Microalgae cultivation and harvesting for bioenergy production

Hydrothermal carbonization of microalgae biomass produced in agro-industrial effluent: Products, characterization and applications

Hydrothermal carbonization is a thermochemical treatment whose objective is to convert carbohydrate components of a given biomass into carbon-rich material in an aqueous medium. Biomass of wastewater grown microalgae is among the various potential biomasses for this route. However, operational parameters of hydrothermal carbonization for different types of biomass are still being investigated. In general, larger temperature ranges (180-260 °C) are applied to woody biomasses, which have fibrous and/or ligneous structures and, therefore, are more thermally stable than algae biomass. This study presents the hydrothermal carbonization of microalgae biomass cultivated in an agro-industrial effluent. For this purpose, a Parr reactor was operated at different temperatures (130, 150 and 170 °C) and retention times (10, 30 and 50 min). Results showed improvements in the properties of the hydrochar, mainly energy yield and carbon concentration, after the thermochemical treatment. Energy recovery was improved, as well as hydrophobicity of the carbonized material. It was observed that in the retention time of 10 min, the increase in temperature provided an increase of 7.53% in the yield of solids. On the other hand, in the retention times of 30 and 50 min, when the temperature was increased, the solid yield decreased 6.70% and 0.92%, respectively. Thus, the highest yield of solids (77.72%) and energy (78.21%) was obtained at the temperature of 170 °C and retention time of 10 min. There was a high ash content in the raw biomass (32.99%) and an increase of approximately 3% in the carbonized material, regardless of the applied treatment. With the exception of potassium and sodium, the other macro and micronutrients were concentrated in the hydrochar after thermochemical treatment, indicating the potential of the material for agriculture application, in addition to energy use. Results showed that the retention time was the most significant operational parameter of the process.

Harvesting freshwater algae with tannins from the bark of forest species: Comparison of methods and pelletization of the biomass obtained

Toxic cyanobacteria growth rates have increased in recent decades due to climate change and human activities. Microalgae, with their ability to produce a large amount of biomass, are considered as a source of energy that can be used to produce biofuels. The aim of this study is to test four different microalgae harvesting methods (sedimentation, coagulation-flocculation, pH variation, and centrifugation) in order to find which is best suited to the A Baxe reservoir, which has been suffering from cyanobacterial blooms in recent years. Centrifugation proved the most efficient method (85.74%-1790 RCF), but it can induce cell rupture. Natural sedimentation and pH variation obtained similar results at 49.36% and 49.02% respectively. Although all four methods have advantages, our results reveal that coagulation-flocculation, using 10 mg/L of Pinus pinaster, results in a removal efficiency of 68.10%, making it the most suitable method, though with 20 mg/L the performance was lower (66.03%). To minimise environmental waste, the microalgae removed were then transformed into pellets to be used as biofuel, with a higher heating value (HHV) of 21,196.96 ± 1602.33 kJ/kg. The pellets obtained from the microalgae residue did not meet all the requirements for use as biofuels, but microalgae biomass could be mixed with other sources and therefore looks like a promising option for the future.