Developing a marine biorefinery for the production of fuels and fertilizer from the hydrothermal liquefaction of macroalgae

Abstract

Hydrothermal liquefaction (HTL) is a promising, low-energy route for the conversion of wet biomass, such as macroalgae, to bio-crude oils, which can be upgraded to advanced biofuels. Co-products of HTL, such as the nutrient-rich aqueous phase, can also be valorised within a biorefinery paradigm. This project sought to explore the effects of feedstock variation and quality on the products from the HTL process.
Initially, a comprehensive screen of a wide range of species of UK macroalgae specific to the South West was undertaken, encompassing all three major macroalgae classes and the correlation between biomass biochemical composition and HTL reactivity was assessed. The complexity of interactions occurring under HTL conditions meant that a simple additive model based on crude biochemical breakdown was insufficient to account for reactivity across all species and predict bio-crude yields. Macroalgae belonging to the genus Ulva gave the highest yields of bio-crude, and would be expected to be a promising feedstock for an HTL biorefinery based in the South West of the UK.
Although Ulva presented a promising HTL feedstock in the UK, geographical and environmental effects are known to affect the biochemical composition of macroalgae. As such, the impact of geographical variability on the production of bio-crude from a single species of macroalgae was assessed. One of the highest bio-crude producers from the UK, Ulva intestinalis, was selected and sampled across a 1,200 km stretch of Swedish coastline before being processed using HTL. Geographical variability in macroalgae composition was substantial across the sampling spectrum, including between sites a short distance apart, resulting in significant levels of variation in bio-crude yield and aqueous phase product composition. As such, suitable feedstock species for future biorefineries will need to be individually assessed for each location, even for locations within relatively close proximity.
A functioning macroalgal biorefinery will also need to have the capacity to handle multiple marine pollutants, including marine plastics. In order to understand the effect of plastics on HTL processing, the effect of simultaneous processing of UK macroalgae with common marine plastic pollutants was assessed. Thermally stable plastics polyethylene and polypropylene were unreactive under the conditions tested, but were more readily degraded under HTL conditions in the presence of macroalgae, and synergistic effects between biomass and plastic conversion were observed. Synergistic effects were also observed for nylon 6, which almost completely depolymerised under HTL conditions to generate the caprolactam monomer, which may constitute an additional revenue source within a marine biorefinery.
Finally, the concept of implementing HTL as a route to simultaneous remediation of marine plastics and algal blooms in the developing world was investigated. Bloom-forming macro- and microalgae harvested in Vietnam were co-liquefied with plastics using similar protocols to those implemented for the UK macroalgae. Due to geographical variation in macroalgae composition, synergistic effects between macroalgae and plastic conversion were stronger than those observed for UK biomass, producing bio-crudes in higher yields and with better fuel properties.
Geographical variability plays a substantial role in dictating feedstock quality and influencing HTL outcomes, and different species are likely to be optimal for different biorefinery locations. The inevitable presence of marine plastic pollutants can affect HTL, but can, in some cases, be beneficial for bio-crude yields and properties. Ultimately, HTL has been demonstrated to be a highly promising route to generating value from marine macroalgal resources, including marine plastic pollutants.

Date of Award	13 Feb 2019
Original language	English
Supervisor	Chris Chuck (Supervisor)