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The understanding of the chemical nature of the oil is important for both the optimization of the process and the design of upgrading strategies for further use as an energy carrier or toward transportation fuels. Hydrothermal treatment (HTT) oil is a complex matrix, whose composition is strongly affected by the feedstock type and by the HTT experimental conditions. In the present work, HTT oil from Desmodesmus sp. was subjected to a detailed chemical analysis. Various characterization techniques (silica gel chromatography, methanolysis, size exclusion chromatography, analytical pyrolysis, elemental analysis, and thermogravimetric techniques) were coupled to gather clearer information on the chemical nature of HTT oil obtained at different reaction times, temperatures, and slurry concentrations. Special attention was paid to the fate of N in the HTT process and the nature of the N-containing species in the oil. By cross-checking results from the chemical characterization of the oil with process data, it was finally possible to identify some different competitive reactions involved in the formation of HTT oil at different conditions. Results show that main compounds obtained at low temperature are still classifiable as lipids, which are extractable without the HTT, together with some short chain algaenan and some hydrophobic protein fragments that end up in the organic solvent phase. At higher temperature (300–375 °C), proteins and cellulose started to break down, giving cyclic dipeptides and amino acids side chains (by pyrolysis-like reactions), carbohydrates derivatives (e.g., furans) and products from the cross reaction of proteins and carbohydrates (e.g., formation of alkyl-pyrrolidinones, pyrazines, pyrroles and melanoidin-like materials). This phenomenon is responsible for the observed increase in oil mass yield with increasing processing temperature, as well as the increase in nitrogen content of the oil. Optimization of the production of fuels and fuel precursors by HTT should be done in conjunction with evaluation of downstream processing options and/or the possibility to recycle unconverted material to the algae cultivation.

The hydrothermal treatment (HTT) technology is evaluated for its potential as a process to convert algae and algal debris into a liquid fuel, within a sustainable algae biorefinery concept in which, next to fuels (gaseous and liquid), high value products are coproduced, nutrients and water are recycled, and the use of fossil energy is minimized. In this work, the freshwater microalgae Desmodesmus sp. was used as feedstock. HTT was investigated over a very wide range of temperatures (175–450 °C) and reaction times (up to 60 min), using a batch reactor system. The different product phases were quantified and analyzed. The maximum oil yield (49 wt %) was obtained at 375 °C and 5 min reaction time, recovering 75% of the algal calorific value into the oil and an energy densification from 22 to 36 MJ/kg. At increasing temperature, both the oil yield and the nitrogen content in the oil increased, necessitating further investigation on the molecular composition of the oil. This was performed in the adjacent collaborative paper with special attention to the nitrogen-containing compounds and to gain insight in the liquefaction mechanism. A pioneering visual inspection of the cells after HTT showed that a large step increase in the HTT oil yield, when going from 225 to 250 °C at 5 min reaction time, coincided with a major cell wall rupture under these conditions. Additionally, it was found that the oil composition, by extractive recovery after HTT below 250 °C, did change with temperature, even though the algal cells were visually still unbroken. Finally, the possibilities of recycling growth nutrients became evident by analyzing the aqueous fractions obtained after HTT. From the results obtained, we concluded that HTT is most suited as post-treatment technology in an algae biorefinery system, after the wet extraction of high value products, such as protein-rich food/feed ingredients and lipids.

In this study, we present the gasification of microalgae (Chlorella Vulgaris) and glycerol in supercritical water (SCW) using batch (quartz capillaries) and continuous flow reactors. Preliminary tests of algae gasification were done with quartz capillaries at varying operating conditions such as temperature (400-700 °C), reaction time (1-15 min), and the addition of catalysts. The dry gas composition of uncatalyzed gasification of algae in SCW mainly comprised of CO 2 , CO, CH 4 , H 2 , and some C 2 -C 3 compounds. Higher temperatures, low algae concentrations, and longer residence times favored the algae gasification efficiency (GE). The addition of catalysts to the capillaries resulted in higher yields of hydrogen and lower CO yields via enhanced water-gas shift activity. The addition of catalysts accelerated the gasification efficiency up to a maximum of 84% at 600 °C and 2 min reaction time with nickel-based catalysts. Complete gasification is achieved at higher temperatures (700 °C) and with excess amounts of (Ru/TiO 2 ) catalyst. To elucidate part of the difficulties related to the SCWG of algae, reforming of a model compound (here glycerol) in SCW was carried out in a continuous flow reactor in the presence of additives like amino acids (L-alanine, glycine, and L-proline) and alkali salt (K 2 CO 3 ) and combinations thereof. The amino acids L-alanine and glycine have a minor effect on the gasification process of glycerol, and a significant reduction of the gasification efficiency was observed in the presence of L-proline. Coke formation and colorization of the reactor effluent were more noticeable with glycerol-amino acid mixtures. In the absence of amino acids, the glycerol solution gasified without any coke formation and colorization of the reactor effluent. Again this effect was more pronounced in the presence of L-proline. The addition of K 2 CO 3 enhanced the glycerol gasification efficiency and increased the hydrogen yields promoting the water-gas shift reaction.

In this paper, we have investigated the possibilities to steer the composition and, thus, the quality of pyrolysis liquids by the reactor temperature and the pyrolysis vapor condenser temperature. Pine wood was pyrolyzed in a 1 kg/h fluidized-bed pyrolysis reactor operated at 330 or 480°C. The pyrolysis vapors produced were condensed using a condenser train of two countercurrent spray columns arranged in series. In this paper, the temperature of the first condenser was varied between 20 and 115°C, while the second condenser temperature was kept at 20°C. To describe the composition of the oils, we have integrated several analytical techniques into a novel characterization scheme that can account for 77À82 wt % of the oils. The effects of the condensation conditions on fractions of light compounds in the oils can be predicted with a simple equilibrium stage condensation model. It has been observed that pyrolysis at 330°C gives a light oil with a low amount of mid-boilers [normal boiling point (nbp) of 150À300°C] and heavy compounds (water insolubles and mono-and oligosugars). Sugars, mid-boilers, and water-insoluble ligninderived oligomers are more present in the oil obtained at 480°C, while the yields of light organics are approximately the same for 330 and 480°C. It can be concluded that fractional condensation is a promising cheap downstream approach to concentrate compounds (classes) and, thus, to control the quality of pyrolysis oils. For instance, operating the first condenser around 70À90°C gives an aqueous liquid in the second condenser containing 40 wt % light organics, which are interesting for extraction (e.g., 10 wt % acetic acid) and supercritical water gasification to produce hydrogen. Under these conditions, the oils from the first condenser have a high content of sugars (20 wt %) and lignin-derived oligomers (40 wt %), which are attractive fractions for fermentation/sugar chemistry and gasoline production via fluidized catalytic cracking (FCC)/hydrotreatment, respectively.