Plastic nuggets aid 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,' said James W. Garthe, instructor in agricultural engineering and co-operative 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. Plastics found on farms and nurseries include polyethylene, polypropylene, polyethylene terephthalate and high-density polyethylene. They would never be mixed in conventional recycling.

Garthe's approach is to convert the various films 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,' said Garthe. 'Even if these plastics could be remelted and remoulded, which they cannot, conventional recycling requires much higher energy input.'

Including the energy required to pre-process 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.

Pre-processing 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 and automatic cutting of the nuggets will eventually be incorporated into the system.