Response surface methodology for optimizing the higher heating value of biochar briquettes prepared by pyrolysis and densification of cottonseed shells (Gossypium hirsutum)

This study aims to determine the optimal conditions for producing biochar briquettes from cottonseed shell residues (Gossypium hirsutum) generated by an industrial vegetable oil processing unit. A Box–Behnken experimental design with three factors—pyrolysis temperature, binder content, and compaction pressure—was employed to evaluate their combined effects on the Higher Heating Value (HHV) of the produced briquettes. Each factor was investigated at three coded levels (−1, 0, and +1). The results revealed that the combined pyrolysis and densification processes significantly enhanced the energy performance of the raw biomass. The HHV increased from 18.507 MJ·kg⁻¹ for the raw material to values ranging between 21.407 and 28.165 MJ·kg⁻¹ for the produced briquettes. These values exceed the minimum requirements for densified solid biofuels specified by the ISO 17225-1:2021 standard, indicating their suitability for domestic and industrial energy applications. The Response Surface Methodology (RSM) results indicated that the maximum HHV within the studied experimental domain was obtained at the coded levels −1, −1, and −1 for pyrolysis temperature, binder content, and compaction pressure, respectively. Under these conditions, the model predicted a maximum HHV of 28.290 MJ·kg⁻¹. The process is not only technically efficient but also economically viable, while complying with the principles of environmental sustainability. This study therefore highlights an innovative pathway for the valorization of this abundant agricultural residue, which has so far been utilized in industry in an unsustainable manner, and contributes to the diversification of renewable energy resources as well as to the promotion of the circular economy.