January 5, 2008
Regenerative thermal oxidizers (RTOs) 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 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. Therefore, understanding these systems and the options available can help mitigate potential problems and ensure reliable, economical and safe operation of thermal oxidation systems.
Regenerative thermal oxidation
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 pre-heated by heat exchanger media to the thermal oxidation temperature, typically 1,400-1,600°F. The air then moves into a combustion chamber for the specified residence time (0.5-2.0 seconds), where an exothermic reaction converts VOCs to carbon dioxide and water vapor.
Before 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: a cycle with incoming cool air into a media bed just heated by hot exhaust, followed by hot exhaust air flowing through the media bed to reheat it.
RTOs can operate at thermal efficiencies of 85-99 percent, 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) has a design similar 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 that enable oxidation at significantly lower temperatures (600-1,000°F). A catalytic system requires VOCs that will oxidize at these lower temperatures. RCOs 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 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 level, to enable the use of an oxidation device that is smaller and/or more energy efficient. Pollutant-laden process exhaust passes through the rotary adsorption unit where VOCs are adsorbed on zeolite or activated carbon media. Purified air is exhausted to the atmosphere, and the solvent is removed from the media by desorption with a smaller stream of hot air, which is then delivered to an oxidation device.
Although 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 fires. Some methods of upstream particulate removal 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 RTO.
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 RTO will collect on the cold face of the media bed. Depending on the design of the media, 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. Capacity of the RTO is reduced, as the media bed becomes less effective at transferring heat, because “dead zones” mean reduced surface area exposed to the air stream and less media mass available to retain heat energy. Moreover, buildup of particulate presents a serious fire hazard.
The only remediation solution for these symptoms is wash-out or bake-out of the media bed, processes that involve costly downtime. Over time, the frequency of wash-out and bake-out procedures typically increases until the only viable solution is a complete media change-out.
Types of media
Over the past few decades, three main categories of heat transfer media have been used for RTOs:
Random packing. In the 1970s, a wide variety of random packing materials were employed in RTOs, including gravel, ceramic balls, and shapes of all kinds. Packing material was randomly dumped into the RTO to form a media bed. Random arrangement was preferred to prevent nesting that would constrict flow and cause dead areas that collect particulate.
In the 1980s, RTO manufacturers discovered that the ceramic “saddles” developed for chemical mass transfer operations provided an optimal shape for RTO random packing. Relative to other types of random packing, the saddle shape minimized pressure drop (for lower electricity consumption by the induction fan) and maximized surface area (for higher heat transfer efficiency). Over the years, RTO media suppliers have refined the design of ceramic saddles. (See Photo 1.)
Several manufacturers coat or impregnate this low pressure drop saddle with a metal catalyst for use in RCOs. Packing may also be available in a glaze-resistant alumina 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 very 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 air stream. However, if particulate plugs a channel at the cold face, where the inflow enters the block, this entire channel becomes a dead zone.
Corrugated structured packing. Corrugated structure packing is an advanced ceramic heat exchange media. (See Photo 2.) The angle of inclination of the corrugations of adjacent sheets is reversed, ensuring optimal distribution of airflow throughout the media bed. Even if an area of the media bed became 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, RTOs with corrugated structured packing consume the same amount of natural gas as RTOs with monolith structured block, although the former has better air flow distribution, and the latter has slightly higher heat storage capacity. The advantage of the corrugated solution becomes dramatic over time because of its ability 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 offers the advantages of low pressure drop, high heat transfer efficiency, reliable operation, long useful life and low energy consumption, all of which can outweigh the additional purchase and installation costs.