Breaking it down

Choosing the right shredder or shredders to properly size feedstock for a variety of waste-to-energy applications can be a complicated decision.

The selection of shredders currently on the market offer varying processing speeds and shredding mechanisms to handle municipal solid waste (MSW) and other forms of postindustrial materials used in waste conversion processes. Speeds range from low to high, with differing torque levels and various numbers of shafts.

Another variable concerns what is being shredded. The average bale or bag of unsorted MSW, for instance, could be compared to a snowflake. No two are ever the same.

For heterogeneous waste used in WTE applications, processors and industry suppliers say slower speed shredders can be more appropriate for a number of operational reasons.
 

Starts and stops

The fact is, unsorted waste contains plenty of unknowns. Terri Ward, solid waste market manager for SSI Shredding Systems Inc., Wilsonville, Oregon, says generally speaking, unsorted waste streams containing tramp metal contamination are best left to slow-speed shredders, which may be more reactive to unpredictable waste.

“While most shredders incorporate some kind of jam relief system to protect against nonshreddable materials, low-speed units generate less inertia and will stop faster, inflicting less potential damage on the cutters, shafts and drive train,” she explains. “Low -speed units also generate less friction and heat, making them better suited for flammable material, sticky or adhesive materials or materials with low melt points.”

Greg Parent, sales representative for equipment manufacturer Vecoplan LLC, Archdale, North Carolina, offers a similar observation about shredding bulky, heterogeneous materials.

“You have to match the material to the technology,” Parent observes. Using a slow-speed shredder is important for these applications, Parent says, in part because of hidden problems.

“In the case of MSW, you could have engine blocks or car axles or propane tanks,” he notes.

Parent says unshreddable items will usually be detected by a preshredder, which may be designed to automatically stop to allow for its removal. Alternatively, some preshredders have reversing mechanisms that help prevent significant downtime.

“That’s one of the reasons you can’t use a high-speed shredder,” Parent adds. “It’s turning so fast, it will cause a tremendous amount of damage.”

Another key difference between lower and high-speed shredders, Parent says, is the shredder’s use of torque.

“A low-speed shredder is working based on torque and muscle to shred the material, whereas a high-speed shredder is typically working on inertia,” he explains. “When you get to the big, bulky stuff, the high-speed shredder doesn’t have the torque or muscle to get through the material.”

Besides having a means to deal with unshreddables, Parent says the low-speed preshredder may be ideal for dealing with loads of MSW or biomass in advance of WTE applications. Here, the preshredder typically serves to liberate and open bales or bags in preparation for separation steps.

On that note, Parent says Vecoplan preshredders use two independent shafts designed to prevent the formation of long stringy strips of film or plastic that could get entangled on equipment. The company’s preshredder also uses a ram to push materials into the rotor, and cutters that bite into material, rather than trying to draw it all in at once. Material leaving the shredder is classified by a designated screen size, forcing materials to remain in the chamber until they reach the desired shred size.

“When dealing with those surprises that come with MSW, you need a certain size shredder and a robustness to handle it,” Parent observes.

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For SSI’s part, Ward says while unsorted MSW is typically processed by primary shredders, available technologies vary drastically in this category.

“Some are single-rotor units that process material in one direction along fixed anvils at one side of the machine,” she says. “Some are twin-shaft units that process by drawing material toward the center, cutting between the shafts. Others, like SSI’s Pri-Max reducer, include two independent shafts that process in both directions.”

Furthermore, Ward says, even the terms high speed and low speed can be confusing, as there is no recognized industry standard.

“SSI specializes in low-speed, high torque shredding technology averaging 20 to 30 rpm (revolutions per minute),” she says. “However we also build what we call ‘medium speed’ rotary grinders operating around 100 rpm.”
 

The second shred

Once materials have passed through the initial preshedding stage, says Parent, reshredding may then occur, after some screening and removal of inert materials. This reshredding may be accomplished with either a lower or higher speed reshredder, Parent says. However, he observes that the use of lower speed reshredders have become more popular in recent years.

“The trend is now going back to the low-speed shredders because of the dust issue, which can lead to explosions, and because of the cost to operate,” he says. This is because dirt or other contaminants in the stream will wear out a high-speed reshredder very quickly. It’s also energy intensive, he says.

“We found ways to produce the high volumes with a low-speed shredder and down to a low particle size,” Parent explains. He refers as well to the company’s V-EBS 2500 shredder for producing refuse-derived fuel, introduced to the WTE industry in 2010. The single-shaft shredder is designed for reshredding light plastics obtained from industrial, commercial and municipal solid waste, and uses an extra-large rotor, multiple counterknives and a feed ram.

Parent says if a third shredding step is needed for a particular application, high-speed shredders may be appropriate in that case, but he adds, “You will need a lot of pieces of equipment to handle the throughput.”
 

High and low experiences

One company that has experience using both low-speed and high-speed shredders for WTE applications is carpetmaker Shaw Industries, headquartered in Dalton, Georgia.

The company is involved in two WTE projects, one dealing with postconsumer carpet, and the other focusing on postindustrial streams of carpet trim.

For its postconsumer project, known as Reclaimed to Energy, or Re2E, the process begins with low-speed shredding, says Jay Henry, director of operations support for Shaw Industries. He explains that the company collects around 30,000 tons of postconsumer polyethylene terephthalate (PET) carpet annually, which is difficult to recycle. However for Shaw, it is serving as a coal alternative to fire a solid fuel boiler that produces steam.

This occurs in the company’s Dalton Re2E plant, which is processing around 150 tons of end-of-life carpet per day, Henry says. Here, the old PET carpet is size-reduced and cleaned, then used to fire the boiler. The resulting steam is used in two adjacent carpet plants.

“We’re using old carpet that would be put in a landfill as fuel to create energy,” Henry says.

The Re2E plant, built in 2010, uses a pair of dual-shaft, slow-speed SSI shredders running at 500 horsepower as the leading edge of the process. The shredders break open and rip apart whole, unbroken bales of postconsumer carpet.

At times, Henry says, the bales of carpet contain tramp items, which he calls “urban debris.” The company has encountered such items as ladders, scissors or even the occasional tricycle.

“When we dump this into the shredder, it’s got to be able to take whatever we put into it,” he explains.

Henry says the low-speed shredders rip the bales apart slowly and indiscriminately. The process is engineered to first size-reduce the carpet, making the stream more manageable for the ensuing metal-removal process. Next, density separation using a shaker table concentrates the carpeting on top of the stream in preparation for reshredding. Here the carpet pieces report to low-speed, high-torque single-shaft Vecoplan shredders running at 270 horsepower. These shredders are designed to reduce the 1-foot pieces of carpet down to 1-inch-thick chunks, he says.

Henry says the company is using slow-speed shredders for this second shredding step as well to minimize the amount of friction and heat created during shredding. The shredded carpet contains a glue system comprising latex, calcium carbonate and other components that, if allowed to melt with the carpet fibers, would make the entire material stream unusable for either recycling or energy recovery.

“Carpet fiber is essentially a plastic, whether it’s nylon or PET polyester,” he says. “If it gets too hot, and if there’s too much surface area and a lot of dwell time, it will heat up too quickly, and then melt and create a big plastic rock in the shredder.” Using the slow-speed shredders allows Shaw to preserve the carpet in a form that can be used by the boiler.

From there, the carpet pieces move to a series of cleaning steps that prepare it to be used as a fuel. Henry says this proprietary process removes the calcium carbonate, latex and dirt which are not desired as part of the fuel product.

Shaw’s other WTE project involves the use of a 50-50 mix of postindustrial carpet edge trim and wood dust to feed a gasifier that’s similarly creating steam. For that process, the trim is shredded with a high-speed Granutech shredder operating at 400 horsepower.

In this case the higher speed shredder works well dealing with the more homogeneous feedstock that contains very little if any contaminants. The high-speed of the shredder helps to create a light and fluffy feedstock which Henry says works well for the gasification process.

“It’s something that flows really quickly through the shredder,” he says. Through this process, Henry says, the company is processing about 7,500 tons of trim per year.

Parent notes that shredding processes necessarily differ based on the material type. Auto shredder fluff, wood, biomass and construction and demolition waste all will require specifically designed reshredder processes.

Parent says while most applications won’t require a third shredding step, those requiring a very small particle size would likely be handled by a high-speed shredder.

“If you have to get to a few millimeters in size, that’s when you’d need to get to high speed to cut that small at any volumes that make sense,” he says. However Parent notes that every material stream is going to shred at a different rate. More homogeneous waste streams, for instance, may be able to reach their final particle size after just one shredding step. On the other hand, heterogeneous streams will likely require more. Therefore, choosing the right shredder setup is key, Parent says, because it can affect the financial feasibility of the entire waste preparation and conversion process that follows.

The author is a managing editor with the Recycling Today Media Group and can be reached at lmckenna@gie.net.