Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 235 of 331

• Pollutant concentration levels: The concentration of pollutants in the waste gas stream can have a major impact on the selection of the type of thermal oxidizer system. Direct flame oxidizers are capable of handling the broadest range of hydrocarbon concentrations, from parts per billion levels to pure hydrocarbon vapors. For waste gas streams with concentrations over 25% LEL, special considerations are routinely taken to prevent flashback from the oxidizer to the waste generating source. The cost of this flexibility is the high fuel cost for this type of oxidizer. Recuperative and regenerative oxidizers are lim- ited gas streams with less than approximately 25% LEL but for differ- ent reasons. For a regenerative system, this restriction is primarily due to the danger a thermal run-away situation. In a thermal run- away, the oxidation of the excessive hydrocarbon concentration caus- es the combustion chamber outlet temperature to rise. This addition- al heat is recovered by the heat exchange system, which increases the combustion chamber inlet temperature, causing a further increase in the combustion chamber outlet temperature and so on until an ex- cessive temperature is reached. A regenerative system is vulnerable to thermal run-away because they are capable of auto-thermal opera- tion. This is a situation where the heat produced by oxidation of the pollutants is enough to operate the system with no additional input from the burner. In auto-thermal operation, the burner can be shut down and the oxidizer will sustain operation as long as the hydrocar- bon loading is high enough. A recuperative thermal oxidizer on the other hand is not capable of self-sustaining operation. In fact, they are purposely designed to avoid a self-sustaining situation because this type of operation will overheat and damage the heat exchanger. The burner must always operate to provide the additional heat to bring the pre-heated waste gas to the full oxidation temperature. As the pollutant loading increases the burner will throttle back by an amount equal to the heat of oxidation. However, if the burner throt- tles back too far, the oxidation reaction will not be properly initiated and the combustion chamber temperature will crash. • Type of Pollutant Process: Exhaust streams that contain high levels of acid or compounds that convert into acids (Chlorine, Fluorine, Bromine, Sulfur, etc.) must be treated with special care. Any of these elements, which are present in many important industrial solvents and cleaning agents will attack metal alloys at high temperatures and can form highly corrosive acids in the presence of water at low temperatures. With special materials of construction and design techniques all types of thermal oxi- dizers can be made to resist low levels of these elements. However, if the levels of acid are high or unpredictable, a direct flame type oxidizer is most preferred. This is because this type of oxidizer has no heat transfer system to be corroded by the acids. • Particulate Emission Levels Process: Exhaust streams containing par- 234

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