Review on synergistic effects during co-pyrolysis of biomass and plastic waste: Significance of operating conditions and interaction mechanism

The quantity of organic solid waste (OSW) discharged by the public has increased significantly in recent years. Herein, we consider bio-waste and plastic waste. Bio-waste, also known as biomass, is a sustainable and abundant energy source available in diverse forms. Plastic waste is a cheap hydrogen source contained within OSW. The transformation of OSW via pyrolysis involves the thermochemical conversion of biomass and plastic. This conversion can mitigate waste accumulation issues and lead to synergistic product improvements for fuels and chemicals. This paper reviews the occurrence and extent of the synergistic/interactive effect during the co-pyrolysis of plastic waste and biomass. The influence of various factors, including the plastic type, biomass type, mixing ratio, reactor type, heating rate, reaction temperature, and catalysts, on the synergistic effect is considered. Furthermore, reasonable interaction mechanisms related to the synergistic effect during co-pyrolysis are presented. The outcome of this review revealed that the interaction mechanisms by which the synergistic effect may occur are the transfer of active hydrogen radicals from plastic to the biomass unstable oxygenated radicals, the catalytic activity of the alkali/alkaline earth metal species in biomass, and the heat and mass transfer during the co-conversion. Biomass pre-treatment, the use of catalysts, and the similarity between the chemical structure of the biomass and the plastic used can strengthen the interactions. Synergistic effects are likely to occur to a great extent at a low heating rate at high temperatures. The conclusions regarding the blend ratio are inconclusive.