Hydrothermal processing of microalgae

Microalgae are regarded as a promising biomass resource for the production of biofuels and chemicals which does not compete with food production. Microalgae contain large amounts of lipids and have faster growth rates than terrestrial biomass. One of the current technological
bottlenecks of biofuels conversion is the economic extraction and processing of microalgae components. Due to their aquatic nature microalgae contain large amounts of water when harvested. Hydrothermal liquefaction (HTL) involves processing the algae as a slurry in hot
compressed water, avoiding drying of the wet feedstock. This is a major energy benefit compared to dry microalgae processing methods.

A detailed characterisation of the microalgae feedstocks investigated for the current work is provided. The main differences between marine, fresh water and cyanobacteria strains are presented. The microalgae strains are investigated for biochemical composition, proximate and
ultimate analysis, thermo-gravimetrical analysis, pyrolysis GC-MS, metal content, pigment analysis and by scanning electron microscopy. The results from the characterisation work are employed throughout the thesis for mass balance calculations and investigation of reaction chemistry.

HTL for bio-crude production is investigated both on laboratory batch systems and a continuous pilot scale facility. Processing at mild conditions results in mainly the lipids of microalgae being extracted resulting in a high quality bio-crude. Higher temperatures are shown to result in higher yields of bio-crude as carbohydrates and proteins increasingly contribute to bio-crude formation. This allows processing of low lipid containing microalgae
which are associated with higher growth rates. Maximum bio-crude yields of around 50 wt.% can be achieved but can contain significant amounts of nitrogen and oxygen. A total of 11 microalgae strains is investigated leading to an average bio-crude yield of 34 %, a heating value of 36 MJ/kg, a nitrogen content of 4.7 wt.% and an oxygen content of 13.6 wt.%.

The use of homogeneous and heterogeneous catalysts is investigated to increase bio-crude quality and yields. Model compounds of protein, lipid and carbohydrates are processed individually to shed light on the HTL behaviour of microalgae components and the reaction pathways involved in bio-crude formation. The effect of sodium carbonate and formic acid as homogeneous catalysts is investigated on various microalgae strains with changing biochemical composition and on model compounds separately. It is shown that biochemical components of microalgae behave additively in respect to bio-crude formation. The trends of
bio-crude formation follow lipids>protein>carbohydrates. It is further shown that carbohydrates are best processed in alkali conditions while protein and lipids are best
processed without the use of catalysts. The same effect is demonstrated for algae high in carbohydrates or proteins and lipids respectively. Heterogeneous catalysts are shown not to increase the bio-crude significantly but result in additional decarboxylation of the bio-crude to
reduce the oxygen level by a further 10%.

The process water composition from HTL is investigated for common nutrients required for algae cultivation. It is shown that nutrients are present in higher concentrations than comparable standard algae growth media. The process water also contains large amounts of organic carbon which is considered a loss, unavailable for bio-crude formation. Growth trials in dilutions of the process water to grow fresh algae demonstrate that growth is sustainable.
The organic carbon in the process water is shown to act as a substrate for mixotrophic growth resulting in increased growth rates and carbon efficiency.

For analysis of the algae obtained from small scale growth trials a new analysis technique for microalgae composition analysis is introduced. This involves Py-GC-MS of model compounds and comparisons to algae pyrolysis products. Promising results are presented, showing the feasibility of detecting protein, carbohydrate and lipid levels of microalgae directly from growth cultures. Additionally the methodology is expanded to detect phytochemical
concentrations such as astaxanthin and chlorophyll a.

An alternative to direct hydrothermal liquefaction involving removal of valuable compounds from microalgae by hydrothermal microwave processing (HMP) is investigated. HMP is shown to remove protein and large amounts of nutrients from the algae biomass which could
be used as a source of nutrients for microalgae cultivation. The cells walls are shown to be
disrupted, leading to increased recovery of lipids by solvent extraction while the lipids‟ degree
of saturation is not affected. This allows effective extraction of high value poly-unsaturated
fatty acids. The residue form HMP is processed using flash pyrolysis and HTL for bio-crude production. The results show that bio-crudes of increased quality are produced. The technique appears especially suitable for marine microalgae strains as the salt content acts as microwave
absorbers, reducing energy consumption and increasing reaction rates.

Overall, the experimental work shows that hydrothermal processing is a low energy intensive wet processing technique for microalgae to produce bio-fuels and chemicals.