Metal Finishing Guide Book

2013

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to be periodically cleaned out. Obviously, provisions must be made in the oxidizer design to allow cleaning. In general, any type of thermal oxidizer is capable of handling purely organic particulate. However, as the total loading increases, increasing amounts of maintenance will be required. One feature of regenerative type systems for these applications is that the can be programmed to perform a thermal self-cleaning or bake out. This process brings heat from the combustion chamber into the lower portions of the heat exchange media and valves and can burn off accumulated organic material. With this feature, regenerative systems are favored in high organic particulate applications because the manpower and disruption to operation is minimal for a bake out compared to cleaning of other types of systems. Any organic particulate that enters the combustion chamber will be oxidized as any other hydrocarbon would. Oxidation of a particle takes longer than a gas because the particle must first be broken down and volatilized before the thermal oxidation reaction can take place. This takes time and therefore, a thermal oxidizer with sufficient residence time to oxidize gaseous compounds, may be inadequate for particulate. In this case, the oxidizer would have elevated hydrocarbons in the exhaust from the partially oxidized particulate and would also show elevated levels of carbon monoxide. If the particulate is fine, less than about 10 micrometers, and of low concentration, less than about 10 grain/standard cubic foot, adequate performance can be achieved with an oxidizer of normal design. It may be necessary to raise the operating temperature by 100°F or so to achieve required emission performance. For significantly higher levels or sizes, some pre-filtration is usually favored. Inorganic particulate presents different challenges. Inorganic particulate can be any of a wide variety of substances ranging from common dust, to soil, metals, paint pigments or salts. Each type has specific characteristics and therefore requires special considerations in oxidizer design. Inorganic compounds can react with oxidizer components, fuse and foul certain parts, accelerate corrosion or cause erosion damage. Because there are such a wide range of possibilities, no general guideline can be given that would cover all inorganic particulate. • Required Pollutant Control Efficiency: Many federal, state and local VOC and HAP emission limits for surface coating operations are expressed in terms of one or more of the following: lb per gallon minus water lb per gallon coating solids as applied (e.g. as sprayed) lb per gallon of applied coating solids (e.g. auto & light truck) These limits may be met either by applying coatings meeting these emission limits without add-on controls or achieving an equivalent limit with add-on controls. For auto and light truck surface coating operations, the paint solids transfer efficiency (TE) is part of the calculation. Some state and local regulations require a minimum TE for certain coating operation in addition to a VOC or HAP content limit. If a catalytic or thermal oxidizer is used to control VOC or HAP emissions, 95% minimum destruction efficiency is generally required. An overall 90% minimum VOC or HAP destruction efficiency is generally required if a carbon or zeolite adsorber is used to concentrate emissions prior to destruction in an oxidizer. 650

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