Fate and reuse of nitrogen-containing organics from the hydrothermal conversion of algal biomass

Hydrothermal (HT) conversion is a promising and suitable technology for the generation of biofuels from microalgae. Besides the fact that water is used as a “green” reactant and solvent and that no biomass drying is required, the technology offers a potential nutrient source for microalgae culture using an aqueous effluent very rich in essential inorganic nutrients. However, upon continuous and multiple recycling of this HT effluent, the recalcitrant organic fraction is likely to increase and may potentially attain toxic thresholds for microalgae use. In this work, we show the presence of recalcitrant N-containing organic compounds (NOC's) in the HT effluent. The most prominent NOC's in the extracts were carefully examined for their effect on microalgae, namely 2-pyrrolidinone and β-phenylethylamine (β-PEA). The first set of experiments consisted in testing these two substances at three different concentrations (10, 50 and 150 ppm) using three different microalgae strains: Phaeodactylum tricornutum, Chlorella sorokiniana and Scenedesmus vacuolatus. The confirmed half maximal inhibitory concentration (IC50) was approximately 75 ppm for all tested species. In the second set of experiments, P. tricornutum was grown using diluted HT effluent. Experimental conditions were set by adjusting the nitrogen concentration in the HT effluent to be equal to a known commercial medium. The concentrations of specific NOC's were lowered to concentrations of 8.5 mg/L 2-pyrrolidinone and 0.5 mg/L β-PEA after dilution. The growth of P. tricornutum using the diluted HT solution was kept constant with no evidence of inhibition or consumption of NOC's, as the concentration of the specific compounds remains the same before and after growth. Therefore, in order to avoid effects of accumulation of NOC's upon continuous recycling, the HT effluent was pumped through the existing hydrothermal gasification unit as a water clean-up step. The conversion of NOC's to ammonium was successfully achieved

Extraction of carotenoids from Chlorella vulgaris using green solvents and syngas production from residual biomass

A combined process for carotenoids extraction and efficient bioenergy recovery from the wet microalgae biomass is proposed. High added-value products could thus be extracted prior a hydrothermal gasification of the algal biomass into synthetic natural gas. The economic sustainability of biofuel production from algal biomass as well as the large energy demands of microalgae cultivation and harvesting is addressed in this paper. Two green solvents, ethanol and 2-methyltetrahydrofuran (MTHF), were used to achieve the maximum extractability of selected carotenoids. Pure MTHF was tested for the first time as an alternative renewable solvent for carotenoid extraction from wet biomass and promising results were obtained (30 min at 110 °C), with 45% of total carotenoids being extracted. The energy content of the residual biomass corresponds to a High Heating Value (HHV) of 18.1 MJ/kg. With a 1:1 mixture of both MTHF and ethanol, more carotenoids were extracted from wet biomass (66%) and the remaining HHV of the residual biomass was 15.7 MJ/kg. The perspectives of combined carotenoid extraction and energy recovery for a better microalgae valorization are discussed

Extraction of carotenoids from Chlorella vulgaris using green solvents and syngas production from residual biomass

A combined process for carotenoids extraction and efficient bioenergy recovery from the wet microalgae biomass is proposed. High added-value products could thus be extracted prior a hydrothermal gasification of the algal biomass into synthetic natural gas. The economic sustainability of biofuel production from algal biomass as well as the large energy demands of microalgae cultivation and harvesting is addressed in this paper. Two green solvents, ethanol and 2-methyltetrahydrofuran (MTHF), were used to achieve the maximum extractability of selected carotenoids. Pure MTHF was tested for the first time as an alternative renewable solvent for carotenoid extraction from wet biomass and promising results were obtained (30 min at 110 °C), with 45% of total carotenoids being extracted. The energy content of the residual biomass corresponds to a High Heating Value (HHV) of 18.1 MJ/kg. With a 1:1 mixture of both MTHF and ethanol, more carotenoids were extracted from wet biomass (66%) and the remaining HHV of the residual biomass was 15.7 MJ/kg. The perspectives of combined carotenoid extraction and energy recovery for a better microalgae valorization are discussed

Algal cellulose, production and potential use in plastics: Challenges and opportunities

The development of new biodegradable plastic materials from renewable sources is a major challenge for plastic industries to provide sustainable alternatives to petroleum plastic. Bio-based plastics are generally obtained from photosynthetic biomass such as higher plants, crops and more recently algae. This review aims to summarise the current state of bioplastic production in general with particular emphasis on algal cellulose and its derivatives (nanocellulose) for bioplastic applications. Despite the potential of algae as a feedstock for nanocellulose, little information is available on strain selection regarding cellulose content and downstream processing. This study lists possible optimization opportunities to increase the cellulose yield of algal biomass and the current status of its conversion to nanocellulose. Moreover, the findings of this review provide insight into existing knowledge and future direction in the algal cellulosic bioplastic domain based on algal bioplastic life cycle assessment studies. Finally, the review gives an overview of the main standards used for biodegradability certifications in view to limit the access to the biodegradable label when the required quality is not reached

A combined hydrothermal gasification - solid oxide fuel cell system for sustainable production of algal biomass and energy

Hydrothermal gasification (HTG) is a promising technology that allows the recovery of nutrients from wet biomass with simultaneous production of an energy-rich gas. In this work, wastewater effluent (leachate) obtained from a composter sanitary system operating without external connection to sewer pipes was used as a feedstock for HTG process. The leachate effluent was characterized by its high moisture and inorganic content. Mainly H2 rich gas was obtained from the HTG of the leachate at 600 °C, 28 MPa with almost full recovery of nitrogen and phosphorus; up to 74.4% and 92% respectively. For an efficient power-generation system with low emissions, experimental results combining solid oxide fuel cells (SOFC) with the obtained gas from the HTG were performed. Thermodynamic calculations were performed on the gas compositions to evaluate the performance and the risk of solid carbon formation at a typical SOFC operation temperature 750 °C. Furthermore, for nutrient recycling purposes, the obtained nutrient rich effluent from the gasification was used as a growth medium for microalgae Chlorella vulgaris. Finally, a complete valorization chain based on both experimental study and model prediction that combine, energy conversion and microalgae valorization was investigated