical composition and the leachability of certain components, MSW ash may
technically be classified as a hazardous waste by the U.S. EPA. As discussed below and in Chapter 11,
the Toxicity Characteristic Leaching Procedure (TCLP) is an extraction procedure used to deter-
mine whether a solid waste may be declared hazardous. If fly ash alone is tested, its constituents
often fail the test and it may be classified as hazardous. Combined with the bottom ash, however,
the mixture often meets the requirements for a nonhazardous waste (Vesilind et al., 2002).

9.6.1 METALS
Ash may contain high concentrations of a number of toxic metals such as Cd, Pb, As, Be, V, and Hg
and other, comparitively less toxic metals, such as Cu, Zn, Fe, and Al (Table 9.6). Fly ash contains
several thousand times more lead and cadmium than bottom ash; however, the levels in bottom ash
still greatly exceed the amounts found in uncontaminated soils. These metals are concentrated in the
ash via the incineration process. Incineration removes the matrix materials such as paper and plastics
that contained the metals and had restricted their release to the biosphere. Once in the form of ash,