Utilization of Coconut Shell for Biohydrogen Generation: Simulation- Based Study

Abstract

The growing need for renewable energy sources has driven attention of researchers in biohydrogen as a sustainable alternative to fossil fuels. This study explored the feasibility of producing biohydrogen from coconut shell waste in Sierra Leone through thermochemical gasification. Coconut shells, which constitute a significant portion of agricultural waste in the country, were characterized through proximate and ultimate analyses to determine their suitability as a feedstock. Aspen Plus was used for simulating the coconut shell biomass, and a dual fluidized-bed gasifier was modeled, incorporating kinetic parameters from validated literatures. The process simulated the decomposition, pyrolysis, oxidation, and reduction stages with steam as the gasifying agent. The sensitivity analysis was performed to assess the impact of key operating parameters, including temperature, pressure, and steam flow rate, on syngas composition. The results showed that the model predicted mole fractions of Hydrogen 0.305 and Carbon monoxide 0.264, aligning closely with experimental values reported by (Yu et al., 2019) and (Agu et al., 2019), though Methane was underpredicted. The Root Mean Square Error values of 0.0449 and 0.0655 confirmed good model reliability and the sensitivity analysis revealed that increasing pressure decreased Hydrogen and Carbon monoxide while increasing Methane and Carbon dioxide, consistent with Le Chatelier’s principle. Higher steam flow rates initially promoted Hydrogen formation but eventually diluted the syngas, lowering overall concentrations of Hydrogen, Carbon monoxide, and Methane. The temperature increased favored endothermic reactions, increasing Hydrogen and Carbon monoxide production while reducing Carbon dioxide and Methane yields. The economic evaluation using Aspen Process Economic Analyzer demonstrated promising feasibility. The process required an estimated capital cost of USD 5.84 million and annual operating and raw material costs of USD 12.92 million and USD 10.9 million, respectively. The annual product sales projected USD 28.53 million, the payback period was 7.04 years, with a rate of return of 20%. These findings confirm that coconut shell gasification in Sierra Leone is both technically and economically viable, offering an environmentally friendly solution to waste management while contributing to renewable energy production.