Feeding the System

Computer programmers have a saying: GIGO. It means “garbage in, garbage out” … if you enter bad data, you’ll get bad results. Waste facilities have a new riff on that. Call it GIFO: garbage in, fuel out. The proper preparation of MSW (municipal solid waste) or postindustrial waste inputs is key to getting a high-quality, burnable product as output.

Most operations are quite careful about the material they prepare for their refuse-derived-fuel (RDF) facility. The size and quality of the material are monitored. Nothing is left to chance. At plants around the country, the goal is to reduce the amount of waste going into landfills while capturing as much burnable energy as possible.

“We produce a high-Btu fuel with little to no chlorine,” says Jay Saxton, general manager at Balcones Fuel Technologies, Little Rock, Ark. Balcones’ typical end product is in excess of 10,500 Btu and typically gets a 12,500 to 13,000 Btu output, according to Saxton.

Balcones is fortunate to have a fairly consistent blend coming into its plant. “Our feedstock hasn’t changed much,” Saxton says. All of its material is postindustrial waste. The bulk of its input product is nonwoven scrap fabrics from filters, baby care products, car covers and items like reusable grocery sacks that do not make the grade.

In addition, Balcones runs some post-industrial food plant waste. “This is not food, itself,” Saxton says. Rather, it is waxed cardboard used to package food, plastic packing bags that do not pass muster for some reason and other packing that is contaminated with breading or fat residue.

Balcones also processes waste from a peanut butter plant. “The residual peanut oil burns great,” Saxton says, adding that the PET (polyethylene terephthalate) jars have great Btu value.

No matter what the material, it is shredded, run through a metering bin, mixed further and then processed. Balcones does not accept material with metals. However, the company runs drum magnets to grab any contaminants that may be in a load.

The company shreds its product to 4-inch minus. Saxton says Balcones found its optimum size by trial and error.

A good shred is one key to the process. “We have slow-speed shredders running at low rpm,” Saxton explains.

Municipal System
Other RDF fuel operations have less specific inputs. One of the oldest waste-to-energy facilities in the country is run by the Solid Waste Authority (SWA) of Palm Beach County, Fla. Its feedstock is varied.

Originally known as its North County Renewable Energy Facility, the operation now is known as Renewable Energy Facility #1 (REF #1). It processes 2,000 tons of curbside collected MSW per day and produces 1,800 tons per day of RDF, six days per week. “We went with RDF to hedge our bets,” says Ray Schauer, SWA director of engineering.

A second facility will soon come online. Called REF #2, its construction is about 30 percent completed. A mass burn facility, it is expected to process 3,000 tons per day with its three 1,000-ton per day boilers.

Palm Beach County got started with RDF in 1989, when waste to energy was just coming into the public eye. Schauer says the county talked about a MRF (material recovery facility), recycling and other options and decided to go the waste-to-energy route.

The front-end of the SWA’s REF #1 facility is a primary shredder. “We size material and send it through trommels,” Schauer says.

“Our raw material is residential and commercial MSW that is collected curbside and in roll-off type containers,” Schauer says. They have an aggressive source-separated dual-stream recycling program that includes newspaper; mixed paper; cardboard; and bottles and containers, whether glass, plastic, aluminum or ferrous.

REF #1 comprises four general areas: the MSW (municipal solid waste) receiving area (tipping floor); the RDF manufacturing building; the fuel (RDF) storage building; and the power block, which includes the fuel feeding equipment, boilers, power generation equipment and the ash handling equipment.

The MSW processing equipment is located on the front end of REF #1. Waste arrives on the tipping floor and is inspected and fed on to a conveyor. Then it is inspected again and conveyed to a flail mill that coarsely shreds the MSW. The output of the flail mill is conveyed through a magnet for ferrous separation to a trommel screen. The first stage of the screen removes the small fraction (less than 2-inch) as precombustion residue. This small fraction is later cleaned with air classification, the light fraction being combined with the RDF and the heavy fraction further processed for metals removal.

Scroll to continue with content

The second stage of the trommel screen removes the MSW fraction that is less than 6 inches and typically consolidates the containers. Nonferrous metals are removed via eddy current separation, and the balance is conveyed to the RDF storage building.

The fraction of the MSW that is larger than 6 inches is conveyed to a secondary shredder for further sizing and is reprocessed to further remove metals prior to being conveyed to the RDF storage building. The resulting RDF product is typically characterized as a loose material between 2 and 6 inches.

“We take what is left over at the RDF facility for further processing,” Schauer says. The front end equipment is designed to size the material to between 2 and 6 inches and remove the ferrous and nonferrous metals. The facility also includes recovery of both metals from the resulting ash on the back end of the combustion process.

The resulting RDF product is a loose product that typically includes the remaining paper, plastic, wood and organics.

Sizing Material
The idea at most RDF sites, including Palm Beach County’s, is to produce a feedstock material of 2 to 4 inches in size. The 2-inch minus comes out the bottom. Anything larger than 6 inches goes back for further shredding.

“Conveyor speeds and feed rates can be adjusted,” Schauer says. However, major adjustments rarely are necessary.

“We are manufacturing RDF specifically to provide for the disposal of solid waste and the generation of clean renewable energy as part of an integrated waste management system,” Schauer says. Other operations have a somewhat different focus.

Vexor Technology, Medina, Ohio, gets its material from single-stream plants. Steven Berry, president of Vexor Technology, emphasizes that its product is not classic RDF but an engineered product designed to meet its customers’ specific requirements. “We do not compete with recyclers,” he says. “We work hard to differentiate ourselves.”

Rather, Vexor takes nonrecyclable plastics, waxy cardboard and other materials that do not have economic value in the traditional recycling stream or cannot be processed economically and mix it with other materials to produce an engineered fuel.

Coming onto the line, the material ranges up to 1 foot in size. Grinders and shredders take the material down to three-quarter-inch-minus particle size. “That’s what our customers request,” Berry says.

The material, he explains, typically gets delivered to cement and power plants.

The fuel Vexor produces provides 10,500 Btu per pound. At most RDF plants that rate their product on a quality output basis, the material typically will be compared to coal for its energy value. Bituminous coal produces from 10,500 to 15,500 Btu per pound, according to the American Coal Foundation. Lower grade lignite coal produces only 4,000 to 8,300 Btu per pound.

At Vexor, magnets remove metallic fractions both at the front end and at the back end of the process.

Metal is a bigger problem at Palm Beach County’s tipping floor. There, they typically must sort out large bulky items such as mattresses, refrigerators, washers, dryers, freezers, engine blocks, etc. Those items are collected and processed for recycling whenever possible at other on-site facilities.

Hazardous materials—typically propane tanks, in this case—are manually picked out on the tipping floor. Ferrous metals are magnetically separated early in the process, and aluminum is removed by eddy current and manual picking. A proprietary system is used to collect coins.

Interestingly, the SWA recovers almost as much metal on the back end of its process by sifting the ash as it recovers in the daily curbside recycling stream.

Testing is an important part of the fuel producing process. Berry notes that Vexor tests 32 different parameters ranging from moisture content to heavy metals. There is quality control (QC) testing both at the start and on the end product.

“We have QC tests to assure the proper mix of material,” Berry says.

Dealing with Feedstock
Inconsistency in the feedstock is a fact of life every facility has to contend with in the process.

Balcones Resources is fortunate to have a relatively consistent feedstock stream, Saxton says. He describes the end product as a cube looking much like a stick of modeling clay.

“Depending on the material and the ratios, it may have a smooth exterior or be ruffled,” Saxton says.

The one thing he does not want coming into his feedstock is moisture. That is a high hurdle in humid Arkansas. “We definitely do not want an abundance of moisture,” Saxton says. “We try to keep it sub-20 percent. Under 15 percent is better. We want it as dry as possible.”

Although the holding area is under roof, the large volume of baby wipes and infant care products makes that a tough standard to meet.

“They use a super-absorbent polymer in their product,” says Saxton. “It grabs moisture out of the air.”

That is only one of the many outside factors that make dealing with feedstock challenging for fuel producers to handle in a consistent and economic way. The challenge is bigger when the mix can change daily.

“MSW, by its heterogeneous nature, is difficult to deal with. The economics are a balancing act,” Schauer says.

SWA’s REF #1 site typically runs at full capacity. Three identical RDF processing sites are providing approximately 150 percent of the required capacity.

“RDF is a good landfill alternative,” Saxton says.

The author, based in Cleveland, is a contributing editor to Renewable Energy from Waste and can be contacted at curt@curtharler.com.