Page 396 - Air Pollution Control Engineering

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Catalytic Oxidation 371

Catalysts typically used for VOC incineration include platinum and palladium; other
formulations are also used, including metal oxides for emission streams containing chlo-
rinated compounds. The catalyst bed (or matrix) in the incinerator is generally a metal
mesh-mat, ceramic honeycomb, or other ceramic matrix structure designed to maximize
catalyst surface area. The catalysts may also be in the form of spheres or pellets.
Recent advances in catalysts have broadened the applicability of catalytic incineration.
Catalysts now exist that are relatively tolerant of compounds containing sulfur or chlo-
rine. These new catalysts are often single or mixed metal oxides and are supported by
a mechanically strong carrier. A significant amount of effort has been directed toward
the oxidation of chlorine-containing VOCs. These compounds are widely used as sol-
vents and degreasers and are often encountered in emission streams. Catalysts such as
chrome/alumina, cobalt oxide, and copper oxide/manganese oxide have been demon-
strated to control an emission stream containing chlorinated compounds. Platinum-based
catalysts are often employed for the control of sulfur-containing VOCs but are sensitive
to chlorine poisoning.
Despite catalyst advances, some compounds simply do not lend themselves well to
catalytic oxidation. These include compounds containing lead, arsenic, and phosphorus.
Unless the concentration of such compounds is sufficiently low or a removal system is
employed upstream, catalytic oxidation should not be considered in these cases.
The performance of a catalytic incinerator is affected by several factors including: (1)
operating temperature, (2) space velocity (reciprocal of residence time), (3) VOC com-
position and concentration, (4) catalyst properties, and, as mentioned earlier, (5) presence
of poisons/inhibitors in the emission stream. When adequate oxygen is present in the
incineration stream, important variables for catalytic incinerator design are the operat-
ing temperature at the catalyst bed inlet, the temperature rise across the catalyst bed, and
the space velocity. The operating temperature for particular destruction efficiency is
dependent on the concentration and composition of the VOC in the emission stream and
the type of catalyst used.
Space velocity (SV) is defined as the volumetric flow rate of the combined gas
stream (i.e., emission stream plus supplemental fuel plus combustion air) entering the
catalyst bed divided by the volume of the catalyst bed. As such, space velocity also
depends on the type of catalyst used. At a given space velocity, increasing the operating
temperature at the inlet of the catalyst bed increases the destruction efficiency. At a
given operating temperature, as space velocity is decreased (i.e., as residence time in the
catalyst bed increases), destruction efficiency increases.
The performance of catalytic incinerators is sensitive to pollutant characteristics and
process conditions (e.g., flow rate fluctuations). In the following discussion, it is
assumed that the emission stream is free from poisons/inhibitors such as phosphorus,
lead, bismuth, arsenic, antimony, mercury, iron oxide, tin, zinc, sulfur, and halogens.
(Note: Some catalysts can handle emission streams containing halogenated compounds,
as discussed above.) It is also assumed that the fluctuations in process conditions (e.g.,
changes in VOC content) are kept to a minimum.
Temperature control in preheat chamber is important to catalytic incineration (cat-
alytic oxidation) systems. High preheat temperatures accompanied by a temperature
increase across the catalyst bed may lead to overheating of the catalyst bed and eventually
loss of its activity (6,7).