Thermochemical conversion of wastewater microalgae: The effects of coagulants used in the harvest process

Abstract

Microalgae are a promising alternative energy source for the future due to their high growth rate, cultivation potential in a wastewater environment, and its higher heating value (HHV). However, until nowadays, only the properties for pure species of microalgae that disregard the presence of coagulants used at the harvesting stage are known. In this study, the effects of these coagulants on the thermochemical conversion of wastewater microalgae were evaluated. The results showed a catalytic effect of coagulants. Tannin-based polymer reduced devolatilization temperature peaked at 308 to 274 °C. Although the maximum devolatilization rate occurred in microalgae without coagulants, 4.57%/min at 308 °C, polyquaternium polymer accelerated the total biomass degradation. At a temperature of 892 °C, 14.1% of microalgae remained to be degraded, while in the presence of this polymer, it was only 7.5%. This coagulant presented the best results, such as low cost, high efficiency, a small reduction in HHV, and improvement in the thermochemical behavior of microalgae biomass. The aliphatic amines polymer was the only coagulant that showed chlorine in the ash analysis, preventing its release as acid gases. Therefore, it was suggested as the better coagulant along with the polyquaternium polymer. For inorganic coagulants, the large amount added prevented an in-depth catalytic assessment, and the results portray the effect of coagulant mass incorporated into microalgae more than the catalytic effect. Iron and aluminum coagulants reduced significantly microalgae HHV from 21.58 MJ/kg to 12.91–14.45 MJ/kg.