The overall aim of the thesis is to investigate the production of bio-crude from microalgae via hydrothermal liquefaction in a sustainable way. It is desirable for the bio-crude to be of a quality which will allow direct combustion as a heavy fuel or upgrading in conventional refineries.
Therefore, a low nitrogen content is favourable as this will decrease NOX emissions upon combustion and require less hydrogen for upgrading via hydrogenation. Additionally a low boiling point range is favourable over high molecular weight fractions as these are of higher commercial value and are associated with better pour behaviour. In order to achieve this aim of producing bio-crude from microalgae a number of objectives with associated goals are outlined below and were investigated in this thesis.
The first chapter provides an introduction to the subject area covered within this thesis. The concept of biomass for bioenergy is explained and the general aspects of biomass and its associated advantages and disadvantages are discussed. The conversion routes available for processing and utilisation of biomass and current research and development are briefly presented. Microalgae as a source of third generation biofuels is introduced. There are a number of different options in cultivation, extraction and processing microalgae which are covered in Chapter 1. The introduction shows the relevance of the work covered in this thesis and the concept of hydrothermal processing of microalgae is introduced. A detailed study of the literature on hydrothermal liquefaction, carbonisation and gasification was performed for the purpose of this thesis. The published literature allows a deeper understanding of the work carried out, identifies areas of research that have been covered and the bottlenecks that require further investigation.
Chapter 3 describes the methodology used. The main objective of this chapter is to describe the methods used allowing others to replicate the experiments. Further, it is important for readers to have an understanding of the sample workup and analysis for the same reason. Each instrument used is described with the manufacturers name and model code. The chemical and physical operation of the instruments is not covered in this chapter as this would be beyond the scope of this thesis. Additionally this information is publicly available and most readers interested in the current work are expected to have some initial knowledge of the technologies involved.
The microalgae feedstocks used in this thesis are presented in Chapter 4. Due to the large number of different microalgae strains and properties investigated, not every parameter is presented for each strain. All the data is however attached in separate data sheets in APPENDIX A. The data in
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Chapter 4 is presented in such a way that the main distinctions in different types of microalgae become apparent. The significance of this is then discussed. The objective of the characterisation is to lay the groundwork of the thesis which is referred back to throughout the thesis. The characterisation of the feedstock is fundamental to the understanding of all the experiments carried out in the subsequent chapters.
Chapter 5 describes the hydrothermal processing of microalgae in water without catalyst. The objective of this chapter was to assess different microalgae strains and operating conditions. In order to achieve this objective the majority of microalgae strains were processed at standard conditions to allow a comparative study of their hydrothermal liquefaction behaviour. Following this, the influence of residence time and operating temperature on hydrothermal liquefaction is investigated. This parametric study determines the optimum operating conditions for maximum bio-crude yield. The bio-bio-crude was additionally analysed for its elemental composition and HHV to assess the bio-crude quality. The process waters resulting from the HTL of microalgae are also analysed in this chapter for common anions, cations, TN, TOC, TIC and pH. This is later referred to in Chapter 8 which investigates nutrient recycling from the process water for microalgae cultivation.
Chapter 6 describes the HTL of microalgae in the presence of catalysts. The objective of this chapter was to increase yields and/or improve the quality of bio-crudes. The chapter is split in two sections; the first includes the use of homogeneous catalysts while the second includes the use of heterogeneous catalysts. The homogenous catalysts include alkali or organic acids. The heterogeneous catalysts include transition metals and precious metals adsorbed on silica and alumina. The effects on yields and bio-crude quality are assessed for both types of catalysts and discussed. The effect of homogeneous catalysts on the nitrogen and carbon distribution in the product fractions was investigated. Analysis of the process water was carried out to evaluate if the homogeneous catalysts affect the concentration of nutrients. The effect of heterogeneous catalysts on the lipid fraction degradation was examined in detail. The fate of triglycerides and free fatty acids and the potential of deoxygenating these to straight chain hydrocarbons was investigated. The yields and composition of bio-crudes produced was investigated and conclusions could be drawn concerning the bio-crude quality by elemental analysis and GC-MS.
Chapter 7 the mechanistic pathways of bio-crude production from microalgae are investigated in detail in order to understand bio-crude formation pathways in more detail. 4 microalgae strains and 7 model compounds were hydrothermally processed in water alone and sodium carbonate and
41 formic acid. The 7 model compounds include several protein, carbohydrate, lipid and amino acid samples as these are the main constituents of microalgae. By processing these and analysing the bio-crude, conclusions could be drawn on the HTL of microalgae. The four selected microalgae differ in biochemical composition; this facilitated comparative examination with results from the model compounds and sheds light on the bio-crude formation and composition from microalgae.
The bio-crudes were analysed for total yield and oil composition and by GC-MS so that typical compounds present in microalgae bio-crudes could be linked to their biochemical origin. The effect of the catalysts was studied on the model compounds and microalgae. Conclusions were drawn on the effects of alkali and acidic catalysts on the HTL behaviour of model compounds and microalgae.
The objective of Chapter 8 was to investigate the use of process water as a source of nutrients for microalgae cultivation. For the purpose of these experiments, the process waters produced at different operating conditions and using different microalgae strains were analysed for its concentration of nutrients. Growth trials of 4 different microalgae were performed using the process water derived from HTL. Growth was assessed by various methods such as chlorophyll a absorbance and cell count and the results compared to growth in standard growth media. The influence of common growth inhibitors such as phenols and nickel are investigated and discussed.
During the cultivation trial used to investigate the nutrient recycling a drawback in the analytical methodologies was identified. The small scale growth trials used result in small amounts of harvested microalgae. This makes analysis by the previously employed techniques (described in Chapter 3) impossible. Therefore, in Chapter 9, a new analytical technique based on Pyrolysis GC-MS was developed to analyse the microalgae allowing analysis of small amounts of sample.
The technique involves pyrolysing less than 1 mg of microalgae and separating the pyrolysis products by GC-MS. By fingerprinting common model compounds of microalgae, marker compounds could be identified for each biochemical component. This has allowed comparison of the chromatograms from microalgae grown under different conditions and sheds light on the composition of the different strains. The new analysis technique was also assessed as a means to determine concentrations of specific high value compounds found in microalgae.
Chapter 10 includes research carried out in collaboration with the University of Sydney, Australia.
Here a state of the art continuous hydrothermal pilot plant processing facility has been used. This allowed experiments to be performed on a large scale (20-40 l/h) for two microalgae strains over the course of the ten week collaborative visit through the WUN network. This was the first time continuous processing has been employed for microalgae on a hydrothermal liquefaction reactor.
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The reactor performance was investigated in regard to feedstock and operating conditions. The results concerning the yields and bio-crude quality are compared to experiments of the same feedstock processed in batch reactors. The objective of this work was to assess the feasibility of continuous processing of microalgae in a HTL reactor in order to make bio-crude production possible on an industrial scale.
In Chapter 11 the use of hydrothermal microwave processing as a pre-treatment method is investigated. The technique is expected to be especially suited for high ash containing marine microalgae samples as the metals in the ash act as microwave absorbers, lowering the activation energy and reducing the energy consumption. Microwave irradiation was assessed as a technique for extraction of protein, polysaccharides and other valuable phytochemicals. Additionally, the technique was evaluated as a pre-treatment for bio-fuel production by hydrothermal processing and flash pyrolysis. The fate of nitrogen during pre-treatment and during bio-fuel production was of particular interest.
The conclusions of the experimental sections are presented in Chapter 12. The overall conclusions of each chapter are discussed separately even though an overall summary on the feasibility of hydrothermal processing for biofuels and chemicals is presented. The limitations of the research performed in this thesis are identified and further work is discussed.
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