|Comparisons of NOx, CO, soot, and engine brake power using different PS blends with the factory-set injection timings. Credit: ACS. Click to enlarge.|
Researchers at Iowa State University are proposing dissolving waste polystyrene (PS) in biodiesel for use as a diesel engine fuel as a mechanism for energy recovery from the waste plastic. Use of polystyrene-biodiesel blends can result in an increase an engine power with polystyrene concentrations of up to 5%, according to a new study by a team of researchers from Iowa State University. At concentrations higher than 5%, engine power decreased.
However, emissions of NOx, soot, CO and hydrocarbons increased with polystyrene concentrations if the injection timing was free to advance due to the increased bulk modulus and fuel viscosity. Varying engine operating parameters, including the fuel injection timing and exhaust gas recirculation resulted in lower NOx emissions but still resulted in higher soot, CO, and HC emissions.
The study, published online in the ACS journal Energy & Fuels, 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, according to researchers Najeeb Kuzhiyil and Song-Charng Kong.
The US generated 251 million tons of municipal solid wastes in 2006, of which plastics accounted for 11.7 wt% or 29.5 million tons. The US also generates approximately 7.6 billion tons of commercial and industrial wastes each year that also contains a considerable quantity of plastics. Polystyrene accounts for 22 wt% of all high volume plastics. Using polystyrene as a fuel in diesel engines is one possible option for energy recovery from these waste plastics.
The polystyrene packing peanuts cannot be burned in a diesel engine in their original form. However, they can be dissolved in a solvent to produce a usable fuel mixture. Biodiesel is an excellent such solvent that is also a proven alternative fuel for diesel engines. Hence, polystyrene packing peanuts dissolved in biodiesel can be a viable fuel. The dissolution properties of polystyrene in various fuels and other solvents can be found in the literature. However, research using dissolved polystyrene as an engine fuel has not been found.—Kuzhiyil and Kong (2009)
In the study, the team used a four-cylinder, 4.5-liter Deere turbocharged diesel engine, modified to incorporate an EGR system to explore strategies for NOx control. A separate fuel handling system enabled the used of multiple fuels during engine testing.
Polystyrene packing peanuts can dissolve in biodiesel at room temperature. To create the fuel blend, the peanuts were added to biodiesel in a stainless steel tank with mild agitation. Polystyrene concentrations of 2, 5, 10, 15, and 20 wt% were tested in the study (denoted as PS2, PS5, PS10, PS15 and PS20, respectively).
Among the findings were:
When the engine was run with the factory-set fuel injection timing, engine power increased with PS2 and PS5 due to the advancement in the fuel injection timing caused by the addition of polystyrene.
Power decreased with PS 10 and PS15, possibly due to the poor spray atomization and deteriorated combustion efficiency using high-viscosity fuel blends. Additionally, high PS blends also resulted in very advanced injection timings that produced early ignition and could cause the piston to work against the expanding gas and thus reduce power output.
As the power increased, the specific fuel consumption decreased and vice versa when power decreased.
CO and HC emissions increased slightly with a slight increase in the PS concentration and then increased significantly as the PS concentration increased to 10 and 15%.
CO and HC are products of incomplete combustion, the authors note, and the advancement in the injection timing will normally result in more premixed burn with higher combustion temperature for reduced CO and HC emissions. However, as the PS concentration increased to 10%, high viscosity of the PS fuel blend could result in poor spray atomization, and the presence of heavy polymer molecules could also further deteriorate combustion, they suggested.
NOx emission increased with the PS concentration up to 10% and then remained nearly the same for PS15.
The trend of soot emissions was similar to that of CO and HC emissions, increasing at high PS concentrations. In the case of PS10 and PS15, poor atomization and incomplete combustion could occur due to the presence of a large number of polymer molecules in the fuel mixture, causing soot emissions to increase.
|Comparisons of emissions and engine performance using DN2, B100, and PS5 with the factory-set injection timings. Credit: ACS. Click to enlarge.|
The authors compared engine performance using No. 2 diesel (DN2), B100 and PS5, which they termed to be the preferable fuel blend. B100 produced higher NOx and lower soot and CO emissions. Engine power decreased slightly using B100 due to the lower energy content. The increase in NOx emissions was approximately 14% and the decrease in soot emissions was approximately 59% without using EGR.
Najeeb Kuzhiyil and Song-Charng Kong (2009) Article Energy Recovery from Waste Plastics by Using Blends of Biodiesel and Polystyrene in Diesel Engines. Energy Fuels, Article ASAP doi: 10.1021/ef801110j