Industrial finishers have taken the necessary steps to reduce or control emissions of Volatile Organic Compounds (VOCs) from their plants. It is time to take a second look at the regulatory horizon.
New EPA proposals intend to implement a new National Ambient Air Quality Standard (NAAQS) for ozone. Because of the stricter air quality standards, it is possible that the number of counties in ozone non-attainment areas will increase from 106 to more than 250. Many areas previously in compliance will now be subject to more stringent standards. These will catapult many minor sources of VOC emissions into major source status and require Best Available Control Technology (BACT) on facilities that are currently subject only to Reasonably Available Control Technology (RACT).
There are a variety of mature, proven technologies and approaches to reduce emissions. Most are readily available to finishers. They include:
- Traditional forms of destroying emissions, such as oxidizers and absorbers;
- Air handling techniques such as recirculation and cascading; and
- Use of modular devices for emission control at individual plant sources.
Regenerative Thermal Oxidation (RTO). RTO technology remains one of the most flexible means of thermal treatment for finishers. RTOs can process a wide variety of VOCs effectively, even if there are fluctuations in the airflow volume. An RTO consists of a combustion chamber located adjacent to several energy recovery chambers. (Fig. 1) The energy recovery chambers are filled with ceramic heat exchange media. The solvent-laden air enters an inlet header and is directed to one of the energy recovery chambers through the inlet control valve. The air passes through the heat exchange media, adsorbing heat from the media. It then enters the combustion chamber at a temperature close to the oxidation temperature. The oxidation process is completed in the combustion chamber. A gas burner maintains a preset oxidation temperature. If the incoming air contains enough solvents, the solvent combustion energy provides the necessary heat to raise the temperature to the combustion set-point.
The clean air leaves the RTO through the heat exchange media of an adjacent chamber. The energy in the clean, hot exhaust air is transferred to the heat exchange media for storage. The clean air then passes through the exhaust manifold and is discharged through a stack to atmosphere. The temperature of the air as it leaves the RTO is close to the temperature of the incoming air. At least one chamber is always on inlet mode and another on outlet mode to allow the RTO to continuously process a solvent-laden air stream.
These are advantages over other types of thermal oxidizers. RTOs also are characterized by low NOx emissions, less susceptibility to the type of VOC, and lower overall operating costs. Its disadvantages include a large size and weight, difficult and expensive installation, higher capital cost than other thermal oxidizers and more moving parts that require maintenance.
Recuperative Thermal Oxidizers. Recuperative thermal oxidizers include a combustion chamber with a primary heat exchanger that recovers waste heat from hot incinerated exhaust air and uses it to preheat incoming air. These oxidizers typically include a shell-and-tube heat exchanger that is capable of about 70 pct primary heat recovery. With an oxidation temperature of approximately 1,400F, sufficient waste heat will be available for secondary heat recovery. This heat is recovered typically for process heating (for ovens or dryers) or to generate steam or hot water. A recuperative thermal oxidizer is best suited for applications where the excess heat can be used. This offsets the relatively higher operating (fuel energy) costs for the unit.
Catalytic Oxidizers. Catalytic oxidizers operate on the same principle as a recuperative and regenerative oxidizer but use a catalyst to aid in the oxidation process. The catalyst reduces the necessary preheat temperature to below 600F, which can reduce the operating cost of the unit when compared with a simple recuperative thermal oxidizer. This allows the oxidizer to be built from less exotic steels as well, resulting in lower fabrication costs.
The major disadvantages of catalytic units are higher maintenance costs due to the need to monitor and maintain the catalyst. Among the many substances that will deactivate catalysts are titanium dioxide, aluminum, halogens, mica, resins, carbon black, barium, sulfate and UV inhibitors, many of which are common to finishers. There are, however, some newer catalysts that can be used with halogenated streams. Finishers must be aware of the various organic or inorganic compounds in an exhaust stream when employing a catalytic oxidizer of any kind.
Rotary Concentration Systems. In airflows that are relatively devoid of VOCs, as in the case of many spray booths, rotary adsorbers can be used to concentrate smaller air streams that can be oxidized more economically.
The rotary concentrator adsorption system (Fig. 2) is designed to continuously adsorb solvent emissions onto activated carbon or zeolite and discharge purified air. This is achieved through the use of a moving adsorbing bed in the shape of a cylinder, a section of which is simultaneously desorbed, or regenerated. This design eliminates the need for dual running and stand-by adsorption beds.
After prefiltration, air passes through the main rotating adsorber. This primary adsorbtion wheel consists of individual segments of activated carbon or zeolite adsorbing media. These segments are stacked together on the periphery of a segmented cylinder to form a revolving adsorption media bed. Solvent-laden air moves through the media on the periphery of the cylinder towards the center of the cylinder. Solvents are adsorbed onto the adsorption media, and the purified air exits through the center of the cylinder.
A portion of the rotating cylinder is simultaneously desorbed by passing hot air through a section of the cylinder. This desorption section is sealed off from the remaining adsorption section of the rotor by seals, so that very high efficiencies can be obtained in the system.
The entire assembly, including the prefilter, rotor and internal desorption ductwork is contained within a reinforced self-supporting module. This results in significantly reduced field assembly time. The modular incinerator unit is designed to be a fully self-supporting reinforced unit with a tubular framework of four-inch by four-inch steel tube. The interior skin is fully seal-welded aluminized steel to prevent leakage.
Handling Multiple Sources. Often the greatest challenge to controlling emissions from finishing sources, such as spray booths, ovens, and coaters, is capturing those emissions at their source and transporting them to a treatment device economically. This is a challenge, because emission sources are commonly scattered throughout a plant, and fume capture ductwork must be designed and installed to convey the solvents to an oxidizer or adsorber on the roof or concrete pad outside the building.
Depending on the location of the emission sources, a finisher may spend more money for the capture of the VOCs than for their destruction. The first-time cost and installation of the fume capture ductwork is in addition to the ongoing power costs to push the air through the duct. These costs must be added to the utility costs of running a control device.
This is where solutions such as cascading and recirculating booth exhaust air pay off. They reduce the total amount of solvent-laden air to be conveyed to a control device and perhaps reduce the diameter of supply ductwork as well.
Recirculating spray booth air can be effective in reducing the air volume to be treated. Depending upon the effect of recirculated air upon the product and the personnel in the booth (if the booths are manual), up to 90 pct of the air volume can be recirculated within the enclosure, while only 10 pct of the air volume is treated by a control device. Similarly, cascading air from one booth to another reduces the overall volume of treated air. Since VOC controls are priced by air volume treated, this has an impact on the overall cost of control.
Another way to treat air from isolated emission sources is to treat each one, or small groups of them with modular control devices. The advent of "off-the-shelf" oxidation and adsorption systems, with a modular pre-packaged design, has made this approach economically feasible, or at least comparable, to installing hundreds of feet of ductwork to tie several emissions sources together for a single control device.
This approach also provides a measure of redundancy. If a control device is out of service, often the spray booths or other sources that feed to it must be idled as well. Modular units serving individual sources treat only the air from that source. Therefore, the control device does not become the limiting factor in the plant's production. Stocking common spare parts also reduces risk that failure of a crucial component will shut down production.
These pre-packaged, modular systems are merely standardized versions of currently available technology such as RTOs and rotary concentrators. RTOs are available in horizontal configurations, which makes possible shipment of a single modular piece and fast installation. This design also makes indoor installation a possibility due to the horizontal RTO's low profile and unitized design.
Rotary concentrator systems can also follow this standardized approach, with assembly and shipment of one or two modules for simple rigging and installation. The advantage of pre-piping and pre-wiring of modular adsorption systems is that such work is performed in the manufacturer's shop instead of the field, where labor is more expensive and quality may be more suspect.
Standardization of design and manufacturing for these pre-packaged, modular units have made them affordable to any size finisher. The use of multiple modules is commonly cost-competitive with the costs of a single, large oxidizer or adsorber and the associated long runs of ductwork to connect far-flung emission sources.
The flexibility of these widely proven VOC destruction techniques will continue to be paramount for finishers as the need for regulatory compliance increases. Often the same technology, such as an RTO, can be applied in different sizes or configurations to solve a complex emissions problem. When coupled with air reduction techniques, the payoff for these solutions is more dramatic