Techno-Economic Assessment of Hydrogen and CO2 Recovery from Broccoli Waste via Dark Fermentation and Biorefinery Modeling

Broccoli waste (Brassica oleracea), comprising non-commercialized stems and leaves, represents a valuable substrate for bioenergy and commodity recovery within agro-industrial systems. This study evaluates the potential of dark fermentation (DF) to produce hydrogen (H2) and carbon dioxide (CO2) from unpretreated broccoli residues. Batch experiments (120 mL) yielded maximum gas production rates of up to 166 mL/L·d, with final compositions of 41.43 mol% and 58.56 mol% of H2 and CO2, respectively. Based on these results, two biorefinery models were simulated using COCO v3.10 and SuperPro Designer® v12.0, incorporating absorption and cryogenic separation technologies in the purification stage. Two scenarios were considered: Option A (169.82 kmol/day; H2: 0.5856 mol fraction, CO2: 0.4143 mol fraction) and Option B (72.84 kmol/day; H2: 0.6808 mol fraction, CO2: 0.3092 mol fraction). In both configurations, the purities of the final streams were the same, being 99.8% and 99.8% for both H2 and CO2, respectively. However, energy consumption was 43.76% higher in the cryogenic H2/CO2 separation system than in the absorption system. Noteworthily, this difference does not depend on the stream’s composition. Furthermore, from a financial standpoint, the cryogenic system is more expensive than the absorption system. These findings confirm the feasibility of designing biorefineries for H2 production with high CO2 recovery from broccoli waste. However, the economic viability of the process depends on the valorization of the secondary effluent from the fermentation reactor, which may require subsequent anaerobic digestion stages to complete the degradation of residual organic matter and enhance overall resource recovery.