The synergistic mechanism between coke depositions and gas for H2 production from co-pyrolysis of biomass and plastic wastes via char supported catalyst

Co-pyrolysis of biomass and polyethylene(PE) wastes with different blending ratios were performed in a bench-scale fixed bed over Ni/char catalyst. The synergistic mechanism between coke depositions and gas products during co-pyrolysis was studied for better regulation of H₂ production. The results showed that feedstock blending ratio played a decisive role in competitive growth of amorphous coke and multi-walled carbon nanotubes (CNTs) on the catalyst surface. For low PE ratio (≤50 wt%) a negative synergy on H₂ yield was generated. It was ascribed to more oxygenates that were more inclined than hydrocarbons to be absorbed by porous char to form amorphous coke, which encapsulated Ni active sites and internal pore channel of catalyst, thus resulting in deactivation of catalyst. For higher PE content, Ni/char catalyst produced more than triple the amounts of H₂ yield (42.28 mmol/gfₑₑdₛₜₒcₖ) as compared to low PE ratio (11.3 mmol/gfₑₑdₛₜₒcₖ). A maximum positive synergy on syngas quality was yielded at 75% PE. Despite the high yield (37.8 wt%) of deposited coke, more hydrocarbon gas from plastic pyrolysis condensed on catalyst and promoted CNTs growth via dehydrogenation and polymerization, simultaneously generating H₂. The unique hollow tubular structure and tip-growth mode of CNTs exposed more Ni active sites and endowed catalyst with lower deactivation extent. The scission of more chain hydrocarbons was subsequently enhanced to interact with oxygenated compounds. Therefore, appropriate PE ratios (>50%) can exert a positive synergy on gaseous conversion by regulating coke nature during co-pyrolysis of biomass and plastics. Furthermore, coke structure rather than content seems to exert more significant effect.