Since the early 1970s, air pollution control regulations have required many different businesses to install emission control systems to destroy volatile organic compounds (VOCs) 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 airstreams are somewhat consistent within each industry, a company’s individual requirements and desires can vary greatly.
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 airstream by converting it to carbon dioxide, water and heat. The rate of reaction is controlled by three interdependent, 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 Oxidizers. This type of oxidizer utilizes ceramic media packed into two or more vertical canisters as a high-efficiency heat exchanger (figure 2). Oxidation is achieved as pollutants pass through the ceramic media, mixed, and held at elevated temperatures from 1,500 to 1,800oF (816 to 982oC) in the combustion chamber for 0.3 to 0.5 sec. The clean (hot) air passes from the combustion chamber through the second canister for energy recovery. Heat generated during thermal oxidation of the air pollutant is adsorbed by the ceramic media (thus cooling the air and preheating the media). The clean (cooled) air is routed to atmosphere through an outlet control (switching) valve, exhaust manifold and the exhaust stack. To maximize the heat exchange, switching the valves alternates the airflow path between the canisters every 3 to 5 min, which continuously regenerates the heat stored within the ceramic media. Older regenerative oxidizers achieved 80 percent to 90 percent thermal efficiency and up to 95 percent destruction efficiency. Modern regenerative oxidizers are designed for up to 97 percent thermal efficiency and more than 99 percent destruction efficiency.
- Removal of the air pollutant from the airstream using a hydrophobic Zeolite rotating wheel.
- Destruction of the concentrated air pollutant using a regenerative thermal oxidizer.
In operation, air pollutants captured from the process via a ductwork collection system are passed through a high efficiency filter; 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, an inorganic, inert material. A slipstream of this air, approximately 10 percent, is routed through a cooling plenum while the remainder is routed directly to the common exhaust stack.
The concentrator wheel rotates at an approximate speed of two to eight revolutions per hour, continuously passing a sector of the wheel with adsorbed air pollutant through a desorbtion plenum for removal by a heated airstream, thus continuously returning a regenerated (or clean) sector back into the main housing for further adsorbtion.
Secondary Heat-Recovery SystemsWhile all of the modern emission control technologies incorporate energy-saving design features, if an even greater level of efficiency is desired, secondary recovery units can be incorporated into new or retrofitted to existing systems.
Secondary recovery units capture the 250 to 1,500oF (140 to 830oC) 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 process makeup air (ovens/dryers, kilns, curing zones, etc.), building comfort heating or, in some cases, can replace the need for natural gas-fired burners in the manufacturing process itself.
Alternatively, 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 air pollution control system’s exhaust stack. Hot water can be returned to the process for use (air preheat, condensation control, etc.) or used for building comfort heating. This coil also could be used to preheat 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 any time 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 backup system.
Selection and Design Criteria
- Determine possible need for a permanent total enclosure, which, when required by EPA, completely surrounds a source of emissions such that all VOC emissions are captured and contained for discharge to a control device. Permitting a facility with a properly designed permanent total enclosure will ensure 100 percent capture of all air pollutants present within the production area. Depending upon the enclosure 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 any available supplemental fuel. 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 airstream. Compounds such as silicones, phosphorus, heavy metals, halogen, sulfur and any particulates could be of 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 should it be needed rather than to install a second system in the future.
With today’s modern equipment incorporating components such as 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 can offer 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 energy savings.