Regenerative Thermal Oxidation of COCs

Contents

•Application to COC Fumes
•Operating Problems
•Application to Acid Fumes
•Industrial Installations
•Typical Industrial Installations of Improved RTO Units
•Conclusion

Dilute VOCs in emissions, such as from the production and application of solvent-based coatings, have been successfully incinerated by regenerative thermal oxidation (RTO) during the past 20 to 25 years. The process has found wide acceptance for its efficient elimination of toxic compounds and odor control and for its spare use of auxiliary fuel. (A large fraction of the net heat required for the process is obtained from the combustion of the organic contaminants, the balance being supplied by the auxiliary fuel to the burners.)

However, condensable organic compounds (COC) have long been troublesome to collect and treat by RTO to meet present emission standards.

Application to COC fumes

To develop the technical feasibility of the process for condensable organics, a three-bed conventional RTO unit was installed on an asphalt-roofing-paper production line in the province of Quebec.

Serious operating problems were soon encountered due to the accumulation of liquid and solid residues in the entrance ducting and valves and the lower regions of the packed beds. These deposits in the beds ignited when allowed to accumulate, exposing the discharge ducting and blower to serious risks of mechanical failure from overheating. Periodic removal of the deposits could only be carried out by ignition of "bake-outs," which were time consuming and, being difficult to control, prone to excessive temperature excursions.

These problems resulted in destruction efficiencies of 84.6% for total organics and of 89.3% for PAHs—well short of expectations.

Destruction efficiency

The inadequate destruction efficiency could be attributed to three possible causes,

• incomplete combustion;
• leakage of control valves; or
• evaporation of condensable deposits in the bed.

Incomplete combustion was ruled out because CO concentrations were less than 10 ppm at the exhaust. Measurements made after adjustments and verification of the valves showed that they could not account for the 15% of unreacted hydrocarbon vapors in the exhaust. That left the re-evaporation of deposits in the lower zones of the beds as well as in the inlet ducting.

The process was modeled with a computer program to determine temperature profiles, deposits, and re-evaporation in the bed from adjusted design and operating parameters. The computer model was based on a method described by A.J.Willmot (J.Heat Mass Transfer, 1964), with adjustments to account for a purging cycle. The exercise indicated the effect of the cycle time and of the purge gas flow and temperature on the removal of deposits in the bed. These and other design parameters were adjusted for optimal conditions and used in the later, Biotox, design.

A gas burner was incorporated to heat the purging gas, and adopted as a distinct feature of the Biotox process.

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Operating problems

To protect the equipment from damage due to unscheduled heating in the bed, a sensitive temperature monitoring-and-alarm system was added. An emergency-control sequence was devised to permit the evacuation of the fumes from the bed directly to the atmosphere by means of a vent trap in the combustion chamber and eliminate the risk of overheating.

The complete elimination of condensables in the raw gases fed to the RTO would be achieved by pre-heating them above the dew point of the condensable phase—a practice proved to prevent fouling of the equipment and all the related production losses and maintenance costs in four recent installations.

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Application to acid fumes

In order to treat the VOCs and odorous reduced-sulfur compounds in the non-condensable gas emissions from kraft pulping, special design considerations went into corrosion prevention, high destruction efficiency, and safety.

The risk of acid corrosion by sulfuric acid condensation on the shell and ducts was, in part, addressed by mixing a recirculated portion of the stack gases into the fumes at the inlet to preheat them. In addition, of a special acid-proof coating was used.

Strict emission standards required a destruction efficiency of the reduced sulfur compounds of 99.5%. Well-sealed valves featuring a special purging step achieved this.

Assurance from explosions in the collection system was achieved by careful attention to separating the high- from the low-concentration stream with separate introduction into the RTO.

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Industrial installations

The process improvements were developed initially to treat CDC emissions from the production of asphalt roofing paper and shingles. Six units were installed in this industry from 1991 to 1998, with an aggregate flow of 97,000 scfm. On the basis of an average emission concentration of 150 mg/Nm3, the organic compounds treated amount to about 550lbs/hr or 2200 tpy.

The process later was applied to treat CDC emission with a high load of PAH (polyaromatic hydrocarbons) produced during the preparation of green anode paste, by the hot-mixing of coal-tar pitch with petroleum coke. This was the first time these fumes were treated by the RTO process. The unit has operated without interruption since the summer of 1997.

The process also was applied, with some additional modifications, to treat the non-condensable VOC and reduce sulfur emissions from various unit processes in a kraft pulping plant. The table lists the main features of one typical installation for each of these applications.

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Typical Industrial Installations of Improved RTO Units

Notes:

1. Stack flow as measured during sampling of emissions.
2. Based on inlet and outlet concentrations times outlet flow.
3. Rates of gas temperature drop in discharge bed over gas temperature rise between inlet and combustion chamber.

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Conclusion

Emissions of condensable organic compounds from hot asphalt-coating and mixing processes can now effectively be incinerated by means of major improvements to the regenerative oxidation process.

The process meets the local emission requirements at a reasonable fuel demand and provides the needed equipment availability to meet environmental standards.

About the author: Jean J.O. Gravel is vice-president, Biothermica, 3333 Cavendish Blvd Suite 440, Montreal, PQ H4B 2M5

This article was adapted from a presentation at the 1999 International Incineration Conference, Orlando FL.