Pyrolytic conversion of waste plastics to energy products: A review on yields, properties, and production costs

In recent years, about 370 million tonnes of waste plastic are generated annually with about 9 % recycled, 80 % landfilled and 11 % converted to energy. As recycling of waste plastics are quite expensive and labour-intensive, the focus has now been shifted towards converting waste plastics into energy products. Pyrolysis of waste plastic generates liquid oil (crude), gas, char and wax among which liquid oil is the most valuable product. In this review, emphasis has been given on the pyrolysis products yield from both thermal and catalytic pyrolysis and the factors that affect pyrolysis products yield. The use of homogenous catalysts, for example AlCl₃, can significantly improve the quality of waste plastic pyrolytic oil (WPPO), reduce time and energy consumption of the process, and help remove the contaminants of waste plastic. This study also thoroughly reviewed physico-chemical properties of WPPO to understand their thermal stability, elemental composition, and functional groups. Although liquid oil exhibits comparable heating value with commercial fuel (diesel/petrol), for example higher heating value of Polypropylene (PP) and Polyethylene (PE) are 50 and 42 MJ/kg which is between 42 and 46 MJ/kg for commercial diesel the other properties depend on several parameters such as plastic and pyrolysis reactor types, temperature, feed size, reaction time, heating rate and catalysts. A techno-economic analysis indicate that the liquid oil production cost could be about 0.6 USD/l if plant capacity is ≥175,000 million litres/year with a breakeven of 1 year. After-treatment of WPPO through distillation and hydrotreatment is recommended for improving the physio-chemical properties comparable to commercial fuel to use in automobile applications. This paper will be a valuable guide for stakeholders, and decision and policy makers for proper utilization of waste plastics.