By CHARLES M. MARTINSON
Regenerative Thermal Oxidizers (RTOs) 97% thermal efficiency and +99% destruction efficiency.
A thermal oxidzer uses a heat exchanger to preheat air in order to achieve oxidation.
Catalytic oxidizers use an industrial-grade catalyst to promote a chemical reaction at 500°-650°F. The lower operating temperatures typically result in lower energy consumption.
A rotary concentrator is a two-step process that first removes the air pollutant from an air stream, then destroys the concentrated material using a RTO.
Since the early 1970 s, air-pollution control regulations have required many different businesses to install emission-control systems to destroy volatile organic compounds (VOCs) and/or hazardous air pollutants (HAPs) or face the possibility of stiff non-compliance fines. While general guidelines regarding the destruction of air pollutants present in process exhaust air streams are somewhat consistent within each industry, a company s individual requirements and desires can vary greatly.
Developing an optimal design for each operation depends on many variables including the type and quantity of air pollutant, the volume and temperature of the air being exhausted, etc. Future manufacturing growth expectations and even the facilities geographical location also should be considered. By understanding the modern, efficient designs available now, and by taking into consideration the selection criteria issues, companies can be better prepared to specify and select a custom-designed air-pollution control system.
Destruction technologies
The basic design concept of both thermal and catalytic oxidizers is to promote a chemical reaction of the air pollutant with oxygen at elevated temperatures. This reaction destroys the pollutant in the air stream by converting it to carbon dioxide, water, and heat. The rate of reaction is controlled by three interdependent and critical factors: time, temperature, and turbulence. What distinguishes one technology from another is the temperature at which the air pollutant is destroyed and the methods used to generate the heat used in the process.
Regenerative Thermal Oxidizer (RTO) To achieve destruction, the air pollutant collected from the manufacturing process is directed by the APCS supply fan into the RTO inlet manifold and through one of the two energy recovery canisters by use of inlet control (switching) valves. The air pollutant passes from the valve assembly vertically upward through the first of the two heat-exchanger canisters, where it adsorbs heat from the ceramic media (thus cooling the media and preheating the air stream). The preheated air then enters into the combustion chamber (typically at a temperature very close to the temperature required for thermal oxidation), where it is heated further to 1,500°F (if necessary) and held at this oxidation temperature for a period of time (minimum 0.5 seconds) sufficient to achieve a high destruction efficiency. Destruction of the air pollutant takes place within the combustion chamber, where auxiliary fuel is introduced if necessary.
The clean (hot) air then passes from the combustion chamber vertically downward through the second energy recovery canister. Heat generated during thermal oxidation of the air pollutant is then adsorbed by the ceramic media (thus cooling the air and preheating the media). Finally, the clean (cooled) air is routed to atmosphere through an outlet control (switching) valve, exhaust manifold and through the APCS exhaust stack. To maximize the heat exchange, switching the valves alternates the airflow path between the canisters every 3 to 5 minutes, which continuously regenerates the heat stored within the ceramic media. Older RTOs achieved from 80% to 90% thermal efficiency and up to 95% destruction efficiency. Modern RTOs are designed for up to 97% thermal efficiency and +99% destruction efficiency.
Thermal Oxidizer Designed to operate at temperatures of 1,400° to 1,500°F, thermal recuperative oxidizers use a tube-in-shell stainless steel heat exchanger as an air pre-heater. Oxidation is achieved as the process exhaust is passed through the heat exchanger, mixed, and held at elevated temperatures in the combustion chamber for a minimum of 0.5 seconds. Older thermal oxidizers achieved only 40-60% thermal efficiency and up to 95% destruction efficiency. Modern thermal oxidizers are designed for up to 80% thermal efficiency and +99% destruction efficiency.
Catalytic Oxidizer Catalytic oxidizers are designed to utilize an industrial-grade catalyst to promote a chemical reaction (oxidation) at lower temperatures, as compared to thermal oxidation, typically between 500° and 650°F. Because of the lower operating temperatures, catalytic oxidation commonly requires less energy to operate. To minimize operating costs, catalytic oxidizers incorporate a high-efficiency; counter-flow plate-type heat exchanger to preheat incoming exhaust fumes from the production process. Oxidation is achieved when the fumes are passed through a bed of industrial-grade catalyst manufactured of pure platinum group metals. Older catalytic oxidizers achieved only 50% to 70% thermal efficiency and up to 95% destruction efficiency. Modern recuperative catalytic oxidizers are designed for up to 80% thermal efficiency and +99% destruction efficiency.
Rotary Concentrator System A Rotary Concentrator System is a hybrid APCS designed to remove and destroy air pollutants efficiently from a process exhaust air stream. Application of this technology is limited to exhaust air steams at or near ambient temperature. The system requires a two-step process:
- Removal of the air pollutant from the air stream using a hydrophobic zeolite rotating wheel.
- Destruction of the concentrated air pollutant using a Regenerative Thermal Oxidizer (RTO).
In operation, air pollutants captured from the process via a ductwork collection system are passed through a high-efficiency filter as particulate can damage the concentrator wheel media. Once filtered, the polluted air passes through the rotating concentrator wheel where the air pollutants are adsorbed onto the hydrophobic zeolite media. A slip-stream of this air, approximately 10%, is routed through a cooling plenum while the remainder is routed directly to the common exhaust stack.
The rotor itself is fabricated from a corrugated mineral fiber substrate to which the manufacturer permanently bonds a proprietary mixture of hydrophobic zeolite and inorganic materials. The hydrophobic zeolite rotor is inorganic, completely inert, and has rigidity, physical integrity, and the ability to withstand thermal stress. In this application the zeolite removes the air pollutant from the manufacturing process exhaust air stream as it passes through the rotor.
The concentrator wheel rotates at an approximate speed of 2-8 revolutions per hour, continuously passing a sector of the wheel with adsorbed air pollutant through a desorbtion plenum for removal by a heated air stream, thus continuously returning a regenerated (or clean) sector back into the main housing for further adsorbtion. Desorbtion is automated as the slipstream of air that was routed through the cooling plenum is sent through a supplemental desorbtion heater where it is elevated to desorbtion temperature (typically ~350°F) and returned to the concentrator housing. This heated desorbtion air is directed back through the wheel via the desorbtion plenum where the concentrated pollutants are removed. Then, the highly concentrated air stream is routed to the RTO for thermal destruction.
Secondary recovery units
While all of the modern emission-control technologies incorporate the latest energy-saving design features, if an even greater level of efficiency is desired secondary recovery units can be incorporated into a new APCS, or retrofitted to an existing system. While these energy-saving benefits are important to any company purchasing a new air-pollution control system, they are essential when replacing older equipment. If a company is currently controlling emissions with an air-pollution control system more than eight to 10 years old, this less-efficient equipment is costing them money every day.
Secondary recovery units capture the 250° to 1,500° of heat energy (depending on the air-pollution control system currently in use) that normally would be vented out the stack to the atmosphere. The unit can be designed for minimal pressure drop so as not to affect the operation of the oxidizer, while returning temperature controlled fresh air for a variety of uses. This heated fresh air can be used for building comfort heating, process make-up air (ovens/dryers, kilns, curing zones, etc), or in some cases, can completely replace the need for natural gas-fired burners in the manufacturing process itself.
Using the same idea of capturing heat from the exhaust stream, a hot water or thermal oil heat-transfer coil can be installed in the APCS exhaust stack. Hot water can be used for building comfort heating or can be returned to the process for use (air preheat, condensation control, etc.) This coil also could be used as a preheat section to pre-heat cool water for a steam generator. Thermal oil is used as a main process heat source where direct flame heating is not desired. Adding a coil in the exhaust stream can reduce or even remove the heat load required from the thermal oil heating system. Depending on the stack temperature, the exhaust from the oxidizer could be routed directly to a low-pressure steam generator. If the plant uses steam for any reason (carbon bed regeneration, humidity control, etc.), this system could supplement steam production capacity anytime the oxidizer is running. In an ideal situation, the steam produced from the oxidizer exhaust would allow the main steam generator to function as a back-up system.
Technology selection/ design criteria
No matter what emission-control technology is finally selected, today s modern air-pollution control systems typically exceed all U.S. EPA, state, and local regulations by destroying +99% of a facility's air pollutants. When a decision is made to install or upgrade an air-pollution control system, the company should provide specifications to be used as a basis for selecting the proper technology and for preparing formal proposals. The following information should be provided:
- Describe the type of production process emitting the air pollutant to be controlled. If possible, include a rough sketch of the building floor plan, showing the location of all pertinent production equipment.
- Provide the geographical location (and elevation level, if known) where the system will be installed. Both the outdoor climate (surface finishes/types of dampers, etc.) and the elevation (fan sizing) could have an effect on system design.
- Estimate the number of hours per day that the system will be operated. The heat exchanger efficiency, chamber design, etc., could change depending upon the operation hours required.
- List the total number of different emission points (exhaust stacks) that are to be controlled by the air-pollution control system. A process control/by-pass tee-damper may be required at each emission point. The system s electrical control design also will change, depending upon the number of damp-ers to be controlled.
- List the exhaust rates and temperatures for each individual emission point. The exhaust rates are important for sizing the unit, but also are used to size the ductwork and dampers. The temperature is used to calculate estimated operating costs and to determine the necessity for ductwork insulation.
- Describe the type of heat source used for any dryers/ovens that are to be controlled. If the heat sources for process dryers or ovens are gas-fired burners, there are NFPA regulations that determine the method of purging and damper control. If the heat source is steam or hot oil, process control/by-pass tee-dampers may not be required at each stack.
- List the air pollutant types and quantity being used. In addition to affecting the choice of catalyst used in catalytic units, solvent type and quantity also will affect the destruction efficiencies, the heat exchanger efficiency, the internal materials of construction, and the estimated operating costs.
- Determine possible need for a Permanent Total Enclosure (PTE). The use of a PTE has become popular in certain industries. Permitting a facility with a properly designed PTE will assure 100% capture of all air pollutants present within the production area. Depending upon the PTE design, it may or may not affect the overall sizing and technology choice of the air-pollution control system.
- Provide the electrical voltage and available power cost. The voltage available determines the type of electronics that are used. The power cost is used to calculate the estimated operating costs.
- Provide the type,, cost, and line pressure of supplemental fuel available. The fuel type available (natural gas, propane, etc.) and the line pressure are used to determine the burner and fuel train design. The fuel cost is used to calculate the estimated operating costs. Compressed air may be required depending upon the design of the air-pollution control system.
- Describe the physical location of the air-pollution control system installation. The actual location determines whether a concrete equipment pad or steel support structure is required. Also, if possible, provide specific site installation plans such as duct run length, exhaust stack height, and gas piping length required.
- Indicate the percent of pollutant destruction efficiency required. The destruction efficiency percentage required will determine the amount of catalyst needed (in catalytic models), as well as the operating temperature and residence time in either technology.
- List any catalyst masking or poisoning agents that could potentially be present in the air stream. Compounds such as silicones, phosphorus, heavy metals, halogen, sulfur, and any particulates could be a concern and should be identified. An air-pollution control system can be designed to handle various levels of most compounds if the user can quantify them in advance.
- Plan ahead. When selecting or sizing an air-pollution control system, the facility's growth expectations for the next two to five years should be considered. Typically, it is less costly to install a system designed to handle additional capacity now than to install a second system in the future.
Summing up
As clean-air regulations become more stringent and/or enforced, companies concerned about emitting pollutants into the atmosphere typically will consider various air-pollution control technology options to meet their individual requirements. As energy costs rise, these companies also should consider all available options to help reduce the energy costs associated with operating an air-pollution control system.
With modern equipment incorporating components like high-efficiency heat exchangers, natural gas-fired burners, industrial-grade blowers, electric or pneumatic actuators and programmable logic controllers, the installation of a new system or replacement of an older system now offers a short-term payback on a company s capital investment. Retrofitting a secondary recovery unit to an older air-pollution control system also can generate significant energy savings. An upgrade or a retrofit could provide substantial operating savings and have a positive impact on a company s bottom line.
Charles M. Martinson is president of The CMM Group LLC, De Pere, WI. He has been involved in the design, fabrication, sales, and installation of air-pollution control systems for over 20 years. The CMM Group manufactures custom-designed systems and provides emergency field service, preventative maintenance programs, engineering services, system retrofits and rebuilds, turnkey installation and spare parts for all makes and models. Contact [email protected], or tel. 920-336-9800.