Integrated marine biorefineries for the production of advanced liquid fuels and value-added materials

Abstract

Marine biorefineries process marine-sourced biomass, such as seaweed (macroalgae) into fuels, chemicals, and new materials. Macroalgae are large multi-cellular and visible plant-like marine algae, commonly referred to as seaweeds. As a source of biomass, macroalgae represents a highly attractive feedstock for the extraction and isolation of high value biomacromolecules, as well as the conversion via thermochemical processing into fuel products, and carbonaceous solids suitable for soil amelioration and carbon sequestration. The potential applications of the marine microalgal biorefinery are well established, however there are limited reports of the technoeconomic feasibility of macroalgal biorefinery processes for the production of fuels, carbonaceous solids, or downstream high-value materials such as composite biopolymers.

Initially a hydrothermal macroalgae-to-biocrude conversion route was investigated. A general purpose macroalgal hydrothermal liquefaction (HTL) process model was developed with a combination of experimental and published data, and this model was used to design and cost a simplified large-scale industrial process with the aim of investigating the economic feasibility of producing HTL-biocrude from a macroalgae feedstock.

While the lab-scale experimental demonstration of the macroalgae-to-biocrude process has been shown previously, this work demonstrated that the minimum selling price of the corresponding bio-barrel of crude (bioBBL) would be 5 to 10 times that of currently available fossil crudes, with reasonable prices only being achieved with significant additional valorisation of wastes and legislative incentives.

Attention was therefore turned towards the using the biorefinery for production of a composite biopolymer product based primarily on the extraction of algal polysaccharides and coupling with a thermochemical conversion process for the production of carbonaceous solids from the extracted macroalgae residues as an additional value-stream. Excitingly, process models, plant design, and technoeconomic analysis of this biorefinery concept demonstrated that a bulk alginate-based polymer could be produced at cost-parity to existing biopolymer products currently available on the market.

Experimental investigations into the coupled biomacromolecule extraction biorefinery and char-production revealed the technical feasibility of combining a number of different extraction steps for isolation of different algal fractions rich in different compounds. Slow pyrolysis was demonstrated as the preferable process for production of a carbon sequestration material and soil ameliorant. However, despite numerous previous reports into suitability of alginate composites for food packaging applications, this work found poor water contact and water barrier properties of these simple polysaccharide films.

In an attempt to improve the water contact properties, agar was functionalised with a C16 fatty-acid residue. This new functionalised material was found to be practically insoluble in all common lab solvents, thus melt-flow processing was successfully demonstrated by way of rheometry assessments and extrusion processing of the functionalised material.

Finally, the end-of-life disposal routes of the demonstrated algal biopolymers was considered in aerobic composting and anaerobic digestion. In both cases, simple alginate materials were found to degrade extremely rapidly. Functionalised agar was significantly slower, however began to show consistent degradation after day 40 in the anaerobic digestion process, and day 56 in the composting trials.

This work has presented two new process modelling methods for the processing of macroalgae in the biorefinery, demonstrated that economic feasibility is achievable when the right products are targeted, and that newly produced biopolymer products are highly degradable in two common end-of-life disposal routes for organic wastes. The macroalgal marine biorefinery remains a highly attractive prospect for the production of new and alternative materials.

Date of Award	18 Jan 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Chuck (Supervisor)