Coal, O&M

Self-cleaning Filters Unclog Wet Scrubbers

Issue 6 and Volume 110.

By Teresa Hansen, Associate Editor

American Electric Power Co.’s (AEP’s) Gen. James M. Gavin Plant in Cheshire, Ohio, contains two 1,300-MW units built in the 1970s. Together, the two units burn about 7.5 million tons of coal per year. The plant uses a super-critical steam cycle which boosts electrical output per ton of coal, resulting in 10 percent better efficiency than the national average. In 1994, AEP spent $700 million to install the world’s largest flue gas desulphurization system, cutting SO2 emissions from a peak of 400,000 tons per year to 25,000 tons. Its electrostatic precipitators cut fly ash by 99.7 percent. In 2001, the plant’s selective catalytic reduction (SCR) system went on line, cutting summer ozone season NOx emissions 90 percent to 3,000 tons.

Maintaining these emission reductions requires proper maintenance of the pollution control equipment. The plant has wet scrubbers in both of its 830-foot stacks. They were doing an adequate job of cleaning particles out of the air, but particulate matter in the water was not good for the equipment.

“We were having a problem with a good bit of solids and small particles going through and plugging up nozzles,” says Randy McFann, Gavin Plant’s production services leader. “We weren’t getting the performance out of them that we needed.”

Because the wet scrubber system didn’t work well on its own, the burden to keep it running was on the plant maintenance staff.

“At least once a month, we would have to shut down the equipment for cleaning; a process that would take two or three people a day and a half to complete,” says McFann.

To reduce the maintenance load, plant staff began experimenting with automatic filters from different manufacturers, but still didn’t find just what they needed.

“We tried several kinds of filters, but they didn’t seem to give the performance we needed,” says McFann. “They were plugging quite often, and were letting material through that shouldn’t make it through.”

The staff continued to experiment with different filters to see how they could improve the process. AEP selected Tekleen filters by Automatic Filters Inc., Los Angeles, for one stack to see how they performed.

“Their specs looked good, so we contacted the vendor, installed their filters and found that they performed a lot better than what we had,” says McFann.

These filters incorporate a self-cleaning mechanism that allows an uninterrupted flow of filtered water even while the cleaning takes place. The self-cleaning procedure uses only a fraction of the water that’s required for normal back-flushing. In this design, the dirty water flows in around the outside of a coarse filter that removes the larger particles. This prefiltered water then flows to the other end of the filter housing where it passes from the inside to the outside of the final filter and then through the outlet. Over time, as the filter removes particles from the water, those particles clog the filter, reducing water flow and water pressure. At the outlet is an adjustable pressure differential switch, typically set at five to seven pounds. When that threshold is crossed, a small flush valve opens, initiating the cleaning procedure.

Rather than use the full water flow to initiate a complete backwash, the Tekleen filters incorporate a number of small cleaning nozzles arrayed around a central shaft. The water pressure within the cleaning unit decreases when the flush valves open, and the nozzles vacuum the dirt from the inside surface of the filter screen. A hydraulic motor and piston cause the nozzles to rotate and move axially, allowing them to cover the entire screen surface in five to ten seconds. At that point, the flush valve closes and the cleaning mechanism returns to its starting position. Except for a small control voltage needed for the differential pressure sensor and to actuate the flush valve, all motions involved in cleaning the filter are performed using the water pressure.

Because the installation on the first stack was successful, AEP installed Tekleen filters on the second stack. The plant now uses two 12-inch Tekleen filters in parallel for each of the stacks, processing a total of 2,500 gallons of water per minute. McFann says that the Tekleen filters operate maintenance-free.

“We don’t have nearly the problems we used to,” he says. The plant is now achieving its goal of reducing scrubber maintenance.

“We have achieved a considerable savings in maintenance and equipment,” says McFann.