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10 June 1999 S AGE-NC Project No 97-0290 ' Page 10 of 26 I unit An internal-combustion(IC) system draws the vapor stream into the carburetor system of the engine ' and mixes the vapor with air prior to combustion Exhaust gasses (oxides of carbon and hydrogen) are further broken down in a catalytic converter before being emitted to the atmosphere These systems require supplemental fuel, such as propane or natural gas, which can greatly increase expenses as vapor concentrations decrease below levels normally required for combustion Typically, internal combustion engines work effectively where in-situ hydrocarbon concentrations approach the 40,000 parts per million- volume,(ppmv)required for peak performance of the engine IAs concentratibns decrease below 20,000-ppmv, supplemental fuel requirements, and therefore expenses, become burdensome Another disadvantage of these systems is the noise generated by the Iengine, however, this can be mitigated by placing the unit inside an enclosure, such as a building Thermal oxidation systems destroy hydrocarbons by open-flame combustion Natural gas is commonly used as supplemental fuel, heating the extracted vapor stream to a combustion temperature of approximately 1400°F Relative to internal combustion systems, these systems may be slightly more expensive to purchase and install,however,they can operate at higher air flow rates, and therefore remediate sites at a faster rate For this system to be effective, hydrocarbon concentrations should range from 5,000 ppmv to 30,000 ppmv They also operate at much lower noise levels than internal combustion systems Catalytic oxidation units provide another option for treating vadose-zone contamination,particularly after other extraction systems have reached their effective limits due to lowered hydrocarbon concentrations These systems operate at temperatures of approximately 700°F, requiring less supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal-combustion and thermal oxidizer systems A carbon-canister system operates by routing hydrocarbon vapor through activated carbon filters I where adsorption of hydrocarbons onto carbon takes place, removing the hydrocarbons from the vapor stream This type of system works best for low concentrations of hydrocarbons, but becomes expensive when high concentrations of hydrocarbon vapor necessitate frequent canister replacement and disposal of spent carbon I7 12 Required Soil Conditions Operation of the blower in a vapor extraction system creates an induced vacuum in the subsurface, generating vapor flow towards the area of lowered air pressure (the extraction point) through the soil pore spaces The efficiency of this process is proportional to soil porosity and intrinsic permeability Intrinsic permeability is the measure of the ability of soils to transmit fluids, porosity is the Advanced GeoEn%ironmental,Inc I