Particulate Emission Control - Mathematics and Statistics for the Environment - page 489

17

Particulate Emission Control

Fresh air is good if you do not take too much of it; most of the achievements and pleasures of life are
in bad air.

—Oliver Wendell Holmes (1809-1894)

17.1 PARTICULATE EMISSION CONTROL BASICS
Particle or particulate matter is defined as tiny particles or liquid droplets suspended in the air that
can contain a variety of chemical components. Larger particles are visible as smoke or dust and
settle out relatively rapidly. The tiniest particles can be suspended in the air for long periods of time
and are the most harmful to human health because they can penetrate deep into the lungs. Some
particles are directly emitted into the air. Constituting a major class of air pollutants, particulates
have a variety of shapes and sizes, and as either liquid droplets or dry dust, they have a wide range
of physical and chemical characteristics. Dry particulates are emitted from a variety of different
sources in industry, mining, construction activities, incinerators, and internal combustion engines—
from cars, trucks, buses, factories, construction sites, tilled fields, unpaved roads, stone crushing,
and wood burning. Dry particulates also come from natural sources—volcanoes, forest fires, pollen,
and windstorms. Other particles are formed in the atmosphere by chemical reactions.
When a flowing fluid (engineering and science applications consider both liquid and gaseous
states as a fluid) approaches a stationary object such as a metal plate, a fabric thread, or a large water
droplet, the fluid flow will diverge around that object. Particles in the fluid (because of inertia) will
not follow stream flow exactly but will tend to continue in their original directions. If the particles
have enough inertia and are located close enough to the stationary object they will collide with the
object, and can be collected by it. This is an important phenomenon and is depicted in Figure 17.1.

17.1.1 i nteraCtion oF p artiCles With g as
To understand the interaction of particles with the surrounding gas, knowledge of certain aspects of
the kinetic theory of gases is necessary. This kinetic theory explains temperature, pressure, mean
free path, viscosity, and diffusion in regard to the motion of gas molecules (Hinds, 1986). The theory
assumes gases—along with molecules being rigid spheres that travel in straight lines—contain a large
number of molecules that are small enough so that the relevant distances between them are discontinu-
ous. Air molecules travel at an average of 1519 ft/s (463 m/s) at standard conditions. Speed decreases
with increased molecule weight. As the square root of absolute temperature increases, molecular
velocity increases. Thus, temperature is an indication of the kinetic energy of gas molecules. When
molecular impact on a surface occurs, pressure develops and is directly related to concentration. Gas
viscosity represents the transfer of momentum by randomly moving molecules from a faster moving
layer of gas to an adjacent slower moving layer of gas. Viscosity of a gas is independent of pressure
but will increase as temperature increases. Finally, diffusion is the transfer of molecular mass without
any fluid flow (Hinds, 1986). Diffusion transfer of gas molecules is from a higher to a lower concentra-
tion. Movement of gas molecules by diffusion is directly proportional to the concentration gradient,
inversely proportional to concentration, and proportional to the square root of absolute temperature.

491