Unraveling the catalytic activity of CaClOH-rich incineration fly ash in the pyrolysis of single-use plastics

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In recent years, increased population and economic growth have resulted in an explosion in demand, production, and disposal of non-biodegradable commodity plastics. Pyrolysis, the anaerobic thermochemical breakdown of plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) into lower molecular weight hydrocarbons with applications as fuels, waxes, and lubricants, has emerged as one of the most promising industrial technologies for the end-of-life treatment of plastics. While a range of inorganic materials have shown efficacy in catalyzing plastic pyrolysis and reducing their energetic demands, the use of existing inexpensive waste materials such as incineration fly ash (IFA) for this application has largely been overlooked. Herein, we demonstrate the viability of IFA as a catalyst for the pyrolysis of PE, PP, and PS, which account for almost 60% of all the waste plastics produced globally. The presence of IFA improved reaction conversions and liquid yields and resulted in positive outcomes for the pyrolysis of real-life plastic waste such as high density polyethylene (HDPE) detergent bottles and low density polyethylene (LDPE) plastic bags. Specifically, the pyrolysis of virgin HDPE pellets with IFA showed increased conversion from 46.7% to 92.8% and liquid yield from 35.1% to 81.6%, respectively. Through a set of control experiments, we determined that the catalytic activity of IFA could be attributed mainly to calcium hydroxychloride (CaClOH), which differed from other common calcium-containing minerals such as calcium oxide (CaO) and calcium chloride (CaCl2). Our findings demonstrate the unique catalytic possibilities offered by overlooked waste materials such as IFA in the upcycling of single-use plastics into liquid hydrocarbons and offer new insights on the use of different calcium-based catalysts for plastic pyrolysis.