Characterization of Biofuels Produced from Hydrothermal Liquefaction of Algae, Its Subsequent Upgrading and Fractional Distillation

Abstract

Crude petroleum is the major source of transportation fuels in the world whose reserves are declining day by day. It has become imperative to look for an alternative that can either replace or substitute the current energy demand and prolong its extinction. Biomass is the only source of renewable energy that can be converted into liquid hydrocarbon fuels. Algae, among various biomass, has a clear advantage because of high productivity, high carbon dioxide sequestration rate and ability to grow in the non-arable land. Algae being an aquatic biomass is well suited for Hydrothermal liquefaction (HTL), a thermochemical process that under hot compressed water conditions can convert wet biomass into biocrude with other by-products being solid char, aqueous phase, and gases and evade intensive energy required for drying in other processes. Filamentous algae are cheaper to grow as compared to microalgae as it is easier to harvest them. Apart from that, algae require a high amount of nitrogen nutrients for their growth which is substituted with cheap source could prove to be economical. The research objective was to compare the product yield and properties obtained from hydrothermal liquefaction of filamentous and microalgae. The filamentous algae were grown in five different nitrogen nutrient conditions that were: sufficient nitrate (A_NO3), sufficient urea (A_Urea), 14- and 21-days starved nitrate (14_NO3 and 21_NO3) and 14 days starved urea (14_Urea). All HTL experiments were carried out at 320˚C for 30 mins residence time and algae loading of 15 wt% with the rest being DI water. Highest oil yield of 64.2 wt% was obtained from 14_Urea which had the highest FAME of 53.2 wt% while 21_NO3 having high carbohydrate content of 57.1 wt% produced the highest char yield of 32.1 wt%. The oil yield for A_Urea, A_NO3, and A_Micro were 44, 34.3, and 49.9 wt%, respectively. Highest heating value (HHV) of HTL oil from all algae ranged from 30.5 to 34.5 MJ/kg. Total acid number (TAN) was lowest for A_Micro (31.36 mg KOH/g) while for other non-stressed algae it was around 40 mg KOH/g. TAN for stressed algae oil was slightly above 100 mg KOH/g which was because of the presence of around 50% of Hexadecanoic acid as observed in GC-MS of oil. The aqueous phase was rich in nutrients with aqueous phase from nitrate algae having about 50% of its nitrogen in the form of ammonium ion. The concentration of heteroatom, like nitrogen (2-5 wt%) and oxygen (11.5-21 wt%) prevents the bio-crude to be used as fuels. In order to reduce heteroatom concentration and TAN values and improve energy density upgrading of oil is necessary. The effect on upgraded oil from the use of polar or aromatic solvents, for product separation, has not been accounted for after upgrading experiments. This study presents a detailed analysis on the yield and properties of oil obtained from upgrading with and without the presence of the catalyst (5% Ru/C and H2 only) and effect of toluene and dichloromethane (DCM) as product separating solvent. Further atmospheric pressure fractional distillation of bio-oil and upgraded oils was carried out to estimate the yield and properties of distilled fractions. Hydrothermal liquefaction bio-crude from Nannochloropsis sp. microalgae was used in this study as other filamentous algae were not available. Mass yields, TAN, HHV, the elemental and chemical composition was evaluated for each oil. Catalytic upgraded oil extracted from toluene had an oxygen content of 0.75 wt% and HHV of 43.36 MJ/kg while DCM extracted oil had 5.93 wt% oxygen and HHV of 37.72 MJ/kg. The catalyst showed better activity for denitrogenation and nitrogen values were not all that different between different solvents. Fractionation produced three distilled fractions (F1 <220˚C, F2 220-350˚C, and F3 >350˚C). Light and middle fractions from toluene extracted upgraded oil had better fuel properties than DCM extracted upgraded oil. Middle and heavy fractions had higher heating value than starting oil for all the treatment conditions. Nitrogen was found to be distributed in the heavy fraction in all cases and prominently as nitrile compounds. Highest heating value of 45.18 MJ/kg was obtained for F2_RuC_Tol which and had TAN value of 1.38 mg KOH/g. Hydrothermal liquefaction of filamentous algae grown on cheap urea produced oil with similar yields and properties as oil from expensive microalgae. This could bring down the cost of production of bio-crude from algae. For upgrading experiments, different solvents during product separation affect the yield and quality of oil produced and do not solely depend on process parameters and activity of the catalyst. Fractional distillation produced distilled fractions with different concentration of heteroatom distribution for which appropriate catalyst could be used for effective denitrogenation or deoxygenation and avoid catalyst poisoning because of the presence of other heteroatoms.