Hydrothermal Liquefaction of Algae for Bio-oil Production
Type of Degreethesis
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Increased demand of energy, depletion of fossil fuel resources, global warming and energy security issues have led to extensive research in renewable energy source. Among the different renewable energy sources, biomass is the only renewable energy source that can be converted to liquid fuels. This research is focused on the conversion of aquatic biomass (microalgae) to produce bio-oil through hydrothermal liquefaction pathways. Hydrothermal liquefaction (HTL) is a thermochemical process in which hot compressed water at a sub- or super-critical stage is used as a reaction medium to convert biomass to bio-oil and co-products (char, aqueous phase and gas). It is mostly suited for wet biomass such as algae. In this study, effects of reaction temperatures and algal strains on HTL product yields were studied with and without using a catalyst (sodium carbonate). HTL was performed on three different algae strains viz. Nannochloropsis, Pavlova and Isochrysis at three reaction temperatures of 250, 300 and 350oC, a holding time of 60 minutes and an algae loading of 14 solid wt.%. The selected algae strains were different from each other in terms of their biochemical composition. Although all the strains used had high protein content, Nannochloropsis had the highest protein content with a low carbohydrate and moderate lipid content, whereas Pavlova had high amount of carbohydrate in comparison to other algae. Isochrysis had a relatively equal distribution of biochemical composition. The non-catalytic study showed that temperature influenced the liquefaction yields; an increase in temperature increased bio-oil and gas yield but reduced solid residue and water soluble product However the variation of product yields depended on the algae strain. Maximum bio-oil yield (48.67 wt.%) was obtained for high protein containing algae strain (Nannochloropsis) at 350oC. The bio-oil produced had high HHV (33-35MJ/kg), moderate pH (6-9), low density (900-950 kg/m3), high TAN (30-65 mg KOH/g) and low moisture content (5-10 wt.% on wet basis). Apart from high nitrogen contents (4-6 wt.%), the elemental analysis resembled that of petroleum crude. A GC study showed that with the increased temperature organic acid content decreased while phenolic content increased. Nitrogen containing compounds like cyclic nitrogen and amides were also present in the bio-oil. A carbon and nitrogen balance showed that most of the carbon was distributed between the bio-oil (30-68 wt.%) and water soluble product (3-25 wt.%). The water soluble product also had a high initial nitrogen distribution, ranging from 3 to 37 wt.% with an increase of temperature. Use of sodium carbonate had a significant role in decreasing the bio-oil yield of protein rich algae and increased the yield of carbohydrate rich algae. Maximum bio-oil yield (47.05 wt.%) was obtained for algae rich in carbohydrate (Pavlova) at 350oC. Physical analysis of the catalytic run resembled that of non-catalytic runs. The chemical composition of bio-oil obtained from catalytic runs at lower reaction temperatures had lower organic acid content and higher hydrocarbons resulting in a lower TAN compared to bio-oil from non-catalytic runs. In addition more cyclic compounds were present in catalytic runs. Water soluble product from catalytic runs contained 5 to 25 wt.% of total initial nitrogen and 6 to 21 wt.% of total initial carbon. Overall, the use of sodium carbonate did not influence bio-oil characteristics, but it did have a significant role in increasing or decreasing bio-oil yield depending on the algae composition. Keywords: Hydrothermal liquefaction, Nannochloropsis, Isochrysis, Pavlova, Algae, Bio-oil.