Thermal decomposition of polypropylene plastics through vacuum pyrolysis

The technique of thermal pyrolysis has a great contribution to waste-to-energy platform and circular economic mission. This study used an internal heating process for the vacuum pyrolysis of plastic waste. Eight heating tubes were placed inside a reactor to accelerate heat transfer to the feedstock. Uniform temperature distribution was achieved in <1 h. After 90 min, three phases of pressure profiles were identified: decomposing samples, maximum feedstock cracking, and stability occurrences. Internal pressure decreased with the reduction of condensable gas. Pyrolytic oil was similar to conventional fuels (gasoline and diesel), with the percentage of lower hydrocarbons C₅–C₁₀ at 73%, C₁₁–C₂₂ at 18%, and >C₂₂ at 9%. Gas chromatography–thermal conductivity detector analysis of noncondensable gases revealed that the main product of PP with 75% area was C₂H₆. Furthermore, the Taguchi method was used to optimize the vacuum pyrolysis of waste to energy. Optimal operating conditions for maximum pyrolytic yields were as follows: temperature 425 °C, heating rate 15 °C/min, residence time 6 h, and output flow rate 0.1 L/min. Moreover, the physical properties, cetane number, soot propensities, and auto-ignition temperature of pyrolytic oil were comparable to petroleum diesel. The heating value of the oil product was 55.96 MJ/kg, which was higher compared with petroleum fuels. Vacuum pyrolysis mass, carbon, and energy balances were also determined. This study demonstrated the novel use of internal heating in vacuum pyrolysis. Ultimately, this study provided a new technique of internal heating method in vacuum pyrolysis.