The HTU 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.

Microalgae strains of Chlorella vulgaris, Scenedesmus dimorphus, and the cyanobacteria Spirulina platensis and Chlorogloeopsis fritschii were processed in batch reactors at 300°C and 350°C. The biocrude yields ranged from 27-47 wt%. The biocrudes were of low O and N con­tent and high heating value, making them suitable for further processing. Growth occurred in heavy dilutions where the amounts of growth inhibitors were not too high. The results show that the closed-loop system using the recovered aqueous phase offers a promising route for sustainable oil production and nutrient management for microalgae (Biller et al., 2012).

Hydrothermal liquefaction (300°C and 10-12 MPa) was used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil. Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280-360 Da; hydrothermal liquefaction 700-1330 Da) and the percentage of low boiling com­pounds (bp <400°C) (pyrolysis 62-66%; hydrothermal liquefaction 45-54%). Analysis of the energy consumption ratio (ECR) also revealed that for wet algal biomass (80% moisture con­tent), hydrothermal liquefaction is more favorable (ECR 0.44-0.63) than pyrolysis (ECR 0.92­1.24) due to required water volatilization in the latter technique (Vardon et al., 2012).

Yu et al (Yu et al., 2011) studied the conversion of a fast-growing, low-lipid, high-protein microalgae species, Chlorella pyrenoidosa, via hydrothermal liquefaction into four products: biocrude oil, aqueous product, gaseous product, and solid residue. The effects of operating conditions (reaction temperature and retention time) on the distributions of carbon and nitro­gen in hydrothermal liquefaction products were quantified. Carbon recovery (CR), nitrogen recovery (NR), and energy recovery in the biocrude oil fraction generally increased with the increase of reaction temperature as well as the retention time. The highest-energy recovery of biocrude oil was 65.4%, obtained at 280°C with 120 min retention time. Both carbon and ni­trogen tended to preferentially accumulate in the hydrothermal liquefaction biocrude oil products as temperature and retention time increased, but the opposite was true for the solid residual product. The NR values of hydrothermal liquefaction aqueous product also in­creased with reaction temperature and retention time. 65-70% of nitrogen and 35-40% of car­bon in the original material were converted into water-soluble compounds when reaction temperature was higher than 220°C and retention time was longer than 10 min. The CR of gas was less than 10% and is primarily present in the form of carbon dioxide.

Garcia et al. used the freshwater microalgae Desmodesmus sp. as feedstock for HTU 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-1. At increasing temperature, both the oil yield and the nitrogen content in the oil increased. A pioneering visual inspection of the cells after HTU shows a large step increase in the HTU oil yield when going from 225-250°C at 5 min reaction time, which coincided with a major cell wall rupture under these conditions. Additionally, it was found that the oil components, by extractive re­covery after HTU 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 HTU. From the results obtained, the au­thors concluded that HTU 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 (Garcia et al., 2012).

Vardon et al. studied the influence of wastewater feedstock compounds on hydrothermal liquefaction biocrude oil properties and physicochemical characteristics. Spirulina algae, swine manure, and digested sludge were converted under hydrothermal liquefaction conditions (300°C, 10-12 MPa, and 30 min reaction time). Biocrude yields ranged from 9.4% (digested sludge) to 32.6% (Spirulina). Although similar higher heating values (32.0-34.7 MJ kg-1) were estimated for all product oils, more detailed characterization revealed significant differences in biocrude chemicals. Feedstock components influenced the individual compounds identified as well as the biocrude functional group chemicals. Molecular weights tracked with obdurate carbohydrate content and followed the order Spirulina < swine manure < digested sludge (Vardon et al., 2011).

Valdez et al. performed hydrothermal liquefaction of Nannochloropsis sp. at 350°C for 60 min and analyzed the gas, crude bio-oil, dissolved aqueous solids, and insoluble residual solids product fractions. Most of the carbon and hydrogen in the algal biomass appear in the crude bio-oil product, as desired. A majority of the original nitrogen appears as ammonia in the aqueous phase. They used both nonpolar solvents (hexadecane, decane, hexane, and cy­clohexane) and polar solvents (methoxycyclopentane, dichloromethane, and chloroform). Hexadecane and decane provided the highest gravimetric yields of bio-oil (39 ± 3 and 39 ± 1 wt%, respectively), but these crude bio-oils had a lower carbon content (69 wt% for decane) than those recovered with polar solvents such as chloroform (74 wt%) and dichloromethane (76 wt%). Fatty acids were the most abundant components, but some aro­matic and sulfur — and nitrogen-containing compounds were also quantified. The amount of free fatty acids in the crude bio-oil significantly depended on the solvent used, with polar solvents recovering more fatty acids than nonpolar solvents. The bio-oil recovered with chloroform, for example, had fatty acid content equal to 9.0 wt% of the initial dry algal biomass (Valdez et al., 2011).

Biller and Ross liquefied a range of model biochemical components, microalgae, and cyanobacteria with different biochemical contents under hydrothermal conditions at 350°C, approximately 200 bar in water, 1 M Na2CO3 and 1 M formic acid. The model com­pounds include albumin and a soya protein, starch and glucose, the triglyceride from sun­flower oil, and two amino acids. Microalgae include Chlorella vulgaris, Nannochloropsis occulata, and Porphyridium cruentum and the cyanobacteria Spirulina. The yields and product distribution obtained for each model compound have been used to predict the behavior of microalgae with different biochemical composition and have been validated using microalgae and cyanobacteria. Broad agreement is reached between predictive yields and actual yields for the microalgae based on their biochemical composition. The yields of biocrude are 5-25 wt% higher than the lipid content of the algae, depending on biochemical composition. The yields of biocrude follow the trend lipids > proteins > carbohydrates (Biller and Ross, 2011).

Valdez et al. investigated hydrothermal liquefaction of Nannochloropsis sp. at different temperatures (250-400°C), times (10-90 min), water densities (0.3-0.5 g mL-1), and biomass loadings (5-35 wt%). Liquefaction produced a biocrude with light and heavy fractions, along with gaseous, aqueous, and solid byproduct fractions. The gravimetric yields of the product fractions from experiments at 250°C, summed to an average of 100 ±4wt%, shows mass balance closure at 250°C. The gravimetric yields of the product fractions are independent of water density at 400°C. Increasing the biomass loading increases the biocrude yield from 36 to 46 wt%; the yields of light and heavy biocrude depend on reaction time and temperature, but their combined yield depends primarily on temperature. Regard­less of reaction time and temperature, the yield of products distributed to the aqueous phase is 51 ± 5 wt% and the light biocrude is 75 ± 1 wt% C. Two-thirds of the N in the alga is immediately distributed to the aqueous phase, and up to 84% can be partitioned there. Up to 85% of the P is distributed to the aqueous phase in the form of free phosphate for nutrient recycling. Up to 80% of the chemical energy in the alga is retained within the biocrude (Valdez et al., 2012).

Biller et al. processed a range of microalgae and lipids extracted from terrestrial oil seed at 350°C at pressures of 150-200 bars in water using heterogeneous catalysts. The results indi­cate that the biocrude yields from the liquefaction of microalgae were increased slightly with the use of heterogeneous catalysts, but the higher heating value (HHV) and the level of de­oxygenation increased by up to 10%. Under hydrothermal conditions, the lipids from microalgae and oil seeds decompose to fatty acids and are hydrogenated to more saturated analogues. The use of heterogeneous catalysts causes an increase in deoxygenation of the biocrude. The Co/Mo/Al2O3 and Pt/Al2O3 appear to selectively deoxygenate the carbohy­drate and protein fractions, whereas the Ni/Al2O3 deoxygenates the lipid fraction. This is illustrated by the presence of alkanes for the Ni/Al2O3 catalyst. The use of a Ni/Al2O3 catalyst also appears to promote gasification reactions (Biller et al., 2011).

Microalgae can be converted to an energy-dense bio-oil via pyrolysis; however, the rela­tively high nitrogen content of this bio-oil presents a challenge for its direct use as fuels. Therefore, hydrothermal pretreatment was employed to reduce the N content in

Nannochloropsis oculata feedstock by removing proteins without requiring significant energy inputs. The effects of reaction conditions on the yield and composition of pretreated algae were investigated by varying the temperature (150-225°C) and reaction time (10-60 min). Compared with untreated algae, pretreated samples had higher carbon contents and enhanced heating values under all reaction conditions and 6-42% lower N contents at 200-225°C for 30-60 min. The pyrolytic bio-oil from pretreated algae contained less N-containing compounds than that from untreated samples, and the bio-oil contained mainly (44.9% GC-MS peak area) long-chain fatty acids (C14-C18), which can be more readily converted into hydrocarbon fuels in the presence of simple catalysts (Du et al., 2012).

Schuping et al. investigated the hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake under various liquefaction temperatures, holding times, and catalyst dosages. It was observed that the maximum bio-oil yield of 25.8% was obtained at a reaction temper­ature of 360°C and a holding time of 50 min using 5% Na2CO3 as a catalyst. The bio-oil is com­posed of fatty acids, fatty acid methyl esters, ketones, and aldehydes. Its empirical formula is CH1.44O0 .29 lue is 30.74 MJ kg 1. The bio-oil product is a possible eco­

friendly green biofuel and chemical (Shuping et al., 2010).

Ross et al. aimed to investigate the conditions for producing high-quality, low-molecular — weight biocrude from microalgae and cyanobacteria containing low lipid contents including Chlorella vulgaris and Spirulina. The influence of process variables such as temperature (300°C and 350°C) and catalyst type has been studied. Catalysts employed include the alkali, potas­sium hydroxide and sodium carbonate, and the organic acids, acetic acid and formic acid. The yields of biocrude are increased using an organic acid catalyst; produced biocrude has a lower boiling point and improved flow properties. The biocrude contains a carbon content of typ­ically 70-75% and an oxygen content of 10-16%. The nitrogen content in the biocrude typi­cally ranges from 4% to 6% and the HHV range was from 33.4 to 39.9 MJ kg-1. Analysis by GC/MS indicates that the biocrude contains aromatic hydrocarbons, nitrogen heterocy­cles, and long-chain fatty acids and alcohols. A nitrogen balance indicates that a large propor­tion of the fuel nitrogen (up to 50%) is transferred to the aqueous phase in the form of ammonium. The remainder is distributed between the biocrude and the gaseous phase, the latter containing HCN, NH3, and N2O, depending on catalyst conditions. The addition of organic acids results in a reduction of nitrogen in the aqueous phase and a corresponding increase of NH3 and HCN in the gas phase. The addition of organic acids has a beneficial ef­fect on the yield and boiling-point distribution of the biocrude produced (Ross et al., 2010).

Shen et al. studied the application of microalgae to the production of acetic acid under hy­drothermal conditions with H2O2 oxidant. Results showed that acetic acid was obtained with a good yield of 14.9% based on a carbon base at 300°C for 80 s with 100% H2O2 supply. This result should be helpful to facilitate studies for developing a new green and sustainable pro­cess to produce acetic acid from microalgae, which are the fastest-growing sunlight-driven cell factories (Shen et al., 2011).

The hydrothermal method includes adding dried and pulverized algae raw material to 0.05-0.15 M base solution or 0.05-0.15 M acid solution, soaking at room temperature for at least 20 h, and adding the soaked liquid and modified natural mordenite catalyst at a mass ratio of 1: 0.02-0.05 to a pressure reactor. The base solution is NaOH, KOH and/or sodium carbonate solution, and the acid solution is sulfuric acid, acetic acid, and/or formic acid (Hu et al., 2011).