• Energy Research

This article presents a comparative evaluation of pyrolysis and hydrothermal liquefaction (HTL) for obtaining biofuel from microalgal biomass (MAB). The research was carried out using biomass of a stable microalgae-bacteria consortium based on Arthrospira platensis. A. platensis was chosen because of its simple cultivation and harvesting. Pyrolysis was carried out at temperatures of 300, 400, 500, and 600 °C with a constant rate of temperature change of 10 °C/min; HTL was carried out at temperatures of 270, 300, and 330 °C. The bio-oil yield obtained by HTL (38.8–45.7%) was significantly higher than that of pyrolysis (up to 21.9%). At the same time, the bio-coal yields using both technologies were almost the same—about 27%. Biochar (bio-coal) can be considered as an alternative strategy for CO2 absorption and subsequent storage since it is 90% geologically stabilized carbon.

Hydrothermal liquefaction of different microalgae samples ( Arthrospira platensis cultivated by our research group) – fresh (directly after harvesting), dried and frozen – have been performed. In hydrothermal liquefaction process, the samples were heated up to 300°C for 30 min and kept at a constant temperature for 60 min. Then dichloromethane was added to the samples to extract the oil fraction. The products obtained after aqueous and dichloromethane solutions evaporation are referred to as water soluble organics and bio-oil correspondingly. The experiments on hydrothermal liquefaction of microalgae pre-treated in different ways were conducted for three independent harvest samples. The average values of bio-oil yield in the experiments with fresh, dried and frozen microalgae were equal to 44.07%, 39.97% and 39.65%, respectively. The average yields of water soluble organics were equal to 19.34%, 29.00% and 21.43% respectively. In all the experiments, the highest yield of bio-oil was reached for fresh microalgae. From this point of view, direct hydrothermal liquefaction processing of fresh microalgae seems to be more preferable that further enhances the advantage of hydrothermal liquefaction in comparison with other biomass-to-biofuel conversion methods.

The effect of thermal treatment of aluminum core-shell particles on their oxidation kinetics in water for hydrogen production was investigated. The samples were obtained by dividing dried aluminum powder, partially oxidized by distilled water, into eight portions, which were thermally treated at temperatures of 120, 200, 300, 400, 450, 500, 550 and 600 °C. Alumina shell cracking at 500–600 °C enhances hydrogen generation due to uncovering of the aluminum cores, while sharp thickening of the protective oxide film on the uncovered aluminum surfaces at 550–600 °C significantly reduces reactivity of the core-shell particles. For these reasons, after reaction with distilled water at 90 °C for two hours, the highest hydrogen yield (11.59 ± 1.20)% was obtained for the sample thermally treated at 500 °C , while the yield for aluminum core-shell powder without heat treatment was only (5.46 ± 0.13)%. Another set of experiments employed multiple consecutive cycles of alternating oxidation by water and thermal treatment at 500 °C of the same powder sample. As predicted, the hydrogen yield gradually decreased with each subsequent experiment. The series of six cycles resulted in a total hydrogen yield of 53.46%.

Direct study of CO2 capture efficiency during microalgae Arthrospira platensis cultivation at high CO2 concentrations was carried out. Microalgae were grown in a 90 L photobioreactor on Zarrouk’s medium prepared with distilled water. Three 15-day experiments were carried out with different initial CO2 concentrations: 1, 5, and 9 vol.%. During the experiments, both the change in the optical density of the microalgae suspension and the direct change in the CO2 concentration in the chamber were measured. The maximum decrease in CO2 concentration due to the growth of microalgae was 0.10 vol.% (CO2)/day in the experiment with an initial CO2 concentration of 5 vol.%. Growth rate of biomass density was 79.4, 76.3, and 48.4 (mg/L)/day at 1, 5, and 9 vol.% CO2 concentrations, respectively. During the experiment with initial CO2 concentrations of 1 and 5 vol.%., pH of the culture medium was increased, but pH was decreased from 9.2 to 8.8 at 9 vol.%. In general, good viability (high quality of biomass and high rate of its growth) of Arthrospira platensis was established at 1 and 5 vol. (CO2)%, while massive death of Arthrospira platensis cells was observed in the experiment with 9 vol. (CO2)%. Biochemical analysis of the resulting biomass revealed a decrease in the content of lipids and proteins with an increase in CO2 concentration.

Hydrothermal liquefaction of microalgae biomass at various temperatures was studied. The products were analyzed by thermogravimetry, elemental analysis, and gas-chromatography—mass-spectromeny. It was concluded that the temperature affected the product yield and composition. The quantitative contents of the major components in the gasoline fraction of the produced bio-oil were determined. The main components in the gasoline fraction were aromatic hydrocarbons, alkanes, and cycloalkanes. The hydrothermal processing products also contained significant quantities of phenols, organic sulfides, and nitrogenous organic compounds that prevented their direct use as fuel components.

Thermochemical methods are regarded as promising approach for managing sludge, that can achieve resources and energy recovery, volume reduction followed by efficient elimination of microorganisms. This review highlights an extensive description of the implementation of thermochemical technologies involving pyrolysis, gasification and hydrothermal liquefaction for valorisation of sludge into bio-fuel thus reducing the issues related to surplus generation and accumulation of sludge in environment affecting human health followed by rapid depletion of energy resources. The paper addresses working mechanism of thermochemical processes, their implementation for sludge conversion to bio-fuel and common factors affecting the process efficiency. Various studies have proved possible potential of thermochemical techniques for conversion of sludge to bio-fuel obtaining a high yield of bio-fuel and syngas. However, few technical challenges are still there that requires further studies and understanding for a better commercialization on industrial-scale and subsequently the future perspectives have also been analysed. Data collected from existing studies revealed that hydrothermal liquefaction has the efficiency to be proved better than other thermochemical technologies for proper valorisation of sludge resulting in high bio-fuel yield.

We have performed a comparative analysis of the bio-oil produced by thermal liquefaction of microalgae in different solvents using high-resolution Orbitrap mass spectrometry and GC-MS approach. Water, methanol, ethanol, butanol, isopropanol, acetonitrile, toluene, and hexane were used as solvents in which the liquefaction was performed. It was observed that all resulting oils demonstrate a considerable degree of similarity. For all samples, compounds containing 1 and 2 nitrogen atoms dominated in the positive ESI spectra, while a relative contribution of other compounds was small. In negative ESI mode, compounds having 2 to 7 oxygens were observed. Statistical analysis revealed that products can be combined in two groups depending on the solvent used for the liquefaction. To the first group, we can attribute the products obtained by using protic (alcohols) and to the second by using aprotic (acetonitrile, toluene) solvents. Nevertheless, based on our results, we concluded that solvent possesses a minor impact on molecular composition of bio-oil. We suggested that the driving force of the liquefaction reaction is the thermal dehydration of the carbohydrate in algae, resulting in water formation, which could be the trigger of the producing of bio-oil. To prove this hypothesis, we performed the reaction with the dry algae in the absence of the solvent and observed the formation of bio-oil. Graphical Abstract.