Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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ticulate must be given special consideration. There are a great num- ber of waste gas sources that contain both gaseous hydrocarbon pol- lutants and particulate pollutants. In most cases, the particulate can be filtered out upstream of the thermal oxidizer. However, in many cases, it is possible to avoid the additional complexity and cost of a filtration system through proper selection of the thermal oxidizer and its operation. Particulate can be broken down into two basic cat- egories, organic and inorganic. An example of an organic particulate is an oil mist from machining operation. This type of pollutant will either accumulate in the ductwork and cooler parts of the thermal oxidizer or penetrate to the combustion chamber. Any particulate that accumulates in the cooler parts may need to be periodically cleaned out. Obviously, provisions must be made in the oxidizer design to al- low cleaning. In general, any type of thermal oxidizer is capable of han- dling purely organic particulate. However, as the total loading increas- es, increasing amounts of maintenance will be required. One feature of regenerative type systems for these applications is that the can be pro- grammed 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 or- ganic material. With this feature, regenerative systems are favored in high organic particulate applications because the manpower and dis- ruption to operation is minimal for a bake out compared to cleaning of other types of systems. Any organic particulate that enters the com- bustion chamber will be oxidized as any other hydrocarbon would. Ox- idation of a particle takes longer than a gas because the particle must first be broken down and volatilized before the thermal oxidation reac- tion 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 elevat- ed hydrocarbons in the exhaust from the partially oxidized particulate and would also show elevated levels of carbon monoxide. If the particu- late 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 differ- ent 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 re- act 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 235

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