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Air Pollution Control Industry Information
Air pollution control equipment removes and eliminates
a wide variety of pollutants, known as volatile organic compounds (VOCs)—including
fumes, gases, odors and vapors—from the atmosphere. Hazardous Air
Pollutants (HAPs) can cause even more serious environmental and biological
damage than other VOCs but can also be destroyed by air pollution control
equipment. Oxidation, a process in which contaminated air pollutants are
broken up and reformed into new, safe compounds, is at the heart of most
of these systems. Automotive, agricultural, oil and gas, mining, woodworking,
chemical and pharmaceutical industries are some of the industries in which
air pollution control equipment is in place. A facility is considered
to have significant air pollution emissions if it releases about one or
more tons per calendar year. To remain in compliance with regulatory requirements,
facilities can use data-providing Continuous Emissions Monitoring Systems
(CEMS) to aid in the control, monitoring and reporting of pollutant emissions.
There is quite a variety of air pollution control equipment from which
to choose. Knowing the amount of airflow and the amount and type of VOCs
being emitted basically determines which technology would be most appropriate.
The %LEL is based on the type and concentration of the contaminants. It
is also helpful to know what to look for in VOC destruction efficiency,
attrition rate and heat exchanger efficiency. What are the requirements
for the inlet/outlet temperature? What are the gas pressure requirements?
Having the answers to these questions is also advantageous when choosing
what type of air pollution control equipment is needed in a given facility.
There are two broad types of oxidizers: thermal and catalytic. Thermal
and catalytic oxidizers are typically either regenerative or recuperative.
Regenerative thermal oxidizers oxidize organics in a retention chamber
and have two or more ceramic heat transfer beds that act as smaller heat
exchangers. Recuperative thermal oxidizers use a plate, shell, tube or
other conventional type of heat exchanger to heat incoming air with air
from the oxidation process. A regenerative catalytic oxidizer preheats
VOC-contaminated process gas in an energy recovery chamber. A catalyst
oxidizes the VOCs, which then release enough energy to allow self-sustained
operation. A catalytic recuperative oxidizer preheats VOC-laden air through
the tube side of the heat exchanger. The air is raised to the operating
temperature and passed through the catalyst, causing a heat releasing
reaction to take place. The contaminant-free air is then released back
into the atmosphere.
There is a variety of other types of air pollution control equipment.
Particulate controls include electrostatic precipitators, which use electrical
fields to remove particulate from boiler flue gas, and fabric filters,
which use tightly woven fabric to sieve flue gas and collect particulate.
Wet scrubbers, which include venturi scrubbers, are effective as acid
gas and SO2 controls but have low efficiencies for smaller
particles. Multiple cyclones have a large number of small cyclones in
parallel to control particulate, but collection efficiencies drop off
rapidly with particle size. NOx controls include the processes of selective
catalytic reduction, which controls emissions of nitrogen oxides from
stationary sources, and selective non-catalytic reduction, which changes
oxides of nitrogen (NOx) into molecular nitrogen (N2). If VOCs
have recovery value, carbon adsorption, scrubbing and condensation are
typical techniques to use. Thermal and catalytic oxidation and biofiltration
are common VOC controls utilized when the VOC stream has no recovery value.
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