Waste Plastics + Biodiesel

Energy Recovery from Waste Plastics by Using Blends of Biodiesel and Polystyrene in Diesel Engines

investigated diesel engine combustion and emissions characteristics using blends of biodiesel and polystyrene. As polystyrene accounts for approximately 22% by weight of all high volume plastics, it is attractive to develop methods to convert these waste plastics into energy. Biodiesel is a biorenewable fuel and a good solvent for certain materials. In this study, biodiesel was used as a recycling agent and polystyrene packing peanuts were dissolved in biodiesel in different concentrations as a means to recover energy from waste plastics. Test results showed that engine power increased initially with the polystyrene concentration and then decreased for concentrations higher than 5%. The initial increase in engine power was mainly due to the injection timing advancement caused by the increased bulk modulus and viscosity of fuel blends. The decline in engine power at high polystyrene concentrations could be caused by the poor spray atomization and deteriorated combustion efficiency due to the high viscosity of polystyrene mixtures. Emissions of NOx, soot, CO, and HC were found to increase with the polystyrene concentration if the injection timing was free to advance due to the increased bulk modulus and fuel viscosity. Parametric study was performed by varying engine operating parameters including the fuel injection timing and exhaust gas recirculation. For the same injection timing, higher polystyrene concentrations still resulted in higher soot, CO, and HC emissions but lower NOx emissions. This study demonstrated that polystyrene-biodiesel blends could be successfully used in diesel engines with minor modifications to the fuel system and appropriate adjustments to engine operating conditions.

Polystyrene is the practically indestructible material used in packing peanuts, foam cups, egg cartons and produce trays.

Like all traditional plastics, polystyrene is made from petroleum and is a non-sustainable source of major pollution. It is ubiquitous, difficult to recycle, does not biodegrade and resists photosynthesis¹.  In a stunning development, a new study has shown that polystyrene not only dissolves in biodiesel fuel, it increases the power output in the process.

Scientists found that polystyrene packing peanuts dissolved in biodiesel can actually boost the power output of the fuel and get rid of garbage at the same time.

Styrofoam™ is a generic term for disposable plates, cups and coolers. Dow Chemical Company trademarked Styrofoam™, a form of polystyrene foam insulation in the 1950s. Styrofoam™, made from extruded polystyrene, is used in building materials, floral and craft products and is mostly blue.

That coffee you drank from a white foam cup this morning was not Styrofoam™; it was polystyrene foam, made from expanded polystyrene beads.

In the recent study, published in the journal, Energy and Fuels², researchers Najeeb Kuzhiyil and Song-Charng Kong of Iowa State University stated that polystyrene accounts for about 22 percent of all high-volume plastics.

Finding a method to convert waste plastics into energy could potentially ease the strain on landfills and generate electricity. Although polystyrene does not break down easily in petroleum-based diesel, it does break down almost instantly in biodiesel.

The study, funded in part by the Department of Defense, was conducted to investigate solutions to trash disposal and power generation under battlefield conditions. According to Song-Charng Kong, a mechanical engineer and co-author of the study, “One can recycle any kind of plastic, but if you are camped in a remote area, recycling is not an option.” Kong adds, “a polystyrene cup will dissolve almost instantly in biodiesel, like a snowflake in water.”

For most materials, recycling is more efficient than converting into energy, however polystyrene is both lightweight and bulky, making it less than economical to ship to recycling facilities and a good candidate for fuel conversion.

The study showed that polystyrene dissolved in biodiesel increases its viscosity, building pressure inside the fuel injector causing fuel to be injected sooner into the engine. This increases the overall output.
Biodiesel is a bio-renewable fuel and a good solvent for certain materials.

The downside is that dissolving polystyrene in biodiesel doesn’t eliminate the problem of harmful emissions. The research team found that adding polystyrene increases the fuel’s emissions of carbon monoxide, soot, and nitrous oxides, which don’t burn completely in the engine.

This is not entirely surprising since polystyrene is derived from petroleum in the first place. However the researchers continue to work on improving the engine’s fuel injection system to achieve a more complete burn with fewer emissions.

Dissolving polystyrene in biodiesel as a means to recover energy from waste plastics brings the idea of fueling a car with waste a step closer. Certainly preferable to land filling this exciting discovery could offer a viable solution to the polystyrene waste problem, if the emissions can be brought into line.

  1. Bandyopadhyay, Abhijit; Chandra Basak, G. “Studies on photocatalytic degradation of polystyrene”, Materials Science and Technology 23 (3): 307–317 (2007)
  2. Najeeb Kuzhiyil and Song-Charng Kong. “Energy Recovery from Waste Plastics by Using Blends of Biodiesel and Polystyrene in Diesel Engines.” J Energy and Fuels. April 21, 2009

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