Pollution Engineering - March 2009 - (Page 32)

Thermal Oxidizers Whether the choice is corrugated packing, monolithic Whether the choice is corrugated packing, monolithic structure block or just plain saddles, thermal oxidation structure block or just plain saddles, ther mal oxidation has come a long way. By PAUL SIMS, global market manager, Environmental at Koch Knight LLC R egenerative thermal oxidizers (RTOs) and other types of thermal oxidation systems can be a highly effective and energy-efficient method of abating volatile organic compounds (VOCs) and other pollutants emitted by industrial plants. However, particulate matter in the emission stream can be a particularly vexing problem, resulting in the fouling and plugging of media beds. Careful attention should be given to selecting the shape and material of the heat exchange media to mitigate potential problems with particulate matter and to ensure reliable, economical and safe operation of thermal oxidation systems. reducing or eliminating the need to burn natural gas in the combustion chamber. RTOs are particularly effective for process streams with low to moderate solvent loading, and can be self-sustaining at moderate lower explosive limit (LEL) levels. In other words, once the system is sufficiently heated, the natural gas burners can be turned off if enough flammable gas is present in the exhaust stream. Other thermal oxidizers For lower solvent loading levels, below 4 percent LEL, a catalytic system is often recommended. A regenerative catalytic oxidizer (RCO) is similar in design to an RTO, except that the ceramic heat exchange media closest to the combustion zone is coated or impregnated with precious metals that function as a catalyst. The metals enable oxidation at significantly lower temperatures (600°F to 1,000°F). A catalytic system requires the presence of the type of VOCs that will oxidize at these lower temperatures. RCOs utilize the same principle as catalytic converters in motor vehicles that oxidize CO and as-yet un-oxidized hydrocarbons to CO2 and water. For exhaust streams with high LEL levels, a simple thermal oxidizer can be used, without any thermal regeneration capability. In such cases, high solvent loading can support combustion without pre-heating, and often with very little or no burning of natural gas. For air streams with relatively low VOC concentrations, rotary adsorbers can be used to concentrate the stream and increase LEL levels, enabling the use of an oxidation device that is smaller and/ or more energy efficient. The pollutant-laden process exhaust passes through the rotary adsorption unit where the VOCs are adsorbed on zeolite or activated carbon media. The purified air is exhausted to the atmosphere, and solvent is then removed from the media by desorption with a smaller stream of hot air, which is then delivered to an oxidation device. Regenerative thermal oxidation Thermal oxidizers are essentially incinerators that thermally or catalytically convert pollutant-laden emissions into CO2 and water vapor. The oxidation process typically achieves better than 99-percent destruction/removal efficiency (DRE) levels for VOCs, hazardous air pollutants (HAPS) and odors. Regenerative thermal oxidizers minimize fuel consumption by regenerating, or reusing, heat generated by the system. Fans draw air from paint-booth collection systems and other sources, and the air is pre-heated by heat exchanger media to the thermal oxidation temperature, typically 1,400°F to 1,600°F. The air then moves into a combustion chamber for the specified residence time (0.5 to 2.0 seconds), where an exothermic reaction takes place, converting the VOCs to CO2 and water vapor. Prior to being exhausted to the atmosphere, the hot, purified air passes through a media bed to capture heat energy that will be used to pre-heat incoming air. Valves continually alternate the flow between media beds through a cycle with incoming cool air into a media bed that has just been heated by hot exhaust, followed by a cycle with hot exhaust air flowing through the media bed to reheat it. RTOs can operate at thermal efficiencies of 85 to 99 percent, 32 Pollution Engineering MARCH2009

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Pollution Engineering - March 2009 Contents The Editor’s Desk EnviroNews PE Events Legal Lookout Green Connections Get Ready to Count Carbon Getting Rid of Foreign Oil Green Goes Underground Thermal Oxidizers Ocean-Front Remediation Pumps, Pipes and Valve Products Air Monitoring Equipment PE Products Classified Marketplace Advertisers Index State Rules

Pollution Engineering - March 2009

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