Regenerative thermal oxidizers and other types of thermal oxidation systems have proven to be a highly effective and energy-efficient method of abating volatile organic compounds (VOCs) and other pollutants. Particulate matter in the emission stream, however, can be a particularly vexing problem, resulting in the fouling and plugging of media beds.
Thermal oxidizers are essentially incinerators that thermally or catalytically convert pollutant-laden emissions into carbon dioxide 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 preheated by heat exchanger media to the thermal oxidation temperature, typically 1,400 to 1,600oF (760 to 871oC). The air then moves into a combustion chamber for the specified residence time (0.5 to 2 sec), where an exothermic reaction takes place, converting the VOCs to carbon dioxide 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 preheat incoming air. Valves continually alternate the flow between media beds: 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.
Regenerative thermal oxidizers can operate at thermal efficiencies of 85 to 99 percent, reducing or eliminating the need to burn natural gas in the combustion chamber. They are particularly effective for process streams with low-to-moderate solvent loading and can being 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.
For lower solvent loading levels (below 4 percent LEL), a catalytic system often is recommended. A regenerative catalytic oxidizer has a similar design as a regenerative thermal oxidizer, except that the ceramic heat exchange media closest to the combustion zone is coated or impregnated with precious metals that function as a catalyst that enable oxidation at significantly lower temperatures (600 to 1,000oF [316 to 538oC]). A catalytic system requires the presence of the type VOCs that will oxidize at these lower temperatures. Regenerative catalytic oxidizers utilize the same principle as catalytic converters in motor vehicles that oxidize carbon monoxide and unburnt hydrocarbons to carbon dioxide 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 preheating and often with very little or no burning of natural gas.
For airstreams with relatively low VOC concentrations, rotary adsorbers can be used to concentrate the stream and increase LEL level to enable 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 atmosphere, and the solvent then is removed from the media by desorption with a smaller stream of hot air, which then is delivered to an oxidation device.
Upstream Particulate RemovalAlthough oxidizer systems are used primarily for the abatement of VOCs, all emission streams contain some quantity of particulate matter, and these particles can lead to bed fouling, performance degradation and even to destructive fires. Some methods of upstream particulate-removal methods include cascade (water wash), baffle and media filtration. Others such as wet and dry electrostatic precipitators (ESP) and cyclone dust collectors can reduce -- but not eliminate -- particulate matter entering the regenerative thermal oxidizer.
Particulate that penetrates deeper into the media bed will tend to burn off. However, chemically reactive particles can cause problems even when they penetrate deep into the media.
A portion of the particulate that enters the regenerative thermal oxidizer will collect on the cold face of the media bed. Depending on the design of the media, the particulate buildup can rapidly lead to plugging of the media bed. Plugging causes several significant problems. Blocking airflow results in a rise in pressure drop, forcing the induced-draft fan to work harder and consume more electricity. The capacity of the regenerative thermal oxidizer is reduced as the media bed becomes less effective at transferring heat. The plugged “dead zones” mean less surface area is exposed to the airstream and less media mass is available to retain heat energy. Moreover, buildup of particulate presents a serious fire hazard.
Remediation solutions for these symptoms are wash-out or bake-out of the media bed, processes that involve downtime. Over time, the frequency of wash-out and bake-out procedures typically increase until the only viable solution is a complete media change out.
Types of MediaOver the past few decades, several different types of heat transfer media have been used for regenerative thermal oxidizers. Three main categories are random packing, monolithic structured block and corrugated structure packing.
Random Packing. In the 1970s, a range of random packing materials were employed in regenerative thermal oxidizers, including gravel, ceramic balls and shapes of all kinds. The packing material was randomly dumped into the regenerative thermal oxidizer to form a media bed. Random arrangement was preferred in order to prevent nesting that would constrict flow and cause dead areas that collect particulate.
In the 1980s, oxidizer manufacturers and owners discovered that the ceramic “saddles” developed for chemical mass transfer operations provided a favorable shape for regenerative thermal oxidizer random packing. Relative to other types of random packing, the saddle shape minimized pressure drop, allowing for lower electricity consumption by the induction fan, and maximized surface area, allowing for higher heat transfer efficiency.
Over the years, oxidizer media suppliers have refined the design of ceramic saddles to optimize features such as low pressure drop and aerodynamic designs. Several manufacturers coat or impregnate low-pressure-drop saddle packing designs with a metal catalyst for use in regenerative catalytic oxidizers. Additional coatings can be applied to the packing such as a glaze-resistant alumina coating to resist exposure to alkaline chemical attack, which may result from cleaning chemical fumes or the metallic salts used in electroplating applications.
Monolith Structured Block. An alternative for clean, low-particulate streams, monolith block is a form of structured packing that is placed in a formal arrangement rather than randomly dumped. Cells extend through the block in a straight channel perpendicular to the cold face. The advantage of this design is that it theoretically provides a straight, aerodynamic channel for the airstream. The disadvantage is that if particulate plugs a channel at the cold face, where the inflow enters the block, the entire channel becomes a dead zone.
Corrugated Structured Packing. Another ceramic heat exchange media for regenerative thermal oxidizers is corrugated structured packing. Constructed of corrugated sheets of ceramic, the packing is designed so the angle of inclination of the corrugations of adjacent sheets is reversed to ensure good airflow distribution throughout the media bed. If an area of the media bed becomes plugged by particulate, the mixing and spreading effect of the alternating corrugation prevents down zones above the plugged area.
Field studies have shown that, upon installation, regenerative thermal oxidizers with corrugated structured packing consume the same amount of natural gas as regenerative thermal oxidizers with monolith structured block, although the former has superior airflow distribution, and the latter has slightly higher heat storage capacity. The corrugated solution is better able to resist fouling caused by particulate buildup.
Owners of thermal oxidizers have a number of options available when installing a new system or replacing the media bed of an existing system. For VOC abatement systems in the finishing industry, where particulates can be a concern, corrugated structured packing should be considered.