Breaking it down: The role shredders play in waste

Waste to energy plays an important role in waste management because of its ability to take discarded material that wouldn't normally be reused and turn it into something that can, such as refuse-derived fuel (RDF).

The shredding of this material plays a critical role in the waste-to-energy process, as the quality of the end product often depends on the feedstock being fed into the system. Given the variety of materials that can enter a conversion facility, there is no one-size-fits-all method for shredding waste prior to processing. Operators must pay special attention to a shredder's specifications to find one that fits their needs.

Waste Today recently spoke with Hartmut Bendfeldt, founder and president of eFactor3 in Charlotte, North Carolina, and Dana Darley, a complex systems manager for Vecoplan in Archdale, North Carolina, which both supply shredders for RDF applications. The two experts discuss in the following Q&A what operators need to consider when getting a shredder.

Waste Today (WT): What should operators look for when considering a shredder for waste-to-energy applications?

Hartmut Bendfeldt (HB): [Operators should consider] a robust shredder that can deal with contaminants like metals, rocks or other forms of debris. They need to consider that no waste stream will be 100 percent clean from contaminants unless it comes directly from a production line. It is important that the design of the shredder, especially on the front end, can deal with contaminants without stopping the equipment or the line.

Dana Darley (DD): Some of the things that an operator should consider is a shredder with a high and consistent output. That way the material that comes out will always be the same. They also need to consider a robust system that is tolerant to scrap metal and other foreign materials, so it can filter them out and have a reduced need for downtime. This leads me to my last point: Operators need to consider a machine that's easy to maintain because short downtimes can sometimes make the difference between a business and its competitors. We can do many things to a shredder to make it tolerant, but we can't make it bulletproof, so it's necessary to ensure it's as durable as possible.

WT: What shredder type(s) are particularly effective in these applications and why?

HB: I am biased, but for preshredding, we’d recommend a twin-shaft shredder for size and material quality. What comes out can then be sorted before the second shred step, so the material is purer.

For secondary shredding, we believe that a single-shaft shredder is the best solution for most streams. A single-shaft shredder that is used as a secondary shredder has a more controllable cost of ownership and is more consistent when cutting material.

Four-shaft shredders are subjected to high wear costs [and] high operational costs. The total cost of ownership on these machines is very high because of a large maintenance bill on parts and long downtimes. I would not recommend a four-shaft because I know of customers that spend about $250,000 a year in replacement parts on these machines. This is, in my opinion, way too high.

DD: We’d recommend a dual-shaft shredder as a first size reduction step to open material for separation and sortation. They can handle some of the most difficult materials because the first step makes the material manageable for separation.

Then, at the second step, operators could consider a single-shaft shredder for precise particle size control for applications like burning or gasification.

WT: What is the difference between low-speed and high-speed shredding and how should it factor into shredding for waste to energy?

HB: The difference between low-speed, medium-speed and high-speed shredding is, of course, the speed of the rotor. The slower the speed, the less temperature pickup you have in the material. There's also a smaller chance of damage because allows for contaminants to be caught and removed safely from the process.

A higher speed gets you more capacity; however, the higher speed of the machine, the higher the risk of nonshreddable material being fed through. As a result, higher speeds lead to a higher risk of damage incurred to the machine.

DD: For us, during the preshredding process, we recommend a low-speed and high-torque [shredder] for a coarse shred of 6 inches to 8 inches. That way you can process the materials effectively and catch any large contaminants or hazardous objects like propane tanks.

During the reshredding process, operators could consider using a high speed, which gives as many cuts per minute for small particle sizes of 1 inch minus.

WT: Are there benefits to using multiple shredders in the process?

HB: Yes, it's beneficial to use multiple steps for the shredding process. Our experience has shown if you have a preshredder that produces a 10-inch to 12-inch particle size, it is easy to mechanically remove metals and other contaminants from the waste stream.

For example, some fuels have restrictions on the chlorine content of materials that are used in RDF. This means operators want to take material like PVC (polyvinyl chloride) and get it out of the stream before the operator shreds them further.

Then, a secondary shredder can bring the material down to the preferred size of the final product. If it has to be smaller, there could be a third step in the process.

DD: Yes, very much so. A course preshred is required to remove large contamination and prepare the material for a final shred. Once that is done, the second shredder works the stream into a final material that should be a small and consistent size for efficient burning.

WT: To what extent does the type and volume of material an operator processes influence selecting a shredder?

HB: The volume is very important to know because, in these applications, we’re talking about pounds per hour. So, typically, these operations are looking at 10-20 tons an hour in alternative fuel production or maybe even more.

In incineration RDF lines, we’re talking 80-90 or 120 tons an hour.

So, knowing an operator's volume is critical to sizing the equipment appropriately. At the end of the day, shredders are volumetric devices, they can only get a certain volume of materials through a specific piece of equipment.

DD: This is one of the main factors that play an important role in the sizing and selection of the shredder.

Material composition determines the style and features of the shredder, and volume determines the size of the shredder. This has to do with what kind of materials are in the stream. For example, if we know there is going to be a certain amount of metal, there are going to be extra provisions that will protect the shredding equipment from metal.

If we know the composition of the material, operators can prepare by getting a machine that can address that composition more efficiently.

WT: Does the method of waste to energy being used play a role in what shredders should be used? If so, why?

HB: Yes, because the method … the company uses has certain requirements that need to be met. If it's incineration, operators [typically] don't need to worry too much about what goes into their plant. The entire pile of municipal solid waste is shredded down in size to make it manageable for the RDF system. They need to get it down to 6-inch minus to 8-inch minus and the system will [typically] use magnets to take out as much ferrous metal as possible, but the rest of it just goes into the burners.

However, depending on the type of fuel a company is making—something like process-engineered fuel—the process is different. This is because they have to sell this fuel to the cement industry, which typically has a requirement to get the fuel free of contaminants. For example, they don't like to have metal in the kilns and they cannot tolerate more than 0.2 percent of PVC going into the kilns due to issues with emission standards.

DD: Knowing what process is being used is necessary to determine what shredders need to be used in an operation. The different waste-to-energy methods have their specifications, and it's important that the system can meet those specifications.

Gasification usually has a tighter material specification than incineration when it comes to material purity and particle size distribution. It all depends on what the operator needs the final product to be.