Thermochemical Recycling of Household Polyolefin Waste

Many plastic wastes, particularly contaminated plastic packaging or fabrics, cannot be easily recycled. Among chemical recycling methods, pyrolysis combines low greenhouse gas emissions with low infrastructure requirements. A shaker reactor was adapted to model rotary kilns for cold start pyrolysis, where the waste is progressively heated, and fluidized beds/stirred reactors for hot start pyrolysis, where the waste is introduced into a hot reactor and kept at a constant temperature. For cold start pyrolysis, no feedstock preprocessing is necessary, and liquid yields of up to 80% by weight (wt %) were achieved, with no observable effect from different vapor residence times. Hot start pyrolysis gave similar yields but is more complex to implement. Thus, cold start pyrolysis is preferred. The hydrocarbon products from both conditions were analyzed by gas chromatography-mass spectroscopy (GC-MS). Cold start product distributions clustered according to polyethylene and polypropylene-based materials but were unaffected by vapor residence times. The reaction start type did not significantly affect the median molecular carbon count, with the hot start yielding more aromatics. The heat requirements for cold start pyrolysis were 1.0 kJ/g for polyethylene-based feedstocks and 0.8 kJ/g for polypropylene-based feedstocks. This heat may be provided by renewable sources or by combusting less than one-third of the pyrolysis gases.