Comparative Assessment of the Thermochemical Conversion of Freshwater and Marine Micro- and Macroalgae

Abstract

The aim of this study was to investigate differences in the thermochemical conversion properties of freshwater and marine algae as well as micro- and macroalgae on the production of biogas, bio-oils, and biochar. The pyrolysis process of all samples involved three main stages consisting of the evaporation of bound water, primary pyrolysis reactions, and the slow decomposition of the remaining carbonaceous matter. There were no obvious differences in the thermal behavior between freshwater micro- and macroalgae, with similar thermogravimetric and apparent specific-heat profiles. However, the marine alga exhibited significantly different thermal behavior to the freshwater algae. The marine alga showed a very significant endothermic reaction of bound water releasing and a dramatic high-temperature endothermic reaction, which were not observed in any of the freshwater algae. The evolution of primary volatiles showed CO2 and CO as the dominant volatiles for all species of algae. At the heating rate of 60 °C/min, the maximum liquid yield for the pyrolysis of the marine Ulva ohnoi was 55 wt %, while the range of 70–75 wt % could be achieved for the pyrolysis of the three species of freshwater algae. The bio-oils collected after heating of the samples to 500 °C under slow and fast heating rates indicated that alcohols (including phenols) and carboxylic acids were the dominant components in the bio-oils produced from the three species of freshwater algae, while nitrogen-containing organics and phenols were overwhelming in the oil from the marine alga. The bio-oils produced at two heating rates presented only minor differences in the bio-oil composition and compound contents.