Precipitator Solves Dryer Problem


Plum Creek Manufacturing, Columbia Falls, Montana, installed a PPC Industries electrostatic precipitator to solve a direct fired dryer dust plugging problem. The electrostatic precipitator was installed in a very congested area and the project had to be done so the dryer was off-line a minimal amount of time. Plum Creek personnel relocated existing equipment, poured concrete footings, ran all of the field wiring, and installed the ductwork. All facets of the project were completed on schedule and the operating unit has achieved the expected goals.

The Coe Manufacturing jet tube dryers at the Plum Creek plywood plant are direct fired with a 25 mmbtu/hr Wellons wood fired fuel cell as a heat source. The fuel cell is followed by a blend chamber to cool the gases from 1800 degrees to a usable temperature using recirculated air returned from the veneer dryers. Hot gases from the blend chamber flow through a Zurn multicone to remove some of the larger particulate and wood fiber picked up in the recirculation air. Even with the multicone in the loop, the ash content of the flue gases was still sufficiently high to cause plugging of the orifices (jets) in the dryer tubes. Production was lost because of ash plugging the dryer jets. As the dryer jets plugged, the drying time increased and production dropped. The ash had to be manually removed with fire hoses from the veneer dryer. Cleaner flue gases were needed to stop ash plugging and regain the lost production from the dryer. The dryers had to be cleaned at least once every week.

A related problem was also caused from deposits of ash on the veneer left during the drying operation. The ash deposits caused a handling problem for plant personnel. If the dryer stopped too long, veneer would be badly discolored with ash and accelerated wear of mechanical parts of downstream machinery.

The most logical device for cleanup of the flue as stream was an electrostatic precipitator. The advantages of low pressure drop, low power consumption and high efficiency overshadowed any other device. Limited fan capacity and downtime also favored the use of an electrostatic precipitator, since electrostatic precipitators have only 0.5” pressure drop and are low maintenance devices.

PPC Industries had built several electrostatic precipitators for Plum Creek Manufacturing at other plants. The units installed on hog fuel boilers have worked very well and have been very reliable. PPC personnel were sure an electrostatic precipitator would clean the gases, but weren’t sure what other problems the electrostatic precipitator would introduce.

This application was unique in that the flue gases were recirculated. It was unknown if the ozone from the electrostatic precipitator would build up and it was uncertain how this would affect the operation of the unit. Other unknowns were high flue gas temperature and the high flue gas moisture. Finally, the level of removal was unknown since it was unclear how much particulate could be tolerated and still keep the orifices from plugging.

PPC had worked with another plant to develop a slip stream unit to remove most of the particulate. However, PPC opted for processing the entire gas stream because of the various unknowns. Gas flow was difficult to measure; however, the gas stream was finally tested to set the design flow rate and temperature.

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