Synergistic production of fuel-grade hydrocarbons and carbon-rich biochar from peat soil–plastic waste co-pyrolysis

Indonesia’s tropical peatlands, particularly in Kalimantan, are major carbon reservoirs but have been severely degraded by drainage and agricultural expansion. Despite ecological challenges, their high organic content offers potential for energy recovery. At the same time, plastic waste remains a critical environmental issue. This study addresses both problems by investigating the co-pyrolysis of Kalimantan peat soil (KPS) and polyethylene terephthalate (PET) waste, a combination rarely explored in previous research. Unlike conventional biomass or plastic pyrolysis, which typically involves lignocellulosic feedstocks or single polymers, this work focuses on tropical peat soil characterized by high ash content and complex humic structures. The novelty lies in applying response surface methodology (RSM) to optimize process conditions for renewable fuel production while revealing distinct synergistic interactions between KPS and PET during thermal conversion. Experiments were conducted in a semi-continuous reactor at 400–600 °C with varying KPS:PET ratios, followed by analysis using TGA, GC–MS, FTIR, SEM–EDS, and RSM optimization. Results demonstrated that PET addition significantly enhanced aromatic hydrocarbon formation (> 60% at 600 °C for 50:50 blends) while reducing oxygenates and improving calorific value. Biochar exhibited enhanced porosity and carbon content (> 80 wt.%) at higher temperatures. The optimized condition (29.10 wt.% KPS at 599.33 °C) yielded 31.35 wt.% bio-oil, 32.93 wt.% biochar, and 27.64 wt.% gas. These findings provide the first systematic evidence of peat–plastic co-pyrolysis, showing its potential for waste reduction, renewable energy recovery, and the production of fuel-grade hydrocarbons suitable for sustainable aviation fuel applications.