Pyrolysis of Household Plastic Wastes

Aims: Thermal cracking of waste plastic (without catalyst) to useful chemicals.
Study Design: To design the experimental procedure, we primarily concentrated on the thermal stability of the materials by bearing in mind the results of thermogravimetric analysis (TGA). Based on the thermogravimetric results the appropriate set-up for the decomposition of the plastic wastes was designed. Three common household plastic wastes – styrofoam dining plates (SDP), shipping protection styrofoam boxes (SPFB), and carrying plastic shopping bags (CPB) – were pyrolized into liquids. GC-MS was used to characterize the sample of the obtained liquids.
Place and Duration of Study: The study was done in the Department of Biological and Physical Sciences at South Carolina State University (SCSU), Orangeburg, SC, USA, during the summer of 2012.
Methodology: The thermal cracking process without catalyst was used to convert household waste plastics into liquids. Three types of waste plastics, SDP, SPFB and CPB were used for these studies. The waste plastics were cut into small slices suitable to fill the reactor. Prior to pyrolysis, the thermal stability of materials were determined by thermogravimetric analysis (from 70ºC to 650ºC) with a heating rate of 10ºC/min while the samples were purged with 10 mL/min argon. The condensed liquids were analyzed by a Shimadzu GC-MS model GCMS-QP 2010s using helium as the mobile phase.
Results: The thermal stability of waste plastics depended on the nature of constituent polymers from which the plastic originated, as was expected. Polystyrene derivatives, SDP and SPFB, both physically soft and hard, had similar thermal stability. The highest decomposition rates were observed at temperatures 418ºC and 423ºC for soft and hard SPFB respectively. No leftover was observed by thermogravimetric analysis. SDP were thermally more stable than SPFB; the decomposition began around 400ºC. The highest weight loss rate was observed at 440ºC. The TGA leftover was about 3% of total mass of SDP. The bulk pyrolysis of SDP and SPFB had 20% to 30% leftover. The GC-MS chromatogram indicated that over 350 chemicals resulted from decomposition of polystyrene based materials; the most abundant compound of pyrolysis was styrene and styrene derivatives as expected. The pyrolysis of CPB yielded hydrocarbons C4 to C24 being both alkanes and alkenes as expected. The TIC picks of CPB were geminals; first being alkene and the next was alkane with the same number of carbons (Figure 9).
Conclusion: The chemical composition of the liquids obtained and the yields depended on the original polymer, quality of the waste, and the engineering of thermolysis procedure. The refinement of liquids resulting from pyrolysis is necessary to obtain a quality fuel. The condensed liquids produced from pyrolysis contained highly reactive chemicals such as vinyl, alkene, and three- and four-member cyclic hydrocarbons, which make the storage life of these materials short. For long time storage, however, these liquids must be stabilized either by stabilizers or hydrogenation of the product promptly after collection.