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Air Pollution Control 101
For the second edition of the Air Pollution Control Equipment Selection Guide, it
is appropriate to include additional information regarding the basics of air pollution
control. More recent applications of air pollution control technologies involve an
environment wherein the carrier gas is different from air—such as in gasication
systems. This and other topics will be addressed in this chapter.
Having spent now more than 40 years in the air pollution control industry, I am
still amazed by how the basics of air pollution control are misunderstood by so many.
Our newspapers have numerous articles regarding the need to control toxic or
carcinogenic substances, but rarely do you see an article explaining how it is done.
In this chapter we will explore the basics of air pollution control and how the devices
work and, in doing so, introduce some of the terminology used in the industry.
1.1 IT IS SEPARATION TECHNOLOGY
Air pollution control can be generally described as a separation technology. The
pollutants, whether they are gaseous, aerosol, or solid particulate, are separated
from a carrier gas, which is usually air. We separate these substances because, if we
dont, these pollutants may adversely affect our health and that of the environment.
Of primary importance is the effect of the pollutants on our respiratory system,
where the impact is most noticeable.
Gaseous pollutants are compounds that exist as a gas at normal environmental
conditions. Usually, normal is dened as ambient conditions. These gases may have,
just moments before release, been in a liquid or even solid form. For the purposes
of the air pollution device, however, the state they are in just prior to entering the
control device is what is most important.
Aerosols are nely divided solid or liquid particles that are typically under 0.5
μm diameter. They often result from the sudden cooling (condensation) of a gaseous
pollutant, through partial combustion, or through a catalytic effect in the gas phase.
In the latter condition, a pollutant in the gas phase may combine to form an aerosol
in the presence of, for example, a metal co-pollutant. Acid aerosols such as SO
3
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for example, can form in the presence of vanadium particulate that may be evolved
through the combustion of oil containing vanadium compounds. Solid metals in a
furnace can sublime (change phase from solid directly to gaseous) in the heat of an
incinerator, then cool sufciently to form a nely divided aerosol.
Solid particulate can be evolved through combustion or through common process-
ing operations such as grinding, roasting, drying, calcining, coating, or metallizing.
Whatever the state of the pollutant, the function of the air pollution control device
is to separate that pollutant from the carrier gas so that our respiratory system does
not have to.
2 Air Pollution Control Equipment Selection Guide
Our respiratory system is our natural separation system. Figure1.1 depicts the
major portions of the human respiratory system. Large particles are removed in the
larger openings of the upper respiratory area, smaller particles are removed in the
more restricted bronchial area, and the tiniest particles are (hopefully) removed in
the tiny alveolar sacs of the lungs.
Air pollution control truly mimics Mother Nature in its separation function. In
general, low energy input wet-type (those using water as the scrubbing medium)
gas cleaning devices remove large particles, higher energy devices remove smaller
particles, and even higher energy (or special technology) devices remove the nest
pollutants. In order of decreasing pollutant size, it goes like this:
Mother Nature Man-Made Devices
Upper Respiratory Low Energy Input
Bronchial Moderate Energy Input
Alveolar High Energy or Special Technology
The larger the particle, or liquid droplet for that matter, the easier it is to separate
from the carrier gas.
Ribs
Lungs
Bronchus
Bronchiole
Ending In
Alveoli
Alveolus
Oxygen
Enters
Carbon
Dioxide
Leaves
Capillaries
Diaphragm
FIGURE 1.1 Respiratory system diagram. (From Marshall, James, The Air We Live In,
Coward, McCann, and Geoghegan, New York, 1968.)

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