ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Mechanistic and optimization studies of NH₄Cl-assisted Co-pyrolysis of microalgae biomass residue for product redistribution and N-doped biochar production

Desy Hikmatul Siami, Hanifrahmawan Sudibyo, Dwi Joko Prasetyo, Joko Wintoko, Muslih Anwar, and Arief Budiman

Chemical Engineering Department, Universitas Gadjah Mada, Yogyakarta, Indonesia

E-mail: abudiman@ugm.ac.id

Received: 6 April 2026  Accepted: 11 June 2026

Abstract:

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 conditions were identified 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 with errors within ± 5%, 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 process conditions.

Keywords: Spirulina platensis residue (SPR); Slow pyrolysis; Ammonium chloride; Additives; RSM; Reaction mechanism

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-026-05204-7

Chemical Papers 80 (11) 13655–13671 (2026)