The simplest method of particle control from stack gas and other gases released
to the atmosphere consists of sedimentation in which particles entrained in stack gas
are allowed to settle by gravity in relatively large chambers. Sedimentation is most
effective for relatively large particles. Inertial mechanisms operate by spinning a gas
in a round chamber such that particles impinge upon the container walls by centrifugal
force. Fabric filters contained in baghouses act to filter particles from air or stack gas.
The mechanism employed provides for periodic shaking of the fabric filters to collect
particles held on their walls, thus restoring gas flow through the fabric. Scrubbers that
spray water or solutions into stack gas are employed to literally wash particles out of
gas. In some cases these are operated with a minimal amount of water, which evaporates,
so that a solid material is collected. One of the most effective means of particle control
consists of electrostatic precipitators. These devices use a very high voltage to impart
a negative charge onto particles from a central electrode, and the particles are attracted
to, and collect on the positively charged walls of the precipitator.
8.6. Pollutant Gaseous Oxides
Carbon Monoxide
Carbon monoxide, CO, is an air pollutant of some concern because of its direct
toxicity to humans. As discussed in Chapter 13, carbon monoxide binds to blood
hemoglobin and prevents the hemoglobin from transporting oxygen from the lungs to
other tissues. Global and regional levels of atmospheric carbon monoxide are too low to
be of concern. However, local levels in areas with heavy automobile traffic can become
high enough to pose a health hazard and on some congested urban streets have reached
levels of 50–100 ppm. The use of exhaust pollution control devices on automobiles
have lowered these levels significantly during the last 30 years. The numerous fatal
cases of carbon monoxide poisoning that occur each year are almost always the result of
improperly vented heating devices in indoor areas.
Carbon monoxide is produced by partial combustion of fuels, largely in the internal
combustion engine. Carbon monoxide emissions can be reduced by careful control of
combustion conditions. Running the exhaust along with pumped air over an exhaust
catalyst can oxidize carbon monoxide to nontoxic carbon dioxide:
2CO + O 2 → 2CO 2 (8.6.1)Modern automobile engines use precise computerized control of engine operating
parameters along with exhaust catalysts to control carbon monoxide emissions.
Sulfur Dioxide
Sulfur dioxide enters the atmosphere as the result of the following:- Direct emissions from volcanoes
Chap. 8. Air and the Atmosphere 207