Computational Framework for Optimal Design of Integrated Biorefineries Using Agricultural Waste Streams in South East Nigeria

The South East geopolitical zone of Nigeria generates diverse agricultural residues including palm kernel shells, rice husks, cassava peels, and yam peels that individually underpin fragmented small-scale processing enterprises, but whose integrated valorisation in a biorefinery framework could generate substantially higher economic and energy returns. This study develops a computational framework for the optimal design of integrated biorefineries that simultaneously maximise economic value and minimise environmental impact from these four waste streams in South East Nigeria. An original multi-product, multi-pathway biorefinery superstructure is formulated as a mixed integer nonlinear programming problem in GAMS, encompassing 47 processing pathways including fermentation, pyrolysis, anaerobic digestion, hydrothermal carbonisation, solvent extraction, and biochemical conversion, producing 18 potential products including bioethanol, biogas, activated carbon, phenolic compounds, and animal feed. Techno-economic parameters were derived from pilot-scale experimental data and literature values adjusted for Nigerian construction, labour, and raw material costs. Life cycle assessment environmental impact categories were embedded as objective function terms using the ReCiPe midpoint methodology with Nigerian electricity grid characterisation. The multi-objective optimisation framework generated a Pareto-optimal surface of 143 solutions. The economically optimal solution achieved a net present value of 2.8 billion Naira over 20 years for a 100-tonne per day dry waste input scale, with a bioethanol-biogas-activated carbon product portfolio. Environmental optimisation shifted the portfolio toward biogas and hydrochar products, reducing global warming potential by 34% at a 22% economic cost. The framework provides decision support for investors and policymakers designing integrated agricultural biorefinery clusters in South East Nigeria. Keywords: biorefinery, superstructure optimisation, MINLP, agricultural waste, South East Nigeria