Refinery ready bio-petroleum via novel catalytic hydrothermal processing of microalgae

Hydrothermal liquefaction of microalgae is recognised as a favourable route to produce renewable fuel from high moisture feedstocks such as microalgae. However, there are still some uncertainties regarding the fate of nitrogen and the best approach to reduce the level of nitrogen in the bio-crude oil.
The aim of this study is to gain a better understanding of the main degradation route of carbohydrates, lipids and proteins and use this to predict the behaviour of microalgae. To reach this goal, different model compounds were selected including vegetable oils containing different degrees of saturation, proteins and carbohydrates (sugars and polysaccharides). The results from the model and study were compared with four different microalgae; stressed and non-stressed strains of P. ellipsoidea, Chlorella v. and Spirulina. Mixtures were prepared using the same model compounds to simulate the composition of microalgae. Reaction variables such as temperature (250, 300 and 350 °C), and influence of additives such as organic acids were investigated.
As the temperature increases, the nitrogen in the bio-crude decreases in general from 250 to 300 °C; in contrast the ammonium compound concentration in the process water increases with temperature. Carbohydrates enhance the formation of carbonaceous residues. Protein and lipids enhance the formation of amides. The protein and carbohydrates enhanced the formation of “heavy molecular weight” materials. A higher yield of 52.9 wt.% of bio-crude containing 1.5 wt.% of nitrogen was achieved with the stressed P. ellipsoidea at 300 °C. The addition of organic acids affected the molecular weight distribution of the bio-crude but had little effect on the heteroatom content.
In a parallel study, the influence of metal doped HZSM-5 on liquefaction behaviour was carried out. Different metal salts were incorporated into HZSM-5 to study the effect on reducing the nitrogen content of the bio-crude. MoZSM-5 was selective in producing aromatics from sunflower oil. In general, NiZSM-5 enhanced the deoxygenation of the bio-crude, but, lower effect on the nitrogen content.