Potential of “Montado” Waste as Feedstock for Thermal Gasification

Abstract

This study examines the feasibility of utilizing acorn wastes from Montado systems in the Alentejo region of southern Portugal as a sustainable feedstock for bioenergy production through thermal gasification. Approximately 45% of the annual acorn harvest remains unused, representing a locally available biomass resource with significant valorization potential. Acorn waste was characterized by proximate, ultimate, and calorimetric analyses, revealing moisture and volatile matter contents within ranges suitable for gasification. The relatively high ash content indicates potential challenges for fuel quality and process performance. Gasification experiments were carried out in a fixed-bed downdraft reactor at temperatures between 600 and 700 °C using air as the gasifying agent. The produced syngas consisted mainly of N₂, H₂, CO, CO₂, and CH₄, with methane concentrations remaining relatively stable across the tested temperatures (3.1–4.0 vol%). Increasing the gasification temperature reduced tar and char formation and improved cold gas efficiency, reaching a maximum of 68.5% at 700 °C, with a syngas lower heating value of up to 5.7 MJ/Nm³. Based on the experimental results, a techno-economic assessment was conducted for a decentralized gasification system with a nominal thermal capacity of 1.0 MWth operating 8,000 h per year. The system requires approximately 244 kg/h of acorn waste and achieves an overall electrical efficiency of 24.0%, converting 80.5% of the input energy into useful outputs. The economic analysis indicates project feasibility, with a positive net present value (0.47 M€), an internal rate of return of 12.1%, a levelized cost of electricity of 0.10 €/kWh, and a payback period of 14.2 years. These results demonstrate that acorn waste from Montado management can support decentralized bioenergy production, contributing to circular economy strategies and renewable energy targets.

Graphical Abstract