Supercritical water gasification of microalgae over a two-component catalyst mixture

Supercritical water gasification (SCWG) of the microalga Chlorella pyrenoidosa was examined with a catalyst mixture of Ru/C and Rh/C in a mass ratio of 1:1. The influences of temperature (380–600 °C), water density (0–0.197 g/cm³), and catalyst loading (0–20 wt%) on the yields and composition of the gaseous products and the gasification efficiency were examined. The temperature and water density significantly affected the SCWG of the microalgae. The hydrogen gasification efficiency was more dependent on the temperature, while the carbon gasification efficiency was more dependent on the water density. The gaseous products mainly consisted of CH₄, H₂, CO, and CO₂, with smaller amounts of C₂-C₃ hydrocarbons. CH₄ made up half of the mole fraction of the gaseous products under most reaction conditions. A synergistic effect between Ru/C and Rh/C existed during the SCWG of the microalgae, and this effect favored the production of CH₄. The role of the catalyst mixture became indistinct at higher temperatures. Hydrogen atoms from the water were transferred to the gaseous products during the SCWG, leading to hydrogen gasification efficiencies that exceeded 100%. The main components of the bio-oil were aromatics and nitrogen-containing compounds, and the main aromatics consisted of azulene and anthracene. The nitrogen-containing compounds are potential poisons to the catalyst mixture.