Metal Finishing Guide Book


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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 limited gas streams with less than approximately 25% LEL but for different reasons. For a regenerative system, this restriction is primarily due to the danger a thermal run-away situation. In a thermal runaway, the oxidation of the excessive hydrocarbon concentration causes the combustion chamber outlet temperature to rise. This additional 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 excessive temperature is reached. A regenerative system is vulnerable to thermal run-away because they are capable of auto-thermal operation. 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 hydrocarbon 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 throttles 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 oxidizers 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 particulate must be given special consideration. There are a great number of waste gas sources that contain both gaseous hydrocarbon pollutants 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 cat692

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