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

Pilot-scale continuous flow hydrothermal liquefaction of mixed textile waste and subsequent bio-oil upgrading

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Hydrothermal liquefaction of ocean plastics from the Great Pacific Garbage Patch

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Hydrothermal liquefaction of food waste: optimization and kinetic modelling

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Comparison of hydrochar fractionation and composition in batch and continuous hydrothermal liquefaction

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Energetic and material valorization of digestate via hydrothermal liquefaction

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Wet Oxidation as an enabling technology for hydrthermal liquefaction

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Multilayer plastic film chemical recycling via sequential hydrothermal liquefaction

Multi-material layered plastic films are used in the food packaging industry due to their excellent properties; however they cannot be mechanically recycled. In this study, a two-stage hydrothermal liquefaction (HTL) process is proposed and tested for chemical recycling of a two-layer film made of LLDPE-PET. Experimental results showed that after a first subcritical stage at 325 ◦C, 94% of terephthalic acid (TPA) is recovered from the PET fraction as a solid and 47% of ethylene glycol in the aqueous phase. The unconverted PE was then used as feedstock for a subsequent supercritical HTL stage at 450 ◦C for 90 min, achieving mass yields of 47% and 29% in a naphtha-gasoline oil and in an alkane-rich gas, respectively. In conclusion, this work proved that a sequential HTL procedure can be used for chemical recycling of multilayer plastics, allowing the recovery of PET monomers to be recycled back to the PET industry and a paraffinic oil and hydrocarbon-rich gas phase that could be used as feedstock for steam cracking to produce virgin materials

Two-stage hydrothermal liquefaction for multilayer plastic valorization

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Hydrothermal liquefaction of organic waste streams on a continuous pilot scale reactor

Hydrothermal liquefaction (HTL) is a promising technology for biofuel production and treatment of organic wastes and biomass. Due to the wet nature of the process where biomass is heated in an aqueous slurry at 350°C and 200 bar, wet biomass and wet wastes are particularly suited for the process. The current study investigates the utilization of wastewater treatment sludges and other organic wastes for the production of sustainable petroleum replacement products. The work has been carried out on a pilot scale continuous hydrothermal liquefaction reactor with a novel oscillating flow system and heat exchanger. The influence of these are discussed in terms of heat recovery and operability of the plant. The reactor was run at 50 L/h with maximum solids loadings of ~25% and short residence times of80% was accomplished, leading to an energy efficient process. During operation of the HTL system, approximately 5 units of energy are created in the form of bio-crude for every unit of energy invested for heating and pumping the slurry (EROI\u3e5). We present and discuss the results of processing diverse samples ranging from high ash (sewage sludge), lignocellulosics (miscanthus) and manure to microalgae. The potential of mixing different waste biomasses such as sludge and lignocellulosics, plastics and lignocellulosics is explored during this research and synergistic effects on bio-crude yields and fuel quality are observed, leading to higher carbon and energy recoveries. Water phase recycling of the HTL process water was employed during the liquefaction of pine where a significant increase in bio-crude yields, energy recovery and energy return on investment could be achieved. Initial results on bio-crude upgrading via catalytic hydrotreatment are also presented, demonstrating the feasibility of the HTL process as a viable pathway towards drop in replacement fuels. The current presentation gives a realistic insight into the processing of diverse biomass feedstocks at pilot scale, showing the potential of the technology while areas for future development and bottlenecks are highlighted