Co-pyrolysis of waste plastic and solid biomass for synergistic production of biofuels and chemicals-A review

The amount of plastics disposed from modern lifestyles have increased sharply in recent years. Solid biomass is an abundant energy resource that exists worldwide. Transformation of these waste plastics and solid biomass feedstock mixtures via co-pyrolysis can provide synergistic product enhancement for fuels and value-added products. The produced products can be used as chemicals and pollutant sorbents to foster eco-friendly pathways for waste management and sustainability. Progress into this avenue of waste disposal and energy production is the focus of this review. Properties of characteristic solid feedstock mixtures are discussed with focus on elemental composition, proximate analysis, and heating value. Effective H to C ratio of the different feedstocks is evaluated for asserting the quality of petrochemical equivalent products produced from co-pyrolysis of plastic wastes and biomass. The characteristics of polyethylene terephthalate (PET), high density and low-density polyethylene (HDPE & LDPE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS) and other major plastic waste components are discussed with focus on synergistic effects attainable by co-pyrolyzing them with biomass. State-of-the-art experimental methods for co-pyrolysis investigation are reviewed in detail using TGA, pyrolyzer, fixed bed reactor, fluidized bed reactor, microwave, and multi-step reactors using GC, MS, and FTIR diagnostics. In addition, different catalytic co-pyrolysis reactors are compared and discussed at different ratios of feedstock to catalyst, reactor temperature, and other operational parameters along with an in-depth understanding of several catalytic processing (ZSM-5 based catalyst, transition metal-based catalyst, multipurpose catalysts and ex-situ catalyst) for favorable products yield. Co-pyrolysis of waste plastic and solid biomass mixtures are reviewed for insights into liquid products for fuels and chemicals, as well as yield and composition of gases and solid residues evolved along with surface characteristics of the solid residues obtained from the selected configurations. The challenges and opportunities envisioned for the development in co-pyrolysis of several solid organic waste and plastic feedstock mixtures are also discussed. The goal was to provide favorable feasible pathways for clean and efficient disposal of plastic wastes with the incorporation of waste biomass for enhanced synergistic effects in waste disposal along with the recovery of energy and value-added products.