Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 234 of 331

CONCENTRATOR/OXIDIZER SYSTEMS Combining technologies creates "Capture & Control" systems that use an inte- grated concentrator and final treatment system to process large volumes of process air, concentrate VOCs in a smaller volume of air, destroy the pollutants in the air and use the heat from the destruction process as part of the concen- tration process. OXIDATION TECHNOLOGY The most reliable and acceptable means of destroying VOCs, HAPs, and odors available today is thermal oxidation. Oxidation, typically, is an energy intensive technology wherein a polluted air stream is heated to a high temperature setpoint that is predetermined by the nature of the pollutant. The simplest form of an ox- idizer is a direct-fired burner that elevates the air temperature from incoming lev- els to combustion levels. Because of the high cost of heating the process exhaust stream to the required oxidation temperature most thermal oxidizers incorpo- rate some type of primary heat recovery. Primary heat recovery transfers energy from the hot clean gas stream exiting the oxidizer into the incoming polluted gas stream. This reduces the amount of additional energy required to achieve the ox- idation temperatures. There are two widely used methods of recovering this thermal energy, recuperative and regenerative. Oxidizer Selection Criteria In order to select which type of oxidizer is most advantageous for a specific ap- plication, the following information must be known: • Process exhaust flow rate: If the process exhaust stream flow rate is below about 3,000 scfm, regenerative systems are generally not practi- cal. This is because the fuel savings gained by the highly efficiency re- generative heat recovery is generally not sufficient to offset the in- creased capital cost and maintenance of the RTO when compared to a recuperative or direct flame system. At flow rates above approxi- mately 25,000 scfm, direct flame oxidizers are at a severe economic disadvantage because of their very high fuel cost. However, it is not unheard of for direct flame systems of this size or larger to be in- stalled where secondary heat recovery boilers can be used to offset the high fuel cost. Another case where direct flame systems are favored for large air volumes is for emergency or stand-by systems which op- erate very few hours per year. • Process exhaust stream temperature: If the polluted waste gas stream temperature is above approximately 600°F, regenerative systems are disfavored because the high temperatures can reduce the reliability and longevity of the valve system. In addition, at these temperatures, there is less difference in fuel consumption to justify the additional cost and complexity. If the exhaust temperature is significantly above 1000°F, recuperative systems are disfavored versus direct flame sys- tems again because the difference in fuel consumption becomes too small to justify the added first cost. 233

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