Influence of hydrothermal pre-treatment on biofuel production from micro algae

Algae are fast becoming a sought-after resource for the production of biofuels and platform chemicals. Macro algae contain high levels of carbohydrates and micro algae contain large amounts of lipids and have much faster growth rates, in comparison to terrestrial biomass. The main issues surrounding biofuels from algae is the processing methods, amount of upgrading required, and the costs associated with this.
Hydrothermal processing is an emerging biomass pre-treatment method, which at temperatures below 200°C, produces a carbonised material and also releases organic and inorganic material into the process waters. At temperatures between 200 and 375°C liquefaction occurs and produces an oil. This work sets out to investigate how to improve the quality of bio-oil produced from algae by studying the fate of heteroatoms, mineral content and biochemical components during hydrothermal processing.
The results show that hydrothermal pre-treatment results in solid algal residues of a higher energy density than the raw algae, with HHV ranging from 12 to 32 MJ/kg (d.a.f.). The process waters from hydrothermal pre-treatment are rich in organic and inorganic material and can be recycled into hydrothermal liquefaction as they are, or after being cleaned using Mg modified bio-chars.
Three conversion routes; pyrolysis, solvent extraction and hydrothermal liquefaction, of the raw and hydrothermally pre-treated algae, are investigated to establish which conversion routes produce the most appropriate bio-oils for use as biofuels. The bio-oils from hydrothermal liquefaction show lower nitrogen and phosphate content and higher HHV, in comparison to the bio-oils from pyrolysis and solvent extraction. Therefore, hydrothermal liquefaction is used as the conversion route for the remainder of the thesis.
Comparison of the bio-crudes from the raw and pre-treated micro algae; autotrophic and heterotrophic Chlorella vulgaris, Spirulina platensis and Chlorogloeopsis fritschii, show that different biochemical components react differently when liquefied. The lipids and proteins mostly contribute to the bio-crude, with some of the carbohydrates contributing to the bio-crude but also being broken down into sugars and acids and released into the process waters.
The effect of formic acid during hydrothermal liquefaction is investigated on the autotrophic Chlorella vulgaris. The addition of formic acid has little effect on the bio-crude yield, but results in further decarboxylation of the bio-crude, which reduces the oxygen content of the bio-crude.
Overall, the results show that the quality of bio-crude from algae can be improved and the process waters from hydrothermal pre-treatment can be valorised during hydrothermal liquefaction.