Hydrothermal Treatment of Algal Biomass: from Batch to Continuous Pilot Plant Operations

Abstract

In the search for large-scale production of renewable fuels and chemicals, hydrothermal liquefaction (HTL) of biomass has emerged as one of the most promising routes. Whilst this conversion route is suitable for various feedstocks, there has been enormous attention directed towards the utilisation of algae. The aim of this thesis is to gain a better understanding of the process scaling effects and to enhance the quality of the bio-crude oil product. A novel feature of this work is that it has successfully tested the HTL process in a continuous-flow (15–70 L/h) pilot-scale plant – by doing so, it has been able to shed light on the process engineering and scaling aspects. A maximum bio-crude yield of ~42 wt.% was achieved from a commercially-available microalga (Chlorella) while being processed with a 10 wt.% solids loading at 350 °C, 200 bar and 3 min residence time. The results indicate that maximal product yields may be obtained in much shorter times under continuous flow hydrothermal processing than batch studies have previously suggested. Collaboration with algal cultivation specialists also allowed links between biomass production and subsequent conversion to be explored. Promising results for a robust microalgae polyculture grown under waste streams from a coal-fired power station are presented and discussed. The opportunity of tailoring the algal growth process, through nutrient starvation, to improve the bio-crude properties was also investigated. Oils obtained from starved Oedogonium macroalgae were shown to contain considerably lower N levels (<2%), compared to those from normal Oedogonium as well as microalgae (>4%). A two-step batch HTL process, also aimed at producing low-N oil, is additionally put forward. In summary, a number of key contributions are contained in this thesis which included providing insights into scaling the HTL process, as well as employing novel approaches to enhance the quality of the derived products.