Experimental validation, computer-aided simulation and techno-economic evaluation of biogas production from herbal extraction waste: A case study of Azadirachta indica leaves residue

The inefficient utilization and disposal of lignocellulosic Post-Extraction Herbal Residues (PEHR) of Azadirachta indica leaves (AIL) have pose environmental challenges and limit their potential as sustainable energy resources. Despite the availability of herbal extraction residues, limited studies have investigated post-extraction Azadirachta indica leaf residues through an integrated experimental, process-simulation, optimization, scale-up, and techno-economic framework. Therefore, this study presents a laboratory proof-of-concept-experimentation and also evaluates the technical performance as well as economic feasibility of biogas production from PEHR of AIL through anaerobic digestion. Aspen Plus simulator, ASPEN Cost analyzer and CAPCOST as well as Excel Microsoft user-defined software were used for the simulation, scale-up, equipment sizing and economic analysis of laboratory-proof-of-concept experimentation. Response Surface Methodology (RSM) was applied for process modelling and optimization study. A process simulation model (PSM) closely biogas yield (194.60 mL/gVS) with a low deviation (8.11%), confirming model reliability. Response Surface Methodology (RSM) based on Box–Behnken Design demonstrated strong predictive capability (R² = 0.9994) and identified temperature and pressure as significant factors influencing biogas yield. Optimal conditions (40°C, 0.5 bar, 22.6 days) produced a maximum yield of 512.43 mL/gVS with high desirability (0.986). Techno-economic analysis indicated strong financial feasibility, with a total capital investment of 456,800.98 USD and annual net profit of 104,434.92 USD. Key indicators—NPV (276,708 USD), IRR (19%), ROI (22.86%), and payback period (4.37 years)—confirm economic attractiveness. Sensitivity analysis further revealed that profitability is most influenced by biogas yield and market price. The findings demonstrate the potential of AIL-PEHR for valorization through anaerobic digestion and provide a basis for further pilot-scale investigation and industrial implementation.