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 Background
and Applications
The Clean Air
Act Amendments lists 189 "Hazardous Air Pollutants"; of them 160 are
VOCs.
Any stationary
source producing more than 10 tons per year of any listed substance, or 25 tons
of any combination of listed substances, fall under Title V permitting requirements.
Smaller process (under 5,000 cfm and 500 ppm) are now required to install controls.
VOCs can be reduced
by switching materials used in a process, such as water based instead of petroleum
based solvents and paints. If that is not practical or possible, exhaust can be
passed through an incineration device, categorically called a thermal oxidizer.
Thermal oxidizers
are used to reduce/remove Volatile Organic Compounds (VOCs) from process exhaust
systems.
Typically large
volumes of low VOC process air is passed into a concentration device that removes
the VOCs from the air before it is exhausted. The VOCs are then driven off of
the concentration device by heat, and burnt by a natural gas flame.
Industries that may need Emissions Controls include
- Adhesive Coating
- Asphalt Manufacturing
- Bakery
- Brewery
- Coating
- Combustion Turbines
- Composite/Synthetic
Materials
- Chemical Processing
- Electronics/Semiconductor
- Food Processing
- Industrial Boilers
- Loading/Unloading
- Metals Finishing
- Odor Control
- Paint Mfg/Paint
Spray
- Petroleum/ Hydrocarbon
- Pharmaceutical
- Plant process
heaters
- Printing/Laminating/Converting
- Roasters
- Sheet Board Manufacturing
- Sheet Coil Coating
- Soil Remediation
- Steel Mills
- Tape Coating
- Utility Boilers
- Vinyl Products
- Waste Heat Cogeneration
Systems
- Waste Treatment
- Wood Furniture
Types
of Thermal Oxidizers
There are several
types and variations of Thermal Oxidizers. Which one is 'right' for a given application
will depend on the following factors:
- level of control
required
- pollutant(s)
to be removed
- concentration
of the pollutant(s) in the air stream
- total volume/flow
of the air stream
- operating costs
- first costs of
equipment
After
Burner
The typical afterburner
consists of a burner, a burner train, a combustion blower and, if necessary, a
process fan. Usually, this scheme is not used unless the concentration of the
organic pollutant is elevated high enough to yield available energy to heat the
products of combustion up to the chosen oxidation temperature. If the gas stream
does not contain enough energy, then a burner is supplied to provide the necessary
heat. Often, this type of system proves to be uneconomical from an operating cost
basis, due to the high gas usage when oxidizing low concentrations of VOC's in
the gas stream.
Recuperative
In a recuperative
unit, the basic operation of the afterburner is retained except that much of the
waste heat is captured. The fact that the system can capture this heat allows
for it to operate very economically. In this type of system, a metallic tube or
plate-type heat exchanger is built into the exhaust end of the combustion chamber
of the oxidation system. Typically, a plate exchanger is used when the exhaust
gas stream does not contain elevated amounts of particulate and the maximum amount
of heat recovery is desired. Recuperative systems generally are smaller and lighter
in weight than other oxidation systems, allowing for skid mounted installations.
Catalytic
Catalytic oxidation
systems are another option for low VOC concentrations. These units are similar
in design to recuperative units, but oxidize solvents with precious metal or metal-oxide
based catalysts, instead of open flames. Operating at about half the temperature
of thermal oxidizers, catalytic units have small footprints and relatively low
operating costs.
Regenerative
A regenerative
system provides extremely high thermal energy recovery. Initially, the incoming
process gas passes through a ceramic heat recovery bed before entering the combustion
chamber. It is preheated to within 5% of the combustion chamber temperature. After
the process stream exits the ceramic bed, the already hot gases are further heated
to the desired combustion chamber temperature. These gases are then sent through
another heat exchange bed, where energy is absorbed and stored to heat the next
cycle of contaminated air. Up to 95 percent of heat energy can be recovered with
this multiple-bed approach. Low VOC concentrations can be processed in a self-sustaining
mode without burning extra fuel.
Regenerative
Catalytic
A more recent
addition to the oxidation technologies available to the process engineer is the
regenerative catalytic oxidizer. This device is very similar in operation to an
RTO, but with a layer of catalyst in the combustion chamber. Both precious metal
and metal oxide-based catalysts are presently in use. This technology has only
recently been developed, with long term success or failure still to be determined.
More Information
See our Air Toxic
Consortium web site at www.AirToxic.com
Source: Energy
TechPro
 
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