Waste valorization through synergistic co-pyrolysis of cashew nut shell and HDPE for enhanced liquid fuel production

Co–pyrolysis of cashew nut shell (CNS) and high–density polyethylene (HDPE) waste was conducted in a fixed-bed pyrolysis system. The aim of the study is to produce fuel oil from CNS and to evaluate the synergistic effect between CNS and HDPE during pyrolysis. Co-pyrolysis with a 1:1 (wt./wt.) mixing ratio unexpectedly resulted in a liquid yield of 43.6 wt%, which is lower than the calculated value of 50.6 wt%. In addition, the liquid product consisted exclusively of wax. To enhance the synergistic effect, catalytic co-pyrolysis was conducted using HZSM-5 at catalyst bed temperatures of 510, 610, and 710 °C. The results indicated that the viscous-phase fraction constituted 53 wt% of the pyrolysis oil, and the oxygen content of the oil was 5.8 wt%, representing the lowest value among all runs conducted at a catalyst bed temperature of 610 °C. Furthermore, cashew nut shell liquid (CNSL), which is known to influence the viscous oil yield in previous studies, was investigated to clarify its effect during decomposition. Acetone–pretreated CNS with partially removed CNSL was pyrolyzed, as well as on the two-stage co-pyrolysis of CNS and HDPE. In the acetone-pretreated CNS, the viscous oil fraction accounted for 3 wt%, whereas the aqueous oil fraction accounted for 97 wt%. The characteristics of the viscous oils were quantitatively analyzed using GC-MS analysis. The results revealed that 3-methylphenol decreased from 13.1 wt% to 1.2 wt% in the viscous oil after CNSL removal. This implies that CNSL contributes to the viscous oil yield during CNS pyrolysis because of its chemical structure.