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1 ' 18 February 1997 AGE-NC Project No 95-0104 ' Page 7 of 22 ' (unsaturated)zone and draw the vapor into extraction wells screened within the contaminant plume The hydrocarbon vapor is drawn from the wells and "treated" in one of several possible treatment units (carbon canisters, internal combustion engines, thermal oxidizers and catalytic oxidizers) Air ' injection wells may be installed outside the contaminant plume to increase air flow to the extraction 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 concentiations 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 As concentrations decrease below 20,000 ppmv, supplemental fuel requirements, and therefore expenses, become burdensome Another disadvantage of these systems is the noise generated by the engine, ' however, this can be mitigated by placing the unit inside an enclosure, such as a building A carbon-canister system operates by i outing hydrocarbon vapor through activated carbon filters lie where adsorption of h}drocarbons onto carbon takes place, removing the hydrocarbons from the vapor stream This type of system v.orks best for low concentrations of hydrocarbons, but becomes expensive when high concentrations of hydrocarbon vapor necessitate frequent canister replacement and disposal of spent carbon I 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 14007 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 anottier 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 thari either thermal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal-combustion and thermal oxidizer systems b 1 2 REQUIRED SOIL CONDITIONS • Operation of the blower in a vapor extractions stem creates a partial vacuum in the subsurface, I