Process intensification of sunflower seed hull pyrolysis via CO2-assisted catalytic upgrading

Abstract

To optimize the thermochemical valorization of sunflower seed hull (SSH), a food-industry residue, this study systematically investigated a CO2-assisted pyrolysis process. The primary objective was to optimize syngas production and fundamentally investigate the reactive behavior of CO2. To achieve this, the process configuration was stepwise upgraded from single-stage (SP) to multi-stage (MP), and ultimately to Ni-based catalytic pyrolysis (CP). In SP, CO2-induced homogeneous reactions increased the CO yield from 7.06 mmol (under N2) to 24.34 mmol (under CO2), while decreasing the bio-oil yield from 45.54 to 39.20 wt%. Although MP was introduced to intensify syngas production, the additional effect of CO2 was limited. By contrast, CP strengthened the CO2 functional role. Under CO2, the H2 and CO yields reached 21.12 mmol and 96.14 mmol, respectively, while the bio-oil yield decreased to 5.63 wt% (compared with 8.49 wt% under N2). CO2 activated on the catalyst surface formed reactive intermediates that reacted with volatiles, promoting deoxygenation and cracking to enhance syngas production. Decreasing the carrier-gas flow rate further enhanced the CO2-over-N2 effect on syngas yield, from 9.30% (800 mL min−1) to 15.57% (200 mL min−1), indicating residence-time-dependent CO2 participation. Consequently, the optimized CP under CO2 (200 mL min−1) achieved a maximum energy recovery of 48.79%, a 5.5-fold increase over SP under N2. To conclude, food-industry residues can be sustainably upgraded into value-added syngas via CO2-assisted catalytic pyrolysis, supporting the progress of clean energy system.