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Technical Article Series

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Circular Vibratory ScreenersProvide Low-Cost Ash Processing

Circular Vibratory Screeners Provide Low-CostAsh Processing

Four 48 in. (1219 mm) diameter Flo-ThruVibroscreen® circular vibratory screeners,two each for bed ash and fly ash, scalpoversize particles that could otherwiseplug pumps moving a slurry of bed ash andfly ash from mix tanks to settling ponds.

JACKSONVILLE, FL The JEA Northside Generating Station had twoexpensive crushers approaching the end of their service lives. JEA knew thatreplacing the aging crushers, a necessary component in the ash-processingsystem, would entail major capital costs. The crushers performed the essentialservice of preventing bed-ash particles larger than 3/16 in. (7.6 cm) fromentering the disposal process, since oversize particles would plug the pumpsthat move the ash slurry to a settling basin. The company wanted to find away around the cost and operational limitations of the crushers while stillensuring that oversized particles were separated.

The benefits JEA targeted were ambitious: (1) avoid the estimated $250,000expenditure to replace two 30 in. (76.2 cm) diameter, double-roll crushers; (2)eliminate disruptive installation of these large pieces of equipment andassociated labor expenses; and (3) reduce future maintenance requirements,which typically incurred downtime of a half-day or more. An ideal newapproach would enable JEA to improve the coal- and petroleum-coke-firedstation's ability to keep its baseload-power operation running with almost noshutdown for crusher-related maintenance.

Reducing Capital and Maintenance Costs

The Northside Generating Station operates two of the largest circulatingfluidized bed (CFBs) combustors in the world, each capable of producingnearly 300 MW and generating a total of 2,000 tons of ash daily, 60 percentbed ash and 40 percent fly ash. During the time JEA was exploringalternatives to the processing of bed ash using double-roller crushers, it wasalso testing a circular vibratory screener for processing fly ash. JEA wanted toreplace a 2 sq ft (0.185 sq m) shaker screen, which required constantmaintenance, with a circular vibratory screener, and reduce maintenancerequirements. While seeking this replacement screener in the fly-ash line, itoccurred to Jerry Kowalski, a structural engineer at JEA, that a circularvibratory screener might also be a good candidate for separating oversizedparticles in the bed-ash line, thus eliminating the expensive crushers.

"Although fly ash is typically as fine and light as talcum powder, moisture in thecirculating gas stream of the boiler can cause ash particles to form clumpslarger than 3/16 in. (7.6 cm)," said Kowalski. "For this reason, oversizedparticles must be screened from the fly ash."

Bed ash, which is coarse, heavy and resembles beach sand, exits the boilersas glowing grains close to 500ºF (260º C) and is carried away from the boilersvia a 100 ft (30 m) drag-chain conveyor. The conveyor deposits the bed ashinto a 600 ft (183 m) long pneumatic line that transports it to a dedicatedbed-ash silo, where the ash arrives cooled to approximately 250ºF (130º C).After the ash cools to about 150ºF (66ºC), it is put through the crusher, thendrops by gravity 8 ft (2.4 m) directly into a mix tank below. The fly ash entersthe mix tank the same way. Both the bed ash and fly ash are mixed into aslurry made up of "on-size" particles in a single tank, then pumped 4,000 ft(1219 m) to settling ponds. There is one bed-ash and one fly-ash silo foreach of the two combustors, totaling four silos.

On-size fly ash and bed ash particles from48 in. (1219 mm) diameter screeners form aslurry in pre-mix tanks.

Slurry of on-size bed ash and fly ashparticles is pumped from mix tanks tosettling ponds.

Separating 1% of 'overs' more efficiently

"Since oversize particles comprised less than 1% of our bed ash, the use ofcrushers was overkill, leading us to consider circular vibratory screening forscalping of bed ash in addition to fly ash," said Kowalski.

Working with Kason technical representative Bernie Petrone, of SouthernProcess Equipment Co., Kowalski installed two 48 in. (1219 mm) diameterFlo-Thru Vibroscreen® screeners, high-flow-rate units fabricated fromepoxy-coated carbon steel and equipped with a single vibrating screen deck.Kowalski concluded that, given the small percentage of "overs" in the bed ash,a single screen would suffice. The same two screeners were installed in thefly-ash line.

Each screener uses two, externally mounted, imbalanced-weight gyratorymotors which provide multi-plane inertial vibration that causes on-sizeparticles to pass through apertures in the screen while oversize particles travelacross the screen surface in controlled pathways to a discharge spout at theperiphery. On-size material passes through the screen at a rate of 50 lb/min(23 kg/min) to the bottom outlet directly below the inlet into the mix tank below.With the bottom outlet located directly below the top inlet, material fallsvertically through the screen at high rates, permitting the screeners to keep upwith the volumes of bed and fly ash the JEA plant processes daily.

Today the two bed-ash and fly-ash lines are equipped with four high flow ratecircular screeners replacing the square shaker screens and crushers. Thecircular screeners have enabled JEA to meet its goal of reducing maintenanceon the bed-ash line and reducing screener maintenance by virtue of theirepoxy-coated, stainless-steel, corrosion-resistant construction.

Major changes yield dramatic benefits

"The 48 in. (1219 mm) diameter screeners met the criteria we needed torationalize switching technologies," Kowalski said. "First, the circular screeneris about one-tenth the cost of a double-roll crusher. This cost reduction alonewould have justified the change, but there were other benefits as well. Thenew unit was small, which meant that we could install it easily and have it upand running quickly. Moreover, the small footprint of the unit would open upthe work area and provide easier access for personnel and greater freedom ofmovement when maintenance needs to be done. There has been nomaintenance requirement in the months since these units were installed, butwhen any maintenance is required it can be accomplished in minutes insteadof hours. All we have to do is remove the single screen and insert areplacement. Rapid maintenance is important, because we run the screeners24/7."

Kowalski reports that the new screeners allow removal of oversized materialcontinuously. The "overs" build up in a special tank, and at one-to-two-weekintervals, are transferred by truck from the tank to a settling pond. "The use ofcircular screeners instead of crushers has proven successful for us," Kowalskisaid, "and they are performing well in both our fly- and bed-ash processingareas."

Oversize material discharged from thescreeners goes into receiving tank.