Co-pyrolysis and characterization of Prosopis juliflora, coffee husk, and LDPE as a strategic waste-to-energy pathway for high-grade bio-oil production

Abstract

The integrated biomass-plastic waste valorization route represents a versatile strategic framework for advancing circular economy solutions tailored to the production of sustainable transportation fuels. The present study evaluated the synergistic non-catalytic co-pyrolytic behavior of a ternary blend comprising Prosopis juliflora , coffee husk, and low-density polyethylene (LDPE). Feedstock characterization through thermogravimetric analysis revealed that high devolatilization rates were observed in 1:1 blends of Prosopis juliflora or coffee husk with LDPE, as well as in ternary blends containing 66.67% LDPE and those with equal proportions of all three constituents. Pyrolysis experiments were conducted at three different temperatures (400, 500, and 600 °C) with a constant heating rate of 10 °C min⁻ 1 and a 15-min isothermal holding time. A blend containing equal proportions of all three feedstocks optimized bio-oil production, achieving a maximum yield of 67 wt.% at 500 °C. Conversely, biochar recovery was maximized at 35.77 wt.% when utilizing a 1:1 binary blend of Prosopis juliflora and coffee husk at 400 °C. At this lower temperature, the ternary blend also yielded the highest gas fraction (40 wt.%). Characterization of the bio-oil showed a moisture content of 31.83–33.07 wt.%, low ash content of 0.23–0.33 wt.%, pH of 4.43–5.29, density of 842–884 kg m⁻ 3 , refractive index of 1.373, boiling point of 111.8–168 °C, dynamic viscosity of 1.65–3.67 mPa·s, and a higher heating value of 17.55 MJ kg⁻ 1 . In conclusion, high bio-oil yields and its characterization profile established this blend a viable refinery feedstock for subsequent upgrading into high-grade transport fuels, providing a scalable pathway for mobility sector decarbonization.