Design and Theoretical Performance Evaluation of a Small-Scale Downdraft Biomass Gasifier Using Groundnut Shells for Rural Energy Access in the Gambia

Energy poverty and rising fossil fuel costs present significant barriers to socio-economic development in rural Gambia. This research presents the design and theoretical performance evaluation of a small-scale, fixed-bed downdraft biomass gasifier specifically engineered to utilize locally abundant groundnut shells ( Arachis hypogaea ) for decentralized power generation. Addressing the technical challenge of high tar content associated with lignin-rich agricultural residues, the study proposes a reactor with an optimized internal diameter of 175 mm, derived from a specific hearth load of 125 kg/h/m². The methodology integrates a high-temperature oxidation zone reaching 1000∘C to facilitate the thermal cracking of tars, coupled with a multi-stage cleaning circuit comprising an 80 mm cyclone separator and a lime-charcoal scrubber. Utilizing thermodynamic modeling and established engineering benchmarks, the system is designed to process 3 kg/h of feedstock at an optimal moisture content of 10.62%. Theoretical validation indicates a Cold Gas Efficiency (CGE) of 70% and a predicted syngas energy density of 2965.6 kJ/Nm³, consisting of approximately 13.0% CO and 8.0% H₂. These parameters satisfy the operational requirements for a 2.5 kVA internal combustion engine while maintaining tar concentrations below the 100 mg/Nm³ safety threshold. Beyond technical feasibility, the study establishes a "zero-waste" framework by repurposing solid bio-char and gypsum byproducts as agricultural soil amendments. This research provides a scalable technical blueprint for converting agricultural liabilities into sustainable energy assets, offering a viable strategy for mitigating deforestation and enhancing energy sovereignty in the North Bank and Lower River regions of The Gambia.