Biofuel Production from Hydrothermal Liquefaction of Algae and Its Subsequent Upgrading

Abstract

Algae are considered as a promising feedstock for biofuel production. The conversion of algae to biofuel was investigated in this study. The study was concentrated in two main areas: hydrothermal liquefaction of algae into bio-oil, and catalytic upgrading of the bio-oil produced from hydrothermal liquefaction of algae to improve the chemical properties of the bio-oil. First, hydrothermal liquefaction (HTL) of nine different algae species was performed to understand the influence of their biochemical composition on product yields and properties at two reaction temperature (280 and 320oC). The biochemical composition of the selected algae species showed a broad range of lipids (13 to 55 wt.%), carbohydrates (9 to 54 wt.%) and proteins (7 to 63 wt.%). The bio-oil yields obtained at 320oC were higher than that obtained at 280oC. The maximum bio-oil yield (66 wt.%) was obtained from the HTL of high lipid containing algae Nannochloropsis sp. at 320oC. A predictive relationship between bio-oil yields and biochemical composition was developed and showed a broad agreement between predictive and experimental yields. The HTL bio-oils obtained from nine algae species were characterized for higher heating value (HHV), total acid number (TAN), ash content, moisture content, boiling point distribution, and elemental composition. The heating values of the bio-oils ranged from 31-36 MJ/kg. The maximum percentage of the bio-oils was in the vacuum gas oil range while high lipid containing algae Nannochloropsis sp. contained a significant portion (33-42%) in the diesel range. The aqueous phase from HTL had a high amount of TOC (12-43 g/L) and COD (35-160 g/L) and showed the potential for carbon recovery. On the other hand, the high amount of ammonium (0.34-12 g/L), and phosphate (0.7-12 g/L) showed the possibility of nitrogen, phosphate and magnesium recovery via struvite production. The bio-oil produced from HTL of algae cannot be used as “drop-in” fuel or blended with petroleum crude because it has high nitrogen content, high oxygen content and it is highly viscous. Thus, upgrading of the bio-oil is necessary to make it applicable as fuel. Second, upgrading of bio-oil produced from HTL of Nannochloropsis sp. was performed. Upgrading was performed with five different catalysts (Ni/C, ZSM-5, Ni/ZSM-5, Ru/C and Pt/C) at two reaction temperatures of 300 and 350oC at a weight hourly space velocity (WHSV) of 0.51 g/gcat.h. The upgraded bio-oil yields obtained were higher at 300oC when compared to 350oC. However, better quality fuel was obtained at 350oC. The maximum upgraded bio-oil yield (61.5 wt.%) at 350oC was obtained using Ni/C, and the lowest yield (47.19 wt.%) was obtained using ZSM5. Among the different catalysts used, Ru/C and Pt/C gave a better-quality fuel. Around 35-40% of the upgraded bio-oils were in the diesel range, and no vacuum residue fraction was found. Hydrogen consumption was the highest for noble metal catalysts. Overall, the catalytic upgrading of algae bio-oil was effective in improving the quality of the bio-oil. Comparing the upgraded bio-oil obtained at 350oC (our study) with the other published literature, which used higher temperatures (>400oC), the properties of the upgraded bio-oil were observed to be similar. However, the drawback of our study was the use of longer residence time (10 h). Residence time is one of the process parameters that is directly related to the energy input and affects the overall process economics. Thus, in our third study, the effect of residence time (2, 4, 6 and 10 h) on the upgrading product yields and its properties were investigated. Upgrading was performed at 350oC using 5% Ru/C catalyst at a catalyst loading of 16.67 wt.%. The maximum upgraded bio-oil yield (60.20 wt.%) was observed at 4 h residence time. The properties of the upgraded bio-oils improved with the increase in residence time. The maximum higher heating value (44.32 MJ/kg), the lowest TAN, viscosity, and nitrogen content was observed at 10 h. However, the maximum energy recovery (44.37%) was obtained at 4 h residence time. The maximum hydrogen consumption (39.87 mg/g of bio-oil) was observed at 10 h residence time. Also, the effect of a binary mixture of CO2 and H2 cold pressure on the upgrading of the bio-oil was investigated. Upgrading was performed at 350oC for 4 h residence time using 5% Ru/C as a catalyst at a catalyst loading of 16.67 wt.%. The cold pressures of the binary mixture were 100 psi CO2 + 900 psi H2, 200 psi CO2 + 800 psi H2, and 300 psi CO2 + 700 psi H2. The upgraded bio-oil yield and properties obtained at 300 psi of CO2+700 psi of H2 were similar to that obtained using 1000 psi of H2 (without CO2), except for the TAN and HHV. The higher heating value decreased with the introduction of CO2 cold pressure and was lowest at 300 psi of CO2+700 psi of H2. Similarly, TAN increased with the introduction of CO2. The concentration of H2S in the gaseous fraction increased with the increase in CO2 pressure. The use of CO2 in the upgrading reaction did not significantly change the upgraded bio-oil quality, but its incorporation in the upgrading system added benefit in terms of process safety as it expands the non-explosive regime. Keywords: Algae, Hydrothermal liquefaction, Upgrading, Catalysis, Supercritical CO2.