Going with the grain

Features - C&D Wood

Choosing the right equipment and facility design plays a key role in the efficient processing of wood waste.

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Transforming wood waste into a revenue stream requires adopting a more efficient processing method. This starts with identifying the processes best suited for the type of material being collected, determining the appropriate equipment to reduce the size of the material and deciding the steps for processing waste.

One of the most important details to monitor during the production process is how frequently operators move material. Every time material is handled, it adds to operational costs. To minimize movement, evaluate the layout of the facility so the material flows one way.

Take steps to ensure clean material

Setting up an efficient wood recycling operation starts with the collection process. Whether the material is coming to a processing facility through a community green waste collection program, is collected at yard waste transfer stations or is dropped off at a landfill, common contaminants in wood debris can include:

  • plastics, such as consumer grocery bags;
  • dirt, which can slow down processing times and degrade the quality of the end product;
  • metals that, when mixed in with wood waste, can damage the processing machinery;
  • concrete, bricks, blocks and other solid materials that can damage machinery and delay recycling; and
  • processed wood material, which often can include other contaminants such as nails, that could create problems, depending on the desired end product being produced and the machinery being used.

Inspection and signage

For larger volumes of material, take a moment to inspect incoming loads, especially if they are from a new customer. Posted signage and written communications also can help keep incoming material clean.

At the facility, it is essential to have incoming organic material separated from nearby garbage collection areas using barriers. This reduces the risk of plastic bags infiltrating the wood piles.

Before running wood through a grinder or shredder, machine operators also should spend time sorting material to remove bulkier contaminants, such as metals and concrete. Sometimes a quick shake of the excavator or loader bucket can dislodge mixed-in contaminants and help protect the machinery.

Metal contaminant protection

Investing in a system that detects metal contaminants ahead of time provides additional protection. Some horizontal and tub grinders automatically can idle the engine, reverse the infeed or tub and disengage the clutch when certain metal contaminants come in contact with the machine’s hammermill. Then, when the contaminant is cleared, regular operations can resume.

It also is important to pay attention to how loaders and excavators handle piled material. Dirt pushed into a pile of organic wood material and then processed by a grinder or shredder can cause premature wear to drums, knives and hammers.

Processed wood material can be recycled. However, because this material has a greater chance of contaminants than organic material does, processors should consider keeping it separate from organic wood debris. Many processed wood materials can be reused to make pallets or biomass for fuel; but, because it does not break down as quickly as organic material, it is best not to use processed wood for compost.

Comparing screen equipment options

Depending on the end product, sorting and separating machinery also might need to be employed.

Screening equipment is widely used to make compost and some biomass/biofuel products. To achieve high-quality products, operators also could need to add an air separator to remove plastic from processed wood.

Screening equipment, such as trommel screens, are used to separate various sizes of material.

While options for screening include deck screens as well as disc and star screens, trommel screens are the preferred method at many facilities because of their efficiency and ease of maintenance. Compared with disc and star screens, trommels rely heavily on gravity to move and screen material versus more mechanical means.

The industry offers two configurations of trommel screens—tensioned screen drums and auger drums. Trommels with a tension screen drum use a lift-and-throw action to separate material. As compost is cycled through the drum, the smaller material passes through the screen’s holes, while larger material exits at the end of the trommel. The slope of the machine dictates the rate at which the material flows through the drum as it’s tumbled. Using tensioned woven-wire screen panels enables smaller gauge wire to increase the total open screen area, which helps maximize production.

Trommels with an auger drum separate material using tumble-and-roll action, which reduces the chance of material piercing the screens. However, auger drums have less open surface area and are more prone to having wet material build up in the drum, and it may be more expensive to change product size because an additional drum might be required.

Choose the most efficient processing option Sizing wood waste is a critical step in the recycling process. Many potential retail markets demand materials within specific size ranges:

  • smaller than 4 inches in length and width for compost;
  • 1½ inches to 2½ inches for much; and
  • biomass/biofuel varies by burner type and size.

Sizing begins during the grinding process. The three most widely used machines for processing wood are high-speed horizontal grinders, high-speed tub grinders and slow-speed shredders. The best machine to use depends on the incoming material type, size and shape; the facility location; and the desired end product.

Horizontal grinders are an excellent option to use at facilities where space is a bit restrictive as they can efficiently handle longer material. Facilities that receive a high volume of long-cut tree branches and limbs should consider using a horizontal grinder to minimize the cutting time before feeding material through grinders. In addition, horizontal grinders are good at handling loose green waste.

Tub grinders can handle bulkier materials such as tree stumps, as well as processed wood material such as pallets. Because they use gravity instead of conveyor belts to feed the hammer mill, tub grinders have fewer moving components than horizontal grinders. Tub grinders are efficient at processing loose green materials.

Thrown-object systems can help reduce the number of items tossed from tub grinders. Items that still are thrown out by the spinning rotor likely will not be thrown as far.

Slow-speed shredders do not have the same production levels as grinders, nor are they as effective at sizing materials. However, they typically are more tolerant when processing material with many contaminants. To achieve the appropriate end product size, many facilities use shredders to presize contaminated material and a grinder to achieve the desired final sizing.

Material moving equipment

While wheel loaders and excavators are some of the most versatile machines at a wood waste facility, they should not be the only way material is moved within a facility. Once raw material is processed through grinders, conveyers are most efficient at moving material around the site.

Smaller machines can provide maneuverability and versatility to retail operations. They can help operators achieve a more cared-for appearance in retail areas and enable large equipment to move bulkier material instead of loading truck beds and trailers with retail products.

Operators also should consider investing in a compost turner if they are producing large volumes of compost or soils. Turning material with wheel loader buckets is time-consuming and might not efficiently stir all the material in a windrow. Turning compost is a critical step in optimizing curing times and producing a quality end product.

Also, if an operation plans to accept food waste, odors are controlled through frequent windrow turning.

In addition, if retailing compost, soils and mulches at a facility, using big wheel loaders in more confined areas can be challenging and less efficient than using a smaller loader in tight retail areas.

Plan for success

Recycling wood waste can provide a positive revenue stream for a business if it properly identifies the best processing methods for the material, concentrates on efficiency, takes steps to ensure material cleanliness and chooses support equipment that best matches the facility’s needs. There is strong market demand, particularly for compost, mulch and biofuel/biomass products. The growth trend will continue as more communities continue to adopt environmental initiatives.

Ted Dirkx is the sales manager for recycling and forestry at Vermeer Corp., Pella, Iowa. He can be reached by email at tdirkx@vermeer.com

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