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The old ceramic media is removed from the heat-recovery baskets. |
A southeastern U.S. pulp and paper mill needed to rebuild the regenerative thermal oxidizer (RTO) used for controlling emissions from its black liquor oxidation exhaust. The stripper off-gases and concentrated noncondensable gases were high concentration, highly corrosive streams. The mill wanted to complete the rebuild during a scheduled mill outage, which would require any oxidizer contractor to work around the clock to finish construction within the planned shutdown.
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Removal of the combustion chamber, transition duct, media and media baskets was completed in the first three days.
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After considering the options, the mill managers approached Nestec Inc., Douglassville, Pa., although the company was not the original equipment manufacturer. After initial meetings and site visits, Nestec recommended the mill remove and replace the existing combustion chamber, followed by the reinstallation of the top platform, burner, injection nozzles, piping and wiring. Furthermore, Nestec suggested the mill externally insulate and clad the combustion chamber. The insulation would maintain an elevated skin temperature on the combustion chamber, which would prevent condensation of corrosive materials on the skin, explained Kevin Nesbitt, national sales manager at Nestec.
Nestec also recommended removing the existing media, fabricating new heat recovery baskets and adding new media. This would be followed by installing insulation on the lower section, externally and internally. The mill would provide the labor, equipment and materials to build a sloped floor in the outlet of the oxidizer. The design, fabrication and installation of a new rotor wash system would be provided by the Nestec.
According to Nestec, one major change made to the system was to replace the original water-quench nozzle with a cold-air quench to prevent overtemperature conditions in the combustion chamber. This change included designing a new purge-air fan system and integrating the valve’s control into the existing oxidizer controls. Nestec also proposed to design, provide and install a modified control system for the burner.
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The old ceramic media had broken down and needed replacing. |
Once the plans were in place and approved, all was in place to complete the job during the planned mill shutdown. Upon arrival at the mill, the Nestec team completed a walkthrough and safety review. As with any complex equipment rebuild project, the contractor was concerned about unseen conditions and damage requiring rapid adjustments in the field.
Once initial concerns were addressed, work began in earnest. Removal of the combustion chamber, transition duct, media and media baskets was completed in the first three days. The new combustion chamber was fabricated from 0.25" thick, 253-MA alloy steel with 310 stainless steel refractory anchors, constructed as two sections to be located above the heat-recovery chambers. The oxidation chamber was designed and constructed with a bolted and davited access door for routine inspection of the burner and internal insulation. Upon final installation, Nesbitt noted, the chamber’s flange connections were bolted and gasketed to ensure airtight construction. This effort was followed by the reinstallation of the top platform, burner, injection nozzles, piping and associated wiring.
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The combustion chamber required internal and external insulation. |
Nesbitt noted the project was completed successfully within in the scheduled downtime. During the complex process, the rebuild team continually worked diligently with vendors and fabricators to maintain the scheduled completion date in spite of unexpected delays. As of August 2011, the rebuilt equipment has been operating efficiently and problem free for six months.
To learn more about Nestec, call (610) 323-7670 or visit www.nestecinc.com.
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