Carbon adsorption is a remediation technology in which pollutants are removed from air by physical adsorption onto the carbon grain. Carbon is 'activated' for this purpose by processing the carbon to create porous particles with a large internal surface area (300 to 2,500 square meters per gram of carbon) that attracts and adsorbs organic molecules as well as certain metal and other inorganic molecules.
Commercial grades of activated carbon are available for specific use in vapor-phase applications. The granular form of activated carbon is typically used in packed beds through which the contaminated air flows until the concentration of contaminants in the effluent from the carbon bed exceeds an acceptable level. Granular activated carbon systems typically consist of one or more vessels filled with carbon connected in series and/or parallel operating under atmospheric, negative, or positive pressure. The carbon can then be regenerated in place, regenerated at an off-site regeneration facility, or disposed of, depending upon economic considerations.
Catalytic oxidation is a relatively new alternative for the treatment of VOCs in air streams resulting from remedial operations. VOCs are thermally destroyed at temperatures typically ranging from 600 to 1,000 °F by using a solid catalyst. First, the contaminated air is directly preheated (electrically or, more frequently, using natural gas or propane) to reach a temperature necessary to initiate the catalytic oxidation of the VOCs. Then the preheated VOC-laden air is passed through a bed of solid catalysts where the VOCs are rapidly oxidized. Oxidation of halogenated VOCs produces acid vapor. Off gas scrubbing may be needed to control the acid vapor.
In most cases, the process can be enhanced to reduce auxiliary fuel costs by using an air-to-air heat exchanger to transfer heat from the exhaust gases to the incoming contaminated air. Typically, about 50% of the heat of the exhaust gases is recovered. Depending on VOC concentrations, the recovered heat may be sufficient to sustain oxidation without additional fuel. Catalyst systems used to oxidize VOCs typically use metal oxides such as nickel oxide, copper oxide, manganese dioxide, or chromium oxide. Noble metals such as platinum and palladium may also be used. However, in a majority of remedial applications, nonprecious metals (e.g., nickel, copper, or chromium) are used. Most commercially available catalysts are proprietary. Catalysts that resist damage from halogenated VOC combustion are available, but cost more than catalysts that are suitable for nonhalogenated VOC combustion.
Thermal oxidation equipment is used for destroying contaminants in the exhaust gas from air strippers and SVE systems. Probably fewer than 100 oxidizers have been sold to treat air stripper effluents; most of these units are rated less than 600 scfm. Typically, the blower for the air stripper or the vacuum extraction system provides sufficient positive pressure and flow for thermal oxidizer operation.
Thermal oxidation units are typically single chamber, refractory-lined oxidizers equipped with a propane or natural gas burner and a stack. Lightweight ceramic blanket refractory is used because many of these units are mounted on skids or trailers. Thermal oxidizers are often equipped with heat exchangers where combustion gas is used to preheat the incoming contaminated gas. If gasoline is the contaminant, heat exchanger efficiencies are limited to 25 to 35% and preheat temperatures are maintained below 530 °F to minimize the possibility of ignition occurring in the heat exchanger. Flame arrestors are always installed between the vapor source and the thermal oxidizer. Burner capacities in the combustion chamber range from 0.5 to 2 million Btus per hour. Operating temperatures range from 1,400 to 1,600 °F, and gas residence times are typically 1 second or less. Like catalytic oxidation, thermal oxidation of halogenated VOCs produces acid vapor. Off gas scrubbing may be needed to control the acid vapor.