CO₂-assisted catalytic pyrolysis of digestate with steel slag

Digestate pyrolysis was investigated in this study. To establish a more sustainable pyrolysis platform, we particularly selected CO₂ as a reactive gas medium. Thus, great emphasis was placed on clarifying the roles of CO₂ in digestate pyrolysis. In addition, a series of thermo-gravimetric analysis (TGA) tests was conducted from 35 to 900 °C (10 °C min⁻¹) to characterize the thermolytic behaviors of digestate in CO₂. The TGA tests demonstrated that the thermolytic patterns of digestate in both atmospheric environments (N₂ and CO₂) were the same at ≥780 °C. However, the homogeneous reactions between volatile matter (VM) from digestate thermolysis and CO₂ were not supported by the TGA tests. To confirm the homogeneous reactions, lab-scale digestate pyrolysis was conducted, which proved that the homogenous reactions initiated at ≥ 480 °C in CO₂ using one-stage and two-stage pyrolyzers. Pyrolytic products in three different phases were analyzed using micro-GC for pyrolytic gases, GC/MS for pyrolytic liquids, and ICP-OES and FE-SEM/EDX for solid residue. The homogeneous reactions resulted in enhanced CO generation while suppressing dehydrogenation. The identified CO₂ role affected the compositional modifications of the pyrolytic oil, which was achieved via shifting the carbon distributions from the pyrolytic oil to gas. However, the reaction kinetics governing the CO₂ roles was not rapid. To expedite the reaction kinetics of the CO₂ roles, steel slag was used as a catalyst. Indeed, the use of steel slag enhanced the reaction kinetics of the homogeneous reactions. As a result, the non-catalytic pyrolysis conducted in the two-stage reactor evolved more gaseous products at comparable conditions. The use of CO₂ and steel slag catalyst generated more pyrolytic gases for the pyrolysis of digestate. Copyright © 2019 Elsevier Ltd. All rights reserved.