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Incineration of MSW 255
occurring within the original waste is simply converted into either a gaseous form or ash. Under
ideal conditions, carbonaceous wastes are converted into innocuous products such as CO and H O
2
2
along with the release of heat energy. The actual composition of flue gases, however, is highly com-
plex and a function of the composition of the original MSW, furnace design, and combustion con-
ditions. Many of the substances emitted from incinerator flue gases are known to negatively impact
human health. The air pollutants of concern from MSW combustion are primarily particulates, acid
gases, and trace gases.

9.4.4 PARTICULATES

Particulates, also known as “fly ash”, occur as either solid particles or liquid droplets composed of
organic or inorganic substances. A number of terms are used to describe atmospheric particles; the
more important of these are summarized in Table 9.1. Particulate matter makes up the most visible
and obvious form of air pollution.
Atmospheric aerosols are solid or liquid particles smaller than 100 µm in diameter. Particles in
the 0.001 to 10 µm range are commonly suspended in the air near sources of pollution such as
industrial facilities and power plants. Aerosols consist primarily of carbonaceous material, metal
oxides and glass, dissolved ionic species (electrolytes), and ionic solids. The predominant con-
stituents are carbon, water, sulfate, nitrate, ammonium nitrogen, and silicon. The composition of
aerosol particles varies significantly with size. Smaller particles tend to be acidic and often origi-
nate from gases such as from the conversion of SO into H SO . Larger particles tend to consist of
2
2
4
materials generated mechanically (Manahan, 1994).
Particulates are a public health concern because large quantities that occur in the respirable frac-
tion (approximately 15 µm in diameter) are commonly emitted from MSW incinerators. In addi-
tion, heavy metals, chlorinated dibenzodioxins, and other trace elements are attached to fly ash. The
rates of particulate emissions from a mass-burn incinerator depend on:
• Ash content, i.e., the percentage of noncombustible materials in the waste. If particle size
is sufficiently small, these can become entrained in the gases passing through the system.
• Furnace design. Some systems create greater degrees of agitation of the waste, thus
releasing particles. Furthermore, entrainment of particulates can result from too much
excess air. Optimal use of overfire and underfire air are important to limit particulate
emissions.
• Temperature. A low-temperature zone occurring in the combustion chamber can result in
the formation of incompletely burned residuals that are often lightweight and easily
transportable.

TABLE 9.1
Terminology Associated with Atmospheric Particles

Term Definition
Aerosol Colloidal-sized atmospheric particle
Condensation aerosol Formed by condensation of vapors or reactions of gases
Dispersion aerosol Formed by grinding of solids, atomization of liquids, or dispersion of dusts
Fog Term denoting high level of water droplets
Haze Denotes decreased visibility due to the presence of particles
Mists Liquid particles
Smoke Particles formed by incomplete combustion of fuel
Source: Manahan, S.E., Environmental Chemistry, 6th ed., CRC Press, Boca Raton, FL. Reproduced with
kind permission. Copyright Lewis Publishers, an imprint of CRC Press. Boca Raton, FL.