Supercritical water gasification of microalgae and their constituents in a continuous reactor

Abstract

The supercritical water gasification of Nannochloropsis gaditana microalgae has been studied in term of process development at 24 MPa and 663 °C. Gasification has been investigated using a 200 cm3 down flow reactor working continuously. The effect of microalgae slurry concentration, with or without alkali catalysts, was studied. Moreover, the role played by different constituents of microalgae on gasification was investigated. Particularly, gasification of amino acids, carbohydrates and model mixtures among them was studied. Nannochloropsis gaditana was successfully gasified up to 97.4 wt% as gasification efficiency and 86.0 wt% as carbon efficiency. The product gas is mainly composed of hydrogen (52.0%), methane (17.9%) and CO2 (23.0%). Gasification of pure amino acids, which are the main components of microalgae, showed a very different resistance to gasification depending on the molecular structure of the molecules. Among the amino acid that have been studied in this work, gasification resistance is in the order glycine > leucine > glutamic acid, whereas CH4 concentration increases in order leucine > glutamic acid > glycine due to the number of methyl groups that are present in the molecules. From this work it can be concluded that gasification of microalgae is feasible in a continuous mode and that, depending on their molecular structure, amino acids offer a different resistance to gasification. These results can be of help in evaluating the feasibility of gasification of defatted microalgae after extraction of the oil.