Anaerobic digestion of açaí seeds hydrolysate integrated with heat and power cogeneration: technical and economic analysis of a net negative emission route

Brazil is a major açaí fruit producer, but the seeds, representing 85% of the fruit mass, often lack adequate destination. These seeds are rich in mannan, a mannose polymer, and can serve as substrate for biomethane production, although few studies have evaluated its economic viability. This work conducts a techno-economic analysis of four açaí seed valorization scenarios using Aspen HYSYS (16 t/h scale), all with energy self-sufficiency via cogeneration: Scenario 1 (benchmark) uses direct combustion for electricity, while Scenarios 2-4 integrate acid hydrolysis, anaerobic digestion of açaí seed hydrolysate, combustion of solid residue for electricity, and biogas upgrading to biomethane and supercritical CO 2 . Digestate concentration is explored in Scenarios 2 and 3 for biofertilizer production. These two scenarios differ only in operating conditions: Scenario 3 increases acid consumption to enhance hydrolysis efficiency, which boosts biomethane output. All options are economically viable, but Scenario 3 is the only one that outperform the benchmark in economic terms and showed resilience to reductions in biomethane price, whereas the decarbonization credit of the Brazilian Biofuels Policy reduced the breakeven price from 23.17 to 21.45 USD/MMBTU. The production of electricity and biomethane avoids 0.451 t CO2eq /t seed of emissions, while the supercritical CO 2 corresponds to 0.256 t CO2 /t seed of biogenic CO 2 removed from atmosphere. Supercritical CO 2 sales and carbon credits contributed significantly to revenues, which highlights the importance of monetizing CO 2 mitigation to negative-emission projects. The present biorefinery concept supports sustainable development in the Amazon region, by valorizing açaí seeds into multiple value-added products while preventing inadequate disposal. • Seeds represents 85% of the fruit's mass, in most cases, without proper destination. • Biomethane, biofertilizer and electricity production with açaí seeds was evaluated. • Techno-economic analysis was performed with simulations using Aspen HYSYS software. • All scenarios are economically viable. CCS promoted negative carbon emissions. • Supercritical CO 2 and decarbonization credits (CBIO) improved economic performance.