Mechanistic and Optimization Studies of NH₄Cl-Assisted Co-Pyrolysis of Microalgae Biomass Residue for Enhanced Bio-Oil and N-Doped Biochar Production

This study investigated the NH₄Cl-assisted slow pyrolysis of Spirulina platensis residue (SPR) as a sustainable strategy for integrated carbon and nitrogen recovery. A central composite design (CCD) was applied to evaluate the interactive effects of pyrolysis temperature (400–600 °C) and NH₄Cl loading (0–4 wt%) on product yields and elemental distributions. Response surface methodology (RSM) was employed to model and optimize multiple responses, including carbon and nitrogen yield in bio-oil, biochar, and aqueous phase, as well as the O/C and N/C atomic ratios in bio-oil. Results showed that moderate NH₄Cl doping significantly enhanced aqueous-phase and biochar yields while promoting nitrogen retention and deoxygenation. The optimal condition was found at 510.67°C and 2.99 wt% NH₄Cl, yielding bio-oil (7.21%), biochar (38.52%), aqueous phase (34.81%), and gas (19.31%). Mechanistic analysis revealed that SPR components undergo convergent transformation via Maillard condensation, Paal–Knorr cyclization, Diels–Alder reaction, and dehydrogenation, forming key bio-oil compounds such as 2-methylfuran, phenolics, indoles, ethylbenzene, and xylene, along with nitrogen-doped biochar. Model predictions were experimentally validated within ±5% error, confirming the robustness of the RSM approach. Overall, this work offers a promising route for converting microalgal waste into value-added fuels and functional materials under optimized catalytic conditions.