Computer programmers use the acronym 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 waste-to-energy facilities are quite careful about their infeed material. 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 more than 10,500 Btu and gets an output of from 12,500 to 13,000 Btu, Saxton says.
Balcones is fortunate to have a fairly consistent blend coming into its plant. “Our feedstock hasn’t changed much,” he says. All of the company’s 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 postindustrial 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 might have breading or fat residue.
The company also processes waste from a peanut butter plant. “The residual peanut oil burns great,” Saxton says. And 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 plant is equipped with drum magnets to remove ferrous contaminants that may be in a load.
Balcones produces a 4-inch-minus product. Saxton says the company 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.
The input at other RDF (refuse-derived fuel) fuel operations may not be as defined.
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.
“We went with RDF to hedge our bets,” recalls Palm Beach County SWA Director of Engineering Ray Schauer.
Originally known as its North County Renewable Energy Facility, the operation is now Renewable Energy Facility #1 (REF #1). It accepts and processes about 2,000 tons of curbside collected MSW per day and produces 1,800 tons per day of RDF, six days per week.
A second facility, called REF #2, will soon come online. A mass burn facility, it is expected to process 3,000 tons per day with 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. The community has an aggressive source-separated dual-stream recycling program that includes newspaper, mixed paper, cardboard, UBCs (aluminum cans) and glass, plastic and steel containers.
REF #1 comprises four areas: the MSW 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 feed 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, which recovers ferrous material, to a trommel screen. The first stage of the screen removes the small fraction (less than 2 inches) as precombustion residue. This small fraction is later cleaned with air classification and combined with the RDF. The heavy fraction is processed further for metals removal.
The second stage of the trommel screen removes the MSW fraction that is less than 6 inches and typically consolidates the containers. The UBCs and other nonferrous metals are removed via eddy current separation and the balance is conveyed to the RDF storage building, Schauer explains.
The MSW fraction that is larger than 6 inches is conveyed to a secondary shredder for further sizing and is reprocessed to 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 to remove the ferrous and nonferrous metals. The facility also includes recovery of metals from the resulting ash on the back-end of the combustion process.
The concluding RDF is a loose product that typically includes the remaining paper, plastic, wood and organics.
The idea at most RDF sites, including Palm Beach County’s, is to produce a feedstock that runs 2 to 4 inches in size.
“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 MRFs. Steven Berry, president of Vexor Technology, emphasizes that his company’s product is not classic RDF, but an engineered product designed to meet customer’s specific requirements.
“We do not compete with recyclers,” Berry 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 mixes it with other materials to produce a high-quality 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 and has a Btu value of about 10,500 per pound.
At most RDF plants that rate their products on a quality output basis, the material typically will be compared with coal for its burn. 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. So Vexor’s product certainly is in good company.
At Vexor, magnets at the front and back end of the system remove metals.
In Palm Beach County, large bulky items, such as mattresses, refrigerators, washers, dryers, freezers, engine blocks and more are recovered on the tipping floor and recycled when possible.
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 collects 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 says Vexor tests 32 different parameters ranging from moisture content to heavy metals. Quality control (QC) testing is performed at the start of the process and on the end product.
“We have QC tests to assure the proper mix of material,” Berry says.
“The material is inconsistent (coming in). But we need a consistent product out,” Berry says.
Dealing With Feedstock
Feedstock inconsistency is a fact of life every facility must contend with.
“We definitely do not want an abundance of moisture,” Saxton says of Balcones’ feedstock. That is a high hurdle in humid Arkansas.
“We try to keep it sub-20 percent—and under 15 percent is better. We want it as dry as possible,” Saxton says.
The large volume of baby wipes and infant care products makes that a tough standard to meet.
“Humidity is naturally high in Arkansas,” Saxton says. “They use a super absorbent polymer in their product. It grabs moisture out of the air.”
Sometimes the product is wet before arrives at the lot. Once moisture is in the product, it is tough to remove.
Moisture is one of the many outside factors that make dealing with feedstock challenging. The challenge is bigger when the feedstock 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 provide approximately 150 percent of the required capacity.
In Arkansas, land is fairly cheap, and landfill costs are lower than they are in many other parts of the country. It can be more economical to landfill material than to make fuel from it. Many of the companies Balcones serves are striving to be as green as possible.
“We only attract companies that want to be as close as they can to landfill free,” Saxton says. “They appreciate the service we provide,” he concludes.
The author is a contributing editor with the Recycling Today Media Group and can be contacted at firstname.lastname@example.org. This article first appeared in the March/April issue of Renewable Energy from Waste, a sister publication of Recycling Today.