Exergy analysis and cold gas efficiency evaluation for hydrogen generation through algae biomass steam gasification coupled with water-gas shift reaction

This research presents a simulated model of hydrogen generation through the application of the steam gasification process (SGP) for algae biomass coupled with the water-gas shift (WGS) reaction. A non-stoichiometric equilibrium-based model is constructed in Aspen Plus and validated against available experimental results in the literature. Four types of algae, namely Spirulina, Chlorella vulgaris, Spirogyra, and Rhizoclonium sp., are utilized as biomass feedstocks. The impact of operating conditions, such as gasifier temperature and the steam-to-biomass ratio (SBR), on the composition of the product gas is investigated. Exergy efficiency and cold gas efficiency (CGE) for each type of algae are calculated and examined within the SBR range of 0.2-1.2 and the gasifier temperature range of 650 degrees C-1050 degrees C. The results indicate that the maximum H-2 concentration is attained at the highest SBR while there exists an optimum value for the gasifier temperature. Exergy efficiency demonstrates an enhancement with an increase in gasification temperature and SBR. Among the algae types, Spirulina exhibits the highest exergy efficiency at a temperature of 1050 degrees C (61.3 %), whereas Spirogyra demonstrates the lowest exergy efficiency at the same temperature (35 %). Furthermore, at an SBR of 0.2, Rhizoclonium sp. demonstrates the highest exergy efficiency (41.8 %), and Spirogyra exhibits the lowest exergy efficiency (27.9 %). The findings also reveal that an increase in SBR leads to a decrease in CGE, whereas an increase in gasifier temperature improves the CGE. The influence of the WGS reaction on CGE and exergy efficiency is investigated. The results indicate that the WGS reaction contributes to an improvement in exergy efficiency, but it has a negative effect on CGE.