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.