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Oxidizer Selection and Technology Options

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Oxidizer Selection and Technology OptionsAs thermal oxidizer technologies have been applied to a wider range of industrial applications, new challenges are continually uncovered that test their efficacy and durability.BY JIM STONE

S electing a control device for industrial or-ganic gaseous emissions is an easier pro-cess today than it was when air pollution control regulations were first introduced.

This is due in part to years of reliable service by a number of mature technologies. Facility owners and project consultants have a variety of options at their disposal to meet regulatory compliance with a mini-mal impact on everyday operations. The most widely used oxidizers employ a successful combination of residence time, conversion temperature, and turbu-lent mixing in the combustion chamber to eliminate gaseous emissions.

A cursory review of these technologies reveals that four main exhaust stream characteristics are key to selecting the right control device: pollutant

type, pollutant concentration, airflow volume, and airflow temperature. It is vital to properly character-ize, identify, and measure process conditions for the simple reason that lower airflow volumes result in a smaller oxidizer and thus lower the capital and oper-ating cost investment. Another advantage to reduced oxidizer size is there are fewer secondary emissions from the oxidation process, meaning that less carbon monoxide, oxides of nitrogen, and carbon dioxide byproducts are produced from the conversion pro-cess. This may aid greatly in the permitting process for the control device.

Thermal and Catalytic OxidationAll thermal oxidizers use heat to accelerate the oxi-dation process of combining organic pollutants with

AIR POLLUTION CONTROL

www.eponline.comEDITORIAL STAFFEDITOR Jerry LawsE-NEWS EDITOR Brent DirksSENIOR EDITOR Lindsay Page CONTENT DEVELOPMENT Matt Holden

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T his month we bring you the second Environmental Protection section of 2016, an extension of the print magazine by that name and the

www.eponline.com website maintained by our company, 1105 Media Inc. The content we showcase at that site—news stories, fea-tures, new products, white papers, videos, and more—is intended to help EHS profes-sionals stay abreast of developments in this all-important regulatory arena.

This section includes a feature article from Anguil Environmental Systems’ Jim Stone on oxidizer technologies for control-ling industrial organic gaseous emissions. It’s a tremendously valuable article because readers can use it to pinpoint the specific technology that’s ideal for their own opera-tion’s exhaust stream.

Something else to keep an eye on this month: EPA has given $4.8 million in grants to six universities to work with their local communities to better understand the eco-

nomic value of water quality. Clark Univer-sity (Worcester, Mass.); Dartmouth College; the University of Connecticut; North Caro-lina State University; Michigan State Uni-versity; and Iowa State University are getting the grants.

“Clean water is a cornerstone of a healthy community. Many communities face chal-lenging decisions about investing in the protection of water resources,” said Thomas Burke, deputy assistant administrator of EPA’s Office of Research and Development. “These grants will help measure the costs and benefits of improving water quality, an important step toward protecting the envi-ronment and human health.”

EPA says the research will “provide a critical link between water quality science and the monetary value of the services that healthy waterways provide, including rec-reational uses.” Jerry Laws (jlaws@1105media.com) is the ed-itorial director of Environmental Protection.

A Focus on Air Pollution Control and Water QualityThis EP section is about selecting the best control technologies for industrial organic gaseous emissions.By Jerry Laws

FROM THE EDITOR

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www.eponline.com NOVEMBER 2016 | Environmental Protection EP3

oxygen in an enclosed chamber. Tem-peratures in excess of 1,400 degrees F are typically sufficient to convert most organic pollutants to carbon dioxide and water va-por at a rate that is usually acceptable by permitting agencies—98 percent or better. Thermal oxidizers are typically steel enclo-sures that feature some sort of combustion chamber with a fuel source, most often a gas burner. The need for fuel from natural gas or some other source often dictates the use of heat recovery in some form.

Direct-fired thermal oxidizer (DFTO)The direct fired version of the ther-

mal oxidizer is a simple “flame in a box” that achieves volatile organic compound (VOC) conversion at a high rate. The lack of any preheat or heat recovery in these models means that the burner is sized to increase the chamber temperature from the inlet temperature to about 1,400 degrees F, which also means that these oxidizers can be copious consumers of natural gas. This consumption is mitigated by the ca-loric content of any incoming pollutants, so DFTOs are applicable to a wide variety of inlet VOC concentrations. Common practice generally restricts their usage to VOC-rich airstreams and air volumes of less than about 10,000 standard cubic feet per minute (SCFM). Anything outside of these parameters is typically more suitable for oxidizers with heat recovery.

Recuperative thermal oxidizerThese oxidizers differ from the direct-

fired version by the use of a metal alloy heat exchanger, which transfers heat from the purified gases leaving the unit to the incoming polluted air stream. By raising the incoming process gas temperature, fuel consumption is reduced by a range of 50-70 percent compared to a similarly sized direct-fired oxidizer. Recuperative types are most effectively applied on airflows up to about 20,000 SCFM with higher incoming pollutant levels, and in cases where process heating can be achieved with the waste heat from the oxidizer.

Catalytic thermal oxidizersThese are similar to recuperatives, ex-

cept that a catalyst bed is inserted in the unit that allows the VOC oxidation to take place at a lower combustion temperature—usu-ally between 500 degrees F and 800 degrees F. Because of the lower temperatures in-volved, there are several benefits: Materials

of construction are less expensive, fuel us-age decreases, and there are lower amounts of secondary emissions from the combus-tion process such as carbon monoxide and oxides of nitrogen (NOX). What’s more, they can often be operated with electric coils as the supplemental heat source for startup and conversion temperature maintenance, eliminating the need for a gas burner.

Typical catalyst types are precious metal or base metal, and they do have a period of maximum efficacy before regeneration or even replacement is required. This window can be in the three- to four-year range, so catalyst cost needs to be figured into the life cycle equation. Finally, there are any num-ber of halogens, metals, non-solvent resins, and other materials in exhaust streams that can contaminate and mask catalyst effec-tiveness, so proper application is vital.

Regenerative thermal oxidizer (RTO)RTOs are wonderfully adaptive oxidiz-

ers. An RTO maximizes heat recovery from combustion to pre-heat incoming pollut-ant-laden air, which reduces auxiliary fuel consumption. These units are character-ized by multiple beds of ceramic heat ex-change material capped by a combustion or conversion chamber where the VOCs are oxidized. Process exhaust air is switched from one bed to another—as one bed re-leases heat to the exhaust stream before the

burner, another one absorbs the heat in the stream after the burner, which results in very high heat recovery (up to 97 percent).

These units can be used for a variety of applications with the proper design—halogen destruction, batch or continuous operation, low VOC inlet loads, high air volumes, and more. They destroy VOCs at a very high rate (99+ percent). However, RTOs can be very large and heavy. While the current generation is very reliable, they do have more moving parts than any other type of oxidizer.

Vapor combustorsThese control devices are similar to flares

but are enclosed so that there is no visible flame. They handle varying emission flow rates and concentrations with a very high destruction efficiency of up to 99.99 per-cent. Vapor combustors can be used on both intermittent and continuous airstreams and are very low-maintenance devices with no heat recovery. Their application is generally limited to airstreams with very high pollut-ant levels. The high emission concentrations generally supply the fuel for conversion in most standard designs.

AdsorptionEmission concentrators

Pollutant concentration level in the ex-haust stream is one of the most important

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This 40,000 SCFM RTO was applied on a pharmaceutical application to destroy emissions from tray ovens, fluid bed dryers, and coating pans.

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AIR POLLUTION CONTROL

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factors in selecting the proper control de-vice, but the airflow volume itself is what determines how large the unit is. There-fore, it is an advantage to reduce the total air volume that actually has to be oxidized in a combustion chamber. By using a hy-brid control system that combines solvent adsorption and thermal oxidation, it is possible to reduce the combusted exhaust by up to 20 times its original volume while enriching the concentration a similar ratio to reduce fuel usage.

Typical solvent emission concentrators operate using hydrophobic zeolites embed-ded on a slowly rotating wheel, adsorbing the pollutant-laden exhaust air continu-ously. At any one time, a small portion of the wheel containing the adsorbed VOCs moves through a heated regeneration, or desorption, zone where the VOCs are stripped from the wheel and conveyed to a small thermal oxidizer for final destruc-tion. This use of temperature swing adsorp-tion means that concentrators are effective only on exhaust airstreams of about 100 de-grees F or less, but typical large air volume applications such as paint spray booths are a good fit, given that they also are relatively dilute in pollutant concentration and will

benefit from the associated auxiliary fuel usage reduction. Emission concentrator systems have few moving parts and con-tribute very minimal byproducts of com-bustion to the purified air stream. They are suitable for airflows of up to 100,000 SCFM or more, and the emission concentrator can be coupled with a variety of thermal oxidiz-ers for final conversion of the VOCs.

Special IssuesAs thermal oxidizer technologies have been applied to a wider range of industrial ap-plications, new challenges are continually uncovered that test their efficacy and dura-bility. In the past, only the most appropri-ate pollutant gas streams were treated by thermal oxidation. However, the oxidizers’ ability to successfully reduce emissions, coupled with new uses of solvents and other VOCs in production processes, have required that they evolve to stay relevant in the air pollution control industry.

For example, the presence of halo-gens in a process exhaust stream presents a number of pitfalls for standard thermal oxidizer designs. When combusted, halo-gen compounds will form acids that will attack and corrode carbon steel shells,

heat exchangers, and internal structures of thermal oxidizers. Thus, proper selection of corrosion-resistant alloy construction materials is the first line of defense against this phenomenon. Additionally, because organic acids will then be present in the otherwise purified exhaust air leaving the oxidizer, a means of neutralizing the acid will be necessary. Wet scrubbers are typi-cally used with great success to first quench the oxidizer exhaust, then neutralize it with a caustic substrate.

Another challenge to the modern ther-mal oxidizer is the presence of silicone in the incoming pollutant air stream. When oxidized, silicones will form solid byprod-ucts that can easily foul and obstruct the passages of both metal and ceramic heat exchangers found in today’s oxidizer units. While it’s not typically possible to avoid the byproduct formation, special attention must be made to the heat exchanger se-lection to ease cleaning and maintenance and to avoid costly downtime due to cata-strophic plugging of the heat exchanger that restricts airflow passage. It is also advisable to design access doors to the thermal oxi-dizer—especially those adjoining combus-tion chambers and heat exchanger matri-ces—with proper care to allow for frequent and effective removal of accumulated solids.

Proper characterization of the pollut-ant airstream is paramount to guarantee that the right design strategies are then employed in selecting and building the thermal oxidizer. Such care puts the end user and oxidizer manufacturer on the same side of the profit equation, virtually ensuring a positive outcome for both par-ties and avoiding costly production shut-downs or regulatory fines associated with non-compliance.

Jim Stone is Senior Sales Manager for An-guil Environmental Systems, a global leader in exhaust air purification. His role includes new and existing account development for the airborne pollution control market, as well as product development of energy-ef-ficiency improvements for pollution control systems worldwide. He has more than 20 years of experience with product and busi-ness development in the air pollution con-trol and hazardous materials industries. To contact him, phone 414-365-6400 or email jim.stone@anguil.com.

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Catalyst within an oxidizer allows oxidation to take place at lower combustion temperatures, which can lower equipment costs, operating expenses, and greenhouse gas emissions.

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