Recycling and Recovery of Energy Stored in Used Plastics

From The Vegetable & Small Fruit Gazette, April 2006, Volume 10, No. 4,
published by Penn State University
William J. Lamont, Department of Horticulture, and James W. Garthe, P.E., Agricultural Engineer and Instructor, Department of Agricultural and Biological Engineering

At Penn State University there is a team dedicated to solving one of the world’s major environmental problems – what to do with the increasing volumes of plastic waste generated by the world population. Although our efforts have been focused on agricultural plastics, we also view the large consumer plastic waste stream as a potential fuel source. This team has been working on this project since 1995 and is on the verge of making a major contribution to solving this problem. A little background on recycling is in order.

In the March 18, 2005 issue of the Centre Daily Times was an article “You Ought to Know About Recycling”. In the article the author stated “recycling is now an international commodities industry driven by market demand and labor costs.” The author stated “Often programs don’t take materials that are available in large enough numbers to make reusing them profitable”. The plastic code is typically identified as triangle of chasing arrows surrounding a number depicting the plastic resin type, such as #1, polyethylene terephthalate or PET, used for making soda and water bottles. The best market for recyclables is the #1 and #2. The demand for these products is insatiable at this point. The bottle caps used on these products are not recycled but removed and enter the trash stream. These are a valuable source of fuel for us.

Although there is strong demand for recycling PET, a soda bottle can be recycled into, for instance, carpet which unfortunately cannot be recycled. The article mentioned garden garbage generated when you buy plants for the garden. But you ask "What does it come in?" The answer is that it comes in a plastic pot or plastic six-pack, and if you buy enough, it also includes a plastic tray or flat to hold the six-packs. You can reuse the pots yourself, of course, and sometimes garden clubs or conservatories need them for plant sales, but most recycling programs do not accept them.

So what are we doing to help solve this problem? We're recovering valuable energy from waste plastic items used in agriculture. Dirty, used or non-recycled plastics can be converted to energy to heat high tunnels, greenhouses, other agricultural structures, commercial buildings, light industrial facilities, or other buildings requiring an environmentally clean, yet safe, fuel. Two fuel-saving technologies are currently being brought to commercial reality.

The First Technology: Plastofuel

A simple process was invented at Penn State University in 1995 to densify waste plastics into a fuel nugget, called Plastofuel. The process, developed in the Department of Agricultural and Biological Engineering, aims to reduce waste plastic buildup on farms around the world. It works by forcing film plastic items, rigid plastic items, or both, through a heated die, thus melting a thin jacket that encapsulates the pieces of plastic and dirt within the extruded material exiting the die. A hot knife cuts the extrudate into dense fuel nuggets that can be easily conveyed, stored and shipped.

The nuggets were originally designed to be co-fired 5-10 percent with coal in existing boilers, allowing the high temperature of coal (around 2000oF or 1100oC) to sustain clean combustion, free of noxious smoke. The end-use is for agricultural boilers or small community boilers designed to burn coal. Plastofuel can be made either on the farm or in small industrial settings, thereby consuming the energy close to the source. The benefit of the system is that it converts an annoying waste into a valuable fuel, with a minimum of energy expended in the process. Non-recycled consumer plastic food and beverage containers can also be used in the process. Many of the plastics not currently recycled can be used a raw material for the Plastofuel.

At the time of this writing in 2006, the Penn State team is scaling-up the prototype Plastofuel process to a machine that will produce 500 lbs/hr (227 kg/hr) through a grant from the Pennsylvania Department of Agriculture. It will be powered by electricity and a hydraulic power unit, all mounted in a trailer to provide mobility. From an energy perspective, calculations reveal that less than one percent of the heat energy contained in the nugget (when combusted) will be used in the process to form the nugget. This system is being instrumented to measure actual energy expenditures, which will better define the economics of the process compared with competing fuels. This pilot-scale system will also provide Plastofuel in quantity for farm-scale tests and demonstrations.

The Second Technology: Korean High Temperature Combustion

Although blending used plastic with coal continues to be an important way to recover energy from non-recyclable plastic, a new Korean technology is being commercialized. Manufactured by GR Technologies Company, Ltd., this hot water boiler heating system burns pea-sized pellets made from waste mulch film plastic. The system preheats a series of combustion chambers to 1650-2000oF (900-1100oC) for 10-15 minutes using fuel oil or kerosene, then automatically switches to the plastic pellets. Field-testing of a 396, 850 Btu/hr (100,000 kcal/hr) unit for heating high tunnels began at the Penn State Horticulture Research Farm in 2004. Eventually, the pellet-fueled unit will be modified to burn the larger and more energy efficient Plastofuel nuggets.

Specifications of the Korean Combustion Unit

Preheat fuel: 2.0 gal (7.6 liters) kerosene or fuel oil for 15 minutes

Fuel: For 20 seconds after preheat, burner dual-fuels with kerosene and plastic, then plastic thereafter indefinitely

Pellet types: Burns polyethylene (#2, #4) or polypropylene (#5). Note: Polystyrene (#6) & ABS pellets require a 50% barrel length increase, currently not available.

Plastic fuel feed rate range: 20 - 33 lb/hr (9-15 kg/hr)

Dioxin (PCDD/F) emissions: 0.119 ng-TEQ/sm³ @ 12% O2 as confirmed by Korea Testing Laboratory (Note: US EPA allows 5.0 ng-TEQ/sm3)

Boiler type: Circulating hot water

Rated capacity: 396,850 Btu/hr (100,000 kcal/hr) on 19.8 lb/hr (9 kg/hr) with 20,686 Btu/lb (11,500 kcal/kg) plastic fuel pellets

Thermal efficiency: 75%. At 150^oF water, thermal efficiency goes to 89%

Combustion chamber operating temperature: 1652-2012oF (900-1100oC)

Boiler cut-in temperature: 140^oF (60^oC)

Boiler cut-out temperature: 176^oF (80^oC)

Boiler heating cycle: 5 hrs comprised of 2 hrs burning & 3 hrs circulating

Boiler heating rate: 1 ton water requires 45 minutes heating time (entire heating system will hold 3 tons of water with 2 hours heating time)

Power needs: 4 kW@ 110 vac with 208 vac for vacuum fuel supply

Future boiler sized for Plastofuel: 7,937,000 Btu / hr (2,000,000 kcal / hr)

Air emissions

Stack testing conforming to U.S. Environmental Protection Agency (EPA) standards were conducted by an independent U.S. testing company in May 2005 that was funded by a grant from the American Plastics Council. The tests compared LDPE (#4) Korean pellets with granulated HDPE (#2) barrels, which had been discarded by a local firm. Three main groups of pollutants were analyzed:

• Gases (sulfur dioxide, oxides of nitrogen, carbon monoxide, carbon dioxide)
• Particulate matter
• Dioxins/furans

Test results proved that this is an extremely clean burning system in all three groups. The Pennsylvania Department of Environmental Protection (DEP) has reviewed the results, noting that combustion units with a heat input rating of 2.5 million Btu / hr or less are exempt from the plan approval and operating permit requirements. This means the burner and associated boiler can be marketed in the Commonwealth of Pennsylvania now and without restraint. However, sites will still have to comply with the DEP opacity regulation, Section 123.41. According to the DEP standard, opacity, or visible air contaminants, cannot be equal or greater than 20 percent for a period or periods aggregating more than 3 minutes in any 1 hour, or greater than 60 percent at any time. For this extremely clean burning system, the opacity regulation testing will most likely not be an issue, hopefully merely a formality.

Please contact us if you are interested in supporting this program or being a part of the program. Bill Lamont, wlamont@psu.edu or James Garthe, jwg10@engr.psu.edu