Hydrothermal liquefaction of macroalgae for the production of renewable biofuels

Abstract

In all biorefinery systems, excess water represents a key challenge, and its removal by drying is often a necessary and crucial pre-treatment. Second-generation feedstocks have often fallen at this hurdle, particularly microalgae-derived biomasses, which require extensive (and costly) dewatering. Hydrothermal liquefaction (HTL) has gained increasing attention in recent years as a technology that uses the water present in the feedstock as a versatile reaction medium, which functions as a solvent, reactant, and catalyst for a cascade of organic reactions. Converting organic biomasses into oil, aqueous, solid, and gas fractions, the development of HTL provides the opportunity to exploit previously unsuitable biomasses in a versatile bio-refinery approach. Marine macroalgae (seaweeds) offer a sustainable source of renewable biomass, which require no land or freshwater to cultivate or harvest. With 70% of the surface of the planet covered in seawater and levels of eutrophication increasing, seaweeds are an underutilized resource with excellent potential for relieving the pressure on fossil resources. Hitherto, this exploitation has been hindered by a lack of suitable and economical processing tools. Here we review the potential for applying HTL to processing marine macroalgae and discuss the potential products and services that can be derived from this potential biorefinery system.

Progress in Hydrothermal Liquefaction of Algal Biomass and Hydrothermal Upgrading of the Subsequent Crude Bio-Oil: A Mini Review

“…It can be seen that a reaction time beyond 30 min is found to be unfavorable for the CBO yield. This limit of reaction time was stressed by Raikova et al to be 15 min. This difference can be ascribed to the other parameters including the biochemical composition of the alga biomass and reaction temperature.…”

“…80 It should be noted that heating rates, reaction times, and reaction temperatures are closely related and inseparable from each other, and thus, slow heating (and cooling) rates lead to long reaction times. 87 High reaction temperatures required a longer heating period. It can be seen that a lower initial pressure is favorable for a high yield of CBO.…”

“…This difference can be ascribed to the other parameters including the biochemical composition of the alga biomass and reaction temperature. In fact, above 30 min reaction time, energy-dense CBO molecules undergo condensation and/or polymerization by interacting with intermediates to form heavy, tarlike substances that will be transferred to the biochar. , The decrease of the CBO yield with increasing reaction time can be attributed to the secondary reaction of solid char with condensable gases in the reactor . It should be noted that heating rates, reaction times, and reaction temperatures are closely related and inseparable from each other, and thus, slow heating (and cooling) rates lead to long reaction times .…”

Key Targets for Improving Algal Biofuel Production

“…Microwave assisted HTL has also been examined [109], as has the application of HTL using macroalgae [110]. However, the economy of the process is still questionable due to the utilization of higher pressure conditions.…”

An integrated biorefinery to produce 5-(hydroxymethyl)furfural and alternative fuel precursors from macroalgae and spent coffee grounds

“…9 Several authors have demonstrated that macroalgae can be used in the hydrothermal liquefaction process. 10 Raikova et al presented a comprehensive study on a broad range of macroalgae species from the UK and the ideal hydrothermal liquefaction conditions to convert these into bio-crude and nutrient partition into the aqueous phase. 11 In a further study the authors also combined plastics present in the ocean with the macroalgae demonstrating that this led to a higher biocrude heating value.…”