Thermodynamic Analysis of Supercritical Water Gasification of Microalgae Biomass for Hydrogen and Syngas Production

Supercritical water gasification (SCWG) is an alternative thermochemical technology for the conversion of wet biomass in a high heat value product gases, such as syngas and hydrogen. Algal biomass has been suggested as one promising option to produce biomass for use as fuels and chemical feedstocks with high area-specific yields. In this study, the thermodynamic analysis of SCWG of microalgae biomass (C. vulgaris and Spirulina Sp.) for hydrogen and syngas production was presented. The effects of pressure (240-300 bar), initial temperature (703.15-1103.15 K), microalgae concentration (5-20 wt%) and coreactant addition (CO2 and CH4) were evaluated with respect to production and final characteristics of the products. The formation of coke and the energetic characteristics of the SCWG were discussed. The analysis was conducted using two non-stoichiometric thermodynamic models, based on Gibbs energy minimization and entropy maximization methods. The virial equation of state was used to represent the non-ideality of the system. Both problems were formulated as optimization problems (non-linear programing) and the software GAMS 23.2.1 in combination with the CONOPT solver, were used to solve them. The results obtained were compared with previously published experimental and simulated data obtained in the literature, with good agreement between them. The syngas production was low for all conditions tested, but the addition of a CO2 and CH4 as a co-reactant proved to be effective way to increase the syngas production with a H2/CO molar ratio close to 2 (ideal for Fischer-Tropsch applications). The SCWG presents exothermic behavior for low initial temperatures. In the region of high initial temperatures the systems showed to be less exothermic.