IN THIS ISSUE |
THERMAL OXIDIZERS CARBON CONTROL FOR COAL JAY SAYS |
THERMAL OXIDIZERS |
Thermal Oxidizers as an Evolving Technology
by Paul Sims
March 1, 2009 Whether the choice is corrugated packing, monolithic structure block or just plain saddles, thermal oxidation has come a long way. Regenerative 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. 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, 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. Upstream particulate removal
Although oxidizer systems are primarily used for the abatement of VOCs, all emission streams contain some quantity of particulate matter, and these particles can lead to bed fouling, performance degradation, or even to dangerous and destructive fires. Some methods of upstream particulate removal methods include cascade (water wash), baffle and media filtration. Others, such as wet or dry electrostatic precipitators (ESP) or cyclone dust collectors, can reduce, but not eliminate particulate matter entering the RTO. Impact of particulate buildup
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, the particulate buildup can rapidly lead to plugging 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 RTO is reduced, as the media bed becomes less effective at transferring heat. This is because of so-called "dead zones," which, as they grow, reduce the surface area exposed to the air stream and result in 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 or bake out the material from the media bed, processes that involve costly downtime. Over time, the frequency of these cleaning procedures typically increases until the only viable solution is a complete media change-out. Types of media
Over the past few decades, several different types of heat transfer media have been used for RTOs. Three main categories are random packing, monolithic structured block and corrugated structure packing.
Random packing. Originally, in the 1970s, a variety of random packing materials were employed in RTOs, including gravel, ceramic balls and shapes of all kinds. The packing material was randomly dumped into the RTO to form a media bed. Random arrangement was preferred in order to prevent nesting that would constrict flow and cause dead areas that collected particulate.
In the 1980s, RTO manufacturers and owners 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. Modern saddles provide a high open area and aerodynamic design that limits nesting and reduces pressure drop by 20 percent from older media. Several manufacturers coat or impregnate today's saddle with a metal catalyst for use in RCOs. Saddle packing is also 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. Another alternative media design for very clean, low particulate streams is the imported Cordierite ceramic honeycomb monolith. 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. The disadvantage is that if particulate plugs a channel at the cold face, where the inflow enters the block, then this entire channel becomes a dead zone.
Corrugated structured packing. The most advanced ceramic heat exchange media for RTOs is corrugated structured packing. Ceramic structured packing is constructed of corrugated sheets of ceramic. The angle of inclination of the corrugations of adjacent sheets is reversed, ensuring excellent 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 superior airflow 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. Lifetime operation costs
Owners of thermal oxidizers have a number of options available when installing a new system or replacing the media bed of an existing system. VOC abatement systems in the finishing industry, where particulates can be a concern, are prime examples of a applications that should consider corrugated structured packing. This solution may cost more to purchase and install, but it will provide lower pressure drop, higher heat transfer efficiency, more reliable operation, and longer useful life compared to the alternative media. The significant long-term reduction in energy consumption alone can far outweigh the additional cost of installing advanced heat exchange media. PE |
CARBON CONTROL FOR COAL |
An EPA ruling on one Utah coal-fired power plant could have sweeping effects on the coal power industry.
The EPA on Nov. 14 released a landmark decision that could halt plans for any new coal-fired power plants that don't have an answer for controlling CO2 releases.
The agency's appeals panel, acting on an appeal by the Sierra Club, rejected a permit by EPA Region 8 for a new coal-fired plant in Utah because it did not stipulate the plant achieve maximum achievable control technology (MACT) for controlling the alleged greenhouse gas.
Sierra Group representatives said the decision could affect dozens of planned coal-fired power plants across the country. The EPA has not said that all new coal power plants would need to plan for cutting carbon, but did mention in the decision that the ruling could have far-reaching effects for the coal power industry. "The Board recognizes that this is an issue of national scope that has implications far beyond this individual permitting proceeding," the agency wrote in its decision.
About 25 coal plants are currently under construction across the United States, according to a Sierra Club release. Another 20 projects have been permitted or are near construction and more than 60 have been announced or are in the early stages of development, the report said.
Industry groups agreed with the environmentalists that this would likely cause delays for future coal-power plants, and that such delays could put the plants in danger of having to meet speculated tougher controls under President-elect Obama and a Democratic Congress. However, they disagreed with the Sierra Club and other environmental groups about the decision's ultimate effect, noting that the decision was more akin to a football "punt," by a regulatory panel looking for the agency to make a final declaration on requiring new plants to have carbon MACT.
According to Reuters, coal-fired power plants generate about half of U.S. power, and are responsible for a third of the country's CO2 emissions, a similar footprint to that of motor vehicles.
Click here to read the complete Bonanza decision. PE
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JAY SAYS |
Dear Reader,
These are troubling economic times. Some manufacturing facilities will not survive. Industries in general must look at the most expensive fuel equipment in their plants to save money. The first place to look is at your air emission control equipment. Ask yourself, “Is my Thermal Oxidizer designed to the most current fuel economic standards? Would an oxidizer modification or a replacement pay for itself in short order?” “Profit Enhancement Sells Equipment”
· Example Client$10 Million Dollars in Revenue$ 1 Million Dollars in Profit, 10% profit margin· To Achieve a 50% Increase inProfit:A. Need a 50% Increase in Sales orB. A 6% reduction in Operating ExpensesThe easiest path to increased profit & survival is Reductions in Energy Consumption
Best regards,
Jay Klaus
JKlaus@KlausEquipment.com
Klaus Equipment Company, Inc.
President
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Klaus Equipment Company
Phone: 724-444-3420
Fax: 724-444-3425
2866 West Bardonner Road,
Gibsonia, PA 15044
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