Microalgae are considered as suitable feedstocks for next-generation biofuel production because of their fast growth rates and prolific yields. Moreover, growing algae has less impact on land-use for food production compared with grain and other lignocellulosic biomass. However, current lipid-to-biodiesel technology primarily focuses on utilizing high-lipid algae, which usually has lower biomass productivities than low-lipid algal strains. In addition, biodiesel production from algae often requires drying of the algal biomass followed by solvent and/or mechanical extraction, which are both energy intensive processes.Hydrothermal liquefaction (HTL) has been demonstrated to be an effective technology to produce bio-crude oil from different biowastes with high moisture content. Therefore, it is suitable to convert algae into bio-crude oil. In this study, two fast-growing, low-lipid, high-protein microalgal species, Chlorella pyrenoidosa and Spirulina platensis, were converted into four products via HTL: bio-crude oil, aqueous product, gaseous product, and solid residue.Effects of the operating parameters, including reaction temperature, retention time and initial pressure on HTL product yields and bio-crude oil quality were investigated. Bio-crude oil yield increased with temperature: it increased from 0.4% to 39.8% as temperature increased from 100°C to 320°C for Chlorella, and increased from 9.7% to 37.3% as temperature increased from 200°C to 300°C for Spirulina. Gaseous product yield, mainly consisting of CO2, also increased with temperature, but the rate of increase was low at temperatures from 240°C to 300°C. Aqueous product yield, which represents the water-soluble fraction of HTL products, first increased and then slightly decreased as temperature increased. Higher reaction temperature resulted in substantially lower yield of solid residue. Solid residue yields were both lower than 5% for Chlorella and Spirulina at 300°C. At five temperature levels (200°C, 220°C, 240°C, 260°C, and 280°C), bio-crude oil yield increased by prolonging retention time. The highest bio-crude oil yield was obtained at 280°C with 120 minutes retention time. Initial pressure provided by nitrogen had a negligible effect on HTL product yields. At 240°C and 280°C with 30 minutes retention time, the effects of five metal catalysts and two alkaline catalysts on HTL of microalgae were investigated. Effects of catalysts at 280°C were more prominent than at 240°C. The highest increase of bio-crude oil yield by adding catalysts compared with uncatalyzed tests was 10% at 280°C. Carbon deposition and mineral mixing was found on the surface of metal catalysts after hydrothermal treatment.Carbon recovery (CR), nitrogen recovery (NR) and energy recovery (ER) in the bio-crude oil fraction increased with reaction temperature and retention time. For instance, from 100°C to 300°C with 30 minutes retention time, CR of bio-crude oil produced from Chlorella increased from 0.1% to 55.3%, NR increased from 0.4% to 23.4%, and ER increased from 0% to 64.1%. Both carbon and nitrogen tended to preferentially accumulate in the bio-crude oil as temperature and retention time increased, but the opposite was true for the solid residual product. The NR value of the 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 fraction when reaction temperature was higher than 220°C and retention time was longer than 10 minutes. At 240°C, with the presence of alkaline catalysts, the CR of bio-crude oil and aqueous product both increased, but the increase of CR of bio-crude oil was mainly due to the increase of its yield. At the same time, NR of bio-crude oil also increased with additions of alkaline catalysts. At 280°C, CR of bio-crude oil increased with the addition of different catalysts. The addition of metal catalysts could decrease the NR of the aqueous product. The energy consumption ratio (ECR), decreased from 0.42 to 0.30 for Chlorella, and from 20.20 to 0.29 for Spirulina, as the temperature increased from 200°C to 300°C, which implies that HTL becomes more energetically favorable at higher temperatures. The fossil energy balance (FEB) value of producing algae bio-crude oil via HTL is higher than that of corn ethanol, algae-biodiesel and petroleum gasoline. The energy balance of algae bio-crude oil is comparable with lignocellulosic ethanol in terms of similar FEB values. The fraction of hydrocarbons, cyclic oxygenates and heterocyclic compounds in bio-crude oil increased with reaction temperature. Some intermediates produced from protein hydrolysis, which can be dissolved in water, degraded at higher temperatures due to their instabilities under hydrothermal conditions. Fraction of amides and N&O heterocyclic compounds in bio-crude oil increased as retention time was prolonged. At the same time, the fraction of organic acids decreased. Additions of heterogeneous catalysts increased the fraction of hydrocarbons as well as decreased the fraction of organic acids in bio-crude oil. The fraction of N&O heterocyclic compounds in bio-crude oil was largely increased with additions of alkaline catalysts. A first-order kinetic model was established for microalgae decomposition under hydrothermal conditions. The activation energy for Chlorella and Spirulina decomposition was 29.8 kJ∙mol-1 and 24.0 kJ∙mol-1, respectively. A simplified reaction pathway including interactions among intermediates produced from protein, lipid and carbohydrates was proposed.
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Hydrothermal liquefaction of low-lipid microalgae to produce bio-crude oil
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