Hydrothermal Liquefaction of Microalgae and Subsequent Bio-Crude Catalytic Upgrading

Abstract

This PhD research project aimed to study the hydrothermal liquefaction of microalgae with subsequent catalytic upgrading of HTL-derived bio-crudes. Five macroalgae (Ascophyllum nodosum, Fucus serrarus, Fucus vesiculosus, Himanthalia elongate and Ulva lactuca) and three microalgae (Arthrospira platensis, Isochrysis galbana and Nannochloropsis sp.) were characterised as a potential feedstock for hydrothermal processing. The results from the characterisation work are utilised for feedstock screening and as starting points for reaction pathways and bio-crude formation. Preliminary HTL experiments for macro and microalgae species included the utilisation of a 25 ml batch reactor followed by larger-scale processing of model components and microalgae in a 300 ml batch reactor over a temperature range of 200-350 oC. Microalgae were found to produce higher bio-crude yields compared to macroalgae species. HTL processing at higher temperatures resulted in higher bio-crude yields with the highest bio-crude yield of 51.6 wt.% achieved at a liquefaction temperature of 300 °C for the microalgae species Nannochloropsis sp.. The bio-crudes were found to have high levels of nitrogen and oxygen which negatively affects the bio-crudes utility as a transportation fuel. Model components (glucose, xylose, oleic acid, stearic acid, L-glutamine, L-leucine and soy protein isolate) chosen to represent the main macromolecular compounds in microalgae were introduced to thermal degradation studies both by the analytical pyrolysis (Py-GC-MS) and HTL processing in a 300 ml batch reactor in order to gain insight into macromolecular reaction pathways. Bio-crude from the HTL treatment of microalgae, model compounds and catalytically upgraded algal oils were analysed using a novel combination of advanced NMR techniques for the chemical characterisation. These included two-dimensional techniques of Heteronuclear Single Quantum Coherence (HSQC), Heteronuclear Multiple Bond Correlation (HMBC) and the spectral edited 13C pulse sequence known as PENDANT (Polarisation Enhancement During Attached Nucleus Testing). The utilisation of these advanced techniques allowed for the characterisation of the whole bio-crude and upgraded liquids samples, not just the fraction that are able to elute from a gas chromatography (GC) column prior to the mass spectrometry (MS) detection. Upgrading of the HTL algal bio-crude was investigated, applying catalysts (Pd/C, Pt/C, Ru/C, HZSM-5 and NiMo) was investigated via supercritical water upgrading at 450 oC. Upgrading resulted in improvements in atomic ratios and HHV. The nitrogen content of the upgraded oils were reduced to levels of 1.9-2.6 wt. %. Oxygen content was dramatically reduced, from 19.9 wt. % to values ranging between 1.3-4.8 wt. %. Analysis of the upgraded oil revealed the content was predominantly composed of C6-C16 alkanes and substituted aromatic compounds. The techno-economic assessment (TEA) proposed using microalgae as the feed to a commercial-scale HTL processing plant designed for a throughput of 2,000 kg/h dry ash-free algae, producing primarily diesel fraction and naphtha co-product liquid fuels. Sensitivity analysis shows the process to be sensitive to feedstock cost, capital expenditure, bio-crude yield and incoming algal slurry concentration. Although technically feasible, the economic results from the TEA propose an MFSP of £2.64/litre, identifying that algae derived biofuels are far from competitive in this scenario.