Co-production of hydrochar, levulinic acid and value-added chemicals by microwave-assisted hydrothermal carbonization of seaweed

Seaweed is considered a third-generation biomass for transport biofuel production with great potential for bioenergy and biorefinery applications. Microwave-assisted hydrothermal carbonization (HTC) offers a conversion route for seaweed valorization to multiple value-added products. However, the HTC products (such as hydrochar and process liquor) are highly dependent on feedstock properties and HTC conditions. This study investigated the valorization of Laminaria digitata to a range of products (primarily including hydrochar, levulinic acid and 5-hydroxymethylfurfural) through microwave-assisted HTC at a range of low temperatures (160, 180 and 200 °C) and levels of dilute acid addition (1, 2 and 4% H₂SO₄ concentration). The work suggests an optimized process at a temperature of 200 °C with 4% H₂SO₄, with levulinic acid and 5-hydroxymethylfurfural in the process liquor achieved the highest yields of 12.5% (12.5 g per 100 g dry L. digitata) and 5.8%, respectively. The obtained hydrochar had a higher heating value of 25.5 MJ/kg volatile solids, comparable to lignite coal. At such a condition the total carbon recovery in the produced hydrochar and process liquor accounted for 84% of carbon in the original seaweed. The 200 °C derived hydrochar presented the highest carbon content of 65.4%, but the lowest abundance in oxygen-containing functional groups compared to the hydrochars obtained under other conditions (160 and 180 °C). The economic benefits of the process might be significantly increased if hydrochar was upgraded into activated carbon instead of being used as a solid fuel.