Performances of syngas production and deposited coke regulation during co-gasification of biomass and plastic wastes over Ni/γ-Al2O3 catalyst: Role of biomass to plastic ratio in feedstock

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In order to improve the quality of syngas and regulate the deposited coke, co-gasification of biomass and polyethylene wastes without and with steam were investigated in a bench-scale fixed bed reactor, where the feedstock was pyrolyzed at the temperature of 600 °C followed by catalytic reforming at 800 °C over Ni/γ-Al2O3 catalyst. The influences of different biomass to plastic ratios in feedstock on the quality of gas products were analyzed by means of gas chromatography. The as-formed deposited coke was examined by TEM, Raman, FTIR and TPO analysis. The results showed a synergistic effect on both gas and tar yields when PE was co-fed to RH, especially at PE content of 50%. For higher PE content (>50%), the total gas and H2 decreased due to more chain hydrocarbons with relatively large molecular size derived from PE volatiles being more difficult to be cracked than oxygenates from pyrolysis of biomass. Two natures of the deposited coke were identified as amorphous carbon and multi-walled carbon nanotubes (CNTs). With increasing PE proportion, CNTs growth exhibited a denser distribution with uniform diameter (~18 nm) and longer tube walls extending to a few microns. In the absence of steam, amorphous carbons was the leading type in coke depositions. The introduction of steam improved the quality and purity of CNTs and decreased amorphous carbon remarkably, owning to a stronger interaction between steam and amorphous carbon rather than CNTs with high graphitization, leading to high CNTs proportion of 61.5%. A significant synergistic effect on the CNT growth was observed particularly for 50–75% of PE. The CNT-predominated deposited coke slowed down the deactivation rate of catalysts, thus producing positive impact to total gas production.