May 2, 2009 -- The energy crisis has given rise to a new source of fuel – the Styrofoam cup. Mechanical engineers at Iowa State University in Ames have demonstrated how to boost the power output of biodiesel simply by adding waste plastic to the fuel.
Song-Charng Kong, a co-author of the study, says the experiment – funded in part by the Department of Defense – was conducted to find a way to dispose of trash and generate power under battlefield conditions.
"One can recycle any kind of plastic, but if you are camped in a remote area, recycling is not an option," Kong says. "Turning plastic into fuel is a way to get rid of garbage and generate electricity."
Kong and colleagues dissolved polystyrene – a polymer used to make disposable foam plates and cups – into biodiesel at concentrations ranging from 2 to 20% polystyrene by weight.
"A polystyrene cup will dissolve almost instantly in biodiesel, like a snowflake in water," Kong says, although the plastic doesn't break down as well in petroleum-based diesel and other liquid fuels.
Tests of the mixed fuel in a tractor engine used for electricity generation showed that as polystyrene concentrations increased to 5%, power output increased at roughly the same rate. However, there was a drop off in output for plastic concentrations above 5%.
Kong thinks the change is due to the fuel's increasing viscosity as more and more polystyrene is added. Initially, he says, the thicker fluid creates greater pressure inside the generator's fuel injector causing earlier injection of fuel into the engine and increasing its output.
But eventually the fluid gets so viscous that it doesn't completely combust in the engine and power output decreases. At 15% polystyrene, the fuel is so thick the fuel injection pump overheats.
The new fuel mix is not without its problems, however – as the concentration of polystyrene increases, so do emissions of carbon monoxide, soot, and nitrous oxides.
"You are putting large polymer compounds in, it's hard to burn them completely," Kong says, adding that he hopes to now work on refining the engine's fuel injection system to yield a more complete burn with fewer emissions.
Robert Malloy of the University of Massachusetts Lowell says as long as emissions can be brought back into line, adding polystyrene into fuel makes sense.
A recent report suggests it's over three times as energy efficient to recycle trash rather than convert it to fuel, but Malloy points out that polystyrene is a special case.
"I think we should try to recycle as much as we can, but there are certain materials that don't lend themselves to recycling in an economic way," he says. Polystyrene is so lightweight and bulky that it's uneconomical to ship to recycling plants. "Technologies like this where you get energy back would be preferable to landfilling," Malloy says.