Pyrolysis of Waste Polypropylene Plastics for Energy Recovery: Optimization of Product Yield and Quality on a Commissioned Pilot-Scale Setup

Plastic recycling via pyrolysis has been identified as one of the promising routes to managing these recalcitrant polymers due to its ability of handling significant levels of contamination and consequently yielding products with huge prospects for fuel and chemical application. Numerous investigations of this process have been reported in the literature. However, most of these investigations were largely performed on microgram and milligram scales. Although these investigations have contributed immensely to understanding fundamental reaction mechanisms and addressing initial process developments, some technical barriers between these stages and industrial-scale applications still remain. Subsequent to our previous study that investigated the pyrolysis of waste polypropylene (PP) at the bench-scale, this work centered on the optimization of waste PP pyrolysis on a 5 kg/h pilot-scale setup that was designed, assembled, and successfully commissioned as part of the study. In addition to the optimization on pilot-scale, condensable products obtained from the scale-up investigations were further subjected to fuel tests to ascertain how their physicochemical properties compare with those of commercial diesel and gasoline fuels. Results revealed that the effects of process parameters, particularly temperature on yield distribution and composition of pyrolytic products recovered from the pilot-scale investigations, differed from bench-scale largely due to variances in configuration of the setups. Nevertheless, these effects on the trends of the product yield distribution were very comparable for both stages. Unlike bench-scale investigations, diesel range compounds predominated all condensable products recovered from the pilot-scale investigations at the expense of gasoline, which was corroborated to the effects of the forward movement of the piston feeder in swiftly removing volatiles from the hot reactor zone, thereby limiting severe cracking of heavier diesel range components into lighter gasoline range fractions. Although the physicochemical properties of these condensable products have demonstrated that they will readily combust in a diesel engine, they have also exhibited hybrid characteristics of both commercial diesel and gasoline fuels and require further downstream processing in a refinery to meet commercially acceptable standards.