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Harding Lawson Associates <br /> • Polymer resin media. <br /> 1. Thermal Oxidation <br /> Oxidizing hydrocarbons with a flame is most effective <br /> ' for treating relatively high concentrations of hydrocarbons <br /> (greater than approximately 8, 000 mg/M3) . As concentrations drop <br /> ' below this level, supplemental propane or natural gas will be <br /> ' necessary to sustain the flare. This is a feasible method for <br /> treating the anticipated initial concentrations but may be cost <br /> prohibitive as extraction continues and concentrations fall. <br /> After the initial soil gas has been removed, remaining soil vapor <br /> concentrations will gradually decline over a long period of time, <br /> lengthening the operation of the emission control equipment. <br /> Operation of a thermal oxidizer with relatively low <br /> ' concentrations will not be cost effective. Cost analyses during <br /> equipment procurement should include a comparative evaluation of <br /> ' the capital and operation costs associated with this alternative. <br /> 2 . Catalytic Oxidation <br /> Hydrocarbon vapors are oxidized by heating them above <br /> ' their ignition temperature and passing them through a catalyst <br /> ' bed (usually made with a noble metal) . Catalytic oxidation is <br /> most effective for moderate hydrocarbon concentrations <br /> ' (approximately 3 , 000 to 8, 000 mg/M3) . High anticipated initial <br /> concentrations may require dilution (which would reduce <br /> ' extraction rates and extend pro3ect life) to keep temperatures in <br /> the catalyst bed below the melting point of the material it is <br /> ' 14 <br />