Catalytic pyrolysis of cotton stalk for bio-oil production: Effectiveness of CaO and Fe 2 O 3 catalysts in ex-situ mode

The study explores ex-situ catalytic pyrolysis of cotton stalk, an abundant agricultural residue, using calcium oxide (CaO) and ferric oxide (Fe₂O₃) to improve bio-oil quality and carbon utilization. Pyrolysis was performed at 450–550 °C, with optimal bio-oil yield at 500 °C. Adding 40 wt.% CaO reduced total bio-oil yield by 22.7% but increased the organic fraction by 139%, achieving an organic phase yield of 15.8 wt.% and a high heating value (HHV) of 30.5 MJ/kg. This enhancement resulted from improved deoxygenation, acid neutralization, catalytic cracking, and CO₂ capture through carbonation. Conversely, 10 wt.% Fe₂O₃ raised the organic fraction by 25.8%, reducing total bio-oil yield to 26.4 wt.%. Fe₂O₃ catalysis promoted Fe3⁺/Fe2⁺ redox cycling, encouraging vapor-phase reforming, oxygen transfer, and water-gas shift reactions, enriching pyro-gas with H₂, CO, and CH₄. Under optimal conditions, the organic phase HHVs were 30.5 MJ/kg for CaO and 28.1 MJ/kg for Fe₂O₃ catalysis. CaO’s strong basicity facilitated decarboxylation, decarbonylation, and dehydration of oxygenated intermediates. Fe₂O₃’s redox properties enhanced oxygen transfer, reforming, and hydrogen generation. Post-reaction analyzes revealed CaO transformed into CaCO₃ and Ca(OH)₂, while Fe₂O₃ partially converted to Fe₃O₄, FeO, and Fe(OH)₃, consistent with carbonation, hydration, and redox processes during pyrolysis. These results highlight the potential of using agro-residues for sustainable waste-to-energy conversion, enabling production of upgraded bio-oil with high calorific value and hydrogen-rich pyro-gas.