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10.6 Control of Volatile Organic Compounds 303

intermittent spikes each time the valves switch the direction of flow, because the
untreated VOC in the gas present in the inlet bed when it is suddenly switched to
the outlet.
Again, the advantage of thermal oxidation in an incinerator is the high
destruction efficiency that can be obtained by proper control of the combustion
chamber design and operation. If temperatures are maintained above 980 °C,
greater than 99 % hydrocarbon destruction is routinely achievable. This efficiency
depends on residence time, temperature, and turbulence (the three Ts) in the
combustion chamber.
Thermal oxidizers can be costly to install because of required support equip-
ment, including high pressure fuel supplies (for example, natural gas), and sub-
stantial process-control and monitoring equipment. In addition, public perception of
a new “incinerator” can make it difficult to locate and permit a new unit.

10.6.5 Catalytic Oxidation

In a catalytic oxidation process, catalysts such as Pt or Pd on a Al 2 O 3 support may
give destruction and removal efficiencies of up to 95 % with small size. The
catalytic oxidation units are, however, much more expensive in operation. The high
cost is caused by frequent replacement of the catalyst after being poisoned by other
pollutants such as soot, particles, chlorine, sulfur, silicon, vanadium, lead, and/or
hydrocarbons with great molecular weight. Temperature excursions also reduce the
lifetime of catalysts. Furthermore, secondary air pollution may be produced by
catalytic oxidation. Gases other than VOCs may be converted to hazardous com-
pounds that require further treatment downstream. From this point of view, catalytic
VOC oxidization is most suitable for the cleaning gases with stable VOC
properties.
As seen in Fig. 10.9, catalysts are loaded on a catalyst bed in the incinerator. The
support structure of catalysts is arranged in a matrix that provides high geometric
surface area, low pressure drop, and uniform flow. Structures providing these
characteristics include honeycombs, grids, and mesh pads. Either a monolith or a
beads/pellets configuration can be employed, depending on the kinetics of VOC
oxidation and the presence of other pollutants. Monolith is a cluster of parallel tubes
and it is used for fast kinetics. Bead or pellet bed is preferred for slow kinetics and it
is less sensitive to fouling or poisoning. The performances of thermal and catalytic
oxidizers are often affected by the presence of CO. Oxidation of CO will be
addressed separately.
Thermal oxidation is not effective for engine exhaust gases mainly due to low
gas temperatures and low concentration of the VOC emission from liquid fuel
combustion. VOC emissions from engines are successfully oxidized in catalytic
converters, where the catalysts are Pt, Pd, or Rh. Pd is more sensitive to poisoning
by lead and sulfur. Most of the time, it is integrated in a three-way catalyst with CO
oxidation and NO x reduction.