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Efficient plastic nuggets key to agricultural plastic waste disposal


A process that would be a plastics recycler’s nightmare may help farmers deal with the disposal of agricultural and domestic plastics by creating burnable, energy-efficient plastic nuggets, according to a Penn State agricultural engineer.

"In plastics recycling there are two unbreakable rules," says James W. Garthe, instructor in agricultural engineering and cooperative extension specialist. "You cannot mix types of plastic, and the plastics must be clean. This process does both since mixed plastics burn just fine and the dirt and debris come out with the coal ash anyway."

Agricultural plastics, such as mulch films, greenhouse films and pots, flats and silage wraps are universally dirty, and the cost of cleaning them before recycling would be expensive. Also, while mulch and greenhouse films are generally low-density polyethylene, nursery pots are polypropylene and soda bottles and milk jugs are polyethylene terephthalate and high-density polyethylene respectively. The plastics found on farms, nurseries and landscape yards are a variety of plastics that would never be mixed in conventional recycling.

Garthe’s unconventional approach is to convert the various film and solid plastics into a plastic nugget that can be burned with coal in coal fired boilers, refuse-derived fuel burners and even cement kilns. The process can mix plastic types, because it does not melt the plastic. Only the outer portion of the nugget is fused forming a melted jacket that contains the compressed plastic waste.

"Because the plastic is not completely melted, calculations show that only about one eighty-fifth of the energy released when the plastic nuggets burn is used to create the nugget," says Garthe. "Even if these plastics could be remelted and remolded, which they cannot, conventional recycling requires much higher energy input."

Including the energy required to preprocess the plastic and to cut the pieces in the calculations; the nuggets still supply 20 times the energy used to make them, according to Garthe.

Preprocessing for the prototype plant is currently done by cutting up the plastics and hand feeding them into the hopper where a feed rod pushes them into the die. The die is heated to melt the outer layer and a snake of compacted, but mostly unmelted plastic emerges. For the prototype, Garthe cuts the snakes into nuggets with a hot knife. Conveyers, a way to produce the proper-size plastic pieces, and automatic cutting of the nuggets will eventually be incorporated into the system.

"The system can be made small enough to be bench scale, but could also be enlarged to industrial scale," says the Penn State researcher.

While dirty and mixed plastics can be used, wet plastics might pose a problem for the system so some type of quality control would be necessary. Also, sandy plastics might cause a problem because the silicate sands might fuse onto the grates.

"We probably will not use polyvinyl chlorides, like PVC pipe or collapsible irrigation tube in the process because of the potential for emissions problems," says Garthe.

Burning the nuggets along with coal probably produces few emissions as the temperatures of 1,500 to 2,000 degrees Fahrenheit allow for more complete combustion. Plastics simply burned in a burn barrel reach only 400 to 700 degrees Fahrenheit and this incomplete combustion can produce unwanted emissions. Although Garthe believes that emissions are not a problem for the nuggets, Penn State’s Energy Institute is currently running tests on the nuggets to characterize the emissions. A grant from the National Watermelon Promotion Board, U.S. Department of Agriculture, supports these tests.

Currently, besides the small amount of plastic bottles and jugs that are recycled, most other plastic wastes end up being land-filled, where the energy potential of the plastic as a fuel is wasted. Nurserymen, landscapers, truck farmers, mushroom growers, hothouse owners and even dairy farmers produce varying amounts of plastic waste.

"Depending on the type of agricultural operation, the generation of plastic waste probably ranges from 10 pounds to 10,000 pounds a year," says Garthe.

The Penn State agricultural engineer has applied for a provisional patent on this process.

Andrea Elyse Messer | EurekAlert!

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