Microwave-assisted co-pyrolysis of rice straw pellets and plastic packaging wastes for value-added products and energy recovery

Abstract

The sustainable utilization of agricultural residues and waste plastics to mitigate environmental pollution, while generating renewable energy through pyrolysis has achieved significant attention through the years. In this study, microwave-assisted pyrolysis and co-pyrolysis of plastic packaging wastes (PPW) and rice straw pellets (RSP) of different mixture compositions (10 % PPW, 20 % PPW, and 30 % PPW) have been carried out to evaluate the synergistic effect of the two different feedstocks on product yields, composition, energy recovery, energy and exergy efficiency. With an intention to avoid pulverization of the feedstocks to fine particle sizes, a unique configuration resembling a chocolate is adopted to mix the biomass pellet with sheets of PPW, while using activated carbon as a susceptor. The calorific value of the co-pyrolysis oil was higher (30–35 MJ kg−1) than the pyrolysis oil obtained from RSP (22–28 MJ kg−1). The maximum oil yield from co-pyrolysis was 40 wt% for a feed-to-susceptor ratio of 2.5:1 w/w, 10 % PPW at 600 °C, and 800 W microwave power. Compared to the pyrolysis of RSP, co-pyrolysis resulted in significant deoxygenation in the oil fraction (50–60 %). The oil composition data revealed an increase in aliphatic hydrocarbons from 37 % to 42 % along with increased aromatics from 13 % to 19 % when PPW content was increased from 10 % to 30 % in the feedstock. High energy recovery (∼91 %) and feed conversion efficiency (∼99 %) were recorded for 20 % PPW at 800 W and 600 °C. The net energy balance for co-pyrolysis was positive (9–16 MJ kg−1) at different reaction conditions, which proves the energetic feasibility of the process. At 800 W and 600 °C, the total exergy efficiency was maximum (57.7 %) for 10 % PPW feedstock, and the exergy efficiencies were lower than the energy efficiency values.